Patent Publication Number: US-2022222775-A1

Title: Sample observation apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a sample observation apparatus. 
     BACKGROUND ART 
     As a background art of the present disclosure, there is, for example, PTL 1. PTL 1 discloses, for example, “a sample observation apparatus including: a charged particle microscope that irradiates and scans a sample placed on a movable table with a charged particle beam to image the sample; an image storage unit that stores a low-quality image with poor image quality and a high-quality image with good image quality of the same portion of the sample acquired by changing observation conditions for imaging the sample with the charged particle microscope; a calculation unit that obtains an estimation processing parameter for estimating a high-quality image from the low-quality image using the low-quality image and the high-quality image stored in the image storage unit; a high-quality image estimation unit that estimates a high-quality image of a desired region by processing the low-quality image of a desired portion of the sample obtained by imaging the desired portion of the sample with the charged particle microscope using the estimation processing parameter obtained by the calculation unit; and an output unit that outputs the estimated high-quality image estimated by the high-quality image estimation unit” (Abstract). 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2018-137275 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, a learning method has been proposed in which a correspondence relationship between a low-quality image and a high-quality image is learned in advance, and the high-quality image is estimated based on the input low-quality image. By using learning type high-quality image estimation processing, it is possible to output a high-quality image even under an observation condition having high throughput. 
     In the learning type high-quality image estimation method as described above, it is important for high throughput observation to reduce time required for the user to acquire an appropriate model for estimating the high-quality image from the low-quality image. 
     Solution to Problem 
     A sample observation apparatus according to one aspect of the invention includes: a microscope that irradiates a sample with a probe, detects a signal from the sample, and outputs a detection signal; and a system that generates an image based on the detection signal received from the microscope. The system receives designation executed by a user for one or more trained models in a model database storing data of a plurality of trained models for estimating a high-quality image based on a low-quality image, generates and displays a current low-quality observation image based on the detection signal, and estimates and displays a high-quality image based on the current low-quality observation image according to each of the one or more trained models. 
     Advantageous Effect 
     According to a typical example of the invention, it is possible to shorten time required for the user to acquire an appropriate model for estimating a high-quality image based on a low-quality image. 
     Problems to be solved, configurations, and effects other than those described above will be apparent based on the description of the following embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a configuration example of a sample observation apparatus including a scanning electron microscope. 
         FIG. 2  shows a configuration example of a control device, a storage device, and an arithmetic device of a control system. 
         FIG. 3  shows a flowchart of an example of a sample observation method. 
         FIG. 4  is a flowchart showing details of a saved image acquisition step. 
         FIG. 5  shows a training image data automatic acquisition setting screen. 
         FIG. 6  shows a screen that receives designation of a training time executed by a user. 
         FIG. 7  is a flowchart showing details of a high quality image estimation processing application step. 
         FIG. 8A  shows a change in a display content of an estimation model selection screen. 
         FIG. 8B  shows a change in a display content of the estimation model selection screen. 
         FIG. 8C  shows a change in a display content of the estimation model selection screen. 
         FIG. 9  shows another example of a method for displaying a high quality image on the estimation model selection screen. 
         FIG. 10  shows a flowchart of another example of the sample observation method. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment will be described with reference to the accompanying drawings. The embodiment is merely an example for implementing the invention, and does not limit the technical scope of the invention. In the drawings showing the embodiment, elements having the same or similar configurations are denoted by the same reference numerals, and a repeated description thereof will be omitted. 
     An example of the sample observation apparatus disclosed below estimates a high-quality image based on a low-quality image, and displays the estimated high-quality image. The sample observation apparatus receives designation of one or more trained learning models (also simply referred to as models) executed by the user, and estimates a high-quality image based on the low-quality image according to the one or more designated trained models. Accordingly, it is possible to efficiently prepare an appropriate model for estimating the high-quality image based on the low-quality image. 
     Hereinafter, a sample observation apparatus according to an embodiment will be described. In an example of a sample observation apparatus described below, a scanning electron microscope (SEM) is used to image a sample. The scanning electron microscope is an example of a charged particle microscope. As the sample observation apparatus, another type of microscope that captures an image of the sample, for example, a microscope using ions or electromagnetic waves as a probe, a transmission electron microscope, or the like can be used. An image quality may change depending on an intensity of the probe and irradiation time. 
       FIG. 1  shows a configuration example of a sample observation apparatus including an SEM according to the present embodiment. The sample observation apparatus  100  includes an SEM  101  that captures an image of a sample, and a control system  120 . The control system  120  includes a control device  102  that controls components of the SEM  101  that captures an image of a sample, a storage device  103  that stores information, an arithmetic device  104  that executes a predetermined arithmetic operation, and an external storage medium interface  105  that communicates with an external storage medium. 
     The control system  120  further includes an input and output interface  106  that communicates information with an input and output terminal  113  used by a user (operator), and a network interface  107  that allows connection with an external network. Components of the control system  120  can communicate with one another via a network  114 . The input and output terminal  113  includes input devices such as a keyboard and a mouse, and output devices such as a display device and a printer. 
     The SEM  101  includes a stage  109  on which a sample  108  is placed, an electron source  110  that generates primary electrons (probes) with which the sample  108  is irradiated, and a plurality of detectors  111  that detect signals from the sample  108 . 
     The stage  109  carries the sample  108  to be observed and moves in an X-Y plane or in an X-Y-Z space. The electron source  110  generates a primary electron beam  115  with which the sample  108  is irradiated. The plurality of detectors  111  detect, for example, secondary electrons  117 , reflected electrons  118 , and X-rays  119  that are generated from the sample  108  irradiated with the primary electron beam  115 . The SEM  101  further includes an electron lens (not shown) that converges the primary electron beam  115  on the sample  108 , and a deflector (not shown) that allows the sample  108  to be scanned with the primary electron beam  115 . 
       FIG. 2  shows a configuration example of the control device  102 , the storage device  103 , and the arithmetic device  104  of the control system  120 . The control device  102  includes a main control unit  200 , a stage control unit  201 , a scan control unit  202 , and a detector control unit  203 . The control device  102  includes, for example, a processor and a memory that stores a program executed by the processor and data used by the program. For example, the main control unit  200  is a program module, and the stage control unit  201 , the scan control unit  202 , and the detector control unit  203  are electric circuits. 
     The stage control unit  201  controls the stage  109  to, for example, move and stop the stage  109  in the X-Y plane or in the X-Y-Z space. By moving the stage  109 , a visual field of an observation image can be moved. The scan control unit  202  controls scanning of the sample  108  with the primary electron beam  115 . Specifically, the scan control unit  202  controls a deflector (not shown) to control a scan region of the primary electron beam  115  on the sample  108  so as to obtain an image of a target visual field and a target imaging magnification. Further, the scan control unit  202  controls a scan speed of the primary electron beam  115  in the scan region. 
     The detector control unit  203  acquires a detection signal from the selected detector  111  in synchronization with scanning with the primary electron beam  115  driven by a deflector (not shown). The detector control unit  203  generates observation image data according to the detection signal from the detector  111 , and transmits the observation image data to the input and output terminal  113 . The input and output terminal  113  displays an observation image based on the received observation image data. The detector control unit  203  automatically adjusts parameters such as a gain and an offset of the detector  111  according to a user instruction from the input and output terminal  113  or according to a detection signal. By adjusting the parameters of the detector  111 , a contrast and brightness of the image are adjusted. The contrast and brightness of the image can also be adjusted in the control device  102 . 
     The storage device  103  may include, for example, one or more nonvolatile storage devices and/or one or more volatile storage devices. Each of the nonvolatile storage device and the volatile storage device includes a non-transitory storage medium that stores information (data). 
     In the configuration example shown in  FIG. 2 , the storage device  103  stores an image database (DB)  204 , observation conditions  205 , sample information  206 , and a model database  207 . The observation condition  205  indicates an apparatus condition of the current observation of the sample  108 . The observation condition  205  includes, for example, an acceleration voltage of the primary electron beam  115  (probe), a probe current, a scan speed, a detector that detects a signal from the sample  108 , a contrast, brightness, an imaging magnification, stage coordinates, and the like. 
     The observation condition  205  indicates an observation condition for each of different imaging modes of a sample image. As will be described later, the imaging mode includes an optical axis adjustment mode for generating a scanned image for optical axis adjustment, a visual field search mode for generating a scanned image for searching of a visual field, and a checking mode for checking a scanned image (to be stored) for observation purpose. 
     The sample information  206  includes information of the current sample  108 , for example, information such as an identifier, a model number, and a category of the sample. Samples having a common model number are prepared based on the same design. The category of the sample includes, for example, a biological sample, a metal sample, a semiconductor sample, and the like. 
     The image database (DB)  204  stores a plurality of images and accompanying information thereof. The accompanying information of the images includes information on the sample to be observed and an apparatus condition (observation condition) in imaging of the image. The information on the sample includes, for example, information such as the identifier of the sample, the model number of the sample, and the category of the sample. The accompanying information of a pair of an input image (low quality image) and a teacher image (high quality image) used for model training associates images constituting the pair with each other. 
     The model database  207  stores configuration data of each of a plurality of trained models and accompanying information of the models. The configuration data of one model includes a learning parameter set updated by training. As the model, for example, a convolutional neural network (CNN) can be used. A type of the model is not particularly limited, and a model (machine learning algorithm) of a type different from that of a neural network can also be used. 
     In the example described below, configurations other than the learning parameter sets of all models are common, and the learning parameter sets may be different for each model. In another example, the model database  207  may store models having different components other than the learning parameter sets. For example, the model database  207  may store neural networks having different hyperparameters or models having different algorithms. 
     The model stored in the model database  207  is trained so as to relatively estimate the high-quality image based on the low-quality image. A low quality and a high quality indicate relative qualities between two images. The input image in the training data is a low-quality image of an observation target captured by the SEM  101 , and the teacher image is a high-quality image of the same observation target captured by the SEM  101 . The training data is stored in the image database  204  as described above. 
     The low-quality image includes an image of a low signal to noise ratio (SNR), and includes, for example, an image generated with a small amount of signal from the sample, an image blurred due to out-of-focus, and the like. The high quality image corresponding to the low SNR image is an image having a higher SNR than the low SNR image. In the example described below, an image pair includes a low-quality image captured in a high-speed scan and a high-quality image captured in a low-speed scan or a high-quality image obtained by frame integration in a high-speed scan is used for model training. The low-speed scan and the high-speed scan show a relative relationship of scan speeds. 
     Each model is trained by a plurality of pairs of the low-quality image and the high-quality image. In an example described below, values of some condition items related to the image quality of the low-quality images of the plurality of pairs are common. For example, the values of the acceleration voltage, the probe current, the scan speed, the type of the detector, the contrast, the brightness, and the like are common. An observation condition model is trained by, for example, low-quality images of different regions under the above-described common conditions of the same sample and high-quality images under the above-described common conditions corresponding to the low-quality images. The high-quality image may be captured under different conditions. 
     Training data of one model can include image data of different samples. For example, the training data can include one or a plurality of image pairs commonly used for a plurality of models, in addition to an image of one sample. A common image pair is an image of a sample of the same category as the above-described one sample, and for example, categories such as a biological sample, a metal sample, and a semiconductor sample are defined. Accordingly, versatility of the model can be improved. The training data may include low-quality images in which values of the condition items do not completely match but are similar to each other. For example, the training data may include a low-quality image in which a difference in the value of each item is within a predetermined threshold value. 
     The arithmetic device  104  includes a high-quality image estimation unit  208  and a model training unit  209 . The arithmetic device  104  includes, for example, a processor and a memory that stores a program executed by the processor and data used by the program. The high-quality image estimation unit  208  and the model training unit  209  are program modules. 
     The high-quality image estimation unit  208  estimates a high-quality image based on the input low-quality image according to the model. The model training unit  209  updates learning parameters of the model using the training data. Specifically, the model training unit  209  inputs the low-quality image of the training data to the high-quality image estimation unit  208  that operates according to the selected model, and acquires the estimated high-quality image. 
     The model training unit  209  calculates an error between the high-quality image which is the teacher image in the training data and the estimated high-quality image, and updates the learning parameter by back propagation so as to reduce the error. The model training unit  209  repeats updating of the learning parameter for each of the plurality of image pairs included in the training data. Training of a machine learning model is a widely known technique, and a detailed description thereof will be omitted. 
     As described above, in one example, the control device  102  and the arithmetic device  104  can include a processor and a memory. The processor executes various types of processing according to programs stored in the memory. The processor operates according to the programs, so that various function units are implemented. The processor can include a single processing unit or a plurality of processing units, and can include a single or a plurality of arithmetic units, or a plurality of processing cores. 
     The program executed by the processor and the data used for the program are stored in, for example, the storage device  103  and loaded into the control device  102  and the arithmetic device  104 . For example, the data of the model executed by the arithmetic device  104  is loaded from the model database  207  into the memory of the arithmetic device  104 . At least a part of the functions of the control device  102  and the arithmetic device  104  may be implemented by a logic circuit different from the processor, and the number of devices in which the functions of the control device  102  and the arithmetic device  104  are implemented is not limited. 
     An example of a sample observation method will be described with reference to the flowchart in  FIG. 3 . In the following description, an instruction of the user is given from the input and output terminal  113  via the input and output interface  106 . First, the user places the sample  108  to be observed on the stage  109  (S 101 ). When the user starts an operation via the input and output terminal  113 , the main control unit  200  displays a microscope operation screen on the input and output terminal  113 . 
     Next, according to a user instruction, the scan control unit  202  irradiates the sample  108  with the primary electron beam  115  (S 102 ). The user adjusts an optical axis while checking the image of the sample  108  at the input and output terminal  113  (S 103 ). According to an instruction from the user to start optical axis adjustment, the main control unit  200  displays an optical axis adjustment screen on the input and output terminal  113 . The user can adjust the optical axis on the optical axis adjustment screen. The main control unit  200  controls an aligner (not shown) for adjusting an optical axis of the SEM  101  according to a user instruction. 
     The optical axis adjustment screen displays the sample image during the optical axis adjustment in real time. The user adjusts the optical axis to an optimum position while viewing the sample image. For example, the main control unit  200  performs wobbling in which the excitation current of an electron lens such as a condenser lens or an objective lens is periodically changed, and the user adjusts the optical axis such that the movement of the sample image is minimized while viewing the sample image. 
     In the optical axis adjustment, the main control unit  200  sets other components of the SEM  101  and the control device  102  according to the observation condition of the optical axis adjustment mode indicated by the observation condition  205 . The detector control unit  203  processes the detection signal from the detector  111  indicated by the observation condition  205  to generate a sample image (scanned image) of the observation region for optical axis adjustment. The main control unit  200  displays the scanned image generated by the detector control unit  203  on the input and output terminal  113 . 
     The scanned image for optical axis adjustment is generated at a high scan speed and is displayed at a high frame rate (high image update speed). The scan control unit  202  moves the primary electron beam  115  on the sample  108  at a scan speed in the optical axis adjustment mode. The scan speed for the optical axis adjustment is higher than the scan speed for generating the sample image to be stored. The generation of one image is completed in, for example, several tens of milliseconds, and the user can check the sample image in real time. 
     After the completion of the optical axis adjustment, the main control unit  200  displays a visual field searching screen including a low-quality sample image (low-quality visual field searching image) on the input and output terminal  113  according to an instruction to start searching for a visual field from the user (S 104 ). A visual field search is an action of searching for a target observation visual field while performing focus adjustment and astigmatism adjustment in parallel. The visual field searching screen displays the sample image during visual field searching in real time. In the visual field searching, the main control unit  200  receives a movement of the visual field and a change of the imaging magnification from the user. 
     In the visual field searching, the main control unit  200  sets other components of the SEM  101  and the control device  102  according to the observation condition of the visual field search mode indicated by the observation condition  205 . The detector control unit  203  processes the detection signal from the detector  111  indicated by the observation condition  205  to generate a sample image (scanned image) of the observation region for visual field searching. The main control unit  200  displays the scanned image generated by the detector control unit  203  on the input and output terminal  113 . 
     The scanned image for visual field searching is generated at a high scan speed and is displayed at a high frame rate (high image update speed). The scan control unit  202  moves the primary electron beam  115  on the sample  108  at a scan speed in the visual field search mode. The scan speed for the visual field searching is higher than the scan speed for generating the sample image to be stored. The generation of one image is completed in, for example, several tens of milliseconds, and the user can check the sample image in real time. 
     Since the scanned image for visual field searching is generated at a high scan speed, the scanned image is a low-quality image, and the SNR thereof is lower than the SNR of the sample image in which the observation target region is stored. When the user determines that it is difficult to appropriately search for a visual field based on a low-quality image for visual field searching (S 105 : YES), the user instructs the control device  102  from the input and output terminal  113  to apply the high-quality image estimation processing to the image for visual field searching. 
     The main control unit  200  executes application of high-quality image estimation processing in response to an instruction from the user (S 106 ). The application of the high-quality image estimation processing will be described later in detail with reference to  FIG. 4 . When the high-quality image estimation processing is applied, the high-quality image estimation unit  208  of the arithmetic device  104  estimates (generates) a high-quality image based on the low-quality scanned image generated by the detector control unit  203  according to the designated high-quality image estimation model. The main control unit  200  displays the high-quality image generated by the high-quality image estimation unit  208  on the visual field searching screen. The generation of the high-quality image is completed in, for example, several tens of milliseconds, and the user can check the high-quality sample image in real time. 
     When the user determines that the appropriate visual field searching is possible based on the low-quality image for visual field searching (S 105 : NO), the main control unit  200  continues displaying the low-quality scanned image generated by the detector control unit  203  on the visual field searching screen. 
     The user executes visual field searching on the visual field searching screen (S 107 ). In the input and output terminal  113 , the user moves the visual field while referring to the sample image (low-quality scanned image or high-quality estimated image) for visual field searching, and changes the imaging magnification as necessary to search for the target observation visual field. The stage control unit  201  moves the visual field by moving the stage  109  in response to a user instruction to move a large visual field. The scan control unit  202  changes the scan region corresponding to the visual field in response to the user instruction to move a small visual field and change the imaging magnification. 
     When a target observation visual field is found, the user performs final focus adjustment and astigmatism adjustment as necessary, and then instructs the input and output terminal  113  to acquire a saved image of the target observation visual field. The main control unit  200  generates a saved image in response to a user instruction, and stores the saved image in the image database  204  of the storage device  103  (S 108 ). 
       FIG. 4  is a flowchart showing the details of the saved image acquisition step  3108 . The main control unit  200  determines whether automatic acquisition of training image data is designated (S 131 ).  FIG. 5  shows a training image data automatic acquisition setting screen  250 . The user previously sets whether training image data is automatically acquired in the input and output terminal  131 . The user sets ON or OFF of automatic acquisition of training image data on the training image data automatic acquisition setting screen  250 . The main control unit  200  holds information of the setting designated on the training image data automatic acquisition setting screen  250 . 
     When automatic acquisition of training image data is not designated (S 131 : NO), the main control unit  200  acquires a saved image (S 132 ). Specifically, the main control unit  200  sets other components of the SEM  101  and the control device  102  according to the observation condition of the checking mode indicated by the observation condition  205 . The observation condition of the checking mode is, for example, the same as the visual field search mode and/or the optical axis adjustment mode in elements other than a scan condition (scan region and scan speed). 
     The scanned image for saving is generated at a low scanning speed. The scan control unit  202  moves the primary electron beam  115  on the sample  108  at a scan speed in the checking mode. The scan speed for generating a saved image (target image) is lower than the scan speed for generating a sample image for optical axis adjustment and visual field searching. The generation of one image is completed in, for example, several tens of seconds. 
     The detector control unit  203  processes the detection signal from the detector  111  indicated by the observation condition  205  to generate a sample image (scanned image) in a designated visual field. The main control unit  200  displays the scanned image generated by the detector control unit  203  on the input and output terminal  113  to allow the user to check the scanned image. The main control unit  200  stores the acquired scanned image in the image database  204  of the storage device  103  in response to an instruction from the user. The main control unit  200  stores the accompanying information including the information on the sample and the observation condition in the image database  204  in association with the image. 
     When automatic acquisition of training image data is designated (S 131 : YES), the main control unit  200  acquires one or a plurality of low-quality images after acquisition (S 133 ) of the saved image (S 134 ). The main control unit  200  may acquire one or a plurality of low-quality images before and/or after acquisition of the saved image. 
     The main control unit  200  generates one or a plurality of low-quality images, and stores the low-quality images in the image database  204  in association with the saved images together with the accompanying information including the observation conditions. Specifically, the main control unit  200  generates a scanned image at a higher scan speed in the same visual field (scan region) as the saved image. The observation condition of the low-quality image is, for example, the same as the observation condition in the visual field search mode or the optical axis adjustment mode. When a plurality of low-quality images are acquired, the low-quality images may include low-quality images whose observation conditions are the same as those in the visual field search mode or the optical axis adjustment mode. As will be described later, a pair of a low-quality image and a high-quality image is used for training of a present or new estimation model (machine learning model). 
     In one example, training of the estimation model is executed outside observation time of the user (background training). Accordingly, it is possible to prevent the training of the estimation model from hindering the observation of the sample executed by the user. For example, the main control unit  200  does not train the estimation model while the user logs in a system for observation. 
     In another example, the main control unit  200  receives designation of the training time executed by the user in order to specify the observation time of the user.  FIG. 6  shows a screen  260  that receives designation of the training time executed by the user. The user inputs a start time and an end time of the background training, and checks the start time and the end time by clicking a setting button. 
     Returning to  FIG. 3 , when the user desires to acquire a saved image of another observation target region (S 109 : NO), the user returns to step S 107  and starts a visual field searching. When all observation images desired by the user are acquired and stored in the image database  204  together with the accompanying information of the observation images (S 109 : YES), the user executes an instruction to stop the irradiation with the primary electron beam  115  via the input and output terminal  113 , and the main control unit  200  stops the irradiation with the primary electron beam  115  in response to the instruction (S 110 ). Finally, the user takes out the sample  108  from the SEM  101  (S 111 ). 
     The high-quality image estimation processing application step S 106  will be described in detail with reference to the flowchart in  FIG. 7 . As described above, when the user determines that it is difficult to search for a visual field based on the low-quality scanned image, the main control unit  200  starts the present step S 106  in response to an instruction from the user. 
     First, the main control unit  200  displays an estimation model selection screen on the input and output terminal  113  (S 151 ). The estimation model selection screen allows the user to designate a model (parameter set) for estimating a high-quality image based on a low-quality scanned image in visual field searching. 
       FIGS. 8A, 8B and 8C  show a change in a display content of the estimation model selection screen  300 . The display content of the estimation model selection screen  300  changes from an image in  FIG. 8A  to an image in  FIG. 8B  and further changes from the image in  FIG. 8B  to an image in  FIG. 8C  in response to the user instruction. 
     As shown in  FIG. 8A , the estimation model selection screen  300  displays a current scanned image (low-quality image)  311  and an observation condition  301  of a current scanned image  311 . In the present example, the observation condition  301  indicates the acceleration voltage of the probe, the probe current, the scan speed, and a detector being used. The estimation model selection screen  300  includes a region  312  in which a high-quality image generated based on the current scanned image  311  according to the designated model is displayed. 
     The estimation model selection screen  300  further displays a candidate model table  320  indicating information on one or more candidate models selected from the model database  207 . The candidate model table  320  includes an ID field  321 , an acquisition date and time field  322 , an acceleration voltage field  323 , a probe current field  324 , a scan speed field  325 , a detector field  326 , a training image field  327 , a temporary application field  328 , and an application field  329 . 
     The ID field  321  indicates the ID of a candidate model. The acquisition date and time field  322  indicates a creation date and time of the candidate model. The acceleration voltage field  323 , the probe current field  324 , the scan speed field  325 , and the detector field  326  indicate observation conditions of an input image (low-quality image) in training image data of the candidate model. The training image field  327  indicates an input image (low-quality image) or a teacher image (high-quality image) in the training data of the candidate model. 
     The temporary application field  328  includes a button for selecting a candidate model to be temporarily applied, and further displays a high-quality image estimated according to the selected candidate model. The application field  329  includes a button for selecting a candidate model to be finally applied. The high-quality image estimated according to the candidate model selected in the application field  329  is displayed in the region  312 . 
     The estimation model selection screen  300  displays a training start button  352 , an end button  353 , and a property button  354 . The training start button  352  is a button for instructing acquisition of new training image data of the current sample  108  and generation of a new model according to the new training data. The end button  353  is a button for ending a selection of the estimation model and checking the estimation model to be applied. The property button  354  is a button for selecting an observation condition or a sample category that is not displayed and adding the selected observation condition or the sample category to a display image. 
     The main control unit  200  selects a candidate model from the model database  207  based on a sample to be observed and/or the observation condition. In the example described below, the main control unit  200  refers to the sample to be observed and the observation condition. The main control unit  200  selects a model of the sample and the observation condition similar to the current sample  108  and the observation condition as the candidate model. Accordingly, it is possible to select a model capable of more appropriately estimating a high-quality image. 
     For example, the main control unit  200  defines a vector representing the value of each item of the category of the sample and the observation condition, and determines a similarity degree based on the distance between the vectors. In another example, the main control unit  200  selects, as the candidate model, a model having a high similarity degree to the current sample  108  and the observation condition among models having the same or similar sample categories. The similarity degree is determined according to, for example, the number of items whose values match or approximate to each other in the observation condition. A range of approximation of the value of the similar category and an item of each category is defined in advance. 
     In the example shown in  FIG. 8A , the observation conditions referred to for determining the similarity degree are the acquisition date and time, the acceleration voltage, the probe current, the scan speed, and the detector. A part of these observation conditions may be omitted, and other observation conditions such as a contrast and brightness may be added. The main control unit  200  may present a predetermined number of models from a model having the highest similarity degree to a current sample and the observation condition, or may present a model having a similarity degree greater than a predetermined value. 
     In the example shown in  FIG. 8A , the candidate model table  320  highlights a cell of the item that matches or is closest to the current observation condition in a record of each candidate model. For example, in the observation condition of the candidate model of an ID “xxx”, the acceleration voltage, the probe current, and the scan speed match the current observation condition. 
     In the observation condition of the candidate model with an ID “yyy”, the detector matches the current observation condition. An acquisition date and time of an ID “zzz” is the latest (closest to the current date and time) in the candidate model, and a cell of the ID “zzz” is highlighted. According to such highlighting, the user can immediately specify a candidate model close to the current observation condition in an item of interest in the current observation condition. A manner of highlighting is any manner. The main control unit  200  may highlight a cell in a predetermined range from the value of each item of the current observation condition. The highlighting may be omitted. 
     When the user selects a check box of one or a plurality of candidate models in the temporary application field  328  and clicks a “start” button, the main control unit  200  displays the high-quality image estimated according to the corresponding candidate model in the cell of the selected check box in the temporary application field  328 . In the example in FIG. BA, the candidate model of the ID “xxx” and the candidate model of the ID “yyy” are selected. 
       FIG. 8B  shows a result of clicking the “start” button in the temporary application field  328  in  FIG. 8A . The estimated high-quality images of the candidate model of the ID “xxx” and the candidate model of the ID “yyy” are displayed in the temporary application field  328 . When the “start” button in the temporary application field  328  is clicked, the main control unit  200  acquires the parameter set of the selected candidate model from the model database  207 . The main control unit  200  sequentially transmits the acquired parameter sets to the arithmetic device  104 , and receives the estimated high-quality image. The main control unit  200  displays the high-quality image in the corresponding cell of the temporary application field  328 . 
     After receiving the parameter set, the high-quality image estimation unit  208  of the arithmetic device  104  acquires the scanned image generated by the detector control unit  203 , and generates a high-quality image based on the scanned image. The high-quality image estimation unit  208  repeats this processing for different parameter sets. 
     When the user selects any of the candidate models in the application field  329 , the main control unit  200  displays the high-quality image according to the selected candidate model in the region  312 . In the example in  FIG. 8B , the candidate model of the ID “xxx” is selected. 
       FIG. 8C  shows a result of selecting the candidate model of the ID “xxx” in the application field  329  in  FIG. 8B . The high-quality image  313  estimated according to the candidate model of the ID “xxx” is displayed side by side with the current scanned image  311 . When one candidate model is selected in the application field  329 , the main control unit  200  acquires a parameter set of the candidate model from the model database  207 . The main control unit  200  transmits the acquired parameter set to the arithmetic device  104 . 
     After receiving the parameter set, the high-quality image estimation unit  208  of the arithmetic device  104  sequentially acquires the scanned image generated by the detector control unit  203 , sequentially generates high-quality images based on the scanned images, and transmits the high-quality images to the control device  102 . The main control unit  200  updates a display image of the region  312  according to the sequentially received high-quality images. 
       FIG. 9  shows an example of another display method of the high-quality image according to the estimation model selected in the application field  329 . The estimation model selection screen  300  shown in  FIG. 9  displays the high-quality image  315  of a partial region in the visual field in a manner of being superimposed on the low-quality scanned image  311 . Accordingly, it is easier for the user to compare the low-quality image and the high-quality image. When the user clicks a “superimpose” button  355 , the main control unit  200  extracts a predetermined region of the estimated high-quality image  313  and superimposes the predetermined region on a corresponding region of the low-quality scanned image  311 . The “superimpose” button  355  may be omitted, and the high-quality image  315  may be always superimposed on the low-quality scanned image  311 . The estimated high-quality image  313  may be omitted. 
     As described above, the high-quality image estimated according to one or more candidate models (parameter sets) stored in the model database  207  is displayed. Accordingly, the user can designate an appropriate high-quality image estimation model in a short time. By presenting the candidate model from the trained model, it is possible to reduce learning time of the machine learning model. 
     When the user determines that an appropriate high-quality image cannot be estimated according to any of the presented candidate models, the user clicks the training start button  352 . In response to the click of the training start button  352 , the main control unit  200  acquires training image data of the current sample  108 , and generates an estimation model suitable for observation of the current sample  108  based on the training data. 
     The main control unit  200  acquires, in each of the different visual fields, a high-quality scanned image captured at a low scanning speed or obtained by frame integration at a high scanning speed and a low-quality scanned image at a high scanning speed. The main control unit  200  moves the visual field by the scan control unit  202  and the stage control unit  201 , and controls the scan speed by the scan control unit  202 . The detector control unit  203  generates a low-quality scanned image and a high-quality scanned image in each visual field. The low-quality scanned image and the high-quality scanned image are included in training data for generating a new estimation model. 
     The training data may include a plurality of representative image pairs of the same category as the current sample  108 . The image pair includes a low-quality scanned image and a high-quality scanned image, and an observation condition of the image pair coincides with a current observation condition or is within a predetermined similarity range. Accordingly, versatility of the estimation model can be improved. 
     The main control unit  200  trains an estimation model having an initial parameter set or a trained parameter set based on the training data. In one example, the main control unit  200  receives selection executed by the user as to whether to use the initial parameter set or the trained parameter set. The main control unit  200  receives selection of the trained parameter set to be re-trained executed by the user. The trained parameter set is selected from, for example, candidate models. The main control unit  200  may select a candidate model (parameter set) having the highest similarity degree to the current sample and the observation condition as a model for re-training. 
     The main control unit  200  transmits a training request including a parameter set to be trained and training data to the arithmetic device  104 . The model training unit  209  of the arithmetic device  104  updates the parameter set using the training data and generates a new estimation model. The model training unit  209  calculates an error between the high-quality scanned image which is the teacher image in the training data and the estimated high-quality image, and updates the parameter set by back propagation so as to reduce the error. The model training unit  209  repeats updating of the parameter set for each of the plurality of image pairs included in the training data. 
     When the training of the estimation model ends, the main control unit  200  acquires a new model (parameter set) from the model training unit  209 , and displays the estimated high-quality image generated by the high-quality image estimation unit  208  using the parameter set in the region  312  or in the visual field searching screen. The main control unit  200  stores the new estimation model in the model database  207  together with the accompanying information, and further stores the training data in the image database  204  together with the accompanying information. 
     As described above, in a case in which an appropriate high-quality image cannot be estimated according to a present estimation model, a new estimation model trained according to the image of the current sample is generated, so that it is possible to more appropriately estimate a high-quality image based on the low-quality image of the current sample. 
     In the observation method described with reference to the flowchart shown in  FIG. 3 , a high-quality image is estimated based on a low-quality scanned image when necessary in the visual field searching after the optical axis adjustment. In another example, the control system  120  may estimate the high-quality image based on the low-quality scanned image in the optical axis adjustment. Accordingly, it is possible to more appropriately adjust the optical axis. 
       FIG. 10  is a flowchart of an observation method for estimating the high-quality image based on the low-quality scanned image in optical axis adjustment and visual field searching when necessary. 
     Steps S 201  and S 202  are similar as steps S 101  and S 102  in  FIG. 3 . In step S 203 , in response to the instruction from the user to start the optical axis adjustment, the main control unit  200  causes the input and output terminal  113  to display an optical axis adjustment screen including a low-quality sample image (optical axis adjustment image). 
     As described with reference to  FIG. 3 , an optical axis adjustment image is a low-quality scanned image. When the user determines that it is difficult to appropriately adjust the optical axis based on the low-quality image for optical axis adjustment (S 204 : YES), the user instructs the control device  102  via the input and output terminal  113  to apply the high-quality image estimation processing to the image for optical axis adjustment. 
     The main control unit  200  executes application of the high-quality image estimation processing in response to the instruction from the user (S 205 ). The high-quality image estimation processing application S 205  is substantially the same as the high-quality image estimation processing application S 105  in  FIG. 3 . The displayed low-quality scanned image is an optical axis adjustment image. When the user determines that appropriate optical axis adjustment is possible based on the low-quality optical axis adjustment image (S 204 : NO), the high-quality image estimation processing application S 205  is omitted. 
     When the high-quality image estimation processing application S 205  is executed, a high-quality image is generated based on the low-quality scanned image and is displayed in the optical axis adjustment S 206  and the visual field searching S 207 . Other points of the optical axis adjustment S 206  are similar as those in the step S 103  in  FIG. 3 . Steps S 207  to S 211  are similar as the steps S 107  to S 111  in  FIG. 3 . 
     The above example is applied to the visual field searching when high-quality image estimation is applied to the optical axis adjustment. In another example, in each of the optical axis adjustment and the visual field searching, the control system  120  may receive, from the user, a designation of whether the high-quality image estimation is applied. When the observation conditions of the scanned image for optical axis adjustment and the scanned image for visual field searching are different from each other, a user designation of the estimation model to be applied may be received in each of the observation conditions (for example, steps S 205  and S 106 ). 
     The invention is not limited to the above embodiments, and includes various modifications. For example, the embodiments described above are described in detail for easy understanding of the invention, and the invention is not necessarily limited to those including all the configurations described above. A part of the configuration according to one embodiment can be replaced with the configuration according to another embodiment, and the configuration according to one embodiment can be added to the configuration according to another embodiment. A part of the configuration according to the embodiment may be added, deleted, or replaced with another configuration. 
     Each of the configurations, functions, processing units, or the like described above may be partially or entirely implemented by hardware by being designed using an integrated circuit. The above configurations, functions, and the like may also be implemented by software by a processor interpreting and executing a program for implementing the functions. Information of programs, tables, files or the like for implementing the functions can be stored in a recording device such as a memory, a hard disk, and a solid state drive (SSD), or a recording medium such as an IC card and an SD card. 
     Control lines and information lines are those that are considered necessary for the description, and not all control lines and the information lines on the product are necessarily shown. In practice, it may be considered that almost all configurations are connected to one another.