EE245 Homework 2

Due 9/13/00


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  1. Here is a paper written by Kirt Williams and Richard Muller on etch rates for micromachining processes.
    1. What is the actual composition (by weight) of "concentrated HF"? 49%(by weight)HF+51%water "5:1 BHF"? Made by 5 parts of NH4F(40%) and 1 part of HF(49%), i.e. 33%NH4F+8.3%HF+58.7%water
    2. Is phosphoric acid a solid, liquid, or gas at room temperature? solid At what temperature do we typically etch with it? 160oC
    3. What two pieces of equipment were used to measure the etch rates? NanoSpec AFT interferometer and Alphastep 200 profiler 
  2. Find two MEMS-related journal and give their names here:
        JMEMS, Sensor and Actuator, and so on
  3. 20# Xerox paper (letter size) is about 100 microns thick. If you were to scale a sheet of Xerox paper down to 2 microns thick how big would it be?
        11inch*8.5inch = 279mm*215.9mm ======> 0.17inch*0.22inch = 5588um*4318um 
  4. Draw the cross-section of the following process flow, along the dash-line direction in the picture. Assume a bare {100} silicon starting substrate. If I release theis cantilever, how will it curl?
        1. LPCVD PSG, 2 micron
        2. spin negative photoresist, expose using mask1, develop
        3. timed HF etch with 50% overetch (how long will this take?)
             Check the etch table, if we use 5:1 BHF, annealed PSG's etching speed is 4400A/min=0.44um/min.
            We need to etch 2um's film and plus 50% overetch, then it gives us a time of
                    2um/(0.44um/min)*1.5 = 6.82mins = 410sec
         4. strip resist in acetone
        5. LPCVD polysilicon, 2 microns, anneal at 900C for 30 minutes
        6. spin positive resist, expose using mask 2, develop
        7. plasma poly etch, 50% overetch (what machine should I use? how long will this take?)
            Cl2 + He (180:400 sccm), Lam Rainbow 4420, etch rate 3200A/min, so the time is
                    2um/(0.32um/min)*1.5 = 9.375mins = 563secs

            Note: For both masks, the area outside of box is considered to be transparent.


    After release, it will curl up.
  5. Draw a cross-section of the same process as in question 2, but assume that positive resist is used for both lithography steps.