Solid Immersion and Evanescent Wave Lithography at Numerical Apertures > 1.60

Solid Immersion and Evanescent Wave Lithography at Numerical Apertures > 1.60 Bruce Smith Y. Fan, J. Zhou, L. Zavyalova, M. Slocum, J. Park, A. Bourov, E. Piscani, N. Lafferty, A. Estroff Rochester Institute of Technology

Outline The imaging limits of materials Pushing the limits of immersion lithography The solid immersion lens Solid immersion lithography (SIL) Evanescent wave lithography (EWL) Imaging 6nm at 1.85NA

Material and Optical Limitations NA = n i sin θ 1. Sin θ increases slowly at large angles (sin 68 =0.93). Hyper-NA will be forced upon material refractive index 3. Resolution will become a function of the lowest index (fluid, optics, photoresist). glass media photoresist substrate hp k1λ n sinθ (0.5 to 0.30)(193nm) n (0.93) min = = = i i 5 n i to 6 n i nm

Technology Limits in Media TIR from Snells Law: θ c = sin -1 (n L /n H ) 100 SIL (1.56) HIF (1.54) Air (1.00) H 0 (1.44) 40 68 84 90 100 SIL (1.70) TIR @ Air (1.00) H HIF 0 (1.44) (1.54) θ c = sin -1 (n L /n H ) 3 49 67 58 Reflectance 90 80 70 60 TE TM Reflectance 90 80 70 60 TE TM 50 50 40 0 10 0 30 40 50 60 70 80 90 Angle (degrees) 40 0 10 0 30 40 50 60 70 80 90 Angle (degrees) Fused silica (n=1.56) Sapphire (n=1.9)

Technology Limits in Media Numerical Aperture 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Half-Pitch (nm) k1=0.5 37 34 3 30 8 7 5 4 k1=0.30 45 44 39 36 34 3 30 9 Angle in media Water (1.44) 65 76 HIF (1.55) 57 65 75 HIF (1.65) 5 58 65 76 Photoresist (1.70) 50 55 6 70 HI PR (1.85) 45 49 54 60 67 77 Fused silica (1.54) 58 65 77 Sapphire (1.9) 43 47 5 56 6 70 TIR

Impact of Angle in Photoresist Simple Approximations Unpolarized modulation 1. 1 0.8 0.6 0.4 0. Polarization and absorption Modulation 1 cos θ Absorption 7 6 5 4 3 1 Absorption scaling DOF scaling 35 30 5 0 15 10 5 Depth of focus 1 sin θ 0 0.85NA 1.0NA 0 0 40 60 80 Angle in resist (degrees) 0 0 0.85NA 1.0NA 0 0 40 60 80 Angle in resist (degrees) Oblique absorption requires low k photoresist Paraxial DOF scales with 1/sin (θ) Angles above 30 (0.85 NA) require attention Oblique reflection becomes an issue > 30

A Solid Immersion Lens - A high index solid immersion lens is placed in close proximity to an image plane - Dry imaging for NA values > 1.0 - Used in optical storage applications - Energy coupled into the thin film decays exponentially: A( z) = e πn upper λ sin n θ n lower upper 1 / + α z n upper = lens n lower = air z = gap

Solid Immersion Lithography Sapphire SIL Breadboard Sapphire Properties: - Hexagonal, single-crystalline Al O 3 - n = 1.9, birefringence ~8x10-3 - Equilateral prism at 60 is 1.67NA - Designed for NA 1.05~1.9 - MgF is ideal AR layer Sapphire prism Turning mirrors prism cylinder Zero-order block Challenges: - Gap and gap control - Birefringence - CAR resist diffusion length limit - Resist/BARC process optimization Grating mask Polarized ArF beam

Optical Coupling in the Prism Laser Detector Laser Detector (a) Baseline (no wafer). (b) Reflection (with wafer). (a) Before pressure is applied. (b) After pressure is applied.

Estimation of Gap Thickness - Reflectance measurement used to estimate gap thickness. - Gap controllable from 0-50nm - 1nm air gap utilized. Reflectance 1 0.8 0.6 0.4 NA=1.66 Theoretical Measurement Immersion solid (sapphire), N 0 =1.9 Air gap, N 1 =1.00, d 1 =0~50 nm Resist, N =1.71-0.399i, d =78 nm BARC, N=1.70-0.1i, 9 nm Substrate N sub =0.87-.76i 0. 0 0 10 0 30 40 50 Air gap thickness (nm) Resist assembly

Solid Immersion Lithography at the Resist Limit 1.4NA, 34nm 1.60NA, 30nm 1.66NA, 9nm ILSim simulations

Beyond the Resist Limit Evanescent Wave Coupling n upper sinθ = NA max Homogeneous propagation θ c Evanescent region n lower sinθ Evanescent region thin film higher n n upper -hi n lower -low Homogeneous propagation NA = n i sinθ Energy coupled into the thin film decays exponentially: A(z) = e πn λ upper sin n θ n lower upper 1 / +α z

Evanescent Wave Lithography Beyond the Resist Limit - 6nm hp at 1.85NA NA (1.85) has been pushed higher than the index of the resist (1.70). Image pattern depth of <10 nm. Sets the stage for new material development toward 5nm. Potential with TSI and hardmask imaging layers. 1.85NA, 6nm

Gap Requirements / Tolerances Assume 50% intensity loss across the image no loss in modulation 1% gap results in ~0.5-1% intensity at 1.70NA dose control issue Gap (nm) for 50% intensity 80 70 60 50 40 30 0 10 8nm hp at 64 in sapphire A(z) = e Water πn λ upper HIF 1.0 1.1 1. 1.3 1.4 1.5 1.6 Gap Index sin Gap (nm) for 50% intensity 90 80 70 60 50 40 30 0 10 0 n θ n 1.65 1.70 1.75 1.80 1.85 1.90 Numerical Aperture lower upper 1 / 8nm hp HIF gap for 0.5k1 +α z 7nm hp

Implications of SIL and Evanescent Wave Lithography 1. SIL / EWL is useful for determining the ultimate limits of optical lithography in the 5nm regime.. NA possible beyond the fluid index. 3. Higher index photoresists may not be necessary if topsurface imaging (TSI) can be employed. 4. SIL may be feasible if small fluid gaps can be maintained.

Technology Limits in Media Numerical Aperture 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Half-Pitch (nm) k1=0.5 37 34 3 30 8 7 5 4 k1=0.30 45 44 39 36 34 3 30 9 Angle in media Water (1.44) 65 76 HIF (1.55) 57 65 75 HIF (1.65) 5 58 65 76 Photoresist (1.70) 50 55 6 70 HI PR (1.85) 45 49 54 60 67 77 Fused silica (1.54) 58 65 77 Sapphire (1.9) 43 47 5 56 6 70 Can be achieved with immersion lithography May be possible with SIL / EWL Not likely Acknowledgements SRC, DARPA/AFRL, Sematech, ASML, Photronics, TOK, JSR, Rohm and Haas, Brewer, NYSTAR, Corning Tropel