This paper is a review of the remarkable progress that has been made during the past few decades in design, modeling, and fabrication of micromachined resonators. Although micro-resonators have come a long way since their early days of development, they are yet to fulfill the rightful vision of their pervasive use across a wide variety of applications. This is partially due to the complexities associated with the physics that limit their performance, the intricacies involved in the processes that are used in their manufacturing, and the trade-offs in using different transduction mechanisms for their implementation. This work is intended to offer a brief introduction to all such details with references to the most influential contributions in the field for those interested in a deeper understanding of the material.

A highly integrated 1.7-2.0 GHz digitally programmable frequency synthesizer using a MEMS resonator as its reference is presented. Due to the dimensions of the MEMS device (e.g., 25 mum by 114 mum), the entire system with a total area of 6.25 mm² can be housed in a small standard chip package. This considerably reduces the form factor and cost of the system, compared to using an external crystal as a reference. The MEMS resonators are clamped-clamped beams fabricated using a CMOS-compatible process. The main structural layer is made of silicon carbide, which provides the resonators with higher power handling capabilities and higher operating frequencies, compared to silicon. The resonators are electrostatically and thermally tunable - an 8.4% frequency tuning is demonstrated for a 9 MHz resonator. The 100 nm vertical transducer gaps of the resonators allow the use of electrostatic actuation voltages as low as 2 V. An integrated high gain-bandwidth trans-impedance amplifier (TIA) is combined with a resonator to generate the synthesizer's input reference signal. The TIA employs automatic gain control to mitigate the inherent low power handling capabilities and the non-linearities of the MEMS device, thus minimizing their effect on phase noise. The fractional- N synthesizer employs a 3rd-order 20-bit delta-sigma modulator to deliver a theoretical output resolution of ~ 11 Hz, in order to allow for high output frequency stability when used with an appropriate feedback loop. A fully integrated on-chip dual path loop filter is used to maintain a high level of system integration. With a supply voltage of 2 V, the phase noise for a 1.8 GHz output frequency and a ~12 MHz reference signal is -122 dBc/Hz at a 600 kHz offset, and -137 dBc/Hz at a 3 MHz offset.

A humidity sensor using suspended carbon nanotubes (CNTs) was fabricated using a low-temperature surface micromachining process. The CNTs were functionalized with carboxylic acid groups that facilitated the interaction of water vapor with the CNTs. The humidity sensor showed a response time of 12 s and a recovery time of 47 s, along with superior hysteresis and stable performance. The hysteresis curve area of the suspended structure is 3.6, a 3.2-fold reduction in comparison to the non-suspended structure. A comparative study between suspended and non-suspended devices highlights the advantages of using a suspended architecture.

A fully integrated current sensing interface chip employing a capacitive-feedback transimpedance amplifier (TIA) is presented. A robust dc current removal block is proposed to prevent the dc portion of the input current from saturating the output voltage. This block allows the TIA to operate in the presence of a wide range of input dc currents, and the cancellation loop is designed to enhance its stability. The TIA is fully integrated in a standard 0.13 μm CMOS technology, and a gain of 170 dBΩ is achieved without requiring any off-chip resistors. The integrated input-referred current noise of the interface circuit is 0.4, 3.8, and 52 pARMS within 0.01, 0.1, and 1 MHz integration bandwidths, respectively.

Piezoelectric energy harvesters have proven to have the potential to be a power source in a wide range of applications. As the harvester dimensions scale down, the resonance frequencies of these devices increase drastically. Proof masses are essential in micro-scale devices in order to decrease the resonance frequency and increase the strain along the beam to increase the output power. In this work, the effects of proof mass geometry on piezoelectric energy harvesters are studied. Different geometrical dimension ratios have significant impact on the resonance frequency, e.g., beam to mass lengths, and beam to mass widths. A piezoelectric energy harvester has been fabricated and tested operating at a frequency of about 4 kHz within the audible range. The responses of various prototypes were studied, and an optimized T-shaped piezoelectric vibration energy harvester design is presented for improved performance.

This paper presents a surface-micromachining technology to fabricate silicon carbide (SiC)-based capacitive micromachined ultrasonic transducers (CMUTs). The use of dc-sputtered amorphous SiC as a structural layer allows the fabrication process to limit the temperature to a thermal budget of 200 °C, which is the lowest reported to date, making this technology ideally suited for above-IC integration. The high Young's modulus of the deposited SiC film, along with its very low residual stress, results in high strength and resilient CMUT membranes. The placement of the suspended aluminum electrode directly at the bottom side of the membrane reduces the effective size of the electrostatic transduction gap, resulting in superior electro-mechanical coupling. Fabricated transducers are tested in air with both continuous-wave and pulsed signals, using a pitch-and-catch configuration. The transducer pair, composed of 110-μm-diameter membrane arrays, exhibits a resonant frequency of 1.75 MHz, a 3 dB-bandwidth of 0.15 MHz, and a transmission gain of -38 dB. The CMUT prototypes showcase the versatility of low-temperature dc-sputtered SiC films applied in the field of MEMS.