Monolithic Integration of ScAlN MEMS Filter on RFSOI Using ALD Al2O3 as VHF Barrier

We present a novel low-temperature process approach that enables the monolithic integration of scandium-doped aluminum nitride (ScAlN)-based radio frequency (RF) micro electromechanical system (MEMS) filters with RF switches. A planarized SiO
2
-based ankle and cavities for the MEMS device were established through a three-step process: 1) swimming pool pattern and etch, 2) passivation of the side wall and bottom of the swimming pool with atomic layer deposited Al
2
O
3
, and 3) swimming pool refill with SiO
2
and planarization. ScAlN film based bulk acoustic resonators (FBAR) filters were then fabricated on the planarized surface, and eventually undercut was formed using vapor hydrofluoric acid (VHF) etch. The highest process temperature is 350 °C in the whole integration flow. The approach enabled successful integration of a ScAlN-based MEMS filter on top of the radio-frequency integrated circuit switch wafer. The integrated FBAR shows a center operation frequency of 2.58 GHz, bandwidth of 117 MHz, and an insertion loss of 3.7 dB. The integrated filter with switch has a footprint of 1mm
²
.