Abstract:
A device for detecting a positional relationship between opposed first and second objects, with respect to a first direction perpendicular to the opposing direction. The device includes a light source for projecting light to the first and second objects, a photodetecting portion for receiving light from one of the first and second objects irradiated with the light from the light source, the photodetecting portion being operable to detect a predetermined parameter related to the light, which parameter is changeable with the positional relationship between the first and second objects with respect to the first direction, a first position detecting system for detecting a positional relationship between the first and second objects, with respect to a second direction perpendicular to the opposing direction of the first and second objects and having an angle with respect to the first direction, and a second position detecting system for detecting the positional relationship between the first and second objects with respect to the first direction, on the basis of the detection by the photodetecting system and the first position detecting system.

Description:
This application is a continuation of prior application, Ser. No. 07/692,975 filed Apr. 29, 1991, which application is a continuation of prior application, Ser. No. 07/403,881 filed Sep. 7, 1989, both now abandoned. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     This invention generally relates to a position detecting method and apparatus suitably usable, for example, in a semiconductor microcircuit device manufacturing exposure apparatus for lithographically transferring a fine electronic circuit pattern formed on the surface of a first object (original) such as a mask or reticle (hereinafter simply &#34;mask&#34;) onto the surface of a second object (workpiece) such as a wafer, for relatively positioning or aligning the mask and the wafer. 
     In exposure apparatuses for use in the manufacture of semiconductor devices, the relative alignment of a mask and a wafer is one important factor with respect to ensuring improved performance. Particularly, for alignment systems employed in recent exposure apparatuses, submicron alignment accuracies or more strict accuracies are required in consideration of the demand for higher degrees of integration of semiconductor devices. 
     In many types of alignment systems, features called &#34;alignment patterns&#34; are provided on a mask and a wafer and, by utilizing positional information obtainable from these patterns, the mask and wafer are aligned. As for the manner of executing the alignment, an example is a method wherein the amount of relative deviation of these alignment patterns is detected on the basis of image processing. Another method is proposed in U.S. Pat. Nos. 4,037,969 and 4,514,858 and Japanese Laid-Open Patent Application, Laid-Open No. Sho 56-157033, wherein so-called zone plates are used as alignment patterns upon which light is projected and wherein the positions of light spots formed on a predetermined plane by lights from the illuminated zone plates are detected. 
     Generally, an alignment method utilizing a zone plate is relatively insensitive to any defect of an alignment pattern and therefore assures relatively high alignment accuracies, as compared with an alignment method simply using a traditional alignment pattern. 
     FIG. 1 is a schematic view of a known type alignment system utilizing zone plates. 
     In FIG. 1, parallel light emanating from a light source 72 passes through a half mirror 74 and is focused at a point 78 by a condensing lens 76. Thereafter, the light illuminates a mask alignment pattern 68a on a mask 68 and an alignment pattern 60a on a wafer 60 which is placed on a support table 62. Each of these alignment patterns 68a and 60a is provided by a reflection type zone plate and functions to form a spot of focused light on a plane perpendicular to an optical axis which contains the point 78. The amount of relative deviation of the positions of these light spots formed on that plane is detected, by directing the focused beams to a detection plane 82 by means of the condensing lens 76 and another lens 80. 
     In accordance with an output signal from the detector 82, a control circuit 84 actuates a driving circuit 64 to relatively align the mask 68 and the wafer 60. 
     FIG. 2 illustrates an imaging relationship of lights from the mask alignment pattern 68a and the wafer alignment pattern 60a shown in FIG. 1. 
     In FIG. 2, a portion of the light divergently advancing from the point 78 is reflectively diffracted by the mask alignment pattern 68a and forms a spot 78a of focused light at or adjacent to the point 78, the spot representing the mask position. Another portion of the light passes through the mask 68 in the form of zeroth order transmission light and is projected upon the wafer alignment pattern 60a on the wafer 60 surface with its wavefront being unchanged. The incident light is reflectively diffracted by the wafer alignment pattern 60a and then again passes through the mask 68 in the form of zeroth order transmission light, and finally, is focused in the neighborhood of the point 78 to form a spot 78b of focused light, representing the wafer position. In the illustrated example, when the light diffracted by the wafer 60 forms a spot, the mask 68 functions merely as a transparent member. 
     The position of the spot 78b formed by the wafer alignment pattern 60a in the described manner represents a deviation Δσ&#39;, in the plane perpendicular to the optical axis containing the point 78, of an amount corresponding to the amount of deviation Δσ of the wafer 60 with respect to the mask 68. 
     Accordingly, by memorizing, in advance, the positional relationship between the positional deviation Δσ of the wafer to the mask and the positional deviation Δσ&#39; of the spot of the focused light into a memory means, for example, and by detecting the deviation Δσ&#39; through the detector 82, it is possible to determine the positional deviation Δσ of the mask and the wafer. 
     In this type of an alignment system, however, if there is a positional deviation between the mask and the wafer in a direction perpendicular to the alignment direction with respect to which the mask and the wafer are going to be aligned, then the following inconveniences arise: 
     (a) The wavefront aberration due to the zone plate changes. 
     (b) The effective aperture area changes, resulting in a change in the spot diameter of diffraction light or a change in the intensity of the light spot. 
     Due to these factors, the amount of deviation Δσ&#39; of the spot of the focused light which should correspond only to the positional deviation between the mask and the wafer in the alignment direction, changes to cause degradation of the precision of alignment. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a position detecting method and apparatus by which the measurement of relative positional deviation between first and second objects is less affected by any positional deviation therebetween in a direction perpendicular to the direction with respect to which the relative positional deviation should be detected, such that the relative positional deviation of the first and second objects can be detected with high precision and very easily. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are schematic representations, illustrating a known type alignment system. 
     FIG. 3 is a schematic representation, illustrating a major part of a position detecting apparatus according to a first embodiment of the present invention. 
     FIGS. 4A and 4B are schematic representations, showing optical paths related to a light flux 10a. 
     FIGS. 4C and 4D are schematic representations, showing optical paths related to a light flux 10b. 
     FIG. 5 is a graph showing the relationship of the positional deviation Δx, Δy with the positional deviation Δδ of the center of gravity of light in the plane of a signal light detection surface. 
     FIG. 6 is a flow chart showing the sequence of deviation detection. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a schematic representation, illustrating a major part of one embodiment of the present invention. In FIG. 3, light emanating from a light source 3 such as a semiconductor laser, for example, is collimated into parallel light by means of a collimator lens 4. The parallel light is converged by a beam reducing projection lens 5 which is used, as required, for optimizing the spot diameter to be formed on a mask 1 surface. Then, the light is deflected by a deflecting mirror 6 to be projected upon a first physical optic element which is provided in a portion of the surface of a first object 1 such as a mask, for example. Diffraction light of a predetermined order or orders from the first physical optic element on the first object 1 is inputted to a second physical optic element which is provided in a portion of the surface of a second object 2 which is a wafer, for example. The inputted light is diffracted by the second physical optic element and the thus produced diffraction light is collected as a signal light by a light receiving lens 7. Then, by using a photodetector 8, the position of the center of gravity of this signal light is detected. Here, the term &#34;center of gravity of light&#34; means such a point that, when in the plane of the light detecting surface a position vector of each point in the plane is multiplied by the light intensity on that point and the thus obtained products are integrated over the entire plane, the integrated value has a &#34;zero vector&#34;. 
     Computing means 11 serves to compute the amount of deviation, by using a signal from the photodetector 8 and a reference signal from a memory means 12, which will be described later. 
     In this embodiment, as the first and second physical optic elements, grating lenses such as Fresnel zone plates, for example, are used. 
     FIGS. 4A and 4B schematically illustrate a path of light 111 which, in this embodiment, emanates from the light source 3 and is incident on the first object 1 and which, after being diffracted, is incident on the second object 2 and, finally, is reflectively diffracted toward a light receiving surface 8. 
     In the illustrated example, what is to be detected is the amount of positional deviation between a mask (first object) 1 and a wafer (second object) 2 in the X direction. 
     The light 111 from the light source 3 is projected on the surface of a first off-axis type grating element 112 (first physical optic element) provided in a portion of the mask 1, with an angle θ 1  of incidence with respect to a normal to the mask 1 surface. Diffraction light of a predetermined order or orders from the grating element 112 emanates perpendicularly from the mask 1 and impinges on a second off-axis type grating element 113 (second physical optic element) provided in a portion of the wafer 2. 
     Here, the term &#34;off-axis type grating element&#34; means such an element wherein, when a light having a given angle of inclination with respect to a normal to the plane in which the element is formed is incident on the element, the property of the element is assured with respect to light of a particular order or orders other than the order of specularly reflected light or rectilinearly transmitted light. 
     The first off-axis type grating element 112, in this embodiment, comprises a zone plate which is adapted to form a one-dimensional image with a light focused at a finite focal distance in the plane of the sheet of FIG. 4B (i.e. the X-Z plane), but not focused in the plane of the sheet of FIG. 4A (i.e. the Y-Z plane), with its chief ray extending substantially in the direction of a normal to the mask 1. 
     On the other hand, the grating element 113 comprises, in this embodiment, a zone plate which is adapted to emit a light (with which an image is formed on the photoreceptor 8 with respect to the plane of the sheet of FIG. 4B), with an angle of emission of θ 2  in the sheet of FIG. 4A with respect to a normal to the wafer 2, with the one-dimensional image formed by the mask 1 being taken as an object point. The reference 10a denotes the light flux to be received. 
     Namely, in the X-Z plane, the grating elements 112 and 113 can be each treated as a lens. In this embodiment, the grating element 112 can serve as a concave lens while the grating element 113 can serve as a convex lens. If, in this structure, the wafer 2 shifts in the X direction, the angle of emission of the light emanating from the grating element 113 changes, as if axial alignment of a lens in an optical system is destroyed. As a result, there occurs a change in the position of incidence of the light upon the photodetector 8. 
     Denoted at 114 is an optical pickup casing (alignment head) in which a light source and optical elements (not shown) for providing the light 111 to be projected as described as well as the photoreceptor 8 are accommodated. Denoted 115 is exposure light for transferring a pattern of the mask 1 to the wafer 2, with an illustrated chain line depicting the boundary of the path thereof. As for the exposure light, ultraviolet light, X-rays or otherwise may be used. 
     When the wafer 2 shifts laterally in the X direction, an illuminance distribution formed on the photoreceptor 8 in the plane of the sheet of FIG. 4B shifts laterally, for the reason described above. 
     The light flux 10a as depicted in FIGS. 4A and 4B is such light as having been diffractively transmitted through the mask 1 with a predetermined order (e.g. first order) and having been diffractively reflected by the wafer 2 with a predetermined order (e.g., first order) and finally, having been transmitted through the mask 1 with zeroth order. For convenience, hereinafter such light will be referred to as &#34;1-1-0 light&#34;. In addition to such 1-1-0 light, there exist many diffraction lights of different orders. Of these lights, such a light flux as having been transmitted through the mask 1 with zeroth order and having been diffractively reflected by the wafer 1 with first order and finally, having been diffractively transmitted through the mask 1 with first order (hereinafter such light will be referred as as &#34;0-1-1 light&#34;), can form a spot on the photoreceptor 8 surface in the neighborhood of a spot provided by the 1-1-0 light. Like the 1-1-0 light (10a), the 0-1-1 light can cause a shift of the position of incidence on the detector 8 as a result of the shift of the wafer 2. 
     Referring to FIGS. 4C and 4D, the path of such 0-1-1 light is depicted at 10b. When a relative deviation Δσ of the mask and the wafer in the X direction (alignment direction with respect to which the mask and the wafer should be aligned) is limited within a relatively small range, each of deviations Δδ110 and Δδ011 of respective gravity centers of the 1-1-0 light and the 0-1-1 light, upon the detecting surface 8, is substantially in a linear relationship with the deviation Δσ, because of a paraxial imaging magnification as determined by the refracting power arrangement of the system provided by the two grating elements 112 and 113. 
     However, with respect to each of the 1-1-0 light and the 0-1-1 light, the corresponding refracting power arrangement is different and, therefore, the imaging magnification is different. 
     As a result, in the process as illustrated in FIGS. 4A-4D, what is detected may be the position of the center of gravity of two combined two spots whose positions are changeable at different magnifications with a positional deviation Δσ between the mask and the wafer. Any change in the position of this center of gravity, to be detected, is substantially in a linear relationship with the mask-to-wafer deviation Δσ and, therefore, by predetecting a proportional constant of the same, it is possible to determine the deviation Δσ on the basis of the shift of the position of the center of gravity. 
     More specifically, at the time of mask setting, trial printing may be made to determine, as a reference position, the position of the center of gravity to be defined when the mask and the wafer do not include any positional deviation. At the time of actual position detection, the amount of positional deviation of the center of gravity from the reference position in the X direction may be detected to determine the amount of relative deviation of the mask and the wafer, on the basis of the aforementioned proportional relationship. 
     In the position detecting process, as described above, if there occurs a positional deviation in a direction perpendicular to the alignment direction (i.e. in the Y direction in FIGS. 4A-4C), because of the difference between the paths of the 1-1-0 light and the 0-1-1 light as illustrated in FIGS. 4A and 4C, there occurs a change in the ratio of the effective aperture areas of these lights. As a result, there occurs a change in the ratio of light quantities of the spots of these lights on the surface of the photoreceptor 8. For this reason, even if the deviations Δδ110 and Δδ011 of the respective gravity centers of these light spots are unchanged with the positional deviation in the Y direction, it is possible that the overall center position of the light changes. This is a factor, in addition to the factors such as a change in the wavefront aberration as described hereinbefore, which causes a possibility of degradation of the alignment precision with respect to the alignment direction due to a positional deviation component in a direction perpendicular to the alignment direction. 
     In the present embodiment, in order to avoid degradation in precision of the detection of a positional deviation Δx in an alignment direction which otherwise might be caused by a positional deviation Δy in a direction perpendicular to the alignment direction, the relationship of the deviations Δy, Δx and Δδ is memorized in preparation into a storing means 12 (as will be described later). Making reference to a signal from such a storing means 12, it is possible to attain high-precision alignment in the alignment direction under the influence of the computing means 11. 
     Details of this alignment method will be explained below. 
     Usually, in an exposure apparatus for the manufacture of semiconductor devices, physical optic elements such as alignment marks are provided at four sites around an exposure area, for detection of two-dimensional positional deviation (lateral shift and rotational error) between a first object (such as a mask) and a second object (such as a wafer). An example is schematically illustrated in FIG. 3. As shown, alignment marks are provided in four regions A-D which are on a scribe line 20 surrounding an exposure area 10. Each of these alignment marks may comprise an off-axis type grating element as described hereinbefore. As for a light projecting system and a light receiving system, those optical systems such as shown in FIG. 3 may be used. 
     In this embodiment, with regard to the regions A and B, any deviation in the X direction is detected, while, on the other hand, with regard to the regions C and D, any deviation in the Y direction is detected. 
     FIG. 5 is an explanatory representation, illustrating the relationship between (i) a positional deviation Δx of the mask and wafer in the alignment direction (X direction) in with respect to the regions A and B shown in FIG. 3 and (ii) a deviation Δδ of the center of gravity of a spot on the detecting surface, from a reference position. The illustrated case corresponds to a case wherein the refracting power and arrangement of the grating element (physical optic element) are set so that the deviation Δδ is in a ratio of about 100 to the deviation Δx. Although, in general, the relationship therebetween is slightly non-linear, because of the aberration of the grating element, for example, this raises substantially no problem with regard to the precision of position detection. 
     The deviation Δδ corresponds, in principle, to the deviation Δx in the alignment direction (X direction). Actually, however, for the reasons described hereinbefore, it can be affected by a positional deviation Δy in a direction (Y direction) perpendicular to the alignment direction. 
     Illustrated in FIG. 5 are three cases where the positional deviation Δy=0, Δy=+10 microns and Δy=-10 microns. When, as illustrated, there is a positional deviation of an amount of about ±10 microns in the Y direction, in a region close to a deviation Δx =3 microns, there occurs a detection error of about ±0.1 micron. In the present embodiment, the relationship of the deviations Δx, Δy and Δδ is predetected by experiments, for example, and is memorized in preparation into the storing means 12. In operation, first the amount of deviation Δy is detected and then, by making reference to the predetected relationship by using the computing means 11, any error resulting from the positional deviation in the direction (Y direction) perpendicular to the alignment direction is corrected. By this, it is possible to enhance the precision of detection of the positional deviation Δx in the alignment direction (X direction). In a particular example, on the basis of the value of Δy, the proportional constant with regard to the proportional relationship used for calculation of Δx from the deviation Δδ may be changed. 
     As regards the precision for detecting the positional deviation Δy, it may be relatively low. As an example, in FIG. 5, if the deviation Δy can be detected with a precision of ±1 micron, then the precision of detecting the deviation Δx (when it is in a range of ±3 microns) can be enhanced to about ±0.01 micron. 
     The described naturally applies to an occasion when, in FIG. 3, the alignment direction is in the Y direction, with regard to the regions C and D. 
     FIG. 6 exemplifies the flow sequence for detecting lateral deviations ΔX and ΔY of a mask and a wafer in the X and Y directions as well as a rotational error Δθ therebetween, by using the computing means 11 and in accordance with the method described hereinbefore. Details of the sequential operations will now be explained. 
     (a) By using the photodetector 8, with regard to each of the regions A-D, any deviation ΔδA, ΔδB, ΔδC or ΔδD of the center of gravity of the spot of light is detected. 
     (b) Assuming that the amount of deviation in a direction perpendicular to the alignment direction (i.e. in the Y direction with regard to the regions A and B, whereas it is in the X direction with regard to the regions C and D) is null, positional deviations Δx A , Δx B , Δy C  and Δy D  in corresponding alignment directions of these points are calculated. 
     (c) Then, from the deviations Δx A , Δx B , Δy C  and Δy D , the two-dimensional positional error of the mask and the wafer, namely, lateral deviations ΔX and ΔY as well as the rotational error Δθ are calculated. 
     (d) From the detected deviations ΔX, ΔY and Δθ, with regard to each point, a positional deviation Δy A , Δy B , Δx C  or Δx D  in the direction perpendicular to the corresponding alignment direction, is calculated. 
     (e) Taking into account the above-described positional deviation in the direction perpendicular to the alignment direction, with regard to each point, a calculation is made again to determine a positional deviation Δx A  &#39;, Δx B  &#39;, Δy C  &#39; or Δy D  &#39; with respect to the corresponding alignment direction. 
     (f) From the deviations Δx A  &#39;, Δx B  &#39;, Δy C  &#39; and Δy D  &#39;, lateral deviations ΔX&#39; and ΔY&#39; as well as rotational error Δθ&#39; are detected. 
     It is a possible alternative that, in the described sequence, the operations made at the steps (d), (e) and (f) are repeated, with appropriate conditions of convergence being predetermined. This assures further enhancement of the precision. 
     In accordance with the embodiments described hereinbefore, when a relative positional error between first and second objects is to be detected by using physical optic element means, reference is made to the positional information related to any positional error which is in a direction perpendicular to the alignment direction. This makes it possible to provide an alignment device having a high precision of positional error detection. 
     The detecting method is not limited to the disclosed one, and various methods can be used with the present invention. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.