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Scanned-Spot-Array Optical Lithography

Scanned-Spot-Array Optical Lithography. A Strategy for Bridging the NGL Technology Gap. Scanned-Spot-Array Optical Lithography.

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Scanned-Spot-Array Optical Lithography

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  1. Scanned-Spot-Array Optical Lithography A Strategy for Bridging the NGL Technology Gap K. Johnson

  2. Scanned-Spot-Array Optical Lithography Objective: Develop a maskless lithography system that overcomes throughput and resolution limitations of prior optical maskless systems. Advantages of the system are: • It eliminates optical proximity effects. • It can use high-NA, wide-field immersion imaging (potentially solid immersion). • Field curvature, distortion, and geometric point-imaging aberrations can be entirely eliminated. • The system can optimally control polarization over the full image field. • The system can be equipped with confocal viewing optics for accurate focus/overlay feedback. The system is similar to Gratings of Regular Arrays and Trim Exposures (GRATE), in that it is applicable primarily to printing periodic structures. But the pattern period is at least two orders of magnitude larger than GRATE, and alternative embodiments could employ a spatial light modulator to eliminate the periodicity constraint. The basic system design concept uses existing, proven technologies, minimizing technological risk, cost, and development time. K. Johnson

  3. Current State-of-the-Art Maskless Optical Lithography: LumArray’s ZPAL System Advantages: Maskless No optical proximity effects Simplified projection optics Simple, planar micro-optics Current state of development: ZP-150A: 405 nm wavelength (I-line), 0.85 NA, 1000 write channels Minimum Feature Size: 150nm Dense, 120nm Isolated Writing Speed: 1.7mm2/sec (~2hrs per Ø150mm wafer) Limitations: Resolution is not competitive with 193-nm immersion. Requires continuous-wave laser. Spatial light modulator limits throughput. K. Johnson

  4. Gratings of Regular Arrays and Trim Exposures (GRATE) Advantages: Maskless (interference lithography), or very simple mask design for critical patterns No optical proximity effects Current state of development: Concept first developed, published by MIT Lincoln Labs in 2001 Being developed by TowerJazz under contract to DARPA, and by IBM under contract to USAF Limitations: Sub-micron periodicity constraint for critical patterns Very simple periodic pattern geometries K. Johnson

  5. Scanned-Spot-Array Lithography: Basic Design Concept laser modulator High print resolution: No image-plane microlens array Spatially-filtered object-plane source spots No optical proximity effects High throughput: No spatial light modulator Limitation: Requires CW or high-rep-rate laser to get high throughput. Low technological risk: Can use existing reduction-lens technology and scanning servomechanisms Uses only low-NA, spatially filtered microlenses Uses only source modulation Low development time and cost beam expander collimating lens low-NA microlens array aperture mask source-spot array reduction lens raster-scanned printing surface focused-spot array K. Johnson

  6. Optical System Alternatives and Tradeoffs Image-plane microlens array, full modulation (e.g., ZPAL): Source-modulated: Object-plane spot array: Simple, low-NA microlens array SLM No SLM No projection lens (just uniform, modulated illumination) Complex, high-NA, wide-field projection lens But the spot-generation optics can greatly simplify the lens requirements. Simple, low-NA projection lens Difficult, high-NA microlens array Difficult, high-NA microlens array K. Johnson

  7. Spot-Generation Optics can Supplant much of the Projection Lens Functionality Eliminate Image Field Curvature: Put microlens foci on curved object surface configured to achieve flat-field imaging. Eliminate Image Distortion: Distribute microlens foci on non-uniform, aperiodic grid configured to achieve strict periodicity of image pattern. Eliminate Point-Imaging Aberrations: Put aberration corrector in region where spot beams do not overlap; define phase profile to achieve aberration-free point imaging. (A Phase-Fresnel surface can correct narrow-band chromatic aberration.) Projection Lens • Implications for Projection Lens: • Less stringent design requirements • Less stringent manufacturing tolerances (with interferometry metrology data used to optimize aberration corrector) • Consequently simpler lens design K. Johnson

  8. Scan Pattern: A Conceptual Illustration Scan field: 25-mm square Focused spots: 25-μm square grid Number of spots: 106 Number of scan lines: 106 Scan line center spacing: 25 nm Addressable grid spacing on scan lines: 25 nm Number of grid points per scan field: 1012 Modulation rate: 1 MHz Scan rate: 1 sec per scan field (25 mm square); roughly 25 wafers (300-mm) per hour Data rate with megapixel SLM: 1012bit per sec Data rate with source modulation (periodic print pattern): only 106bit per sec focused spot scan line Periodicity constraint is, e.g., 25-micron square (versus submicron for GRATE). surface scan direction Requires CW or high-rep-rate laser source. K. Johnson

  9. Laser Options • Available from Coherent : • 266 nm (frequency-quadrupled variant of Paladin laser system) • "Quasi-CW": 120 MHz • Currently 500mW ($120K). Will be upgraded to 1.5W in 12-18 months (~$150K); can potentially be boosted to 6-10W at 80 MHz. • 193-nm excimer laser options : • Currently only available up to 6 kHz (Cymer, GigaPhoton) • Multiplex N synchronized, low-power, low-rep-rate lasers to effectively increase power and rep rate both by a factor of N. • Use more image spots (e.g. reduce pattern period from 25 μm to 10 μm – equivalent to 6X rep-rate gain). K. Johnson

  10. Y scan line focus spot spot-generation optics (mechanically scanned) projection lens last lens element (solid immersion) cover plate wafer Solid-Immersion Lithography • Scanning mechanism: • Mechanicalactuation of spot-generation optics (or beam scanner in projection optics) • Wafer only moves for field stepping. • Optical coupling: • Wafer contacts cover plate. • Last lens element contacts cover plate during scan, is released during field stepping. • High-n glass options: • LuAG • n>2 • good transmittance @ 266 nm • Sapphire • n =1.9 @ 193 nm, 2.1 @ 157 nm • good transmittance @ 193 nm and 157 nm • Requires polarization control because of birefringence. X bidirectional X-Y scan: K. Johnson

  11. Focus and Overlay Feedback via Confocal Imaging • Merge Exposure and Confocal Viewing Light Paths: • Split-aperture beam combiner, or • Dichroic beam combiner, or • Interleaved exposure/viewing microlens arrays DUV (modulated) • Achromatization: • Common light path can be approximately achromatized with a single-glass phase-Fresnel lens. • Aberration correctors eliminate residual chromatic aberration. • A phase-Fresnel lens can be simultaneously blazed for two wavelengths that have an approximate harmonic ratio, e.g.: 633/266 ≈ 12/5. 633 nm confocal imaging system K. Johnson

  12. Modulation Options Intermittent Spot Blanking Full Modulation with SLM high-speed SLM imaging optics micromechanical shutters Shutters only operate intermittently; do not limit throughput. Spot blanking can relax pattern-periodicity constraint. microlens array K. Johnson

  13. SiO2 grating (stationary) d x Stacked-Grating Light Modulator Pixel optics: • ON- and OFF-state reflectance very insensitive to x and air gap. • Can be used for gray-scale modulation. Illumination/ Reflection OFF position Al grating (MEMS-actuated) Zero-Order Reflectance versus x(with ±5-nm tolerance range on air gap): 8-micron pitch (d) 1-micron pitch (d) x/d x/d K. Johnson

  14. EUV Spot-Generation Optics EUV (13.5 nm) • Microlens array • Free-standing Mo zone-plate lenses (85 nm thick; minimum line/space period is 135 nm if the object-plane NA is 0.1) • Aberration correction is designed into the zone structure. Cross-section: Microchannel plate • Aperture array • Passes first order from zone plates • Blocks scatter and extraneous orders • Slightly underfilled to accommodate corrective aberration • Stationary aperture array replaces the scanning photomask of mask-projection EUV. Zone-plate lens (with spider vanes), plan view: Limitation: EUV source rep rate (e.g. 100 KHz) will limit throughput (but will also limit power requirement). K. Johnson

  15. Absorbance modulation optical lithography (AMOL) Potential advantages: Order-of-magnitude improvement in print resolution without multiple patterning. Optical wavelengths (with CW laser  high throughput) Current state of development: Published: 36 nm lines printed with exposure wavelength of 325 nm, masking wavelength 633 nm No demonstration yet of good-quality printing (incl. dense patterns) with AMOL Photochromic materials are being researched by LumArray and U. Arizona (with DARPA support). AMOL has generally only been considered for use with ZPAL and interference lithography. (Other optical system variations are possible.) Limitation: Robust photochromic materials are still undeveloped. K. Johnson

  16. Relevant Patents • Almogy, et al., "Spot grid array imaging system," U.S. Patent 6,639,201, issued October 28, 2003, assigned to Applied Materials, Inc. • Almogy, "Optical spot grid array printer ," U.S. Patent 6,897,941, issued May 24, 2005, assigned to Applied Materials, Inc. • Menonet al., "System and method for absorbance modulation lithography," U.S. Patents 7,713,684 and 7,714,988, issued May 11, 2010, assigned to Massachusetts Institute of Technology. • Johnson, "Optical Systems and Methods for Absorbance Modulation," U.S. Patent Application No. 13/103,874, filed May 9, 2011, unassigned. • Johnson, "Scanned-Spot-Array Optical Lithography," U.S. Provisional Patent Applications No. 61/498,427, filed June 17, 2011, and No. 61/521,684, filed August 9, 2011, unassigned. • Johnson, "Stacked-Grating Light Modulator," U.S. Patent Application No. 13/198,512, filed August 4, 2011, unassigned. • Johnson, "Spot-Array Imaging System for Maskless Lithography and Parallel Confocal Microscopy," U.S. Provisional Patent Applications No. 61/525,125, filed August 18, 2011, 61/531,981,filed September 7, 2011, and 61/549,158, filed October 19, 2011, unassigned. K. Johnson

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