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An ultra-deep (40-120um) electrical isolation trench without void has been fabricated. The process combines DRIE (deep reactive ion etch), LPCVD insulating material refilling, RIE (reactive ion etching), and TMAH or KOH backside etching technology. With the alternation of refilling and etching, the profile of the opening of the trenches has been modified; the keyholes in trenches are prevented; as a result the mechanical strength and reliability of isolation trenches are improved and the cost is saved. This technology has been successfully applied in the fabrication of the monolithic integrated bulk micromachining MEMS.