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Paschen curve analysis to optimise pulsed d.c. sputtering plasmas

The behaviour of the breakdown voltage of a pulsed d.c. magnetron sputtering system under various operating conditions has been studied with reference to Paschen’s Law. This can help to understand the conditions necessary for a stable breakdown at low voltage and hence improve sputtering performance and target life. The breakdown voltage decreases with increasing pressure in constant current mode, and at a certain pressure it reaches a minimum value and then increases thereafter. The behaviour of breakdown voltage versus pressure does not follow previous models based on Paschen’s curve, but instead follows an exponential linear functional form. The breakdown voltage decreases and the graph minima move towards the lower pressure region when the frequency is increased. It is possible that the combined effect of metastable atoms or ions that remain from the previous pulse-on time and the high mean free path at lower pressures results in this effect. The pressure at which the minimum value of breakdown voltage occurred was calculated by applying an exponential linear function for different electrode separations. From this analysis, it is clear that the pressure at which the minimum breakdown occurs is independent of the electrode separation. The breakdown voltage minima shift towards higher pressures when the operating current is increased. The behaviour of the breakdown voltage with pulsing frequency at different pressures and constant pulse-off time was recorded and revealed that the breakdown voltage decreased consistently as the frequency increased up to 70 kHz. Above this frequency, perturbation in the breakdown voltage was noted, possibly due to the rise in pre-breakdown current during the few microseconds of pulse-on time. The breakdown voltage was seen to decrease when the pulse-off time was increased while keeping the total period of the pulse constant.