STIFFNESS TRIMMING OF HIGH Q MEMS RESONATORS BY EXCIMER LASER ANNEALING OF GERMANIUM THIN FILM ON SILICON

This paper reports on a new method to optically trim the resonance frequency of a low-loss micromechanical silicon (Si) resonator coated with a thin-film germanium (Ge) layer. A focused ultra-violet (UV) laser beam (349 nm wavelength) locally heats Ge to high temperatures, enabling selective micro-crystallization of SiGe. The stiffness of the optically crystallized SiGe regions decreases with increasing concentration of Ge. These stiffness variations lead to an overall downward shift of the resonance frequency. Large trimming range (similar to 5000ppm) is demonstrated for low-frequency out-of plane resonant modes. In addition, fine frequency trimming of in-plane Lame mode resonators is repeatedly achieved (similar to 4 ppm per scan) without introducing any damping throughout the trimming process (Q similar to 300,000), enabling fine frequency control of high-Q micro resonators. The resonance frequency of a trimmed germanium-coated silicon resonator remains stable after being heated to 450 degrees C for 30 minutes in a Rapid Thermal Annealing (RTA) chamber.