Electrical isolation of bulk silicon MEMS devices via thermomigration

Electrical isolation of bulk micromachined single crystal silicon MEMS devices is demonstrated using through-wafer junction isolation. Through-wafer npn junctions are fabricated using "temperature gradient zone melting" or "thermomigration" of aluminum in n-type silicon. The npn structures isolate various regions of the single crystal silicon from one another by acting as back-to-back diodes. Thermomigration is a potentially high-throughput process that is consistent with batch fabrication principles, avoids the necessity of a handle wafer, and retains the mechanical integrity of single crystal silicon. By use of this process, electrically isolated sensors and actuators can be fabricated from a single wafer of silicon. Breakdown voltages of multiple thermomigrated npn junctions in excess of 1500 V are demonstrated. The utility of this technique is shown by fabricating a comb-drive electrostatic actuator from a single silicon wafer and driving it at 162 Vpp.