Plasma potential of a moving ionization zone in DC magnetron sputtering

Abstract

Using movable emissive and floating probes, we determined the plasma and floating potentials of an ionization zone (spoke) in a direct current magnetron sputtering discharge. Measurements were recorded in a space and time resolved manner, which allowed us to make a three-dimensional representation of the plasma potential. From this information we could derive the related electric field, space charge, and the related spatial distribution of electron heating. The data reveal the existence of strong electric fields parallel and perpendicular to the target surface. The largest E-fields result from a double layer structure at the leading edge of the ionization zone. We suggest that the double layer plays a crucial role in the energization of electrons since electrons can gain several 10 eV of energy when crossing the double layer. We find sustained coupling between the potential structure, electron heating, and excitation and ionization processes as electrons drift over the magnetron target. The brightest region of an ionization zone is present right after the potential jump, where drifting electrons arrive and where most local electron heating occurs. The ionization zone intensity decays as electrons continue to drift in the Ez × B direction, losing energy by inelastic collisions; electrons become energized again as they cross the potential jump. This results in the elongated, arrowhead-like shape of the ionization zone. The ionization zone moves in the –Ez × B direction from which the to-be-heated electrons arrive and into which the heating region expands; the zone motion is dictated by the force of the local electric field on the ions at the leading edge of the ionization zone. We hypothesize that electron heating caused by the potential jump and physical processes associated with the double layer also apply to magnetrons at higher discharge power, including high power impulse magnetron sputtering.

Direct observation of spoke evolution in magnetron sputtering

“…In contrast, spokes drift in the opposite direction for dcMS, indicating that at least two opposing mechanisms are responsible for the motion or shift of spokes. Briefly, one can argue that a spoke shift in the E× B direction (as observed in dcMS) should be the expected, the "natural" direction, because drifting electrons arriving at a spoke are accelerated at the spoke edge's electric field, 6 while ions formed there are accelerated in the E× B direction, displacing the electric double layer in the E× B direction. Additionally, those ions are subsequently accelerated to the target where they emit secondary electrons on the E× B side of the spoke's edge.…”

“…Additionally, those ions are subsequently accelerated to the target where they emit secondary electrons on the E× B side of the spoke's edge. 6 In dcMS there are always plenty (gas) neutrals available to be ionized by energetic electrons, however, the situation is different in HiPIMS: neutrals may be locally depleted due to rarefaction and intense ionization. As was argued in ref.…”

“…At intermediate current levels, between dcMS and HiPIMS, spoke direction reversal can be observed. 7,8 Additional information on spokes has been gained by numerous probe studies, including those monitoring the potential distribution of dcMS spokes, 6 the merging and splitting of spokes in HiPIMS, 9 and simultaneous oscillations of the potential of all spokes, labeled as "breathing mode". 10 In this contribution we add a powerful approach to such research by using a "linear" magnetron,…”

“…Assuming that the electron drift is dominated by the E× B drift mechanism, and further considering that the B-field some mm above the target is about 50 mT, the electric field in the magnetic presheath is, on average, of the order 500 V/m, a reasonable value. 6 Energetic electrons cause ionization and excitation (the shape and position of the corresponding cross sections are similar), and electrons drift away from the locations of ionization faster than ions are removed by the local electric field, thus the plasma potential is locally enhanced where more ionization occurs. This, in turn, is the mechanism that gives electrons arriving at his location the extra "kick" in energy, allowing them to cause inelastic collisions.…”

Синтез Высокоориентированных Пленок Оксида Цинка На Аморфных Подложках Методом Магнетронного Распыления На Постоянном Токе

“…Очевидно, что в такой пространственно неоднородной плазме плавающий потенциал U f в разных точках про-странства (следовательно, в разных точках подложки a, b, c, рис. 1) принимает различные значения и является функцией параметров r и ϕ, т. е. U f = F(r, ϕ) [11]. Поэтому плавающий потенциал, до которого заряжается подложка в МРС, является технологическим параметром, от которого существенно зависит структурное совершенство получаемых пленок.…”