High power impulse sputtering of chromium: correlation between the energy distribution of chromium ions and spoke formation

Abstract

High power magnetron sputtering (HiPIMS) discharges generate ions with high kinetic energies in comparison to conventional dc magnetron sputtering. The peculiar shape of the ion energy distribution function (IEDF) is correlated to the formation of localized ionization zones (IZ) in the racetrack of a HiPIMS discharge, so called spokes. This is explained by a local maximum of the electrical potential inside these localized IZ. By using ion energy mass spectrometry, probe experiments and plasma spectroscopy the connection between IZ and IEDFs is evaluated with high temporal resolution. The data of a floating probe next to the target is used to directly monitor the movement of the spokes in the × E B direction. Chromium is used as target material, because the plasma undergoes a sequence from stochastic spoke formation, to regular spoke pattern rotating in the × E B direction to a homogeneous plasma torus with increasing plasma power. In particular, the analysis of the transition from the regular spoke pattern to the homogeneous plasma torus at very high plasma powers shows that the high energy part of the IEDF is not affected and only the low energy part is modified. Consequently, one could consider the homogenous plasma torus at very high plasma powers as a a single ionization zone localized over the complete torus, which is formed by merging individual spokes with increasing power. Details and consequences of that model are discussed.

Single-shot spatial-resolved optical emission spectroscopy of argon and titanium species within the spoke

“…This contribution focuses on the investigation of the spoke's inner parameters using OES. The aim is to study the spokes with well-defined shapes, as they possess clearly visible borders on the probe and the camera signals as presented in [17,26,48,49]. Clear distinction of borders allows the precise determination of the position where the spoke starts and where it ends.…”

“…As the spokes are associated with a potential hump [28], any spokes passing over the Langmuir probe induce changes in the floating potential. This is demonstrated by periodic fluctuations observed on the floating potential corresponding to periodic passing of spokes [41,48,50].…”

The statistics of spoke configurations in high-power impulse magnetron sputtering discharges

“…A few years ago, it was observed that the plasma is not always homogeneously distributed over the racetrack area of the target in high-power impulse magnetron sputtering (HiPIMS) discharges [1][2][3], but under certain conditions it is self-organised into ionisation zones called spokes [2,4]. Several non-perturbing diagnostic techniques have been used to investigate spokes; these include photo-multiplier tubes [3,[5][6][7][8], high-speed 2D imaging of broadband emission [1][2][3][5][6][7][8][9][10][11][12][13][14][15][16][17][18], spectrally filtered high-speed 2D imaging [19][20][21], streak cameras [21][22][23][24], laser-induced fluorescence [20] and embedded probes [13,25,26]. Furthermore, more perturbing methods such as the Langmuir probe (single as well as multiprobe) [3,[5][6][7][8], emissive probe [2], magnetic sensor probe [27] or triple probe…”

Engineered ion-bombardment as a tool in thin film deposition

“…Hecimovic et al [63] investigated in detail the origin of the IEDF in HiPIMS and found that the high energy tail can be fitted with a Maxwell distribution, which indicates that the sputtered ions are also thermalized, though this will depend a lot on the pressure and the distance between the target and the substrate [62]. It is also common to see a distinct energy shoulder between the thermal ion population and the high energy tail [64,67,102,[104][105][106][107] and this was argued by Maszl et al [103] to be due to the presence of self-organizing ionization zones called spokes [108][109][110][111], which results to a potential hump that can accelerate ions [103,112]. An example of the IEDF of Ti + ions in a HiPIMS discharge is shown in Fig.…”