Niobium thin film thickness profile tailoring on complex shape substrates using unbalanced biased High Power Impulse Magnetron Sputtering

The authors report in this paper the possibility to control the thickness profile of a thin film deposited by High Power Impulse Magnetron Sputtering (HiPIMS). It is shown that the combination between a HiPIMS discharge, an unbalanced magnetic configuration and the application of a negative bias onto the surface to coat enables tailoring on demand the coating thickness profile. This effect is hereafter used to coat complex shapes such as low-beta accelerating cavities with a niobium layer. The authors first present the magnetic design proposed to obtain an unbalanced cylindrical sputtering source. Numerical simulations are then used to predict the electron density and energy spatial distributions that can subsequently be correlated to the ionization region shape. Finally, the authors present the effect of such technique comparing Direct Current Magnetron Sputtering (DCMS), HiPIMS and biased HiPIMS using, respectively, a balanced and an unbalanced magnetic configuration, as well as detailing the effect of modifying either the magnetic field lines distribution or the magnetic strength.