Critical processing issues for micromachined sacrificial layer etching andsealing

Integrating on-chip circuitry with micromachined devices poses many problem s, from clearing of the sacrificial layer to sealing of the structural laye r after sacrificial etch. In this paper, the repeatability of sacrificial e tch of phosphosilicate glass (PSG) and the sealing of etch channels using d ifferent layers are investigated. The sacrificial etch rate changes with ph osphorus weight percent concentration in the PSG. Thirteen typical structur es in 10 different runs were monitored for etch front distance after 60 s i n concentrated HF (49%). There was a maximum run to run coefficient of vari ance of 21%. Four different layers were investigated to determine the most effective for sealing. TEOS sealed less than 5% of 100 mu m devices and 30% of 130 mu m devices, LPCVD nitride sealed 25% of 100 mu m devices and 55% of 130 mu m devices. LPCVD polysilicon sealed all the devices. A reoxidatio n 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 t o '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 thes e stages in the process allows optimisation of the production of sealed cav ity pressure sensors. (C) 1999 Elsevier Science S.A. All rights reserved.