Bipolar HiPIMS: The role of capacitive coupling in achieving ion bombardment during growth of dielectric thin films
Hao Du, Michal Zanáška, N. Brenning, Ulf Helmersson
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 ( U p ), during the interval in-between the HiPIMS-pulses, using a positive reversed voltage ( U rev ). As long as the film surface potential ( U s ) is maintained low, close to ground potential, this increase in U p results in ion-acceleration as ions approach the film surface. The effect of U rev on the ion bombardment is demonstrated by the growth of dielectric (Al,Cr) 2 O 3 films on two sets of substrates, Si (001) and sapphire (0001) utilizing a U rev 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 U s when the substrate is subjected to the bombardment of positive ions . For a film surface that has a high capacitance to ground, U s remains close to ground potential for an extended time in-between the HiPIMS pulses, while if the capacitance is low, U s quickly attains floating potential ( U float ) close to U p . The simulated temporal evolutions of U s for the films by using capacitors show that for a 1 μm thick (Al,Cr) 2 O 3 film on a conductive substrate, U s is maintained close to ground potential during the entire 20 μs that U rev is applied after the HiPIMS pulse. On the other hand, when a capacitance corresponding to the 0.5 mm thick sapphire substrate is used, U s rapidly attains a potential close to U rev .