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LPCVD Silicon Dioxide Sacrificial Layer Etching for Surface Micromachining

Published online by Cambridge University Press:  15 February 2011

David J. Monk ,
David S. Soane  and
Roger T. Howe
David J. Monk
Affiliation:
Department of Chemical Engineering, University of California, Berkeley, California 94720. (510) 642-0958, FAX (510) 642-4778
David S. Soane
Affiliation:
Department of Chemical Engineering, University of California, Berkeley, California 94720. (510) 642-0958, FAX (510) 642-4778
Roger T. Howe
Affiliation:
Berkeley Sensor & Actuator Center, Department of Electrical Engineering & Computer Science, University of California, Berkeley, California 94720
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Abstract

The key step in producing a movable microstructure is the controllable sacrificial layer etching process. For the current study, a 1 μm silicon-rich silicon nitride structural layer was deposited on a 2 μm phosphosilicate glass (PSG) sacrificial layer that had been deposited on a single-crystal silicon wafer. Silicon nitride, unlike the more common polycrystalline silicon (poly-Si) structural material, is transparent and, therefore, allows the observation of an etch front in the underlying PSG sacrificial layer. The PSG is an amorphous silicon dioxide, with phosphorus added as a controlled impurity, that can be selectively etched in hydrofluoric acid. Several experiments have been performed with four HF concentrations using these test structures. Diffusion limited etching is observed at long etching times for each concentration. Additional experimental work has been done to determine appropriate kinetic expressions for the PSG/HF reactions.

The sacrificial layer etching process has been modeled as a chemical reaction/diffusion system. Constants for the kinetic expression have been found from independent experimental work. This Deal-Grove type model assumes isothermal conditions, constant density solution, onedimensional etching, and the equality of the diffusion and reaction fluxes. The model calculates the HF surface concentration and the length of PSG underetched as a function of time. It fits the data well but requires the use of an unphysical diffusion coefficient. An extension of the Deal-Grove model to nonfirst order kinetics allows for a reasonable estimate of the reactant/product mixture diffusivity in water to be used when fitting the model to experimental data.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 276: Symposia U/Y – Smart Materials Fabrication and Materials for Micro-Electro-Mechanical Systems , 1992 , 303
Copyright
Copyright © Materials Research Society 1992

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