Digital image correlation of nanoscale deformation fields for local stress measurement in thin films

In this paper, the application of an in situ stress measurement technique to a silicon nitride thin film deposited onto a thick silicon substrate is presented. The method is based on the measurement of the displacement field originated when a slot is milled into the material under study. Displacements are obtained by digital correlation analysis of scanning electron microscope (SEM) images, whereas milling is performed by ion milling in focused ion-beam equipment. Due to the mechanical constraint introduced by the substrate and the small thickness of the tested layer, the displacements generated by the milling process are in the range 0-5 nm, which is one of the smallest displacement ranges measured up to now in a relaxation-based measurement technique. The local stress value determined with this new method is in good agreement with values obtained by a classical method like the wafer bending test