Electrostatic measurement of plasma plume characteristics in pulse laser ablated carbon

A triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (approx. 3.0 J/cm²) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0×10¹³-2.0×10¹⁴ cm^-3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of approx. 10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions play an important role in DLC film deposition. Electron temperature is found to consistently reside in the range 0.5-3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N₂ fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of T_e suggests expulsion of background gas by the energetic plume. The leading edge ion plume speed is measured via temporal displacement of spatially separated probe signals on consecutive PLE pulses. Flow speeds as high as 5.0 × 10⁴ m/s are observed, corresponding to approx. 156 eV in C⁺. The ion flow speed is found to be a strongly decreasing function of fill pressure from an average high of approx. 126 eV in vacuum to approx. 0.24 eV at 600 mTorr N₂. Raman scattering spectroscopy indicates DLC film quality also degrades with fill pressure suggesting the importance of high ion kinetic energy in producing good quality films. Optical emission indicates an increase in C₂ molecular light intensity with fill gas pressure implying a reduced, if any, role of these species in DLC production. Ion current signal anomalies in high pressure pulses indicate the formation of high mass carbon clusters during plume evolution in the presence of background gas. Mass diffusivity estimates, based on density decay, suggest the presence of C₂⁺ under these conditions. Demonstration and control of such cluster formation may provide method(s) for controlling novel advanced materials properties.