Examination of the pulsing phenomena in pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS) of Cr–Al–N thin films

Abstract

Pulsed DC magnetron sputtering was developed initially as a means of suppressing the arcing at the target during reactive sputtering. More recently, pulsing the magnetrons has also been shown to modify the intrinsic plasma parameters to bring higher ion energy and ion flux to the growing film at the substrate, which, in turn may have a strong influence on film structure and properties. This paper examines the effect of pulsing the unbalanced magnetrons in a two-cathode closed field configuration on the plasma properties, in particular, the ion energy distributions (IEDs) and ion flux, as determined using a Hiden electrostatic quadrupole plasma mass spectrometer (EQP). Chromium and aluminum targets were reactively sputtered in an Ar–N 2 atmosphere in this two-cathode pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS) system. The pulsing parameters examined included: pulsed power (400 W to 1400 W), working pressure (0.13 Pa/1.0 mTorr to 0.53 Pa/4 mTorr), nitrogen to argon flow ratio, substrate to target distance (127 mm/5 in. and 203 mm/8 in.), duty cycle (40 kHz to 350 kHz frequency, 0.4 μs to 5 μs reverse time), operated under synchronized (voltage-time wave forms of the two magnetrons were in phase) and asynchronized (voltage-time wave forms of the two magnetrons were out of phase) conditions. The effect of the pulsing regimes on the ion energy and ion flux was determined for reactive sputtering of Cr–Al–N thin films. It was shown that pulsing both magnetrons in this P-CFUBMS configuration had a large effect on both the ion energies and ion fluxes generated within the plasma. The positive voltage overshoot and the positive pulse voltage developed on each target during the reverse voltage (positive) period increased the plasma potential and thereby increased the overall extended ion energy range. Three main ion energy regions in the IEDs were identified and related to different positive voltage values during the reverse pulse period. In addition, the extent of synchronization of the voltage waveforms of the two magnetrons also substantially influenced both the IEDs and the ion fluxes, while varying the pulsing frequencies and reverse times (duty cycle) in either synchronized or asynchronized modes also resulted in different ion energies and ion fluxes within the plasma. It was also found that the ion flux increased on increasing the EQP orifice to the chamber wall distance from 127 mm to 203 mm, and the ion energy range decreased as the working pressure increased from 0.13 Pa to 0.53 Pa. The ion energies and fluxes are shown to be strongly dependent on pulsing frequency, duty cycle and waveform.