Dry etching method using double-layered etching mask for modulating shape of deep-etched quartz surface

This paper describes a fabrication method to control 3D microstructures having deep etched structures with precisely modulated surface shape for 3D quartz MEMS. The method to realize the microstructures 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) was used for deep etching of quartz and soft mask (photoresist) was used for modulating shape of the etched quartz surface. Because of the difference in the etch selectivity between hard mask (>;30) and soft mask (0.3-2), the limited range of 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.