Plasma diagnostics of a DC glow discharge using tuneable diode laser absorption spectroscopy

The behaviour of plasmas is influenced by thermodynamic quantities such as gas and electron temperatures and their corresponding number densities. For applications such as thin-film deposition, material synthesis, sputter coating, spark plugs, and spacecraft thrusters that depend upon plasma processes it is of practical interest to quantify these physical properties in order to optimise their performance and efficiency. In this work, time-resolved tuneable diode laser absorption spectroscopy (TD-LAS) is used to characterise an Argon glow discharge in a Dy-navac pulsed DC sputter coater. This is achieved by current scanning a vertical-cavity surface-emitting laser (VCSEL) at a known neutral Argon transition wavelength of 794.8nm that corresponds to an energy level transition from the 3s²3p⁵4s metastable state to the excited upper state 3s²3p⁵4p [4]. The pressure and current conditions of the sputter coater are kept constant at 11Pa and 24mA respectively during the experiment. A Doppler-broadened absorption profile is assumed from which both time-resolved temperature and number density are calculated. It was found that the plasma temperature is constant to within measurement uncertainty for a given duty cycle, while the number density variation over a cycle follows the voltage characteristics of the device.