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The Materials Science of “Permeable Polysilicon” Thin Films

Published online by Cambridge University Press:  15 March 2011

George M Dougherty ,
Timothy Sands  and
Albert P. Pisano
George M Dougherty
Affiliation:
Department of Materials Science and Engineering andDepartment of Mechanical Engineering University of California – Berkeley Berkeley, CA 94720
Timothy Sands
Affiliation:
Department of Materials Science and Engineering andDepartment of Mechanical Engineering University of California – Berkeley Berkeley, CA 94720
Albert P. Pisano
Affiliation:
Department of Mechanical Engineering University of California – Berkeley Berkeley, CA 94720
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Abstract

Polycrystalline silicon thin films that are permeable to fluids, known as permeable polysilicon, have been reported by several researchers. Such films have great potential for the fabrication of difficult to make MEMS structures, but their use has been hampered by poor process repeatability and a lack of physical understanding of the origin of film permeability and how to control it. We have completed a methodical study of the relationship between process, microstructure, and properties for permeable polysilicon thin films. As a result, we have determined that the film permeability is caused by the presence of nanoscale pores, ranging from 10-50 nm in size, that form spontaneously during LPCVD deposition within a narrow process window. The unusual microstructure within this process window corresponds to the transition between a semicrystalline growth regime, exhibiting tensile residual stress, and a columnar growth regime exhibiting compressive residual stress. A simple kinetic model is proposed to explain the unusual morphology within this transition regime. It is determined that measurements of the film residual stress can be used to tune the deposition parameters to repeatably produce permeable films for applications. The result is a convenient, single-step process that enables the elegant fabrication of many previously challenging structures.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 687: Symposium B – Materials Science of Microelectromechanical Systems (MEMS) Devices IV , 2001 , B7.3
Copyright
Copyright © Materials Research Society 2002

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