Effect of residence time on microwave plasma chemical vapor deposition of diamond

Abstract

We have investigated the effects of reactant residence time on the properties of microwave-assisted chemical vapor deposited diamond films. Using a constant gas pressure of 40 Torr and gas composition of 1% CH4 in H2, the total gas flow rate was adjusted from 25 to 800 sccm which corresponds to apparent chamber residence times of 3–100 s for our reactor. The flow rate is one of the few parameters in the microwave plasma deposition system that is decoupled from all others, making this a relatively clean study on the effects of a single process variable. Two distinct types of diamond films were prepared in this work. One set was deposited directly on diamond-seeded silicon wafers. A second set was prepared separately, and under otherwise identical diamond film growth conditions, but deposited on a layer of microcrystalline diamond (MCD). The 5-h growths resulted in continuous polycrystalline diamond films of ∼2 μm thickness which were analyzed with scanning electron microscopy, Raman scattering and photoluminescence (PL) spectroscopies, infrared (IR) transmission, and x-ray diffraction. The Raman and PL spectra, and x-ray diffraction data exhibited flow-dependent behavior, while the electrical resistivity, IR transmission spectra, and deposition rate of the films, as well as the optical emission spectra of the deposition plasmas, were relatively insensitive to the flow rate. The x-ray diffraction results revealed that the diamond films were textured, although the preferred orientation was not particularly related to the surface morphology. The surfae preparation strongly affected the texture, with films deposited on MCD exhibiting a [110] texture while those on Si had a [111]-preferred orientation. Furthermore, the extent of texture for the films on Si varied with the gas flow rate, unlike that of the films on MCD. The Raman and PL spectra varied with flow for both the films on Si and on MCD. Flow modeling showed the importance of convective flow in our reactor, and the insensitivity of deposition rate on gas flow rate can be rationalized by the flow pattern. The presence of vortices above the growth surface complicates the concept of residence time for this system. We argue that variation in the concentrations of gas-phase diamond growth species, arising from changes in gas residence time, are responsible for the flow-dependent variation in diamond film material properties.