Growth of nanostructured tungsten using pulsed direct current and bipolar high power impulse magnetron sputtering systems

Nanostructured tungsten (W) has been studied for the first time in pulsed direct current (DC) and bipolar-high power impulse-magnetron sputtering (HiPIMS) systems operating in helium (He). In pulsed DC magnetron sputtering (DCMS), a nano-fiber-form layer was produced over the pulse frequency range (from 100 to 150 kHz), surface temperatures (Ts) of 1075 and 1175 K, and helium ion fluences (ΦHe) of 5.84 × 1024 and 5.29 × 1024 m−2. Time-resolved measurements of ion bombarding energy (EHe) and helium ion flux (ΓHe) were evaluated during both the pulsed-DCMS and bipolar-HiPIMS pulses, with a net width of 833 μs, utilizing the advanced Langmuir Probe Instrument. The experimental results indicated that the fuzz thickness (hfuzz) decreased by approximately 25% at 1075 K and 45% at 1175 K as the system pulse frequency increased from 100 to 150 kHz. The time-averaged growth rate, dhfuzz/dt, of a fuzz layer produced in a bipolar-HiPIMS with a positive cathode pulse voltage range, Vrev, from 90 to 100 V, was found to be 30% lower than that observed in the HiPIMS system at the same temperature range (Ts). In this scenario, the reduced (dhfuzz)/dt in the bipolar-HiPIMS system can be attributed to the fuzzy layer's exposure to highly energetic ionized metal particles and helium ions [of energies above the sputtering threshold of tungsten (W) atoms] during the fuzz formation process.