University of California s18

EE 147/247A Prof. Pister Fall 2016 Homework Assignment #12v2 Due by online submission Wednesday 12/7/2016 (Thursday 9am)

1. Take a look at (BSAC grad) Kirt Williams’ etch table [1] A. What wet etch would you use for silicon nitride? How long would it take to etch through a 0.2um film? What could you use as a mask? B. How many different answers are there to the question “What is the etch rate of silicon dioxide in HF”? I don’t want to know the etch rates, I want you to see how many different HF etches you can find, and how many different silicon dioxides. C. Estimate the selectivity of the DRIE etch between X and (100) silicon, for X={aluminum, photoresist, silicon nitride, silicon dioxide}. Explain your reasoning. (Note that there are several different variations of both the etch conditions and the materials, which is one reason why it is difficult to say definitively that “the selectivity of X to Y in DRIE is Z”. Which exact etch? What kind of silicon dioxide? How was it annealed? …) 2. 13Final problem 5 3. 13Final problem 8a 4. 13Final problem 12 5. 13Final problem 13 6. Your goal is to make a sub-lithography-limit comb drive. Assume that you have a lithography setup which allows you to make 1um line and space reliably, but nothing smaller. Assume that alignment between lithography steps is better than 0.25um, that LPCVD films are perfectly conformal, and that RIE etches are perfectly vertically anisotropic with infinite selectivity. A. You start with a 2um undoped poly film on a 1um sacrificial oxide, and pattern 1um line and space fingers (the “mandrel”, or mold for the sidewalls) in the poly. You then deposit 0.1um HTO, and 0.2um undoped poly, followed by an unmasked 0.2um poly RIE. A.i. Draw a cross-section showing two fingers (four sidewalls). A.ii. If you could electrically contact the individual sidewalls, how much more force per gap, and force per unit layout area, would this sidewall structure give you than the original lithography-limited fingers? B. Design a process so that the sidewalls are doped and released (no oxide or undoped poly fingers) and can be separately electrically contacted. You don’t need to show how to make electrical or mechanical contact to them (although you might think about it) B.i. Show process cross sections at key steps (at least two) B.ii. Show the layout, indicating how a second mask is used to expose and etch the undoped poly mandrel, and third mask to electrically isolate sidewalls from one another. (your mask order may be different) B.iii. Note that the finger spacing (size of the gaps) in part A is not equal. If you could tune your lithography to overexpose the polysilicon and narrow the undoped polysilicon mandrel lines (while maintaining 2um center-to-center spacing), can you make the ultimate comb spacing equal on both sides?

[1] K. Williams, K. Gupta, M. Wasilik, “Etch rates for micromachining processing – Part II”, JMEMS V12N6, 2003.