Coherent Gradient Sensing for Measuring Curvature

Coherent gradient sensing (CGS) is a diffraction-based, noncontact optical technique for measuring the slight curvature of a nearly flat thin film or specimen surface. CGS is especially useful for measuring curvatures of micromechanical structures and of thin films in electronic devices, to enable the determination of stresses in, and mechanical properties of, such structures and films. CGS is a full-field, real-time technique: Unlike in some other techniques, it is not necessary to acquire images of the specimen surface at different times under different conditions of curvature, nor is it necessary to scan a narrow beam of light over the surface; instead, CGS yields data on curvature over the entire surface area of interest, in as little time as it takes to acquire, digitize, and process a video image. Moreover, CGS is insensitive to rotation or displacement of the specimen.