Peculiarities of the Acoustic Wave Propagation in Diamond-Based Multilayer Piezoelectric Structures as “Me1/(Al,Sc)N/Me2/(100) Diamond/Me3” and “Me1/AlN/Me2/(100) Diamond/Me3” under Metal Thin-Film Deposition
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-01-07 |
| Journal | Electronics |
| Authors | Г. М. Квашнин, B. P. Sorokin, Nikita O. Asafiev, V.M. Prokhorov, A. V. Sotnikov |
| Institutions | Leibniz Institute for Solid State and Materials Research, Moscow Institute of Physics and Technology |
| Citations | 8 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research details the development and analysis of diamond-based High Overtone Bulk Acoustic Resonators (HBARs) for use as ultra-sensitive microwave acoustic sensors.
- Core Value Proposition: Creation of a robust, fifth-layered multilayer piezoelectric structure (MPS) utilizing single-crystalline diamond substrates, enabling high-sensitivity mass sensing in the microwave frequency band (1 GHz up to 20 GHz).
- Structure and Materials: The MPS configurations studied were “Me1/(Al,Sc)N/Me2/(100) diamond/Me3” and “Me1/AlN/Me2/(100) diamond/Me3,” where Me3 is the deposited sensing film (Sc, Mo, or Pt).
- High Q-Factor Retention: The diamond substrate ensures exceptionally low acoustic attenuation, maintaining a high quality factor (Q) of up to 12,500 at 20 GHz, even after metal film deposition.
- Sensitivity Mechanism: Sensor sensitivity (frequency shift ∆f/f) is highly dependent on the ratio of the acoustic impedance of the deposited film (ZMe3) relative to the diamond substrate (Zdiam).
- Optimal Sensitivity Conditions:
- For ZMe3 < Zdiam (e.g., Sc film), maximum sensitivity occurs when the film thickness (hMe3) is approximately an odd multiple of a quarter wavelength (nλ/4).
- For ZMe3 > Zdiam (e.g., Pt film), maximum sensitivity occurs when hMe3 is approximately a half wavelength (mλ/2).
- Modeling Validation: Finite Element Method (FEM) simulations using COMSOL Multiphysics showed satisfactory agreement with experimental data regarding resonant frequency shifts and Q-factor changes.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operational Frequency Range | 1 to 20 | GHz | HBAR sensing band (up to 40 GHz demonstrated previously) |
| Maximum Quality Factor (Q) | ~12,500 | Dimensionless | Observed at ~20 GHz (diamond substrate) |
| Substrate Material | Type IIa Synthetic Diamond | Single Crystal | (100) orientation |
| Piezoelectric Layer (AlN) | 930 to 1120 | nm | Thickness range for AlN (0% Sc) |
| Piezoelectric Layer (AlScN) | 1013 | nm | Thickness for Sensor #C (13% Sc content) |
| Diamond L-BAW Velocity | 17,542 | m/s | In [100] direction |
| Diamond Density (ρ) | 3.5 | g/cm3 | Substrate material |
| Diamond Acoustic Impedance (Zdiam) | 61.7 x 106 | kg/m2·s | Reference impedance |
| Sc Acoustic Impedance (ZSc) | 16.74 x 106 | kg/m2·s | ZSc < Zdiam (Low impedance film) |
| Mo Acoustic Impedance (ZMo) | 63 x 106 | kg/m2·s | ZMo ≈ Zdiam (Matched impedance film) |
| Pt Acoustic Impedance (ZPt) | 84.7 x 106 | kg/m2·s | ZPt > Zdiam (High impedance film) |
| Film Thickness Measurement Uncertainty | 3 to 10 | nm | Using Atomic Force Microscopy (AFM) |
| Mean Film Growth Rate | ~100 | nm/h | During magnetron sputtering |
Key Methodologies
Section titled “Key Methodologies”The HBAR sensors were fabricated and tested using a combination of thin-film deposition and high-frequency acoustic characterization.
- Substrate Preparation: Plates of Type IIa synthetic single-crystalline diamond were used as the acoustic substrate.
- HBAR Fabrication (Me1/Piezo/Me2):
- Thin-film piezoelectric transducers (TFPT) were deposited, consisting of AlN or Al1-xScxN (up to 15% Sc).
- Metallic electrodes (Me1, Me2) of Al, Mo, or Pt were deposited.
- Deposition was performed using AJA Orion 8 magnetron sputtering equipment.
- Me3 Film Deposition (Sensing Layer):
- Films of Sc, Mo, or Pt (Me3) were deposited onto the free side of the diamond substrate.
- Substrate holder rotation speed was 1 rad/min (increased to 2 rad/min for layers < 20 nm) to ensure thickness uniformity (< 5% unevenness).
- Thickness Control and Measurement:
- Film thickness during sputtering was monitored using a Quartz Crystal Microbalance (QCM) sensor.
- Precise thickness measurement was verified post-deposition using Atomic Force Microscopy (AFM) on accompanying Si samples.
- Acoustic Characterization:
- The frequency response of the HBAR was measured using an E5071C Agilent network analyzer.
- Operational checkpoint frequencies were selected based on overtones exhibiting the highest Q-factor or highest impedance response.
- Relative frequency shift (∆f/f) and Q-factor were recorded as a function of deposited film thickness (h).
- Modeling and Simulation:
- Acoustic wave propagation was simulated using the Finite Element Method (FEM) via COMSOL Multiphysics software.
- 2D MPS models (10 µm width) were used with periodic boundary conditions to visualize the L-BAW vertical Y-component of elastic displacements.
Commercial Applications
Section titled “Commercial Applications”The unique properties of diamond-based HBARs—specifically their high operating frequency, high Q-factor, and material robustness—make them highly prospective for next-generation sensor platforms.
| Application Area | Technical Advantage | Specific Use Cases |
|---|---|---|
| Ultra-Sensitive Mass Sensing | Operation in the microwave band (up to 20 GHz) significantly enhances gravimetric sensitivity compared to lower-frequency SAW or QCM devices. | Detection of single gas molecules (ppb level), biological agents (viruses, proteins), and explosives. |
| Harsh Environment Monitoring | Diamond’s chemical inertness, resistance to temperature load, and abrasive wear resistance. | Sensors for high-temperature industrial processes, chemical analysis in corrosive media, and aerospace applications. |
| Fine Physicochemical Studies | High Q-factor allows for precise measurement of acoustic attenuation and velocity changes in deposited films. | Spectroscopy of metal film properties, studying solid-liquid interfacial layers, and monitoring oxidation processes in situ. |
| Multipurpose Sensor Platforms | The five-layered structure provides a prospective platform for developing sensors capable of multiple applications (e.g., mass, temperature, and pressure sensing). | Integrated acousto-electronic devices for complex atmospheric or biological analysis. |
| High-Frequency RF Components | Utilization of high-quality piezoelectric films (AlScN) on diamond allows for effective electromechanical excitation of L-BAW at extremely high frequencies. | Development of stable, high-frequency filters and oscillators for 5G/6G communication systems. |
View Original Abstract
New theoretical and experimental results of microwave acoustic wave propagation in diamond-based multilayer piezoelectric structures (MPS) as “Me1/(Al,Sc)N/Me2/(100) diamond/Me3” and “Me1/AlN/Me2/(100) diamond/Me3” under three metal film depositions, including the change in the quality factor Q as a result of Me3 impact, were obtained. Further development of our earlier studies was motivated by the necessity of creating a sensor model based on the above fifth layered MPS and its in-depth study using the finite element method (FEM). Experimental results on the change in operational checkpoint frequencies and quality factors under the effect of film deposition are in satisfactory accordance with FEM data. The relatively small decrease in the quality factor of diamond-based high overtone bulk acoustic resonator (HBAR) under the metal layer effect observed in a wide microwave band could be qualified as an important result. Changes in operational resonant frequencies vs. film thickness were found to have sufficient distinctions. This fact can be quite explained in terms of the difference between acoustic impedances of diamond and deposited metal films.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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