1Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05(1) Contact to source/drain of MOSFET.n+SiO2SiO2Al“ideal”p-SiSiO2SiO2Alp-Sin+“short”, ohmic contactAlignment error∆2Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05SiO2n+SiO2Solution: Design n+ region larger than contact holeAl3Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05(2) Poly-Gate Overlap over FOXFox“Ideal”poly-gateS/D implantn+Electrical short“With alignment error”4Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Solution: Make poly gate longer to overlap the FOX5Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Total Overlay Toleranceσσ22totalii=∑σi= std. deviation of overlay error for ithmasking stepσtotal= std. deviation for total overlay errorLayout design-rule specification should be > σtotal6Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Standing WavessubstratePositivePhotoresisthvsubstrateAfter developmentPositivePhotoresist.*Photoresist has a finite thicknessHigher IntensityLower IntensityFaster Development rateSlower Development rate7Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05reflectingsurfaceOxidePhotoresistAirE1E2E3E4x=0 x=d x=drI23 (x) = 1T ⌡⌠ 0 T (E2(x)+E3(x))2dt = 12 (E2 - E3)2 + 2E2E3sin2[k(d-x)] ∴ I23 (max) = 12(E2 + E3)2 ; I 23(min) = 12(E2 - E3)2 I (max)23I (min)23dx Intensity minima occur at : 2πnλ(d-x) =0, π, 2π, .......Intensity maxima occur at : 2πnλ(d-x) = π/2, 3π/2, 5π/2, .......Resist profile and energydeposition depend onoxide thickness underneath(see handout for derivation)Resist profile and energydeposition depend onoxide thickness underneath(see handout for derivation)Standing wave effect8Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05P.R.Intensity = minimum whennmdx2λ−=xdm = 0, 1, 2,...Intensity = maximum whennmdx4λ−=m = 1, 3, 5,...n = refractive index of resistSiO2/Si substrate9Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Simulated Resist Cross-section as function of development time10Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Proximity Scattering11Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Approaches for Reducing Substrate Effects• Use absorption dyes in photoresist• Use anti-reflection coating (ARC)• Use multi-layer resist process1: thin planar layer for high-resolution imaging2: thin develop-stop layer, used for pattern transfer to 33: thick layer of hardened resist(imaging layer)(etch stop)(planarization layer)12Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Electron-Beam LithographyV312.=λAngstroms for V in VoltsExample: 30 kV e-beam => λ= 0.07 AngstromsNA = 0.002 – 0.005Resolution < 1 nmBut beam current needs to be 10’s of mA for a throughput of more than 10 wafers an hour.13Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Low Throughputfor both raster and vector scanning (SerialProcess)VariableBeam-shapeEBLStencilMaskEBL14Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05e-beam lithography resolution factors • beam quality ( ~1 nm) • secondary electrons ( la te ra l ra nge : fe w n m ) pe rforma nce re cords organic resist PMMA ~ 7 nm inorganic resist, b.v. AlF3 ~ 1-2 nm15Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05The Proximity EffectMonte Carlo simulation of electron trajectories16Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05SynchrotronRadiationX-Ray Source17Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05X-Ray Proximity Printing10≈λlgm∝λAngstroms18Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05X-Ray Projection Lithography19Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Stulen & SweenyAlpha-Machine for EUV Lithography20Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05Resist patterned by Extreme UV Lithography21Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F0522Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05•A liquid with index of refraction n>1 is introduced between theimaging optics and the wafer. Immersion LithographyAdvantages1) Resolution is improved proportionately to n. For water, the index of refraction at λ = 193 nm is 1.44, improving the resolution significantly, from 90 to 64 nm.2) Increased depth of focus at larger features, even those that are printable with dry lithography.23Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05B.J. Lin Sept. 02Drag a drop schemes also being considered by UTA.24Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F0525Professor N. Cheung, U.C. BerkeleyLecture 12EE143 F05For your reference onlyFor your reference
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