Laser Spectroscopy Diagnostics of the gas Phase During Diamond CVD
Resonance Enhanced MultiPhoton Ionisation (REMPI)
REMPI is a laser diagnostic technique used to study the gas phase species within a diamond reactor. A UV laser beam is passed into a CVD chamber and focused at a point (see schematic diagram, below). The position of the focus can be changed by moving the entire chamber vertically with respect to the laser beam. The laser wavelength is chosen so as to ionise a selected species, say H atoms, to produce H+ and electrons. The electrons can then be collected by a nearby biased metal probe and recorded as an electric current. The arrival time of the current at the probe is monitored as the laser pulses occur and a Gaussian lineshape is seen. This shows that the assumption that the local gas within the sampling region is in thermal equilibrium and can be approximated by a Maxwell-Boltzman velocity distribution.
The FWHM of the spectral line can be converted to an absolute measure of the average kinetic temperature of the Hatoms, and hence to their KE and velocity. The area beneath the line is proportional to the relative number density of H atoms at that position.
We are currently using REMPI to obtain these data as a function of filament temperature, spatial distance from the filament, presence or absence of different types of substrate, etc.
Cavity Ring Down Spectroscopy
Cavity Ring Down Spectroscopy is a pulsed laser technique used to analyse the composition of gaseous sources, such as the methane/hydrogen gas mixtures within a diamond CVD reactor. It is extremely sensitive, and is one of the only spectroscopic techniques that can measure the absolute concentrations of certain species within the gas mixture. It utilises 2 highly reflecting (99.9%) mirrors which form a cavity containing the gas under investigation. The laser passes into the cavity and bounces back and forth between the 2 mirrors many times, producing an absorbance path length of several km. This very long path length is the reason for the sensitivity to low species concentrations. Eventually some of the laser light leaks out of the cavity and is detected by a photomultiplier. Depending on the reflectivity of the mirrors, each laser pulse decays at a known rate, called the ring-down time. If any species within the cavity absorbs the laser light, the ring-down time decreases. Thus by tuning the laser wavelength to one that is absorbed by one of the species in the cavity (e.g. H atoms or CH3), we can measure the concentration of the species.
We are currently employing this technique to look at the gases used in CVD diamond films made in a hot filament reactor and a MW plasma reactor, using CH4/H2 mixtures, and the effect upon H atom concentrations of N2 additions.