The effluent of a micro-scaled atmospheric pressure plasma jet (μ-APPJ) operated in helium with admixtures of water vapor (10 4 ppm) has been analyzed by means of cavity ring-down laser absorption spectroscopy and molecular beam mass spectrometry to measure hydroxyl (OH) radical densities, and by two-photon absorption laser-induced fluorescence spectroscopy to measure atomic oxygen (O) densities. Additionally, the performance of the bubbler as a source of water vapor in the helium feed gas has been carefully characterized and calibrated. The largest OH and O densities in the effluent of × − 2 10 cm 14 3 and × − 3.2 10 cm 13 3 , respectively, have been measured at around 6000 ppm. The highest selectivity is reached around 1500 ppm, where the OH density is at ∼63% of its maximum value and is 14 times larger than the O density. The measured density profiles and distance variations are compared to the results of a 2D axially symmetric fluid model of species transport and reaction kinetics in the plasma effluent. It is shown that the main loss of OH radicals in the effluent is their mutual reaction. In the case of O, reactions with other species than OH also have to be considered to explain the density decay in the effluent. The results presented here provide additional information for understanding the plasma-chemical processes in non-equilibrium atmospheric pressure plasmas. They also open the way to applying μ-APPJ with He/H 2 O as a selective source of OH radicals.

We report on the results obtained using time-resolved Langmuir probe measurements in high-power pulsed dc magnetron sputtering discharges. Time evolutions of the electron energy distribution and the local plasma parameters were investigated at a substrate position of 100 mm from a planar target of 100 mm diameter during a high-rate deposition of copper films. The average target power density in a pulse was 500 W cm −2 at a repetition frequency of 1 kHz, a voltage pulse duration of 200 µs and an argon pressure of 1 Pa. The electron energy distributions with two energy groups and sharply truncated high-energy tails were observed during a pulse. After a fast rise in a 50 µs initial stage of the pulse, the kinetic temperature of electrons, defined using the mean electron energy, remained in the range from 10 500 to 12 200 K till the pulse termination. The temperature of weakly populated hot electrons decreased rapidly in the initial stage of the pulse approaching the kinetic temperature approximately 100 µs after a pulse initiation. High plasma densities, being in the range 1 × 10 12 -2 × 10 12 cm −3 for 100 µs, were achieved at the substrate position with a 50 µs delay after establishing the 125 µs steady-state discharge regime with the target power density of 650-680 W cm −2 during a pulse. The plasma potential slowly increased from 0.5 to 3.5 V during the pulse and 25 µs after its termination.

A bipolar HiPIMS discharge with a rectangular positive voltage pulse (with controllable amplitude, delay after the main negative pulse and positive pulse length) was systematically investigated by mass spectroscopy. The time-averaged spectra of ions measured at the substrate position exhibit a prominent high-energy peak. It is shown that the position of the peak can be varied by the positive pulse amplitude, its magnitude scales with the pulse length and its width can be slightly influenced by the length of the delay interval. Measurements of the plasma potential at the mass spectrometer position and time-resolved mass spectroscopy clearly show that the high-energy peak is formed by ions accelerated by the elevated plasma potential during the positive pulse. In addition, fine details of the ion energy distribution functions related to the plasma potential transients at the start of the positive pulse are identified. The presented results are beneficial for the optimisation of the parameters of the positive pulse in experiments implementing the bipolar HiPIMS technology.

We have determined the local plasma parameters using the Langmuir probe measurements with a sub-microsecond time resolution during positive voltage pulses of a bipolar high-power impulse magnetron sputtering discharge using an unbalanced magnetron with a titanium target. The effects of the positive voltage pulse amplitude and the delay between the negative voltage pulse end and the positive voltage pulse initiation are investigated as well as the spatial dependence of the plasma parameters at three distances from the target. From the results, the values of the average energy flux of ions during the positive voltage pulse to the substrate are estimated. We have found that the time evolution of the plasma parameters has similar developments which are independent of the positive voltage pulse parameters and the distance from the target, although the values of the plasma parameters are different. During the initial part of the positive voltage pulse, a large difference (up to 200 V) between the plasma and the floating potential accompanied by a high electron temperature (up to 150 eV) and a significant decrease of electron density (up to one order of magnitude) is registered. After this part, the difference of the potentials and the electron temperature are low (<2 V and ≲1 eV, respectively). The short delays between the negative voltage pulse end and the positive voltage pulse initiation as well as the higher positive voltage amplitudes have a beneficial effect on the average energy flux of ions during the positive voltage pulse to the grounded and insulated substrates.

We present a non-stationary model proposed for high power impulse magnetron sputtering discharges, which is based on a global description of the plasma processes. The model takes into account a typical structure of magnetron discharges by dividing the plasma volume into two zones, the magnetically confined high-density zone above the target racetrack and the bulk plasma zone, where the transport of particles onto the substrate and the chamber walls dominates. The comparisons of the calculated data with measured results for distinct experimental conditions in two different high power impulse magnetron sputtering systems show a good agreement, suggesting that all relevant plasma processes were correctly incorporated into the model equations. The model can be used to gain a more detailed insight into the complicated processes in such types of discharges and to predict the influence of various process parameters on the deposition characteristics.