Abstract:
An object of the invention is to provide a defect inspection method which can prevent the failure in detecting a defect, caused by saturation of a pattern signal obtained by inspecting an inspected object, so that the investigation of the cause for defect occurrence can be done earlier. To achieve this object, according to an embodiment of the invention, there is provided a defect inspection that irradiates laser light on an inspected object having a pattern formed thereon, detects a signal from the inspected object and thereby detects a defect, the inspection including: inputting pattern information contained in layout data on the inspected object; determining based on the inputted pattern information, at least one of arrangement, repetitiveness and density for each of a plurality of inspected areas of the inspected object; estimating a saturation level of the detected signal based on the determination result; and determining a transmittance condition so that the signal does not saturate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a defect inspection method and defect inspection system for detecting a foreign matter or pattern defect in a semiconductor wafer, a photomask, a magnetic disk, a liquid crystal substrate or the like. 
     2. Background Art 
     In the process of fabricating semiconductor devices, a foreign matter or occurrence of a defect or the like in a circuit pattern results in a defective product. This is also the case with a magnetic disk and a liquid crystal substrate. Further, in the case of a photomask used to form patterns in a semiconductor wafer or a liquid crystal substrate, also, the presence of a defect causes wrong patterns to be formed in the semiconductor wafer or liquid crystal substrate, thus resulting in a defective product. 
     Descriptions will be given below by taking semiconductor wafer as an example. In inspecting a semiconductor wafer for defective appearance caused by a foreign matter or defective circuit pattern, the defective appearance must be quantified to check at all times the occurrence of a problem in the fabricating equipment or fabricating environment. Further, the shape of defective appearance is observed to check whether the defective appearance exerts critical effects in the product, whereby the degree of effect by the defective appearance can be determined. Instead of determining from the image of defective appearance by visual inspection whether or not the defective appearance is critical, there has recently been introduced a technique of ADC (Automatic Defect Classification) that automatically classifies the defective appearance (for example, refer to JP Patent Publication (Kokai) No. 2006-269489A). 
     As the defective appearance inspection apparatus, there is known an optical pattern inspection apparatus using a dark-field optical microscope (for example, refer to JP Patent Publication (Kokai) No. 05-45862A). The defect inspection mechanism which inspects a semiconductor wafer will be briefly described below. Chips constituting multiple semiconductor devices are formed on a single semiconductor wafer. These inspected chips are each typically constituted of a group of multiple patterns based on the functions of the memory area, peripheral circuit area, logic area and the like. When laser light is irradiated on the inspected object, the incident light is diffracted by the patterns, but the incident light irradiated on a defect is scattered by the defect. The diffracted light and scattered light pass through a field lens of the inspection apparatus and are adjusted to an appropriate light intensity by a variable-transmittance filter. Thereafter, the diffracted light from a pattern having a high repetitiveness such as a memory area pattern is eliminated by a spatial filter. However, the diffracted light from a pattern having a low repetitiveness such as a peripheral circuit area pattern or logic circuit area pattern, and the scattered light from a defect, which are not eliminated by the spatial filter, enters a signal detector and the signals are stored in the memory of the inspection apparatus. Then, a difference between the diffracted light signal and scattered light signal and a reference chip signal preinstalled in the memory is calculated by a difference circuit, and the difference signal is compared with a predetermined threshold level by a comparator, so that the signal greater than the threshold level is detected as a defect signal. 
     SUMMARY OF THE INVENTION 
     Diffracted light produced, as described above, when laser is irradiated on an inspected object depends greatly on pattern repetitiveness and pattern density. While the signal from a high-repetitiveness pattern is effectively eliminated by the spatial filter, the signal from a low-repetitiveness pattern such as a logic circuit pattern or peripheral circuit pattern is not eliminated by the spatial filter and enters the signal detector, thus causing detection signal saturation. When detection signal saturation occurs, the defect signal cannot be separated, so that the defect cannot be detected. In order to address this problem, a method can be used which uses multiple detectors having different saturation levels, but this method takes high cost. There can also be used a method which performs multiple inspections under different optical conditions for each area with respect to spatial filter, laser power, signal transmittance and the like, and combines the inspection results; but this method results in low throughput and it also takes much labor and time to optimize the multiple conditions, and it is difficult to learn the operation method easily. Further, the high sensitivity of the inspection apparatus makes the image data voluminous, and it takes much time to sort out the data to check detected defects, thus posing a large problem. 
     An object of the present invention is to provide a defect inspection apparatus and defect inspection method which can prevent the failure in detecting a defect, caused by saturation of a pattern signal obtained by inspecting an inspected object, so that the investigation of the cause for defect occurrence can be done earlier. 
     To achieve this object, according to an embodiment of the invention, there is provided a defect inspection that irradiates laser light on an inspected object having a pattern formed thereon, detects a signal from the inspected object and thereby detects a defect, the inspection including: inputting pattern information contained in layout data on the inspected object; determining based on the inputted pattern information, at least one of arrangement, repetitiveness and density for each of a plurality of inspected areas of the inspected object; estimating a saturation level of the detected signal based on the determination result; and determining a transmittance condition so that the signal does not saturate. 
     According to the present invention, there can be provided a defect inspection apparatus and defect inspection method which can prevent the failure in detecting a defect, caused by saturation of a pattern signal obtained by inspecting an inspected object, so that the investigation of the cause for defect occurrence can be done earlier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view illustrating an example of pattern arrangement in an inspected chip. 
         FIG. 2  is a schematic view illustrating a pattern and defect in the inspected chip. 
         FIG. 3  is a system configuration diagram of a related art defect inspection apparatus. 
         FIG. 4  is a graph illustrating a signal level at the A scan cross-section of  FIG. 2 . 
         FIG. 5  is a schematic configuration diagram of a detector, for which the transmittance is variable, used by the present invention. 
         FIG. 6  is a schematic configuration diagram of a defect inspection system according to the present invention. 
         FIG. 7  is a schematic configuration diagram of a defect inspection system according to the present invention. 
         FIG. 8  is a schematic configuration diagram of a defect inspection system according to the present invention. 
         FIG. 9  is a user interface screen view displayed on a monitor of a data processing system. 
         FIG. 10  is a graph illustrating the number of DOI defects detected under five inspection conditions. 
         FIG. 11  is a schematic configuration diagram of a defect inspection system according to the present invention. 
         FIG. 12  is a graph illustrating a signal level at the A scan cross-section of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a plan view illustrating an example of pattern arrangement in an inspected chip. An inspected chip  1  is typically constituted of a memory section  2 , logic section  3  and the like provided for each function; these sections are connected via wires. Usually, a memory pattern  4  has a high-repetitiveness, dense pattern configuration; and a logic pattern  5  has a low-repetitiveness pattern configuration. 
     A defect inspection method will be described with reference to  FIGS. 2 ,  3  and  4 .  FIG. 2  is a schematic view illustrating a pattern and defect in the inspected chip  1 . Now, assume that there is a defect  6  on the memory pattern  4 , and a defect  7  on the logic pattern  5 .  FIG. 3  is a system configuration diagram of a defect inspection apparatus. The defect inspection apparatus  9  mainly includes an apparatus controller  10 , apparatus drive unit  11 , signal detection unit  12 , signal processing unit  13  and stage  27 . When a laser  15  is irradiated on an inspected wafer  14  on the stage  27 , the incident light is diffracted by a pattern  16 , but when irradiated on a defect  17 , the incident light is scattered. The diffracted light and scattered light pass through a field lens  18 , and are adjusted to a given light intensity by a variable-transmittance filter  19 . Thereafter, the diffracted light from a pattern having a high repetitiveness such as the memory area pattern is eliminated by a spatial filter  20 . However, the diffracted light from a pattern having a low repetitiveness such as the peripheral circuit area pattern or logic circuit area pattern, and the scattered light from the defect, which are not eliminated by the spatial filter, enters a signal detector  21  and the signals are stored in a first memory  22  of the signal processing unit  13 . Then, a difference between the diffracted light signal and scattered light signal and a reference chip signal preinstalled in a second memory  23  is calculated by a difference circuit, and the difference signal is compared with a predetermined threshold level by a comparator  25 , so that the signal greater than the threshold level is detected as a defect signal. The detection result is displayed on an interface  26  of the apparatus controller  10 . This laser scan is applied to the whole inspection area by driving the stage  27  by means of the apparatus drive unit  11 . 
       FIG. 4  is a graph illustrating a signal level at the A scan cross-section  8  of  FIG. 2 ; the position is plotted along the abscissa and the signal level along the ordinate. In the case of a high-repetitiveness pattern such as the memory pattern  4 , the signal is effectively eliminated by the spatial filter  20 . However, in the case of a low-repetitiveness pattern such as the logic pattern  5 , the spatial filter  20  is ineffective, and thus the pattern signal is not sufficiently eliminated, so that the detector  21  saturates. Consequently, the defect  6  in the memory section  2  can be detected but the defect  7  in the logic section  3  cannot be detected. 
     In  FIG. 4(A) , the signal from the inspected chip is plotted along the ordinate. Since the detection signal from the memory pattern  4  in the memory section  2  of  FIG. 2  does not reach the saturation level, the signal from the defect  6  appears. However, since the signal from the logic pattern  5  in the logic section  3  of  FIG. 2  exceeds the saturation level, the signal from the defect  7  does not appear. In  FIG. 4(B) , the signal from a reference chip is plotted along the ordinate. As in  FIG. 4(A) , the detection signal from the memory pattern  4  in the memory section  2  of  FIG. 2  does not reach the saturation level, but the signal from the logic pattern  5  in the logic section  3  of  FIG. 2  exceeds the saturation level. In  FIG. 4(C) , the difference signal, i.e., the difference between the inspected chip signal of  FIG. 4(A)  and the reference chip signal of  FIG. 4(B)  is plotted along the ordinate. As illustrated in  FIG. 4(C) , the signal from the memory pattern  4  in the memory section  2  of  FIG. 2  disappears as a result of calculating the difference, and only the signal from the defect  6  illustrated in  FIG. 4(A)  can be observed; if this signal exceeds the threshold level, the defect  6  can be detected. However, the signal from the logic pattern  5  in the logic section  3  saturates and the signal from the defect  7  does not appear, so the defect  7  cannot be detected by calculating the difference. 
       FIG. 5  is a schematic configuration diagram of a detector, for which the transmittance is variable, used by the present invention. As the detector  21  illustrated in  FIG. 3 , there are provided a detector  31  and detector  32 . The detector  31  is a CCD (Charge Coupled Devices) sensor, and the detector  32  is a TDI (Time Delay and Integration) sensor. To arrange a low-transmittance section  29  in one half of the sensor surface and a high-transmittance section  30  in the other half of the sensor surface, there is provided a variable-transmittance filter  28 . 
       FIG. 6  is a system configuration diagram of a defect inspection system according to the present invention. A layout data storage system  33  is connected to the defect inspection apparatus  9  illustrated in  FIG. 3 . The layout data storage system  33  includes a layout database  34  and a monitor  35  with a microprocessor. The defect inspection apparatus  9  includes the apparatus controller  10  with a microprocessor, and the apparatus drive unit  11 , signal detection unit  12  and signal processing unit  13 . The apparatus controller  10  stores inspection condition information such as information on the inspected chip  1 , laser power, spatial filter  20  configuration and the transmittance of the variable-transmittance filter  19 . The apparatus drive unit  11  drives based on the information stored in the apparatus controller  10 , the driven sections such as the stage  27 , variable-transmittance filter  19  and spatial filter  20 . The signal detection unit  12  includes the laser  15 , field lens  18 , variable-transmittance filter  19 , spatial filter  20 , detector  21  and the like, and detects a pattern signal and defect signal. The signal processing unit  13  includes an image processing substrate and the like, and distinguishes the defects  6  and  7  from the patterns. Pattern information  36  including the pattern arrangement, pattern configuration and the like in the inspected chip is sent from the layout data storage system  33  to the apparatus controller  10  of the defect inspection apparatus  9 . The apparatus controller  10  determines the pattern repetitiveness and density based on the pattern information  36 . In driving the stage  27  by means of the apparatus drive unit  11  to inspect the inspected area, the spatial filter  20  is effective for a pattern area determined to have a high repetitiveness, so the signal from the high-transmittance section  30  of the detector  31  or  32  illustrated in  FIG. 5  is used. For a pattern area determined to have a low repetitiveness, the signal from the low-transmittance section  29  of the detector  31  or  32  is used. These signals are compared with the threshold levels in the signal processing unit  13  to determine the presence/absence of the defects  6  and  7 . 
       FIG. 7  is a schematic configuration diagram of a defect inspection system according to the present invention. This system is obtained by adding a data processing system  40  to the configuration of  FIG. 6 . The data processing system  40  includes a data storage memory  38  and a monitor  39  with a microprocessor. Pattern information  36  including the pattern arrangement, pattern configuration and the like in the inspected chip is sent from the layout data storage system  33  to the external data processing system  40 . The data processing system  40  determines information on pattern repetitiveness and density  37  based on the pattern information  36 . This information is sent to the apparatus controller  10  of the defect inspection apparatus  9 . In driving the stage  27  by means of the apparatus drive unit  11  to inspect the inspected area, the spatial filter  20  is effective for a pattern area determined to have a high repetitiveness, so the signal from the high-transmittance section  30  of the detector  31  or  32  illustrated in  FIG. 5  is used. For a pattern area determined to have a low repetitiveness, the signal from the low-transmittance section  29  of the detector  31  or  32  is used. These signals are compared with the threshold levels in the signal processing unit  13  to determine the presence/absence of the defects  6  and  7 . 
       FIG. 8  is a schematic configuration diagram of a defect inspection system according to the present invention. This system is obtained by adding a data processing system  40  to the configuration of  FIG. 6 , but the functions of constituent devices are different from those of  FIG. 7 . The pattern information  36  including the pattern arrangement, pattern configuration and the like in the inspected chip is sent from the layout data storage system  33  to the apparatus controller  10  of the defect inspection apparatus  9 . The apparatus controller  10  determines information on pattern repetitiveness and density  37  based on the pattern information  36 . In driving the stage  27  by means of the apparatus drive unit  11  to inspect the inspected area, the spatial filter  20  is effective for a pattern area determined to have a high repetitiveness, so the signal from the high-transmittance section  30  of the detector  31  or  32  illustrated in  FIG. 5  is used. For a pattern area determined to have a low repetitiveness, the signal from the low-transmittance section  29  of the detector  31  or  32  is used. These signals are compared with the threshold levels in the signal processing unit  13  to determine the presence/absence of the defects  6  and  7 . The inspection for each inspected area is performed using multiple transmittance values, and the image signal and coordinates information  41  of a defect candidate is sent to the external data processing system  40 . The data processing system  40  determines an optimum transmittance condition based on a combination of the multiple transmittance values and the number of defect candidates. The determination of transmittance will be described later with reference to  FIG. 10 . 
       FIG. 9  is a user interface screen view displayed on the monitor  39  of the data processing system  40 . There will be described below: the content of processing inspection data including defect feature quantity and image data outputted from the defect inspection apparatus  9 ; and the method of displaying the inspection data. 
     An icon on the desk top of the data processing system  40  is double-clicked to start up the data processing system, so that a screen  50  illustrated in  FIG. 9  is displayed on the monitor  39 .As a result of being associated with corresponding coordinates data, the following pieces of information are displayed in parallel on the screen  50 : multiple images  59  sent from the defect inspection apparatus when the inspection is performed by varying the transmittance; feature quantity data  55  including transmittance parameter setting and the luminance of defect part; multiple ADR images  60  and ADC information  63  sent from a review apparatus (not illustrated); and CAD data image  61  clipped at a given location from layout data sent from the layout database  34 . A scroll bar  62  is displayed depending on the number of coordinates data, so that information corresponding to given coordinates can be displayed. In each column, each information can be displayed in ascending order or in descending order by clicking on the title sections  51 ,  52 ,  53  and  54 . 
     Multiple inspection data displayed on the screen  50  each have defect ID  58 . However, defect ID  58  is assigned during inspection independently of the data processing and thus is meaningless during the analysis by the screen  50 . Accordingly, serial number  57  is automatically assigned in addition to defect ID  58 , so that all the data inputted to the data processing system can be managed using serial number  57 . Further, on the screen  50 , the titles for the three inspection conditions, image data corresponding to two reviews, CAD data, and ADC result  53  is displayed so that these data can be perceived. A defect contained in a review file sent to the review apparatus can be arbitrarily selected by ticking a defect selection section  56  and then clicking a review data output button  65 . 
       FIG. 10  is a graph illustrating the number of DOI defects detected under five inspection conditions. The number of inspection conditions is three in  FIG. 9  but in this case, the number is set to five and after the screen  50  is displayed, a DOI inspection rate graph button is clicked, whereby the screen of  FIG. 10  is displayed. DOI (Defect Of Interest) means a defect in which the operator of the data processing system has an interest. In the example of  FIG. 10 , a largest number of DOIs are detected under condition  5 . That is, the transmittance value used at condition  5  is the optimum setting. In this case, the relationship between the DOI at condition  5  and the degradation of product yield ratio can be checked using defect images and ADC results displayed on the screen  50  illustrated in  FIG. 9 . The optimum transmittance parameter  42  thus selected is, as illustrated in  FIG. 8 , sent from the data processing system  40  to the apparatus controller  10  of the defect inspection apparatus  9 . 
       FIG. 11  is a schematic configuration diagram of a defect inspection system according to the present invention. The data communication content between the data processing system  40  and defect inspection apparatus  9  in the system of  FIG. 11  is different from that of  FIG. 7 . Pattern information  36  including the pattern arrangement, pattern configuration and the like in the inspected chip is sent from the layout data storage system  33  to the data processing system  40 . The data processing system  40  determines information on pattern repetitiveness and density  37  based on the pattern information  36 . This information is sent to the apparatus controller  10  of the defect inspection apparatus  9 . In driving the stage  27  by means of the apparatus drive unit  11  to inspect the inspected area, the spatial filter  20  is effective for a pattern area determined to have a high repetitiveness, so the signal from the high-transmittance section  30  of the detector  31  or  32  illustrated in  FIG. 5  is used. For a pattern area determined to have a low repetitiveness, the signal from the low-transmittance section  29  of the detector  31  or  32  is used. These signals are compared with the threshold levels in the signal processing unit  13  to determine the presence/absence of the defects  6  and  7 . The inspection for each inspected area is performed using multiple transmittance values, and the image signal and coordinates information  41  of a defect candidate is sent to the external data processing system  40 . The data processing system  40  determines an optimum transmittance condition based on a combination of the transmittance values and the number of defect candidates. The transmittance parameter  42  determined to be optimum is sent to the apparatus controller  10  of the defect inspection apparatus  9 . Here, “optimum transmittance” means, as described with reference to  FIG. 10 , a transmittance with which the number of DOIs is largest. 
       FIG. 12  is a graph illustrating a signal level at the A scan cross-section of  FIG. 2 ; the position is plotted along the abscissa and the signal level along the ordinate. Signal saturation can be prevented by optimizing the transmittance depending on the pattern, so there can also be detected the defect  7  of the logic section illustrated in  FIG. 2  which cannot be detected according to the related art. Referring to  FIG. 12(A) , the inspected chip signal is plotted along the ordinate, whereby the signal level at the A scan cross-section of  FIG. 2  is illustrated. According to the related art illustrated in  FIG. 4(A) , the signal from the logic pattern  5  of the logic section of  FIG. 2  exceeds the saturation level, so the defect  7  cannot be detected. However, when the signal detection is performed under the optimum transmittance condition, as illustrated in  FIG. 12(A) , the signal from the logic pattern  5  of the logic section of  FIG. 2  does not exceed the saturation level, so the signal from the defect  7  appears. Referring to  FIG. 12(B) , the reference chip signal is plotted along the ordinate. According to the related art illustrated in  FIG. 4(B) , the signal from the logic pattern  5  of the logic section of  FIG. 2  exceeds the saturation level. However, when the signal detection is performed under the optimum transmittance condition, as illustrated in  FIG. 12(B) , the signal from the logic pattern  5  of the logic section of  FIG. 2  does not exceed the saturation level. Referring to  FIG. 12(C) , plotted along the ordinate is a difference signal, i.e., a difference between the inspected chip signal of  FIG. 12(A)  and the reference chip signal of  FIG. 12(B) . According to the related art illustrated in  FIG. 4(C) , the defect  6  of the memory section  2  of  FIG. 2  can be detected, but the defect  7  of the logic section cannot be detected. In contrast, referring to  FIG. 12(C) , since the signal detection is performed under the optimum transmittance condition, so that the signal from the logic pattern  5  of the logic section does not reach the saturation level, when the difference between the signal of  FIG. 12(A)  and the signal of  FIG. 12(B)  is calculated, the signal from the defect  7  appears; when this signal exceeds the threshold level as illustrated in  FIG. 12(C) , the defect  7  can be detected. 
     As described above, according to the present embodiment, the signal detection can be performed using, depending on the design layout data of semiconductor pattern, the level at which detected signal saturation does not occur. Thus, defects in the logic pattern can be unfailingly detected. As a result, the investigation of the cause for defect occurrence in the logic pattern can be started earlier, thus contributing to product yield rate improvement. Further, with the same inspected object, even when the defect detection sensitivity varies depending on individual characteristics of defect inspection apparatuses, proper adjustments can be made so that the defect detection is performed at the same level; thus in performing the defect inspection using multiple defect inspection apparatuses, inspection level equalization can be done. Further, in setting the inspection condition, the condition corresponding to the optimum signal transmittance can be easily determined, thus shortening the length of time taken to set the inspection condition.