title: Plasma diagnosis of reactive high power impulse magnetron sputtering (HiPIMS) discharges
Creators: name: Bowes, Michael
Publisher: name: University of Liverpool Repository
Year: 2014

description: Reactive HiPIMS discharges have been investigated by employing a selection of plasma diagnostic techniques. Plasma dynamics in a reactive HiPIMS discharge were studied by means of a single Langmuir probe which revealed electron and positive ion densities of the order of 10^17 to 10^18 m^-3 in typical substrate positions, the temporal evolutions of which exhibited a dual-peak structure attributed to the propagation of an ion acoustic wave or the compression and subsequent rarefaction of the process gas caused by the intense 'sputter wind'. The compression phase is also thought to be the cause of the quenching of the effective electron temperature observed during the on-time with the rarefaction phase being responsible for the increase in the effective electron temperature toward the end of the voltage pulse. Negative ion dynamics in the afterglow of a reactive HiPIMS discharge were also studied by means of a Langmuir probe for a range of oxygen partial pressures. The extended afterglow was found to be highly electronegative with the negative ion-to-electron ratio (alpha) at 3 milliseconds after the pulse termination reaching values of almost 400 for the highest oxygen partial pressures. By comparing results to a simple plasma-chemical model, it is speculated that increased negative ion formation occurs for higher oxygen partial pressures due to the increased availability of oxygen metastables that are formed in the active phase. Despite exhibiting a strong correlation, a comparison to the alpha values determined by photodetachment revealed an overestimation by a factor of 8-15 when employing the Langmuir probe method. Furthermore, from photodetachment measurements the O- ion density was observed to peak in the early afterglow at values of the order of 10^17 m^-3. It was also concluded that a negative ion flux of approximately 10^17-10^18 m^-2 s^-1 could be expected at the chamber walls and substrate surface once the plasma afterglow transitions into an ion-ion state, which could have implications for many plasma processing methods. Time-averaged energy distributions of oxygen negative ions obtained using energy-resolved mass spectrometry during reactive HiPIMS of Ti in an Ar/O2 gas mixture revealed three distinct populations of O- ions at varying energies. The peak of the high-energy population was found to correspond to the value of average applied target potential during the pulse on-time. Comparison to a Thompson energy distribution of sputtered particles suggested that O- ions are sputtered from the target surface before undergoing acceleration in the cathode sheath. A study of the attenuation of the high-energy O- ion population as a function of the pressure-distance product yielded an effective total cross section of 2 x 10^-19 m^2 for O- interacting with the background Ar/O2 gas mixture for both Ti and Nb targets. During reactive HiPIMS of different target materials, a correlation between O- emission and the ion-induced secondary electron emission coefficient was found. In addition, large differences in the high-energy O- yield were also observed when employing different inert gases mixed with O2, which was also attributed to changes in the ion-induced secondary electron emission coefficient. The deposition rates in reactive HiPIMS of TiO2 using different inert gases were measured by means of a quartz crystal microbalance. In contrast to the trends predicted by SRIM as well as those measured in DCMS, the power normalized deposition rate, D_n, was found to increase with the mass of inert gas in both metallic and reactive modes, with the exception of the Xe/O2 gas mixture. The observed increase of D_n with the mass of inert gas was partially attributed to a decreased return effect in the heavier gases. For the case of Kr/O2, the normalized deposition rate measured in HiPIMS was found to be 87% of that measured in equivalent DCMS operation despite no attempt at optimization.