Combined HF+MW CVD Approach for the Growth of Polycrystalline Diamond Films with Reduced Bow
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-02-07 |
| Journal | Coatings |
| Authors | Vadim Sedov, А. Ф. Попович, Stepan Linnik, Artem Martyanov, Junjun Wei |
| Institutions | University of Science and Technology Beijing, MIREA - Russian Technological University |
| Citations | 5 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research presents a novel, combined Chemical Vapor Deposition (CVD) approach—Microwave Plasma CVD (MW CVD) followed by Hot Filament CVD (HF CVD)—to produce high-quality polycrystalline diamond (PCD) films on silicon (Si) substrates, specifically targeting thermal management applications.
- Thermal Conductivity Enhancement: The combined 110 µm film achieved a Thermal Conductivity (TC) of 210 W/mK, representing a 60% improvement compared to the pure 93 µm HF CVD film (130 W/mK).
- Bow Reduction: The overall substrate bow was reduced by 57% (from -45 µm to -19 µm) by leveraging the opposite stress trends inherent in the two growth methods (MW CVD is typically compressive; HF CVD is typically tensile).
- Structural Quality Control: The initial 25 µm high-quality MW CVD layer promoted larger diamond grain growth, which propagated through the subsequent 85 µm HF CVD layer, mitigating the negative TC effects usually associated with the small-grained nucleation layer.
- Cost Efficiency: The use of cost-effective HF CVD for the majority (85 µm) of the film thickness allows for the production of large-area, high-TC PCD plates, overcoming the size and cost limitations of pure MW CVD.
- Surface Quality: The combined MW+HF film exhibited the lowest root-mean-squared roughness (Rrms = 1.8 µm) compared to both pure MW CVD (3.2 µm) and pure HF CVD (2.7 µm) films.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Combined Film Thickness | 110 | µm | MW (25 µm) + HF (85 µm) layers |
| Thermal Conductivity (MW+HF) | 210 ± 25 | W/mK | Measured via Laser Flash Technique (LFT) |
| Thermal Conductivity (Pure HF) | 130 ± 15 | W/mK | 93 µm thick film (Baseline comparison) |
| Bow (Delta h) Reduction | 57 | % | Reduction from pure HF (-45 µm) to MW+HF (-19 µm) |
| Curvature Radius (MW+HF) | -1.9 | m | Lowest bow achieved; closer to the HF CVD trend |
| Surface Roughness (Rrms, MW+HF) | 1.8 | µm | Root-mean-squared roughness |
| MW CVD Growth Rate | ~1 | µm/h | Used for the initial layer |
| HF CVD Growth Rate | ~1.1 | µm/h | Used for the bulk layer |
| Diamond Raman Peak (MW+HF) | 1334.3 | cm-1 | Indicates highly compressive stress |
| FWHM (MW+HF) | 7.8 | cm-1 | Full Width at Half Maximum of diamond peak |
| Substrate Size (Final) | 17 x 17 | mm2 | Laser-cut from 19 x 19 mm2 Si (111) plates |
Key Methodologies
Section titled “Key Methodologies”The combined MW+HF CVD process was executed in two distinct stages on seeded Si (111) substrates:
1. Initial Layer Growth (MW CVD - 25 µm)
Section titled “1. Initial Layer Growth (MW CVD - 25 µm)”- Reactor Type: Microwave Plasma CVD (MW CVD), ARDIS-100 (2.45 GHz).
- Power: 4.8 kW (Microwave).
- Gas Mixture: CH4/H2 (3% CH4 concentration).
- Total Gas Flow: 500 sccm.
- Pressure: 55 Torr.
- Substrate Temperature: 850 ± 25 °C.
- Purpose: To establish a high-quality, large-grain nucleation layer critical for high TC and to introduce compressive stress.
2. Bulk Layer Growth (HF CVD - 85 µm)
Section titled “2. Bulk Layer Growth (HF CVD - 85 µm)”- Reactor Type: Laboratory-built Hot Filament (HF) reactor (Tungsten filaments, 0.16 mm diameter).
- Gas Mixture: H2/CH4 (6 vol.% CH4 concentration).
- Pressure: 20 ± 1 Torr.
- Substrate Temperature: 850 ± 20 °C.
- Filament Parameters: Average current 6 ± 0.1 A; distance 12 ± 1 mm.
- Purpose: To provide cost-effective, high-rate bulk growth while introducing tensile stress to compensate for the initial MW CVD compressive stress.
3. Characterization Techniques
Section titled “3. Characterization Techniques”- Structure and Morphology: Scanning Electron Microscopy (SEM) in both Secondary Electrons (SE) and Backscattered Electrons (BSE) modes (BSE clearly revealed the MW→HF interface).
- Phase Composition and Stress: Micro-Raman Spectroscopy (473 nm laser) to analyze the diamond peak shift and the presence of non-diamond phases (t-PA peak at 1490 cm-1).
- Bow and Roughness: Optical Profilometer (NewView 5000) used to measure curvature radius (r) and Rrms from the substrate side.
- Thermal Conductivity (TC): Laser Flash Technique (LFT) applied to freestanding PCD films (Si substrate removed) to determine thermal diffusivity (D) perpendicular to the surface.
Commercial Applications
Section titled “Commercial Applications”The development of high-TC, low-bow, cost-effective PCD films is critical for next-generation thermal management in high-power density electronics.
- Power Electronics:
- GaN-on-Diamond Devices: Direct application for creating efficient thermal management layers for Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) and other high-frequency communications devices, overcoming GaN’s low intrinsic TC.
- High-Power RF/Microwave Systems: Used as heat spreaders to manage localized heat flux dissipation in high-performance regimes.
- Thermal Management:
- Large-Area Heat Sinks: Enables the production of wider-sized PCD plates (surpassing the 2-inch limit of standard 2.45 GHz MW CVD) for industrial heat sink applications where cost and size are primary constraints.
- Semiconductor Disk Lasers (SDLs): Heat spreaders for high-power laser systems.
- Protective Coatings:
- Cutting Tools: Improving the quality and TC of HF CVD diamond films used for hard or protective coatings, benefiting from the enhanced grain structure initiated by the MW layer.
- General CVD Diamond Industry:
- The technique provides a blueprint for tuning intrinsic stress (tensile/compressive) in PCD films by combining different CVD regimes, allowing for precise control over substrate bow for various thin-film applications.
View Original Abstract
A combination of two methods of chemical vapor deposition (CVD) of diamond films, microwave plasma-assisted (MW CVD) and hot filament (HF CVD), was used for the growth of 100 µm-thick polycrystalline diamond (PCD) layers on Si substrates. The bow of HF CVD and MW CVD films showed opposite convex\concave trends; thus, the combined material allowed reducing the overall bow by a factor of 2-3. Using MW CVD for the growth of the initial 25 µm-thick PCD layer allowed achieving much higher thermal conductivity of the combined 110 µm-thick film at 210 W/m·K in comparison to 130 W/m·K for the 93 µm-thick pure HF CVD film.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2018 - Thermal Conductivity of High Purity Synthetic Single Crystal Diamonds [Crossref]
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