Collective fabrication of BAW resonators on 4 ″ SC-cut quartz wafers

In this paper we report new results on collective fabrication of resonators, designed to work at about ten MHz, on 4” SC-cut quartz wafer. In this frame, their spherical curvature has been changed into discretized shape composed on two (or three) steps. For fabrication of the proposed design, two independent methods were investigated: - deep reactive ion dry etching (DRIE) and - wet chemical etching in two different etchants. For DRIE we applied inductively coupled plasma RIE (ICP RIE). This method allows quartz etching of 40 μm depth with an etch rate of about 0.7 μm/min. The homogeneity of the depth obtained over the entire 4” wafer is better than ± 5%. Moreover, the fabricated structures present very small roughness (less than a few nm) and nearly vertical walls. Wet chemical etching was done with two different etchants: NaOH and NH
₄
F-HF. The first one is used at about 180°C and allows a very good roughness when we start from polished surface (from 5 to 10 nm after 20 to 30 μm removed). The removal material rate (RMR) is ranged between 2.6 to 5.8 μm per hour, different on the 2 surfaces due to the anisotropy of the SC-cut. In contrary, the 2d etchant (acid) which allows a strong anisotropy etching, can be used to remove more material during one hour with a temperature close to 65°C (5.2 to 10 μm/h). But, unfortunately, the obtained roughness is not so good (45 nm instead of 11 nm with NaOH in the same time). Furthermore, after a strong etching, we have problem of adhesion of the mask and etch channels can appear. Generally, the mask is made by sputtered gold deposition. The homogeneity of the wet etching process is also discussed. We show that it is strongly linked to the homogeneity of the fluid flow in the bath.