Abstract:
A method of overlaying two images and from this overlay observe and measure the accuracy of the alignment of the wafer. Wafer misalignment can be readily corrected based on the results of these observations. Alignment marks are provided on the surface of the wafer that is being validated for accuracy of alignment. The position of this mark relative to a pattern provided on the surface of a control wafer is measured and forms an indication of the alignment of the wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the fabrication of integrated circuits and, more specifically, to a method and apparatus for measuring and calibrating the registration between overlying layers on the surface of a semiconductor wafer. 
     2. Description of the Prior Art 
     The fabrication of complex semiconductor devices involves multiple processing steps. Multiple patterned layers of different materials are applied to a substrate to create the desired electronic semiconductor device. The different layers overlie each other and must be accurately registered to ensure proper operation of the semiconductor device. Displacement between corresponding features on different layers can degrade the performance of the device or can cause the device to be totally inoperative. As semiconductor devices have become increasingly complex, the feature dimensions have been correspondingly reduced. This reduction in feature dimensions has reduced acceptable tolerances or displacements between the various layers within a semiconductor device. 
     Most semiconductor devices are now made using photolithographic techniques. Such techniques involve the exposure of the surface of a semiconductor body to a particular pattern, and the subsequent formation or development of that pattern into permanent form through the use of wet or dry etching techniques that create various regions and structures on the surface of the semiconductor body. As is well known in the state of the art, photolithographic procedures require that a mask be used to define those portions of the semiconductor material where various elements of semiconductor devices are to be located. Because different parts or elements of these semiconductor devices must be located at precisely defined distances from each other, it is desirable that each of the masks used in forming the semiconductor device is aligned with respect to the other masks as precisely as possible both in vertical and in horizontal directions. 
     These operations of alignment are typically performed visually by examining the surface of the semiconductor wafer and the mask under a microscope. 
     Standard practice in positioning and aligning a wafer during wafer processing operations is to use an inspection pattern on the semiconductor wafer to determine the degree of alignment of a first device layer. 
     A great number of the processing steps used during the manufacturing of semiconductor devices are based on the application of photolithographic exposures. Of key importance in making successive photolithographic exposures is that successive layers of patterns are accurately aligned with respect to each other. The degree of misalignment that can occur between successive photolithographic exposures is known as pattern overlay, overlay than represents the degree of misalignment that occurs between successive layers of patterns on thin film electronic structures and the preceding layer. 
     The term overlay represents the relative location of features formed during different steps of the semiconductor wafer processing sequence. The overlay is a numeric quantity that is defined at every location on the substrate as the difference between a numerical value indicative of a position or location on the first formed portion of a semiconductor structure on a substrate and a numeric quantity of the corresponding point on a following or second formed portion of a semiconductor structure. Perfect alignment between the first and the second portion of the semiconductor structure requires that the overlay, as defined here, be equal to zero. 
     One approach in aligning wafer is to use an independent process layer, the so-called zeroth layer, as the source of reference and to align all process layers to this zeroth layer. Techniques and measuring tools are provided to measure the degree of shift that occurs in the overlay of the successive layers and patterns. All these techniques use alignment patterns of a particular design that are applied to both successive and preceding layers. The first layer used in this alignment sequence does, by its very nature, not have a reference point or pattern. This may lead to considerable problems of alignment in subsequent alignment steps. 
     FIG. 1 shows the Prior Art method of placing reference marks  10  on the surface of wafer  12 , this top view of the wafer surface represents the previously highlighted zeroth layer process. This process places the reference marks on the surface of the substrate. Successive formations of patterns use marks  10  as alignment marks, it is a given that the overlay of the marks  10  for the successive patterns that are formed on the semiconductor substrate is zero. That is the marks  10  are, going from the preceding to the following deposition of patterns, in perfect alignment. Measured is the overlay within the successive patterns while these patterns are being created. 
     FIG. 2 a  shows the Prior Art creation of a preceding pattern  14  formed on the wafer  12  by use of prior art method of chip manufacturing using two intersecting patterns  18  and  20 . FIG. 2 b  shows a magnification of the pattern  14  as representative of the first pattern that is created on the surface of the semiconductor substrate. This pattern is created at scribe lines within the surface of the semiconductor substrate and serves as the reference pattern for the measurement of the alignment of the following patterns. 
     FIG. 3 a  shows the Prior Art formation of a following or second pattern  16  on the surface of the semiconductor substrate as representative of the second pattern that is created on the surface of the semiconductor substrate. The pattern  16  is created by use of prior art method of chip manufacturing using two intersecting patterns  22  and  24 . The pattern  16  (FIG. 3 a ) is roughly in the same geometric location on the wafer surface as the previously highlighted first pattern (pattern  14 , FIG. 2 a ). FIG. 3 b  shows a magnified image where the reference pattern (pattern  14 , FIG. 2 a ) is superimposed over the pattern that is representative of the second pattern (pattern  16 , FIG. 3 a ). The smaller square  26  is patterned in the second pattern in the same geometric location as the reference square  28 . 
     It is clear from FIG. 3 b  that in measuring values for x 1 , x 2 , y 1  and y 2  accurate conclusions can be drawn relating to the relative position of the second pattern (pattern  16 , FIG. 3 a ) with respect to the first or reference pattern (patter  14 , FIG. 2 a ). It is also clear that the alignment accuracy of following layers can be determined in the same manner. 
     The use of marks on the mask and on the wafer is known to facilitate alignment between the various masks. This approach however can be very time consuming and is influenced by human error. The present invention therefore teaches a method that is both simple and dependable, thus contributing to increased product reliability and a considerable improvement in product yield. 
     U.S. Pat. No. 5,617,340 (Cresswell et al.) shows a method for measuring overlay and for calibrating image tools. 
     U.S. Pat. No. 5,280,437 (Corliss) shows a method for direct calibration of registration measurement systems. 
     U.S. Pat. No. 4,571,538 (Chow) shows a mask alignment measurement structure. 
     U.S. Pat. No. 4,538,105 (Ausschnitt) discloses an overlay test wafer. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, it is an objective of the present invention to facilitate wafer alignment procedures. 
     Another objective of the present invention is to eliminate the need for using standard or reference wafers presently used for wafer alignment. 
     Yet another objective of the present invention is to allow for quick and dependable validation of wafer alignment during scheduled Preventive Maintenance. 
     Yet another objective of the present invention is to allow for frequent and dependable verification of wafer alignment during semiconductor manufacturing operations. 
     Yet another objective of the present invention is to verify the reliability of wafer alignment. 
     Yet another objective of the present invention is to decrease the overall cost of manufacturing semiconductor devices. 
     Yet another objective of the present invention is to simplify wafer alignment procedures by requiring no more that one reference mark on the control wafer. 
     In accordance with the indicated objectives of the present invention, the invention teaches a method of overlaying two images and from this overlay observe and measure the accuracy of the alignment of the wafer. Wafer misalignment can be readily corrected based on the results of these observations. Alignment marks are provided on the surface of the wafer that is being validated for accuracy of alignment. The key point of the invention is that double exposure is performed on the same wafer to validate alignment between two reference marks. The relative positioning of the two reference marks with respect to each other determines the amount of overlay of the wafer that is being evaluated for proper positioning. The term overlay as used here indicates the accuracy by which a wafer handling tool positions a wafer within that tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a wafer surface with Prior Art zeroth layer. 
     FIG. 2 a  shows a wafer surface with Prior Art first pattern. 
     FIG. 2 b  shows a wafer surface with a magnification of the Prior Art first pattern. 
     FIG. 3 a  shows a wafer surface with Prior Art second pattern. 
     FIG. 3 b  shows a wafer surface with a magnification of the Prior Art second pattern superimposed over the Prior Art first pattern. 
     FIG. 4 shows the alignment marks on top of the surface of the wafer that is being validated for alignment accuracy. 
     FIGS. 5 a  through  5   d  show the process that is being used to perform the alignment procedure, as follows: 
     FIG. 5 a  shows the alignment measurement tool used during the alignment measurement procedure. 
     FIG. 5 b  shows the image of the small box alignment marks on top of the wafer that is being checked for alignment accuracy. 
     FIG. 5 c  shows the image of the big frame image on top of the wafer that is being checked for alignment accuracy. 
     FIG. 5 d  shows the pattern that is developed during the alignment procedure. 
     FIGS. 6 a  and  6   b  show a top view of the image obtained during the alignment procedure, as follows: 
     FIG. 6 a  shows a top view of the image obtained for a wafer which is aligned since the small box  34  is centered in the middle of the big frame  32 . 
     FIG. 6 b  shows a top view of the image obtained for a wafer which is not aligned since the small box  34  is not centered in the middle of the big frame  32 . 
     FIGS. 7 a  and  7   b  show how the alignment procedure of the present invention can be applied for increased accuracy of alignment, as follows: 
     FIG. 7 a  shows a multiplicity of big frames with the adjustments D-x that have to be made in order to center the small box in the center of the big frame. Where, for instance D-60 is indicated, the X-Y distance have to be adjusted by 60 nano-meter in order for the small box to be within the center of the big frame. 
     FIG. 7 b  shows the various adjustments that can be made to the small box in order to center the small box within the big frame. 
     FIG. 8 shows the image or mask used for the creation of the big frame. 
     FIG. 9 shows the big frame after its creation. 
     FIG. 10 shows the small box after its creation. 
     FIG. 11 shows the image or mask used for the creation of the small box. 
     FIG. 12 shows the overlay of the big frame with the small box. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now specifically to FIG. 4, there is shown production wafer  20  with two alignment marks  10  and  15  together with the wafer orientation notch  25 . Wafer  20  is the wafer that is being validated for alignment accuracy, that is it is the wafer that has been entered into a wafer handling tool whereby the alignment of the wafer within this tool needs to be determined. The alignment marks  10  and  15  are at opposite sites of the wafer  20  with the line connecting these two marks running through the geometric center of the wafer. The two alignment marks  10  and  15  have the shape of a square, the size of the sides of this square are not basic to the intent of the present invention. The two alignment marks  10  and  15  are of equal size, that is approximately 0.5×0.5 mm. 
     FIG. 5 a  shows the alignment tool  30  used to observe the wafer alignment. Wafer alignment is, in most instances, performed by validating the alignment of the photo-resist pattern. 
     the procedure for checking wafer alignment within a particular wafer handling tool is as follows. As first step, the wafer is inserted into the tool that is being evaluated for wafer alignment accuracy. The alignment-measuring tool  30  creates a first exposure  40  of the “big frame”,  42 , FIG. 5 b . Wafer alignment tool  30  (FIG. 5 a ) makes a second exposure  50 , FIG. 5 c , creating the image of the alignment “small box”  52 , FIG. 5 c . The position of the “small box”  52  is compared with the previously obtained image  42  of the “big frame”. These steps will be further highlighted in the following figures. 
     FIG. 5 b  shows a cross section view of the first exposure  40  of the “large frame”  42 . The accuracy or resolution setting of this alignment tool is at this time in the explanation not of great importance, it will later become apparent how this accuracy can further be used to improve the accuracy of measuring the wafer alignment. 
     FIG. 5 c  shows the cross section of the second exposure  50  obtained by the alignment tool  30  of the “small box”  52 . The areas  60  (FIG. 5 c ) are visually lined up with the areas  42  (FIG. 5 b ). 
     FIG. 5 d  shows the combined image of the “large frame” ( 42 , FIG. 5 b ) exposure  57  and the “small box” image ( 52 , FIG. 5 c ) exposure  59 . Since the image of the “large frame”  57  is used as the reference, it can readily understood that the degree of centering of image of the “small box”  59  within the large frame is a direct indication as to how well the wafer is aligned. This alignment than can be measured directly by measuring the parameters X 1  and X 2  of FIG. 5 d . This will be further highlighted in the following figures. 
     FIG. 6 a  shows a top view of the large frame  32 , which is the same as large frame  57  shown in FIG. 5 d , and the alignment small box  34 , which is the same as the small box  59  shown in FIG. 5 b . The relative positioning of the alignment small box  34  within the large frame  32  as indicated in FIG. 6 a  shows that the marker is centered in the middle of the large frame which is the condition where the production wafer is in perfect alignment with the standard or reference wafer. 
     FIG. 6 b  illustrates the case where the alignment marker  34  is not in the center of the large frame  32  in which case the production wafer is not in alignment with the reference wafer. 
     FIGS. 7 a  and  7   b  indicate a range of alignment settings where, for the ideal case, there is no shift of the alignment marker with respect to the sides of the large frame. The images of the large frame, FIG. 7 a , can be quickly adjusted with respect to the images of the alignment small box, FIG. 7 b . By centering the alignment marker within the large frame, the scale shown adjacent to the alignment markers provides a direct reading as to the alignment of the wafer. 
     Prior Art alignment measurements have used a production wafer in combination with a reference wafer. For instance, D+60 indicates that, by moving the wafer within the wafer handling tool and centering the alignment marker within the large frame after this movement, the X and Y coordinates of the production wafer have increased with 60 nano-meter (60 nm) with respect to its previous position. By therefore fixing the position of the reference wafer, the production wafer can be adjusted to perfect alignment with the reference wafer. Inversely, by not adjusting the production wafer with respect to the reference wafer, it can be measured how far the production wafer is out of alignment with respect to the reference wafer. This measurement reflects the previously indicated AB measurement, that is the measurement that indicates how far a wafer that is positioned in a wafer handling tool is out of alignment with respect with its desired position, that is the position of the standard wafer. 
     The present invention teaches the elimination of AB matching and the replacement of the AB alignment with the AA alignment. AB alignment used a production wafer and a reference wafer for monitoring of the overlay. The AA measurement is the measurement of wafer alignment accuracy by performing the above detailed double exposure of one wafer and from this double exposure determine how well that wafer is aligned within the wafer handling tool. 
     FIG. 8 shows the exposure  56  by the alignment analysis tool of the mark or image that is attached to the top of the standard or reference wafer. The mark in this case consists of a square  54 ; the cross section of the sides of this square is indicated as  52 . These sides are recorded by the failure analysis tool as a big frame. 
     FIG. 9 shows the frame  62  recorded by the wafer alignment analysis tool. This frame serves as a reference structure into which an image of the wafer that is being measured for alignment is projected. The positioning of this latter image with respect to frame  62  gives a measure of how the wafer that is being evaluated is positioned with respect to this frame. The areas, which are not exposed or show no image in the wafer analysis tool, are indicated as  64 . 
     FIG. 10 shows the recorded small box  82 , all the other areas within this image are not recorded by the image analysis tool. 
     FIG. 11 gives a cross section view of the frame or image that is provided on the top surface of the wafer that is being evaluated for alignment. The areas  72  are used for alignment with the big frame that has previously been observed (see above under FIG.  9 ), the wafer alignment tool does not record the areas  72  and areas  74 . The wafer alignment tool records the area  76 ; this area forms the so-called small box. 
     FIG. 12 shows the combination of the big frame  62  superimposed over the small box  82 . From this figure it is apparent that the dimensions  92 ,  94 ,  96 , and  98  are indicative as to how well the small box  82  is centered within the big frame  62  where the big frame  62  forms the standard or source of reference. 
     Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art, that various modifications may be made without departing f rom the spirit and scope of the present invention which is limited solely by the appended claims.