Jie Zou

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 (k2eff). 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 k2eff of 0.19%. On the contrary, the Lamb wave resonators with the electrically floating bottom electrode show the largest average k2eff 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 k2eff of 0.78% and a similar average loaded Q of 758 at the series resonance frequency.

Theoretical study of thermally stable SiO2/AlN/SiO2 Lamb wave resonators at high temperatures

Aluminum nitride (AlN) and silicon dioxide (SiO2) bilayer structure has been widely utilized in temperature-compensated micromechanical resonators as SiO2 has unique positive temperature coefficients of elasticity. However, the thermal expansion mismatch would cause large bending deformation and stress distribution in the resonant plate. In this study, a symmetrical SiO2/AlN/SiO2 sandwiched structure is proposed to reduce the temperature-induced deformation in the asymmetrical AlN/SiO2 bilayer plate. The thermal compensation at high temperatures for the Lamb wave resonators utilizing the lowest-order symmetric (S0) mode in the SiO2/AlN/SiO2 sandwiched structure is theoretically investigated herein. While operation temperature rises from room temperature to 600 °C, the temperature-induced bending deformation in the symmetrical SiO2/AlN/SiO2 composite plate is much less than that in the AlN/SiO2 composite plate conventionally used for temperature compensation. Furthermore, the different material properties ...

Quality factor enhancement in Lamb wave resonators utilizing butterfly-shaped AlN plates

A novel technique to enhance the quality factor (Q) of Lamb wave resonator by utilizing an aluminum nitride (AlN) plate formed in a butterfly shape is investigated in this paper. In the conventional design, the Qs of the micromachined Lamb wave resonators are largely harmed by the energy dissipation through the support tethers. The finite element analysis (FEA) simulation results show that the butterfly-shaped topology can efficiently change the displacement field in the AlN plate and reduce the vibration in the support tethers. The unloaded Q of the resonator is raised from 3,360 to 4,758 by simply using of the butterfly-shaped AlN plate with a tether-to-plate angle α = 59°, representing a 1.42× increase. The experimental Qs are also in good agreement with the anchor loss Qs computed using the PML-based FEA method.

Transducer design for AlN Lamb wave resonators

AlN Lamb wave resonators enjoy advanced and attractive properties for enabling the next-generation single-chip radio frequency front-end, but their moderate effective electromechanical coupling coefficient (k2eff) poses a limit to their application in filters and multiplexers. Despite the fact that the reported k2eff enhancement techniques of doped AlN thin films which are expensive and trade off the quality factor (Q), the transducer topology itself extensively impacts the k2eff value. Although an AlN cross-sectional Lame mode resonator exhibiting a k2eff of 6.34% has been demonstrated without the need for changing the piezoelectric material, a detailed study of transducer design for AlN Lamb wave resonators has not been conducted. In this work, we investigate the impact of (i) transducer configurations, (ii) electrode materials, (iii) electrode thicknesses, and (iv) interdigital transducer duty factors on the k2eff dispersive characteristics of one-port AlN Lamb wave resonators by using the finite eleme...

High-frequency and low-resonance-impedance lamb wave resonators utilizing the S 1 mode

In this work, a high-frequency (f_s) and low-resonance-impedance (Z_min) Lamb wave resonator utilizing the first-order symmetric (S₁) mode propagating in highly textured aluminum nitride (AlN) plates is presented. In order to achieve the larger electromechanical coupling coefficient and smooth phase velocity dispersion as well as to avoid the negative group velocity in the S₁ Lamb wave mode, the 4-μm-thick AlN layer and 3-μm-wide finger electrodes are employed in the Lamb wave resonator design. The experimental results show that despite the S₁ mode shows a slightly lower quality factor than the lowest-order symmetric (S₀) mode, the S₁ mode presents a Z_min of 94

Thermally stable SiO 2 /AlN/SiO 2 Lamb wave resonators utilizing the lowest-order symmetric mode at high temperatures

Thermal compensation at high temperatures for Lamb wave resonators utilizing the lowest symmetric (S₀) mode in a SiO₂/AlN/SiO₂ sandwiched structure is theoretically investigated in this work. When temperature raises from room temperature to 600°C, the deformation and displacement caused by the thermal expansion mismatch in the SiO₂/AlN/SiO₂ symmetric composite plate is much less than a conventional temperature compensation plate, i.e. the AlN/SiO₂ composite plate. Acoustic characteristics of the SiO₂/AlN/SiO₂ symmetric membrane with three main electrode configurations are investigated, exhibiting higher phase velocity and larger electromechanical coupling coefficient than the common AlN/SiO₂ layered structure since the symmetric sandwiched plate traps more acoustic waves in the AlN layer. In addition, with proper thicknesses of the AlN and SiO₂ layers, the S₀ mode can simultaneously achieve a turnover temperature at high temperatures and a large intrinsic k² in the SiO₂/AlN/SiO₂ sandwiched structure.

Anchor loss suppression using butterfly-shaped plates for AlN Lamb wave resonators

The use of butterfly-shaped thin plates, formed by reducing the tether-to-plate angle, can raised the quality factor (Q) of aluminum nitride (AlN) Lamb wave resonators (LWRs) by eliminating the anchor loss. The finite element analysis (FEA) simulation results show that the butterfly-shaped plate can efficiently keep the vibration far from the edges at the tether-to-plate plane, so that the acoustic wave leaky through the supporting tethers is reduced. Specifically, the rounded butterfly-shaped resonators show more efficient suppression in the anchor loss compared to the beveled butterfly-shaped resonators. The measured frequency response for a 863-MHz AlN LWR with 45° beveled tether-to-plate transition yields a Q of 1,979 which upwards 30% over a conventional rectangular resonator; another AlN LWR on the butterfly-shaped plate with rounded tether-to-plate transition yields a Q of 2,531, representing a 67% improvement.

Temperature compensation of the AlN Lamb wave resonators utilizing the S 1 mode

The temperature compensation techniques for the first-order symmetric (S₁) Lamb wave mode in the AlN Lamb wave resonators are firstly investigated in this paper. The S₁ mode simultaneously offers very high phase velocity (ν_p) and large coupling coefficient (k²) when h_AlN/λ is smaller than 0.4, but its thermal stability needs further improvement. The AlN/SiO₂ bilayer and SiO₂/AlN/SiO₂ sandwiched temperature compensation structures are investigated and compared in this study. The SiO₂/AlN/SiO₂ symmetric structure shows higher v_p and larger k² than the lowest-order quasi-symmetric (QS₁) mode traveling in the AlN/SiO₂ bilayer structure because the symmetric structure trapes more acoustic energy inside the AlN piezoelectric layer. Despite the trade-off between first-order temperature coefficient of frequency (TCF) and k², the SiO₂/AlN/SiO₂ structure can provide large k² and near-zero TCF at the same time with proper thickness selection of AlN and SiO₂. The temperature-compensated resonator utilizing the S₁ mode in the symmetrical SiO₂/AlN/SiO₂ sandwiched membrane can simultaneously offer excellent thermal compensation, and large k² at super-high frequency.

High-Q piezoelectric Lamb wave resonators based on AlN plates with chamfered corners

A novel approach to the boost quality factor (Q) of Lamb wave resonators by chamfering the aluminum nitride (AlN) plate is investigated for the first time. It is well-known that the Qs of the AlN Lamb wave resonators are degraded due to energy dissipation through the support tethers. In this work, similar to the beveled edges used in AT-cut quartz resonators, the chamfered corners are utilized to trap vibration energy in the AlN plate to enhance the anchor Qs. Based on finite element analysis (FEA) simulated results, the AlN plate with chamfered corners can efficiently reduce mechanical vibrations in support tethers and trap more mechanical energy in the plate. The experimental results demonstrate that the loaded Q of the Lamb wave resonator is boosted from 2,041 to 3,016 and the minimum impedance is reduced from 87.4 O to 61.6 O by simply chamfering the AlN rectangular plate, showing a 1.48× increase in measured Qs.

Transverse mode suppression in the AlN lamb wave resonators by “piston mode”

Type I Lamb wave modes exhibit a strong affinity toward multimode behavior, especially the high-transduction-efficiency modes: S0 and S1 mode. Apodization, the standard technique to suppress the transverse modes for IDT-excited resonators, suffers from drawbacks such as additional loss and reduction of the effective coupling coefficient (k2eff). Most Lamb wave modes in AlN show a positive slope in the dispersion branch, so that a border region of lower eigenresonance frequency is required for spurious mode suppression. Based on dispersion calculations and finite element method (FEM) simulations, we demonstrate that by changing the transducer layout, the guiding can be improved and a “piston mode” can be obtained for the type I Lamb wave modes.