Critical processing issues for micromachined sacrificial layer etching and sealing

Abstract

Integrating on-chip circuitry with micromachined devices poses many problems, from clearing of the sacrificial layer to sealing of the structural layer after sacrificial etch. In this paper, the repeatability of sacrificial etch of phosphosilicate glass (PSG) and the sealing of etch channels using different layers are investigated. The sacrificial etch rate changes with phosphorus weight percent concentration in the PSG. Thirteen typical structures in 10 different runs were monitored for etch front distance after 60 s in concentrated HF (49%). There was a maximum run to run coefficient of variance of 21%. Four different layers were investigated to determine the most effective for sealing. TEOS sealed less than 5% of 100 μm devices and 30% of 130 μm devices, LPCVD nitride sealed 25% of 100 μm devices and 55% of 130 μm devices. LPCVD polysilicon sealed all the devices. A reoxidation of the polysilicon membrane at atmospheric pressure was also investigated but the film seemed to cause the devices to lift and none appeared sealed. Scanning electron microscopy revealed the nitride and polysilicon layers to `fill in' the etch channel. The TEOS formed a `lip' over the channel and the reoxidation did not close off the etch channel gap. Characterising these stages in the process allows optimisation of the production of sealed cavity pressure sensors.