Thermal conductivity of ultrathin nano-crystalline diamond films determined by Raman thermography assisted by silicon nanowires

At a Glance

Metadata	Details
Publication Date	2015-06-01
Journal	Applied Physics Letters
Authors	J. Anaya, Stefano Rossi, M. Alomari, E. Kohn, Lajos Tóth
Institutions	Universität Ulm, Institute for Technical Physics and Materials Science
Citations	53
Analysis	Full AI Review Included

6CCVD Technical Analysis: Thermal Transport in Ultrathin Nano-Crystalline Diamond (c-NCD)

Reference: Anaya Calvo, J., et al. (2015). Thermal conductivity of ultrathin nano-crystalline diamond films determined by Raman thermography assisted by silicon nanowires. Applied Physics Letters, 106(22), 223101.

Executive Summary

This paper presents crucial data on the anisotropic thermal properties of ultra-thin columnar nano-crystalline diamond (c-NCD) films, solving a critical knowledge gap concerning heat flow near the nucleation interface necessary for advanced thermal management of AlGaN/GaN HEMTs.

Anisotropic Thermal Transport: Demonstrated significant anisotropy, finding the cross-plane thermal conductivity (κ_⊥) to be up to three times higher than the in-plane thermal conductivity (κ_||) near the Si substrate interface.
High Cross-Plane Performance: Measured peak cross-plane thermal conductivities (κ_⊥) up to 300 W/mK for 1 µm thick films, suggesting superior vertical heat extraction compared to the lateral spread near the nucleation zone.
Depth-Dependent Lateral κ: Determined a strong depth-dependence of lateral thermal conductivity, increasing from 65 W/mK (near the nucleation interface) up to 150 W/mK in the top 320 nm layer, driven by columnar grain expansion.
Interface Bottleneck Identified: Quantified the large Thermal Boundary Resistance (TBR) at the c-NCD/Si interface at 42.2 m²K/GW, confirming that the defective 5-25 nm transition zone dominates cross-plane resistance.
Advanced Metrology: Utilized a robust, steady-state technique relying on Raman thermography combined with custom-fabricated Silicon Nanowires (NWs) acting as highly accurate, localized surface nano-thermometers.
Application Impact: Results are directly applicable to optimizing the integration and thickness of diamond layers grown directly onto GaN devices to maximize heat dissipation efficiency.

Technical Specifications

Hard data extracted from the experimental results and parameters:

Parameter	Value	Unit	Context
Cross-Plane κ (1 µm film)	~300	W/mK	Vertical heat transport component
Cross-Plane κ (680 nm film)	~250	W/mK	Vertical heat transport component
In-Plane κ (Full 680 nm layer)	65 ± 5	W/mK	Lateral heat transport (dominated by small grains)
In-Plane κ (Top 320 nm layer)	150	W/mK	Lateral heat transport (shows grain growth impact)
TBR (c-NCD/Si Interface)	42.2 ± 1.0	m²K/GW	Thermal bottleneck resistance
Vertical Thermal Resistance (1 µm NCD)	3.3 ± 1.3	m²K/GW	Contribution of the NCD film itself
Thermal Anisotropy Ratio (κ_⊥ / κ_\|\|)	~3	N/A	Cross-plane is higher than in-plane near nucleation
Film Thicknesses Tested	1.0 and 0.68	µm	Ultra-thin columnar NCD
Heater Metal Stack	Ti (10 nm) / Au (100 nm)	nm	Adhesion/Conductive layers
CVD Growth Temperature	750	°C	Hot Filament CVD process
CVD Pressure	1.5	kPa	Growth environment parameter
Si Nanowire Diameter	~50	nm	Used for surface thermometry
Si Nanowire Length	2-5	µm	Used for surface thermometry

Key Methodologies

The study employed specialized CVD growth and sophisticated measurement techniques to isolate and quantify thermal parameters in the ultrathin diamond films.

Substrate Preparation & Nucleation: Single crystalline Si (100) wafers were used as substrates. Polycrystalline diamond nucleation was achieved via Bias Enhance Nucleation (BEN).
Diamond Growth: Columnar nano-crystalline diamond (c-NCD) films were grown using Hot Filament Chemical Vapor Deposition (HFCVD) at a temperature of 750 °C and a pressure of 1.5 kPa, utilizing a gas mixture of 0.4% CH₄ diluted in H₂.
Test Structure Fabrication (Heaters): Metal heaters were deposited onto the diamond surface using a 10 nm thick Titanium (Ti) adhesion layer followed by 100 nm of Gold (Au).
- In-Plane κ Measurement: Used thin membrane structures with line heaters (100 µm x 500 µm) etched free of the underlying Si substrate.
- Cross-Plane κ Measurement: Used annular (ring) heaters to establish a radially symmetric, near-isothermal heat source, minimizing lateral heat spread.
Raman Thermography: A Renishaw InVia system with a 488 nm Ar-ion laser and a 0.65 N.A. 50x objective (spot size < 1 µm) was used for temperature measurement.
Nano-Thermometry: Silicon Nanowires (NWs, 50 nm diameter) were deposited onto the metal heaters. The temperature dependence of the Si NW phonon frequency (Raman shift) was used as a precise, stress-free method for local temperature determination.
Data Extraction: Measured temperature profiles were fitted against a highly precise three-dimensional (3D) Finite Element Model (FEM) to simultaneously determine both κ_||, κ_⊥, and the Thermal Boundary Resistance (TBR) values.

6CCVD Solutions & Capabilities

This research highlights the critical importance of material engineering—specifically controlling grain size evolution and managing interfacial defects—to optimize diamond integration for high-power thermal management applications like AlGaN/GaN HEMTs. 6CCVD offers the specialized materials and customization required to replicate, extend, or improve upon this research.

Applicable Materials

The foundation of this research is ultra-thin nano-crystalline diamond (c-NCD), which is a specific morphology of Polycrystalline Diamond (PCD).

Ultra-Thin PCD: 6CCVD specializes in high-quality MPCVD Polycrystalline Diamond (PCD) films available across the required thickness range, from 0.1 µm up to 500 µm. We can supply materials with controlled grain structures to facilitate the study of columnar growth effects (as seen by the κ_|| depth dependence).
Optical Grade SCD: For maximum lateral heat spreading (necessary for integration near the channel), 6CCVD Single Crystal Diamond (SCD) offers κ > 2000 W/mK, far exceeding the 150 W/mK achieved even in the best-performing NCD layers of the study. SCD plates (up to 500 µm thick, Ra < 1 nm) provide the ultimate foundation for low-TBR interfaces.
Substrates: The study showed that the Si substrate interface is the thermal bottleneck (TBR = 42.2 m²K/GW). 6CCVD provides thick, high-purity SCD or PCD substrates (up to 10 mm thick) for superior heat sinking, ideal for replacing the limiting Si substrate in next-generation HEMT designs.

Customization Potential

The experimental success hinges on precise material fabrication, including specific dimensions and metal contacts. 6CCVD provides comprehensive custom engineering support:

Requirement from Paper	6CCVD Capability	Value Proposition
Thin Film Control	PCD layers from 0.1 µm	Enables direct replication of the 680 nm and 1 µm films studied.
Custom Dimensions	Plates up to 125 mm; Custom laser cutting	Provides wafers and precision cutting for complex line heater, ring heater, or membrane geometries (e.g., the required 100 µm x 500 µm membranes).
Metalization Layers	Au, Pt, Pd, Ti, W, Cu (Internal capability)	Direct application of the required Ti/Au (Titanium adhesion layer/Gold conductor) stack used for heater fabrication, eliminating outsourcing complexity.
Surface Quality	Polishing to Ra < 1 nm (SCD) or Ra < 5 nm (PCD)	Ensures atomically smooth surfaces necessary for accurate Raman thermography and stable integration of high-resolution sensors like the Si NWs.
Global Logistics	Global shipping (DDU default, DDP available)	Secure, reliable supply chain for time-sensitive R&D projects anywhere in the world.

Engineering Support

The observed anisotropy and depth-dependence of thermal conductivity are complex effects highly sensitive to CVD recipe parameters (e.g., CH₄ concentration, pressure) which affect the grain size and transition layer quality.

6CCVD’s in-house PhD engineering team can assist clients with material selection and design consultation for similar AlGaN/GaN HEMT thermal management projects. We offer guidance on optimizing material characteristics—such as target grain size, interface preparation, and metal stack selection—to minimize TBR and maximize heat spreading efficiency tailored to the specific device requirements.

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

View Original Abstract

The thermal transport in polycrystalline diamond films near its nucleation region is still not well understood. Here, a steady-state technique to determine the thermal transport within the nano-crystalline diamond present at their nucleation site has been demonstrated. Taking advantage of silicon nanowires as surface temperature nano-sensors, and using Raman Thermography, the in-plane and cross-plane components of the thermal conductivity of ultra-thin diamond layers and their thermal barrier to the Si substrate were determined. Both components of the thermal conductivity of the nano-crystalline diamond were found to be well below the values of polycrystalline bulk diamond, with a cross-plane thermal conductivity larger than the in-plane thermal conductivity. Also a depth dependence of the lateral thermal conductivity through the diamond layer was determined. The results impact the design and integration of diamond for thermal management of AlGaN/GaN high power transistors and also show the usefulness of the nanowires as accurate nano-thermometers.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4922035