CMOS-MEMS Variable Capacitors for Reconfigurable RF Circuits

Microelectromechanical systems (MEMS) variable capacitors have demonstrated excellent performance in terms of tuning ratio and quality factor ( Q) at radio frequencies (RF). These characteristics can enhance existing RF circuits and enable frequency-reconfigurable circuits. However, MEMS are typically not monolithically integrated with complementary metal-oxide-semiconductor (CMOS) processes. To achieve integration with CMOS, this work uses maskless post-CMOS processing to create two types of CMOS-MEMS variable capacitors, the gap-tuning capacitor and the digital capacitor. To show that these capacitors enable frequency-reconfigurable circuits, the digital capacitor was used to develop the first CMOS-MEMS phase shifter.;The CMOS-MEMS gap-tuning capacitor uses lateral electrothermal actuation to control the gap between electrodes. A mechanical latch maintains the capacitance without consuming static power. This capacitor achieved series and shunt tuning ratios up to 6.9:1 and 5.7:1, respectively, with a Q of 80 at 1 GHz. Millions of actuation cycles were demonstrated without failure. However, the gap-tuning capacitor had several disadvantages, specifically large device variation, a non-linear distribution of capacitance values, and low capacitance density, which inspired development of the digital capacitor.;The CMOS-MEMS digital capacitor consists of many switched capacitors that are organized into capacitive bits to produce a linear distribution of capacitance values. This capacitor uses both vertical electrothermal actuation and lateral electrostatic actuation to achieve large tuning ratios without sacrificing capacitance density. The maximum capacitance state is maintained by electrostatic force to prevent static power consumption. The digital capacitor achieved series and shunt tuning ratios up to 73:1 and 19:1, respectively, with a Q of 369 at 1 GHz. Compared with that of the gap-tuning capacitor, the capacitance density was increased by 5x and the device variation was reduced by 4x. Reliability tests revealed evidence of mechanical fatigue after millions of actuation cycles, but no catastrophic failures occurred.;The CMOS-MEMS phase shifter consists of a micromachined transmission line periodically loaded with digital capacitors. The digital capacitors were modified to achieve a shunt tuning ratio of 7.2:1 with a Q of 18.8 at 30 GHz. The phase shifter produced an average insertion loss and maximum phase shift of 2.9 dB and 337.5°, respectively, at 32 GHz.