Bipolar HiPIMS: The role of capacitive coupling in achieving ion bombardment during growth of dielectric thin films

Abstract

Abstract Bipolar high-power impulse magnetron sputtering (HiPIMS) is used to achieve ion acceleration for ion bombardment of dielectric thin films. This is realized by increasing the plasma potential (Up), during the interval in-between the HiPIMS-pulses, using a positive reversed voltage (Urev). As long as the film surface potential (Us) is maintained low, close to ground potential, this increase in Up results in ion-acceleration as ions approach the film surface. The effect of Urev on the ion bombardment is demonstrated by the growth of dielectric (Al,Cr)2O3 films on two sets of substrates, Si (001) and sapphire (0001) utilizing a Urev ranging from 0 to 300 V. A clear ion bombardment effect is detected in films grown on the conductive Si substrate, while no, or a very small, effect is observed in films grown on the dielectric sapphire substrate. This is ascribed to the changes in Us when the substrate is subjected to the bombardment of positive ions. For a film surface that has a high capacitance to ground, Us remains close to ground potential for an extended time in-between the HiPIMS pulses, while if the capacitance is low, Us quickly attains floating potential (Ufloat) close to Up. The simulated temporal evolutions of Us for the films by using capacitors show that for a 1 μm thick (Al,Cr)2O3 film on a conductive substrate, Us is maintained close to ground potential during the entire 20 μs that Urev is applied after the HiPIMS pulse. On the other hand, when a capacitance corresponding to the 0.5 mm thick sapphire substrate is used, Us rapidly attains a potential close to Urev.