Thermal Transport and Mechanical Stress Mapping of a Compression Bonded GaN/Diamond Interface for Vertical Power Devices

At a Glance

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

Technical Analysis: GaN/Diamond Interface for Vertical Power Devices

This document analyzes the thermal transport and mechanical stress mapping of a compression-bonded Gallium Nitride (GaN) to Diamond interface, a critical structure for next-generation vertical power electronics requiring superior thermal management.

Executive Summary

Application Focus: The research successfully demonstrates a high-performance thermal interface for vertical GaN power devices, which require robust heat dissipation due to extremely high heat fluxes (>10,000 W/cm²).
Thermal Achievement: Room Temperature (RT) compression bonding of GaN to Diamond, utilizing a thin Au/Ti interlayer, achieved an ultra-high Thermal Boundary Conductance (TBC) of >100 MW/m²K in fully bonded regions.
Material Requirement: The use of diamond as a heat spreader is essential to achieve the required thermal performance, significantly outperforming traditional direct-bonded copper solutions.
Interface Variability: Frequency Domain Thermoreflectance (FDTR) mapping revealed TBC variability spanning two orders of magnitude (from >100 MW/m²K down to 0.141 MW/m²K) across the interface.
Mechanical Correlation: Raman stress mapping confirmed an average compressive stress of 50 MPa at the interface, and high mechanical stress areas were found to correlate directly with regions of low thermal boundary conductance.
Failure Mechanism: TEM analysis identified that unbonded regions resulted from delamination occurring specifically at the Ti layer, leading to the formation of pressurized air nanogaps that severely limit local heat transfer.
6CCVD Value Proposition: 6CCVD specializes in the high-quality SCD substrates and custom Ti/Au metalization required to replicate and optimize this high-TBC interface for scalable power device manufacturing.

Technical Specifications

The following hard data points were extracted from the analysis of the GaN/Diamond interface:

Parameter	Value	Unit	Context
Peak Heat Flux (Target)	>10,000	W/cm²	Required for Vertical GaN Diode/MOSFET operation
Bonded Region Interface Conductance (TBC)	>100	MW/m²K	Achieved via Au/Ti interlayer compression bonding
Unbonded Region Interface Conductance (TBC)	0.141	MW/m²K	Limited by pressurized air nanogaps
Transition Region Interface Conductance (TBC)	1.1	MW/m²K	Intermediate thermal performance
Average Interfacial Stress	50	MPa	Compressive stress measured by Raman spectroscopy
Bonding Force Applied	2	kN	Used for Room Temperature (RT) compression bonding
Vertical GaN Breakdown Voltage (Target)	≈5	kV	Compared to lateral architecture (≈1.2 kV)
FDTR Sensing Depth (Max)	115.28	µm	Measured at 1 kHz pump frequency
FDTR Sensing Depth (Min)	14.27	µm	Measured at 162 kHz pump frequency
Delamination Location	Ti Layer	N/A	Identified via TEM/EDS mapping

Key Methodologies

The experiment combined advanced fabrication techniques with high-resolution thermal and mechanical characterization to analyze the GaN/Diamond interface:

Substrate Metalization: Thin film layers of Titanium (Ti) and Gold (Au) were deposited onto both the GaN and Diamond substrates (GaN/Ti/Au and Dia./Ti/Au) using sputtering techniques.
Room Temperature Compression Bonding: The metalized substrates were aligned and bonded at room temperature using a high compression force of 2 kN.
Thermal Mapping (FDTR): Frequency Domain Thermoreflectance was utilized to map subsurface thermal properties and quantify the Interface Conductance (G) across the bonded and unbonded regions at various pump frequencies (1 kHz to 162 kHz).
Mechanical Stress Mapping (Raman): Micro-Raman spectroscopy was employed to map mechanical stress in the GaN layer by measuring the shift in the E₂^H phonon mode, revealing compressive stress profiles.
Structural Analysis (TEM/C-SAM): C-SAM (Scanning Acoustic Microscopy) was used to identify bonded (gray) and unbonded (black) regions. High-resolution TEM (Transmission Electron Microscopy) with Energy Dispersive X-ray Spectroscopy (EDS) confirmed the material stack and identified delamination at the Ti layer in unbonded regions.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality MPCVD diamond substrates and custom metalization services necessary to replicate and advance this critical GaN/Diamond thermal management technology. Our capabilities directly address the material and processing requirements identified in this research.

Applicable Materials

To achieve the ultra-high TBC (>100 MW/m²K) demonstrated in this study, the highest quality diamond material is required:

Optical Grade Single Crystal Diamond (SCD): Recommended for maximum thermal conductivity (up to 2200 W/mK) and superior surface quality (Ra < 1 nm), which is essential for minimizing voids during compression bonding and maximizing thermal performance in high-power vertical devices.
High-Purity Polycrystalline Diamond (PCD): Available for applications requiring larger area coverage (up to 125 mm diameter) where cost-effectiveness and scalability are primary concerns, while still offering excellent thermal properties.

Customization Potential

The success of the GaN/Diamond bond relies heavily on the precise preparation and metalization of the diamond surface. 6CCVD offers comprehensive in-house services:

Requirement from Research Paper	6CCVD Solution & Capability	Technical Advantage
Custom Metalization Stack (Ti/Au)	Internal Metalization Services: We offer deposition of Ti, Au, Pt, Pd, W, and Cu. We can precisely deposit the required Ti/Au stack, ensuring optimal adhesion and purity for compression bonding.	Eliminates external processing risks and guarantees clean, high-quality interfaces critical for achieving high TBC.
Precise Substrate Dimensions	Custom Dimensions & Laser Cutting: Plates/wafers available up to 125 mm (PCD). We provide custom laser cutting for specific die sizes and geometries required for vertical device integration.	Supports both R&D scale (small dies) and pilot production (inch-size wafers).
Ultra-Smooth Surface Finish	Advanced Polishing: SCD polished to Ra < 1 nm and inch-size PCD polished to Ra < 5 nm.	Minimizes surface roughness, which is crucial for reducing air gaps and maximizing the contact area during RT compression bonding, thereby preventing the low TBC regions observed in the study.
Thickness Optimization	SCD/PCD Thickness Control: SCD (0.1 µm - 500 µm) and Substrates (up to 10 mm).	Allows engineers to fine-tune the thermal path length and mechanical compliance of the diamond heat spreader.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD diamond growth and surface engineering. We can assist researchers and engineers with:

Material Selection: Optimizing diamond grade (SCD vs. PCD) and doping (BDD) based on specific electrical and thermal requirements for GaN/Diamond thermal management projects.
Interface Optimization: Consulting on metalization stack design (e.g., adhesion layers, barrier layers) to mitigate delamination issues observed at the Ti layer and ensure robust, high-TBC interfaces.
Global Logistics: Providing global shipping (DDU default, DDP available) to ensure rapid delivery of custom diamond materials worldwide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.2172/2563890