Thermomigration-based junction isolation of bulk silicon MEMS devices

Electrical isolation of bulk-micromachined single crystal silicon (SCS) microelectromechanical systems (MEMS) devices is demonstrated using through-wafer junction isolation. Through-wafer junctions of alternating n-type and p-type silicon (npn junctions) are fabricated using 'temperature gradient zone melting' (TGZM) or thermomigration of aluminum in n-type silicon. The npn regions electrically isolate various regions of the SCS from one another by acting as series connected backbiased diodes. Thermomigration is a potentially high throughput process that is consistent with batch fabrication principles. Practical demonstration of this technique is shown by fabricating and testing a full wafer thickness (~300 [micro]m) electrostatic actuator fabricated from a single silicon wafer, and driven at 360 Vpp at resonance of 1933 Hz. Breakdown voltages of single thermomigrated npn junctions in excess of 189 V with leakage currents less than 70 nA were measured. For multiple junctions in series, overall breakdown voltages greater than 1500 V were demonstrated. Compatibility of thermomigration with standard p-channel metal-oxide-semiconductor (PMOS) transistors is also demonstrated. [1345] Index Terms--Bulk micromachining, deep reactive ion etching (DRIE), diodic isolation, electrical isolation, inductively coupled plasma (ICP), junction isolation, temperature gradient zone melting (TGZM), thermomigration.