A comparative study of plasma conditions, microstructure and residual stress in sputtered thin films

Restricted until 03 Nov. 2012. The correlation between external (e.g. power, pressure) and internal (e.g. plasma conditions, impurities) chamber conditions to the film residual stress and microstructure has been studied for copper and tantalum sputtered thin films. To investigate the effect of external parameters on the film residual stress and microstructure, the stress evolution in copper and tantalum films at various sputtering powers and pressures was measured in-situ using Multi-beam Optical Stress Sensor (MOSS) technique. The results demonstrate that for both materials, during deposition, a compressive stress initially develops at low sputtering pressures, while at the highest sputtering pressures the stress is always tensile. In addition, profilometry measurements are compared to the final instantaneous stress value at the end of deposition measured by MOSS. Comparing the total residual stress measured ex-situ with the growth stress measured in-situ, emphasizes on the significant contribution of the relaxations mechanism after deposition. The measurements demonstrate that the contribution of the relaxation mechanism is quite significant and can be as large or larger than the growth stress and might change the stress state on the film.; Meanwhile, the effect of internal chamber conditions in relation to the phase formation and stress state of tantalum films was investigated as a function of external chamber parameters. The formation of stable body centered cubic (BCC) and metastable tetragonal tantalum was observed in films deposited on Si substrates at pressures between 0.3 and 1.4 Pa. The results demonstrate the formation of low resistivity BCC tantalum at 0.7 Pa sputtering pressure and various powers ranging from 50 to 200 watts. The variation in hardness, residual stress and texture development are investigated for all deposition conditions. In addition, the effect of internal chamber conditions such as the plasma kinetics and impurities levels was studied in respect to the phase formation at a given sputtering condition. Results show that the formation of alpha tantalum was related to the lower amount of impurities during deposition as well as to an increase in the ion bombardment energy. Therefore, the internal chamber conditions have a significant effect on the tantalum film’s microstructure. Moreover, the results demonstrated that alpha tantalum films can be deposited at room temperature having either compressive or tensile stress states. Hence, the stress evolution in tantalum films can be independent of a particular phase formation and is directly related to the kinetics of ions and atoms inside the chamber.