A Fast-Switching (1.35- $\mu \text{s}$ ) Low-Control-Voltage (2.5-V) MEMS T/R Switch Monolithically Integrated With a Capacitive Micromachined Ultrasonic Transducer

This paper describes the design and fabrication of an electrostatic microelectromechanical systems (MEMS) switch that can be co-fabricated on the same substrate with a capacitive micromachined ultrasonic transducer (CMUT) as a transmit/receive switch. The structure of the switch is modified from a single CMUT cell. An interrupted transmission line is defined across the center of the cell with control electrodes on both sides to pull a movable plate down. The plate has an insulation layer underneath, and a metal bump is formed on the insulation layer and aligned to the transmission line gap, so that the switch could be turned ON by pulling down the plate with electrostatic force and making the metal bump close the gap in the transmission line. The switch was designed using a finite-element model and fabricated on a glass substrate using anodic bonding. A static characterization was first performed on a switch test structure, which showed that the dc switching voltage was 68 V and the ON-resistance was 50 Ω. The RFin-to-RFout isolation was measured as approximately 66 dB and insertion loss was approximately 4.85 dB for the frequency range commonly used for medical ultrasound imaging. Then, we performed the dynamic characterization in immersion. By setting the dc bias at 67 V, we found that the switch could be operated with a control-voltage as low as 2.5 V. The switching and release times are related to the rise time and fall time of the control signal, respectively. The minimum switching time was measured as 1.34 μs with a control signal rise time of 300 ns, and the minimum release time was measured as 80 ns with a control signal fall time of 20 ns. We further demonstrated that a 1-kHz control signal with the optimized rise and fall times can be used to conduct and block a sinusoidal signal with 1-MHz frequency and 300-mVpp amplitude, as well as unipolar pulses with 5-Vpp amplitude, 500-ns pulse width, and 2-kHz repetition rate. The presented MEMS switch could potentially eliminate the high-voltage process requirement for the on-chip front-end electronics of a CMUT-based ultrasound imaging system and thus improve the overall system efficiency.