High-Frequency Surface Acoustic Wave Resonator with Diamond/AlN/IDT/AlN/Diamond Multilayer Structure

At a Glance

Metadata	Details
Publication Date	2022-08-28
Journal	Sensors
Authors	Lei Liang, Bo Dong, Yuxuan Hu, Yisong Lei, Zhizhong Wang
Institutions	Shenzhen Technology University
Citations	15
Analysis	Full AI Review Included

Executive Summary

This research presents a novel, high-performance Surface Acoustic Wave (SAW) resonator utilizing a multilayer structure with a sandwiched Interdigital Transducer (IDT). The design, Diamond/AlN/IDT/AlN/Diamond on a Si substrate, is optimized via Finite Element Method (FEM) simulation to achieve superior frequency and coupling characteristics compared to traditional IDT-free surface structures.

Novel Structure: The IDT is embedded (sandwiched) within two layers of Aluminum Nitride (AlN) and capped by two layers of high-velocity Diamond film, all resting on a Silicon (Si) substrate.
Ultra-High Frequency: The optimized structure successfully excites the higher-order M2 mode (Love wave), achieving an operational resonance frequency ($f_r$) of 6.15 GHz.
High Phase Velocity: The phase velocity ($v_p$) is significantly enhanced, reaching 12,470 m/s, which is crucial for high-frequency operation and miniaturization.
Strong Coupling: The electromechanical coupling coefficient ($K^{2}$) is high at 5.53%, enabling wider bandwidth and improved acoustoelectric conversion efficiency.
Thermal Stability: The device exhibits excellent temperature stability, characterized by a Temperature Coefficient of Frequency (TCF) of -6.3 ppm/°C.
Performance Gain: This design offers substantial improvements over the traditional IDT/AlN/diamond/Si structure (which yielded $f_r$ = 1.26 GHz, $v_p$ = 7,620 m/s, and $K^{2}$ = 1.55%).

Technical Specifications

The following specifications are based on the optimized M2 mode (Love wave) simulation results for the Diamond/AlN/IDT/AlN/Diamond structure.

Parameter	Value	Unit	Context
Operation Frequency ($f_r$)	6.15	GHz	Optimized M2 mode resonance
Antiresonance Frequency ($f_{ar}$)	6.32	GHz	Optimized M2 mode
Electromechanical Coupling ($K^{2}$)	5.53	%	Optimized M2 mode performance
Phase Velocity ($v_p$)	12,470	m/s	Optimized M2 mode
Temperature Coefficient of Frequency (TCF)	-6.3	ppm/°C	Measured between 25 °C and 40 °C
Wavelength (λ)	2.0	µm	IDT periodicity used in simulation
AlN Layer Thickness ($h_{AlN}$)	1.8 (0.9λ)	µm	Optimized piezoelectric layer thickness
Diamond Layer Thickness ($h_{Dia}$)	1.0 (0.5λ)	µm	High sound speed layer thickness
Al Electrode Thickness ($h_{Al}$)	0.15 (0.075λ)	µm	Optimized metal thickness
Substrate Material	Si	N/A	Base material
Piezoelectric Material Orientation	AlN (002)	N/A	Euler angles (0°, 0°, 0°)
Comparison $K^{2}$ (Traditional M1)	1.55	%	Traditional IDT/AlN/diamond/Si structure
Comparison $v_p$ (Traditional M1)	7,620	m/s	Traditional IDT/AlN/diamond/Si structure

Key Methodologies

The SAW resonator characteristics were determined using the Finite Element Method (FEM) via COMSOL Multiphysics software.

Modeling Environment: The structural mechanics module was used to solve the coupled piezoelectric constitutive relations and the general partial differential equation for the quasi-static state.
Structure Definition: A unit cell model was established, representing the Diamond/AlN/IDT/AlN/Diamond stack on a Si substrate. Aluminum (Al) was selected for the IDT electrode material.
Boundary Conditions (Mechanical):
- A Perfect Matching Layer (PML) with a thickness of 0.5λ was added to the bottom of the Si substrate to absorb wave reflections.
- The bottom boundary (PML) was set as a fixed constraint.
- Periodic boundary conditions were applied to the front, back, left, and right boundaries to simulate an infinite, uniform IDT array.
Boundary Conditions (Electrical):
- One IDT electrode ($\Gamma_6$) was set to 1 V.
- The adjacent electrode ($\Gamma_7$) and the Si substrate ($\Gamma_5$) were connected to ground.
Material Parameters: Published elastic constants, piezoelectric constants, relative dielectric constants, and their temperature coefficients for AlN, Diamond, Al, and Si were incorporated, including mechanical and dielectric losses ($\eta_{CE}$ and $\eta_{\epsilon S}$) set at 0.010 for AlN.
Geometric Optimization: The ratios $h_{AlN}/\lambda$ and $h_{Al}/\lambda$ were systematically varied to identify the optimal geometry that maximized $K^{2}$ and $v_p$ while maintaining low dispersion, focusing specifically on the higher-order M2 mode.
Performance Metrics: Phase velocity ($v_p$) and electromechanical coupling coefficient ($K^{2}$) were calculated from the simulated resonant ($f_r$) and anti-resonant ($f_{ar}$) frequencies. TCF was derived by comparing $f_r$ at 25 °C and 40 °C.

Commercial Applications

The high frequency, high phase velocity, and excellent thermal stability achieved by this sandwiched multilayer structure make it highly valuable for advanced electronic and sensing applications.

5G/6G Wireless Communication: The 6.15 GHz operating frequency is directly applicable to high-frequency RF filters and duplexers required for modern cellular and broadband communication systems.
High-Performance RF Front-Ends: The high electromechanical coupling coefficient ($K^{2}$ = 5.53%) allows for the design of wide-bandwidth filters, essential for minimizing signal loss and improving data throughput in mobile devices.
Thermal Management in RF Devices: Utilizing diamond, which has thermal conductivity 2.5-3 times greater than traditional materials, significantly improves heat dissipation, enhancing the power handling and reliability of high-power RF components.
Stable SAW Sensors: The low TCF (-6.3 ppm/°C) ensures frequency stability across temperature variations, making the device suitable for highly reliable wireless sensors (e.g., pressure, chemical, and biological) operating in industrial or automotive environments.
Acoustofluidics and Micro-Manipulation: The high phase velocity and frequency enable precise, high-speed acoustic streaming and particle manipulation in miniaturized lab-on-chip systems.

View Original Abstract

A high-frequency surface acoustic wave (SAW) resonator, based on sandwiched interdigital transducer (IDT), is presented. The resonator has the structure of diamond/AlN/IDT/AlN/diamond, with Si as the substrate. The results show that its phase velocity and electromechanical coupling coefficient are both significantly improved, compared with that of the traditional interdigital transduce-free surface structure. The M2 mode of the sandwiched structure can excite an operation frequency up to 6.15 GHz, with an electromechanical coupling coefficient of 5.53%, phase velocity of 12,470 m/s, and temperature coefficient of frequency of −6.3 ppm/°C. This structure provides a new ideal for the design of high-performance and high-frequency SAW devices.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s22176479

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