C245 Homework 2

Due 2/11/05


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    1. Go to the LAVA web page (or try the backup) and under "Applications" run the "deposition and metalization" simulation.  You should see a yellow "substrate" with a 1 micron trench etched into it, and some straight lines above it representing line-of-sight deposition.
      1. Don't change any defaults, just press "Submit to Sample2D".  You should see that the trench is filled in, but that the corners didn't fill in well.  Using one of the red dots on the trench, or the sliders on the side, change the top aperture to 1.1 microns and run the simulation again.  Assuming that this is a metal deposition onto an insulator, will there be electrical connectivity between the metal on top and the metal in the hole?  Change the top aperture to 0.9 microns and re-run the simulation.  Will there be electrical continuity now?
      2. Reset the simulation so that the trench is 1 micron square again, and change the source type to "Hemispherical", and run the simulation.  Using the mouse, determine the deposition rate in the field, and in the center of the bottom of the trench.  Change the deposition time to 400s and graph the two deposition rates on the same plot.  Put the URL for this plot here:  What can you say qualitatively about the deposition rates in the field, at the bottom of the trench, and on the sidewalls?
      3. Reset the simulation to a 1 micron square trench, use hemispherical deposition with cosine flux, and set the positive and negative angles to +-90.  How long of a deposition is necessary before the top of the keyhole is less than 0.1 microns wide (use the mouse to measure)? What film thickness does this correspond to?
      4. Reset the simulation to a unidirectional source, and select a "line" profile.  Change the base of the line to a width of 0.75 microns, leaving the top at 1micron.  Run the simulation twice, once with a 200second deposition, and once with 100 seconds.  If the "line" were made of photoresist, and the film deposited were gold, which of these two thicknesses would likely work if the goal were to pattern the metal by liftoff?
      5. Reset the simulation to hemispherical source, uniform flux, and +-90 angles, a line profile, and 100s deposition.  What fraction of "blue sky" can the top and side of the beam see (in degrees):    What deposition rate would you predict for the top and sidewalls of the beam? What are the deposition rates on the top of the beam, on the side of the beam, and in the lower/inner corners of the beam?   Does the simulated deposition rate fit with what you predicted? Try the same simulation with longer depositions, and think about how reflow would affect an oxide shaped like this.
    2.  Figure 3.19 in Senturia is a clear conceptual illustration of a surface micromachining process, but it is somewhat lacking in practicality.  Lets assume that the structural layer is polysilicon, the sacrificial is LTO, and the substrate is bare SCS.  Assume that:
      • the LTO etch is an RIE plasma with 10:1 selectivity to silicon and silicon nitride
      • the polysilicon etch is an RIE plasma with 10:1 selectivity to silicon dioxide and silicon nitride
      • the polysilicon and oxide films are 2 microns thick each, and the poly is perfectly conformal
      Re-draw the third and fourth cross-sections of Figure 3.19 with these assumptions, using the following hints:
      • Calculate the vertical thickness of the polysilicon where it goes over the edge of the oxide? 
      • Based on that thickness, and the fact that you don't want to leave stringers around your oxide, how much overetch must be used in the polysilicon etch?  In microns:  In minutes (assume that you are using the chlorine and helium etch in the Lam Rainbow4420 etcher from Kirt Williams' paper): 
      • In this simplified process, don't forget that the polysilicon was deposited directly on single crystal silicon, which will etch just like the poly!
      • URL for cross-sections:
      Now assume that we modify the process by adding a uniform (unpatterned) 0.5 micron silicon nitride layer before any other processing.  Re-draw all of the cross-sections from Figure 3.19 with this nitride layer and the assumptions and your calculations above.  URL for cross-sections:
      • Comment on these cross sections.  How could you fix the layout to minimize these problems? 
    3. Calculate all of the possible angles of intersection between planes in the {110} family.  Do the same for members of the {111} family.
    4. 20# Xerox paper 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?
    5. Extra for Enthusiasts: Given a single sheet of 20# Xerox copier paper and a 30 cm length of half inch wide Scotch tape, design a structure to support as many pennies as possible at least 50 cm above the surface of a flat, level, low-friction table top.

      6. You may:
        1. use scissors, razor blades, etc. to cut the paper and tape.
        2.  apply tape to the table top
        3.  choose how, when, and where the pennies will be placed on your design
        Not encouraged:
        1. Rolling the paper (tough to do with polysilicon).
        2.  Cutting separate pieces and assembling them later.
        Turn in a printout of your solution with solid lines where cuts are to be made, short dashed lines where the paper is to be folded up, long dashed lines where the paper is to be folded down, and outlines of tape attachment points. Attachment points should be numbered in order of suggested assembly, and with ``abc...'' markings to indicate which points are to be connected.  Also turn in a URL pointing to a PDF version of your design  .
         
      1. Bring your solutions to class on the due date for a live competition after class. Your structure may be partly or completely assembled when you bring it, but you must be able to place it anywhere (without any additional supports).  In other words, you can't have it built and taped onto your own flat surface. Winning designs must carry the load for at least 10 seconds without collapse. Winners get a free lunch at the Northside Shops location of his/her choice.  You may work in teams of two.  Your partners name (if you have a partner):  .


or 

(Kris Pister)