Main influencing factors in the diamond deposition process of diamond deposition equipment
Main influencing factors in the diamond deposition process of diamond‑depositing equipment
Diamond growth is a highly complex process, influenced by numerous factors, such as the CH4/H2 gas mixture, carbon source concentration, deposition temperature, and growth pressure. These factors are discussed individually below:
Knowledge of Ellipsoidal Resonant Cavity–Type Diamond Deposition Equipment
Ellipsoidal resonant‑cavity diamond deposition equipment emerged around 1997, designed and developed by the Fraunhofer Institute in Germany. Its structure closely resembles that of quartz‑bell‑jar‑type diamond deposition systems: both feature a large quartz bell jar and employ a coaxial‑antenna‑based converter for microwave mode conversion. The primary distinction between the two lies in their respective resonant cavities.
The History and Present of Diamond Deposition Equipment
The distinctive feature of MPCVD deposition lies in the fact that its plasma is sustained by microwave energy, allowing the discharge to proceed without metal electrodes. This eliminates the contamination of diamond that can occur in other CVD methods due to electrode use. Furthermore, the gas‑discharge region in MPCVD is highly localized, enabling the generation of a high‑density plasma and the activation of a greater number of reactive species, while also ensuring exceptional discharge stability. These advantages collectively facilitate the long‑term, stable growth of high‑quality diamond. Consequently, MPCVD has long been the preferred technique in the industry for producing both high‑quality single‑crystal and optical‑grade polycrystalline diamond.
Characterization of Diamond Grown on Diamond-Deposition Equipment
Characterization of Diamond Grown on Diamond‑Deposition Equipment
Diamond samples grown using deposition equipment must first undergo Raman spectroscopy, which is an extremely important and critical characterization technique. It allows for a preliminary assessment of whether the epitaxial single‑crystal sample is indeed diamond and whether it contains carbon in the sp² hybridized state. If Raman spectroscopy confirms that the sample is a diamond single crystal, further characterization of its crystalline quality can be performed: X‑ray rocking curves are used to evaluate the crystal quality, while photoluminescence, infrared absorption, and ultraviolet absorption spectra can be employed to assess the optical properties of the single‑crystal diamond and to make a rough determination of the presence of defects or impurities within the crystal.
Pre-treatment of Seed Crystals Prior to Single-Crystal Diamond Growth in MPCVD Equipment
During the MPCVD growth of diamond, in addition to process parameters such as power, gas pressure, temperature, leakage rate, and the types and ratios of gas sources, the quality of the seed crystal itself is also a critical process parameter. Currently, the primary crystallographic orientations used for diamond growth are (100), (110), (111), and (113). Among these four orientations, the (100) face exhibits a relatively low probability of defect and twin formation during growth, making it well suited for producing high‑quality, large‑area single‑crystal thick films. Moreover, seed crystals with this orientation are easier to polish than those with other faces and can readily yield substrates with few defects.
The growth process of diamond in MPCVD equipment
The growth of diamond in an MPCVD reactor requires maintaining a high vacuum within the system. The reactor chamber is first cleaned, and pre‑treated seed crystals are placed on the substrate holder. The chamber door is then closed, and pumping is initiated to achieve the desired vacuum level. Once the required vacuum is attained, pumping is stopped, and the process gases are introduced. Under microwave excitation, these gases are ionized into a plasma state, generating reactive species that deposit carbon onto the seed crystal, thereby enabling diamond growth.
Main components of an MPCVD system
MPCVD equipment can be broadly divided into four major subsystems: the microwave system, the gas supply and vacuum system, the water-cooling system, and the associated control system.
Working Principle of Diamond Growth in MPCVD Equipment
In summary, the growth of diamond in MPCVD equipment is a typical quasi‑equilibrium process. The plasma’s energy is sustained by microwaves. The microwave source emits microwaves that propagate in the rectangular waveguide in the transverse electric (TE) mode; a mode‑conversion antenna then transforms this TE mode into the transverse magnetic (TM) mode before coupling it into the reaction chamber. When the frequency and mode are appropriately matched, a standing wave forms inside the reaction chamber, creating a strong electric‑field region above the substrate holder. This highly localized electric field not only generates a high‑density plasma but also ensures a remarkably stable discharge.
Why is MPCVD equipment considered an ideal system for diamond deposition?
The principle behind diamond deposition in MPCVD equipment is that a microwave generator produces microwaves, which are transmitted via a waveguide into the vacuum chamber. There, reaction gases such as methane and hydrogen are ionized, forming a plasma that subsequently deposits on the substrate as diamond. Under microwave excitation, the reaction gases transition into a plasma state. This occurs because microwaves constitute a high-frequency electromagnetic field; electrons, subjected to this field, collide with other gas-phase species, significantly enhancing gas ionization and generating a high‑density plasma. The plasma serves two primary functions: first, it raises the substrate temperature, bringing the diamond substrate to an optimal level for epitaxial growth; second, it supplies reactive species that participate in the chemical reactions.
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