Patent ID: 7570358

Claim:
A lithographic cell, comprising: a coater arranged to coat substrates with a radiation sensitive layer; a lithographic apparatus arranged to expose images onto the radiation sensitive layer of the substrates coated by the coater; a developer arranged to develop images exposed by the lithographic apparatus; and an angularly resolved scatterometer configured to determine a value related to a parameter of a target pattern, corresponding to the images, printed on the substrates, comprising, an acousto-optical tunable filter comprising: an acousto-optical crystal configured to receive an input beam of radiation from a radiation source configured to produce the input beam having a first wavelength, and a transducer coupled to the acousto-optical filter and configured to excite acoustic waves in the acousto-optical crystal, a beam selecting device arranged to select as an output beam one of a plurality of beams, output by the acousto-optical crystal in response to the input beam and the acoustic waves, having a second wavelength, the second wavelength being narrower than the first wavelength, an optical system comprising an objective lens configured to direct the output beam onto the target pattern and to project radiation reflected or scattered by the target pattern onto a detector to obtain a scatterometric spectrum, and a driver electrically coupled to the transducer and configured to generate a drive signal for the transducer, the driver being adapted to control a frequency of the drive signal so as to control the second wavelength.