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, Université Sorbonne Paris Nord
Citations	10
Analysis	Full AI Review Included

Executive Summary

This study demonstrates the successful synthesis of high-quality, transparent Nanocrystalline Diamond (NCD) films on temperature-sensitive glass substrates using a Distributed Antenna Array (DAA) Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) system.

Low-Temperature Synthesis: NCD films were grown effectively at substrate temperatures as low as 265 °C, enabling deposition on low melting-point materials like borosilicate and soda-lime glass.
High Optical Purity: The resulting films exhibit a high diamond sp³ fraction (up to 84%) and a wide optical band gap (3.55 eV), ensuring high transparency in the visible range.
Excellent Transparency: Transmittance of the NCD/glass system is consistently greater than 75% across the 400-900 nm wavelength range, with a low absorption coefficient (less than 10³ cm^-1).
Ultra-Smooth Surface: The films exhibit extremely low root mean square (Rms) roughness, typically below 10 nm, which minimizes light scattering and is ideal for optical coatings.
Scalability: The DAA system, featuring 16 coaxial sources arranged in a matrix, provides a large and homogeneous plasma (>600 cm²), promising scalability for industrial applications.
Surface Modification: NCD coating successfully shifts the surface wettability from hydrophilic (bare glass) to a more hydrophobic regime (contact angle increased to ~76°).

Technical Specifications

Parameter	Value	Unit	Context
Reactor Type	Distributed Antenna Array (DAA) MPACVD	N/A	16 coaxial microwave plasma sources
Microwave Power	3	kW	Total input power
Operating Pressure	0.35	mbar	Low-pressure synthesis
Substrate Temperature (T_sub)	265 - 400	°C	Range investigated; best results <300 °C
Gas Mixture Composition	96.4% H₂, 2.6% CH₄, 1% CO₂	%	Standard NCD growth mixture
Total Gas Flow Rate	50	sccm	Constant flow rate
Maximum Growth Rate	55	nm·h^-1	Achieved at 400 °C
Typical Rms Roughness	<10	nm	Range 4.9-10.5 nm
Diamond Grain Size (XRD)	12 ± 3	nm	Characteristic of nanocrystalline structure
Diamond Purity (sp³ fraction)	79 - 84	%	Calculated from Raman spectra
Optical Transmittance (T)	>75	%	Measured between 400 and 900 nm
Optical Band Gap (E_g)	3.55 ± 0.35	eV	High value indicating high purity
Absorption Coefficient (α)	<10³	cm^-1	Low absorption in the visible range (480-900 nm)
Water Contact Angle (NCD)	~76	°	Shift from hydrophilic (bare glass) to hydrophobic

Key Methodologies

The NCD films were synthesized using a Distributed Antenna Array (DAA) microwave reactor, which utilizes 16 coaxial microwave plasma sources arranged in a 2D matrix to achieve large-area, homogeneous plasma (>600 cm²).

Substrate Preparation: Borosilicate (250 µm thick) and soda-lime (1 mm thick) glass slides were used.
Seeding: Substrates were seeded by spin coating a colloidal solution containing 25 nm diamond powder (SYP-GAF-0-0.05) diluted in water with Polyvinyl Alcohol (PVA) to prevent coagulation.
Deposition Parameters:
- Gas mixture: 96.4% H₂, 2.6% CH₄, 1% CO₂ (50 sccm total flow).
- Pressure: 0.35 mbar.
- Microwave Power: 3 kW.
- Substrate Temperature Control: Regulated via a graphite heater embedded in a molybdenum holder (T_sub varied 265-400 °C).
Morphological and Structural Characterization:
- Scanning Electron Microscopy (SEM): Used to assess surface homogeneity and the presence of nanometric spherulitic aggregates.
- Atomic Force Microscopy (AFM): Used to analyze surface topography and measure Rms roughness (tapping mode, 1 x 1 µm² area).
- X-ray Diffraction (XRD): Used to confirm crystalline diamond presence and estimate grain size (~12 nm) using a modified Scherrer equation.
- Raman Spectroscopy (473 nm excitation): Used to determine phase purity, identify sp³ diamond and sp² non-diamond phases (graphite D/G bands, TPA bands), and calculate the sp³ fraction (Equation 1).
Optical and Surface Characterization:
- UV-Visible Reflectometry: Used to determine film thickness, transmittance (T_sys), and reflectance (R_sys).
- Absorption Coefficient (α) and Band Gap (E_g): Calculated iteratively from T and R measurements (Equations 3-6), followed by Tauc’s model fitting (Equation 7).
- Wettability: Measured using a drop shape analyzer (KRÜSS DSA25) to record the water contact angle (2 µL dH₂O drop).

Commercial Applications

The ability to deposit highly transparent, ultra-smooth, and protective diamond films on large areas of low-temperature substrates (like glass) makes this DAA technology highly relevant for several optical and protective coating markets.

Optical Devices and Windows:
- High-power CO₂ laser windows (leveraging diamond’s transparency and thermal properties).
- Protective coatings for optical materials, including optical fibers and lenses.
Consumer Optics:
- Ophthalmic glasses (lenses) requiring scratch resistance and high clarity.
- Outdoor operating optics and displays (due to high transparency and enhanced hydrophobicity).
Energy and Photovoltaics:
- Anti-reflecting and protective coatings for solar cells, improving efficiency and durability.
Large-Scale Industrial Coating:
- The DAA system’s inherent scalability (large plasma area >600 cm²) allows for the industrial manufacturing of diamond coatings on large glass panels or complex 3D substrates.

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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