Abstract:
A method of forming a mark in an IC fabricating process is described. Two parts of the mark each including a plurality of linear patterns are respectively defined by two exposure steps that either belong to two lithography processes respectively or constitute a double-exposure process including X-dipole and Y-dipole exposure steps.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to fabrication of integrated circuits, and more particularly to a method of forming a mark in an IC-fabricating process, wherein the mark is defined by two exposure steps and can serve as a basis of an overlay mark. 
     2. Description of the Related Art 
     Through an IC-fabricating process, various marks are formed on the wafer for different purposes. For example, to check the alignment accuracy between the patterns of lower and upper wafer layers that is more and more important as the linewidth gets smaller increasingly, a wafer is formed with many overlay marks in some non-die areas. A conventional overlay mark is the so-called box-in-box (BIB) overlay mark, but such overlay marks are too sensitive to certain factors other than the alignment so that the alignment accuracy cannot be checked accurately. Hence, the overlay mark of AIM (advanced imaging mark) type is provided in the prior art. 
     Referring to  FIG. 1 , a conventional AIM-type overlay mark  100  is disposed in a non-die area of the wafer (not shown) including four adjacent regions  102 - 108 , which are arranged in a 2×2 array and include a first region  102  and a second region  104  arranged diagonally and a third region  106  and a fourth region  108  arranged diagonally, and includes a portion of the lower layer defined by a lithography process for defining the die patterns of the lower layer and a patterned photoresist layer formed in a lithography process for defining the die patterns of the upper layer. The portion of the lower layer includes a first set of x-directional linear patterns  110  arranged in the y-direction in one half of the first region  102 , a second set of x-directional linear patterns  112  arranged in the y-direction in one half of the second region  104 , a first set of y-directional linear patterns  114  arranged in a x-direction in one half of the third region  106 , and a second set of y-directional linear patterns  116  arranged in the x-direction in one half of the fourth region  108 . The patterned photoresist layer includes a first set of x-directional photoresist bars  118  arranged in the y-direction in the other half of the first region  102 , a second set of x-directional photoresist bars  120  arranged in the y-direction in the other half of the second region  104 , a first set of y-directional photoresist bars  122  arranged in the x-direction in the other half of the third region  106 , and a second set of y-directional photoresist bars  124  arranged in the x-direction in the other half of the fourth region  108 . 
     The linear patterns  110 - 116  and photoresist bars  118 - 124  are designed such that when the lower layer is fully aligned with the upper layer, the central line of respective central lines of the first set of x-directional linear patterns  110  and the second set of x-directional linear patterns  112  coincides with that of respective central lines of the first set of x-directional photoresist bars  118  and the second set of x-directional photoresist bars  120 , and the central line of respective central lines of the first set of y-directional linear patterns  114  and the second set of y-directional linear patterns  116  coincides with that of respective central lines of the first set of y-directional photoresist bars  122  and the second set of y-directional photoresist bars  124 . 
     To check the alignment, the y-coordinate “y 1a ” of the central line of the first set of x-directional linear patterns  110 , the y-coordinate “y 1b ” of the central line of the second set of x-directional linear patterns  112 , the x-coordinate “x 1a ” of the central line of the first set of y-directional linear patterns  114 , the x-coordinate “x 1b ” of the central line of the second set of y-directional linear patterns  116 , the y-coordinate “y 2a ” of the central line of the first set of x-directional photoresist bars  118 , the y-coordinate “y 2b ” of the central line of the second set of x-directional photoresist bars  120 , the x-coordinate “x 2a ” of the central line of the first set of y-directional photoresist bars  122  and the x-coordinate “x 2b ” of the central line of the second set of y-directional photoresist bars  124  are derived at first. 
     The method of deriving the x- and y-coordinates is exemplified by the following process of deriving x 1a  that is shown in  FIG. 2 . The first y-directional linear patterns  114  are scanned by a light beam (not shown) in the direction  200  to obtain a reflectivity curve  202 , and respective x-coordinates of the six first y-directional linear patterns  114  are determined based on the reflectivity curve  202 . When the linear patterns  110 - 116  are, for example, trenches in the portion of the lower layer, x 1a  is calculated as the average of the x-coordinates x 1a1 , x 1a2 , x 1a3 , x 1a4 , x 1a5  and x 1a6  of the six locally minimal points of the reflectivity curve  202 . 
     Then, the x-directional alignment error of the die patterns of the upper layer with those of the lower layer near the overlay mark is calculated as “(x 2a +x 2b )/2−(x 1a +x 1b )/2”, and the y-directional alignment error of the die patterns of the upper layer with those of the lower layer near the overlay mark is calculated as “(y 2a +y 2b )/2−(y 1a +y 1b )/2”. After the x-directional alignment errors and y-directional alignment errors at different areas of the wafer are determined using the overlay marks thereat, overlay analyses can be done for better control of the exposure system. 
     Moreover, when the lower layer is defined by two exposure steps, in the prior art, two above overlay marks have to be formed for the two exposure steps respectively, so that the x-directional and y-directional alignments of the die patterns defined by the first exposure step as well as those of the die patterns defined by the second exposure step with the die patterns of the upper layer can be checked. 
     However, when one exposure step uses X-dipole off-axis light to define patterns requiring higher resolution in the x-direction and the other uses Y-dipole off-axis light to define patterns requiring higher resolution in the y-direction, each set of x-directional linear patterns arranged in the y-direction in the overlay mark  100 ′ for the X-dipole exposure merge together, as shown in  FIG. 3(   a ), and each set of y-directional linear patterns arranged in the x-direction in the overlay mark  100 ″ for the Y-dipole exposure step merge together, as shown in  FIG. 3(   b ). Therefore, a half area of each overlay mark cannot be utilized in the alignment check and is wasted. 
     SUMMARY OF THE INVENTION 
     Accordingly, this invention provides a method of forming a mark in an IC-fabricating process, which can be modified to be a method of forming an overlay mark that allows the number of overlay marks required to be reduced by 50%. 
     In the method, a first part and a second part of the linear patterns of the mark are formed from a portion of a material layer and are respectively defined by two exposure steps that either belong to two lithography processes respectively or constitute a double-exposure process including X-dipole and Y-dipole exposure steps. 
     In an embodiment of the method of this invention, a plurality of first trenches is formed in a portion of a material layer on a wafer through a first lithography process and a first etching process. A plurality of second trenches is formed in the portion of the material layer through a second lithography process and a second etching process. The first trenches and the second trenches are arranged alternately, and the width of a first or second trench is substantially equal to the distance between a pair of first and second trenches that are closest to each other. 
     By forming over the portion of the material layer a plurality of photoresist bars in a lithography process for defining an upper layer above the material layer after the first and second trenches are formed, an overlay mark can be obtained. 
     In another embodiment of the method of this invention, the IC fabricating process includes a double exposure process including an X-dipole exposure step and a Y-dipole exposure step. A plurality of y-directional linear patterns in parallel is defined in a photoresist layer over a portion of a material layer on a wafer in the X-dipole exposure step. A plurality of x-directional linear patterns in parallel is defined in the photoresist layer over the portion of the material layer in the Y-dipole exposure step. The y-directional and x-directional linear pattern are developed in the development step of the photoresist layer, and then the portion of the material layer is patterned using the photoresist layer thereon as an etching mask. In this embodiment, the region for forming the x-directional linear patterns is masked in the X-dipole exposure step, and the region for forming the y-directional linear patterns is masked in the Y-dipole exposure step. 
     Similarly, by forming a plurality of x-directional photoresist bars and a plurality of y-directional photoresist bars in a lithography process for defining an upper layer above the material layer after the portion of the material layer is patterned, an overlay mark can be obtained. 
     In the former embodiment, since one mark formed includes a first and a second parts that are respectively defined by two exposure steps belonging to two lithography processes, the relationship between the material layer defined by the first exposure step and that defined by the second exposure step can be derived from one mark. In the latter embodiment, since the y-directional linear patterns in parallel are defined in the X-dipole exposure step that well resolves x-directionally separated patterns and the x-directional linear patterns in parallel are defined in the Y-dipole exposure step that well resolves y-directionally separated patterns, no linear patterns in parallel are merged together in one mark. Therefore, all the area of each mark can be utilized. 
     In a case where the mark of the former or latter embodiment is further made into an overlay mark, since the overlay mark includes a first and a second parts respectively defined by two exposure steps that belong to two lithography processes or a double exposure process including X- and Y-dipole exposure steps, the alignment accuracy between the lower layer defined by one exposure step and the upper layer as well as that between the lower layer defined by the other exposure step and the upper layer both can be checked with only one of the overlay mark. 
     It is noted that both the foregoing general description and the following detailed description are just exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conventional AIM-type overlay mark in the prior art. 
         FIG. 2  illustrates a method of deriving the x-coordinate of the central line of a set of y-directional linear patterns arranged in the x-direction in the overlay mark of  FIG. 1 . 
         FIG. 3(   a )/( b ) illustrates that a set of the x-/y-directional linear patterns arranged in the y-/x-direction merge together in an overlay mark for X-/Y-dipole exposure. 
         FIGS. 4A-4C  illustrate a method of forming an overlay mark according to a first embodiment of this invention, wherein FIG.  4 C(b) illustrates the overlay mark. 
         FIG. 5  illustrates exemplary use of the overlay mark of FIG.  4 C(b). 
         FIG. 6  shows a method of deriving the x-coordinates of respective central lines of two sets of y-directional linear patterns respectively defined by the first and second exposure steps and intermixed with each other according to the first embodiment. 
         FIGS. 7A-7C  illustrate a first method of forming an overlay mark according to a second embodiment of this invention, wherein FIG.  7 C(b) shows the overlay mark. 
         FIGS. 8A-8C  illustrate a second method of forming an overlay mark according to the second embodiment of this invention, wherein FIG.  8 C(b) shows the overlay mark. 
         FIGS. 9A-9C  illustrate a third method of forming an overlay mark according to the second embodiment of this invention, wherein FIG.  9 C(b) shows the overlay mark. 
         FIGS. 10A-10C  illustrate a fourth method of forming an overlay mark according to the 2 nd  embodiment of this invention, wherein FIG.  10 C(b) shows the overlay mark. 
         FIG. 11A  illustrates an example of die patterns suitably defined by X-dipole and Y-dipole exposure steps and checked with the overlay mark of the second embodiment, and FIG.  11 B/C illustrates the photomask patterns for the X-/Y-dipole exposure step. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is noted that the following embodiments are intended to further explain this invention but not to restrict the scope of this invention. 
     First Embodiment 
       FIGS. 4A-4C  illustrate a method of forming an overlay mark according to the first embodiment of this invention, wherein FIG.  4 C(b) illustrates the overlay mark. 
     Referring to  FIG. 4A , after a lower layer including a portion  402  for forming the overlay mark is formed on a substrate  400 , a first lithography process and a subsequent etching process are conducted. The first lithography process includes a first exposure step that uses a first photomask having die patterns (not shown) and an overlay mark area  40  with trench patterns  42  for forming an overlay mark. Through the first lithography process and the subsequent etching process, the die patterns on the first photomask are transferred to the lower layer in the die areas (not shown), and trenches corresponding to the trench patterns  42  are formed in the portion  402  of the lower layer in the overlay mark area on the wafer. 
     The overlay mark area on the wafer includes four regions that are arranged in a 2×2 array and include a first region  404  and a second region  406  arranged diagonally and a third region  408  and a fourth region  410  arranged diagonally. The trenches in the portion  402  of the lower layer include a first set of x-directional trenches  412  arranged in the y-direction in one half of the first region  404 , a second set of x-directional trenches  414  arranged in the y-direction in one half of the second region  406 , a first set of y-directional trenches  416  arranged in the x-direction in one half of the third region  408 , and a second set of y-directional trenches  418  arranged in the x-direction in one half of the fourth region  410 . In this embodiment, the photomask trench patterns corresponding to the trenches in each of the above sets are equally spaced, and each of the trenches has a width smaller than that of one linear pattern in a conventional AIM-type overlay mark, such as about ⅓ of the latter. 
     Referring to  FIG. 4B , a second lithography process and a subsequent etching process are performed, wherein the former includes a second exposure step that uses a second photomask having die patterns (not shown) and an overlay mark area  44  with trench patterns  46  for forming the overlay mark. Through the second lithography process and the subsequent etching process, the die patterns on the second photomask are transferred to the lower layer in die areas (not shown), and trenches corresponding to the trench patterns  46  are formed in the portion  402  of the lower layer. The trenches include a third set of x-directional trenches  420  arranged in the y-direction in the one half of the first region  404 , a fourth set of x-directional trenches  422  arranged in the y-direction in the one half of the second region  406 , a third set of y-directional trenches  424  arranged in the x-direction in the one half of the third region  408 , and a fourth set of y-directional trenches  426  arranged in x-direction in the one half of the fourth region  410 . In this embodiment, the photomask trench patterns corresponding to the trenches in each of the above sets are equally spaced and each of the trenches has a width smaller than that of one linear pattern in a conventional AIM-type overlay mark, such as about ⅓ of the latter. 
     The trench patterns  42  on the first photomask and the trench patterns  46  on the second photomask are positioned such that the x-directional trenches  412  and the x-directional trenches  420  defined by different exposure steps are arranged alternately in the one half of the first region  404 , the x-directional trenches  414  and the x-directional trenches  422  defined by different exposure steps are arranged alternately in the one half of the second region  406 , the y-directional trenches  416  and the y-directional trenches  424  defined by different exposure steps are arranged alternately in the one half of the third region  408 , and the y-directional trenches  418  and the y-directional trenches  426  defined by different exposure steps are arranged alternately in the one half of the fourth region  410 . 
     The distance between a trench  420  ( 422 ,  424  or  426 ) defined by the second lithography process and the trench  412  ( 414 ,  416  or  418 ) defined by the first one to be closest to the former may also be about ⅓ of the width of one linear pattern in a conventional AIM-type overlay mark. That is, the width of a trench  412 ,  414 ,  416 ,  418 ,  420 ,  422 ,  424  or  426  may be substantially equal to the distance between a pair of trenches  412  and  420  ( 414  and  422 ,  416  and  424 , or  418  and  426 ) that are closest to each other. 
     After an upper layer (not shown) is formed over the wafer, a lithography process is conducted using a third photomask that has die patterns (not shown) and an overlay mark area  48  with line patterns  49  for forming the overlay mark. With the lithography process, the die patterns on the third photomask are transferred to the die areas (not shown) on the wafer, and a patterned photoresist layer as a part of the overlay mark is formed on the portion  402  of the lower layer. The patterned photoresist layer includes a first set of x-directional photoresist bars  444   a  arranged in the y-direction in the other half of the first region  404 , a second set of x-directional photoresist bars  444   b  arranged in the y-direction in the other half of the second region  406 , a first set of y-directional photoresist bars  444   c  arranged in the x-direction in the other half of the third region  408 , and a second set of y-directional photoresist bars  444   d  arranged in the x-direction in the other half of the fourth region  410 . The width of a y- or x-directional photoresist bar may be about 1 W and the pitch between the photoresist bars about 2 W, as in a conventional AIM-type overlay mark. 
     The trench patterns  42  and  46  and the line patterns  49  on the above first to third photomasks are arranged such that when the lower layer is fully aligned with the upper layer, the following relationships are made. First, the central line of respective central lines of the x-directional trenches  428  and the x-directional trenches  432 , that of respective central lines of the x-directional trenches  430  and the x-directional trenches  434  and that of respective central lines of the x-directional photoresist bars  444   a  and x-directional photoresist bars  444   b  coincide with each other. Second, the central line of respective central lines of the y-directional trenches  436  and the y-directional trenches  440 , that of respective central lines of the y-directional trenches  438  and the y-directional trenches  442  and that of respective central lines of the y-directional photoresist bars  444   c  and the y-directional photoresist bars  444   d  coincide with each other. 
     Moreover, the trench patterns  42  and  46  and the line patterns  49  on the above first to third photomasks are preferably configured such that the set of x-directional trenches  428  or  430 , the set of y-directional trenches  436  or  438 , the set of x-directional trenches  432  or  434  and the set of y-directional trenches  440  or  442  are not arranged adjacently but the set of x-directional photoresist bars  444   a , the set of y-directional photoresist bars  444   c , the set of x-directional bars  444   b  and the set of y-directional photoresist bars  444   d  are arranged adjacently. 
     Further, the lithography process using the first photomask and that using the second photomask can alternatively be conducted in the reverse order without changing the patterns of the overlay mark. 
     Referring to  FIG. 5 , to check the alignment between the lower layer and the upper layer, the y-coordinate “y 1a ” of the central line of the x-directional trenches  428 , the y-coordinate “y 2a ” of the central line of the x-directional trenches  430 , the y-coordinate “y 1b ” of the central line of the x-directional trenches  432 , the y-coordinate “y 2b ” of the central line of the x-directional trenches  434 , the x-coordinate “x 1a ” of the central line of the y-directional trenches  436 , the x-coordinate “x 2a ” of the central line of the y-directional trenches  438 , the x-coordinate “x 1b ” of the central line of the y-directional trenches  440 , the x-coordinate “x 2b ” of the central line of the y-directional trenches  442 , the y-coordinate “y 3a ” of the central line of the x-directional photoresist bars  444   a , the y-coordinate “y 3b ” of the central line of the x-directional photoresist bars  444   b , the x-coordinate “x 3a ” of the central line of the y-directional photoresist bars  444   c  and the x-coordinate “x 3b ” of the central line of the y-directional photoresist bars  444   d  are derived. The method of deriving the above x-coordinates and y-coordinates may be the one illustrated in  FIG. 6 , wherein the derivation of x 1a  and x 2a  is exemplified. 
     Referring to  FIG. 6 , respective x-coordinates of the trenches  436  and  438 , x 1a1 -x 1a6  and x 2a1 -x 2a6 , are determined, possibly by scanning the trenches with a light beam in the direction  600  and simultaneously detecting the reflected light and then analyzing the variation of the reflectivity. When the lower layer having the trenches  436  and  438  therein is a dielectric layer and the upper layer is a conductive layer, or when the lower layer is a conductive layer and the upper layer is a dielectric layer, for example, a trench in the lower layer lowers the reflectivity so that the reflectivity variation has a profile  602 , from which the coordinates x 1a1 -x 1a6  and x 2a1 -x 2a6  can be determined by locating the locally minimal points thereof. Thereafter, x 1a  is calculated as the average of the six x-coordinates of the six locally minimal points of the curve  602  corresponding to the six trenches  436 , and x 2a  is calculated as the average of the six x-coordinates of the six locally minimal points corresponding to the six trenches  438 . 
     After the coordinate y 1a , y 2a , y 1b , y 2b , x 1a , x 2a , x 1b , x 2b , y 3a , y 3b , x 3a  and x 3b  are derived similarly, the x-directional alignment error of the die patterns of the upper layer with the die patterns of the lower layer defined by the first exposure step is calculated as “(x 3a +x 3b )/2−(x 1a +x 1b )/2”, and the y-directional alignment error of the same is calculated as “(y 3a +y 3b )/2−(y 1a +y 1b )/2”. The x-directional alignment error of the die patterns of the upper layer with the die patterns of the lower layer defined by the second exposure step is calculated as “(x 3a +x 3b )/2−(x 2a +x 2b )/2”, and the y-directional alignment error of the same is calculated as “(y 3a +y 3b )/2−(y 2a +y 2b )/2”. In addition, the x-directional alignment error of the die patterns of the lower layer defined by the second exposure step with those defined by the first exposure step is calculated as “(x 2a +x 2b )/2−(x 1a +x 1b )/2”, and the y-directional alignment error of the same is calculated as “(y 2a +y 2b )/2−(y 1a +y 1b )/2”. 
     Since two sets of x-directional trenches and two sets of y-directional trenches defined by the first exposure step as well as two sets of x-directional trenches and two sets of y-directional trenches defined by the second exposure step are dispose in one overlay mark of the first embodiment, the x-directional alignment and y-directional alignment of the die patterns defined by the first exposure step as well as those of the die patterns defined by the second exposure step with the upper layer can be checked based on only one overlay mark. The x-directional and y-directional alignments of the die patterns defined by the first exposure step with those defined by the second exposure step can also be checked based on the same overlay mark. 
     Second Embodiment 
     Embodiment 2.1 
       FIGS. 7A-7C  illustrate a first method of forming an overlay mark according to the second embodiment of this invention, wherein FIG.  7 C(b) shows the overlay mark. 
     Referring to  FIG. 7A , after a positive photoresist layer  702  is formed on a lower layer that is on a substrate  700  and includes a portion  701  for forming the overlay mark, an X-dipole exposure step is conducted using a first photomask that has die patterns (not shown) and an overlay mark area  70  with x-directionally separated y-directional line patterns  72  and light-blocking layers  73 . Through the X-dipole exposure step, the die patterns on the first photomask are transferred to the photoresist layer  702  in the die areas (not shown), and linear unexposed regions corresponding to the line patterns  72  are formed in the photoresist layer  702  on the portion  701  of the lower layer in the overlay mark area on the wafer. 
     The overlay mark area on the wafer includes four regions arranged in a 2×2 array that include a first region  704  and a second region  706  arranged diagonally and a third region  708  and a fourth region  710  arranged diagonally. The linear unexposed regions corresponding to the line patterns  72  include a first set of y-directional linear unexposed regions  712  arranged in the x-direction in one half of the third region  708  and a second set of y-directional linear unexposed regions  714  arranged in the x-direction in one half of the fourth region  710 . Unexposed regions  715  are also formed occupying the whole first region  704  and the whole second region  706  because of the light-blocking layers  73 . The line patterns  72  corresponding to the linear unexposed regions in each of the above sets are equally spaced. 
     Referring to  FIG. 7B , a Y-dipole exposure step is conducted using a second photomask that has die patterns (not shown) and an overlay mark area  74  with y-directionally separated x-directional line patterns  76  and light-blocking layers  77 . Through the Y-dipole exposure step, the die patterns on the second photomask are transferred to the photoresist layer  702  in the die areas (not shown), and linear unexposed regions corresponding to the line patterns  76  are formed in the photoresist layer  702  on the portion  701  of the lower layer. The linear unexposed regions include a first set of x-directional linear unexposed regions  716  arranged in the y-direction in one half of the first region  704  and a second set of x-directional linear unexposed regions  718  arranged in the y-direction in one half of the second region  706 . The linear unexposed regions  712  and  714  formed previously in the X-dipole exposure step are masked by the light-blocking layers  77  in the Y-dipole exposure step. It is noted that the photoresist layer  702  on the portion  701  of the lower layer except all the linear unexposed regions constitutes an exposed region. The line patterns  76  corresponding to the linear unexposed regions in each of the above sets are also equally spaced. 
     Referring to  FIG. 7C , since the photoresist material in the unexposed regions  712 ,  714 ,  716  and  718  is retained together with that in the unexposed regions in the die areas (not shown) in the subsequent development, the portion  701  of the lower layer is patterned, together with the lower layer in the die areas, into corresponding line patterns in the subsequent etching process for patterning the lower layer. The line patterns include a first set of x-directional line patterns  701   a  arranged in the y-direction in the one half of the first region  704  and defined by the Y-dipole exposure step, a second set of x-directional line patterns  701   b  arranged in the y-direction in the one half of the second region  706  and defined by the Y-dipole exposure step, a first set of y-directional line patterns  701   c  arranged in the x-direction in the one half of the third region  708  and defined by the X-dipole exposure step, and a second set of y-directional line patterns  701   d  arranged in the x-direction in the one half of the fourth region  710  and defined by the X-dipole exposure step. 
     After an upper layer (not shown) is formed over the wafer, a lithography process is conducted using a third photomask that has die patterns (not shown) and an overlay mark area  78  with line patterns  79 . With the lithography process, the die patterns on the third photomask are transferred to die areas on the wafer, and a patterned photoresist layer as a part of the overlay mark is formed within the overlay mark area on the wafer. The patterned photoresist layer includes a first set of x-directional photoresist bars  720   a  arranged in the y-direction in the other half of the first region  704 , a second set of x-directional photoresist bars  720   b  arranged in the y-direction in the other half of the second region  706 , a first set of y-directional photoresist bars  720   c  arranged in the x-direction in the other half of the third region  708 , and a second set of y-directional bars  720   d  arranged in the x-direction in the other half of the fourth region  710 . 
     It is noted that in the X-dipole exposure step, not only the region for forming the first set of x-directional linear unexposed regions  716  and the region for forming the second set of x-directional linear unexposed regions  718  but also the region for forming the first set of x-directional photoresist bars  720   a  and the region for forming the second set of x-directional photoresist bars  720   b  are masked by the light-blocking layers  73 , while the region for forming the first set of y-directional photoresist bars  702   c  and the region for forming the second set of y-directional photoresist bars  720   d  are not masked. In the Y-dipole exposure step, not only the region for forming the first set of y-directional linear unexposed regions  712  and the region for forming the second set of y-directional linear unexposed regions  714  but also the region for forming the first set of y-directional photoresist bars  720   c  and the region for forming the second set of y-directional photoresist bars  720   d  are masked by the light-blocking layers  77 , while the region for forming the first set of x-directional photoresist bars  720   a  and the region for forming the second set of x-directional photoresist bars  720   b  are not masked. 
     Moreover, it is preferred that the first set of x-directional line patterns  701   a , the first set of y-directional line patterns  701   c , the second set of x-directional line patterns  701   b  and the second set of y-directional line patterns  701   d  are not arranged adjacently and the first set of x-directional photoresist bars  720   a , the first set of y-directional photoresist bars  720   c , the second set of x-directional photoresist bars  720   b  and the second set of y-directional photoresist bars  720   d  are arranged adjacently. 
     The line patterns  72 ,  76  and  79  on the first to third photomasks are arranged such that when the patterns of the lower layer defined by the X-dipole exposure step and those defined by the Y-dipole exposure step are aligned with the upper layer in the x-direction and in the y-direction, respectively, the following relationships are satisfied. First, the central line of respective central lines of the x-directional line patterns  701   a  and the x-directional line patterns  701   b  and that of respective central lines of the x-directional photoresist bars  720   a  and the x-directional photoresist bars  720   b  coincide with each other. Second, the central line of respective central lines of the y-directional line patterns  701   c  and the y-directional line patterns  701   d  and that of respective central lines of the y-directional photoresist bars  720   c  and the y-directional photoresist bars  720   d  coincide with each other. 
     Accordingly, the y-directional alignment error of the patterns of the upper layer with those of the lower layer defined by the second exposure step can be derived as the y-directional shift of the central line of respective central lines of the bars  720   a  and the bars  720   b  relative to that of respective central lines of the line patterns  701   a  and the line patterns  701   b . The x-directional alignment error of the patterns of the upper layer with those of the lower layer defined by the first exposure step is derived as the x-directional shift of the central line of respective central lines of the bars  720   c  and the bars  720   d  relative to that of respective central lines of the line patterns  701   c  and the line patterns  701   d.    
     In a preferred case of the second embodiment, one line pattern  701   a/b/c /d and one photoresist bar  720   a/b/c /d have substantially the same width, and the pitch between the line patterns  701   a/b/c /d or the photoresist bars  720   a/b/c /d is about two times the width of one line pattern  701   a/b/c /d or one photoresist bar  720   a/b/c /d in each set of the line patterns or the photoresist bars, as in a conventional AIM-type overlay mark. 
     In addition, though the X-dipole exposure step is conducted before the Y-dipole exposure step in the above embodiment, it is also possible to alternatively conduct the Y-dipole exposure step before the X-dipole exposure step. This is also true for the following three embodiments 2.2-2.4. 
     Embodiment 2.2 
       FIGS. 8A-8C  illustrate a second method of forming an overlay mark according to the second embodiment of this invention, wherein FIG.  8 C(b) shows the overlay mark. 
     Referring to  FIG. 8A , after a positive photoresist layer  802  is formed on a lower layer that is on a substrate  800  and includes a portion  801  for forming the overlay mark, an X-dipole exposure step is conducted using a first photomask that has die patterns (not shown) and an overlay mark area  80  with x-directionally separated y-directional line patterns  82  and light-blocking layers  83 . Through the X-dipole exposure step, the die patterns on the first photomask are transferred to the photoresist layer  802  in the die areas (not shown), and linear unexposed regions corresponding to the line patterns  82  are formed in the photoresist layer  802  on the portion  801  of the lower layer in the overlay mark area on the wafer. 
     The overlay mark area on the wafer includes four regions arranged in a 2×2 array that include a first region  804  and a second region  806  arranged diagonally and a third region  808  and a fourth region  810  arranged diagonally. The linear unexposed regions corresponding to the line patterns  82  include a first set of y-directional linear unexposed regions  812  arranged in the x-direction in one half of the third region  808  and a second set of y-directional linear unexposed regions  814  arranged in the x-direction in one half of the fourth region  810 . Unexposed regions  815  are also formed occupying a half of the first region  804  and a half of the second region  806  for forming x-directional linear unexposed regions later because of the light-blocking layers  83 . The line patterns  82  corresponding to the unexposed regions in each of the above sets are equally spaced. 
     Referring to  FIG. 8B , a Y-dipole exposure step is conducted using a second photomask that has die patterns (not shown) and an overlay mark area  84  with y-directionally separated x-directional line patterns  86  and light-blocking layers  87 . Through the Y-dipole exposure step, the die patterns on the second photomask are transferred to the photoresist layer  802  in the die areas (not shown), and linear unexposed regions corresponding to the line patterns  86  are formed in the photoresist layer  802  on the portion  801  of the lower layer. The linear unexposed regions include a first set of x-directional linear unexposed regions  816  arranged in the y-direction in one half of the first region  804  and a second set of x-directional linear unexposed regions  818  arranged in the y-direction in one half of the second region  806 . The linear unexposed regions  812  and  814  formed previously are screened by the light-blocking layers  87  in the Y-dipole exposure step. The line patterns  86  corresponding to the linear unexposed regions in each of the above sets are also equally spaced. 
     Referring to  FIG. 8C , since the photoresist material in the unexposed regions  812 ,  814 ,  816  and  818  is retained together with that in the unexposed regions in the die areas (not shown) in the subsequent development, the portion  801  of the lower layer is patterned, together with the lower layer in the die areas, into corresponding line patterns in the subsequent etching process of the lower layer. The line patterns include a first set of x-directional line patterns  801   a  arranged in the y-direction in the one half of the first region  804  and defined by the Y-dipole exposure step, a second set of x-directional line patterns  801   b  arranged in the y-direction in the one half of the second region  806  and defined by the Y-dipole exposure step, a first set of y-directional line patterns  801   c  arranged in the x-direction in the one half of the third region  808  and defined by the X-dipole exposure step, and a second set of y-directional line patterns  801   d  arranged in the x-direction in the one half of the fourth region  810  and defined by the X-dipole exposure step. 
     After an upper layer (not shown) is formed over the wafer, a lithography process is conducted using a third photomask that has die patterns (not shown) and an overlay mark area  88  with line patterns  89 . With the lithography process, the die patterns on the third photomask are transferred to die areas on the wafer, and a patterned photoresist layer as a part of the overlay mark is formed within the overlay mark area on the wafer. The patterned photoresist layer includes a first set of x-directional photoresist bars  820   a  arranged in the y-direction in the other half of the first region  804 , a second set of x-directional photoresist bars  820   b  arranged in the y-direction in the other half of the second region  806 , a first set of y-directional photoresist bars  820   c  arranged in the x-direction in the other half of the third region  808 , and a second set of y-directional bars  820   d  arranged in the x-direction in the other half of the fourth region  810 . 
     It is noted that in the X-dipole exposure step, only the region for forming the first set of x-directional linear unexposed regions  816  and the region for forming the second set of x-directional linear unexposed regions  818  are entirely masked. In the Y-dipole exposure step, only the region for forming the first set of y-directional linear unexposed regions  812  and the region for forming the second set of y-directional linear unexposed regions  814  are masked. The regions for forming the x-directional and y-directional photoresist bars  820   a ,  820   b ,  820   c  and  820   d  are all exposed through the X-dipole exposure step and the Y-dipole exposure step. 
     Moreover, it is preferred that the first set of x-directional line patterns  801   a , the first set of y-directional line patterns  801   c , the second set of x-directional line patterns  801   b  and the second set of y-directional line patterns  801   d  are not arranged adjacently and the first set of x-directional photoresist bars  820   a , the first set of y-directional photoresist bars  820   c , the second set of x-directional photoresist bars  820   b  and the second set of y-directional photoresist bars  820   d  are arranged adjacently. 
     The line patterns  82 ,  86  and  89  on the first to third photomasks are arranged as in the case of the line patterns  72 ,  76  and  79  in  FIGS. 7A-7C , and are thus not described here for their arrangement. Meanwhile, the alignment error of the patterns of the upper layer with those of the lower layer can be derived as in the case of Embodiment 2.1, and the preferred width of one line pattern and the preferred pitch of parallel line patterns can be the same as in Embodiment 2.1. 
     Embodiment 2.3 
       FIGS. 9A-9C  illustrate a third method of forming an overlay mark according to the second embodiment of this invention, wherein FIG.  9 C(b) shows the overlay mark. 
     Referring to  FIG. 9A , after a positive photoresist layer  902  is formed on a lower layer that is on a substrate  900  and includes a portion  901  for forming the overlay mark, an X-dipole exposure step is conducted using a first photomask that has die patterns (not shown) and an overlay mark area  90  with x-directionally separated y-directional trench patterns  92  in a light-blocking layer  93 . Through the X-dipole exposure step, the die patterns on the first photomask are transferred to the photoresist layer  902  in the die areas (not shown), and linear exposed regions corresponding to the trench patterns  92  are formed in the photoresist layer  902  on the portion  901  of the lower layer in the overlay mark area on the wafer. 
     The overlay mark area on the wafer includes four regions arranged in a 2×2 array that include a first region  904  and a second region  906  arranged diagonally and a third region  908  and a fourth region  910  arranged diagonally. The linear exposed regions corresponding to the trench patterns  92  include a first set of y-directional linear exposed regions  912  arranged in the x-direction in one half of the third region  908  and a second set of y-directional linear exposed regions  914  arranged in the x-direction in one half of the fourth region  910 . The rest of the photoresist layer  902  on the portion  901  of the lower layer constitutes an unexposed region. The trench patterns  92  corresponding to the linear exposed regions in each of the above sets are equally spaced. 
     Referring to  FIG. 9B , a Y-dipole exposure step is conducted using a second photomask that has die patterns (not shown) and an overlay mark area  94  with y-directionally separated x-directional trench patterns  96  in a light-blocking layer  97 . Through the Y-dipole exposure step, the die patterns on the second photomask are transferred to the photoresist layer  902  in the die areas (not shown), and linear exposed regions corresponding to the trench patterns  96  are formed in the photoresist layer  902  on the portion  901  of the lower layer. The linear exposed regions include a first set of x-directional linear exposed regions  916  arranged in the y-direction in one half of the first region  904  and a second set of x-directional linear exposed regions  918  arranged in the y-direction in one half of the second region  906 . The regions for forming the linear exposed regions  912  and  914  formed previously in the X-dipole exposure step are masked by the light-blocking layer  97  in the Y-dipole exposure step. The trench patterns  96  corresponding to the linear exposed regions in each of the above sets are also equally spaced. 
     Referring to  FIG. 9C , since the photoresist material in the exposed regions  912 ,  914 ,  916  and  918  is removed together with that in the exposed regions in the die areas (not shown) in the subsequent development, the portion  901  of the lower layer is patterned, together with the lower layer in the die areas, to form corresponding trenches therein in the subsequent etching process of the lower layer. The trenches include a first set of x-directional trenches  903   a  arranged in the y-direction in the one half of the first region  904  and defined by the Y-dipole exposure step, a second set of x-directional trenches  903   b  arranged in the y-direction in the one half of the second region  906  and defined by the Y-dipole exposure step, a first set of y-directional trenches  903   c  arranged in the x-direction in the one half of the third region  908  and defined by the X-dipole exposure step, and a second set of y-directional trenches  903   d  arranged in the x-direction in the one half of the fourth region  910  and defined by the X-dipole exposure step. 
     After an upper layer (not shown) is formed over the wafer, a lithography process is conducted using a third photomask that has die patterns (not shown) and an overlay mark area  98  with line patterns  99 . With the lithography process, the die patterns on the third photomask are transferred to die areas on the wafer, and a patterned photoresist layer as a part of the overlay mark is formed on the portion  901  of the lower layer. The patterned photoresist layer includes a first set of x-directional photoresist bars  920   a  arranged in the y-direction in the other half of the first region  904 , a second set of x-directional photoresist bars  920   b  arranged in the y-direction in the other half of the second region  906 , a first set of y-directional photoresist bars  920   c  arranged in the x-direction in the other half of the third region  908 , and a second set of y-directional bars  920   d  arranged in the x-direction in the other half of the fourth region  910 . 
     It is noted that in the X-dipole exposure step, not only the region for forming the first set of x-directional linear exposed regions  916  and the region for forming the second set of x-directional linear exposed regions  918  but also all the regions for forming the x-directional and y-directional photoresist bars  920   a ,  920   b ,  920   c  and  920   d  are masked by the light-blocking layer  93 . In the Y-dipole exposure step, not only the region for forming the first set of y-directional linear exposed regions  912  and the region for forming the second set of y-directional linear exposed regions  914  but also all the regions for forming the x-directional and y-directional photoresist bars  920   a ,  920   b ,  920   c  and  920   d  are masked by the light-blocking layer  97 . Accordingly, the regions for forming the x-directional and y-directional photoresist bars  920   a ,  920   b ,  920   c  and  920   d  are all masked through the X-dipole exposure step and the Y-dipole exposure step. 
     Moreover, it is preferred that the first set of x-directional trenches  903   a , the first set of y-directional trenches  903   c , the second set of x-directional trenches  903   b  and the second set of y-directional trenches  903   d  are not arranged adjacently and the first set of x-directional photoresist bars  920   a , the first set of y-directional photoresist bars  920   c , the second set of x-directional photoresist bars  920   b  and the second set of y-directional photoresist bars  920   d  are arranged adjacently. 
     The trench patterns  92  and  96  and the line patterns  99  on the above first to third photomasks are arranged similar to the case of the line patterns  72  and  76  and the line patterns  79  in  FIGS. 7A-7C , and are thus not described here for their arrangement. Meanwhile, the alignment error of the patterns of the upper layer with those of the lower layer can be derived by a method analogous to that described in Embodiment 2.1 and is different from the latter only in that the position measurement about the lower layer is done to trenches rather than line patterns. The preferred width of one trench and the preferred pitch of parallel trenches can be substantially the same as the preferred width of one line pattern and the preferred pitch of parallel line patterns in Embodiment 2.1. 
     Embodiment 2.4 
       FIGS. 10A-10C  illustrate a fourth method of forming an overlay mark according to the 2 nd  embodiment of this invention, wherein FIG.  10 C(b) shows the overlay mark. 
     Referring to  FIG. 10A , after a positive photoresist layer  1002  is formed on a lower layer that is on a substrate  1000  and includes a portion  1001  for forming the overlay mark, an X-dipole exposure step is conducted using a first photomask that has die patterns (not shown) and an overlay mark area  130  with light-blocking layers  132  and  136  and x-directionally separated y-directional trench patterns  134  in the light-blocking layers  132 . Through the X-dipole exposure step, the die patterns on the first photomask are transferred to the photoresist layer  1002  in the die areas (not shown), and unexposed regions corresponding to the light-blocking layers  132  and  136  as well as linear exposed regions, which correspond to the trench patterns  134  and are surrounded by unexposed regions, are formed in the photoresist layer  1002  on the portion  1001  of the lower layer in the overlay mark area on the wafer. 
     The overlay mark area on the wafer includes four regions arranged in a 2×2 array that include a first region  1004  and a second region  1006  arranged diagonally and a third region  1008  and a fourth region  1010  arranged diagonally. The unexposed regions  1015  corresponding to the light-blocking layers  136  are in the first region  1004  and the second region  1006 , reserved for the formation of x-directional exposed regions later. The unexposed regions  1013  corresponding to the light-blocking layers  132  are in the third region  1008  and the fourth region  1010 , surrounding linear exposed regions that correspond to the trench patterns  134  and include a first set of y-directional linear exposed regions  1012  arranged in the x-direction in one half of the third region  1008  and a second set of y-directional linear exposed regions  1014  arranged in the x-direction in one half of the fourth region  1010 . The rest of the photoresist layer  1002  on the portion  1001  of the lower layer constitutes a pinwheel-like exposed region. The trench patterns  134  corresponding to the linear exposed regions in each of the above sets are equally spaced. 
     Referring to  FIG. 10B , a Y-dipole exposure step is conducted using a second photomask that has die patterns (not shown) and an overlay mark area  140  with light-blocking layers  142  and  146  and y-directionally separated x-directional trench patterns  144  in the light-blocking layers  142 . Through the Y-dipole exposure step, the die patterns on the second photomask are transferred to the photoresist layer  1002  in the die areas (not shown), and linear exposed regions corresponding to the trench patterns  144  are formed in the previously unexposed regions  1015  in the photoresist layer  1002  on the portion  1001  of the lower layer. The linear exposed regions include a first set of x-directional linear exposed regions  1016  arranged in the y-direction in one half of the first region  1004  and a second set of x-directional linear exposed regions  1018  arranged in the y-direction in one half of the second region  1006 . The regions for forming the linear exposed regions  1012  and  1014  formed previously in the X-dipole exposure step are masked by the light-blocking layers  146  in the Y-dipole exposure step. The trench patterns  144  corresponding to the linear exposed regions in each of the above sets are also equally spaced. 
     Referring to  FIG. 10C , since the photoresist material in the exposed regions  1012 ,  1014 ,  1016  and  1018  and the pinwheel-like exposed region of the photoresist layer  1002  on the portion  1001  of the lower layer is removed together with that in the exposed regions in the die areas (not shown) in the subsequent development, the portion  1001  of the lower layer is patterned, together with the lower layer in the die areas, into corresponding islands with trenches therein in the subsequent etching step for patterning the lower layer. The islands include a first island  1001   a  with a first set of x-directional trenches  1003   a  arranged in the y-direction and defined by the Y-dipole exposure step therein in the one half of the first region  1004 , a second island  1001   b  with a second set of x-directional trenches  1003   b  arranged in the y-direction and defined by the Y-dipole exposure step therein in the one half of the second region  1006 , a third island  1001   c  with a first set of y-directional trenches  1003   c  arranged in the x-direction and defined by the X-dipole exposure step therein in the one half of the third region  1008 , and a fourth island  1001   d  with a second set of y-directional trenches  1003   d  arranged in the x-direction and defined by the X-dipole exposure step therein in the one half of the fourth region  1010 . 
     After an upper layer (not shown) is formed over the wafer, a lithography process is conducted using a third photomask that has die patterns (not shown) and an overlay mark area  150  with line patterns  152 . With the lithography process, the die patterns on the third photomask are transferred to die areas on the wafer, and a patterned photoresist layer as a part of the overlay mark is formed on the substrate  1000  between the islands  1001   a ,  1001   b ,  1001   c  and  1001   d . The patterned photoresist layer includes a first set of x-directional photoresist bars  1020   a  arranged in the y-direction in the other half of the first region  1004 , a second set of x-directional photoresist bars  1020   b  arranged in the y-direction in the other half of the second region  1006 , a first set of y-directional photoresist bars  1020   c  arranged in the x-direction in the other half of the third region  1008 , and a second set of y-directional bars  1020   d  arranged in the x-direction in the other half of the fourth region  1010 . 
     It is noted that in the X-dipole exposure step, only the region for forming the first set of x-directional linear exposed regions  1016  and the region for forming the second set of x-directional linear exposed regions  1018  are entirely masked, while all the regions for forming the x-directional and y-directional photoresist bars  1020   a ,  1020   b ,  1020   c  and  1020   d  are exposed. In the Y-dipole exposure step, only the region for forming the first set of y-directional linear exposed regions  1012  and the region for forming the second set of y-directional linear exposed regions  1014  are entirely masked, while all the regions for forming the x-directional and y-directional photoresist bars  1020   a ,  1020   b ,  1020   c  and  1020   d  are exposed. Accordingly, all the regions for forming the x-directional and y-directional photoresist bars  1020   a ,  1020   b ,  1020   c  and  1020   d  are exposed through the X-dipole exposure step and the Y-dipole exposure step. 
     Moreover, it is preferred that the first set of x-directional trenches  1003   a , the first set of y-directional trenches  1003   c , the second set of x-directional trenches  1003   b  and the second set of y-directional trenches  1003   d  are not arranged adjacently and the first set of x-directional photoresist bars  1020   a , the first set of y-directional photoresist bars  1020   c , the second set of x-directional photoresist bars  1020   b  and the second set of y-directional photoresist bars  1020   d  are arranged adjacently. 
     The trench patterns  134  and  144  and the line patterns  152  on the above first to third photomasks are arranged similar to the case of the line patterns  72  and  76  and the line patterns  79  in  FIGS. 7A-7C , and are thus not described here for their arrangement. Meanwhile, the alignment error of the patterns of the upper layer with those of the lower layer can be derived by a method analogous to that described in Embodiment 2.1 and is different from the latter only in that the position measurement about the lower layer is done to trenches rather than line patterns. The preferred width of one trench and the preferred pitch of parallel trenches can be substantially the same as the preferred width of one line pattern and the preferred pitch of parallel line patterns respectively in Embodiment 2.1. 
       FIG. 11A  illustrates an example of die patterns suitably defined by X-dipole and Y-dipole double exposure and checked with an overlay mark of the second embodiment, and FIG.  11 B/C illustrates the photomask patterns for the X-/Y-dipole exposure step. 
     Referring to  FIG. 11A , the patterns in this example are conductive line patterns  1100  including patterns  1102  requiring a higher resolution in the Y-direction and therefore suitably defined by Y-dipole exposure, and patterns  1104  requiring a higher resolution in the X-direction and therefore suitably defined with X-dipole exposure. The corresponding upper layer in this example is a dielectric layer (not shown) covering the patterns  1100 , and the patterns of the upper layer are contact hole patterns. The lithography process including the X-dipole and Y-dipole double exposure process uses a positive photoresist layer. 
     Referring to  FIG. 11B , the X-dipole off-axis light source  10  has two illumination regions  12  arranged in the X-direction with the central axis as the center of symmetry. The corresponding photomask includes patterns  1120  corresponding to the patterns  1104  to be defined in the die area and a pattern  1140  for screening the portion of the die area for forming the patterns  1102 . 
     Referring to  FIG. 11C , the Y-dipole off-axis light source  14  has two illumination regions  16  arranged in the Y-direction with the central axis as the center of symmetry. The corresponding photomask includes patterns  1160  corresponding to the patterns  1102  to be defined in the die area and patterns  1180  for screening the portions of the die area for forming the patterns  1104 . The two photomasks are respectively used in the two exposure steps, and each of them is further formed, in an area thereof corresponding to a non-die area of the wafer, with patterns for defining a portion of the lower layer to form a part of the overlay mark of the second embodiment of this invention. 
     In a case where the overlay mark of FIG.  7 C(b) is to be formed, the patterns for forming the overlay mark on the photomask having the die patterns  1120  and  1140  in  FIG. 11B  have to be those shown in the left of  FIG. 7A , for y-directionally separated linear patterns can&#39;t be correctly transferred by X-dipole exposure. On the other hand, the patterns for forming the overlay mark on the photomask having the die patterns  1160  and  1180  in  FIG. 11C  have to be those in the left of  FIG. 7B , for x-directionally separated linear patterns cannot be correctly transferred by Y-dipole exposure. 
     Thereby, the x-directional alignment between the contact pads of the patterns  1104  and corresponding contact holes in the upper layer can be checked based on the x-coordinates of the line patterns  701   c  and  701   d  and the photomask bars  720   c  and  720   d , as mentioned above. Similarly, the y-directional alignment between the contact pads of the patterns  1102  and corresponding contact holes in the upper layer can be checked based on the y-coordinates of the line patterns  701   a  and  701   b  and the photomask bars  720   a  and  720   b.    
     Since y-directionally separated x-directional linear patterns are well transferred with Y-dipole off-axis exposure light and x-directionally separated y-directional linear patterns are well transferred with X-dipole off-axis exposure light, the whole area of the overlay mark can be utilized in the alignment check. The x-directional alignment of the die patterns defined by the X-dipole exposure step and the y-directional alignment of those defined by the Y-dipole exposure step with the die patterns of the upper layer can be checked based on only one overlay mark. 
     Since the linear patterns defined by the first exposure step and those defined by the second exposure step are disposed together in one overlay mark of any of the above embodiments, the wafer area required for forming the overlay marks can be reduced by one half as the alignment accuracy between a lower layer defined by two exposure steps and an upper layer is to be checked. 
     This invention has been disclosed above in the preferred embodiments, but is not limited to those. It is known to persons skilled in the art that some modifications and innovations may be made without departing from the spirit and scope of this invention. Hence, the scope of this invention should be defined by the following claims.