K. Johnson 1

Scanned-Spot-Array Optical Lithography

A Strategy for Bridging the NGL Technology Gap

10/19/2011

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:1. It eliminates optical proximity effects.2. It can use high-NA, wide-field immersion imaging (potentially solid immersion).3. Field curvature, distortion, and geometric point-imaging aberrations can be entirely

eliminated.4. The system can optimally control polarization over the full image field.5. 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.

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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.

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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

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K. Johnson 5

Scanned-Spot-Array Lithography: Basic Design Concept

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 focused-spotarray

source-spotarray

laser

modulatorbeam expander

collimating lenslow-NAmicrolens array

aperture mask

reduction lens

raster-scannedprinting surface

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K. Johnson 6

Optical System Alternatives and Tradeoffs

Image-plane microlens array, full modulation (e.g., ZPAL):

Source-modulated: Object-plane spot array:

SLM

Simple, low-NA projection lens

Difficult, high-NA microlens array

No projection lens (just uniform, modulated illumination)

No SLM

Difficult, high-NA microlens array

Simple, low-NA microlens array

Complex, high-NA, wide-field projection lensBut the spot-generation optics can greatly simplify the lens requirements.

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K. Johnson 7

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.)

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.

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

Spot-Generation Optics can Supplant much of the Projection Lens Functionality

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ProjectionLens

K. Johnson 8

• 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: 1012 bit per sec

• Data rate with source modulation (periodic print pattern): only 106 bit per sec

Scan Pattern: A Conceptual Illustrationfocused spotscan line

surface scandirection

Periodicity constraint is, e.g., 25-micron square (versus submicron for GRATE).

Requires CW or high-rep-rate laser source.

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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).

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K. Johnson 10

Solid-Immersion Lithography

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cover plate

projection lens

last lens element (solid immersion)

spot-generation optics(mechanically scanned)

wafer

Y

scan linefocus spot

X

bidirectional X-Y scan:

Scanning mechanism:• Mechanical actuation 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.

K. Johnson 11

Focus and Overlay Feedback via Confocal Imaging

DUV (modulated)

633 nm confocal imaging system

Merge Exposure and Confocal Viewing Light Paths:• Split-aperture beam combiner, or• Dichroic beam combiner, or• Interleaved exposure/viewing

microlens arrays

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.

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K. Johnson 12

Modulation Options

Shutters only operate intermittently; do not limit throughput.

Spot blanking can relax pattern-periodicity constraint.

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micromechanical shutters

Intermittent Spot Blanking Full Modulation with SLM

high-speed SLM

imaging optics

microlens array

K. Johnson 1310/19/2011

Stacked-Grating Light Modulator

Zero-Order Reflectance versus x (with ±5-nm tolerance range on air gap):

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

s

R[s]

x/d

8-micron pitch (d)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

s

R[s]

x/d

1-micron pitch (d)

Pixel optics:

d

x

Illumination/Reflection SiO2 grating

(stationary)

Al grating(MEMS-actuated)

OFF position

• ON- and OFF-state reflectance very insensitive to x and air gap.

• Can be used for gray-scale modulation.

K. Johnson 14

Zone-plate lens (with spider vanes), plan view:

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.

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.

Cross-section:

EUV (13.5 nm)

EUV Spot-Generation Optics

Limitation: EUV source rep rate (e.g. 100 KHz) will limit throughput (but will also limit power requirement).

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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.

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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.

• Menon et 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.

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