Microwave plasma modelling in clamshell chemical vapour deposition diamond reactors

At a Glance

Metadata	Details
Publication Date	2022-02-17
Journal	Diamond and Related Materials
Authors	Jerome A. Cuenca, Soumen Mandal, Evan L. H. Thomas, Oliver A. Williams
Institutions	Cardiff University
Citations	31
Analysis	Full AI Review Included

Technical Documentation & Analysis: MPCVD Reactor Optimization

Executive Summary

This research provides critical insights into achieving spatial homogeneity in Microwave Plasma Chemical Vapor Deposition (MPCVD) diamond growth, a core challenge for scaling high-quality materials.

Core Challenge Addressed: Optimization of the sample holder (puck) geometry to ensure uniform plasma density and substrate temperature, which directly dictates diamond film quality and growth rate uniformity across the wafer.
Methodology: A sophisticated multi-physics Finite Element Modeling (FEM) approach coupling Electromagnetic (EM), Plasma Fluid, and Heat Transfer solutions was used to simulate the TM_0(n>1)p clamshell reactor style.
Key Finding (Thermal Management): Substrate temperature is the dominant factor determining spatial uniformity. Shallow pucks (5 mm) are heavily cooled by the stage, while excessively tall pucks (20 mm) severely perturb the resonant frequency, preventing stable plasma ignition.
Optimal Result: The 10 mm Molybdenum (Mo) puck yielded the most consistent diamond quality and uniformity (highest sp³/sp² ratio across the center), correlating with a stable growth temperature (~790 °C).
Material Relevance: The study validates the necessity of precise thermal and electromagnetic engineering for producing high-quality Polycrystalline Diamond (PCD) films, essential for large-area electronic and thermal applications.
6CCVD Value Proposition: 6CCVD specializes in providing custom SCD and PCD substrates, along with the necessary engineering support (custom pucks, metalization, and thermal modeling consultation) required to replicate and scale these optimized growth conditions.

Technical Specifications

Data extracted from the modeling and experimental validation of the MPCVD process.

Parameter	Value	Unit	Context
Reactor Topology	TM_0(n>1)p Clamshell	N/A	Seki Diamond 6K style (Carat CTS6U)
Substrate Material	Silicon (Si)	N/A	1” diameter, 0.5 mm thickness
Sample Holder Material	Molybdenum (Mo)	N/A	Puck diameter modeled 40 to 60 mm
Optimal Puck Height ($h_{puck}$)	10	mm	Yielded best sp³/sp² uniformity
High Microwave Power Density (MWPD)	5.0	kW	Growth condition
High Pressure (Growth)	160	mbar	Growth condition
Gas Mixture	3% CH₄ in H₂	N/A	Total flow rate 300 sccm
Initial Resonant Frequency	~2.45	GHz	Unperturbed EM model
Max Frequency Perturbation ($\Delta f$)	~-66	MHz	Observed at $h_{puck}$ = 20 mm
Target Substrate Temperature	~800	°C	Typical CVD diamond growth temperature
Measured Substrate Temperature (Optimal)	~790	°C	Measured for 10 mm puck
Max Electron Density (High MWPD)	10 x 10¹⁷	m^-3	Calculated in plasma fluid model
Diamond Quality Metric	sp³/sp² Ratio	N/A	Measured via Raman d/G line scans

Key Methodologies

The experimental and modeling approach relied on a tightly coupled multi-physics simulation validated by physical growth and characterization.

Multi-Physics FEM: The process utilized COMSOL Multiphysics to sequentially solve three coupled models: Electromagnetic (EM) Eigenfrequency, Frequency-Transient EM/Plasma Fluid, and Transient Heat Transfer.
EM Modeling: Calculated the E-field distribution and resonant frequency shifts caused by the Mo puck. This step identified the TM₀₁₁ mode and demonstrated that taller pucks significantly perturb the chamber frequency.
Plasma Fluid Dynamics: Employed a simplified H₂ reaction cross-section set (Itikawa database) to model electron density ($n_e$) and plasma shape as a function of power and pressure, showing that high MWPD results in a smaller, focused elliptical plasma.
Heat Transfer Solution: Calculated the spatial gas and substrate temperature profiles, confirming that temperature variation (not just electron density) is the primary driver of non-uniform growth rate.
Substrate Preparation: 1” Si wafers (0.5 mm thick) were seeded using an ultrasonic nanodiamond colloidal solution process.
Growth Parameters: Fixed growth time (30 minutes) at 5 kW forward power and 160 mbar pressure, using 3% CH₄ in H₂.
Characterization: Raman spectroscopy (532 nm laser) line scans were used to map the sp³/sp² ratio (d/G ratio) across the wafer, complemented by Scanning Electron Microscopy (SEM) to observe grain size variation.

6CCVD Solutions & Capabilities

This research underscores the critical role of precise material engineering and thermal management in MPCVD. 6CCVD is uniquely positioned to supply the necessary materials and technical expertise to advance this research into scalable, high-performance diamond products.

Applicable Materials for Replication and Scaling

The paper focused on thin film PCD on Si. 6CCVD offers materials optimized for both uniformity and specific applications:

Electronic Grade PCD Wafers: For applications requiring large-area uniformity (up to 125 mm diameter), 6CCVD provides highly polished PCD substrates (Ra < 5nm) that minimize surface defects and promote homogeneous nucleation, directly addressing the uniformity challenges highlighted in the paper.
Optical Grade SCD Substrates: For high-value applications like quantum sensing (NV centers) or high-power optics, where even minor spatial variations are unacceptable, 6CCVD supplies Single Crystal Diamond (SCD) plates with exceptional purity and thickness control (0.1 µm to 500 µm).
Boron-Doped Diamond (BDD): If the research were extended to electrochemical applications (as referenced in the paper’s citations), 6CCVD provides custom BDD films and substrates with tailored doping levels.

Customization Potential & Engineering Services

The success of the 10 mm Mo puck demonstrates that precise reactor component geometry is non-negotiable for optimal growth. 6CCVD offers comprehensive customization capabilities:

Research Requirement	6CCVD Custom Solution	Technical Specification
Custom Sample Holders	Precision machining of Mo, W, or Ta pucks/stages.	Custom dimensions (diameter, height) to match specific reactor EM profiles and thermal mass requirements.
Substrate Dimensions	Custom plates and wafers for SCD and PCD.	Plates/wafers up to 125 mm (PCD). Thickness control from 0.1 µm to 10 mm.
Thermal Management	Custom metalization layers for improved heat spreading or electrical contact.	Internal capability for Au, Pt, Pd, Ti, W, Cu metalization stacks.
Surface Finish	Ultra-low roughness polishing for homoepitaxy.	Ra < 1nm (SCD) and Ra < 5nm (Inch-size PCD).
Logistics	Global supply chain management.	Global shipping (DDU default, DDP available) ensures rapid delivery of custom components.

Engineering Support for Uniformity Optimization

The paper concluded that monitoring spatial temperature is paramount. 6CCVD’s in-house PhD team offers consultation services to assist engineers and scientists in optimizing their MPCVD processes for spatial homogeneity:

Thermal Modeling Consultation: Assistance in correlating pyrometer measurements with substrate temperature profiles, crucial for replicating the optimal 790 °C growth conditions identified in this study.
Material Selection Guidance: Expert advice on selecting the optimal substrate material (SCD vs. PCD) and holder material (Mo vs. W) based on thermal conductivity, EM properties, and target application (e.g., high-rate growth vs. quantum purity).
Process Scaling Support: Guidance on transitioning from small 1” samples (used in this study) to larger diameter wafers (up to 125 mm PCD) while maintaining the required plasma and thermal uniformity.

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

View Original Abstract

A microwave plasma model of a chemical vapour deposition (CVD) reactor is presented for understanding spatial heteroepitaxial growth of polycrystalline diamond on Si. This work is based on the TM0(n>1) clamshell style reactor (Seki Diamond/ASTEX SDS 6K, Carat CTS6U, ARDIS-100 style) whereby a simplified H2 plasma model is used to show the radial variation in growth rate over small samples with different sample holders. The model uses several steps: an electromagnetic (EM) eigenfrequency solution, a frequency-transient EM/plasma fluid solution and a transient heat transfer solution at low and high microwave power densities. Experimental growths provide model validation with characterisation using Raman spectroscopy and scanning electron microscopy. This work demonstrates that shallow holders result in non-uniform diamond films, with a radial variation akin to the electron density, atomic H density and temperature distribution at the wafer surface. For the same process conditions, greater homogeneity is observed for taller holders, however, if the height is too extreme, the diamond quality reduces. From a modelling perspective, EM solutions are limited but useful for examining electric field focusing at the sample edges, resulting in accelerated diamond growth. For better accuracy, plasma fluid and heat transfer solutions are imperative for modelling spatial growth variation.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.diamond.2022.108917

References

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