Effect of Substrate Holder Design on Stress and Uniformity of Large-Area Polycrystalline Diamond Films Grown by Microwave Plasma-Assisted CVD

At a Glance

Metadata	Details
Publication Date	2020-09-30
Journal	Coatings
Authors	Vadim Sedov, Artem Martyanov, A. S. Altakhov, А. Ф. Попович, Mikhail Shevchenko
Institutions	Cascade Technologies (United States), MIREA - Russian Technological University
Citations	44
Analysis	Full AI Review Included

Executive Summary

This research focused on optimizing substrate holder geometry in Microwave Plasma-Assisted CVD (MPCVD) to produce large-area (2-inch), thick (100 µm), low-stress polycrystalline diamond (PCD) films on thin silicon (Si) substrates.

Holder Comparison: Three molybdenum (Mo) holder geometries—Flat, Pocket, and Pedestal—were designed based on E-field simulations to control plasma homogeneity above the 2-inch wafer.
Optimal Design: The Pedestal geometry (4.5 kW, 55 Torr) provided the most homogeneous and intensive electric field distribution over the substrate area, minimizing edge effects.
Quality Improvement: The Pedestal holder successfully produced highly homogeneous microcrystalline diamond (MCD) films across the entire 2-inch diameter. In contrast, the Pocket design resulted in poor homogeneity and the formation of low-quality nanocrystalline diamond (NCD) phases near the edges.
Stress Reduction: The Pedestal design dramatically reduced compressive stress in the PCD film to 1.1-1.4 GPa, compared to 3.2 GPa measured in the Pocket-grown film.
Curvature Minimization: Low stress translated directly to minimal plate bending. The resulting “diamond-on-Si” plate displacement (Ah) was reduced to an extremely low 50 µm (Radius of Curvature R = 6.2 m) using the Pedestal holder, a significant improvement over the Pocket holder (470 µm).
Thermal Performance: PCD films grown using the optimized Pedestal geometry achieved thermal conductivities of 10 W/cm·K (100 µm thick) and 15 W/cm·K (200 µm thick) at room temperature, suitable for high-end heat sinks.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Si (111)	N/A	Mirror-polished monocrystalline
Substrate Size	2 (50.8)	inches (mm)	Wafer diameter
Substrate Thickness	0.35	mm	Thin Si wafer
Target PCD Thickness	100 ± 10	µm	Final film thickness
Target Growth Rate	~1	µm/hour	Used for 100-hour deposition runs
Substrate Temperature	850 ± 25	°C	Controlled via pyrometer
Microwave Frequency	2.45	GHz	ARDIS-100 reactor
MW Power (Pedestal Opt.)	4.5	kW	Optimized setting
Gas Pressure (Pedestal Opt.)	55	Torr	Optimized setting
Total Gas Flow Rate	500	sccm	CH₄/H₂ mixture
Methane Concentration	3	%	CH₄ in H₂
Pedestal/Pocket Stage Height	2	mm	Optimized based on E-field simulation
Final Plate Displacement (Pedestal)	50	µm	Curvature Ah (R = 6.2 m)
Compressive Stress (Pedestal)	1.1-1.4	GPa	Deduced from Raman shift (low stress)
Diamond Raman Peak (Pedestal)	1335.1 ± 0.3	cm^-1	High crystalline quality
Thermal Conductivity (100 µm PCD)	10	W/cm·K	Measured at room temperature
Thermal Conductivity (200 µm PCD)	15	W/cm·K	Measured at room temperature

Key Methodologies

E-Field Simulation: The finite element method (using >9000 elements) was applied to a generalized axisymmetric model of the ARDIS-100 reactor to calculate E-field strength and uniformity above the substrate surface for various holder heights (0 to 5 mm).
Holder Fabrication: Three molybdenum (Mo) holders (Flat, Pocket, Pedestal) were fabricated, with the pedestal stage height and pocket protective ring height set to 2 mm based on simulation results.
Substrate Seeding: Mirror-polished 2-inch Si (111) wafers (0.35 mm thick) were seeded using detonation nanodiamond slurries (5 nm average particle size) via 10 minutes of ultrasonic treatment, followed by spin-coating.
MPCVD Synthesis: Films were grown in an ARDIS-100 reactor using CH₄/H₂ gas mixtures (3% CH₄, 500 sccm total flow) at 850 ± 25 °C. Growth parameters (MW power and pressure) were slightly adjusted for each holder type to maintain a target growth rate of ~1 µm/hour.
In Situ Monitoring: Film thickness and growth rate were controlled using laser interferometry (λ_exc = 655 nm).
Characterization:
- Morphology/Phase: Scanning Electron Microscopy (SEM) and micro-Raman spectroscopy (λ = 473 nm) were used to assess crystalline quality and homogeneity (MCD vs. NCD phases).
- Stress/Curvature: Curvature (R) and displacement (Ah) of the final “diamond-on-Si” plates were measured using white light interferometry (NewView 5000).
- Thermal Conductivity (TC): The Si substrate was chemically removed (HNO₃-HF). TC was measured using the Laser Flash Technique (LFT) on 10x10 mm samples, requiring thin Ti layers (~400 nm) deposited for enhanced absorption/emissivity.

Commercial Applications

The successful synthesis of large-area, low-stress, high-quality PCD films using the optimized Pedestal holder design directly addresses critical needs in high-performance electronics and thermal management.

High-End Heat Sinks: The PCD layers (TC up to 15 W/cm·K for 200 µm thick films) are ideal for thermal management applications requiring record-high heat dissipation capabilities.
Power Electronics (GaN Devices): The low-stress nature (1.1-1.4 GPa) and minimal curvature (50 µm displacement) are crucial for integrating diamond thermal management layers with Gallium Nitride (GaN) devices, overcoming GaN’s inherent low thermal conductivity (2 W/cm·K).
High-Frequency Communications: Provides necessary thermal stability for electronic devices operating in extreme, high-power regimes.
Large-Area Substrates: The ability to produce 2-inch PCD plates with high homogeneity and low distortion meets the size and quality standards required by the modern electronic and optical industries.
CVD Reactor Technology: The findings provide a blueprint for designing and improving novel MPCVD reactors specifically aimed at manufacturing large-area, thick, homogeneous PCD plates for commercial use.

View Original Abstract

In this work, the substrate holders of three principal geometries (flat, pocket, and pedestal) were designed based on E-field simulations. They were fabricated and then tested in microwave plasma-assisted chemical vapor deposition process with the purpose of the homogeneous growth of 100-μm-thick, low-stress polycrystalline diamond film over 2-inch Si substrates with a thickness of 0.35 mm. The effectiveness of each holder design was estimated by the criteria of the PCD film quality, its homogeneity, stress, and the curvature of the resulting “diamond-on-Si” plates. The structure and phase composition of the synthesized samples were studied with scanning electron microscopy and Raman spectroscopy, the curvature was measured using white light interferometry, and the thermal conductivity was measured using the laser flash technique. The proposed pedestal design of the substrate holder could reduce the stress of the thick PCD film down to 1.1-1.4 GPa, which resulted in an extremely low value of displacement for the resulting “diamond-on-Si” plate of Δh = 50 μm. The obtained results may be used for the improvement of already existing, and the design of the novel-type, MPCVD reactors aimed at the growth of large-area thick homogeneous PCD layers and plates for electronic applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings10100939

References

2018 - Thermal conductivity of high purity synthetic single crystal diamonds [Crossref]
2019 - High Power (>27 W) Semiconductor Disk Laser Based on Pre-Metalized Diamond Heat-Spreader [Crossref]
2019 - A diamond made microchannel heat sink for high-density heat flux dissipation [Crossref]
2016 - Multifinger Indium Phosphide Double-Heterostructure Transistor Circuit Technology With Integrated Diamond Heat Sink Layer [Crossref]
2019 - Polycrystalline diamond films with tailored micro/nanostructure/doping for new large area film-based diamond electronics [Crossref]
2016 - Single crystal diamond wafers for high power electronics [Crossref]
1998 - Multilayer diamond heat spreaders for electronic power devices [Crossref]
2012 - Reduced Self-Heating in AlGaN/GaN HEMTs Using Nanocrystalline Diamond Heat-Spreading Films [Crossref]