Patent ID: 7001247

Claim:
A processing apparatus having a chuck table adapted to hold a workpiece, a backpressure sensor adapted to detect a position of a surface to be processed of the workpiece held on the chuck table, and a processing unit adapted to process the surface of the workpiece held on the chuck table, wherein: the backpressure sensor includes a blowout nozzle adapted to blow air onto the workpiece, the blowout nozzle being freely movable in the air blowout direction and in the direction opposite thereto, a driving unit adapted to drive the blowout nozzle in the air blowout direction or in the direction opposite to the air blowout direction so as to move the blowout nozzle toward or away from the workpiece, an air supply source adapted to supply the air to the blowout nozzle, a first path connecting the blowout nozzle and air supply source together, a second path connected to the air supply source and arranged to discharge the air to the atmosphere, a differential pressure sensor connected to the first and second paths and adapted to output a voltage corresponding to a difference between the pressure in the first path and the pressure in the second path, a control unit adapted to recognize a value of a voltage output from the differential pressure sensor, and a free-movement detecting sensor adapted to detect an actual free-movement of the blowout nozzle, wherein the free-movement detecting sensor has a function to notify an actual free-movement of the blowout nozzle to the blowout nozzle driving unit, and the blowout nozzle driving unit has a function to drive the blowout nozzle in the direction to move away from the workpiece on receiving such a notification.