| AVS 63rd International Symposium & Exhibition | |
| Plasma Science and Technology | Tuesday Sessions |
| Session PS-TuM |
| Session: | Plasma Diagnostics, Sensors and Control |
| Presenter: | Matjaz Panjan, Lawrence Berkeley National Laboratory, Slovenia |
| Authors: | M.P. Panjan, Lawrence Berkeley National Laboratory, Slovenia A. Anders, Lawrence Berkeley National Laboratory |
| Correspondent: | Click to Email |
In the magnetron discharges formation of dense plasma structures, called ionization zones or spokes, have been extensively studied over the last few years. Ionization zones were first observed in high power impulse magnetron sputtering (HiPIMS) [1] and later in DC magnetron sputtering (DCMS) [2]. In DCMS discharges operated at low-currents and low-pressure a single ionization zone forms with the shape of an elongated arrowhead and rotates in the direction opposite to the electron drift (i.e., in the -E×B direction). In this work we used emissive and floating probes to measure plasma and floating potentials of rotating ionization zone for a magnetron with a 3” niobium target operated at 2 mTorr (0.27 Pa), -270 V and 100 mA. Both probes showed strong temporal and spatial variations of the signals. From the measurements in the radial and axial directions we reconstructed full three-dimensional distributions of the plasma potential. The potential distribution was compared with the images recorded by an intensified CCD camera. Strongest light intensities in the zone corresponded to maximum plasma potential (i.e., ~0 V for probe positioned over the racetrack and for axial distances above 5 mm), whereas weaker light intensities corresponded to negative potentials (e.g., -70 V for the probe positioned over the racetrack and 5 mm away from the cathode). The plasma potential distribution matches with a previously suggested potential hump model [3]. Sharp drops in the light intensity are associated with large potential gradients, which result in strong in-plane electric fields. The largest in-plane fields are found in the azimuthal direction at the edge of the ionization zone (up to 10 kV/m). Weaker electric fields also form in the radial direction. The presence of the in-plane electric fields changes the paradigm of predominantly axially-directed electric fields. From the plasma potential we calculated the space charge distribution. A double layer is present around the edge of the ionization zone with higher ion density inside the zone and higher electron density just behind the zone's edge. From the difference between the plasma and floating potentials we also reconstructed the three-dimensional distribution of electron temperature. Electrons have largest energies in the area of highest light intensity (i.e., inside the zone and close to the edge) whereas their energy decreases along the drift direction in correlation with the fading light intensity.
[1] A. Anders et al., J. Appl. Phys., 111 (2012) 053304
[2] M. Panjan et al., Plasma Sources Sci. Technol., 24 (2015)065010
[3] A. Anders et al., Appl. Phys. Lett., 103 (2013) 144103