Electron transport in high power impulse magnetron sputtering at low and high working gas pressure

Abstract

The magnetic field of a magnetron serves to increase the residence time of electrons in the ionization region and thereby enables the discharge to be sustained at low working gas pressures. This hinders the electrons to reach the anode which is necessary to close the electrical circuit. At high atom densities in the ionization region, and in the presence of an electric field, collisions of electrons with heavy species consecutively push electrons across the magnetic field lines, which is known as the classical cross-field transport mechanism. At low atom densities in the ionization region, collisions are rare and the classical cross-field transport mechanism is insufficient to carry the discharge current. This gives rise to plasma instabilities, called spokes, that locally provide pathways for electrons to escape from the near-target region and across the magnetic field lines. Here, we show experimentally, for the case of a high power impulse magnetron sputtering discharge with an aluminum target, how spokes gradually disappear with the increase in local gas density. We present an analytical model that shows that under these high gas density conditions, the classical electron transport mechanism is indeed strong enough to solely carry the discharge current. This highlights the importance of the local gas density in the ionization region for the intensity of spokes in a magnetron sputtering discharge and suggests ways for process optimization.

Generating spokes in direct current magnetron sputtering discharges by an azimuthal strong-to-weak magnetic field strength transition

“…This avoids the necessity for spokes for providing an additional mechanism for electron transport across the magnetic field lines. A similar effect can be achieved by increasing the working gas pressure [28]. While typical spokes are triangularly shaped, also other shapes have been observed, depending on discharge conditions [26,29].…”

“…A gated camera (Pi-Max 4, Teledyne Princeton Instruments) was used to record images with an exposure time of 250 ns. We installed an optical bandpass filter having a center wavelength of λ c = (480 ± 5) nm for spectroscopic imaging, which had been previously shown to enhance the contrast of spokes in magnetron sputtering discharges [28,32]. By this technique, the camera dominantly gathers light from a transition of Ar + ions (from the 4p to the 4s manifold with λ = 480.6 nm).…”

Influence of the magnetic field on the extension of the ionization region in high power impulse magnetron sputtering discharges

“…Due to a high electron conductivity along the magnetic field lines, the electron properties, notably the density and temperature, are close to being constant along a magnetic field line. Conversely, the electron conductivity across the magnetic field lines is limited [48][49][50], so that electron properties can change drastically in the direction orthogonal to the magnetic field lines. It is therefore understandable that the shape of the IR closely follows the magnetic field lines.…”

“…Note that on shorter time scales than those used to record the images, one can observe plasma instabilities, so called spokes. They can be observed with gated cameras looking laterally and axially onto the discharge [50,53,54] and through simulations [55,56]. Such instabilities further complicate the description of the IR.…”

Formation mechanism of the rotating spoke in partially magnetized plasmas

“…Experiments demonstrated that rotating spokes are regions where the electric potential is locally enhanced (potential humps) and electrons are energized in a double layer surrounding the potential hump region [22,18,23]. The spokes and associated electron transport and heating have attracted significant interests recently [24,25,26,27,28,29,17,30,21,31]. Unfortunately, the rotating spoke characteristics (rotation velocity, mode number, potential hump etc.)…”