Scanning Deposition Method for Large-Area Diamond Film Synthesis Using Multiple Microwave Plasma Sources

At a Glance

Metadata	Details
Publication Date	2022-06-08
Journal	Nanomaterials
Authors	Seung Pyo Hong, Kang-Il Lee, Hyun Jong You, Soo Ouk Jang, Young Sup Choi
Institutions	Korea Institute of Fusion Energy
Citations	8
Analysis	Full AI Review Included

Executive Summary

This study presents a novel scanning deposition method utilizing an array of multiple surface-wave plasma (SWP) sources to overcome the area limitations inherent in traditional resonant cavity Microwave Plasma Chemical Vapor Deposition (MPCVD) for diamond film synthesis.

Core Innovation: Development of a scalable, large-area diamond synthesis technique using a linear array of compact, ball-shaped SWP sources combined with a reciprocating substrate motion.
Uniformity Achievement: A minimum unit array of three 700 W plasma sources achieved a diamond film thickness uniformity of ±6.25% across a 70 mm wafer width.
Material Quality: Optimized single-source deposition yielded high-purity microcrystalline diamonds (MCDs) up to 1 µm in size, with an excellent sp³/sp² carbon ratio (I_Dia/I_G) reaching 2.75.
Process Environment: The method operates at relatively low pressures (600 mTorr) and high substrate temperatures (950 °C) to ensure high crystallinity, unlike typical low-pressure SWP methods that yield nanodiamonds.
Seeding Improvement: Ultrasonic seeding using nanodiamond powder significantly increased nucleation density (approximately 2.5 times higher) compared to mechanical scratching.
Scalability Potential: The method is theoretically infinitely scalable by extending the linear array of plasma sources and increasing the substrate scanning distance.
Current Limitations: The scanning process resulted in a decreased deposition rate, power loss due to the coaxial cable microwave transmission, and slightly reduced diamond crystallinity compared to the optimized fixed single-source process.

Technical Specifications

Parameter	Value	Unit	Context
Film Uniformity (Thickness)	±6.25	%	Achieved over 70 mm width using triple source scanning.
Film Thickness (Center)	570	nm	Result of 12 h scanning deposition.
Maximum Grain Size	1	µm	Microcrystalline diamond (MCD) size after optimization.
sp³/sp² Carbon Ratio (I_Dia/I_G)	2.75	Ratio	Optimized single-source deposition purity indicator.
Diamond Peak FWHM	18.58	cm^-1	Full Width at Half Maximum for the optimized single source.
Substrate Temperature (Optimized)	950	°C	Required for high-purity MCD synthesis.
Operating Pressure (Optimized)	600	mTorr	Low-pressure regime for SWP operation.
Microwave Frequency	2.45	GHz	Standard solid-state power amplifier (SSPA) frequency.
Microwave Power (Per Source)	700	W	Power used for each of the three sources in the array.
CH₄/H₂ Gas Ratio (Optimized)	0.50	%	Optimal ratio for clear diamond peak synthesis.
Substrate Size (Max)	150	mm	Maximum wafer size accommodated by the system.
Source-Substrate Gap	15	mm	Distance maintained during scanning deposition.
Source Spacing (Center-to-Center)	50	mm	Distance between adjacent plasma sources in the array.
Substrate Scanning Speed	0.2	mm/s	Reciprocating motion speed during deposition.
Average Crystallite Size	14.9	nm	Calculated from the (111) XRD peak FWHM.

Key Methodologies

The diamond synthesis process involved two main phases: single-source optimization and multi-source scanning deposition.

1. Single Plasma Source Optimization (Fixed Substrate)

Plasma Source: Improved single-launcher surface-wave plasma (SWP) source, designed to extract plasma in a concentrated, ball-shaped form (20 mm inner diameter, 20 mm depth) to enhance gas temperature and crystallinity.
Microwave Coupling: Utilized a 2.45 GHz solid-state power amplifier and a three-stub tuner, coupled via a coaxial cable.
Temperature Study: Substrate temperature was varied from 700 °C to 980 °C.
- 700-800 °C: Produced elongated amorphous carbon structures.
- 900 °C: Formed spherical 1 µm diamonds within an amorphous matrix (low density).
- 980 °C: Produced vertically oriented graphene nanowalls.
- Optimal Temperature: 950 °C was selected for high-crystallinity MCD growth.
Gas Ratio Study (CH₄/H₂): Ratios were varied from 1.00% down to 0.40%.
- 1.00%: Graphene nanowall formation.
- 0.75%: Transition region (graphene nanowall and nanodiamond).
- Optimal Ratio: 0.50% (1.5 sccm CH₄ / 300 sccm H₂) yielded clear diamond peaks.
Nucleation Enhancement: Compared mechanical scratching (3 µm abrasive) versus ultrasonic seeding (5% nanodiamond solution). Ultrasonic seeding was adopted due to its 2.5 times higher nucleation density.

2. Multi-Source Scanning Deposition (Large Area)

Array Configuration: A minimum unit array consisting of three SWP sources was mounted linearly in the upper chamber section, spaced 50 mm apart.
Substrate Handling: A 4-inch Si wafer was heated by a SiC heater to 950 °C and loaded via a load lock system.
Scanning Motion: The substrate was reciprocated perpendicular to the source array direction over a 50 mm distance at a speed of 0.2 mm/s for 12 hours.
Power and Pressure: Each source operated at 700 W microwave power, maintaining 600 mTorr pressure.
Analysis: FE-SEM cross-sectional measurements confirmed 570 nm film thickness and ±6.25% uniformity over the 70 mm measurement area. UV-Raman spectroscopy confirmed diamond presence (I_Dia/I_G ratio 1.0 to 2.0), though crystallinity was slightly lower than the fixed-source optimum.

Commercial Applications

The ability to synthesize large-area, high-quality diamond films is critical for next-generation electronic and thermal management devices.

High-Performance Power Semiconductors: Diamond substrates are ideal for high-power devices (like those used in 5G and beyond wireless communication) due to their exceptional thermal conductivity, enabling efficient heat dissipation and improved device reliability.
Advanced Electronic Devices: Utilization in high-electron and hole mobility devices, suitable for extreme environments (e.g., space electronics) and high-frequency applications.
Quantum Computing and Sensing: Diamond containing nitrogen-vacancy (NV) centers is a leading material for quantum sensors and qubits. Large-area synthesis is necessary for scaling up quantum device manufacturing.
Micro-Electro-Mechanical Systems (MEMS): Diamond films offer superior mechanical hardness and chemical inertness, making them valuable for robust MEMS components, processing tools, and mechanical structures.
Optical Windows and Coatings: Application in high-power optics where diamond’s wide bandgap and thermal stability are essential for transparent, durable coatings.

View Original Abstract

The demand for synthetic diamonds and research on their use in next-generation semiconductor devices have recently increased. Microwave plasma chemical vapor deposition (MPCVD) is considered one of the most promising techniques for the mass production of large-sized and high-quality single-, micro- and nanocrystalline diamond films. Although the low-pressure resonant cavity MPCVD method can synthesize high-quality diamonds, improvements are needed in terms of the resulting area. In this study, a large-area diamond synthesis method was developed by arranging several point plasma sources capable of processing a small area and scanning a wafer. A unit combination of three plasma sources afforded a diamond film thickness uniformity of ±6.25% at a wafer width of 70 mm with a power of 700 W for each plasma source. Even distribution of the diamond grains in a size range of 0.1-1 μm on the thin-film surface was verified using field-emission scanning electron microscopy. Therefore, the proposed novel diamond synthesis method can be theoretically expanded to achieve large-area films.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12121959

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