Curvature of substrates is measured by means of a self-mixing scheme

We present a new method for in-situ measurement of substrate camber, the deformation introduced by the epitaxial layer being grown on the substrate. Similar to the currently employed laser beam deflection method, we measure the radius of curvature from the inclination angle α(z) of the wafer surface, during the wafer rotation under the observation window of a metal-organic chemical vapor deposition (MOCVD) chamber. But, instead of the commonly used triangulation, based on a laser plus a position-sensitive detector, we employ a noncontact self-mixing interferometric detector, based on a diode laser projecting a spot onto the wafer surface and detecting the backreflection with the laser itself. Advantage of the new method is that separation of source and detector is eliminated and a smaller MOCVD window can be used. Ultimate sensitivity is improved because the angular resolution is better than with a normal detector. The experimental data show that the range of curvature radius measurable with the new method spans from ≈10 m up to >10 km.