An investigation of the mechanical strengthening effect of hydrogen anneal for silicon torsion bar

This paper reports on the use of hydrogen anneal to enhance the torsional fracture strength of dry-etched single crystal silicon (SCS) microstructures. Moving-magnet-type scanning mirrors with torsion bars were employed as fracture test specimens. Two types of device were fabricated using SCS and silicon-on-insulator (SOI) wafers by deep reactive ion etching (DRIE). For the SCS-wafer-based device, scalloping on DRIE sidewalls were smoothed out and the fracture strength of the torsion bar was improved by a factor of three by 120 min hydrogen anneal. For the SOI-wafer-based device, hydrogen anneal introduced surface irregularity onto the Si sidewalls by hydrogen-induced etching with the existence of SiO₂. As a result, the fracture strength of the torsion bar was degraded contrarily. Therefore, hydrogen anneal is effective in improving the mechanical reliability of SCS microstructures without SiO₂.