A Three-Dimensional Finite Element Analysis Model of SAW Torque Sensor with Multilayer Structure

At a Glance

Metadata	Details
Publication Date	2022-03-29
Journal	Sensors
Authors	Zhipeng Li, Xu Meng, Bonan Wang, Chao Zhang
Institutions	Northeast Forestry University
Citations	8
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Frequency SAW Torque Sensors

Executive Summary

This research validates the use of diamond thin films in multi-layer Surface Acoustic Wave (SAW) structures to significantly enhance operating frequency and performance, positioning MPCVD diamond as a key enabling material for next-generation sensor technology.

High-Frequency Achievement: The multi-layer IDT/LiNbO₃/Diamond/Si(100) structure successfully boosted the characteristic frequency of the SAW resonator to approximately 960 MHz, far exceeding traditional 433 MHz quartz sensors.
Performance Metrics: The optimized structure achieved an exceptional Quality Factor ($Q_m$) of 8370 and a strong reflection coefficient ($S_{11}$) of -20.5 dB.
Material Optimization: The study identified optimal diamond film thickness ($h_{dia} = 0.4 \mu m$) and LiNbO₃ thickness ($h_{LN} = 1.2 \mu m$) to maximize the electromechanical coupling coefficient ($K^2$) at 0.105.
Mechanical Superiority: Finite Element Analysis (FEA) confirmed that the new sensor design reduced surface shear stress by 17% compared to traditional designs, improving stability and safety.
Application Relevance: This work directly supports the development of high-performance, high-frequency SAW devices required for emerging technologies, such as 5G communication and advanced automotive steering systems.
6CCVD Value Proposition: 6CCVD specializes in the precise growth and processing of the required thin-film diamond layers, offering custom thickness control and metalization services essential for replicating and advancing this research.

Technical Specifications

The following hard data points were extracted from the simulation results and material definitions:

Parameter	Value	Unit	Context
Characteristic Frequency	~960	MHz	High-frequency operation target
Resonant Frequency ($f_{M+}$)	966.2	MHz	Fitted linear equation result
Anti-Resonant Frequency ($f_{M-}$)	1002.7	MHz	Fitted linear equation result
Maximum Quality Factor ($Q_m$)	8370	-	Under initial structural parameters
Reflection Coefficient ($S_{11}$)	-20.5	dB	At 959.6 MHz
Maximum Coupling Coefficient ($K^2$)	0.105	-	Optimized value
Optimal Diamond Thickness ($h_{dia}$)	0.4	µm	For maximum $K^2$
Optimal LiNbO₃ Thickness ($h_{LN}$)	1.2	µm	For maximum $K^2$
Working Wavelength ($\lambda$)	4	µm	SAW device design parameter
Torque Measurement Range	$\pm 40$	Nm	Tested range for linearity
Substrate Material	Si(100)	-	Matrix layer
Piezoelectric Material	128° Y-X LiNbO₃	-	Piezoelectric layer
Diamond Density ($\rho$)	3515	kg/m³	Material parameter
Diamond Young’s Modulus (E)	105 $\times$ 10¹⁰	Pa	Material parameter

Key Methodologies

The research utilized a three-dimensional Finite Element Analysis (FEA) model to simulate the performance of the multi-layer SAW torque sensor.

Layered Structure Definition: A multi-layer stack was modeled: Interdigital Transducer (IDT, Aluminum)/Piezoelectric Layer (128° Y-X LiNbO₃)/Growth Layer (Diamond)/Matrix Layer (Si(100)).
Structural Parameter Variation: The simulation systematically varied the thickness of the two critical thin films:
- Diamond film thickness ($h_{dia}$): Ranged from 0.4 µm to 4.0 µm.
- LiNbO₃ film thickness ($h_{LN}$): Ranged from 0.4 µm to 4.0 µm.
Optimization Criteria: The primary goal was to analyze the effects of these thickness variations on the characteristic frequency, electromechanical coupling coefficient ($K^2$), and mechanical quality factor ($Q_m$).
Torque Sensor Modeling: A new 3D SAW torque sensor structure was established, suitable for small diameter torsion bars (10 mm), utilizing two SAW resonators placed at $\pm 45$° to the axis.
Boundary and Electrical Conditions:
- Mechanical conditions included Free, Fixed, and Periodic boundaries.
- Electrical conditions included Zero charge, Continuity, Ground, and $\pm 1$ V applied to the IDT electrodes.
Performance Testing: The model tested the sensor’s response to external torque ($\pm 40$ Nm), analyzing strain linearity and the resulting shift in resonant and anti-resonant frequencies.

6CCVD Solutions & Capabilities

The successful replication and advancement of this high-frequency SAW sensor technology rely critically on the quality and precision of the MPCVD diamond layer. 6CCVD is uniquely positioned to supply the required materials and engineering support.

Applicable Materials

The research requires a high-quality, high sound velocity diamond layer to function as the growth layer in the multi-layer stack.

Recommended Material: Optical Grade Single Crystal Diamond (SCD) or Electronic Grade Polycrystalline Diamond (PCD).
- Rationale: SCD offers the highest purity and lowest defect density, ensuring optimal acoustic wave propagation and minimal loss, which is essential for maximizing the Quality Factor ($Q_m$). PCD offers a cost-effective solution for large-scale production while maintaining high sound velocity.

Customization Potential

6CCVD’s core capabilities directly address the critical dimensional and material requirements identified in the FEA model.

Requirement from Research Paper	6CCVD Solution & Capability	Technical Advantage
Precise Diamond Film Thickness	SCD/PCD Thickness Control (0.1 µm to 500 µm)	The optimal $h_{dia}$ was found to be $0.4 \mu m$. 6CCVD guarantees sub-micron precision necessary for maximizing $K^2$ and tuning the characteristic frequency.
Large-Scale Production	PCD Plates up to 125 mm Diameter	Enables the transition from simulation to mass production, supporting the semiconductor planar process required for commercial viability.
IDT Metalization Stability	Custom Metalization Services (Ti, Pt, Au, W, Cu)	While the paper used Al IDTs, high-frequency operation generates heat. 6CCVD can deposit high-stability metal stacks (e.g., Ti/Pt/Au) to prevent IDT melting and ensure long-term reliability under high-power conditions.
Surface Quality for Deposition	Ultra-High Polishing (Ra < 1 nm for SCD)	An ultra-smooth diamond surface is crucial for the subsequent uniform deposition of the 128° Y-X LiNbO₃ piezoelectric layer, minimizing scattering losses and maximizing device performance.

Engineering Support

The optimization of multi-layer acoustic stacks is complex, involving trade-offs between film thickness, material properties, and operating frequency.

Specialized Consultation: 6CCVD’s in-house PhD team offers expert consultation on material selection and stack optimization for high-frequency acoustic applications, including SAW, BAW, and FBAR devices.
Application Focus: We provide engineering support to researchers and manufacturers working on similar high-frequency torque sensors and other 5G-compatible acoustic wave devices, helping to bridge the gap between simulation and experimental testing.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

A three-dimensional finite element analysis model of surface acoustic wave (SAW) torque sensor based on multilayer structure is proposed in this paper. Compared with the traditional saw torque sensor with quartz as piezoelectric substrate, the SAW torque sensor with multilayer structure has the advantages of fast propagation speed and high characteristic frequency. It is a very promising torque sensor, but there is very little related research. In order to successfully develop the sensor, it is essential to understand the propagation characteristics and torque sensing mode of SAW in multilayer structure. Therefore, in this study, we first established a multi-layered finite element analysis model of SAW device based on IDT/128° Y-X lithium niobate/diamond/Si (100). Then, the effects of different film thicknesses on the characteristic frequency, electromechanical coupling coefficient, s parameter, and mechanical quality factor of SAW device without changing the wavelength are analyzed. Then, based on the finite element analysis, a three-dimensional research model of a new SAW torque sensor suitable for small diameter torsion bar (d = 10 mm) is established, and the relationship between saw device deformation and torque under the condition of small torque (±40 Nm) is tested. The shape variable is introduced into the finite element analysis model of multi-layer SAW device. Finally, the relationship between saw torque sensor with multi-layer structure and torque is established by using the deformation relationship, which shows the perfect curve of sensor performance.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s22072600

References

2020 - Design and characterization of high-Q SAW resonators based on the AlN/Sapphire structure intended for high-temperature wireless sensor applications [Crossref]
2020 - A Sensitive Inexpensive SAW Sensor for Wide Range Humidity Measurement [Crossref]
2016 - Surface Acoustic Wave (SAW) Ammonia Gas Sensor Based on the ZnO Nanorod Array [Crossref]
2019 - Highly Sensitive Surface Acoustic Wave Flexible Strain Sensor [Crossref]
2018 - AlN/ZnO/LiNbO3 Packageless Structure as a Low-Profile Sensor for Potential On-Body Applications [Crossref]
2018 - Surface Acoustic Wave Viscosity Sensor with Integrated Microfluidics on a PCB Platform [Crossref]
2017 - GaN Membrane Supported SAW Pressure Sensors With Embedded Temperature Sensing Capability [Crossref]