Patent ID: 7202491

Claim:
A method for detecting a plurality of semiconductor wafers in a closeable cassette configured to hold the semiconductor wafers during transport of the semiconductor wafers, comprising: providing the semiconductor wafers in the closeable cassette; sensing a position of the wafers in wafer slots of the cassette using a sensor disposed outside of the cassette, wherein sensing is performed while the cassette is closed, wherein sensing the position of the wafers comprises: providing a radiation source disposed outside the cassette, wherein at least part of the cassette is at least partially transparent to radiation directed to the wafers from the radiation source; directing radiation from the radiation source through the at least partially transparent part of the cassette to edge portions of the wafers inside the cassette; providing the sensor, wherein the sensor is sensitive to radiation reflected from the wafers; and aligning the sensor to detect radiation reflected from the wafers, wherein aligning the sensor comprises vertically moving the sensor relative to the cassette and detecting radiation reflected from the wafers with the sensor at different vertical positions relative to the cassette, wherein the sensor is an image sensor and wherein detecting radiation reflected from the wafers at different vertical positions comprises capturing a plurality of images of the wafers, wherein the plurality of images are captured with the image sensor positioned at a plurality of vertical positions relative to the cassette, wherein each image covers two or more wafers and wherein the images mutually overlap.