Determining the position of a semiconductor substrate on a rotation device

A device for determining the position of a semiconductor substrate on a rotation device having a rotational axis, said semiconductor substrate having a peripheral edge, said device comprising:

• a detecting unit for determining the rotational state of the rotation device;
• at least one light source, said at least one light source comprising a modulating device that produces a light beam having a specific variation over time;
• at least one receiver which is photo-sensitive to the light from the light source, said at least one receiver comprising a demodulating device configured to demodulate said light beam detected by said at least one receiver;
• wherein during rotation of said semiconductor substrate on said rotation device, said at least one light source emits a light beam toward said peripheral edge of said semiconductor substrate such that said light beam partially intersects and partially passes over said peripheral edge, and wherein said at least one receiver detects said light beam which has partially passed over said peripheral edge and determines whether there is any temporary variation over time in the intensity of said detected light beam.