Direct Integration of Polycrystalline Diamond With 3C‐SiC for Enhanced Thermal Management in GaN HEMTs - Impact of Grain Structure and Interface Engineering
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-07-10 |
| Journal | Advanced Materials Technologies |
| Authors | Chiharu Moriyama, Zhe Cheng, Zifeng Huang, Yutaka Ohno, Koji Inoue |
| Institutions | Peking University, Tohoku University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”The research demonstrates a significant advancement in thermal management for high-power GaN electronics by successfully integrating polycrystalline diamond (PCD) substrates.
- Scalable Integration Breakthrough: Direct bonding of 2-inch PCD wafers (surface roughness Ra: 2.48 nm) to 3C-SiC layers for GaN High-Electron-Mobility Transistors (HEMTs) was achieved using Surface-Activated Bonding (SAB) at room temperature.
- Superior Substrate Conductivity: The PCD growth surface exhibited a thermal conductivity (k) of 2088 W/mK, which is higher than the k of the Single-Crystal Diamond (SCD) tested (1971 W/mK).
- Thermal Bottleneck Identified: Despite high bulk conductivity, the thermal resistance (RTH) of GaN HEMTs on PCD was 27% higher than on SCD, primarily due to phonon scattering caused by the fine-grained (2.2 µm average diameter) nucleation layer of the PCD.
- Interface Engineering Success: High-temperature annealing (1100 °C) transformed the initial 7 nm amorphous bonding layer into a uniform 13 nm polycrystalline SiC layer, demonstrating robust structural integrity without cracks or separations.
- Performance Improvement: GaN HEMTs on PCD showed a 60% reduction in RTH compared to Si substrates, confirming the diamond’s effectiveness in reducing localized heating and improving thermal uniformity across the 2-inch wafer.
- Future Optimization Path: Full utilization of PCD’s thermal advantages requires removing the fine-grained nucleation layer (e.g., via mechanical polishing) to minimize phonon scattering and maximize heat dissipation efficiency.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| PCD Wafer Size | 2 | inch | Maximum size demonstrated for direct bonding. |
| PCD Growth Surface Roughness (Ra) | 2.48 | nm | Roughness accommodated by SAB technique. |
| PCD Thermal Conductivity (k) | 2088 | W/mK | Measured on the coarse-grained growth surface via TDTR. |
| SCD Thermal Conductivity (k) | 1971 | W/mK | Measured on the reference single-crystal diamond. |
| Al/PCD Thermal Boundary Conductance (TBC) | 121 | MW/m2K | TBC at the Al/PCD interface. |
| HEMT RTH on Si | 41.3 | K-mm/W | Thermal resistance reference (gate edge measurement). |
| HEMT RTH on PCD | 14.7 | K-mm/W | Thermal resistance on polycrystalline diamond. |
| HEMT RTH on SCD | 9.93 | K-mm/W | Thermal resistance on single-crystal diamond. |
| RTH Reduction (PCD vs Si) | 60 | % | Improvement in heat dissipation efficiency. |
| RTH Difference (PCD vs SCD) | 27 | % higher | Attributed to the fine-grained PCD nucleation layer. |
| PCD Growth Surface Avg. Grain Diameter | 55.9 | µm | Coarse-grained structure (95% > 10 µm). |
| PCD Nucleation Surface Avg. Grain Diameter | 2.2 | µm | Fine-grained structure (98% < 5 µm). |
| Initial Amorphous Interface Thickness | ~7 | nm | Formed after room-temperature bonding. |
| Post-Anneal Polycrystalline SiC Layer Thickness | ~13 | nm | Formed after 1100 °C annealing. |
| Ohmic Contact Annealing Temperature | 800 | °C | Rapid Thermal Annealing (RTA) step. |
Key Methodologies
Section titled “Key Methodologies”The integration relies on a combination of advanced material synthesis, surface preparation, and Surface-Activated Bonding (SAB).
- HEMT Layer Growth: AlGaN/GaN/3C-SiC heterostructures were grown on 6-inch Czochralski (Cz)-Si (111) wafers using Metal-Organic Chemical Vapor Deposition (MOCVD).
- PCD Synthesis: PCD wafers (400 µm thick) were synthesized on Si substrates via Microwave Plasma Chemical Vapor Deposition (MPCVD) using a methane-hydrogen gas mixture (2-5% CH4) at 200 Torr and 5.6 KW.
- Surface Preparation: The Si substrate was removed via fluoric acid etching. The exposed 3C-SiC layer and the PCD growth surface were chemically mechanically polished (CMP) and cleaned (sulfuric acid-hydrogen peroxide mixture).
- Surface-Activated Bonding (SAB): The 3C-SiC and PCD surfaces were simultaneously irradiated by an Argon Fast Atom Beam (FAB) (1.6 kV, 160 mA) in high vacuum (1.0 x 10-7 Pa).
- Room-Temperature Bonding: The activated surfaces were brought into contact under a load of 1 GPa for 180 s at room temperature, successfully transferring the HEMT layers onto the PCD.
- Ohmic Contact Formation: Source and drain contacts (Ti/Al/Ti/Au) were formed, followed by Rapid Thermal Annealing (RTA) at 800 °C for 60 s in N2 ambient.
- Interface Analysis: Structural integrity and elemental distribution were analyzed using Transmission Electron Microscopy (TEM) and Energy-Dispersive X-ray Spectroscopy (EDS) before and after a 1100 °C annealing step.
- Thermal Measurement: Thermal conductivity was assessed using Time-Domain Thermoreflectance (TDTR), and device operating temperature was measured using micro-photoluminescence (µ-PL) at the gate edge.
Commercial Applications
Section titled “Commercial Applications”This technology addresses the critical thermal bottleneck in high-power wide bandgap semiconductor devices, enabling higher power density and improved reliability.
- High-Frequency and RF Systems: Essential for 5G/6G base stations, satellite communications, and advanced radar systems, where GaN HEMTs operate at high power fluxes (often exceeding the Sun’s surface heat flux).
- Power Electronics: Used in compact and efficient power converters for electric vehicles (EVs), renewable energy infrastructure, and energy storage systems, requiring minimal thermal resistance.
- High-Reliability Devices: The enhanced heat spreading provided by PCD mitigates localized hot spots, significantly extending the median lifetime of GaN devices.
- Cost-Effective Substrates: Utilizing scalable Polycrystalline Diamond (PCD) instead of prohibitively expensive, size-limited Single-Crystal Diamond (SCD) enables industrial mass production of GaN-on-diamond wafers up to 8 inches.
- Next-Generation Thermal Management: Provides a platform for advanced thermal co-design, allowing devices to operate closer to their electronic limits rather than being constrained by thermal limitations.
View Original Abstract
Abstract The direct integration of polycrystalline diamond (PCD) with semiconductors is crucial for enhancing heat dissipation in high‐power electronics. However, achieving low surface roughness (<1 nm) remains challenging. In this study, the direct bonding of PCD to 3C‐SiC for GaN high‐electron‐mobility transistors (HEMTs) on a 2‐inch PCD wafer is demonstrated using an advanced bonding technique. The PCD wafer (surface roughness: 2.48 nm) is bonded at room temperature, forming a 7 nm‐thick amorphous layer, which transformed into a 13 nm‐thick polycrystalline SiC layer after annealing at 1100 °C without cracks or separations. Thermal analysis revealed higher thermal conductivity of PCD’s growth surface than single‐crystal diamond (SCD). However, the thermal resistance ( R TH ) of GaN HEMTs on PCD is 27% higher than on SCD, attributed to phonon scattering from smaller grain sizes on the nucleation surface. Removing the fine‐grained nucleation layer can enhance heat dissipation. This successful direct bonding of PCD with 3C‐SiC overcomes key integration challenges, enabling improved thermal transport and high‐power device reliability. To fully utilize PCD’s thermal advantages, grain size optimization and interface engineering are essential to reduce phonon scattering, improve thermal transport efficiency, and maximize device performance for next‐generation high‐power electronics.