Thermal Performance Improvement of AlGaN/GaN HEMTs Using Nanocrystalline Diamond Capping Layers

At a Glance

Metadata	Details
Publication Date	2022-09-07
Journal	Micromachines
Authors	Huaixin Guo, Yizhuang Li, Xinxin Yu, Jianjun Zhou, Yuechan Kong
Citations	12
Analysis	Full AI Review Included

Technical Documentation & Analysis: Nanocrystalline Diamond for GaN HEMT Thermal Management

Executive Summary

This research validates the use of Nanocrystalline Diamond (NDC) capping layers as superior heat spreaders for AlGaN/GaN High-Electron Mobility Transistors (HEMTs), a critical component in high-power RF electronics. 6CCVD’s Polycrystalline Diamond (PCD) materials are ideally suited to replicate and scale this technology.

Thermal Performance: NDC capping layers reduced the device thermal resistance (R) by 21.4% compared to conventional SiN passivation, confirming diamond’s role as an effective top-side heat spreader.
Electrical Enhancement: The improved thermal management resulted in a 27.9% increase in maximum drain current (I_DS,max) at V_GS = 1 V (950.45 mA/mm vs. 743.28 mA/mm for SiN).
RF Gain Improvement: Small signal gain at 10 GHz showed an average improvement of 36.7% (10.83-11.80 dB vs. 7.91-8.55 dB for SiN).
Process Compatibility: The “diamond-before-gate” approach successfully integrated 500 nm thick NDC layers with sub-0.5 µm gate structures, demonstrating feasibility for advanced RF device fabrication.
6CCVD Solution: 6CCVD offers high-quality, thermal-grade Polycrystalline Diamond (PCD) wafers up to 125mm in diameter, along with custom thickness control (0.1 µm to 500 µm) and integrated metalization services required for scaling this technology.

Technical Specifications

The following performance metrics highlight the significant advantages of NDC capping layers over standard SiN passivation for GaN HEMTs.

Parameter	NDC-GaN HEMTs Value	SiN-GaN HEMTs Value	Unit	Context / Improvement
Thermal Resistance (R)	20.54	26.12	K/W	21.4% Reduction (NDC vs. SiN)
Max Drain Current (I_DS,max)	950.45	743.28	mA/mm	27.9% Improvement (at V_GS = 1 V)
Cut-Off Frequency (f_T)	34.6	34.0	GHz	1.8% Improvement
Small Signal Gain (10 GHz)	10.83-11.80	7.91-8.55	dB	36.7% Average Improvement
NDC Layer Thickness	500	N/A	nm (0.5 µm)	Optimized for heat spreading
SiN Isolation Layer Thickness	20	200	nm	Used for AlGaN barrier protection
Gate Length (L_g)	0.3247	N/A	µm	Sub-0.5 µm RF device requirement
NDC Growth Temperature	710	N/A	°C	MPCVD process stability
NDC Growth Rate	80	N/A	nm/h	Controlled for high thermal conductivity

Key Methodologies

The successful fabrication of the NDC-GaN HEMTs relied on precise MPCVD growth and advanced multi-step etching techniques compatible with sub-micron features.

Mesa Isolation and Ohmic Metal: Conventional processing was used, defining source and drain metal thickness at 200 nm.
SiN Isolation Layer Deposition: A 20 nm thick SiN layer was deposited via Plasma Enhanced Chemical Vapor Deposition (PECVD) to protect the AlGaN barrier and minimize interfacial thermal resistance.
Nanocrystalline Diamond (NDC) Growth: The NDC capping layer (500 nm thick) was grown using Microwave Chemical Vapor Deposition (MPCVD) at 710 °C. The low growth rate (80 nm/h) ensured high quality and high thermal conductivity.
Hard Mask Deposition: A 150 nm SiN film was deposited by PECVD to serve as a hard mask for the subsequent diamond etching process in the gate region.
Multi-Step Diamond Etching: A three-step Inductively Coupled Plasma (ICP) etching technique was employed to define the sub-0.5 µm gate region:
- Step 1 (Rapid Etch): ICP with O₂/Ar atmosphere for verticality, removing ~80% of the NDC.
- Step 2 (Surface Quality): ICP with O₂ atmosphere to smooth the surface and ease burrs.
- Step 3 (Isolation Etch): Low power ICP with O₂ atmosphere and over-etching to remove the SiN isolation layer without damaging the underlying AlGaN barrier.
Schottky Gate Preparation: The gate metal (520 nm thick) was prepared using an e-beam evaporation method.
Metal Interconnection: Final source, gate, and drain circuit interconnection was implemented using Au deposition.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and fabrication services required to replicate and commercialize the high-performance GaN HEMT technology demonstrated in this research.

Applicable Materials

The research utilized Nanocrystalline Diamond (NDC) for its high thermal conductivity and compatibility with thin-film deposition. 6CCVD provides the ideal commercial equivalent:

Thermal Grade Polycrystalline Diamond (PCD): Our high-quality PCD films are optimized for thermal management applications, offering the necessary thermal conductivity to act as an effective heat spreader, directly replacing the NDC layer used in the study.
Custom Thickness Control: The paper specified a 500 nm (0.5 µm) NDC layer. 6CCVD routinely delivers PCD films with precise thickness control from 0.1 µm up to 500 µm, ensuring exact replication of the critical thermal path dimensions.

Customization Potential

Scaling this technology from 1 cm² research samples to commercial production requires advanced material handling and integration capabilities, which 6CCVD provides in-house.

Research Requirement	6CCVD Capability	Value Proposition
Substrate Size	Plates/wafers up to 125mm (PCD)	Enables high-volume manufacturing and integration onto industry-standard wafer sizes, far exceeding the 1 cm² samples used in the study.
Layer Thickness	SCD/PCD thickness from 0.1 µm to 500 µm	Guarantees precise control over the 500 nm NDC capping layer thickness, critical for thermal performance (R = 20.54 K/W).
Metalization Stack	In-house deposition of Au, Pt, Pd, Ti, W, Cu	Supports the complex ohmic and Schottky contacts (e.g., Au interconnection, Ti/Pt/Au stacks) required for GaN HEMTs.
Surface Finish	Polishing to Ra < 5 nm (Inch-size PCD)	Ensures ultra-smooth surfaces necessary for subsequent high-resolution lithography and deposition steps, especially for sub-0.5 µm gate fabrication.
Etching/Patterning	Custom laser cutting and patterning services	Facilitates the precise definition of the diamond layer in the gate region, supporting the “diamond-before-gate” process flow.

Engineering Support

The successful integration of diamond capping layers requires expertise in managing interfacial stress and thermal boundary resistance (TBR).

TBR Optimization: The research highlighted the importance of the 20 nm SiN isolation layer to avoid large interfacial thermal resistance. 6CCVD’s in-house PhD team specializes in optimizing diamond growth recipes and interface engineering to minimize TBR, ensuring maximum heat transfer efficiency for similar GaN HEMT Thermal Management projects.
Process Consultation: We offer consultation on material selection (e.g., choosing the optimal PCD grain size and quality) and process parameters (e.g., growth temperature compatibility with existing ohmic metals, as noted in the paper).

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

View Original Abstract

Nanocrystalline diamond capping layers have been demonstrated to improve thermal management for AlGaN/GaN HEMTs. To improve the RF devices, the application of the technology, the technological approaches and device characteristics of AlGaN/GaN HEMTs with gate length less than 0.5 μm using nanocrystalline diamond capping layers have been studied systematically. The approach of diamond-before-gate has been adopted to resolve the growth of nanocrystalline diamond capping layers and compatibility with the Schottky gate of GaN HEMTs, and the processes of diamond multi-step etching technique and AlGaN barrier protection are presented to improve the technological challenge of gate metal. The GaN HEMTs with nanocrystalline diamond passivated structure have been successfully prepared; the heat dissipation capability and electrical characteristics have been evaluated. The results show the that thermal resistance of GaN HEMTs with nanocrystalline diamond passivated structure is lower than conventional SiN-GaN HEMTs by 21.4%, and the mechanism of heat transfer for NDC-GaN HEMTs is revealed by simulation method in theory. Meanwhile, the GaN HEMTs with nanocrystalline diamond passivated structure has excellent output, small signal gain and cut-off frequency characteristics, especially the current-voltage, which has a 27.9% improvement than conventional SiN-GaN HEMTs. The nanocrystalline diamond capping layers for GaN HEMTs has significant performance advantages over the conventional SiN passivated structure.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi13091486

References

2017 - Nanocrystalline diamond integration with III-Nitride HEMTs [Crossref]
2021 - A numerical investigation of heat suppression in HEMT for power electronics application [Crossref]
2021 - Thermal stress modelling of diamond on GaN/III-Nitride membranes [Crossref]
2020 - Integration of GaN and diamond using epitaxial lateral overgrowth [Crossref]
2017 - Effect of self-heating on electrical characteristics of AlGaN/ GaN HEMT on Si (111) substrate [Crossref]
2019 - Selective area deposition of hot filament CVD diamond on 100 mm MOCVD grown AlGaN/GaN wafers [Crossref]
2013 - Impact of intrinsic stress in diamond capping layers on the electrical behavior of AlGaN/GaN HEMTs [Crossref]