Homework #2

Assigned: Jan. 20, 2004
Due: Jan. 27, 2004
Points: 30

1. Points: 10
   Consider the two-level surface micromachining process consisting of the following steps:
   • Silicon dioxide layer 1 over the single crystal silicon substrate
   • Patterning of the oxide layer
   • Conformal deposit of polysilicon 1 layer
   • Patterning of poly 1 layer
   • Conformal deposit of oxide 2 layer
   • Patterning of oxide 2 layer
   • Conformal deposit of poly 2 layer
   • Patterning of poly 2 layer
   • Sacrifice of oxide layers 1 and 2
   You have seen how a "substrate-hinge" with the axis of rotation vertical to the plane of the wafer can be made with this process. Now, create a "floating-hinge: with axis parallel to that of the substrate hinge. Please submit mask drawings as well as some 3-D drawings of the created hinge to convey your idea clearly. Card-board or origami models are also fine in lieu of 3-D drawings/sketches. Remember that the hinge you create should not fall off when you flip it up side down.

2. Points: 10
   A (100) silicon wafer of thickness 300 um has a rectangular mask opening of 50 um x 200 um. The 200 um long edges are aligned with [110] direction. This wafer is subjected to DRIE so that a vertical trench of 100 um depth is first formed. Assume that the mask is intact during the DRIE process. The wafer is later wet-etched in KOH solution with the same mask in place for 30 minutes. Assume that the etch rate in [100] direction is 1.1 um/min and 1.6 um/min in the [110] direction. If you need etch rates on other planes, look into the literature. Sketch the cross-section of the resulting pit as seen in the plane formed by [110] and [100] vectors and passing through the middle of the rectangular opening. Mark all the exposed planes in this cross-section view.

3. Points: 10
   The cover of our textbook shows an image of the piezoresistive pressure sensor made at MIT using a "sealed-cavity" process developed there. Search the literature (IEEE Xplore is a good place to do it online. Try ieeexplore.ieee.org) to know more about this device and how it is made. Write a detailed verbal description of this process and sketch the step-by-step process flow.
   An extra credit of 5 points if you also draw the masks. The dimensions need not be exact and the mask drawings need not be to scale.