Photoresist Profile 4-15 Undercut: negative slope, common for negative resist; oxygen diffusion prohibits cross-linking; good for lift-off undercut overcut Overcut: positive slope, common to positive resist, cannot perform lift-off.
Image Reversal Positive resist has better resolution but cannot perform lift-off. After UV exposure, exposure to an amine vapor (e.g. triethanolamine) in a vacuum oven. The amine neutralize the product of photodecomposition and makes the exposed area resistant to further UV light and developer. Then a flood exposure makes the areas adjacent to the neutralized image soluble in conventional positive PR developer. triethanolamine Negative resist has reversed by oxygen flooding during low-intensity UV exposure to scavenge all photogenerated azide polymerization precursors, making the exposed area soluble in a developer. Subsequent flood high intensity UV exposure under a N 2 blanket initiates the polymerization in the previous nonexposed areas. 4-16
Bilayer Liftoff Wafer cleaning, dehydration bake 200 C-5 min LOR/PMGI adhere well on Si, GaAs; without HMDS LOR/PMGI film needs to be 25% thicker than metal film. LOR/PMGI film needs to be 25% thicker than metal film. LOR or PMGI Metal deposition Positive PR Soft bake Lift-off 1: acetone Lift-off 2: Remover PG UV exposure Developing 4-18
Single-layer Liftoff Single-layer Liftoff vs. Bi-layer Liftoff PR16-05 PR16-06 PR10-10 LOR after development Bi-layer Liftoff PR18-10 PR22-1 4-17
Limit on Photolithography - diffraction Finite slit leads to Fraunhofer diffraction in the far field limit dd kkaa 2 and Fresnel diffraction in the near field limit dd kkaa 2. 2 I In the far field limit, light through a finite slit: ( θ ) sinα = where πa sinθ α = a: aperture I( 0) α λ The Fourier transform of a square wave is a sinc. ssssss ππππ ππππ ff Rayleigh resolution criterion R = 0.61λ NA NA: numerical aperture determines how much the lens can capture the diffracted light. Ideal NA=1, best ones ~0.5. R: distance between two objects NA = nsinθ θ f Ref: Campbell: 7.2 4-19
4-20 Direct write EBL: E-beam Lithography Ref: Campbell: 9.2 Beat the diffraction limit Maskless, control of dose and energy Electron irradiation breaks polymer bonds or cross-link polymers. Sensitivity, tone, resolution, and etching resistance PMMA,Poly(methyl methacrylate), (positive resist), HSQ, hydrogen silsesquioxane, (negative, monomer H 8 Si 8 O 12 ) Electron source: thermionic emission, field emission <100 kev Limited by beam size, aberrations, space charge, forward and backward scattering, low throughput
4-21 E-beam Lithography Electron projection lithography EPL- scattering with angular limitation projection SCALPEL Masks are transparent to100 kev electrons. Low atomic number membrane scatters electrons weakly to small angles. High atomic number patterns scatters electrons strongly to high angles. An aperture in the back-focal plane blocks the strongly scattered electrons, forming a contrast image at the wafer plane. Beam is 2-3 mm, but image is still limited to 1x1 mm at the mask level. Ref: Campbell: 9.8 25 nm 100 nm
Physical Deposition- Evaporation (I) Cha evaporator Materials: power of the evaporator, source Thickness control: crystal microbalance cosθ cosφ Evaporation rate 2 πr Tooling factor Density, Z θ Contamination φ R Temperature increases on sample Deposition evenness Source materials, configurations Crucible heater crucible Mo, Ta graphite BN, Alumina coil basket boat 5-1
5-2 Edwards Auto 306 Physical Deposition- Evaporation (I) Atomic force microscopy holder tooling Edge/center thickness 3 1.53 0.94 4 2.80 0.76
5-3 Physical Deposition- Evaporation (II) E-beam evaporation: Need UHV Reduce crucible contamination Charge has to be of particular shape to fit in crucible Reduce component outgassing by water cooling Multiple pockets allow multi-layer coating of different materials without breaking vacuum. Multicomponent film of certain stoichiometry is difficult due to different vapor pressure of materials.
Step Coverage and Shadowing Metal evaporation is highly directional. Aspect Ratio = height diameter Cannot form a continuous film if AR>1 Marginal if 1 > AR > 0.5 Improvement Raise sample temperature - enhance diffusion Sample rotation Shadow evaporation Create lines and gaps much smaller than lithography 14 nm Al/AlO x /Al tunnel junctions made G. J. Doland, APL 31, 337 (1977). 5-4
Physical Deposition- Sputtering Breakdown voltage: Paschen law P: pressure, d: electrode spacing V = Pd ln( Pd) + b d Angular dependence Threshold energy ~10-30 ev 3 2 V Ion flux J 2 m d ion Less directional coverage No melting point limit Inert gas pressure ~0.1 torr d~ 10 cm Maximum yield E~ 1 kev E > 10 kev implantation Ref: Campbell: 10.6 5-5
5-6 CVD Metal Deposition Ref: Campbell: 13.8 Plug can reduce contact area Cold wall LPCVD WF + H W 6HF 6 3 2 +