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A. Muhammad, C. Eisenmenger-Sittner:
"Investigation of Target Poisoning in Reactive Sputtering by Optical Reflectometry";
Vortrag: 20th International Vacuum Congress (IVC-20), Busan/Korea; 21.08.2016 - 26.08.2016.

We present an experimental approach for the in situ monitoring of the morphological evolution of the surface of a target during reactive magnetron sputtering. The target surface was characterized by laser reflectometry without breaking the vacuum of the sputter chamber. Cylindrical Aluminum and Titanium targets were reactively sputtered in the presence of Nitrogen and Oxygen, separately. From the hysteresis loops of reactive gas flow rate and discharge voltage, the threshold flow rates of Nitrogen and Oxygen were determined for the transition of the Al and Ti targets from metallic to fully poisoned mode.
After the determination of the poisoning above thresholds the targets were sputtered in fully poisoned mode for different times, up to 30 minutes. After a given sputtering time, the discharge was turned off and the eroded regions of the cylindrical targets were subjected to illumination by a laser beam of 3 mm diameter. The reflected images of the beam were captured on a screen which was carefully protected from being coated during the sputtering process by a specially designed moveable shutter.
Based on ray tracing optics, the reflected intensity distribution of the laser was utilized for the assessment of the morphology of the poisoned surface. The mean tilt angle of the surface roughness for both nitrided and oxidized Al and Ti targets was measured from the reflected intensity profiles. A gradual change in surface roughness of the poisoned targets was observed with increasing poisoning time. This change in surface roughness was in good agreement with the same measurements taken from a tactile profilometer. In general, prolonged sputtering in poisoned mode led to smoother surfaces as was shown by both, the in-situ reflectometry measurements as well as the ex situ profilometer measurements on the target surface.
It can therefore be concluded that, although sputtering voltage and process gas flow are immediately influenced by the transition from metallic to reactive mode, the target surface morphology during reactive sputtering still changes even after several tens of minutes. In addition it could also be shown that the smoothing effect of reactive sputtering can completely be erased by removing the reactively modified surface layer by non-reactive sputtering with an inert process gas.