Ultra-Fine Polishing Technique for Diamond Substrates and Its Application to Semiconductor Devices with Improved Heat Dissipation Performance
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-06-04 |
| Journal | Journal of the Japan Society for Precision Engineering |
| Authors | Shuichi HIZA, Akihisa Kubota |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the development and validation of ultra-high precision processing techniques for diamond substrates, enabling the creation of high-performance GaN-on-diamond High Electron Mobility Transistors (HEMTs) for advanced thermal management.
- Core Value Proposition: Solves the critical thermal bottleneck in high-power GaN devices by utilizing diamond, the material with the highest thermal conductivity, as the substrate.
- Ultra-Precision Polishing: Achieved arithmetic mean surface roughness (Sa) of approximately 0.1 nm on mosaic single-crystal diamond substrates using novel chemical-assisted polishing methods.
- Scalability Demonstrated: The polishing techniques successfully maintained uniform surface quality across grain boundaries in large-area (15 mm square) mosaic diamond wafers.
- Integration Method: Successfully bonded the polished GaN layer and diamond substrate using a modified Surface Activated Bonding (SAB) technique, incorporating an ultra-thin (< 10 nm) amorphous Si intermediate layer.
- Thermal Performance: The resulting GaN-on-diamond devices demonstrated significant thermal improvement, suppressing the gate electrode temperature rise by approximately 1/3 compared to reference devices fabricated on standard Si substrates.
- Future Outlook: The technology is positioned for the practical realization of âinch-classâ large-area diamond wafers for high-volume manufacturing of high-power, high-reliability devices.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Substrate Type | Mosaic Single Crystal | - | Used for large-area processing trials. |
| Substrate Size Tested | 15 x 15 | mm2 | Size used for bonding and performance validation. |
| Target Surface Roughness (Sa) | < 0.1 | nm | Required for high-quality, void-free bonding (Ref 13). |
| Achieved Sa (Mechanical Polishing) | 0.32 | nm | Initial state before chemical-assisted refinement (Fig 2a). |
| Achieved Sa (Transition Metal/H2O2) | 0.11 | nm | Roughness achieved using the transition metal polishing method (Fig 2b). |
| Achieved Sa (Oxide Polishing Plate) | 0.12 | nm | Roughness achieved using the oxide polishing method (Fig 2c). |
| Bonding Layer Thickness | < 10 | nm | Thickness of the amorphous Si layer used in the SAB process. |
| Operating Power Density (DC) | ~10 | W/mm | Power density applied during thermal evaluation. |
| Temperature Rise Suppression | ~1/3 | - | Reduction factor in gate electrode temperature rise compared to Si substrate. |
| Bonding Interface Gap | < 10 | nm | Observed thickness of the integrated bonding layer (Fig 4). |
Key Methodologies
Section titled âKey MethodologiesâThe core achievement relies on two distinct chemical-assisted polishing methods for diamond and a modified Surface Activated Bonding (SAB) technique for integration.
1. Diamond Ultra-Precision Polishing
Section titled â1. Diamond Ultra-Precision PolishingâTwo primary chemical-assisted methods were investigated to achieve Sa < 0.15 nm, significantly surpassing traditional mechanical grinding (Sa = 0.32 nm).
Method A: Transition Metal Plate and Oxidizing Agent (Fig 1a)
Section titled âMethod A: Transition Metal Plate and Oxidizing Agent (Fig 1a)â- Setup: A transition metal polishing plate (e.g., Iron or Nickel) is immersed in an oxidizing solution (e.g., Hydrogen Peroxide, H2O2).
- Reaction: The transition metal (Fe2+) reacts with the oxidizing agent (H2O2) to generate highly reactive hydroxyl radicals (OHâ˘).
- Polishing: The diamond workpiece is pressed against the metal plate and moved relatively. The OH radicals chemically attack the diamond surface carbon atoms, enabling material removal and ultra-smooth finishing.
Method B: Oxide Polishing Plate (Fig 1b)
Section titled âMethod B: Oxide Polishing Plate (Fig 1b)â- Setup: An oxide polishing plate (e.g., Quartz or Sapphire) is used. The plate surface is heated and/or irradiated with Deep UV (VUV) light or Ozone to promote efficient OH termination on the oxide surface.
- Reaction: The diamond workpiece is pressed against the heated oxide plate. A thermochemical reaction occurs at the interface, primarily dehydration condensation between the OH groups on the diamond and oxide surfaces.
- Polishing: This reaction facilitates the extraction of carbon atoms from the diamond surface, achieving high-precision polishing without relying on abrasive particles.
2. GaN Layer Preparation
Section titled â2. GaN Layer Preparationâ- Epitaxial Growth: GaN thin films were grown epitaxially on temporary growth substrates (Si or SiC).
- Device Fabrication: HEMT structures were fabricated on the GaN layer.
- Substrate Removal: The original Si or SiC growth substrate was removed.
- Surface Refinement: The exposed GaN surface (previously facing the growth substrate) was polished to high precision using Chemical Mechanical Polishing (CMP).
3. GaN-on-Diamond Integration
Section titled â3. GaN-on-Diamond Integrationâ- Bonding Technique: A modified Surface Activated Bonding (SAB) method was employed, suitable for room-temperature, high-vacuum integration.
- Surface Activation: The polished diamond and GaN surfaces were activated (e.g., using atomic beams) within a high-vacuum chamber.
- Intermediate Layer: An ultra-thin layer of amorphous Si (a-Si) was deposited or inserted at the interface to enhance bonding quality and uniformity.
- Integration: The activated surfaces were brought into contact under pressure at room temperature, forming a unified structure with a bonding layer thickness observed to be less than 10 nm.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology is critical for next-generation high-power and high-frequency electronics where thermal management dictates performance and reliability.
- High-Power RF Amplifiers: Essential for increasing the output power and efficiency of GaN HEMTs used in communication systems.
- Satellite Communication Systems: Enables smaller, lighter, and more reliable high-power amplifiers for space-based applications.
- 5G/6G Mobile Infrastructure: Supports the high-density, high-frequency requirements of advanced base stations by managing heat effectively.
- Advanced Thermal Management: Provides a scalable method for integrating high-thermal-conductivity diamond into complex semiconductor stacks.
- High-Reliability Electronics: By significantly lowering the operating temperature of the active region, the technology extends the lifespan and reliability of high-power devices.