In situ assessment of target poisoning evolution in magnetron sputtering

We present an experimental approach for the in situ monitoring of target poisoning through roughness evolution during reactive sputtering by laser reflectometry. Aluminum (Al) and Titanium (Ti) targets were reactively sputtered in the presence of Nitrogen (N2) and Oxygen (O2), separately. From the hysteresis loops of reactive gas flow and discharge voltage, the threshold flow rates of N2 and O2 were determined for partial and complete poisoning of the targets, respectively. These targets were then sputtered up to 60 min in metallic and poisoning modes at different flow rate of the reactive gases. For given time intervals, the eroded regions of the cylindrical sputtering targets were hit by a laser beam and reflected images were captured on a screen. Based on ray tracing optics, the reflected intensity distribution of the laser was utilized for the assessment of the poisoned surface. The mean tilt angle of the surface roughness for both nitride and oxide formation on the Al and Ti targets was measured from the reflected intensity profiles. A change in surface roughness of poisoned targets was observed after different sputtering times. As a proof of concept, the deposition rate and de-poisoning time were measured at different poisoning levels. The change in surface roughness of the targets was found to be in good agreement with depoisoning time. This in situ method can be applied for the assessment of target poisoning evolution during reactive sputtering.