Synergy of experiment and model for reactive HiPIMS: effect of discharge parameters on WO_x composition and deposition rate

Abstract

Reactive high-power impulse magnetron sputtering of tungsten oxide films using metallic tungsten target (72 mm in diameter) in argon-oxygen atmosphere (total pressure of 0.75 Pa) was carried out. The effect of various discharge parameters on the deposition rate and film oxygen concentration was investigated. Moreover, a model combining a reactive high-power impulse magnetron sputtering model and a discharge plasma model for the ionization region was successfully used for deeper insight into the effect of particular discharge parameters such as voltage pulse length (from 100 –800 µs), oxygen partial pressure (from 0.25–0.50 Pa) or the value of pulse-averaged target power density (from 2.5–500 W cm−2). The results of the presented model, most notably trends in the target- and substrate oxide fraction, composition of particle fluxes onto the substrate, degree of W atom ionization or degree of O2 molecule dissociation are discussed and put into context with experimentally measured quantities.

Pulse length dependence of a reactive high power impulse magnetron (HiPIMS) discharge

“…As a consequence, the fraction of implanted oxygen ions and atoms decreases with pulse duration and the oxygen composition of the SSL becomes smaller for long compared to short pulses. Calculations by Rezek et al also show a decrease of the target oxide fraction as function of pulse length [18]. However, the calculated decline is rather weak and quantitatively deviates from our observations.…”

“…For a typical discharge current of 10 A, the surface coverage reduces within 54 µs by half (θ = 0.5) and to less than θ = 0.12 within 200 µs. The surface coverage is a pronounced function of the discharge current (power) which is in line with calculations performed by Rezek et al [18] who concluded that the O/W flux ratio decreases with increasing discharge power. The surface coverage recovers during the off time.…”

“…There is an ongoing challenge to further enhance the energy-influx during film deposition making use of so-called bipolar pulsing, a positively biased anode, and the use of an external anode [13][14][15]. Another approach is to vary the employed pulse length which influences ion composition and ion energy during the discharge and thus affects the deposition rate, the composition, and other properties of deposited thin films [16][17][18][19][20][21][22][23][24][25][26][27][28]. An alternative approach was employed by Magnus et al [29] varying the pulse frequency and, hence, the time between two subsequent pulses.…”

Modeling of high power impulse magnetron sputtering discharges with tungsten target

“…Tungsten (7.98 V) has a higher ionization potential than Cu (7.73 V), Ti (6.83 V), and Al (5.99 V), and is therefore more difficult to ionize, while it has a sputter yield slightly higher than titanium, and has a higher atomic mass. Earlier, a volume-averaged model has been developed to describe a reactive HiPIMS discharge and to model the deposition of WO 3 films from a tungsten target in Ar/O 2 mixture [28]. The model results indicate that the degree of tungsten ionization increases from 50% to 80% as the power density is increased from 100-500 W cm −2 , and it also increases with increased pulse length.…”

“…The model results indicate that the degree of tungsten ionization increases from 50% to 80% as the power density is increased from 100-500 W cm −2 , and it also increases with increased pulse length. Furthermore, it is claimed that W + ions are re-implanted into the target which effectively decreases the target oxide coverage during reactive sputtering [28].…”