Fundamentals of pulsed plasmas for materials processing

Pulsed plasmas use much higher power during each pulse compared to continuously operated plasmas. This feature and the appearance of additional new variables such as pulse duty cycle can lead to new, flexible, sometimes enabling processes for surface modification and thin film deposition, as discussed for plasma immersion ion processing, high power pulsed sputtering, thin film deposition with pulsed cathodic arcs and metal plasma immersion ion implantation and deposition. In all of these processes, transient sheaths are of great importance. The fundamentals of plasma sheaths are briefly reviewed. It is argued that the often-considered ion matrix sheath is only an educational case, whereas most realistic transient sheaths obey a time-dependent Child law. The recovery of pulsed sheaths is usually less considered than the formation of sheaths. If the pulse fall time normalized by the inverse ion plasma frequency is much greater than unity, sheath collapse can be simply described by a retreating Child law sheath. In the opposite case, electrons refill the ion space charge and complete recovery of the boundary layer is determined by ambipolar diffusion from the bulk plasma. The case of high power pulsed sputtering is discussed in greater detail, especially the appearance of a kink in the current–voltage characteristic, which changes its slope for current densities exceeding approximately 600 mA/cm2. It is suggested that high pulse power opens the possibility that self-sputtering can occur during each pulse, especially for target materials of high sputter yield. The necessary condition for self-sputtering is αβγ>1, where α, β and γ are the ionization probability, ion return probability and sputter yield, respectively.