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6CCVD Research

Low-pressure diamond - from the unbelievable to technical products

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2021-03-16
JournalChemTexts
AuthorsRoland Haubner
InstitutionsTU Wien
Citations21
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Industrial Maturity: Low-pressure Chemical Vapor Deposition (CVD) diamond synthesis has transitioned from a scientific curiosity (1950s idea, 1982 realization) to an industrially manufactured product used across diverse technical fields.
  • Metastable Growth Mechanism: The process relies on non-equilibrium reactions driven by Atomic Hydrogen (at.H), which is generated by high temperature or plasma. At.H selectively etches sp2 (graphitic) carbon much faster than sp3 (diamond) carbon, enabling direct diamond growth.
  • Primary Synthesis Methods: Hot-Filament CVD (HF-CVD) is favored for wear applications and complex geometries despite lower growth rates (approx. 1 ”m/h). Microwave Plasma CVD (MW-CVD) is used for high-purity diamond required for optical and thermal applications (growth rates >10 ”m/h).
  • Key Material Properties: CVD diamond is utilized for its extreme hardness, highest thermal conductivity, high transparency (multispectral), and chemical inertness.
  • Doping and Functionality: Boron doping creates p-type semiconducting diamond, essential for high-performance electrodes used in electrochemical processes, such as industrial wastewater treatment.
  • Substrate Challenges: Coating hardmetals (WC-Co) requires specialized pre-treatment (e.g., etching or intermediate layers) to mitigate the negative catalytic effects and migration of the Cobalt (Co) binder phase.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Hot-Filament Temperature (TFil)2200 - 2500°CTypical range for H2 activation.
Substrate Temperature (Tsub)600 - 1000°CRequired range; must be <1000 °C to prevent graphitization.
Hot-Filament Growth Rate~1”m/hTypical rate for wear applications (lower purity).
Microwave Plasma Growth Rate>10”m/hHigher rate achieved with increased microwave power.
Gas Pressure (HF-CVD)20 - 200TorrOptimized range for at.H generation and transport.
Characteristic Diamond Raman Peak1332cm-1Peak intensity correlates with diamond crystallinity.
Boron Concentration (Gas Phase)<500ppm B/CConcentration range that improves crystallinity and growth rate.
Boron Incorporation (111 face)Up to 3at.%Maximum incorporation observed in (111) growth sectors.
Boron Incorporation (100 face)<0.3at.%Lower incorporation observed in (100) growth sectors.

Key Methodologies

Section titled “Key Methodologies”

  1. Atomic Hydrogen (at.H) Generation:

    • H2 gas is activated using high temperature (Hot-Filament, Acetylene Torch) or plasma (Microwave, DC Glow Discharge) to generate at.H.
    • The at.H concentration must be high enough to ensure the selective etching of non-diamond sp2 carbon, which is crucial for diamond stability in the metastable region.

  2. Gas Phase Composition Control (C/H/O Ratio):

    • Hydrocarbon precursors (e.g., CH4, acetone, ethanol) are mixed with H2. The ratio of at.H to carbon dictates the resulting morphology and quality.
    • High at.H/C ratio yields well-faceted, high-quality diamond crystals (low defect density).
    • Low at.H/C ratio yields un-faceted ballas, nanocrystalline diamond (NCD), or ultra-nanocrystalline diamond (UNCD) films, which have higher sp2 content.

  3. Substrate Pre-treatment and Nucleation:

    • Diamond nucleation is the first critical step. Nucleation rates can be increased by mechanical roughening (grinding) or seeding the substrate surface with nano-diamond particles.
    • For hardmetal (WC-Co) substrates, the Co binder must be managed:
      • Etching: Selective etching of Co using acids (e.g., Murakami/Caro etching) reduces surface Co concentration.
      • Stabilization: Heat treatment with elements like B or Si forms stable Co compounds, reducing Co migration and catalytic sp2 deposition.

  4. Process Parameter Optimization:

    • Substrate temperature must be tightly controlled (600-1000 °C) to maximize growth rate while preventing the transformation of diamond into graphite.
    • Gas pressure and activation temperature are optimized to maximize at.H transport to the substrate surface while maintaining a reasonable deposition rate.

  5. Doping for Specific Functionality:

    • Boron (B): Added for p-type semiconducting properties and electrochemical applications. Small B additions (<500 ppm B/C) also improve diamond crystallinity.
    • Nitrogen (N): Even small contaminations drastically influence crystal orientation and morphology, and change electrical/thermal properties.

Commercial Applications

Section titled “Commercial Applications”
  • Wear-Resistant Tool Coatings (HF-CVD):
    • Products: Diamond coatings on hardmetal (WC-Co) inserts and tools.
    • Benefit: High hardness and wear resistance, offering a cost-effective alternative to sintered polycrystalline diamond (PCD).
  • Thermal Management/Heat Spreaders (MW-CVD):
    • Products: High-purity, free-standing CVD diamond sheets.
    • Benefit: Highest thermal conductivity at room temperature, ideal for packaging semiconductors and high-power electronic devices.
  • Electrochemical Applications (Boron-Doped Diamond, BDD):
    • Products: BDD electrodes on substrates (e.g., Ti).
    • Benefit: Large electrochemical window and electrical conductivity used for industrial wastewater treatment (converting organic carbon completely to CO2).
  • Optical Windows and Lenses (High Transparency):
    • Products: Optically clean, single crystalline or high-purity polycrystalline diamond films.
    • Applications: X-ray detector windows (SEM, X-ray tubes), mid-infrared attenuated total reflectance (ATR) spectroscopy, and high-strength optical lenses.
  • Gemstones and Jewelry:
    • Products: CVD-grown and polished single crystalline diamonds (up to several carats).
    • Benefit: Economic production of high-quality synthetic gemstones.
View Original Abstract

Abstract The idea to grow diamond from the gas phase was born in the 1950s but it took about 30 years until first diamond layers directly grown from the gas phase on substrates were shown in Japan by Matsumoto and co-workers. During the first years of research the function of atomic hydrogen, various growth methods and process parameters were investigated. Research was primarily focused on applications for wear-resistant tools. For this topic the interactions of substrates like hardmetals and ceramics, with diamond deposition gas atmosphere, were investigated. Beside its superior hardness, diamond exhibits the highest heat conductivity, high transparency, high chemical inertness and suitable semiconducting properties. The various requirements for the areas of application of diamond required a division of diamond research into corresponding sub-areas. The hot-filament method is used mainly for wear applications, because it is highly suited to coat complex geometries, but the diamond contains some impurities. Another method is the microwave plasma system which allows the growth of pure diamond used for optical windows and applications requiring high thermal conductivity. Other research areas investigated include doped diamond for microelectronic or electrochemical applications (e.g. waste water treatment); ballas (polycrystalline, spherical diamond), NCD (nanocrystalline diamond) and UNCD (ultra-nanocrystalline diamond) for wear applications. It should be noted that CVD (chemical vapour deposition) diamond synthesis has reached the stage of industrial production and several companies are selling different diamond products. This work is intended to convey to the reader that CVD diamond is an industrially manufactured product that can be used in many ways. With correspondingly low costs for this diamond, new innovative applications appear possible.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1971 - Physical-chemical synthesis of diamond from vapor phase

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