The Science Behind Lab-Grown Diamonds: How HORIBA OES Ensures Purity

Lab grown diamonds are more prevalent in today’s jewelry industry due to technological advancements in Chemical Vapor Deposition (CVD), a newer and easier method compared to traditional synthetic diamond growing methods such as the High Pressure,  High Temperature (HPHT) method which attempts to mimic natural diamond formation conditions that occur in the earth’s mantle.

MPCVD (Microwave Plasma Chemical Vapor Deposition) process is used primarily for lab grown diamonds. The process involves placing a small diamond seed in a vacuum chamber, introducing carbon carrier gas into the chamber, and applying microwave energy to dissociate the gas molecules into carbon atoms which then deposit onto the diamond seed, layer by layer forming a diamond.

High-quality diamonds require precise control of the process parameters during the growth phase as purity, crystallinity and size of the diamond is highly sensitive to minor fluctuations in gas composition, gas flow rate or chamber pressure. A lab grown diamond grown in the presence of a nitrogen leak in the chamber or nitrogen contamination in the feed gas will exhibit a brown coloration due to the substitution of carbon atoms with nitrogen in the diamond lattice. It is therefore essential to detect and eliminate any contamination in the feed gas or leak in the chamber during the process.

HORIBA Optical Emission Spectroscopy or OES has a proven record in the industry for plasma monitoring, diagnostics and process optimization using advanced software and algorithms for leak detection, chamber health and plasma monitoring. HORIBA’s OES EV2.0 offers a wide range of wavelengths and spectral resolution tailored for the lab grown diamond industry as shown in Figure 1.

HORIBA OES EV2.0 (shown in Figure 2) is available in two configurations, an API shown in Figure 2 and a PC configuration. The API system includes the OES spectrometer, a SMA fiber cable for delivering the signal from the optical window of the chamber to the interface of the spectrometer and monitoring software. The collected signal is displayed in a spectrograph that illustrates the plasma emission variation vs. wavelength. It can be used to detect and track commonly used gases in the diamond growth process such as methane (CH₄), argon (Ar), hydrogen (H₂), oxygen (O₂) and nitrogen (N₂) along with their corresponding radicals generated inside the plasma, as well as impurities such as nitrogen leak in the chamber.

Plasma monitoring using HORIBA OES is a powerful technique to identify critical radicals such as carbon from the methane (CH4) source gas during the diamond growth process. Figure 3 shows the emission spectra obtained during the diamond growth process from two separate chambers using the HORIBA EV2.0 HR system. The emission spectra of chamber 1 (Figure 3a) shows hydrogen (H) and carbon (C) radicals such as Hβ at 457 nm, Hα at 657 nm, CH at 430 nm and C2 in the range of 450-475 nm and 500-570 nm whereas that of chamber 2 (Figure 3c) shows hydrogen (H), carbon (C) and CN radicals. The presence of CN emission peak indicates a nitrogen leak as the excited carbon and nitrogen can react to form CN radicals. Additionally, the variation of the intensity of the various species of radicals with time is also available from the HORIBA OES system (Figures 3b and 3d).

The HORIBA OES system is proven to be a highly reliable, in-situ and non-invasive monitoring method with proven capability to detect impurities and leaks in the process chamber thereby ensuring a high level of purity in the diamond growth process.

EV 2.0 Series Endpoint / Chamber Health Monitor based on Optical Emission Spectroscopy and MWL Interferometry - HORIBA