Patent ID: 8810798

Claim:
Apparatus for measurement of a curvature of a surface of a wafer in a multi-wafer epitaxial reactor, comprising: means adapted for irradiating a first light beam and a second light beam onto a surface of a sample, wherein the first light beam and the second light beam are parallel to each other; a detector comprising at least one detector plane and being adapted to detect a first position of a reflected first light beam in the at least one detector plane and a first position of a reflected second light beam in the at least one detector plane at a first sampling time and to detect a second position of a reflected first light beam in the at least one detector plane and a second position of a reflected second light beam in the at least one detector plane at a second sampling time; means adapted to synchronize the detector sampling time with the sample main rotation (ω main ); means adapted to determine: a first distance between the first position of the reflected first light beam and the first position of the reflected second light beam measured at the first sampling time; a second distance between the second position of the reflected first light beam and the second position of the reflected second light beam measured at the second sampling time; a third distance between the first position of the reflected first light beam measured at the first sampling time and the second position of the reflected first light beam measured at the second sampling time; and a fourth distance between the first position of the reflected second light beam measured at the first sampling time and the second position of the reflected second light beam measured at the second sampling time; means adapted to determine a mean spherical curvature (C sph ) of the surface and an azimuthal aspherical curvature deviation (ΔC asph ) from the first to fourth distances.