WO2020249353A1

WO2020249353A1 - Lithographic apparatus

Info

Publication number: WO2020249353A1
Application number: PCT/EP2020/063398
Authority: WO
Inventors: Arthur Winfried Eduardus Minnaert; Sander Baltussen; Johannes Hubertus Josephina Moors
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2019-06-13
Filing date: 2020-05-14
Publication date: 2020-12-17
Anticipated expiration: 2021-12-13
Also published as: CN113939773A

Abstract

A protection device for an optical system of a lithographic apparatus, the optical system utilising electromagnetic radiation having a predetermined wavelength, the protection device comprising a film that is transparent to radiation of the predetermined wavelength and is positioned adjacent a component of the optical system during use of the lithographic apparatus.

Description

LITHOGRAPHIC APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19179946.9 which was filed on June 13, 2019 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to lithographic apparatus. The present invention has particular, but not exclusive, use in connection with EUV lithographic apparatus and EUV lithographic tools.

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 that can be formed on that substrate. A lithographic apparatus that uses EUV radiation, that is electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a lithographic apparatus using DUV radiation (for example with a wavelength of 193 nm).

[0005] As the size of features to be formed in a lithographic reduces, the performance requirements on all aspects of the lithographic apparatus become stricter. Consequently, the precision and accuracy of sensors that are used to measure parameters of the lithographic apparatus during calibration or in operation must be increased.

SUMMARY

[0006] According to a first aspect of the invention there is provided a protection device for an optical system of a lithographic apparatus, the optical system utilising electromagnetic radiation having a predetermined wavelength, the protection device comprising a film that is transparent to radiation of the predetermined wavelength and is positioned adjacent a component of the optical system during use of the lithographic apparatus.

[0007] According to a second aspect of the invention there is provided a method of manufacturing devices using a lithographic apparatus, the method comprising:

providing a film to protect a component of an optical system of the lithographic apparatus, the optical system using electromagnetic radiation to measure a parameter of the lithographic apparatus; and using the optical system to measure the parameter while the electromagnetic radiation passes though the film.

[0008] Features of different aspects of the invention may be combined with features of other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] 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 lithographic apparatus and a radiation source;

Figure 2 is a schematic side view of an arrangement for protecting a sensor component of the lithographic apparatus of Figure 1;

Figure 3 is a schematic view from below of part of the arrangement of Figure 2; and

Figure 4 is schematic view of an arrangement for feeding a protective film.

DETAIFED DESCRIPTION

[0010] Figure 1 is a schematic illustration of a lithographic system. The lithographic system comprises a radiation source SO and a lithographic apparatus FA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus FA comprises an illumination system IF, a support structure MT configured to support a patterning device MA, a projection system PS and a substrate table WT configured to support a substrate W.

The illumination system IF is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the patterning device 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 with a pattern previously formed on the substrate W.

[0011] The radiation source SO, illumination system IF, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in the illumination system IF and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IF and/or the projection system PS.

[0012] The radiation source SO shown in Figure 1 is of a type that may be referred to as a laser produced plasma (FPP) source. A laser 1, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) that is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, for example, in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de excitation and recombination of ions of the plasma.

[0013] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure that is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[0014] In other embodiments of a laser produced plasma (LPP) source the collector 5 may be a so- called grazing incidence collector that is configured to receive EUV radiation at grazing incidence angles and focus the EUV radiation at an intermediate focus. A grazing incidence collector may, for example, be a nested collector, comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be disposed axially symmetrically around an optical axis O.

[0015] The radiation source SO may include one or more contamination traps (not shown). For example, a contamination trap may be located between the plasma formation region 4 and the radiation collector 5. The contamination trap may for example be a rotating foil trap, or may be any other suitable form of contamination trap.

[0016] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.

[0017] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[0018] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The facetted field mirror device 10 and facetted pupil mirror device 11 together provide the radiation beam B with a desired cross- sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA (which may for example be a mask) reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the facetted field mirror device 10 and facetted pupil mirror device 11.

[0019] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 that are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The mirrors 13, 14 which form the projection system may be configured as reflective lens elements. 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 1, the projection system may include any number of mirrors (e.g. six mirrors).

[0020] The lithographic apparatus may, for example, be used in a scan mode, wherein 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 substrate W (i.e. a 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 demagnification and image reversal characteristics of the projection system PS. The patterned radiation beam that is incident upon the substrate W may comprise a band of radiation. The band of radiation may be referred to as an exposure slit. During a scanning exposure, the movement of the substrate table WT and the support structure MT may be such that the exposure slit travels over an exposure field of the substrate W.

[0021] The radiation source SO and/or the lithographic apparatus that is shown in Figure 1 may include components that are not illustrated. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[0022] In other embodiments of a lithographic system the radiation source SO may take other forms. For example, in alternative embodiments the radiation source SO may comprise one or more free electron lasers. The one or more free electron lasers may be configured to emit EUV radiation that may be provided to one or more lithographic apparatuses.

[0023] Lithographic apparatus using EUV radiation to expose substrates have been introduced to enable the formation of smaller features without the necessity to use techniques such as double patterning and self-assembly. However, to reduce the size of features that can be formed (or in other words to increase resolution) it is not sufficient merely to reduce the wavelength of the exposure radiation; other subsystems of the lithographic apparatus have to be upgraded as well. In particular, improved control of the lithographic apparatus requires improved sensor systems that provide inputs to the control system.

[0024] For example, a lithographic apparatus may use an interferometric displacement measuring system in order to measure the position of the substrate stage and hence a substrate held thereon. An interferometric measuring system may use reflectors (mirrors) mounted on the substrate stage WT to reflect a coherent beam of electromagnetic radiation so that it forms interference fringes with a reference beam. Movements of the substrate stage WT change the path length of the beam that reflects from the mirror and hence cause the interference fringes to move in a way that can be detected by a light sensor. The interferometric displacement measuring system may also use fixed mirrors in the vicinity of the substrate stage. For example, to measure displacements of the substrate stage in the Z-direction (parallel to the optical axis of the projection system) fixed mirrors may be provided adjacent the projection system and opposing the upper surface of the substrate stage. The electromagnetic radiation used by the interferometric displacement measuring system conveniently has a wavelength in the visible range, but other wavelengths are possible.

[0025] The substrate stage of a lithographic apparatus may also include components of other sensor systems using electromagnetic radiation in the visible and other wavelength ranges. For example, reference targets (often referred to as fiducials) for an alignment system may be provided in the upper surface of the substrate stage. An alignment system may use one or a plurality of beams of different wavelengths (colours). Various sensor elements may also be mounted in or on the substrate stage to make measurements using the exposure radiation B or other radiation beams. Examples include: transmission and reflection image sensors, aberration sensors, energy sensors.

[0026] The present inventors have determined that a possible source of error in the measurements obtained by a sensor system, for example an interferometric displacement measuring system, is contamination of components thereof, for example mirrors. In an EUV lithographic apparatus, the substrate is exposed whilst in a vacuum or low pressure environment. This means that it is possible for contaminants that out-gas from the photosensitive layer to travel further from the substrate than they do in lithographic apparatus in which the substrate is exposed in an environment at atmospheric pressure.

[0027] In particular, the inventors have determined that contamination build up on components of a measuring system that uses electromagnetic radiation (e.g. light) may cause long term drift effects as well as stochastic errors. Contamination of components which interact with the electromagnetic radiation (which may be referred to as optical components), and particularly contamination of surfaces on which the electromagnetic radiation is incident, is most likely to cause errors. Optical components may include mirrors, windows, gratings, lenses, fiducials. Errors may be caused by a variety of mechanisms, for example scattering or absorption of the electromagnetic radiation, refraction and/or changes in optical path length.

[0028] Cleaning of contaminated surfaces is the obvious approach to addressing this problem. However, such cleaning requires the lithographic apparatus to be opened up, which then necessitates a lengthy process of evacuating and reconditioning the lithographic apparatus before it can be used again. Cleaning to remove such contamination can be performed when the lithographic apparatus is opened for other reasons but this can still increase downtime. A general maintenance schedule for a lithographic apparatus may not be suitable to remove contamination from an optical component of a sensor system. Cleaning may not always be effective, since only limited solvents and non-abrasive materials can be used, or may damage the optical component. Therefore, it is desirable to prevent the optical component from becoming contaminated.

[0029] According to an embodiment of the invention, a transparent film is provided adjacent a surface of a component of a sensor system in a lithographic apparatus that is at risk of contamination. An example is illustrated in Figure 2, which is a side view showing protective films 100 provided parallel and close to the operative surfaces of interferometer mirrors 101. Interferometer mirrors 101 are provided adjacent the projection system PS, opposing the surface of the substrate stage WT. Interferometer mirrors 101 form part of an interferometric displacement measuring system and in particular are used to measure changes in position of the substrate stage WT in the Z direction and/or rotations about axes parallel to the X and Y directions (often referred to as Rx and Ry). A measurement beam of an interferometer may be directed onto the interferometer mirror by a 45° mirror (not shown) mounted on the edge of the substrate stage WT.

[0030] Contaminants 102 outgassing from the photo-sensitive layer (e.g. resist) on substrate W will deposit on the film 100, rather than the interferometer mirror which it protects. Therefore the need to clean the protected optical component is reduced or eliminated.

[0031] Figure 3 is a view from below of the interferometer mirrors 101 showing that the film 100 covers only parts of the interferometer mirrors 101. The parts of the interferometer mirrors that are protected are the parts closest to the projection system PS. These parts experience the most contamination since out-gassing from the photosensitive layer is increased when the projection B is incident thereon. In other embodiments, the whole of an optical component is covered by the film. However, if contamination of a component to be protected is not uniform then it can save costs and space in the lithographic apparatus to protect only those parts of the component that experience the heaviest contamination load. If the film 100 covers only part of the component then a difference in response of the sensor system between parts of the component that are covered and parts that are not may need to be corrected for by calibration.

[0032] The film of the invention can be considered to have a similar function to a pellicle used to protect a reticle or mask. However, a pellicle is normally required to be spaced form the reticle being protected by a distance sufficient to ensure that any particulate contaminants deposited on the pellicle are out of focus. That requirement does not apply in the present invention since the film of the present invention may be used in sensor system where the measurement beam is not focused and or the contaminants deposit in thin layers rather than as particles.

[0033] In an embodiment of the invention, the film is replaced when it has become contaminated.

It is possible to measure the level of contamination in order to determine when to replace the film or simply to replace the film according to a predetermined schedule (e.g. after a fixed period or number of exposures). Where the film is inexpensive and replacement easy, a scheduled approach is preferable as avoiding the need to provide means for measuring contamination. Scheduled replacement can be automatic or manually triggered.

[0034] Figure 4 depicts a mechanism for allowing the film to be replaced automatically and without opening the vacuum chamber of the lithographic apparatus. The film is provided in the form of an elongate ribbon 100a which is provided on a supply reel 102 and taken up by a take-up reel 103. Guides (e.g. rollers) 104 ensure that the film is correctly positioned in front of the interferometer mirror 101. An actuator (e.g. electric motor) 105 drives the supply and/or take-up reels according to the schedule to advance the film by a set distance so that a clean area of the film is in front of the component to be protected. An active brake may be provided to maintain a constant tension in the protective film.

[0035] A wide variety of materials are suitable for use as the film, for example plastics films such as PET (Polyethylene terephthalate), BoPET (bi-axially oriented Polyethylene terephthalate), BoPP (bi-axially oriented polypropylene) or BoN (bi-axially oriented nylon).

[0036] Desirable properties of the film are that it is at least substantially transparent to the wavelength of radiation used in the sensor system to which the component to be protected belongs. Desirably the film has a transmission of greater than 90%, more desirably greater than 95%. In some cases, depending on the type of sensor, it is desirable that the film has a uniform optical thickness so that optical path length variations are not introduced. Desirably, the variation in optical thickness is no more than 10%, more desirably no more than 1%. If the sensor system uses multiple wavelengths or broadband radiation, it may be desirable that the film has a uniform refractive index at relevant wavelengths. The thickness of the film may be in the range of from 1 pm to 10 pm.

[0037] In an embodiment of the invention the film is in the form of a coating applied to the surface of the component to be protected. The coating is selected so as to be easier to clean from the component than the contamination. For example the coating may be easily soluble in ultra-pure water. The coating may be a gel or aerogel. The coating may be in the form of a sticker which is easily peel-able off the surface to be protected.

[0038] Desirably the film is vacuum compatible, e.g. having minimal out-gassing. In most cases the intensity of the radiation used in the sensor system is not large so that heat resistance and physical robustness are not particularly important. However, if the film is close to or in the path of the projection beam at any point then additional robustness may be desirable.

[0039] Desirably the film is formed of a material that has an affinity for substances out-gassed from the photo-sensitive layer. For example the film may adsorb or absorb substances out-gassed from the photo- sensitive layer. For example the material film may react with substances out-gassed from the photo- sensitive layer to form other substances that either remain in gaseous form so as to be purged from the lithographic apparatus or stick to the material. For example the film may catalyse reactions that convert substances out-gassed from the photo-sensitive layer into other substances that either remain in gaseous form so as to be purged from the lithographic apparatus or stick to the material. [0040] The component to be protected may be any form of optical component, for example mirrors, windows, gratings, lenses, fiducials in any sensor system using electromagnetic radiation.

[0041] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0042] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[0043] 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. Possible other applications include 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.

[0044] 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

1. A protection device for an optical system of a lithographic apparatus, the optical system utilising electromagnetic radiation having a predetermined wavelength, the protection device comprising a film that is at least substantially transparent to radiation of the predetermined wavelength and is positioned adjacent a component of the optical system during use of the lithographic apparatus.

2. A protection device according to claim 1, is formed of a material that has an affinity for substances out-gassed from the photo- sensitive layer.

3. A protection device according to claim 1 or 2, wherein the film is formed of a plastics material.

4. A protection device according to claim 1, 2 or 3, wherein the film has an optical thickness variation in a part thereof that is traversed by the electromagnetic radiation that is less than 5%, desirably less than 1%.

5. A protection device according to any one of claims 1 to 4, wherein the film is spaced apart from the component.

6. A protection device according to any one of claims 1 to 4, wherein the film is in contact with the component.

7. A protection device according to any one of the preceding claims, further comprising a film replacement mechanism for replacing with clean film a part of the film that has become contaminated.

8. A protection device according to claim 7, wherein the film is elongate and the film replacement mechanism comprises a supply reel for storing clean film, a take-up reel for storing contaminated film and an actuator for advancing the film from the supply reel to the take-up reel.

9. A protection device according to claim 7 or 8, wherein the replacement mechanism is configured to replace the part of the film according to a predetermined schedule.

10. A protection device according to any one of the preceding claims, wherein the film is positioned between the component and a substrate stage so as to protect the component from substances out-gassed from a photosensitive layer provided on substrate held on the substrate stage.

11 A protection device according to any one of the preceding claims, wherein the component is a reflector, in particular a reflector in an interferometric displacement measuring system.

12. A protection device according to any one of the preceding claims, wherein the component is a sensor.

13. A lithographic apparatus comprising:

an optical system including a component; and a protection device according to any one of the preceding claims for protecting the component.

14. A lithographic apparatus according to claim 13 further comprising:

an illumination system configured to illuminate a patterning device with EUV radiation; and a projection system configured to project radiation patterned by the patterning device onto a substrate.

15. A method of manufacturing devices using a lithographic apparatus, the method comprising: providing a film to protect a component of an optical system of the lithographic apparatus, the optical system using electromagnetic radiation to measure a parameter of the lithographic apparatus; and

using the optical system to measure the parameter while the electromagnetic radiation passes though the film.

16. A method according to claim 15, further comprising replacing a contaminated part of the film with clean film.