Resonant Body Transistors in IBM's 32 nm SOI CMOS Technology

This paper presents unreleased CMOS-integrated MEMS resonators fabricated at the transistor level of IBM's 32SOI technology and realized without the need for any post-processing or packaging. In this technology, resonant body transistors (RBTs) are driven capacitively and sensed piezoresistively using an n-channel field effect transistor (FET). Acoustic Bragg Reflectors (ABRs) are used to localize acoustic vibrations in the unreleased resonators completely buried under the CMOS metal stack and surrounded by low-κ dielectric. FET sensing is analytically compared with alternative active and passive sensing mechanisms to benchmark CMOS-MEMS resonator performance with frequency scaling. Experimental results from the first generation hybrid CMOS-MEMS RBTs show RBTs operating above 11 GHz with Qs of 24-30 and footprints of 5 x 3 μm. Comparative behavior of devices with design variations is used to demonstrate the effect of ABRs on spurious mode suppression. In addition, the performance of the RBTs is compared with passive electrostatic resonators, which show no discernible peak. Finally, temperature stability of <3 ppm/K due to complimentary materials in the CMOS stack is analytically and experimentally verified.