TECHNICAL FIELD 
       [0001]    The present invention relates to a method and device for observing defects caused in manufacture of a semiconductor wafer. 
       BACKGROUND ART 
       [0002]    In manufacture of a semiconductor wafer, it is important for securing a profit to start a manufacture process rapidly and make a shift to mass production system of high yield early. 
         [0003]    In order to achieve this object, various inspection/measurement devices are introduced into manufacture lines. In the process start stage, it is conducted to, for example, change process conditions intentionally, making a plurality of wafers or chips, inspecting the wafers or chips, and determining process conditions on the basis of a result of the inspection, with the object of determining process conditions capable of forming a desired circuit pattern early. 
         [0004]    On the other hand, wafer inspection in a mass production stage is conducted for monitoring. In other words, in an intermediate stage of wafer manufacture, a wafer is sampled and inspected, and it is checked whether a defect occurs on a surface of the wafer or whether there is an abnormality in a circuit pattern formed on the wafer surface. In a case where a defect or an abnormality in the circuit pattern is detected because of inspection, a cause thereof is investigated and a necessary countermeasure is taken. 
         [0005]    As a representative inspection device used in such a process start stage or a mass production stage, there is an optical wafer inspection device. For example, JR-A-2000-105203 (Patent Literature 1) discloses a technique of picking up an optical image of a wafer surface by means of bright visual field illumination and inspecting a defect by comparing the optical image of the wafer surface with an image of a good article region (for example, an image of an adjacent chip). However, such an optical inspection device undergoes influence of an illumination wavelength thereof, and a resolution limit of an acquired image becomes approximately several hundred nanometers. As regards a defect of several ten nanometers order on the wafer, therefore, the optical inspection device can detect only whether there is a defect. When conducting a detailed defect analysis, a different defect observation device having a higher imaging resolution is needed. 
         [0006]    The defect observation device is a device that picks up an image in a defect position on a wafer by using an output of the inspection device and outputs the image. In the semiconductor manufacture process, size shrinking is promoted. As a result, a defect size reaches an order of several nm. In order to observe a defect in detail, therefore, a resolution of several nm order is needed. Accordingly, an observation device (hereafter referred to as review SEM) using a scanning electron microscope (SEM) has been widely used in recent years. In a mass production line of semiconductor, automation of the observation work is desired. The review SEM mounts a function of ADR (Automatic Defect Review) for automatically collecting images in defect positions in a sample and ADC (Automatic Defect Classification) for automatically classifying approximately several hundred defect images acquired by the ADR according to occurrence causes or features of exterior view. By the way, a defect position coordinate (coordinate information indicating a position of a defect on the sample) output by the inspection device includes an error. Therefore, the ADR has a function of re-detecting a defect from an SEM image picked up with a wide visual field around a defect position coordinate output by the inspection device and picking up an image in the re-detected defect position with high magnification. As a method for detecting defects from the SEM image, JPA-2001-189358 (Patent Literature 2) states a method of using an image obtained by picking up an image of an area where the same circuit pattern as that of a defect region is formed, as a reference image and detecting a defect by comparing an image obtained by picking up an image of defect region with the reference image. Furthermore, JP-A-2007-40910 (Patent Literature 3) states a method of detecting a defect from one image obtained by picking up an image of a defect region. As one method of automatic classification, JP-A-21803 (Patent Literature 4) states a method of quantizing an exterior view feature quantity of a defect region by conducting image processing and classifying defects by using a neural network. Furthermore, as a method capable of easily coping with even in a case where the number of kinds of defects to be classified is many, JP-A 2007-225531 (Patent Literature 5) states a method of classifying defects by combining a rule base classification method with a teaching classification method. 
       CITATION LIST 
     Patent Literature 
       [0007]    PATENT LITERATURE 1: JP-A-2000-105203 
         [0008]    PATENT LITERATURE 2: JP-A-2001-189358 
         [0009]    PATENT LITERATURE 3: JP-A-2007-40910 
         [0010]    PATENT LITERATURE 4: JP-A-8-21803 
         [0011]    PATENT LITERATURE 5: JP-A-2007-225531 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    As described above, the defect position coordinate output by the inspection device includes an error. In the defect observation device as well, therefore, it is necessary to re-detect the defect position from a picked up image. Patent Literature 1 states a method of calculating a variation of image signals as to corresponding pixels of structures having the same shape or neighboring pixels on the basis of detected image signals, setting a determination criterion (threshold) of a pixel signal level for determining whether there is a defect such as a foreign matter in accordance with the calculated variation, and determining whether there is a defect as to the image signal on the basis of the determination criterion. 
         [0013]    With size shrinking and increased complexity in the semiconductor manufacture process, however, lowering of the defect detection rate poses a problem. For example, with the size shrinking, a size of a fatal defect also becomes minute. Therefore, it becomes difficult to distinguish a defect from a manufacture tolerance (such as, for example, line edge roughness) of a circuit pattern. Furthermore, if a device has a three-dimensional structure, a signal quantity from a lower layer becomes small, and consequently it becomes difficult to distinguish noise from a defect signal. 
         [0014]    For conducting the distinguishing with high precision, an analysis using a feature quantity is effective. Specifically, first, an area having a large light and shade difference between a defect image and a reference image (good article image) is extracted as a defect candidate by conducting light and shade comparison (as this method, a method stated in Patent Literature 1, Patent Literature 2, or Patent Literature 3 may be used). Defect candidates include regions where a manufacture tolerance or the like is detected falsely (hereafter referred to as nuisance) besides true defect regions. As regards each of extracted defect candidates, therefore, a feature (such as, for example, a light and shade value, an area, or a circularity) obtained from an image is quantified, and a defect and a nuisance are discriminated by using a discriminator. By the way, the discriminator may be configured by machine learning using a technique of pattern recognition. As the pattern recognition technique, for example, a neural network, an SVM (Support Vector Machine) or the like may be used. Or a discrimination rule may be generated automatically. 
         [0015]    In general, for configuring a high precision discriminator by machine learning, it is necessary to indicate a large number of defect candidates labeled with “defect” and “nuisance.” For a user to provide defect candidates detected by the light and shade comparison inspection with labels of “defect” and “nuisance,” much labor is needed and it is not realistic. Therefore, it is an object of the present invention to provide a method and device for easily extracting defect candidates that can be labeled with “defect” and “nuisance” from samples of observation object and making parameters concerning observation processing adjustable simply. 
       Solution to Problem 
       [0016]    In order to solve the above-described problem, for example, a configuration stated in claims is adopted. 
         [0017]    The present application includes a plurality of means that solve the above-described problem. For example, a defect observation method includes an imaging process for imaging an inspection object on the basis of defect information from an inspection device and obtaining a defect image and a reference image corresponding to the defect image, a parameter determination process for determining a first parameter to be used in defect extraction by using first feature quantity distribution obtained from the defect image picked up in the imaging process and the reference image and second feature quantity distribution obtained from the reference image, and an observation process for conducting observation by using the first parameter determined in the parameter determination process. 
       Advantageous Effects of Invention   
       [0018]    According to the present invention, it becomes possible to extract easily defect candidates that can be labeled with defect and nuisance from samples of the observation object. 
         [0019]    Problems, configurations, and effects other than those described above will be clarified by ensuing description of embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is a configuration diagram of a defect observation device; 
           [0021]      FIG. 2  is a configuration diagram of a control unit, a storage unit, and an arithmetic unit in the defect observation device; 
           [0022]      FIG. 3  is a configuration diagram for acquiring a defect observation image; 
           [0023]      FIG. 4  is a processing flow for acquiring a defect observation image; 
           [0024]      FIG. 5  is a schematic diagram representing an SEM image and a defect candidate extraction result; 
           [0025]      FIG. 6  is a schematic diagram representing a feature quantity space; 
           [0026]      FIG. 7  is a configuration diagram for adjusting parameters; 
           [0027]      FIG. 8  is a processing flow for adjusting parameters; 
           [0028]      FIG. 9  shows an example in which a defect is extracted by using a plurality of reference images; 
           [0029]      FIG. 10  shows an example in which a nuisance is extracted by using a plurality of reference images; 
           [0030]      FIG. 11  is a processing flow for adjusting parameters; 
           [0031]      FIG. 12  is a processing flow for controlling execution of parameter adjustment; 
           [0032]      FIG. 13  is a GUI for displaying a result of parameter adjustment; 
           [0033]      FIG. 14  is a GUI for confirming a result of defect detection; and 
           [0034]      FIG. 15  is a processing flow for adjusting parameters. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
       [0035]    Hereafter, defect observation processing concerning the present invention will be described. In the present embodiment, a method for acquiring an observation image of a defect with an imaging device having a scanning electron microscope (SEM) will be described. However, the imaging device concerning the present invention may be other than the SEM, and may be an imaging device using a charged particle beam such as ions. 
         [0036]      FIG. 1  represents a configuration diagram of a device according to the present invention. The device includes an SEM  101  to conduct picking up an image, a control unit  102  to exercise general control, a storage unit  103  to store information into a magnetic disk, a semiconductor memory or the like, an arithmetic unit  104  to conduct arithmetic operations in accordance with a program, an external storage media input/output unit  105  to conduct inputting/outputting of information to/from external storage media connected to the device, a user interface unit  106  to control inputting/outputting of information to/from a user, and a network interface unit  107  to conduct communication with another device or the like via a network. Furthermore, an input/output terminal  113  configured to have a keyboard, a mouse, a display, and so forth is connected to the user interface unit  106 . The SEM  101  includes a movable stage  109  to mount a sample wafer  108 , an electron source  110  to irradiate the sample wafer  108  with an electron beam, detectors  111  to detect secondary electrons, reflected electrons, and so forth generated from the sample wafer, an electron lens (not illustrated) to focus the electron beam onto the sample, a deflector (not illustrated) to scan the top of the sample wafer with the electron beam, and an image generation unit  112  to conduct digital conversion on a signal from the detector  111  and generate a digital image. By the way, these components are connected via a bus  114 , and these components can conduct information giving/receiving with each other. 
         [0037]      FIG. 2  shows a configuration of the control unit  102 , the storage unit  103 , and the arithmetic unit  104 . The control unit includes a wafer conveyance control unit  201  to control conveyance of a wafer, a stage control unit  202  to control a stage, a beam shift control unit  203  to control a position of irradiation with an electron beam, abeam scan control unit  204  to control scanning with the electron beam, and an image acquisition unit  205 . The storage unit  103  includes an image storage unit  206  to store acquired image data, a recipe storage unit  207  to store imaging conditions (such as, for example, an acceleration voltage, a probe current, the number of added frames, and imaging visual field size), processing parameters, and so forth, and a measurement coordinate storage unit  208  to store a coordinate of an observed place. The arithmetic unit  104  includes an imaging place setting unit  209  to set a coordinate of a place to be imaged, a comparative inspection unit  214  to conduct comparative inspection on an image, a defect feature quantity distribution extraction unit  212  to extract feature quantity distribution in a defect region, and a distribution analysis unit  213  to analyze feature quantity distribution. Furthermore, the comparative inspection unit  214  includes an image difference quantification unit  210  to quantify a difference between images, and a defect region selection unit  211  to select a defect region from an image. By the way, the units  209  to  213  may be configured as hardware designed to conduct respective arithmetic operations. Or the units  209  to  213  may be configured to be mounted as software and executed by using a general-purpose arithmetic device (such as, for example, a CPU or a GPU). 
         [0038]    A method for acquiring an image of a specified coordinate by using the image acquisition unit  205  will now be described. First, a robot arm places the wafer  108  that becomes a measurement object on the stage  109  under control of the wafer conveyance control unit  201 . Subsequently, the stage control unit  202  moves the stage  109  to cause a beam irradiation range to include an imaging visual field. At this time, the stage position is measured to absorb a movement error of the stage. The beam control unit  203  adjusts the beam irradiation position to cancel the movement error. The electron beam is emitted from the electron source  110 . The beam scan control unit  204  scans in the imaging visual field with the electron beam. Secondary electrons and reflected electrons generated from the wafer by irradiation with the beam are detected by the detector  111 , and converted to a digital image by the image generation unit  112 . The image storage unit  206  stores the picked up image together with incidental information such as the imaging condition and imaging date and hour. 
         [0039]    A defect observation method according to the present invention will now be described with reference to  FIGS. 3 and 4 . First, a defect position coordinate, which is output by a different defect inspection device, is read from the coordinate storage unit. All of the read defect position coordinates may be set to be observation object, or defect position coordinates sampled on the basis of conditions specified by the user may be set to be observation object. Subsequently, an imaging coordinate of a reference image is set by using the imaging place setting unit  209  (S 401 ). As for the reference image, it is necessary to pick up an image of a place designed to form the same circuit pattern as that around the defect position. In the semiconductor wafer, a plurality of chips designed to form a similar circuit pattern thereon are disposed on the wafer. As the simplest method, therefore, a coordinate displaced from the defect position coordinate by a coordinate corresponding to one chip can be set as an imaging coordinate of the reference image. Then, the reference image and the defect image are picked up by using the image acquisition unit  205  (S 402 , S 403 ). Then, in the image difference quantification unit  210 , areas that become defect candidates are extracted by using a detect candidate extraction unit  301 , and feature quantities of respective defect candidate areas are calculated by using a feature quantity calculation unit  302  (S 404 ). Then, a defect is selected from among defect candidates by using the defect selection unit  211  (S 405 ). It is determined whether there is the selected defect (S 406 ). An image of the selected defect position is picked up and an image for defect observation is obtained (S 407 ). By the way, parameters of each processing are stored in the recipe storage unit  207 , and are read as occasion demands. For example, in the image acquisition unit  205 , electro-optical system conditions (such as, for example, a probe current and an acceleration voltage), the number of added frames, and so forth are read out ( 303 ). Furthermore, in the defect candidate extraction unit  301 , comparative processing parameters (such as, for example, a detection threshold and an image mixture rate) are read out ( 304 ). Furthermore, in the feature quantity calculation unit  302 , image processing parameters (such as, for example, light and shade smoothing degree) are read out ( 305 ). Furthermore, in the defect selection unit, selection conditions (such as, for example, discrimination surfaces of defect and nuisance, an origin and shape of nuisance distribution, and a threshold for distance) are read out ( 306 ). These processing parameters concerning the image acquisition unit, the defect candidate extraction unit, the feature quantity calculation unit, and the defect selection unit are parameters concerning observation processing. 
         [0040]    The defect candidate extraction unit  301  will now be described with reference to  FIG. 5 . Image  501  schematically represents a defect image. Image  502  schematically represents a reference image. In the image  501 , a defect exists on a coordinate (dx, dy). Image  503  represents extraction results of defect candidates. Areas where a light and shade difference between the defect image and the reference image is large are represented by a white color. Each of the extracted areas is a defect candidate. Defect candidates caused by the line edge roughness are extracted in areas other than the defect region (dx, dy). The feature quantity calculation unit  302  in the subsequent stage calculates a feature quantity with respect to each defect candidate. 
         [0041]    The defect selection unit  211  will now be described as supplement with reference to  FIG. 6 .  FIG. 6  represents an example in which respective defect candidates are plotted in a feature quantity space. The defect selection unit  211  selects defects on the basis of feature quantities of respective defect candidates.  FIG. 6  shows how defects and nuisances stored in the recipe storage unit  207  are discriminated by using a separating hyper-plane  601 . By the way, defects may be selected by using a method other than the method of discriminating by using the separating hyper-plane. For example, as a simpler method, it is possible to store the center of nuisance distribution in the recipe storage unit  207  and judge a candidate having the largest distance from the center to be a defect. Or it is possible to use a configuration in which n higher order defect candidates are judged as defects and n images for defect observation are acquired. Furthermore, it is not necessary to use all feature quantities calculated by the feature quantity calculation unit  302 , but feature quantity selection techniques may be combined and used. 
         [0042]    Heretofore, the defect observation method according to the present invention has been described. In the present method, it is necessary to set processing parameters suitably for re-detecting defects with high sensitivity. For example, in a case where a detection threshold is set to be higher than a proper value (a sensitivity is set to be a lower value) in the image difference quantification unit  210 , a defect candidate (area) is not extracted in a defect region. To the contrary, in a case where the detection threshold is set to be lower (the sensitivity is set to be higher), a large number of defect candidates are extracted and false discrimination is caused in the defect selection unit  211 . Furthermore, in a case where the separating hyper-plane between defects and nuisances is not set suitably in the defect selection unit  211 , overlooking of defects and false detection are caused. The present invention provides a method that makes it possible for the user to set parameters in the image difference quantification unit  210  and the defect selection unit  211  easily. Hereafter, a concrete method, and device configuration will be described. 
         [0043]    A configuration, and method, for adjusting parameters in the detect selection unit  211  among processing parameters according to the present invention will now be described with reference to  FIGS. 7 and 8 . First, a defect position coordinate that is output by the different inspection device is read. Then, with respect to a defect that becomes an object, coordinates of R places where a reference image is to be picked up are set on the basis of the defect position coordinate by using the imaging place setting unit  209  (S 801 ). Then, a defect image Di is picked up by using the image acquisition unit  205  (S 802 ). Then, with respect to the set imaging places of the reference image, reference images Ri,j (j=1 to R) are acquired by using the image acquisition unit  205 . Then, feature quantity distribution of a defect is calculated. Specifically, the image difference quantification unit  701  extracts defect candidates from the defect image Di and the reference images Ri,j (S 804 ), and calculates a feature quantity Fd(i, j) of each defect candidate area (S 805 ). Then, feature quantity distribution Fd(i) of a detect is extracted by using the defect feature quantity distribution extraction unit  212  (S 806 ). Then, feature quantity distribution of a nuisance is calculated by using a combination (j, k) of reference images Ri (i=1 to R) picked up. Specifically, the image difference quantification unit  702  extracts defect candidates from a reference image Ri,j (j=1 to R) and a reference image Ri,k (k=j+1 to R) (S 807 ), and calculates a feature quantity Fr(i, j, k) of each defect candidate area (S 808 ). 
         [0044]    The imaging place setting unit  209  calculates an area designed to form the same circuit pattern as that around a defect position. As a simple method, it is possible to select R chips located nearest the chip including the defect position coordinate and calculate a coordinate corresponding to the defect position in each chip. If the size of one chip is already known, the coordinate can be calculated easily by conducting addition and subtraction on the defect position coordinate. By the way, a search may be conducted from around the defect position by using design information of the sample. 
         [0045]    The defect feature quantity distribution extraction unit  212  will now be described with reference to  FIG. 9 . An image  901  schematically represents a defect image (Di). Images  902  to  904  schematically represent reference images (Ri, 1  to Ri, 3 ) in a case where R=3. An image  908  is a result of defect candidate extraction  905  using the defect image  901  and the reference image  902 . Images  909  and  910  are also results of defect candidate extraction  906  and  907  in the same way. Manufacture tolerances such as line edge roughness occur at random. If only an area common to defect candidate extraction results  908  to  910  is extracted ( 911 ), therefore, defect candidates occurring at random are excluded and it becomes possible to extract only a defect ( 912 ). By the way, the detect candidate judged to be a detect may not be common to all defect detection results. In other words, if the defect candidate coincides in extraction results of at least a determinate number among a plurality of defect candidate extraction results, the defect candidate may be judged as a defect. Furthermore, the determinate number may be specified by a ratio to the number R of defect candidate extraction results. From feature quantities Fd(i, j) (j=1 to 3) calculated on the basis of this result, a feature quantity Fd(i) labeled with “defect” can be extracted. By the way, a method for extracting the feature quantity Fd(i) labeled with “defect” is not restricted to this, but, for example, distribution that is close in the feature quantity space may be extracted. 
         [0046]    The method for calculating feature quantity distribution of a nuisance by using a combination (j, k) of reference images RI (i=1 to R) picked up will now be described as supplement with reference to  FIG. 10 . Images  1001  to  1003  schematically represent the reference images (Ri, 1  to Ri, 3 ). An image  1004  represents a defect candidate extraction result using the reference image  1001  and the reference image  1002 . In the same way, an image  1005  represents a defect candidate extraction result using the reference image  1001  and the reference image  1003 , and an image  1006  represents a defect candidate extraction result using the reference image  1002  and the reference image  1003 . A defect is not included in a defect candidate extracted from a combination of reference images. Therefore, feature quantity distribution calculated from defect candidates included in the images  1004  to  1006  is feature quantity distribution labeled with “nuisance.” 
         [0047]    Referring back to  FIG. 8 , description will be continued. The above-described processing S 801  to S 808  is executed repeatedly with respect to defect position coordinates of M points of the observation object. As a result, defect feature quantity distribution Fd(i) and nuisance feature quantity distribution Fr(i, j, k) are obtained (where i=1 to M, and (j, k) is a combination of reference images). A separating hyper-plane for discriminating defects and nuisances is learned on the basis of the obtained distribution by using the distribution analysis unit  213  (S 809 ). As a method of learning, the technique of pattern recognition can be used. Specifically, the neural network may be used or the SVM may be used. Or an algorithm (such as, for example, C4.5) for automatically generating a discrimination rule may be used. Or the separating hyper-plane is not calculated, but it is possible to store labeled feature quantity distribution and discriminate by using the k-nearest neighbor algorithm or the like at the time of discrimination. By the way, the calculated separating hyper-plane or parameters required for discrimination are stored in the recipe storage unit  207 . 
         [0048]    Heretofore, the method for adjusting the parameters in the defect selection unit  211  has been described. A method for adjusting parameters in the image difference quantification unit  210  will now be described. Previously, an outline will be described. Defect detection is conducted by using a combination of parameters (such as, for example, a detection threshold and an image mixture rate) concerning the image difference quantification unit  210 . A parameter having a high defect detection rate is searched. At this time, it becomes possible to calculate a defect detection rate having a high probability by using a plurality of reference images. Hereafter, details of the processing will be described with reference to  FIG. 11 . By the way, the parameter adjustment in the defect selection unit  211  and the parameter adjustment in the image difference quantification unit  210  may be conducted in parallel simultaneously, or may be conducted one after another. By the way, in a case the parameter adjustments are conducted one after another, either of them may be conducted earlier in execution order. Or only one of them may be conducted. 
         [0049]    First, with respect to an object defect, image pickup places of reference images are set by using the imaging place setting unit  209  (S 1101 ). A defect image Di is acquired by using the defect acquisition unit  205  (S 1102 ). By the way, in a case where it can be judged in the defect acquisition unit  205  that a defect image of a pertinent defect ID is already picked up, an image may be read from the image storage unit. Then, R reference images Ri,j (j=1 to R) are acquired by using the image acquisition unit  205  (S 1103 ). Then, defect candidates Cd(i, j) are extracted by using the defect image Di and the reference images Ri,j (S 1104 ). Then, a defect candidate Cd(i) is extracted from a plurality of obtained defect candidates Cd(i, j) (j=1 to R) (S 1105 ). As one method for extraction, only a common defect candidate area can be extracted as shown in  FIG. 9 . A defect candidate included in Cd(i) can be supposed to be a defect by using a result of a plurality of reference images as described above. Then, defect candidates Cr(i, j, k) are extracted by using a combination (j, k) of reference images Ri (i=1 to R) (S 1106 ). Because of extraction results using only reference images, a defect candidate included in the defect candidates Cr(i, j, k) can be supposed to be a nuisance. Then, detection reliability DRp is calculated (S 1107 ). DR(p) is an index representing reliability of a defect detection result at the time when a parameter set p is used. DR(p) can be calculated according to, for example, (MATH. 1). S 1101  to S 1107  described heretofore are conducted for combinations of parameters of search objects, and parameters that maximize DR(p) are found. As a result, parameters reducing defect candidates that become nuisances and raising the defect detection rate are obtained. Furthermore, parameters and DR(p) may be associated with each other and output to make it possible for the user to conduct selection. 
         [0000]        DR ( p )=Sum total of the numbers of defect candidates included in  Cd ( i )/(sum total of the numbers of defect candidates included in  Cr ( i, j, k ))  (MATH. 1)
 
         [0050]    Heretofore, the method for adjusting parameters in the image difference quantification unit  210  and the defect selection unit  211  has been described. By, the way, it can be implemented easily to acquire an observation image of a defect at the same time that parameters are adjusted, by utilizing the fact that a defect can be detected with high reliability by using a plurality of reference images. However, it becomes a cause of throughput lowering to pick up a plurality of reference images. After the parameter adjustment has been completed, therefore, it is desirable to conduct ordinary observation processing (using one reference image) ( FIG. 4 ). 
         [0051]      FIG. 12  shows a method for automatically changing over between the parameter adjustment processing ( FIGS. 8 and 11 ) and ordinary observation processing ( FIG. 4 ). First, reliability of a recipe stored in the recipe storage unit  207  is judged (S 1201 ). By the way, in a case where a recipe corresponding to the sample of the observation object does not exist, the reliability is also judged as low. In a case where the reliability is low, the recipe is adjusted by using the above-described recipe adjustment processing. Then, reliability of the created recipe is judged (S 1202 ). Reliability of parameters concerning the defect selection unit  211  can be judged by conducting n-fold cross validation and evaluating stability of the defect detection rate. Furthermore, reliability of parameters concerning the image difference quantification unit  212  is can be judged by evaluating whether the defect detection rate has reached a predetermined threshold on the basis of a result of the defect candidate extraction processing (S 1105 ) using a plurality of reference images and a defect image. In a case where reliability is judged as high in the recipe reliability decision processing, ordinary observation processing (S 1204 ) is conducted using adjusted parameters when observing samples of observation object thereafter. By the way, it is also possible to record date and hour of recipe creation and judge the reliability as low in the recipe reliability decision processing (S 1201 ) in a case where a determinate period has elapsed from the time of recipe creation. 
         [0052]    The parameter adjustment method in the case where defect candidates are extracted on the basis of the defect image and the reference images picked up has been described. However, the method can also be utilized in parameter adjustment in a case where defect detection is conducted from one defect image by using, for example, the method stated in Patent Literature 3. In this case, in the processing (S 807 , S 1106 ) for extracting defect candidates that become nuisances, defect candidates should be extracted by using reference images synthesized from reference images picked up and the reference images picked up. Furthermore, in the processing (S 804 , S 1104 ) for extracting defect candidates from a defect image, defect candidates may be extracted by using reference images synthesized from reference images picked up and the reference images picked up. By doing so, it becomes also possible to cope with a case where the feature quantity distribution of nuisances changes according to the synthesis precision of the reference image. Furthermore, it also becomes possible to adjust parameters concerning synthesis processing of the reference images. 
         [0053]    Furthermore, an image generated on the basis of design information of the sample may be used as a reference image. At this time, a plurality of reference images generated by changing parameters at the time when generating an image from design information may be used. By doing so, it becomes possible to adjust parameters at the time when detecting defect candidates by using a defect image picked up and an image generated on the basis of design information of the sample. 
         [0054]    A user interface of the defect observation device according to the present invention will now be described.  FIG. 13  shows an example of a GUI, which displays a list of parameters searched for, in association with detection reliability, the number of detected defects, and the number of detected nuisances. In a parameter list display portion  1301 , the user can select a parameter. Furthermore, a button  1302  is provided to call a GUI for confirming a defect detection result in a case where a selected parameter is used.  FIG. 14(   a ) shows an example of a GUI for confirming a defect detection result. A selection portion  1401  is provided to select a defect ID, and a display portion  1402  is provided to display an image and calculated feature quantities. In the display portion  1402 , display contents can be changed over by using an “image tab”  1403  and a “feature quantity tab” 1404 . In a case where the image tab is effective, a selection portion  1405  is provided to select a displayed image, a selection portion  1406  is provided to specify a displayed image detector (a secondary electron image or a reflected electron image), a selection portion  1407  is provided to specify whether to overlay display a defect candidate detection result on an image, and a display portion  1408  is provided to display an image. Furthermore, it may be made possible to display two images or feature quantities side by side and confirm them by providing a second display portion  1409  to display an image and a calculated feature quantity.  FIG. 14(   b ) shows an example of a GUI in a case where the “feature quantity tab” is made effective in the second display portion  1408 . A specification portion  1410  is provided to specify an abscissa axis and an ordinate axis of a displayed feature quantity space, and a display portion  1411  is provided to display a result obtained by plotting defect candidates in the feature quantity space. Furthermore, a separating hyper-plane  1412  in the feature quantity space may be displayed. By the way, not the feature quantity space, but a histogram of a specified feature quantity may be displayed. 
         [0055]    As described heretofore, it becomes possible to extract defect candidates that can be labeled easily with “defect” and “nuisance” from the sample of observation object by using a defect image and a plurality of reference images. As a result, it becomes possible to adjust easily the parameter used to extract defect candidates and the separating hyper-plane used to separate defect candidates into defects and nuisances. Furthermore, it becomes possible to re-detect defects with high sensitivity by storing adjusted parameters into a recipe. 
       Embodiment 2 
       [0056]    In embodiment 1, the method of extracting defect candidates that can be labeled easily with “defect” and “nuisance” from the sample of observation object on the basis of a defect image and a plurality of reference images and adjusting parameters concerning defect re-detection has been described. In embodiment 2, a method of extracting defect candidates that can be labeled easily with “nuisance” from the sample of observation object on the basis of a plurality of reference images and adjusting parameters concerning defect re-detection will be described. 
         [0057]    A device configuration according to the present embodiment is similar to that shown in  FIGS. 1 and 2 . Furthermore, as regards a user interface as well, a user interface similar to that shown in  FIGS. 13 and 114  is provided. The present embodiment differs from embodiment 1 in a flow concerning parameter adjustment in the defect selection unit  211 . Hereafter, only a portion where the present embodiment differs from embodiment 1 will be described. 
         [0058]    First, as for places designed to form similar circuit patterns, R points are extracted and imaging places are set (S 1501 ). At this time, imaging places are set to prevent a defect from being included in images picked up, on the basis of defect position coordinates that are output by a different defect inspection device. Then, images Ri,j (j=1 to R) in set imaging places are acquired by using the image acquisition unit  205  (S 1502 ). Then, with respect to a combination (j, k) of images picked up, defect candidates are extracted and feature quantity distribution Fr(i, j, k) of the extracted defect candidates is calculated by using the image difference quantification unit  210 . S 1501  to S 1504  described heretofore are conducted repeatedly with respect to N points (S 1505 ). Since a defect is not included in acquired images, the feature quantity distribution Fr(i, j, k) becomes feature quantity distribution labeled with “nuisance.” 
         [0059]    Therefore, it becomes possible to obtain a separating hyper-plane for discriminating defects and nuisances, by using the nuisance feature quantity distribution Fr(i, j, k) and the technique of pattern recognition and configuring a one-class classifier (S 1506 ). Furthermore, parameters (such as, for example, the origin position and variance) of nuisance distribution may be found without finding a separating hyper-plane. 
         [0060]    As described heretofore, it becomes possible to extract defect candidates that can be labeled easily with “nuisance” from the sample of observation object by using a plurality of reference images. As a result, it becomes possible to adjust easily the separating hyper-plane that distinguishes nuisances from defect candidates. Furthermore, it becomes possible to re-detect defects with high sensitivity by storing adjusted parameters into the recipe. 
       REFERENCE SIGNS LIST 
       [0061]      101 : Scanning electron microscope (SEM) 
         [0062]      112 : Image generation unit 
         [0063]      206 : Image storage unit 
         [0064]      207 : Recipe storage unit 
         [0065]      208 : Coordinate storage unit 
         [0066]      209 : Imaging place setting unit 
         [0067]      210 : Image difference quantification unit 
         [0068]      211 : Defect selection unit 
         [0069]      212 : Defect feature quantity distribution extraction unit 
         [0070]      213 : Distribution analysis unit 
         [0071]      301 : Defect candidate extraction unit 
         [0072]      302 : Feature quantity calculation unit 
         [0073]    S 404 : Step of conducting defect candidate extraction and feature quantity calculation 
         [0074]    S 405 : Step of selecting a defect 
         [0075]    S 802 : Step of picking up a defect image 
         [0076]    S 803 : Step of picking up a plurality of reference images 
         [0077]    S 805 : Step of finding a comparative inspection result between a reference image and a defect image 
         [0078]    S 808 : Step of finding a comparative inspection result between a reference image and a reference image 
         [0079]    S 809 : Step of calculating a separating hyper-plane 
         [0080]    S 1202 : Step of adjusting parameters
The purpose of the present invention is to easily extract, from samples to be observed, defect candidates that can be labeled as a defect or "nuisance" (a part for which a manufacturing tolerance or the like is erroneously detected) and to allow parameters pertaining to observation processing to be easily adjusted. This defect observation method comprises: an imaging step to image, on the basis of defect information from an inspection device, an object to be inspected and obtain a defect image and a reference image corresponding to the defect image; a parameter determining step to determine a first parameter to be used in the defect extraction by using a first feature set distribution acquired from the reference image and the defect image captured in the imaging step and a second feature net distribution acquired from the reference image; and an observing step to observe using the first parameter determined in the parameter determining step. The present invention can be applied to a method of observing defects generated during the manufacturing of semiconductor wafers.