A triple-wafer-bonded AlScN driven quasi-static MEMS mirror with high linearity and large tilt angles

In the previous work a quasi-static MEMS mirror with a novel design and powerful piezoelectric driving material of AlScN was shown, which possesses large mechanical tilt angles of up to ±12.5°, high frequency of about 1 kHz, high fill-factor (aperture diameter is 0.8 mm and die size is 1.3 x 1.1 mm²), great long-term stability and great linearity. Further developments have been done for improving the material properties of AlScN, moreover, to integrate the mirror plate onto actuators by BEOL bonding process instead of hybrid assembly, since for the high fill-factor and good mechanical linearity the mirror plate and actuators are on different planes. Additionally, a third wafer of TSV wafer is used for the vertical electrical contacts. This unique technology includes not only triple-wafers-bonding, after which the wafer stacks also have to withstand grinding and pattering via DRIE. This paper shows the process efforts for realizing the triple-wafer-stack and discusses the technological challenges and also achieved results.