Patent Publication Number: US-11043369-B2

Title: Sample analyzer and sample analysis method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to Japanese Patent Application No. 2018-044494, filed Mar. 12, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a sample analyzer and a sample analysis method. 
     BACKGROUND 
     An atomic probe device can be used as an analysis device at the nanometer level or the atomic level. The atomic probe device can specify the material of a sample by applying a high voltage to a needle-like sharpened sample and performing mass spectrometry of ions field-evaporating from the tip portion of the sample. A laser atomic probe device can induce field evaporation by irradiating the tip portion of a sample with a laser beam. Such an atomic probe device can analyze the structure of the tip of a sample in three dimensions. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating a configuration example of a laser atomic probe device according to a first embodiment. 
         FIG. 2  is a schematic diagram illustrating a state of a tip portion of a sample in a circle indicated by a broken line in  FIG. 1 . 
         FIG. 3A  and  FIG. 3B  are perspective views illustrating configurations of blocks in model information and analysis information. 
         FIG. 4  is a flowchart illustrating an example of a sample analysis method according to the first embodiment. 
         FIG. 5  is a flowchart illustrating an example of a sample analysis method of a probe device according to a second embodiment. 
         FIG. 6A  and  FIG. 6B  are conceptual diagrams illustrating a case where a sample is irradiated with a laser beam in one direction. 
         FIG. 7A  and  FIG. 7B  are conceptual diagrams illustrating a case where a tip portion of the sample includes a plurality of materials. 
         FIG. 8  is a conceptual diagram illustrating a configuration example of a probe device according to a third embodiment. 
         FIG. 9A  and  FIG. 9B  are conceptual diagrams illustrating a detection surface of a detector and ions detected on the detection surface. 
         FIG. 10  is a conceptual diagram illustrating the detection surface of the detector, which is divided into four, and ions detected on the detection surfaces. 
         FIG. 11  is a flowchart illustrating an example of a sample analysis method according to a third embodiment. 
         FIG. 12  is a conceptual diagram illustrating a configuration example of a probe device according to Modification Example 1 of the third embodiment. 
         FIG. 13  is a conceptual diagram illustrating a configuration example of a sample analyzer according to a fourth embodiment. 
         FIG. 14  is a diagram illustrating a configuration example of the tip portion of the sample. 
         FIG. 15  is a diagram illustrating a tip portion of the sample when being irradiated with a laser beam from a direction D 2 . 
         FIG. 16  is a flowchart illustrating a sample analysis method according to the fourth embodiment. 
         FIG. 17  is a conceptual diagram illustrating a configuration example of a probe device according to Modification Example 2 of the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In a case where a tip portion of a sample includes a plurality of materials having different electrical fields (e.g., evaporation field) when performing field evaporation, the sample may not be uniformly evaporated due to the difference of the evaporation field, and the shape of the tip portion may be distorted from a hemispherical shape. In such a case, in three-dimensional reconstruction processing using measurement data, if three-dimensional reconstruction is performed on the premise that the tip of a sample is hemispherical, reconstructing the structure of the sample with accuracy is not possible. 
     Moreover, since irradiation with a laser beam is performed from one side of the sample, field evaporation easily occurs from a laser irradiation surface, but field evaporation occurs less from an opposite side of the laser irradiation surface. Thus, the laser irradiation surface of the sample is smoothened, and thus, the tip shape of the sample is deformed from the hemispherical shape. Such deformation of the tip shape of the sample hinders accurate analysis of the structure of the tip of the sample, which is called “local magnification effect.” 
     In the present disclosure, embodiments provides a sample analyzer and a sample analysis method in which it is possible to accurately analyze the structure of a sample by using field evaporation even when the tip shape of the sample deforms, and to prevent deformation of the tip shape of the sample. 
     According to one embodiment, a sample analyzer may include a voltage source that applies a voltage to a sample. A laser irradiation unit may irradiate the sample with a laser beam. A detection unit (or a detector) may detect a particle emitted from the sample. An operation unit (or operation device) may specify a material of the particle detected by the detection unit, by mass spectrometry of the particle and analyze the structure of the sample. The operation unit may calculate a ratio in structure between model information indicating the structure of the sample, which is prepared in advance, and analysis information indicating the structure of the sample, which is prepared or obtained by the mass spectrometry, and apply the ratio to the analysis information so as to correct the analysis information. 
     Hereinafter, embodiments of the disclosure will be described with reference to the drawings. The embodiments are not limited to the disclosure. The drawings are schematic or conceptual, and the ratio and the like between the components are not necessarily the same as those in practice. In the specification and the drawings, components similar to those described above with reference to the drawings are denoted by the same reference signs, and detailed descriptions thereof will be appropriately omitted. 
     First Embodiment 
       FIG. 1  is a conceptual diagram illustrating a configuration example of a laser atomic probe device  1  according to a first embodiment. The laser atomic probe device (also may be simply referred to as a probe device)  1  includes a voltage source  10 , a laser irradiation unit (or a laser irradiator)  20 , a detector  30 , and an operation unit  40  (or operation device). The components of the probe device  1  may be disposed in a vacuum chamber or on the outside of the vacuum chamber. 
     The voltage source  10  may apply a high voltage to a sample  2 . For example, the voltage source  10  applies a high voltage of 1 kV to 10 kV so as to generate a high electric field between the sample  2  and the detector  30 . For example, the sample  2  is a semiconductor material which is cut out from a semiconductor substrate, a semiconductor chip, or the like, and is sharpened to have a needle shape. The tip portion of the sample  2  may be formed to be hemispherical. 
     The probe device  1  may perform mass spectrometry of ions scattering from the tip portion of the sample  2  to the detector  30  in the high electric field by field evaporation. Thus, the probe device  1  can specify an element or an ion type of the tip portion of the sample  2  and analyze the material or the structure of the tip portion of the sample  2 . 
     The laser irradiation unit  20  may induce field evaporation of atoms from the surface of the tip portion of the sample  2  by irradiating the tip portion of the sample  2  with a laser beam  21 . For example, the laser beam  21  is an ultraviolet laser beam and is a pulsed laser which is generated periodically. 
     The detector  30  may detect ions field-evaporated from the sample  2 . For example, the detector  30  may be a position sensitive detector having a two-dimensional detection surface. The detector  30  can recognize the position of the tip portion of the sample  2 , from which the ion scatters, by detecting the ion on the detection surface. That is, the detector  30  can detect two-dimensional position information of an atom on the surface of the tip portion of the sample  2 . 
     The operation unit  40  may specify the material of a particle detected by the detector  30 , by mass spectrometry of the particle, and analyze the structure of the sample. The operation unit  40  may obtain the two-dimensional position information of an atom detected for each pulse of the laser beam  21 , from the detector  30 , and reconstruct the structure of the tip portion of the sample  2  in three dimensions. Thus, the operation unit  40  can reconstruct both the three-dimensional structure of the tip portion of the sample  2  and an element constituting the structure, at resolution of an atomic level on data. In some embodiments, the operation unit or operation device  40  may include a processor and memory configured to specify the material of a particle detected by the detector  30 , analyze the structure of the sample, obtain the two-dimensional position information, or reconstruct the structure of the tip portion of the sample  2 . 
       FIG. 2  is a schematic diagram illustrating a state of the tip portion of the sample  2  in a circle C indicated by a broken line in  FIG. 1 . A high voltage may be applied to the sample  2  by the voltage source  10 . If the tip portion of the sample  2  is irradiated with the laser beam  21 , an atom A at the tip portion may come to an ion I and may be field-evaporated. At this time, the atom A on the surface of the tip portion may be field-evaporated by approximately one atom for each pulse of the laser beam  21 . The ion I may scatter to the detector  30  in  FIG. 1  and may be detected on the detection surface of the detector  30 . At this time, the ion I may be detected at the position on the detector  30 , in accordance with the position of the atom A on the surface of the sample  2 . Thus, the atom A constituting the surface of the sample  2  and the two-dimensional position of the atom A on the surface of the sample  2  can be determined. If the two-dimensional positions of atoms A are accumulated for a plurality of pulses of the laser beam  21 , the three-dimensional configuration of the tip portion of the sample  2  can be obtained as data. 
     At this time, basically, the operation unit  40  may generate the three-dimensional configuration of the tip portion of the sample  2  as data, on the premise that the tip portion of the sample  2  is hemispherical, as illustrated in  FIG. 2 . However, as described above, in a case where the tip portion of the sample  2  includes a plurality of materials or an irradiation direction of the sample  2  with the laser beam  21  is deviated, the shape of the tip portion of the sample  2  may become a dented shape or a distorted shape from a hemispherical shape. In such a case, in three-dimensional reconstruction processing using measurement data, if three-dimensional reconstruction is performed on the premise that the tip of a sample is hemispherical, it may not be possible for the laser atomic probe device to reconstruct the structure of the sample  2  with accuracy. 
     The probe device  1  according to the first embodiment may correct analysis information obtained by the mass spectrometry, by using design data or observation data of the sample  2  as model information. The design data or observation data may be prepared in advance. The design data may be the two-dimensional structure or the three-dimensional structure of a portion of the sample  2  from design data of a semiconductor substrate or a semiconductor chip as a base of the sample  2 . The observation data may be an image of the sample  2  which is two-dimensional or three-dimensional and may be an appearance image or a transparent image. The image of the sample  2  may be obtained by capturing using any of a scanning electron microscope (SEM), a transmission electron microscope (TEM), a scanning transmission electron microscope (STEM), a scanning ion microscope (SIM), and a scanning probe microscope (SPM). Referring to such model information, a user can determine an actual surface on which density of the sample  2  changes, and an actual surface on which the material of the sample  2  changes. That is, a schematic structure provided on the surface of the sample  2  or in the sample  2  may be known. For example, the structure of a gate electrode, an impurity diffusion layer, a contact plug, or the like may be known to some extent. However, determining the structure (for example, material or density) of the sample  2  at an atomic level may not be possible with such schematic structures. 
     Analysis information obtained by mass spectrometry may be data on a two-dimensional or three-dimensional structure of the tip portion of the sample  2 , which is generated based on information of ions I detected by the detector  30 . Thus, the material or density of the sample  2  can be detected at an atomic level by using the analysis information. However, in a case where the shape of the tip portion of the sample  2  is distorted, the analysis information does not accurately indicate the structure of the sample  2 . 
     The operation unit  40  may compare model information and analysis information and calculate a ratio in structure of the sample  2 , in order to correct such analysis information. The ratio is calculated, for example, as follows. 
     Firstly, the operation unit  40  virtually divides the sample  2  into a plurality of blocks in the model information and the analysis information.  FIG. 3A  and  FIG. 3B  are perspective views illustrating configurations of blocks in model information and analysis information. A block Bm of the model information, which is illustrated in  FIG. 3A , and a block Be of the analysis information, which is illustrated in  FIG. 3B  are blocks having the same position and the same shape in the sample  2 . For example, the blocks Bm and Be may have a three-dimensional shape such as a cube or rectangular parallelepiped. The length of one side of each of the blocks Bm and Be is about 5 nm, for example.  FIG. 3A  and  FIG. 3B  illustrate the three-dimensional structure of the sample  2 . In some embodiments, the two-dimensional structure of the sample may be illustrated. 
     Reference signs F 1  to F 4  in the blocks Bm and Be indicate equal density surfaces of a certain element. The element may be any of silicon, oxygen, nitrogen, boron, phosphorus, or arsenic, for example. The element may be, for example, metal such as copper, tungsten, or the like. 
     Referring to  FIG. 3A , a first equal density surface F 1  and a second equal density surface F 2  in the block Bm may indicate different equal density surfaces of the same element. Alternatively, the first equal density surface F 1  and the second equal density surface F 2  may indicate the equal density surface (or interface) of different elements. Referring to  FIG. 3B , a third equal density surface F 3  and a fourth equal density surface F 4  in the block Be may indicate different equal density surfaces of the same element. Alternatively, the third equal density surface F 3  and the fourth equal density surface F 4  may indicate the equal density surface (or interface) of different elements. 
     The first equal density surface F 1  in the block Bm and the third equal density surface F 3  in the block Be may indicate the same equal density surface of the same element. The second equal density surface F 2  in the block Bm and the fourth equal density surface F 4  in the block Be may indicate the same equal density surface of the same element. Thus, in a case where the analysis information coincides with the model information, the first equal density surface F 1  and the third equal density surface F 3  may be displayed as the same surface at the substantially same position in the blocks Bm and Be. The second equal density surface F 2  and the fourth equal density surface F 4  may be also displayed as the substantially same surface at the substantially same position in the blocks Bm and Be. 
     However, if the tip portion of the sample  2  comes to a dented shape or a distorted shape from a hemispherical shape, the third and fourth equal density surfaces F 3  and F 4  in the analysis information may be displayed as different surfaces at different positions in the blocks Bm and Be from the first and second equal density surfaces F 1  and F 2  in the model information, respectively. 
     Thus, the operation unit  40  may be calculate a first distance Dm between the first equal density surface F 1  and the second equal density surface F 2 , and calculate a second distance De between the third equal density surface F 3  and the fourth equal density surface F 4 . The operation unit  40  may calculate a ratio (Dm/De) between the first distance Dm and the second distance De and may set the ratio (Dm/De) as a ratio between the model information and the analysis information. 
     The first distance Dm is a distance between points P 1  and P 2  which are intersection points of a line segment Lm in a direction in the block Bm with the first and second equal density surfaces F 1  and F 2 . For example, the line segment Lm may be a diagonal line passing through the center of the block Bm. Similarly, the second distance De is a distance between points P 3  and P 4  which are intersection points of a line segment Le in a direction in the block Be with the third and fourth equal density surfaces F 3  and F 4 . The line segment Le corresponds to the line segment Lm. For example, the line segment Le may be a line segment in the same direction as that of the line segment Lm and may be a diagonal line passing through the center of the block Be. The line segments Lm and Le are line segments corresponding to the blocks Bm and Be, respectively. 
     The line segments Lm and Le may be another diagonal lines or be a center line passing through the center between any facing surfaces, in the blocks Bm and Be. The operation unit  40  may select a line segment Lm which is approximately vertical to the first equal density surface F 1  or the second equal density surface F 2  from the block Bm of the model information, which is obtained in advance. The operation unit may set a line segment in the block Be, which corresponds to the selected line segment Lm, as Le. 
     The operation unit  40  may use the ratio (Dm/De) for correcting the analysis information. For example, the operation unit  40  may correct the position of data in the block Be by multiplying the entirety of data in the block Be of the analysis information by the ratio (Dm/De). The reference point for the correction may be any vertex of the block Bm or may be the center point of the block Bm. The reference point may be a point (for example, any of P 1  to P 4 ) in the equal density surface. For example, in a case where the ratio (Dm/De) is 0.95 and the center point of the block Be is set as the reference, the operation unit  40  multiplies the distance from the center point of the block Be to the position of each piece of data in the block Be by 0.95. Thus, the configuration of the block Be in the analysis information can approach the configuration of the block Bm in the model information. 
     The operation unit  40  may calculate the ratio (Dm/De) for other blocks (not illustrated) of the sample  2  and correct the position of data for each block of the analysis information by using the calculated ratio (Dm/De), in the similar manner. That is, the operation unit  40  may calculate the ratio (Dm/De) for each block of the sample  2  and correct data for each block. As a result, the analysis information can indicate a structure similar to the actual shape of the sample  2 . That is, even when the shape of the tip portion of the sample  2  is distorted, the operation unit  40  can correct the analysis information by using the model information, so as to have a configuration similar to the actual shape of the sample  2 . Thus, the probe device  1  can accurately analyze the structure of the sample  2  by mass spectrometry even when the tip portion of the sample  2  deforms. 
     In the embodiment, F 1  to F 4  in  FIG. 3A  and  FIG. 3B  indicate the equal density surfaces. However, F 1  to F 4  in  FIG. 3A  and  FIG. 3B  may be change surfaces of an element or an ion type, that is, may be interfaces between different materials. In this case, the operation unit  40  may calculate the first distance Dm between the first change surface F 1  and the second change surface F 2  on which the element or the ion type changes, in the block Bm of the model information. The operation unit  40  may calculate the second distance De between the third change surface F 3  and the fourth change surface F 4  on which the element or the ion type changes, in the block Be of the analysis information. That is, the first and second distances Dm and De may be set as distances between the interfaces. The operation unit  40  may calculate the ratio (Dm/De) between the first distance Dm and the second distance De and multiply coordinates of data in the block Be by the calculated ratio so as to correct the position of the data in the block Be. As described above, the operation unit  40  may correct the analysis information by using the surface on which the material changes. 
     Next, a sample analysis method using the probe device  1  according to the first embodiment will be described. 
       FIG. 4  is a flowchart illustrating an example of the sample analysis method according to the first embodiment. 
     Firstly, a sample  2  may be prepared from a semiconductor substrate by using a focused ion beam (FIB) method, an electrolytic polishing method, or the like (S 10 ). The tip portion of the sample  2  may be sharpened to have a needle shape. 
     Then, it may be determined whether or not observation data obtained by a microscope such as a SEM, a TEM, a STEM, and an SPM can be used as model information of the sample  2  (S 20 ). For example, in a case where density or the material changes to be small in the sample  2 , or the structure of the sample  2  is too complicated, using the observation data itself obtained by the microscope as the model information has difficulty (NO in S 20 ). In such a case, simulation data other than the observation data may be used as the model information. The sample analysis method using the simulation data will be described later with reference to  FIG. 5 . 
     In a case where the observation data obtained by the microscope can be used as the model information of the sample  2  (YES n S 20 ), the observation data in the microscope may be created as the model information (S 30 ). At this time, the model information may be created as a model of a three-dimensional structure of the sample  2 . 
     Then, the model information may be virtually divided into a plurality of boxes (on data) (S 40 ). The box is, for example, a cube having a side of about 1 nm. The operation unit  40  may calculate the composition in each box and calculates a three-dimensional equal density surface for a certain element from the composition (S 50 ). 
     Then, the operation unit may calculate a distance Dm between a plurality of equal density surfaces F 1  and F 2  in a state where the plurality of boxes is set as one block Bm (S 60 ). For example, the operation unit  40  may set 5×5×5 pieces of boxes (total 125 boxes) as one block Bm and calculate the distance Dm between the equal density surfaces F 1  and F 2 , as described with reference to  FIG. 3A  and  FIG. 3B . 
     Then, the operation unit may perform mass spectrometry of the sample  2  using the probe device  1  (S 70 ). The operation unit  40  may specify an element detected by the detector  30 , by the mass spectrometry of the element, and analyze the structure of the sample  2  based on position information detected by the detector  30  (S 80 ). Thus, the operation unit  40  can reconstruct both the structure of the tip portion of the sample  2  and the element constituting the sample  2  in three dimension on data. Data of the sample  2 , which is reconstructed by the mass spectrometry in this manner may be referred to as analysis information (or atom map). 
     Then, processing similar to processing of Steps S 40  to S 60  may be also performed on the analysis information. That is, the analysis information may be virtually divided into a plurality of boxes (on data) (S 82 ). The operation unit  40  may calculate the composition in each box and calculate a three-dimensional equal density surface for a certain element from the composition (S 84 ). The operation unit may calculate a distance De between a plurality of equal density surfaces F 3  and F 4  in a state where the plurality of boxes is set as one block Be (S 86 ). 
     Then, the operation unit  40  may calculate a ratio (Dm/De) from the distance Dm and the distance De (S 100 ). 
     The operation unit  40  may correct the analysis information by applying the ratio (Dm/De) to the analysis information (atom map) (S 110 ). For example, the operation unit  40  may obtain the corrected analysis information by multiplying the analysis information by the ratio (Dm/De). 
     Steps S 100  and S 110  may be also performed on other blocks of the tip portion of the sample  2  (NO in S 120 ). In a case where correction of the analysis information for all blocks has ended (YES in S 120 ), correction of the analysis information may end. As described above, the operation unit  40  can obtain ratio variation distribution for the entirety of the tip portion of the sample  2  and can correct the analysis information by using the ratio variation distribution. A user can analyze the structure of the sample  2  with reference to the corrected analysis information, for example, the atom map. 
     As described above, according to the first embodiment, the operation unit  40  may calculate the ratio (Dm/De) in structure of the sample  2  and correct data of the analysis information by using the calculated ratio (Dm/De). Thus, the analysis information can indicate a structure similar to the actual shape of the sample  2 . That is, even when the shape of the tip portion of the sample  2  is distorted, the operation unit  40  can correct the analysis information by using the model information, so as to have a configuration similar to the actual shape of the sample  2 . As a result, the user can accurately analyze the structure of the sample  2 . 
     According to the first embodiment, the operation unit  40  can calculate the ratio (Dm/De) of the sample  2  for each block, and locally correct data of the analysis information for each block by using the ratio (Dm/De). Thus, even when the sample  2  is locally distorted, the operation unit  40  can correct the analysis information so as to approach the actual shape of the sample  2 . The size of the block may be reduced, and the resolution may increase, in order to accurately correct the analysis information. 
     In the first embodiment, the observation data may be used as the model information. However, in a case where design data can be obtained, the design data can be used as the model information. In this case, the operation unit may divide the design data into a plurality of boxes and calculate an equal density surface for each of the boxes. The operation unit may calculate the distance Dm from the equal density surface. As described above, it is possible to correct the analysis information even by using the design data. The analysis information may be created and corrected in a personal computer or the like which is separate from the probe device  1 . 
     Second Embodiment 
       FIG. 5  is a flowchart illustrating an example of a sample analysis method of a probe device  1  according to a second embodiment. The second embodiment represents a sample analysis method in a case where using the observation data as the model information is not possible in Step S 20 . The configuration of the probe device  1  according to the second embodiment may be similar to that in the first embodiment. 
     For example, in a case where density or the material changes to be small in the sample  2 , or the structure of the sample  2  is complicated, using the observation data itself obtained by the microscope as the model information may be difficult (NO in S 20 ). In a case where obtaining the design data is not possible, creating model information from the design data also may be difficult. In such a case, in the second embodiment, observation data may be obtained by using any of the above-described microscopes, and a field evaporation simulation is performed based on the observation data (S 21 ). Data estimated by the simulation may be used as the model information (S 31 ). 
     The field evaporation simulation may include a simulation in the process of field evaporation and a simulation of calculating a scattering trajectory of an ion after the field evaporation. Thus, the ion type (e.g., chemical element) detected by the detector  30 , the detection position of the ion, and detection procedures of the ion may be simulated based on data. As the field evaporation simulation, for example, a method disclosed in “C. Oberdorfer et al., Ultramicroscopy 128 (2013) 55-67” can be used. 
     Then, the operation unit may perform Steps S 40  to S 100  in the first embodiment. 
     Then, the operation unit  40  may determine whether or not the ratio (Dm/De) of each block, which is calculated in Step S 100  is within a predetermined range (S 101 ). The predetermined range is shown by Expression 1, for example.
 
0.9&lt;Dm/De&lt;1.1  (Expression 1)
 
     In a case where the ratio (Dm/De) for a certain block is not within the range by Expression 1 (NO in S 101 ), the operation unit  40  may determine that the field evaporation simulation is not proper (S 103 ), and perform the field evaporation simulation in S 21  again. At this time, the operation unit may change the numerical value and the like of a parameter input for the field evaporation simulation (S 105 ). For example, as the parameter, at least one of the film thickness, the width, the height of a device structure in the sample  2 , the evaporation electric field of a material, or the like are provided. 
     In a case where the ratios (Dm/De) for all blocks are within the range by Expression 1 (YES in S 101 ), the operation unit  40  may perform Step S 110  and correct analysis information (atom map), similar to the first embodiment. 
     The subsequent operations may be similar to those in the first embodiment. 
     According to the second embodiment, even when using the observation data itself as the model information is not possible, three-dimensional structure data can be created by performing the field evaporation simulation with respect to the observation data. The operation unit  40  can calculate the ratio (Dm/De) and correct the analysis information, by using such three-dimensional structure data as the model information. Thus, in the second embodiment, it is possible to obtain effects similar to those in the first embodiment. 
     As the sample  2 , a sample which is capable of being analyzed may be provided. The entirety of the sample may have a needle-like shape or a protrusion shape formed on a flat surface such as a substrate. The probe device  1  may use a voltage pulse instead of the pulsed laser beam or along with the pulsed laser beam. The probe device  1  may be mounted in any of the SEM, the TEM, the STEM, and the SPM. 
     Third Embodiment 
     As described above, in a case where the tip portion of the sample  2  includes a plurality of materials or the irradiation direction of the sample  2  with the laser beam  21  is deviated, the shape of the tip portion of the sample  2  may become a dented shape or a distorted shape from a hemispherical shape by the field evaporation. 
     For example,  FIG. 6A  and  FIG. 6B  are conceptual diagrams illustrating a case where the sample  2  is irradiated with the laser beam  21  in one direction. As illustrated in  FIG. 6A , if the sample is irradiated with the laser beam  21  from one direction, field evaporation of an atom may occur from an irradiation portion. Thus, as illustrated in  FIG. 6B , a dent or distortion DST may occur at the tip portion of the sample  2 . 
     As illustrated in  FIG. 8 , a laser irradiation unit  20  according to a third embodiment may include a plurality of laser beam sources LS 1  and LS 2 .  FIG. 8  is a conceptual diagram illustrating a configuration example of a probe device  1  according to the third embodiment. The plurality of laser beam sources LS 1  and LS 2  may irradiate the sample  2  with laser beams  21  from substantially symmetric directions. The number of laser beam sources may be equal to or greater than 3. The laser beam sources may be preferably arranged around the tip of the sample  2  substantially symmetrically and uniformly. Thus, the tip portion of the sample  2  can be uniformly irradiated with the laser beam  21  and thus the laser beam  21  can cause atoms to be uniformly field-evaporated from the surface of the tip portion of the sample  2 . 
     In a case where the tip portion of the sample  2  includes a plurality of materials having different evaporation fields, even when the tip portion of the sample  2  is substantially uniformly irradiated with the laser beam  21 , a dent or distortion DST may occur at the tip portion of the sample  2  due to the difference of the evaporation field. For example,  FIG. 7A  and  FIG. 7B  are conceptual diagrams illustrating a case where the tip portion of the sample  2  includes a plurality of materials. As illustrated in  FIG. 7A , in a case where the tip portion of the sample  2  includes a plurality of materials  2   a  and  2   b  having different evaporation fields, the number of atoms subjected to field evaporation in the material  2   a  having a smaller evaporation field is more than the number of atoms subjected to field evaporation in the material  2   b  having a larger evaporation field. Thus, as illustrated in  FIG. 7B , a dent or distortion DST may occur at the tip portion of the sample  2 . 
     The operation unit  40  according to the third embodiment may change an output of each of the plurality of laser beam sources LS 1  and LS 2  based on the number of ions detected by the detector  30  or density of the ions. 
     For example,  FIG. 9A  and  FIG. 9B  are conceptual diagrams illustrating the detection surface of the detector  30  and ions I 1  and I 2  detected on the detection surface. The ion I 1  is an ion from the material  2   a  in  FIG. 7A  and  FIG. 7B . The ion I 2  is an ion from the material  2   b  in  FIG. 7A  and  FIG. 7B . In  FIG. 9A , outputs of the laser beam sources LS 1  and LS 2  may be substantially equal to each other, and the tip portion of the sample  2  may be uniformly irradiated with the laser beam  21 . However, the ion I 1  may be field-evaporated more than the ion I 2  because of the difference of the material at the tip portion of the sample  2 . In this case, the number of ions I 2  detected by the detector  30  may be smaller than the number of ions I 1 . This may result in the occurrence of a dent or distortion DST in  FIG. 7B . 
     On the contrary, the operation unit  40  in the third embodiment may perform feedback control of the outputs of the laser beam sources LS 1  and LS 2  based on the ion types (I 1  and I 2 ) detected by the detector  30  and in-surface distribution of the number of detected ions of each of the ion types. For example, as illustrated in  FIG. 9A , the detection surface of the detector  30  may be virtually divided into first and second detection regions R 1  and R 2  so as to correspond to the laser beam sources LS 1  and LS 2 , respectively. The first detection region R 1  may be a half detection surface on the laser beam source LS 1  side. The second detection region R 2  may be a half detection surface on the laser beam source LS 2  side. The sample  2  may be disposed such that the ions I 1  are detected in the first detection region R 1  and the ions I 2  are detected in the second detection region R 2 . 
     In a case where the number of detected ions I 2  or detection density thereof in the second detection region R 2  of the detector  30  is smaller than the number of detected ions I 1  or detection density thereof in the first detection region R 1 , the operation unit  40  may control the output of the laser beam source LS 2  to increase and thus increases the light intensity of the laser beam  21  from the laser beam source LS 2 . The operation unit  40  may control the outputs of the laser beam sources LS 1  and LS 2  in real time while the detector  30  detects the ions I 1  and I 2 . Thus, the number of detected ions I 2  or the detection density thereof in the detector  30  may increase. As illustrated in  FIG. 9B , the operation unit  40  may control the outputs of the laser beam sources LS 1  and LS 2  such that the number of detected ions I 1  or the detection density thereof is substantially equal to the number of detected ions I 2  or the detection density thereof. 
     As described above, according to the third embodiment, the operation unit  40  may perform feedback control of the outputs of the laser beam sources LS 1  and LS 2  based on the number of ions I 1  and I 2  detected by the detector  30  and or density of ions I 1  and I 2 . At this time, the operation unit  40  may virtually divide the detection surface of the detector  30  into a plurality of detection regions R 1  and R 2  and control the outputs of the laser beam sources LS 1  and LS 2  such that the number of ions detected in the detection region R 1  or density of the detected ions is substantially equal to the number of ions detected in the detection region R 2  or density of the detected ions. Thus, it is possible to prevent the occurrence of a dent or distortion DST on the sample  2  including a plurality of materials as illustrated in  FIG. 7B . 
     The number of divisions of the detection surface of the detector  30  may be equal to the number of laser beam sources capable of being independently controlled. For example,  FIG. 10  is a conceptual diagram illustrating the detection surface of the detector  30 , which is divided into four, and ions I 1  and I 2  detected on the detection surface. In the example of  FIG. 10 , the tip portion of the sample  2  is irradiated with the laser beam  21  from four laser beam sources LS 11  to LS 14 . The detection surface of the detector  30  is virtually divided into four, that is, first to fourth regions R 11  to R 14  so as to correspond to the four laser beam sources LS 11  to LS 14 , respectively. The operation unit  40  performs feedback control of an output of each of the laser beam sources LS 11  to LS 14  based on the number of ions detected by the first to fourth regions R 11  to R 14  and density of the detected ions. As described above, the number of laser beam sources and the number of divided regions in the detector  30  may have any values and may not be particularly limited. Thus, it is possible to prevent the occurrence of a dent or distortion DST on the sample  2 . 
     In some embodiments, the number of divisions of the detection surface of the detector  30  may be different from the number of laser beam sources capable of being independently controlled. The number of divisions of the detection surface may also be smaller than the number of laser beam sources in accordance with the structure or the material of the sample  2 . In some embodiments, the number of divisions of the detection surface may be greater than the number of laser beam sources in accordance with the structure or the material of the sample  2 . As the number of divisions of the detection surface increases, the sample  2  can be analyzed finer. 
     Next, a sample analysis method according to the third embodiment will be described. 
       FIG. 11  is a flowchart illustrating an example of the sample analysis method according to the third embodiment. For example, as illustrated in  FIG. 8  to  FIG. 9B , the sample  2  is irradiated with the laser beam  21  of two laser beam sources LS 1  and LS 2  from both sides of the sample  2 . 
     Firstly, the sample  2  may be substantially symmetrically irradiated with laser beam  21  from the laser beam sources LS 1  and LS 2 . The laser beam sources LS 1  and LS 2  may perform irradiation simultaneously or alternately (S 12 ). Then, the detector  30  may detect ions field-evaporated from the sample  2  (S 22 ). The operation unit  40  may calculate the number of ions, the number of detected atoms, density, and the like in each of the detection regions R 1  and R 2  (S 32 ). The operation unit  40  may compare the number of ions detected in the second detection region R 2 , the number of detected atoms, density, or the like to the number of ions detected in the first detection region R 1 , the number of detected atoms, density, or the like (S 42 ). A timing at which the number of ions, the number of detected atoms, the density, or the like are compared between the detection regions R 1  and R 2  may be arbitrarily determined. Regarding the comparison timing, for example, the comparison may be performed every time a predetermined period elapses, the number of detected atoms reaches a predetermined value, or the number of times of irradiation with laser reaches a predetermined value. 
     In a case where the number of ions detected in the second detection region R 2  or the density thereof is substantially equal to the number of ions detected in the first detection region R 1  or the density thereof (YES in S 42 ), the operation unit  40  may totally adjust both of the outputs of the laser beam sources without changing an output ratio between the laser beam sources LS 1  and LS 2  (S 52 ). 
     In a case where the number of ions detected in the second detection region R 2  or the density thereof is not equal to the number of ions detected in the first detection region R 1  or the density thereof (NO in S 42 ), the operation unit  40  may increase the output of the laser beam source LS 1  (or LS 2 ) on the side on which the number of ions or the density is smaller, or decrease the output of the laser beam source LS 2  (or LS 1 ) on the side on which the number of ions or the density is greater (S 62 ). 
     Steps S 12  to S 62  may repeat until the entirety of an analysis target (tip portion) of the sample  2  is subjected to field evaporation (NO in S 72 ). If the entirety of an analysis target (tip portion) of the sample  2  is subjected to field evaporation (YES in S 72 ), the analysis may end. 
     As described above, the probe device  1  may perform feedback control of the outputs of the laser beam sources LS 1  and LS 2  based on the number of ions detected by the detector  30  and or the density of the ions. Thus, the operation unit  40  can adjust the number of detected ions or the density of the ions such that the number of ions detected in the first detection region R 1  or the density of the ions is substantially equal to the number of ions detected in the second detection region R 2  or the density of the ions. As a result, it is possible to prevent deformation of the tip portion of the sample  2  from the substantially hemispherical shape. 
     MODIFICATION EXAMPLE 1 
       FIG. 12  is a conceptual diagram illustrating a configuration example of a probe device  1  according to Modification Example 1 of the third embodiment. In the third embodiment, the plurality of laser beam sources may be provided. However, in Modification Example 1, a laser beam  21  from the same laser beam source LS 3  may be divided into a plurality of laser beams  21   a  and  21   b , and the sample  2  may be irradiated with the plurality of laser beams. 
     Referring to  FIG. 12 , a laser irradiation unit  20  according to Modification Example 1 includes at least one laser beam source LS 3 , a spectroscope  25 , and mirrors M 1  to M 3 . The laser beam source LS 3  may generate a laser beam  21 . The spectroscope  25  may divide the laser beam  21  into a plurality of laser beams  21   a  and  21   b . The mirrors M 1  to M 3  as an optical system may guide the divided laser beam  21   b  and cause the sample  2  to be irradiated with the laser beams  21   a  and  21   b  from substantially symmetrical directions to each other. That is, the sample  2  may be irradiated with the laser beams  21   a  and  21   b  from both the sides of the sample  2 . 
     The operation unit  40  may control the spectroscope  25  based on the number of ions detected by the detector  30  and the density of the ions, and change the ratio of light intensity between the plurality of divided laser beams  21   a  and  21   b . That is, the operation unit  40  may perform feedback control of a spectral ratio in the spectroscope  25  based on the number of ions I 1  and I 2  detected by the detector  30  or the density thereof. Thus, in Modification Example 1, it is possible to obtain effects similar to those in the third embodiment. 
     Fourth Embodiment 
       FIG. 13  is a conceptual diagram illustrating a configuration example of a sample analyzer  1  according to a fourth embodiment. In the fourth embodiment, the sample analyzer may control a laser irradiation position for the sample  2  or a laser irradiation angle for the sample  2  based on model information which is prepared in advance and indicates the structure of the sample  2 . The model information may be similar to that in the first embodiment. 
     The sample analyzer  1  according to the fourth embodiment may further include a sample holder  4 , a sample stage  5 , and a control unit  8 . The sample holder  4  may be used for fixing the sample  2  and is attached to the sample stage  5 . 
     The sample stage  5  as a driving unit may include a first driving unit  6  and a second driving unit  7 . The first driving unit  6  may include a three-dimensional step motor driving by a piezoelectric element, for example, and thus can move the position of the sample holder  4 , that is the position of the sample  2  in any of X, Y, and Z directions. The Z direction is a direction in which the tip portion of the sample  2  is directed (or direction of a center axis). The X and Y directions indicate orthogonal coordinates perpendicular to the Z direction. 
     The second driving unit  7  may be a motor that rotates the first driving unit  6  around a Z axis. The second driving unit  7  may further include a piezoelectric element for inclining an axial direction of the sample  2  from the Z axis. Although not illustrated, a mechanism that finely adjusts an optical path of the laser beam  21  may be provided in the laser irradiation unit  20 . 
     The control unit  8  may control the first and second driving units  6  and  7  to adjust the position or the orientation of the sample  2 . 
     In the fourth embodiment, the control unit  8  may move the sample  2 , rotate the sample  2 , or changes the orientation (or direction of the center axis or tilt angle) of the sample  2 , and thus can determine an irradiation position of the laser beam  21  on the sample  2 . For example, as described with reference to  FIG. 14 , the sample stage  5  can move the sample  2  such that the position or the orientation of an interface B between different materials of the sample  2  is set to be substantially parallel to the laser beam  21 . 
       FIG. 14  is a diagram illustrating a configuration example of the tip portion of the sample  2 . The tip portion of the sample  2  may include a first material  201  and a second material  202 . The second material  202  may be sandwiched by the first material  201 . An interface B may be provided between the first material  201  and the second material  202 . 
     In this case, the control unit  8  may control the sample stage  5  such that the laser beam  21  is incident to the sample  2  so as to be substantially parallel to the interface B. Thus, the sample  2  may be irradiated with the laser beam  21  from a direction D 1  along the interface B. The direction D 1  is a direction directed toward the sample  2  in a plane which is substantially parallel to the interface B. The direction D 1  may be a direction (paper surface vertical direction in  FIG. 14 ) which is substantially perpendicular to a Z-axis direction. However, the direction D 1  may be inclined from the vertical direction to some extents so long as the direction is in a plane which is substantially parallel to the interface B. 
       FIG. 15  is a diagram illustrating the tip portion of the sample  2  when being irradiated with the laser beam  21  from a direction D 2 . As illustrated in  FIG. 15 , if the sample is irradiated with the laser beam  21  from the direction D 2  perpendicular to the interface B, a side surface of the first material  201  may be irradiated with the laser beam  21 , and thus the first material  201  may be mainly field-evaporated. In this case, field evaporation may occur from the first material  201  on one side. Thus, as illustrated in  FIG. 15 , the side surface of the first material  201  on the one side may be dented. Since the sample  2  deforms, the position and the shape of the interface, which are obtained by the operation unit  40  may be changed or distorted from those of the interface B in the original sample  2 . The manner of changing the composition in the vicinity of the interface B, which is obtained by the operation unit  40 , may also differ from that in the original sample  2 . 
     In the fourth embodiment, the orientation of the interface B may be specified in advance by using the model information such as observation data, which is obtained by the microscope. In the probe device  1 , the sample stage  5  may cause the position or the orientation of the sample  2  to be adjusted, and thus the sample  2  may be irradiated with the laser beam  21  from the direction D 1  which is substantially parallel to the interface B of the sample  2 . Thus, atoms may be field-evaporated from both the first and second materials  201  and  202 , and thus the interface B may be clearly detected by the detector  30 . Since the sample  2  is irradiated with the laser beam  21  to be substantially parallel to the interface B, even when the sample  2  locally deforms at the irradiation portion with the laser beam, the operation unit  40  can accurately obtain the position or the shape of the interface B. That is, according to the fourth embodiment, it is possible to reduce a negative effect (or local magnification effect) occurring due to local deformation of the sample  2  and to accurately analyze the material or the structure of the sample  2 . 
     In a case where the model information is not provided, the probe device  1  may start mass spectrometry by irradiating the sample  2  with the laser beam  21  while changing the laser irradiation position or the laser irradiation angle. When the laser irradiation position or the laser irradiation angle which allows the interface B to be accurately detected are determined, the sample stage  5  may fix the position of the sample  2 . Then, the probe device  1  may continue mass spectrometry in a state where the laser irradiation position or the laser irradiation angle is fixed. Even in this manner, it is possible to reduce the negative effect occurring by local deformation of the sample  2  and to accurately analyze the material or the structure of the sample  2 . 
     The probe device  1  according to the fourth embodiment can accurately analyze the material or the structure of the sample  2  by irradiating the sample  2  with the laser beam  21  from the direction suitable for the structure of the sample  2 . The fourth embodiment may be combined with any of the first to third embodiments or the modification example. Thus, in the fourth embodiment, it is possible to obtain the effects of any of the first to third embodiments and the modification example together. 
       FIG. 16  is a flowchart illustrating a sample analysis method according to the fourth embodiment. Firstly, the orientation of the interface B may be specified in advance by using model information (S 13 ). Then, the sample stage  5  may adjust the position or the orientation of the sample  2  such that the sample is irradiated with the laser beam  21  from the direction D 1  which is substantially parallel to the interface B of the sample  2  (S 23 ). Then, the laser irradiation unit  20  may irradiate the sample  2  with the laser beam  21  so as to perform mass spectrometry (S 33 ). 
     MODIFICATION EXAMPLE 2 
       FIG. 17  is a conceptual diagram illustrating a configuration example of a probe device  1  according to Modification Example 2 of the fourth embodiment. In the fourth embodiment, the irradiation position of the laser beam  21  on the sample  2  may be adjusted by controlling the sample stage  5  to change the position or the orientation of the sample  2 . 
     On the contrary, the probe device  1  according to Modification Example 2 may further include an adjustment unit  26  that adjusts the position or the angle of laser irradiation with the laser beam  21 . Other components in Modification Example 2 may be similar to the corresponding components in the fourth embodiment. Thus, according to Modification Example 2, it is possible to obtain effects similar to those in the fourth embodiment. In Modification Example 2, either or both of the first driving unit  6  and the second driving unit  7  may be omitted. 
     The irradiation direction of the laser beam  21  on the sample  2  may be adjusted similar to the fourth embodiment. Further, the adjustment unit  26  can finely adjust the irradiation angle of the laser beam  21 . Thus, in Modification Example 2, the sample  2  can be irradiated with the laser beam  21  in more various directions than that in the fourth embodiment. Thus, in Modification Example 2, it is possible to reduce the negative effect occurring by local deformation of the sample  2  and to accurately analyze the material or the structure of the sample  2 . 
     At least a portion of the sample analysis method according to the above embodiments may be configured with hardware or be configured with software. In a case where the portion of the sample analysis method is configured with software, a program implementing the function of at least a portion of the sample analysis method may be stored in a recording medium such as a flexible disk or a CD-ROM, may be read in a computer, and may be executed. The recording medium is not limited to a detachable medium such as a magnetic disk and an optical disk and may be a fixed type recording medium such as a hard disk device or a memory. The program implementing the function of at least a portion of the sample analysis method may be distributed via a communication line (also including a wireless communication) such as the Internet. The program may be encrypted or modulated. The program may be stored in a recording medium and be distributed over a wired line such as the Internet or a wireless line or stored in a recording medium for distribution. 
     While certain embodiments have been described, these embodiments have been presented byway of example only, and are not intended to limit the scope of the present disclosure. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the present disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the present disclosure.