When a plasma process is performed while a backside gas such as He is supplied to the backside of the semiconductor wafer held on the lower electrode for plasma generation in the plasma processing chamber by the electrostatic chuck, a plasma is generated between the lower electrode and the ground member through the backside gas introduction pipe In order to prevent the obtained discharge, a conical flow path member made of two kinds of electric insulating materials having a plurality of small-diameter through-flow holes extending in the axial direction in the gas introduction pipe at a position in the insulator between the lower electrode and the ground member . As a result, since the diameter of the gas channel of the backside gas is small, the discharge starting voltage becomes high and discharge can be prevented, and a large conductance can be ensured since a large number of through holes are formed. The backside gas is discharged through the gas introduction pipe after the plasma treatment.;This prevents moisture from remaining between the wafer and the electrostatic chuck and moisture remaining in the processing chamber, thereby preventing electrification of the wafer and shortening the exhaust time of the processing chamber.
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