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The mechanical strength of polysilicon films : Part 1. The influence of fabrication governed surface conditions

Chasiotis, Ioannis and Knauss, Wolfgang G. (2003) The mechanical strength of polysilicon films : Part 1. The influence of fabrication governed surface conditions. Journal of the Mechanics and Physics of Solids, 51 (8). pp. 1533-1550. ISSN 0022-5096.

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In an effort to explain the considerable variations in measured mechanical strength of polysilicon films doped with phosphorous for use in MEMS applications, the influence of the specimen manufacturing processes on the mechanical properties has been examined in connection with varying exposure to 49% hydrofluoric acid (HF). It was found that surface roughness as characterized by groove formation along grain boundaries depends on the HF release time. Surface undulations and crevasses related to grain structure result thus in reduced fracture strength and, in addition, induce errors into the determination of the effective elastic modulus - especially when the latter is determined from flexure configurations. Extensive exposure to HF results in pervasive material degradation, as evidenced by a transition from transgranular to intergranular fracture, and a correspondingly precipitous drop of the film strength with attendant increase in grain boundary material removal. Short times of exposure to HF can result in delamination of a thin surface layer, which is suffcient to initiate an "early" failure. Longer exposure allows HF permeation into the intergranular domains, degrading the body of the material significantly. On the other hand, tests on material from a different source that has undergone different doping and post-processing demonstrated a suppression of this degradation resulting in film strengths that are higher by a factor of two or more. Thus, consideration of additional influences of doping and electro-chemical phenomena during the HF wet release, in association with silicon-metal contacts, is necessary.

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Additional Information:Received 8 February 2002; accepted 7 March 2003. The authors gratefully acknowledge the financial support by the Air Force Office of Scientific Research (AFOSR) through grant F49629-97-1-0324 (Round Robin Program) and under grant F49620-99-1-0091, which included funds from the National Science Foundation (NSF). Over the duration of the programs Major Brian Sanders, Drs. O. Ochoa, D. Segalman, and T. Hahn were the monitors. We would also like to thank Dr. D. LaVan and the Sandia National Laboratories for repeated discussions and for kindly providing the tensile specimens.
Funding AgencyGrant Number
Air Force Office of Scientific Research (AFOSR)F49629-97-1-0324 (Round Robin Program)
Air Force Office of Scientific Research (AFOSR)F49620-99-1-0091
Subject Keywords:Mechanical strength; Polycrystalline silicon; Hydrofluoric acid; Transgranular fracture; Intergranular fracture; Phosphorous doping; MEMS
Record Number:CaltechAUTHORS:CHAjmps03a
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:22
Deposited By: Wolfgang Gustav Knauss
Deposited On:10 Aug 2004
Last Modified:14 Sep 2016 00:05

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