Post-CMOS MEMS capacitive pressure sensor: Porous ALD membrane for sacrificial layer release and diaphragm sealing
In this work a nanoporous ALD composite layer is used as an intermediate membrane in order to support the releasing and sealing of diaphragms for the fabrication of absolute capacitive pressure sensor MEMS on top of a CMOS substrate. The nanoporous properties are achieved due to the principle of island growth which occurs at small numbers of ALD cycles. By alternately applying less than 10 ALD cycles for Al2O3 and ZnO, a homogeneous composite is achieved instead of a stack of Al2O3 and ZnO nanolaminates. Exposing the ALD composite layer to vapour phase HF/H2O (vHF/H2O) mixture, the ZnO is selectively texture etched and an etch-resistant membrane remained, while the sacrificial layer is etched simultaneously. Further, the remaining ALD composite layer successfully prevents deposition of the sealing layer inside the cavity. Thus the principle suitability of a porous ALD composite layer for the sacrificial layer release and sealing of diaphragms is successfully demonstrated.