||In the MEMS process, a thin film is likely to be coated on a substrate. Residual stress is established in the coating upon cooling to room temperature because thermal expansion coefficients of the thin film and the substrate are different. The residual stress in the coating not only causes bending but also affects the quality of coating. This thesis discusses the relationship of the residual stress and its distribution with the parameters of the process and the thickness of the ITO thin film that is coated on the Si-substrate.|
Four parameters of coating process - coating time, sputtering power, working pressure, and argon flow rate – are considered here. In the experiment, each parameter is set to one of three levels, consistent with the Taguchi Method, yielding nine combinations. For each of the nine combinations, three Si-substrates are used. Measurements are made at nine points on each Si-substrate. Two sets of the in-plane displacement of the coating from two different angles of the screen can be measured by using digital image correlation technique. The obtained sets of in-plane displacements can be used to calculate the out-of-plane displacement, the components of residual stress in the x-direction and the y-direction can be calculated using the modified Stoney’s equation. The equivalent residual stress is calculated from these components of residual stresses, and the coefficient of variation is calculated from the average and standard deviation of the equivalent residual stress at the nine points on the Si-substrate. Finally, the distribution of residual stress in coating is discussed with reference to the experimentally obtained coefficient of variation.
The experimental results indicate that the coating time and the sputtering power importantly affect the magnitude of residual stress, but the working pressure and the argon flow rate have very little effect. Additionally, the experimental results show that the residual stress distribution is affected by coating time, sputtering power, argon flow rate and working pressure, in order of declining strength of the effect. The coating time most strongly affects the coating process. As the coating thickness increases, the residual stress decreases and the variation among measurements made of three Si-substrates of same combination declines. Restated, the precision of the experimental measurements improves with the coating thickness, but the distribution of residual stress becomes less uniform.