Influence of surface properties on the fracture of MEMS beams

The effect of surfaces on the fracture strength statistics of Si MEMS beams is investigated through experiments and simulations. 180 nm thick, doubly clamped beams were fabricated from single crystal silicon (111) using photolithography and wet etching. The surfaces of the beams were varied by using two different aqueous base etchants, KOH and TMAH; the latter resulted in much smoother surfaces with smaller mean step heights than the former. The RMS roughness values were 2.3 nm for KOH and 0.8 nm for the TMAH etch. The experiment consisted of loading the samples with a carefully calibrated AFM cantilever until fracture. The Weibull strengths were estimated to be 12.8 GPa for beams with rough surfaces and 15.7 GPa for beams with smooth surfaces. The dependence of fracture strength on surface was analyzed using a Monte-Carlo simulation. Typical KOH and TMAH surface profiles were measured and used to generate simulated surfaces. A corner stress singularity approach was used to calculate strength statistics that are compared to experimental results.