ULTRA-SMALL SCALE MECHANICAL PROPERTIES MEASUREMENT
Gaither, Michael Scott
Lloyd, Isabel K
Cook, Robert F
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In order for the microelectromechanical systems (MEMS) industry to continue to grow and advance, it is critical that methods are developed to determine the mechanical reliability of MEMS devices. This is particularly so for advanced devices with contacting, moving components, for which component strength is a key factor in determining reliability. The etching processes used to produce MEMS devices leave residual surface features that typically limit device strength and, consequently, device lifetime and reliability. In order to optimize MEMS device reliability, it is therefore necessary to understand and characterize the effects these etching processes have on MEMS-scale device strengths. At the micro and nano scales, however, conventional strength testing methods cannot be used, and a standardized test method for MEMS-scale strength measurement has yet to be established. The micro-scale theta specimen, shaped like the Greek-letter theta, acts as a tensile test specimen when loaded in compression by generating a uniform tensile stress in the central web of the specimen. Utilizing the theta specimen for strength measurements allows for simple micro-scale strength testing and assessment of etching effects, while removing the difficulties associated with gripping and loading specimens as well as minimizing potential misalignment effects. Micro-scale silicon theta samples were fabricated using techniques relevant to MEMS processing. Processing-structure relationships were determined with microscopy techniques measuring sample dimensional variations, etch quality, and surface roughness. Structure-properties relationships were determined using three techniques. Samples were tested by instrumented indentation testing (IIT) and finite element analysis determined sample strength. Sample set strength data were examined via Weibull statistics. Fractographic analysis determined initial fracture locations and fracture propagation behavior. Key scientific findings included: (1) directly relating the processing-induced etching quality of fabricated samples to sample strength, and (2) critical flaw size calculations from sample strength measurements that were consistent with sample surface roughness. Technical contributions included development of the micro-scale theta specimen fabrication methodology, super-resolution dimensional measurements, and extension of IIT to strength measurements. The micro-scale theta specimen and corresponding testing methodology have enabled successful determination of processing-structure-mechanical properties relationships for three processing approaches. This is vital to the determination of properties-performance relationships in MEMS devices.