Micromechanical fracture strength of silicon

In order to test the statistical influence of some process and micromachining parameters on the fracture strength of silicon microelements, arrays of identical microsized cantilever beams were bulk micromachined in single-crystalline silicon wafers. The beams were exposed to various surface treatments (diamond polishing with different grades, oxidization, stripping of oxide) in different combinations. The influence on fracture strength was investigated by bending the beams to fracture in a micromanipulator mounted in situ in a scanning electron microscope while registering force-versus-deflection curves. Average fracture strengths, standard deviations, Weibull moduli, crack-initiating flaw sizes, and in some cases elastic moduli were evaluated. Diamond polishing was found to decrease the fracture strength drastically, but polishing followed by oxidization not only restored the original strength, but actually increased it, due to crack healing. Polishing, oxidization, and subsequent stripping of oxide resulted in fracture strengths slightly higher than the original strength. The Weibull modulus was diminished from 10 to 6–9 by the polishing. The initiating flaw sizes were theoretically evaluated, and found to agree with previous results of cross-sectional transmission electron microscopy studies of polished silicon surfaces. The elastic moduli determined were significantly lower (30%–40%) than the corresponding module of pure, single-crystalline silicon, probably due to high dopant contents in the specimens investigated here.