Development of Minimal RF-magnetron Sputtering Machine

At a Glance

Metadata	Details
Publication Date	2023-01-01
Authors	Satoshi Fujii, Yuki Odo, Hiroshi Nishizato
Institutions	Toyohashi University of Technology, Horiba (Japan)
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond SAW Resonators

Executive Summary

This document analyzes the development of a minimal RF-magnetron sputtering system used to fabricate high-frequency Surface Acoustic Wave (SAW) resonators on Single Crystal Diamond (SCD) substrates.

Application Focus: Development of high-performance filters for 5G/6G mobile communication systems, utilizing the high sound velocity of diamond.
Material Stack: Aluminum Nitride (AlN) piezoelectric thin film deposited via sputtering onto a Single Crystal Diamond (100) substrate.
Key Achievement: Successful fabrication and evaluation of a 5.2 GHz one-port diamond SAW resonator.
Film Quality: Optimized AlN films achieved a c-axis rocking curve FWHM of 2.66° and thickness uniformity of ± 0.95%.
Performance Metrics: The device yielded a Q-value of 1737 and an electromechanical coupling coefficient (K²) of 0.89%.
Critical Limitation: The measured K² was 74% lower than the theoretical value, primarily attributed to insufficient AlN orientation and defects caused by the lack of a dedicated substrate heating mechanism.
6CCVD Value Proposition: 6CCVD provides the high-quality, low-defect SCD substrates and custom metalization required to overcome the identified performance limitations and enable optimal AlN film growth.

Technical Specifications

Parameter	Value	Unit	Context
Resonant Frequency (f_r)	5.2	GHz	Diamond SAW Resonator
Electromechanical Coupling (K²)	0.89	%	Measured performance
Q-Value	1737	-	Measured performance
SAW Phase Velocity	11000	m/s	Theoretical/Design (Sezawa mode)
AlN Film Thickness	0.70	µm	Piezoelectric layer
Al Electrode Thickness	90	nm	Interdigital Transducer (IDT)
SAW Wavelength (λ)	2.0	µm	IDT Design
AlN Rocking Curve FWHM	2.66	°	Optimized film quality on Si(001)
AlN Thickness Uniformity	± 0.95	%	Optimized film quality
Optimized RF Power	100	W	Sputtering condition
Optimized Process Pressure	3	mTorr	Sputtering condition
Substrate Temperature	210	°C	Plasma heating only (not dedicated)

Key Methodologies

The study focused on optimizing the sputtering process for AlN thin films and subsequent device fabrication on diamond substrates.

Sputtering System: A minimal RF-magnetron sputtering system (13.56 MHz RF plasma) was developed within a standard minimal fab enclosure (144 cm high, 30 cm wide).
Target Material: A one-inch pure 4N Aluminum (Al) or Al-Sc alloy target was used.
Substrate Preparation: Half-inch Single Crystal Diamond (100) substrates were used for device fabrication; Si(001) substrates were used for initial film optimization.
Vacuum Conditions: Achievable vacuum was approximately 1×10^-5 Pa.
Optimized Sputtering Recipe:
- Process Gas Mixture: 30% N₂/Ar.
- RF Power: 100 W.
- Process Pressure: 3 mTorr.
- Target-Substrate Distance (TSD): 730 mm.
- Deposition Time: 1560 sec.
- Substrate Temperature: 210 °C (achieved via plasma heating only).
Crystallinity Evaluation: X-ray diffraction (XRD) was used to determine the c-axis orientation of the AlN film, yielding an optimized FWHM of 2.66°.
Device Fabrication: Aluminum (Al) electrodes (90 nm thick) were formed on the AlN/Diamond stack using Electron Beam (EB) lithography to create a one-port 5 GHz SAW resonator.

6CCVD Solutions & Capabilities

6CCVD specializes in providing the high-quality MPCVD diamond materials and custom processing required to advance high-frequency acoustic and RF-MEMS research, directly addressing the limitations identified in this study (insufficient crystallinity and electrode defects).

Applicable Materials for Replication and Extension

To replicate and extend this high-frequency SAW research, the highest quality Single Crystal Diamond (SCD) is essential.

Research Requirement	6CCVD Material Solution	Technical Advantage
Substrate Material	Optical Grade Single Crystal Diamond (SCD)	Low defect density and high purity maximize the intrinsic sound velocity (11,000 m/s) and Q-factor potential of the device.
Orientation	SCD (100) Orientation	Standard orientation matching the requirements for optimal piezoelectric film deposition and acoustic wave propagation.
Surface Quality	Precision Polished SCD	Achievable surface roughness Ra < 1 nm ensures an atomically smooth template, critical for achieving the highly oriented c-axis AlN film growth necessary to maximize K².
Substrate Size	Custom SCD Plates	We supply SCD plates in custom dimensions, including the half-inch format used, up to 500 µm thick, or substrates up to 10 mm thick.

Customization Potential & Process Enhancement

The authors noted that adding a substrate heating mechanism is necessary to improve AlN crystallinity and device performance. 6CCVD capabilities directly support this necessary improvement.

Custom Metalization for Thermal Stability: The integration of heating elements or high-temperature processing requires robust metal contacts. 6CCVD offers in-house metalization services, including Ti/Pt/Au or W stacks, which provide superior adhesion and thermal stability compared to simple Al electrodes, ensuring reliability during high-temperature deposition or operation.
Precision Dimensions and Shaping: We offer laser cutting and shaping services to provide custom geometries for minimal fab tools, ensuring perfect fit and alignment for specialized sputtering chambers.
Advanced Polishing for Epitaxy: While the paper used sputtering, future high-performance devices may require epitaxial AlN growth. Our Ra < 1 nm polishing capability on SCD provides the near-perfect surface termination required for high-quality heteroepitaxy.

Engineering Support

6CCVD’s in-house PhD team specializes in diamond material science and its application in high-power electronics and RF devices. We can assist researchers with:

Material Selection: Consulting on the optimal SCD grade (purity, orientation, and defect density) to maximize the Q-factor and minimize acoustic losses for similar Diamond SAW/FBAR projects.
Interface Optimization: Advising on pre-treatment and metalization schemes to ensure robust interfaces between the diamond substrate and piezoelectric thin films (AlN, ScAlN).
Global Logistics: We offer reliable global shipping (DDU default, DDP available) to ensure timely delivery of custom diamond wafers worldwide.

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

View Original Abstract

A minimal RF-magnetron sputtering system was developed for AlN and ScAlN thin film deposition in elastic wave devices, and the sputtering conditions for this system were optimized. X-ray diffraction measurement results after optimization showed that the c-axis rocking curve of thin AlN films on Si(001) substrates had an FWHM of 2.66 under the best conditions, and the thickness distribution measured using optical thickness measurement was less than 1 %. A diamond surface acoustic wave (SAW) resonator with an electrode of 2 μm wavelength (0.5 μm comb electrode width) was fabricated by depositing a 700-nm thick AlN film on a single-crystal diamond (100) substrate using this apparatus. The measured resonance characteristics yielded a resonant frequency of 5.2 GHz, an electromechanical coupling coefficient (K 2 ) of 0.89 %, and a Q-value of 1737. However, the K 2 value was 74% smaller than the theoretically obtained one. This deviation from the theoretical value can be attributed to the insufficient orientation of AlN as well as defects in the SAW electrode formation, as several anti-resonance peaks were observed. However, these can be improved by adding a substrate heating mechanism. The newly developed system exhibits satisfactory performance as a minimal RF sputtering device.

Tech Support

Original Source

DOI: https://doi.org/10.12792/iciae2023.014