143s2006-final

# B 10 points a 500 μ m thick bare si wafer is

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(B) (10 points) A 500 μ m -thick bare Si wafer is originally flat. After a 300nm-thick oxide deposition, the wafer radius of curvature is measured to be +200 m. A 600nm nitride film is then deposited on top of the oxide and the wafer radius of curvature becomes infinity ( i.e., flat, no curvature). Calculate the stress of the nitride film alone. Is the nitride stress compressive or tensile ? (Given: ν Si = 0.272, E Si = 1.9 x 10 11 Newton/m 2 ) RADIUS OF CURVATURE RELATIONSHIP : σ f = E s × t s 2 ( 1- ν ) s × 6 × r × t f )

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10 Problem 5 CMOS on Silicon-on-insulator (35 points total) Design a process flow to produce the following poly-Si gate CMOS on SOI (silicon on insulator) structure. The starting material is 1 μ m-thick n-type single-crystalline silicon on SOI with N d = 2 × 10 15 / cm 3 . (a) (10 points) List major processing advantages for CMOS on SOI as compared with CMOS on bulk Si (b) (25 points) Describe your process flow in a left column and sketch and label the cross-sections of the structure after each lithography step in a right column. Process Description Cross-sections Starting Material: SOI substrate Buried Oxide (SiO2) Si substrate CVD oxide Gate oxide (thermal SiO2) CVD oxide CVD oxide
11 Problem 6 Nonplanar Device Processing (25 points total) You found a conceptual process flow of the DELTA (DEpleted Lean-Channel TrAnsistor) SOI MOSFET in a published paper. The key idea is to form a vertical Si pillar which is electrical isolated from the substrate by lateral thermal oxidation of the bottom part of the pillar. The top Si pillar is then used as the active MOSFET region. Fabrication of this non-planar device structure is expected to be challenging for several process modules. In the table below, explain concisely the difficulties you expect to encounter. Process Module Difficulties Optical Lithography Reactive Ion Etching Thermal Oxidation Source and Drain Doping Metal Contact to Source and Drain
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