Thin-film Piezoelectric-on-silicon Resonator Research Articles

A piezoelectric mechanically coupled Lamé mode resonator with ultra-high Q

A piezoelectrically actuated high quality factor (Q) mechanically coupled Lamé mode resonator based on a thin-film piezoelectric-on-silicon (TPoS) platform is demonstrated. The reported coupled resonator consists of two identical square plates of single-crystal silicon (SCS) coupled with a SCS beam having thin-film piezoelectric stacked layers on top. The coupled resonator is piezoelectrically driven into resonance, and the two square plates are excited into vibrations in a coupled Lamé mode through mechanical coupling. An impressive passive Q-enhancement is obtained by such a scheme based on mechanical coupling of two SCS square plates with a piezoelectrically transduced beam. A fabricated mechanically coupled Lamé mode resonator exhibits an ultra-high Q of 309 894 while still maintaining a moderate impedance (Rm) of 6652 Ω at its resonant frequency of 18.2838 MHz, corresponding to the record highest f-Q product of 5.67 × 1012 for TPoS resonators. The reported high Q microelectromechanical system (MEMS) resonator overcomes the inherent low-Q nature of piezoelectric MEMS resonators, indicating an application for providing a high-performance timing solution in a MEMS timing field.

Design, Modeling and Fabrication of TPoS MEMS Resonators With Improved Performance at 1 GHz

In this work, the effects of physical dimensions such as length, width and thickness as well as the mode of vibration and the number of anchors on the performance of longitudinal thin film piezoelectric on silicon (TPoS) MEMS resonators have been studied. TPoS resonators, designed for a resonant frequency of around 1 GHz, were fabricated with a 4 mask CMOS compatible process. A 225 μm wide resonator excited in its 23 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order had an unloaded quality factor of 9453 (in vacuum), which is the highest value reported so far for similar resonators, motional resistance of 107 Ω and linear thermal coefficient of frequency of -28.4 ppm. We have also studied and modeled the different loss mechanisms in these devices. The model matches well with measured results for resonators of different geometries, modes of vibrations and number of anchors. [2020-0397]

Q-Factor Enhancement of Thin-Film Piezoelectric-on-Silicon MEMS Resonator by Phononic Crystal-Reflector Composite Structure.

Thin-film piezoelectric-on-silicon (TPoS) microelectromechanical (MEMS) resonators are required to have high Q-factor to offer satisfactory results in their application areas, such as oscillator, filter, and sensors. This paper proposed a phononic crystal (PnC)-reflector composite structure to improve the Q factor of TPoS resonators. A one-dimensional phononic crystal is designed and deployed on the tether aiming to suppress the acoustic leakage loss as the acoustic wave with frequency in the range of the PnC is not able to propagate through it, and a reflector is fixed on the anchoring boundaries to reflect the acoustic wave that lefts from the effect of the PnC. Several 10 MHz TPoS resonators are fabricated and tested from which the Q-factor of the proposed 10 MHz TPoS resonator which has PnC-reflector composite structure on the tether and anchoring boundaries achieved offers a loaded Q-factor of 4682 which is about a threefold improvement compared to that of the conventional resonator which is about 1570.

Fully-Differential TPoS Resonators Based on Dual Interdigital Electrodes for Feedthrough Suppression

As one of the core components of MEMS (i.e., micro-electro-mechanical systems), thin-film piezoelectric-on-silicon (TPoS) resonators experienced a blooming development in the past decades due to unique features such as a remarkable capability of integration for attractive applications of system-on-chip integrated timing references. However, the parasitic capacitive feedthrough poses a great challenge to electrical detection of resonance in a microscale silicon-based mechanical resonator. Herein, a fully-differential configuration of a TPoS MEMS resonator based on a novel structural design of dual interdigital electrodes is proposed to eliminate the negative effect of feedthrough. The fundamental principle of feedthrough suppression was comprehensively investigated by using FEA (i.e., finite-element analysis) modeling and electrical measurements of fabricated devices. It was shown that with the help of fully-differential configuration, the key parameter of SBR (i.e., signal-to-background ratio) was significantly enhanced by greatly suppressing the in-phase signal. The S-parameter measurement results further verified the effectiveness of this novel feedthrough suppression strategy, and the insertion loss and SBR of proposed TPoS resonators were improved to 4.27 dB and 42.47 dB, respectively.

Thin-Piezo on Single-Crystal Silicon Reactive Etched RF MEMS Resonators

This paper demonstrates how a single crystal silicon wafer can be used to fabricate thin-film piezoelectric-on-silicon (TPoS) resonators by utilizing a modified version of Single Crystal Silicon Reactive Etch and Metallization (SCREAM) process. The developed process enables the fabrication of MEMS resonators with varied device layer thicknesses ranging from sub-micrometer to tens of micrometers (one thickness per die) from a single bulk silicon wafer, while avoiding the need of costly silicon-on-insulator (SOI) substrates. The thin-film piezoelectric on single-crystal silicon reactive etched technique allows batch fabrication of TPoS resonators, while also retaining the same number of photolithography steps. To maintain a good resonator body sidewall roughness, a conformal Al2O3 thin film was deposited by atomic layer deposition to act as the sidewall protection layer. Through the developed process, resonators with varied silicon layer ranging from $0.1~\mu \text{m}$ to $47~\mu \text{m}$ have been successfully implemented. The measured results under different ZnO-to-Si thickness ratios have been studied, in terms of motional impedance ( $R_{\mathrm {m}}$ ), quality factor ( $Q$ ), and resonance frequency. It is noted that TPoS MEMS resonators operating in fundamental and higher lateral extensional modes exhibit their best performance under an optimal ZnO-to-Si thickness ratio. Resonators fabricated by the modified TPoS process with a Si device layer thickness of 4- $20~\mu \text{m}$ exhibits optimal performance. The highest $Q$ of 1,567 for a disk resonator and the lowest motional impedance of 791 $\Omega $ for a square plate resonator were achieved with Si layer thicknesses of $20~\mu \text{m}$ and $4~\mu \text{m}$ , respectively.

A 10 MHz thin-film piezoelectric-on-silicon MEMS resonator with T-shaped tethers for Q enhancement

Thin-film piezoelectric-on-silicon (TPoS) MEMS (i.e. micro-electro-mechanical system) resonators have attracted much attention due to its unique features, such as CMOS-compatible fabrication process, however, its applications require, in particular, the enhancement of quality factor (Q). Herein, a 10 MHz TPoS MEMS resonator with T-shaped tethers coupled with reflecting blocks was proposed. Finite-element analysis was employed for modeling the proposed TPoS resonators to investigate the underlying mechanism of Q enhancement by the introduction of T-shaped tethers and reflecting blocks. It was figured out that the combination of T-shaped tethers and reflecting blocks can significantly reduce the energy leakage through the anchor substrate by reflecting mechanical waves and trapping the energy. The measured results of fabricated TPoS resonators further verified the effectiveness of Q enhancement by the as-presented new design, and the insertion loss and the Q were improved to 3.27 dB and 1903, respectively.

A Microfluidic MEMS-Microbalance Platform With Minimized Acoustic Radiation in Liquid.

In this article, the microfluidic channels that deliver liquid to a microscale thin-film piezoelectric-on-silicon (TPoS) gravimetric resonant sensor are incorporated into the backside of the silicon-on-insulator (SOI) wafer on which the resonator is fabricated. Specifically, a microwell is embedded at the bottom of the disk -shaped TPoS resonator, while a very thin layer of parylene covering the backside of the resonator and the microwell forms an isolation layer between the liquid and the top device-layer features. In this way, the liquid is in contact with the backside of the resonator, while the device-defining trenches and the electrical connections to the resonator stay clear, thus mitigating the acoustic energy loss and undesirable feedthroughs. The impact of the parylene layer thickness on a few symmetric ( S ) and antisymmetric ( A ) Lamb wave modes of the resonator is experimentally studied, and the performance of such modes in the liquid is characterized by filling the microwells through a PDMS-based microfluidic channel. The parylene layer, while marginally affecting the resonator in the air, is found to substantially enhance its performance in the liquid media. Strong resonance peaks with high quality factors ( Q ) are observed for the S modes, among which Q values above 400 are recorded for a specific mode named S (4, 2) (among the highest ever reported). This article can potentially facilitate the realization of highly stable and sensitive resonant mass sensors (i.e., microbalance) for real-time applications. Additionally, the effect of the acoustic energy radiation in the form of evanescent shear and longitudinal waves in liquid on the Q and resonance frequency of the disk resonators is experimentally validated.

Frame structure for thin-film piezoelectric-on-silicon resonator to greatly enhance quality factor and suppress spurious modes

Thin-film piezoelectric-on-silicon (TPoS) bulk acoustic wave resonator provides an attractive vision of system-on-chip integrated circuits by micro/nano-engineering the timing reference. However, its quality factor (Q) at high frequency is a great challenge due to the mechanical loss. Herein, a novel suspended frame structure on tethers was proposed to significantly enhance the Q and suppress spurious modes of TPoS resonator working in the 30.4 MHz fundamental mode. Compared with the pristine one, the measured Q of fabricated resonator with frame structure showed a maximum 136-fold enhancement. The systematical investigation by both finite-element-analysis (FEA) simulation and experimental comparison revealed that the existing of frame structure effectively decreases the effective coupling coefficient (k2eff) of resonator and further results in the suppression of spurious modes and the enhancement of Q. As the width of frame structure increases, the k2eff maintains a consistent value of 0.15% while the anchor loss significantly reduced, which results in the reduction of insertion loss (IL) and the enhancement of Q.

Study of a 10 MHz MEMS oscillator with a TPoS resonator

This paper reports a low phase noise 10MHz oscillator based on a lateral-extensional mode thin-film piezoelectric-on-silicon (TPoS) resonator. A novel design of interdigital electrodes is introduced to improve both the insertion loss and quality factor of the resonator. The average quality factor gets improved by 43% by this method. The equivalent circuit model of the fundamental lateral-extensional mode is analysed. An oscillator is also designed and manufactured. The whole circuit includes a single common emitter BJT amplifier, LC network filter, a phase shifting unit and a MEMS resonator. Tuning the phase shifter in the feedback circuit, the MEMS resonator works at the best operating point while the phase shifting of the peripheral circuit is 90°. With an operating input power around 0dBm, the resonator is not driven at its bifurcation point. Little nonlinearity is observed in the output waveform. The phase-noise is measured below −105dBc/Hz@100Hz, −135dBc/Hz@1kHz offset and the far-from-carrier noise reaches below −150dBc/Hz.

Effects of cryogenic cooling on the quality factor of lamb wave mode aluminium nitride piezoelectric-on-silicon MEMS resonators

This work investigates the temperature dependence of the quality factors (Q) of Aluminium Nitride (AlN) thin-film piezoelectric-on-silicon (TPoS) resonators as the devices are cryogenically cooled from room temperature to 78K. Four designs of TPoS resonators with resonant frequencies in the range of 48MHz to 142MHz were considered. We found that in all but one of the resonators, Q appeared to be mostly constant in the measurement range of room temperature to 78K. The only exception was a 90μm wide resonator vibrating at its fundamental mode at 48MHz (referred to as Design A in this work). At room temperature, Design A at the fundamental mode had a Q substantially higher than another resonator of the same size but different support design (referred to as Design B in this work: 9500 compared to 2400). This notable difference in Q observed at room temperature due to differences in the support design corresponds well with differences in the trends of Q as a function of temperature when the designs were cryogenically cooled from room temperature to 78K. More specifically, the Q of Design A doubled while Q of Design B was constant. Assuming that anchor loss as an energy loss mechanism is independent of temperature, the cryogenic cooling results thus further illustrate the dominance of anchor loss in Design B but not Design A at room temperature. Designs A and B were then excited at higher order modes (142MHz) along with Designs C and D (100MHz). Measured values of Q were much lower than Design A at the fundamental mode, and Q likewise did not increase with cryogenic cooling to 78K. These experimental results taken as a whole illustrate the usefulness of cryogenic cooling as a practical approach to ascertain the dominance of temperature-independent energy losses (e.g. anchor loss) in TPoS resonators at room temperature.

Turnover Temperature Point in Extensional-Mode Highly Doped Silicon Microresonators

This paper demonstrates the existence of a local zero temperature coefficient of frequency (i.e., turnover point) in extensional-mode silicon microresonators, fabricated on highly n-type-doped substrates and aligned to the [100] crystalline orientation. It is shown through both theoretical analysis and finite-element simulation that the turnover point in thin-film piezoelectric-on-silicon (TPoS) resonators is a function of doping concentration and orientation. Moreover, the turnover point can be adjusted by changing the thickness ratio of Si and the piezoelectric film (e.g., AlN) in the resonant structure. In order to experimentally validate this result, similar resonators are fabricated on silicon-on-insulator substrates, and the temperature variation of frequency is measured. An overall temperature-induced frequency variation of less than 245 ppm is measured over the range of -40 °C-85 °C for an ~ 25-MHz TPoS resonator aligned to the [100] plane. This is more than a 15-times reduction with respect to the uncompensated conventional silicon resonators reported before. This work is a significant step toward strengthening silicon's position as an alternative resonator technology in the quartz-dominated stable oscillator market.