Synthesis of High Quality Transparent Nanocrystalline Diamond Films on Glass Substrates Using a Distributed Antenna Array Microwave System

At a Glance

Metadata	Details
Publication Date	2022-09-20
Journal	Coatings
Authors	Chaimaa Mahi, Ovidiu Brinza, Riadh Issaoui, Jocelyn Achard, Fabien Bénédic
Institutions	Sorbonne Université, Laboratoire des Sciences des Procédés et des Matériaux
Citations	10
Analysis	Full AI Review Included

Technical Documentation & Analysis: Transparent Nanocrystalline Diamond Films for Optical Devices

Executive Summary

This research successfully demonstrates the synthesis of high-quality, highly transparent Nanocrystalline Diamond (NCD) films on low-melting-point glass substrates using a Distributed Antenna Array (DAA) Microwave Plasma Chemical Vapor Deposition (MPCVD) system.

Low-Temperature Synthesis: NCD films were successfully grown on borosilicate and soda-lime glass at low substrate temperatures (T_sub 265-400 °C), enabling deposition on temperature-sensitive materials.
High Optical Performance: The resulting NCD/glass systems achieved high transmittance (>75%) across the visible and near-infrared spectrum (400-900 nm), confirming suitability for optical windows and coatings.
Material Purity: Films exhibited high diamond purity, with sp³ fractions reaching 84% and a wide optical band gap estimated at 3.55 ± 0.35 eV, indicating minimal sp² non-diamond absorption.
Ultra-Smooth Surface: The as-grown films displayed very low surface roughness (Rms 4.9-10.5 nm), characteristic of nanocrystalline structure, which eliminates the need for costly post-deposition polishing.
Surface Functionalization: Diamond deposition drastically modified the surface wettability, transitioning the substrates from hydrophilic (Contact Angle ~40-54°) to a more hydrophobic regime (Contact Angle ~76°).
Industrial Scalability: The DAA MPCVD technique provides a large, homogeneous plasma area (>600 cm²), supporting the industrial scale-up of transparent diamond coatings for large optical devices.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Temperature (T_sub) Range	265 - 400	°C	Low-temperature deposition range
Maximum Growth Rate	55 ± 6	nm·h^-1	Achieved at 400 °C (Sample S-3)
Film Thickness Range	79 - 153	nm	Investigated film thickness
RMS Roughness (Rms)	4.9 - 10.5	nm	As-grown surface quality (typically < 10 nm)
Diamond Grain Size	12 ± 3	nm	Estimated via XRD (Nanocrystalline structure)
Maximum sp³ Fraction	84	%	Achieved at 265 °C (Sample S-1)
Optical Band Gap (E_g)	3.55 ± 0.35	eV	High value indicating high purity
Transmittance (Visible/NIR)	>75	%	Measured between 400-900 nm
Absorption Coefficient (Visible)	<10³	cm^-1	From 480 nm up to 900 nm
Contact Angle (Hydrophobicity)	76.5 ± 3.8	°	NCD film on soda-lime glass (Sample S-2)
Reactor Plasma Area	>600	cm²	Distributed Antenna Array (DAA) system capability

Key Methodologies

The Nanocrystalline Diamond (NCD) films were synthesized using a specialized Distributed Antenna Array (DAA) MPCVD system, optimized for low-temperature, large-area deposition.

Reactor Technology: DAA Microwave Plasma CVD system composed of 16 coaxial microwave plasma sources arranged in a 2D matrix.
Substrate Preparation: Borosilicate (250 µm) and Soda-lime (1 mm) glass substrates were seeded via spin coating using a colloidal solution of 25 nm diamond powder (SYP-GAF-0-0.05) stabilized with polyvinyl alcohol (PVA).
Gas Mixture Composition: A specific gas mixture was employed to promote NCD growth:
- Hydrogen (H₂): 96.4%
- Methane (CH₄): 2.6%
- Carbon Dioxide (CO₂): 1%
Process Parameters:
- Total Gas Flow Rate: 50 sccm
- Operating Pressure: 0.35 mbar
- Microwave Power: 3 kW
Temperature Control: Substrate temperature was regulated using a graphite heater embedded in a molybdenum holder, with T_sub varied between 265 °C and 400 °C.
Characterization Techniques: Film properties were assessed using UV-visible reflectometry (thickness, reflectance), Field Emission Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) (roughness), Raman Spectroscopy (sp³ fraction, purity), X-ray Diffraction (XRD) (grain size), and Drop Shape Analysis (contact angle).

6CCVD Solutions & Capabilities

The successful synthesis of high-quality, transparent NCD films on glass highlights a critical need for specialized, high-purity polycrystalline diamond materials in the optical and protective coating sectors. 6CCVD is uniquely positioned to supply the materials and customization required to replicate and advance this research.

Requirement from Research Paper	6CCVD Solution & Capability	Value Proposition
Nanocrystalline Diamond (NCD) Films	Optical Grade PCD Wafers: 6CCVD provides high-purity Polycrystalline Diamond (PCD) optimized for optical transmission, serving as the direct material equivalent to the NCD films synthesized.	Guaranteed high sp³ content and low non-diamond phase absorption for maximum transparency (E_g > 3.5 eV).
Ultra-Low Roughness (Rms 4.9-10.5 nm)	Precision Polishing Services: We guarantee ultra-smooth surfaces on inch-size PCD wafers with Rms roughness < 5 nm, exceeding the as-grown quality reported in this study.	Minimizes light scattering and attenuation, critical for high-performance optical windows and anti-reflective coatings.
Thin Film Deposition (79-153 nm)	Custom Thickness Control: 6CCVD offers precise thickness control for both SCD and PCD materials, ranging from 0.1 µm up to 500 µm.	Enables exact replication of thin-film optical stacks and protective layers required for glass substrates and complex optics.
Large Area Requirements	Large Format PCD: We supply custom plates and wafers up to 125 mm in diameter (PCD), supporting the scale-up required for industrial optical device manufacturing.	Facilitates transition from R&D scale to commercial production volumes for large optical components.
Future Functionalization (e.g., Electrodes)	In-House Metalization: We offer internal capabilities for depositing standard and custom metal stacks (Au, Pt, Pd, Ti, W, Cu), enabling functionalization for integrated optical devices or sensors.	Streamlines the supply chain for integrated optical components requiring subsequent processing steps.
Alternative Materials	Boron-Doped Diamond (BDD): For applications requiring transparent conductive coatings or electrochemical functionality, 6CCVD supplies highly conductive BDD films (SCD or PCD).	Allows researchers to extend the NCD platform into electro-optical or sensing applications while maintaining high transparency.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD diamond growth kinetics and material characterization. We can assist engineers and scientists with material selection, optimizing surface termination (e.g., hydrogen termination for enhanced hydrophobicity, as discussed in the paper), and defining custom specifications for similar optical window, protective coating, or anti-reflective coating projects.

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

View Original Abstract

Diamond is a material of choice for the fabrication of optical windows and for protective and anti-reflecting coatings for optical materials. For these kinds of applications, the diamond coating must have a high purity and a low surface roughness to guarantee a high transparency. It should also be synthesized at low surface temperature to allow the deposition on low melting-point substrates such as glasses. In this work, the ability of a Distributed Antenna Array (DAA) microwave system operating at low temperature and low pressure in H2/CH4/CO2 gas mixture to synthesize nanocrystalline diamond (NCD) films on borosilicate and soda-lime glass substrates is investigated aiming at optical applications. The influence of the substrate temperature and deposition time on the film microstructure and optical properties is examined. The best film properties are obtained for a substrate temperature below 300 °C. In these conditions, the growth rate is around 50 nm·h−1 and the films are homogeneous and formed of spherical aggregates composed of nanocrystalline diamond grains of 12 nm in size. The resulting surface roughness is then very low, typically below 10 nm, and the diamond fraction is higher than 80%. This leads to a high transmittance of the NCD/glass systems, above 75%, and to a low absorption coefficient of the NCD film below 103 cm−1 in the visible range. The resulting optical band gap is estimated at 3.55 eV. The wettability of the surface evolves from a hydrophilic regime on the bare glass substrates to a more hydrophobic regime after NCD deposition, as assessed by the increase of the measured contact angle from less than 55° to 76° after the deposition of 100 nm thick NCD film. This study emphasizes that such transparent diamond films deposited at low surface temperature on glass substrate using the DAA microwave technology can find applications for optical devices.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings12101375

References

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