A method for in situ measurement of the residual stress in thin films by using the focused ion beam

A new method for the in situ measurement of the residual stresses, σ_R, in thin films, thickness h, is described. It is based on the combined capability of the focused ion beam (FIB) imaging system and of high-resolution strain mapping software (VIC-2D). The method can be used for any film material (whether amorphous or crystalline) without thinning the substrate. In the method, a region of the film surface is highlighted and scanning electron images of that region taken before and after a long slot, depth a, is introduced using the FIB. The mapping software evaluates the displacement of the surface normal to the slot, u_x, by using naturally occurring features on the film surface. Numerical results relate the displacements to σ_R. The accuracy of the method has been assessed by performing measurements on thin (sub-micron) films of diamond-like carbon on glass substrates. Independent measurements of the residual stress exist for this system. The new method determines σ_R to within 5% of the magnitude determined independently. New possibilities enabled by the method are discussed, such as the ability to perform local measurements and to ascertain through thickness gradients.