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4.2 3DMEMS Process Refinement
The long 525 mm DRIE step has been identified as an possible weakness in the 3DMEMS process. The thermal resistance of the wafer stack is very high due to the bonded wafer stack thickness, the two buried oxide layers, and the numerous cavities. This large thermal resistance may prevent the chuck from adequately cooling the top wafer even when a good bond is achieved. As discussed, neither wafer set survived DRIE during the first fabrication run. Although the bonding failure is probably due to the sealed cavity issue described in Chapter 2, the long DRIE step could also lead to overheating.
One possible process revision is the reduction of the cap thickness by chemical-mechanical polishing from 525 mm to about 20 mm before the final DRIE step. Besides reducing DRIE length and possible overheating, this revision would truncate the disproportionately tall caps to a more modest height. Because alignment marks would be removed during polishing, this revision would prevent the cap layer from being used as an alignment layer. Nevertheless, the wafers could still be aligned by using the substrate of the 50 mm SOI as an alignment layer.
Methods should also be considered for lifting the micromanipulator platform off the substrate after fabrication is completed. Individual manipulation by means of a probe station, while acceptable for testing, is unacceptable for true batch fabrication. Electrostatic attraction is one option for lifting many platforms simultaneously.