Discharge runaway in high power impulse magnetron sputtering of carbon: the effect of gas pressure, composition and target peak voltage

Abstract

Pressure and target voltage driven discharge runaway from low to high discharge current density regimes in high power impulse magnetron sputtering of carbon is investigated. The main purpose is to provide a meaningful insight of the discharge dynamics, with the ultimate goal to establish a correlation between discharge properties and process parameters to control the film growth. This is achieved by examining a wide range of pressures (2–20 mTorr) and target voltages (700–850 V) and measuring ion saturation current density at the substrate position. We show that the minimum plasma impedance is an important parameter identifying the discharge transition as well as establishing a stable operating condition. Using the formalism of generalized recycling model, we introduce a new parameter, ‘recycling ratio’, to quantify the process gas recycling for specific process conditions. The model takes into account the ion flux to the target, the amount of gas available, and the amount of gas required for sustaining the discharge. We show that this parameter describes the relation between the gas recycling and the discharge current density. As a test case, we discuss the pressure and voltage driven transitions by changing the gas composition when adding Ne into the discharge. We propose that standard Ar HiPIMS discharges operated with significant gas recycling do not require Ne to increase the carbon ionization.

A Strategy for Alleviating Micro Arcing during HiPIMS Deposition of DLC Coatings

“…In the reported process, change of the processing atmosphere by addition of either Ne or of C 2 H 2 had a similar effect on the electrical behaviour of HiPIMS pulses. In both cases, an increase of the peak current at a constant discharge voltage was observed, in agreement with earlier reports [21]. The probability of arc ignition increased proportionally with the discharge current, as it was shown in [29], due to the creation of hot spots on the target that provided the seeds for arc formation.…”

“…When comparing different process conditions it is very common to use the average power of the discharge as a constant parameter. In the case of HiPIMS on the other hand, it was previously shown that the peak current is the key parameter governing the sputtering regime and the ionization degree of the sputtered species [21,30]. We discuss therefore in the present paper, the use of the peak current as the main control parameter, to compare different deposition processes.…”

“…One practical solution to increase the electron temperature is to change the working gas to one with higher ionization energy than the commonly used Ar (Ei = 15.76 eV). In this respect, Ne (E i = 21.5 eV) was shown to be a good candidate for increasing T e [18] and obtaining higher peak current values both to the target and to the substrate [21]. The use of Ne as sputtering gas was also investigated in a semi-industrial setup [22].…”

“…A common issue in C sputtering is the occurrence of micro-arcs. The frequency of arcing depends on the target material quality and its surface state during sputtering [27] and also on the peak current used for sputtering [21,28]. A stable operating regime can be established in a so called "mixed mode" sputtering [28,29], where a transition to arc occurs towards the end of the pulse and provides potentially higher current intensities and ionisation degrees.…”

“…Nevertheless, the tendency for arcing and relatively difficult control of the process presents potential additional issues regarding the quality of the thin films, the formation of droplets on the growing film increasing the surface roughness. Since the increase of ionization degree and consequently of sp 3 content can be achieved by increasing the peak current to the target above a certain critical level [21,30], the occurrence of uncontrolled micro-arcs becomes a limiting factor. Further increasing of the current and potential increase of sp 3 content and hardness can come with unwanted higher density of defects on the surface.…”