Thermal stress modelling of diamond on GaN/III-Nitride membranes

At a Glance

Metadata	Details
Publication Date	2020-11-27
Journal	Carbon
Authors	Jerome A. Cuenca, Matthew D. Smith, D. Field, Fabien Massabuau, Soumen Mandal
Institutions	University of Cambridge, University of Glasgow
Citations	43
Analysis	Full AI Review Included

Thermal Stress Management in GaN/III-N Diamond Integration: A 6CCVD Technical Analysis

Executive Summary

This research investigates the critical challenge of thermal stress and membrane deformation (bow) when integrating MPCVD diamond directly onto GaN/III-Nitride (III-N) heterostructures for high-power electronic heat spreading applications.

Core Challenge: The significant mismatch in the Coefficient of Thermal Expansion (CTE) between diamond, GaN, and the Si frame induces large residual tensile stresses (up to 1.0 ±0.2 GPa) in the GaN membrane upon cooling from CVD growth temperatures (720-750 °C).
Deformation Risk: Membrane bow reached up to 58 µm for 5 mm diameter samples, posing a major obstacle for subsequent device fabrication steps like contact lithography.
Material Requirement: Polycrystalline Diamond (PCD) grown via MPCVD is necessary for large-area integration, offering high thermal conductivity (k > 2000 W/mK in SCD, high k in PCD).
Critical Thickness: Numerical modeling confirms that a CVD diamond layer as thin as 1 µm is sufficient to “lock” the membrane deformation, highlighting the extreme stiffness of diamond (Young’s Modulus E_Dia ≈ 1000 GPa).
Interface Engineering: An unexpected SiO₂ layer was identified at the diamond/AlN interface, potentially facilitating stable carbide bonding and requiring further investigation for adhesion strength.
Mitigation Strategy: The key to reducing final bow is pre-stressing the GaN/III-N membrane in the opposite direction prior to diamond deposition, ensuring a flat platform at CVD growth temperatures.

Technical Specifications

Parameter	Value	Unit	Context
Maximum Measured Tensile Stress (GaN)	1.0 ±0.2	GPa	Residual stress in GaN/III-N membrane (0.5 mm diameter)
Maximum Measured Membrane Bow	58	µm	5 mm diameter membrane after cooling
Diamond Thickness (Grown)	38 to 57	µm	Final Polycrystalline Diamond (PCD) layer
CVD Growth Temperature (T_g)	720 to 750	°C	Low-temperature MPCVD recipe
Plasma Power (Forward)	5.5	kW	MPCVD growth conditions
Methane Concentration	3	%	CH₄ in H₂ gas mixture
Typical Growth Rate	2 to 3	µm/hour	Rate achieved under specified conditions
Critical Locking Thickness (Modelled)	1	µm	Minimum diamond thickness required to lock membrane deformation
GaN Tensile Strength (Reported)	4 to 7.5	GPa	Mechanical limit of GaN at room temperature
Diamond Young’s Modulus (E_Dia)	1000	GPa	Value used in analytical/numerical models

Key Methodologies

The integration of CVD diamond onto GaN/III-N membranes involved precise etching, surface preparation, and controlled MPCVD growth:

Wafer Preparation: Commercially obtained GaN/III-N on Si wafers were diced into 15 mm x 15 mm squares.
Membrane Etching (High Power): A high-power Inductively Coupled Plasma (ICP) etch (900 W) was used from the Si side to thin the substrate and define the membrane pattern via photolithography.
Membrane Etching (Low Power): A subsequent lower-power ICP etch (600 W) completely removed the remaining Si, exposing the AlN layer on the GaN/III-N membranes.
Plasma Pre-treatment: A N₂/H₂ microwave plasma pre-treatment (1.5 kW, 20 Torr) was applied to the exposed membrane to control seeding density and increase surface oxygen content on the AlN.
Seeding: A non-ultrasonic nano-diamond colloid solution was pipetted onto the exposed GaN/III-N membrane side, followed by rinsing and drying at 115 °C.
MPCVD Growth: Diamond was grown in a CH₄/H₂ microwave plasma (5.5 kW, 110-120 Torr, 3% CH₄) at a deposition temperature of 720-750 °C for 19 hours.
Characterization: Residual stress was measured using Raman spectroscopy (514 nm laser), and membrane bow was quantified using surface profilometry (12 µm tip).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to support and advance research into GaN-on-diamond heat spreading by providing high-quality, customizable MPCVD diamond materials and advanced processing services.

Applicable Materials

To replicate and extend this research, high-quality Polycrystalline Diamond (PCD) is the required material, specifically optimized for thermal management applications.

High Thermal Conductivity PCD: 6CCVD offers high-purity PCD wafers with controlled grain size and minimal non-diamond carbon content, crucial for achieving the thermal conductivity necessary for effective GaN HEMT heat spreading (k > 2000 W/mK is achievable in high-quality SCD, and high k PCD is essential here).
Thickness Control: The study demonstrated that precise thickness control (38-57 µm) is necessary, but even 1 µm is sufficient to lock deformation. 6CCVD provides PCD films with thickness control ranging from 0.1 µm up to 500 µm, allowing researchers to precisely tune the mechanical locking effect.

Customization Potential

The limitations observed in the paper (small 5 mm membranes, damage due to thermal runaway, need for free-standing stacks) are directly addressed by 6CCVD’s advanced manufacturing capabilities:

Research Requirement	6CCVD Capability	Benefit to Researcher
Large Area Growth: Need for commercially viable, larger wafers.	PCD Wafers up to 125 mm diameter.	Enables scaling beyond the small 15 mm squares used in the study, facilitating commercial device production.
Free-Standing Stacks: Need to remove the Si frame after growth.	Precision Laser Cutting Services.	Allows for the creation of custom-shaped, free-standing GaN/III-N/Diamond stacks, ready for final device processing.
Interface Engineering: Need for stable, low-stress interfaces (e.g., SiO₂ layer).	Custom Metalization Services (Au, Pt, Ti, W, Cu).	We can deposit custom interlayers or adhesion layers (e.g., Ti/W/Au) to manage thermal boundary resistance (TBR) and adhesion strength, potentially replacing or optimizing the serendipitous SiO₂ layer.
Surface Quality: Need for smooth surfaces for subsequent lithography.	Advanced Polishing: Ra < 5 nm (Inch-size PCD).	Ensures the diamond surface is compatible with high-resolution contact lithography, mitigating challenges imposed by membrane bow.

Engineering Support

The primary challenge identified in this work is the management of thermal stress and membrane bow through pre-stressing. 6CCVD’s in-house PhD team specializes in the thermomechanical properties of CVD diamond integration.

Stress Mitigation Consultation: We offer consultation on material selection and deposition parameters (temperature, pressure, gas mixture) to minimize intrinsic and extrinsic stresses, supporting the development of pre-stressed GaN-on-Si wafers required to achieve a flat membrane during CVD growth.
Interface Optimization: Our experts can assist in designing and characterizing custom interlayers to optimize adhesion and reduce the high stresses (up to 1.0 GPa) observed at the GaN/III-N interface, crucial for preventing delamination.
Application Focus: We provide material selection support for similar GaN HEMT Heat Spreading projects, ensuring the chosen diamond grade meets both thermal conductivity and mechanical stability requirements.

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

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.carbon.2020.11.067

References

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2019 - Thermal conductivity of GaN, 71GaN, and SiC from 150 K to 850 K [Crossref]
2014 - Low thermal resistance GaN-on-diamond transistors characterized by three-dimensional Raman thermography mapping [Crossref]
1993 - Thermal conductivity of isotopically modified single crystal diamond [Crossref]
2011 - AlGaN/GaN high-electron mobility transistors with low thermal resistance grown on single-crystal diamond (111) substrates by metalorganic vapor-phase epitaxy [Crossref]
2012 - Reduced self-heating in AlGaN/GaN HEMTs using nanocrystalline diamond heat-spreading films [Crossref]
2017 - Thermal characterization of polycrystalline diamond thin film heat spreaders grown on GaN HEMTs [Crossref]
2015 - Thermal management of hotspots using diamond heat spreader on Si microcooler for GaN devices [Crossref]
2018 - Transfer of AlGaN/GaN RF-devices onto diamond substrates via van der Waals bonding [Crossref]