WO2015161934A1

WO2015161934A1 - A lithographic apparatus, radiation source, and lithographic system

Info

Publication number: WO2015161934A1
Application number: PCT/EP2015/052043
Authority: WO
Inventors: Pieter-Jan VAN ZWOL; Andrei Mikhailovich Yakunin; Florian Didier Albin DHALLUIN
Original assignee: Asml Netherlands B.V.
Priority date: 2014-04-23
Filing date: 2015-02-02
Publication date: 2015-10-29
Also published as: TW201541196A

Abstract

A lithographic apparatus for projecting a pattern from a patterning device onto a substrate comprises a patterning device support structure constructed to support a patterning device and a substrate support constructed to hold a substrate. The apparatus is configured to receive at an input to the lithographic apparatus a beam of radiation at an operating wavelength and to direct the beam of radiation at the operating wavelength along a radiation path, such that in operation when a patterning device is supported by the patterning device support structure and a substrate is held by the substrate support a pattern from the patterning device is projected onto the substrate. The operating wavelength is one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 7.1nm, approximately 22.8nm, or in a range of approximately 22.8nm to approximately 25.2nm.

Description

A Lithographic Apparatus, Radiation Source, and Lithographic System

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to EP Patent Application No. 14165680.1, filed April 23, 2014 which is incorporated by reference herein in its entirety.

FIELD

[0002] The present invention relates to a lithographic apparatus, radiation source and lithographic system, in particular a lithographic apparatus, radiation source and lithographic system for using or providing radiation at wavelengths below 40nm.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. Lithographic apparatus which use EUV radiation with a wavelength of 13.5 nm are commercially available, and provide smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm). Lithographic apparatus which use a wavelength of 6.7 nm have also been proposed.

[0005] It is an object of the invention to provide a lithographic apparatus which overcomes or mitigates a disadvantage associated with lithographic apparatus known from the prior art.

SUMMARY

[0006] In a first, independent aspect of the invention there is provided a lithographic apparatus for projecting a pattern from a patterning device onto a substrate, the lithographic apparatus comprising a patterning device support structure constructed to support a patterning device and a substrate support constructed to hold a substrate, wherein the apparatus is configured to receive at an input a beam of radiation at an operating wavelength and to direct the beam of radiation at the operating wavelength along a radiation path, such that in operation when a patterning device is supported by the patterning device support structure and a substrate is held by the substrate support a pattern from the patterning device is projected onto the substrate; and the operating wavelength is one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 17.1nm, approximately 22.8nm, or in range of approximately 22.8nm to approximately 25.2nm.

[0007] A wavelength of approximately 4.37nm may, for example, comprise any wavelength in a range 4.37nm + 0.5nm, optionally any wavelength in a range 4.37nm + 0.3nm, optionally a wavelength in a range 4.37nm + O. lnm, optionally a wavelength substantially equal to 4.37nm.

[0008] A wavelength of approximately 9.49nm may, for example, comprise any wavelength in a range 9.49nm + lnm, optionally any wavelength in a range 9.49nm + 0.5nm, optionally a wavelength in a range 9.49nm + 0.3nm, optionally a wavelength substantially equal to 9.49nm.

[0009] A wavelength of approximately 10.5nm may, for example, comprise any wavelength in a range 10.5nm + lnm, optionally any wavelength in a range 10.5nm + 0.5nm, optionally a wavelength in a range 10.5nm + 0.3nm, optionally a wavelength substantially equal to 10.5nm.

[00010] A wavelength of approximately 11.3nm may, for example, comprise any wavelength in a range 11.3nm + 0.5nm, optionally any wavelength in a range 11.3nm + 0.3nm, optionally a wavelength substantially equal to 11.3nm.

[00011] A wavelength of approximately 17.1 nm may, for example, comprise any wavelength in a range 17. lnm + 0.5nm, optionally any wavelength in a range 17. lnm + 0.3nm, optionally a wavelength in a range 17. lnm + O. lnm, optionally a wavelength substantially equal to 17. lnm.

[00012] A wavelength of approximately 22.8nm may comprise any wavelength in a range 22.8nm + 0.5nm, optionally any wavelength in a range 22.8nm + 0.3nm, optionally a wavelength in a range 22.8nm + O. lnm, optionally a wavelength substantially equal to 22.8nm.

[00013] The lithographic apparatus may comprise a plurality of multi-layer mirrors, each multi-layer mirror comprising a plurality of layers formed of a first material and a plurality of further layers formed of a second, different material. The layers and the further layers may be alternating layers, and may be provided in any suitable order such that either one of the layers or one of the further layers may be the outermost of the layers and further layers. The multi-layer mirrors may be arranged to direct radiation at the operating wavelength along at least part of the radiation path, for example a part of the radiation path between the patterning device support structure and the substrate support.

[00014] For at least one of the multi-layer mirrors the first material may comprise carbon, a nitride, oxide or other compound of carbon, or a carbon-containing alloy.

[00015] For at least one of the multi-layer mirrors the first material may comprise graphene or a graphene compound.

[00016] For at least one of the multi-layer mirrors the second material may comprise at least one of:- Li, Ti, V, Ca, Co, Cr, Mn, Fe, La, Nd, Pd, Ag, In, Ce, Ni or an oxide, nitride or other compound, or alloy, thereof. The operating wavelength may be approximately equal to 4.37nm.

[00017] For at least one of the multi-layer mirrors the first material may comprise at least one of:- lithium; an alloy, or nitride, oxide or other compound of, lithium; beryllium; an alloy, or nitride, oxide or other compound of, beryllium.

[00018] For at least one of the multi-layer mirrors the second material may comprise at least one of:- Be, Al, Si, La, B, or an alloy, or nitride, oxide or other compound, thereof. The operating wavelength may be approximately equal to 22.8nm.

[00019] The operating wavelength may be approximately equal to 4.37nm and at least one of the multi-layer mirrors may comprise C/Li, C/U, C/Ti, C/V, C/Ca, C/Co, C/Cr, C/Mn, C/Fe, C/La, C/Nd, C/Pd, C/Ag, C/In, C/Ce or C/Ni bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00020] The operating wavelength may be approximately equal to 22.8nm, or in a range 22.8nm to 25.2nm, and at least one of the multi-layer mirrors may comprise Li/Be, Li/Al, Li/Si, La/Li or B/Li bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00021] The operating wavelength may be approximately equal to 9.49nm and at least one of the multi-layer mirrors may comprise Pd/Sr, Ag/Sr, Rh/Sr, Pd/Eu, Rh/Eu or Eu/Ag bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00022] The operating wavelength may be approximately equal to 10.5nm and at least one of the multi-layer mirrors may comprise Rh/Sr, Pd/Sr, Ru/Sr, Ag/Sr, or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof. [00023] The operating wavelength may be approximately equal to 11.3nm and at least one of the multi-layer mirrors may comprise Ru/Be, Be/Rh, Nb/Be, Mo/Be, Ru/Sr, Rh/Sr, Be/Pd, Be/Zr, B/Be, Ag/Be or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00024] The operating wavelength may be approximately equal to 17.1nm and at least one of the multi-layer mirrors may comprise Al/Sr, Y/Al, Be/Al, Al/Zr, Ca/Al, Nb/Al, B/Al, Al/Si, Al/Mo, La/Al or Ti/Al bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00025] The apparatus may comprise a plurality of reflectors, wherein a calculated maximum reflectivity of at least one of the reflectors at the operating wavelength may be greater than or equal to 60%, optionally greater than or equal to 70%.

[00026] The calculated maximum reflectivity of at least one, optionally each, reflector may be in a range 60% to 99%, optionally in a range 70% to 99%. At least one of the reflectors, optionally each reflector, may comprise a multi-layer mirror. At least one of the multi-layer mirrors may comprise a multi-layer mirror selected as having a calculated maximum reflectivity at the operating wavelength of greater than or equal to 60%, optionally greater than or equal to 70%, in accordance with reflectivity values provided in the tables of Figures 7a and 7b.

[00027] The lithographic apparatus may comprise at least one absorber comprising or forming part of a reticle or resist. The reticle may comprise or form part of the patterning device. The resist may form part of the substrate or cover at least part of the substrate.

[00028] The lithographic apparatus may be configured for operation at approximately 4.37nm and the absorber may comprise Hf, Ir, Re, Os, Pt, W, Au, Ta, Mo, Cu, Ni or Zi or an alloy or compound of one or more thereof.

[00029] The lithographic apparatus may be configured for operation at approximately 9.49nm and the absorber may comprise Cu, Ni, Co, Zn, ZnCu, Fe, La, brass or other Cu-Zn alloy, W, Os, Al, Ta or Hf or an alloy or compound of one or more thereof.

[00030] The lithographic apparatus may be configured for operation at approximately 10.5nm and the absorber may comprise Cu, Ni, Co, Zn, brass or other Cu-Zn alloy, Fe, Ta, Re, Al, Hf, Os or Cr or an alloy or compound of one or more thereof.

[00031] The lithographic apparatus may be configured for operation at approximately 11.3nm and the absorber may comprise Ni, Cu, Co, Zn, Te, Fe, Ta, W, Re, Hf, Os, Pt or Al or an alloy or compound of one or more thereof. [00032] The lithographic apparatus may be configured for operation at approximately 17.1nm and the absorber may comprise Pt, Ag, Pd, Rh, Ir, Co, Ni, Os, Au, Re, Ti, Cu, W, Te, Cr, Hf, Fe or Zn or an alloy or compound of one or more thereof.

[00033] The lithographic apparatus may be configured for operation at approximately 22.8nm and the absorber may comprise Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti, Cr or Hf or an alloy or compound of one or more thereof.

[00034] The lithographic apparatus may be configured for operation at a wavelength in the range 22.8nm to 25.1nm and the absorber may comprise Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti, Cr or Hf or an alloy or compound of one or more thereof

[00035] The apparatus may comprise at least one pellicle, and the material and thickness of the pellicle may be selected to provide a transmissivity of the pellicle at the operating wavelength of greater than or equal to 90%.

[00036] The transmissivity at the operating wavelength may be in a range 90% to

99%. The pellicle or at least one of the pellicles may comprise a pellicle comprising a material listed in one of the tables of Figures 9a and 9b as having a transmissivity of at least 90% at the operating wavelength for a thickness of at least 20nm, optionally at least 50nm, optionally at least lOOnm.

[00037] The lithographic apparatus may comprise at least one pellicle comprising at least one of:- C; Ti; Sc; an alloy, or oxide, nitride, carbide or other compound of Ti or Sc; or a carbon compound or carbon-containing alloy. The operating wavelength may be approximately equal to 4.37nm.

[00038] The lithographic apparatus may comprise at least one pellicle comprising at least one of:- Al; Si; Al strengthened with B; Si, strengthened with B; an alloy or a nitride, oxide or other compound of Al; or an alloy or nitride of Si. The operating wavelength may be approximately equal to 22.8nm.

[00039] The lithographic apparatus may be configured for operation at approximately 4.37nm and the pellicle may comprise C, Ti, Sc or La or an alloy or compound including one or more thereof.

[00040] The lithographic apparatus may be configured for operation at approximately 9.49nm and the pellicle may comprise B, C, Zr, Nb, Mo, or Eu or an alloy or compound including one or more thereof.

[00041] The lithographic apparatus may be configured for operation at approximately 10.5nm and the pellicle may comprise B, Zr, C, Nb or Mo or an alloy or compound including one or more thereof. [00042] The lithographic apparatus may be configured for operation at approximately 11.3nm and the pellicle may comprise B, Zr, C, Nb or Mo or an alloy or compound including one or more thereof.

[00043] The lithographic apparatus may be configured for operation at approximately 17.1nm and the pellicle may comprise Al, Si, La, B or Zr or an alloy or compound including one or more thereof.

[00044] The lithographic apparatus may be configured for operation at approximately 22.8nm, or in a range approximately 22.8nm to 25.2nm, and the pellicle may comprise Al, Si or an alloy or compound including one or more thereof.

[00045] The lithographic apparatus may comprise an illumination system configured to condition the beam of radiation, and a projection system configured to project the patterned radiation beam onto the substrate, wherein the projection system comprises four or fewer multi-layer mirrors.

[00046] The operating wavelength may be approximately equal to 4.37nm, and the apparatus may comprise at least one multilayer mirror comprising C/Li bilayers, at least one pellicle comprising C, and at least one reticle comprising Re.

[00047] The operating wavelength may be approximately equal to 22.8nm and the apparatus may comprise at least one multilayer mirror comprising Li/Be bilayers, at least one pellicle comprising Al, and at least one reticle comprising Ru.

[00048] The operating wavelength may be approximately equal to 9.49nm and the apparatus may comprise at least one multilayer mirror comprising Pd/Sr bilayers, at least one pellicle comprising B, and at least one reticle comprising Cu.

[00049] The operating wavelength may be approximately equal to 10.5nm and the apparatus may comprise at least one multilayer mirror comprising Rh/Sr bilayers, at least one pellicle comprising B, and at least one reticle comprising Cu.

[00050] The operating wavelength may be approximately equal to 11.3nm and the apparatus may comprise at least one multilayer mirror comprising Ru/Be bilayers, at least one pellicle comprising B, and at least one reticle comprising Ni.

[00051] The operating wavelength may be approximately equal to 17.1nm and the apparatus may comprise at least one multilayer mirror comprising Al/Sr bilayers, at least one pellicle comprising Al, and at least one reticle comprising Pt.

[00052] The operating wavelength may be in a range from approximately 22.8nm to approximately 25.2nm and the apparatus may comprise at least one multilayer mirror comprising Li/Be bilayers, at least one pellicle comprising Al, and at least one reticle comprising Ru.

[00053] In a further aspect of the invention, which may be provided independently, there is provided a radiation source configured to provide a beam of radiation at an operating wavelength to at least one lithographic apparatus, the radiation source comprising a free electron laser for generating the beam of radiation, wherein the operating wavelength is approximately equal to one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 17.1nm or approximately 22.8nm, or in a range approximately 22.8nm to approximately 25.2nm.

[00054] The lithographic system may further comprise at least one optical element between the source and the lithographic apparatus, arranged to condition the beam of radiation and/or to direct the beam of radiation, wherein the optical element comprises at least one of a grazing mirror, a convex mirror, a concave mirror, an astigmatic or aspherical mirror or other reflector, or a beam splitter element.

[00055] The operating wavelength may be approximately equal to 4.37nm and the grazing mirror may comprise U, Co, Cr, Mn, Fe, V, or Ni or an alloy or compound including one or more thereof.

[00056] The operating wavelength may be approximately equal to 22.8nm and the grazing mirror may comprise Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U or Ti or an alloy or compound including one or more thereof.

[00057] The operating wavelength may be approximately equal to 9.49nm and the grazing mirror may comprise Pd, Rh, Ag, Ru, Mo, Nb, Cd, B, C, Au or Zr or an alloy or compound including one or more thereof.

[00058] The operating wavelength may be approximately equal to 10.5nm and the grazing mirror may comprise Rh, Ru, Pd, Ag, Mo, Nb, B, Zr, C or Au or an alloy or compound including one or more thereof.

[00059] The operating wavelength may be approximately equal to 11.3nm and the grazing mirror may comprise Ru, Rh, Mo, Nb, Pd, Ag, Zr, B, C, Y or Au or an alloy or compound including one or more thereof.

[00060] The operating wavelength may be approximately equal to 17.1nm and the grazing mirror may comprise Y, Zr, Nb, Sr, Mo, Be, B, Ti, U, C, Sc, Ru or La or an alloy or compound including one or more thereof. [00061] The operating wavelength may be in a range from approximately 22.8nm to approximately 25.2nm and the grazing mirror may comprise Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U, Ti or an alloy or compound including one or more thereof.

[00062] In another aspect of the invention, which may be provided independently, there is provided a multi-layer mirror for use at an operating wavelength in a lithographic apparatus, wherein at least one of:-

[00063] the operating wavelength is approximately equal to 4.37nm and the multi-layer mirror comprises C/Li, C/U, C/Ti, C/V, C/Ca, C/Co, C/Cr, C/Mn, C/Fe, C/La, C/Nd, C/Pd, C/Ag, C/In, C/Ce or C/Ni bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

[00064] the operating wavelength is approximately equal to 22.8nm and the multilayer mirror comprises Li/Be, Li/Al, Li/Si, La/Li or B/Li bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

[00065] the operating wavelength is approximately equal to 9.49nm and the multi- layer mirror comprises Pd/Sr, Ag/Sr, Rh/Sr, Pd/Eu, Rh/Eu or Eu/Ag bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

[00066] the operating wavelength is approximately equal to 10.5nm and the multilayer mirror comprises Rh/Sr, Pd/Sr, Ru/Sr, Ag/Sr, or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00067] the operating wavelength is approximately equal to 11.3nm and the multilayer mirror comprises Ru/Be, Be/Rh, Nb/Be, Mo/Be, Ru/Sr, Rh/Sr, Be/Pd, Be/Zr, B/Be, Ag/Be or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

[00068] the operating wavelength is approximately equal to 17.1nm and the multi- layer mirror comprises Al/Sr, Y/Al, Be/Al, Al/Zr, Ca/Al, Nb/Al, B/Al, Al/Si, Al/Mo, La/Al or Ti/Al bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

[00069] the operating wavelength is in a range from approximately 22.8nm to approximately 25.2nm and the multi-layer mirror comprises Li/Be, Li/Al, Li/Si, La/Li or B/Li bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

[00070] In another aspect of the invention, which may be provided independently, there is provided an absorber for use at an operating wavelength in a lithographic apparatus, wherein the absorber comprises a reticle or mask and at least one of:- [00071] the operating wavelength is approximately 4.37nm and the absorber comprises Hf, Ir, Re, Os, Pt, W, Au, Ta, Mo, Cu, Ni or Zr or an alloy or compound including one or more thereof;

[00072] the operating wavelength is approximately 9.49nm and the absorber comprises Cu, Ni, Co, Zn, ZnCu, Fe, La, brass or other Cu-Zn alloy, W, Os, Al, Ta or Hf or an alloy or compound including one or more thereof;

[00073] the operating wavelength is approximately 10.5nm and the absorber comprises Cu, Ni, Co, Zn, brass or other Cu-Zn alloy, Fe, Ta, Re, Al, Hf, Os or Cr or an alloy or compound including one or more thereof;

[00074] the operating wavelength is approximately 11.3nm and the absorber comprises Ni, Cu, Co, Zn, Te, Fe, Ta, W, Re, Hf, Os, Pt or Al or an alloy or compound including one or more thereof;

[00075] the operating wavelength is approximately 17.1nm and the absorber comprises Pt, Ag, Pd, Rh, Ir, Co, Ni, Os, Au, Re, Ti, Cu, W, Te, Cr, Hf, Fe or Zn or an alloy or compound including one or more thereof;

[00076] the operating wavelength is approximately 22.8nm and the absorber comprises Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti, Cr or Hf or an alloy or compound including one or more thereof ;

[00077] the operating wavelength is in the range approximately 22.8nm to approximately 25.1nm and the absorber comprises Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti, Cr or Hf or an alloy or compound including one or more thereof .

[00078] In another aspect of the invention, which may be provided independently, there is provided a pellicle for use at an operating wavelength in a lithographic apparatus, wherein at least one of:- the operating wavelength is approximately 4.37nm and the pellicle comprises C, Ti,

Sc or La or an alloy or compound including one or more thereof;

the operating wavelength is approximately 9.49nm and the pellicle comprises B, C, Zr, Nb, Mo, or Eu or an alloy or compound including one or more thereof;

the operating wavelength is approximately 10.5nm and the pellicle comprises B, Zr, C, Nb or Mo or an alloy or compound including one or more thereof;

the operating wavelength is approximately 11.3nm and the pellicle comprises B, Zr, C, Nb or Mo or an alloy or compound including one or more thereof;

the operating wavelength is approximately 17.1nm and the pellicle comprises Al, Si, La, B or Zr or an alloy or compound including one or more thereof; the operating wavelength is approximately 22.8nm and the pellicle comprises Al, Si or an alloy or compound including one or more thereof;

the operating wavelength is in a range from approximately 22.8nm to approximately 25.2nm and the pellicle comprises Al, Si or an alloy or compound including one or more thereof.

[00079] In another aspect of the invention, which may be provided independently, there is provided a grazing mirror for use at an operating wavelength in a lithographic system and at least one of:- the operating wavelength is approximately equal to 4.37nm and the grazing mirror comprises or an alloy or compound of U, Co, Cr, Mn, Fe, V, Ni or an alloy or compound including one or more thereof;

the operating wavelength is approximately equal to 22.8nm and the grazing mirror comprises Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U or Ti or an alloy or compound including one or more thereof;

the operating wavelength is approximately equal to 9.49nm and the grazing mirror comprises Pd, Rh, Ag, Ru, Mo, Nb, Cd, B, C, Au or Zr or an alloy or compound including one or more thereof;

the operating wavelength is approximately equal to 10.5nm and the grazing mirror comprises Rh, Ru, Pd, Ag, Mo, Nb, B, Zr, C or Au or an alloy or compound including one or more thereof.

the operating wavelength is approximately equal to 11.3nm and the grazing mirror comprises Ru, Rh, Mo, Nb, Pd, Ag, Zr, B, C, Y or Au or an alloy or compound including one or more thereof;

the operating wavelength is approximately equal to 17.1nm and the grazing mirror comprises Y, Zr, Nb, Sr, Mo, Be, B, Ti, U, C, Sc, Ru or La or an alloy or compound including one or more thereof;

the operating wavelength is in a range from approximately 22.8nm to approximately 25.2nm and the grazing mirror comprises Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U, Ti or an alloy or compound including one or more thereof.

[00080] In a further aspect of the invention, which may be provided independently, there is provided a method of projecting a pattern from a patterning device onto a substrate, the method comprising generating a beam of radiation at an operating wavelength using a free electron source, and providing the beam of radiation to a patterning device of a lithographic apparatus such that a pattern is projected from the patterning device onto the substrate, wherein the operating wavelength is one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 17.1nm, or approximately 22.8nm, or in the range approximately 22.8nm to approximately 25.2nm.

[00081] Various aspects and/or features of the invention set out above or below may be combined with various other aspects and/or features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[00082] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 is a schematic illustration of a lithographic system comprising a radiation source and a plurality of lithographic apparatus;

Figure 2 is a schematic illustration of a lithographic apparatus that forms part of the lithographic system of Figure 1 ;

Figure 3 is a schematic illustration of a free electron laser;

Figure 4 is a schematic illustration of a lithographic system including a radiation source comprising two free electron lasers;

Figure 5 is a schematic illustration of an optical system;

- Figure 6 is a graph of maximum attainable reflectivity as a function of wavelength for a multi-layer mirror formed from any of a selected set of materials;

Figures 7a and 7b are tables of periods and reflectivity values at different wavelengths for multi-layer mirrors formed from the selected set of materials;

Figures 8a and 8b are tables of absorption coefficient values for various elements at different wavelengths; and

Figures 9a and 9b are tables of maximum thickness value to obtain transmission value of 90% for various elements at different wavelengths

Figures 10a and 10b are tables of calculated grazing incidence reflectance a at grazing incidence angle of 5 degrees, for various materials.

DETAILED DESCRIPTION

[00083] Figure 1 shows a lithographic system LS, comprising: a radiation source SO, a beam splitting apparatus 20 and a plurality of lithographic apparatus LA1-LA2₀. The radiation source SO comprises at least one free electron laser and is configured to generate radiation beam B (which may be referred to as a main beam). The main radiation beam B is split into a plurality of radiation beams Bi-B₂o (which may be referred to as branch beams), each of which is directed to a different one of the lithographic apparatus LA1-LA2₀, by the beam splitting apparatus 20. The branch radiation beams B 1-B2₀ may be split off from the main radiation beam B in series, with each branch radiation beam being split off from the main radiation beam B downstream from the preceding branch radiation beam. The beam splitting apparatus may, for example, comprise a series of mirrors (not shown) which are each configured to split off a portion of the main radiation beam B into a branch radiation beam B 1-B20.

[00084] The branch radiation beams B1-B2₀ are depicted in Figure 1 as being split off from the main radiation beam B such that the branch radiation beams B 1-B2₀ propagate in directions which are approximately perpendicular to the direction of propagation of the main radiation beam B. However, in some embodiments the branch radiation beams Bi-B₂o may instead be split off from the main radiation beam B such that an angle between the direction of propagation of each branch radiation beam B1-B2₀ and the direction of propagation of the main radiation beam is substantially less than 90 degrees. This may allow mirrors of the beam splitting apparatus to be arranged such that the main radiation beam B is incident on the mirrors at an angle of incidence which is less than normal. This may advantageously decrease the amount of radiation which is absorbed by the mirrors and therefore increase the amount of radiation which is reflected from the mirrors and which is provided to the lithographic apparatus LA1-LA2₀ via the branch radiation beams B 1-B2₀.

[00085] The lithographic apparatus LA1-LA2₀ may all be positioned on the same vertical level. The vertical level on which the lithographic apparatus LA1-LA2₀ are positioned may be substantially the same vertical level as the vertical level on which the beam splitting apparatus 20 is positioned and on which the main beam B is received from the radiation source SO. Alternatively, the beam splitting apparatus 20 may direct at least some of the branch radiation beams B 1-B2₀ to one or more different vertical levels on which at least some of the lithographic apparatus LA1-LA2₀ are positioned. For example, the main radiation beam B may be received by the beam splitting apparatus on a basement or ground floor vertical level. The beam splitting apparatus 20 may direct at least some branch radiation beams B 1-B2₀ to a vertical level which is positioned above the beam splitting apparatus and on which at least some of the lithographic apparatus LA1-LA2₀ are positioned. The lithographic apparatus LA1-LA2₀ may be positioned on multiple vertical levels and as such the beam splitting apparatus 20 may direct the branch radiation beams B₁-B₂₀ to different vertical levels in order to be received by the lithographic apparatus LAi-LA₂₀.

[00086] The radiation source SO, beam splitting apparatus 20 and lithographic apparatus LAi-LA₂o may all be constructed and arranged such that they can be isolated from the external environment. A vacuum may be provided in at least part of the radiation source SO, beam splitting apparatus 20 and lithographic apparatus LAi-LA₂o so as to minimise the absorption of radiation. Different parts of the lithographic system LS may be provided with vacuums at different pressures (i.e. held at different pressures which are below atmospheric pressure).

[00087] Figure 2 is a schematic depiction of a lithographic apparatus LAi of the lithographic system LS shown in Figure 1. The lithographic apparatus LAi comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask, also referred to as a reticle), a projection system PS and a substrate table WT configured to support a substrate W.

[00088] The illumination system IL is configured to condition the branch radiation beam Bi that is received by the lithographic apparatus LAi before it is incident upon the patterning device MA. The projection system PS is configured to project the branch radiation beam Bi (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B i with a pattern previously formed on the substrate W.

[00089] The branch radiation beam Bi that is received by the lithographic apparatus LAi passes into the illumination system IL from the beam splitting apparatus 20 through the opening 8 in an enclosing structure of the illumination system IL. Optionally, the branch radiation beam Bi may be focused to form an intermediate focus at or near to the opening 8.

[00090] The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam Bi with a desired cross-sectional shape and a desired angular distribution. The radiation beam Bi passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam to form a patterned beam Bii. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11. The illumination system IL may for example include an array of independently moveable mirrors. The independently moveable mirrors may for example measure less than 1mm across. The independently moveable mirrors may for example be MEMS devices.

[00091] Following reflection from the patterning device MA the patterned radiation beam Bn enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam Bn onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 2, the projection system may include any number of mirrors (e.g. six mirrors).

[00092] It can be understood from the above description that in operation, a radiation beam is received at an input, in the form of opening 8, of the lithographic apparatus LAi and is directed along a radiation path to the substrate W at the substrate table WT via the patterning device MA, by mirror devices 10, 11 and mirrors 13, 14.

[00093] In some embodiments a lithographic system LS may include one or more mask inspection apparatus (not shown). A mask inspection apparatus may include optics (e.g. mirrors) configured to receive a branch radiation beam B₁-B₂₀ from the beam splitting apparatus 20 and direct the branch radiation beam at a mask MA. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect radiation reflected from the mask and form an image of the mask at an imaging sensor. The image received at the imaging sensor may be used to determine one or more properties of the mask MA. The mask inspection apparatus may, for example, be similar to the lithographic apparatus LAI shown in Figure 2, with the substrate table WT replaced with an imaging sensor.

[00094] In some embodiments a lithographic system LS may include one or more Aerial Image Measurement System (AIMS) which may be used to measure one or more properties of a mask MA. An AIMS may, for example, be configured to receive a branch radiation beam Bi-B₂o from the beam splitting apparatus 20 and use the branch radiation beam Bi-B₂o to determine one or more properties of a mask MA.

[00095] The radiation source SO comprises a free electron laser FEL which is operable to produce a beam of radiation. Optionally, the radiation source SO may comprise more than one free electron laser FEL.

[00096] A free electron laser comprises an electron source, which is operable to produce a bunched relativistic electron beam, and a periodic magnetic field through which the bunches of relativistic electrons are directed. The periodic magnetic field is produced by an undulator and causes the electrons to follow an oscillating path about a central axis. As a result of the acceleration caused by the magnetic fields the electrons spontaneously radiate electromagnetic radiation generally in the direction of the central axis. The relativistic electrons interact with radiation within the undulator. Under certain conditions, this interaction causes the electrons to bunch together into microbunches, modulated at the wavelength of radiation within the undulator, and coherent emission of radiation along the central axis is stimulated.

[00097] Figure 3 is a schematic depiction of a free electron laser FEL comprising an electron source 21, a linear accelerator 22, a steering unit 23 and an undulator 24. The electron source 21 may alternatively be referred to as an injector.

[00098] The electron source 21 is operable to produce a beam of electrons E. The electron source 21 may, for example, comprise a photo-cathode or a thermionic cathode and an accelerating electric field. The electron beam E is a bunched electron beam E which comprises a series of bunches of electrons. The electron beam E is accelerated to relativistic energies by the linear accelerator 22. In an example, the linear accelerator 22 may comprise a plurality of radio frequency cavities, which are axially spaced along a common axis, and one or more radio frequency power sources, which are operable to control the electromagnetic fields along the common axis as bunches of electrons pass between them so as to accelerate each bunch of electrons. The cavities may be superconducting radio frequency cavities. Advantageously, this allows: relatively large electromagnetic fields to be applied at high duty cycles; larger beam apertures, resulting in fewer losses due to wakefields; and for the fraction of radio frequency energy that is transmitted to the beam (as opposed to dissipated through the cavity walls) to be increased. Alternatively, the cavities may be conventionally conducting (i.e. not superconducting), and may be formed from, for example, copper. Other types of linear accelerators may also be used. For example, the linear accelerator 22 may comprise a laser accelerator, wherein the electron beam E passes through a focused laser beam and the electric field of the laser beam causes the electrons to accelerate.

[00099] The relativistic electron beam E which exits the linear accelerator 22 enters the steering unit 23. The steering unit 23 is operable to alter the trajectory of the relativistic electron beam E so as to direct the electron beam E from the linear accelerator 22 to the undulator 24. The steering unit 23 may, for example, comprise one or more electromagnets and/or permanent magnets configured to generate a magnetic field in the steering unit 23. The magnetic field exerts a force on the electron beam E which acts to alter the trajectory of the electron beam E. The trajectory of the electron beam E upon leaving the linear accelerator 22 is altered by the steering unit 23 so as to direct the electrons to the undulator 24.

[000100] In embodiments in which the steering unit 23 comprises one or more electromagnets and/or permanent magnets, the magnets may be arranged to form one or more of a magnetic dipole, a magnetic quadrupole, a magnetic sextupole and/or any other kind of multipole magnetic field arrangement configured to apply a force to the electron beam E. The steering unit 23 may additionally or alternatively comprise one or more electrically charged plates, configured to create an electric field in the steering unit 23 such that a force is applied to the electron beam E. In general the steering unit 23 may comprise any apparatus which is operable to apply a force to the electron beam E to alter its trajectory.

[000101] The steering unit 23 directs the relativistic electron beam E to the undulator 24. The undulator 24 is operable to guide the relativistic electrons along a periodic path so that the electron beam E interacts with radiation within the undulator 24 so as to stimulate emission of coherent radiation. Generally the undulator 24 comprises a plurality of magnets, which are operable to produce a periodic magnetic field which causes the electron beam E to follow a periodic path. As a result the electrons emit electromagnetic radiation generally in the direction of a central axis of the undulator 24. The undulator 24 may comprise a plurality of sections (not shown), each section comprising a periodic magnet structure. The electromagnetic radiation may form bunches at the beginning of each undulator section. The undulator 24 may further comprise a mechanism for refocusing the electron beam E such as, for example, a quadrupole magnet in between one or more pairs of adjacent sections. The mechanism for refocusing the electron beam E may reduce the size of the electron bunches, which may improve the coupling between the electrons and the radiation within the undulator 24, increasing the stimulation of emission of radiation.

[000102] As electrons move through the undulator 24, they interact with the electric field of the electromagnetic radiation in the undulator 24, exchanging energy with the radiation. In general the amount of energy exchanged between the electrons and the radiation ill oscillate rapidly unless conditions are close to a resonance condition, given by:

where X_em is the wavelength of the radiation, X_u is the undulator period, γ is the Lorentz factor of the electrons and K is the undulator parameter. A is dependent upon the geometry of the undulator 24: for a helical undulator A=\, whereas for a planar undulator A=2. In practice, each bunch of electrons will have a spread of energies although this spread may be minimised as far as possible (by producing an electron beam E with low emittance). The undulator parameter K is typically approximately 1 and is given by:

where q and m are, respectively, the electric charge and mass of the electrons, Bo is the amplitude of the periodic magnetic field, and c is the speed of light.

[000103] The resonant wavelength X_em is equal to the first harmonic wavelength spontaneously radiated by electrons moving through the undulator 24. The free electron laser FEL may operate in self-amplified stimulated emission (SASE) mode. Operation in SASE mode may require a low energy spread of the electron bunches in the electron beam E before it enters the undulator 24. Alternatively, the free electron laser FEL may comprise a seed radiation source, which may be amplified by stimulated emission within the undulator 24.

[000104] Electrons moving through the undulator 24 may cause the amplitude of radiation to increase, i.e. the free electron laser FEL may have a non-zero gain. Maximum gain may be achieved when the resonance condition is met or when conditions are close to but slightly off resonance.

[000105] An electron which meets the resonance condition as it enters the undulator

24 will lose (or gain) energy as it emits (or absorbs) radiation, so that the resonance condition is no longer satisfied. Therefore, in some embodiments the undulator 24 may be tapered. That is, the amplitude of the periodic magnetic field and/or the undulator period _u may vary along the length of the undulator 24 in order to keep bunches of electrons at or close to resonance as they are guided though the undulator 24. Note that the interaction between the electrons and radiation within the undulator 24 produces a spread of energies within the electron bunches. The tapering of the undulator 24 may be arranged to maximise the number of electrons at or close to resonance. For example, the electron bunches may have an energy distribution which peaks at a peak energy and the tapering maybe arranged to keep electrons with this peak energy at or close to resonance as they are guided though the undulator 24. Advantageously, tapering of the undulator has the capacity to significantly increase conversion efficiency. The use of a tapered undulator may increase the conversion efficiency (i.e. the portion of the energy of the electron beam E which is converted to radiation in the radiation beam B) by more than a factor of 2. The tapering of the undulator may be achieved by reducing the undulator parameter K along its length. This may be achieved by matching the undulator period _u and/or the magnetic field strength % along the axis of the undulator to the electron bunch energy to ensure that they are at or close to the resonance condition. Meeting the resonance condition in this manner increases the bandwidth of the emitted radiation.

[000106] After leaving the undulator 24, the electromagnetic radiation is emitted as a radiation beam B'. The radiation beam B' may form all or part of the radiation beam B which is provided to the beam splitting apparatus 20 (depicted in Figure 1) and which forms the branch radiation beams Bi_₂o which are provided to the lithographic apparatus LAi_₂o.

[000107] In the embodiment of a free electron laser which is depicted in Figure 3, the electron beam E' which leaves the undulator 24 enters a second steering unit 25. The second steering unit 25 alters the trajectory of the electron beam E' which leaves the undulator 24 so as to direct the electron beam E' back through the linear accelerator 22. The second steering unit 25 may be similar to the steering unit 23 and may, for example, comprise one or more electromagnets and/or permanent magnets. The second steering unit 25 does not affect the trajectory of the radiation beam B' which leaves the undulator 24. The steering unit 25 therefore decouples the trajectory of the electron beam E' from the radiation beam B'. In some embodiments, the trajectory of the electron beam E' may be decoupled from the trajectory of the radiation beam B' (e.g. using one or more magnets) before reaching the second steering unit 25.

[000108] The second steering unit 25 directs the electron beam E' to the linear accelerator 22 after leaving the undulator 24. Electron bunches which have passed through the undulator 24 may enter the linear accelerator 22 with a phase difference of approximately 180 degrees relative to accelerating fields in the linear accelerator 22 (e.g. radio frequency fields). The phase difference between the electron bunches and the accelerating fields in the linear accelerator 22 causes the electrons to be decelerated by the fields. The decelerating electrons E' pass some of their energy back to the fields in the linear accelerator 22 thereby increasing the strength of the fields which accelerate the electron beam E arriving from the electron source 21. This arrangement therefore recovers some of the energy which was given to electron bunches in the linear accelerator 22 (when they were accelerated by the linear accelerator) in order to accelerate subsequent electron bunches which arrive from the electron source 21. Such an arrangement may be known as an energy recovering LIN AC.

[000109] Electrons E' which are decelerated by the linear accelerator 22 are absorbed by a beam dump 26. The steering unit 23 may be operable to decouple the trajectory of the electron beam E' which has been decelerated by the linear accelerator 22 from the trajectory of the electron beam E which has been accelerated by the linear accelerator 22. This may allow the decelerated electron beam E' to be absorbed by the beam dump 26 whilst the accelerated electron beam E is directed to the undulator 24. The steering unit 23 may, for example, be operable to generate a periodic magnetic field which has a substantially constant phase relationship with the electron bunches which form the accelerated electron beam E and the decelerated electron beam E' . For example at times at which electron bunches from the accelerated electron beam E enter the steering unit 23, the steering unit 23 may generate a magnetic field which acts to direct the electrons to the undulator 24. At times at which electron bunches from the decelerated electron beam E' enter the steering unit 23, the steering unit 23 may generate a magnetic field which acts to direct the electrons to the beam dump 26. Alternatively, at times at which electron bunches from the decelerated electron beam E' enter the steering unit 23, the steering unit 23 may generate little or no magnetic field such that the electrons pass out of the steering unit 23 and to the beam dump 26.

[000110] Alternatively the free electron laser FEL may comprise a beam splitting unit (not shown) which is separate from the steering unit 23 and which is configured to decouple the trajectory of the accelerated electron beam E from the trajectory of the decelerated electron beam E' upstream of the steering unit 23. The beam splitting unit may, for example, be operable to generate a periodic magnetic field which has a substantially constant phase relationship with the electron bunches which form the accelerated electron beam E and the decelerated electron beam E'.

[000111] Alternatively the trajectory of the accelerated electron beam E may be decoupled from the trajectory of the decelerated electron beam E' by generating a substantially constant magnetic field. The difference in energies between the accelerated electron beam E and the decelerated electron beam E' causes the trajectories of the two electron beams to be altered by different amounts by the constant magnetic field. The trajectories of the two electron beams will therefore become decoupled from each other.

[000112] The beam dump 26 may, for example, include a large amount of water or a material with a high threshold for radioactive isotope generation by high energy electron impact. For example, the beam dump 26 may include aluminium with a threshold for radioactive isotope generation of approximately 15MeV. By decelerating the electron beam E' in the linear accelerator 22 before it is incident on the beam dump 26, the amount of energy the electrons have when they are absorbed by the beam dump 26 is reduced. This reduces the levels of induced radiation and secondary particles produced in the beam dump 26. This removes, or at least reduces, the need to remove and dispose of radioactive waste from the beam dump 26. This is advantageous since the removal of radioactive waste requires the free electron laser FEL to be shut down periodically and the disposal of radioactive waste can be costly and can have serious environmental implications.

[000113] When operating as a decelerator, the linear accelerator 22 may be operable to reduce the energy of the electrons E' to below a threshold energy. Electrons below this threshold energy may not induce any significant level of radioactivity in the beam dump 26.

[000114] In some embodiments a decelerator (not shown) which is separate to the linear accelerator 22 may be used to decelerate the electron beam E' which has passed through the undulator 24. The electron beam E' may be decelerated by the decelerator in addition to being decelerated by the linear accelerator 22 or instead of being decelerated by the linear accelerator 22. For example, the second steering unit 25 may direct the electron beam E' through a decelerator prior to the electron beam E' being decelerated by the linear accelerator 22. Additionally or alternatively the electron beam E' may pass through a decelerator after having been decelerated by the linear accelerator 22 and before being absorbed by the beam dump 26. Alternatively the electron beam E' may not pass through the linear accelerator 22 after leaving the undulator 24 and may be decelerated by one or more decelerators before being absorbed by the beam dump 26.

[000115] Optionally, the free electron laser FEL may comprise one or more bunch compressors (not shown). A bunch compressor may be disposed downstream or upstream of the linear accelerator 22. A bunch compressor is configured to bunch electrons in the electron beam E and spatially compress existing bunches of electrons in the electron beam E. One type of bunch compressor comprises a radiation field directed transverse to the electron beam E. An electron in the electron beam E interacts with the radiation and bunches with other electrons nearby. Another type of bunch compressor comprises a magnetic chicane, wherein the length of a path followed by an electron as it passes through the chicane is dependent upon its energy. This type of bunch compressor may be used to compress a bunch of electrons which have been accelerated in a linear accelerator 22 by a plurality of conductors whose potentials oscillate at, for example, radio frequencies.

[000116] It may be desirable for electron bunches entering the undulator 24 to be tightly bunched and therefore have a very small spread in position. It may therefore be desirable to compress the electron bunches before they pass into the undulator 24 using one or more bunch compressors in order to reduce the spread in position of electron bunches in the undulator 24. A separate bunch compressor (not shown) may therefore be disposed between the steering unit 23 and the undulator 24. Alternatively, or additionally, the steering unit 23 itself may act to compress the electron bunches in the electron beam E. An electron bunch which is accelerated by the linear accelerator 22 may have a spread of energies. For example, some electrons in an electron bunch may have energies which are higher than an average energy of the electron bunch and some electrons in the bunch may have energies which are lower than the average energy. The alternation of the trajectory of an electron which is caused by the steering unit 23 may be dependent on the energy of the electrons (e.g. when the trajectory is altered by a magnetic field). Electrons of different energies may therefore have their trajectories altered by different amounts by the steering unit 23, which may be difference in trajectories may be controlled to result in a compression of an electron bunch.

[000117] The free electron laser FEL shown in Figure 3 is housed within a building

31. The building 31 may comprise walls which do not substantially transmit radiation which is generated in the free electron laser FEL whilst the free electron laser FEL is in operation. For example, the building 31 may comprise thick concrete walls (e.g. walls which are approximately 4 metres thick). The walls of the building 31 may be further provided with radiation shielding materials such as, for example, lead and/or other materials which are configured to absorb neutrons and/or other radiation types. Providing walls of a building 31 with radiation absorbing materials may advantageously allow the thickness of the walls of the building 31 to be reduced. However adding radiation absorbing materials to a wall may increase the cost of constructing the building 31. A relatively cheap material which may be added to a wall of the building 31 in order to absorb radiation may, for example, be a layer of earth.

[000118] In addition to providing walls of the building 31 which have radiation shielding properties. The building 31 may also be configured to prevent radiation generated by the free electron laser FEL from contaminating ground water below the building 31. For example, the base and/or foundations of the building 31 may be provided with radiation shielding materials or may be sufficiently thick to prevent radiation from contaminating ground water below the building 31. In an embodiment the building 31 may be positioned at least partly underground. In such an embodiment ground water may surround portions of the exterior of the building 31 as well as being below the building 31. Radiation shielding may therefore be provided around the exterior of the building 31 in order to prevent radiation from contaminating ground water which surrounds the building 31.

[000119] In addition to or as an alternative to shielding radiation at the exterior of the building 31 , radiation shielding may also be provided inside of the building 31. For example, radiation shielding may be provided inside the building 31 at locations proximate to portions of the free electron laser FEL which emit large amounts of radiation.

[000120] The building 31 has a width W and a length L. The width W and the length L of the building 31 is partly determined by the size of a loop 32 which the electron beam E follows through the free electron laser FEL. The loop 32 has a length 33 and a width 35.

[000121] The length 33 of the loop 32 is determined by the length of the linear accelerator 22 and the length of the undulator 24. A given length of linear accelerator 22 may, for example, be required in order to accelerate the electron beam E to high enough energies such that the electrons emit radiation of the desired wavelength in the undulator 24. For example, a linear accelerator 22 may have a length of greater than about 40 metres. In some embodiments a linear accelerator 22 may have a length of up to about 80 metres. Additionally a given length of undulator 24 may be required in order to stimulate emission of coherent radiation in the undulator 24. For example, an undulator 24 may have a length of greater than about 40m. In some embodiments an undulator 24 may have a length of up to about 60 metres.

[000122] The width of the loop is determined by the radius of curvature with which the steering unit 23 adjusts the trajectory of the electron beam E. The radius of curvature of the electron beam E in the steering unit 23 may depend, for example, on the velocity of the electrons in the electron beam E and on the strength of a magnetic field which is generated in the steering unit 23. An increase in the strength of a magnetic field which is generated in the steering unit 23 will decrease the radius of curvature of the electron beam E whereas an increase in the velocity of the electrons will increase the radius of curvature of the electron beam E. The radius of curvature of the electron beam E through the steering unit 23 may, for example, be approximately 12m. In some embodiments the radius of curvature of the electron beam E through the steering unit 23 may be less than 12m. For example, the radius of curvature of an electron beam E through the steering unit 23 may be approximately 7m.

[000123] The loop 32 which the electron beam E follows through the free electron laser FEL may have a length 33 which is greater than about 60 metres. In some embodiments the loop 32 may have a length 33 which is up to about 120 metres. The loop 32 may have a width 35 which is greater than about 12 metres. In some embodiments the loop 32 may have a width 35 which is up to about 25 metres.

[000124] The building 31 may also house other components. For example, electrical cabinets 37 which contain electrical components which supply electrical power to, for example, the undulator 24, the steering units 23, 25 and/or other components of the free electron laser FEL may be housed within the building 31. It may be advantageous to provide the electrical cabinets 37 in close proximity to the undulator 24 as is shown in Figure 3. However electrical cabinets 37 may be positioned in other positions relative to the components of the free electron laser FEL.

[000125] Additionally cryogenic cooling cabinets 39 which contain apparatus which is configured to provide cryogenic cooling to components of the free electron laser FEL may be housed within the building 31. Cryogenic cooling may, for example, be provided to the linear accelerator 22 and may cool superconducting cavities of the linear accelerator 22. It may be advantageous to provide the cryogenic cooling cabinets 39 in close proximity to the linear accelerator 22. This may reduce any energy loss between the cryogenic cooling cabinets 39 and the linear accelerator 22.

[000126] It may be desirable to provide electrical cabinets 37 and cryogenic cooling cabinets 39 on the outside of the loop 32 which the electron beam E follows through the free electron laser FEL (as is shown in Figure 3). Providing the cabinets 37, 39 on the outside of the loop 32 may allow easy access to the cabinets, for example, to monitor, control, maintain and/or repair components which are housed within the cabinets 37, 39. As will be appreciated from Figure 3, positioning the cabinets 37, 39 on the outside of the loop 32 may increase the minimum width W of the building 31 which is required to house the components of the free electron laser FEL within the building 31. The building 31 may also house other components which are not shown in Figure 3 which may also determine the dimensions of the building 31.

[000127] As is shown in Figure 3, a wall 47 is positioned between the loop 32 which the electron beam follows through the free electron laser FEL and the electric cabinets 37. A wall 47 is also positioned between the loop 32 and the cryogenic cooling cabinets 39. The walls 47 may shield the electric cabinets 37 and the cryogenic cabinets 39 from radiation which is generated by the electron beam E in the free electron laser FEL. This protects the components in the cabinets 37, 39 from being damaged by radiation and may allow maintenance workers to access the cabinets 37, 39 whilst the free electron laser FEL is in operation without being exposed to dangerous levels of radiation.

[000128] In the embodiment depicted in Figure 3 the cabinets 37, 39 are shown as being housed in the same building 31 as the loop 32 which the electron beam follows through the free electron laser FEL whilst being shielded from the loop 32 by the walls 47. The cryogenic cooling components which are housed within the cabinets 39 may generate vibrations which may be transferred to components of the free electron laser FEL and may adversely affect components of a free electron laser FEL which are sensitive to vibrations. In order to prevent vibrations which are generated by cryogenic cooling components from transferring to sensitive parts of the free electron laser, a portion of the building 31 in which the cryogenic cooling cabinets 39 are housed may be mechanically isolated from the portion of the building in which sensitive components are housed. For example, the cryogenic cooling cabinets 39 may be mechanically isolated from the linear accelerator 22, the steering unit 23 and the undulator 24. In order to provide mechanical isolation the portion of the building 31 in which the cryogenic cooling cabinets 39 are housed may, for example, have separate foundations to a portion of the building in which the linear accelerator 22, the steering unit 23 and the undulator 24 are housed.

[000129] Alternatively the cryogenic cooling cabinets 39 and/or the electrical cabinets 37 may be housed in one or more buildings which are separate from the building 31. This may ensure that the cabinets 37, 39 are shielded from radiation which is produced by the electron beam E and that sensitive components of the free electron laser FEL are mechanically isolated from the cryogenic cooling cabinets 39.

[000130] A lithographic system LS may comprise a single free electron laser FEL.

The free electron laser FEL may supply a radiation beam to a beam splitting apparatus 20 which provides branch radiation beams to a plurality of lithographic apparatus. The radiation source SO may comprise an optical system which includes dedicated optical components configured to direct a radiation beam B' output from a free electron laser FEL to a beam splitter 20 of a lithographic system LS. Since radiation at the wavelengths of interest is generally well absorbed by all matter, reflective optical components are generally used (rather than transmissive components) so as to minimise losses. The dedicated optical components of the optical system may adapt the properties of the radiation beam produced by the free electron laser FEL so that it is suitable for acceptance by the illumination systems IL of the lithographic apparatus LA1-LA2₀ and/or a mask inspection apparatus.

[000131] Alternatively a radiation source SO may comprise a plurality of free electron lasers (e.g. two free electron lasers) which may each provide a radiation beam to an optical system which also forms part of the radiation source SO. The optical system may receive a radiation beam from each of a plurality of free electron lasers and may combine the radiation beams into a composite radiation beam which is provided to a beam splitting apparatus 20 in order to provide branch radiation beams B1-B2₀ to lithographic apparatus LA1-LA20. [000132] Figure 4 is a schematic depiction of a lithographic system LS which includes a radiation source SO comprising a first free electron laser FEL' and a second free electron laser FEL". The first free electron laser FEL' outputs a first radiation beam B' and the second free electron laser FEL" outputs a second radiation beam B". The first free electron laser FEL' is housed within a first building 31 '. The second free electron laser FEL" is housed within a second building 31".

[000133] The first and second radiation beams B', B" are received by an optical system 40. The optical system 40 comprises a plurality of optical elements (e.g. mirrors) which are arranged to receive the first radiation beam B' and the second radiation beam B" and output a main radiation beam B. At times at which both the first and second free electron lasers are operating, the main radiation beam B is a composite radiation beam which comprises radiation from both the first and second radiation beams B', B". The composite radiation beam B is provided to the beam splitting apparatus 20 which provides branch radiation beams B1-B2₀ to lithographic apparatus LA1-LA2₀.

[000134] The arrangement which is depicted in Figure 4 in which two free electron lasers are arranged to provide radiation beams B', B" to form a main radiation beam B, may allow one of the free electron lasers to be turned off whilst radiation is continuously provided to the lithographic apparatus LAi-LA₂o. For example, one of the free electron lasers may be taken out of operation in order to, for example, allow the free electron laser to be repaired or to undergo maintenance. In this event the other free electron laser may continue to provide a radiation beam which is received by the optical system 40. In the event that only one of the free electron lasers provides radiation to the optical system 40, the optical system 40 is operable to form a main radiation beam B which comprises radiation from the free electron laser which is providing radiation to the optical system 40. This allows for continuous operation of the lithographic apparatus LAi-LA₂o even when one of the free electron lasers is taken out of operation.

[000135] Figure 5 is a schematic depiction of an embodiment of an optical system

40 according to an embodiment of the invention which is arranged to receive a beam of radiation B', B" from each of the free electron lasers FEL', FEL" and to output an output radiation beam B. The radiation beam B that is output by the optical system 40 is received by the beam splitting apparatus 20 (see Figure 1).

[000136] The optical system 40 comprises four optical elements: first and second optical elements 132, 134 associated with a first one of the free electron lasers FEL'; and first and second optical elements 136, 138 associated with a second one of the free electron lasers FEL". The optical elements 132, 134, 136, 138 are arranged to alter the size and shape of the cross section of the radiation beams B', B" from the free electron lasers FEL', FEL".

[000137] In particular, the first optical elements 132, 136 are convex mirrors, which act to increase the cross sectional area of the radiation beams B', B" from the free electron lasers FEL', FEL". Although in Figure 5 the first optical elements 132, 136 appear to be substantially flat in the x-y plane they may be convex both in this plane and in the z direction. Since the first optical elements 132, 136 are convex, they will increase the divergence of the radiation beams B', B", thereby decreasing the heat load on mirrors downstream of them. The first optical element 132 is therefore a diverging optical element arranged to increase the cross sectional area of the radiation beam B ' received from the first free electron laser FEL' . The first optical element 136 is a diverging optical element arranged to increase the cross sectional area of the radiation beam B" received from the second free electron laser FEL. This may allow mirrors downstream to be of a lower specification, with less cooling, and therefore less expensive. Additionally or alternatively, it may allow the downstream mirrors to be nearer to normal incidence. In practice, the radiation beam B output by the radiation source SO may be split by a plurality of consecutive, static, knife edge mirrors arranged in series in the path of the beam B. Increasing the size of the beam B (by, for example, using convex mirrors as the first optical elements 132, 136) reduces the accuracy with which the mirrors must be located in the beam B path. Therefore, this allows for more accurate splitting of the output beam B by the splitting apparatus 20.

[000138] The second optical elements 134, 138 are concave and are complementary in shape to the first optical elements such that the beams leaving the second optical elements 134, 138 have substantially zero divergence. Therefore, downstream of the second optical elements 134, 138 the beams are substantially collimated. Again, although in Figures 5 the second optical elements 134, 138 appear to be substantially flat in the x-y plane they are in fact concave both in this plane and in the z direction.

[000139] It may be preferable for the output beam B, which is received by the beam splitting apparatus 20, to have a different shape and/or intensity distribution to that output by the free electron lasers FEL', FEL". For example, a rectangular shape may be preferable to a circular beam for consecutive knife edge extraction mirrors within the beam splitting apparatus 20. Therefore, in addition to increasing the cross sectional area of the radiation beams B', B", the optical elements 132, 134, 136, 138 may act to alter the cross sectional shape of the radiation beams B', B". In particular, the optical elements 132, 134, 136, 138 may be astigmatic or aspherical and may be shaped so as to ensure that the radiation beams Β', B" leaving the second optical elements 134, 138 are more rectangular in shape than the radiation beams B', B" produced by the free electron lasers FEL', FEL". For example, the optical elements may be shaped so that the beams B', B" leaving the second optical elements 134, 138 are generally rectangular but with rounded corners, although other shapes are also possible. The two dimensions of such a rectangular shape may be related to radii of curvature of the optical elements in two perpendicular directions such as, for example, in the x-y plane and in the z direction. Advantageously, this allows the mirrors that are used to split the output radiation beam B into branch radiation beams B 1-B20 (see Figure 1) before they enter the lithographic apparatuses LA1-LA20, to be identical or at least very similar. This is especially beneficial from a manufacturing point of view.

[000140] When both of the free electron lasers FEL', FEL" are on, the optical system 40 is operable to combine their radiation beams B ' , B" to form a composite radiation beam B. In this embodiment, this is achieved by offsetting the first and second optical elements 132, 134 of the first free electron laser FEL' from those 136, 138 of the second free electron laser FEL" in the x-direction so that the beams B', B" leaving the second optical elements 134, 138 are both adjacent to each other and mutually parallel. In particular, the first and second optical elements 132, 134 of the first free electron laser FEL' are disposed "downstream" (with respect to the direction of propagation of the laser beams B', B") of those 136, 138 of the second free electron laser FEL".

[000141] In such an arrangement, the optical system 40 is operable to combine the two radiation beams B', B" to form a composite radiation beam. The composite beam is the output radiation beam B output by the optical system 40. It will be appreciated that Figure 5 is merely exemplary and that the optical system 40 may be implemented other than as shown in Figure 5.

[000142] Referring again to Figure 4, the buildings 31 ', 31" are configured to substantially prevent radiation (other than the radiation beams B ' , B") which is generated by an operating free electron laser from propagating out of the buildings 3 , 31". Housing the first and second free electron lasers inside separate buildings therefore allows maintenance and/or repair to be safely carried out on one of the free electron lasers whilst the other free electron laser continues to operate. For example, the first electron laser FEL' may be taken out of operation in order to allow the first free electron laser FEL' to be repaired or to undergo maintenance. During this time the second free electron laser FEL" may continue to operate in order to provide radiation to the optical system 40 and to the lithographic apparatus LAI-LA₂Q. Radiation will therefore be generated in the second building 31" due to the operation of the second free electron laser FEL". Dangerous levels of radiation do not however leave the second building 31" and do not enter the first building 3 due to the radiation shielding which is provided by the walls of the second building 31". The first building may therefore be safely entered by maintenance workers in order to repair or carry out maintenance to the first free electron laser FEL' .

[000143] It is a feature of lithographic systems such as that of Figures 1 to 5, based upon use of a free electron laser to produce radiation that, in general, any desired wavelength within the range of operation of the system may be produced. This is achieved by providing electrons in the free electron laser with an energy which corresponds with the desired wavelength. The system is thus not limited to using only one particular wavelength such as 13.5 nm or 6.7 nm as may be the case for plasma sources. For example any desired wavelength in the range 4nm to 40nm may be produced. Thus, for example, a wavelength may be selected which allows multi-layer mirrors to be constructed that have a higher reflectivity than multi-layer mirrors configured to reflect 13.5 nm radiation.

[000144] It is recognised herein that by suitable choice of wavelength, it is possible that a range of materials with desired or acceptable characteristics may be available to form optical devices, for example multi-layer mirrors, pellicles or absorbers, forming part of a lithographic apparatus or other component of a lithographic system. For example, by suitable choice of wavelength it is found herein that a range of materials with acceptable characteristics may be available for use in forming optical devices forming part of a lithographic system, or that for at least some materials optical or other characteristics may be optimized or improved by suitable choice of operating wavelength.

[000145] Figure 6 is a graph of maximum attainable reflectivity values as a function of wavelength for a multi-layer mirror formed from any of a selected set of pairs of materials. The reflectivity values in this case are reflectivity values at normal incidence determined theoretically, for a temperature of 300K, from values of refractive index for various selected pairs of materials having calculated layer periods (e.g. thicknesses) from which the multilayer mirror may be formed. Temperature-dependent variations of the reflectivity values are expected to be relatively low for temperatures at which the materials remain in the solid state, although in practice diffusion between layers in multi-layer mirror devices may increase as temperature increases. It will be understood that, in practice, actual reflectivity values may be less than the maximum attainable reflectivities due to imperfections that may occur in real devices, however Figure 6 shows clearly how attainable reflectivity values may vary with wavelength. An example of a calculation of theoretical reflectivity values for multi-layer mirrors is provided in A.V. Vinogradov & B.Ya. Zeldovich, Applied Optics, 16, 1, 89-93, 1977 and the calculation of the reflectivities shown in Figures 6, 7a and 7b was performed in accordance with the teaching of that document, however any suitable method can be used to calculate or otherwise determine multi-layer mirror reflectivity values.

[000146] Two lines are plotted in Figure 6, the first line 140 is a plot of the maximum attainable reflectivity value as a function of wavelength for the selected set of pairs of materials under consideration. The second line 142 is a plot of the second highest attainable reflectivity values as a function of wavelength for the selected set of pairs of materials under consideration.

[000147] The pairs of materials that provide the maximum 140 and second highest

142 reflectivity values change as the wavelength changes, and particular pairs of materials for which the maximum and second highest reflectivity values are obtained are indicated on the graph of Figure 6 for selected ones of the wavelengths. For example, it is indicated in Figure 6 that at a wavelength of 4.37nm the maximum attainable reflectivity value for a multi-layer mirror constructed from the pairs of materials under consideration is obtained for UC (e.g. a multi-layer mirror having alternating layers of uranium and carbon), and the second highest reflectivity value is obtained for LiC. Similarly, at a wavelength of 9.49nm, the maximum reflectivity value is obtained for PdSr and the second highest reflectivity value is obtained for AgSr at a wavelength of 10.5nm, the maximum reflectivity value is obtained for RhSr and the second highest reflectivity value is obtained for PdSr, at a wavelength of 11.3nm, the maximum reflectivity value is obtained for RuBe and the second highest reflectivity value is obtained for BeRh, at a wavelength of 17.1nm the maximum reflectivity value is obtained for AlSr and the second highest reflectivity value is obtained for YAl, and at a wavelength of 22.8nm the maximum reflectivity value is obtained for BeLi and the second highest reflectivity value is obtained for Bali.

[000148] Figures 7a and 7b show tables indicating values of maximum attainable reflectivity at certain selected wavelengths for multi-layer mirrors constructed from at least some of the pairs of materials for which the plot of Figure 6 was obtained. Calculations were performed for many different combinations of solid, pure materials (e.g. solid elements), with certain solid materials not being considered if they seemed to be too hazardous or otherwise impracticable, for example plutonium. A selection of relevant results is shown in Figures 6 and 7a and 7b. The tables also show the period of the multi-layer mirror (e.g. thickness of each repeating arrangement of a first layer and a second layer) and the ratio of the top layer (e.g. outermost of the first and second layers) of each repeating arrangement of a first layer and a second layer, for the pairs of materials under consideration. Although the tables of Figures 7a and 7b refer to a top layer and a bottom layer, either of the layers may be the top or bottom layer, and either of the layers may be the outermost layer, in embodiments. For the purposes of the calculations, the total thickness of the mirrors, and the total number of layers, are presumed to be effectively infinite, although in practice the total number of layers may in the tens or hundreds, for example between 40 and 400 depending on the operating wavelength and the particular mirror in question. The period of the repeating arrangement of layers, and the relative thickness of each layer depends on the refractive index of each material and the wavelength for which the multi-layer mirror is under consideration. In the tables of Figures 7a and 7b, the period and relative thickness for a pair of materials at a particular wavelength are the period and relative thickness that would provide the maximum calculated reflectivity value for a multi-layer mirror constructed of that pair of materials at that wavelength.

[000149] In the tables of Figures 7a and 7b, at each indicated wavelength the pairs of materials are ordered by calculated maximum attainable multi-layer mirror reflectivity value, with the highest reflectivity values at the left hand side. It has been found that, although a maximum attainable reflectivity value can be calculated as being obtainable for a particular pair of materials at a particular wavelength, other pairs of materials can also provide acceptable reflectivity values at that wavelength. For example, for a wavelength of 4.37nm, the highest calculated reflectivity value for the pairs of materials in Table 7 is 84% for LiC, but various pairs of materials provide maximum theoretical reflectivity values of greater than 60%, which may be acceptable.

[000150] The reflectivity that may be acceptable for particular embodiments can vary in dependence on the operating wavelength, the number of mirrors or other properties of the lithographic apparatus, and/or properties of the lithographic operation to be performed or lithographic structure to be formed. Depending on the lithographic apparatus in question, for example the number of multi-layer mirrors provided on a radiation path, it is possible that at 4.37nm a theoretical maximum reflectivity value of approximately 60% or greater may be acceptable and thus a choice may be available between various possible pairs of multi-layer mirror materials when constructing a lithographic apparatus for operation at 4.37nm in accordance with particular embodiments. A higher threshold value of reflectivity of approximately 70%, or approximately 90%, is applicable for at least some embodiments, for instance at least some embodiments having higher operating wavelengths. [000151] In practice the actual reflectivity value for a real device is likely to be lower than the theoretical maximum, so when specifying that a theoretical maximum reflectivity value should be approximately 60% or some other value when selecting materials for multi-layer mirrors for particular embodiments, it is assumed that the actual reflectivity values obtained from the actual multi-layer mirrors may be lower than the theoretical maximum. For example, the actual reflectivity values achieved by devices in practice may be 10% to 15% lower, or 10% to 25% lower, than the theoretical maximum values in some cases, with the difference between actual and theoretical values generally tending to be higher at lower wavelengths. For example, at 4.37nm, actual values of reflectivity of around 60% or so may be achieved for some mirrors that have theoretical values of reflectivity of around 80% or so.

[000152] Although a high reflectivity value may be important when choosing an operating wavelength and choosing materials for multi-layer mirrors for a lithographic apparatus, the properties of other optical components at the chosen wavelength, and other properties of chosen materials, are also important as is discussed further below in relation to particular embodiments. For example, it is found that the optical performance of materials that can be used as absorbers (for example, reticles or resists) or as pellicles can also be important when making a choice of operational wavelength.

[000153] Figures 8a and 8b are tables of absorption coefficient values for various elements at the wavelengths that were the subject of the tables of Figures 7a and 7b. Absorption coefficient values were obtained for various solid elements at 300K but only values for a selected sub-set of those elements are included in Figures 7a and 7b, for clarity. At each wavelength the materials identified in the tables are ordered by absorption coefficient value such that, for each wavelength, the materials with the highest absorption coefficient values are at the left hand side of the tables.

[000154] Absorption coefficient values are usually expected to fall with wavelength. However it can be seen from the tables of Figures 8a and 8b that the highest absorption coefficient values at 4.37nm (for example, obtained for Ir, Os, Re, W, Pt) are similar to the highest absorption coefficient values at 6.62nm (for example, obtained for Os, W, Re, Cu, Ta, Ni).

[000155] A high absorption coefficient value may be important when choosing materials for absorbers (for example, reticles or resists) for a lithographic apparatus as it can enable a thinner layer of material to be used to obtain a required absorption level. The availability of materials with sufficiently high absorption coefficient values can be a significant factor when selecting a suitable operating wavelength. The availability of a range of materials with acceptable values of absorption coefficient can allow for selection amongst those materials of an absorber material having a particular desired property or properties, for example mechanical or thermal stability, non-toxicity, non-radioactivity or ease of processing, manufacture or storage.

[000156] The availability of materials with other desired properties, in addition to the availability of materials with suitable absorption or reflectivity properties, can also be a significant consideration when selecting a suitable operating wavelength. For example, the availability of materials suitable for use as pellicles, for instance for use as a protective layer covering the mask MA (also referred to as a reticle), and having desired properties can also be a significant consideration.

[000157] Figures 9a and 9b are tables of required material thicknesses (in nm) to obtain a 90% transmission ratio for the elements that are the subject of Figures 8a and 8b if used to form pellicles. The tables of Figures 9a and 9b can be considered effectively to represent the inverse of the tables of Figures 8a and 8b, presented such that thicknesses are given for materials to yield 90% transmission at a given wavelength. It may be important to take into account available pellicle materials and maximum thicknesses of such materials to obtain acceptable pellicle performance and properties when selecting an operational wavelength for lithography.

[000158] It is recognised herein, based for example on the results shown in Figures

6 to 9, that although a free electron laser source can be used to obtain radiation at any selected operational wavelength, it may be important to take into account the properties of available materials at the possible wavelengths, in particular the availability of materials with suitable reflectivity, absorption, transmission or other properties, when selecting a wavelength for lithographic applications.

[000159] Turning to details of particular embodiments, in certain embodiments a lithographic system such as that of Figures 1 to 5 is configured to operate at a wavelength of 4.37nm.

[000160] By reducing the operational wavelength to 4.37nm, in contrast for example to conventionally used or proposed operational wavelengths of 13.5nm or 6.7nm, the minimum size of lithographic features that can be produced can be reduced. It can also be seen from Figure 6 that there is a strong peak in maximum reflectivity value at a wavelength of 4.37nm for multi-layer mirrors formed of pairs of materials that have been considered herein. [000161] Furthermore, it is known that by reducing wavelength, as well as potentially enabling the production of smaller lithographic features, the minimum numerical aperture requirement for optical components in the projection system of a lithographic apparatus can in principle be reduced (see for example, P.Kuerz et al, "Optics for EUV Lithography", 2008 International Symposium on Extreme Ultraviolet Lithography, 28 September 2008 - 1 October 2008, Lake Tahoe, California; and P.Kearney, "High-NA EUV Challenges and Promise", IEUVI Mask TWO meeting, 24 February 2013. It is a feature of certain of the embodiments configured to operate at a wavelength of 4.37nm that, as the numerical aperture requirement may be relaxed due to the short wavelength, the number of multi-layer mirrors 13, 14 included in the projection system PS can be reduced to four or fewer. By reducing the number of mirrors, the minimum reflectivity value required for each individual multi-layer mirror can in turn be reduced, as the cumulative reflectivity value reduces with the number of mirrors in the radiation path.

[000162] Thus, in certain of the embodiments configured to operate at approximately 4.37nm, the materials for the multi-layer mirrors 13, 14 are selected such that each multi-layer mirror has a minimum value of maximum theoretical attainable reflectivity of around 60% calculated for an ideal version of the multi-layer mirror (in practice, the actual values of maximum reflectivity obtained will usually be lower than the theoretical maximum). In contrast, other embodiments discussed below and configured to operate at higher wavelengths, for example at approximately 9.49nm, 10.5nm, 11.3nm, 17.1nm, 22.8nm, or between approximately 22.8nm and approximately 25.2nm, for instance approximately 25.2nm, can have constraints on minimum value of maximum attainable multi-layer mirror reflectivity that are higher, for example around 70% or around 90%.

[000163] Considering in more detail embodiments configured to operate at approximately 4.37nm, it can be seen from, for example, the tables of Figures 7a and 7b that there are various pairs of materials that can provide a maximum calculated multi-layer reflectivity of at least around 60%. Embodiments are not limited to including multi-layer mirrors formed only of pairs of materials identified in the tables of Figures 7a and 7b. Instead, any suitable combination of materials that can provide the required reflectivity can be used for the multi-layer mirrors.

[000164] In certain embodiments configured for operation at around 4.37nm, there is provided a lithography apparatus with a projection system PS that has four multi-layer mirrors each multi-layer mirror being a C/Li multi-layer mirror (comprising alternating layers of carbon and lithium). Similarly, the mirrors 10, 11 of the illumination system IL may also be C/Li multi-layer mirrors in some of those embodiments. The C/Li multi-layer mirrors have periods and thickness ratios substantially as given in the tables of Figures 7a and 7b (period 2.19nm and a ratio of 0.49 of the top (Li) layer to the period in this case), and in this case have a thickness of around 400 bi-layers (i.e. a total thickness of the C and Li layers together of around 400x2.19nm). Either a C or a Li layer of the C/Li multi-layer mirrors may be the outermost of the C or Li layers, with the outermost layer being the one of those layers that the radiation at the operating wavelength would reach first in operation. The multi-layer mirrors also include an outermost protective layer, for example a cap layer of a substantially transparent metal oxide or nitride, such as Zr0₂ or ZrN. The cap layer may protect the underlying layers from hydrogen gas or oxidation. The projection system in this case may have a numerical aperture (NA) of less than 0.5, and only 4 mirrors are required in the projection system PS instead of 6 or 8 mirrors. In alternative embodiments a larger number of mirrors may be included in the projection system PS, if desired. For example, six mirrors may be included in the projection system PS in some embodiments.

[000165] Although it can be seen from the table of Figure 7a that the C/Li mirrors of the embodiments of the preceding paragraph do not provide as high a theoretical reflectivity as, for example, C/U mirrors the reflectivity value is acceptable.

[000166] In alternative embodiments, C/U embodiments are used in place of the

C/Li mirrors to obtain reflectivity of greater than 80% per mirror, bandwidth of around 0.75% and numerical aperture of around 0.5.

[000167] As can also be seen from the tables of Figures 7a and 7b there are various pairs of materials, as well as C/Li or C/U, which can provide acceptable multi-layer mirror reflectivity values at a wavelength of 4.37nm and a choice amongst such acceptable pairs of materials can be made in dependence on other factors as well as on the value of reflectivity. For example, a choice of materials for the multi-layer mirrors can be made in dependence on one or more of the availability, cost, ease of handling or processing, ease of manufacture of the multi-layer mirror, toxicity level, radioactivity level, strength, robustness, resistance to degradation or other factors.

[000168] At lower wavelengths, such as 4.37nm, carbon and oxygen tend to be more transparent to radiation than at higher wavelengths, which means that carbon contamination on mirrors and oxidation of materials, which may occur during operation, may cause less reflectivity loss at lower wavelengths than at higher wavelengths.

[000169] Alternative carbon-based mirrors are used in the lithographic apparatus, beam splitting apparatus 20 or optical system 40 in other embodiments, for example other embodiments configured for operation at around 4.37nm. For instance, in some further embodiments the multi-layer mirrors are C/Ti, C/V, C/Ca, C/Co, C/Cr, C/Mn, C/Fe, C/La, C/Nd, C/Pd, C/Ag, C/In, C/Ce or C/Ni multi-layer mirrors. As with certain of the C/Li multilayer embodiments that have been described, the total number of bi-layers may be around 400 for operation at a wavelength of around 4.37nm. However, any suitable number of bi-layers may be used. In other embodiments, configured for operation at other wavelengths, and which may comprise other materials, the total number of bi-layers may be different. For example, in some embodiments for an operating wavelength of approximately 9.49nm, 10.5nm, 11.3nm or 17.1 nm the total number of bi-layers may be around 50 to 100. In some embodiments for an operating wavelength of approximately 22.8nm the total number of bi- layers may be around 100.

[000170] The multi-layer mirrors of certain embodiments can include other layers, for instance spacer, protective or further reflective layers, in addition to the pairs of reflective layers, for example the reflective layers identified in the preceding paragraph, in accordance with known methods of construction of multi-layer mirrors. Thus, for example, at least some of the C/U, C/Li, C/Ti, C/V, C/Ca, C/Co, C/Cr, C/Mn, C/Fe, C/La, C/Nd, C/Pd, C/Ag, C/In, C/Ce or C/Ni multi-layer mirrors can include additional protective, spacer, further reflective layers or other layers or components.

[000171] The multi-layer mirrors of embodiments are not limited to the materials indicated in Figures 7a and 7b, and other materials can be used. For example, in some further embodiments, the multi-layer mirrors include alloy layers, or oxide or nitride layers, or boride or carbide layers, or other layers of carbon compounds or metal compounds, for example metal oxide or metal nitride layers, or metal boride or metal carbide layers, in place of the metal layers indicated in the preceding paragraph. Thus, in some embodiments one of the repeating layers of the multi-layer mirrors comprises carbon and the other of the repeating layers comprises an oxide, nitride or other compound, or an alloy, of one of Li, Ti, V, Ca, Co, Cr, Mn, Fe, La, Nd, Pd, Ag, In, Ce, or Ni, or other metal. In general, oxides and nitrides may become more absorbing of radiation at higher wavelengths.

[000172] In the embodiments configured for operation at 4.37nm there is also a choice of various materials that provide suitable optical properties for use as absorbers in the lithographic system, for example as masks or reticles or as resists. The absorption properties of various materials can be seen from the tables of Figures 8a and 8b to be similar at 4.37nm for at least some materials as at 6.7nm. Thus, in some embodiments, known masks and reticles developed for use at 6.7nm can also be used at 4.37nm, and resist and reticle/mask fabrication may be similar to that for fabrication of resists and reticles/masks for us at 6.7nm.

[000173] For instance, hafnium or hafnium oxide-based masks or reticles, or resists, are used in certain of the embodiments configured for operation at 4.37nm. For instance, hafnium oxide based resists such as described in M.Trikeriotis et al, 6^th International Symposium on Immersion Lithography Extensions, 22-23 October 2009, or M.Trikeriotis et al, Development of an Inorganic Photoresist for DUV, EUV and Electron Beam Imaging, SPIE 7639, Advances in Resist Materials and Processing Technology, XXVII, 76390E (26 March 2010) can be used as resists in certain embodiments configured for operation at around 4.37nm. Any other suitable absorber materials, for example mask, reticle or resist materials suitable for use at 4.37nm can also be used.

[000174] In some embodiments, Hf, Ir, Re, Os, Pt, W or Au or alloys or compounds thereof, for example oxides thereof, are used to form masks or reticles for use at a wavelength of 4.37nm. Similarly, those materials are also used in resists for use at a wavelength of 4.37nm in some embodiments. In some embodiments, Ta, Mo, Cu, Ni, Zr or alloys or compounds thereof, are used to form masks, reticles or resists for use at a wavelength of 4.37nm, although potentially some of those materials may oxidise more readily than at least some of the other named materials. Protective layers or other measures to avoid or reduce oxidation may be used.

[000175] Various pellicle materials can also be used in embodiments configured for operation at 4.37nm. The tables of Figures 9a and 9b provide the maximum thickness of various materials to provide light transmission of 90% at 4.37nm. Any suitable material from the table, or derivative thereof, can be selected from the table for use as a pellicle at 4.37nm. The choice of pellicle materials is not limited to those indicated in the tables of Figures 9a and 9b and any suitable pellicle material that provides desired transmission properties at an acceptable thickness level can be used. For example, oxides or carbides of at least some of the materials listed in the tables of Figures 9a and 9b may be used as pellicles in some embodiments.

[000176] In some embodiments configured for operation at around 4.37nm, one or more pellicles of the lithographic system comprise carbon, scandium, titanium, lanthanum or an alloy or oxide or other compound of one or more of those materials, for example titanium oxide or titanium carbide. Such pellicles can be made thicker than at least some silicon based pellicles currently under development for 13.5nm, whilst retaining acceptable transmission properties at 4.37nm (for example having a transmission ratio of 90%). Thicker pellicles are generally expected to be thermo-mechanically stronger than thinner pellicles. The pellicles may include one or more additional passivation or other layers. In some embodiments configured for operation at around 4.37nm, the pellicle is a carbon-based pellicle, for example C or TiC, with a thickness of 280nm and a transmission ratio of 90%.

[000177] In one particular embodiment configured for operation at approximately

4.37nm, the lithographic apparatus comprises four multilayer mirrors each comprising approximately 400 bilayers of C/Li, a pellicle comprising C, a reticle comprising Re, and a resist comprising hafnium oxide.

[000178] It can be understood from the description relating to embodiments configured for operation at approximately 4.37nm, that for at least some such embodiments relatively high reflectivity of multi-layer mirrors used in the lithographic system may be obtained, the number of multi-layer mirrors may be reduced, and there may be a choice of various materials acceptable for use as multi-layer mirrors, absorbers or pellicles. The bandwidth of multi-layer mirrors for radiation at 4.37nm is similar to that obtainable at 6.7nm for at least some embodiments. Furthermore, in principle, embodiments that use source radiation at 4.37nm may be able to produce smaller scale lithographic features than embodiments that use higher wavelength radiation. In addition, carbon and oxide deposits or contamination, for example deposits or contamination on mirror surfaces, tend to become increasingly transparent at lower wavelengths. Therefore, by operating at or near 4.37nm, carbon cleaning cycles may be reduced or eliminated in the case of at least some embodiments.

[000179] Although in some embodiments there can be benefits to operating at

4.37nm, shot noise may be higher at 4.37nm than is the case at higher wavelengths, for example 13.5nm, by a factor of three in some cases. Alternative embodiments are configured to operate at other, higher wavelengths, for example wavelengths approximately equal to wavelengths at which peaks are seen in the reflectivity plots of Figure 6.

[000180] In some embodiments a lithographic system, such as that of Figures 1 to

5, is provided that is configured to operate at a wavelength of 22.8nm. It can be seen from the plot of Figure 6 and the tables of Figures 7a and 7b that the maximum attainable multi- layer mirror reflectivity for the materials considered reaches its highest value at 22.8nm (87% for Li/Be). The multi-layer mirror materials for use in embodiments configured to operate at 22.8nm are not limited to Li/Be, and any suitable combination of materials that provides acceptable reflectivity values can be used in such embodiments configured to operate at 22.8nm. [000181] In certain embodiments configured to operate at 22.8nm at least six or eight multi-layer mirrors are provided in the projection system PS of the lithographic apparatus. In some such embodiments the minimum reflectivity value for each multi-layer mirror is constrained to be greater than or equal to around 70%, and any suitable multi-layer mirror materials may be chosen in such embodiments subject to that constraint.

[000182] In certain embodiments configured to operate at 22.8nm, multi-layer mirrors comprising any of Li/Be (e.g. e.g. comprising alternating layers of lithium and beryllium), Li/Al, Li/Si, La/Li or B/Li are provided as the mirrors of the projection system PS and/or illumination system IL. The periods and ratios of top layer thickness to period for those multi-layer mirrors may be as indicated in the table of Figure 7a. Thus, a high numerical aperture (>0.5) Li-based multi-layer mirror projection system PS of the lithographic apparatus may be provided.

[000183] The Li/Be mirrors may each provide a theoretical reflectivity of up to

87%, and in practice a gain of 10% in reflectivity per mirror in comparison to mirrors of the same materials configured for operation at lower wavelengths, for example 4.37nm, may be obtainable (assuming real mirror performance of around 80% for example). It is possible that the actual reflectivity values of real devices at higher wavelengths, for example 22.8nm, may be closer to the maximum theoretical values than may be the case for lower wavelengths. In some embodiments, the thickness of the stack of bi-layers of the multi-layer mirror may be greater in embodiments configured for operation at 22.8nm than at lower wavelengths, and in some cases this may mean that diffusion effects and roughness effects at layer interfaces may cause lower reflection losses and/or flare.

[000184] At least some of the Li/Be, Li/Al, Li/Si, La/Li or B/Li multi-layer mirrors or other multi-layer mirrors configured for operation at or near wavelengths of 22.8nm, can include additional protective, spacer, further reflective layers or other layers or components in some embodiments.

[000185] The multi-layer mirrors of embodiments are not limited to the materials indicated in Figures 6 to 9, and other materials can be used. For example, in some embodiments configured to operate at approximately 22.8nm, the multi-layer mirrors include alloy layers, or layers of metal compounds, in place of the layers indicated in the preceding paragraph. Thus, in some embodiments one or both of the repeating layers comprises a compound, or an alloy, of one or other of Li, Be, Al, Si, La, B or other metal.

[000186] In the embodiments configured for operation at 22.8nm there is also a choice of various materials that provide suitable optical properties for use as absorbers in the lithographic system, for example as masks or reticles or as resists. For instance, hafnium or hafnium oxide based masks or reticles, or resists, may be used, and it can be seen from Figures 8a and 8b that Hf provides around three times improved absorptance performance at 22.8nm than at 4.37nm.

[000187] Any other suitable absorber materials, for example mask, reticle or resist materials can also be used in alternative embodiments and, given the generally higher absorption coefficient values at 22.8nm than at lower wavelengths, the choice of possible absorber materials may be wider than at lower wavelengths. Less thick material may be required to produce, for example, a reticle due to the higher absorption coefficients at 22.8nm and so, in some cases, reticle design may be more straightforward. In some embodiments, Re or Os or Re-based or Os-based materials, for example an alloy or oxide or other compound of Re or Os may be used to form the reticle or mask. In other embodiments, the reticle or mask may comprise one or more of Ru, Rh, Os, W, Pd, Re, Ag, Pt, Ti or Cr or an alloy or an oxide or other compound of one or more of those materials. Other metal oxides, particularly metal oxides included highly absorbing metals, may also be suitable for use in reticles or masks for wavelengths of approximately 22.8nm, as many oxides absorb significant amounts of radiation at such wavelengths.

[000188] Any suitable pellicles may be used in the lithographic systems configured for operation at 22.8nm. For example, pellicles comprising Al or Si, or suitable Al or Si compounds or alloys (for example, Al strengthened with B, Si strengthened with B; an alloy of Al, or an alloy of Si) may be used in some embodiments configured for operation at 22.8nm. A thin layer of silicon nitride may be used as a protective layer of the pellicle in some embodiments. Similarly a thin layer of aluminium oxide may be used as a protective or additional pellicle layer in some embodiments. In one embodiment configured for operation at around 22.8nm, the pellicle is an Al pellicle with a thickness of 65nm and a transmission ratio of 90%.

[000189] In one particular embodiment configured for operation at approximately

22.8nm, the lithographic apparatus comprises multilayer mirrors each comprising approximately 100 bilayers of Li/Be, a pellicle comprising Al, a reticle comprising Ru, and a resist comprising hafnium oxide.

[000190] The periods of the multi-layer mirrors configured for use at 22.8nm are generally larger than the periods of multi-layer mirrors of the same materials configured for operation at 13.5nm, for the materials indicated in the tables of Figures 7a and 7b. In some cases a larger period for a multi-layer mirror can lead to lower flare and lower radiation loss due, for example, to diffusion. Thus in some cases less loss per mirror can be obtained at an operating wavelength of 22.8nm than at an operating wavelength of 13.5nm, for corresponding multi-layer mirror materials. Furthermore, multi-layer mirrors with larger periods can, in some cases, be easier to manufacture than multi-layer mirrors with smaller periods, for example the multi-layer mirrors of the tables of Figures 7a and 7b configured for operation at 4.37nm, which have periods of around 2.2nm.

[000191] Although more mirrors (for example 6 or 8 mirrors) may in some cases be required in the projection systems PS of lithographic apparatus configured for operation at 22.8nm, than may be the case for corresponding systems configured for operation at lower wavelengths, the reduction in loss from each mirror may in some cases reduce the impact of the increase in the number of mirrors. Higher numerical aperture systems may be a possibility.

[000192] If there is less loss per mirror then the throughput of power from the source at the operating wavelength may be higher, due to reduced losses at the mirrors. That can relax the power requirements for the source, for example by up to a factor of five in some cases. If there is less loss per mirror, and in turn if the applied power can be reduced, then degradation of mirrors may also be reduced in some cases, for example due to a reduction in blistering, radiation damage, heat load or other degradation mechanisms. That could, in some cases, lead to longer lifetime of components.

[000193] In some embodiments the higher numerical apertures (for example >0.5) that may be obtainable for Li-based or other multi-layer mirrors at an operating wavelength of around 22.8nm can offset a 70% lower resolution (e.g. worsening of resolution of around 70% that could occur if the numerical aperture was the same) that may be obtained, in some cases, in comparison to mirrors configured for operation at lower wavelengths, for example 13.5nm.

[000194] In general, shot noise is significantly lower when operating at 22.8nm than when operating at lower wavelengths, for example 13.5nm or 4.37nm. Furthermore, double patterning is possible at 22.8nm, which can provide for formation of smaller features. As transmission loss per mirror can be low, radiation dose at the wafer can be high, and consequently throughput of wafers can be high, for at least some embodiments configured for operation at 22.8nm double patterning can be economically viable. At some lower wavelengths, feature sizes available from single patterning are lower, and thus double patterning may be used less often, if at all. [000195] Although there can be significant benefits in some cases in configuring a lithographic apparatus for operation at around 22.8nm, such lithographic apparatus provides lower resolution compared to operation at 13.5nm. Furthermore, carbon, oxides and nitrides in general absorb more light at 22.8nm than at lower wavelengths, therefore carbon cleaning of multi-layer mirrors may be required more frequently. Also, oxidation related reflectivity loss may, in some cases, be more likely to occur at 22.8nm than at 13.5nm.

[000196] The choice of operating wavelength to be used for a particular embodiment can take into account the particular characteristics of operating at that wavelength, including the properties of materials at that wavelength, and the potential benefits or drawbacks, bearing in mind desired properties of the device or devices that are intended to be formed using lithographic techniques by the embodiment, for example minimum feature size and tolerances.

[000197] Specific embodiments have been described that are configured for operation at wavelengths of around 4.37nm and 22.8nm. In further embodiments, the lithographic system of Figures 1 to 5 is configured for operation at other operating wavelengths, based on selection of suitable materials for multi-layer mirrors and other components for use at such other operating wavelengths. For instance, in some further embodiments the lithographic system is configured for operation at other wavelengths in the 4nm to 40nm range that show peaks in multi-layer mirror reflectivity in the plot of Figure 6 or other wavelengths for which materials may have desired properties, such as at around 9.49nm, 10.5nm, 11.3nm or 17.1nm, or at wavelengths for which materials have acceptable reflectivity or absorption properties and have desirable material or other properties.

[000198] In some embodiments the lithographic system is configured for operation at any suitable wavelength in the range approximately 22.8nm to approximately 25.2nm. It can be seen that the maximum multi-layer mirror reflectivity values fall after the peak at 22.8nm, but they remain relatively high and a range of materials is available for use as multilayer mirrors, absorbers and pellicles at those wavelengths, such as those materials listed in Figures 7 to 9 with suitable values of absorption coefficient or reflectivity, or oxides, carbides, nitrides or other derivatives of such materials.

[000199] Any suitable materials may be selected for the multi-layer mirrors, pellicles or absorbers (e.g. masks or resists) for the operating wavelengths of approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, or approximately 17.1nm, or approximately 22.8nm to approximately 25.2nm. [000200] In some embodiments configured for operation at approximately 9.49nm, multi-layer mirrors are formed of, for example, Pd/Sr, Ag/Sr, Rh/Sr, Pd/Eu, Rh/Eu or Eu/Ag bilayers or, for one of both layers of each bilayer, alloys or compounds thereof. In some embodiments pellicles can be formed of, for example, one or more of B, C, Zr, Nb, Mo, Eu or alloys or compounds of any one or more of those materials. In some embodiments reticles can be formed of, for example, one or more of Cu, Ni, Co, Zn, ZnCu, Fe La, brass or other Cu-Zn alloy, W, Os, Al, Ta, Hf or alloys or compounds of any one or more of those materials. The resist may, for example, be formed of material including Hf, for example including hafnium oxide. In one particular embodiment configured for operation at approximately 9.49nm, the multi-layer mirrors comprise approximately 50-100 Pd/Sr bilayers, the pellicle comprises B, the reticle comprises Cu, and the resist is formed of material comprising hafnium oxide.

[000201] In some embodiments configured for operation at approximately 10.5nm, multi-layer mirrors comprise, for example, Rh/Sr, Pd/Sr, Ru/Sr, Ag/Sr, or Mo/Sr bilayers or, for one of both layers of each bilayer, alloys or compounds thereof. In some embodiments pellicles can be formed of, for example, one or more of B, Zr, C, Nb or Mo, or alloys or compounds of any one or more of those materials. In some embodiments reticles can be formed of, for example, one or more of Cu, Ni, Co, Zn, brass or other Cu-Zn alloy, Fe, Ta, Re, Al, Hf, Os or Cr or alloys or compounds of any one or more of those materials. The resist may, for example, be formed of material including Hf, for example including hafnium oxide. In one particular embodiment configured for operation at approximately 10.5nm, the multi-layer mirrors comprise approximately 50-100 bilayers of Rh/Sr, the pellicle comprises B, the reticle comprises Cu, and the resist is formed of material comprising hafnium oxide.

[000202] In some embodiments configured for operation at approximately 11.3nm, multi-layer mirrors comprise, for example, Ru/Be, Be/Rh, Nb/Be, Mo/Be, Ru/Sr, Rh/Sr, Be/Pd, Be/Zr, B/Be, Ag/Be or Mo/Sr bilayers or, for one of both layers of each bilayer, alloys or compounds thereof. In some embodiments pellicles can be formed of, for example, one or more of B, Zr, C, Nb or Mo, or alloys or compounds of any one or more of those materials. Some of those pellicles may need to be thin for use at 11.3nm, for example around 24nm for C, or around 20nm for Mo. In some embodiments reticles can be formed of, for example, one or more of Ni, Cu, Co, Zn, Te, Fe, Ta, W, Re, Hf, Os, Pt or Al or alloys or compounds of any one or more of those materials. The resist may, for example, be formed of material including Hf, for example including hafnium oxide. In one particular embodiment configured for operation at approximately 11.3nm, the multi-layer mirrors comprise approximately 50-100 bilayers of Ru/Be, the pellicle comprises B, the reticle comprises Ni, and the resist is formed of material comprising hafnium oxide.

[000203] In some embodiments configured for operation at approximately 17.1nm, multi-layer mirrors comprise, for example, Al/Sr, Y/Al, Be/Al, Al/Zr, Ca/Al, Nb/Al, B/Al, Al/Si, Al/Mo, La/Al or Ti/Al bilayers or, for one of both layers of each bilayer, alloys or compounds thereof. In some embodiments pellicles can be formed of, for example, one or more of Al, Si, La, B or Zr or alloys or compounds of any one or more of those materials. In some embodiments reticles can be formed of, for example, one or more of Pt, Ag, Pd, Rh, Ir, Co, Ni, Os, Au, Re, Ti, Cu, W, Te, Cr, Hf, Fe or Zn or alloys or compounds of any one or more of those materials. The resist may, for example, be formed of material including Hf, for example including hafnium oxide. In one particular embodiment configured for operation at approximately 17.1nm, the multi-layer mirrors comprise approximately 50-100 bilayers of Al/Sr, the pellicle comprises Al, the reticle comprises Pt, and the resist is formed of material comprising hafnium oxide.

[000204] In embodiments configured for operation in the range between approximately 22.8nm and approximately 25.2nm, in general the same or similar materials for multi-layer mirrors, pellicles, reticles and resists can be used as are used for embodiments configured for operation at 22.8nm, although magnesium may also be used in multi-layer mirrors at higher wavelengths of the range, for example at around 25.2nm.

[000205] In general, hafnium oxide is suitable to include in resist material for embodiments configured for operation at all of the operating wavelengths discussed herein. In general, oxides of any of the metals described as being used for reticles at a particular operating wavelength may also potentially be used as resist materials at that operating wavelength.

[000206] It has been mentioned in connection with various embodiments that Sr, Be or Y may be used in certain components of a lithographic system, for example in multi-layer mirrors. It is noted that Be is toxic, Sr is highly reactive and Y burns readily in air so those materials may need particular care in handling during manufacture of components. Anti- oxidation or other protection, for example the use of one or more protective layers on the multi-layer mirrors, is likely to be provided for embodiments that include Sr or Y. Similarly, Zr can oxidise easily and so anti-oxidation measures, for example protective layers on multilayer mirrors may be used in connection with that material.

[000207] It has been mentioned in connection with various embodiments that C or carbon compounds may be used in certain components of a lithographic system, for example in multi-layer mirrors or pellicles. Any suitable form of carbon may be used, for example graphene or graphene compounds can be used if so desired.

[000208] There is description above concerning selection of materials for components of a lithographic apparatus such that the apparatus is configured for operation at a selected wavelength, for example at around 4.37nm or 22.8nm, or at around 9.49nm, 10.5nm, 11.3nm or 17.1nm or between 22.8nm and 25.2nm. Material for components of other parts of the lithographic system, as well as the lithographic apparatus LA, can also selected in dependence on the particular wavelength at which the lithographic system is configured for operation. For example, the materials of which grazing mirrors or other reflectors of the optical system 40, beam splitting apparatus 20 or source SO are formed can be selected based on the intended operating wavelength of the lithographic system, and in some embodiments the same or similar materials that are used to form the multi-layer mirrors or other reflectors of the lithographic apparatus are also used to form grazing mirrors or other reflectors of the optical system 40 or beam splitting apparatus 20.

[000209] Tables of calculated grazing incidence reflectance at grazing incidence angle of 5 degrees, calculated using Fresnel reflection coefficients, are provided in Figures 10a and 10b for various materials.

[000210] It can be seen from the tables of Figures 10a and 10b that at a wavelength of 4.37nm, uranium has the highest grazing reflectance of 89%, and may be used in grazing mirrors in some embodiments. Co, Cr, Mn, Fe, V, Ni, which could also be used for grazing mirrors in some embodiments, have reflectance>60% which represents a significant loss, although lowering the grazing angles to 2 or 3 degrees improves the situation for those materials. If grazing mirrors are used in particular embodiments at a wavelength of approximately 4.37nm using the materials considered then, in some cases, one may have to accept large losses or go to lower grazing angles, or use uranium.

[000211] With regard to other embodiments, at an operating wavelength of approximately 9.49nm, Pd, Rh, Ag, Ru, Mo, Nb, Cd, B, C, Au, Zr have reflectance (R) >85% at a 5 degree grazing angle and can be used in grazing mirrors in some embodiments. At an operating wavelength of approximately 10.5nm, Rh, Ru, Pd, Ag, Mo, Nb, B, Zr, C, Au have reflectance (R) >85% at a 5 degree grazing angle and can be used in grazing mirrors in some embodiments. At an operating wavelength of approximately 11.3nm, Ru, Rh ,Mo, Nb, Pd, Ag, Zr, B, C, Y, Au have reflectance (R) >85% at a 5 degree grazing angle and can be used in grazing mirrors in some embodiments. At an operating wavelength of approximately 17.1nm, Y, Zr, Nb, Sr, Mo, Be, B, Ti, U, C, Sc, Ru, La have reflectance (R) >85% at a 5 degree grazing angle and can be used in grazing mirrors in some embodiments. At an operating wavelength of approximately 22.8nm, Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U, Ti have reflectance (R) >85 at a 5 degree grazing angle and can be used in grazing mirrors in some embodiments.

[000212] For each of the operating wavelengths, mixtures or compounds of two or more of the listed materials for that wavelength can be used in grazing mirrors in some embodiments.

[000213] The selection of materials for multi-layer mirrors, absorbers and pellicles in particular embodiments for use at described operating wavelengths have been described. The multi-layer mirrors, reticles, resists and pellicles of various embodiments can be formed using any suitable technique, for example known multi-layer mirror, reticle, resist or pellicle manufacturing techniques. For example, the multi-layer mirrors may be manufactured using sputtering techniques to deposit the layers of material.

[000214] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS:

1. A lithographic apparatus for projecting a pattern from a patterning device onto a substrate, the lithographic apparatus comprising a patterning device support structure constructed to support a patterning device and a substrate support constructed to hold a substrate,

wherein the apparatus is configured to receive at an input a beam of radiation at an operating wavelength and to direct the beam of radiation at the operating wavelength along a radiation path, such that in operation when a patterning device is supported by the patterning device support structure and a substrate is held by the substrate support a pattern from the patterning device is projected onto the substrate; and

the operating wavelength is one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 17.1nm, approximately 22.8nm, or in range of approximately 22.8nm to approximately 25.2nm.

2. A lithographic apparatus according to Claim 1, comprising a plurality of multi-layer mirrors, each multi-layer mirror comprising a plurality of layers formed of a first material and a plurality of further layers formed of a second, different material. 3. A lithographic apparatus according to Claim 2, wherein for at least one of the multilayer mirrors the first material comprises carbon; a nitride, oxide or other compound of carbon; or a carbon-containing alloy.

4. A lithographic apparatus according to Claim 2 or 3, wherein for at least one of the multi-layer mirrors the first material comprises graphene or a graphene compound.

5. A lithographic apparatus according to any of Claims 2 to 4, wherein for at least one of the multi-layer mirrors the second material comprises at least one of:- Li, Ti, V, Ca, Co, Cr, Mn, Fe, La, Nd, Pd, Ag, In, Ce, Ni or an oxide, nitride or other compound, or alloy, thereof.

6. A lithographic apparatus according to any of Claims 2 to 5, wherein the operating wavelength is approximately equal to 4.37nm.

7. A lithographic apparatus according to Claim 2, wherein for at least one of the multilayer mirrors the first material comprises at least one of:- Li; an alloy of, or nitride, oxide or other compound of, Li; Be; an alloy of, or nitride, oxide or other compound of, Be. 8. A lithographic apparatus according to Claim 2 or 7, wherein for at least one of the multi-layer mirrors the second material comprises at least one of:- Be, Al, Si, La, B, or an alloy of, or nitride, oxide or other compound of, Be, Al, Si, La, or B.

9. A lithographic apparatus according to any of Claims 2, 7 or 8, wherein the operating wavelength is approximately equal to 22.8nm

10. A lithographic apparatus according to Claim 2, wherein at least one of:- the operating wavelength is approximately equal to 4.37nm and at least one of the multi-layer mirrors comprises C/Li, C/U, C/Ti, C/V, C/Ca, C/Co, C/Cr, C/Mn, C/Fe, C/La, C/Nd, C/Pd, C/Ag, C/In, C/Ce or C/Ni bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 22.8nm, or in a range approximately 22.8nm to approximately 25.2nm, and at least one of the multi-layer mirrors comprises Li/Be, Li/Al, Li/Si, La/Li or B/Li bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 9.49nm and at least one of the multi-layer mirrors comprises Pd/Sr, Ag/Sr, Rh/Sr, Pd/Eu, Rh/Eu or Eu/Ag bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 10.5nm and at least one of the multi-layer mirrors comprises Rh/Sr, Pd/Sr, Ru/Sr, Ag/Sr, or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 11.3nm and at least one of the multi-layer mirrors comprises Ru/Be, Be/Rh, Nb/Be, Mo/Be, Ru/Sr, Rh/Sr, Be/Pd, Be/Zr, B/Be, Ag/Be or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 17.1nm and at least one of the multi-layer mirrors comprises Al/Sr, Y/Al, Be/Al, Al/Zr, Ca/Al, Nb/Al, B/Al, Al/Si, Al/Mo, La/Al or Ti/Al bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

11. A lithographic apparatus according to any preceding claim, comprising a plurality of reflectors, wherein a calculated maximum reflectivity of at least one of the reflectors at the operating wavelength is greater than or equal to 60%, optionally greater than or equal to 70%.

12. A lithographic apparatus according to any preceding claim, wherein the lithographic apparatus comprises at least one absorber comprising or forming part of a reticle or resist.

13. A lithographic apparatus according to Claim 12, wherein at least one of:- the lithographic apparatus is configured for operation at approximately 4.37nm and the absorber comprises Hf, Ir, Re, Os, Pt, W, Au, Ta, Mo, Cu, Ni or Zr or an alloy or compound of one or more thereof;

the lithographic apparatus is configured for operation at approximately 9.49nm and the absorber comprises Cu, Ni, Co, Zn, ZnCu, Fe, La, brass or other Cu-Zn alloy, W, Os, Al, Ta or Hf or an alloy or compound of one or more thereof;

the lithographic apparatus is configured for operation at approximately 10.5nm and the absorber comprises Cu, Ni, Co, Zn, brass or other Cu-Zn alloy, Fe, Ta, Re, Al, Hf, Os or Cr or an alloy or compound of one or more thereof;

the lithographic apparatus is configured for operation at approximately 11.3nm and the absorber comprises Ni, Cu, Co, Zn, Te, Fe, Ta, W, Re, Hf, Os, Pt or Al or an alloy or compound of one or more thereof;

the lithographic apparatus is configured for operation at approximately 17.1nm and the absorber comprises Pt, Ag, Pd, Rh, Ir, Co, Ni, Os, Au, Re, Ti, Cu, W, Te, Cr, Hf, Fe or Zn or an alloy or compound of one or more thereof;

the lithographic apparatus is configured for operation at approximately 22.8nm and the absorber comprises Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti, Cr or Hf or an alloy or compound of one or more thereof ;

the lithographic apparatus is configured for operation at wavelength in the range 22.8nm to 25.1nm and the absorber comprises Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti, Cr or Hf or an alloy or compound of one or more thereof .

14. A lithographic apparatus according to any preceding claim, comprising at least one pellicle, wherein the material and thickness of the pellicle are selected to provide a transmissivity of the pellicle at the operating wavelength of greater than or equal to 90%.

15. A lithographic apparatus according to any preceding claim, comprising at least one pellicle wherein at least one of:- the lithographic apparatus is configured for operation at approximately 4.37nm and the pellicle comprises C, Ti, Sc or La or an alloy or compound including one or more thereof; the lithographic apparatus is configured for operation at approximately 9.49nm and the pellicle comprises B, C, Zr, Nb, Mo, or Eu or an alloy or compound including one or more thereof;

the lithographic apparatus is configured for operation at approximately 10.5nm and the pellicle comprises B, Zr, C, Nb or Mo or an alloy or compound including one or more thereof;

the lithographic apparatus is configured for operation at approximately 11.3nm and the pellicle comprises B, Zr, C, Nb or Mo or an alloy or compound including one or more thereof;

the lithographic apparatus is configured for operation at approximately 17.1nm and the pellicle comprises Al, Si, La, B or Zr or an alloy or compound including one or more thereof;

the lithographic apparatus is configured for operation at approximately 22.8nm, or in a range approximately 22.8nm to 25.2nm, and the pellicle comprises Al, Si or an alloy or compound including one or more thereof.

16. A lithographic apparatus according to Claim 15, wherein the lithographic apparatus is configured for operation at approximately 22.8nm and the at least one pellicle comprises at least one of Al strengthened with B, Si strengthened with B, a nitride or oxide of Al, or a nitride of Si.

17. A lithographic apparatus according to any preceding claim, comprising an illumination system configured to condition the beam of radiation, and a projection system configured to project the patterned radiation beam onto the substrate, wherein the projection system comprises four or fewer multi-layer mirrors.

18. A lithographic apparatus according to any preceding claim, wherein one of:- the operating wavelength is approximately equal to 4.37nm, and the apparatus comprises at least one multilayer mirror comprising C/Li bilayers, at least one pellicle comprising C, and at least one reticle comprising Re;

the operating wavelength is approximately equal to 22.8nm and the apparatus comprises at least one multilayer mirror comprising Li/Be bilayers, at least one pellicle comprising Al, and at least one reticle comprising Ru;

the operating wavelength is approximately equal to 9.49nm and the apparatus comprises at least one multilayer mirror comprising Pd/Sr bilayers, at least one pellicle comprising B, and at least one reticle comprising Cu;

the operating wavelength is approximately equal to 10.5nm and the apparatus comprises at least one multilayer mirror comprising Rh/Sr bilayers, at least one pellicle comprising B, and at least one reticle comprising Cu;

the operating wavelength is approximately equal to 11.3nm and the apparatus comprises at least one multilayer mirror comprising Ru/Be bilayers, at least one pellicle comprising B, and at least one reticle comprising Ni;

the operating wavelength is approximately equal to 17.1nm and the apparatus comprises at least one multilayer mirror comprising Al/Sr bilayers, at least one pellicle comprising Al, and at least one reticle comprising Pt;

the operating wavelength is in a range from approximately 22.8nm to approximately 25.2nm and the apparatus comprises at least one multilayer mirror comprising Li/Be bilayers, at least one pellicle comprising Al, and at least one reticle comprising Ru.

19. A radiation source configured to provide a beam of radiation at an operating wavelength to at least one lithographic apparatus, the radiation source comprising a free electron laser for generating the beam of radiation, wherein the operating wavelength is one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 17.1nm or approximately 22.8nm, or in a range approximately 22.8nm to approximately 25.2nm. 20. A lithographic system comprising a radiation source according to Claim 19, and at least one lithographic apparatus according to any of Claims 1 to 18.

21. A lithographic system according to Claim 20, further comprising at least one optical element between the source and the lithographic apparatus, arranged to condition the beam of radiation and/or to direct the beam of radiation, wherein the optical element comprises at least one of a grazing mirror, a convex mirror, a concave mirror, an astigmatic or aspherical mirror or other reflector, or a beam splitter element. 22. A lithographic system according to Claim 21, wherein the optical element comprises a grazing mirror and at least one of:- the operating wavelength is approximately equal to 4.37nm and the grazing mirror comprises U, Co, Cr, Mn, Fe, V, or Ni or an alloy or compound including one or more thereof;

the operating wavelength is in a range from approximately 22.8nm to approximately

25.2nm and the grazing mirror comprises Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U, Ti or an alloy or compound including one or more thereof.

23. A multi-layer mirror for use at an operating wavelength in a lithographic apparatus, wherein at least one of:- the operating wavelength is approximately equal to 4.37nm and the multi-layer mirror comprises C/Li, C/U, C/Ti, C/V, C/Ca, C/Co, C/Cr, C/Mn, C/Fe, C/La, C/Nd, C/Pd, C/Ag, C/In, C/Ce or C/Ni bilayers or, for one of both layers of each bilayer, an alloy or compound thereof; the operating wavelength is approximately equal to 22.8nm and the multi-layer mirror comprises Li/Be, Li/Al, Li/Si, La/Li or B/Li bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 9.49nm and the multi-layer mirror comprises Pd/Sr, Ag/Sr, Rh/Sr, Pd/Eu, Rh/Eu or Eu/Ag bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 10.5nm and the multi-layer mirror comprises Rh/Sr, Pd/Sr, Ru/Sr, Ag/Sr, or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof.

the operating wavelength is approximately equal to 11.3nm and the multi-layer mirror comprises Ru/Be, Be/Rh, Nb/Be, Mo/Be, Ru/Sr, Rh/Sr, Be/Pd, Be/Zr, B/Be, Ag/Be or Mo/Sr bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is approximately equal to 17.1nm and the multi-layer mirror comprises Al/Sr, Y/Al, Be/Al, Al/Zr, Ca/Al, Nb/Al, B/Al, Al/Si, Al/Mo, La/Al or Ti/Al bilayers or, for one of both layers of each bilayer, an alloy or compound thereof;

the operating wavelength is in a range from approximately 22.8nm to approximately 25.2nm and the multi-layer mirror comprises Li/Be, Li/Al, Li/Si, La/Li or B/Li bilayers or, for one of both layers of each bilayer, an alloy or compound thereof. 24. An absorber for use at an operating wavelength in a lithographic apparatus, wherein the absorber comprises a reticle or mask and at least one of:- the operating wavelength is approximately 4.37nm and the absorber comprises Hf, Ir, Re, Os, Pt, W, Au, Ta, Mo, Cu, Ni or Zr or an alloy or compound including one or more thereof;

the operating wavelength is approximately 9.49nm and the absorber comprises Cu,

Ni, Co, Zn, ZnCu, Fe, La, brass or other Cu-Zn alloy, W, Os, Al, Ta or Hf or an alloy or compound including one or more thereof;

the operating wavelength is approximately 10.5nm and the absorber comprises Cu, Ni, Co, Zn, brass or other Cu-Zn alloy, Fe, Ta, Re, Al, Hf, Os or Cr or an alloy or compound including one or more thereof;

the operating wavelength is approximately 11.3nm and the absorber comprises Ni, Cu, Co, Zn, Te, Fe, Ta, W, Re, Hf, Os, Pt or Al or an alloy or compound including one or more thereof; the operating wavelength is approximately 17.1nm and the absorber comprises Pt, Ag, Pd, Rh, Ir, Co, Ni, Os, Au, Re, Ti, Cu, W, Te, Cr, Hf, Fe or Zn or an alloy or compound including one or more thereof;

the operating wavelength is approximately 22.8nm and the absorber comprises Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti or Cr or an alloy or compound including one or more thereof ; the operating wavelength is in the range approximately 22.8nm to approximately 25.1nm and the absorber comprises Ru, Rh Os, W, Re, Pd, Ag, Pt, Ti or Cr or an alloy or compound including one or more thereof . 25. A pellicle for use at an operating wavelength in a lithographic apparatus, wherein at least one of:- the operating wavelength is approximately 4.37nm and the pellicle comprises C, Ti, Sc or La or an alloy or compound including one or more thereof;

the operating wavelength is approximately 17.1nm and the pellicle comprises Al, Si,

La, B or Zr or an alloy or compound including one or more thereof;

the operating wavelength is approximately 22.8nm and the pellicle comprises Al, Si or an alloy or compound including one or more thereof;

26. A grazing mirror for use at an operating wavelength in a lithographic system and at least one of:- the operating wavelength is approximately equal to 4.37nm and the grazing mirror comprises or an alloy or compound of U, Co, Cr, Mn, Fe, V, Ni or an alloy or compound including one or more thereof; the operating wavelength is approximately equal to 22.8nm and the grazing mirror comprises Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U or Ti or an alloy or compound including one or more thereof;

the operating wavelength is in a range from approximately 22.8nm to approximately 25.2nm and the grazing mirror comprises Be, La, Y, B, Sc, Sr, Zr, Si, C, Pr, Nb, U, Ti or an alloy or compound including one or more thereof. 27. A method of projecting a pattern from a patterning device onto a substrate, the method comprising generating a beam of radiation at an operating wavelength using a free electron source, and providing the beam of radiation to a patterning device of a lithographic apparatus such that a pattern is projected from the patterning device onto the substrate, wherein

the operating wavelength is one of approximately 4.37nm, approximately 9.49nm, approximately 10.5nm, approximately 11.3nm, approximately 17.1nm, or approximately 22.8nm, or in a range approximately 22.8nm to approximately 25.2nm.