Wafer processing apparatus, method of operating the same and wafer detecting system

A wafer processing apparatus handling wafers from cassettes having a cassette body with an internal space maintainable at a pressure approximately equal to atmospheric pressure, with a transparent part made of a transparent material and with a cassette opening of the cassette body on one side of the cassette body, shelves formed on an inner wall of the cassette body, and a lid for hermetically closing the cassette opening of the cassette body, thereby hermetically sealing the cassette body, said processing apparatus comprising:

• a loading region where a wafer is transferred out from within a cassette;
• a working region;
• a wall separating the working region from the loading region, the wall being provided with a wall opening;
• a door movable in a sliding fashion along the wall for closing and opening the wall opening;
• a cassette stand disposed at a position corresponding to the wall opening of the wall in the working region and having a cassette table for supporting the cassette thereon such that the cassette opening is fitted to the wall opening;
• and
• a wafer detecting device disposed in the working region for detecting a number and positions of wafers contained in the cassette, disposed in the working region, from outside the cassette, the wafer detecting device including
• a light emitting device for emitting a light beam toward the cassette body of the cassette supported on the cassette table, and a photodetector for receiving the light beam emitted by the light emitting device;
• and
• a lifting mechanism for vertically moving the cassette supported on the cassette table relative to the light emitting device and the photodetector.