Demonstration of Sub-Rayleigh Lithography Using a Multi-Photon Absorber

1. Demonstration of Sub-Rayleigh Lithography Using aMulti-Photon Absorber Heedeuk Shin, Hye Jeong Chang*, Malcolm N. O'Sullivan-Hale, Sean Bentley# , and Robert W. Boyd The Institute of Optics, University of Rochester, Rochester, NY 14627, USA * The Korean Intellectual Property Office, DaeJeon 302-791, Korea # Department of Physics, Adelphi University, Garden City, NY Presented at OSA annual meeting, October 11th, 2006

2. Outline • Motivation • Quantum lithography • Proof-of-principle experiments • Multi-photon lithographic recording material • Experimental results • Non-sinusoidal 2-D patterns • Conclusion & future work 1/16

3. Motivation • In optical lithography, the feature size is limited by diffraction, called the ‘Rayleigh criterion’. - Rayleigh criterion : ~ l/2 • Quantum lithography using an N-photon lithographic recording material & entangled light source was suggested to improve optical lithography. • We suggest PMMA as a good candidate for an N-photon lithographic material. 2/16

4. Quantum lithography • Classical interferometric lithography - , where K = l/(2sinq) • Resolution : ~ l/2 at grazing incident angle • Quantum interferometric lithography uses entangled N-photon light source. - • Resolution : ~ l/2N • Advantage : high visibility is possible even with large resolution enhancement. Boto et al., Phys. Rev. Lett. 85, 2733, 2000 3/16

5. Enhanced resolution with a classical light source Phase-shifted-grating method • Fringe patterns on an N-photonabsorber with M laser pulses. • The phase of mth shot is given by 2pm/M. • The fringe pattern is • Example One photon absorber Single shot Two-photon absorber Two shots S.J. Bentley and R.W. Boyd, Optics Express, 12, 5735 (2004) 4/16

6. PMMA as a multi-photon absorber • PMMA is a positive photo-resist, is transparent in visible region and has strong absorption in UV region. • 3PA in PMMA breaks chemical bonds, and the broken bonds can be removed by developing process. (N = 3 at 800 nm) PMMA is excited by multi-photon absorption 800 nm UV absorption spectrum of PMMA 5/16

7. Experiment – material preparation • Sample preparation 1) PMMA solution PMMA (Aldrich, Mw ~120,000) + Toluene : 20 wt% 2) PMMA film : Spin-coat on a glass substrate Spin coating condition : 1000 rpm, 20 sec, 3 times Drying : 3 min. on the hot plate • Development 1) Developer : 1:1 methyl isobutyl ketone (MIBK) to Isopropyl Alcohol 2) Immersion : 10 sec 3) Rinse : DI water, 30 sec 4) Dry : Air blow dry 6/16

8. Experimental setup with regenerative amplifcation 120 fs, 1 W, 1 kHz, at 800 nm (Spectra-Physics) WP : half wave plate; Pol. : polarizer; M1,M2,M3 : mirrors; BS : beam splitter; f1,f2 : lenses; PR : phase retarder (Babinet-Soleil compensator) 7/16

9. Experimental process Interference pattern shifted by l/4 Path length difference l/2 Developing l/2 Phase retarder l/4 PMMA Substrate (Glass) 8/16

10. Demonstration of writing fringes on PMMA Recording wavelength = 800 nm Pulse energy = 130 mJ per beam Pulse duration = 120 fs Recording angle, θ = 70 degree Period λ/(2sinθ) = 425nm 425 nm 9/16

11. Sub-Rayleigh fringes ~l/4 (M = 2) Recording wavelength = 800nm Two pulses with phase shift Pulse energy = 90 mJ per beam Pulse duration = 120 fs Recording angle, θ = 70 degree Fundamental period λ/(2sinθ) = 425 nm Period of written grating = 213 nm p 213 nm 10/16

12. Threefold enhanced resolution (M = 3) Recording wavelength = 800nm Three pulses with 2π/3 & 4π/3 phase shift Pulse energy = 80 mJ per beam Pulse duration = 120 fs Recording angle, θ = 8.9 degree Fundamental period λ/(2sinθ) = 2.6 mm Period of written grating = 0.85 mm 0.8 mm 1.67 mm 213 nm 2.6 mm 11/16

13. 141 nm Non-sinusoidal fringes • PMMA is a 3PA at 800 nm. (N=3) • Illumination with two pulses. (M=2) • If the phase shift of the second shot is , where , the interference fringe is • Numerical calculation is similar to the experimental result. • This shows the possibility of non-sinusoidal fringe patterns. 12/16

14. Non-sinusoidal Patterns • Different field amplitudes on each shot can generate more general non-sinusoidal patterns. For example, if N = 3 , M = 3 A1 = 1 A2 = 0.75 A3 = 0.4 ∆1 = 0 ∆2 = π/2 ∆3 = π 13/16

15. Two Dimensional Patterns • Method can be extended into two dimensions using four recording beams. For example, N=8, M=14 14/16

16. Conclusion • The possibility of the use of PMMA as a multi-photon lithographic recording medium for the realization of quantum lithography. • Experimental demonstration of sub-Rayleigh resolution by means of the phase-shifted-grating method using a classical light source. - writing fringes with a period of l/4 • Quantum lithography (as initially proposed by Prof. Dowling) has a good chance of becoming a reality. Future work • Higher enhanced resolution (M = 3 or more) • Build an entangled light source with the high gain optical parametric amplification. • Realization of the quantum lithography method. 15/16

17. Acknowledgement & Dr. Samyon Papernov Supported by - the US Army Research Office through a MURI grant - the Post-doctoral Fellowship Program of Korea Science and Engineering Foundation (KOSEF) and Korea Research Foundation (KRF) 16/16

18. Thank you for your attention! http://www.optics.rochester.edu/~boyd

19. Two Dimensional Patterns • Illuminate one shot, N = 3, M = 1 • Rotate the sample • Illuminate the second shot, N = 3, M = 1 • Experimental 2-D pattern