Stress development and relaxation during sputter deposition film growth
The stress development and relaxation of magnetron sputtered copper and amorphous-silicon (a-Si) films at room temperature are studied. Samples were prepared as a function of pressure and deposition power. In-situ stress measurements with the wafer curvature method were made using a helium neon gas laser system with a 10mm beam splitter. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to perform post-growth microstructural and surface analysis. SEM cross-section analysis was used to determine the final film thickness. Phase compositions were studied by X-ray diffraction. The growth rates of copper films decreased with increasing pressure. Copper film stress development followed a non-monotonic compressive, tensile then tensile relaxation curve. In order to investigate further the nature of the stress relaxation, stress curves both after deposition was stopped and after it is restarted were also measured. Correlations between growth rate and pressure were also observed in a-Si sputter deposition. In some contrast to what was observed for Cu deposition, stress measurement during a-Si deposition showed a trend of tensile development and relaxation at all pressures studied. In a new approach to understanding stress relaxation during film growth, an acoustic emission (AE) system is introduced to measure the AE energy during sputter deposition. Evidence shows a certain relation between the strain energy of films calculated using the measured stresses and AE energy recorded during the deposition. AE energy occurs immediately after deposition starts and follows the trend of stress development (increasing hits and energies) and relaxation (decreasing hits and energies). No further signal was detected after deposition, matching the results of stress curve measurements showing that stress magnitude after deposition stays at the same level as before deposition stopped. Results also show a lower AE energy magnitude with increasing deposition pressure.