The Fracture Strength of Brittle Films for Micro-Electro-Mechanical Systems (MEMS) Devices

This work investigates the fracture behavior of brittle thin films used as structural materials for Micro-Electro-Mechanical Systems (MEMS) devices. Static fracture is a possible mode of failure for MEMS devices, and as such needs to be well characterized for designers to make reliable products using this technology. Challenges lie in characterizing brittle fracture in MEMS devices because well known bulk properties do not necessarily hold, and traditional material testing methods are often inadequate or difficultto perform on the microscopic scale.
The approach is to measure the strain at fracture using a test device made from the material under study. A suspended shuttle was designed and fabricated, which laterally deformed an array of cantilever beams in a controlled manner until each beam failed. Beams varied in length from 50 to 120 um and in width from 2 to 5 um, with film thicknesses ranging from 2 to 50 um. Non-linear beam theory and finite element analysis were used to ascertain the strain state in the beam at the time of failure.
Weibull statistical analysis was used to characterize the fracture behavior of the brittle materials studied. After demonstrating that the Weibull distribution successfully models the fracture behavior of the MEMS beams, two effects on the apparent strength of a material were anticipated. These effects are an apparent decrease in the average strain at fracture as the size of the specimen is increased and an apparent change in the average strain at fracture depending on the manner in which the load is applied.
The primary material of interest for this research was polycrystalline silicon, however other materials such as polycrystalline germanium and single-crystal silicon were also investigated. Polycrystalline silicon films made fiom different foundries were tested for comparison of the effects of processing conditions. The effects of surface oxidation, parylene coatings, and self-assembled monolayer coatings (SNs) on the strength of polycrystalline silicon were also investigated in an attempt to improve the strength of a given material.
Peter Truxtun Jones
George C. Johnson
Lisa A. Pruitt
Publication date: 
December 31, 1999
Publication type: 
Ph.D. Dissertation
Jones, P. T. (1999). The Fracture Strength of Brittle Films for Micro-electro-mechanical Systems (MEMS) Devices. United States: University of California, Berkeley.

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