Abstract: Focus performance of a lithographic apparatus is measured using pairs of targets that have been exposed (1110) with an aberration setting (e.g. astigmatism) that induces a relative best focus offset between them. A calibration curve (904) is obtained in advance by exposing similar targets on FEM wafers (1174, 1172). In a set-up phase, calibration curves are obtained using multiple aberration settings, and an anchor point (910) is recorded, where all the calibration curves intersect. When a new calibration curve is measured (1192), the anchor point is used to produce an adjusted updated calibration curve (1004?) to cancel focus drift and optionally to measure drift of astigmatism. Another aspect of the disclosure (FIGS. 13-15) uses two aberration settings (+AST, ?AST) in each measurement, reducing sensitivity to astigmatism drift. Another aspect (FIGS. 16-17) uses pairs of targets printed with relative focus offsets, by double exposure in one resist layer.

Abstract: Focus performance of a lithographic apparatus is measured using pairs of targets that have been exposed (1110) with an aberration setting (e.g. astigmatism) that induces a relative best focus offset between them. A calibration curve (904) is obtained in advance by exposing similar targets on FEM wafers (1174, 1172). In a set-up phase, calibration curves are obtained using multiple aberration settings, and an anchor point (910) is recorded, where all the calibration curves intersect. When a new calibration curve is measured (1192), the anchor point is used to produce an adjusted updated calibration curve (1004?) to cancel focus drift and optionally to measure drift of astigmatism. Another aspect of the disclosure (FIG. 13-15) uses two aberration settings (+AST, ?AST) in each measurement, reducing sensitivity to astigmatism drift. Another aspect (FIG. 16-17) uses pairs of targets printed with relative focus offsets, by double exposure in one resist layer.

Abstract: A transmission image sensor is provided that is optimized for detection of an alignment marker that includes gratings, which are used to detect the position of a part of an overlay marker, e.g. of box-in-box type. Both the alignment markers and the overlay markers may be used to derive a measurement of magnification of a projection system. Magnification values obtained from the overlay marker components using the transmission image sensors link the values conventionally obtained using the transmission image sensor to detect alignment markers and an off-line tool to detect overlay markers.

Abstract: FM-radio receiver having an audio amplifier stage, provided with switching means, and a squelch circuit which is coupled to the output of the receiver demodulator, this squelch circuit comprising two circuits which are constructed in such a manner that one circuit produces at an early stage an output signal when co-channel interference occurs whereas the other circuit produces an output signal which controls said switching means and which occurs only at an impermissible signal/noise ratio and/or an impermissible degree of co-channel interference.