Patent Publication Number: US-2022216032-A1

Title: Charged particle beam device

Description:
TECHNICAL FIELD 
     The present invention relates to a charged particle beam apparatus that irradiates a sample with a charged particle beam. 
     BACKGROUND ART 
     In a manufacturing process of a semiconductor device, in-line inspection and measurement by using a scanning electron microscope (SEM) is an important inspection item for a purpose of improving a yield. In particular, a low voltage SEM (LV SEM) using an electron beam having an acceleration voltage of several kV or less is extremely useful in inspection and measurement of a two-dimensional shape such as a resist pattern in a lithography process and a gate pattern in a previous process because a penetration depth of the electron beam is shallow and an image having rich surface information can be acquired. However, since organic materials such as a resist and an anti-reflection film used in the lithography process have compositions similar to each other, or silicon-based semiconductor materials constituting a transistor have compositions similar to each other, it is difficult to obtain a difference in secondary electron emission from the materials. Since a sample made of such materials has a low image contrast of the SEM, visibility of an ultrafine pattern or a defect of a semiconductor device is reduced. As a visibility improving method of the SEM, a method for adjusting observation conditions such as an acceleration voltage and an irradiation current and a technique for discriminating energy of electrons emitted from a sample are known, but a resolution and an imaging speed are problems depending on the conditions. 
     PTL 1 discloses a technique for controlling an image contrast of an SEM by irradiating an observation region of the SEM with light. Since an exciting carrier is generated by light irradiation, conductivity of a semiconductor or an insulator changes. A difference in conductivity between materials is reflected in a potential contrast of an SEM image. A conduction failure location of a semiconductor device or the like can be detected by controlling the potential contrast of the SEM by the light irradiation. 
     PTL 2 discloses a method for controlling an image contrast of an SEM by selecting a light wavelength for a sample configured with a plurality of layers, focusing on a difference in light absorption characteristics depending on a wavelength of light to be emitted. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2003-151483 
     PTL 2: Japanese Patent Application No. 2010-536656 
     SUMMARY OF INVENTION 
     Technical Problem 
     In both PTL 1 and PTL 2, the image contrast of the SEM is controlled according to the difference in the absorption characteristics of materials depending on the light wavelength. Both can enhance the image contrast in the materials having a large difference in wavelength dependence of the absorption characteristics. However, similar wavelength dependence of the absorption characteristics exists in many materials of similar types such as silicon materials having different dopant types and densities, or organic materials having similar compositions. In a sample composed of these materials, it may be difficult to obtain a sufficient difference in the absorption characteristics. 
     The invention has been made in consideration of the above problem, and an object of the invention is to provide a charged particle beam apparatus capable of acquiring an observation image having a high contrast even in a sample whose light absorption characteristic depends on a light wavelength. 
     Solution to Problem 
     A charged particle beam apparatus according to the invention irradiates a sample with light and generates an observation image of the sample, and generates a plurality of the observation images having different contrasts by changing an irradiation intensity per unit time of the light. 
     Advantageous Effect 
     According to the charged particle beam apparatus according to the invention, an amount of secondary electrons emitted from the sample can be controlled by adjusting a light irradiation intensity per unit time according to a light absorption characteristic. As a result, contrasts of the observation images even in materials of similar types having similar light absorption characteristics with respect to a light wavelength can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a charged particle beam apparatus  1  according to a first embodiment. 
         FIG. 2  is a configuration example of an absorption characteristic measuring unit  13 . 
         FIG. 3  is a flowchart illustrating a procedure in which the charged particle beam apparatus  1  acquires an observation image of a sample  8 . 
         FIG. 4  is a graph showing a relation between a light irradiation intensity I r  per unit time and a light absorption intensity I a . 
         FIG. 5  is a graph showing a relation between the light irradiation intensity I r  per unit time and an emission amount of secondary electrons. 
         FIG. 6  is an example of a GUI  61  displayed by an image display unit  25 . 
         FIG. 7  is an example of a cross-sectional view of the sample  8 . 
         FIG. 8  is an example of observation images acquired under three conditions of a light irradiation intensity per unit time. 
         FIG. 9  is a configuration diagram of the charged particle beam apparatus  1  according to a second embodiment. 
         FIG. 10  is a flowchart illustrating a procedure in which the charged particle beam apparatus  1  acquires an observation image of the sample  8 . 
         FIG. 11  is a configuration diagram of a pulse laser  10  and a light intensity adjusting unit  11  in the second embodiment. 
         FIG. 12  is an example of a relation between a light absorption characteristic measured in S 1001  and a light irradiation intensity per unit time. 
         FIG. 13  is an example of a cross-sectional view of the sample  8 . 
         FIG. 14  is a graph showing a relation of a correction amount AC of a secondary electron detection signal with respect to the light irradiation intensity per unit time in the second embodiment. 
         FIG. 15  is an example of observation images acquired under three conditions of the light irradiation intensity per unit time. 
         FIG. 16  is a time chart showing an electron beam irradiation timing/a pulse laser irradiation timing/a secondary electron detection timing, respectively. 
         FIG. 17  is an example of the GUI  61  displayed by the image display unit  25  in a third embodiment. 
         FIG. 18  is an example of a cross-sectional view of the sample  8 . 
         FIG. 19  is an example of observation images acquired by electron beams under irradiation conditions. 
         FIG. 20  is a configuration example of an absorption characteristic measuring unit  13 . 
         FIG. 21  is a configuration example of the absorption characteristic measuring unit  13 . 
         FIG. 22  is an example of observation images acquired under three conditions of the light irradiation intensity per unit time. 
         FIG. 23  is a configuration diagram of the charged particle beam apparatus  1  according to a fifth embodiment. 
         FIG. 24  is a graph showing an energy distribution of the secondary electrons when a light pulse is emitted with various light irradiation intensities. 
         FIG. 25  is an example of observation images acquired by an energy filter  231  under two conditions of the light irradiation intensity per unit time. 
         FIG. 26  is a configuration diagram of the charged particle beam apparatus  1  according to a sixth embodiment. 
         FIG. 27  is an example of a cross-sectional view of the sample  8 . 
         FIG. 28  is an example of observation images acquired under two conditions of the light irradiation intensity. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Regarding Basic Principle of Invention 
     Hereinafter, first, a basic principle of the invention is described, and then specific embodiments of the invention are described. The invention irradiates a sample to be observed with light to excite a carrier inside the sample. In this case, the sample is in an excited state. An emission amount of secondary electrons in the excited state increases according to a light absorption amount. Meanwhile, when photoelectrons are emitted from the sample by light irradiation, the sample is in a depleted state where electrons are deficient. An emission amount of the secondary electrons in the depleted state decreases according to the light absorption amount. 
     An increase and decrease amount ΔS of the secondary electrons due to the light irradiation is expressed by Formula 1. A represents a light absorption amount, and z represents a distance to a light intrusion direction. 
       [Formula 1] 
       ΔS∝+∫dA/dz·dz   (1)
 
     An intrusion direction dependence of the light absorption amount dA/dz is expressed by Formula 2. α 1  to α 3  represent absorption coefficients of a material, α 1  represents a linear absorption term, and α 2  and α 3  represent a second-order and third-order non-linear absorption terms. Here, the terms up to the third-order are described, but higher-order terms are also confirmed. I r  represents a light irradiation intensity per unit time on the sample. Parameters that control the light irradiation intensity per unit time include an average output of a pulse laser, an energy per pulse, a peak intensity per pulse, a pulse width of the pulse laser, the number of light pulses to be emitted per unit time, a frequency of the light pulse, an area of a light spot, a light wavelength, a polarization, and the like. 
       [Formula 2] 
         dA/dz=α   1   ·I   r +α 2   ·I   r   2 +α 3   ·I   r   3    (2)
 
     When the light irradiation intensity is low, a linear absorption term based on single photon absorption is dominant, and if the light wavelength is in an absorption band of the material, the sample absorbs light and comes into an excited state. In the excited state, an emission efficiency of the secondary electrons becomes high. When the light irradiation intensity is high, a non-linear absorption term based on multiphoton absorption is dominant, and even if the light wavelength is not in the absorption band of the material, the sample absorbs light and changes from the excited state to a depleted state where photoelectrons are emitted. In the depleted state, the emission efficiency of the secondary electrons becomes low. That is, the emission amount of the secondary electrons can be controlled by controlling an absorption characteristic as the single photon absorption or the multiphoton absorption according to the light irradiation intensity. Photophysical property parameters for confirming non-linear absorption include an absorption coefficient, a reflection coefficient, a polarization modulation, a wavelength modulation, a photoelectron emission, and the like. 
     The invention provides a charged particle beam apparatus in which the above principle is used, and even in materials having similar absorption characteristics with respect to light wavelengths, a highly visible observation image in which a contrast of patterns or defects is enhanced can be acquired by adjusting an irradiation intensity per unit time of light. 
     FIRST EMBODIMENT 
     A first embodiment of the invention describes a charged particle beam apparatus that irradiates an observation region with a pulse laser whose light irradiation intensity per unit time is controlled according to a light absorption characteristic of a sample, and enhances an observation image contrast. 
       FIG. 1  is a configuration diagram of a charged particle beam apparatus  1  according to the first embodiment. The charged particle beam apparatus  1  is configured as a scanning electron microscope that irradiates a sample  8  with an electron beam (primary charged particles) to acquire an observation image of the sample  8 . The charged particle beam apparatus  1  includes an electro-optical system, a stage mechanical system, a light pulse irradiation system, a light absorption characteristic measurement system, a control system, an image processing system, and an operation system. A storage device  27  will be described later. 
     The electro-optical system includes an electron gun  2 , a deflector  3 , an electron lens  4 , and an electron detector  5 . The stage mechanical system includes an XYZ stage  6  and a sample holder  7 . An inside of a housing  9  is controlled to a high vacuum, and is provided with the electro-optical system and the stage mechanical system. The light pulse irradiation system includes a pulse laser  10  and a light intensity adjusting unit  11 . Light is emitted to the sample  8  through a light pulse introduction unit  12  provided in the housing  9 . An absorption characteristic measuring unit  13  detects a light pulse reflected from the sample  8 . 
     The control system includes an electron gun control unit  14 , a deflection signal control unit  15 , an electron lens control unit  16 , a detector control unit  17 , a stage position control unit  18 , a pulse laser control unit  19 , a light intensity adjustment control unit  20 , an absorption characteristic measurement control unit  21 , a control transmission unit  22 , and a detection signal acquisition unit  26 . The control transmission unit  22  writes and controls a control value to each of the control units based on input information input from an operation interface  23 . The image processing system includes an image forming unit  24  and an image display unit  25 . 
     An electron beam accelerated by the electron gun  2  is focused by the electron lens  4  and emitted to the sample  8 . The deflector  3  controls an irradiation position of the electron beam on the sample  8 . The electron detector  5  detects emission electrons (secondary charged particles) emitted from the sample  8  by irradiating the sample  8  with the electron beam. The operation interface  23  is a functional unit for a user to specify and input an acceleration voltage, an irradiation current, a deflection condition, a detection sampling condition, an electron lens condition, and the like. 
     A light pulse emitted from the pulse laser  10  is emitted to a position on the sample  8  irradiated with the electron beam. The light intensity adjusting unit  11  is a device that controls an irradiation intensity per unit time of a light pulse laser. The electron detector  5  detects secondary electrons emitted from the sample  8 . The secondary electrons include both low-energy emission electrons from a sample and high-energy backscattered electrons. The image forming unit  24  forms an SEM image (observation image) of the sample  8  using a detection signal detected by the electron detector  5 , and the image display unit  25  displays the image. 
       FIG. 2  is a configuration example of the absorption characteristic measuring unit  13 . A pulse laser whose irradiation intensity is adjusted by the light intensity adjusting unit  11  is split by a beam splitter  30  before being emitted to the sample  8 . An irradiation light detector  31  detects a signal according to an intensity of light emitted to the sample  8 . In this case, the light intensity is calibrated according to a split ratio of the beam splitter  30 . The pulse laser emitted to the sample  8  is reflected in the sample  8 , and a reflection light detector  32  installed opposite to the beam splitter detects a signal according to the light intensity. A subtractor  33  obtains a difference signal of the signals detected by the irradiation light detector  31  and the reflection light detector  32 . A signal detector  34  digitizes a light absorption intensity based on the difference signal. 
       FIG. 3  is a flowchart illustrating a procedure in which the charged particle beam apparatus  1  acquires the observation image of the sample  8 . Hereinafter, each step in  FIG. 3  will be described. 
     FIG.  3 : Steps S 301  to S 303   
     The stage mechanical system moves the sample  8  to an observation position (S 301 ). The control transmission unit  22  sets the acceleration voltage, the irradiation current, a magnification, and a scanning time as basic electron beam observation conditions according to the specification and input from the operation interface  23  (S 302 ). The pulse laser control unit  19  sets a wavelength of the pulse laser (S 303 ). The laser wavelength is desired to be set based on a wavelength band in which the sample  8  absorbs light. 
     FIG.  3 : Step S 304   
     The control transmission unit  22  measures a light absorption characteristic of the sample  8  while changing an irradiation intensity per unit time of light. The light irradiation intensity is controlled by the light intensity adjusting unit  11 . A light absorption measurement is performed by the absorption characteristic measuring unit  13 . The control transmission unit  22  stores, in the storage device  27 , data describing a correspondence relation between the light irradiation intensity and the light absorption characteristic based on the measurement result. An example of the correspondence relation in this step is described with reference to  FIG. 4  described later. 
     FIG.  3 : Step S 305   
     The control transmission unit  22  sets a threshold of the light irradiation intensity per unit time based on the result of step S 304 . The threshold here can be determined based on, for example, which of the light absorption characteristics of Formula 2 is dominant, the linear absorption term (α 1 ) or the non-linear absorption term (from α 2 ). A specific example of a criteria for determining the threshold is described with reference to  FIG. 4  described later. 
     FIG.  3 : Steps S 304  and S 305 : Supplement No. 1 
     In this flowchart, an analysis result in S 304  is stored in the storage device  27  and used, and the correspondence relation between the light irradiation intensity and the light absorption characteristic under various conditions is analyzed in advance and a result thereof can be stored in the storage device  27  as a database. As a result, it is unnecessary to carry out steps S 304  and S 305  every time the observation image is acquired. 
     FIG.  3 : Steps S 304  and S 305 : Supplement No. 2 
     The storage device  27  can be configured with an appropriate device that stores the measurement result and the correspondence relation. For example, if the measurement result and the correspondence relation are stored as a database in advance and used, the storage device  27  can be configured with a non-volatile storage device. If the measurement result and the correspondence relation are acquired each time this flowchart is executed, the storage device  27  can be configured with a memory device or the like that temporarily stores the measurement result and the correspondence relation. These devices may be combined. 
     FIG.  3 : Steps S 306  to S 308   
     The control transmission unit  22  sets one or more light irradiation intensities as an observation condition according to the results of S 304  and S 305  (S 306 ). The observation condition described here does not have to be the threshold itself set in S 305 , and may be an appropriate value close to the threshold as described later. The control transmission unit  22  adjusts the irradiation intensity by the light intensity adjusting unit  11  such that the irradiation intensity is the light irradiation intensity set as the observation condition (S 307 ). The control transmission unit  22  irradiates the sample  8  with a light pulse and an electron beam whose irradiation intensities per unit time are adjusted, and acquires an observation image by the image forming unit  24  (S 308 ). 
       FIG. 4  is a graph showing a relation between a light irradiation intensity I r  per unit time and a light absorption intensity I a . In S 304 , the relation as illustrated in  FIG. 4  is measured. Here, the relation between the light absorption characteristic and the light irradiation intensity per unit time when the sample  8  is composed of silicon (Si) and silicon nitride (SiN) is illustrated. In an absorption characteristic  41  of silicon, it can be seen that the light absorption intensity I a  changes from a linear characteristic to a non-linear characteristic when the light irradiation intensity I r  per unit time is about 150 MW/cm 2 /μs. In an absorption characteristic  42  of silicon nitride, a linear characteristic is maintained until the light irradiation intensity I r  becomes about 300 MW/cm 2 /μs. 
     In S 305 , the control transmission unit  22  can set an irradiation intensity at which the absorption characteristic  41  (Si) changes from linear to non-linear as a threshold I rth(Si) , and can set an irradiation intensity at which the absorption characteristic  42  (SiN) changes from linear to non-linear as a threshold I rth(SiN) . Significances of these thresholds are described with reference to  FIG. 5 . 
       FIG. 5  is a graph showing a relation between the light irradiation intensity I r  per unit time and an emission amount of the secondary electrons. As an amount of I r  increases, an emission amount of secondary electrons  51  of silicon increases, and gradually decreases when I r  reaches about 150 MW/cm 2 /μs or more. An emission amount of secondary electrons  52  of silicon nitride increases to about 300 MW/cm 2 /μs. In the present description, the phenomenon of increase and decrease in the emission amount of secondary electrons is referred to as a modulation effect of secondary electrons. The present inventors have discovered that the modulation effect occurs when the absorption characteristic changes from linear to non-linear. Therefore, in  FIG. 5 , the irradiation intensities at which the emission amounts of secondary electrons start to decrease correspond to the threshold I rth(Si)  and the threshold I rth(SiN) , respectively. 
     In order to enhance the contrast of the observation image for each of the materials, it is desirable to set the observation condition such that the emission amounts of secondary electrons differ greatly for the materials. This corresponds to a large difference between the emission amounts of secondary electrons  51  and  52  in  FIG. 5 . It is considered that such an observation condition with a high contrast generates at the irradiation intensities closing to boundaries which are the thresholds at which the emission amounts of secondary electrons start to decrease. Therefore, in  FIG. 5 , three observation conditions for comparing the contrast are set: condition a (0 MW/cm 2 /μs), condition b (70 MW/cm 2 /μs), and condition c (350 MW/cm 2 /μs), respectively. An example of observation images using these conditions will be described later. 
       FIG. 6  is an example of a graphical user interface (GUI)  61  displayed by the image display unit  25 . Basic observation conditions of an acceleration voltage  62 , an irradiation current  63 , a magnification  64 , and a scanning speed  65  can be set on the GUI  61 . The image display unit  66  displays the observation image. An irradiation condition setting unit  67  includes (a) a wavelength setting unit  68  that sets a wavelength of a light pulse, (b) an absorption characteristic analysis unit  69  that acquires (or calls from a database) an absorption characteristic of a sample, (c) an absorption characteristic display unit  70  that displays the absorption characteristic, and (d) an irradiation intensity setting unit that sets an average output  71  of a light pulse, a pulse width  72 , a frequency  73  of the light pulse, and an irradiation diameter  74  of the light pulse based on the conditions of the light irradiation intensity per unit time determined on the absorption characteristic display unit  70 . In  FIG. 6 , two wavelengths can be selected as the wavelengths of the light pulse. Further, three conditions can be set as the conditions of the light irradiation intensity per unit time. Other parameters can also be set on the GUI  61 . 
       FIG. 7  is an example of a cross-sectional view of the sample  8 . Here, as illustrated in  FIG. 4 , an example composed of silicon  75  and silicon nitride  76  is shown. A thin film of the silicon nitride  76  is patterned in a line on the silicon  75 . Observation conditions of an electron beam include an acceleration voltage of 0.5 kV, an irradiation current of 100 pA, an observation magnification of 100 K times, and a scanning speed of a TV scanning speed. A wavelength of a light pulse is 355 nm. As illustrated in  FIG. 5 , the light irradiation intensities per unit time are set to 0 MW/cm 2 /μs, 70 MW/cm 2 /μs, and 350 MW/cm 2 /μs. Light average outputs are 0 mW, 44 mW, and 220 mW for irradiation intensities, respectively. 
       FIG. 8  is an example of observation images acquired under three conditions of the light irradiation intensity per unit time. The conditions a to c have been described in  FIG. 5 . In the observation image acquired under the condition a, the silicon  75  and the silicon nitride  76  show the same image brightness, and visibility of a pattern is low. In the observation image acquired under the condition b, a high image brightness is obtained for both the silicon  75  and the silicon nitride  76 , and the visibility of the pattern is high. In the observation image acquired under the condition c, the image brightness of the silicon  75  is low, and the image brightness of the silicon nitride  76  is high. It can be seen that the observation image acquired under the condition c can obtain the highest contrast. 
     The same effect can be obtained even if the charged particle beam apparatus  1  according to the first embodiment is implemented in a returning system in which a voltage is applied to the XYZ stage  6 , the sample holder  7 , and the sample  8  to reduce an electron energy applied to the sample. 
     Overview of First Embodiment 
     The charged particle beam apparatus  1  according to the first embodiment can control the amount of the secondary electrons emitted from the sample  8  by adjusting the irradiation intensity of actually emitted light per unit time according to the light absorption characteristic that depends on the light irradiation intensity per unit time. Therefore, even if the materials are of the same type and have similar absorption characteristics with respect to the light wavelength, the observation image contrast can be enhanced, and thus the visibility of the defect and the pattern of the sample  8  is improved. 
     SECOND EMBODIMENT 
     When the sample  8  is irradiated with light, photoelectrons may be emitted from the sample  8 . The photoelectrons act as noise for the secondary electrons. Therefore, in the second embodiment of the invention, a configuration example for removing an influence of the photoelectrons on a detection result of the secondary electrons is described. 
       FIG. 9  is a configuration diagram of the charged particle beam apparatus  1  according to the second embodiment. The charged particle beam apparatus  1  according to the second embodiment includes the configuration described in the first embodiment, and further includes a photoelectron detector  91 , a photovoltaic current measuring device  92 , a circuit breaker  93 , and a signal corrector  94 . The photoelectron detector  91  detects the photoelectrons from the sample  8  by light pulse irradiation. The photovoltaic current measuring device  92  measures a current flowing through the sample  8  by irradiating the sample  8  with the light. The circuit breaker  93  has a function of blocking an electron beam. The signal corrector  94  corrects a detection signal of the secondary electrons or brightness of an observation image based on the detection signal of the photoelectrons which is detected by the photoelectron detector  91 . Since other configurations are the same as those in the first embodiment, a difference will be mainly described below. 
       FIG. 10  is a flowchart illustrating a procedure in which the charged particle beam apparatus  1  acquires an observation image of the sample  8 . In the flowchart of  FIG. 10 , S 1002  is added between S 307  and S 308  in addition to the flowchart illustrated in  FIG. 3 , and S 304  is replaced with S 1001 . Other steps are the same as those in  FIG. 3 . 
     FIG.  10 : Step S 1001   
     The control transmission unit  22  measures the light absorption characteristic of the sample  8  while changing the irradiation intensity per unit time of light. The light absorption characteristic can be measured based on an emission amount of photoelectrons detected by the photoelectron detector  91  or a photovoltaic current measured by the photovoltaic current measuring device  92 . A relation between the emission amount of the photoelectrons and a light absorption amount, or a relation between the photovoltaic current and the light absorption amount may, for example, be measured in advance and the measurement result may be stored in the storage device  27 . 
     FIG.  10 : Step S 1002   
     The signal corrector  94  corrects the detection signal of the secondary electrons based on the light absorption characteristic measured in S 1001 . That is, the influence of the light irradiation on the secondary electron detection signal is removed by subtracting the secondary electron detection signal when the sample  8  is irradiated with the light and not irradiated with the electron beam from the secondary electron detection signal when the sample  8  is irradiated with the electron beam and light. The secondary electron detection signal when the sample  8  is irradiated with the light and not irradiated with the electron beam can be acquired from the detection result in S 1001 . 
       FIG. 11  is a configuration diagram of the pulse laser  10  and the light intensity adjusting unit  11  in the second embodiment. A laser oscillator (or laser amplifier)  111  emits the light pulse. A wavelength converter  112  is configured with a non-linear optical element and the like, and controls the wavelength of the light pulse. A pulse picker  113  is configured with an electro-optic effect device and a magneto-optic effect device, and controls the frequency of the light pulse. A pulse dispersion controller  114  is configured with a pair of prisms and the like, and controls the pulse width of the light pulse. A polarization controller  115  is configured by using a birefringent element or the like, and controls a polarization plane of the light pulse. An average output controller  116  is configured with a neutral density (ND) filter or the like whose density can be changed, and adjusts the average output of the light pulse. Further, the light pulse introduction unit  12  can be configured with a zoom lens or the like, so that an irradiation diameter of the light pulse can be controlled. 
       FIG. 12  is an example of a relation between the light absorption characteristic measured in S 1001  and the light irradiation intensity per unit time. Here, absorption characteristics of P-type silicon and N-type silicon which have different types of impurities are analyzed. The measurement is carried out by detecting the photoelectrons using the photoelectron detector  91 . 
     In this case, the electron beam is blocked by the circuit breaker  93 . The wavelength of the light pulse is 405 nm. At this wavelength, there is no light energy (eV) that reaches a vacuum level of silicon, and thus the photoelectrons are not emitted when the light pulse is linearly absorbed. As the light irradiation intensity per unit time increases, the photoelectrons are emitted through the multiphoton absorption, which is a non-linear process. 
       FIG. 12  shows a relation between the light irradiation intensity I r  per unit time and an emission intensity S ph  of the photoelectrons in the P-type silicon and the N-type silicon. P-type silicon  121  emits the photoelectrons with a light irradiation intensity per unit time of 4 MW/cm 2 /μs as a threshold, whereas N-type silicon  122  emits the photoelectrons with a threshold of 12 MW/cm 2 /μs.  FIG. 12  shows an example of the photoelectrons detected by using the photoelectron detector  91 , but when the photovoltaic current measuring device  92  is used, the photoelectron current emitted from the sample  8  can be measured, and thus the same thresholds as in  FIG. 12  can be extracted. 
       FIG. 13  is an example of a cross-sectional view of the sample  8 . N-type silicon  132  is joined and formed on a surface of P-type silicon  131 , and a hole pattern of a silicon oxide film  133  is further formed on the surface. A defect  134  is a portion where the N-type silicon  132  and the hole pattern of the silicon oxide film  133  are out of alignment. 
     In the second embodiment, the same GUI as in the first embodiment is used. SEM observation conditions include an acceleration voltage of 1.0 kV, an irradiation current of 500 pA, an observation magnification of 200 K times, and a scanning speed of twice the TV scanning speed. 0.0 MW/cm 2 /μs is made as the condition a of the light irradiation intensity per unit time. 4 MW/cm 2 /μs is made as the condition b. 12 MW/cm 2 /μs is made as the condition c. The condition b further includes a light pulse frequency of 100 MHz, an average output of 16 mW, a pulse width of 1000 femtoseconds, and an irradiation diameter of 50 μm. The condition c further includes a light pulse frequency of 50 MHz, an average output of 54 mW, a pulse width of 800 femtoseconds, and an irradiation diameter of 60 μm. 
       FIG. 14  is a graph showing a relation of a correction amount ΔC of the secondary electron detection signal with respect to the light irradiation intensity per unit time in the second embodiment. The correction amount ΔC is determined by an area ratio of the P-type silicon  131  to the N-type silicon  132  in the sample  8  in addition to the relation between the light irradiation intensity I r  per unit time and the emission intensity S ph  of the photoelectrons shown in  FIG. 12 . In the second embodiment, the ratio is set to 50%. 
       FIG. 15  is an example of observation images acquired under three conditions of the light irradiation intensity per unit time. In the observation image acquired under the condition a, the P-type silicon  131  and the N-type silicon  132  show the same image brightness, the visibility of the pattern is low, and the defect portion cannot be visually recognized. In the observation image acquired under the condition b, the visibility of the P-type silicon  131  and the N-type silicon  132  is improved, but a defect detection is insufficient. In the observation image acquired under the condition c, the image brightness of the P-type silicon  131  is low, and the pattern contrast is the highest. A defect  156  can be sufficiently visually recognized if the observation image is acquired under the condition c. 
     As a method for removing the influence of photoelectrons from the secondary electron signal, by controlling a voltage applied to an energy filter included in the electron lens control unit  16 , the influence of photoelectrons may be removed from the secondary electron signal detected by the electron detector  5 . 
     Overview of Second Embodiment 
     The charged particle beam apparatus  1  according to the second embodiment corrects the secondary electron detection signal by removing, from the secondary electron detection signal, the influence of the photoelectrons emitted from the sample  8  by irradiating the sample  8  with the light. As a result, the contrast of the observation image of the sample  8  can be formed more accurately, so that the visibility of the defect and the pattern can be improved. 
     THIRD EMBODIMENT 
     In a third embodiment of the invention, an example of intermittently irradiating the sample  8  with the electron beam is described. The visibility of the sample  8  can be improved by comparing the observation image when the electron beam is emitted with the observation image when the electron beam is not emitted. The configuration of the charged particle beam apparatus  1  is the same as that according to the second embodiment. By blocking the electron beam with the circuit breaker  93 , an irradiation period and a non-irradiation period (interval period) of the electron beam can be controlled. 
       FIG. 16  is a time chart showing an electron beam irradiation timing/a pulse laser irradiation timing/a secondary electron detection timing, respectively. The control transmission unit  22  controls an irradiation period  161  and an interval period  162  of the electron beam by controlling the circuit breaker  93 . In the third embodiment, a light pulse  163  of the pulse laser is controlled at a constant frequency regardless of the irradiation period  161  and the interval period  162 . The light pulse  163  may be emitted in synchronization with the irradiation period  161  or may be emitted in synchronization with the interval period  162 . A timing  164  for detecting the secondary electrons is synchronized with the irradiation period  161 . The timing  164  for detecting the secondary electrons needs to be synchronized with the irradiation period  161  in consideration of a traveling time of the secondary electrons and a delay time based on a circuit delay of the electron detector  5 . 
       FIG. 17  is an example of the GUI  61  displayed by the image display unit  25  in the third embodiment. In the third embodiment, in addition to the GUI  61  described in the first embodiment, an irradiation period setting unit  171  and an interval period setting unit  172  are added. 
       FIG. 18  is an example of a cross-sectional view of the sample  8 . N-type silicon  182  is joined and formed on the surface of P-type silicon  181 . The silicon oxide film  183  is provided on the surface and the hole pattern is formed in the silicon oxide film  183 . A contact plug  184  of polysilicon is formed in the hole pattern. A defect  185  is injected with the N-type silicon of a high density. A defect  186  has a thin residual film between the contact plug  184  and the N-type silicon  182 . A defect  187  has a thicker residual film than that of the defect  186 . 
     In the third embodiment, the observation conditions include an acceleration voltage of 0.3 kV, an irradiation current of 50 pA, an observation magnification of 50 K times, and a scanning speed of TV scanning speed. When emitting the electron beam intermittently, an irradiation time is 200 ns and an interval time is 3.2 μs. In the third embodiment, a relation between the light absorption characteristic of the sample  8  and the light irradiation intensity per unit time is acquired by using the photovoltaic current measuring device  92 . As shown in the absorption characteristic display unit  70  in  FIG. 17 , the conditions a to c are set as the light irradiation intensities per unit time based on the absorption characteristics. The condition a is 0.0 MW/cm 2 /μs. The condition b is 16 MW/cm 2 /μs. The condition c is 30 MW/cm 2 /μs. Conditions corresponding to these conditions are set in the irradiation condition setting unit  67 . 
       FIG. 19  is an example of observation images acquired by the electron beam under irradiation conditions. In the observation image acquired by continuously irradiating the sample with the electron beam for 5 μs or more under the condition a, the contact plug  192  can be identified, but the defect cannot be identified. In the observation image acquired by continuously irradiating the sample with the electron beam for 5 μs or more under the condition b, a depletion layer of junction comes into conductive due to linear absorption of the light pulse, so that a normal contact plug  194  becomes bright. However, a defect having the N-type silicon of a high density with weak linear absorption (the defect  185  in  FIG. 18 ) and defects with residual film (defects  186  and  187  in  FIG. 18 ) are charged by being irradiated with the electron beam, and thus brightness of the contact plug remains low. In the observation image acquired by continuously irradiating the sample with the electron beam for 5 μs or more under the condition c, the depletion layer of the junction having the N-type silicon of a high density is also made conductive by the non-linear absorption, and thus a defect  196  becomes bright. In the observation image acquired by continuously being intermittently irradiated for an irradiation time of 200 ns and an interval time of 3.2 μs of the electron beam under the condition c, a defect  198  which has a thin residual film between the contact plug and the N-type silicon and a defect  199  which has a thicker residual film than that of the defect  198  can be recognized as a grayscale contrast. Under this condition, the defect  198  with a high capacitance is brighter than the defect  199  with a low capacitance. 
     A difference image  200  is formed by a difference between the two observation images (condition b: 5 μs) (condition c: 5 μs) in the middle of  FIG. 19 . From the difference image  200 , the defect of the junction on the bottom of the contact plug can be extracted. A difference image  201  is formed by a difference between the two observation images (condition c: 5 μs) (condition c: 200 ns) in the lower part of  FIG. 19 . From the difference image  201 , the residual film defects having different film thicknesses on the bottom of the contact plug can be extracted. 
     Overview of Third Embodiment 
     The charged particle beam apparatus  1  according to the third embodiment generates an observation image while intermittently irradiating the sample  8  with the electron beam by switching between a period in which the sample  8  is irradiated with the electron beam and a period in which the sample is not irradiated with the electron beam. As a result, it is possible to acquire an observation image having a contrast different from an observation image acquired while continuously irradiating the sample  8  with an electron beam. In this way, an electrical defect having different electrical characteristics can be discriminated and detected. 
     FOURTH EMBODIMENT 
       FIG. 20  is a configuration example of the absorption characteristic measuring unit  13 . Here, a configuration for detecting a polarization plane of light is shown. The light pulse reflected by the sample  8  is elliptically polarized by a wave plate  211 , and is divided into an S-polarized light and a P-polarized light by a birefringent element  212 . A photodetector  213  detects a light intensity of the S-polarized light, and a photodetector  214  detects a light intensity of P-polarized light. A subtractor  215  calculates a difference between the light intensity of the S-polarized light and the light intensity of the P-polarized light. A signal detector  216  converts the calculation result into data as an intensity of elliptically polarized lights. A digital processing may be used instead of an analog circuit to acquire a difference signal. 
       FIG. 21  is a configuration example of the absorption characteristic measuring unit  13 . Here, a configuration for detecting a harmonic generated by the non-linear absorption is shown. A harmonic light pulse generated in the sample  8  is spectrally decomposed by a diffraction grating  217 . A light intensity for each spectrum is detected by a light intensity sensor  218  having a plurality of detection elements made by a silicon process on a line. The light intensity of each wavelength acquired by the light intensity sensor  218  is converted into data by a signal detector  219 . In the fourth embodiment, a light pulse to be emitted is a circularly polarized light, and a wavelength is 700 nm. The threshold of the light irradiation intensity per unit time at which the linear is changed into the non-linear is an irradiation intensity at which the light pulse is changed into the elliptically polarized light or an irradiation intensity at which a second harmonic with 350 nm is generated. 
     In the fourth embodiment, the flowchart in  FIG. 3  and the GUI in  FIG. 6  are used. As a sample in the fourth embodiment, a sample formed by an organic-inorganic hybrid material in which a dielectric is mixed with an organic substance is used. According to the threshold of the light irradiation intensity per unit time at which the polarization plane from the sample  8  changes due to the light pulse irradiation or the second harmonic is generated, the condition a to the condition c are set as the light irradiation intensity per unit time. The condition a is 0.0 MW/cm 2 /μs. The condition b is 4 MW/cm 2 /μs. The condition c is 10 MW/cm 2 /μs. The condition b further includes the light pulse frequency of 100 MHz, an average output of 14 mW, a pulse width of 220 femtoseconds, and an irradiation diameter of 100 μm. The condition c further includes the light pulse frequency of 100 MHz, an average output of 35 mW, the pulse width of 220 femtoseconds, and the irradiation diameter of 100 μm. 
       FIG. 22  is an example of observation images acquired under the three conditions of the light irradiation intensity per unit time. In the observation image acquired under the condition a, an organic substance  222  and a dielectric  223 , which are bases of a hybrid material, show the same image brightness, and visibility of a dielectric domain is low. In the observation image acquired under the condition b, the dielectric is excited by the linear absorption, so that secondary electrons emitted from a dielectric  225  increase, and the dielectric domain can be clearly seen. In the observation image under the condition c, the non-linear absorption occurs in each of the dielectrics having different complex dielectric constants, so that the emission of the secondary electrons is reduced. In the observation image acquired under the condition c, dielectrics  227  having different complex dielectric constants can be inspected on a gray scale according to a difference in the complex dielectric constants. 
     According to the charged particle beam apparatus  1  according to the fourth embodiment, domains having different dielectric constants of the sample  8  can be discriminated and detected. In the fourth embodiment, two configuration examples for detecting the polarization plane and the wavelength are shown as the absorption characteristic measuring unit  13 , but it is unnecessary to detect both of the two characteristics, and the polarization plane may be detected or the wavelength may be detected. 
     FIFTH EMBODIMENT 
     In a fifth embodiment of the invention, in addition to the configurations described in the first to fourth embodiments, a configuration example in which the contrast of the observation image is enhanced by energy discrimination of the secondary electrons is described. Other configurations are the same as those in the first to fifth embodiments. 
       FIG. 23  is a configuration diagram of the charged particle beam apparatus  1  according to the fifth embodiment. Here, in addition to the configuration described in the first embodiment, a configuration example including an energy filter  231  that discriminates an energy of the secondary electrons and an energy filter control unit  232  that controls a voltage applied to the energy filter  231  is shown. The user specifies a voltage to be applied to the energy filter  231  via the operation interface  23 , and the energy filter control unit  232  controls the voltage according to the specification. An energy spectrometer such as a spectrum meter using a Wien filter can be used instead of the energy filter  231 . 
     In the fifth embodiment, the sample  8  shown in  FIG. 7  is used. The observation conditions include the acceleration voltage of 0.5 kV, the irradiation current of 100 pA, the observation magnification of 100 K times, and the scanning speed of the TV scanning speed. The wavelength of the light pulse is 355 nm. As for the light irradiation intensity per unit time, the condition a and the condition b are set as the light irradiation intensities based on the relation between the absorption characteristic and the light irradiation intensity per unit time as in the first embodiment. 0 MW/cm 2 /μs is made as the condition a and 350 MW/cm 2 /μs is made as the condition b. Further, the average outputs are adjusted based on the two set conditions of the light irradiation intensity per unit time. The average outputs are 0 mW and 220 mW, respectively. 
       FIG. 24  is a graph showing an energy distribution of the secondary electrons when a light pulse is emitted according to the light irradiation intensities. In the light pulse of 0 MW/cm 2 /μs (that is, no light irradiation), almost no difference exists between silicon  241  and silicon nitride  242 . When the light pulse of 350 MW/cm 2 /μs is emitted, the silicon nitride is in a linear absorption state, and an emission efficiency of the secondary electrons is high. It can be seen that in an energy distribution of secondary electrons of silicon nitride  243  in this state, a peak intensity is high and a peak is shifted to a low energy side. Silicon irradiated with the light pulse 350 MW/cm 2 /μs is in a non-linear absorption state, and emission of the secondary electrons is suppressed. It can be seen that in an energy distribution of secondary electrons of silicon nitride  244  in this state, a peak intensity is low and a peak is shifted to a high energy side. From  FIG. 24 , it can be seen that in addition to the difference in the emission efficiencies of the secondary electrons, a difference in secondary electron yields can be expanded by the energy filter  231 . In the fifth embodiment, a filter voltage V EF  is set to 4V. 
       FIG. 25  is an example of observation images acquired by the energy filter  231  under two conditions of the light irradiation intensity per unit time. In the observation image acquired under the condition a, silicon  252  and silicon nitride  253  show the same image brightness, and the visibility of the pattern is low. In the observation image acquired under the condition b, a difference in the image brightness between the silicon  252  and the silicon nitride  253  is widened, and the visibility of the pattern is improved. In the observation image acquired by using the energy filter  231  (filter voltage is 4V) under the condition b, it can be seen that an image contrast between the silicon  252  and the silicon nitride  253  is enhanced by the energy discrimination, and the visibility of the pattern is further improved. 
     Overview of Fifth Embodiment 
     According to the charged particle beam apparatus  1  according to the fifth embodiment, in addition to adjusting the light irradiation intensities per unit time described in the first to fourth embodiments, the contrast of the observation image can be enhanced by using the energy discrimination of the secondary electrons. 
     SIXTH EMBODIMENT 
       FIG. 26  is a configuration diagram of the charged particle beam apparatus  1  according to a sixth embodiment of the invention. The sixth embodiment describes a configuration example for identifying the characteristic of the sample  8  by using the secondary electron detection signal or the observation image itself instead of using the absorption characteristic measuring unit  13  and the absorption characteristic measurement control unit  21 . A configuration shown in  FIG. 26  is the same as the configuration described in the first embodiment except that the absorption characteristic measuring unit  13  and the absorption characteristic measurement control unit  21  are not provided. 
     In the sixth embodiment, the condition a and the condition b are set as the conditions of the light irradiation intensity per unit time. The condition a is 10.0 MW/cm 2 /μs. The condition b is 100 MW/cm 2 /μs. The condition a further includes an average output of the light pulse of 400 mW. The condition b further includes an average output of the light pulse of 4000 mW. 
       FIG. 27  is an example of a cross-sectional view of the sample  8 . N-type silicon  272  with a low density and N-type silicon  273  with a high density are formed on a surface of P-type silicon  271 . An N-type silicon well  274  with a low density is further formed on the surface of the P-type silicon  271 . P-type silicon  275  with a low density and P-type silicon  276  with a high density are formed on a surface of the N-type silicon well  274 . 
       FIG. 28  is an example of observation images acquired under two conditions of the light irradiation intensity. In the observation image acquired under the condition a, N-type silicon  282  and P-type silicon  283  can be clearly distinguished. From the observation image acquired under the condition a, types of impurities and an energy band of a material can be known. In the observation image acquired under the condition b, a difference in density can be distinguished from a difference in image brightness of N-type silicon  285  with a low density and N-type silicon  286  with a high density. Similarly, P-type silicon  287  with a low density and P-type silicon  288  with a high density can be distinguished from a difference in the image brightness. From the observation image acquired under the condition b, a density of the impurities and an electronic state of the material can be known. 
     According to the charged particle beam apparatus  1  according to the sixth embodiment, different types of the characteristics of the sample  8  can be discriminated and visualized from the observation images acquired under different conditions of the light irradiation intensity per unit time. 
     Modifications of Invention 
     The invention is not limited to the embodiments described above, and includes various modifications. For example, the embodiments described above have been described in detail for easily understanding the invention, and the invention is not necessarily limited to those including all the configurations described above. In addition, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. A part of the configuration of each of the embodiments may be added to, deleted from, or replaced with another configuration. 
     In the embodiments described above, one or more wavelengths can be selected by using, as the pulse laser  10 , a tunable laser whose wavelength can be selected by parametric oscillation. A single wavelength pulse laser may be used, or a wavelength conversion unit that generates a harmonic of light may be used. Since an image with a uniform image contrast can be acquired in an irradiation region of the light pulse, the irradiation region of the light pulse is desired to be wider than a deflection region of the electron beam controlled by the deflector  3 , but the invention is not limited to a difference between the irradiation region of the light pulse and the deflection region. The light pulse and the electron beam may be emitted simultaneously in time, or may be emitted at different timings in time. 
     In the embodiments described above, the ND filter capable of changing a density for controlling an average output of a laser can be used as the light intensity adjusting unit  11 . In addition, an optical attenuator can be used as an optical system for controlling an average output. The following can also be used as the light intensity adjusting unit  11 : (a) a pulse picker or the like that uses the electro-optic effect device and the magneto-optic effect device and is used to control a frequency of pulses and an irradiation number of the pulses; (b) a pulse dispersion control optical system or the like that is configured with a pair of prisms and is used to control a pulse width; and (c) a condenser lens that is used to control an irradiation region of a light pulse. In addition, an optical branching device, a pulse stocker, a light wavelength conversion device, a polarization control device, and the like can also be used. These devices can be used in combination. 
       FIG. 2  illustrates that an absorption intensity is obtained from a difference signal between an irradiation light and a reflection light as the light absorption characteristic, but the light intensity of the reflection light may be used. In order to acquire the difference signal, a difference may be obtained by the digital processing instead of the analog circuit. 
     In the second embodiment, the photoelectron detector  91  can be shared with the electron detector  5 . In the second embodiment, the photoelectron detector  91  and the photovoltaic current measuring device  92  are used in combination as means for measuring the photoelectrons from the sample  8 , but only one of them may be used. As the absorption characteristic measuring unit  13 , a reflection light detector from the sample  8 , a polarization plane detector of the reflection light from the sample  8 , a wavelength detector of the reflection light from the sample  8 , and the like can also be used. 
     The circuit breaker  93  can be configured with an electron beam blocking portion including a parallel electrode and a diaphragm. In the deflector  3 , the electron beam may be blocked, or a shield such as a valve on an optical axis of the electron beam may be operated. 
     In the embodiments described above, the control transmission unit  22  can be configured by using hardware such as a circuit device where a function is implemented, or can be configured by using a calculation device to execute software where a function is implemented. The same applies to the functional units (the electron gun control unit  14 , the deflection signal control unit  15 , the electron lens control unit  16 , the detector control unit  17 , the stage position control unit  18 , the pulse laser control unit  19 , the light intensity adjustment control unit  20 , the absorption characteristic measurement control unit  21 , and the like) controlled by the control transmission unit  22 . The same applies to the image forming unit  24 . 
     In the embodiments described above, the example in which the charged particle beam apparatus  1  is configured as the scanning electron microscope has been described as the configuration example for acquiring the observation images of the sample  8 , but the invention can also be used in other charged particle beam apparatuses. That is, the invention can be applied to other charged particle beam apparatuses that adjust an emission efficiency of secondary charged particles by irradiating the sample  8  with light. 
     REFERENCE SIGN LIST 
       1  charged particle beam apparatus 
       2  electron gun 
       3  deflector 
       4  electron lens 
       5  electron detector 
       6  XYZ stage 
       7  sample holder 
       8  sample 
       9  housing 
       10  pulse laser 
       11  light intensity adjusting unit 
       12  light pulse introduction unit 
       13  absorption characteristic measuring unit 
       14  electron gun control unit 
       15  deflection signal control unit 
       16  electron lens control unit 
       17  detector control unit 
       18  stage position control unit 
       19  pulse laser control unit 
       20  light intensity adjustment control unit 
       21  absorption characteristic measurement control unit 
       22  control transmission unit 
       23  operation interface 
       24  image forming unit 
       25  image display unit 
       30  beam splitter 
       31  irradiation light detector 
       32  reflection light detector 
       33  subtractor 
       34  signal detector 
       51  silicon 
       52  silicon nitride 
       61  GUI 
       66  image display unit 
       67  irradiation condition setting unit 
       68  wavelength setting unit 
       69  absorption characteristic analysis unit 
       70  absorption characteristic display unit 
       75  silicon 
       76  silicon nitride 
       91  photoelectron detector 
       92  photovoltaic current measuring device 
       93  circuit breaker 
       94  signal corrector 
       111  laser oscillator (or laser amplifier) 
       112  wavelength converter 
       113  pulse picker 
       114  pulse dispersion controller 
       115  polarization controller 
       116  average output controller 
       121  P-type silicon 
       122  N-type silicon 
       131  P-type silicon 
       132  N-type silicon 
       133  silicon oxide film 
       134  defect 
       152  P-type silicon 
       153  N-type silicon 
       156  defect 
       161  irradiation period 
       162  interval period 
       163  light pulse 
       171  irradiation period setting unit 
       172  interval period setting unit 
       181  P-type silicon 
       182  N-type silicon 
       183  silicon oxide film 
       184  contact plug 
       185  defect 
       186  defect 
       187  defect 
       192  contact plug 
       194  contact plug 
       196  defect 
       198  defect 
       199  defect 
       200  difference image 
       201  difference image 
       211  wave plate 
       212  birefringent element 
       213  photodetector 
       214  photodetector 
       215  subtractor 
       216  signal detector 
       217  diffraction grating 
       218  light intensity sensor 
       219  signal detector 
       222  organic substance 
       223  dielectric 
       225  dielectric 
       227  dielectric 
       231  energy filter 
       232  energy filter control unit 
       252  silicon 
       253  silicon nitride 
       271  P-type silicon 
       272  N-type silicon 
       273  N-type silicon 
       274  N-type silicon well 
       275  P-type silicon 
       276  P-type silicon 
       282  N-type silicon 
       283  P-type silicon 
       285  N-type silicon 
       286  N-type silicon 
       287  P-type silicon 
       288  P-type silicon