A fabrication method of high-Q quartz crystal resonator using double-layered etching mask for DRIE

This paper reports a fabrication method of high-Q quartz-crystal resonator having deep etched structures with precisely modulated surface shape. The method to realize the structures bases on the use of multi-layered mask having different etch selectivity to quartz during deep reactive ion etching (DRIE) process. Here, hard mask (nickel) and soft mask (photoresist) was used for deep etching and modulating etched surface shape, respectively. Because of the large difference in the etch selectivity between quartz/hard-mask (>30) and quartz/soft-mask (0.3–2), the limited range size of modulating shapes in the vertical direction was improved. Inverted-mesa quartz resonators combined with a spherical shape are demonstrated using the proposed method showing the improvement of the device performance.