Thin-film Piezoelectric-on-silicon Research Articles

Quality Factor Improvement of a Thin-Film Piezoelectric-on-Silicon Resonator Using a Radial Alternating Material Phononic Crystal.

This paper studies the radial alternating material phononic crystal (RAM-PnC). By simulating the band gap structure of the phononic crystal, a complete acoustic band gap was verified at the resonant frequency of 175.14 MHz, which can prevent the propagation of elastic waves in a specific direction. The proposed alternately arranged radial phononic crystal structure is applied to the thin-film piezoelectric-on-silicon (TPOS) MEMS resonator. The finite element simulation method increases the anchor quality factor (Qanchor) from 60,596 to 659,536,011 at the operating frequency of 175.14 MHz, which is about 10,000 times higher. The motion resistance of the RAM-PnC resonator is reduced from 156.25 Ω to 48.31 Ω compared with the traditional resonator. At the same time, the insertion loss of the RAM-PnC resonator is reduced by 1.1 dB compared with the traditional resonator.

Modal-Transition-Induced Valleys of K2 in Piezoelectric Bilayer Laterally Vibrating Resonators.

Piezoelectric Laterally Vibrating Resonators (LVRs) have attracted significant attention as a potential technology for next-generation wafer-level multi-band filters. Piezoelectric bilayer structures such as Thin-film Piezoelectric-on-Silicon (TPoS) LVRs which aim to increase the quality factor (Q) or aluminum nitride and silicon dioxide (AlN/SiO2) composite membrane for thermal compensation have been proposed. However, limited studies have investigated the detailed behaviors of the electromechanical coupling factor (K2) of these piezoelectric bilayer LVRs. Herein, AlN/Si bilayer LVRs are selected as an example, we observed notable degenerative valleys in K2 at specific normalized thicknesses using two-dimensional finite element analysis (FEA), which has not been reported in the previous studies of bilayer LVRs. Moreover, the bilayer LVRs should be designed away from the valleys to minimize the reduction in K2. Modal-transition-induced mismatch between electric and strain fields of AlN/Si bilayer LVRs are investigated to interpret the valleys from energy considerations. Furthermore, the impact of various factors, including electrode configurations, AlN/Si thickness ratios, the Number of Interdigitated Electrode (IDT) Fingers (NFs), and IDT Duty Factors (DFs), on the observed valleys and K2 are analyzed. These results can provide guidance for the designs of piezoelectric LVRs with bilayer structure, especially for LVRs with a moderate K2 and low thickness ratio.

A Wide-Dynamic Range (-40 °C to 180 °C) Active Wireless MEMS Temperature Sensor With 7m °C Sensitivity

We demonstrate a highly-accurate active wireless microelectromechanical (MEMS) temperature sensor with the largest reported dynamic range (-40°C to 180°C) among active MEMS temperature sensors. The sensor uses a 27.2 MHz high quality factor (Q > 3,200) AlN-on-Si thin-film piezoelectric-on-silicon (TPoS) MEMS resonator to enable highly accurate temperature sensing, with minimum sensitivity (S), better than 0.011 °C across this wide temperature range. The wireless sensor uses an innovative frequency shift keying (FSK) 900 MHz direct up-conversion transmitter. The transmitter uses frequency multipliers for up-conversion, obviating the need for a temperature-stable frequency reference clock, thereby preserving the temperature coefficient of frequency (TCF) of the resonator, and consequently the sensitivity of the sensor. The experimental results show a very high correlation (<0.03% error) between the TCF of the MEMS oscillator and that of the transmitter output signal, demonstrating the viability of the proposed direct up-conversion technique. A detailed analysis of the Allan variance for MEMS temperature sensors which can be used to design for a given minimum sensitivity, is also presented in the paper.

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.

Q-Factor Enhancement of Coupling Bragg and Local Resonance Band Gaps in Single-Phase Phononic Crystals for TPOS MEMS Resonator.

This paper presents a type of single-phase double “I” hole phononic crystal (DIH-PnC) structure, which is formed by vertically intersecting double “I” holes. By using the finite element method, the complex energy band curve, special point mode shapes, and different delay lines were calculated. Numerical results showed that DIH-PnC yielded ultra-wide band gaps with strong attenuation. The formation mechanism is attributed to the Bragg-coupled local resonance mechanism. The effects of the pore width in DIH-PnC on the band gaps were further explored numerically. Significantly, as the pore width variable, the position of the local resonance natural frequency could be modulated, and this enabled the coupling between the local resonance and the Bragg mechanism. Subsequently, we introduced this DIH-PnC into the thin-film piezoelectric-on-silicon (TPOS) resonator. The results illustrated that the anchor loss quality factor () of the DIH-PnC resonator was 20,425.1% higher than that of the conventional resonator and 3762.3% higher than the of the cross-like holey PnC resonator. In addition, the effect of periodic array numbers on was researched. When the reached 1.12 × 106, the number of the period array in DIH-PnC only needed to be 1/6 compared with cross-like holey PnC. Adopting the PnC based on the coupling Bragg and local resonance band gaps had a good effect on improving the of the resonator.

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.

Near-Carrier Phase Noise Suppression at Turnover Temperature in a Thin-Film Piezoelectric-on-Silicon Oscillator

In this paper, the near-carrier enhancement of phase-noise (PN) at turnover temperature ( ${T} _{to}$ ) in a quasi-thickness-Lame (QTL) mode thin-film piezoelectric-on-silicon (TPoS) oscillator is reported for the first time. QTL-TPoS resonators fabricated on degenerately-doped n-type silicon offer a ${T} _{to}$ greater than 80°C and are suitable for implementation of highly-stable ovenized oscillators. In this work, a ~123MHz QTL-TPoS resonator is heated up to ${T} _{to}$ (~90°C) by injecting current through the silicon body of the resonator. It is experimentally observed that at turnover temperature, the phase-noise slope close to the carrier frequency ( $\Delta $ f 100Hz ) decreases substantially in contrast to the expected trends. A ~10dB improvement in phase noise at 10Hz offset and a ~25dB improvement at 1Hz offset is recorded when the oscillator is operating at ${T} _{to}$ compared to room temperature. [2020-0206]

A PM2.5 Sensor Module Based on a TPoS MEMS Oscillator and an Aerosol Impactor

In this work, a real-time PM2.5 sensor module with high mass resolution and wide detection range is successfully demonstrated while the module consists of a two-stage aerosol impactor, a Thin-film Piezoelectric-on-Silicon (TPoS) MEMS oscillator, and a micro-pump. The aerosol impactor's collection efficiency of 51% for $2.54~\mu \text{m}$ and 50% for $1.03~\mu \text{m}$ is verified for stage 1 and stage 2, respectively. A length-extensional mode TPoS MEMS resonator is implemented, featuring a resonant frequency of 4.05 MHz and a quality factor (${Q}$ ) of 438 in air. To achieve a portable sensor module, a board-level sustaining circuit, which includes a transimpedance amplifier (TIA), a phase shifter, a band-pass filter, and a buffer circuit are integrated to fulfill the condition of Barkhausen criteria for oscillation. Phase noise of -111.92 dBc/Hz at 1 kHz offset was recorded under ambient pressure and room temperature for a carrier frequency of 4.05 MHz. The mass resolution for the PM2.5 sensor implemented in this work is 0.71 pg. Finally, an incense was used to produce the liquid type particles continuously for the aerosol measurement. The coefficient of determination (${R} ^{2}$ ) of 0.81 between the frequency slope of the proposed sensor and the aerosol concentration of the commercial optical aerosol sensor within a range of $10~\mu $ g/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to $1000~\mu $ g/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . This work successfully integrated MEMS, circuit, and air flow technologies to realize a complete mass sensing module, and has the potential to be implemented in the particulate matter concentration monitoring applications of air.

Liquid-Loaded Piezo-Silicon Micro-Disc Oscillators for Pico-Scale Bio-Mass Sensing

In this work, we report on the implementation of a closed-loop liquid-loaded resonant mass sensor as a test vehicle for the realization of highly sensitive miniaturized biomarker assays. Using thin-film piezoelectric-on-silicon (TPoS) contour-mode disc resonators that are specifically designed and optimized for liquid-phase operation, a liquid-loaded low power (~1.6 mW) oscillator at ~17 MHz is demonstrated with a single transistor. By proper placement of the electrodes and the supporting tethers, a high liquid-loaded quality factor (~380) and a low insertion loss (~19 dB) are concurrently achieved which enable the accurate frequency tracking of the potential mass microbalance. The frequency shifts due to the liquid-loading and the preliminary characterized frequency stability of the oscillator imply a minimum mass detection resolution in the picograms range. [2020-0177]

A Thin-Film Piezoelectric-on-Silicon MEMS Oscillator for Mass Sensing Applications

In this study, a Thin-film Piezoelectric-on-Silicon (TPoS) MEMS oscillator is successfully demonstrated that uses lead zirconate titanate (PZT) thin-film piezoelectric layer deposited on a single-crystal silicon substrate to realize a length-extensional mode resonator, having a resonant frequency of 5.12 MHz and a quality factor (Q) of 311. For closed-loop implementation, a commercial Lock-in amplifier with a Phase Locked Loop (PLL) facility was used to fulfill the Barkhausen criteria for oscillation. The mass resolution and sensitivity of the proposed mass sensor are 0.54 pg and 4.96 Hz/pg, respectively. Finally, a board-level sustaining circuit, which includes a transimpedance amplifier (TIA) and an attenuator is integrated with the TPoS MEMS resonator for oscillation to achieve a portable sensor. Phase noise of -104.11 dBc/Hz at 1 kHz offset was recorded under ambient pressure and room temperature for a carrier frequency of 5.12 MHz. The mass resolution for the board-level mass sensor implemented in this work is 0.15 pg.

Technique and Circuit for Contactless Readout of Piezoelectric MEMS Resonator Sensors.

A technique and electronic circuit for contactless electromagnetic interrogation of piezoelectric micro-electromechanical system (MEMS) resonator sensors are proposed. The adopted resonator is an aluminum-nitride (AlN) thin-film piezoelectric-on-silicon (TPoS) disk vibrating in radial contour mode at about 6.3 MHz. The MEMS resonator is operated in one-port configuration and it is connected to a spiral coil, forming the sensor unit. A proximate electronic interrogation unit is electromagnetically coupled through a readout coil to the sensor unit. The proposed technique exploits interleaved excitation and detection phases of the MEMS resonator. A tailored electronic circuit manages the periodic switching between the excitation phase, where it generates the excitation signal driving the readout coil, and the detection phase, where it senses the transient decaying response of the resonator by measuring through a high-impedance amplifier the voltage induced back across the readout coil. This approach advantageously ensures that the readout frequency of the MEMS resonator is first order independent of the interrogation distance between the readout and sensor coils. The reported experimental results show successful contactless readout of the MEMS resonator independently from the interrogation distance over a range of 12 mm, and the application as a resonant sensor for ambient temperature and as a resonant acoustic-load sensor to detect and track the deposition and evaporation processes of water microdroplets on the MEMS resonator surface.

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.

Thin Film Piezoelectric-on-Silicon Elliptical Resonators With Low Liquid Phase Motional Resistances

Thin film piezoelectric-on-silicon (TPoS) elliptical resonators are presented in this paper. The micro-resonators are composed of two rectangular resonant plates vibrating out-of-phase at the first length-extensional mode, which are coupled through arches on sides. In this way, extensional motions of rectangles are converted to rotational motions via arches which make the resonator suitable for operation under water, where all sidewalls of the resonator vibrate in parallel with liquid to maintain high liquid phase quality factors ( ${Q}$ ). Thanks to wide electrode area available on rectangular plates, strong piezoelectric transduction is achieved leading to low liquid phase motional resistances. Different variations of the resonators were designed and fabricated to operate at different resonance frequencies. The TPoS elliptical resonators exhibit high ${Q}\text{s}$ up to 7400 for operation in air while showing liquid phase ${Q}\text{s}$ as high as 200. An elliptical resonator operating at 4 MHz with the lowest motional resistance of 5.6 $\text{k}\Omega $ was involved in an oscillation circuit with a single-stage sustaining amplifier exhibiting frequency stability of $2\times 10^{-7}$ ( $\Delta f/f_{0}$ ). Such high stability makes such resonators an excellent choice for high-resolution mass or viscosity sensing at liquid phase such as biosensing applications.

Piezoelectric MEMS resonant dew point meters

This work presents a new class of dew point meters using Thin-film Piezoelectric-on-Silicon (TPoS) resonators. The proposed approach utilizes microelectromechanical resonant balances coupled with a cooling element in which a standard chilled surface technique is used to deposit layers of moisture on the resonator surface. Reducing the temperature causes the resonance frequency of the resonator to increase due to its negative temperature coefficient of frequency (TCF). However, when the temperature of the cooled resonator reaches the dew point (DP) of the surrounding environment, a thin layer of frost starts to form on the resonator surface causing an abrupt negative shift in the resonant frequency due to mass-loading. Therefore, the resonance frequency will be at its maximum value when the dew or frost starts to form i.e., at DP. A custommade setup which includes a MEMS dew point sensor head, a dew point generator and a measurement/control/data collection interface is assembled to enable automated measurements. Using the proposed setup, dew points as low as -41 °C i.e., a dry air containing 145 ppmv H2O, are measured with a standard deviation of 0.1 °C. Long-term reliability and accuracy of the sensors presented here are investigated by conducting 35,000 cycles of uninterrupted measurements. The long-term stability, precision, and durability of the presented dew point meters make them suitable solutions for a wide variety of industrial applications.

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.

A Lorentz force magnetometer based on a piezoelectric-on-silicon square-extensional mode micromechanical resonator

In this letter, we present a Lorentz force magnetic field sensor based on a thin-film piezoelectric-on-silicon (TPoS) CMOS-compatible resonator for the detection of an out-of-plane (perpendicular to the plane of fabrication) magnetic field. We here exploit the fundamental breathing mode of vibration in a suspended square plate, which is commonly referred to as the square-extensional (SE) mode. The symmetric stress profile of the SE mode avails stresses in the two orthogonal in-plane axes to be effectively coupled into a charge output through the piezoelectric transducers. This in turn enhances the output motional current from the device, which effectively determines the responsivity of the device. In this context, the responsivity has been defined as a ratio of output motional current to the external magnetic field, which has been further normalized against the input reference current of the device. The reported device has recorded a responsivity of 6950 ppm/T (μA/A.T) at a resonant frequency of 5.28 MHz and a reasonable mechanical quality (Q) factor of 1056 in air.