Diamond is an allotropic form of carbon. Synthetic diamond is generally synthesized in high-pressure and high-temperature conditions similar to the conditions under which natural diamond is formed. However, it is also possible to obtain high-quality diamond film by chemical vapor deposition from a carbon precursor and hydrogen gas mixture.
The critical parameter of diamond growth is obtaining a high hydrogen radical concentration near the substrate surface. It contributes to the stabilization of the sp3 dangling bonds. Without this stabilizing effect, these bonds would not be maintained and the diamond plane would collapse into the graphite structure. The other function of atomic hydrogen is to remove graphite selectively. The etch rate of graphite is twenty times higher than the diamond one.
Several techniques are used in order to activate the gas phase.
Due to the high surface energy of diamond, its nucleation occurs through the formation of islands or clusters (Volmer-Weber mode), which then grow to impinge upon each other and coalesce. Nucleation is a critical phase in the diamond deposition process due to its strong influence on film roughness and pinhole formation.
The substrate surface must be treated before the deposition to achieve high nuclei density. Several techniques have been studied including ion bombardment, diamond powder scratching, and ultrasonic treatment with a diamond solution.
Except on a single-crystal diamond substrate, the diamond film exhibits a polycrystalline structure. As the process temperature is around 800-900°C and high nuclei density is easier to achieve on carbide-forming material, diamond films are generally deposited on silicon-based material (Si, SiC, Si3N4) or refractory metals and alloys.
To change the conductivity value of the diamond film, a boron-based component is added to the gas mixture during diamond deposition. A fine adjustment of boron precursor flow allows us to deposit diamond film with variable conductivity.
1. The effect of nitrogen on the growth of diamond film:
(1) With higher microwave power, when the substrate temperature is 8700 °C, with the increase of nitrogen volume concentration, the deposition rate of diamond film will show a maximum value, but the quality has been decreasing.
(2) The introduction of nitrogen can simultaneously improve the deposition rate and increase the secondary nucleation for the deposition of diamond films. At higher deposition temperatures, the effect of nitrogen will be mainly manifested in increasing the deposition rate. As the temperature of the substrate decreases, the effect of introducing nitrogen will obviously increase the secondary nucleation rate of the diamond film, thereby reducing the grain size of the diamond film.
(3) When introducing nitrogen gas to deposit diamond film at lower substrate temperature, nano-diamond film with higher crystallinity can be obtained by controlling the volume concentration of nitrogen gas. When the substrate temperature is 7500 ℃, the volume concentration of nitrogen gas is kept at 0.03%--0.07%, and the nano-diamond film with the grain size of about 50nm and high crystallinity can be obtained.
2. The effect of carbon dioxide on the growth of diamond films:
(1) Proper increase of microwave power can reduce the secondary nucleation in the growth process of diamond film, improve the quality of diamond film, and help to improve the orientation growth of diamond film (100).
(2) The introduction of an appropriate amount of CO2 can improve the quality of the diamond film and maintain the growth of the diamond (100) orientation. With the increase of the introduced CO2 concentration, the orientation of the diamond film (100) becomes worse and the etching is more serious.
(3) In order to obtain a diamond film with high quality, high orientation and good crystal form, it is an ideal method to appropriately increase the microwave power and add an appropriate amount of CO2.
---EDITOR: Anna Wang/Cynthia Lee
---POST: Cynthia Lee