Method for improved detection of anomalous substrates in automated material-handling systems

A method for distinguishing between tilted and multi-stacked substrates in Automated Material-Handling Systems comprising:

• providing a SEMI standard substrate cassette carrier having at least one substrate in a cassette slot, the SEMI standard substrate cassette carrier is positioned within the material handling system, the substrate having a top surface and a bottom surface;
• moving the SEMI standard substrate cassette carrier with the substrate vertically via an elevator at a known rate;
• producing a signal profile from a first optical path between a first light beam and a first detector and a second optical path of a second light beam and a second detector while vertically moving the SEMI standard substrate cassette with the substrate at the known rate, wherein the first light beam is positioned above the substrate and the first detector is positioned below the substrate and wherein the second light beam is positioned below the substrate and the second detector is positioned above the substrate,
• wherein the first detector and the second detector are positioned at opposing angles across the substrate;
• determining the presence of the substrate or substrates within the cassette slot or cassette slots of the SEMI standard substrate cassette carrier using said produced signal profile;
• and
• determining the relative thickness of the substrate or substrates within the cassette slot or cassette slots of the SEMI standard substrate cassette carrier using the produced signal profile.