Pattern Detector

1. Field of the Invention

The present invention relates to a pattern detector used in precise position measurement of a wafer or a mask in a mask aligner during the manufacture of a semiconductor device.

2. Description of the Prior Art

FIG. 1 shows a pattern detector in a prior art reduction projection aligner system. A similar device is disclosed in Japanese Patent Application Laid-Open No. 162227/80 (corresponding to U.S. Pat. No. 4,380,395).

In the reduction projection aligner system of FIG. 1, a circuit pattern of a reticle 2 to be newly formed is superimposed on a circuit pattern on a wafer 4 formed in a previous step through an exposing condenser lens 1 and a reduction lens 3. Usually, the superimposing step is repeated for a certain number of reticles to form a desired circuit pattern on the wafer. Usually, a high precision of no less than 1 .mu.m is required for the registration of the two circuit patterns to be superimposed. In the reduction projection aligner system shown in FIG. 1, the registration is effected by detecting a positioning pattern on the wafer and relatively moving the reticle having the pattern to be newly formed so that it coincides with the wafer. An area containing the positioning pattern (not shown) on the wafer 4 is locally illuminated by a light ray emanated from a positioning pattern detection light source (not shown) and guided by a light guide 10, and a light reflected thereby is magnified and focused through the reduction lens 3, the reticle 2, a mirror 100 and a magnifying optical system 5 on a moving plane of a reciprocating table 7 having a slit 6. The reciprocating table 7 is reciprocated in the direction of arrow shown in the figure and the magnified image of the area containing the positioning pattern on the wafer 4 is scanned by the slit 6 reciprocating in accordance with the movement of the reciprocating table 7. The intensity of illumination of the magnified image is applied to a photo-multiplier 9 as a transmitted light from the slit and it is photo-electrically converted. An output of the photo-multiplier 9 is digitized by a circuit including an A-D converter, a digital counter and a CPU and an illumination-intensity data of points corresponding to slit positions of the magnified image is produced. Numeral 8 denotes a portion of a mechanism for generating an input signal to the digital counter. The wafer position (that is, a center position of the positioning pattern on the wafer 4) can be determined in the following manner (see Japanese Patent Application Laid-Open No. 69063/78, corresponding to U.S. Pat. No. 4,115,762). An arbitrary position Xi of the slit is assumed as a virtual center and 2m intensity data Yi on both sides thereof are superimposed to calculate Zi= ##EQU1## A point which provides a minimum one of the values of Z is determined as the center position of the positioning pattern on the wafer. In this device, the center position X.sub.c of the positioning pattern on the wafer relative to an origin sensor (not shown) located at a position on the reciprocating table 7 corresponding to a center position of the reticle reference pattern is determined and the new pattern is superimposed on the pattern on the wafer in accordance with the result of the determination.

In the above reduction projection aligner system, when the pattern position is detected through the reduction lens 3, a detecting light wavelength is an exposure wavelength of monochromatic light because the reduction lens 3 is compensated for aberration for the exposure wavelength. When a monochromatic light is applied to a wafer having an optically transparent thin film photoresist applied thereon, the light reflected by the surface of the wafer is repeatedly reflected between the wafer surface and the photoresist surface and goes out of the photoresist surface. An interference occurs among the light from the photoresist surface, the light from the wafer surface and the repeatedly reflected light in the photoresist, and a so-called interference fringe of equal thickness occurs. Accordingly, when the monochromatic light such as exposure light is used as the pattern detecting light, the interference fringe of equal thickness occurs depending on the thickness of the photoresist. This results in a reduction of a contrast of the detection signal by dark areas of the interference fringe and hence the precision of the pattern positioning is lowered. Further, since the reflection of the exposure light by the wafer impedes the formation of a fine pattern, an anti-reflection film is overlayed on the wafer or light absorbing agent for the exposure light wavelength is added in the photoresist. In such a case, a sufficient intensity of reflected light from the wafer is not obtained if the detection is made using the exposure light wavelength. In order to resolve this problem, it is necessary to detect the pattern by a light of different wavelength than the exposure light wavelength. To this end, a color aberration correction lens is disposed between the reduction lens 3 and the reticle 2 or a special pattern detecting optical system adapted exclusively to a color aberration of the reduction lens 3 is prepared to detect the pattern only by a light of a specific wavelength. As a result, a pattern detection error occurs due to a reproduction error of a correction lens position. Even if the light of different wavelength than the exposure light wavelength is used, the pattern detecting magnifying optical system for a single wavelength cannot vary the detecting wavelength depending on the thickness of the photoresist on the wafer, the material of the photoresist and the material of the wafer to detect the pattern at an optimum wavelength. Namely, it cannot detect the pattern at any desired wavelength.

It is an object of the present invention to attain a high precision of pattern positioning.

In accordance with one aspect of the present invention, in a pattern position detector which illuminates a positioning pattern on a wafer by a light from a light source, magnifies and projects a light reflected from the positioning pattern by a magnifying optical system and detects the position of the positioning pattern on the wafer on the basis of the magnified projected image, a plurality of optical filters are exchangeably arranged in an optical path between the light source and the magnified projected image imaged through the positioning pattern on the wafer so that a monochromatic light of a desired wavelength is available by selecting one of those filters, and magnifying optical systems are provided one for each of the available monochromatic lights whereby a magnified projected image having a highest contrast is obtained. Thus, the positioning pattern on the wafer can be detected by a desired light having the optimum wavelength to the wafer.

In accordance with another aspect of the present invention, the wavelength of the monochromatic light is varied to effectively utilize an interference fringe of equal thickness so that the positioning pattern is illuminated by intensity-emphasized area of the interference fringe.

In accordance with a further aspect of the present invention, the pattern position is detected without using a color aberration correction lens for correcting a color aberration of the reduction lens. Therefore, a pattern detection error due to a reproduction error of the correction lens position by exchanging the correction lenses does not occur.

FIG. 1 is a basic configuration of a reducing projection aligner system having a prior art pattern detector,

FIG. 2 is a conceptual view illustrating a color aberration of a reduction lens,

FIG. 3 shows one embodiment of the present invention,

FIG. 4 illustrates a principle of color aberration correction or image position correction of the reduction lens by vertical movement of a wafer, and

FIG. 5 shows another embodiment of the present invention.

Referring to FIG. 2, a color aberration of a reduction lens is explained. A positioning pattern 4' on a wafer 4 is imaged in magnified scale on a reticle 2 as shown by solid lines 21 by a violet exposure light, which is used to project a circuit pattern on the reticle 2 onto the wafer 4 through an exposing condenser lens 1 and a reduction lens 3 as shown in FIG. 3. When a monochromatic light of different wavelength than the exposure light wavelength, for example green light is used, the pattern is not imaged on the reticle 2 but imaged in magnified scale above the reticle 2 as shown by broken lines 22.

FIG. 3 shows one embodiment of the present invention, which has detecting optical systems, one for the magnified image by the exposure light wavelength and one for the magnified image by the green light. The like elements to those of FIG. 1 are designated by the like numerals.

In the embodiment of FIG. 3, the pattern on the wafer imaged on the reticle by the exposure light wavelength is magnified and projected on the moving plane of the reciprocating table 7 by the reflection mirror 100 and the magnifying optical system 5 and the intensity of the magnified projected image is transmitted through the slit 6 and photo-electrically converted by the photo-multiplier 9. On the other hand, when a wafer which does not provide a sufficient amount of reflection at the exposure light wavelength is to be detected, the green monochromatic light is used and the pattern imaged above the reticle is magnified and projected to the moving plane of the reciprocating table 7 by a reflection mirror 100' and a magnifying optical system 5' provided for the green monochromatic light, and the intensity of the green magnified projected image is transmitted through a slit 6' and photo-electrically converted by a photo-multiplier 9'. Thus, a detection signal having a sufficient contrast is produced. The wavelength of the detecting monochromatic light (green in the present embodiment), that is, the wavelength of the monochromatic light for forming the magnified projected image of the pattern can be selected by exchanging an optical filter 200 disposed at an inlet or an exit of the light guide 10 made by an optical fiber or exchanging an optical filter 300 or 300' disposed in the magnifying optical system 5 or 5' to extract the monochromatic light of the desired wavelength from the detecting light source (not shown) disposed in front of the light guide 10.

As described above, when the color aberration of the reduction lens is so large that the imaging position varies from wavelength to wavelength, the magnifying optical systems are provided one for each of the monochromatic lights of different wavelengths extracted through the optical filters of different optical characteristics so that the magnified image by the monochromatic light of the desired wavelength is formed on the moving plane of the reciprocating table 7.

Where the color aberration of the reduction lens is small, only one magnifying optical system may be used and a desired one of the optical filters is selected in the same manner as the present embodiment to detect the pattern position by the monochromatic light of the desired wavelength.

In the present embodiment, two reflection mirrors are used and the magnifying optical systems are arranged in parallel. Alternatively, at least one of the reflection mirrors 100, 100' may be formed as a beam splitter. Of the lights reflected by the positioning pattern on the wafer 4, the monochromatic light of the selected wavelength is extracted and reflected by the beam splitter to detect the pattern and the lights of other wavelengths are transmitted so that two detection light axes per positioning pattern on the wafer are formed. The light of other wavelengths may be passed through the optical filters to produce the monochromatic lights of desired wavelengths or a plurality of beam splitters may be superimposed on the above beam splitter to produce the monochromatic lights of other wavelengths. The beam splitters are followed by the corresponding magnifying optical systems. When the beam splitters are used, only one transparent pattern on an edge of the reticle through which the light reflected by the wafer is transmitted is needed.

Where the color aberration of the reduction lens is so large that the imaging position of the positioning pattern on the wafer by the reduction lens varies from wavelength to wavelength of the detecting monochromatic light, a distance between the wafer and the reduction lens may be changed to correct the imaging position to precisely detect the positioning pattern. For example, the wafer is vertically moved depending on the wavelength used so that the pattern on the wafer is imaged on the reticle irrespective of the wavelength used. FIG. 4 illustrates a principle of an optical system which attains the above concept. As shown in FIG. 4, when the green light is used, the wafer is descended by a predetermined distance from the wafer position at which the pattern on the wafer is imaged on the reticle when the exposure light wavelength is used. Thus, the pattern on the wafer is imaged on the reticle by the green light. One embodiment of the present invention based on this principle is shown in FIG. 5, in which the like elements to those of FIGS. 1 and 3 are designated by the like numerals. In the present embodiment, the wafer 4 is mounted on a moving table comprising an X-direction moving table 13, a Y-direction moving table and a Z-direction moving table 14 so that it is movable in X, Y and Z directions. A filter device 11 having a plurality of filters is arranged in the magnifying optical system. The pattern detection by the exposure light wavelength is carried out in a conventional manner. When the green light is used, the wafer is lowered by a predetermined distance so that the image focused on the reticle is focused on the slit 6 through the green optical filter. Since no additional optical part is required in the focusing optical system between the wafer and the reticle due to the change of the wavelength, no pattern detection error due to the addition of the optical part occurs. In the present embodiment, the wafer is vertically moved to correct the color aberration (and hence the imaging position) due to the change of the wavelength. Alternatively, the reduction lens, the reticle or the pattern detecting optical system may be raised to focus the pattern on the wafer onto the reticle. When the color aberration of the reduction lens is so small that the variation of the imaging position can be neglected, the detecting light wavelength may be selected by merely changing the optical filter.

As described hereinabove, according to the present invention, the pattern detection by various wavelengths is attained. Thus, by selecting the detecting wavelength depending on the thickness of the photoresist on the wafer, the material of the photoresist and the material of the wafer substrate, the detection signal of high contrast can be always produced. Since the optical part such as correction lens need not be inserted, a high precision of pattern detection is attained.

Further, by effectively utilizing the interference fringe of equal thickness which occurs when monochromatic detecting light is used, a high contrast detection signal is produced.