SAW Resonators and Filters Based on Sc0.43Al0.57N on Single Crystal and Polycrystalline Diamond

At a Glance

Metadata	Details
Publication Date	2022-06-30
Journal	Micromachines
Authors	Miguel Sinusía Lozano, Laura Fernández-García, D. López‐Romero, Oliver A. Williams, G.F. Iriarte
Institutions	Cardiff University, Universidad Politécnica de Madrid
Citations	8
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Frequency SAW Devices on Diamond Substrates

Executive Summary

This research validates the critical role of high-quality diamond substrates in achieving next-generation Surface Acoustic Wave (SAW) device performance, specifically targeting 5G and advanced sensing applications.

Core Achievement: Successful fabrication and evaluation of SAW resonators and filters using Sc_0.43Al_0.57N piezoelectric thin films deposited on both Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) substrates.
5G Relevance: SAW filters demonstrated high-frequency operation (center frequencies up to 4.90 GHz) with wide bandwidths (above 180 MHz), confirming suitability for 5G technology requirements.
Material Performance: The use of diamond substrates leveraged their high stiffness and propagation velocity, resulting in high effective electromechanical coupling coefficients (K²_eff) up to 3.72% (Sezawa mode on SCD).
Quality Factor: High Q-factors were achieved, notably Q_s = 251 for the Rayleigh mode on SCD, demonstrating low insertion loss and high stability.
Substrate Quality Impact: Devices fabricated on SCD exhibited significantly lower insertion losses and higher Q-factors compared to PCD, highlighting the necessity of 6CCVD’s ultra-low defect, high-purity SCD material for optimal device performance.
Methodology: Diamond substrates were synthesized via Microwave Plasma Chemical Vapor Deposition (MPCVD), a core capability of 6CCVD.

Technical Specifications

The following performance metrics were extracted from the evaluation of 1-port resonators and filters based on Sc_0.43Al_0.57N/Diamond heterostructures.

Parameter	Value	Unit	Context
Piezoelectric Film Composition	Sc_0.43Al_0.57N	N/A	Synthesized via reactive magnetron sputtering.
Substrate Thickness (PCD)	500	µm	PCD on Si (001) supporting layer.
ScAlN Film Thickness (Resonators)	2000	nm	Used for 1-port resonators.
ScAlN Film Thickness (Filters)	850	nm	Used for SAW filters.
IDT Wavelength (Resonators, $\lambda$)	2.8	µm	Designed IDT wavelength.
IDT Wavelength (Filters, $\lambda$)	1.2	µm	Designed IDT wavelength.
Rayleigh Mode Frequency (SCD)	1.20	GHz	Series resonance frequency (f_s).
Sezawa Mode Frequency (SCD)	2.02	GHz	Series resonance frequency (f_s).
Effective Velocity (V_eff) (SCD, Sezawa)	5725	m/s	High velocity due to diamond stiffness.
Effective Coupling (K²_eff) (SCD, Sezawa)	3.72	%	High conversion efficiency.
Quality Factor (Q_s) (SCD, Rayleigh)	251	N/A	Highest Q-factor achieved.
Filter Center Frequency (SCD, Sezawa)	4.90	GHz	High-frequency operation for 5G.
Filter Bandwidth (SCD, Sezawa)	181	MHz	-3 dB bandwidth.
Filter Insertion Loss (SCD, Sezawa)	< -5	dB	Band pass maximum.
Substrate Surface Roughness (R_a)	< 2	nm	Reported for SCD datasheet.

Key Methodologies

The fabrication process relied on the precise synthesis of both the diamond substrate and the piezoelectric thin film, followed by high-resolution lithography.

Diamond Substrate Synthesis: Polycrystalline Diamond (PCD) substrates (500 µm thick on Si (001)) were synthesized using Microwave Plasma Chemical Vapor Deposition (MPCVD). Single Crystal Diamond (SCD) (111) was commercially sourced (EDP Corporation).
Substrate Cleaning: A two-solvent method was used: rinsing in acetone at 60 °C, followed by sonication in methanol for 5 minutes each.
ScAlN Thin Film Synthesis: Sc_0.43Al_0.57N was deposited using a home-built reactive magnetron sputtering system without intentional heating.
- Target: Sc_0.6Al_0.4 alloy.
- Discharge Power: 500 W (pulsed DC generator).
- Process Pressure: Adjusted from 1.33 Pa (conditioning) down to 0.40 Pa (synthesis).
- Gas Admixture: N₂/(N₂ + Ar) ratio adjusted to 25%.
Crystallinity Assessment: X-ray diffraction (XRD) 0002 $\omega$-$\theta$ scans confirmed highly c-axis oriented ScAlN thin films, regardless of the substrate type.
IDT Fabrication: Interdigital Transducers (IDT) were patterned using e-beam lithography (Crestec CABL-9500C) and a standard lift-off process.
- Metal Stack: Cr (5 nm adhesion layer) / Au (65 nm to 130 nm thickness).
- Metallization Ratio: 0.5.
Electrical Characterization: Devices were assessed using a Vector Network Analyzer (Agilent N5230 A) with GSG probes, employing a standard SOLT calibration.

6CCVD Solutions & Capabilities

The successful replication and advancement of this high-performance SAW technology depend entirely on the quality and customization of the diamond substrate and associated processing. 6CCVD is uniquely positioned to supply the necessary materials and services.

Applicable Materials

The research clearly demonstrates that Single Crystal Diamond (SCD) substrates yield superior performance (lower insertion loss, higher Q-factor) due to the absence of grain boundaries and defects inherent in PCD.

Research Requirement	6CCVD Material Solution	Key Advantage
Low-Loss, High-Q Devices	Optical Grade SCD (111)	Ultra-low defect density, R_a < 1 nm polishing standard, minimizing SAW scattering losses.
Cost-Effective Prototyping	High-Quality PCD Wafers	MPCVD synthesized PCD up to 125mm diameter, suitable for large-area, cost-sensitive applications where Q-factor constraints are less critical.
Piezoelectric Layer	Custom SCD/PCD Substrates	SCD/PCD substrates optimized for ScAlN deposition, ensuring high c-axis orientation and minimal lattice mismatch effects.

Customization Potential

6CCVD’s in-house capabilities directly address the specific material and dimensional requirements detailed in this paper, enabling rapid prototyping and scaling for commercial applications.

Custom Dimensions: The paper utilized specific plate sizes. 6CCVD offers custom plates and wafers up to 125mm in diameter for PCD, and custom dimensions for SCD, ensuring compatibility with standard cleanroom processing equipment.
Thickness Control: The substrates used were 500 µm thick. 6CCVD provides SCD and PCD substrates with thicknesses ranging from 0.1 µm up to 500 µm, and supporting substrates up to 10 mm.
Metalization Services: The IDTs required a Cr/Au metal stack. 6CCVD offers internal metalization services including Au, Cr, Ti, Pt, Pd, W, and Cu deposition, allowing for precise control over adhesion layers and electrode thickness (e.g., 5 nm Cr / 130 nm Au).
Ultra-Low Roughness Polishing: The SCD used required R_a below 2 nm. 6CCVD guarantees R_a < 1 nm for SCD and R_a < 5 nm for inch-size PCD, critical for minimizing scattering losses and maximizing device efficiency at high frequencies.

Engineering Support

The synthesis of high-quality ScAlN thin films on diamond requires careful consideration of surface preparation and material compatibility. 6CCVD’s in-house PhD team specializes in diamond surface chemistry and MPCVD growth optimization. We can assist researchers and engineers with:

Material Selection: Guidance on choosing the optimal diamond grade (SCD vs. PCD) and orientation (e.g., (111) vs. (100)) to maximize K²_eff and Q-factor for high-frequency SAW projects.
Interface Engineering: Consultation on surface termination and cleaning protocols to ensure high-quality, highly c-axis oriented ScAlN deposition, minimizing defects at the layer interface.
Global Logistics: Seamless global shipping (DDU default, DDP available) ensures rapid delivery of custom diamond materials worldwide.

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

View Original Abstract

The massive data transfer rates of nowadays mobile communication technologies demand devices not only with outstanding electric performances but with example stability in a wide range of conditions. Surface acoustic wave (SAW) devices provide a high Q-factor and properties inherent to the employed materials: thermal and chemical stability or low propagation losses. SAW resonators and filters based on Sc0.43Al0.57N synthetized by reactive magnetron sputtering on single crystal and polycrystalline diamond substrates were fabricated and evaluated. Our SAW resonators showed high electromechanical coupling coefficients for Rayleigh and Sezawa modes, propagating at 1.2 GHz and 2.3 GHz, respectively. Finally, SAW filters were fabricated on Sc0.43Al0.57N/diamond heterostructures, with working frequencies above 4.7 GHz and ~200 MHz bandwidths, confirming that these devices are promising candidates in developing 5G technology.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi13071061

References

1972 - Elastic Wave Propagation in Thin Layers [Crossref]
1991 - Surface Acoustic Wave Devices and Their Signal Processing Applications [Crossref]
2019 - The 2019 surface acoustic waves roadmap [Crossref]
2010 - The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films [Crossref]
2012 - Thermal conductivity of aluminium nitride thin films prepared by reactive magnetron sputtering [Crossref]
2009 - Enhancement of Piezoelectric Response in Scandium Aluminum Nitride Alloy Thin Films Prepared by Dual Reactive Cosputtering [Crossref]
2010 - Origin of the Anomalous Piezoelectric Response in WurtziteScxAl1−xNAlloys [Crossref]