RF sputtering: A viable tool for MEMS fabrication

Abstract

Fabrication of Micro-Electro-Mechanical-Systems (MEMS) requires deposition of films such as SiO₂, Si₃N₄, ZnO, polysilicon, phosphosilicate glass (PSG), Al, Cr-Au, Pt, etc. for use as structural, sacrificial, piezoelectric and conducting material. Deposition of these materials at low temperature is desirable for fabricating sensors/actuators on temperature-sensitive substrates and also for integrating MEMS structures on silicon in post-CMOS processing procedures. Plasma enhanced chemical vapour deposition (PECVD) and sputtering are amongst potential techniques for preparing films for MEMS fabrication at comparatively low temperatures. The sputtering technique has an added advantage that the process is carried out in an inert ambient (argon) and chemically sensitive substrate/sacrificial layers can be used in realization of MEMS. Furthermore, the same system can be used for depositing dielectric, piezoelectric and conducting materials as per requirement in the fabrication sequence. This enables rapid low-cost prototyping of MEMS with minimum fabrication facilities.

In the present work, we report preparation, characterization and application of RF sputtered SiO₂, Si₃N₄ and ZnO films for MEMS fabrication. The effect of RF power, sputtering pressure and target-to-substrate spacing was investigated on the structural and other properties of the films. The residual stress in the films was obtained usingwafer curvature measurement technique. The deposition parameters are optimized to obtain low stress films of SiO₂ and Si₃N₄. The self-heating of the substrate during deposition was advantageously exploited to obtain highly c-axis oriented films of ZnO without any external heating. A variety of MEMS structures such as cantilever beams, micro-bridges, diaphragms, etc. are demonstrated using bulk, surface and surface-bulk micromachining techniques.