it is noted that the following embodiments are intended to further explain this invention but not to restrict the scope of this invention . fig4 a - 4c illustrate a method of forming an overlay mark according to the first embodiment of this invention , wherein fig4 c ( b ) illustrates the overlay mark . referring to fig4 a , 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 fig4 b , 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 fig5 , 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 fig6 , wherein the derivation of x 1a and x 2a is exemplified . referring to fig6 , 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 . fig7 a - 7c illustrate a first method of forming an overlay mark according to the second embodiment of this invention , wherein fig7 c ( b ) shows the overlay mark . referring to fig7 a , 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 fig7 b , 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 fig7 c , 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 . fig8 a - 8c illustrate a second method of forming an overlay mark according to the second embodiment of this invention , wherein fig8 c ( b ) shows the overlay mark . referring to fig8 a , 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 fig8 b , 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 fig8 c , 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 fig7 a - 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 . fig9 a - 9c illustrate a third method of forming an overlay mark according to the second embodiment of this invention , wherein fig9 c ( b ) shows the overlay mark . referring to fig9 a , 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 fig9 b , 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 fig9 c , 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 fig7 a - 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 . fig1 a - 10c illustrate a fourth method of forming an overlay mark according to the 2 nd embodiment of this invention , wherein fig1 c ( b ) shows the overlay mark . referring to fig1 a , 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 fig1 b , 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 fig1 c , 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 fig7 a - 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 . fig1 a 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 fig1 b / c illustrates the photomask patterns for the x -/ y - dipole exposure step . referring to fig1 a , 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 fig1 b , 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 fig1 c , 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 fig7 c ( b ) is to be formed , the patterns for forming the overlay mark on the photomask having the die patterns 1120 and 1140 in fig1 b have to be those shown in the left of fig7 a , 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 fig1 c have to be those in the left of fig7 b , 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 .