Stress mechanisms of SiO2 and Nb2O5 thin films sputtered on flexible substrates investigated by finite element method

The stress mechanisms of SiO2 and Nb2O5 thin films sputtering on BK-7 glass, PET, and PC flexible substrates were investigated by the numerical analysis of the finite element method (FEM). Residual stress is a combination of thermal and intrinsic stresses during the sputtering process. The thermal stress results from the difference in thermal expansion coefficients and the substrate temperature between the film and substrate. The sputtering process is complicated; hence, the intrinsic stress was simulated by an equivalent-room-temperature (ERT) technique of FEM and a polynomial fitting curve, which could reduce the error ratio to less than 2%. The experimental verification of residual stresses has been conducted using the self-made Twyman-Green interferometer and shadow moiré interferometer with Stoney and modified Stoney formulas for glass and flexible substrates, respectively. However, in examining the stress mechanism of the BK-7 hard substrate, it was found that the degree of intrinsic stress was directly proportional to the D factor (Unit cell ratio) of SiO2 and Nb2O5 thin films, and that the error ratio was about 3%; while the stress mechanisms of PET and PC flexible substrates show that the residual stresses were directly proportional to the D factor, and the error ratio was about 6%.