WO2013152921A1 - Pellicle, reticle assembly and lithographic apparatus - Google Patents

Abstract

Pellicles or films are disclosed that are suitable for use as protective covers for EUV device lithography reticles (patterning structures). The pellicles pass radiation of wavelength 5nm to 20nm whilst acting as a barrier to particulate deposits on reticles, which would otherwise lead to defects in devices patterned using the reticles. Also disclosed are reticle assemblies and lithographic apparatus including such pellicles, as well as methods for forming the pellicles. The pellicles may have a multilayer configuration, with the central region having two or more layers of silicon alternating with layers of the refractory material. Silicon oxide or nitride may be used as an interfacial layer for adhesion/anti-diffusion between the silicon and the refractory material. The pellicles are capable of self-support when tensioned over a reticle, without need of a support grid, even when sufficiently thin to permit high EUV transmissivity.

Description

PELLICLE, RETICLE ASSEMBLY AND LITHOGRAPHIC APPARATUS

BACKGROUND OF THE INVENTION

Cross-reference to related applications

[0001] This application claims the benefit of US provisional application 61/623,207, which was filed on April 12^th, 2012, and which is incorporated herein in its entirety by reference.

Field of the Invention

[0002] The present invention relates to films, referred to herein as pellicles, for use in reducing debris deposition onto reticles for EUV device lithography. The invention also relates to reticles and lithographic apparatus that include such pellicles.

Background Art

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

[0005] A lithographic apparatus typically includes an illumination system configured to condition a radiation beam; a support structure constructed to hold a patterning device, such as a reticle or mask, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

[0006] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

CD = k *—^- (1)

NA_PS

where λ is the wavelength of the radiation used, NA_PS is the numerical aperture of the projection system used to print the pattern, ki is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAps, or by decreasing the value of k_r

[0007] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of about 5-20 nm, for example within the range of about 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of about 5-10 nm such as about 6.7 nm or about 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0008] EUV radiation may be produced using a plasma. A radiation system for producing

EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source-collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source-collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source. In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are configured to output a radiation wavelength from 5 to 20 nm, such as of about and/or below 13.5 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.

SUMMARY OF THE INVENTION

[0009] Pellicles or films are used in EUV lithography to shield or protect the patterned surfaces of reticles that are used to impart patterns to an EUV beam incident on the reticle's patterned surface for use in device patterning. It is desirable that such pellicles are highly transmissive to the EUV radiation whilst also acting as a barrier to particulate deposits transferring onto the patterned surface of the reticles. Such deposits could lead to defects in devices patterned using the reticles, and so it is desirable to maintain low levels of such defects to sustain high production yields. In practice, no particles in excess of a certain particle size (say about 20nm) may be tolerated on a reticle surface. The use of a pellicle may increase the tolerated particle size to say about 500nm, and this also allows for inspection and monitoring of particle contamination, on the pellicle surface, to be facilitated. Larger particles are more easily monitored.

[0010] Contaminant particles may arise, in an EUV lithography apparatus, from various sources. Fast-moving particles can be produced from the EUV radiation source, particularly when the source includes a plasma for generation of EUV radiation, and these may pass through the intermediate focus aperture of the source-collector assembly into the illuminator assembly and eventually may reach reticles after elastic collisions with mirrors, following the path of the EUV beam.

[0011] Slow-moving particles may be lifted by system vibration and moving parts inside an EUV apparatus and may randomly deposit on patterned surfaces of reticles.

[0012] Prior art pellicles are typically made of a silicon membrane or film, as silicon has a high transmissivity for EUV radiation. However, the low thickness required for the silicon pellicle may require a grid or honeycomb support structure to hold the pellicle in place over a patterned surface of a reticle. In order to reduce the impact of the grid pattern interfering with the EUV patterning by the reticle, it may be necessary for the pellicle and grid to be supported several mm, such as 5mm, spaced from the patterned surface of the reticle. In lithographic apparatus, clearance space is at a premium and so it is desirable to have pellicles positioned close to the patterned reticle surface, say as close as about 2.5 mm or less from the patterned surface. However, even with spacings of about 5mm or more for the pellicle, from the reticle, the grid pattern still may have a negative impact on high resolution patterning.

[0013] As EUV lithography is typically carried out under high vacuum conditions, in order to avoid absorption of EUV radiation beams by gases in their path, the temperature of a pellicle may increase considerably over the duration of an EUV radiation pulse, as EUV energy is absorbed by the pellicle and converted to heat energy, with heat loss predominantly occurring by radiative loss. Thus it is desirable that a pellicle should be able to withstand such temperature cycling without excessive sagging, which could in turn lead to risk of the pellicle contacting other EUV tool components, or wrinkling, which could affect imaging resolution.

[0014] Hence, it is an object of the invention to provide pellicles, suitable for shielding reticles from particulate deposition, which address or overcome the problems in the prior art, as set out above or otherwise.

[0015] An aspect of the invention provides a pellicle for shielding reticles for EUV device lithography, the pellicle comprising a central region comprising at least one layer of silicon, and the central region having opposed faces of silicon,

wherein the central region is sandwiched between cap layers provided on the opposed faces of silicon, the cap layers forming outermost faces of the pellicle, and

wherein each cap layer is of a refractory material selected from the group consisting of:

the elements Nb, Zr, Y, La, Ce,

alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

silicides of Mo, Nb, Ru, Zr, Y, La and Ce,

silicides of such alloys,

oxides of Mo, Nb, Ru, Zr, La, Ce,

oxides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of such alloys, nitrides of Mo, Nb, Ru, Zr, La, Ce and

nitrides of alloys of No, Nb, Ru, Zr, La, Y, Ce.

[0016] An aspect of the invention provides a reticle assembly for EUV device lithography, the reticle assembly comprising:

a reticle having a patterned surface adapted to impart a pattern to an EUV radiation beam incident thereon,

a frame, and

a pellicle according to an aspect of the invention,

wherein the frame is arranged to hold the pellicle tensioned over the patterned surface.

[0017] An aspect of the invention provides an EUV lithographic projection apparatus arranged to project a pattern from a reticle assembly onto a substrate with a radiation beam having a wavelength from about 5nm to about 20nm, wherein the reticle assembly is a reticle assembly according to an aspect of the invention.

[0018] An aspect of the invention provides a method of forming a pellicle configured to transmit radiation having a wavelength from about 5nm to about 20nm, the method comprising:

providing a silicon membrane having opposed outermost silicon faces, providing interfacial layers of silicon oxide or silicon nitride on each of the outermost silicon faces,

providing cap layers of a refractory material on the interfacial layers, wherein the refractory material is selected from the group consisting of:

the elements Nb, Zr, Y, La, Ce,

alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

silicides of Mo, Nb, Ru, Zr, Y, La and Ce,

silicides of such alloys,

oxides of Mo, Nb, Ru, Zr, La, Ce,

oxides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of such alloys,

nitrides of Mo, Nb, Ru, Zr, La, Ce and

nitrides of alloys of No, Nb, Ru, Zr, La, Y, Ce. [0019] Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of others. The term "consisting essentially of or "consists essentially of means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, a composition consisting essentially of a set of components will comprise less than about 10% by weight, typically less than about 5% by weight, more typically less than about 3% by weight, such as less than about 1% by weight of non-specified components. The term "consisting of or "consists of means that other components are specifically excluded.

[0020] Whenever appropriate, the use of the term "comprises" or "comprising" may also be taken to include the meaning or "consisting essentially of or "consisting of.

[0021] When it is said, in this specification, that a material is "X," or is "of X" it means that the material consists essentially of "X".

[0022] Where mention is made in this specification of compounds such as oxides carbides, nitrides or silicides, it is to be understood that these terms refer to both stoichiometric and non- stoichiometric compounds, the latter being formed in circumstances where the compound is formed by a process, such as sputtering or chemical vapour deposition, that may evidently lead to the provision of non- stoichiometric compounds in layers. For instance, silicon nitride may be the stoichiometric form S1₃N4, or it may be SiN_x, where x is a number, for instance from about 0.1 to about 1.4. Similarly, Mo silicide and Nb silicide may refer to MoSi₂ and NbSi₂ respectively, or to non- stoichiometric compounds.

[0023] The optional and/or preferred features set out in this specification, either in the description or in the claims, may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional and/or preferred features for each aspect of the invention set out herein are also applicable to any other aspects of the invention, where appropriate.

[0024] As set out above, an aspect of the invention provides a pellicle for shielding reticles for EUV device lithography. By EUV radiation in this specification is meant radiation having a wavelength from about 5nm to about 20nm, such as about 1 lnm to about 16nm, or for instance from about 6.4nm to about 7.2nm. The pellicle suitably has a high transmissivity to such EUV radiation, by which is meant that the intensity maintained on single pass transmission through the pellicle for normal incidence is 80% or more, say 85% or more or even 90% or more. It will be understood that in use, EUV will undergo dual passage through the pellicle, first to impinge upon a reticle and second when leaving the reticle after a pattern has been imparted to the EUV beam as a result of interaction with a patterned surface of the reticle.

[0025] The pellicle comprises a central region comprising at least one layer of silicon, and having opposed faces of silicon. The central region will typically be in the form of a thin sheet or membrane. The central region may be a silicon membrane or sheet, or may be a multilayer structure, as set out hereinbelow, with opposed faces of silicon.

[0026] The central region is sandwiched between cap layers provided on the opposed faces of silicon of the central region. Hence, the cap layers form outermost faces of the pellicle, enclosing the central region. The cap layers may be directly in contact with the opposed silicon faces of the central region, or an intermediate layer or layers may be present between the cap layers and the silicon faces of the central region, as is set out hereinbelow.

[0027] Each cap layer is suitably of a refractory material selected from:

the elements Nb, Zr, Y, La, Ce, alloys of Mo, Nb, Ru, Zr, Y, La, Ce, silicides of Mo, Nb, Ru, Zr, Y, La, Ce and alloys thereof, oxides of Mo, Nb, Ru, Zr, La, Ce, oxides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce, carbides of Mo, Nb, Ru, Zr, Y, La, Ce and alloys thereof, nitrides of Mo, Nb, Ru, Zr, La, Ce and nitrides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce.

[0028] By a refractory material, in this specification, is meant a material having a melting point in excess of about 800°C, preferably in excess of about 2000°C, more preferably in excess of about 2400°C. Preferably the refractory material is selected from the elements Nb and alloys of Mo or Nb, silicides of Mo, Nb and alloys thereof, oxides of Mo, Nb and alloys thereof, carbides of Mo, Nb and alloys thereof, nitrides of Mo, Nb and alloys thereof.

[0029] A particularly useful refractory material is selected from Nb, alloys of Mo or Nb, molybdenum silicide, niobium silicide and silicides of alloys of Mo and Nb.

[0030] A layer of silicon oxide (Si0₂ or a non-stoichiometric oxide layer) or silicon nitride

(S1₃N4 or a non stoichiometric silicon nitride) may be provided as an interfacial layer between each silicon face of the central region and its respective cap layer. [0031] The central region of the pellicle of the invention may comprise a plurality of layers of silicon, such as from 2 to 5 layers of silicon, alternating with a layer or layers of the refractory material, arranged so that the central region has silicon layers providing opposed silicon faces of the central region. In other words, the central region is defined as having silicon layers as its outermost layers, whether it is a single layer of silicon, or a multilayer with silicon layers as outermost layers of the central region.

[0032] Although the layers of refractory material (including cap layers) are preferably of the same refractory material, it is also possible to put the invention into effect with layers of refractory material that differ from each other.

[0033] A layer of silicon oxide or silicon nitride, as already described herein, may be provided as an interfacial layer between each silicon layer and respective adjacent layer of refractory material.

[0034] Suitably, the silicon nitride or silicon oxide layers, if present, each may have a thickness from about O.lnm to 5nm.

[0035] The total thickness of silicon layers in the pellicle may suitably be from about lOnm to about 100 nm such as from about 20nm to about 60nm.

[0036] The thickness of each layer of refractory material in the pellicle may be from about lnm to about 8nm.

[0037] The layer of silicon, or at least one layer of silicon, when a plurality of silicon layers is present in the pellicle, may be doped, for instance n- or p- doped, to provide electrical conductivity therein.

[0038] An aspect of the invention provides a reticle assembly for EUV device lithography.

The reticle assembly includes a reticle having a patterned surface adapted to impart a pattern to an EUV radiation beam incident thereon, a frame, and a pellicle according to an aspect of the invention. The frame is arranged to hold the pellicle tensioned over the patterned surface. Suitably, the frame may be mounted to the reticle, for instance directly bonded thereto.

[0039] Typically, the patterned surface of the reticle will be arranged to impart a pattern to an EUV beam reflected from the patterned surface of the reticle, with the EUV beam passing through the pellicle twice: first prior to patterning and second after patterning.

[0040] The frame may be adapted to hold the pellicle tensioned in a pellicle plane lying parallel to a reticle plane defined by the patterned face of the reticle. However, it may be advantageous to have the frame adapted to hold the pellicle tensioned in a pellicle plane lying at an angle from about 0.01° to about 1° to a reticle plane defined by the patterned face of the reticle. Preferably, the angle may be from about 0.02° to about 0.1°.

[0041] An aspect of the invention provides an EUV lithographic projection apparatus arranged to project a pattern from a reticle assembly onto a substrate with a radiation beam having a wavelength from about 5nm to about 20nm, wherein the reticle assembly is a reticle assembly according to an aspect of the invention. The EUV lithographic projection apparatus may further include an illumination system configured to condition the radiation beam; a support structure constructed to hold the reticle assembly, the reticle assembly being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

[0042] An aspect of the invention provides a method of forming a pellicle configured to transmit radiation having a wavelength from about 5nm to about 20nm. The method comprises providing a silicon membrane having opposed outermost silicon faces. The membrane may be provided by suitable methods known in the prior art for silicon membrane formation. Optionally, but preferably, interfacial layers of silicon oxide or silicon nitride may be provided on each of the outermost silicon faces of the silicon membrane. Subsequently, layers of a refractory material may be provided, on the faces of the silicon membrane or on the interfacial layers, if present. The refractory material is as set out hereinbefore in relation to the other aspects of the invention. The process to this stage, as described, provided cap layers of refractory material on a central region, as set out for a pellicle aspect of the invention.

[0043] The method of this aspect of the invention may further comprise optionally, but preferably, providing first further interfacial layers of silicon oxide or silicon nitride on each of the cap layers. Further silicon layers may be provided on each of the cap layers, or on each of the first further interfacial layers, if provided. Second further interfacial layers of silicon oxide or silicon nitride may then be optionally, but preferably, provided on each of the outermost silicon faces, and further layers of the refractory material may be provided on the second further primer layers, if present, or on the further silicon layers, if not. The further layers of refractory material may act as cap layers, as set out hereinbefore, or the process steps set out in the previous paragraph may be repeated once more to provide further layers of silicon and refractory material with optional intermediate layers therebetween.

[0044] The method of the invention may be effected by providing the various layers by means of deposition such as sputtering or chemical vapour deposition.

[0045] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0047] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the present invention;

[0048] Figure 2 schematically depicts a side view of an embodiment of an EUV illumination system and projection system of the lithographic projection apparatus of

Figure 1;

[0049] Figure 3 schematically depicts a view of a laser produced plasma source-collector module/assembly of the apparatus of Figure 1 in accordance with an embodiment of the present invention;

[0050] Figure 4 schematically depicts a reticle assembly that may be used in the lithographic apparatus of Figure 1 or Figure 2 in accordance with an embodiment of the present invention;

[0051] Figure 5 schematically depicts a multilayer pellicle that may be used in the reticle assembly of Figure 4 in accordance with an embodiment of the present invention; and

[0052] Figure 6 schematically depicts a cross-sectional view through another embodiment of a pellicle 2, which may be used as pellicle in the reticle assembly of Figure 4. [0053] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION OF THE INVENTION

[0054] Figure 1 schematically depicts a lithographic apparatus 100 according to an embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0055] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0056] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0057] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0058] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam that is reflected by the mirror matrix.

[0059] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. Some gas may be provided in some parts of the lithographic apparatus, for example to allow gas flow to be used to reduce the likelihood of contamination reaching optical components of the lithographic apparatus.

[0060] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0061] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0062] Referring to Figure 1, the illuminator IL receives an extreme ultra violet (EUV) radiation beam from the source-collector module/assembly SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP"), the desired plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source-collector module/assembly SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source-collector module/assembly. The laser and the source-collector module/assembly may be separate entities, for example when a C0₂ laser is used to provide the laser beam for fuel excitation.

[0063] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source-collector module/assembly with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases, the source may be an integral part of the source-collector module/assembly or assembly, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0064] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0065] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0066] The depicted apparatus could be used in at least one of the following modes: [0067] 1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

[0068] 2. In scan mode, the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0069] 3. In another mode, the support structure (e.g., mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0070] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0071] Figure 2 shows the apparatus 100 in more detail, including the source-collector module/assembly SO, the illumination system IL, and the projection system PS. The source-collector module/assembly SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source-collector module/assembly SO. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0072] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 may include a channel structure. Contamination trap 230 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure, as known in the art.

[0073] The collector chamber 211 may include a radiation collector CO, which may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source-collector module/assembly is arranged such that the intermediate focus IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0074] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT.

[0075] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Figure 2.

[0076] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254 and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0077] Alternatively, the source-collector module/assembly SO may be part of an LPP radiation system as shown in Figure 3. A laser LA is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma 210 with electron temperatures of several lO's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near normal incidence collector optic CO and focused onto the opening 221 in the enclosing structure 220.

[0078] Figure 4 schematically illustrates a cross-sectional view through an embodiment of a reticle assembly according to an aspect of the invention, the reticle assembly being suitable for use in the lithographic apparatus as set out in Figure 1 or Figure 2. A reticle 1 is shown with patterned surface upmost, having a frame 5 mounted on the patterned surface of the reticle 1. A pellicle 2 according to an embodiment of an aspect of the invention is mounted tensioned on the frame 5 over the reticle.

[0079] In use, an EUV radiation beam 3 passes through pellicle 2 to impinge upon a patterned surface of reticle 1 , acquiring a pattern therefrom, and reflecting from reticle 1 as a patterned EUV beam 3a, which passes back through the pellicle 2. Debris particles 4 are prevented from contacting the patterned surface of reticle 1 by being collected on the outer surface of the pellicle 2.

[0080] Figure 5 schematically illustrates a cross-sectional view through an embodiment of a pellicle 2, which may be used as pellicle in the reticle assembly of Figure 4.

[0081] The pellicle 2 has a central region in the form of a single layer 6 of silicon, with the silicon layer 6 sandwiched between cap layers 7 of molybdenum silicide. Although once again the cap layers 7 are of molybdenum silicide for this depicted embodiment, other refractory materials, as set out herein, may be used as replacements. Intermediate layers 8 of silicon nitride, are located directly between the silicon layer 6 and the cap layers 7. The intermediate layers of silicon nitride may be replaced by silicon oxide in other embodiments according to the invention. [0082] Figure 6 schematically illustrates a cross-sectional view through another embodiment of a pellicle 2, which may be used as pellicle in the reticle assembly of Figure 4.

[0083] The pellicle 2 has a central region in the form of two layers 6 of silicon, having a layer of refractory material 7, in this case molybdenum silicide, located between the two silicon layers 6. Intermediate silicon nitride layers 8 are provided between the innermost layer 7 of silicon nitride, and the silicon layers 6.

[0084] Cap layers 7 of molybdenum silicide form the outermost surfaces of the pellicle 2, sandwiching the central region with further intermediate layers 8 of silicon nitride provided between the cap layers 7 and the silicon faces of the central region. Although once again the cap layers 7 are of molybdenum silicide for this depicted embodiment, other refractory materials, as set out herein, may be used as cap or intermediate layers instead. Similarly, the intermediate layers of silicon nitride may be replaced by silicon oxide.

[0085] The invention provides a number of technical benefits over the prior art. The pellicle suitably has a high transmissivity to such EUV radiation, by which is meant that the intensity maintained on single pass transmission through the pellicle for normal incidence is 80% or more. It is important that the pellicle transmissivity be high as it will be understood that in use, EUV will undergo dual passage through the pellicle, first to impinge upon a reticle and second when leaving the reticle after a pattern has been imparted to the EUV beam as a result of interaction with a patterned surface of the reticle. High transmissivity prevents overheating of the pellicle arising from adsorbed EUV energy, and this in turn reduces risk of the pellicle sagging as the pellicle heats up in use. Also, high EUV intensity is desirable for lithographic patterning, so losses in EUV intensity arising from absorption in the pellicle are undesired.

[0086] A particularly useful refractory material for use in the pellicles of the invention is selected from Nb, alloys of Mo, Nb, molybdenum silicide, niobium silicide and silicides of alloys of Mo and Nb. These materials have a high transmissivity to EUV radiation and a high melting point. The refractory cap layers also act to protect the central region of silicon from decomposition and degradation in use.

[0087] A layer of silicon oxide (Si0₂ or a non-stoichiometric oxide layer) or silicon nitride

(S1₃N4 or a non stoichiometric silicon nitride) may be provided as an interfacial layer between each silicon face of the central region and its respective cap layer in order to act as a primer or adhesive or bonding layer between the silicon layer and a layer of refractory material. Not only may such intermediate layers improve mutual bonding of silicon and layers of refractory material, they may also act as anti-diffusive barriers within the pellicle. When the central region comprises a plurality of silicon layers, alternating with layers of refractory material, interfacial layers of silicon nitride or silicon oxide may be provided between the silicon layers and the layers of refractory material to provide the same effect as for the interfacial layers at the cap layers of the pellicle.

[0088] The central region of the pellicle of the invention may comprise a plurality of layers of silicon, such as from 2 to 5 layers of silicon, alternating with a layer or layers of the refractory material, wherein the central region has silicon layers providing opposed faces of the central region. In other words, the central region is defined as having silicon layers as its outermost layers prior to the cap layers and any intermediate layers being provided on the central region.

[0089] This symmetrical structure of the pellicle of the invention, having a central region that is either a single silicon layer, or a central region that has alternating silicon layers and layers of refractory material as set out herein, sandwiched between outer cap layers of refractory material, seems to provide a flat pellicle that resistant to bending and capable of self-support when tensioned over a frame to protect a reticle, without need of a support grid, even when the pellicle is sufficiently thin to permit high overall EUV transmissivity. This symmetrical structure also acts to prevent wrinkling, particularly when the pellicle is subjected to a pulsed EUV radiation beam used for patterning.

[0090] The total thickness of silicon layers in the pellicle may suitably be from about lOnm to about 100 nm such as from about 20nm to about 60nm. Greater thickness may lead to excessive EUV adsorption by the silicon layers, whereas insufficient thickness may lead to the pellicle lacking structural strength.

[0091] The thickness of each layer of refractory material in the pellicle is suitably from about 3nm to about 8nm in order to provide a balance between providing adequate structural strength and thermal resistance for the pellicle without the refractory material absorbing too much EUV radiation.

[0092] At least one layer of silicon may be n- or p- doped to provide electrical conductivity in the layer sufficient to allow the pellicle to be electrically earthed through the dep silicon layer being connected to an earth or ground connection, whereby static charging of the pellicle in use may be reduced or avoided.

[0093] The frame of the reticle assembly, arranged to hold the pellicle over a patterned surface of a reticle, may be adapted to hold the pellicle tensioned in a pellicle plane lying at an angle from about 0.01° to about 0.5° to a reticle plane defined by the patterned face of the reticle. Preferably, the angle may be from about 0.01° to about 0.1°. This arrangement may be used to prevent visible or infrared radiation, reflected from the pellicle, being parallel to the radiation reflected from the surface of the reticle. Such an arrangement may be of use in preventing reticle positional sensor arrangements being confounded by radiation reflected from the pellicle, rather than responding to radiation directly from reflected from the reticle surface under the pellicle.

[0094] The method of the invention, to form the pellicles of the invention, may be effected by providing the various layers by means of deposition such as sputtering or chemical vapour deposition.

[0095] The layers of refractory material (including cap layers) are preferably of the same refractory material, for ease of manufacture of the pellicle by the methods set out herein. However, it is also possible to put the invention into effect with layers of refractory material that are different refractory materials for different layers. Preferably, the pellicle symmetry is maintained about its central plane.

[0096] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0097] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography.

[0098] The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of about 5nm to about 20nm), as well as particle beams, such as ion beams or electron beams. [0099] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practised otherwise than as described. For example, the pellicle of the invention may be used in any application in which reflection of radiation having a wavelength in the range of about 5 nm to about 20 nm is desirable or required (e.g., in a radiation source, an alignment system, or the like).

[00100] 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

WHAT IS CLAIMED IS:

A pellicle for shielding reticles for EUV device lithography, the pellicle comprising: a central region comprising at least one layer of silicon, and the central region having opposed faces of silicon; and

cap layers that sandwich the central region, the cap layers forming outermost faces of the pellicle, and

wherein each cap layer is of a refractory material selected from the group consisting of:

the elements Nb, Zr, Y, La, Ce,

alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

silicides of Mo, Nb, Ru, Zr, Y, La and Ce,

silicides of such alloys,

oxides of Mo, Nb, Ru, Zr, La, Ce,

oxides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of such alloys,

nitrides of Mo, Nb, Ru, Zr, La, Ce and

nitrides of alloys of No, Nb, Ru, Zr, La, Y, Ce.

The pellicle of claim 1, wherein, the refractory material is selected from the group consisting of:

the elements Nb and alloys of the elements Mo, Nb,

silicides of Mo, Nb and alloys thereof,

oxides of Mo, Nb and alloys thereof,

carbides of Mo, Nb and alloys thereof,

nitrides of Mo, Nb and alloys thereof.

The pellicle of claim 1 or 2, wherein the refractory material is selected from the group consisting of Nb, alloys of Mo, Nb, molybdenum silicide, niobium silicide and silicides of alloys of Mo and Nb.

4. The pellicle of claim 1, 2, or 3, wherein a layer of silicon oxide or silicon nitride is provided as an interfacial layer between each silicon face of the central region and its respective cap layer.

5. The pellicle of any preceding claim, wherein the central region comprises from 2 to 5 layers of silicon alternating with a layer or layers of the refractory material, and wherein the central region has silicon layers providing opposed faces of the central region.

6. The pellicle of claim 5, wherein a layer of silicon oxide or silicon nitride is provided as an interfacial layer between each silicon layer and respective adjacent layer of refractory material.

7. The pellicle of claim 4, wherein the silicon nitride or silicon oxide layers each have a thickness from about O.lnm to about 5nm.

8. The pellicle of any preceding claim, wherein the total thickness of silicon layers in the pellicle is from about 10 to about 100 nm.

9. The pellicle of any preceding claim, wherein the thickness of each layer of refractory material is from about lnm to about 8 nm.

10. The pellicle of any preceding claim, wherein at least one layer of silicon is doped to provide electrical conductivity therein.

11. A reticle assembly for EUV device lithography, the reticle assembly comprising:

a reticle having a patterned surface adapted to impart a pattern to an EUV radiation beam incident thereon;

a frame; and a pellicle comprising: a central region comprising at least one layer of silicon, and the central region having opposed faces of silicon, and

cap layers that sandwich the central region, the cap layers forming outermost faces of the pellicle, and wherein each cap layer is of a refractory material selected from the group consisting of:

the elements Nb, Zr, Y, La, Ce,

alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

silicides of Mo, Nb, Ru, Zr, Y, La and Ce,

silicides of such alloys,

oxides of Mo, Nb, Ru, Zr, La, Ce,

oxides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of such alloys,

nitrides of Mo, Nb, Ru, Zr, La, Ce and

nitrides of alloys of No, Nb, Ru, Zr, La, Y, Ce.

wherein the frame is arranged to hold the pellicle tensioned over the patterned surface.

12. The reticle assembly of claim 11, wherein the frame is mounted to the reticle.

13. The reticle assembly of claim 11 or 12, wherein the frame is adapted to hold the pellicle tensioned in a pellicle plane lying at an angle from about 0.01° to about 0.5° to a reticle plane defined by the patterned face of the reticle.

14. An EUV lithographic projection apparatus arranged to project a pattern from a reticle assembly onto a substrate with a radiation beam having a wavelength from about 5nm to about 20nm, wherein the reticle assembly is a reticle assembly according to any one of claims 11 - 13.

15. The EUV lithographic projection apparatus of claim 14, further comprising:

an illumination system configured to condition the radiation beam; a support structure constructed to hold the reticle assembly, the reticle assembly being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate; and

a projection system configured to project the patterned radiation beam onto a target portion of the substrate. A method of forming a pellicle configured to transmit radiation having a wavelength from about 5nm to about 20nm, the method comprising:

providing a silicon membrane having opposed outermost silicon faces;

providing interfacial layers of silicon oxide or silicon nitride on each of the outermost silicon faces;

providing layers of a refractory material on the interfacial layers;

wherein the refractory material is selected from the group consisting of:

the elements Nb, Zr, Y, La, Ce,

alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

silicides of Mo, Nb, Ru, Zr, Y, La and Ce,

silicides of such alloys,

oxides of Mo, Nb, Ru, Zr, La, Ce,

oxides of alloys of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of Mo, Nb, Ru, Zr, Y, La, Ce,

carbides of such alloys,

nitrides of Mo, Nb, Ru, Zr, La, Ce and

nitrides of alloys of No, Nb, Ru, Zr, La, Y, Ce.

The method of claim 16, further comprising:

providing first further interfacial layers of silicon oxide or silicon nitride on each of the cap layers;

providing further silicon layers on each of the first further interfacial layers;

providing second further interfacial layers of silicon oxide or silicon nitride on each of the outermost silicon faces; and

providing further layers of the refractory material on the second further primer layers.