Impact of diamond seeding on the microstructural properties and thermal stability of GaN-on-diamond wafers for high-power electronic devices

At a Glance

Metadata	Details
Publication Date	2016-10-14
Journal	Scripta Materialia
Authors	Dong Liu, Daniel Francis, Firooz Faili, Callum Middleton, J. Anaya
Institutions	Element Six (United States), University of Bristol
Citations	50
Analysis	Full AI Review Included

Executive Summary

This documentation analyzes the key findings of the research on microstructural control and thermal stability in GaN-on-diamond wafers, focusing on implications for high-power electronic devices and connecting the requirements to 6CCVD’s advanced MPCVD capabilities.

Critical Parameter Identified: The size and density of initial diamond seeding nanoparticles critically determine the GaN/diamond interface quality and subsequent thermal performance.
Optimal Seeding: Use of 30 nm diamond seeds yielded superior material structure, preventing the formation of Type A (voids/through-holes) and Type B (pinholes/cavities) microscopic defects at the critical interface.
Enhanced Thermal Performance: Wafers fabricated with optimal seeding demonstrated a highly homogeneous thermal boundary resistance (TBR$_{\text{eff}}$) of 23 ± 3 m²K/GW across the entire 4-inch diameter.
Structural Reliability: The defect-free GaN-on-diamond material maintained structural integrity and stability even after high-temperature annealing at 825 °C in a nitrogen atmosphere, crucial for typical device processing steps (e.g., ohmic contact formation).
Suboptimal Results: Wafers seeded with 100 nm particles exhibited high TBR$_{\text{eff}}$ (up to 46 m²K/GW) and suffered structural degradation (void enlargement and crack formation) upon annealing.
Scaling Potential: The ability to produce homogeneous, high-stability GaN-on-diamond on 4-inch wafers confirms the viability of MPCVD diamond for ultra-high power microwave electronic devices.

Technical Specifications

Key material and thermal parameters extracted from the research for GaN-on-Diamond systems.

Parameter	Value	Unit	Context
Wafer Diameter	4	Inch	Large-scale integration for MMIC/PA
Diamond Growth Technique	Microwave Plasma CVD (MPCVD)	N/A	Used to deposit the 100 µm diamond layer
Diamond Layer Thickness	100	µm	Typical thickness analyzed
GaN Layer Thickness	900	nm	Full GaN layer thickness
Dielectric Seed Layer Thickness	30	nm	Amorphous layer (SiN) deposited via low-pressure CVD
Optimal Seeding Size	30	nm	Resulted in defect-free interface
Suboptimal Seeding Size	100	nm	Resulted in Type A & B defects
Optimal TBR$_{\text{eff}}$ (30 nm seeds)	23 ± 3	m²K/GW	Homogeneous thermal properties
Suboptimal TBR$_{\text{eff}}$ (100 nm seeds)	39 - 46	m²K/GW	Inhomogeneous thermal properties
Annealing Stability Temperature	825	°C	Stability maintained by 30 nm seeded material
Optimal Seeding Density	> 1 x 10¹¹	cm^-2	Required for uniform nucleation (30 nm seeds)
SiC Thermal Conductivity (Reference)	450	W/mK	Standard competitor material at 300K
Diamond Thermal Conductivity (Reference)	Up to 2000	W/mK	Required for high waste heat extraction

Key Methodologies

The following methods were critical to achieving and characterizing high-quality, thermally stable GaN-on-diamond wafers:

Starting Material Preparation: AlGaN/GaN heterostructures were grown on Si substrates (via MOCVD). The Si substrate and AlGaN strain relief layer were subsequently removed via grinding, lapping, and cleaning.
Interface Deposition: A 30 nm thin amorphous SiN dielectric layer was deposited via low-pressure CVD onto the exposed GaN N-face to protect the GaN surface during subsequent diamond growth.
Diamond Seeding Strategy: Diamond nanoparticles of two distinct average diameters (30 nm and 100 nm) were utilized to control crystal grain size at the nucleation layer.
Bulk Diamond Growth: A 100 µm thick PCD layer was deposited using Microwave Plasma CVD (MPCVD), ensuring a harsh, high-temperature environment.
Thermal Reliability Testing: Samples underwent post-manufacture annealing at 825 °C in a nitrogen atmosphere, simulating conditions typical for ohmic contact formation.
Microstructural Analysis: Focused Ion Beam (FIB) milling was used to create precise cross-sections, followed by Scanning Electron Microscopy (SEM) imaging to evaluate defects (voids, cracks, pinholes).
Thermal Performance Mapping: Transient Thermoreflectance (TTR) was employed using a 355 nm Nd:YAG heating laser to map the effective Thermal Boundary Resistance (TBR$_{\text{eff}}$) across the 4-inch wafers non-destructively.

6CCVD Solutions & Capabilities

The findings underscore the paramount importance of controlling the diamond nucleation layer to achieve high-integrity GaN-on-diamond interfaces necessary for robust thermal management in high-power RF devices. 6CCVD provides the specialized MPCVD materials and processing services required to replicate and scale this advanced research.

Applicable Materials

To achieve the benchmark thermal performance demonstrated in this research (TBR$_{\text{eff}}$ of 23 ± 3 m²K/GW), the highest purity, thermal-grade diamond is required:

Thermal Grade Polycrystalline Diamond (PCD): 6CCVD specializes in high thermal conductivity (K > 1800 W/mK) PCD wafers grown via MPCVD, matching the material type and performance standards necessary for efficient heat spreading beneath GaN heterostructures.
Custom PCD Thickness: We offer PCD layers from 0.1 µm up to 500 µm. Researchers requiring the tested 100 µm thick layer, or those exploring thicker heat spreaders (up to 500 µm), can utilize our bespoke growth capability.

Customization Potential

The success of this research hinges on precision control over wafer dimensions and interfacial layers. 6CCVD’s fabrication capabilities directly support scaling and optimization:

Research Requirement	6CCVD Capability	Direct Benefit to Client
4-Inch Wafer Scale	Custom PCD Wafers up to 125mm	Enables high-volume, large-scale production of GaN MMICs/PAs.
Thin Diamond Layer (100 µm)	Thickness control from 0.1 µm to 500 µm	Precise material management for optimizing thermal performance and minimizing material costs.
Interface Control	SCD/PCD Polishing to Ra < 5 nm	Ensures optimal surface roughness for uniform dielectric layer deposition and subsequent nanoparticle seeding.
Post-Process Functionalization	Custom Metalization Services (Au, Pt, Ti, W, Cu)	Enables integration of high-reliability ohmic contacts and thermal layers post-GaN transfer/annealing.
Substrate Removal/Handling	Substrates available up to 10 mm thickness	Provides rigid handling support for thin-film GaN processing prior to device lift-off or transfer.

Engineering Support

This study highlights that the choice of seeding methodology is a crucial CVD recipe parameter influencing the final device performance. 6CCVD offers consultation services to optimize this complex process:

Interface Optimization: Our in-house PhD team can assist engineers and scientists in selecting and developing the appropriate CVD parameters (pressure, temperature, gas mixture) and initial surface preparation techniques (such as specific nanoparticle seeding methods) to ensure a defect-free GaN/diamond interface.
Thermal Management Analysis: We provide materials specifications and support for projects aiming to minimize TBR$_{\text{eff}}$ for similar ultra-high power microwave electronic devices.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. 6CCVD offers global shipping (DDU default, DDP available) for your advanced diamond material needs.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.scriptamat.2016.10.006