Abstract:
A method for defect segmentation in features on semiconductor substrates is disclosed. After acquisition of an image of a semiconductor substrate, identical features or feature elements are subtracted from one another. The resulting difference function is compared with an upper and a lower threshold in order to identify defects.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of the German patent application 103 31 593.4 which is incorporated by reference herein.  
       FIELD OF THE INVENTION  
       [0002]     The invention concerns a method for defect segmentation in features on semiconductor substrates.  
       BACKGROUND OF THE INVENTION  
       [0003]     Defects can be accentuated by determining the difference between images of equivalent semiconductor features. The difference image is disrupted by noise. Defects can be distinguished from defect-free regions using an (adaptive) threshold. Dilation and erosion of the defect image does not always produce the desired result. Images of continuous faults, e.g. scratches or bubbles, on semiconductor structures can result in various deviations from a reference image. The amplitude of the fault signal can vary depending on the substrate. A threshold determines which fault signal is to be evaluated as a fault. If that threshold is set too low, pseudo-defects then occur as a result of the noise. If it is set too high, it may happen that continuous defects are broken down by noise into numerous individual defects.  
         [0004]     In semiconductor manufacturing, wafers are sequentially processed in a plurality of process steps during the manufacturing process. With increasing integration density, requirements in terms of the quality of the features configured on the wafers become more stringent. To allow the quality of the configured features to be checked, and any defects to be found, a corresponding requirement exists in terms of the quality, accuracy, and reproducibility of the components and process steps used on the wafers. This means that during production of a wafer, with the many process steps and many layers of photoresist, or the like, to be applied, early and reliable detection of defects in the individual features is particularly important. As a result of the patterning, in certain regions of the patterning faults may occur that are discovered and detected by a comparison of mutually corresponding features or feature elements.  
       SUMMARY OF THE INVENTION  
       [0005]     It is the object of the invention to create a method that makes possible a segmentation of defects in difference images of equivalent features on semiconductor substrates, and simultaneously prevents the breakdown of large defects into multiple individual defects.  
         [0006]     This object is achieved by a method for the inspection of features on semiconductor substrates, characterized by the following steps: 
        acquiring an image of at least one semiconductor substrate that encompasses a plurality of elements having identical recurring features;     creating a difference profile from two mutually corresponding features or feature regions of the imaged semiconductor substrate; and     determining a defect on the basis of a lower threshold and an upper threshold, both being spaced away from and parallel to one another.        
 
         [0010]     It has proven advantageous if firstly an image of at least one semiconductor substrate is acquired, the image encompassing a plurality of elements that have identical recurring features. From the acquired images or image data, a difference function is determined from two mutually corresponding features or feature regions. The difference profile is compared with two thresholds in order to allow regions with a high difference amplitude to be classified as fault regions. A possible fault region is determined by the fact that the value of the difference function everywhere exceeds the lower threshold. It qualifies as a real defect region, however, only if the difference profile also exceeds the upper threshold at at least one point in that region. The fault regions, their extent, and their property of being deemed real, are automatically calculated using a computer program that is implemented in a computer of the system.  
         [0011]     The lower threshold defines, by intersections with the peaks of the difference profile, at least one region in the lower threshold that indicates possible defects. Application of the upper threshold allows regions in the upper threshold to be determined by way of intersections with the peaks of the difference profile, the possible defects being characterized as real defects if, for that purpose, the respective peak of the difference profile exceeds the lower threshold, and thus the region in the upper threshold lies above the region in the lower threshold. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The subject matter of the invention is depicted schematically in the drawings and will be described below with reference to the Figures, in which:  
         [0013]      FIG. 1  schematically depicts a system for detecting faults on wafers or patterned semiconductor substrates;  
         [0014]      FIG. 2   a  depicts the manner in which the images or image data of a wafer are acquired;  
         [0015]      FIG. 2   b  is a schematic plan view of a wafer;  
         [0016]      FIG. 3  schematically shows a comparison of two mutually corresponding features on a semiconductor substrate;  
         [0017]      FIG. 4  schematically depicts a pattern element with no defects;  
         [0018]      FIG. 5  schematically depicts a pattern element having several defects;  
         [0019]      FIG. 6  schematically depicts the difference between what is depicted in  FIG. 4  and in  FIG. 5 ;  
         [0020]      FIG. 7  schematically depicts the difference with a section line along which the determination of the defects is explained;  
         [0021]      FIG. 8  schematically depicts the application of the lower threshold to the difference profile;  
         [0022]      FIG. 9  schematically depicts the application of the upper threshold to the difference profile;  
         [0023]      FIG. 10  depicts a conventional threshold according to the existing art that is used in order to evaluate the difference signal with regard to defects; and  
         [0024]      FIG. 11  depicts the same difference signal as in  FIG. 10 , segmentation being performed here by means of a dual threshold. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 1  shows a system  1  for the inspection of features on semiconductor substrates. System  1  comprises, for example, at least one cassette element  3  for the semiconductor substrates or wafers. In a measurement unit  5 , images or image data of the individual wafers or patterned semiconductor substrates are acquired. A transport mechanism  9  is provided between cassette element  3  for the semiconductor substrates or wafers and measurement unit  5 . System  1  is enclosed by a housing  11 , housing  11  defining a base outline  12 . Also integrated into system  1  is a computer  15  that receives and processes the images or image data of the individual measured wafers. System  1  is equipped with a display  13  and a keyboard  14 . By means of keyboard  14 , the user can input data in order to control system  1 , or can also make parameter inputs in order to evaluate the image data of the individual wafers. On display  13 , several user interfaces are displayed to the user of the system.  
         [0026]      FIG. 2   a  is a schematic view of the manner in which images and/or image data of a wafer  16  are sensed. Wafer  16  is placed on a stage  20  that is movable in housing  11  of system  1  in a first direction X and a second direction Y. First and second direction X, Y are arranged perpendicular to one another. An image acquisition device  22  is provided above surface  17  of wafer  16 , the image field of image acquisition device  22  being smaller than the entire surface  17  of wafer  16 . In order to sense the entire surface  17  of wafer  16  with image acquisition device  22 , wafer  16  is scanned in meander fashion. The successively sensed individual image fields are assembled into an overall image of surface  17  of a wafer  16 . This is done also using computer  15  provided in housing  11 . In order to produce a relative motion between stage  20  and image acquisition device  22 , an X-Y scanning stage that can be displaced in the coordinate directions X and Y is used in this exemplary embodiment. Image acquisition device  22  is here installed immovably with respect to stage  20 . Conversely, of course, stage  2  can also be installed immovably, and image acquisition device  22  can be moved over wafer  16  in order to acquire images. Also possible is a combination of motion of image acquisition device  22  in one direction and of stage  20  in the direction perpendicular thereto. A variety of systems can be used as image acquisition devices  22 . On the one hand, both area cameras and linear cameras, which create microscopic or macroscopic images, can be used. The resolution of the camera is generally coordinated with the imaging optical system, e.g. the objective of a microscope or macroscope. For macroscopic images, the resolution is e.g. 50 μm per pixel. Wafer  16  is illuminated with an illumination device  23  which illuminates at least regions on wafer  16  that correspond to the image field of image acquisition device  22 . The concentrated illumination, which moreover can also be pulsed with a flash lamp, allows images to be acquired on the fly, i.e. with stage  20  or image acquisition device  22  being displaced without stopping to acquire the image. This allows a high wafer throughput. It is also possible, of course, to stop the relative motion between stage  20  and image acquisition device  22  for each image acquisition, and also to illuminate wafer  16  over its entire surface  17 . Stage  20 , image acquisition device  22 , and illumination device  23  are controlled by computer  15 . The acquired images can be stored by computer  15  in a memory  15   a,  and also retrieved again therefrom as necessary. As a rule, the wafer is moved beneath image acquisition device  22 . It is also conceivable, however, for image acquisition device  22  to be moved relative to the wafer. This motion is continuous. The individual images are achieved by the fact that a shutter is opened and a corresponding flash is triggered. The flash is triggered as a function of the relative position of the wafer, which is reported by way of corresponding position parameters of the stage that moves the wafer.  
         [0027]      FIG. 2   b  shows a plan view of a wafer  16  that is placed onto a stage  20 . Layers are applied onto wafer  16  and are then patterned in a further operation. A patterned wafer encompasses a plurality of elements  25  that, as a rule, comprise features  24  that are identical and recur in all elements  25 .  
         [0028]     As depicted in  FIG. 3 , a patterned semiconductor wafer or a semiconductor substrate comprises multiple stepper area windows (SAWs)  32  that in turn contain multiple dice  33 . “Streets”  34  are provided between dice  33 . A certain number of dice are exposed simultaneously using a stepper. The same recurring features or pattern elements  35  are present in the various dice  33 . A difference function  55  (see  FIG. 6  or  FIG. 7 ) is obtained by subtraction  36  of the image data of a first pattern element  37   1  from a second corresponding pattern element  37   2 . Identical features are always compared to one another for the determination of difference function  55 . If a fault is present on a pattern element, this results in a fluctuation or peak  70  in difference function  55 .  
         [0029]      FIG. 4  shows, by way of example, a pattern element  45  that encompasses several sub-elements  40 . Pattern element  45  is free of faults.  FIG. 5  shows a pattern element  46  that encompasses several faults or defects  47 .  FIG. 6  is a schematic depiction of the difference between pattern element  45  (without faults) and pattern element  46  (with faults  47 ). Difference image  48  substantially comprises the background and faults  47 , which emerge more clearly as a result of the differentiation. In  FIG. 7 , a line  49  is drawn to represent, by way of example, a section line along which an exemplifying graphical depiction of difference profile  55  (a brightness profile) is reproduced in  FIG. 8  and  FIG. 9 , and to illustrate application of the lower and upper thresholds. The brightness profile of the difference image is acquired along line  49 .  FIG. 8  depicts the application of a lower threshold  62  (see  FIG. 11 ) to difference image  48 . The intersection of lower threshold  61  with difference image  48  emphasizes faults  47 , and the extent of fault  47  at the level of lower threshold  62  is depicted as a first uniform, at least partly continuous surface  47   1 . When upper threshold  61  in  FIG. 9  is used, faults  47  are emphasized and the extent of fault  47  at the level of upper threshold  61  is depicted as a second uniform, at least partly continuous surface  47   2 .  
         [0030]      FIGS. 10 and 11  illustrate more clearly the manner in which the defects are ascertained. The three-dimensional difference profile or difference image along line  49  from  FIG. 7  is depicted for that purpose by way of example (a projection of the difference profile onto the drawing plane being depicted for illustrative purposes).  FIG. 10  shows the determination of a defect by means of a single threshold. Detection of a defect depends on the distance of the threshold from abscissa  63 . A first threshold  51 , second threshold  52 , and third threshold  53  are depicted, each leading to a different result upon detection of a defect. When one threshold  51 ,  52 , or  53  is used, correct segmentation of the defects in the context of a given difference signal  55  (as shown in  FIG. 10 ) is not possible. For example, if first threshold  51  located farthest away from abscissa  63  is selected, then not all defects will be found. With third threshold  53 , which is at the shortest distance from the abscissa, all defects are found but small fluctuations in difference signal  55  additionally result in incorrect detections, as labeled with the number  57  in  FIG. 10 . For second threshold  52 , its distance from the abscissa is selected in such a way that incorrect detections do not occur, but the detected defects break down into a plurality of individual defects labeled with the number  59  in  FIG. 10 .  
         [0031]      FIG. 1  shows the same difference signal  55  as in  FIG. 10 . Here the defects are segmented and detected by means of an upper threshold  61  and a lower threshold  62 . In the depiction selected in  FIG. 11 , upper and lower thresholds  61  and  62  are reproduced as lines. It is self-evident to one skilled in the art that when the thresholds are applied in a three-dimensional defect profile, the respective threshold becomes a plane. The defect profile can moreover encompass more than three dimensions.  
         [0032]     Upper and lower thresholds  61  and  62  are parallel to abscissa  63 . The distance between them, and their distances from abscissa  63 , can be defined by the user. The user utilizes, for example a mouse (not depicted) or keyboard  14  to move upper and lower thresholds  61  and  62  into positions favorable for the detection of defects. The user can also input a numerical value in the user interface and thereby define the positions of first and second thresholds  61  and  62  with respect to abscissa  63 .  
         [0033]     Two thresholds are considered in the example depicted in  FIG. 11 . The regions in which the difference profile exceeds lower threshold  62  are shown in  FIG. 8  and marked accordingly; the regions in which the difference profile exceeds upper threshold  61  are shown in  FIG. 9 . Only one peak  70  of difference profile  55  will be singled out for description. Upper threshold  61  intersects difference profile  55  at, among others, a first and a second intersection point  63  and  64 , whereas lower threshold  62  intersects difference profile  55  at the corresponding intersection points  73  and  74 . A real defect exists between intersection points  73  and  74 , since within region  66  there are points at which the upper threshold is exceeded by peak  70  of difference profile  55 , namely in the vicinity of region  65  between points  63  and  64 .  
         [0034]     Incorrect detections  57 , as evident e.g. from  FIG. 10 , are thus not detected as defects. The defects become somewhat larger as a result of upper and lower thresholds  61  and  62 . This is not a disadvantage, however, since more information is thus available for later classification of the defects. With the use of upper and lower thresholds  61  and  62 , breakdown into multiple individual defects can be prevented. Upper threshold  61  determines whether any defect at all is present. A defect is present only when at least one peak  70  of difference profile  55  exceeds upper threshold  61 . Lower threshold  62  determines the extent of the defect. Lower threshold  62  is evaluated in all directions of the selected pattern element. Merging of two individual defects, in cases where the interstice is characterized by a very small difference signal, can thus be prevented. Individual defects are likewise combined when the difference between them lies below upper threshold  61  and above lower threshold  62  solely as a result of noise. A further variant of this principle consists in adapting lower threshold  62  as a function of the distance from the nearest point above upper threshold  61 .