Preparation of nanocrystalline diamond films for optical coating applications using a pulsed microwave plasma CVD method

It is well understood that surface roughness has a significant impact on optical transmission of a sample. Unlike the case for perfectly smooth surfaces, reflected and transmitted light wavelets from rough surfaces and grain boundaries are scattered in various directions. This incoherent light scattering phenomenon leads to surface haziness and poor transparency, particularly for samples with high refractive index such as diamond (n = 2.4). Hence, nanocrystalline diamond films have been prepared for optical coating applications using a unique pulsed microwave plasma CVD technique. Diamond grain size can be tailored by controlling the substrate temperature, the time duration of each plasma pulse (Δt) and the number of the pulses (p). Films with fine grains(≤ 3000 A) have been obtained on quartz substrates at temperature as low as 400" C and at a rate of approximately 1.2 - 1.5 µ.m/hr. Typical deposition conditions are: gas mixture, CH₄ : H₂ : 0₂ = 0.3 : 99.5 : 0.2; total flow rate = 100 seem; pressure= 40 mbar; microwave power= 400 W. The surface roughness of the grown films is on the order of 50-200 A. The optical transparency of the films is over 60% in the range of 0.6-2 µm wavelength, which is comparable to that of type IIa natural diamond. The crystalline structure of the films is confirmed by a diffraction pattern obtained from a transmission electron microscope (TEM). Twin and stacking fault defects are also observed under a high resolution microscope. Using a block-on-ring tribotester, it is found that the diamond films adhere well to quartz substrates.