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.

A semiconductor focusing exposure apparatus in which an opposite face of a mask to a face to be illuminated by exposure light is illuminated with alignment light so that the light reflected from said opposite face may be used for alignment and which is equipped with a second moving arrangement which is separate from a moving arrangement for an x-y moving table supporting a wafer, for aligning the mask and the wafer in an orthogonal direction with respect to the optical axis of a focusing lens. Moreover, the center of the flux of alignment pattern light for illuminating the wafer is made incident upon a line of intersection on which a plane containing the optical axis of an alignment optical system and the optical axis of said focusing lens and the incident plane of said focusing lens intersect with each other. Still moreover, the optical path of the alignment light beam is aligned in parallel with a straight line joining an alignment mark formed on the diffraction pattern and the center of the entrance pupil of said focusing lens.

An exposure apparatus comprises a light source, a mask plate having an exposure pattern area section and an alignment/reflection area section, a projection lens, a movable stage for holding a workpiece having a workpiece alignment mark, an alignment control and a driver for the movable stage. Before the exposure pattern area section is illuminated by the light source to be projected through the projection lens onto the workpiece, the workpiece is properly aligned with the mask. Alignment between the mask plate and the workpiece is performed by the effective use of the alignment/reflection area section specifically arranged and having a specific structure. The alignment/reflection area section is on that surface of the mask plate which does not face the light source and includes a reflection portion for conducting light from another light source to the workpiece and conducting light scattered from the workpiece and passing through the projection lens to the alignment control and a mask alignment mark portion of providing, when illuminated, an image of the mask alignment mark portion to the alignment control so that it detects the positional relation between the mask alignment mark portion and the workpiece alignment mark and produces a control signal for achieving alignment between the mask plate and the workpiece.

In a method for aligning first and second objects relative to each other, according to this invention, the first and second objects are arranged opposite to each other, and are aligned in a direction perpendicular to their opposing direction. A grating pattern is formed, as an alignment mark, on the first object, and a checkerboard-like grating pattern is formed, also as an alignment mark, on the second object. A light beam emitted from an alignment light source is radiated onto the checkerboard-like grating pattern of the second object. The light beam diffracted by the checkerboard-like grating pattern is guided onto the grating pattern of the first object. The light beam diffracted by the grating pattern of the first object is detected by a detector. Since the light beam emitted from the light source is diffracted by the checkerboard-like grating pattern, a relative position of the first and second objects can be detected, irrespective of the distance therebetween. The first and second objects are accurately aligned, based on the detection result. This invention can be applied to a method for aligning a mask and a wafer when a circuit pattern pre-formed on the mask is to be transferred onto the wafer.