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M. Arif:
"In Situ Assessment of Sputter Target Erosion and Poisoning by Laser Reflectometry";
Betreuer/in(nen), Begutachter/in(nen): C. Eisenmenger-Sittner, M. Cekada, P.H. Mayrhofer; E-138, 2017; Rigorosum: 09.08.2017.

We present an experimental approach for the in-situ characterization of the surface roughness of a uniformly eroded cylindrical magnetron target based on the reflectance pattern of a laser beam. The surface of the eroded region was modeled as a random arrangement of tilted plane facets with large surface extensions as compared to the wavelength of the laser. The target surface was hit by a laser beam and the reflection of the laser was captured on the screen. A simple algorithm was used to reconstruct the surface profiles form the intensity values reflected from the facets of the eroded surface. Aluminum (Al), Copper (Cu) and Titanium (Ti) targets were sputtered for different time intervals and the surface profiles were reconstructed. From these generated profiles, the roughness parameters, RMS roughness, correlation length, skewness and kurtosis of different target materials were calculated. The results measured with this in-situ technique under high vacuum conditions without disturbing the processing parameters of the magnetron sputtering plant are in good agreement with comparable measurements using a mechanical profilometer outside the sputtering system. However, some deviation was observed for surfaces with higher roughness and mean tilt angles.
As a proof of concept, we demonstrate that this approach can be used for the in-situ monitoring of poisoning of the target during reactive magnetron sputtering. The Al and Ti targets were reactively sputtered in the presence of Nitrogen (N2) and Oxygen (O2), separately. From hysteresis loops of reactive gas flow and discharge voltage, the threshold flow rates of N2 and O2 were determined for complete poisoning of Al and Ti targets, respectively. These targets were then sputtered up to 60 minutes in metallic and poisoned modes at different flow rate of the reactive gases. For each time interval, a laser beam hit the eroded regions of the cylindrical sputtering targets and reflected images were captured. The reflected intensity distribution of the laser was utilized for the characterization of the poisoned surface. The mean tilt angle of the surface roughness for both nitride and oxide formation on the Al and Ti targets were measured from the reflected intensity profiles. A change in surface roughness of the poisoned targets was observed after different poisoning time. After poisoning, the targets were sputter cleaned in the absence of reactive gas and de-poisoning time was measured. The de-poisoning time was found to be in good agreement with the change in surface roughness of the targets.
As a step further in this direction, we investigated the effect of the target poisoning level on the microstructure, preferred orientation and composition of the deposited films.
Aluminum nitride (AlN) and Titanium nitride (TiN) films were grown on natively oxidized Silicon (Si) wafers after different poisoning levels of the respective targets. For each target, the nitride films were deposited during the first 10 min of poisoning and after 60 min of poisoning again for 10 minutes. Higher substrate temperatures were observed for both AlN and TiN samples prepared after long-term poisoning. A change in surface morphology and film structure was also observed at different poisoning levels of the targets. For AlN, for long-term poisoning, the nitrogen content was increased and the XRD peak of maximum intensity became the [103] plane. This is characteristic for piezo electric AlN material. In the case of TiN, the deposited films were transformed to Ti-rich coatings with higher compressive stress and a preferred orientation of [220] for samples prepared after long-term poisoning of the target. It was found that the different target poisoning levels and the corresponding energy of the incident particles influence the properties of the deposited films.
The method of laser reflectometry can be used for in-situ monitoring the condition of sputter targets during reactive and non-reactive sputtering to extract the surface features which may correlate with sputtering parameters. These results can also be utilized for in-situ monitoring the target poisoning and respective changes in the properties of the sputter deposited films.