| 摘要 |
<p>PURPOSE: To provide a means capable of detecting the state of wafer attraction with high precision, when the amplitude or the frequency of an AC voltage for detecting the state of attraction changes or both of them change. CONSTITUTION: A DC voltage for attraction holding and an AC voltage 21 for detecting the state of attraction are applied to an electrostatic chuck which attracts and holds a wafer. An attraction state signal 20 is extracted through an electrostactic capacity of the electrostatic chuck, and changes according to the electrostatic capacity of the electrostatic chuck. The state of wafer attraction is detected by comparing the level of the attraction state signal 20 with a threshold level 25. The erroneous decision of the state of wafer attraction which is caused by the variation of the amplitude or the frequency of the AC voltage 21 for detecting the state of attraction or the variation of both of them is reduced, by controlling the level of the attraction state signal 20 or the threshold value level 25 to be compared with the attraction state signal 20 or both of them, by the amplitude or the frequency of the AC voltage 21 or both of them.</p> |
