International Business Machines Corporation

Guenter Makosch

Walter Jaerisch

Gerd Haeusler

Arno Schmackpfeffer

A phase measuring method and apparatus are described for determining the phase difference between two polarized light beams which, in contrast to prior phase measuring techniques, permits simultaneous signal evaluation. The orthogonally polarized light beams with the phase difference are initially split by a beam splitter into several partial beam pairs which, by means of a lens are focused as parallel beams into a phase shifter, a polarizer, and an array of light sensors. Phase differences of the light beams create intensity differences between the beams received by the different light sensors. High measuring speed and accuracy are thus provided. When combined with means for directing two spaced orthogonally polarized beams on a surface, the method and apparatus can be used to determine height differences along the surface.

Interferometric phase measurement

To align a grating on a mask with respect to an equivalent grating of a wafer in a photolithographic system where the mask is imaged by an imaging system onto the wafer, symmetrical diffraction orders (u.degree..sub.+1, u.degree..sub.-1) are focussed on the wafer grating and diffracted a second time to return colinear with the optical axis and to be deflected by a beam splitter to a photo detector. The intensity of the superimposed outbeams depends on the relative phase differences of the diffracted beams, and, hence, on the displacements of the mask and wafer gratings. The phase of the electrical output signal is determined by introducing periodic phase differences in the diffracted beams of the mask grating by a wobbling parallel glass plate. For simultaneous X-, Y- alignment, crossed gratings are used that operate on two pairs of diffracted beams. The polarization direction of one of these pairs is rotated by 90.degree. before impinging on the second grating so that each pair of diffracted beams can be fed to a separate photodetector. In a step-and-repeat photolithographic system, optical fine alignment can be performed by adjusting a fixed tilt angle of the glass plate after having determined the amount of misalignment.

Interferometric mask-wafer alignment

Optical imaging systems are used for making microstructures, and have to be very precise. In order to test these imaging systems, a new method, and a device for carrying out this method, are described in which two interferograms are made and compared. In a first step, an interferogram of an original pattern is made, followed by a second step in which an interferogram of a copy of the original pattern is produced using the identical conditions used to form the first interferogram. The pattern copy is made in the imaging system to be tested. In a third step, the two interferograms are compared with one another to provide a measure of the accuracy of the imaging system. This technique can be used for testing imaging systems which produce only a mirror image.

Method and device for testing optical imaging systems

In an interferometric measuring system, a collimated monochromatic and coherent beam of light (1, I.sub.1 -I.sub.n)impinges on a grating 1 disposed parallel to the test surface 2. It has been found that in the above arrangement, a diffraction order (preferably the first diffraction order S.sub.1 -S.sub.4) of the light reflected from the face of the grating opposite to the test surface is always parallel to three diffraction orders of the radiation which after being first diffracted upon its first passage through the grating and reflected from the test surface is again diffracted upon its second passage through the grating. These four radiations (S.sub.1 to S.sub.4) generate two interference fields, the combination of which generates a beat pattern. According to the invention, the angle of incidence of the radiation impinging onto the face of the grating opposite to the test surface is chosen in such a way (preferably from 0.5.degree. to 5.degree.) that the areas in which the beat patterns generate an additional field of interference fringes, related to .lambda./4 distances, from the test surface are a maximum. When properly selecting pairs of angles of incidence and of distances between test surface and grating the resolution of the resulting fringe pattern (interference fringes symmetrically interleaved by beat fringes) is improved by the factor of 2, i.e. the distance between two fringes is related to a .lambda./4 distance from the test surface, as opposed to the interferometric methods known heretofore where the maximum resolution is defined by fringe distances related to .lambda./2 distances from the test surface.

Interferometric measuring system

Disclosed is an interferometric apparatus and method for the inspection and detection of overlay errors characterized in that two plane polarized laser beams are directed onto the surface to be inspected, the angle included by these two beams being dimensioned in such a way that the radiation generating the plus or minus first order of the diffraction pattern of one beam is parallel to the other beam, thus generating an interference field which in the absence of overlay errors consists of an homogeneous fringe pattern while in the presence of such errors the fringe pattern is locally distorted. In a first embodiment, a first component of a laser beam is deflected onto a viewing screen by a beam splitter while a second component passes through the beam splitter to a mirror. The position and angle of the mirror is determined by the first order diffraction characteristics of the grating (object). The second component provides a first order of diffraction which interferes with the first component, producing an interference pattern on the viewing screen.

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Sindelfingen-Maichingen, Germany

Guenter Makosch