Abstract:
A method is provided for fabricating a photolithography alignment mark structure. The method includes providing a substrate; forming a first grating, a second grating, a third grating and a fourth rating in the substrate; forming a photoresist layer on a surface of the substrate; obtaining a first alignment center along a first direction and a second alignment center alone a second direction based on the first grating and the fourth grating, respectively; providing a mask plate having a fifth grating pattern and a sixth grating pattern; aligning the mask plate with the substrate by using the first alignment center as an alignment center along the first direction and the second alignment center as an alignment center along the second direction; reproducing the fifth grating pattern and the sixth grating pattern in the photoresist layer; and forming a fifth grating and a sixth grating on the substrate by removing a portion of photoresist layer.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Chinese patent application No. 201410505490.0, filed on Sep. 26, 2014, the entirety of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to the field of semiconductor manufacturing technology and, more particularly, relates to a photolithographic alignment mark structure and the fabricating process thereof. 
       BACKGROUND 
       [0003]    With the development of semiconductor technology, semiconductor chip area is getting smaller and smaller while the line width inside semiconductor chip is also shrinking. Therefore, semiconductor process capability is facing a growing challenge, the precision of the process and the control of process variations also became increasingly important. Among the processes for fabricating semiconductor chips, photolithography technique is one of the most important processes. Photolithography is a technological process to transfer mask patterns of a mask plate onto a wafer through a series of steps including alignment, exposure, development, etc. Therefore, the quality of the photolithography process directly affects the performance of the ultimately formed semiconductor chip. 
         [0004]    During photolithography process, to accurately transfer a mask pattern on a mask plate onto a wafer, a key step is to align the mask plate with the wafer, that is, to calculate the position of the mask plate with respect to the wafer to meet the requirements of registration accuracy. As the feature size getting smaller and smaller, the requirement of registration accuracy and thereby the requirement of alignment accuracy also becomes more and more strict. 
         [0005]    In current technology, there are two methods for performing photolithography alignment. One method is a through the lens (TTL) alignment technique: using a laser beam to light up an alignment mark on a mask plate and simultaneously imaging the alignment mark onto the surface of a wafer through an objective lens; then moving the wafer base station to let a reference mark on the wafer base station scan the image of the alignment mark; in the meantime, sampling the intensity of the image and finally reaching the correct alignment position when the detector receives a maximum intensity. The other method is an off-axis (OA) alignment technique: first, by using an off-axis alignment system to gauge multiple alignment marks on a wafer and reference marks on a reference plate located on a wafer base station, alignment between the wafer and the wafer base station is thus realized; then the reference marks on the wafer base station is aligned with the alignment marks on a mask plate so that the alignment between the mask plate and the wafer base station is also realized. As such, the relative position of the mask plate with respect to the wafer is determined and alignment between the mask plate and the wafer is then realized. 
         [0006]    According to the present disclosure, at present, most of the mainstream photolithography facilities use grating diffraction. Grating diffraction refers to that, when a light beam is illuminated on a grating type alignment mark on a wafer, the beam is then diffracted and the diffracted light carries all the information about the alignment mark. The multi-level diffracted light spread out from the grating alignment mark from different angles. After using a spatial filter to filter out the zeroth-level light, the interference image of the ±n levels of the diffracted light on the reference plane is collected. As the feature size is getting smaller and smaller, an interference image of more levels of the diffracted light on the reference plane may be collected. Further, using a corresponding reference grating to scan the image along a certain direction and the signal is simultaneously detected by a photoelectric detector. After signal processing, the position of the alignment center is then determined. The position of the alignment center may be defined in the coordinate system of the wafer base station. Then, the position of the alignment center is aligned with the alignment marks on the mask plate to realize the alignment between the mask plate and the wafer. 
         [0007]    Grating diffraction may be used in a double exposure type double patterning process. Referring to  FIG. 1 , a first grating  11  along the x-axis and a second grating  12  along the y-axis are formed in a substrate  1 . The first grating  11  is an alignment mark for the direction along the x-axis while the second grating is an alignment mark for the direction along the y-axis. Referring to  FIG. 2 , a device layer (not shown) is formed on the substrate  1  and then a photoresist layer  2  is formed on the surface of the device layer. Both the first grating  11  and the second grating  12  are covered by the photoresist layer  2  and cannot be seen from the top, thus they are represented by dashed lines. Referring to  FIG. 3 , using grating diffraction, the first alignment center x 0  along the x-axis direction is obtained based on the first grating  11  while the second alignment center y 0  along the y-axis direction is obtained based on the second grating  12 . Further, reference marks on a mask plate which contains a first device pattern  3  are then aligned with the first alignment center x 0  and the second alignment center y 0 , respectively. Afterwards, the photoresist layer  2  is exposed for the first time to define the first device pattern  3 . The first device pattern  3  includes a number of parallel and equally spaced first strip lines  31 . 
         [0008]    Referring to  FIG. 4 , reference marks on a mask plate which contains a second device pattern  4  are aligned with the first alignment center x 0  and the second alignment center y 0 , respectively. The second device pattern  4  includes a number of second strip lines  42  parallel to the first strip lines  31 . Every neighboring pair of first strip lines  31  correspond to a second strip line  42 , and all the first strip lines  31  and the second strip lines  42  are arranged in a staggered way and are spaced equally. Then the photoresist layer  2  is exposed for the second time to define the first device pattern  4 . Finally, the photoresist layer  2  is developed, and then the developed photoresist layer  2  is used as a mask to etch the device layer and thus form a semiconductor structure. The ultimately formed semiconductor structure includes a number of the first strip lines  31  and a number of the second strip lines  42 . 
         [0009]    However, referring to  FIG. 3  and  FIG. 4 , during the first exposure process, the position of the first device pattern  3 , with respect to the first alignment center x 0 , may have an overlay shift. Correspondingly during the second exposure process, the position of the second device pattern  4 , with respect to the second alignment center y 0 , may also have an overlay shift. Therefore, as shown in  FIG. 4 , the distances between each second strip line  42  and the neighboring two first strip lines  31  of the second strip line  42  may not be the same, i.e. w 1 ≠w 2 . As such, on one hand, the second device pattern  4  may have an overlay shift with respect to the first alignment center x 0 , thus the actual position of the second device pattern  4  on the substrate may also have an alignment error with respect to its intended position; on the other hand, due to the two overlay shifts, precise alignment between the first strip lines  31  and the second strip lines  42  may not be able to achieve, thus reducing the registration accuracy between the second strip lines  42  and the first strip lines  31 . All of the above factors further affect subsequent semiconductor fabrication processes and the performance of the semiconductor structure containing the second strip lines  42  and the first strip lines  31 . 
         [0010]    In view of the above problems, the present disclosure provides a new alignment strategy to reduce the alignment error and improve the performance of semiconductor structures formed by a double exposure type double patterning process using grating diffraction. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0011]    The present disclosure includes a method for forming a photolithography alignment mark structure. The method includes providing a semiconductor substrate; forming a first grating, a second grating, a third grating and a fourth grating in the substrate; and forming a photoresist layer on a surface of the substrate. The method also include obtaining a first alignment center along a first direction and a second alignment center along a second direction based on the first grating and the fourth grating, respectively, by using grating diffraction; providing a mask plate having a fifth grating pattern and a sixth grating pattern on the mask plate; aligning the mask plate with the substrate by using the first alignment center as an alignment center along the first direction and the second alignment center as an alignment center along the second direction. The method further includes reproducing the fifth grating pattern and the sixth grating pattern in the photoresist layer on the substrate through an exposure process; and forming a fifth grating and a sixth grating on the surface of the substrate by removing the portion of denatured photoresist layer. 
         [0012]    The present disclosure also includes a photolithography alignment mark structure. The photolithography alignment mark structure includes a substrate and a first grating, a second grating, a third grating, and a fourth grating formed in the substrate. The photolithography alignment mark structure also includes a fifth grating and a sixth grating formed on a surface of the substrate. The first grating, the second grating, and the third grating firmed in the substrate are along a first direction; the fourth grating formed in the substrate is along a second direction; the first direction and the second direction are perpendicular to each other; the fifth grating and the sixth grating formed on the mask plate are along the first direction; the grating constant of the first grating is smaller than the grating constant of the second grating; and the second grating, the third grating, the fifth grating, and the sixth grating have a same grating constant. 
         [0013]    The present disclosure also includes a method for fabricating semiconductor structures using a photolithography alignment mark structure. The method includes providing a semiconductor substrate having the photolithography alignment mark structure; forming a device layer on the surface of the substrate to cover the substrate and the photolithography alignment mark structure; and forming a photoresist layer on the surface of the device layer. The method also includes using grating diffraction to obtain a first alignment center x 0  along a first direction based on a first grating, a third alignment center x 1  along the first direction based on a second grating and a fifth grating, and a fourth alignment center x 2  along the first direction based on a third grating and a sixth grating; and performing a first exposure to define a first device pattern in the photoresist layer by using the first alignment center x 0  as an alignment center along the first direction for the alignment prior to the first exposure process. The first device pattern includes a number of parallel first strip lines along the first direction. The method further includes performing a second exposure to define a second device pattern in the photoresist layer by using x′=((x 1 +x 2 )/2+x 0 )/2 as the alignment center along the first direction for the alignment prior to the second exposure process, where the second device pattern includes a number of parallel second strip lines along the first direction, and the second strip lines are interlaced with the first strip lines. 
         [0014]    Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1-4  illustrate schematic top views of semiconductor structures corresponding to certain stages of a current double exposure type double patterning process; 
           [0016]      FIGS. 5-14  illustrate schematic views of photolithography alignment mark structures corresponding to certain stages of an exemplary fabrication process consistent with the disclosed embodiments; and 
           [0017]      FIG. 15  illustrates an exemplary fabrication process of a lithography alignment mark structure in one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0019]    Referring to  FIG. 1-4 , a detailed description of a representative double exposure type double patterning process of the prior art has been provided in above background section. In the existing double exposure type double patterning process, an overlay shift after each exposure process may be unavoidable and the two overlay shifts may further affect subsequent semiconductor fabrication processes and the performance of the ultimately formed semiconductor structure. 
         [0020]    In view of the problems described above, the disclosed embodiments provide a method to form a photolithography alignment mark structure for a double exposure type double patterning process, improving the alignment accuracy and the performance of the later-formed semiconductor structure. 
         [0021]      FIG. 15  illustrates an exemplary fabrication process of a lithography alignment mark structure in one embodiment of the present disclosure. As shown in  FIG. 15 , at the beginning of the fabrication process for the photolithography alignment mark structure, a substrate  10  is provided (S 1502 ).  FIG. 5  shows a top view of the substrate. 
         [0022]    In the one embodiment, the substrate  10  is made of silicon. In certain other embodiments, the substrate  10  may also be made of any other appropriate materials, such as germanium, silicon germanium, silicon on insulator (SIO), or germanium on insulator (GOI), etc. 
         [0023]    In one embodiment, the substrate  10  is placed on a base station or a wafer stage. The base station has an x-y coordinate system. The x-y coordinates are used to mark the position of alignment center. A first direction ‘A’ is defined as along the x-axis while a second direction ‘B’ is defined as along the y-axis. In addition, the base station also has a reference mark. The reference mark is used to locate the position of the substrate  10  so that the position of the substrate  10  on the base station may be determined. In the following, description of an overlay shift occurred along the first direction ‘A’ during a photolithography process sets an example to illustrate the technical scheme of the present embodiment. 
         [0024]    Referring to  FIG. 5 , a first grating  11 , a second grating  12 , and a third grating  13  are formed in the substrate  10  along the first direction ‘A’ (S 1504 ). The grating constant of the first grating  11  is smaller than the grating constant of the second grating  12  while the grating constant of the second grating  12  is the same as the grating constant of the third grating  13 . The grating constant of a given grating is the distance between any two neighboring reticles of the grating. The second grating  12  and the third grating  13  are spaced along the first direction ‘A’ and are arranged parallel to each other. The first grating  11  is spaced away from the second grating  12  and the third grating  13  along the second direction ‘B’. A fourth grating  14 , a seventh grating  17 , and an eighth grating  18  are formed in the substrate  10  along the second direction ‘B’. The grating constant of the fourth grating  14  is smaller than the grating constant of the seventh grating  17  while the grating constant of the seventh grating  17  is the same as the grating constant of the eighth grating  18 . The seventh grating  17  and the eighth grating  18  are spaced along the second direction ‘B’ and parallel to each other. The fourth grating  14  is spaced away from the seventh grating  17  and the eighth grating  18  along the first direction ‘A’. 
         [0025]    In any other embodiments, the relative position of the first grating  11 , the second grating  12 , and the third grating  13  may not be limited to the situation in the present embodiment and all the three gratings may be spaced either along the first direction ‘A’ or along the second direction ‘B’. Correspondingly, the relative position of the fourth grating, the seventh grating, and the eighth grating may not be limited to the situation in the present embodiment and all the three may be spaced either along the first direction ‘A’ or along the second direction ‘B’. 
         [0026]    In one embodiment, the first grating  11 , the second grating  12 , the third grating  13 , the fourth grating  14 , the seventh grating  17 , and the eighth grating  18  are all scribe grooves and the scribe grooves may be formed by any appropriate techniques, such as mechanical scribing, holographic photolithography, e-beam lithography, laser interference lithography, focused ion-beam lithography, etc. 
         [0027]    When a holographic photolithography technique is used, the fabrication process starts with coating the substrate with a layer of photoresist. After baking the photoresist, the substrate is placed into an interference optical system. Then an exposure process is performed. During the process, light waves passing through a mask plate (object wave) interference with a parallel light beam (reference beam) and the exposure leads to an interference fringe recorded on the photoresist layer. A portion of the photoresist layer exposed to interference lights with a relatively high intensity is denatured. After development, the portion of the denatured photoresist is then removed. Further, scribe grooves are formed by etching the substrate  10  and the scribe grooves are the reticles of the corresponding grating. Finally, the rest of the photoresist layer is removed. 
         [0028]    When laser interference lithography technique is used, the laser interferometry uses the characteristics of optical interference and diffraction, and controls the distribution of light intensity in an interference field through a certain combination of light beams. The distribution of light intensity is then recorded by using a photosensitive material. A portion of the photoresist layer exposed to interference light with a relatively high intensity is denatured and thus a photolithography pattern is obtained. The pattern is then reproduced onto the substrate  10 . 
         [0029]    The scribe grooves may be formed by other techniques. For example, when an e-beam lithography or focused ion-beam lithography technique is used, electron beam (e-beam) or focused ion-beam bombardment may be used to denature the property of a portion of the photoresist layer and thus define the lithography pattern. 
         [0030]    In one embodiment, the grating constant of the second grating  12  is identical to that of the third grating  13 . If the two grating constants a not the same, when forming a fifth grating corresponding to the second grating  12  and a sixth grating  16  corresponding to grating  13  in a subsequent process, the difference in the grating constant may be an obstructive factor during the formation of the fifth grating and the sixth grating. Specifically, the difference in the grating constant may cause the overlay shift of the fifth grating along the first direction ‘A’ with respect to the second grating  12  not equal to the overlay shift of the sixth grating along the first direction ‘A’ with respect to the third grating  13 , thus leading to an inaccurate registration precision correction. Correspondingly, similar situation also applies to the seventh grating  17  and the eighth grating  18 . 
         [0031]    In one embodiment, the grating constant of the first grating  11  is the same as the grating constant of the fourth grating  14 . However, the identical grating constant of the first grating  11  and the fourth grating  14  should not limit the scope of the present disclosure. In other embodiments, the grating constant of the first grating  11  may not be the same as the grating constant of the fourth grating. Correspondingly, whether the grating constant of the second grating is the same as the grating constant of the seventh grating  17  or not should not limit the scope of the present disclosure. 
         [0032]    Referring to  FIG. 5 , the second grating  12  and the third grating  13  are arranged parallel to each other along the first direction ‘A’. The spacing between the two gratings, w 0 , may be less than or equal to 100 μm. The spacing w 0  refers to the distance between the two closest reticles with one from the second grating  12  and the other from the third grating  13  along the first direction ‘A’. If the spacing between the second grating  12  and the third grating  13  is larger than 100 μm, a relatively large alignment error and a relatively large registration error may be induced during the formation of the fifth grating and the sixth grating, thus affecting the precision in adjusting the registration accuracy in the present embodiment. Correspondingly, the second grating  17  and the eighth grating  18  are arranged parallel to each other along the second direction B. The spacing between the two gratings is not greater than 100 μm. Further, the length relation between w 0  and w′ is not limited, w 0  and w′ may or may not have the same length. 
         [0033]    Returning to  FIG. 15 , after the formation of the gratings in the substrate  10  described above, a photoresist layer  30  is formed on the substrate  10  (S 1506 ).  FIG. 6  shows a top view of the structure. The photoresist layer  30  may be formed using any appropriate method. 
         [0034]    Referring to  FIG. 6 , the photoresist layer  30  may be formed on the substrate  11  via a spin-coating process and may cover the substrate  11 , the first grating  11 , the second grating  12 , the third grating  13 , the fourth grating  14 , the seventh grating  17 , and the eighth grating  18 . Moreover, because the first grating  11 , the second grating  12 , the third grating  13 , the fourth grating  14 , the seventh grating  17 , and the eighth grating  18  are covered by the photoresist layer  30 , the gratings cannot be seen in the top view, thus they are shown by dashed lines in the figure. 
         [0035]    Referring to  FIG. 15 , after forming the photoresist layer  30 , a first alignment center x 0  is obtained using grating diffraction based on the first grating  11  along the first direction ‘A’, while a second alignment center y 0  is also obtained using grating diffraction based on the fourth grating  14  along the second direction ‘B’ (S 1508 ).  FIG. 6  schematically indicates the positions of the two alignment centers x 0  and y 0 . 
         [0036]    Referring to  FIG. 6 , the alignment center x 0  corresponds to a point on the x-axis of the x-y coordinates of the base station and the alignment center y 0  corresponds to a point on the y-axis of the x-y coordinates of the base station. 
         [0037]    As an example, a detailed description on determining x 0  is now given to illustrate the process of locating an alignment center by using grating diffraction. First, a light beam is used to illuminate the first grating  11 . The illumination light beam may be a laser beam. Diffraction then occurs when the light beam passes through the first grating  11  and the diffracted light carries all the information about the first grating  11 . The multi-level diffracted light spread out from the first grating  11  from different angles and an interference image is then formed on the reference plane by collecting the multi-level diffracted light through a spatial filter. A reference grating is placed symmetrically on the reference plane with respect to the center of the main optical axis of the illumination light beam. The reference grating and the first grating  11  have a same grating period. A corresponding probe optical fiber is placed behind the reference grating. The probe optical fiber guides the intensity signal of the light passing through the reference grating to a photoelectric conversion device. The photoelectric conversion device converts and processes the intensity signal of the light. 
         [0038]    Referring to  FIG. 7 , based on the principle of Fourier optics, a sinusoidal signal corresponding to the intensity signal of the light with a certain period is generated in the detector. The period of the sinusoidal signal corresponds to the grating period of the first grating  11  and the center of the sinusoidal signal is thus the position of the first alignment center x 0 . Using a similar method, another sinusoidal signal corresponding to the fourth grating  14  may be obtained. The second alignment center y 0  corresponding to the fourth grating  14  may then be determined based on the sinusoidal signal. 
         [0039]    Referring to  FIG. 15 , further, a mask plate  21  is provided (S 1510 ).  FIG. 8  shows a schematic top view of the mask plate  21 . Referring to  FIG. 8 , the mask plate has a fifth grating pattern  15 ′, a sixth grating pattern  16 ′, and a first reference mark x 0 ′ that corresponds to the first alignment center x 0  and a ninth grating pattern  19 ′, a tenth grating pattern  20 ′, and a second reference mark y 0 ′ that corresponds to the second alignment center y 0 . 
         [0040]    The mask plate  21  is used to define intended positions on the substrate  10  for the grating patterns on the mask plate  21 .  FIG. 9  shows a schematic view of the predesigned positional relationships between the fifth grating pattern  15 ′and the second grating  12  and between the sixth grating pattern  16 ′ and the third grating  13 . 
         [0041]    According to the predesign shown in  FIG. 9 , the fifth grating pattern  15 ′ has the same grating constant as the second grating  12  and the fifth grating pattern  15 ′ is stacked against the second grating  12 . That is, the reticles of the fifth grating pattern  15 ′ are interlaced with the reticles of the second grating  12 . The fifth grating pattern  15 ′ has an offset of a first distance dx along the first direction ‘A’ with respect to the second grating  12 . That is, the center axis of a reticle of the fifth grating pattern  15 ′ between two neighboring reticles of the second grating  12  has an offset of the first distance dx along the first direction ‘A’ with respect to the center line of the two neighboring reticles on the second grating  12 . 
         [0042]    Referring to  FIG. 9 , the sixth grating pattern  16 ′ has the same grating constant as the third grating  13  and the sixth grating pattern  16 ′ is stacked against the third grating  13 . That is, the reticles of the sixth grating pattern  16 ′ are interlaced with the reticles of the third grating  13 . The sixth grating pattern  16 ′ has an offset of a first distance dx along a third direction ‘C’ with respect to the third grating  13 . The third direction ‘C’ is the opposite direction of the first direction ‘A’. Therefore, the center axis of a reticle of the sixth grating pattern  16 ′ between two neighboring reticles of the third grating  13  has an offset of the first distance dx along the third direction ‘C’ with respect to the center line of the two neighboring reticles on the third grating 
         [0043]    Accordingly,  FIG. 10  shows a schematic view of the predesigned position relationships between the ninth grating pattern  19 ′and the seventh grating  17  and between the tenth grating pattern  20 ′ and the eighth grating  18 . 
         [0044]    According to the predesign shown in  FIG. 10 , the ninth grating pattern  19 ′ has the same grating constant as the seventh grating  17  and the ninth grating pattern  19 ′ is stacked against the seventh grating  17 . That is, the reticles of the ninth grating pattern  19 ′ are interlaced with the reticles of the seventh grating  17 . The ninth grating pattern  19 ′ has an offset of a second distance dy along the second direction ‘B’ with respect to the seventh grating  17 . That is, the center axis of a reticle of the ninth grating pattern  19 ′ between two neighboring reticles of the seventh grating  17  has an offset of the second distance dy along the second direction ‘B’ with respect to the center line of the two neighboring reticles on the seventh grating  17 . 
         [0045]    Referring to  FIG. 10 , the tenth grating pattern  20 ′ has the same grating constant as the eighth grating  18  and the tenth grating pattern  20 ′ is stacked against the eighth grating  18 . That is, the reticles of the tenth grating pattern  20 ′ are interlaced with the reticles of the eighth grating  18 . The tenth grating pattern  20 ′ has an offset of a second distance dy along a fourth direction ‘D’ with respect to the eighth grating  18 . The fourth direction ‘D’ is the opposite direction of the second direction ‘B’. Therefore, the center axis of a reticle of the tenth grating pattern  20 ′ between two neighboring reticles of the eighth grating  18  has an offset of the second distance dy along the fourth direction ‘D’ with respect to the center line of the two neighboring reticles on the eighth grating  18 . 
         [0046]    In one embodiment, the first distance dx may be predefined. Therefore, in a subsequent lithography process, the overlay shift information of a fifth grating along the first direction ‘A’ with respect to the second grating  12  and the overlay shift information of a sixth grating along the first direction ‘A’ with respect to the third grating  13  may also be increased. This makes the actual offset of the fifth grating with respect to the second grating  12  and the actual offset of the sixth grating with respect to the third grating  13  measurable. The first distance dx may be relatively small. For example, the first distance dx may be approximately in a range of 1 nm˜10 nm. Therefore, overlay shifts of the fifth grating and the sixth grating may take place during a subsequent lithography process and the values of the overlay shifts depend linearly on dx, allowing linear addition or subtraction be performed on the offset of the alignment center in subsequent processes, thus ensuring the implementation of the method of the present embodiment. 
         [0047]    Correspondingly, the second distance dy may also be predefined. Thus, in a subsequent lithography process, the overlay shift information of a ninth grating along the second direction ‘B’ with respect to the seventh grating  17  and the overlay shift information of a ninth grating alone the first direction ‘A’ with respect to the fluid grating  13  may also be increased. 
         [0048]    Also referring to  FIG. 15 , further, an alignment process for the mask plate  21  is then performed on the top of the photoresist layer  30  (S 1512 ). Specifically, the first reference mark x 0 ″ on the mask plate  21  is aligned with the first alignment center x 0  while the second reference mark y 0 ′ on the mask plate  21  is aligned with the second alignment center y 0 , thus the position of the mask plate  21  with respect to the base station is then determined. The alignment lets the fifth grating pattern  15 ′ be aligned with the second grating  12  along a direction perpendicular to the top surface of the photoresist layer  30  and the sixth grating pattern  16 ′ be aligned with the third grating  13  along the direction perpendicular to the top surface of the photoresist layer  30 . The alignment also simultaneously lets the ninth grating pattern  19 ′ be aligned with the seventh grating  17  along the direction perpendicular to the top surface of the photoresist layer  30  and tenth grating pattern  20 ′ be aligned with the eighth grating  18  along the direction perpendicular to the top surface of the photoresist layer  30 . 
         [0049]    Further, referring to  FIG. 15 , an exposure process is performed after the mask plate  21  is aligned (S 1514 ).  FIG. 11  shows a schematic top view of the structure of the substrate after the exposure process. Referring to  FIG. 11 , the fifth grating pattern  15 ′ is reproduced in the photoresist layer  30  to define a fifth grating  15 , the sixth grating pattern  16 ′ is reproduced in the photoresist layer  30  to define a sixth grating  16 , the ninth grating pattern  19 ′ is reproduced in the photoresist layer  30  to define a ninth grating  19 , and the tenth grating pattern  20 ′ is reproduced in the photoresist layer  30  to define a tenth grating  20 . Due to the influence of the lithography equipment and other factors, the fifth grating  15  with respect to the second grating  12  and the sixth grating with respect to the third grating  13  may have an overlay shift along the first direction ‘A’, or equivalently along the third direction ‘C’. In one embodiment, the direction of the overlay shift is along the third direction ‘C’. During the photolithography process, the fifth grating  15  with respect to the second grating  12  and the sixth grating  16  with respect to the third grating  13  may have an additional overlay shift Δx (not shown) along the first direction ‘A’ corresponding to the focus depth of the photolithography apparatus. 
         [0050]    Accordingly, the ninth grating  19  with respect to the seventh grating  17  and the tenth grating with respect to the eighth grating  18  may have an overlay shift along the second direction ‘B’, or equivalently along the fourth direction ‘D’. In one embodiment, the direction of the overlay shift is along the fourth direction “D”. 
         [0051]    Returning back to  FIG. 9 , because the predefined fifth grating pattern  15 ′ has an offset along the first direction ‘A’ with respect to the second grating  12  while the sixth grating pattern has an offset along the third direction ‘C’ with respect to the third grating  13 . Therefore, after exposure, referring to  FIG. 11 , the fifth grating  15  has an offset along the first direction ‘A’ with respect to the second grating  12  while the sixth grating  16  has an offset along the third direction ‘C’ with respect to the third grating  13 . Correspondingly, the ninth grating  19  has an offset along the second direction ‘B’ with respect to the seventh grating  17  while the tenth grating  20  has an offset along the fourth direction ‘D’ with respect to the eighth grating  18 . Also, both the fifth grating  15  with respect to the second grating  12  and the sixth grating  16  with respect to the third grating  13  may have another overlay shift Δy (not shown) along the second direction ‘B’ corresponding to the focus depth of the photolithography apparatus. 
         [0052]    Finally, also referring to  FIG. 15 , a development process is performed to remove the denatured portion of photoresist layer (S 1516 ). After the development process, the photoresist layer except for the fifth grating  15 , the sixth grating  16 , the ninth grating  19 , and the tenth grating  20  is removed and the surface of the substrate  10  is exposed.  FIG. 12  shows a schematic view of the structure after the development process. 
         [0053]    Accordingly, the photolithography alignment mark structure includes the first grating  11 , the second grating  12 , the third grating  13 , the fourth grating  14 , the seventh grating  17 . and the eighth grating  18  formed in the substrate  10  and the fifth grating  15 , the sixth grating  16 , the ninth grating  19 , and the tenth grating  20  formed on the surface of the substrate  10 . 
         [0054]    Referring to  FIG. 13 , with the photolithography alignment mark structure, a third alignment center x 1  of a grating that consists of the second grating  12  and the fifth grating  15  and a fourth alignment center x 2  of a grating that consists of the third grating  13  and the sixth grating  16  may be obtained by using grating diffraction. The reference grating used in the process includes a first segment corresponding to the first grating  11 , a second segment corresponding to the grating formed by the second grating  12  and the fifth grating  15 , and a third segment corresponding to the grating formed by the third grating  13  and the sixth grating  16 . A probe optical fiber is placed behind of each of the segments to collect the intensity signal of the light passing through the reference gratings. 
         [0055]    Correspondingly, referring to  FIG. 13 , a fifth alignment center y 1  of a grating that consists of the seventh grating  17  and the ninth grating  19  and a sixth alignment center y 2  of a grating that consists of the eighth grating  18  and the tenth grating  20  may be obtained by using grating diffraction. The reference grating used in the process includes a first segment corresponding to the fourth grating  14 , a second segment corresponding to the grating formed by the seventh grating  17  and the ninth grating  19 , and a third segment corresponding to the grating formed by the eighth grating  18  and the tenth grating  20 . A probe optical fiber is placed behind of each of the segments to collect the intensity signal of the light passing through the reference gratings. 
         [0056]    Returning back to  FIG. 7 , because of the first distance dx and the overlay shift Δx, the third alignment center x 1  has an offset with respect to the first alignment center x 0  while the fourth alignment center x 2  also has an offset with respect to the first alignment center x 0 . In addition, because the first distance dx is relatively small, the overlay shift Δx and the first distance dx have a linear relationship, the offset of the third alignment center x 1  with respect to the first alignment center x 0  corresponds to but is not equal to −(dx+Δx) while the offset of the fourth alignment center x 2  with respect to the first alignment center x 0  corresponds to but is not equal to dx+Δx. The minus sign wherein indicates that the offset is along the third direction ‘C’. 
         [0057]    Accordingly, because the second distance dy is relatively small, the overlay shift Δy and the second distance dy have a linear relationship, the offset of the fifth alignment center y 1  with respect to the second alignment center y 0  corresponds to but is not equal to −(dy+Δy) while the offset of the sixth alignment center y 2  with respect to the second alignment center y 0  corresponds to but is not equal to dy+Δy. The minus sign wherein indicates that offset is along the fourth direction ‘D’. 
         [0058]    In one embodiment, the first grating  11 , the grating formed by the second grating  12  and the fifth grating  15 , and the grating formed by the third grating  13  and the sixth grating  16  are used as alignment marks along the first direction ‘A’; the fourth grating  14 , the grating formed by the seventh grating  17  and the ninth grating  19 , and the grating formed by the eighth grating  18  and the tenth grating  20  are used as alignment marks along the second direction ‘B’. 
         [0059]    The photolithography alignment mark structure of the embodiments of the present disclosure may then be used in a double exposure type double patterning process to improve the alignment accuracy. Specifically, a double exposure type double patterning process using the photolithography alignment mark structure disclosed in the embodiments may include the following steps: 
         [0060]    First, during the first exposure, the first alignment center x 0  is used as the alignment center along the first direction ‘A’ and the second alignment center y 0  is used as the alignment center along the second direction ‘B’. After the first exposure, the first device pattern has an alignment offset with respect to the intended position on the substrate. 
         [0061]    Further, the alignment center along the first direction ‘A’ for the second exposure may be adjusted based on the third alignment center x 1  and the fourth alignment center x 2 . Referring to  FIG. 14 , because of the linear relationship between the first distance dx and the overlay shift Δx, linear operation may be performed by using the third alignment center x 1  and the fourth alignment center x 2 : first, the center position between the third alignment center x 1  and the fourth alignment center x 2  may be calculated and the result is x″=(x 1 +x 2 )/2. the value of x″ is regarded as the actual offset value of the first device pattern with respect to the intended position on the substrate after the first exposure; then, based on the offset value of the first device pattern with respect to the substrate after the first exposure, an average value of the offset of x″ with respect to the first alignment center x 0  may be calculated and the average value is (x″−x 0 )/2; then, an alignment center x′=x 0 +(x″−x 0 )/2=(x″+x 0 )/2=((x 1 +x 2 )/2+x 0 )/2 may be used for the second exposure. That is, during the second exposure process, the first alignment center x 0  is no longer used as the alignment center; instead, the adjusted position x′ is used as the alignment center. 
         [0062]    By choosing the center position x′ between x″ and x 0  as the new alignment center, after the second exposure, the alignment error of the actual position of the second device pattern along the first direction ‘A’ with respect to the intended position of the second device pattern on the substrate may be reduced. In the meantime, the registration error between the second device pattern and the first device pattern may also be reduced. Thus, the alignment error between the intended position of the second device pattern and the actual position formed on the substrate after the second exposure may be reduced due to compensation, and the registration offset value of the second device pattern with respect to the first device pattern may also be reduced. Therefore, the registration accuracy of the second device pattern with respect to the first device pattern may be greatly improved, e.g., about 40%. The improvement may not only ensure that subsequent semiconductor manufacturing processes can be normally performed but also ensure that the semiconductor structure containing the second device pattern and the first device pattern has good performance. 
         [0063]    In addition, during the second exposure, the alignment center of the second exposure along the second direction ‘B’ may also be adjusted based on the fifth alignment center y 1  and the sixth alignment center y 2 . Referring to the above description, the alignment center of the second exposure along the second direction ‘B’ after the adjustment is y′=((y 1 +y 2 )/2+y 0 )/2. Using the adjusted alignment center y′, after the second exposure, the alignment error between the actual position of the second device pattern and the intended position of the second device pattern on the substrate along the second direction ‘B’ may be reduced. 
         [0064]    The above detailed descriptions only illustrate certain exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention. Those skilled in the art can understand the specification as whole and technical features in the various embodiments can be combined into other embodiments understandable to those persons of ordinary skill in the art. Any equivalent or modification thereof, without departing from the spirit and principle of the present invention, falls within the true scope of the present invention.