Abstract:
A system for aligning a mask and a semiconductor wafer comprises radiation source for producing a radiation beam, a dual focus linear Fresnel zone plate provided on the mask for focusing the radiation beam incident thereto on a surface of the semiconductor wafer which comprises a first part having a first focal length and a second part having a second focal length substantially smaller than the first focal length, a diffraction grating provided on the surface of the semiconductor wafer in correspondence to the dual focus linear Fresnel zone plate for diffracting the radiation beam focused thereon, a detector held with a predetermined relationship with respect to the radiation source and the mask for detecting the diffracted beam, a movable stage for supporting the semiconductor wafer, and a controller for moving the stage means responsive to an output signal of the detection means.

Description:
This is a divisional of copending application Ser. No. 07/471,008, now U.S. Pat. No. 4,948,983, filed on Jan. 25, 1990, a continuation of Ser. No. 07/349,077 filed May 9, 1989 (now abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to fabrication of semiconductor devices and in particular to a system used for achieving alignment between a mask carrying a semiconductor pattern and a semiconductor wafer by using a linear Fresnel zone plate. 
     Conventionally, there is a system for achieving proper alignment between a mask carrying a desired semiconductor pattern and a semiconductor wafer by using a circular Fresnel zone plate (Fay, B. and Novak, W.T., &#34;Automatic X-Ray Alignment System for Submicron VLSI Lithography. Solid State Technology, May, 1985). The circular Fresnel zone plate works similarly to a convex lens and focuses a radiation such as light or X-ray passing therethrough at a predetermined focal length as a result of diffraction caused at the edge between alternating transparent and opaque concentric rings constituting the plate. In this prior art system, a pair of circular Fresnel zone plates are provided on the mask and another circular Fresnel zone plate is provided on the wafer. In operation, they are illuminated obliquely from a top so that the radiation is incident to the wafer after passing through the mask. The radiation thus incident to the wafer is then reflected and passed through the mask in the opposite direction. At the time of reflection by the wafer and at the time of passing through the mask in the second time, the radiation is focused by the circular Fresnel zone plates on the wafer and on the mask. When the mask and the wafer are properly aligned, the spot of the radiation thus focused by these circular Fresnel zone plates are aligned on a straight line. Therefore, one can determine the state of alignment of the mask to the wafer by observing the relation between these three spots thus formed by suitable means such as a CCD device or camera. 
     In this prior art system, however, there is a problem in that a large area on the mask and the wafer are occupied by the circular Fresnel zone plates. In fact, at least a pair of such set of Fresnel zones, each including the three circular Fresnel zone plates therein, two of which on the mask and one on the wafer, have to be used in order to achieve a two dimensional alignment of the wafer and the mask. As such a set of Fresnel zone plates have to be provided on each of the masks, the area occupied by the circular Fresnel zone plates is further increased when a number of such masks are used for the patterning as is usually practised in the actual patterning of the semiconductor devices. 
     Alternatively it is proposed to use a linear Fresnel zone plate in the system for aligning the wafer and the mask. In this system, the Fresnel zone plate referred to hereinafter as LFZP is provided on the mask so as to focus a parallel beam of the radiation such that the radiation passed through the mask is focused on the wafer as a line-shaped image similarly to the case of a cylindrical lens. The wafer on the other hand carries a diffraction grating which may be a row of projections or depressions extending along the line-shaped image. Such a grating diffracts the radiation to a predetermined direction when irradiated by the focused radiation through the LFZP. 
     FIG. 1(A) shows a typical conventional LFZP comprising a plurality of alternating transparent and opaque bands arranged in one direction. Such a structure behaves similarly to the cylindrical lens as schematically illustrated in FIG. 1 (B). The LFZP is symmetrical about its central line 1 and there holds a relation: 
     
         r.sub.n.sup.2 =nfλ+(nλ).sup.2 /4 n=1,2,      (1) 
    
     where r n  stands for a distance measured from the central line 1 to a boundary between an nth band which may be opaque or transparent and an (n+1)th band which may be transparent or opaque, f stands for the focal length, and λ stands for the wavelength of the radiation. 
     As a result, there occurs an interference of the diffracted light in a focal plane which is separated from the LFZP by the distance f as schematically illustrated by an image m which represents the distribution of intensity of the diffracted radiation on the focal plane. When such a diffracted radiation is incident to a diffraction grating, there occurs a diffraction in a predetermined direction. 
     FIG. 2 is a diagram showing a conventional system used for aligning a mask 10 and a wafer 11. Referring to the drawing, a radiation 12 produced by a radiation source 13 is focused on a first LFZP 14a having a first focal length via a mirror 15. The wafer 11 is provided with a diffraction grating (not shown) and is transported horizontally while maintaining proper focusing until a strong diffraction is observed by a detector (not shown) held with a predetermined angular relation with respect to the source 13. As a result, a coarse alignment of the mask and wafer is achieved. Next, both of the radiation source 13 and the mirror 15 are moved horizontally to a position indicated by a broken line. In this state, the radiation falls on a second LFZP 14b having a second focal length which is substantially shorter than the first focal length. At the same time, the wafer 11 is raised such that the radiation diffracted by the second LFZP 14b is properly focused on the wafer 11. In correspondence to the LFZP 14b, there is provided a second grating (not shown) on the wafer 11 such that there appears a strong diffraction to a predetermined direction when the radiation correctly falls on the second grating. In order to detect the properly aligned position, the wafer is moved to and fro horizontally until the detector detects the diffraction from the second grating. By doing so, a precise alignment between the mask and wafer is achieved. In a typical example, the LFZP 14a has a focal length of 50 μm and the LFZP 14b has a focal length of 10 μm. 
     In such a prior art construction, however, there is a problem in that one has to use at least two LFZPs in order to achieve the alignment in one direction such as X or Y (FIG. 2). In correspondence to this, two diffraction gratings must be provided on the surface of the wafer for each direction. As a plurality of masks are usually employed for the patterning in the actual semiconductor devices, the number of the LFZPs on the mask and the number of corresponding gratings on the wafer increases with a rate twice as large as the number of the masks. For example, ten LFZPs in total and ten corresponding gratings have to be provided on the mask and on the wafer when patterning a device comprising five semiconductor layers. Thus, substantial area on the wafer and the mask is occupied by the marks such as the LFZP and the grating for the alignment. 
     Further, one has to move the radiation source and the associated optical system such as the mirror 15 horizontally each time the coarse alignment is completed. Associated with the movement, there arises a problem of achieving proper alignment between the thus moved optical system and the mask. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful system for aligning a mask and a wafer wherein the problems aforementioned are eliminated. 
     Another and more specific object of the present invention is to provide a system for aligning a mask and a wafer by using a linear Fresnel zone plate on the mask and a corresponding diffraction grating on the wafer, wherein the number of the linear Fresnel zone plate and the diffraction grating is reduced. 
     Another object of the present invention is to provide a system for aligning a mask and a wafer by using a dual focus linear Fresnel zone plate, wherein the linear Fresnel zone plate comprises a first part having a first focal length and a second part having a second focal length which is substantially smaller than the first focal length and formed inside the first part. According to the present invention, the same linear Fresnel zone plate can be used for coarse alignment and for precise alignment. As a result, one can eliminate the hitherto required shifting of optical system and a cooperating detector from a first position corresponding to a first linear Fresnel zone plate for coarse alignment to a second position corresponding to a second linear Fresnel zone plate for precise alignment. Further, the number of the linear Fresnel zone plate and corresponding grating to be provided on the mask and on the wafer is reduced to one half. 
     Other objects and further features of the present invention will become apparent from the following detained description when read in conjunction with attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(A) and (B) are a front view and a plan view showing a prior art linear Fresnel zone plate; 
     FIG. 2 is a perspective view schematically showing a prior art alignment system for aligning a mask and a wafer; 
     FIG. 3 is a perspective view showing an overall construction of the alignment system according to an embodiment of the present invention; 
     FIGS. 4(A) and (B) are plan view and perspective view showing a part of the system of FIG. 3 in detail; 
     FIGS. 5(A) and (B) are front views showing first embodiment dual focus linear Fresnel zone plate used in the system of FIG. 3 and its modification; 
     FIG. 6 is a diagram similar to FIGS. 5(A) and (B) showing another embodiment of the dual focus linear Fresnel zone plate; and 
     FIG. 7 is a flowchart showing the steps of aligning the mask and the wafer by the system of FIG. 3. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 shows an overall construction of the alignment system for achieving alignment between a mask and a wafer according to an embodiment of the present invention. It should be noted that the term &#34;mask&#34; used herein means a medium carrying a desired semiconductor pattern to be transferred to the wafer and includes those used for photolithography wherein the pattern is transferred to the wafer by optical irradiation, those used for X-ray lithography wherein the pattern in transferred to the wafer by X-ray irradiation or those used for projection of the pattern by any known radiation beam. 
     Referring to FIG. 3, a semiconductor wafer 20 is held by a wafer chuck 21 which in turn is mounted on a stage 22. The stage 22 is moved in X-X&#39; direction, Y-Y&#39; direction and Z--Z&#39; direction as indicated in the drawing by a controller 23. Over the wafer 20, there is positioned a mask 24 which carries a semiconductor pattern (not shown) to be transferred to the wafer 20. When the wafer 20 is moved together with the stage 22 by the controller 23, the relative position between the wafer 20 and the mask 24 is changed. In order to achieve a proper alignment between the mask 24 and the wafer 20, the mask is formed with a dual focus LFZP 24a  to be described later. Further, the wafer 20 is formed with a diffraction grating 20a which may be a row of projections or depressions extending in a predetermined direction. The LFZP comprises a plurality of parallel bands or stripes and the grating 20a extends parallel to the direction of the individual bands. The LFZP 24a is illuminated by a laser 25 for producing a laser beam 25a which is coherent and incident to the LFZP 24a along a suitable optical path including optical element such as a mirror 26. When the laser beam 25a is incident to the LFZP 24a, the LFZP 24a works similarly to a cylindrical lens and focuses the laser beam on the wafer 20 in a form of a linear image extending along the grating 20a. In order to achieve a proper focusing of the laser beam 25a on the wafer 20, the controller 23 moves the wafer 20 up and down if necessary as will be described. The laser beam 25a incident to the LFZP 24a is diffracted to a predetermined direction as a diffraction beam 25b, and the diffraction beam 25b thus produced is detected by a photodiode 27. The photodiode 27 produces an output signal proportional to the intensity of the diffraction beam 25b and the controller 13 moves the stage in the X-Y plane responsive to the output signal until a strong diffraction beam 25b is detected by the photodiode 27. 
     FIGS. 4(A) and (B) show the arrangement for focusing of the laser beam 25a on the dual focus LFZP 24a. Referring to the drawings, the mask 24 is uniformly coated with an absorbent 24b except for a patterning region 24c formed with a semiconductor pattern (not shown). Further, one or more a transparent windows 24d are formed in the absorbent 24b located outside of the patterning region 24c. Further, another laser 28 produces a laser beam 28a which is incident on the wafer 20 through the transparent window 24d. The incident laser beam 28a is partially reflected by the surface of the mask 24 and partially transmitted. This transmitted beam causes multiple reflections between the mask 24 and the wafer 20. These reflected beams interfere and there is formed an interference pattern P as shown in FIG. 4(B). This interference pattern takes a form of a periodical stripe having a wave length which is inversely proportional to a distance between the wafer 20 and the mask 24. The interference pattern is detected by a charge-coupled device (CCD) 29 which produces an output signal carrying information about the distance between the wafer 20 and the mask 24. This output signal of the CCD 29 is supplied to the controller 23 for analysis, and the controller moves the wafer 20 up or down responsive to the output signal of the CCD 29 by driving the stage 22 in the Z--Z&#39; direction. 
     In the illustrated example, the optical path for the optical beams 28a and 28b intersects with the optical path for the optical beams 25a and 25b in the plan view as illustrated in FIG. 4(A). Actually, the optical beams 28a and 28b pass below the beams 25a and 25b as shown in the perspective view of FIG. 4(B). In order to achieve the alignment of the wafer in the X-Y plane, there are provided a plurality of the dual focus LFZPs 24b and corresponding gratings 20a. In the example of the drawing, there are provided four sets of the LFZPs and gratings. Further, there are provided four transparent windows 24d for focusing of the beam 25a on the LFZP 24a on the mask 24. In each of the set of the LFZPs, the alignment of the wafer to the mask in the X-Y plane is performed as already described. In the system of the present invention, lateral shifting of the laser and cooperating optical system hitherto necessary when switching from the coarse alignment to the precise alignment can be eliminated by using a single LFZP. 
     Next, a first embodiment of the dual focus LFZP 24a used in the system of FIG. 3 will be described with reference to FIG. 5(A). 
     Referring to the drawing, the dual focus LFZP 24a comprises a first LFZP part I having a first focal length f1 and a second LFZP part II having a second focal length f2. The first LFZP I has an opaque central zone C similar to the LFZP of FIG. 1 (A), and the second LFZP II having a transparent central zone D is formed in this central zone C. In other words, the central zone C. of the first LFZP I would be totally opaque when the second LFZP II is not formed therein. Each of the first and second LFZP parts comprises a plurality of alternating transparent and opaque bands 1 and 2 extending parallel to each other and disposed symmetrically about a central line l of the central zone C. In other words, the bands 2 for the LFZP part II are located centrally relative to bands 1 for the LFZP part I. In the illustrated example, the number of the bands for the LFZP parts I and II is chosen to be seven. It should be noted that the second LFZP II need not necessarily be formed in the central zone C but may be formed in a zone E of the first LFZP I shown in FIG. 5(A) with substantially a same result. 
     The dimension of the bands for the LFZP part I and II is determined by Eq.(1). Thus, in the case where it is assumed that the wave length λ of the used radiation is 780 nm and that the first and second focal lengths f1 and f2 are assumed to be 80 μm and 10 μm respectively, the distance from the central line l of the dual LFZP 24a to a boundary between a transparent band and an adjacent opaque band is calculated as follows: 
     
                       TABLE I______________________________________n               rn    Rn______________________________________1               2.8    7.92               4.0   11.23               5.0   13.74               5.8   15.95               6.5   17.86               7.2   19.57               7.9   21.1______________________________________ 
    
     where r n  and R n  are defined in FIG. 5(A) and represent the distance represented in microns from the center of the dual LFZP 24a to the boundary between an nth band and an (n+1)th band in the first LFZP I and in the second LFZP II, respectively. 
     As already noted, the first LFZP part I has the opaque central zone C in which the second LFZP part II is not formed. In such a construction, the intensity of the radiation focused by the first LFZP part I remains the same as in the case in which there is no second LFZP part II in the central zone C. 
     FIG. 5(B) shows a modification of the dual focus LFZP of FIG. 5(A) in which the second LFZP part II&#39; having the transparent central zone D&#39; is provided in a transparent central zone C&#39; of the first LFZP I&#39; having bands 1&#39; of which transparency is reversed relative to the bands (i.e., of FIG. 5A). In this case there occurs a decrease in the intensity of radiation focused by the first LFZP part I&#39; because of the existence of the second LFZP II&#39; in the central zone C&#39; which would otherwise be transparent. 
     FIG. 6 shows a second embodiment of the dual focus LFZP used in the system of FIG. 3. In this embodiment, the central band C&#39; is transparent and a second LFZP part II&#39; having an opaque central zone D&#39; and bands 2&#39; of which transparency is similarly reversed  is formed in this transparent zone C&#39;. In order to facilitate the understanding, the LFZP part I&#39; and LFZP part II&#39; are drawn separately. In this embodiment, the intensity of the radiation focused by the LFZP part I&#39; is reduced to about one half as compared to the case in which there is no second LFZP part II&#39; provided in the central zone C&#39;. Such a decrease in the intensity of the focused radiation causes no substantial problem in the coarse alignment of the wafer when used in the system of FIG. 3. 
     Further, the first LFZP to be combined with the second LFZP II&#39; having the opaque central zone D&#39; is not limited to those illustrated in FIG. 6 having a transparent central band but may be those having an opaque central band C. It should be noted that the LFZP works generally identically even if the transparency of the opaque band and transparent band is reversed. 
     FIG. 7 shows the procedure for achieving the alignment by the system of FIG. 3. 
     In a step 1, the distance between the wafer 20 and the mask 24 is determined by the CCD device 29 of FIGS. 4(A) and (B). Next, the stage 22 is moved up or down by the controller 23 in a step 2 so that the distance between the mask 24 and the wafer 20 is set to the focal length of the first LFZP part I of the dual focus LFZP 24a. In a step 3, the stage 22 is moved in the X--X&#39; and Y--Y&#39; directions by the controller 23 until a strong diffraction is detected by the detector 27. Next, the stage 22 is moved further upwards in a step 4 while measuring the distance between the wafer 20 and the mask 24 in a step 5 until the distance is correctly set to the focal length of the second LFZP II part in a step 6. Then, the stage 22 is moved in the X--X&#39; and Y--Y&#39; directions until the strong diffraction is detected by the detector 27. 
     According to this procedure, one can achieve a precise alignment between the wafer and the mask while using a single LFZP and without shifting the position of the detector and optical system cooperating thereto at the time of switching from the coarse alignment to the precise alignment. 
     Further, the present invention is not limited to these embodiments but various variations and modifications may be made without departing from the scope of the present invention.