Abstract:
A charged particle microscope system with a charged particle microscope including an irradiation unit that irradiates a subject to be inspected with a charged particle beam and a detection unit having a detector that detects a charged particle signal from the subject to be inspected irradiated by the irradiation unit; a signal processing unit that converts the charged particle signal detected by the detector of the charged particle microscope into an image signal; and an arithmetic processing unit that corrects the image signal converted by the signal processing unit with the use of signal conversion characteristics.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a charged particle microscope system and a measurement method using the same. 
       BACKGROUND ART 
       [0002]    Background art in this technical field is disclosed in JP 2000-67797 A (PTL 1). PTL 1 describes that image quality obtained by a plurality of detection optical systems relative to one kind of input signal is quantitatively evaluated, and the evaluation results are used to adjust image processing parameters for inspection such that the detection optical systems are equivalent in inspection sensitivity. 
       CITATION LIST 
     Patent Literature 
       [0003]    PTL 1: JP 2000-67797 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    PTL 1 discloses a technique by which image quality obtained by a plurality of detection optical systems relative to one kind of input signal is quantitatively evaluated, and the evaluation results are used to adjust image processing parameters for inspection such that the detection optical systems are equivalent in inspection sensitivity. However, PTL 1 does not disclose a relationship between an input signal and an output signal in a certain range is quantitatively evaluated between the plurality of detection optical systems. 
         [0005]    According to the technique disclosed in PTL 1 relating to inspection devices, the evaluation of an output signal relative to one specific input signal is considered as effective. However, if the technique is applied to measurement devices in which signal waveform information obtained from a subject to be imaged by a charged particle microscope is used to measure the dimensions of the subject to be imaged, it is important that the relationship between an input signal and an output signal in a certain range is equivalent between the devices. 
         [0006]    In addition, if, regardless of the magnitude of the input signal, there are certain differences in magnitude of the output signal between the plurality of detection optical systems or the plurality of devices, the method described in PTL 1 is effective. In actuality, however, the differences in magnitude of the output signal between the plurality of detection optical systems or the plurality of devices depend on the magnitude of the input signal. 
       Solution to Problem 
       [0007]    To solve the foregoing problem, a configuration described in the claims is utilized, for example. The present invention includes a plurality of means for solving the foregoing problem. As an example, one of the means is a charged particle microscope system including: a charged particle microscope including an irradiation unit that irradiates a subject to be inspected with a charged particle beam and a detection unit that detects a charged particle signal from the subject to be inspected irradiated by the irradiation unit; a signal processing unit that converts the charged particle signal detected by the detector of the charged particle microscope into an image signal; and an arithmetic processing unit that corrects the image signal converted by the signal processing unit with the use of signal conversion characteristics. 
       Advantageous Effects of Invention 
       [0008]    According to the present invention, it is possible to provide a charged particle microscope having the function of correcting differences in signal amount between devices and a measurement method using the same. 
         [0009]    Problems, configurations, and advantages other than the foregoing ones will be more clarified by the following descriptions of embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  illustrates an example of a configuration diagram of a scanning electron microscope system according to the present invention. 
           [0011]      FIG. 2  illustrates an example of signal conversion characteristics in the scanning electron microscope system according to the present invention. 
           [0012]      FIG. 3  illustrates an example of a difference in signal conversion characteristics between devices in the scanning electron microscope system. 
           [0013]      FIG. 4  illustrates an example of a difference in output waveform between the devices in the scanning electron microscope system. 
           [0014]      FIG. 5  illustrates an example of a flowchart of a data acquisition process for calculating signal conversion characteristics according to the present invention. 
           [0015]      FIG. 6  illustrates an example of a flowchart of a process for deciding image acquisition conditions without signal saturation. 
           [0016]      FIG. 7  illustrates an example of data for deciding the image acquisition conditions without signal saturation. 
           [0017]      FIG. 8  illustrates an example of a flowchart of a signal conversion process on an image for dimension measurement according to the present invention. 
           [0018]      FIG. 9  illustrates an example of experimentally acquired signal conversion characteristics in the scanning electron microscope system. 
           [0019]      FIG. 10  illustrates an example of a table of input signals corresponding to image signals. 
           [0020]      FIG. 11  illustrates an example of a GUI for executing data acquisition for calculating signal conversion characteristics according to the present invention. 
           [0021]      FIG. 12  illustrates an example of a GUI for displaying evaluation results of signal conversion characteristics and a signal conversion table in the scanning electron microscope system according to the present invention. 
           [0022]      FIG. 13  illustrates an example of a flowchart of a data acquisition process for calculating signal conversion characteristics according to the present invention. 
           [0023]      FIG. 14  illustrates an example of a flowchart of a data acquisition process for calculating signal conversion characteristics according to the present invention. 
           [0024]      FIG. 15  illustrates an example of a flowchart of a data acquisition process for calculating signal conversion characteristics according to the present invention. 
           [0025]      FIG. 16  illustrates an example of a flowchart of a data acquisition process for calculating signal conversion characteristics according to the present invention. 
           [0026]      FIG. 17  illustrates an example of a flowchart of a signal conversion process on the image for dimension measurement according to the present invention. 
           [0027]      FIG. 18  illustrates an example of a GUI for displaying evaluation results of temporal changes in signal conversion characteristics and the temporal changes in a signal conversion table in the scanning electron microscope system according to the present invention. 
           [0028]      FIG. 19  illustrates an example of a flowchart of a data acquisition process for calculating a difference in signal conversion characteristics between devices according to the present invention. 
           [0029]      FIG. 20  illustrates an example of a flowchart of a signal conversion process on an image for dimension measurement according to the present invention. 
           [0030]      FIG. 21  illustrates an example of experimentally acquired signal conversion characteristics in the scanning electron microscope system. 
           [0031]      FIG. 22  illustrates an example of a table of image signals in a reference device corresponding to image signals in an evaluation device according to the present invention. 
           [0032]      FIG. 23  illustrates an example of a GUI for displaying evaluation results of differences in signal conversion characteristics between devices and a signal conversion table in the scanning electron microscope system according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0033]    According to this embodiment, descriptions are hereinafter given as to an example of a scanning electron microscope system for dimension measurement in which non-linearity of an output signal is corrected relative to an input signal in a certain range as a reference in a signal processing unit. 
         [0034]      FIG. 1  illustrates an example of a configuration diagram of the scanning electron microscope system in this embodiment. 
         [0035]    The scanning electron microscope system in this embodiment is configured to include a scanning electron microscope main body  10 , a signal processing unit  11 , a general control unit  12 , and a PC  13 , which are connected to a data server  14  via a network. 
         [0036]    The scanning electron microscope main body  10  is configured to include an electron gun  101 , acceleration electrodes  103  that accelerate an electron beam  102  emitted from the electron gun  101 , focusing lenses  104 , deflection electrodes  105  that deflect the trajectory of the electron beam  102 , an objective lens  106  that controls the focus position of the electron beam  102  into which the electron beam  102  converges such that the focus position falls on a patterned surface of a sample  107 , a table  108  on which the sample  107  is placed, and a detector  109  that detects some of secondary electrons from the sample  107  irradiated with the electron beam  102 , which are controlled by the general control unit  12 . 
         [0037]    A signal detected by the detector  109  is converted into image data at the signal processing unit  11  under instructions from the general control unit  12 . 
         [0038]    The PC  13  includes a storage unit  131 , an arithmetic processing unit  132 , and an input/output unit  133  with a display screen. 
         [0039]    The arithmetic processing unit  132  in the PC  13  processes the image data converted at the signal processing unit  11  to extract signal conversion characteristics as information related to a relationship between an input signal and an output signal, stores the signal conversion characteristics as extraction results in the storage unit  131 , and then displays the same on the display screen. The input signal here refers to a signal detected by the detector  109 , and the output signal here refers to a signal processed and output by the signal processing unit  11 . The results displayed on the display unit are sent to the data server  14  accessible to a plurality of devices via a communication line, and are stored in the data server  14 . 
         [0040]    When another image is acquired by imaging, the information related to the relationship between an input signal and an output signal is read from the storage unit  131  of the PC  13  or the data server  14 , the other image data is corrected by the arithmetic processing unit  132  based on the read information, the dimensions of the imaged subject are measured, and then the processed image data, the dimension measurement results, the information related to the dimension measurement are stored in the storage unit  131  and displayed on the display screen. 
         [0041]      FIG. 2  illustrates an example 20 of a relationship  203  between an input signal  201  and an output signal  202  in a certain range (hereinafter, referred to as signal conversion characteristics) in the scanning electron microscope system illustrated in  FIG. 1 . The signal of secondary electrons detected by the detector  109  of the scanning electron microscope main body  10  is amplified and biased in the detector  109  and the signal processing unit  11 , and then the signal is output as an image to the PC  13 . 
         [0042]    In general, a signal is acquired with an increased amplification factor such that an output signal does not exceed the upper and lower limits of image gradation, because an output image (corresponding to the output signal) is more favorable in appearance with a higher signal contrast and the impact of a noise signal generated after the amplification of the signal in the detector  109  can be relatively suppressed. Meanwhile, a signal of secondary electrons (corresponding to the input signal) acquired by the detector  109  of the scanning electron microscope main body  10  includes a large proportion of noise relative to the magnitude of the signal. Thus, if the amplification factor of the signal is increased, the signal becomes saturated with the noise content beyond the upper and lower limits of image gradation. Accordingly, the output signal becomes low mainly due to the saturation of the input signal with the noise content. As a result, the relationship  203  between the input signal  201  and the output signal  202  takes on a non-linear form. 
         [0043]      FIG. 3  illustrates an example of a difference in signal conversion characteristics between devices in the scanning electron microscope system. Since the non-linearity varies depending on the magnitude of a noise signal or the like, the tendency of the non-linearity may be different between a plurality of devices as illustrated in  FIG. 3 . The signal conversion characteristics are represented by the relationship between the input signal  201  and the output signal  202  as illustrated in  FIG. 2 . It is noted that a signal conversion characteristic  203 A in a device A and a signal conversion characteristic  203 B in a device B are different from each other. 
         [0044]      FIG. 4  illustrates an example of a difference in output waveform between the devices in the scanning electron microscope system. As illustrated in  FIG. 4 , it is noted that output signal waveforms  402 A and  402 B relative to a coordinate  401  in the direction of dimension measurement acquired by imaging one and the same subject to be measured are different between the devices, which causes a difference in dimension measurement results calculated from the signal waveforms. In this embodiment, it is possible to decrease the difference in signal conversion characteristics between the devices by making corrections at the devices with consideration given to the signal conversion characteristics indicating the non-linearity of the output signal relative to the input signal. 
         [0045]      FIG. 5  illustrates a flow of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system illustrated in  FIG. 1 . The calculation of the signal conversion characteristics  20  described in  FIG. 5  is carried out at the PC  13  including the arithmetic processing unit illustrated in  FIG. 1 . 
         [0046]    First, image acquisition conditions for acquiring the input signal  201  in a certain range as a reference are decided (S 501 ). This step is needed to, in the case where it is difficult to measure the absolute value of the input signal  201  (the number of secondary electrons detected by the detector  109  in this embodiment) in the scanning electron microscope main body  10 , use instead of the input signal  201  an output signal acquired under image acquisition conditions without signal saturation for retention of linearity in a more reliable manner. The flow of deciding the image acquisition conditions without signal saturation will be described later in more detail with reference to  FIG. 6 . Under the image acquisition conditions without signal saturation, the amplification factor of the signal is lower than that at acquisition of the image for dimension measurement. 
         [0047]    When the image acquisition conditions for acquisition of the input signal  201  are decided, a sample wafer for use in acquisition of the input signal  201  and the output signal  202  is loaded onto the table  108  of the scanning electron microscope main body  10  (S 502 ). In this embodiment, a solid film sample of silicon or the like without a pattern or a crystalline pattern on a wafer is used. 
         [0048]    Then, the amount of current in the electron beam  102  as one of the image acquisition conditions is set to the i-th value (S 503 ). To obtain the signal conversion characteristics  20  illustrated in  FIG. 2 , it is necessary to acquire a plurality of input signals  201  and a plurality of output signals  202  corresponding to the input signals  201 . In this embodiment, the signals are acquired with changes in the amount of current in the electron beam  102  as a means for changing the magnitude of the input signal  201 . The amount of current in the electron beam  102  is desirably changed such that the input signals  201  can be acquired within the range including the input signal acquired from the subject of dimension measurement. 
         [0049]    Then, the image acquisition conditions for acquirement of the input signal  201  except for the amount of current in the electron beam  102  are set (S 504 ). 
         [0050]    Then, the image is acquired (S 505 ). 
         [0051]    Then, the average value of the acquired image signals of a plurality of pixels is calculated and set as an input signal B(i)  201  (S 506 ). 
         [0052]    Then, the acquired input signal B(i)  201  is stored in the storage unit  131  (S 507 ). 
         [0053]    Then, the image acquisition condition for acquisition of the j-th output signal  202  except for the amount of current in the electron beam  102  is set (S 508 ). The image acquisition condition for acquisition of the output signal  202  is the same as the image acquisition condition for acquisition of the image for dimension measurement. When the image for dimension measurement is to be acquired under a plurality of image acquisition conditions, the output signal  202  is acquired under each of the conditions. 
         [0054]    Then, the image is acquired (S 509 ). 
         [0055]    Then, the average value of the acquired image signals of a plurality of pixels is calculated and set as an output signal S(i, j)  202  (S 510 ). 
         [0056]    Then, the acquired output signal S(i, j)  202  is stored in the storage unit  131  (S 511 ). 
         [0057]    The steps S 508  to S 511  are repeatedly executed until the last number j of the image acquisition condition for acquisition of the output signal  202  is reached (S 512 ). 
         [0058]    In addition, the steps S 503  to S 512  are repeatedly executed until the last number i of the amount of current in the electron beam  102  is reached (S 513 ). 
         [0059]    After execution of the foregoing steps, the data acquisition of the input signal B(i)  201  and the output signal S(i, j)  202  with the amount of current i in the electron beam  102  is terminated. 
         [0060]      FIG. 6  illustrates a flow of the deciding image acquisition conditions without signal saturation described above in relation to S 501  of  FIG. 5 . 
         [0061]    First, imaging conditions for acquisition of the input signal  201  are set (S 601 ). 
         [0062]    Then, while the scanning electron microscope main body  10  is set so as not to detect a signal from a sample, images are acquired with changes in signal amplification factor (g) and bias addition amount (b) included in the image acquisition conditions (S 602 ), and the average value of the acquired image signals is calculated (S 603 ). The calculated result is set as S 0 ( g, b ). Since there is no signal detected by the detector  109 , the signal amount S 0 ( g, b ) at that time is ideally represented as in (Equation 1) shown below. 
         [0000]        S 0( g,b )= g× 0+ b   (Mathematical Formula 1)
 
         [0063]    Then, a sample wafer to be used in acquisition of data for calculating the signal conversion characteristics  20  is loaded onto the table  108  of the scanning electron microscope main body  10  (S 604 ). 
         [0064]    Then, images are acquired under the same conditions of signal amplification factor (g) and bias addition amount (b) as those at S 602  (S 605 ), and the average value of the acquired image signals is calculated (S 606 ). The calculated result is set as S(g, b). When the signal detected by the detector  109  is designated as s, the signal amount S(g, b) at that time is ideally represented as in (Mathematical Formula 2) shown below. 
         [0000]        S ( g,b )= g×s+b   (Mathematical Formula 2)
 
         [0065]    Then, the difference between the signal S(g, b) of the sample wafer acquired with changes in the signal amplification factor (g) and the bias addition amount (b) and the signal S 0 ( g, b ) without detection of a signal from the sample is calculated (S 607 ). The calculated result is set as dS(g, b). The signal amount difference dS(g, b) at that time is ideally represented as in (Mathematical Formula 3) shown below according to (Mathematical Formula 1) and (Mathematical Formula 2). 
         [0000]        dS ( g,b )= g×s   (Mathematical Formula 3)
 
         [0066]    That is, the signal amount difference dS(g, b) ideally takes a constant value without dependence on the bias addition amount (b). In actuality, however, the signal S(g, b) of the sample wafer becomes saturated depending on the conditions of the signal amplification factor (g) and the bias addition amount (b). Thus, as in the example of  FIG. 7 , the signal amount difference dS(g, b)  701  varies with changes in the bias addition amount (b)  702 .  FIG. 7  illustrates an example of data for deciding the image acquisition conditions without signal saturation. 
         [0067]    Taking these characteristics into account, the conditions of the signal amplifier factor (g)  703  and the bias addition amount (b)  702  are automatically decided such that, even if the bias addition amount (b)  702  varies, the signal does not become saturated while changes in the signal amount difference dS(g, b)  701  are small (S 608 ). 
         [0068]    By performing the foregoing steps, the image acquisition conditions without signal saturation are decided. 
         [0069]      FIG. 8  illustrates a flow of signal conversion in an image for dimension measurement using the data acquired in the flow of data acquisition for calculating the signal conversion characteristics  20  illustrated in  FIG. 5 . 
         [0070]    First, image acquisition conditions for acquisition of the image for dimension measurement are read (S 801 ). 
         [0071]    Then, the input signal B(i)  201  or the output signal S (i, j)  202  acquired in the flow of data acquisition ( FIG. 5 ) for calculating the signal conversion characteristics  20  under the target image acquisition condition (j=J) is read (S 802 ).  FIG. 9  illustrates a relationship  901  between the input signal B(i)  201  and the output signal S(i, J)  202 . 
         [0072]    Then, the relationship  901  between the input signal B(i)  201  and the output signal S(i, J)  202  is approximated by a function B=f(S) (S 803 ). The function for use in approximation may be a sigmoid function or a quadratic function, for example.  FIG. 9  also illustrates an approximate function  902 . The approximate function is adopted as the signal conversion characteristics  20 . 
         [0073]    Then, an input signal B 201  in each of the image signals S is calculated by the calculated approximate function B=f(S)  902 , and table data  100  illustrated in  FIG. 10  is created (S 804 ). 
         [0074]    Then, the image signal S for dimension measurement is converted into an input signal B 201  according to the created table data  100  (S 805 ). 
         [0075]    By performing the foregoing steps, the signal of the image for dimension measurement is converted. This conversion allows the image signal for dimension measurement to be corrected in the non-linearity  203  of the output signal  202  relative to the input signal  201 . 
         [0076]      FIG. 11  illustrates an example of a GUI  110  for executing data acquisition for calculation of the signal conversion characteristics  20 . The GUI includes an image acquisition condition selection field  1101  for selecting image acquisition conditions for acquiring the output signal  201  and a data acquisition execution button  1102  for executing data acquisition. 
         [0077]    The GUI  110  is displayed on an output screen of the input/output unit  133  to allow the user to select arbitrary image acquisition conditions or confirm execution of data acquisition. 
         [0078]      FIG. 12  illustrates an example of a GUI  120  for displaying calculation results of the signal conversion characteristics  20 . The GUI  120  includes a signal conversion characteristic data selection button  1201  for selecting signal conversion characteristic data to be displayed, a display field  1202  for displaying the relationship  901  between the acquired input signal  201  and output signal  202  and the function approximation  902  of the relationship  901 , and a display field  1203  for displaying the table data  100  illustrated in  FIG. 10 . 
         [0079]    The corrected difference in the image signals to be measured in dimensions between the devices is smaller than that before the correction. Accordingly, the difference in results of dimension measurement using these image signals between the devices also becomes smaller. In addition, when a plurality of detectors  109  exists in one device, the difference in image signals and the difference in dimension measurement results between the detectors  109  can be reduced by executing data acquisition for calculating the signal conversion characteristics  20  described in  FIG. 5  for each of the detectors  109  and performing signal conversion in the image for dimension measurement described in  FIG. 8  for each of the detectors  109 . 
         [0080]    According to the scanning electron microscope system in this embodiment, it is possible to not only reduce the difference in dimension measurement results between the devices but also correct the non-linearity  203  of the output signal  202  relative to the input signal  201 . Accordingly, the improvement in accuracy of dimension measurement can be expected by using library matching by which the dimensions of a target subject are measured by matching a signal waveform directly to library data of a simulation waveform, for example. 
         [0081]    The GUI  120  is displayed on the output screen of the input/output unit  133  and allows the user to select arbitrary signal conversion characteristic data or view signal conversion characteristics and signal correction LUT. 
       Embodiment 2 
       [0082]    According to this embodiment, descriptions are hereinafter given as to another example of a flow of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system described above in relation to the first embodiment with reference to  FIG. 5 . 
         [0083]      FIG. 13  illustrates an example 2 of a flow of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system according to the present invention. The flow of data acquisition in this embodiment is basically the same as the flow of data acquisition for calculating the signal conversion characteristics  20  illustrated in  FIG. 5 , and thus descriptions are given only as to the difference from the flow of data acquisition illustrated in  FIG. 5 . 
         [0084]    In the flow of  FIG. 5 , the image acquisition conditions without signal saturation are decided to acquire the input signal  201  at S 501 . The flow of  FIG. 13  is different from the flow of  FIG. 5  in that, since the input signal  201  is considered as being proportional to the amount of current in the electron beam  102 , the amount of current in the electron beam  102  is saved as the input signal  201  in the storage unit  131  at S 1301 . 
         [0085]      FIG. 14  illustrates an example 3 of a flow of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system according to the present invention. The flow of data acquisition in this embodiment is basically the same as the flow of data acquisition for calculating the signal conversion characteristics  20  illustrated in  FIG. 5 , and thus descriptions are given only as to the difference from the flow of data acquisition illustrated in  FIG. 5 . 
         [0086]    In the flow of  FIG. 5 , the magnitude of the input signal  201  is changed with variations in the amount of current in the electron beam  102  at S 503 . Meanwhile, in this embodiment, a plurality of samples is prepared from materials different in the yield of secondary electrons, and is used in a switched manner at  51401  and  51402  to change the magnitude of the input signal  201 . Alternatively, instead of using the materials different in the yield of secondary electrons, a plurality of samples may be prepared from material different in inclination relative to the incident angle of the electron beam  102  and used in a switched manner to change the magnitude of the input signal  201 . This harnesses the property of changing the yield of secondary electrons depending on the angle formed by the incident direction of the electron beam  102  and the inclination of the subject to be irradiated. 
         [0087]    Alternatively, when the yield of secondary electrons in each of the samples or the magnitude of the input signal  201  in each of the samples is known in advance, the step of deciding imaging conditions for acquisition of the input signal  201  (S 501 ) and the steps related to the acquisition of the input signal (S 504  to S 507 ) may be omitted to use a known input signal instead. 
         [0088]      FIG. 15  illustrates an example 4 of a flow of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system according to the present invention. The flow of data acquisition in this embodiment is basically the same as the flow of data acquisition for calculating the signal conversion characteristics  20  illustrated in  FIG. 5 , and thus descriptions are given only as to the difference from the flow of data acquisition illustrated in  FIG. 5 . 
         [0089]    In the flow of  FIG. 5 , the magnitude of the input signal  201  is changed with variations in the amount of current in the electron beam  102  at S 503 . Meanwhile, in this embodiment, a sample with a pattern is used to change the magnitude of the input signal  201 . As described above in relation to the example of  FIG. 14 , since the yield of secondary electrons varies depending on the angle formed by the incident direction of the electron beam and the inclination of the subject to be irradiated, the magnitude of the input signal  201  varies depending on the irradiated position of the pattern with asperities. 
         [0090]    Thus, the sample wafer with a pattern is loaded at  51501 , and an image with the target pattern is acquired under the imaging conditions for acquisition of the input signal at S 504  and S 505 . Then, the input signal B(i)  201  is calculated at a position coordinate i of the acquired image at  51502 . In addition, an image with the same pattern as that at S 505  is acquired under the imaging conditions for acquisition of the j-th output signal  202  at S 509 . Then, the output signal S(i, j)  202  is calculated at the position coordinate i of the acquired image at  51503 . 
         [0091]    When the subject is imaged plural times at one and the same point, the signals vary under influence of electrical charging or the like. Thus, for acquisition of the output signal  202 , it is preferred to acquire images in a pattern of a different point formed in the same manner as the target pattern in which the images are acquired at acquisition of the input signal  201 . At that time, the change of imaging positions may displace the position of the pattern in the images. Thus, it is preferred that the value of the position coordinate i of the output signal  202  is corrected in alignment with the position coordinate of the input signal  201 . 
         [0092]    In each of the flows of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system described above in relation to the second embodiment, the means for changing the magnitude of the input signal  201  may be combined with changing the amount of current in the electron beam  102 . 
       Third Embodiment 
       [0093]    According to this embodiment, descriptions are hereinafter given as to an example of a flow of data acquisition for calculating the signal conversion characteristics  20  and an example of a flow of signal conversion of the image for dimension measurement in the scanning electron microscope system illustrated in  FIG. 1 , which are different from those in the first embodiment. In the first embodiment, the output signal  202  is acquired under each of the image acquisition conditions for acquisition of the image for dimension measurement. In this embodiment, taking into account the case where the data for calculating the signal conversion characteristics  20  is acquired and then the imaging conditions for acquisition of the image for dimension measurement are decided, the output signal  202  corresponding to arbitrary imaging conditions for acquisition of the image for dimension measurement is acquired to calculate the signal conversion characteristics  20 . 
         [0094]      FIG. 16  illustrates an example of a flow of data acquisition for calculating the signal conversion characteristics  20  in the scanning electron microscope system according to the present invention. The flow of data acquisition in this embodiment is basically the same as the flow of data acquisition for calculating the signal conversion characteristics  20  illustrated in  FIG. 5 , and thus descriptions are given only as to the difference from the flow of data acquisition illustrated in  FIG. 5 . 
         [0095]    At  51602  and  51603  of  FIG. 16 , an imaging parameter p and an imaging parameter q are set as image acquisition conditions for acquisition of the image for dimension measurement that are considered to vary depending on a subject to be measured in dimensions. Then, the image is acquired at S 509 . The imaging parameter p and the imaging parameter q are each set by dividing the possible setting range as appropriate. Specific examples of the imaging parameter p and the imaging parameter q are signal amplification factor, bias addition amount, and the like. The output signal calculated from the acquired image is designated as S(i, j, k). 
         [0096]      FIG. 17  illustrates a flow of signal conversion in the image for dimension measurement with the use of the data acquired in the flow of data acquisition for calculating the signal conversion characteristics  20  described in  FIG. 16 . 
         [0097]    First, image acquisition conditions for acquisition of the image for dimension measurement are read (S 1701 ). In particular, the value of the imaging parameter p is designated as P and the value of the imaging parameter q as Q. 
         [0098]    Then, the input signal B(i) and the output signal S(i, j, k) acquired in the flow of data acquisition for calculating the signal conversion characteristics  20  are read (S 1702 ). 
         [0099]    Then, an output signal S(i, P, Q) is calculated by interpolation of the output signal S(i, j, k) with the imaging parameter p as P and the imaging parameter q as Q (S 1703 ). 
         [0100]    The subsequent steps are the same as those at S 803  and subsequent steps illustrated in  FIG. 8 , and thus descriptions thereof are omitted. 
         [0101]    By performing the foregoing steps, the signal of the image for dimension measurement is converted. This conversion allows the image signal of the image for dimension measurement acquired under arbitrary imaging conditions to be corrected in the non-linearity  203  of the output signal  202  relative to the input signal  201 . 
       Fourth Embodiment 
       [0102]    According to this embodiment, descriptions are hereinafter given as to an example of a scanning electron microscope system configured to not only correct a difference in the signal conversion characteristics  20  between a plurality of devices but also correct temporal changes in the signal conversion characteristics  20 . 
         [0103]    In this embodiment, the scanning electron microscope system in the first to third embodiments is used to execute regularly the flow of data acquisition for calculating the signal conversion characteristics  20  and the flow of signal conversion in the image for dimension measurement with the use of the data acquired in the flow of data acquisition in the first to third embodiments. The results obtained by the regular execution are stored chronologically in the storage unit  131 . 
         [0104]    The stored signal conversion characteristics  20  are compared in chronological order, and if there is a change larger than a predetermined value, alarm display is provided. 
         [0105]      FIG. 18  illustrates an example of a GUI  180  for displaying evaluation results and temporal changes in the signal conversion characteristics  20 . The GUI  180  includes a signal conversion characteristic data selection field  1801  for selecting signal conversion characteristic data to be displayed, an input field  1802  for inputting imaging conditions under which the signal conversion characteristics  20  are to be displayed, a data display field  1803  for displaying the signal conversion characteristics  20  under the input imaging conditions, a display field  1805  for displaying the table data  100  of the signal conversion characteristics  20  in chronological order, and the like. In the display field  1805  for displaying the table data  100  of the signal conversion characteristics  20  in chronological order, if there is a change in the signal conversion characteristics  20  in chronological order larger than a predetermined value, the applicable chronological data is colored to inform the user of the change. The GUI  180  is displayed on the output screen of the input/output unit  133  and allows the user to select arbitrary signal conversion characteristic data or input arbitrary imaging conditions. 
       Fifth Embodiment 
       [0106]    According to this embodiment, descriptions are hereinafter given as to an example of the scanning electron microscope system described above in relation to the first to fourth embodiments in which the scanning electron microscope main body  10  has a dedicated sample holder. 
         [0107]    In particular, when data acquisition for calculating the signal conversion characteristics  20  is to be regularly executed as in the fourth embodiment, the data acquisition can be performed more easily by placing a permanent evaluation sample in the device than by preparing an evaluation sample at each execution of data acquisition. The scanning electron microscope main body  10  in this embodiment is configured in the same manner as that illustrated in  FIG. 1 . The scanning electron microscope main body  10  has on the table  108  a holder dedicated for an evaluation sample to be used in data acquisition for calculating the signal conversion characteristics  20 . The evaluation sample is attached to the holder. 
       Sixth Embodiment 
       [0108]    In the first to fourth embodiments, the non-linearity  203  of the output signal  202  relative to the input signal  201  is corrected by the flow of conversion of the output signal of the image for dimension measurement. Meanwhile, according to this embodiment, descriptions are hereinafter given as to an example of correction of the non-linearity  203  of the output signal  202  relative to the input signal  201  at the time of outputting the image for dimension measurement. 
         [0109]    In the scanning electron microscope main body  10  of  FIG. 1 , the signal detected by the detector  109  is processed and output as an image by the signal processing unit  11 . At the signal processing unit  11 , signal conversion is performed based on the calculation results of the signal conversion characteristics  20  and the imaging conditions for the image for dimension measurement saved in the storage unit  131 , and then the image after the signal conversion is output. The signal conversion is performed on the PC  13  after the acquisition of the image in the first to fourth embodiments, whereas the signal conversion is performed before the output of the image in this embodiment. 
       Seventh Embodiment 
       [0110]    According to this embodiment, descriptions are hereinafter given as to an example of a scanning electron microscope system for dimension measurement configured to correct a difference between devices in non-linearity of an output signal relative to an input signal in a specific range as a reference. 
         [0111]    An example of a configuration diagram of the scanning electron microscope in this embodiment is the same as that of  FIG. 1 , and thus descriptions thereof are omitted. 
         [0112]    In this embodiment, an example 20 of the relationship  203  between the input signal  201  and the output signal  202  in a specific range (hereinafter, referred to as signal conversion characteristics) in the scanning electron microscope system illustrated in  FIG. 1  is the same as that of  FIG. 2 , and thus descriptions thereof are omitted. 
         [0113]      FIG. 19  illustrates a flow of data acquisition for calculating a difference in signal conversion characteristics between devices in the scanning electron microscope system illustrated in  FIG. 1 . 
         [0114]    First, a sample wafer to be used in acquisition of the output signal  202  is loaded onto the table  108  of the scanning electron microscope main body  10  (S 1901 ). In this embodiment, a solid film sample of silicon or the like without a pattern or a crystalline pattern on a wafer is used. 
         [0115]    Then, the amount of current in the electron beam  102  as one of the image acquisition conditions is set to the i-th value (S 1902 ). To obtain a difference in signal conversion characteristics between the devices, it is necessary to acquire the output signal  202  with a plurality of magnitudes. In this embodiment, as a means for changing the magnitude of the signal, the signal is acquired with changes in the amount of current in the electron beam  102 . It is desired that the amount of current in the electron beam  102  is changed to obtain the signal in the range including the signal from the subject to be measured in dimensions. 
         [0116]    Then, an image acquisition condition for acquiring the j-th output signal  202  except for the amount of current in the electron beam  102  is set (S 1903 ). The image acquisition condition for acquiring the output signal  202  is set to be the same as the image acquisition condition for acquiring the image for dimension measurement. When the image for dimension measurement is to be acquired under a plurality of image acquisition conditions, the output signal  202  is acquired under each of the conditions. 
         [0117]    Then, the image is acquired (S 1904 ). 
         [0118]    Then, the average value of acquired image signals with a plurality of pixels is calculated and set as an output signal S(i, j)  202  (S 1905 ). 
         [0119]    Then, the acquired output signal S (i, j)  202  is stored in the storage unit  14  (S 1906 ). 
         [0120]    The steps S 1903  to S 1906  are repeatedly performed until the last number j of the image acquisition condition for acquisition of the output signal  202  is reached. 
         [0121]    In addition, the steps S 1902  to S 1907  are repeatedly performed until the last number i of the amount of current in the electron beam  102  is reached (S 1908 ). 
         [0122]    After the execution of the foregoing steps, the data acquisition of the output signal S(i, j)  202  with the amount of current i in the electron beam  102  is terminated. 
         [0123]    The data acquisition for calculating a difference in signal conversion characteristics between devices in the flow of  FIG. 19  is performed at a plurality of devices to be corrected in the difference, and then the results obtained by the devices are stored together with device identification information in the storage unit  14 . 
         [0124]      FIG. 20  illustrates a flow of signal conversion in the image for dimension measurement, with the use of the data for calculating the signal conversion characteristics of the plurality of devices that is acquired in the flow of data acquisition for calculating the signal conversion characteristics described in  FIG. 19 . 
         [0125]    First, the image acquisition conditions for acquiring the image for dimension measurement acquired at the devices to be corrected in the difference of signal conversion characteristics between the devices are read (S 2001 ). 
         [0126]    Then, of the output signals S(i, j)  202  acquired in the flow of data acquisition for calculating the signal conversion characteristics, the output signal S(i, j)  202  acquired under a target image acquisition condition (i=J) is read into a reference device. The read result is set as a reference signal B(i, J) (S 2002 ). 
         [0127]    Then, of the output signals S(i, j)  202  acquired in the flow of data acquisition for calculating the signal conversion characteristics, the output signal S(i, j)  202  acquired under the target image acquisition condition (i=J) is read into the devices to be corrected in the difference of signal conversion characteristics between the devices. The read result is set as a reference signal S (i, J) (S 2003 ).  FIG. 21  illustrates a relationship  2103  between the reference signal B(i, J)  2101  and the output signal S(i, J)  202  of the devices to be corrected in the difference of signal conversion characteristics between the devices. 
         [0128]    Then, the relationship illustrated in  FIG. 21  is approximated by the function B=f(S) (S 2004 ).  FIG. 21  illustrates an experimentally acquired example of a difference in signal conversion characteristics between the devices in the scanning electron microscope system. The function for use in the approximation may be a sigmoid function or a quadratic function, for example.  FIG. 21  also illustrates an approximate function  2104 . 
         [0129]    Then, a reference signal B 2101  is calculated for each of the output signals S from the calculated approximate function B=f(S), and table data  220  is created as illustrated in  FIG. 22  (S 2005 ).  FIG. 22  illustrates an example of a table of image signals in the reference device corresponding to image signals in an evaluation device according to the present invention. 
         [0130]    Then, the image signal S for dimension measurement acquired at the devices to be corrected in difference of signal conversion characteristics between the devices is converted into the reference signal B 2101  according to the created table data  220  (S 2006 ). 
         [0131]    By performing the foregoing steps, the signal of the image for dimension measurement acquired at the devices to be corrected in difference of signal conversion characteristics between the devices is converted. According to this conversion, the image signal  202  for dimension measurement acquired at the devices to be corrected in difference of signal conversion characteristics between the devices, can be corrected in non-linearity  2103  of the output signal  2101  of the reference device. 
         [0132]    An example of a GUI necessary for data acquisition for calculating a difference in signal conversion characteristics between the devices is the same as that illustrated in  FIG. 11 , and thus descriptions thereof are omitted. 
         [0133]      FIG. 23  illustrates an example of a GUI  230  for displaying calculation results of the difference in signal conversion characteristics between the devices. The GUI  230  includes a selection field  2301  for selecting signal conversion characteristic data to be displayed, a field  2302  for illustrating the relationship  2103  between the reference signal  2101  and the output signal  202  from the devices to be corrected in difference of signal conversion characteristics between the devices and a result  2104  obtained by subjecting the relationship  2103  to function approximation, and a display field  2303  for displaying the table data  220  illustrated in  FIG. 22 . 
         [0134]    The corrected difference in the image signals to be measured in dimensions between the devices is smaller than that before the correction. Accordingly, the difference in results of dimension measurement using these image signals between the devices also becomes smaller. In addition, when a plurality of detectors  109  exists in one device, the difference in image signals and the difference in dimension measurement results between the detectors  109  can be reduced by executing data acquisition for calculating the signal conversion characteristics described in  FIG. 19  for each of the detectors  109  and performing signal conversion in the image for dimension measurement described in  FIG. 20  for each of the detectors  109 . 
         [0135]    The GUI  110  is displayed on an output screen of the input/output unit  133  to allow the user to select arbitrary image acquisition conditions or confirm execution of data acquisition. 
       Eighth Embodiment 
       [0136]    The second to sixth embodiments as modification examples of the first embodiment are also applicable to the seventh embodiment. 
       Ninth Embodiment 
       [0137]    According to this embodiment, descriptions are hereinafter given as to an example of a system for correcting the non-linearity  203  of the output signal  202  relative to the input signal  201  and a difference  1904  between the devices, which is connected to the scanning electron microscope main body  10 , the signal processing unit  11 , and the general control unit  12 . 
         [0138]    The configuration of the system is equivalent to the PC  13  illustrated in  FIG. 1 , and can be realized by connecting the PC  13  to the scanning electron microscope system. 
         [0139]    The contents of the processes performed in the system are as described above in relation to the first to eighth embodiments, and thus detailed descriptions thereof are omitted. 
       Tenth Embodiment 
       [0140]    In this embodiment, descriptions are hereinafter given as to an example of an execution program for correcting the non-linearity  203  of the output signal  202  relative to the input signal  201  and the difference  1904  between the devices, which is installed in the PC  13  connected to the scanning electron microscope main body  10 . 
         [0141]    The contents of the processes performed in the system are as described above in relation to the first to eighth embodiments, and thus detailed descriptions thereof are omitted. 
       Eleventh Embodiment 
       [0142]    Each of the first to tenth embodiments is limited to a scanning electron microscope device for dimension measurement. However, the scanning electron microscope device is not necessarily to be used in dimension measurement as far as it is a device, a system, or a program configured in the same manner. In addition, the present invention is not limited to a scanning electron microscope device but may be applied to a transmission electron microscope device or another charged particle microscope device. 
         [0143]    As described above, according to the present invention, it is possible to provide the function of correcting a difference in signal amount between devices by which to quantitatively evaluate output signals (image quality) acquired from the devices relative to an input signal in a specific range as a reference, and use the evaluation results to adjust the output signals from the devices to be equivalent in magnitude. 
         [0144]    The present invention is applicable to a charged particle microscope system and a measurement method using the same. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  scanning electron microscope main body 
           101  electron gun 
           102  electron beam 
           103  acceleration electrode 
           104  focusing lens 
           105  deflection electrode 
           107  sample 
           106  objective lens 
           108  table 
           109  detection unit  109   
           12  general control unit  12