A Simulation of Thermal Management Using a Diamond Substrate with Nanostructures

At a Glance

Metadata	Details
Publication Date	2023-08-05
Journal	Micromachines
Authors	Tingting Liu, Kaiwen Zheng, Tao Tao, Wenxiao Hu, Kai Chen
Institutions	Nanjing University, Nanjing University of Posts and Telecommunications
Citations	3
Analysis	Full AI Review Included

Executive Summary

Core Challenge Addressed: Mitigating the severe self-heating effect in high-power GaN devices, which limits reliability and performance due to increased integration and power density.
Solution Proposed: Utilizing single-crystal diamond substrates, which possess significantly higher thermal conductivity (2000 W/(m·K)), optimized with surface micro/nanostructures.
Thermal Performance Gain: COMSOL simulations demonstrated a reduction in maximum device temperature from 121.82 °C (conventional sapphire substrate) to 114.88 °C (optimized diamond substrate) under 1 W heat load.
Optimal Structure Identified: The best heat dissipation performance was achieved using square nanopillars with a height of 2000 µm, corresponding to the Size 2 parameters in the simulation study.
Mechanism of Enhancement: Nanostructures effectively increase the contact interface area between the GaN device and the diamond substrate, thereby enhancing heat transfer efficiency.
Fabrication Demonstrated: Single-crystal diamond substrates with nanostructures (cylindrical, hemispherical, square column designs) were experimentally prepared using Microwave Plasma Chemical Vapor Deposition (MPCVD) followed by top-down plasma etching techniques.

Technical Specifications

Parameter	Value	Unit	Context
Optimal Max Temperature (T_max)	114.88	°C	Diamond substrate, Size 2 Square Nanopillars
T_max (Sapphire Substrate, Ideal)	121.82	°C	Baseline comparison, no dielectric layer
T_max (Diamond Substrate, Ideal)	116.65	°C	Conventional diamond, no dielectric layer
T_max (Diamond + PMMA Interlayer)	134.82	°C	Non-ideal contact, PMMA thermal conductivity 0.192 W/(m·K)
Diamond Thermal Conductivity	2000	W/(m·K)	Material coefficient used in simulation
Sapphire Thermal Conductivity	25.12 (@100 °C)	W/(m·K)	Material coefficient used in simulation
GaN Thermal Conductivity	130	W/(m·K)	Material coefficient used in simulation
Heat Generation Rate (Simulated)	1	W	Distributed throughout the GaN device
Air Flow Rate (Simulation)	0.1	m/s	Ambient condition
Optimal Square Nanopillar Height (H)	2000	µm	Size 2 structure (L: 2097 nm)
Radiator Base Dimensions (L x W x H)	8 x 8 x 0.8	mm	Copper heat sink geometry
GaN Device Dimensions (R x H)	1000 x 100	µm	Simplified model dimensions
Diamond Density	3515	g/cm³	Material coefficient

Key Methodologies

The study combined computational modeling using COMSOL Multiphysics with experimental fabrication of nanostructured diamond substrates.

1. COMSOL Simulation (v4.2)

Modeling Focus: Solid heat transfer and fluid heat transfer (air flow) to simulate thermal distribution in a natural environment.
Device Setup: GaN device (1 W heat source) attached to a substrate (Sapphire or Diamond) and mounted on a copper heat sink with four fins.
Boundary Conditions: Ambient temperature set at 25 °C; air flow rate set at 0.1 m/s; all external surfaces (except air inlet/outlet) were thermally insulated.
Contact Modes: Simulated both ideal (no dielectric) contact and non-ideal contact using a PMMA dielectric interlayer (0.192 W/(m·K)).
Nanostructure Optimization: Three shapes (cylindrical, hemispherical, square column) were simulated across three size groups (Size 1, 2, 3), maintaining a consistent contact interface area for comparison.

2. Diamond Substrate Fabrication (MPCVD)

Growth Method: Single-crystal diamond substrate growth via Microwave Plasma Chemical Vapor Deposition (MPCVD, Opto-Systems ARDIS-300).
Seed Pretreatment: Meticulous cleaning and H₂ plasma etching (900 °C, 250 Torr pressure, 3000 W microwave power for 30 min) to eliminate surface impurities.
Polycrystalline Suppression: A circular Molybdenum (Mo) bracket was incorporated in the central region of the CVD chamber to suppress polycrystalline nucleation along the substrate edges.

3. Nanostructure Etching (Top-Down Plasma)

Masking: A self-organized nickel (Ni) mask was created using rapid thermal treatment.
Cylindrical Nanopillar Etching (ICP):
- Process: Combined Inductively Coupled Plasma (ICP) and MPCVD etching.
- Conditions: RF/ICP power: 100/800 W. Gas flow: O₂ 20 sccm. Pressure: 200 mtorr.
Hemispherical Nanostructure Etching (H₂ Plasma):
- Process: Hydrogen plasma etching.
- Conditions: Microwave power: 2000 W. Gas flow: H₂ 100 sccm. Pressure: 150 torr.
Cleaning: All nanostructured substrates were cleaned in a 1:1 dilute nitric acid solution to remove residual Ni particles.

Commercial Applications

This technology is critical for advancing devices requiring high power density and superior thermal management, particularly those utilizing GaN wide-bandgap semiconductors.

High Power RF Devices: Essential for GaN High Electron Mobility Transistors (HEMTs) and Monolithic Microwave Integrated Circuits (MMICs) used in radar, electronic warfare, and satellite communications.
5G/6G Infrastructure: Enables the development of smaller, more powerful, and more reliable base station amplifiers and transmitters by managing the intense heat generated by high-frequency operation.
Power Electronics: Applicable to high-efficiency power converters, inverters, and motor drives where GaN devices replace silicon, requiring robust thermal pathways for high-temperature operation (e.g., electric vehicles, industrial power supplies).
High-Density IC Packaging: Provides a solution for thermal bottlenecks in highly compacted integrated circuits where multiple functional units are integrated, requiring chip-level heat removal.
Aerospace and Defense: Supports the creation of lightweight, high-power electronic systems where thermal stability and reliability under extreme conditions are paramount.

View Original Abstract

In recent years, the rapid progress in the field of GaN-based power devices has led to a smaller chip size and increased power usage. However, this has given rise to increasing heat aggregation, which affects the reliability and stability of these devices. To address this issue, diamond substrates with nanostructures were designed and investigated in this paper. The simulation results confirmed the enhanced performance of the device with diamond nanostructures, and the fabrication of a diamond substrate with nanostructures is demonstrated herein. The diamond substrate with square nanopillars 2000 nm in height exhibited optimal heat dissipation performance. Nanostructures can effectively decrease heat accumulation, resulting in a reduction in temperature from 121 °C to 114 °C. Overall, the simulation and experimental results in this work may provide guidelines and help in the development of the advanced thermal management of GaN devices using diamond micro/nanostructured substrates.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi14081559

References

2008 - GaN-Based RF Power Devices and Amplifiers [Crossref]
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2011 - AlGaN/GaN high-electron mobility transistors with low thermal resistance grown on single-crystal diamond (111) substrates by metalorganic vapor-phase epitaxy [Crossref]
1986 - Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer [Crossref]
2004 - 30-W/mm GaN HEMTs by Field Plate Optimization [Crossref]