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Silicon-Based Epitaxial Films for Mems

Published online by Cambridge University Press:  10 February 2011

E.A. Fitzgerald ,
K.C. Wu ,
M. Currie ,
N. Gerrish ,
D. Bruce  and
J. Borenstein
E.A. Fitzgerald
Affiliation:
Dept. of Materials Science and Engineering, MIT, Cambridge, MA 02139, eafitz@mit.edu
K.C. Wu
Affiliation:
Dept. of Materials Science and Engineering, MIT, Cambridge, MA 02139, eafitz@mit.edu
M. Currie
Affiliation:
Dept. of Materials Science and Engineering, MIT, Cambridge, MA 02139, eafitz@mit.edu
N. Gerrish
Affiliation:
Dept. of Materials Science and Engineering, MIT, Cambridge, MA 02139, eafitz@mit.edu
D. Bruce
Affiliation:
Dept. of Materials Science and Engineering, MIT, Cambridge, MA 02139, eafitz@mit.edu
J. Borenstein
Affiliation:
Charles Stark Draper Laboratories, Cambridge, MA 02139
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Abstract

The objective of this work was to develop an improved wet etch-stop technology for silicon micromachining. To establish a reference for process improvement, the diffusion process currently used to fabricate p++ Si:B etch stops was comprehensively investigated. Subsequently, a novel germanium- based epitaxial etch-stop technology was developed.

A range of techniques were used to study p++ silicon layers created by boron diffusion into (001) silicon wafers. The results revealed gradients in boron and lattice constant, as well as a graded three- dimensional dislocation array from lattice-mismatch stress. The gradients in boron concentration and dislocation density can lead to curl in micromachined structures. Although annealing steps can remove the boron gradient, a flat membrane will be a tenuous balance.

Epitaxial films of p++Si:B and strain-compensated p++Si1-xGex:B can remove composition gradients and improve process control. However, it is undesirable to depend on p-type layers doped at levels near the solubility limit to prevent etching. We have therefore developed a unique etch stop created from relaxed SiGe alloys. Etch-stop behavior quite similar to heavily boron-doped silicon has been demonstrated in undoped silicon-germanium. Neither strain nor defects are responsible for the etch-stop behavior. A model is proposed based on energy band structure and a SiOx passivation mechanism.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 518: Symposium N – Microelectromechanical Structures for Materials Research , 1998 , 233
Copyright
Copyright © Materials Research Society 1998

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References

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