Coherent Raman Spectroscopic Monitoring of Pulsed Radio Frequency PECVD of Silicon Nitride Thin Films

Abstract

During pulsed plasma enhanced chemical vapor deposition, (PECVD), of silicon nitride thin films, depletion of silane reactant was measured by coherent anti-Stokes Raman scattering, (CARS), spectroscopy as a function of radio frequency pulse width, peak power, and delay time after the rf pulse. The results were correlated with “goodness of deposition parameters” including film thickness, deposition rate, and N-H and Si-H film content. The pulse width and peak power affected the plasma similarly, as silane depletion, film thickness, and rate of film growth all increased with both pulse width and peak power for a 10 Hz repetition rate. The CARS measured silane depletion also increased proportionally with pulse width for short rf pulses but not for long ones. Although the film properties changed with both power and pulse width, there were differences in the effects. A decreasing SiH/NH ratio resulted from increasing peak power, but increases in pulse width lead to an increasing ratio in some cases. The delay studies showed the CARS-measured silane depletion was higher 2 ms after the rf pulse had ended than 0.5 ms within the pulse. This result was accounted for by the flow rate, shower head design, and placement of the focus of the laser beams. Based on the results of the CARS-measured silane depletion as a function of pulse width and delay time, it is evident that the reactants move in a segregated-flow with a significant proportion of the velocity component directed toward the lower electrode, at least for the conditions of this experiment. Thus differences noted between pulsed and continuous PECVD processing likely depend largely on the transit time of the molecules in the plasma region.