In a vacuum processing chamber, a monitoring arrangement for detecting when a substrate is sufficiently dechucked by an electrostatic clamp to allow safe movement thereof by a transfer mechanism such as a pin lifter. The monitoring arrangement includes an acoustic generator which outputs a sound wave at the resonant frequency of the substrate and the dechucking condition is detected when the sound wave is adsorbed by the substrate. The arrangement can be used during processing of wafers such as plasma etching or chemical vapor deposition.