MICRO AND NANOPROCESSING TECHNOLOGIES LECTURE 4 Optical lithography Concepts and processes Lithography systems Fundamental limitations and other issues Photoresists Photolithography process Process parameter issues Image reversal issues Chapt.7. Optical lithography 1/43
CONCEPTS AND PROCESSES Chapt.7. Optical lithography 2/43
A photolithography system consists of a projection system, a mask, and a surface coated with a photo-sensitive film. The light changes the properties of the film locally. This allows the film to be partly removed ( by development ) and the substrate to be selectively exposed to etching or deposition. Chapt.7. Optical lithography 3/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 4/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 5/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 6/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 7/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 8/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 9/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 10/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 11/43
Spinning resist Baking Aligning mask Exposing resist Soft-bake Develop resist Hard bake Etch / Deposit Remove resist Chapt.7. Optical lithography 12/43
LITHOGRAPHY SYSTEMS Illumination issues Chapt.7. Optical lithography 13/43
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Chapt.7. Optical lithography 19/43
LITHOGRAPHY SYSTEMS Basic setups Chapt.7. Optical lithography 20/43
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FUNDAMENTAL LIMITATIONS - diffraction - resolution and DOF - MTF and coherence Chapt.7. Optical lithography 25/43
Diffraction w 2 >> λ g 2 + r 2 (1) w 2 << λ g 2 + r 2 (2) (1) Fresnel diffraction ( near field ). (2) Fraunhoffer diffraction ( far field ). Chapt.7. Optical lithography 26/43
Chapt.7. Optical lithography 27/43
5:1 10:1 Printing High NA implies: Smaller throughput. More complex lens system. Very mechanical demanding. EXPENSIVE! w = kλ / min NA NA: numerical aperture λ: light wavelength k: constant depending on resist process Practical limit: < 0.2 µm. Expensive, high resolution. Large volume production. No mask wear. Chapt.7. Optical lithography 28/43
Resolution vs Depth of Focus ( DOF ) k1λ w = NA DOF = k2λ 2 NA (1) (2) CONTRADICTIVE!!! Chapt.7. Optical lithography 29/43
Chapt.7. Optical lithography 30/43
1:1 Printing Gap: zero to a few 100µm. Resolution limit (diffraction). w min = k λ g g: proximity gap λ: light wavelength k: constant depending on resist process Practical limit: 0.7 1 µm. Cheap, medium resolution. Small volume. Mask wear due to mask-wafer contact. Chapt.7. Optical lithography 31/43
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Chapt.7. Optical lithography 33/43
MTF Modulation Transfer Function MTF = I I MAX MAX + I I MIN MIN I MAX =5; I MIN =1; MTF=0.67 Chapt.7. Optical lithography 34/43
Coherence concept NA = n* sin( α) NA - Numerical Aperture NA = n* sin( α) S = source pupil diameter diameter S - Spatial coherence Chapt.7. Optical lithography 35/43
nmtf normalized Modulation Transfer Function ( normalized resolving power of the projector ) ν = 1 NA 0.61* λ o = W o ν ο - Spatial frequency normalized to Rayleigh criterion ( more @ http://micro.magnet.fsu.edu/primer/java/imageformation/rayleighdisks/ ) Chapt.7. Optical lithography 36/43
OTHER ISSUES - reflections - standing waves - phase shifters Chapt.7. Optical lithography 37/43
Topological reflections Chapt.7. Optical lithography 38/43
Standing waves effect Chapt.7. Optical lithography 39/43
Phase shifters concept Chapt.7. Optical lithography 40/43
Phase shifters concept Chapt.7. Optical lithography 41/43
HOME ASSIGNMENT Plot resolution and depth of field as a function of exposure wavelength for a projection mask aligner with 100nm < λ < 500nm. Use k=0.75 and NA=0.26. On the same plots, recalculate these functions for NA=0.41. Discuss the implications of these plots for the technologist that must manufacture transistors with 0.5μm features. Chapt.7. Optical lithography 42/43
THAT S ALL FOLKS! Chapt.7. Optical lithography 43/43