Patent Publication Number: US-2023161053-A1

Title: Radiation Analysis System, Charged Particle Beam System, and Radiation Analysis Method

Description:
TECHNICAL FIELD 
     The present invention relates to a radiation analysis system, a charged particle beam system, and a radiation analysis method. 
     BACKGROUND ART 
     There is an energy dispersive X-ray detector (Energy Dispersive Spectroscopy, hereinbelow referred to as “EDS”) and a wavelength dispersive X-ray detector (Wavelength Dispersive Spectroscopy, hereinbelow referred to as “WDS”) as radiation analyzers capable of discriminating energy of a radiation. The EDS converts energy of an X-ray taken into a detector to an electric signal in the detector, and calculates energy based on the magnitude of the electrical signal. The WDS monochromatizes the X-ray using a spectrometer and detects the monochromatic X-ray using a proportional counter or the like. 
     Widely used as the EDS are semiconductor detectors such as a silicon lithium detector, a silicon drift detector, and a germanium detector. For example, the silicon lithium or silicon drift detector is used for an elemental analyzer in an electron microscope, which can detect energy in a range of about 0.1 keV to 20 keV. However, because its performance depends on a bandgap of the silicon used in the detector (about 1.1 eV), it is difficult to improve an energy resolution to about 120 eV or lower, the energy resolution being more than ten times inferior to that of the WDS. 
     That the energy resolution being one of indices indicative of the performance of the X-ray detector is, for example, 120 eV means that the energy can be detected with an uncertainty of 120 eV when the X-ray detector is irradiated with an X-ray. The smaller the uncertainty is, the higher the energy resolution is. In a case of detecting an X-ray consisting of two adjacent spectra with about 20 eV difference, it is possible to separate two peaks as long as the energy resolution is about 20 eV to 30 eV. 
     As an alternative to the semiconductor detector used for the EDS, a superconductive X-ray analyzer of the energy dispersive type and also having the energy resolution equivalent to that of the WDS has been gathering attention. Among superconductive X-ray analyzers, a detector that includes a superconductive transition edge sensor (Transition Edge Sensor, hereinbelow referred to as “TES”) is a highly sensitive calorimeter using a sudden change in resistance value between superconductivity and normal conductivity of a thin metal film (e.g., a change in resistance value due to a temperature change of a few mK is 100 mΩ). The TES is also referred to as a microcalorimeter. 
     The TES analyzes a sample by detecting the temperature change of the TES which occurs when a fluorescent X-ray or a characteristic X-ray generated by exposure of a radiation such as a primary X-ray or a primary electron beam enters the TES. The TES has an energy resolution higher than that of the semiconductor detector, for example, the energy resolution of 10 eV or lower with respect to an X-ray of 5.9 keV. 
     Although it is significant to maintain the baseline current flowing through the TES constant in order to achieve the high energy resolution of the TES, it is technically difficult to make variation of the baseline current zero, and therefore there are proposed various sensitivity correction methods. 
     For example, Patent Literature 1 discloses an X-ray analyzer including “a sensitivity correction operation unit  7  that corrects a current detected by a current detection mechanism  4  or a peak value measured by a pulse height analyzer  5  depending on a variation width in a case in which the baseline current varies out of a default value” (see abstract in Patent Literature 1). 
     Patent Literature 2 discloses a radiation analyzer including “a sensitivity correction operation unit  7  that corrects sensitivity of a TES  1  from the relation between an output from a first heater  20  acquired in advance and a peak value measured by a pulse height analyzer  5 ” (see abstract in Patent Literature 2). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-271016 
         Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2014-038074 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, with the X-ray analyzer according to Patent Literature 1, it is required to constantly monitor the baseline current and acquire additional correction data every time the baseline current flowing through the TES varies. 
     The radiation analyzer according to Patent Literature 2 acquires correlation characteristics between the output of the heater and the sensitivity of the TES in advance and then corrects the peak value of the signal pulse of the TES using the sensitivity of the TES corresponding to the output of the heater when acquiring the signal pulse of the TES in an actual measurement. However, since the output of the heater has slow responsiveness, it is not possible to acquire a high energy resolution when the baseline current varies fast. 
     Therefore, the present disclosure provides a radiation analysis system that does not need to acquire additional correction data and that can acquire a stably high energy resolution. 
     Solution to Problem 
     A radiation analysis system according to the present disclosure includes: a superconductive transition edge sensor that detects a radiation; a current detection mechanism that detects a current flowing through the superconductive transition edge sensor; and a computer subsystem that processes a detection signal of the current from the current detection mechanism, wherein the computer subsystem performs a process of calculating a baseline current of the detection signal of the current, a process of calculating a peak value of a signal pulse generated in the detection signal when the superconductive transition edge sensor detects the radiation, a process of acquiring correlation data based on the baseline current and the peak value, and a process of correcting the peak value of the signal pulse or an energy value calculated from the peak value on the basis of the baseline current before the signal pulse is generated when a radiation having unknown energy is detected by the superconductive transition edge sensor and the correlation data. 
     Further features related to the present disclosure will be apparent from the description of the present specification and appended drawings. Moreover, aspects of the present disclosure will be achieved and realized by elements and combinations of various elements as well as the following detailed descriptions and modes of appended claims. 
     The description in the specification merely presents typical examples, and does not limit the claims or application examples of the present disclosure in any way. 
     Advantageous Effects of Invention 
     According to the radiation analysis system in the present disclosure, it is possible to acquire a stably high energy resolution without having to acquire additional correction data. 
     Other problems, configurations, and effects will become apparent from the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows a configuration of a scanning electron microscope system according to a first embodiment; 
         FIG.  2    is a schematic diagram showing a configuration of a radiation analyzer; 
         FIG.  3    is a schematic diagram showing a configuration of a TES; 
         FIG.  4    is a schematic diagram showing a configuration of a part of the radiation analyzer; 
         FIG.  5    is a flowchart showing a measurement preparation method by the radiation analyzer; 
         FIG.  6    is a graph showing a change in the baseline current from the start of measurement preparation until the end of the measurement preparation; 
         FIG.  7    is a schematic diagram showing a correlation data acquisition screen for the measurement preparation; 
         FIG.  8    is a flowchart showing a method of correcting a peak value by a correction unit; 
         FIG.  9    is a graph showing a change in a current acquired by the correction unit; 
         FIG.  10    shows correlation data between the baseline current and the peak value of the radiation analyzer; 
         FIG.  11    shows the correlation data when energy of a radiation entering the TES of the radiation analyzer is proportional to the peak value; 
         FIG.  12    shows the correlation data when the energy of the radiation entering the TES of the radiation analyzer is not proportional to the peak value; and 
         FIG.  13    shows a correlation data acquisition screen for acquiring correlation data according to a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of the present disclosure are described with reference to drawings. 
     Although an example of using a scanning electron microscope as an electron microscope (charged particle beam irradiation subsystem) in a charged particle beam system of the embodiments will be described, it is merely an example of a technology disclosed herein and the technology of the present disclosure is not limited to the embodiments described below. In the present disclosure, the electron microscope should include a wide variety of devices that takes an image of a sample using an electron beam. For example, the technology of the present disclosure can also be applied to a scanning electron microscope, a transmission electron microscope, and a sample processing device or a sample analyzer equipped with the scanning electron microscope. 
     Moreover, examples of an X-ray analyzer using the electron beam include an inspection device, a review device, a pattern measurement device, and the like using the scanning electron microscope. The X-ray analyzer using the electron microscope should include a system in which respective devices including the above-described electron microscope are connected via a network and an apparatus combining the respective devices described above. 
     As used herein, a “sample” includes a wide variety of objects to be observed and analyzed. For example, the “sample” includes a semiconductor wafer formed of silicon or the like, a high-performance material such as those used for a lithium battery, a biological sample, and the like. 
     First Embodiment 
     &lt;Configuration of Scanning Electron Microscope System&gt; 
       FIG.  1    shows a configuration of a scanning electron microscope system  100  (charged particle beam system) according to a first embodiment. As shown in  FIG.  1   , the scanning electron microscope system  100  includes a radiation analysis subsystem  200  (radiation analysis system), a scanning electron microscope  300  (charged particle beam irradiation subsystem), a high voltage power supply  400 , and a computer subsystem  500 . 
     The scanning electron microscope  300  includes an electron source  301 , a condenser lens  303 , a deflection scanning coil  304 , an objective lens  305 , a sample stage  307 , a backscattered electron detector  308 , and a secondary electron detector  309 . 
     The computer subsystem  500  is a computer system that controls operations of the scanning electron microscope  300  and the high voltage power supply  400 , and includes a total control unit  501 , an electron optical system control unit  502 , a stage control unit  503 , an A/D conversion unit  504 , an image operation unit  505 , a storage device  506 , a display unit  507 , and an input device  508 . 
     A sample  306  to be observed is placed on the sample stage  307 . The sample stage  307  moves in an X-Y direction on the basis of an instruction signal from the stage control unit  503 . Connected to the electron source  301  is the high voltage power supply  400 , and a voltage is applied from the high voltage power supply  400  to the electron source  301  on the basis of the instruction signal from the total control unit  501 . 
     An electron beams  302  (charged particle beam) released from the electron source  301  is converged by the condenser lens  303  and the objective lens  305  on the basis of the instruction signal from the electron optical system control unit  502 , and scanned over the sample  306  by the deflection scanning coil  304 . 
     By the electron beam  302  falling onto the sample  306 , backscattered electrons and secondary electrons are generated from the sample  306 . The backscattered electrons reaching the backscattered electron detector  308  and the secondary electrons reaching the secondary electron detector  309  are converted to a current, output to the A/D conversion unit  504 , and converted to a digital signal. The image operation unit  505  performs generation of an SEM image and image processing using the digital signal generated by the A/D conversion unit  504 . 
     Connected to the total control unit  501  are the storage device  506 , the display unit  507 , and the input device  508 , and the SEM image generated by the image operation unit  505  is stored in the storage device  506  and displayed on the display unit  507  via the total control unit  501 . 
     The display unit  507  also displays a GUI screen for a user of the scanning electron microscope system  100  to input an instruction. The user sends the instruction to the total control unit  501  via the GUI screen displayed on the display unit  507  by operating the input device  508 . The total control unit  501  controls each unit in such a manner as changing an imaging condition or imaging position of the SEM image, changing a detection condition of the radiation analysis subsystem  200 , and the like by sending an instruction to the high voltage power supply  400 , the electron optical system control unit  502 , the stage control unit  503 , the image operation unit  505 , or the radiation analysis subsystem  200  on the basis of the user input. 
     The radiation analysis subsystem  200  is a device that can be used as a composition analyzer such as, for example, an electron microscope, an ion microscope, an X-ray microscope, a fluorescent X-ray microscope, and the like. The radiation analysis subsystem  200  detects a characteristic X-ray (radiation) emitted from the sample by irradiation with the electron beam  302 , and calculates its energy. Since the characteristic X-ray has energy specific to the element, the radiation analysis subsystem  200  can analyze what element is present at the irradiated position of the electron beam  302  on the sample  306  by generating a spectrum in which the horizontal axis indicates energy and the vertical axis indicates a count of X-rays. 
     The radiation analysis subsystem  200  is configured to be able to transmit and receive (communicate) the data and the instruction signal to and from the total control unit  501  in the computer subsystem  500 , and the result of the elemental analysis by the radiation analysis subsystem  200  is output to the storage device  506  and the display unit  507  via the total control unit  501 . The total control unit  501  can also transmit information of the detected element to a data management server (not shown) via the network. 
       FIG.  2    is a schematic diagram showing a configuration of the radiation analysis subsystem  200 . As shown in  FIG.  2   , the radiation analysis subsystem  200  includes a TES  201 , a sensor circuit unit  202 , a bias current source  203 , a current detection mechanism  206 , a first thermometer  209 , a heater  210 , a refrigerator  211 , and a computer subsystem  250 . 
     Upon receiving a radiation, the TES  201  detects its energy as a temperature change, and outputs the temperature change as a current signal. Detailed configuration of the TES  201  will be described later. The bias current source  203  applies to the sensor circuit unit  202  a current for artificially driving the sensor circuit unit  202  at a constant voltage. The sensor circuit unit  202  is connected to the TES  201 , and includes a shunt resistor  204  and an input coil  205 . 
     The shunt resistor  204  is connected in parallel with the TES  201  and presents a resistance value smaller than that of the TES  201 . The input coil  205  is connected to the TES  201  in series. When a bias current is applied from the bias current source  203  to the sensor circuit unit  202 , the current is bifurcated at a resistance ratio between the resistance value of the shunt resistor  204  and the resistance value of the TES  201 . That is, a voltage value of the TES  201  is determined by the voltage value determined by the current flowing through the shunt resistor  204  and the resistance value of the shunt resistor  204 . 
     The current detection mechanism  206  includes a SQUID amplifier  207  (SQUID: Superconducting Quantum Interference Device, superconducting quantum interference device) and a room temperature amplifier  208 . The SQUID amplifier  207  detects a magnetic field generated by the input coil  205 , generates an electric signal, and outputs the electric signal to the room temperature amplifier  208 . The room temperature amplifier  208  acquires a signal pulse of the current flowing through the TES  201  by amplifying and forming the electric signal output from the SQUID amplifier  207 . In this manner, the current detection mechanism  206  can detect an extremely minor change in the current flowing through the TES  201  by using the SQUID amplifier  207 . It is to be noted that, although the SQUID amplifier  207  using the input coil  205  and the room temperature amplifier  208  are used as the current detection mechanism  206 , other configurations may be employed as long as the change in the current flowing through the TES  201  can be detected. 
     In  FIG.  2   , the refrigerator  211  is schematically represented by a region surrounded by a one dot chain line, and it is shown that the TES  201 , the sensor circuit unit  202 , the SQUID amplifier  207 , the first thermometer  209 , and the heater  210  are installed inside the refrigerator  211 . Moreover, a cold head (not shown in  FIG.  2   ) is installed inside the refrigerator  211 , and the first thermometer  209  and the heater  210  are installed in the cold head. 
     The computer subsystem  250  includes a pulse height analyzer  251 , a correction unit  252 , a spectrum generation unit  253 , and a temperature control unit  254 . Although not shown in the figures, the computer subsystem  250  includes a display unit such as a display, a processor (operation circuit) such as a central processing unit (CPU), and a storage device such as a memory for storing a program and the like. Functions of respective modules of the computer subsystem  250  (pulse height analyzer  251 , the correction unit  252 , the spectrum generation unit  253 , and the temperature control unit  254 ) can be realized by, for example, program processing by the processor. It is to be noted that each module of the computer subsystem  250  may be incorporated into the above-described total control unit  501 , or each module may be incorporated into a different personal computer. 
     The pulse height analyzer  251  receives an input of a detection signal of the current detected by the current detection mechanism  206  and calculates the peak value of the signal pulse of the current flowing through the TES  201 . It is to be noted that the “peak value” as used herein includes a wide variety of the signal pulses calculated to improve precision of the analysis. For example, the “peak value” includes a height component of the signal pulse, an integrated value of the signal pulse, the signal pulse convoluted with a filter such as a band filter, and the like. 
     The correction unit  252  corrects the peak value calculated by the pulse height analyzer  251 . Details of correcting the peak value by the correction unit  252  according to the present embodiment will be described later. Moreover, the correction unit  252  receives an input of a detection signal of the current detected by the current detection mechanism  206  and calculates an average value of the baseline current. 
     The spectrum generation unit  253  generates an energy spectrum of the radiation detected by the TES  201  using the peak value corrected by the correction unit  252 . The spectrum generation unit  253  generates the spectrum by repeating an operation of incrementing the count of the peak value by one in a histogram in which the horizontal axis indicates the peak value and the vertical axis indicates the count. Moreover, when data for converting the peak value to the energy is taken into the pulse height analyzer  251 , the correction unit  252 , or the spectrum generation unit  253  in advance, a histogram in which the horizontal axis indicates the energy and the vertical axis indicates the count can be displayed on the display unit. 
     The temperature control unit  254  controls the output of the heater  210 . 
       FIG.  3    is a schematic diagram showing a configuration of the TES  201 . As shown in  FIG.  3   , the TES  201  includes an absorber  212 , a second thermometer  213 , and a membrane  214 . The absorber  212  is a metal, a semimetal, a superconductor, or the like for absorbing a radiation such as an X-ray, which is formed of, for example, gold, copper, or bismuth. The second thermometer  213  is formed of the superconductor, and detects heat generated in the absorber  212  as the temperature change. The second thermometer  213  is formed of a stack including two layers of titanium and gold, for example. The membrane  214  is formed of silicon nitride, for example. The membrane  214  loosely thermally connects between the second thermometer  213  and the cold head  215 , and controls the flow of the heat flowing to the cold head  215 . 
       FIG.  4    is a schematic diagram showing a configuration of a part of the radiation analysis subsystem  200 . As shown in  FIG.  4   , the TES  201 , the shunt resistor  204  (not shown in  FIG.  4   ), and the SQUID amplifier  207  are provided at a tip of the cold head  215 . A substrate having the TES  201  and a substrate having the SQUID amplifier  207  are connected with a superconductive wiring  216 . The cold head  215  is surrounded by a heat shield  217 . 
     Provided inside the cold head  215  are the first thermometer  209  that monitors temperature of the cold head  215  and the heater  210 . A resistive thermometer can be used as the first thermometer  209 , and the material of the sensor may be, for example, a semiconductor such as germanium, a superconductor, or a metal oxide such as ruthenium oxide. The first thermometer  209  has a resistance value that changes in accordance with the temperature of the cold head  215 . Accurate temperature information of the cold head  215  can be acquired by correlating the temperature and the electric signal output from the first thermometer  209  and storing them in the temperature control unit  254  in advance. 
     The cold head  215  is cooled by the refrigerator  211  to 50 mK-400 mK. Specifically, the TES  201  needs to be cooled to a temperature lower than the temperature at which the super conductive transition occurs. Examples of the means of cooling the refrigerator  211  include a dilution refrigerator and an insulated demagnetized refrigerator (Adiabatic Demagnetization Refrigerator, hereinbelow referred to as “ADR”). The former employs a technology of cooling using enthalpy change which occurs when 3He melts from a dense phase into a dilute phase in a mixing chamber. The latter employs a technology of cooling an object connected to a magnetic body using entropy change which occurs when aligning a spinning direction by applying a magnetic field to the magnetic body and removing the magnetic field. In both cases, the cold head  215  is installed at a position which is cooled the most. 
     In the case of stabilizing the temperature of the cold head  215  with the dilution refrigerator, once a target temperature is set to the temperature control unit  254 , the temperature control unit  254  controls the output of the heater  210  on the basis of the temperature of the first thermometer  209 . It is to be noted that, in the case of the ADR, the temperature of the cold head  215  is maintained constant by controlling the strength of the magnetic field applied to the magnetic body on the basis of the temperature of the first thermometer  209 . 
     &lt;Operation Principle of TES&gt; 
     The TES  201  uses superconductive transition in the superconductor, and retains an operating point in an intermediate state between normal conductivity and superconductivity during an operation of detecting the radiation. When a single radiation is absorbed by the TES  201 , in a state in which the operating point is retained during the superconductive transition, this allows for acquiring, for example, a change of a few mQ in the resistance value with respect to 100 μK temperature variation, and acquiring a signal pulse on the order of ρA. Moreover, by calculating a relation between the energy of the radiation and the peak value of the signal pulse in advance, when a radiation having unknown energy enters the TES  201 , it is possible to detect the energy of the incident radiation from the peak value of the signal pulse. 
     When having the TES  201  retained at the operating point during the superconductive transition, the operating point of the TES  201  is determined by a thermal balance between the current flowing through the TES  201  (hereinbelow, referred to as “TES current It”) and a thermal link to the cold head  215 . An energy resolution of the TES  201  is correlated with the temperature, and the temperature is preferably as low as possible. The temperature of the cold head  215  is left at, for example, about 50 mK-400 mK. The TES current It is determined by the following formula (1). 
       [Formula 1] 
         It   2   Rt ( T )= G ( T−Tb )  (1)
 
     In the formula (1), the TES current It is described with a dynamic resistance Rt of the TES  201 , a thermal conductivity G of the thermal link for thermally connecting the second thermometer  213  and the cold head  215  provided in the TES  201 , a temperature T of the second thermometer  213 , and a temperature Tb of the cold head  215 . Here, the baseline current means the TES current It in a state where the TES  201  is not irradiated with the radiation. 
     Furthermore, a relation between the TES current It and the signal pulse ΔI is given by the following formula (2). Ideally, when the TES current It is constant, the always constant signal pulse ΔI is acquired. 
     
       
         
           
             
               
                 
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     In the formula (2), the TES current It and the signal pulse ΔI are described with a sensitivity a of the TES  201 , a heat capacity C, an energy E of the radiation to be irradiated, and the temperature T of the second thermometer  213 . As can be seen from the formula (2), when the baseline current flowing through the TES  201  changes, the signal pulse ΔI varies even if the TES  201  is irradiated with the radiation having the same energy. Moreover, as can be seen from the formula (1), the baseline current changes as the temperature of the cold head  215  changes. That is, when the temperature of the cold head  215  varies, the signal pulse ΔI varies. 
     The signal pulse ΔI when the TES  201  is irradiated with the radiation changes in the increasing direction with the increase of the current flowing through the SQUID amplifier  207  (equal to the TES current It) according to the above-described formula (2). The signal pulse ΔI is convoluted with the filter by the pulse height analyzer  251 , and the peak value which is calculated therefrom is corrected by the correction unit  252  and transmitted to the spectrum generation unit  253 . At this time, the spectrum is generated by the spectrum generation unit  253  in which the horizontal axis indicates the peak value and the vertical axis indicates the count. For example, when the peak value is 100, one is counted at a position of 100. 
     That the signal pulse varies despite irradiation with the radiation having the same energy means that the peak value fluctuates. The degree of this fluctuation is equivalent to the energy resolution described above. That is, in order to achieve a high energy resolution, it is required to reduce the fluctuation of the peak value with respect to the radiation having the same energy. 
     When the TES  201  is retained in the intermediate state between the normal conductivity and the superconductivity, Joule heat generated in the second thermometer  213  is thermally balanced with a heat flow flowing to the cold head  215  through the membrane  214 . The thermal balance between the Joule heat and the heat flow is given by the above-described formula (1). Here, taking into account that the TES current It is affected by heat Pex from the outside of the TES  201 , the above-described formula (1) is rewritten as the following formula (3). 
     
       
         
           
             
               
                 
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     As the heat Pex from the outside of the TES  201  increases, δIt in the second term on the left side is reduced so as to satisfy the formula (3). That is, the baseline current varies as the heat Pex from the outside of the TES  201  varies, and the signal pulse ΔI varies as the baseline current varies. Since the peak value varies as its signal pulse ΔI varies, the energy resolution is degraded. Example variations of the heat Pex from the outside of the TES  201  include a temperature variation of the cold head  215  that cools the TES  201 , a variation of heat radiation due to the temperature variation in the heat shield  217  surrounding the cold head  215 , a variation of heat conduction from the heat shield  217  to the TES  201  due to residual gas present in the refrigerator  211 , or the like. 
     &lt;Operation of Radiation Analysis Subsystem&gt; 
     Accordingly, the present embodiment employs the method of correcting the peak value as described below. In summary, the correction unit  252  acquires the baseline current and the peak value in a case of irradiation with radiation having predetermined energy (first energy) under at least two different temperature conditions, and stores them as the correlation data in advance (before starting the analysis). When analyzing the actual radiation (when the scanning electron microscope system  100  is in operation), the correction unit  252  measures the baseline current regularly flowing through the TES  201  immediately before the TES  201  detects the signal pulse. It is then possible to acquire an accurate peak value by correcting the peak value of the signal pulse using the correlation data between the baseline current and the peak value acquired in advance. It is to be noted that the correlation data only needs to be based on the baseline current and the peak value, and the baseline current and the energy value calculated from the peak value may be taken as the correlation data. 
     The baseline current has a statistical fluctuation because it is a current that regularly flows through the TES  201 . Therefore, it is possible to average about 100 points of sampling data, for example, and to use the averaged value. As an example, it is possible to monitor a current value that is the output of the room temperature amplifier  208  using a 1 MS/sec analog-to-digital-converter (ADC), acquire multiple data at a 1 μsec sampling interval, and average them. 
     In the radiation analysis subsystem  200 , by suppressing the variation of the baseline current during operation of the scanning electron microscope system  100  (during sample analysis) within ±2.0 μA, it is possible to make the variation of the measured radiation energy no higher than 1 eV, which is used as a bin width of the energy spectrum, and to constantly acquire a high energy resolution. 
     (Measurement Preparation: Acquisition of Correlation Data) 
       FIG.  5    is a flowchart showing a measurement preparation method by the radiation analysis subsystem  200 . The aforementioned correlation data is acquired in this measurement preparation. Although the operation shown in  FIG.  5    is in fact performed by the computer subsystem  250  executing a program to implement the function of each module, the description may be made below taking each module of the computer subsystem  250  as a subject of each operation. 
     During the measurement preparation, the TES  201  needs to be irradiated with the radiation having the same energy (energy E0 (first energy)). The energy of the radiation entering the TES  201  is determined by an element contained in the sample  306  placed on the sample stage  307  of the scanning electron microscope  300  shown in  FIG.  1   . Thus, the measurement preparation is performed using the sample  306  that allows for acquiring a radiation having the desired energy E0. Placed on the sample stage  307  may be, for example, a sample for the measurement preparation and a sample to be observed by the scanning electron microscope  300 . The element to be irradiated with the electron beam  302  can be changed by moving the sample stage  307  in the X-Y direction using a sample containing multiple types of elements for the measurement preparation. Alternatively, the element to be irradiated with the electron beam  302  may be changed by scanning, with the electron beam  302 , the sample for the measurement preparation containing multiple types of elements using the deflection scanning coil  304 . As an example, in a case in which silicon is irradiated with the electron beam  302 , a radiation having 1740 eV energy is generated. 
     First, the computer subsystem  250  confirms that the refrigerator  211  is sufficiently cooled on the basis of the temperature acquired by the first thermometer  209 . The temperature control unit  254  then sets a reference set temperature to T0, and adjusts the output of the heater  210  on the basis of the temperature acquired by the first thermometer  209 . After the temperature of the cold head  215  reaches TO, the output of the first thermometer  209  varies around the temperature T0. 
     At Step S 11 , the temperature control unit  254  confirms that the variation in temperature acquired by the first thermometer  209  is reduced to be smaller than ±0.1 mK, and the correction unit  252  confirms that the variation of the baseline current is reduced to be smaller than ±0.2 μA. 
     At Step S 12 , upon detecting the signal pulse due to irradiation with the radiation having the energy E0, the pulse height analyzer  251  calculates its peak value, and outputs the peak value to the correction unit  252  as a peak value PH0. The correction unit  252  stores the baseline current BL0 and the peak value PH0 at this time as the correlation data. 
     At Step S 13 , the temperature control unit  254  adjusts the output of the heater  210  by changing the set temperature to T+, and increases the temperature so that the baseline current flows about 2.0 μA more than BL0. The temperature control unit  254  confirms that the variation in temperature has become smaller than ±0.1 mK and the correction unit  252  confirms that the variation of the baseline current has become smaller than ±0.2 μA. 
     At Step S 14 , upon detecting the signal pulse due to irradiation with the radiation having the energy E0, the pulse height analyzer  251  calculates its peak value, and outputs the peak value to the correction unit  252  as a peak value PH+. The correction unit  252  stores the baseline current BL+ and the peak value PH+ at this time as the correlation data. 
     At Step S 15 , the temperature control unit  254  changes the set temperature to T−, adjusts the output of the heater  210 , and reduces the temperature so that the baseline current flows about 2.0 μA less than BL0. The temperature control unit  254  confirms that the variation in temperature has become smaller than ±0.1 mK and the correction unit  252  confirms that the variation of the baseline current has become smaller than ±0.2 μA. 
     At Step S 16 , upon detecting the signal pulse due to irradiation with the radiation having the energy E0, the pulse height analyzer  251  calculates its peak value, and outputs the peak value to the correction unit  252  as a peak value PH−. The correction unit  252  stores the baseline current BL− and the peak value PH− at this time as the correlation data. 
     At Step S 17 , the temperature control unit  254  adjusts the output of the heater  210  so that the temperature is at a reference value TO. The temperature control unit  254  confirms that the variation in temperature has become smaller than ±0.1 mK. In this manner, the measurement preparation is completed, and analysis with a constantly high energy resolution is made possible. 
       FIG.  6    is a graph showing a change in the baseline current from the start of the measurement preparation until the end of the measurement preparation. In  FIG.  6   , the horizontal axis indicates time and the vertical axis indicates the baseline current. In the example shown in  FIG.  6   , the baseline current BL0 at the set temperature T0 is 10 μA, the baseline current BL+ at the set temperature T+ is 12 μA, and the baseline current BL− at the set temperature T− is 8 μA. The graph shown in  FIG.  6    can be displayed on the display unit or the computer subsystem  250  or on the display unit  507  of the computer subsystem  500 . 
     Moreover, the display unit of the computer subsystem  250  can display a GUI screen for the user to input the energy E0 of the radiation, the set temperatures T0, T+, and T− of the temperature control unit  254  set in the aforementioned measurement preparation and for displaying the acquired correlation data. 
       FIG.  7    is a schematic diagram showing a correlation data acquisition screen  255  (GUI screen) for the measurement preparation. As shown in  FIG.  7   , the correlation data acquisition screen  255  has an input box for a user to input three temperatures (T0, T+, T−) and the energy of the radiation (E0), for example. The correlation data acquisition screen  255  has a start button, and when the user clicks the start button, the computer subsystem  250  performs the measurement preparation in accordance with the flowchart shown in  FIG.  5    and acquires the correlation data between the baseline current and the peak value. The computer subsystem  250  displays the obtained correlation data between the baseline current and the peak value on the correlation data acquisition screen  255 . Since the correlation data acquisition screen  255  allows the user to confirm the correlation data between the baseline current and the peak value in this manner, the system is made user-friendly. 
     (Method of Correcting Peak Value when Analyzing Radiation) 
     Described below is a method of correcting the peak value when analyzing a radiation having unknown energy (second energy). 
       FIG.  8    is a flowchart showing the method of correcting the peak value by the correction unit  252 . As described above, the baseline current can be less affected by the fluctuation by averaging, for example, about 100 points of sampling data and using the averaged value. Accordingly, at Step S 21 , the correction unit  252  acquires one point of current value sampling data with respect to each ρsec, and acquires the current value sampling data to have 110 points in total. 
     At Step S 22 , the correction unit  252  acquires one point of the current value sampling data. At Step S 23 , the correction unit  252  determines whether the latest current value acquired at Step S 22  exceeds a threshold and determines whether the signal pulse is detected. Steps S 22  and S 23  are repeated until the signal pulse is detected. 
     If the latest current value exceeds the threshold (Yes), the process proceeds to Step S 24 , where the correction unit  252  calculates the average value of the 100 current values in total from 110 points before to 11 points before the point that exceeded the threshold, the average value being regarded as the baseline current BL. 
       FIG.  9    is a graph showing the change in the current acquired by the correction unit  252 . In  FIG.  9   , the horizontal axis indicates a count of sampled points and the vertical axis indicates the baseline current. In the example shown in  FIG.  9   , the current value exceeds the threshold at the 526th point, and the baseline current BL is 9.042 μA taking an average value from the 416th point to the 515th point. 
     Returning to  FIG.  8   , the correction unit  252  corrects the peak value PH of the signal pulse due to the irradiation with the radiation having the unknown energy using the baseline current BL acquired at the same time and the correlation data acquired in advance by the method shown in  FIG.  5   . Specifically, an accurate peak value PH′ after correction is calculated using the baseline current (BL0, BL+, BL−) acquired in the measurement preparation and the peak value (PH0, PH+, PH−) of the signal pulse of the radiation having the energy E0. 
     The correction unit  252  first calculates the peak value PH0′ of the radiation having the energy E0 when the baseline current is BL using the following formula (4). The correction unit  252  then calculates the peak value PH′ after the correction using a relation in which a proportion of the peak value PH and the peak value PH0′ is equal to a proportion of the peak value PH′ and the peak value PH0 using the following formula (5). The correction unit  252  or the spectrum generation unit  253  can acquire a value of the unknown energy by converting the peak value PH′ after the correction to the energy. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                         
                     4 
                   
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                     PH 
                     ⁢ 
                     
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                                 ) 
                               
                             
                           
                         
                         
                           
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                                 ⁢ 
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                                 ⁢ 
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                   4 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
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                     Formula 
                     ⁢ 
                         
                     5 
                   
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                     PH 
                     ′ 
                   
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                   5 
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       FIG.  10    shows an example of the correlation data between the baseline current and the peak value of the signal pulse of the radiation having the energy E0, and a relation among PH0, PH0′, PH, and PH′. In  FIG.  10   , the horizontal axis indicates the baseline current and the vertical axis indicates the peak value. As shown in  FIG.  10   , although the correlation data is an approximate calculated with three points of data, there may be at least two points of data for acquiring the correlation data. It is to be noted that, as described above, when the energy calculated from the peak value (PH0, PH+, PH−) and the peak value (PH0, PH+, PH−) is used as the correlation data, at Step S 24 , the correction unit  252  can acquire the energy value after the correction by correcting the value of the energy calculated from the peak value PH. 
     Returning to  FIG.  8   , at Step S 25 , the correction unit  252  acquires, for example, 900 points of the current value sampling data to wait until the current flowing through the TES  201  returns to its static state. 
     At Step S 26 , the correction unit  252  acquires one point of the current value sampling data. At Step S 27 , the correction unit  252  determines whether the latest current value is lower than the threshold. Steps S 26  and S 27  are repeated until the latest current value becomes lower than the threshold. 
     When it is determined at Step S 27  that the latest current value is lower than the threshold (Yes), the process returns to Step S 21  and performs the same process as described above. 
     The method of correcting the peak value according to the present embodiment as described above is applicable when the energy of the radiation entering the TES  201  is proportional to the peak value of the signal pulse of the TES  201 . 
       FIG.  11    is a graph showing an example of the correlation data when the energy of the radiation entering the TES  201  is proportional to the peak value. In  FIG.  11   , the horizontal axis indicates the energy of the radiation and the vertical axis indicates the peak value. As shown in  FIG.  11   , when the energy of the radiation is proportional to the peak value, the proportion among the peak values PH0, PH+, and PH− of the signal pulse stays constant regardless of the energy value. 
     Accordingly, it is possible to acquire the correlation data using any one radiation having the energy E0, and to calculate the peak value PH′ after the correction as described above using the formulas (4) and (5). 
     &lt;Technical Effect&gt; 
     As described above, the radiation analysis subsystem  200  according to the present embodiment corrects the peak value in accordance with the baseline current flowing through the TES  201  immediately before the peak value is measured by the pulse height analyzer  251  at the time of actual analysis, using the correlation data based on the baseline current and the peak value acquired in advance. Since this makes it possible to acquire a constant peak value with respect to the radiation having the same energy regardless of the variation of the baseline current without acquiring the additional correction data, it is possible to constantly acquire a high energy resolution. 
     Second Embodiment 
     In the first embodiment, the technology was described to correct the peak value on the basis of the fact that the energy of the radiation entering the TES  201  is proportional to the peak value of the signal pulse of the TES  201 . However, in an actual radiation analyzer, the energy of the radiation may not be proportional to the peak value of the signal pulse. Even in such a case, it is possible to acquire an accurate peak value by acquiring the correlation data based on the energy of the radiation and the peak value of the signal pulse in advance. Accordingly, proposed in a second embodiment is an operation of the radiation analysis subsystem in a case in which the energy of the radiation is not proportional to the peak value. 
     A configuration of the radiation analysis subsystem in the present embodiment can be the same as the configuration in the first embodiment, and therefore description thereof is omitted. 
     &lt;Operation of Radiation Analysis Subsystem&gt; 
     (Measurement Preparation: Acquisition of Correlation Data) 
       FIG.  12    is a graph showing an example of the correlation data when the energy of the radiation entering the TES  201  is not proportional to the peak value. In  FIG.  12   , the horizontal axis indicates the energy of the radiation and the vertical axis indicates the peak value. As shown in  FIG.  12   , when the energy of the radiation is not proportional to the peak value, it is required to acquire the correlation data using radiations having two or more energies. 
     The method of acquiring the correlation data according to the present embodiment is substantially the same as in the first embodiment (flowchart in  FIG.  5   ) except the following points. That is, when acquiring the peak value (PH0, PH+, PH−) at the time of acquiring the correlation data (Steps S 12 , S 14 , and S 16 ), the pulse height analyzer  251  detects the signal pulse corresponding to the two or more radiation energies, and the correction unit  252  stores the peak value. The energy of the radiation can be changed by changing the sample  306  (element) to be irradiated with the electron beam  302  by moving the sample stage  307  of the scanning electron microscope  300  shown in  FIG.  1    or scanning the scanning coil  304 . Moreover, the correction unit  252  creates a correction curve (function f0, f+, f−) of the data including the acquired radiation energy and the peak value interpolated by an n-order function (n is an integer) or a spline curve, and stores the correction curve as the correlation data. 
       FIG.  13    shows a correlation data acquisition screen  256  (GUI screen) for acquiring correlation data according to the second embodiment. As shown in  FIG.  13   , the correlation data acquisition screen  256  is a GUI screen for acquiring the baseline current and the correlation data between the radiation energy and the peak value, and has an input box to input three temperatures (T0, T+, T−), two or more radiation energies, and an interpolation method, for example. 
     For example, up to about 20 radiation energies can be set. The correlation data acquisition screen  256  shown in  FIG.  13    is configured so that nine energies can be set. Moreover, as the interpolation method, for example, either the n-order function (n is an integer) or the spline curve can be selected. The correlation data acquisition screen  256  is provided with the start button, and when the user clicks the start button, the computer subsystem  250  performs the measurement preparation in accordance with the flowchart shown in  FIG.  5   , and stores the data of the baseline current and the peak values corresponding to all the radiation energies. The computer subsystem  250  then creates the correction curve of the radiation energy and the peak value and stores the correction curve as the correlation data. The computer subsystem  250  displays the acquired baseline current as well as the correlation data of the radiation energy and the peak value at each baseline current on the correlation data acquisition screen  256 . 
     (Method of Correcting Peak Value when Analyzing Radiation) 
     The method of acquiring the correlation data according to the present embodiment is substantially the same as in the first embodiment (flowchart in  FIG.  8   ) except Step S 24  in the following points. 
     The correction unit  252  corrects the peak value PH of the signal pulse of the radiation having the unknown energy E (second energy) measured by the pulse height analyzer  251  in the following manner using the baseline current BL measured at the same time and the correlation data. Specifically, the accurate peak value PH′ after the correction is calculated using the baseline current (BL0, BL+, BL−) acquired in the measurement preparation and the correction curve of the radiation energy and the peak value at each baseline current (function f0, f+, f−). At this time, the relation between the unknown energy E and the peak value PH is given by the following formula (6). The correction unit  252  then calculates the peak value PH′ after the correction using an inverse function of the correction curve using the following formula (7). 
     
       
         
           
             
               
                 
                                    
                   
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                             ( 
                             
                               
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                                 ⁢ 
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                                 ⁢ 
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                   ( 
                   7 
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     In the formula (7), the expression in parentheses after f0 −1  indicates that, when a single radiation is detected, an energy value of the radiation can be calculated from the peak value of the pulse and the baseline current. The formula (7) is for calculating an accurate peak value PH′ after the correction by using an inverse function of the energy value. 
     &lt;Technical Effect&gt; 
     As described above, the radiation analysis subsystem according to the second embodiment acquires the peak values at  254  confirms least two baseline currents with respect to each energy value using radiations having at least two known energy values, thereby acquiring the correlation data (correction curve) of the peak value and the energy value. This correlation data is used to correct the peak value in accordance with the baseline current flowing through the TES immediately before the peak value is measured by the pulse height analyzer. This makes it possible to acquire a constant peak value with respect to the radiation having the same energy regardless of the variation of the baseline current without acquiring the additional correction data, and therefore it is possible to constantly acquire a high energy resolution. 
     Modification 
     The present disclosure is not limited to the aforementioned embodiments but may include various modifications. For example, the aforementioned embodiments are described in detail for easy comprehension of the present disclosure, and the invention does not necessarily include all the configurations described herein. A part of one embodiment may be replaced by a configuration of another embodiment. A configuration of one embodiment may also be added to a configuration of another embodiment. Moreover, a part of a configuration of one embodiment may be added to, deleted from, or replaced by a part of a configuration of another embodiment. 
     It is needless to say that, although each configuration, function, control unit, or the like described above is described focusing on an example of creating a program for implementing a part or all thereof, the part or all may be implemented on a hardware by designing an integrated circuit, for example. That is, all or a part of the function of the control unit may be implemented by an integrated circuit such as an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), and the like, for example, instead of the program. 
     LIST OF REFERENCE SIGNS 
     
         
           100 : Scanning electron microscope system 
           200 : Radiation analysis subsystem 
           201 : TES 
           202 : Sensor circuit unit 
           203 : Bias current source 
           204 : Shunt resistor 
           205 : Input coil 
           206 : Current detection mechanism 
           207 : SQUID amplifier 
           208 : Room temperature amplifier 
           209 : First thermometer 
           210 : Heater 
           211 : Refrigerator 
           212 : Absorber 
           213 : Second thermometer 
           214 : Membrane 
           215 : Cold head 
           216 : Superconductive wiring 
           217 : Heat shield 
           250 : Computer subsystem 
           251 : Pulse height analyzer 
           252 : Correction unit 
           253 : Spectrum generation unit 
           254 : Temperature control unit 
           255 ,  256 : Correlation data acquisition screen 
           300 : Scanning electron microscope 
           301 : Electron source 
           302 : Electron beam 
           303 : Condenser lens 
           304 : Deflection scanning coil 
           305 : Objective lens 
           306 : Sample 
           307 : Sample stage 
           308 : Backscattered electron detector 
           309 : Secondary electron detector 
           400 : High voltage power supply 
           500 : Computer subsystem 
           501 : Total control unit 
           502 : Electron optical system control unit 
           503 : Stage control unit 
           504 : A/D conversion unit 
           505 : Image operation unit 
           506 : Storage device 
           507 : Display unit 
           508 : Input device