Aluminum Nitride Lamb Wave Resonators Research Articles

A MEMS Fabrication Process with Thermal-Oxide Releasing Barriers and Polysilicon Sacrificial Layers for AlN Lamb-Wave Resonators to Achieve fs·Qm > 3.42 × 1012.

This paper presents a micro-electro-mechanical systems (MEMS) processing technology for Aluminum Nitride (AlN) Lamb-wave resonators (LWRs). Two LWRs with different frequencies of 402.1 MHz and 2.097 GHz by varying the top interdigitated (IDT) periods were designed and fabricated. To avoid the shortcomings of the uncontrollable etching of inactive areas during the releasing process and to improve the fabrication yield, a thermal oxide layer was employed below the platted polysilicon sacrificial layer, which could define the miniaturized release cavities well. In addition, the bottom Mo electrode that was manufactured had a gentle inclination angle, which could contribute to the growth of the high-quality AlN piezoelectric layer above the Mo layer and effectively prevent the device from breaking. The measured results show that the IDT-floating resonators with 12 μm and 2 μm electrode periods exhibit a motional quality factor (Qm) as high as 4382 and 1633. The series resonant frequency (fs)·Qm values can reach as high as 1.76 × 1012 and 3.42 × 1012, respectively. Furthermore, Al is more suitable as the top IDT material of the AlN LWRs than Au, and can contribute to achieving an excellent electrical performances due to the smaller density, smaller thermo-elastic damping (TED), and larger acoustic impedance difference between Al and AlN.

Narrowband Impedance Transformer With Extremely High Transformation Ratio of 200

This letter presents a hybrid narrowband radio frequency (RF) impedance transformer using an aluminum nitride (AlN) microelectromechanical system (MEMS) resonator and lumped elements. The hybrid transformer consists of an AlN Lamb wave resonator, a parallel inductor, and a set of three lumped-element networks on a printed circuit board (PCB). The AlN resonator and the parallel inductor form the narrowband response while the lumped element networks fulfill the high impedance transformation. The transformer has been theoretically predicted and experimentally shown to achieve a narrow fractional bandwidth (FBW) of 2.2% and an extremely high impedance transformation ratio (ITR) of 200.

A humidity sensor based on AlN Lamb wave resonator coated with graphene oxide of different concentrations

A high-performance humidity sensor based on an AlN (aluminum nitride) Lamb wave resonator coated with graphene oxide (GO) is presented in this paper. With the porous structure and oxygen-containing groups of GO as the sensing layer, the humidity sensor exhibits a high sensitivity of 26 kHz/%RH at 10%–90% relative humidity. However, more than half of the frequency variation occurs in the range of 70%RH to 90%RH. In this paper, we propose a vacuum film-coating method to improve the performance of the GO film. With this GO film, the sensitivity is increased by 160% in the humidity range between 10% to 60%RH compared to conventional ones. The sensor also demonstrates a low temperature coefficient of frequency, small hysteresis, and short response (~9 s)/recovery (~11 s) time. Therefore, the humidity sensor with GO shows great potential in practical applications.

High- ${Q}$ Butterfly-Shaped AlN Lamb Wave Resonators

Butterfly-shaped aluminum nitride (AlN) plates with anchor-to-plate angle smaller than 90° are proposed to boost the quality factor ( ${Q}$ ) of Lamb wave resonators (LWRs) thanks to the elimination of the anchor loss. Finite-element method simulation shows that the butterfly-shaped plate can efficiently reduce mechanical energy leakage via the supporting anchors and accordingly enhance the mechanical ${Q}$ . Furthermore, the rounded butterfly-shaped LWRs show better suppression in the anchor loss in comparison with the beveled butterfly-shaped LWRs. More specifically, the measured frequency response of an 863-MHz beveled butterfly-shaped AlN LWR yields an unloaded ${Q}$ of 4189, representing 25% improvement over an LWR on a rectangular AlN plate; another AlN LWR based on the rounded butterfly-shaped plate shows an unloaded ${Q}$ of 5352, enabling 60% improvement in unloaded ${Q}$ values.

GHz spurious mode free AlN lamb wave resonator with high figure of merit using one dimensional phononic crystal tethers

This letter reports a spurious mode free GHz aluminum nitride (AlN) lamb wave resonator (LWR) towards high figure of merit (FOM). One dimensional gourd-shape phononic crystal (PnC) tether with large phononic bandgaps is employed to reduce the acoustic energy dissipation into the substrate. The periodic PnC tethers are based on a 1 μm-thick AlN layer with 0.26 μm-thick Mo layer on top. A clean spectrum over a wide frequency range is obtained from the measurement, which indicates a wide-band suppression of spurious modes. Experimental results demonstrate that the fabricated AlN LWR has an insertion loss of 5.2 dB and a loaded quality factor (Q) of 1893 at 1.02 GHz measured in air. An impressive ratio of the resistance at parallel resonance (Rp) to the resistance at series resonance (Rs) of 49.8 dB is obtained, which is an indication of high FOM for LWR. The high Rp to Rs ratio is one of the most important parameters to design a radio frequency filter with steep roll-off.

Spurious-free Lamb wave resonators with protrusion structures

In this letter, we demonstrate a technique to eliminate the spurious modes in aluminum nitride Lamb wave resonators (LWRs). The transverse acoustic wave characteristics are examined, and a resonance modulation theory on the regulation of mechanical boundary conditions is deducted. As examples of embodiments, vertical and lateral protrusion structures are proposed for the suppression. Finite element analysis verifies that the employment of these structures effectively restrains the transverse modes, and the measured electrical performance of the LWR with protrusions demonstrates an 11 dB reduction in the spurious response.

Transverse Mode Spurious Resonance Suppression in Lamb Wave MEMS Resonators: Theory, Modeling, and Experiment

This paper presents a transverse mode suppression theory and its experimental verification through aluminum nitride Lamb wave resonators (LWRs) operating at 142 MHz. An effective 2-D approximation model of the LWR is proposed, based on which the origin of transverse modes in LWR is investigated. The displacement distribution, resonant frequencies, and electromechanical coupling coefficients $( k_{t}^{2})$ of the main mode and its auxiliary transverse modes are obtained. A spurious mode suppression theory in terms of the expression of $k_{t}^{2}$ in the 2-D model is proposed. Three kinds of electrodes are designed to suppress the transverse mode adjacent to the main mode, including a novel interdigital transducer gap technique that is reported for the first time. With the applicable geometries, these methods reduce the spurious response from 11.8 to <0.5 dB, without significantly affecting the figure of merit of the resonator.

Lamb Wave AlN Micromechanical Filters Integrated With On-chip Capacitors for RF Front-End Architectures

This paper reports on the implementation of an ultraminiature 140 MHz narrowband filter based on aluminum nitride Lamb wave resonators. Monolithically integrated with a pair of on-chip capacitors and cascaded with a pair of inductors, the filter is well matched to 50 ohm, showing a remarkably high performance. A low pass-band insertion loss of 2.78 dB and steep filter skirts are achieved. The form factor of the monolithic microelectromechanical systems filter is more than ten times smaller than its surface acoustic wave counterpart in the intermediate frequency band and it involves much simpler matching circuits.

Design and fabrication of aluminum nitride Lamb wave resonators towards high figure of merit for intermediate frequency filter applications

A design guideline for one-port aluminum nitride (AlN) Lamb wave resonators (LWRs) working at S0 mode with high performance is reported. A fabricated 252 MHz LWR, with an aperture of 200 μm, 12 fingers, and 1.5 μm thick AlN, is found to have a remarkably high figure of merit (FOM), which exhibits a high ratio of the resistance at parallel resonance (Rp) to the resistance at series resonance (Rs) of 1317 and a corresponding product of the effective coupling coefficient (k2eff) and quality factor (Q) exceeding 52. Consisting of such resonators, a 6-stage ladder filter with a low pass-band insertion loss (IL) of 4.5 dB and steep filter skirts is achieved, offering significant advantage of size savings.

Micromachined One-Port Aluminum Nitride Lamb Wave Resonators Utilizing the Lowest-Order Symmetric Mode

The characteristics of one-port aluminum nitride (AlN) Lamb wave resonators utilizing the lowest-order symmetric mode with electrically open, grounded, and floating bottom electrode configurations are theoretically and experimentally investigated in this paper. Finite element analysis is performed to take an insight into the static capacitance characteristics of the AlN Lamb wave resonators with various bottom surface conditions. Without sacrificing the transduction efficiency, the floating bottom electrode is capable of reducing the static capacitance in the AlN thin plate and then promotes an efficient improvement in the effective coupling coefficient (k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ). In addition, in comparison with the grounded bottom electrode, the employment of the floating bottom electrode offers simple fabrication processes for the micromachined Lamb wave resonators. Experimentally, the AlN Lamb wave resonators without the bottom electrode exhibit an average loaded quality factor (Q) as high as 2676 at the series resonance frequency, but a low average k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> of 0.19%. On the contrary, the Lamb wave resonators with the electrically floating bottom electrode show the largest average k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> up to 1.13% among the three topologies but a low average loaded Q of 800 at the series resonance frequency. In contrast to the floating bottom electrode, the Lamb wave resonators with the electrically grounded bottom electrode show a smaller average k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> of 0.78% and a similar average loaded Q of 758 at the series resonance frequency.

High-Q aluminum nitride Lamb wave resonators with biconvex edges

A Lamb wave resonator utilizing an aluminum nitride (AlN) plate with biconvex edges to enhance the quality factor (Q) is demonstrated. The simulation results based on finite element analysis verify that the use of the biconvex edges, instead of the conventional flat edges, can efficiently confine mechanical energy in the AlN Lamb wave resonator. Specifically, the measured frequency response of a 491.8-MHz AlN Lamb wave resonator with biconvex edges yields a Q of 3280 which represents a 2.6× enhancement in Q over a 517.9-MHz Lamb wave resonator on the same AlN plate but with the suspended flat edges.

Thermally compensated aluminum nitride Lamb wave resonators for high temperature applications

In this letter, temperature compensation for aluminum nitride (AlN) Lamb wave resonators operating at high temperature is presented. By adding a compensating layer of silicon dioxide (SiO2), the turnover temperature can be designed for high temperature operation by varying the normalized AlN film thickness (hAlN/λ) and the normalized SiO2 film thickness (hSiO2/λ). With different designs of hAlN/λ and hSiO2/λ, the Lamb wave resonators were well temperature-compensated at 214 °C, 430 °C, and 542 °C, respectively. The experimental results demonstrate that the thermally compensated AlN Lamb wave resonators are promising for frequency control and sensing applications at high temperature.