| 摘要 |
<p>PROBLEM TO BE SOLVED: To remove unstable variations of a sensor by finding an offset characteristic which recognizes the offset between a perfect incident light signal and a perfect nonincident light signal of a sensor and setting true threshold values for judging the existence of a substance taking the offset charactristic into consideration. SOLUTION: True threshold values 71, 72 for preventing an erroneous judgment are set for each light emitting and detecting element LED. This is performed by finding an offset characteristic to be recognized as an offset at the time of a perfect incident light signal for each light emitting and detecting element LED, and setting true threshold values for judging the existence of a wafer taking this offset characteristic obtained into consideration. A true threshold value can be obtained by 'true threshold value = (perfect incident light signal-perfect nonincident light signal)×0.92 + perfect nonincident light signal', at the time of a standard threshold value of 92%. Consequently, it becomes possible to perform a stable wafer detection without a detection error, and to perform a precise detection, even if there is a slight detected output difference like the case of a transparent wafer and a semitransparent wafer.</p> |
