Abstract: The invention relates to a level sensor for use in a lithographic apparatus that determines a surface height of a substrate. The level sensor includes an emitter and a receiver, wherein the emitter is arranged to emit a signal directed to a predetermined position on the surface of the substrate, such that the signal is at least partially reflected by the substrate to render a reflected signal. The receiver is arranged to receive at least part of the reflected signal, and the level sensor is arranged to determine the surface height of the substrate with respect to the level sensor based on the emitted and received signal. The signal includes a pressure wave.

Abstract: An embodiment of the invention may be applied to measure the positions of features in a alignment region on a mask with a sensor. The positions of the features in the alignment region are known from the design. A feature of which the position is measured, is identified by comparing the relative positions between measured features with relative positions known from the design. The known position of the identified feature is subtracted from a measured position of the identified feature to give the position of the mask.

Abstract: A lithographic apparatus includes a measurement system including at least one optical component and at least one electrical component. The electrical component is configured to dissipate heat. The optical component is mounted on a first frame of the apparatus, and the electrical component is mounted on a second frame of the apparatus that is thermally and mechanically decoupled from the first frame. An optical coupling is provided between the first frame and the second frame.