CVD Technologies
Chemical Vapor Deposition (CVD) may be defined as the deposition of a solid on heated surface from a chemical reaction in the vapor phase. In the case of diamond deposition, the gas mixture contains a carbon precursor as CH4 or CO2, and hydrogen.The carbon species and hydrogen must be activated since at low pressure, graphite is thermodynamically stable, and without activation, only graphite would be formed. Activation is obtained by two basic methods:
- High temperature: A filament is heated up 2400-2600K. Atomic hydrogen is formed and the carbon species become activated in the vicinity of the hot metal. The most common materials for filament are tungsten or tantalum.
- Plasma: The gas phase is ionized that leads to important concentrations of atomic hydrogen and activated carbon species. Two types of plasma are used glow discharge plasma (non-isothermal) and arc plasma (isothermal).
The following table summarizes the main characteristics of each CVD technologies, which have advantages and drawbacks.
| Hot Filament | Glow Discharge Plasma | DC Arc Plasma | |
| Gas temperature | 2000K | 3000K | 5000K |
| Substrate temperature | 750-850°C | 600-900°C | > 900°C |
| Surface | > 0.5 m² | < Φ200mm | < Φ20mm |
| Growth rate | 0.2-0.5 µm/h | 1-10 µm/h | 10-100 µm/h |
| Structure | Microcrystalline or nanocrystalline | Mono, micro or ultra-nanocrystalline | Large grained |
In order to deposit diamond film on large surface (up to 0.5 m²) and with a cost-effective process, NeoCoat has focused its process development on hot-filament CVD technique.