International Business Machines Corporation

Walter Jarisch

Gunter Makosch

Gerd Hausler

In an alignment method for lithographic devices of the proximity printing type two light beams (9a,d;9b,c) are made to impinge symmetrically on gratings (3) associated to each alignment direction on mask (1) and wafer (2) such that diffracted beams return along the optical axis of the alignment system. The relative phase of the diffracted beams is measured with a phase compensation technique using an electro-optic modulator (15). The alignment method is performed in two steps, the first being a normalizing step where a smooth portion of the wafer is brought under the alignment grating in the fixed mask (2) and a measuring step yielding the phase difference as a position control signal when the alignment grating on the wafer has been brought under the alignment grating of the mask. The four partial beams required for alignment in two orthogonal directions are generated from a single laser beam (15) in a compact set (14) of birefringent crystals or Wollaston plates and half-wave retarders.

Phase-sensitive interferometric mask-wafer alignment

Flat objects (6), e.g. non-transparent wafers, are tested over their entire surface for thickness variations in an interferometric set-up (FIG. 1) where symmetrical opposite points on the front and rear surface, respectively are sensed by the same light ray (10). A laser beam (3) impinges under oblique angle of incidence on a beam splitter (e.g. a grating 4) to generate a reference beam (3a) and a measurement beam (3b). The latter is directed by a first of a pair of plane mirrors (5a) to the front surface of sample (6) from which it is reflected again to plane mirror (5a) to reach blazed grating (7) which deflects it symmetrically to plane mirror (5b). The rear surface of sample (6) is thus illuminated under the same angle of incidence in a way that each ray (10) is directed to the point on the rear surface that is exactly opposite to its reflection point at the front surface. The beam reflected at the rear surface passes after further reflection at plane mirror (5b) through beam splitter (4) in the same direction as reference beam (3a) thus generating an interference pattern on screen (8). The fringe separation in the interference pattern is representative of the local thickness of sample (6) and can be evaluated electronically or by pattern recondition methods to thickness readings with an accuracy of .lambda./200. A second embodiment of the invention uses a folded mirror instead of blazed grating (7) for the deflection.

Interferometric thickness analyzer and measuring method

Interferometric devices wherein two beams (the object and reference beams) interfere are rendered insensitive to deformations in the ideally planar wave fronts of the two beams by making the deformations identical through optical phase mixing. For arrangements wherein the beams to be superposed upon each other are symmetrically guided, phase symmetrized output beams are formed from a common input beam by making the input beam perpendicularly incident upon a partially reflecting hypotenuse face of a roof prism or axicon. Rays positioned symmetrically with respect to the center plane of the prism or the center axis of the axicon become mixed because the portion of each reflected by the hypotenuse face becomes superposed with the transmitted portion of the other after total reflection inside the prism or axicon, yielding two output beams with a symmetrical phase distribution with respect to the center plane of the prism or the center axis of the axicon.

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Boeblingen, Germany

Walter Jarisch