Diamond features excellent impact resistance, high thermal conductivity, and high transmittance over a wide wavelength range, which makes it an ideal material for infrared optical windows, high-power laser windows, X-ray windows, and microwave windows. Microwave plasma chemical vapor deposition (MPCVD) is the most commonly employed method to prepare optical grade diamond films, but the optical quality and deposition rate are often mutually constraining factors with the increasing deposition size. Currently, high diamond quality and high deposition rate are difficult to be achieved spontaneously. The growth rate of high-quality optical diamond films is typically in the range of 1-2 μm/h. Fast growth can be economically valuable for preparing diamond optical windows, significantly saving costs, and improving preparation efficiency. This is particularly important for applications that typically require the utilization of thicknesses in the millimeter range.

The substrate adopted in this study is a p-type (100) silicon wafer with a diameter of 35 mm and thickness of 3 mm and is pretreated by grinding with 5 μm diamond powder for 15 min to disperse nucleated crystalline species. The wafer is then ultrasonically cleaned in acetone and alcohol for 15 min each before diamond film deposition in a 2.45 GHz and 6 kW MPCVD system. The orthogonal experimental method is leveraged to investigate the effects of substrate temperature, methane volume fraction, and oxygen volume fraction on growth, and the growth parameters are optimized by comparing the full width at half peak of the diamond at different parameters through Raman spectrum analysis. After optimizing the process parameters, the power and cavity pressure are increased, and the methane volume fraction is adjusted to deposit the diamond films. Preliminary observations of the diamond films are conducted by an optical microscope in transillumination mode, and diamond quality is determined through Raman spectrum analysis. X-ray diffractometry is utilized to analyze the crystal structure of the resulting samples, while ultra-violet-visible-near-infrared (UV-VIS-NIR) spectrum and Fourier transform infrared spectrum are employed to measure the transmittance of polished diamond films in the visible and infrared spectrum, respectively. The optical emission spectrum is also adopted to study the trends of each reactive group in the plasma at different power densities, CH₄ volume fractions, and O₂ volume fractions to reveal the rapid growth mechanism of high-quality diamonds.

According to the analysis of the orthogonal experiments, it is evident that the greatest influence on the FWHM is the substrate temperature, followed by the oxygen flow rate, and finally the methane flow rate. Meanwhile, by comparing the magnitude of k values, the optimal levels are obtained as substrate temperature 850 ℃, oxygen flow rate 1×10^-3 L/min, and methane flow rate 9×10^-3 L/min. However, the maximum growth rate of the diamond films deposited under this optimized process is 1.5 μm/h, and significant mass inhomogeneity is observed. In the above process, the power density and CH₄ flow rate are further improved, while the temperature and O₂ flow rate are kept constant. With the parameters of 18.47 kPa, 4700 W, 850 ℃, CH₄ flow rate of 12×10^-3 L/min, and O₂ flow rate of 1×10^-3 L/min, the diamond film is deposited with a thickness of 300 μm after being polished on both sides (Fig. 5). The film exhibits uniform quality without cracks, and the growth rate reaches 3.1 μm/h, 2.1 times higher than the previous rate without compromising quality. The diamond Raman peak full width at half maximum (FWHM) is 3.16 cm^-1, and the highest transmission rate reaches 70.9% in the visible band and 68.9% at 10.6 μm. The plasma diagnostic results indicate that the rapid growth of high-quality diamonds is mainly due to the H-atom excitation and CH₄ decomposition at high power densities. The addition of oxygen also contributes to CH₄ decomposition and produces an etching effect on the non-diamond phases, thereby leading to the rapid deposition of high-quality diamond films.

We study the effects of substrate temperature, methane volume fraction, and oxygen volume fraction on the quality and growth rate of diamonds by orthogonal experiments, and the growth parameters are optimized. The FWHM of the diamond Raman peak is 3.16 cm^-1, and the transmission rate is up to 70.9% in the visible band and 68.9% in the infrared at 10.6 μm. Additionally, the peaks of H-atom excitation and C-related groups are characterized by the OES technique. The results show that the promotion of H-atom excitation and CH₄ decomposition process at high power densities significantly increase the volume fraction of H-atoms and C-active chemicals in the plasma, and the addition of auxiliary gas oxygen can promote CH₄ decomposition and produce an etching effect on non-diamond phases, which improves the growth rate and crystal quality of diamond films.