CMOS-MEMS Resonator Platform
We have developed several CMOS-MEMS resonator platforms co-fabricating mechanical resonators and their amplifier circuits for MEMS/IC integration.
The foundry-oriented CMOS-MEMS platform provides ease of use, low cost, fast prototyping, and circuit integrated features for vibrating RF-MEMS applications.
This platforms can be mainly categorized into two different release methods - oxide removal and metal removal maskless post-CMOS processes, implemented in both 0.35um 2-Poly-4-Metal (2P4M) and 0.18um 1-Poly-6-Metal (1P6M) CMOS technologies.
Advanced CMOS-MEMS Resonator
To address the issue of high motional impedance, we transfer this oxide removal process into a 0.18um CMOS where the cross-sectional and physical views are shown in figure.
The electrode-to-resonator gap spacing is reduced via the minimal feature size of the 0.18um CMOS (1.8X smaller than the 0.35um CMOS) while the transducer area can be increased by the 6-metal stacking (4-metal stacking in the 0.35um CMOS).
The combined merit of the gap and area contributes to much lower motional impedance as compared to the 0.35um CMOS-MEMS resonators.
CMOS-MEMS Oxide Resonator
Although the oxide removal process only requires one maskless wet etching step, the metal-rich feature resonators places a bottleneck on quality factor Q since the structural aluminum is often treated as a high-loss acoustic material.
A simple way to overcome this structural loss is to increase the constituent ratio of the SiO2 (i.e., low-loss material) inside the resonator, thus enabling high Q.
The oxide-rich CMOS-MEMS resonators feature 3-4X higher Q than that of the metal-rich versions
RF CMOS-MEMS Switch
A micromechanical actuator fabricated using a foundry-oriented CMOS-MEMS process has been demonstrated with low actuation voltage via the pull-in aided frame design for RF-MEMS switch applications.
With such a two-stage mechanical support design, the actuation voltage of the proposed actuator has been reduced by 35% as compared to that of the conventional one-stage design with the same offstate gap spacing.
The switch design methodology and foundry-type fabrication technology bring mechanical on/off switching capability into high-Q CMOS-MEMS circuits for future multimode, multi-band wireless communication systems.
Co-integrated CMOS-MEMS Microsystems
The micromechanical resonator and its sustaining amplifier can form a closed loop to ensue oscillation once Barkhausen criteria are satisfied.
In our group, a single-chip CMOS-MEMS oscillator has been successfully demonstrated in vacuum, exhibiting the output waveform and phase noise performance, which is comparable to the silicon-based oscillators.
Live Demo: CMOS-MEMS ocsillator operating in air !
SOI Thermal-Piezoresistive Resonators
We proposed a methodology to enhance thermal stability of MEMS resonators by the use of a constant-structural-resistance control where temperature coefficient of resistivity (TCR) of the MEMS resonator serves as an instrinsic temperature sensor, thus leading to a constant structural temperature to greatly alleviate the frequency drifts due to change of ambient temperature.
The SOI-based thermal-piezoresistive MEMS resonators feature simple fabrication process, high Q in air and liguid,
and high power efficiency when miniaturization for sensor applications.
