EE143 S06 Review 2 Qualitative Explanation of image degradation by lens 2 Mask lens wafer plane 1 0 1 parallel optical beam 2 grating with spatial frequency 1 P L L P Professor N Cheung U C Berkeley P sin n n 0 1 2 sin NA of lens P 2L 1 EE143 S06 Why Mask Intensity lm NA Review 2 We want small lm The lens has to collect at least the n 1 diffracted beams to show any spatially varying intensity on wafer Imax O Image on wafer x optical system x For advanced reading see Microlithography Science and Technology by Sheats and Smith Professor N Cheung U C Berkeley 2 EE143 S06 Review 2 Depth of Focus DOF We want LARGE DOF off point best Professor N Cheung U C Berkeley 3 EE143 S06 Review 2 Normalized remianing thickness after development Positive Resist 1 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 1 10 100 1000 Exposure energy dose mJ cm2 This is log scale Professor N Cheung U C Berkeley 4 EE143 S06 Review 2 Why High Contrast Resist is desirable Optical image Infinite contrast resist resist substrate Finite contrast resist resist substrate Position x Professor N Cheung U C Berkeley 5 EE143 S06 Review 2 Past Exam Question Resist cross section after development Mormalized resist thickness Normalized remianing thickness after development Positive Resist 1 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 1 10 100 1 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 1000 1 Photon energy dose mJ cm2 Answer 3 Lateral position x in um Exposure energy dose mJ cm2 160 140 120 100 90 80 70 60 50 40 30 20 10 0 2 Any dose 20mJ cm2 will work Any dose 20mJ cm2 will work 0 1 2 3 Lateral position x in um Professor N Cheung U C Berkeley 6 EE143 S06 Review 2 Overlay Error is never zero for ALL lateral positions wafer photomask plate Professor N Cheung U C Berkeley Alignment marks from previous masking level 7 EE143 S06 Review 2 Deposited thickness not uniform across wafer D H Wafer Emission flux cos n Evaporation Source Area A Professor N Cheung U C Berkeley 8 EE143 S06 Review 2 Evaporation deposited film photo resist Step coverage problem is not the same as thickness nonuniformity problem substrate Sputtering deposited film photo resist substrate Professor N Cheung U C Berkeley 9 EE143 S06 Review 2 Thermal Oxidation Model CG stagnant layer Cs Note Note CCs CCo s o SiO2 Si Co Ci X0x F2 F1 gas transport flux Professor N Cheung U C Berkeley diffusion flux through SiO2 F3 reaction flux at interface 10 EE143 S06 Review 2 CVD Deposition Rate Grove Model by setting Xox 0 film Si F1 F3 F1 F3 Professor N Cheung U C Berkeley D CG CS D hG k s ko e E kT thickness of stagnant layer F1 F3 kS CS 11 EE143 S06 Review 2 ks depends only on temperature hG depends on temperature pressure and flow velocity ks hG hG reduced Professor N Cheung U C Berkeley 12 EE143 S06 Review 2 Degree of Anisotropy dm hf etching mask Af film substrate Bias B d f d m B can be 0 or 0 df B 1 2h f 0 Af 1 dm substrate df Professor N Cheung U C Berkeley 13 EE143 S06 Review 2 Etching profile depends on 1 mask film configuration 2 Film etching rate and anisotropy 3 Maks etching rate and anisotropy SIMPLE EXAMPLE Film etching 100 anisotropic NO mask etching etching mask film final 1 m x 1 m substrate Professor N Cheung U C Berkeley substrate substrate 14 EE143 S06 Review 2 Etching Selectivity S S AB v A vertical etching velocity of materal A v B vertical etching velocity of materal B Wet Etching S is controlled by chemicals concentration temp RIE S is controlled by plasma parameters plasma chemistry gas pressure flow rate temperature Professor N Cheung U C Berkeley 15 EE143 S06 Review 2 Importance of Etching Selectivity A simple Example Thickest SiO2 on wafer 1 1 m Thinnest SiO2 on wafer SiO2 0 9 m When ALL SiO2 is just cleared Si substrate Professor N Cheung U C Berkeley Si substrate 16 EE143 S06 Review 2 How to Control Anisotropy 1 ionic bombardment to damage expose surface 2 sidewall coating by inhibitor prevents sidewall etching Professor N Cheung U C Berkeley 17 EE143 S06 Review 2 How to Control Selectivity E g SiO2 etching in CF4 H2 plasma Rate SiO2 S Rate Si S Rates P R SiO2 Si SiO2 H2 Si H2 in CF4 H2 Reason F H HF F content SiF 4 Professor N Cheung U C Berkeley The F reaction with Si alone is a chemical etching more isotropic 18 EE143 S06 Review 2 Effect of RIE process variables on etching characteristics Control variable effect Professor N Cheung U C Berkeley 19 EE143 S06 Review 2 Example Dual Damascene Process Etch mask for top insulator layer Slope due to resist erosion Etch stop for top insulator layer Etch mask for bottom insulator layer Etch stop for bottom insulator layer Final insulator cross section Overetch of bottom insulator layer Overetch of bottom Cu layer Professor N Cheung U C Berkeley 20
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