Abstract:
A method for measuring overlay shift is disclosed. An image is acquired of at least one reference element that comprises at least one first pattern element in a first plane and at least one second pattern element in a second plane. An image of a measurement element is likewise acquired. The shift value between the reference element and measurement element is ascertained by comparing the image of the reference element with the image of the measurement element. An output on a user interface indicates whether a predefined tolerance value is being exceeded.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of the German patent application 103 37 767.0 which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention concerns a method for measuring overlay shift. 
     BACKGROUND OF THE INVENTION 
     In the production of a semiconductor module, its patterns are fabricated in a variety of planes. A completed semiconductor module encompasses a plurality of planes in which the individual patterns are located. The orientation of the individual planes with respect to one another is of considerable importance. If a plane were shifted too much with respect to a previous or subsequent plane, this could result in an interruption of the connection between elements in one plane and the next. The orientation, shifting, and alignment of two successive planes is referred to as “overlay shift.” In semiconductor production, wafers are sequentially processed during the production process in a plurality of process steps. As integration density increases, requirements in terms of the quality of the patterns configured on the wafers become more stringent. To allow the quality of the configured patterns to be checked and any defects to be discovered, commensurate demands are placed on the quality, accuracy, and reproducibility of the components and process steps with which the wafers are handled. This means that in the production of a wafer, with the many process steps and many layers of photoresist or the like that must be applied, reliable and prompt detection of defects is particularly important. Equally significant for the quality of a semiconductor component is the overlay of the individual planes in the semiconductor component. It is thus particularly important that the shift of the individual planes remain within a tolerance range. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to create a method with which the overlay (the shift of successive planes) of a semiconductor substrate can be determined in simple fashion. 
     This object is achieved by way of a method for measuring overlay shift, comprising the following steps:
         acquiring an image of at least one reference element that has at least one first pattern element in a first plane and at least one second pattern element in a second plane;   traveling to at least one measurement element and acquiring an image of the measurement element;   ascertaining a shift value between the reference element and the at least one measurement element by comparing the image of the reference element with the image of the measurement element; and   generating an output if the shift value between the reference element and the measurement element exceeds a predefined tolerance value.       

     It is particularly advantageous if the following steps are performed in order to measure the overlay shift. Firstly, at least one image is acquired of a reference element that comprises at least one first pattern element in a first plane and at least one second pattern element in a second plane. Then at least one measurement element is traveled to, and an image of the measurement element is acquired. A shift value between the reference element and the at least one measurement element is then ascertained by comparing the image of the reference element with the image of the measurement element. If a predefined tolerance value is exceeded, an output to an operator is made on a user interface. 
     Several reference elements on one substrate can also be imaged, an average for evaluation of the measurement elements then being determined therefrom. 
     It is particularly advantageous if the reference element comprises a first pattern element that surrounds the second pattern element. The first pattern element and the second pattern element can each be constructed from an n-sided polygon. It is particularly suitable for determination of the overlay if the first pattern element and the second pattern element are each constructed from a regular rectangle or a square. 
     The operator selects a reference element, for example, via a user interface in such a way that a border is drawn around the reference element. The inspection arrangement encompasses a microscope that is equipped with a camera which acquires an image of a substrate region, of the reference element, and/or of the measurement element. The comparison of the image of the reference element with the image of the measurement element is performed by sub-pixel-accuracy pattern matching. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the invention is depicted schematically in the drawings and will be described below with reference to the Figures, in which: 
         FIG. 1  schematically depicts a system for ascertaining the overlay in semiconductor substrates; 
         FIG. 2  schematically depicts a user interface with which a user performs the overlay check; 
         FIG. 3   a  is a schematic view of a first embodiment of a reference element with which the overlay is determined; 
         FIG. 3   b  is a schematic view of the first embodiment of the reference element with which the overlay is determined, the matrix of a CCD being superimposed; 
         FIG. 4   a  is a schematic view of the first embodiment of the reference element with which the overlay is determined, the first plane being shifted with respect to the second plane; 
         FIG. 4   b  is a schematic view of the first embodiment of the reference element, the first plane being shifted with respect to the second plane and the matrix of a CCD being superimposed. 
         FIG. 5   a  is a view of a second embodiment of a reference pattern or reference element with which the overlay is determined; 
         FIG. 5   b  is a schematic view of the second embodiment of the reference pattern or reference element with which the overlay is determined, the first plane being shifted in the X direction with respect to the second plane; and 
         FIG. 6  is a view of a third embodiment of a pattern on the basis of which the overlay is checked. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary embodiment of an inspection arrangement  1  with which planar substrates S, for example wafers, can be investigated microscopically. In the context of the invention described here, for example, the shift of two successive planes of a wafer is investigated in order to ascertain any misalignment of the individual planes. Inspection arrangement  1  is equipped, for that purpose, with a microscope  2 . For image processing, microscope  2  can be equipped with a camera  3  having a CCD chip, the imaged microscopic subregion of the wafer being digitized. 
     Microscope  2  of inspection arrangement  1  can be directed onto a substrate S, in this case a wafer, located at an inspection location I. Inspection location I is enclosed by a housing  4  in which microscope  2  is simultaneously also received. Also provided in housing  4  is a conveying device  5  for transporting substrates S to and from inspection location I. 
     Inspection arrangement  1  furthermore encompasses a first magazine  6  for receiving several substrates S. Additionally provided is a transfer device  7  which transfers substrates S from first magazine  6  to conveying device  5 . After inspection, substrates S are collected in a second magazine  8 . A further transfer device  9  serves to transfer substrates S from conveying device  5  into second magazine  8 . Magazines  6  and  8  are preferably embodied as replaceable magazines in which substrates S are stacked one above another. Each of magazines  6  and  8  is, for that purpose, coupled separately onto housing  3 . 
     Inspection arrangement  1  furthermore encompasses an operating console  10  that is arranged on one side of housing  4  at operating position P. Provided for that purpose, on viewing port  12  for microscope  2  projecting out of housing  4 , are two eyepieces  14  that extend over operating console  10 . 
     In addition to viewing port  12  for microscope  2 , a first viewing field  16  (display) for displaying an image or image area of substrate S, and a second viewing field  18  for direct viewing of substrate S or a subregion of substrate S, are provided on housing  4 . The two viewing fields  16  and  18  are arranged at an inclination with respect to an operator  20  in such a way that operator  20 , located in front of viewing port  12  of microscope  2 , looks at the respective viewing field  16  and  18  in substantially perpendicular fashion. Also provided in housing  4  is at least one computer  22  that is also used, among other purposes, for processing the images acquired with microscope  2 . 
       FIG. 2  schematically depicts a user interface  22  with which a user performs the overlay check or adjusts inspection arrangement  1  for the overlay check. On user interface  22 , an overview image of substrate S is displayed in a first window  24 . Substrate S is subdivided into multiple image windows  26  that can be imaged by microscope  2  of inspection arrangement  1 . It is self-evident to one skilled in the art that the size of image window  26  depends on the selected magnification of microscope  2 . Image window  28  currently being imaged by microscope  2  is displayed on user interface  22  as a solid rectangle. The center of substrate S is identified by a cross  30 . A further cross  30  identifies an image window in which a pattern for determining the shift of two planes on substrate S is also located. 
     In a second window  32  on user interface  22 , an image  34  of the current image window  28  imaged by means of camera  3  of microscope  2  is displayed. The acquired image encompasses at least one reference element  36  or measurement element on which the shift of two planes with respect to one another is to be determined. Reference element  36  encompasses at least one first pattern element  36   a  in a first plane  38 , and at least one second pattern element  36   b  in a second plane  40 . Although the description mentions only two planes whose overlay is to be determined, this is not to be construed as a limitation. It is equally conceivable for the measurement elements or reference elements  36  to comprise more than two pattern elements that are arranged in more than two different planes. The task is thus to ascertain the shift of the individual planes with respect to one another. Operator  20  selects the reference element in such a way that a border  42  is drawn around reference element  36 . Operator  20  can do this by way of operating console  10  or a mouse (not depicted). 
     Provided above second and first windows  32  and  24  is a bar  44  that encompasses several click buttons  45 . Each of click buttons  45  stands for a tool that operator  20  can call. The callable tools can encompass, for example, saving, calculation, measurement, magnification selection, image acquisition, etc. User interface  22  furthermore encompasses several subregions  46   a ,  46   b ,  46   c ,  46   d  that are provided for controlling the inspection arrangement or for outputting information for operator  20 . A first subregion  46   a  concerns input and output of a substrate S into inspection arrangement  1 . The data already saved in inspection arrangement  1  can also be managed here. Data already saved for overlay checks of previous substrates S can be retrieved, new data saved, or other data deleted. A second subregion  46   b  concerns focus and position determination for a substrate S. Here, for example, it is possible to select between a laser focus and a TV focus. A third subregion  46   c  concerns the detection and programming mode. Here, for example, the inspection arrangement can be used to program in an overlay shift that is then utilized for further measurements on substrates S of a batch. The limit values within which an overlay shift is still regarded as acceptable are defined in the programming mode. A fourth subregion  46   d  concerns the inspection position. Here operator  20  can store or edit several operating positions so that inspection arrangement  1  travels to the corresponding positions on the substrate. 
     A control element  47  is depicted on user interface  22  below first window  24 . With control element  47 , operator  20  can displace substrate S in such a way that a specific region is imaged by microscope  2  and camera  3 . The displacement of substrate S can be accomplished with a conventional motor-controlled XYZ stage (not depicted). Also provided in the vicinity of control element  47  are several windows  48  which display, for example, the X position and Y position of the image window of substrate S that is currently located in the observation position of microscope  2 . Further windows  49  display to operator  20  the row and column on substrate S in which the image window of substrate S currently being imaged is located. 
     First window  24  is moreover equipped with a plurality of tabs  50 . Using the tabs, operator  20  can make selections such as Wafer Boat, Wafer Map, Statistic, Info, Gallery, etc. 
       FIG. 3   a  is a schematic view of a first embodiment of a reference pattern or reference element  36  with which the overlay is determined. Reference element  36  encompasses at least one first pattern element  36   a  in a first plane  38 , and at least one second pattern element  36   b  in a second plane  40 . Note that first plane  38  lies below second plane  40 .  FIG. 3   b  is a schematic view of the first embodiment of reference element  36  with which the overlay is determined, a matrix  50  of a CCD of camera  3  being superimposed on reference element  36 . Matrix  50  of the CCD comprises a plurality of pixels  52  that acquire the image of reference element  36 . As compared with  FIG. 3   a ,  FIG. 4   a  depicts a schematic view of the first embodiment of reference element  36  with which the overlay is determined, first plane  38  having been shifted with respect to second plane  40 . The difference as compared with  FIG. 3   a  results from a shift of second pattern element  36   b  in the X direction with respect to first pattern element  36   a . A shift in the X direction and Y direction is likewise possible, but is not mentioned here for reasons of simplicity. 
       FIG. 4   b  is a schematic view of the first embodiment of reference element  36 , first plane  38  having been shifted with respect to second plane  40 , and matrix  50  of the CCD of camera  3  being superimposed. The signals of individual pixels  52  of the CCD are employed to ascertain the shift. Determination of the overlay requires the presence of at least one substrate S or wafer that comprises reference elements having either a correct alignment or a known misalignment. From that substrate S or wafer, an image of the reference element is grabbed. This has already been described in  FIGS. 3   b  and  4   b . For example, individual pixels  52  of matrix  50  of a CCD acquire the image of reference element  36 . Reference element  36  possesses patterns that are contained in both layers or planes whose mutual alignment is to be measured. Operator  20  must define which patterns belong to which layers. In the exemplary embodiment disclosed in  FIG. 3   a , this is a so-called box-in-box pattern, and definition is performed by drawing the rectangular border  42  (see  FIG. 2 ). Patterns of any desired complexity are also, however, possible as reference elements (see  FIG. 5  and  FIG. 6 ). For determination of a shift value between reference element  36  and the at least one measurement element, a comparison is made between the image of reference element  36  and the image of the measurement element. The comparison is performed for each of the two planes  38  and  40  by sub-pixel-accuracy pattern matching against the image of reference element  36 . Only the pattern elements of one plane or layer are searched for in each case. The misalignment M is calculated in accordance with equation 1:
   M =(( A−A   0 )−( B−B   0 ))×(pixel size)+ M   0 ,  (Equation 1) 
where A denotes the position of first pattern element  36   a  in first plane  38  and B the position of second pattern element  36   b  in second plane  40  of pattern element  36  ( FIG. 4   a ) in the measured image. Similarly, A 0  denotes the position of first pattern element  36   a  in first plane  38 , and B 0  the position of second pattern element  36   b  in second plane  40  of pattern element  36  ( FIG. 4   a ) in the reference image. M 0  is the misalignment of reference element  36  on substrate S or the reference wafer.
 
       FIG. 5   a  is a view of a second embodiment of a reference element (or reference pattern)  60  with which the overlay is determined. Reference pattern  60  comprises a plurality of first pattern elements  60   a  and a plurality of second pattern elements  60   b . Reference element  60  is a comb-like pattern, first pattern elements  60   a  being arranged in a first plane and second pattern elements  60   b  in the second plane. Reference pattern  60  comprises a first sub-pattern  62 , a second sub-pattern  63 , a third sub-pattern  64 , and a fourth sub-pattern  65 . First and second sub-patterns  62  and  63  are arranged in such a way that longitudinal axes of first and second pattern elements  60   a  and  60   b  are parallel to the Y direction. Third and fourth sub-patterns  64  and  65  are arranged in such a way that longitudinal axes of first and second pattern elements  60   a  and  60   b  are parallel to the X direction. The depiction in  FIG. 5   a  shows reference pattern  60  in which no shift exists between the first and second planes. 
       FIG. 5   b  is a schematic view of the second embodiment of reference pattern (or reference element)  60  with which the overlay is determined, the first plane being shifted in the X direction with respect to the second plane. The shift is evident from the fact that in first and second sub-patterns  62  and  63 , second pattern elements  60   b  are shifted more toward first pattern elements  60   a . In third and fourth sub-patterns  64  and  65 , second pattern elements  60   b  and first pattern elements  60   a  are pulled apart in the X direction relative to one another. The magnitude of the shift is determined, as in the first exemplary embodiment, with sub-pixel accuracy. 
       FIG. 6  is a view of a third embodiment of a pattern on which the overlay of a first and a second plane is checked. Any pattern on a substrate S or wafer that has defined pattern elements in different planes is suitable for overlay checking. In the exemplary embodiment depicted in  FIG. 6 , reference pattern  70  comprises a first pattern element  70   a  and a second pattern element  70   b . First pattern element  70   a  comprises a flat portion  72  and an angled extension  73 . First pattern element  70   a  is arranged in a first plane. Adjoining the first pattern element is a second pattern element  70   b  that extends substantially parallel to the X direction. The second pattern element is arranged in a plane that differs from the first plane. A shift of the first plane with respect to the second would result, in this embodiment, in a defective transition from first pattern element  70   a  to second pattern element  70   b.