Wen-Chien Chen

A generalized CMOS-MEMS platform for micromechanical resonators monolithically integrated with circuits

A generalized foundry-oriented CMOS-MEMS platform well suited for integrated micromechanical resonators alongside IC amplifiers has been developed for commercial multi-user purpose and demonstrated with a fast turnaround time of only 3 months and a variety of design flexibilities for resonator applications. With this platform, different configurations of capacitively-transduced resonators monolithically integrated with their amplifier circuits, spanning frequencies from 500 kHz to 14.5 MHz, have been realized with resonator Q’s ranging between 700 and 3500. This platform, specifically featured with various configurations of structural materials, multi-dimensional displacements, different arrangements of mechanical boundary conditions, tiny supports of resonators, large transduction areas, well-defined anchors and performance enhancement scaling with IC fabrication technology, offers a variety of flexible design options targeted for sensor, timing reference, and RF applications. In addition, resonators consisting of metal-oxide composite structures fabricated by this platform offer an effective temperature compensation scheme for the first time in CMOS-MEMS resonators, showing TCf six times better than that of resonators merely made by CMOS metals. (Some figures in this article are in colour only in the electronic version)

High-

Integrated CMOS-MEMS free-free beam resonators using pull-in mechanism to enable deep-submicrometer electrode- to-resonator gap spacing without interference in their mechanical boundary conditions (BCs) have been demonstrated simultaneously with low motional impedance and high Q. The key to attaining high Q relies on a decoupling design between pull-in frames for gap reduction and mechanical BCs of resonators. In addition, the use of metal-SiO2 composite structures has been proved to greatly benefit the thermal stability of CMOS-MEMS resonators. Furthermore, tuning electrodes underneath pull-in frames were designed to offer “quasi-linear” frequency tuning capability where linear relationship between tuning voltage and frequency was achieved. In this paper, CMOS-MEMS free-free beam resonators with gap spacings of 110, 210, and 275 nm, respectively, were tested under direct one-port measurement in vacuum, demonstrating a resonator Q greater than 2000 and a motional impedance as low as 112 kΩ and, at the same time, allowing quasi-linear frequency tuning to achieve a total tuning range of 5000 ppm and a sensitivity of 83.3 ppm/V at 11.5 MHz with zero dc power consumption. Such a resonator monolithically integrated with a CMOS amplifier, totally occupying a die area of only 300 μm × 130 μm, was also tested with enhanced performance, benefiting future timing reference and RF synthesizing applications.

Q

Integrated CMOS-MEMS free-free beam resonator arrays operated in a standard two-port electrical configuration with low motional impedance and high power handling capability, centered at 10.5 MHz, have been demonstrated using the combination of pull-in gap reduction mechanism and mechanically coupled array design. The mechanical links (i.e., coupling elements) using short stubs connect each constituent resonator of an array to its adjacent ones at the high-velocity vibrating locations to accentuate the desired mode and reject all other spurious modes. A single second-mode free-free beam resonator with quality factor Q >; 2200 and motional impedance Rm <; 150 kΩ has been used to achieve mechanically coupled resonator arrays in this work. In array design, a 9-resonator array has been experimentally characterized to have performance improvement of approximately 10× on motional impedance and power handling as compared with that of a single resonator. In addition, the two-port electrical configuration is much preferred over a one-port configuration because of its low-feedthrough and high design flexibility for future oscillator and filter implementation.

Integrated CMOS-MEMS Resonators With Deep-Submicrometer Gaps and Quasi-Linear Frequency Tuning

A generalized foundry CMOS-MEMS platform suited for integrated micromechanical resonator circuits have been developed for commercial multi-user purpose and demonstrated with a fast turnaround time and a variety of design flexibilities for resonator applications. With this platform, different configurations of capacitively-transduced resonators monolithically integrated with their associated amplifier circuits, spanning frequencies from 500kHz to 14.5MHz, have been realized with resonator Qs around 2,000. This platform specifically featured with various configurations of structural materials, different arrangements of mechanical boundary conditions, large transduction area, well-defined anchors, and performance enhancement scaling with IC fabrication technology, offers a variety of flexible design options suited for sensor and RF applications.

Mechanically coupled CMOS-MEMS free-free beam resonator arrays with enhanced power handling capability

A fully differential CMOS-MEMS oxide resonator fabricated using 0.18 μm CMOS-MEMS platform via metal wet-etching post process has been demonstrated with Q >; 10,000, first time ever in any CMOS-MEMS resonators, and more than 25 dB signal-to-feedthrough ratio at 47.9 MHz. Key to attaining such performance attributes to (1) the bulk-mode vibration to enable exceptional Q and much higher frequencies and (2) the oxide-rich structure with embedded metal electrodes for capacitive transduction, where SiO2 offers better mechanical properties than metals to minimize intrinsic energy loss and where flexible electrical routing facilitates fully differential configuration to suppress capacitive feedthroughs. In addition, a previously developed metal wet-etching technique capable of releasing large device areas has been successfully transferred from 0.35 μm 2-Poly-4-Metal (2P4M) CMOS process to a new 0.18 μm 1-Poly-6-Metal (1P6M) technology node, therefore greatly lowering the motional impedance of the capacitively-transduced resonators due to smaller electrode-to-resonator gap spacing and larger transduction areas. This technology paves a way to realize fully-integrated CMOS-MEMS oscillators and filters which might benefit future single-chip transceivers for wireless communications.

A generalized foundry CMOS platform for capacitively-transduced resonators monolithically integrated with amplifiers

A fully differential CMOS-MEMS double-ended tuning-fork (DETF) oxide resonator fabricated using a 0.18-μm CMOS process has been demonstrated with a Q greater than 4800 and more-than-20-dB stopband rejection at 10.4 MHz. The key to attaining such a performance attributes to the use of oxide structures with embedded metal electrodes, where SiO2 offers a Q enhancement (at least a 3-times-higher Q) as compared to other CMOS-MEMS-based composite resonators with similar structures and vibrating modes and where flexible electrical routing facilitates fully differential configuration to suppress capacitive feedthroughs. In addition, the resonators developed in this work possess a positive temperature coefficient of frequency (TCf) and mode-splitting capability, therefore indicating a great potential for temperature compensation and spurious-mode suppression, respectively. This technology paves a way to realize fully integrated CMOS-MEMS oscillators and filters which might benefit future single-chip transceivers for wireless communications.

VHF CMOS-MEMS oxide resonators with Q > 10,000

This study demonstrates the torque-enhancement design for a 2-axis magnetostatic SOI scanner driven by a double-side electroplating ferromagnetic film. The present design has two merits: (1) the slender ferromagnetic material patterns with higher length-to-width ratio enhance the magnetization, (2) the backside electroplating of the ferromagnetic film increases the volume of the ferromagnetic materials. This study also establishes the fabrication processes to implement the proposed design. The processes also have two merits: (1) the handle-layer of the SOI wafer is exploited as the shadow mask to pattern the seed-layer at the backside of the device layer, (2) the device layer of the SOI wafer acts as the cathode to enable simultaneous double-side electroplating. In applications, a 2-axis SOI scanner was implemented and characterized. Measurements show a 149% torque enhancement from the double-side electroplating design. The vertical slender ferromagnetic material patterns further increase the magnetostatic torque to 211%. This study also successfully demonstrates the Lissajous scanning using the presented 2-axis SOI scanner.

A Fully Differential CMOS–MEMS DETF Oxide Resonator With

Integrated CMOS-MEMS array resonators have been demonstrated that takes advantage of pull-in effect to surmount limitations of CMOS foundry process and attains electrode-to-resonator gap spacing at a deep-submicron range, leading to much smaller motional impedance compared to conventional CMOS-MEMS technologies, while possessing unique frequency tuning capability by modulating their mechanical boundary conditions. With the increase of applied dc-bias which simultaneously serves for functions of pull-in and resonator operation, the upward frequency shift of resonance caused by boundary condition (“BC”) change offers opposite tuning mechanism to well-known effect of electrical stiffness. As a result, frequency variation induced by BC-modulation and electrical-stiffness would yield a frequency-insensitive region under a certain dc-bias.

Q > \hbox{4800}

This letter presents the design of a micromechanical oscillator based on a vertically coupled (VC) CMOS-microelectromechanical systems (MEMS) resonator pair for phase noise reduction. The prototyped resonator pair consists of two vibrating plates coupled with each other, thus providing enhanced power handling while keeping compact footprint. The proof-of-concept oscillator based on a 6.5-MHz, VC mode resonator achieves a phase noise of -97 dBc/Hz at 1-kHz offset and -118 dBc/Hz at 1-MHz offset from carrier with a resonator size of 30 × 30 μm2. Compared with its nonresonance-coupled saddle mode counterpart, the VC-mode-based oscillator features 10-dB phase noise reduction using the same oscillator circuit setup. The design concept of this letter can be extended to three dimensional (3-D) mechanically coupled resonator designs in the future.

and Positive TCF

A novel metal wet etching process to release CMOS-MEMS resonators has been developed and demonstrated for the first time to enable SiO2-rich resonator structure and embedded metal electrodes with well-defined anchors for Q enhancement. In virtue of exceptional selectivity of metal wet etchant to SiO2 among CMOS layers, the use of release holes needed for most of isotropic etching processes could be eliminated, hence substantially preserve the integrity of resonator structures and their anchors. With such a maskless metal release process, capacitively-transduced oxide resonators monolithically integrated with readout circuitry using standard CMOS 0.35 µm 2P4M process have been fabricated and tested, showing resonator Qs up to 4,400, stopband rejections from 40 to 80 dB, centered at 3 MHz with maximum breakdown voltage of 250 V and better temperature coefficient of frequency (TCƒ) compared with that of mere-metal CMOS-MEMS counterparts due to “SiO2-rich” resonator configuration.