Abstract:
In a first step, first trenches are formed to constitute alignment marks. In a second step, second trenches are formed, and the first and second trenches are filled with metal. When detecting alignment marks, the second trenches filled with metal prevent the position of the first trenches from being detected. In a third step, third trenches of the same shape as the first trenches are formed. In a fourth step, fourth trenches are formed, and the third and fourth trenches are filled with metal. When detecting alignment marks, the fourth trenches filled with metal prevent the position of the third trenches formed in a lower layer from being detected. The third and fourth steps are repeated with an increase in the number of stacked layers. Consequently, influences caused by detection of alignment marks formed in a lower layer are reduced while controlling an increase in the area occupied by alignment marks.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of forming an alignment mark, and is applicable to, for example, alignment when exposing a pattern in a lithography process.  
         [0003]     2. Description of the Background Art  
         [0004]     In the manufacture of semiconductor devices, alignment marks have conventionally been formed for aligning patterns in a lithography process. To control an increase in the area occupied by alignment marks, the alignment marks have been so formed as to conform with those already formed on a target (cf. Japanese Patent Application Laid-Open No. 2002-25888).  
         [0005]     Japanese Patent Application Laid-Open No. 10-150085 (1988) discloses a technique in which alignment marks having a greater size than those formed in a lower layer are used for a resist pattern.  
         [0006]     However, it has been difficult to accurately align alignment marks of the same shape, resulting in misalignment. When detecting the position of alignment marks exposed at a surface, the position of alignment marks formed in a lower layer is also detected through an insulating layer and a resist. Therefore, it has been difficult to define the position of the alignment marks exposed at the surface.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to reduce influences caused by an alignment mark formed in a lower layer when detecting an alignment mark formed in an upper layer, while controlling an increase in the area occupied by the alignment marks.  
         [0008]     According to a first aspect of the present invention, a method of forming an alignment mark includes the steps (a) and (b). The step (a) is to form a first pattern and a first alignment mark in parallel in an insulating layer. The step (b) is to form a second pattern and a second alignment mark in parallel in the insulating layer after the step (a). The second alignment mark covers the first alignment mark in the insulating layer in a first predetermined position.  
         [0009]     An increase in the area for forming the alignment marks is controlled. Further, the second alignment mark covers the first alignment mark, so that detection of the first alignment mark is prevented while the second alignment mark is detected.  
         [0010]     According to a second aspect of the invention, a method of forming an alignment mark includes the steps (a) to (c). The step (a) is to form a first pattern and a first alignment mark in parallel in a first insulating layer. The step (b) is to form a second insulating layer to cover the first insulating layer and the first alignment mark. The step (c) is to form a second pattern and a second alignment mark in parallel in the second insulating layer. The second alignment mark is located inside the first alignment mark in a predetermined position.  
         [0011]     An increase in the area for forming the alignment marks is controlled. Further, even when the first alignment mark is detected when detecting the second alignment mark, influences caused by the first alignment mark are reduced since the positional relationship between the first and second alignment marks is clear.  
         [0012]     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIGS. 1A  to  1 C are a conceptual sectional view, a top view and a waveform chart, respectively, for describing a first preferred embodiment of the present invention;  
         [0014]      FIGS. 2A  to  2 C are a conceptual sectional view, a top view and a waveform chart, respectively, for describing the first preferred embodiment;  
         [0015]      FIGS. 3A  to  3 C are a conceptual sectional view, a top view and a waveform chart, respectively, for describing the first preferred embodiment;  
         [0016]      FIGS. 4A  to  4 C are a conceptual sectional view, a top view and a waveform chart, respectively, for describing the first preferred embodiment;  
         [0017]      FIGS. 5A  to  5 C are a conceptual sectional view, a top view and a waveform chart, respectively, for describing a second preferred embodiment of the invention;  
         [0018]      FIGS. 6A  to  6 C are a conceptual sectional view, a top view and a waveform chart, respectively, for describing the second preferred embodiment;  
         [0019]      FIGS. 7A and 7B  are a conceptual sectional view and a top view, respectively, for describing a third preferred embodiment of the invention;  
         [0020]      FIGS. 8A and 8B  are a conceptual sectional view and a top view, respectively, for describing the third preferred embodiment;  
         [0021]      FIGS. 9A and 9B  are a conceptual sectional view and a top view, respectively, for describing the third preferred embodiment; and  
         [0022]      FIGS. 10A and 10B  are a conceptual sectional view and a top view, respectively, for describing the third preferred embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Preferred Embodiment  
       [0023]     The present embodiment will describe a method of forming alignment marks in the case of employing a dual damascene method among damascene methods used for forming plugs and interconnect lines. Here, alignment marks formed by arranging rectangles in parallel to one another at a regular space shall be used.  FIGS. 1A through 4C  sequentially illustrate the steps of forming interconnect lines.  FIGS. 1A, 2A ,  3 A and  4 A are conceptual sectional views of alignment marks as formed,  FIGS. 1B, 2B ,  3 B and  4 B are top views of alignment marks as formed, and  FIGS. 1C, 2C ,  3 C and  4 C conceptually illustrate one-dimensional waveforms obtained when detecting the position of alignment marks by image processing.  
         [0024]     In a first step, an insulating layer  11  is formed on a substrate  100 . Then, trenches  31  to serve as alignment marks are formed in a region  111  by lithography and etching. Formation of the trenches  31  is conducted in parallel with that of a hole  51 . The hole  51  is a via hole formed in the insulating layer  11 , and is used for forming a first plug to be connected to a first interconnect line which will be described later ( FIG. 1A ). Each of the plug and interconnect line can be considered as a pattern. This applies throughout the present specification.  
         [0025]     Next, the position of the trenches  31  is detected by an image processor from the side opposite to the substrate  100 , i.e., at a surface  11   a  of the insulating layer  11 .  FIG. 1C  shows a waveform detected by scanning the trenches  31  along a scanning line L. Straight lines  91  and  92  in pair constitute a rectangle presented by each of the trenches  31  at the surface  11   a  and cross the scanning line L. Peak positions  91   a  and  92   a  of the waveform correspond to the positions of the lines  91  and  92 , respectively ( FIG. 1B ).  
         [0026]     In a second step, the position of a mask pattern to be used in a subsequent lithography step is determined with reference to the peak positions  91   a  and  92   a . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  32  for alignment marks in the insulating layer  11  on the surface  11   a  side. Straight lines  93  and  94  in pair constitute a rectangle presented by each of the trenches  32  at the surface  11   a . The lines  93  and  94  of a trench  32  crossing the scanning line L are arranged so as not either to locate between the lines  91  and  92  that constitute a trench  31  communicating to the trench  32  or to overlap the lines  91  and  92 , respectively. Formation of the trenches  32  is conducted in parallel with that of a hole  52  to be used for forming the first interconnect line ( FIGS. 2A and 2B ). Then, metal burying is carried out, whereby the lines  93  and  94  in pair constitute a rectangle presented by an alignment mark  2  at the surface  11   a.    
         [0027]     More specifically, the holes  51  and  52  are filled with a metal  102  to form the first plug and first interconnect line, respectively. In parallel with this, the trenches  31  and  32  are filled with a metal  101 . Then, the trenches  32  filled with the metal  101  become alignment marks  2  and are exposed at the surface  11   a.    
         [0028]     Next, the position of the alignment marks  2  is detected at the surface  11   a .  FIG. 2C  shows the waveform detected by scanning the alignment marks  2  along the scanning line L. When detecting the position of the alignment marks  2 , the alignment marks  2  themselves filled with the metal  101  prevent the trenches  31  from being detected. That is, only the position of the alignment marks  2  is detected. Accordingly, only the positions of the lines  93  and  94  are detected as peak positions  93   a  and  94   a  of the waveform.  
         [0029]     In a third step, an insulating layer  12  is formed over the insulating layer  11 . Then, the position of a mask pattern to be used in a subsequent lithography step is determined with reference to the peak positions  93   a  and  94   a . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  33  for alignment marks in the insulating layer  12  on the alignment marks  2 . Straight lines  95  and  96  in pair constitute a rectangle presented by each of the trenches  33  at a surface  12   a  of the insulating layer  12 . The lines  95  and  96  of a trench  33  crossing the scanning line L are arranged so as to locate between the lines  93  and  94  that constitute a trench  32  communicating to the trench  33  and not to overlap the lines  93  and  94 , respectively. The trenches  33  may be of the same size as the trenches  31 , for example. Formation of the trenches  33  is conducted in parallel with that of a hole  53 . The hole  53  is a via hole formed in the insulating layer  12 , and is used for forming a second plug for connecting a second interconnect line which will be described later and the above-mentioned first interconnect line ( FIGS. 3A and 3B ).  
         [0030]     Next, the position of the trenches  33  is detected at the surface  12   a . At this time, the position of the alignment marks  2  formed in the lower layer is also detected through the insulating layer  12 , because the lines  95  and  96  are located inside the alignment mark  2 .  FIG. 3C  shows the waveform detected by scanning the trenches  33  along the scanning line L. Peak positions  95   a  and  96   a  of high strength respectively correspond to the positions of the lines  95  and  96  crossing the scanning line L. The peak positions  93   a  and  94   a  of low strength respectively correspond to the lines  93  and  94  constituting each of the alignment marks  2 .  
         [0031]     In a fourth step, the position of a mask pattern to be used in a subsequent lithography step is determined with reference to the peak positions  95   a  and  96   a . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  34  for alignment marks in the insulating layer  12  on the surface  12   a  side. Straight lines  97  and  98  in pair constitute a rectangle presented by each of the trenches  34  at the surface  11   a . The lines  97  and  98  of a trench  34  crossing the scanning line L are arranged so as not either to locate between the lines  93  and  94  that constitute a trench  32  communicating to the trench  34  or to overlap the lines  93  and  94 , respectively. Formation of the trenches  34  is conducted in parallel with that of a hole  54  to be used for forming the second interconnect line ( FIGS. 4A and 4B ). Then, metal burying is carried out, whereby the lines  97  and  98  in pair constitute a rectangle presented by an alignment mark  4  at the surface  12   a.    
         [0032]     More specifically, the holes  53  and  54  are filled with a metal  104  to form the second plug and second interconnect line, respectively. In parallel with this, the trenches  33  and  34  are filled with a metal  103 . Then, the trenches  34  filled with the metal  103  become alignment marks  4  and are exposed at the surface  12   a.    
         [0033]     Next, the position of the alignment marks  4  is detected at the surface  12   a .  FIG. 4C  shows the waveform detected by scanning the alignment marks  4  along the scanning line L. When detecting the position of the alignment marks  4 , the alignment marks  4  themselves filled with the metal  103  prevent the position of the alignment marks  2  and trenches  33  from being detected. Accordingly, only the positions of the lines  97  and  98  are detected as peak positions  97   a  and  98   a  of the waveform.  
         [0034]     Thereafter, the third and fourth steps are repeated, so that alignment marks are formed in parallel with formation of a plug and an interconnect line.  
         [0035]     Through the above-described method of forming alignment marks, a region in which alignment marks are to be formed can be limited to a certain region regardless of the number of stacked insulating layers. Further, when detecting the position of alignment marks, the position of alignment marks formed in a lower layer is not detected.  
         [0036]     Although the position of alignment marks in a lower layer is detected as mentioned in the description of the third step, making clear the positional relationship between alignment marks present in the lower layer and alignment marks desired to be detected can reduce influences caused by the alignment marks present in the lower layer.  
         [0037]     The invention of the present embodiment is also applicable to a single damascene method. That is, an interconnect line and a plug are formed in each layer, and in parallel with this, alignment marks are formed. In this case, the waveforms illustrated in  FIGS. 1C, 2C ,  3 C and  4 C are also detected.  
       Second Preferred Embodiment  
       [0038]     The present embodiment will describe a method of forming alignment marks different from that of the first preferred embodiment with respect to the third step ( FIGS. 3A  to  3 C) and fourth step ( FIGS. 4A  to  4 C).  FIGS. 5A through 6C  sequentially illustrate the steps of forming a second plug and a second interconnect line.  FIGS. 5A and 6A  are conceptual sectional views of alignment marks as formed,  FIGS. 5B and 6B  are top views of alignment marks as formed, and  FIGS. 5C and 6C  conceptually illustrate one-dimensional waveforms obtained when detecting the position of alignment marks by image processing.  
         [0039]     In a third step, the insulating layer  12  is formed over the insulating layer  11 , and the position of a mask pattern to be used in a subsequent lithography step is determined with reference to the peak positions  93   a  and  94   a . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  35  to serve as alignment marks in the insulating layer  12  over the alignment marks  2 . Straight lines  81  and  82  in pair constitute a rectangle presented by each of the trenches  35  at the surface  12   a  of the insulating layer  12 . The lines  81  and  82  of a trench  35  crossing the scanning line L are arranged so as not either to locate between the lines  93  and  94  that constitute a trench  32  communicating to the trench  35  or to overlap the lines  93  and  94 , respectively. Formation of the trenches  35  is conducted in parallel with that of a hole  55 . The hole  55  is a via hole formed in the insulating layer  12 , and is used for forming the second plug for connecting the second interconnect line which will be described later and the first interconnect line formed in the hole  52  ( FIGS. 5A and 5B ).  
         [0040]     Next, the position of the trenches  35  is detected at the surface  12   a . At this time, the alignment marks  2  are exposed at the surface  11   a  which forms the bottom of each of the trenches  35 , and therefore, the position of the alignment marks  2  is also detected.  FIG. 5C  shows the waveform detected by scanning the trenches  35  along the scanning line L. Peak positions  81   a  and  82   a  of high strength respectively correspond to the positions of the lines  81  and  82  crossing the scanning line L. The peak positions  93   a  and  94   a  of low strength respectively correspond to the lines  93  and  94  constituting each of the alignment marks  2 .  
         [0041]     In a fourth step, the position of a mask pattern is determined with reference to the peak positions  81   a  and  82   a . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  36  for alignment marks in the insulating layer  12  on the surface  12   a  side. Straight lines  83  and  84  in pair constitute a rectangle presented by each of the trenches  36  at the surface  12   a . The lines  83  and  84  of a trench  36  crossing the scanning line L are arranged so as not either to locate between the lines  81  and  82  that constitute a trench  35  communicating to the trench  36  or to overlap the lines  81  and  82 , respectively. Formation of the trenches  36  is conducted in parallel with that of a hole  56  to be used for forming the second interconnect line ( FIGS. 6A and 6B ). Then, metal burying is carried out, whereby the lines  83  and  84  in pair constitute a rectangle presented by an alignment mark  6  at the surface  12   a.    
         [0042]     More specifically, the holes  55  and  56  are filled with a metal  106  to form the second plug and second interconnect line, respectively. In parallel with this, the trenches  35  and  36  are filled with a metal  105 . Then, the trenches  36  filled with the metal  105  become alignment marks  6  and are exposed at the surface  12   a.    
         [0043]     Next, the position of the alignment marks  6  is detected at the surface  12   a .  FIG. 6C  shows the waveform detected by scanning the alignment marks  6  along the scanning line L. When detecting the position of the alignment marks  6 , the alignment marks  6  themselves filled with the metal  105  prevent the position of the alignment marks  2  and trenches  35  from being detected. Accordingly, only the positions of the lines  83  and  84  are detected as peak positions  83   a  and  84   a  of the waveform.  
         [0044]     Thereafter, the above-described third and fourth steps are repeated, so that alignment marks are formed in parallel with formation of a plug and an interconnect line. This technique is also applicable to the single damascene method.  
         [0045]     Through the method of forming alignment marks according to the present embodiment, similar effects to those achieved by the first preferred embodiment can be obtained. In the first and second preferred embodiments, the size relationship between trenches for forming alignment marks shall be determined in each step of forming a pair of a plug and an interconnect line which is in contact with the plug on the side opposite to the substrate, but trenches for forming alignment marks have any size relationship between steps of forming different pairs of a plug and an interconnect line.  
       Third Preferred Embodiment  
       [0046]     The present embodiment will describe a method of forming alignment marks in which alignment marks are formed in a region in parallel with formation of a via hole for forming a plug and alignment marks are formed in a separate region in parallel with formation of a trench for forming an interconnect line.  FIGS. 7A through 10B  conceptually illustrate the method of forming alignment marks according to the present embodiment.  FIGS. 7A, 8A ,  9 A and  10 A are sectional views, and  FIGS. 7B, 8B ,  9 B and  10 B are top views.  
         [0047]     In a first step, the insulating layer  11  is formed over the substrate  100 . Trenches  37  to serve as alignment marks are formed in a region  112  for forming alignment marks by lithography and etching. Formation of the trenches  37  is conducted in parallel with that of a hole  57 . The hole  57  is a via hole formed in the insulating layer  11 , and is used for forming a first plug to be connected to a first interconnect line which will be described later ( FIG. 7A ). Then, the position of the trenches  37  is detected at the surface  11   a  along the scanning line L ( FIG. 7B ).  
         [0048]     In a second step, the position of a mask pattern is determined with reference to the position of the trenches  37 . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  38  for alignment marks in a region  113  for forming alignment marks in the insulating layer  11 . The region  113  is different from the region  112 . Formation of the trenches  38  is conducted in parallel with that of a hole  58  to be used for forming the first interconnect line ( FIG. 8A ).  
         [0049]     Then, the holes  57  and  58  are filled with a metal  108  to form the first plug and first interconnect line, respectively. In parallel, the trenches  37  and  38  are filled with a metal  107 . Then, the trenches  38  filled with the metal  107  become alignment marks  8  and are exposed at the surface  11   a . The position of the alignment marks  8  is detected at the surface  11   a  along the scanning line L ( FIG. 8B ).  
         [0050]     In a third step, the insulating layer  12  is formed over the insulating layer  11 . Then, the position of a mask pattern is determined with reference to the alignment marks  8 . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  39  to serve as alignment marks in a region  114  for forming alignment marks in the insulating layer  12 . The region  114  is different from the regions  112  and  113 . Formation of the trenches  39  is conducted in parallel with that of a hole  59 . The hole  59  is a via hole formed in the insulating layer  12 , and is used for forming a second plug for connecting the second interconnect line which will be described later and the above-mentioned first interconnect line ( FIG. 9A ). Then, the position of the trenches  39  is detected at the surface  12   a  along the scanning line L ( FIG. 9B ).  
         [0051]     In a fourth step, the position of a mask pattern is determined with reference to the position of the trenches  39 . Thereafter, exposure and etching is carried out using the mask pattern, thereby forming trenches  40  for alignment marks in the region  113  in the insulating layer  12  over the alignment marks  8 . Straight lines  87  and  88  in pair constitute a rectangle presented by each of the trenches  40  at the surface  12   a . The lines  87  and  88  of a trench  40  are arranged so as not either to locate between lines  85  and  86  ( FIG. 9B ) that constitute an alignment mark  8  communicating to the trench  40  or to overlap the lines  85  and  86 , respectively. Formation of the trenches  40  is conducted in parallel with that of a hole  60  to be used for forming the second interconnect line ( FIG. 10A ).  
         [0052]     As described, alignment of the mask pattern to be used for forming the trenches  40  for alignment marks is conducted with reference to the position of the trenches  39  formed in the region  114 . When detecting the position of the trenches  39 , the position of the trenches  37  is detected in the region  112  but does not appear in the region  114 , which therefore causes no problem.  
         [0053]     Then, the holes  59  and  60  are filled with a metal  110  to form the second plug and second interconnect line, respectively. In parallel with this, the trenches  39  and  40  are filled with a metal  109 . Accordingly, the trenches  40  filled with the metal  109  become alignment marks  9  and are exposed at the surface  12   a . The position of the alignment marks  9  is detected at the surface  12   a  along the scanning line L ( FIG. 10B ).  
         [0054]     The insulating layers  11  and  12  may be considered as one insulating layer  10  in combination. That is, the insulating layer  12  covers the insulating layer  11  and alignment marks  8 , and constitutes the insulating layer  10  in combination with the insulating layer  11 . The alignment marks  8  can be considered as being formed within the insulating layer  10 .  
         [0055]     Through the above-described method of forming alignment marks, a region in which alignment marks are formed to overlap alignment marks formed underneath in parallel with formation of trenches for interconnect lines can be limited to the region  113 . In the region  113 , when detecting the alignment marks  9  at the surface  12   a , the position of the alignment marks  8  formed in the lower insulating layer  11  can be prevented from being detected. Further, in the first to third steps, only the position of rectangles represented at the surface is detected while the position of alignment marks in a lower layer is not detected.  
         [0056]     Furthermore, the size relationship between the trenches  38  and  40  for forming the alignment marks  8  and  9 , respectively, formed in the region  113  shall be determined, but trenches  37  and  39  formed in the regions  112  and  114 , respectively, may have any size relationship with the trenches  38  and  40 . Similarly, the trenches  37  and  39 , formed in different regions, may have any size relationship with each other.  
         [0057]     The present embodiment is also applicable to the case of forming the trenches  37  and  39  in the same region in the first and third steps. In this case, it is preferable to determine the size relationship between the trenches  37  and  39 . However, the trenches  37  and  39  may have any size relationship with the trenches  38  and  40 .  
         [0058]     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.