Fast, Efficient Tailoring Growth of Nanocrystalline Diamond Films by Fine-Tuning of Gas-Phase Composition Using Microwave Plasma Chemical Vapor Deposition

At a Glance

Metadata	Details
Publication Date	2024-06-18
Journal	Materials
Authors	C.J. Tang, A.J.S. Fernandes, M. Facão, Alexandre F. Carvalho, Weixia Chen
Institutions	University of Aveiro, Changshu Institute of Technology
Citations	2
Analysis	Full AI Review Included

Executive Summary

High-Rate NCD Growth: Successfully achieved Nanocrystalline Diamond (NCD) films with exceptionally high average growth rates, ranging from 2.1 µm/h up to 6.7 µm/h, using high-power Microwave Plasma Chemical Vapor Deposition (MPCVD).
Efficient Tailoring: Demonstrated a simple and effective method to tailor NCD growth characteristics by fine-tuning small amounts of N₂ and O₂ additives in a standard 4% CH₄/H₂ plasma mixture.
Extended Parameter Range: Established a new operating parameter window for NCD formation, confirming stable NCD growth with O₂/N₂ flow ratios up to 3:1 (1 sccm N₂, 3 sccm O₂).
Superior Surface Quality: The resulting NCD films exhibited low Root Mean Square (RMS) surface roughness (as low as 291 nm) and small average grain sizes (approx. 31 nm), crucial for low-friction applications.
Crystallographic Control: The NCD films consistently showed a <110> preferred crystallographic orientation, a common feature for NCD films grown under these conditions.
Process Stability: The study validates the stability and reproducibility of the high-power MPCVD system across a wide range of substrate temperatures (inferred 672 °C to 864 °C), adjusted via substrate thickness and holder design.

Technical Specifications

Parameter	Value	Unit	Context
Maximum NCD Growth Rate (DNO2)	6.7	µm/h	1:1 N₂:O₂, 3.5 kW power
NCD Growth Rate Range (New Regime)	2.1 to 6.7	µm/h	N₂:O₂ ratios 1:1 to 1:3
Conventional MCD Growth Rate (D4)	2.5	µm/h	Pure 4% CH₄/H₂ mixture
Microwave Power Range	3.2 to 3.5	kW	Used for main NCD depositions
Operating Pressure	105	Torr	Fixed for all experiments
CH₄ Concentration	4	%	Fixed in H₂ mixture
N₂ Additive Flow Rate	1.0	sccm	Fixed for second and third series
O₂ Additive Flow Rate Range	1.0 to 3.0	sccm	O₂/N₂ ratios 1:1 and 3:1
Inferred Substrate Surface Temp. Range	672 to 864	°C	Simulated range based on Si thickness (1 mm to 5 mm)
RMS Surface Roughness (Sq) Low	291	nm	Sample DNO7 (5 mm thick Si)
RMS Surface Roughness (Sq) High	524	nm	Sample DNO9 (Long deposition time)
Average Grain Size (DNO2)	31 ± 10	nm	Calculated from XRD (220) peak FWHM
Dominant Crystallographic Texture	<110>	N/A	Observed in NCD films
Diamond Raman Peak (D4) FWHM	3.5	cm^-1	High-quality Microcrystalline Diamond (MCD) reference

Key Methodologies

MPCVD System: All depositions were performed using a high-power 5 kW MPCVD reactor (ASTeX PDS-18) under a fixed pressure of 105 Torr.
Gas Chemistry: The base gas mixture was fixed at 4% CH₄ in H₂. N₂ and O₂ were introduced as additives, primarily fixing N₂ at 1 sccm and varying O₂ from 1 sccm (1:1 ratio) to 3 sccm (1:3 ratio).
Substrate Preparation: <100>-oriented single-crystal Si substrates were pre-scratched with 0-0.5 µm diamond powder to ensure high nucleation density.
Substrate Temperature Control: Substrate surface temperature was indirectly controlled and varied systematically by three means:
- Adjusting microwave power (3.2 kW vs. 3.5 kW).
- Using Si wafers of different thicknesses (1 mm, 3 mm, 5 mm).
- Employing two types of Molybdenum (Mo) holders with distinct thermal contact areas to the water-cooled base.
Temperature Inference: Substrate surface temperatures were estimated using COMSOL Multiphysics 5.3a heat transfer simulations, confirming a successful NCD growth range between 672 °C and 864 °C.
Structural and Morphological Analysis:
- Growth Rate and Thickness: Measured via cross-sectional Scanning Electron Microscopy (SEM).
- Crystalline Quality: Assessed using micro-Raman spectroscopy (442 nm laser) to quantify the diamond (1333 cm^-1) peak relative to non-diamond sp² components (1500 cm^-1 band).
- Texture and Grain Size: Determined by X-ray Diffraction (XRD) (0-2θ scans). Average grain size was calculated using the Scherrer equation on the dominant (220) peak.

Commercial Applications

The ability to rapidly deposit uniform NCD films with high quality and low roughness opens avenues for industrial applications where diamond properties are needed but traditional MCD roughness is prohibitive.

Advanced Tribological Coatings:
- Wear-resistant coatings for high-speed moving parts, tools, and dies, leveraging diamond’s hardness combined with NCD’s low friction coefficient.
- Anti-friction coatings for Micro- and Nano-Electromechanical Systems (MEMS/NEMS) components, where surface smoothness is critical for device longevity and performance.
High-Volume Industrial Production:
- The high growth rates (up to 6.7 µm/h) significantly reduce deposition time, making the technology economically viable for large-area coating applications and scaling up manufacturing processes.
Biomedical Devices:
- Coatings for biocompatible implants and surgical tools, benefiting from diamond’s chemical inertness and NCD’s smooth surface finish, minimizing biological response and wear.
Electronic and Optical Components:
- Fabrication of diamond-based heat spreaders and windows where uniform thickness and high thermal conductivity are required, particularly in high-power electronics.

View Original Abstract

Nanocrystalline diamond (NCD) films are attractive for many applications due to their smooth surfaces while holding the properties of diamond. However, their growth rate is generally low using common Ar/CH4 with or without H2 chemistry and strongly dependent on the overall growth conditions using microwave plasma chemical vapor deposition (MPCVD). In this work, incorporating a small amount of N2 and O2 additives into CH4/H2 chemistry offered a much higher growth rate of NCD films, which is promising for some applications. Several novel series of experiments were designed and conducted to tailor the growth features of NCD films by fine-tuning of the gas-phase compositions with different amounts of nitrogen and oxygen addition into CH4/H2 gas mixtures. The influence of growth parameters, such as the absolute amount and their relative ratios of O2 and N2 additives; substrate temperature, which was adjusted by two ways and inferred by simulation; and microwave power on NCD formation, was investigated. Short and long deposition runs were carried out to study surface structural evolution with time under identical growth conditions. The morphology, crystalline and optical quality, orientation, and texture of the NCD samples were characterized and analyzed. A variety of NCD films of high average growth rates ranging from 2.1 μm/h up to 6.7 μm/h were successfully achieved by slightly adjusting the O2/CH4 amounts from 6.25% to 18.75%, while that of N2 was kept constant. The results clearly show that the beneficial use of fine-tuning of gas-phase compositions offers a simple and effective way to tailor the growth characteristics and physical properties of NCD films for optimizing the growth conditions to envisage some specific applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma17122976

References

2021 - Achieving Large Uniform Tensile Elasticity in Microfabricated Diamond [Crossref]
2022 - Plastic Deformation in Silicon Nitride Ceramics via Bond Switching at Coherent Interfaces [Crossref]
2023 - In Situ Transmission Electron Microscopy Investigation on Oriented Attachment of Nanodiamonds [Crossref]
1997 - Nanocrystalline Diamond Films: Transmission Electron Microscopy and Raman Spectroscopy Characterization [Crossref]