International Business Machines Corporation

Gunter Makosch

Franz J Schedewie

Walter Jarisch

Gerd Hausler

Arno Schmackpfeffer

Jorg Ziller

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

A surface to be tested is illuminated with two beams polarized perpendicularly to each other, extending symmetrically to an optical axis and focussed on the surface by a lens. The light scattered at the surface is separated from the directly reflected light by a reflecting diaphragm, and by means of electro-optical compensation (with an electro-optical modulator and photodetector) the scattered light is tested for phase difference which is a function of the distance of a scattering element of the surface from the optical axis. The phase difference within the illuminated spot is a unique function of the distance of the scattering element from the optical axis, if a diffraction-limited optical system is used.

Interferometric measuring methods for surfaces

A surface is subjected to a light field whose intensity is periodically modulated in a vertical direction to evaluate a scattered light pattern. The light field is obtained from interference between a first or central beam with vertical incidence and two symmetrical laterally disposed beams with oblique incidence. All beams are focussed to a common spot if the profile is obtained by scanning the surface or are superimposed as collimated beams if large area profiling is desired. The surface is imaged onto a diaphragm which selects appropriate locations of the surface for measurements by a photodetector to record sinusoidal intensity variations that are obtained when the light is periodically shifted in a vertical direction by phase modulating the first or central beam with respect to the lateral beams.

Method and arrangement for optically determining surface profiles

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

A method and apparatus is described for making absolute and relative distance measurements optically. A grating, or gratings, is located on the object whose displacement is to be measured, or on a surface in contact with the object. Two perpendicularly polarized light beams interact with the grating to produce a phase difference therebetween. This phase difference is a measure of the displacement of the object, and can be compensated for in order to determine the exact displacement of the object. The relative displacement between two objects can be measured in a similar way, there being gratings on the two objects. One of the light beams strikes one grating while the other strikes the second grating. If there is perfect alignment between the gratings on the two objects, there will be no phase difference in the diffracted light from the gratings. If there is misalignment between the objects, a phase difference between the diffracted signals will be produced indicative of this misalignment.

Method and arrangement for optical distance measurement

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.

Apparatus for phase symmetrizing optical wave fronts

For the point-by-point interferometric measuring of surfaces which are scanned along lines, or which are subjected to a processing procedure (e.g. etching) a focused laser beam is directed with oblique incidence as a measuring beam onto the respective measured points. The respective height of a measured point determines the path difference (phase difference) between the measuring beam and a reference beam which has been suitably split off the input beam. The oblique incidence (angle of incidence .gtorsim.80.degree.) makes it possible to measure interferometrically very rough surfaces. The measuring range may be extended if the reference beam is directed at an oblique angle of incidence, differing only slightly from that of the measuring beam onto the surface to be examined.

Method and means for interferometric surface topography

In an interferometer wherein there is provided means for directing two linearly polarized beams having their polarizations perpendicular to each other onto a surface at different angles of incidence and means for recombining the beams reflected from the object into a combined beam, the improvement wherein means for periodically varying the phase of one component of the combined beam is provided and wherein the amount of change in phase between the two component beams of the combined beam when an amplitude sensing means senses a maximum or a minimum amplitude is a measure of the topographic height variations of the object surface which is irradiated.

Interferometric measurement apparatus and method having increased

The gaussian energy distribution common to all laser beams is a serious handicap in most laser beam applications for machining material, such as for drilling, welding and the like. This is especially true in welding extremely thin metal sheets, as in the manufacture and assembly of the transducers of modern magnetic disk files. According to the subject invention, a laser beam is directed to a glass cone or axicon which converts the gaussian energy distribution of the laser beam to one more uniform in cross-section. Depending on the distance of the workpiece from the exit side of the axicon, the energy distribution of the radiation impinging onto the workpiece will be either uniform, saddle shaped or annular cross-section. It has been discovered that saddle shaped or annular energy distributions are optimal for welding very thin metal sheets by a laser beam since the heat dissipation in the center of a welding spot is a minimum and that at the periphery of the laser beam depends on the material, size and shape of the workpiece, which parameters can be matched by adjusting the distance between the axicon and the workpiece.

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Sindelfingen, Germany

Gunter Makosch