Dynamics of bipolar HiPIMS discharges by plasma potential probe measurements

Abstract

The plasma potential at a typical substrate position is studied during the positive pulse of a bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) discharge with a Cu target. The goal of the study is to identify suitable conditions for achieving ion acceleration independent on substrate grounding. We find that the time-evolution of the plasma potential during the positive pulse can be separated into several distinct phases, which are highly dependent on the discharge conditions. This includes exploring the influence of the working gas pressure (0.3 – 2 Pa), HiPIMS peak current (10 – 70 A corresponding to 0.5 – 3.5 A/cm2), HiPIMS pulse length (5 – 60 μs) and the amplitude of the positive voltage U+ applied during the positive pulse (0 – 150 V). At low enough pressure, high enough HiPIMS peak current and long enough HiPIMS pulse length, the plasma potential at a typical substrate position is seen to be close to 0 V for a certain time interval (denoted phase B) during the positive pulse. At the same time, spatial mapping of the plasma potential inside the magnetic trap region revealed an elevated value of the plasma potential during phase B. These two plasma potential characteristics are identified as suitable for achieving ion acceleration in the target region. Moreover, by investigating the target current and ion saturation current at the chamber walls, we describe a simple theory linking the value of the plasma potential profile to the ratio of the available target electron current and ion saturation current at the wall.

Temporal, spatial and spectroscopic study of plasma emission on Cu target in bipolar HiPIMS

“…The result section describes three representative cases where the HiPIMS pulse was followed by the positive pulse. The plasma potential evolution obtained by the emissive probe determined the state of a discharge, according to the following classification established in a previous paper [22]. Phase A is defined as a phase where the plasma potential is close to the potential applied on the target, and a rather large target current is present.…”

“…An emissive probe was positioned 13 cm downstream from the target (i.e. outside the magnetic trap of the magnetron) to monitor the plasma potential evolution during the positive pulse; for further details, see [22]. Signals were captured by a 4-channel Keysight Infiniium DSO-S 204A oscilloscope working in the single-shot mode.…”

“…The aim of this paper is to significantly extend the bipolar HiPIMS research of our previous paper [22] and obtain a complex picture of processes occurring during different stages of the positive pulse. Plasma emission screening with a fast camera and a spectrometer was chosen as a diagnostic method, showing a positive pulse discharge in spatial, temporal and spectroscopic resolution.…”

“…Additionally, using a HiPIMS generator capable to supply positive pulses is technically less complex and often a cheaper alternative. In the literature, different names for the same method of adding a positive voltage pulse after the HiPIMS pulse have been used; bipolar HiPIMS [3,[22][23][24], positive kick [25,26], and HiPIMS with cathode voltage reversal [27]. The aim of introducing the positive voltage after the negative pulse in bipolar HiPIMS is to accelerate ions remaining from the negative pulse in the ionization zone towards the substrate.…”

Ion energy analysis of a bipolar HiPIMS discharge using a retarding field energy analyser

“…In the sputter deposition of thin films, controlling the bombarding positive ion energy at the substrate is useful for tailoring the specific microstructures, crystallographic orientations and phases [1]. For electrically conducting samples, this is commonly achieved through substrate biasing, in which a negative potential (typically <150 V) is applied to accelerate positive ions to significant energies upon bombardment of the growing film.…”