Realizing Deep-Submicron Gap Spacing for CMOS-MEMS Resonators

Integrated complementary metal-oxide-semiconductor (CMOS)-microelectro mechanical systems (MEMS) beam-array resonators that take advantage of the pull-in effect to surmount limitations of standard CMOS foundry processes have been demonstrated to attain electrode-to-resonator gap spacing at a deep-submicrometer range. Such deep-submicrometer gaps lead to much larger electromechanical coupling coefficient and smaller motional impedance as compared with conventional CMOS-MEMS technologies. In addition, a unique frequency tuning capability by modulating the mechanical boundary conditions (“BC”) of the resonators has also been proposed. Furthermore, the mechanically coupled array approach used in this paper not only provides five times smaller motional impedance compared with that of a single resonator but offers superior power handling capability due to higher stiffness of the arrayed resonator. In such an array design, high velocity coupling for beam-array resonators effects a single resonance behavior without spurious modes. With the increase of an applied dc-bias which simultaneously serves for functions of pull-in and resonator operation, the upward frequency shift of resonance caused by boundary condition change offers opposite tuning mechanism against the well-known effect of electrical stiffness. As a result, frequency variation induced by the BC-modulation and electrical stiffness would yield a frequency-insensitive region under a certain dc-bias voltage. Furthermore, the use of metal/oxide composite materials for resonator structures in this paper substantially alleviates residual stress often seen in the CMOS layers as compared to mere-metal resonators.