Adjustment of high-energy ion flux in BP-HiPIMS via pulsed coil magnetic field: plasma dynamics and film deposition

As an emerging and extraordinary plasma source, bipolar pulsed high power impulse magnetron sputtering (BP-HiPIMS) discharge has been widely concerned by academia and industry due to the ability to control the deposited ion energy. In the present work, with the intension of increasing the high-energy ion fraction and flux during deposition, the BP-HiPIMS is operated together with a solenoidal coil installed in front of the target. This intension is achieved by applying a pulsed coil current so that the ions generated during the high-power negative pulse can be manipulated to diffuse towards substrate and then arrive at the substrate surface during the positive pulse. Systematic investigations of discharge characteristics and plasma parameters for Ti target discharge in Ar gas have been made, illustrating that applying a pulsed coil current prior to the positive pulse onset for ∼50 μs is an optimal selection to obtain a larger fraction of high-energy ions. The complex plasma dynamics has also been explored using the particle-in-cell/Monte Carlo collision approach. To verify the statements realized by plasma measurements, the Ti films have been deposited on a floating Si substrate, whose microstructure and surface morphology are characterized by field emission scanning electron microscope and atomic force microscope. The deposition illustrates that applying a pulsed coil current prior to the positive pulse onset for ∼50 μs can prepare a thicker, denser, and smoother Ti film. The link between the plasma parameters and film microstructure is studied using the molecular dynamics simulations which show that the high-energy ions contribute to optimizing the adatom diffusion and mobility on growing film surface, which is more beneficial to get a smaller grain size and decrease the film surface roughness.