Abstract:
A two dimensional vernier is provided along with a method of fabrication. The two dimensional vernier has a reference array patterned into a substrate, or a material overlying the substrate. An active array is patterned into photoresist overlying the substrate or the material. Both the reference array and the active array each comprise a two dimensional array of shapes. A difference between a combination of size or spacing of the shapes in each array determines vernier resolution. Vernier range is determined by a combination of vernier resolution and an integer related to a total number of shapes along a given axis. The two dimensional vernier allows an operator to readily measure the misalignment of a pattern to be processed relative to a previous pattern in two dimensions using a microscope. The two dimensional vernier reduces, or eliminates, repositioning of the microscope to determine both x-axis misalignment and y-axis misalignment. If a significant misalignment is detected the photoresist can be stripped and the lithography step repeated prior to subsequent processing, and possible yield reduction.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to manufacturing processes requiring lithography, and more particularly to an X/Y vernier for ascertaining misalignment of a lithographic step relative to an underlying pattern.  
           [0002]    Lithography has a broad range of industrial applications, including the manufacture of integrated circuits, flat-panel displays, and micromachines.  
           [0003]    The lithographic process transfers a mask, or reticle, pattern onto a substrate. Usually, the pattern is formed in a photoresist layer overlying the substrate. The pattern may then be etched into a material underlying the photoresist layer. For example, in the case of inlaid copper, copper is deposited into trenches formed by the etch step and then polished using CMP to form the desired copper lines. This layer will include a reference pattern. A second device layer to be formed will be patterned using photoresist. The second layer will include an active pattern in addition to the desired device related features. Prior to completion of the process associated with the second device layer, the photoresist pattern is compared to the underlying reference pattern to confirm alignment, or measure misalignment.  
           [0004]    Referring now to FIG. 1 (prior art), a typical vernier  10  is shown. The vernier is comprised of two patterns aligned adjacent to each other. The first pattern  12 , which is also called the reference pattern, is assimilated into the substrate, or an overlying layer, depositing material, by etching, or otherwise delineating the pattern. The second pattern  14 , which is also called the active pattern, is a layer of photoresist that has been patterned. This vernier could be used, for example, to check the alignment during the formation of multiple metal layers. It is necessary to check the alignment of the second pattern  14  relative to the first pattern  12  prior to continuing with the subsequent process steps. If the alignment is beyond a predetermined tolerance, the second pattern  14  can be removed and redone, prior to additional processing. The alignment may be checked by viewing the pattern under a microscope.  
           [0005]    A proper alignment would be shown by proper alignment of the first centerline  16 , and the second centerline  18 . As shown in FIG. 1 (prior art) the patterns are out of alignment as the two most apparently aligned features are reference pattern mark  20  and active pattern mark  22 . If this misalignment were outside of an acceptable range, the second pattern would have to be removed, re-exposed, and re-checked. If the wafer is not reworked while out of alignment there a significant likelihood of producing a wafer with poor yield.  
           [0006]    A disadvantage of this type of vernier is that only one direction is available for inspection at a time. As shown in FIG. 1 (prior art) only the x-direction can be determined. A second vernier, rotated 90-degrees relative to the one shown, will need to be provided in order to inspect alignment along the y-direction. Although these verniers are usually formed within a scribe sheet, in order to avoid interference with device structures, they may not be easy to find. The x-direction and the y-direction may be a relatively large distance from each other. Even if the x-direction and y-direction are in close proximity, it will require reading two verniers to determine proper alignment in both axes. This wastes time during inspections, and may slow wafer fab processing. In some cases where the vernier is not within a scribe line, it may also waste valuable wafer area that could be used for constructing devices.  
         SUMMARY OF THE INVENTION  
         [0007]    A two-dimensional vernier formed on a substrate is provided. The vernier comprises a first two-dimensional array of spaced shapes and a second two-dimensional array of spaced shapes overlying the first two-dimensional array of spaced shapes. The first two-dimensional array has a first distance across each shape and a first distance between adjacent shapes. The first two-dimensional array has a first pitch defined by the first distance across each shape and the first distance between adjacent shapes. The second two-dimensional array has a second distance across each shape and a second distance between adjacent shapes. The second two-dimensional array has a second pitch defined by the sum of second distance across each shape and the second distance between adjacent shapes. The second pitch is different from the first pitch and this difference determines the resolution of the vernier.  
           [0008]    The vernier can be symmetrical in the x-axis and the y-axis having the same number, size and spacing of shapes in both axes. Alternatively, the vernier may be asymmetrical with a different number, size or spacing of shapes in one direction versus another.  
           [0009]    In addition to the two arrays of shapes, the vernier may further comprise measurement guides. The measurement guides may include centerline marks, direction marks, both positive and negative, and alignment marks. The measurement guides are preferably formed adjacent to the first array of spaced shapes. Alternatively, the measurement guides are formed adjacent to the second array of spaced shapes.  
           [0010]    The first array of spaced shapes may be patterned as either a dark field pattern, in which spaced shapes appears bright, or a light field pattern, in which the spaced shapes appear dark. Likewise the second array of spaced shapes can be either dark field or light field regardless of whether the underlying first array of spaced shapes is dark field or light field.  
           [0011]    A method of forming the vernier described above comprises the steps of forming a reference pattern on the substrate. This step of forming the reference pattern on the substrate may be accomplished by etching the substrate, or by depositing a material over the substrate and etching the material. The reference pattern preferably includes the first array of spaced shapes and the measurement guides. Following the step of forming the reference pattern, a step of depositing a layer of photoresist over the reference completed. Then, the photoresist is patterned to produce an active pattern, which comprises the second array of spaced shapes.  
           [0012]    The two-dimensional vernier may be used to readily determine misalignment of two patterns formed over a substrate in two dimensions simultaneously, that is without looking at two different verniers. A method of determining misalignment between two patterns formed over a substrate comprising the steps of: positioning a two dimensional vernier under a microscope, determining an alignment region, identifying a pair of overlapped shapes that are most fully aligned, and ascertaining the position of the pair of overlapped shapes.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 (prior art) is a plan view of a one dimensional vernier.  
         [0014]    [0014]FIG. 2 is a plan view of a reference layer with a light field.  
         [0015]    [0015]FIG. 3 is a plan view of an active layer light field.  
         [0016]    [0016]FIG. 4 is a plan view of a reference layer with a dark field.  
         [0017]    [0017]FIG. 5 is a plan view of an active layer with a dark field.  
         [0018]    [0018]FIG. 6 is a cross-sectional view of a reference layer that is flat.  
         [0019]    [0019]FIG. 7 is a cross-sectional view of a reference layer that is indented.  
         [0020]    [0020]FIG. 8 is a cross-sectional view of a reference layer that is raised.  
         [0021]    [0021]FIG. 9 is a cross-sectional view of an active layer overlying the reference layer of FIG. 6.  
         [0022]    [0022]FIG. 10 is a cross-sectional view of an active layer overlying the reference layer of FIG. 7.  
         [0023]    [0023]FIG. 11 is a cross-sectional view of an active layer overlying the reference layer of FIG. 8.  
         [0024]    [0024]FIG. 12 is a cross-secional view of an active layer with a dark field overlying the reference layer in FIG. 6.  
         [0025]    [0025]FIG. 13 is a plan view showing an active layer overlying a reference layer in perfect alignment.  
         [0026]    [0026]FIG. 14 is a plan view showing an active layer overlying a reference layer in an out of alignment condition.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Referring to FIG. 2 and FIG. 3, a two dimensional vernier, also referred to as an x/y vernier, is provided. A first part of the vernier, a reference pattern  50 , is shown in FIG. 2. The reference pattern  50  comprises a first array of shapes  52 . In addition, x-axis measurement guides  53  (indicated by a dotted line) and y-axis measurement guides  54  (indicated by a dotted line) may be added adjacent the first array of shapes  52 . Each set of either x-axis measurement guides  53 , or y-axis measurement guides  53  comprise a set of centerline marks  55 , a set of negative direction marks  56 , a set of positive direction marks  58 , and miscellaneous alignment marks  60 , which may be used to indicate acceptable ranges of misalignment. The various alignment marks are preferable, but are not necessary to the formation or use of the vernier. The first array of shapes  52  is shown as small squares. They may be any desired shapes but squares or circles are preferable.  
         [0028]    [0028]FIG. 3 shows a second part of the vernier, an active pattern  64 . The active pattern  64  comprises a second array of shapes  66 . Just as with the first array of shapes  52 , the second array of shapes  66  is shown as small squares. As discussed above, other shapes may be used including circles. The second array of shapes  66  need not be comprised of the same shapes as those used for the first array of shapes. As shown, there are no alignment marks on the active pattern  64 . It is possible to incorporate measurement guides into the active pattern  64 , either in addition to, or instead of, the x-axis measurement guides  53  and the y-axis measurement guides  54  incorporated into the reference pattern shown in FIG. 2.  
         [0029]    The vernier is formed by delineating the reference pattern  50  onto the substrate, or a layer overlying the substrate, and superimposing the active pattern  64  over the reference pattern  50  by patterning the active pattern  64  into an overlying photoresist layer.  
         [0030]    The design of the vernier will now be discussed. Referring again to FIG. 2, the total distance across the reference pattern  50  is shown at  70  as D ref . Reference linewidth, L ref , is shown at  72 . Reference spacewidth, S ref , is shown at  74 . Reference pitch, P ref , is shown at  76 . The reference pitch is equal to the sum of the reference linewidth and the reference spacewidth (P ref= L ref+ S ref ). The reference center, C ref ,  78  is shown as corresponding to the reference linewidth, L ref . The reference center, C ref ,  78  may correspond to the reference spacewidth, S ref , where the number of shapes on a given axis are even, instead of odd.  
         [0031]    The number of shapes, N, that traverse the vernier is an integer value.  
           N =( D   ref   +C   ref )/ P   ref    
         [0032]    where C ref  equals L ref  or S ref . For example referring to FIG. 2, where N equals 11, L ref  equals S ref , and C ref  equals L ref , it is possible to solve for the distance across D ref .  
           D   ref =21 ×L   ref    
         [0033]    So, if the linewidth L ref  equals 3 micrometers, the total distance across the vernier, D ref , will be 63 micrometers.  
         [0034]    Referring again to FIG. 3, the total distance across the actvive pattern  64  is shown at  80  as D. Active linewidth, L, is shown at  82 . Active spacewidth, S, is shown at  84 . Active pitch, P, is shown at  86 . The active pitch is equal to the sum of the active linewidth and the active spacewidth (P=L + S). The active center, C,  88  is shown as corresponding to the active linewidth, L. The active center, C,  88  may correspond to the active spacewidth, S, where the number of shapes on a given axis are even, instead of odd.  
         [0035]    The most important aspects of the vernier, are the vernier resolution, Res, and the vernier range, Range. The resolution is defined as the smallest unit of distance of misalignment that can be determined with the vernier and is equal to the difference between the reference pitch and the active pitch.  
         
       Res=|P−P 
       ref| 
     
         [0036]    The range of the vernier is defined as the maximum misalignment that can be determined.  
         Range= Res× ( N− 1)  
         [0037]    The range and the resolution of the vernier can be independently designed. For a given resolution, the range can be increased by increasing N. N may be increased without needing to increase the distances across either vernier, D ref  or D. For a given range, the resolution can be adjusted by controlling the relative pitches between the reference pattern and the active pattern.  
         [0038]    As shown the reference pattern  50  and the active pattern  64  are symmetrical, having the same D ref  or D for both the x-direction and the y-direction. Although a symmetrical pattern is generally preferred, in some applications an asymmetrical pattern may be used for example one direction could be longer than another, have a different resolution, or a different range than the other.  
         [0039]    The reference pattern  50  shown in FIG. 2 is referred to as a light field because the array of shapes  52  are viewed as dark on a light background. FIG. 4 shows a reference pattern  50  with the opposite polarity, also referred to as a dark field. Likewise, FIG. 3 showed an active pattern  64  with a light field. FIG. 5 shows an active pattern  64  with a dark field. Depending upon the materials being used and the method selected to delineate either the reference pattern  50  or the active pattern  64 , any combination of light field or dark field arrangements can be used. The reference pattern can be dark field or light field. The active pattern can be dark field or light field, regardless of the polarity of the underlying reference pattern.  
         [0040]    [0040]FIG. 6 shows a cross section of a portion of the reference pattern  50  formed using an underlying substrate  92 . The portion of the reference pattern  50  comprises two adjacent shapes  94 . The adjacent shapes  94  correspond to individual shapes within the first array of shapes  52 . As shown in FIG. 6, the adjacent shapes  94  are level with an upper surface of the substrate. This is a typical arrangement following CMP. For example, in the case of copper metal lines, copper will be inlaid and then poslished.  
         [0041]    [0041]FIG. 7 shows a cross section of a portion of the reference pattern  50  formed using an underlying substrate  92 . The portion of the reference pattern  50  comprises two adjacent shapes  94 . The adjacent shapes  94  correspond to individual shapes within the first array of shapes  52 . As shown in FIG. 7, the adjacent shapes  94  are etched into the substrate  92 . Etching is a common method of patterning a wide variety of materials used in the semiconductor industry.  
         [0042]    [0042]FIG. 8 shows a cross section of a portion of the reference pattern  50  formed using an underlying substrate  92 . The portion of the reference pattern  50  comprises two adjacent shapes  94 . The adjacent shapes  94  correspond to individual shapes within the first array of shapes  52 . As shown in FIG. 8, the adjacent shapes  94  are formed by depositing a material overlying the substrate  92  and patterning it. Deposition and patterning, by etching, are common methods of producing semiconductor device structures.  
         [0043]    [0043]FIG. 9 shows a portion of the active pattern  64  overlying the portion of the reference pattern  50  shown in FIG. 6. The portion of the active pattern comprises overlying shapes  100 . The overlying shapes  100  correspond to individual shapes within the second array of shapes  66 . As shown in FIG. 9, the overlying shapes  100  are formed by depositing photoresist and patterning to form the overlying shapes  100 . The overlying shapes  100  have an upper surface  102  that is relatively flat, because the overlying shapes are overlying adjacent shapes  94  that are level with the upper surface of the substrate.  
         [0044]    [0044]FIG. 10 shows the portion of the active pattern  64  overlying the portion of the reference pattern  50  shown in FIG. 7. The portion of the active pattern comprises overlying shapes  100 . The overlying shapes  100  correspond to individual shapes within the second array of shapes  66 . As shown in FIG. 10, the overlying shapes  100  are formed by depositing photoresist and patterning to form the overlying shapes  100 . The upper surface  102  of the overlying shapes  100  are not flat, because the overlying shapes are overlying adjacent shapes  94  that are etched, and the overlying shapes follow the contour of the underlying adjacent shapes  94 .  
         [0045]    [0045]FIG. 11 shows the portion of the active pattern  64  overlying the portion of the reference pattern  50  shown in FIG. 8. The portion of the active pattern comprises overlying shapes  100 . The overlying shapes  100  correspond to individual shapes within the second array of shapes  66 . As shown in FIG. 11, the overlying shapes  100  are formed by depositing photoresist and patterning to form the overlying shapes  100 . The upper surface  102  of the overlying shapes  100  are not flat, because the overlying shapes are overlying adjacent shapes  94 , are deposited and etched to produce positive relief, and the overlying shapes follow the contour of the underlying adjacent shapes  94 .  
         [0046]    [0046]FIG. 12 shows the portion of the active pattern  64  overlying the portion of the reference pattern  50  similar to that shown in FIG. 9 but with a dark field. The portion of the active pattern comprises overlying shapes  100 . The overlying shapes  100  correspond to individual shapes within the second array of shapes  66 . As shown in FIG. 10, the overlying shapes  100  are formed by depositing photoresist and patterning to form the overlying shapes  100 . However, in this case the overlying shapes  100  are formed as trenches by etching into the photoresist, which remains to act as the dark field. Dark field active patterns can also be used with reference patterns shown in FIG. 10 and FIG. 11.  
         [0047]    [0047]FIG. 13 shows a top view of the active pattern  64  overlying the reference pattern  50 . Following formation of the reference pattern  50  the active pattern  64  is overlaid on it. An operator can view the complete vernier through a microscope. The operator looks down at the vernier and sees a view similar to that shown in FIG. 13. As shown in FIG. 13 the alignment is essentially perfect. The active pattern  64  is aligned over the reference pattern  50  such that the region of apparent optimum alignment of the first array of shapes and the second array of shapes is in an apparent alignment region  110 . More specifically optimum shape alignment is apparent at a pair of overlapped shapes  112 . Since overlapped shapes  112  are in line with the centerline marks in both the x-axis and the y-axis the patterns are properly aligned. The operator is able to determine the alignment by looking at a single vernier and determining x and y alignment, without the need to significantly reposition the microscope.  
         [0048]    [0048]FIG. 14 shows a top view of the active pattern  64  overlying the reference pattern  50 . Again, the operator can view the complete vernier through a microscope. The operator looks down at the vernier and sees a view similar to that shown in FIG. 13, but this time there is a misalignment apparent. As shown in FIG. 14 the alignment is off slightly in both the x-axis and the y-axis. The overlapped shapes  112  in the apparent alignment region  110  that is most aligned is two positions up and three positions over from the optimal centered position  116 . Each integer shift in the apparent position corresponds to the absolute difference in the reference pitch and the active pitch as discussed above. So for example, if the difference in pitch were 0.05 micrometers, the operator would determine that the misalignment was +0.15 micrometers in the x-direction and −0.10 micrometers in the y-direction. The operator would then confirm whether this was an acceptable misalignment.  
         [0049]    It is thus possible using this two dimensional vernier, for the operator to calculate the level of misalignment in two dimensions simultaneously. Simultaneously as used herein means that no realignment of the microscope to a second vernier is required when taking a reading for two axes of alignment.