When thin films are deposited at high temperature or annealed at high temperature, intrinsic stresses develop in the film due to mismatch of thermal expansion coefficients between the film and substrate material. The wafer will visibly bow or bend to a measurable degree based on the stress developed in the film. If the bow or radius of curvature of the wafer can be reliably measured, the stress developed in the film can be found out. Tensile stress will bow the wafer down making it concave while compressive stress will bow the wafer up making it convex.
The design interface can be used to determine the stress in the film if the bow of the wafer is known before and after measurement. A positive bow is considered wafer bending up and a negative bow is considered bending down. If the change in bow (bow2-bow1) is negative, the stress is negative and tensile, while a positive stress is compressive.