The etching processes used to produce microelectromechanical systems (MEMS) 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 component strength. 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 enables 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. Three sets of single-crystal silicon theta specimens are fabricated using two deep reactive ion etching recipes and a temperature-controlled cryogenic plasma etching recipe, each set resulting in a different specimen surface quality. Each sample is tested by instrumented indentation and finite element analysis is used to determine sample strength; this testing protocol allows many measurements to be performed in a time-effective manner, enabling statistically relevant numbers of strength measurements to be obtained. The resulting strength distributions are analyzed in two ways. First, the strength data are fit to a three-parameter Weibull distribution function to determine the lower bound, or threshold value, of each distribution. Second, the strength data are used in conjunction with various loading schemes to assess their effect on the lifetime spectrum of the device. In both approaches, the theta specimen is used to great effect to gain quantitative insight into the role of etching-induced surface features on the manufacturing yield and operational reliability of MEMS components.
Journal of Microelectromechanical Systems
Fracture, materials testing, microelectromechanical systems, reliability, silicon