Time-resolved investigation of plasma parameters during deposition of Ti and TiO2 thin films

Pulsed DC magnetron sputtering is now an established process for the reactive deposition of dielectric films. One important feature of pulsed DC is the suppression of arcs at the target. This supports stable processes, and therefore leads to films with improved properties, compared to the same film deposited in a continuous DC system. However, recent studies have shown that pulsing the plasma also modifies the intrinsic plasma parameters, especially the plasma potential VP and the electron temperature Te. This study focuses on Ti and TiO2 films produced in a sputtering magnetron deposition system driven by pulsed DC. For different frequencies from 5 to 350 kHz and reverse times from 1.1 to 5 μs, these films were deposited on polished substrates of stainless steel, aluminium pin stubs and float glass. For all films a constant power of 1 kW and a pressure of 0.2 Pa was set. During the deposition process, time-resolved Langmuir probe measurements were taken and the driving waveforms on the target were recorded with a digital oscilloscope. After deposition the surface roughness, argon content, structure, refractive index and wear resistance were analysed. For Ti films the substrate potential was recorded after deposition. Assuming a simple model for VP, it was then possible to calculate the bombarding ion energy at the substrate. It was found that this energy is modulated with the target driving voltage waveform, and can increase during the ‘reverse phase’, when positive potential is applied to the target, to values well above 200 eV, more than 10-fold the value in the ‘duty phase’, when the target is driven with a negative potential. The film properties show a strong increase in Ar contamination with pulsing frequency, up to 3.2% for films deposited at 350 kHz, and the lowest value for surface roughness, Ra≈50 nm, was obtained for frequencies from 100 to 200 kHz. For the TiO2 films, the time-resolved Langmuir probe measurements reveal modulation of VP and Te during each pulse and show how these parameters relate to the driving waveform. The film properties show a decrease in the refractive index, from 2.51 at 50 kHz to 2.41 at 350 kHz, and early failure during wear testing for pulsing frequency of 300 and 350 kHz.