Patent ID: 7391676

Claim:
A lithographic apparatus comprising: an illumination system that conditions a radiation beam; a support that supports a patterning device, the patterning device pattering the beam; a projection system that projects the patterned beam onto a target portion of a substrate; and an ultrasonic distance sensor that monitors a height of a surface of the substrate, the sensor comprising, an ultrasonic emitter system that emits a first transmitted beam towards the substrate surface and a second transmitted beam, an ultrasonic receiver system that receives a first reflected beam produced by reflection of the first transmitted beam from the substrate surface and a second reflected beam produced by reflection of the second transmitted beam, and a signal processor that processes signals received from the ultrasonic emitter system and the ultrasonic receiver system indicative of time differences between the emission and the receipt of the transmitted and reflected beams in order to monitor the height of the substrate surface.