Plasma characteristics in deep oscillation magnetron sputtering of chromium target

Abstract

A global model for deep oscillation magnetron sputtering (DOMS) discharge is established to investigate the plasma characteristics in the ionization region. Target voltage and current waveforms with micropulse on-time <i>τ</i><sub>on</sub> of 2–6 μs and charging voltage of 300–380 V are acquired and used as an input of the proposed model. The effects of micropulse on-time and charging voltage on the plasma are investigated. At <i>τ</i><sub>on</sub> = 2 μs, the DOMS plasma density oscillates with the discharge current waveform. The plasma is mainly composed of Ar<sup>+</sup> ions though the ionization fraction of Ar is only 2%. The proportion of Cr<sup>+</sup> ions is lower but has a relatively high ionization fraction of 12%, and Cr<sup>2+</sup> ions are negligible. The peak plasma density increases from 1.34×10<sup>18</sup> m<sup>–3</sup> at <i>τ</i><sub>on</sub> = 2 μs to 2.64×10<sup>18</sup> m<sup>–3</sup> at <i>τ</i><sub>on</sub> = 3 μs and the metal ionization fraction increases to 20%. Further increasing the on-time leads the peak density and ionization fraction to slightly change. When the charging voltage increases from 300 V to 380 V at <i>τ</i><sub>on</sub> = 6 μs, the peak plasma density increases linearly from 2.67×10<sup>18</sup> m<sup>–3</sup> to 3.90×10<sup>18</sup> m<sup>–3</sup>, and the metal ionization fraction increases from 21% to 28%. The gas rarefaction occurs in the ionization region for DOMS discharge. The gas density oscillates in the initial stage of macropulse, and 5–6 micropulses later it reaches dynamic equilibrium. The Ar density dynamics shows that the Ar consumption is mainly caused by electron impact ionization, followed by electron impact excitation, and the consumption rate caused by sputter wind is about 10% of the electron impact ionization. The typical metal self-sputtering phenomenon of high power impulse magnetron sputtering (HiPIMS) also appears in the DOMS discharge. The peak value of self-sputtering parameter increases linearly with the peak power density rising. This suggests that the peak power density is one of the important parameters to manipulate the metal self-sputtering process in the DOMS discharge. The peak value of self-sputtering parameter reaches up to 0.20, indicating that a certain degree of metal self-sputtering occurs. The plasma density and the ionization fraction of the depositing flux are improved, which relieves the shadowing effect during conventional magnetron sputtering as a result of low ionization degree of sputtered metal.