Full-field Liquid Immersion Interference Lithography

James Jacob1, John Hoffnagle1, John Burnett2, Eric Benck2, Darrell Armstrong3, Arlee Smith4

Acknowledgment: Michael Fritze, DARPA

1. Actinix, Scotts Valley, CA2. NIST, Gaithersburg, MD3. Sandia National Labs, Albuquerque, NM4. A-S Photonics, Albuquerque, NM

M2M1

G

P

W

Liquid immersion interference lithography (LIIL)

• Why LIIL? Potentially very low cost, maskless process for 35-25 nm hp using water and gen 2 and 3 fluids

• Main Issues: Full-field exposures and new circuit libraries (2D to 1D)

• Enabling technology: long coherence length, far-UV laser (=197 nm) being developed by Actinix/Sandia under a DARPA STTR for low volume nano-fab

• This laser development project is related to upcoming DARPA program, GRATE: Gratings of Regular Arrays and Trim Exposures

• Pending proposal on a full-field LIIL R&D tool for testing new nano-scale metrology methods and protocols essential for next generation semiconductor device production

• Tool will support investigations into fringe doubling and directed self-assembly (DSA) as viable next generation litho processes (< 16 nm hp).

http://www.sematech.org/meetings/announcements/8653/

http://www.sematech.org/meetings/announcements/8653/

Interference Lithography Tools - Some Examples

• 257 nm - IBM, UNM• 248 nm - RIT• 213 nm - UNM• 193 nm - RIT (Amphibian), IBM Nemo• 157 nm - MIT LL• EUV - Intel, Hanyang Univ., Paul Scherrer Inst., Univ. of Wisconsin

Micro-exposure Tools

• Meter scale gratings • 351 nm (Ar ion), Dry• High precision metrology• Transparent substrates• Scanned small field, 1 mm• Approx. 100 nm hp

MIT NanoRuler - Scanning beam interference litho (SBIL)

IBM Nemo

http://snl.mit.edu/

LIIL Tool: IBM Nemo

• Laser: 193 nm, 7 pm, 25 mW, 5 kHz• High spatial coherence, TEM00 • Liquid Immersion: H2O, Gen II• 35 - 30 nm hp• Millimeter field size• High contrast > 90%

197 nm vs 193 nm : High index fluids have less transmission loss

3X lower absorption in IF-132 @ 197 nm

Second Generation Fluids for 193 nm Immersion Lithography, R. French, et al. SPIE Microlithography 2007.

Lower Less heating Lower n Less Distortion

• Increase source by 2%

• Move away from band edge

• Higher index liquidsmore feasible

Technical Challenges for LIIL to Print Full-field Nano-scale Patterns

• Long coherence length - Lc of 5 cm prints 35 nm hp over 1 cm field @ 98% V• High spatial coherence - pattern fidelity, uniformity• Power - @ 50 mJ/cm2 dose, 1 watt => 49 w/hr, assuming 50% overall optical efficiency• Wavelength stability - pitch control• Energy stability - dose control• Wavefront flatness - suppression of hyperbolic fringes• Rectangular fields - mask circular/elliptical exposure beams• Metrology - measure fringe straightness, pitch, registration• Optical quality - minimize figure errors

Modify the Nemo design for full field patterning:• Increase beam dia. to >3 cm, or • Use elliptical pattern, 0.5 x 2.6 cm, and scan• Mask to form rectangular exposures• Employ metrology for registration

33 mm

26

mm or

Single full-field Scanned sub-field

Coherence Properties of Light Source

Ray A is symmetrical - paths are equalRay B lacks symmetry - paths are unequalCoherence length needs to accommodate OPD

BW Pitch Contrast Field size

0.26 pm 70 nm 98% 10 mm

0.26 pm 70 nm 92% 22 mm

0.26 pm 70 nm 83% 33 mm

M2M1

AB

Solid-state 193 nm laser mode quality

TEM00, M2=1.2 Spatial Coherence Test

High spatial coherence is necessary for pattern fidelity and uniformity

Spatial Coherence

Temporal Coherence

Lc = c/

@ 197 nm: Lc of 4.7 cm => of 2 GHz => of 0.26 pm

(assumes Lorentzian lineshape)

Interferometer - symmetrical grating-mirror design

• - fine freq control• G - Grating• Mirrors M1, M2 control pitch• D - dose monitor• S - shutter• L - Inspection lens • C - CCD camera• P - coupling prism• W - wafer• MG - Metrology grating

(Deformable mirror)

0 +11

M2M1

WP

L

G

S

D

Laser

beam conditioning optics

C

MG

Beam shapingSuper-gaussian profiles

Second lens corrects the wavefrontdistortion due to the first lens,

First lens redistributes rays to transform the intensity profile

Collimated beam with planar wavefront

Efficiency vs uniformity

Keplerian-type anamorphic telescope

p=16

p=8

p=4

p=2 (Gaussian)

p=2

p=4

p=8

p=12

Metrology – Pitch Control• Rectangular interference pattern illumination - full chip field (or scanned over field)

33 mm

L=N=26 mm

pitch ofinterference pattern

• Registration requirements (~10% of pitch) = 0.1 x 70 nm = 7 nm

distortion L 7 nm over field

tolerance on pitch

• Tolerance on: , n, T,

• Can use precise control of to help maintain registration across field

Metrology – Registration with Reference Grating• Need to control pattern pitch, straightness, position, and orientationOverlap laser interference pattern with reference reflection grating having desired pitch reflected Moiré pattern

interference patternwith low-order curvature

reflection/absorption grating

• View Moiré pattern with CCD (e.g., 100 fringes per pixel) - unresolved interference• Eliminate Moiré nulls with stage rotation, translation, control, and adaptive optics (for low-order curvature) can verify registration with grating across field

• Achieve registration on wafer with help of wafer alignment gratings

Moiré pattern

nulls where patterns of phase

Long Coherence Length Light Source - Key Enabler

• Fiber laser based light source -- Long storage time in fiber amps, pulsed oscillator appears CW- Pulse repetition frequency can therefore be easily increased 4X- Requires higher pump diode power - current design only 6 W/amp

• CLBO mixing crystal used for 197 nm generation - Low absorption- Can be non-critically phase-matched (no walkoff)

• Other fiber laser based sub-200 nm systems:- Nikon - 8th harmonic of Er fiber (193 nm)- Megaopto - Er/Yb fibers mixing to 198 nm

Initial Specs : 197 nm; Avg. power = 250 mW; BW = 0.26 pm; PRF = 1 MHz

Scaled Specs : 197 nm; Avg. power = 1 W; BW = 0.09 pm; PRF = 4 MHz

Far-UV moduleDUV Module

Infra-red front end

Solid-state Far-UV Generation

Fiber Amplifiers: Design for Low Self Phase Modulation

x

Pulse duration = 2 ns; Energy = 2.6 J (P = 2.6 watts)Power scalable - increase PRF to 4 KHz, P = 10 watts

Modeled phase distortion for our design

0.75 rad

Modeled spectrum assuming 2.0 rad phase

Our operating point300 MHzFWHM

In birefringent crystalsPoynting vector walk-off reduces effective length

L

deff

• Low absorption, •High non-linear coefficient, deff

• Long crystal length, L• No walkoff, non-critical PM, = 0• Small spot size, w

Efficient NLO Lower Power Fiber Amp Narrower Bandwidth

~ E2 , 1/t2, L2, d2, 1/, 1/w2

E, t

w

Efficiency relation for NLO interaction For highest efficiency:

E and 1/t chosen to minimize SPM

Far UVMixer

Er amps

Yb amps

Switch

Switch

Er CW fiber laser197 nm

Isolator

Isolator

Yb CW fiber laser

Pulse generator

• CW sources: 50 MHz frequency stability (7 fm @ 197 nm)• 50 MHz frequency tuning resolution• Multiplexed fiber laser architecture

allows power scaling while maintaining bandwidth• Efficient frequency conversion scheme

All NLO crystals non-critically phase-matched (or quasi-PM)

Fiber Laser System Architecture

DUV Module

Summary

• Liquid immersion interference lithography is a tractable low cost solution for single exposure patterning offull field grating structures down to 25 nm hp

• 197 nm fiber-based laser will provide coherence length, beam quality and power suitable for printing high contrast gratings at reasonable throughput

• Interferometer design needs to incorporate metrology and possibly adaptive optical techniques to control fringes over full-field