UV Lithography System
A multifunction UV or DUV (ultraviolet/deep-ultraviolet) lithography system uses a modified Schwarzschild flat-image projection system to achieve diffraction-limited, distortion-free and double-telecentric imaging over a large image field at high numerical aperture. A back-surface primary mirror enables wide-field imaging without large obscuration loss, and additional lens elements enable diffraction-limited and substantially distortion-free, double-telecentric imaging. The system can perform maskless lithography (either source-modulated or spatially-modulated), mask-projection lithography (either conventional imaging or holographic), mask writing, wafer writing, and patterning of large periodic or aperiodic structures such as microlens arrays and spatial light modulators, with accurate field stitching to cover large areas exceeding the image field size.
This application claims the benefit under 35 U.S.C. § 119(e) of the following two applications, both of which name Kenneth C. Johnson as the inventor, and both of which are incorporated by reference in their entirety for all purposes:
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- U.S. Patent Application No. 63,050,850, filed Jul. 12, 2020 for “UV Lithography System” (hereafter “the '850 application”); and
- U.S. Patent Application No. 63,087,302, filed Oct. 5, 2020 for “UV Lithography System” (hereafter “the '302 application”).
This application pertains to ultraviolet (UV) and deep-ultraviolet (DUV) lithography, including mask-projection and maskless lithography, in the context of semiconductor and microsystems manufacture. For the purpose of this disclosure, the acronym “UV” will be used generically to include DUV. Although the focus of the disclosure is on UV lithography, the devices and methods disclosed herein are equally applicable to lithography at visible-light wavelengths, or at any wavelength that can be focused with optical glass lenses such as fused silica (SiO2), calcium fluoride (CaF2), etc.
Background patents and non-patent literature references relevant to this application are listed at the end of the disclosure in the References section.
UV lithography systems operate at wavelengths down to 193 nm and provide wide-field, diffraction-limited imaging at a numerical aperture (NA) of up to 1.35 (with immersion). These systems require very complex projection lenses with more than forty optical surfaces (Ref's. 1, 2). Projection optics for extreme ultraviolet (EUV) lithography (Ref. 3) require only six surfaces (all mirrors), in part because they operate at lower NA (up to 0.55), they only cover a narrow ring field, and the surfaces are all aspheric. (EUV lenses cannot be used because there are no EUV-transmitting optical materials, except in very thin films such as EUV mirror coatings.)
References 4-6 disclose a maskless EUV lithography scanner, illustrated in
The maskless scanner can use a spatial light modulator (MEMS microshutters at the microlens foci) to individually modulate the focus spots. Alternatively, the spots can be collectively modulated by a single modulator at the EUV illumination source so that all spots generate identical exposure patterns in a periodic array matching the spot array's periodicity. These two scan modes are termed “spatially-modulated” and “source-modulated”, respectively. (Microlens array layouts and scan patterns for maskless lithography are discussed in Ref. 8, Section 7.)
The two-mirror Schwarzschild projection system can also be used for “holographic” mask-projection lithography, which uses a diffractive photomask displaced some distance from the projection system's object plane. A holographic mask, like the microlenses in a maskless system, can correct projection system aberrations. Also, holographic masks can achieve very high exposure dose levels for sparse patterns, and they would be relatively insensitive to defects because the defects are not in focus at the image plane.
Analogous two-mirror, obscured projections systems for UV operation are known in the prior art, e.g., as disclosed in Ref. 9. These systems are more complex than the Schwarzschild apparatus in
The Schwarzschild EUV projection optics described in Ref's. 4-6 can be adapted for lithography at UV wavelengths (and more generally for visible light as well) with incorporation of lens elements to improve performance and functionality. The primary mirror (M1 in
Additionally, lens elements can be incorporated in the optical path between the object plane and the primary mirror, as illustrated in
After the microlens array is formed, it can be used to manufacture of other types of periodic structures, again via source-modulated maskless writing. In particular, it can be used to make spatial light modulator arrays for use in spatially-modulated maskless writing. With a spatial light modulator, the system would be capable of printing aperiodic structures such as photomasks, which can then be used for production of specialized semiconductors, MEMS, micro-optics, etc. via high-throughput, mask-projection lithography. Thus, the projecting system's imaging capabilities enable it to operate as a multi-function tool for performing maskless lithography (either source-modulated or spatially-modulated), mask-projection lithography (either conventional imaging or holographic), mask writing, wafer writing, and patterning of large periodic or aperiodic structures via field stitching to cover large areas exceeding the image field size.
Lithography Projection Optics and Image Generation
An array of MEMS microshutters 406 can be added to the microlens array to modulate the individual beams for spatially-modulated scanning. Without the shutters, the microlens array can print periodic patterns, using a single modulator at the source to collectively modulate the beams in source-modulated scanning mode.
Holographic mask-projection lithography is similar to maskless lithography, but with a diffractive mask 501 replacing the microlens array, as illustrated in
Conventional (non-holographic) mask-projection lithography uses a transmission mask 601 located at object plane 101, as illustrated in
In each of these imaging modes either the image pattern generator (the microlens array or mask) or the projection system needs to block or suppress any zero-order (undiffracted) light that is directed straight into the M1B transmission window without intercepting the reflective surface. A zero-order stop 106 (
An optical positioning sensor unit 301 (
Optical Design Data
Following is an outline of illustrative optical design data for
The projection system comprises the following optical elements and surfaces:
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- Lens L1 (front and back surfaces L1F and LIB)
- Lens L2 (front and back surfaces L2F and L2B)
- Back-surface primary mirror M1 (front and back surfaces M1F and M1B)
- Front-surface secondary mirror M2 (one surface only)
The optical light path intercepts the surfaces in the following order:
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- 1. Object plane 101
- 2. L1F transmission
- 3. L1B transmission
- 4. L2F transmission
- 5. L2B transmission
- 6. M1F transmission
- 7. M1B reflection
- 8. M1F transmission
- 9. M2 reflection
- 10. M1F transmission
- 11. M1B transmission
- 12. Image plane 103
Predetermined design specifications are tabulated in
The design parameters in
The performance data outlined in
The above design could be improved to increase the working distance and/or reduce the obscuration, e.g., by adding another lens in the space between M1 and M2. Alternatively, a diffractive, phase-Fresnel mirror surface can be used for M1B (for the reflecting area; the window need not be diffractive). A design with a diffractive M1B surface is illustrated in
The diffractive surface provides additional design degrees of freedom, and it improves the working distance and/or reduces the obscuration by flattening surface M1B. The design needs to be constrained to maintain sufficient clearance for the zero-order stop; otherwise, the clearance b. in
The illustrative design in
The projection system design makes use of strongly aspheric mirrors, which can present challenges for manufacturing, especially for measuring the surface form error. The secondary mirror M2 can be tested using a flat-plate spherical aberration corrector, as illustrated in
For the
The primary mirror's back surface M1B can be similarly tested by retroreflecting a test beam from its back side, as illustrated in
The transmitting front surface M1F can be tested in the fully assembled projection system (e.g.,
In a conventional PDI (Ref. 12), an attenuated transmission surface with a clear pinhole filter is placed at or near the focal point of a convergent optical beam to generate a far-field interference pattern, which can be analyzed to determine the beam aberrations. A variety of alternative interferometer types (e.g., Ronchi-grating test, Foucault knife-edge test, phase Foucault knife-edge test, Zernike phase-contrast test) can also be used for aberration measurement (Ref. 13). For example, the Zernike phase test (Ref's. 14, 15) is a type of PDI that incorporates an optical phase shift in the pinhole. In all of these interferometer types, the focused beam interacts with a diffracting object (e.g., a pinhole filter), which can be scanned in up to three dimensions as the far-field diffraction pattern is measured. The scanning operation and data analysis are similar to ptychography, which can reconstruct unknown attributes of both the aberrated beam and the diffracting object based on interferometric data (Ref's 16, 17). Interferometric characterization of a lithography projection system's point-imaging performance at multiple image field points can be used to simultaneously resolve surface shape errors in all of the optical surfaces, as well as positional alignment errors in the optics, the laser source point, and the pinhole or diffracting object.
The projection optics illustrated in
The phase-Fresnel diffractive M1B mirror in
The following additional patent and literature references are referred to in this disclosure and are incorporated by reference:
- Ref 1: Matsuyama, Tomoyuki, Yasuhiro Ohmura, and David M. Williamson. “The lithographic lens: its history and evolution.” Optical Microlithography XIX. Vol. 6154. International Society for Optics and Photonics, 2006. http://www.lithoguru.com/scientist/litho_history/Lithographic_lens_history_and_evolution_Matsuyama_2006.pdf
- Ref 2: Shafer, David, et al. “Catadioptric projection objective.” U.S. Pat. No. 8,208,198. 26 June 2012.
- Ref 3: S. Migura. “Optics for EUV Lithography”, in 2019 EUVL Workshop, P24, EUV Litho, Inc. https://www.euvlitho.com/2019/P24.pdf
- Ref 4: Johnson, Kenneth C. “Scanned-spot-array EUV lithography system.” U.S. Pat. No. 9,097,983. 4 Aug. 2015.
- Ref 5: Johnson, Kenneth C. “EUV Lithography System with Diffraction Optics”, 2019. https://vixra.org/pdf/1911.0361v1.pdf
- Ref 6: K. Johnson. “EUV Lithography Design Concepts using Diffraction Optics”, in 2020EUVL Workshop, P22, EUV Litho, Inc. https://vixra.org/pdf/2007.0167v2.pdf
- Ref 7: M. Bass, ed., Handbook of Optics, 2nd ed. (Optical Society of America, Washington, DC, 1995), Vol. 2. (p. 18.15)
- Ref 8: Johnson, K. C. (2019). Maskless EUV lithography, an alternative to e-beam. Journal of Micro/Nanolithography, MEMS, and MOEMS, 18(4), 043501. https://doi.org/10.1117/1.JMM.18.4.043501
- Ref 9: Shafer, David R., Yung-ho Chuang, and B. Tsai Bin-ming. “Broad spectrum ultraviolet catadioptric imaging system.” U.S. Pat. No. 5,717,518. 10 Feb. 1998.
- Ref 10: Coherent, Inc., Azure NX laser product information. https://www.coherent.com/lasers/laser/azure
- Ref 11: Oxide Corp., 266-nm laser product information. https://www.opt-oxide.com/en/products/266 nm/Ref.
- 12: Smartt, R. N., and W. H. Steel. “Theory and application of point-diffraction interferometers.” Japanese Journal of Applied Physics 14, no. S1 (1975): 351. https://doi.org/10.7567/JJAPS.14S1.351 https://wp.optics.arizona.edu/jcwyant/wp-content/uploads/sites/13/2016/08/10-JJAPS-14S1-351.pdf
- Ref 13: Barakat, Richard. “General diffraction theory of optical aberration tests, from the point of view of spatial filtering.” JOSA 59, no. 11 (1969): 1432-1439. https://doi.org/10.1364/JOSA.59.001432
- Ref 14: Golden, Lewis J. “Zernike test. 1: Analytical aspects.” Applied optics 16, no. 1 (1977): 205-213. https://doi.org/10.1364/AO.16.000205
- Ref 15: Golden, Lewis J. “Zernike test. 2: Experimental aspects.” Applied optics 16, no. 1 (1977): 214-217. https://doi.org/10.1364/AO.16.000214
- Ref 16: Wojdyla, Antoine, Ryan Miyakawa, and Patrick Naulleau. “Ptychographic wavefront sensor for high-NA EUV inspection and exposure tools.” In Extreme Ultraviolet (EUV) Lithography V, vol. 9048, p. 904839. International Society for Optics and Photonics, 2014. https://doi.org/10.1117/12.2048386
- Ref 17: Dwivedi, Priya, Silvania F. Pereira, and H. Paul Urbach. “Ptychography as a wavefront sensor for high-numerical aperture extreme ultraviolet lithography: analysis and limitations.” Optical Engineering 58, no. 4 (2019): 043102. https://doi.org/10.1117/1.0E.58.4.043102
- Ref 18: Dumas, Paul R., Robert W. Hallock, and Alex Pisarski. “Applications and benefits of ‘perfectly bad’ optical surfaces.” Optical Fabrication, Testing, and Metrology III. Vol. 7102. International Society for Optics and Photonics, 2008. https://doi.org/10.1117/12.797718
Ref 19: “Fabrication of Diffractive Optical Elements using Direct Laser Writing.” University of Stuttgart, Institute of Applied Optics. https://www.ito.uni- stuttgart.de/en/res earch/group-ido/fabrication-of-diffractive-optic al-elements/Ref.
- Ref. 20: Reichle, R.; Yu, K.; Pruss, C.; Osten, W. “Spin-coating of photoresist on convex lens substrates.” In DGaO-Proceedings 2008, 109. ISSN: 1614-8436 (2008) https://www.dgao-proceedings.de/download/109/109 p44.pdf
- Ref 21: Voronov, D. L., E. M. Gullikson, and H. A. Padmore. “Ultra-low blaze angle gratings for synchrotron and free electron laser applications.” Optics express 26.17 (2018): 22011-22018. https://doi.org/10.1364/0E.26.022011
- Ref 22: Siewert, F., et al. “Gratings for synchrotron and FEL beamlines: a project for the manufacture of ultra-precise gratings at Helmholtz Zentrum Berlin.” Journal of synchrotron radiation 25.1 (2018): 91-99. https://journals.iucr.org/s/issues/2018/01/00/x15026/x15026.pdf
Claims
1. An optical lithography exposure apparatus comprising an image pattern generator and a projection system, wherein
- the projection system images an object plane onto an image plane without forming an intermediate image between the object and image planes;
- the image pattern generator generates an optical image pattern on the object plane, and the projection system conveys the image pattern to a printing surface at the image plane;
- the image pattern generator is either a microlens array, which condenses illuminating radiation onto an array of focus spots on the object plane, or a holographic projection mask, which is displaced from the object plane and produces a diffractive image pattern on the object plane, or a non-holographic mask located at the object plane;
- the projection system comprises primary and secondary mirrors in a flat-image Schwarzschild configuration, wherein the primary mirror is a back-surface reflector with a transmitting front surface and a reflecting back surface;
- the primary mirror's back surface has a central, non-reflective window for beam transmission, and the secondary mirror has a central open hole for beam transmission;
- optical radiation from the image pattern on the object plane transmits through the secondary mirror's central hole, is then reflected from the primary mirror, then reflects from the secondary mirror and transmits through the primary mirror's transmission window to the image plane.
2. The lithography exposure apparatus of claim 1 wherein the primary mirror's front and back surfaces are convex and the secondary mirror is concave.
3. The lithography exposure apparatus of claim 1 wherein the primary mirror's front surface is convex, its back surface is diffractive, and the secondary mirror is concave.
4. The lithography exposure apparatus of claim 1, further comprises transmitting lens elements between the object plane and the secondary mirror's central hole, wherein the lenses and mirrors are configured to provide substantially distortion-free, double-telecentric imaging from the object plane to the image plane.
5. The lithography exposure apparatus of claim 1, wherein the image pattern generator is a microlens array, the focus spots are individually modulated by MEMS shutters proximate the object plane, and the printing surface is scanned in the image plane as the focus spots are modulated to synthesize a digital exposure image on the printing surface.
6. The lithography exposure apparatus of claim 1, wherein the image pattern generator is a microlens array, the focus spots are collectively modulated by a single modulator that modulates the illuminating radiation, and the printing surface is scanned in the image plane as the focus spots are modulated to synthesize a digital exposure image on the printing surface.
7. The lithography exposure apparatus of claim 1, further comprising a zero-order stop in the projection system for blocking radiation that would, in the absence of the stop, be directed from the object plane directly through the primary mirror's transmission window without being reflected by the primary mirror.
8. The lithography exposure apparatus of claim 1, further comprising a focus/alignment sensor system attached to the primary mirror's back surface for sensing and controlling the positional relationship between the printing surface and the projection system.
9. The lithography exposure apparatus of claim 8, wherein the focus/alignment sensor system comprises a multi-level confocal sensor for focus sensing.
10. The lithography exposure apparatus of claim 8, wherein the focus/alignment sensor system focuses radiation onto the printing surface and detects far-field, reflective scattering of the radiation by alignment targets on the surface for alignment sensing.
Type: Application
Filed: Jul 10, 2021
Publication Date: Jan 12, 2023
Inventor: Kenneth Carlisle Johnson (Santa Clara, CA)
Application Number: 17/372,446