Cheng-Syun Li

Advanced CMOS–MEMS Resonator Platform

Deep-submicrometer-gap CMOS-MEMS “composite” resonators fabricated using 0.18- μm-1-poly-6-metal foundry CMOS technology have been demonstrated for the first time to substantially improve their electromechanical coupling coefficient, hence leading to a motional impedance of only 880 kΩ at 15.3 MHz. A simple maskless wet release process has been successfully transferred from a 0.35- μm platform to an advanced 0.18-μm version, capable of offering enhanced gap spacing and transduction area for CMOS-MEMS resonators monolithically integrated with high-performance CMOS circuitry. This proposed platform offers ease of use, fast turnaround time, low cost, convenient prototyping, and inherent MEMS-circuit integration, therefore showing great potential toward future integrated sensing and single-chip RF applications.

A Monolithic CMOS-MEMS Oscillator Based on an Ultra-Low-Power Ovenized Micromechanical Resonator

A fully monolithic complimentary metal-oxide- semiconductor-microelectormechanical systems (CMOS-MEMS) oscillator comprised of an ovenized double-ended tuning fork resonator to enable ultra-low heater power operation of only 0.47 mW over entire temperature span (-40 °C to 85 °C) and a low noise sustaining circuit to achieve low phase noise has been demonstrated in a Taiwan Semiconductor Manufacturing Company (TSMC) 0.35-μm CMOS process. The combination of low thermal conductivity material and high thermal isolation design is the key to attaining ultra-low-power heater operation in a sub-mW level. Passive temperature compensation scheme is also conducted in the proposed CMOS-MEMS resonator by an oxide-metal composite structure, showing a low temperature coefficient of frequency (TC f ) of only +5.1 ppm/°C, which is suited for the use in ovenized oscillator systems. By implementing a constant-resistance temperature control scheme, the frequency drift of the resonator smaller than 120 ppm from -40 °C to 85 °C is demonstrated in this paper, indicating an equivalent TC f smaller than 1 ppm/°C, a record-low value against its CMOSMEMS counterparts. The CMOS-MEMS oscillator operating at 1.2 MHz demonstrates a phase noise of -112 dBc/Hz at 1-kHz offset and -120 dBc/Hz at 1-MHz offset while drawing less than 1.3 mW. The entire power consumption of the ovenized oscillator system is confirmed to be less than 1.8 mW (oscillator + micro-oven), verifying the great potential of low power oven-controlled MEMS oscillators realized in CMOS-MEMS technology.

Foundry-CMOS integrated oscillator circuits based on ultra-low power ovenized CMOS-MEMS resonators

A cutting-edge ovenized micromechanical resonator circuit comprised of a double-ended tuning fork (DETF) resonator and serpentine-shaped heaters to enable ultra-low heater power of only 0.47 mW over the entire temperature range (-40°C to 85°C) has been reported for the first time in a low-cost, foundry CMOS-based fabrication platform. The combination of low thermal conductivity materials (i.e., SiO2 and poly-Si) and high thermal isolation designs is key to attaining low heater power consumption in a sub-mW level. An ovenized 1.2-MHz CMOS-MEMS oscillator with a phase noise lower than -103 dBc/Hz at 1-kHz offset and -110 dBc/Hz at 1-MHz offset was also demonstrated in this work, verifying the great potential of low power oven-controlled MEMS oscillators realized using the well-established CMOS-MEMS technology.

Differentially Piezoresistive Sensing for CMOS-MEMS Resonators

A foundry-oriented capacitively driven CMOS-MEMS resonator using differentially piezoresistive sensing is successfully demonstrated to enable effective feedthrough cancellation with more than 20-dB feedthrough floor reduction as compared to its capacitive readout. The resonator is mainly formed by high-Q SiO2 structure utilizing metal wet etching and XeF2 release processes, while the polysilicon layer (originally CMOS gate poly material) embedded inside the resonator structure serves as a piezoresistor for vibratory detection. In addition, such a composite structure enabling electrical isolation realizes decoupling of the capacitive and piezoresistive transductions, allowing the selection (or switching) of the preferred readout scheme using the same resonator device. The proposed resonator consists of only one single capacitor for driving and a simple beam structure for both vibration and detection, therefore greatly simplifying the device design and facilitating future CMOS-MEMS implementation. This paper achieves resonator , more than 28-dB signal-to-feedthrough ratio, and two-times smaller motional impedance than that of the single-ended piezoresistive detection using the same device and driving condition. Furthermore, the piezoresistive operation offers a simple temperature compensation scheme for CMOS-MEMS resonators via the adjustment of the dc current through the piezoresistor, therefore showing 1.4-times improvement on thermal stability as compared to their capacitive readout.

A CMOS-MEMS Resonator Integrated System for Oscillator Application

This paper focuses on the design and development of a CMOS-MEMS resonator integrated with an on-chip amplifier with emphasis on its single-chip frequency reference oscillator implementation. A flexural-mode ring resonator with a desired mode shape featuring an inherent fully-differential mode of mechanical operation is designed using both analytical and finite element models. Two such resonators in low- and high-frequency domains, centered at 1.39 and 9.34 MHz respectively, are individually modeled using first principals, equations, and simulation tools to evaluate and improve device performance. In this paper, the device is also shown to offer a potential benefit of capacitive feedthrough cancelation up to 30 dB attributed to differential signaling scheme. Subsequently, both rapidly prototyped devices integrated with their on-chip transimpedance amplifiers are demonstrated using a commercially available TSMC 0.35-μm CMOS technology. A low-frequency resonator integrated with its on-chip amplifier exhibited decent overall performance capabilities in terms of much higher transmission spectra (closer to 0 dB), greater feedthrough suppression, higher signal-to-feedthrough ratio (35 dB), and exact phase shift (0°) at resonance frequency, therefore being a potential candidate for a single-chip oscillator system.

Design and Characterization of a Dual-Mode CMOS-MEMS Resonator for TCF Manipulation

A novel complimentary metal-oxide-semiconductor-microelectormechanical systems (CMOS-MEMS) composite ring resonator capable of a dual-mode operation has been proposed to enable temperature coefficient of frequency (TCf

Optimizing the close-to-carrier phase noise of monolithic CMOS-MEMS oscillators using bias-dependent nonlinearity

manipulation. To study the temperature dependence between dual modes, two resonant modes of a single resonator vibrating in the orthogonal axes (i.e., in-plane and out-of-plane) are chosen to enable a large difference of their TCfs while not to sacrifice its form factor. By adjusting the constituent ratio and position of the composed metals and dielectrics through the computer-aided-design layout, different TCfs have been successfully demonstrated in a single CMOS-MEMS resonator. By concurrently measuring the TCfs of the in-plane and out-of-plane modes with a divider-based scaling concept, estimated minimum first- and second-order temperature sensitivities (0.53 and 0.29 ppm/°C2, respectively) of their beat frequency can be obtained under proper scaling numbers for temperature-compensated clock applications. This paper also suggests that the first-order temperature coefficient of the beat frequency could be maximized under proper divider numbers. The process variations of the CMOS-MEMS resonators in terms of frequency, quality factor, and transmission magnitude are also intensively studied with an applicable amount of devices. The characterization result shows 1-σ frequency variations of 2,574 and 5,414 ppm for in-plane and out-of-plane modes, respectively.

Capacitively-driven and piezoresistively-sensed CMOS-MEMS resonators

A fully monolithic 1.12-MHz CMOS-MEMS nonlinear oscillator comprising a double-ended tuning fork (DETF) resonator and a transimpedance sustaining amplifier has been proposed to enable significant close-to-carrier phase noise (PN) reduction while maximizing its output power for far-from-carrier phase noise improvement. The best-case PN of -77 dBc/Hz at 10-Hz offset, -97 dBc/Hz at 100-Hz offset, and -113 dBc/Hz at 1-kHz offset is realized in a monolithic CMOS-MEMS flexural-mode resonator oscillator for the first time, which is on par with bulk-mode MEMS oscillators using resonator Q > 100,000.

Combined electrical and mechanical coupling for mode-reconfigurable CMOS-MEMS filters

A foundry-oriented capacitively-driven CMOS-MEMS resonator using differentially piezoresistive sensing has been demonstrated for the first time to enable feedthrough cancellation with more than 20 dB noise floor reduction as compared to purely capacitive transduction. The resonators are formed by high-Q SiO2 structure (Q >; 5,500) using metal wet etching and XeF2 release processes while polysilicon (originally CMOS gate poly) embedded inside the resonator structure serves as piezoresistive element for vibratory detection. In addition, such composite structure enabling electrical isolation accomplishes decoupling of capacitive and piezoresistive transductions, allowing the selection (or switching) of the preferred transduction scheme using the same resonator device. The resonators with capacitive drive and differentially piezoresistive sense configuration have been demonstrated with Q >; 4,000 and more than 28 dB signal-to-feedthrough ratio. CMOS-MEMS oxide resonators with differentially piezoresistive sensing provide an excellent alternative to purely capacitive transduction for integrated oscillator applications.

A 17.6-MHz 2.5V ultra-low polarization voltage MEMS oscillator using an innovative high gain-bandwidth fully differential trans-impedance voltage amplifier

This work presents a novel filter scheme which combines both electrical and mechanical coupling mechanisms implemented in a CMOS-MEMS filter to simultaneously attain small percent bandwidth through weakly mechanical link and decent stopband rejection via differentially electrical configuration. As compared to the traditional parallel-class (i.e., electrically-coupled) filters and mechanically-coupled filters, the proposed oxide-rich filter structure features flexible electrical routing and non-conductive mechanical filter couplers, hence enabling common-mode to differential (CIDO) and differential to common-mode (DICO) reconfigurable modes all within a single device. The proposed 8.6-MHz CMOS-MEMS filter has been successfully demonstrated with a narrow passband of 35 kHz (0.41% bandwidth) and stopband rejection more than 20 dB under proper termination.