Effects of the dynamic cathode sheath on electron transport at the initial period of HiPIMS pulse studied by Langmuir probe measurements and 2D PIC-MCC simulation

Abstract

In high power impulse magnetron sputtering (HiPIMS) discharge, the cathode sheath is a particularly vital section which determines the spatial distribution of electric field and the energy and transport of charged particles. In this work, a single Langmuir probe is employed to explore the effects of dynamic cathode sheath on electron transport at the initial period of HiPIMS pulse. Measurements show that the electron temperature is as high as ~20 eV and the plasma density is much lower than ~1015–1016 m−3 at the beginning of HiPIMS pulse t < 3 μs; correspondingly, a feature of small low-energy amplitude and extended high-energy tail is found in the electron energy distribution function (EEDF). And the high-energy electrons can escape the magnetic trap and gradually diffuse to further axial positions, even to z = 140 mm with only several microseconds delay. The diffusion coefficient of electrons D⊥ is larger than the typical Bohm diffusion at z > 32 mm in our discharge case. The two-dimension Particle-in-cell Monte Carlo collision (2D PIC-MCC) simulation results are well in agreement with the experimental results. The net positive charge density ∆n near the target is lower than ~+9.2×1014 m−3 at t = 0.5 μs, and the charge separation is up to ~0.4. In this case, the cathode sheath has a large thickness and the axial electric field EZ outside the sheath is as strong as dozens of 10 kVm−1. The simulation results confirm, for the case of the expanding sheath, (i) the electrons can be accelerated and escape the magnetic trap with weak constraint; (ii) the electron group can maintain their kinetic energy when they diffuse from the ionization region (IR) to bulk plasma (BP). When the net charge density increases to ~+3×1016 m−3 at t = 2.5 μs, the sheath thickness is condensed to ~1 mm with ~600 V voltage drop.