Fabrication of keyhole-free ultra-deep high-aspect-ratio isolation trench and its applications

An ultra-deep (40–120 μm) keyhole-free electrical isolation trench with an aspect ratio of more than 20:1 has been fabricated. The process combines DRIE (deep reactive ion etch), LPCVD insulating materials refilling and TMAH or KOH backside etching technologies. Employing multi-step DRIE with optimized etching conditions and a sacrificial polysilicon layer, the keyholes in trenches are prevented; as a result the mechanical strength and reliability of isolation trenches are improved. Electrical tests show that such an isolation trench can electrically isolate the MEMS structures effectively from each other and from on-chip detection circuits. The average resistance in the range of 0–100 V is more than 1012 , and the breakdown voltage is above 205 V. This technology has been successfully employed in the fabrication of the monolithic integrated bulk micromachining MEMS gyroscope. (Some figures in this article are in colour only in the electronic version)