Evaluating Interface Effect On Stresses In Thin Films By A Local Curvature Metrology With High Accuracy And Resolution

Novel local curvature test structures combined with a sub-nanometer optical interferometry measuring setup have been developed to detect stresses in nanometer-scale films as well as ultra low stresses in thin films. Stress difference as small as 1.5 MPa within a 30 nm film can be resolved with this metrology, which is among the best in the present reports. Residual stresses in thermal SiO₂ and LPCVD Si₃N₄ thin films are measured with repeatability better than 1%, which are in good agreement with those extracted by other methods. In addition, residual stresses in Si₃ N₄ thin films with different types of substrates are evaluated. Taking the advantage of the high accuracy and high resolution of this metrology, interface effect on stresses in Si₃N₄ films has been experimentally revealed, which is acute in nanometer-scale film but relaxes with the increase of film thickness. Stress differences as high as 42% suggest that Si₃N₄ deposited on oxide has a much higher residual stress than that grown on silicon, which is crucial for stringent MEMS/NEMS design