Patent ID: 7880896

Claim:
An interferometer comprising: a transparent substrate having a transmission grating and a reflection grating arranged on one side thereof, each of the transmission and reflection gratings adapted to form a zero order beam advancing straight and a first order beam diffracted at a predetermined angle; a single mode vertical-cavity surface-emitting laser for generating a beam of light perpendicular to a wafer, the vertical-cavity surface-emitting laser surface-mounted on the other side of the transparent substrate so that a portion the beam passes through the transmission grating of the substrate; a mirror cube movably arranged on a path of the beam portion to displace the zero order beam transmitted through the transmission grating to a predetermined length in parallel and reflect the replaced zero order beam toward the reflection grating, wherein the mirror cube is configured to be moved in a direction perpendicular to the one side having the transmission grating and the reflection grating; and a photodetector arranged on paths of the first order beams from the transmission grating and from the reflection grating to detect the first order beam reflected from the mirror cube and diffracted by the reflection grating of the substrate and the first order beam diffracted by the transmission grating thereby locating the mirror cube based on an interference fringe created by a difference in optical paths of the two first order beams.