Fracture strength characterization for 25 micron and 125 micron thick SOI-MEMS structures

Abstract

MEMS structural devices are often fabricated by bulk-micromachining using a deep reactive ion-etching process (DRIE) for silicon. DRIE creates the high aspect ratio silicon features used as sensors and actuators in MEMS devices. This paper characterizes fracture strength distributions for DRIE etched MEMS test structures with device layer thicknesses of 25 μm and 125 μm using pull-table style strength test structures. The 25 μm thick measurements offered a direct comparison with previous investigations; whereas, the 125 μm thick measurements are unique to this study. The fracture strength distributions measured for 125 μm thick specimens ranged to significantly lower values and had a narrower distribution compared to previous and current 25 μm thick device layer fracture strength distributions. scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize and correlate surface roughness features with the observed fracture strength measurements. Characterization revealed that although both the 25 μm and 125 μm thick device layer structures predominantly exhibited scalloping sidewall morphology, the 125 μm thick structures transitioned to rougher curtaining sidewall morphology towards the bottom of the device layer. The difference in strength distributions is attributed to this increased roughness present in the curtaining region and on the bottom surfaces of thicker device layers.