The Effect of Interlayer Microstructure on the Thermal Boundary Resistance of GaN-on-Diamond Substrate

At a Glance

Metadata	Details
Publication Date	2022-05-14
Journal	Coatings
Authors	Jia Xin, Lu Huang, Miao Sun, Xia Zhao, Junjun Wei
Institutions	University of Science and Technology Beijing, Jiangsu Institute of Metrology
Citations	8
Analysis	Full AI Review Included

Technical Documentation & Analysis: GaN-on-Diamond Thermal Management

Executive Summary

This research successfully demonstrates a novel method for significantly reducing the Thermal Boundary Resistance (TBR) in high-power Gallium Nitride (GaN)-on-Diamond devices, a critical step for next-generation High Electron Mobility Transistors (HEMTs).

Core Achievement: Implementation of a periodic (patterned) Silicon Nitride (SiN_x) interlayer microstructure to enhance thermal transport across the GaN/Diamond interface.
Quantitative Improvement: The patterned SiN_x interlayer reduced the effective TBR (TBReff,Dia/GaN) by approximately 20%, achieving a low value of 32.2 ± 1.8 m²KGW^-1.
Mechanism: The periodic structure (20 nm x 20 nm pits) increases the interface contact area, improves diamond nucleation density, and enhances the film-base bonding strength (critical load increased from 8 N to 15 N).
Material Requirement: The study utilized high-quality polycrystalline diamond (PCD) films grown via Microwave Plasma Chemical Vapor Deposition (MPCVD).
Measurement Technique: Thermal properties were accurately characterized using the Time-Domain Thermoreflectance (TDTR) technique, requiring precise thin-film deposition (e.g., Al transducer layer).
Future Direction: The authors suggest further optimization by thinning the SiN_x interlayer (currently 100 nm) and refining the periodic structure to achieve even lower TBR values, demonstrating the potential of diamond as the ultimate thermal diffusion substrate.

Technical Specifications

The following hard data points were extracted from the research detailing the material properties and thermal performance of the GaN-on-Diamond structures.

Parameter	Value	Unit	Context
Target Application	GaN HEMT Thermal Management	N/A	High-power electronic devices
Diamond Film Type	Polycrystalline (PCD)	N/A	Grown via MPCVD
Diamond Film Thickness	~2	µm	Deposited on GaN/SiN_x
Interlayer Material	SiN_x	N/A	Used to mitigate lattice/thermal mismatch
SiN_x Interlayer Thicknesses Tested	80, 100	nm	Comparative samples
SiN_x Periodic Structure Dimensions	20 x 20	nm	Cubic pits and step length
Lowest Thermal Boundary Resistance (TBR)	32.2 ± 1.8	m²KGW^-1	Achieved using SiN_x(Peri) structure
Highest Thermal Boundary Resistance (TBR)	40.5 ± 2.5	m²KGW^-1	SiN_x(orig) structure (100 nm)
TBR Reduction (Periodic vs Original)	~20	%	Significant reduction achieved by patterning
Diamond Nucleation Temperature	750	°C	5 min duration, 12% CH₄ concentration
Diamond Growth Temperature	800	°C	120 min duration, 5% CH₄ concentration
Critical Adhesion Load (Periodic)	15	N	Micro-scratch test, indicating superior film-base bonding
Critical Adhesion Load (Original)	8	N	Control sample adhesion

Key Methodologies

The experimental success hinges on precise control over the interface preparation and the MPCVD growth environment.

Substrate Preparation: Standard GaN-on-Si wafers were used, followed by ultrasonic cleaning in acetone and ethanol.
SiN_x Interlayer Deposition: SiN_x films (80 nm or 100 nm) were deposited via Radio Frequency (RF) magnetron sputtering to act as a protective layer and mismatch buffer.
Periodic Patterning: The 100 nm SiN_x layer was etched using Inductively Coupled Plasma (ICP) etching combined with a precise mask control method to create the 20 nm x 20 nm periodic cubic pit structure.
Nanodiamond Seeding: Samples were ultrasonically soaked in nanodiamond solution (grain size ~5 nm) to achieve high nucleation density, with seeds embedded in the SiN_x layer.
MPCVD Diamond Growth: A polycrystalline diamond layer (~2 µm thick) was grown using a self-made quartz bell structure MPCVD system.
- Nucleation Phase: 5 min at 750 °C (12% CH₄).
- Growth Phase: 120 min at 800 °C (5% CH₄).
Thermal Measurement: A 100 nm Aluminum (Al) film was deposited via electron beam evaporation to serve as the transducer/sensor layer for Time-Domain Thermoreflectance (TDTR) analysis, allowing for accurate extraction of TBReff,Dia/GaN.

6CCVD Solutions & Capabilities

The findings confirm that high-quality MPCVD diamond is essential for advanced thermal management in GaN devices. 6CCVD is uniquely positioned to supply the necessary materials and customization required to replicate and advance this research into commercial applications.

Applicable Materials

To replicate the high-performance thermal stack described, researchers require diamond material optimized for thermal conductivity and interface quality.

Material Recommendation: Optical Grade Polycrystalline Diamond (PCD).
- Justification: The paper utilized PCD grown via MPCVD. 6CCVD’s PCD offers thermal conductivity exceeding 2000 W/mK, crucial for maximizing heat dissipation from the GaN HEMT channel.

Customization Potential

The study highlights the need for precise control over film thickness, substrate handling, and post-processing steps like patterning and metalization.

Research Requirement	6CCVD Capability	Engineering Support
Large-Scale Integration	Custom Dimensions up to 125mm	6CCVD supplies PCD plates/wafers up to 125mm, enabling scale-up from research samples to commercial inch-size GaN-on-Diamond wafers.
Precise Film Thickness	Thickness Control (0.1 µm to 500 µm)	We can match the required 2 µm diamond film thickness precisely, or supply thinner films (as suggested for future optimization) to minimize thermal resistance.
Interface Preparation	Advanced Polishing (Ra < 5 nm)	Our high-precision polishing ensures the diamond surface is ready for subsequent patterning (ICP etching) or direct device integration, minimizing surface roughness effects on TBR.
Thermal Measurement & Device Integration	Internal Metalization Services	We offer deposition of critical metal layers (Au, Pt, Pd, Ti, W, Cu). This is essential for both TDTR transducer layers (e.g., Al/Ti/Au stacks) and for subsequent device contact fabrication.
Substrate Flexibility	Custom Substrate Handling	6CCVD can perform diamond deposition directly onto customer-supplied GaN-on-Si or GaN-on-SiC wafers, facilitating direct integration into existing fabrication flows.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing MPCVD growth parameters and material selection for high-power electronics. We can assist researchers and engineers with:

Material Selection: Determining the optimal PCD grain size and quality necessary to minimize phonon scattering at the GaN/Diamond interface.
Thermal Stack Design: Consulting on the ideal diamond thickness and surface preparation techniques to achieve the lowest possible TBReff,Dia/GaN for similar GaN HEMT thermal management projects.
Process Integration: Providing guidance on post-processing steps, including metalization recipes (e.g., Ti/Pt/Au contacts) compatible with diamond surfaces.

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

View Original Abstract

Diamond has the highest thermal conductivity of any natural material. It can be used to integrate with GaN to dissipate heat from AlGaN/GaN high electron mobility transistor (HEMT) channels. Much past work has investigated the thermal properties of GaN-on-diamond devices, especially the thermal boundary resistance between the diamond and GaN (TBReff,Dia/GaN). However, the effect of SiNx interlayer structure on the thermal resistance of GaN-on-diamond devices is less investigated. In this work, we explore the role of different interfaces in contributing to the thermal boundary resistance of the GaN-on-diamond layers, specifically using 100 nm layer of SiNx, 80 nm layer of SiNx, 100 nm layer of SiNx with a 20 nm × 20 nm periodic structure. Through combination with time-domain thermoreflectance measurement and microstructural analysis, we were able to determine that a patterning SiNx interlayer provided the lower thermal boundary resistance (32.2 ± 1.8 m2KGW−1) because of the diamond growth seeding and the diamond nucleation surface. In addition, the patterning of the SiNx interlayer can effectively improve the interface bonding force and diamond nucleation density and reduce the thermal boundary resistance of the GaN-on-diamond. This enables significant improvement in heat dissipation capability of GaN-on-diamond with respect to GaN wafers.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings12050672

References

1993 - High electron mobility transistor based on a GaN/AlxGa1−xN heterojunction [Crossref]
2009 - An internally-matched GaN HEMTs device with 45.2 W at 8 GHz for X-band application [Crossref]
2021 - Fabrication of low stress GaN-on-diamond structure via dual-sided diamond film deposition [Crossref]
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2006 - Self-heating in a GaN based heterostructure field effect transistor: Ultraviolet and visible Raman measurements and simulations [Crossref]
2017 - Barrier-layer optimization for enhanced GaN-on-diamond device cooling [Crossref]
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2018 - Low Thermal boundary resistance interfaces for GaN-on-diamond devices [Crossref]
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