Thermal Transport and Mechanical Stress Mapping of a Compression Bonded GaN/Diamond Interface for Vertical Power Devices
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-04-01 |
| Authors | William Delmas, Amun Jarzembski, Matthew Bahr, Anthony E. McDonald, Wyatt Hodges |
| Institutions | Sandia National Laboratories |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research focuses on solving the critical thermal management challenge in high-power vertical Gallium Nitride (GaN) devices by bonding them directly to diamond using a thin gold (Au) interlayer.
- Core Problem Addressed: GaN power devices generate high heat fluxes, and conventional bonding methods result in high thermal resistance, limiting performance.
- Solution & Achievement: GaN was successfully bonded to diamond via room temperature compression bonding (2 KN force) using a Ti/Au interlayer stack.
- Exceptional Thermal Performance: The bonded region achieved an interface thermal conductance (G) of >100 MW/m2K, demonstrating excellent heat transfer efficiency.
- Interface Variability: Interface conductance was found to vary by two orders of magnitude across the sample, dropping to 0.141 MW/m2K in unbonded regions.
- Mechanical State: Raman stress mapping revealed an average 50 MPa compressive stress at the bonded GaN/Diamond interface.
- Key Finding (Combined Mapping): The unbonded regions are characterized by pressurized air nanogaps, which limit thermal conductance and are bordered by areas of high mechanical stress.
- Structural Failure Mode: TEM analysis confirmed that delamination in the unbonded regions occurs specifically at the Titanium (Ti) layer.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Vertical Device Breakdown Voltage | 5 | kV | Target performance for vertical architecture |
| Lateral Device Breakdown Voltage | 1.2 | kV | Comparison baseline |
| Bonding Force | 2 | KN | Applied during compression bonding |
| Bonded Interface Conductance (G) | >100 | MW/m2K | High-performance region (Thermal Model) |
| Transition Interface Conductance (G) | 1.1 | MW/m2K | Measured at the bonded/unbonded boundary |
| Unbonded Interface Conductance (G) | 0.141 | MW/m2K | Limited by pressurized air nanogaps |
| Average Interface Stress | 50 | MPa | Compressive stress measured via Raman mapping |
| Thermal Performance Improvement | 3 | x | Potential gain for vertical devices with optimal thermal management |
| FDTR Pump Frequency Range | 1 to 162 | kHz | Used to vary thermal sensing depth |
| FDTR Sensing Depth Range | 14.27 to 115.28 | ”m | Corresponds to 162 kHz and 1 kHz frequencies, respectively |
| Raman Stress Measurement Mode | EH2 | cm-1 | Phonon mode used to detect stress shift in GaN |
Key Methodologies
Section titled âKey MethodologiesâThe study relied on a specialized room-temperature compression bonding process and advanced multi-modal characterization techniques to analyze the resulting GaN/Diamond interface.
Fabrication Method (Compression Bonding)
Section titled âFabrication Method (Compression Bonding)â- Material Preparation: GaN and Diamond substrates were prepared with thin metal interlayers (Ti and Au).
- Surface Cleaning: Surfaces were treated using Argon (Ar) plasma cleaning prior to bonding to remove contaminants and activate the surfaces.
- Bonding: The GaN/Ti/Au stack was aligned and compressed against the Au/Ti/Diamond stack using a 2 KN force at room temperature (RT).
Characterization Techniques
Section titled âCharacterization Techniquesâ- Frequency Domain Thermoreflectance (FDTR):
- Used to map the thermal phase shift, which is inversely related to interface conductance.
- Measurements were taken across the bonded-to-unbonded transition region.
- Varying the pump modulation frequency allowed for depth-resolved thermal analysis (from 14 ”m to 115 ”m).
- Raman Spectroscopy:
- Used for non-destructive mechanical stress mapping.
- Stress was quantified by measuring the shift in the GaN EH2 phonon mode relative to a zero-stress bare GaN reference.
- A line scan across the transition region revealed oscillatory stress patterns in the unbonded area.
- C-SAM (C-mode Scanning Acoustic Microscopy):
- Used for initial quality assessment, distinguishing bonded (gray) from unbonded (black) regions.
- TEM/EDS (Transmission Electron Microscopy / Energy-Dispersive X-ray Spectroscopy):
- Provided high-resolution structural and elemental analysis of the interface.
- Confirmed that the unbonded regions were caused by delamination specifically occurring at the Ti layer.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology is directly applicable to the development and manufacturing of next-generation high-power semiconductor devices, where thermal management is the primary bottleneck for performance scaling.
- High Power RF and Switching:
- Enables the use of GaN in high-frequency, high-power applications (e.g., 5G/6G base stations, radar systems) by effectively dissipating heat.
- Electric Vehicles (EVs) and Power Conversion:
- Critical for developing high-efficiency, compact power electronics (inverters, converters) required for EV drivetrains and charging infrastructure.
- Vertical Device Architecture:
- Supports the transition to vertical GaN devices, which offer significantly higher breakdown voltages (5 kV) compared to traditional lateral devices (1.2 kV).
- Advanced Thermal Substrates:
- Establishes a reliable, high-conductance bonding method for integrating wide bandgap semiconductors (like GaN) with ultra-high thermal conductivity materials (like diamond).
- Aerospace and Defense:
- Applicable to systems requiring robust, high-density power modules that must operate reliably under extreme thermal loads.