Abstract: An imaging apparatus, for instance a wafer stepper, has an optical system for forming an image of an object such as a reticle and it has a focusing device. The focusing device has detector means for determining deviations between the actual image surface of the optical system and a second surface (surface of the semiconductor wafer) on which the image is to be formed. The focusing device includes two sets of detector means, each said set having means fast with the optical system and constructed to focus a spot of monochromatic light under a large angle of incidence onto said actual image surface and means for collecting the light reflected by said second surface and focusing it on a differential light detector so located that the image of the spot is centered on the detector when the two surfaces coincide and means for summing differential signals supplied by the detectors of both sets, said sets being arranged for the light path in the two sets of detector means to be substantially reverse from each other.

Abstract: A machine for drawing on a photographic plate and especially for integrated ircuit masks comprises an electro-optical imager having an optical axis x-x', means for controlling the optical state of each point which has dimensions corresponding to the smallest detail to be drawn on the plate, the entire light beam issuing from the imager being applied to an objective which provides a magnification n and has an axis corresponding to the optic axis. Means for producing the relative displacement of the imager and of the photosensitive plate in a plane at right angles to the optic axis and in two orthogonal directions are controlled as a function of indications supplied by means for measuring the relative displacement.