Fracture strength of silicon torsional mirror resonators fully coated with submicrometer-thick PECVD DLC film

Abstract

In this work, we attempted to enhance the torsional fracture strength of single crystal silicon (SCS) resonators for micro mirrors application by introducing a 300 nm-thick diamond-like carbon (DLC) coating. The SCS torsional beams of the resonators were 20 μm long, 9 μm wide and 9 μm thick, and fully coated with DLC films using plasma enhanced chemical vapor deposition (PECVD) at three different deposition bias voltages. The resonators were driven by a piezoelectric actuator and their angular amplitude was measured by a custom-made torsional test system. Average nominal torsional fracture strength of DLC coated resonators was 11.1–30.0% higher than that of bare SCS, reaching a value of 2.93 GPa. The torsional fracture strength of resonators exhibited a good agreement with the tensile fracture strength. Deviations in torsional strength were reduced with increasing deposition bias voltage due to the compressive residual stress of DLC films.