Investigation of plasma spokes in reactive high power impulse magnetron sputtering discharge

Abstract

Spokes, localised ionisation zones, are commonly observed in magnetron sputtering plasmas, appearing either with a triangular shape or with a diffuse shape, exhibiting self-organisation patterns. In this paper, we investigate the spoke properties (shape and emission) in a high power impulse magnetron sputtering (HiPIMS) discharge when reactive gas (N2 or O2) is added to the Ar gas, for three target materials; Al, Cr, and Ti. Peak discharge current and total pressure were kept constant, and the discharge voltage and mass flow ratios of Ar and the reactive gas were adjusted. The variation of the discharge voltage is used as an indication of a change of the secondary electron yield. The optical emission spectroscopy data demonstrate that by addition of reactive gas, the HiPIMS plasma exhibits a transition from a metal dominated plasma to the plasma dominated by Ar ions and, at high reactive gas partial pressures, to the plasma dominated by reactive gas ions. For all investigated materials, the spoke shape changed to the diffuse spoke shape in the poisoned mode. The change from the metal to the reactive gas dominated plasma and increase in the secondary electron production observed as the decrease of the discharge voltage corroborate our model of the spoke, where the diffuse spoke appears when the plasma is dominated by species capable of generating secondary electrons from the target. Behaviour of the discharge voltage and maximum plasma emission is strongly dependant on the target/reactive gas combination and does not fully match the behaviour observed in DC magnetron sputtering.

Ion formation in an argon and argon-oxygen gas mixture of a magnetron sputtering discharge

“…Figure 8 [29]. It can be partly explained by a smaller sputtering rate from an oxidised target [17,18].…”

“…Film properties are frequently linked to particular properties of deposition plasma [9,10]. In this context, the ion and energy influx into the growing film is of particular importance [11][12][13][14][15][16][17][18]. New and highly reactive species appear with the addition of oxygen to the discharge.…”

Spokes in high power impulse magnetron sputtering plasmas

“…At moderate reactive gas concentrations, the plasma is considered to be in the transition mode (T), but enters in the so-called poisoned mode (P) if the complete target is covered by the compound. This transition can be followed by optical diagnostics as a transition from metal to reactive gas dominated emission [21].…”

Micro instabilities and rotating spokes in the near-anode region of partially magnetized plasmas

“…Instabilities in a large range of frequency and wavelength, from large scale "rotating spokes" to submillimeter structures have been reported experimentally or by simulations in the partially magnetized plasmas of Hall thrusters 7,8,[11][12][13][14][15][16][17][18][19][20][21][22][23][24] , magnetrons 8,[25][26][27][28][29] , ion sources for neutral beam injection 30 , cylindrical magnetized plasma column [31][32][33][34][35][36] and Penning ion sources [37][38][39] . These instabilities are very dependent on the specific conditions or regions of a particular device.…”