Low temperature growth of stress-free single phase α-W films using HiPIMS with synchronized pulsed substrate bias

Abstract

Efficient metal-ion-irradiation during film growth with the concurrent reduction of gas-ion-irradiation is realized for high power impulse magnetron sputtering by the use of a synchronized, but delayed, pulsed substrate bias. In this way, the growth of stress-free, single phase α-W thin films is demonstrated without additional substrate heating or post-annealing. By synchronizing the pulsed substrate bias to the metal-ion rich portion of the discharge, tungsten films with a ⟨110⟩ oriented crystal texture are obtained as compared to the ⟨111⟩ orientation obtained using a continuous substrate bias. At the same time, a reduction of Ar incorporation in the films are observed, resulting in the decrease of compressive film stress from σ = 1.80-1.43 GPa when switching from continuous to synchronized bias. This trend is further enhanced by the increase of the synchronized bias voltage, whereby a much lower compressive stress σ = 0.71 GPa is obtained at U s = 200 V. In addition, switching the inert gas from Ar to Kr has led to fully relaxed, low tensile stress (0.03 GPa) tungsten films with no measurable concentration of trapped gas atoms. Room-temperature electrical resistivity is correlated with the microstructural properties, showing lower resistivities for higher U s and having the lowest resistivity (14.2 μΩ cm) for the Kr sputtered tungsten films. These results illustrate the clear benefit of utilizing selective metal-ion-irradiation during film growth as an effective pathway to minimize the compressive stress induced by high-energetic gas ions/neutrals during low temperature growth of high melting temperature materials.

Modeling of high power impulse magnetron sputtering discharges with tungsten target

“…The cold (or primary) argon ground state density (denoted Ar C (3p 6 ) in figure 4) decreases steadily to a minimum at the end of the pulse, which indicates working gas rarefaction [56]. Working gas rarefaction is known to occur and be rather significant in HiPIMS discharges as has been demonstrated experimentally [15,[57][58][59]. Actually, Shimizu et al [15] noted that the working gas rarefaction is particularly pronounced for the case of tungsten.…”

“…The details of the experimental setup and some measured discharge properties as well as the resulting thin tungsten films are discussed by Shimizu et al [15]. Based on the earlier work [15] the discharge current waveforms were measured for this current work for four different discharge voltages 500 V, 600 V, 700 V, and 800 V. There are slight differences in the discharge current characteristics compared to the earlier report as the target is somewhat more eroded. The measured discharge current waveforms for the various applied discharge voltages are shown in figure 2.…”

“…The discharge parameters were measured for a discharge with a planar circular unbalanced magnetron assembly with a tungsten (99.999% in purity) disk of diameter 75 mm, and a thickness of 5 mm, as the cathode target, and with argon working gas of purity of 99.997% at a constant flow rate of 100 sccm which was maintained at a constant working gas pressure of 1.0 Pa by adjusting the pumping speed via the main gate valve. Unipolar HiPIMS pulses, 100 μs in length, were supplied by a HiPSTER 1 pulsing unit fed by a 1 kW HiPSTER 1-DCPSU DC power supply (Ionautics AB, Sweden) [15]. The details of the experimental setup and some measured discharge properties as well as the resulting thin tungsten films are discussed by Shimizu et al [15].…”

“…Unipolar HiPIMS pulses, 100 μs in length, were supplied by a HiPSTER 1 pulsing unit fed by a 1 kW HiPSTER 1-DCPSU DC power supply (Ionautics AB, Sweden) [15]. The details of the experimental setup and some measured discharge properties as well as the resulting thin tungsten films are discussed by Shimizu et al [15]. Based on the earlier work [15] the discharge current waveforms were measured for this current work for four different discharge voltages 500 V, 600 V, 700 V, and 800 V. There are slight differences in the discharge current characteristics compared to the earlier report as the target is somewhat more eroded.…”

“…Here we extend the IRM to include a reaction set for a tungsten discharge. We apply the model to study a HiPIMS discharge formed with argon as the working gas and a tungsten cathode target in the discharge setup that was earlier explored experimentally by Shimizu et al [15] and used to deposit thin tungsten films. The IRM is described in section 2, where the reaction set for tungsten is also discussed.…”