Abstract:
An alignment mark set is provided, which facilitates the formation of a desired contour of each alignment mark and which suppresses the degradation of measurement accuracy for alignment of patterns. This alignment mark set comprises: (a) a first alignment mark formed in an exposure area; the area having a periphery, first central axis, and a second central axis perpendicular to the first axis; the first alignment mark being located near the first central axis and apart from the second axis; (b) a second alignment mark formed in the exposure area; the second alignment mark being located near the second central axis and apart from the first axis; and (c) when the exposure areas are regularly arranged in such a way as to have a same orientation in a plane, each of the first and second alignment marks in one of the sets is not located close to the first and second alignment marks in another of the sets, thereby ensuring irradiation of exposing light to all the areas. It is preferred that each of the first and second marks is square.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to alignment marks and more particularly, to an alignment mark set comprising alignment marks that are formed in an exposure area and used to measure the alignment or overlay accuracy of patterns in the lithographic process for semiconductor device fabrication, and a method of measuring alignment accuracy of patterns using alignment mark sets. 
     2. Description of the Related Art 
     Generally, semiconductor devices comprise a lot of layers that form electronic elements, wiring lines, contacts, and so forth, which are stacked to each other along with interlayer dielectric layers. These stacked layers constitute an integrated circuit. Thus, to fabricate semiconductor devices, the stacked layers need to be patterned to form desired electronic elements, wiring lines, and so forth using well-known lithography and etching techniques. 
     With the lithography and etching techniques, it is very important not only to transfer a desired minute pattern to a specific layer on or over a semiconductor wafer as closely as possible but also to overlay an upper pattern to a lower one as correctly as possible. Thus, in the lithography process, the pattern of a resist layer, which is formed on a layer to be etched or patterned (i.e., a target layer), needs to be aligned on a desired lower pattern located below the target layer with high accuracy. In particular, circuits and elements provided in a recent semiconductor device have been increasingly miniaturized and therefore, the need to raise the overlay accuracy (i.e., alignment accuracy) of patterns has been becoming stronger. 
     Conventionally, to meet the above-described need, “alignment marks” have been usually formed along with a pattern for desired circuits and/or elements, thereby measuring the alignment accuracy using the alignment marks. 
     FIGS. 1 and 2 show an example of the conventional alignment marks of this sort, which is formed on a semiconductor substrate or wafer. 
     The conventional alignment mark  100  shown in FIGS. 1 and 2, which has been usually used for this purpose, comprises two mark elements  118  and  119 . The inner element  119  is located in the outer element  118 . Each of the elements  118  and  119  is square in plan shape. The mark  100  is formed in the following way. 
     First, as shown in FIG. 2, a first layer  122  is formed on the surface of a semiconductor substrate or wafer  121  and then, a patterned resist layer (not shown) is formed on the first layer  122 . The patterned resist layer is formed by the lithography technique. Then, using the patterned resist layer as a mask, the first layer  122  is selectively etched, thereby forming a first or lower circuit pattern (not shown) and the outer square element  118  of the alignment mark  100  in the layer  122 . As clearly seen from FIG. 2, the element  118  is a square hole or opening of the layer  122 . 
     Subsequently, a second layer  123  is formed on the first layer  122  thus patterned so as to contact the bottom and side faces of the outer element  118  (i.e., the hole of the first layer  122 ). Then, a resist layer (not shown) is formed on the second layer  123  thus formed, and is patterned by the lithography technique, thereby forming a second or upper circuit pattern (not shown) and the inner square element  119  of the alignment mark  100  on the second layer  123  in the hole  118  of the first layer  122 . 
     As clearly seen from FIG. 2, the inner element  119  of the mark  100  is a square part of the resist layer and located in the hole or outer element  118 . The patterned resist layer thus formed is used as a mask in the next etching process for patterning the underlying second layer  123 . 
     The alignment mark  110 , shown in FIG. 3, comprising the outer and inner elements  118  and  119  thus formed is used to measure the alignment accuracy between the first circuit pattern formed by the first layer  122  and the second circuit pattern of the resist layer. In this case, the relative positional relationship between the outer and inner elements  118  and  119  is measured. 
     For example, as shown in FIG. 2, the distance d 1  between the inner side face  118   a  of the outer element  118  and the facing side face  119   a  of the inner element  119  is measured. At the same time as this, the distance d 2  between the opposite inner side face  118   b  of the outer element  118  and the facing side face  119   b  of the inner element  119  is measured. If the values of the distances d 1  and d 2  are equal, it is judged that the first or lower circuit pattern is overlaid with the second or upper circuit pattern correctly (i.e., with desired alignment accuracy). Sometimes, it is judged whether or not the value of the difference (d 1 −d 2 ) between the distances d 1  and d 2  is within a specific range. In any of these cases, no subsequent process is applied unless it is judged that the first or lower circuit pattern is overlaid with the second or upper circuit pattern with desired alignment accuracy. 
     FIG. 3 shows the layout or arrangement of the conventional alignment marks in an exposure area on a semiconductor wafer. As seen from FIG. 3, four alignment marks  110 ,  111 ,  112 , and  113  are provided in one of rectangular one-shot exposure areas  107  arranged on a semiconductor wafer  10 . Each of the marks  110 ,  111 ,  112 , and  113  has the same structure as the conventional mark  100  shown in FIGS. 1 and 2. Specifically, each of the marks  110 ,  111 ,  112 , and  113  comprises the square outer and inner mark elements  118  and  119 . The set of the four alignment marks  110 ,  111 ,  112 , and  113  may be termed the “conventional alignment mark set”. 
     In the rectangular exposure area  107  in FIG. 3, the two marks  110  and  112  are located on the longitudinal, central axis  115  of the area  107 , where the direction along the axis  115  is defined as the Y direction. Since the marks  110  and  112  are on the axis  115 , they are positioned at the middle of the short sides  107   a  and  107   c  of the area  107 . The mark  110  is close to the upper short side  107   a  while the mark  112  is close to the lower short side  107   c . The other marks  111  and  113  are located on the lateral, central axis  114  of the area  107 , where the direction along the axis  114  is defined as the x direction. Since the marks  111  and  113  are on the axis  114 , they are positioned at the middle of the long sides  107   b  and  107   d  of the area  107 . The mark  111  is close to the right long side  107   b  while the mark  113  is close to the left long side  107   d . A desired circuit or element pattern (not shown) is typically located among the four marks  110 ,  111 ,  112 , and  113  in the area  107 . 
     To measure the alignment accuracy along the X direction, the alignment marks  111  and  113  located on the lateral axis  114  are used. Specifically, the distances d 1  and d 2  between the outer and inner elements  118  and  119  along the X direction is measured for each of the marks  111  and  113 . Then, the difference (d 1 −d 2 ) of the distances d 1  and d 2  is calculated. Thus, the alignment accuracy along the X direction is determined by the value of the difference (d 1 −d 2 ) thus calculated. 
     Similarly, the alignment accuracy along the Y direction is measured using the alignment marks  110  and  112  located on the longitudinal axis  115 . Specifically, the distances d 3  and d 4  between the outer and inner elements  118  and  119  along the Y direction is measured for each of the marks  110  and  112 . Then, the difference (d 3 −d 4 ) of the distances d 3  and d 4  is calculated. Thus, the alignment accuracy along the Y direction is determined by the value of the difference (d 3 −d 4 ) thus calculated. 
     Actually, a lot of the rectangular exposure areas  107  shown in FIG. 3, each of which includes the conventional alignment mark set comprising the four marks  110 ,  111 ,  112 , and  113 , are regularly arranged on the semiconductor wafer  121 , as shown in FIG.  4 . In FIG. 4, the exposure areas  107  are arranged in a matrix array on the wafer  121 . Needless to say, the alignment accuracy is measured in each of the areas  107  using the marks  110 ,  111 ,  112 , and  113 . 
     With the conventional alignment mark set comprising the four marks  110 ,  111 ,  112 , and  113 , some of the marks  110 ,  111 ,  112 , and  113  are located close to each other. For example, as shown in FIG. 4, the mark  111  in one of the areas  107  and the mark  113  in another of the areas  107 , which are surrounded by an ellipse  116 , are close to each other. Also, the mark  112  in one of the areas  107  and the mark  110  in another of the areas  107 , which are surrounded by an ellipse  117 , are close to each other. In this case, there arises a problem that the inner elements  119  of the marks  110 ,  111 ,  112 , and  113  do not have desired contours or edges, which is explained in detail below with reference to FIG.  5 . 
     For example, with the inner element  119  of the alignment mark  111  surrounded by the ellipse  116 , the outer side face  119   b  tends to be tilted, as shown in FIG.  5 . The reason why the outer side face  119   b  is made oblique is not known clearly; however, it is thought in the following way. 
     Specifically, it is assumed that the exposing light irradiated to the resist layer for the inner element  119  of the mark  111  is affected by the next mark  113  to the mark  111 . Thus, the resist layer is not exposed to the light as desired. As a result, the top corner of the side face  119  is broken or deformed in the development process of the resist layer. 
     If the element  119  has the oblique side face  119   b  shown in FIG. 5, the contour or outline of the side face  119   b  is unable to be detected or observed correctly. Thus, the distance d 2  between the side face  119   b  and the opposing inner face  118   b  of the outer element  118  tends to be measured d 2 ′. As a consequence, the alignment accuracy thus observed tends to include some divergence or error of [(d 1 −d 2 ′)/2], which degrades the measurement accuracy. 
     Moreover, there is a possibility that the inner element  119  of the mark  110 ,  111 ,  112 , or  113  is deformed due to applied etching action and/or applied heat in a subsequent process or processes. In this case, because of synergism of the deformation and the tilting/breaking of the side face  119   b , the measurement accuracy deteriorates more. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an alignment mark set that facilitates the formation of a desired contour of each alignment mark, and a method of measuring alignment of patterns using the set. 
     Another object of the present invention is to provide an alignment mark set that suppresses the degradation of measurement accuracy for alignment of patterns, and a method of measuring alignment of patterns using the set. 
     Still another object of the present invention is to provide an alignment mark set that ensures high measurement accuracy, and a method of measuring alignment of patterns using the set. 
     The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description. 
     According to a first aspect of the present invention, an alignment mark set is provided. The set comprises: 
     (a) a first alignment mark formed in an exposure area; 
     the area having a periphery, first central axis, and 
     a second central axis perpendicular to the first axis; 
     the first alignment mark being located near the first central axis and apart from the second axis; 
     (b) a second alignment mark formed in the exposure area; 
     the second alignment mark being located near the second central axis and apart from the first axis; and 
     (c) when the exposure areas are regularly arranged in such a way as to have a same orientation in a plane, each of the first and second alignment marks in one of the sets is not located close to the first and second alignment marks in another of the sets, thereby ensuring irradiation of exposing light to all the areas. 
     With the alignment mark set according to the first aspect of the present invention, the first alignment mark is formed in the exposure area in such a way as to be located near the first axis of the area and apart from the second axis thereof. On the other hand, the second alignment mark is formed in the exposure area in such a way as to be located near the second axis of the area and apart from the first axis thereof. 
     Thus, when the alignment mark sets according to the first aspect are arranged to be adjacent to each other in a plane on use, each of the first and second alignment marks in one of the set is not located close to the first and second alignment marks in another of the sets. Accordingly, even if exposing light is irradiated to all the exposure areas thus arranged, none of the first and second alignment marks in one of the sets are affected by the first and second marks in another of the sets. 
     As a result, the formation of a desired contour of each of the first and second marks is facilitated, which suppresses the degradation of measurement accuracy for alignment of patterns. This means that high measurement accuracy is ensured. 
     It is preferred that each of the first and second marks is square. 
     In a preferred embodiment of the alignment mark set according to the first aspect, there are additionally provided with; 
     (c) a third alignment mark formed in the exposure area; 
     the third alignment mark being located near the first central axis and apart from the second axis; 
     the third alignment mark being shifted from the first central axis along the second central axis in an opposite direction to the first alignment mark; and 
     (d) a fourth alignment mark formed in the exposure area; 
     the fourth alignment mark being located near the second central axis and apart from the first axis; 
     the fourth alignment mark being shifted from the second central axis along the first central axis in an opposite direction to the second alignment mark. 
     In another preferred embodiment of the alignment mark set according to the first aspect, there are additionally provided with; 
     (c) a third alignment mark formed in the exposure area; 
     the third alignment mark being located near the first central axis and apart from the second axis; 
     the third alignment mark being located on the first central axis along with the first alignment mark; and 
     (d) a fourth alignment mark formed in the exposure area; 
     the fourth alignment mark being located near the second central axis and apart from the first axis; 
     the fourth alignment mark being located on the second central axis along with the second alignment mark. 
     In this embodiment, it is preferred that the first and third marks are apart from the periphery on opposite sides at equal distances while the second and fourth marks are apart from the periphery on opposite sides at equal distances. More preferably, the first and third marks are apart from the periphery at equal distances of 100 μm or greater while the second and fourth marks are apart from the periphery at equal distances of 100 μm or greater. 
     According to a second aspect of the present invention, a method of measuring alignment of patterns is provided, where alignment mark sets are regularly arranged in such a way as to have a same orientation in a plane on use. This method comprises the steps of: 
     (a) arranging the alignment mark sets on the plane so as to be adjacent to each other; 
     each of the sets comprising 
     (a-1) a first alignment mark formed in an exposure area; 
     the area having a periphery, first central axis, and a second central axis perpendicular to the first axis; 
     the first alignment mark being located on the first axis near the periphery; 
     (a-2) a second alignment mark formed in the exposure area; 
     the second alignment mark being located on the second axis near the periphery; 
     (a-3) a third alignment mark formed in the exposure area; 
     the third alignment mark being located on the first axis near the periphery on an opposite side to the first mark; 
     (a-4) a fourth alignment mark formed in the exposure area; 
     the fourth alignment mark being located on the second axis near the periphery on an opposite side to the second mark; and 
     (b) measuring alignment accuracy using the first mark in a first one of the sets and the third mark in a second one of the sets adjacent to the first one of the sets and the second mark in one of the first one of the sets and the fourth mark in the second one of the sets; 
     wherein unopposed sides of the first and third marks located on opposite sides to each other with respect to the periphery of the area and unopposed sides of the second and fourth marks located on opposite sides to each other with respect to the periphery of the area are used for measuring. 
     With the method according to the second aspect of the invention, unopposed sides of the first and third marks located on opposite sides to each other with respect to the periphery of the area and unopposed sides of the second and fourth marks located on opposite sides to each other with respect to the periphery of the area are used for measuring. Thus, even if exposing light is irradiated to all the exposure areas thus arranged, none of the first and second alignment marks in one of the sets are affected by the first and second marks in another of the sets. 
     As a result, the formation of a desired contour of each of the first and second marks is facilitated, which suppresses the degradation of measurement accuracy for alignment of patterns. This means that high measurement accuracy is ensured. 
     It is preferred that each of the first and second marks is square. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic plan view of a conventional alignment mark set, which is formed on a semiconductor substrate or wafer. 
     FIG. 2 is a schematic cross-sectional view of the conventional alignment mark set shown in FIG. 1, which is taken along the line II—II in FIG.  1 . 
     FIG. 3 is a schematic plan view showing the layout of the alignment marks in the conventional set shown in FIGS. 1 and 2, which are formed in a rectangular exposure area on a semiconductor wafer. 
     FIG. 4 is a schematic plan view showing the layout of the conventional alignment mark sets shown in FIGS. 1 to  3  on a semiconductor wafer. 
     FIG. 5 is a schematic cross-sectional view of the conventional alignment mark set shown in FIG. 1 taken along the line II—II in FIG. 1, which schematically shows the unwanted oblique side face of the inner element of the mark. 
     FIG. 6 is a schematic plan view showing the layout of alignment marks in an alignment mark set according to a first embodiment of the invention, which are formed in an exposure area on a semiconductor wafer. 
     FIG. 7 is a schematic plan view showing a method of measuring alignment accuracy in the exposure area with the alignment mark set according to the first embodiment of FIG.  6 . 
     FIG. 8 is a schematic plan view showing the layout of the alignment mark sets according to the first embodiment of FIG. 6 on a semiconductor wafer. 
     FIG. 9 is a schematic cross-sectional view of the alignment mark of the set according to the first embodiment of FIG. 6, which is taken along a line like the line II—II in FIG.  1 . 
     FIG. 10 is a schematic plan view showing a method of measuring alignment accuracy in the exposure area with an alignment mark set according to a second embodiment of the invention. 
     FIG. 11 is a schematic plan view showing the layout of the alignment mark sets according to the second embodiment of FIG. 10, which are formed in each of the exposure areas on a semiconductor wafer. 
     FIG. 12 is a schematic plan view showing the layout of alignment mark sets according to a third embodiment of the invention, which are formed in each of the exposure areas on a semiconductor wafer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. 
     FIRST EMBODIMENT 
     As shown in FIG. 8, a lot of rectangular exposure areas  7  are regularly formed or defined on a semiconductor wafer  21 ; in other words, the areas  7  are arranged in a matrix array on the wafer  21 . In each of the areas  7 , an alignment mark set according to a first embodiment of the invention is formed, which comprises four alignment marks  1 ,  2 ,  3 , and  4 , as shown in FIG.  6 . 
     The four alignment marks  1 ,  2 ,  3 , and  4  are arranged in such a manner as shown in FIG.  6 . Specifically, the two marks  1  and  3  are located near the longitudinal, central axis  6  of the rectangular exposure area  7 , where the direction along the axis  6  is defined as the Y direction. The mark  1  is near the upper short side  7   a  of the area  7  while the mark  3  is near the lower short side  7   c  thereof. The mark  1  is shifted (i.e., offset) from the axis  6  toward the left-hand side in FIG. 6 by a distance h 1 . The mark  3  is shifted (i.e., offset) from the axis  6  toward the right-hand side in FIG. 6 by the same distance h 1 . Thus, it is said that the marks  1  and  3  are shifted from the axis  6  in opposite directions. The distance h 1  is measured from the axis  6  to the center of the mark  1  or  3 . 
     On the other hand, the remaining two marks  2  and  4  are located near the lateral, central axis  5  of the rectangular exposure area  7 , where the direction along the axis  5  is defined as the X direction. The mark  2  is near the right long side  7   b  of the area  7  while the mark  4  is near the left long side  7   d  thereof. The mark  2  is shifted (i.e., offset) from the axis  5  upward in FIG. 6 by a distance h 2 . The mark  4  is shifted (i.e., offset) from the axis  5  downward in FIG. 6 by the same distance h 2 . Thus, it is said that the marks  2  and  4  are shifted from the axis  5  in opposite directions. The distance h 2  is measured from the axis  5  to the center of the mark  2  or  4 . 
     All the marks  1 ,  2 ,  3 , and  4  are the same in shape and structure and therefore, the shape and structure of the mark  1  is explained in detail below. 
     As shown in FIG. 6, the mark  1  comprises a square outer element  24  and a square inner element  25 . The inner element  25  is entirely located in the outer element  24  so as to be concentric with the outer element  24 . Each of the four sides of the inner element  25  (the length of which is L 2 ) is parallel to a corresponding one of the four sides of the outer element  24  (the length of which is L 1 ), where L 1 &gt;L 2 . 
     The offset distance h 1  of the mark  1  or  3  is determined in such a way as to be larger than half of the side length L 1  of the mark  1  or  3 , i.e., h 1 &gt;[(L)/2]. Thus, the marks  1  and  3  are located apart from the longitudinal, central axis  6 . 
     The offset distance h 2  of the mark  2  or  4  is determined in such a way as to be larger than half of the side length L 1  of the mark  2  or  4 , i.e., h 2 &gt;[(L 1 )/2]. Thus, the marks  2  and  4  are located apart from the lateral, central axis  5 . 
     FIG. 9 shows a cross-sectional view showing the structure of the mark  1 , which is taken along a line like the line II—II in FIG.  1 . The other marks  2 ,  3 , and  4  have the same cross-section as shown in FIG.  9 . The mark  1  is formed in the following way. 
     First, a first layer  22  is formed on the surface of the semiconductor wafer  21 . Next, a resist layer (not shown) is formed on the first layer  22  and is patterned to have a specific shape by the lithography technique. Thereafter, using the patterned resist layer as a mask, the underlying first layer  22  is selectively etched, thereby forming a first or lower circuit pattern (not shown) and the outer square element  25  of the alignment mark  1  in the layer  22 . As clearly seen from FIG. 9, the element  25  is a square hole formed in the layer  22 . 
     Subsequently, a second layer  23  is formed on the first layer  22  thus patterned so as to contact the bottom and side faces of the element  25  (i.e., the hole of the first layer  22 ). Then, a resist layer (not shown) is formed on the second layer  23  and is patterned by the lithography technique, thereby forming a second or upper circuit pattern (not shown) and the inner square element  24  of the alignment mark  1  on the layer  23  in the hole  25  of the layer  22 . As clearly seen from FIG. 6, the inner element  24  is a square part of the resist layer and located in the hole or outer element  25 . The patterned resist layer thus formed is used as a mask in the next etching process for the second layer  23 . 
     The alignment mark set comprising the marks  1 ,  2 ,  3 , and  4  according to the first embodiment is used to measure the alignment accuracy between the first circuit pattern formed by the first layer  22  and the second circuit pattern formed by the patterned resist layer. In this case, the relative positional relationship between the inner and outer elements  24  and  25  is measured. 
     For example, as shown in FIGS. 7 and 9, the distance d 1  between the inner side face  25   a  of the outer element  25  and the facing side face  24   a  of the inner element  24  is measured. At the same time, the distance d 2  between the opposite inner side face  25   b  of the outer element  25  and the facing side face  24   b  of the inner element  24  is measured. If the values of the distances d 1  and d 2  are equal, it is judged that the first or lower circuit pattern is overlaid with the second or upper circuit pattern correctly (i.e., with desired alignment accuracy). Sometimes, it is judged whether or not the value difference (d 1 −d 2 ) between the distances d 1  and d 2  is within a specific range. Thus, the alignment accuracy in the X direction is obtained. 
     Similarly, the distance d 3  between the inner side face of the outer element  25  and the facing side face of the inner element  24  is measured. At the same time, the distance d 4  between the opposite inner side face of the outer element  25  and the facing side face of the inner element  24  is measured. If the values of the distances d 3  and d 4  are equal, it is judged that the first or lower circuit pattern is overlaid with the second or upper circuit pattern correctly (i.e., with desired alignment accuracy). Sometimes, it is judged whether or not the value difference (d 3 −d 4 ) between the distances d 3  and d 4  is within a specific range. Thus, the alignment accuracy in the Y direction is obtained. 
     No subsequent process is applied unless the first or lower circuit pattern is overlaid with the second or upper circuit pattern with desired alignment accuracy. 
     The distances d 1 , d 2 , d 3 , and d 4  are measured by the known laser-scanning method or image-processing method with a known suitable apparatus. 
     With the alignment mark set according to the first embodiment of FIG. 6, as explained above, the two marks  1  and  3  are respectively located in the vicinity of the short sides  7   a  and  7   c  of the rectangular exposure area  7  in such a way as to be shifted by the same distance h 1  in the opposite directions with respect to the longitudinal, central axis  6 . Similarly, the remaining two marks  2  and  4  are respectively located in the vicinity of the long sides  7   b  and  7   d  of the area  7  in such a way as to be shifted by the same distance h 2  in the opposite directions with respect to the lateral, central axis  5 . 
     Accordingly, when a lot of the exposure areas  7  each including the alignment mark set of the first embodiment are arranged in a matrix array, as shown in FIG. 8, none of the alignment marks  1 ,  2 ,  3 , and  4  belonging in the two adjoining exposure areas  7  are located close to each other. For example, as shown in FIG. 8, the marks  1  and  3  surrounded by an ellipse  9  are shifted in opposite directions (i.e., downward and upward) along the common short side of the adjoining areas  7 . This means that these marks  1  and  3  are sufficiently apart from each other; in other words, none of these marks  1  and  3  apply any bad effect to each other due to exposing light used in the lithography process for pattering the resist layer on the second layer  23 . 
     Similarly, the remaining two marks  2  and  4  surrounded by an ellipse  8  are shifted in opposite directions (i.e., toward left and right sides) along the common long side of the adjoining areas  7 . This means that these marks  2  and  4  are sufficiently apart from each other; in other words, none of these marks  2  and  4  apply any bad effect to each other due to the same exposing light. 
     As a result, unlike the previously-explained conventional alignment mark set, no breaking or deformation of the square inner element  25  occurs at its side face and desired shape and/or contour can be easily formed. Thus, the contour of the side faces of the inner element  25  can be detected correctly, decreasing the measurement error and raising the measurement accuracy. This leads to improvement of fabrication yield of semiconductor devices. 
     SECOND EMBODIMENT 
     FIG. 10 shows an alignment mark set according to a second embodiment of the invention, which comprises four square alignment marks  31 ,  32 ,  33 , and  34 . This alignment mark set is formed in each of the rectangular exposure areas  7  arranged in a matrix array on the wafer  21 , which has been referred in the first embodiment. 
     With the alignment mark set according to the second embodiment shown in FIG. 10, the alignment marks  31 ,  32 ,  33 , and  34  are formed in the following way: 
     As shown in FIG. 10, unlike the first embodiment, both the marks  31  and  33  are located on the longitudinal central axis  6  of the rectangular exposure area  7 . In other words, these two marks  31  and  33  have no offset. Also, the mark  31  is apart from the upper short side  7   a  of the area  7  at a distance k 1  while the mark  33  is apart from the lower short side  7   c  thereof at the same distance k 1 . The distance k 1  is measured from the short side  7   a  or  7   c  to the opposing side of the mark  31  or  33 . The marks  31  and  33  are apart from each other at a distance k 3  along the axis  6 , which is measured between the opposing sides of the marks  31  and  33  in the Y direction. 
     Preferably, the distance k 1  is equal to or more than 50 μm. More preferably, the distance k 1  is equal to or more than 100 μm. The distance k 3  between the opposing sides of the marks  31  and  33  is set to be 100 μm or more. 
     On the other hand, both the marks  32  and  34  are located on the lateral, central axis  5  of the rectangular exposure area  7 . In other words, the marks  32  and  34  have no offset. Also, the mark  32  is apart from the right long side  7   b  of the area  7  at a distance k 2  while the mark  34  is apart from the left long side  7   c  thereof at the same distance k 2 . The distance k 2  is measured from the long side  7   b  or  7   d  of the area  7  to the opposing side of the mark  32  or  34 . The marks  32  and  34  are apart from each other at a distance k 4  along the axis  5 , which is measured between the opposing sides of the marks  32  and  34  in the X direction. 
     Preferably, the distance k 2  is equal to or more than 50 μm. More preferably, the distance k 2  is equal to or more than 100 μm. The distance k 4  between the opposing sides of the marks  32  and  34  is set to be 100 μm or more. 
     Since the four marks  31 ,  32 ,  33 , and  34  are arranged in the area  7  in such a manner as above, they are located at the shifted positions toward the center of the area  7  (i.e., the intersection of the axes  5  and  6 ) compared with the conventional alignment mark set comprising the marks  110 ,  111 ,  112 , and  113  and the alignment mark set comprising the marks  1 ,  2 ,  3 , and  4  according to the first embodiment. 
     All the marks  31 ,  32 ,  33 , and  34  are the same in shape and structure as the marks  1 ,  2 ,  3 , and  4  of the first embodiment. 
     The alignment mark set comprising the marks  31 ,  32 ,  33 , and  34  according to the second embodiment is used to measure the alignment accuracy between the first circuit pattern formed by the first layer  22  and the second circuit pattern. In this case, the relative positional relationship between the elements  24  and  25  is measured. 
     For example, as shown in FIG. 10, the distances d 1  between the outer and inner elements  24  and  25  of the marks  32  and  34  are measured while the distances d 2  between the outer and inner elements  24  and  25  of the marks  31  and  33  are measured. Thus, the alignment accuracy in the Y direction is obtained. Similarly, the distances d 3  between the outer and inner elements  24  and  25  of the marks  31  and  33  are measured while the distances d 4  between the inner and outer elements  24  and  25  are measured. Thus, the alignment accuracy in the X direction is obtained. According to the value of the difference (d 1 −d 2 ) and the value of the difference (d 3 −d 4 ), i.e., the alignment accuracy in the Y and X directions, it is judged whether or not the first or lower circuit pattern is overlaid with the second or upper circuit pattern correctly (i.e., with desired alignment accuracy). 
     With the alignment mark set according to the second embodiment of FIG. 10, the marks  31  and  33  are respectively located on the axis  6  at the positions shifted toward the center of the exposure area  7  in such a way as to be at the same distance k 1  from the short sides  7   a  and  7   c  of the area  7 . Similarly, the marks  32  and  34  are respectively located on the axis  5  at the positions shifted toward the center of the area  7  in such a way as to be at the same distance k 2  from the long sides  7   b  and  7   d  thereof. 
     Accordingly, when a lot of the exposure areas  7  each having the alignment mark set of the second embodiment are arranged in a matrix array, as shown in FIG. 11, all the alignment marks  31 ,  32 ,  33 , and  34  in the two adjoining exposure areas  7  are arranged at sufficient long distances to each other. For example, as shown in FIG. 11, the marks  31  and  33  surrounded by the ellipse  39  are shifted in opposite directions along the common long side of the adjoining areas  7 . This means that the marks  31  and  33  are sufficiently apart from each other; in other words, none of these marks  31  and  33  apply any bad effect to each other due to the exposing light. 
     Similarly, the marks  32  and  34  surrounded by the ellipse  38  are shifted in opposite directions along the common short side of the adjoining areas  7 . This means that the marks  32  and  34  are sufficiently apart from each other; in other words, none of these marks  32  and  34  apply any bad effect to each other due to the exposing light. 
     As a result, like the first embodiment, no breaking or deformation of the inner element  25  occurs at its side face and desired shape and/or contour can be formed. Thus, the contour of the side faces of the inner element  25  can be detected correctly, decreasing the measurement error and raising the measurement accuracy. 
     THIRD EMBODIMENT 
     FIG. 12 shows an alignment mark set used in a method of measuring alignment accuracy according to a third embodiment of the invention. This set is formed in each of the rectangular exposure areas  7  arranged in a matrix array on the wafer  21 , which has been referred in the first embodiment. This set comprises four alignment marks  41 ,  42 ,  43 , and  44 . 
     As shown in FIG. 12, like the second embodiment, both the marks  41  and  43  are located on the longitudinal, central axis  6  of the rectangular exposure area  7 . In other words, the marks  41  and  43  have no offset with respect to the axis  6 . Also, the mark  41  is close to the upper short side  7   a  of the area  7  at a distance while the mark  43  is close to the lower short side  7   c  thereof at the same distance. 
     On the other hand, both the marks  42  and  44  are located on the lateral, central axis  5  of the area  7 . In other words, the marks  42  and  44  have no offset with respect to the axis  5 . Also, the mark  42  is close to the right long side  7   b  of the area  7  at a distance while the mark  44  is apart from the left long side  7   c  thereof at the same distance. 
     All the marks  41 ,  42 ,  43 , and  44  are the same in shape and structure as the marks  1 ,  2 ,  3 , and  4  of the first embodiment. 
     Next, the method of measuring the alignment accuracy according to the third embodiment is explained below, in which the alignment mark set comprising the marks  41 ,  42 ,  43 , and  44  is used. To measure the alignment accuracy in the X-direction, as shown in FIG. 12, the marks  41  and  43  aligned on the axis  6  (not the marks  42  and  44  aligned on the axis  5 ) are used. Specifically, the distance d 1  between the inner side face of the outer element  24  of the mark  41  and the facing side face  25   d  of the inner element  25  thereof is measured. At the same time, the distance d 2  between the opposite inner side face of the outer element  24  of the mark  41  and the facing side face  25   b  of the inner element  25  thereof is measured. Similarly, the distance d 1  between the inner side face of the outer element  24  of the mark  43  and the facing side face  25   d  of the inner element  25  thereof is measured. At the same time, the distance d 2  between the opposite inner side face of the outer element  24  of the mark  43  and the facing side face  25   b  of the inner element  25  thereof is measured. 
     According to the value of the difference (d 1 −d 2 ) of the marks  41  and  43 , it is judged whether or not the first or lower circuit pattern is overlaid with the second or upper circuit pattern with desired alignment accuracy in the X direction. 
     To measure the alignment accuracy in the Y-direction, the marks  42  and  44  aligned on the axis  5  (not the marks  41  and  43  aligned on the axis  6 ) are used. Specifically, the distance d 3  between the inner side face of the outer element  24  of the mark  42  and the facing side face  25   a  of the inner element  25  thereof is measured. At the same time, the distance d 4  between the opposite inner side face of the outer element  24  of the mark  42  and the facing side face  25   c  of the inner element  25  thereof is measured. Similarly, the distance d 3  between the inner side face of the outer element  24  of the mark  44  and the facing side face  25   d  of the inner element  25  thereof is measured. At the same time, the distance d 4  between the opposite inner side face of the outer element  24  of the mark  44  and the facing side face  25   b  of the inner element  25  thereof is measured. 
     According to the value of the difference (d 3 −d 4 ) of the marks  42  and  44 , it is judged whether or not the first or lower circuit pattern is overlaid with the second or upper circuit pattern with desired alignment accuracy in the Y direction. 
     As seen from FIG. 12, the marks  41  and  43  are aligned in the Y direction and close to each other while the marks  42  and  44  are aligned in the x direction and close to each other. Thus, similar to the conventional method of measuring alignment accuracy as explained with reference to FIGS. 3 and 4, there is a possibility that breaking or deformation occurs at the side face  25   a  of the inner element  25  of the mark  41 , at the side face  25   c  of the inner element  25  of the mark  43 , at the side face  25   b  of the inner element  25  of the mark  42 , and at the side face  25   d  of the inner element  25  of the mark  44 . 
     However, as explained above, the broken or deformed faces  25   a  and  25   c  of the marks  41  and  43  are not used (i.e., the faces  25   b  and  25   d  of the marks  41  and  43  are used) to measure the Y-direction accuracy. Also, the broken or deformed faces  25   b  and  25   d  of the marks  42  and  44  are not used (i.e., the faces  25   a  and  25   c  of the marks  42  and  44  are used) to measure the X-direction accuracy. Thus, there is almost no possibility that the measurement accuracy degrades. In other words, the alignment can be measured with higher accuracy than the conventional method. 
     With the above-described method according to the third embodiment, the alignment mark set is approximately the same in structure as the conventional mark set. However, it is needless to say that any alignment mark set (including the sets according to the first and second embodiments) may be used for the method of the third embodiment. 
     In the above-described first to third embodiments, the inner and outer elements  25  and  24  of every alignment mark in the exposure area  7  are square. This is to facilitate the alignment measuring operation with a measuring apparatus. However, the invention is not limited to these cases. It is needless to say that the elements  25  and  24  may have any other shape according to the necessity. 
     Moreover, in the above-described first to third embodiments, the alignment mark set comprises four alignment marks in each exposure area. This is to facilitate the alignment measuring operation with a measuring apparatus and to simplify the pattern for the alignment mark set. However, the invention is not limited to this case. It is sufficient for the set to include at least two alignment marks. Needless to say, the set may comprise five or more alignment marks in each exposure area. 
     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the present invention, therefore, is to be determined solely by the following claims.