Patent Publication Number: US-2022230842-A1

Title: Pattern measurement system and pattern measurement method

Description:
TECHNICAL FIELD 
     The present invention relates to a pattern measurement system and a pattern measurement method of measuring a 3D profile of a pattern formed on a semiconductor wafer or the like. 
     BACKGROUND ART 
     In order to increase capacity of memory devices and reduce bit costs, process shrinkage and high-level integration of semiconductor devices have been progressed so far. In recent years, in order to meet a demand for higher integration, the development and manufacture of 3D structured devices have been developed. When a planar structure is made to be three-dimensional, the device becomes thicker. For example, 3D-NAND and DRAM, the number of the stacked film layers increases. Therefore, in the process the ratio of the depth to the area in horizontal plane (aspect ratio) of a hole or a trench increases. In addition, the kinds of materials used in the device also tend to increase. 
     For example, in order to etch a very high aspect ratio hole or trench having a diameter of 50 nm to 100 nm and a depth of 3 pm or more, firstly, it is necessary to open a thick mask using a material with high selectivity. This is a process to make a template that guides a subsequent etching step, and the requirement for the process accuracy is extremely high. Subsequently, the etched mask is used as a template to perform etching to form the hole or the trench by dividing a stacked film made of different materials into one or more parts. When etching is performed in a state where a wall surface penetrating the mask or stacked film of different materials is not perpendicular to a surface, stable device performance may not be finally obtained. Therefore, confirmation of the shape of the hole of trench during and after an etching process is very important. 
     In order to know the 3D profile of the pattern, it is possible to obtain an accurate cross-sectional shape by cutting the wafer and measuring the cross-sectional shape. However, it takes time and cost to check wafer-level uniformity. Therefore, a nondestructive method of accurately measuring, in a method for measuring the dimension, the cross-sectional profile, or a 3D profile at a desired height of a pattern formed on different materials is desired. 
     Here, a general method using microscopes such as an electron microscope to observe a 3D profile without breaking a wafer includes two methods: stereo observation and top-down observation. 
     For example, in stereo observation described in PTL 1, the tilt angle of an electron beam relative to the surface of the sample is changed by inclining a sample stage or the electron beam, and measurement such as a height of a pattern and an sidewall angle of a side wall is performed by using a plurality of images obtained by different incident angles. 
     In addition, when the aspect ratio of a deep hole or a trench increases, efficiency of detecting secondary electrons emitted from a bottom decreases, and therefore, PTL 2 describes a method of measuring the depth of the hole by detecting a backscattered electron (BSE) generated by a high-energy primary electron, and using a phenomenon that the amount of BSE signals decreases with increasing the depth of the hole. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-T-2003-517199 
     PTL 2: JP-A-2015-106530 
     SUMMARY OF INVENTION 
     Technical Problem 
     In an etching step of a pattern having a high aspect ratio, it is difficult to control the shape of the side wall or the bottom. The change of dimension at the interface of different materials, taper, bowing, and twisting may appear. Therefore, not only a dimension of an upper surface or a bottom surface of the hole or the trench, but also a cross-sectional profile is an important evaluation item. In addition, since the wafer -level uniformity is required at a high accuracy level, it can be said that the key to improving a yield is to inspect and measure a wafer-level variation and to give a feedback to a device manufacturing process (for example, etching tool). 
     However, in PTL 1, measurement from a plurality of angles is indispensable, and there are problems such as an increase in measurement time and complexity of an analysis method. Moreover, since only information on edges (ends) of the pattern can be obtained, measurement of a continuous 3D profile cannot be performed. 
     In addition, PTL 2 discloses that based on a standard sample or actual measurement data with known hole depth, the depth of the bottom of the hole is measured by using a phenomenon that an absolute signal amount of transmitted backscattered electrons decreases when a hole bottom is deep. However, an intensity of a backscattered electron signal detected from a hole formed in different materials is influenced by both continuous 3D profile information inside the hole (a height to an upper surface of the pattern) and material information (the intensity of the backscattered electron signal depending on the material). Therefore, in order to obtain the depth information and a three-dimensional profile based on the intensity of the backscattered electron signal, it is not possible to measure a highly accurate cross-sectional shape or a three-dimensional shape unless these two information are separated. PTL 2 does not explain separation of the two information. 
     Solution to Problem 
     A pattern measurement system which is an embodiment of the invention is a pattern measurement system configured to measure a 3D profile of a pattern formed on a sample obtained by stacking a plurality of different materials, and includes: a storage unit configured to store, for each of these materials constituting the pattern, an attenuation coefficient indicating a probability of an electron being scattered at a unit distance in the material; and a calculation unit configured to extract an interface position where different materials are in contact with each other, an upper surface position, and a bottom surface position of the pattern in a BSE image created by detecting a backscattered electron emitted by scanning the pattern with a primary electron beam, and calculate a depth from the upper surface position to a specified position of the pattern, in which the calculation unit calculates the depth from the upper surface position to the specified position of the pattern based on a ratio of a contrast between the specified position and the bottom surface position of the pattern to a contrast between the upper surface position and the bottom surface position of the pattern in the BSE image, and an attenuation coefficient of a material at the bottom surface position of the pattern and an attenuation coefficient of a material at the specified position of the pattern, which are stored in the storage unit. 
     A pattern measurement system which is another embodiment of the invention is a pattern measurement system configured to measure a 3D profile of a pattern formed on a sample obtained by stacking a plurality of different materials, and includes: an electron optical system configured to irradiate the sample with a primary electron beam; a first electron detector configured to detect a secondary electron emitted by scanning the pattern with the primary electron beam; a second electron detector configured to detect a backscattered electron emitted by scanning the pattern with the primary electron beam; an image processing unit configured to form an image based on a detection signal of the first electron detector or the second electron detector; and a calculation unit configured to compare a cross-section profile of a side wall of the pattern extracted from a cross-sectional image of the pattern and a BSE profile which indicates a backscattered electron signal intensity from the side wall of the pattern along a predetermined direction and which is extracted from a BSE image formed by the image processing unit based on the detection signal of the second electron detector, distinguish the BSE profile according to the pattern formed in each of the materials, and obtain an attenuation coefficient of the material based on a relationship between a depth from an upper surface position of the pattern and a backscattered electron signal intensity in the distinguished BSE profile. 
     A pattern measurement method which is yet another embodiment of the invention is a pattern measurement method of measuring a 3D profile of a pattern formed on a sample obtained by stacking a plurality of different materials, and includes: previously storing, for each of materials constituting the pattern, an attenuation coefficient indicating a probability that the material and an electron are scattered at a unit distance in the material; and extracting an interface position where different materials are in contact with each other, an upper surface position, and a bottom surface position of the pattern in a BSE image created by detecting a backscattered electron emitted by scanning the pattern with a primary electron beam; and calculating a depth from the upper surface position to a specified position of the pattern based on a ratio of a contrast between the specified position and the bottom surface position of the pattern to a contrast between the upper surface position and the bottom surface position of the pattern in the BSE image, an attenuation coefficient of a material at the bottom surface position of the pattern, and an attenuation coefficient of a material at the specified position of the pattern. 
     Advantageous Effect 
     It is possible to accurately measure a cross-sectional shape or a 3D profile of a 3D structure such as a deep hole or a deep trench formed in different materials. 
     Other problems and novel features will become clear from the description of the present specification and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a pattern measurement system. 
         FIG. 2  is a diagram illustrating a principle of measuring a 3D profile of a pattern. 
         FIG. 3  is a flowchart showing a sequence of measuring the 3D profile of the pattern. 
         FIG. 4  is an example of a GUI. 
         FIG. 5A  is a diagram illustrating a method of estimating an attenuation coefficient μ using a cross-sectional image. 
         FIG. 5B  is a diagram illustrating the method of estimating the attenuation coefficient μ using the cross-sectional image. 
         FIG. 5C  is a diagram illustrating the method of estimating the attenuation coefficient μ using the cross-sectional image. 
         FIG. 6A  is a diagram illustrating a method of estimating the attenuation coefficient μ using material information. 
         FIG. 6B  is a diagram illustrating the method of estimating the attenuation coefficient μ using the material information. 
         FIG. 7A  is an example (schematic diagram) of a BSE differential signal waveform (dI/dX). 
         FIG. 7B  is a diagram illustrating a method of calculating an interface depth and a dimension. 
         FIG. 8A  is an example of a GUI. 
         FIG. 8B  is an example of an output screen of a 3D profile measurement result. 
         FIG. 8C  is an example of the output screen of the 3D profile measurement result. 
         FIG. 9A  is a flowchart of an SEM showing a sequence of measuring the 3D profile of the pattern offline. 
         FIG. 9B  is a flowchart of a calculation server showing the sequence of measuring the 3D profile of the pattern offline. 
         FIG. 10A  is an example of a pattern formed on a sample obtained by stacking a plurality of materials. 
         FIG. 10B  is an example of a pattern formed on a sample obtained by periodically stacking a plurality of materials. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a measurement system and a measurement method of measuring a cross-sectional shape or a 3D profile of a hole pattern or a trench pattern having a high aspect ratio formed in a stack made of different materials in observation or measurement of a semiconductor wafer or the like in a semiconductor manufacturing process will be described. An example of a sample to be observed is a semiconductor wafer on which a pattern is formed, but the sample is not limited to a pattern on a semiconductor and any sample that can be observed by an electron microscope or other microscopes can be applicable. 
       FIG. 1  shows a pattern measurement system of the present embodiment. An example of using a scanning electron microscope (SEM) is shown as one embodiment of the pattern measurement system. A scanning electron microscope main body is composed of an electron optical column  1  and a sample chamber  2 . As main components of an electron optical system, an electron gun  3 , which generates an electron and is an emission source of a primary electron beam energized with a predetermined acceleration voltage, a condenser lens  4  configured to focus an electron beam, a deflector  6  configured to scan a wafer (sample)  10  with the primary electron beam, and an objective lens  7  configured to focus the primary electron beam and irradiate the sample are provided inside the column  1 . In addition, a deflector  5  that deviates the primary electron beam from an ideal optical axis  3 a and deflects the deviated beam in a direction inclined with respect to the ideal optical axis  3 a to obtain an inclined beam is provided. These optical elements constituting the electron optical system are controlled by an electron optical system control unit  14 . The wafer  10 , which is a sample, is placed on an XY stage  11  installed in the sample chamber  2 , and the wafer  10  is moved according to a control signal provided by a stage control unit  15 . A system control unit  20  of a control unit  16  scans an observation region of the wafer  10  with the primary electron beam by controlling the electron optical system control unit  14  and the stage control unit  15 . 
     In the present embodiment, in order to measure a 3D profile of a deep hole or a deep trench having a high aspect ratio, the wafer  10  is irradiated with a high-energy (high acceleration voltage) primary electron beam that can reach a deep part of the pattern. The electron generated by scanning the wafer  10  with the primary electron beam is detected by a first electron detector  8  and a second electron detector  9 . Detection signals output from the detectors are separately signal-converted by an amplifier  12  and an amplifier  13 , and are input to an image processing unit  17  of the control unit  16 . 
     The first electron detector  8  mainly detects a secondary electron generated by irradiating the sample with the primary electron beam. The secondary electron is an electron excited from an atom constituting the sample by inelastically scattering a primary electron in the sample, and energy thereof is 50 eV or less. Since an emission amount of the secondary electron is sensitive to a surface shape of a sample surface, the detection signal of the first electron detector  8  mainly indicates pattern information of a wafer surface (upper surface). On the other hand, the second electron detector  9  detects a backscattered electron generated by irradiating the sample with the primary electron beam. The backscattered electron (BSE) is obtained by emitting the primary electron, with which the sample is irradiated, from the sample surface in the process of scattering the primary electron. When a flat sample is irradiated with the primary electron beam, a BSE emission rate mainly reflects material information. 
     The control unit  16  includes an input unit (not shown) and a display unit (not shown), and information necessary for measuring the 3D profile is input and the information is stored in a storage unit  19 . As will be described in detail later, cross-section information about a measurement target pattern, a material information database about materials constituting the measurement target pattern, and the like are stored in the storage unit  19 . In addition, an image output from the image processing unit  17  is also stored in the storage unit  19 . 
     As will be described in detail later, a calculation unit  18  computes an attenuation coefficient, which is a parameter for measuring a 3D profile pattern of the measurement target pattern using an image captured by the SEM (BSE image, secondary electron image) and the cross-section information about the measurement target pattern, and calculates a depth and a dimension of the measurement target pattern. 
     Although the pattern measurement system of the present embodiment can construct a three-dimensional model of a pattern, since the construction of the three-dimensional model requires high processing capability of a computer, a calculation server  22  connected to the control unit  16  via a network  21  may be provided. This enables quick three-dimensional model construction after image acquisition. Providing the calculation server  22  is not limited to the purpose of constructing a three-dimensional model. For example, when pattern measurement is performed offline, computation resources of the control unit  16  can be effectively used by causing the calculation server  22  to perform computation processing in the control unit  16 . In this case, more efficient operation becomes possible by connecting a plurality of SEMs to the network  21 . 
     A principle of measuring the 3D profile of the pattern in the present embodiment will be described with reference to  FIG. 2 . A measurement target in this embodiment is a hole pattern provided at a predetermined density in a sample  200  in which two kinds of materials having different average atomic numbers are stacked. For the sake of clarity, the figure shows only one hole pattern and a shape of the hole pattern is exaggerated. 
     In pattern shape measurement of the present embodiment, when a side wall of the hole  205  is irradiated with the primary electron beam, an electron is scattered inside the sample, and a BSE that has passed through the sample surface and jumped out is detected. When the pattern is a deep hole or a deep trench having a depth of 3 μm or more, such as 3D-NAND or DRAM, the acceleration voltage of the primary electron beam is 5 kV or more, and preferably 30 kV or more.  FIG. 2  schematically shows a state where a BSE  221  is emitted with respect to a primary electron beam  211  emitted on the sample surface (the upper surface of the pattern), a state where a BSE  222  is emitted with respect to a primary electron beam  212  emitted on an interface  201  between a material  1  and a material  2 , and a state where a BSE  223  is emitted with respect to a primary electron beam  213  emitted on a bottom surface of the hole  205 . 
     Here, a volume of a hole or a trench having a high aspect ratio, which is a cavity formed in the sample  200 , is much smaller than that in an electron scattering region in the sample, and an influence on an electron scattering trajectory is extremely small. In addition, it has been found that the primary electron beam is incident on an inclined side wall of the hole  205  at a predetermined incident angle, but when the primary electron beam has high acceleration and a small incident angle, an influence of a difference in incident angle on the electron scattering trajectory is negligible. 
     Further, it is known that the hole  205  is formed in a sample obtained by stacking different materials, and an amount of BSE generated depends on average atomic numbers of the materials. 
     That is, a BSE signal intensity  230  obtained by scanning the hole  205  with the primary electron beam depends on an average distance from an incident position of the primary electron beam to a surface, and also depends on an average atomic number of materials in the electron scattering region. A magnitude of a BSE signal intensity I can be expressed by (Equation 1). 
     [Math. 1] 
       I=I 0 e −μh   (Equation 1)
 
     Here, an initial BSE signal intensity Io is a BSE signal intensity generated at an irradiation position of the primary electron beam, and depends on the acceleration voltage of the primary electron beam, that is, the energy of the primary electron. An attenuation coefficient μ is a physical quantity that indicates a speed of attenuation, and indicates a probability that an electron and a solid material are scattered at a unit distance through which the electron passes. The attenuation coefficient μ has a value that depends on the material. A passing distance h is a depth from the sample surface (the upper surface of the pattern) to the irradiation position of the primary electron beam. 
     The detected BSE signal intensity I can be expressed as a function of an average distance h from the irradiation position of the primary electron beam to the sample surface, and the attenuation coefficient μ in this way. That is, as the irradiation position of the primary electron beam approaches the bottom surface of the hole, a distance that the electron passes through the solid becomes longer, and therefore, an energy loss increases and the BSE signal intensity decreases. In addition, a degree to which the BSE signal intensity decreases depends on materials constituting the sample. This is because for the two kinds of materials constituting the sample  200 , when the material  2  has more atoms per unit volume than the material  1 , a scattering probability of the material  2  is greater than a scattering probability of the material  1  and the energy loss also increases. In this case, there is a relationship of μ 1 &lt;μ 2  between an attenuation coefficient μ 1  of the material  1  and an attenuation coefficient μ 2  of the material  2 . 
     In other words, the detected BSE signal intensity I includes both information about a depth position at which the BSE is emitted and information about a material in the electron scattering region. Therefore, it is possible to accurately calculate depth information (stereoscopic information) of the pattern by acquiring in advance the attenuation coefficient μ for each of the materials constituting the hole pattern or the trench pattern, which is the measurement target, to remove an influence of the difference in materials included in the BSE signal intensity obtained by scanning these patterns with the primary electron beam. 
       FIG. 3  is a sequence of measuring the 3D profile of the pattern using the pattern measurement system of the present embodiment. Firstly, the wafer on which the pattern, which is the measurement target, is formed is introduced into the sample chamber of the SEM (step S 1 ). Next, it is determined whether the pattern, which is the measurement target, is a new sample for which measurement conditions need to be set (step S 2 ). In the case of a sample whose pattern can be measured according to an existing measurement recipe, the 3D profile is measured according to the measurement recipe and a measurement result is output (step S 9 ). In the case of a sample without a measurement recipe, firstly, appropriate optical conditions (acceleration voltage, beam current, beam aperture angle, etc.) are set to image the pattern (step S 3 ). Next, the number of the kinds of materials constituting the measurement target pattern is input using a GUI (step S 4 ). Imaging conditions for each of a low-magnification image and a high-magnification BSE image of the measurement target pattern are set, and the images are acquired and registered (step S 5 ). Then, structure information of the measurement target pattern is input using the GUI (step S 6 ). It is desirable to use a cross-sectional image of the measurement target pattern, but considering that such a cross-sectional image may not always be available, a plurality of structure information input methods are provided. Based on the input structure information, the attenuation coefficient μ of each of the materials constituting the target pattern is calculated and stored (step S 7 ). Subsequently, a measurement item of a three-dimensional pattern to be measured is set (step S 8 ). By the steps mentioned above, the measurement recipe for measuring the 3D profile of the pattern is ready. 
     The 3D profile is measured according to the measurement recipe, and a result of measuring the shape is output (step S 9 ). Then, it is determined whether the sample is the last sample (step S 10 ), and if the sample is not the last sample, the sequence returns to step S 1  and measurement of the next sample is started. If the sample is the last sample in step S 10 , the measurement ends. 
       FIG. 4  is an example of a GUI  400  for executing the sequence shown in  FIG. 3 . The GUI  400  has two parts including an optical condition input unit  401  and a measurement target pattern registration (Registration of target pattern) unit  402 . 
     Firstly, in setting the optical conditions (step S 3 ), the optical condition input unit  401  is used to set an optical condition currently set (Current) or an optical condition number (SEM condition No) appropriate for imaging the measurement target pattern. A plurality of optical conditions (a combination of acceleration voltage, beam current, beam aperture angle, etc.) for imaging the pattern are stored in the SEM in advance, and a user can set the optical conditions by specifying any one of the optical conditions. 
     Subsequently, the user uses the measurement target pattern registration unit  402  to register the measurement target pattern. Firstly, the number of the kinds of materials constituting the measurement target pattern is input to a material constituent input unit  403  (step S 4 ). In this example, “two kinds” is selected. 
     Subsequently, each of the low-magnification image and the high-magnification BSE image is registered as the image of the measurement target pattern (step S 5 ). A top-view image registration unit  404  includes a low-magnification image registration unit  405  and a high-magnification BSE image registration unit  408 . Firstly, the low-magnification image registration unit  405  specifies that the measurement target pattern is arranged in a center of a field of view by an imaging condition selection box  406 , and a low-magnification image  407  is imaged and registered. It is desirable that the low-magnification image  407  is a secondary electron image suitable for observing the shape of the sample surface. In addition, it is desirable to set an imaging field of view wider than a scattering region of the primary electron beam according to the acceleration voltage set in the optical conditions. For example, when measuring a periodic pattern formed on a material SiO 2 , the field of view is set to 5 μm×5 μm or more. Subsequently, the high-magnification BSE image registration unit  408  specifies that the measurement target pattern is arranged in the center of the field of view by an imaging condition selection box  409 , and a high-magnification BSE image  410  is imaged and registered. For example, the imaging conditions selected by the imaging condition selection box  409  include focus, scan mode, incident angle of a primary beam, and the like. 
     Subsequently, the structure information of the measurement target pattern is input using a structure input unit  411  (step S 6 ). As described above, a plurality of input methods for the structure information of the measurement target pattern are provided, and the user selects one of the input methods for input. 
     A first method is a method of inputting the cross-sectional image. For example, the user images a cross-sectional structure of the target pattern in advance by using SEM, FIB-SEM (focused ion beam microscope), STEM (scanning transmission electron microscope), AFM (atomic force microscope), etc., and registers the cross-sectional image from a cross-sectional image input unit  412 . A second method is a method of inputting design data. The design data of a device (CAD drawing) is registered from a design data input unit  413 . Alternatively, a file that stores the cross-sectional shape of the device maybe used, which is neither of the two methods. In this case, the file is read from a cross-section information input unit  414 . 
     On the other hand, when it is not possible to input an image including the cross-sectional structure and a cross-sectional image such as the design data, a manual input unit  415  sequentially specifies the kind of a material and film thickness at a region including an upper surface to a lower surface of the target pattern. The manual input unit  415  is provided with a layer-based input box  416 , so that material information for each layer constituting the target pattern can be input. The material information database of the material is provided in advance, and a material selection unit  417  selects a material constituting a layer, so that physical parameters of the material are automatically input from the material information database. When it is desired to actually measure and use the physical parameters of the material, the physical parameters are individually input from a user definition unit  418 . The physical parameters required for input are physical parameters required to calculate the average atomic number of the material of the layer. In addition, the film thickness of the layer is input from a film thickness input unit  419 . 
     The attenuation coefficient μ for each layer is estimated and stored based on the input structure information of the measurement target pattern, and is displayed on an attenuation coefficient display unit  420  (step S 7 ). Hereinafter, a method of estimating the attenuation coefficient μ will be described. 
     The method of estimating the attenuation coefficient μ when a cross-sectional image is input as the structure information of the measurement target pattern will be described with reference to  FIGS. 5A to 5C . Firstly, as shown in  FIG. 5A , a cross-section profile  501  of the measurement target pattern is acquired from a cross-sectional image  500 . A cross-section profile of the measurement target pattern is data obtained by representing a cross section of the pattern by coordinates (X, Z) when a width direction of the pattern is an X-axis and a depth direction perpendicular to the upper surface of the pattern is a Z-axis. The cross-section profile can be obtained by using, as a contour extraction method, a well-known method such as signal differential processing or processing by a high-pass filter. In the case of a two-dimensional image, a high-level differentiation may be used so as to react sharply to an edge. Left and right inclined portions  502  in the cross-section profile  501  are side walls of the measurement target pattern. The coordinates (X, Z) between the upper surface of the pattern and the bottom surface of the pattern corresponding to cross-section profiles of the side walls (inclined portions  502 ) of the measurement target pattern are extracted. The coordinates (X, Z) corresponding to the side walls of the measurement target pattern may be extracted by using a machine learning model. 
     Next, as shown in  FIG. 5B , a BSE profile  511  of the measurement target pattern is acquired from a high-magnification BSE image  510  along a specified orientation  512 . A BSE profile of the measurement target pattern is data obtained by representing a BSE signal intensity (X, I) along a certain direction with coordinates of a specified orientation (as an X-axis) on the horizontal axis and a BSE signal intensity I on the vertical axis. Positions of an upper surface and a bottom surface of a hole in the BSE profile  511  are determined. A first threshold value Th 1  for determining an upper surface position of the pattern and a second threshold value Th 2  for determining a bottom surface position of the pattern are set for the BSE profile  511 . The threshold values are set such that a variation of the BSE signal intensity I due to noise is minimized. For example, the first threshold value Th 1  is set as 90% of the total height of a signal waveform in the BSE profile  511 , and the second threshold value Th 2  is set as 0% of the total height of the signal waveform. It should be noted that the above-mentioned values of the threshold values are examples. 
     If a high-magnification secondary electron image is acquired at the same time as the high-magnification BSE image  510  is acquired, it is desirable to determine the upper surface position by using the high-magnification secondary electron image. Since the edges of the pattern appear in high contrast in the secondary electron image, the upper surface position can be determined with higher accuracy. Therefore, in step S 5  (see  FIG. 3 ) or step S 9 , it is desirable to simultaneously acquire the BSE image generated based on a signal detected by the second electron detector  9  and the secondary electron image generated based on a signal detected by the first electron detector  8 . When positions of the upper surface and the bottom surface of the pattern are determined in the BSE profile  511  in this way, a BSE signal waveform  515  between an upper surface position  513  and a bottom surface position  514 , that is, between the side walls of the measurement target pattern is extracted. 
     Subsequently, the side wall coordinates (X, Z) extracted from the cross-section profile  501  and the BSE signal waveform (X, I) of the side wall extracted from the BSE profile  511  are used to create a BSE profile  521  with the X coordinate as a key, the Z coordinate on the horizontal axis, and the BSE signal intensity I on the vertical axis. The BSE profile  521  (schematic diagram) thus obtained is shown in  FIG. 5C . At this time, since a pixel size in an X direction of the cross-sectional image  500  and a pixel size in an X direction of the high-magnification BSE image  510  are usually different from each other, it is necessary to adjust these pixel sizes such that these pixel sizes have the same size. For example, when a pixel size of the cross-section profile  501  is large, the data may be increased and matched by an interpolation method. 
     The BSE profile  521  has the depth direction on the horizontal axis and the BSE signal intensity on the vertical axis, and a BSE signal waveform  522  has a portion having different slopes depending on the material. Therefore, the attenuation coefficient μ of each material is calculated by classifying the BSE signal waveform in a range  523  from the upper surface to the interface and the BSE signal waveform in a range  524  from the bottom surface to the interface and fitting each BSE signal waveform to (Equation 1), and the calculated attenuation coefficient μ is stored. It should be noted that  FIG. 5C  is a schematic diagram, and in practice, there is a possibility that a clear inflection point as shown in  FIG. 5C  cannot be seen near the interface due to the influence of a plurality of material layers included in a BSE scattering region. Therefore, weighting of data near the interface may be lowered upon fitting. 
     Next, a method of estimating the attenuation coefficient μ when the structure information of the measurement target pattern is manually input will be described with reference to  FIGS. 6A and 6B . In this case, for a material often used in a semiconductor device in advance, a material density and an attenuation factor p 0  at each acceleration voltage are calculated in advance by Monte Carlo simulation and are stored in a database. The calculation is made on the material as a single layer with no pattern formed.  FIG. 6A  schematically shows a relationship between the material density and the attenuation factor μ0 when the acceleration voltage is 15 kV, 30 kV, 45 kV, 60 kV for a certain material. The attenuation factor μ0 may be stored as a table or as a relational expression. 
     The device to be measured is a device in which a pattern such as a deep hole or a deep trench is periodically formed on a stack made of different materials. The densely formed pattern influences the scattering of an electron, that is, the detected BSE signal intensity, by reducing the material density. Therefore, when the “pattern density” is defined as a ratio of an opening area of a pattern (for example, a deep hole or a deep trench) to the minimum unit area in the periodically formed pattern, it can be said that as the pattern density increases, an average density of the sample decreases due to an increase in a vacuum portion in the material. Even under the same passing distance of the scattered electron, the energy loss due to scattering with a material atom is reduced, so that the detected BSE signal intensity is increased. That is, the attenuation coefficient μ and the average density of the material are in inverse proportional relation to each other. 
     Using this relation, the pattern density is calculated based on the low-magnification image  407  of the registered measurement target pattern, and the average density of the material of each layer constituting the sample can be calculated based on the density of the material in the case of no pattern and the pattern density of the sample.  FIG. 6B  is a binarized image  601  (schematic diagram) of the low-magnification image  407 . A pixel value of the sample surface is set as 1, and a pixel value of an opening of a hole, which is a pattern, is set as 0. The pattern density is calculated by defining an individual unit  602  of a periodic pattern (defining the individual unit such that the periodic pattern is formed by being covered with the individual unit  602 ) for the binarized image  601  and calculating a ratio of the pixel having a pixel value of 0 to pixels of the entire individual unit  602 . 
     By the above procedure, the user can obtain the attenuation coefficient μ of the material for each layer constituting the pattern regardless of whether the structure information of the measurement target pattern is input as a cross-sectional image or is manually input. 
     A method of measuring the depth information (3D profile) of the pattern by using the attenuation coefficient μ of each material constituting the measurement target pattern will be described. Firstly, the BSE profile is acquired from the BSE image of the pattern formed on the sample which is the measurement target, and the positions of the upper surface and the bottom surface of the hole in the BSE profile are determined. A method of determining the positions of the upper surface and the bottom surface of the hole in the BSE profile is the same processing as described with reference to  FIG. 5B  in the creation of the measurement recipe, and the duplicated explanation will be omitted. When the upper surface position and the bottom surface position are determined, the BSE signal waveform (X, I) between the upper surface position and the bottom surface position, that is, between the side walls of the measurement target pattern is obtained, and the BSE signal waveform (X, I) is differentiated.  FIG. 7A  shows an example (schematic diagram) of a BSE differential signal waveform (dI/dX)  701  obtained by differentiating the BSE signal waveform (X, I). A discontinuous point of the BSE differential signal waveform occurs at an interface between layers of different materials, and this discontinuous point is an interface coordinate XINT in the X direction. In obtaining the interface coordinate XINT, high-level differentiation may be used so as to react sharply, or other signal processing of determining a discontinuity of a slope of the BSE signal intensity from the side wall may be performed. 
     A method of calculating an interface depth hint (distance from the upper surface of the pattern) and a dimension d thereof by using a BSE signal intensity I INT  at the interface corresponding to the interface coordinate X INT , the acquired attenuation coefficient μ 1  of the material  1  and attenuation coefficient μ 2  of the material  2  will be described with reference to  FIG. 7B . The dimension d can be obtained based on a difference between X coordinates of two points of the BSE signal waveform  711  having the BSE signal intensity I INT  On the other hand, a BSE relative signal intensity nI INT  at the interface can be represented by (Equation 2). Here, the BSE relative signal intensity nI is a signal intensity obtained by normalizing the BSE signal intensity on the upper surface of the pattern as 1 and normalizing the BSE signal intensity on the bottom surface of the pattern as 0, and is a ratio of a contrast between an interface position and the bottom surface position of the pattern to a contrast between the upper surface position and the bottom surface position of the pattern. In addition, a depth of the entire pattern is set to H. 
     
       
         
           
             
               
                 
                   
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     Thereby, a ratio of the interface depth hint to the total depth H can be obtained. Although the details are omitted here, a BSE image is acquired by obliquely emitting the primary electron beam on the sample surface, and the total depth H can be obtained based on a relationship between a tilt angle of the primary electron beam and a magnitude of a positional deviation of the bottom surface of the hole in a BSE image acquired by emitting the primary electron beam perpendicular to the sample surface and the BSE image acquired by obliquely emitting the primary electron beam. The interface depth hint can be obtained by obtaining an absolute value of the total depth H. 
     A measurable depth is not limited to the interface depth, and a dimension and a depth at any position can be obtained. Alternatively, the cross-sectional shape can be obtained by continuously obtaining the dimension and the depth. Thus, a pattern depth h at any position can be calculated using (Equation 3). 
     
       
         
           
             
               
                 
                   
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     Here, an attenuation coefficient μ* is the attenuation coefficient μ 1  when a desired depth is located above the interface, and is the attenuation coefficient μ 2  when the desired depth is located below the interface. 
     A cross-section in the X direction has been described above, but it is also possible to obtain cross-section information in a plurality of orientations by changing the orientation in which the BSE signal intensity is extracted, and it is also possible to obtain a three-dimensional model by integrating the cross-section information in a large number of orientations. 
       FIG. 8A  shows an example of a GUI  800  for executing step S 8  (item setting of shape measurement) in the sequence shown in  FIG. 3 . A dimension at a measurement position specified by a measurement position specification unit  801  is measured. In order to specify the measurement position, an interface specification unit  802  configured to specify an interface between the layers constituting the pattern and a depth specification unit  803  configured to instruct dimension measurement at a specific depth are provided. At this time, it is desirable to display the cross-section information on a pattern display unit  804  and display the specified measurement position by using a cursor  805 . In this case, the cursor  805  is moved by the user such that the measurement position can be specified based on the cross-section information. In addition, the measurement position may be specified by a side wall angle on the cross-section profile, a maximum dimension, and a depth located at the maximum dimension, and the like. Further, the measurement position specification unit  801  makes it possible to measure a plurality of positions for one pattern by adding a tag  806 . Furthermore, an orientation of the cross-section to be measured can be specified by an orientation specification unit  807 , and when a 3D profile selection unit  808  is selected, it is possible to perform measurement in a plurality of orientations and obtain a three-dimensional model. 
     An example of an output screen of a shape measurement result in the pattern measurement system according to the present embodiment will be described.  FIG. 8B  is an example of an output screen that displays a wafer-level variation of the measurement target pattern. A square in a wafer map  810  represents a region (for example, a chip)  811  in which each measured pattern is present. For example, if a measured shape is appropriate, the square is displayed in a light color, and if a degree of deviation from an appropriate value is large, the square is displayed in a dark color. Thus, it is possible to display the wafer-level variation in a list by mapping and displaying measurement results at different locations on the wafer. 
     Further, if the user wants to know the details of the measurement results, a specific region is specified on the wafer map  810 , and a dimensional value measurement result, depth (height) information, cross-section profile information, three-dimensional profile information, and the like obtained from the captured image of the measurement target pattern are displayed as shown in  FIG. 8C . In addition, it is also possible to display, in a map, a location where a measured value exceeds a specified threshold value range based on a design value. The user can efficiently obtain information by performing such various displays. 
       FIG. 1  shows an example of connecting the SEM to the calculation server  22  via the network  21 , and  FIGS. 9A and 9B  show a flow in which an image is acquired and stored by the SEM and is transferred to the connected calculation server  22 , and the calculation server  22  creates a measurement recipe and measures the 3D profile of the sample offline. The steps common to those in  FIG. 3  are indicated by the same reference numerals as those in  FIG. 3 , and the duplicated explanation will be omitted.  FIG. 9A  shows a flow executed by the control unit  16  of the SEM. The SEM main body exclusively acquires an image necessary for measurement. When there is no measurement recipe for the measurement target pattern, the acquired image is transferred to the calculation server  22  together with an image for obtaining the attenuation coefficient μ (step S 11 ). In addition, when a secondary electron image is acquired together with the BSE image, the secondary electron image is also transferred to the calculation server  22 . 
       FIG. 9B  shows a flow executed by the calculation server  22 . The image transferred from the SEM connected to the network is loaded (step S 12 ). When it is necessary to set a measurement recipe for the transferred image, steps S 4  to S 8  are executed for the low-magnification image and the high-magnification BSE image included in the transferred image, and the measurement recipe is set. According to the set measurement recipe, the 3D profile of the measurement target pattern is measured based on the BSE image acquired in step S 11  by the SEM, and the shape measurement result is output to a display unit provided in the calculation server  22  or the like (step S 13 ). In addition, when the measurement recipe is already present, only the BSE image acquired in step S 11  is transferred from the SEM, so that the 3D profile of the measurement target pattern is measured according to the existing measurement recipe, and the shape measurement result is output (step S 13 ). 
     Although the present embodiment has been described by taking a sample obtained by stacking two kinds of materials as an example, there is no limitation on the number of layers constituting the pattern for the measurement target pattern.  FIG. 10A  shows a pattern formed on a sample  900  obtained by stacking two or more kinds of materials and a BSE signal intensity (ln (I/I 0 )) thereof.  FIG. 10B  shows a pattern formed on a sample  910  obtained by alternately stacking a material A and a material B and a BSE signal intensity (ln (I/I 0 )) thereof. There is no limit to the number of layers. In each case, an interface between materials is clearly indicated by the BSE signal intensity, and the 3D profile can be effectively measured by the measurement method of the present embodiment. 
     In contrast, the interface between different materials may be obscured. The first case is a case where atomic numbers and densities of a first material and a second material forming two adjacent layers are similar. In this case, attenuation coefficients of the two materials are similar, and it is difficult to separate the two materials. The second case is a case where a film thickness is thin. When a film thickness of a layer is thin and a distance traveled until the electron is scattered once in the sample involves a plurality of layers of materials, even when attenuation coefficients of the materials are significantly different from each other, the interface cannot be clearly indicated. When a difference in the attenuation coefficients with respect to the height of the side wall cannot be distinguished in this way, it is preferable to treat the layers as one layer and measure the 3D profile. 
     The invention has been described above with reference to the drawings. However, the invention should not be interpreted as being limited to description of the embodiments described above, and the specific configuration of the invention can be changed without departing from the spirit or gist of the invention. That is, the invention is not limited to the described embodiments, and may include various modifications. The described embodiments are described in detail in the configuration in order to clearly describe the invention, but the invention is not necessarily limited to an embodiment that includes all the configurations that have been described. In addition, a part of the configuration of each embodiment can be added to, deleted from, or replaced with the other configurations as long as no conflict arises. 
     Further, the position, size, shape, range, etc. of each configuration shown in the drawings and the like may not represent the actual position, size, shape, range, etc. so as to facilitate understanding of the invention. Therefore, the invention is not limited to the position, size, shape, range, etc. disclosed in the drawings and the like. 
     Furthermore, the embodiments show the control line and information line considered as necessary for the explanation, and all the control lines and information lines on the product are not always shown. For example, all of the configurations maybe mutually connected. 
     Moreover, the configurations, functions, processing units, processing means, and the like described in the present embodiments may partially or entirely be implemented by hardware by, for example, designing in the form of an integrated circuit. Alternatively, the configurations, functions, processing units, processing means, and the like may partially or entirely be implemented by program codes of software. In this case, a storage medium on which the program codes are recorded is provided to a computer, and a processor that the computer is provided with reads the program codes stored on the storage medium. In this case, the program codes themselves read from the storage medium realize the functions according to the embodiments mentioned above, and the program codes themselves and the storage medium storing the program codes constitute the invention. 
     REFERENCE SIGN LIST 
       1  electron optical column 
       2  sample chamber 
       3  electron gun 
       3   a  ideal optical axis 
       4  condenser lens 
       5 ,  6  deflector 
       7  objective lens 
       8  first electron detector 
       9  second electron detector 
       10  wafer 
       11  XY stage 
       12 ,  13  amplifier 
       14  electron optical system control unit 
       15  stage control unit 
       17  image processing unit 
       18  calculation unit 
       19  storage unit 
       20  system control unit 
       21  network 
       22  calculation server 
       200 ,  900 ,  910  sample 
       201  interface 
       205  hole 
       211 ,  212 ,  213  primary electron beam 
       221 ,  222 ,  223  BSE 
       230  BSE signal intensity 
       400 ,  800  GUI 
       401  optical condition input unit 
       402  measurement target pattern registration unit 
       403  material constituent input unit 
       404  top-view image registration unit 
       405  low-magnification image registration unit 
       406 ,  409  imaging condition selection box 
       407  low-magnification image 
       408  high-magnification BSE image registration unit 
       410 ,  510  high-magnification BSE image 
       411  structure input unit 
       412  cross-sectional image input unit 
       413  design data input unit 
       414  cross-section information input unit 
       415  manual input unit 
       416  layer-based input box 
       417  material selection unit 
       418  user definition unit 
       419  film thickness input unit 
       420  attenuation coefficient display unit 
       500  cross-sectional image 
       501  cross-section profile 
       502  inclined portion 
       511  BSE profile 
       512  orientation 
       513  upper surface position 
       514  bottom surface position 
       515  BSE signal waveform 
       521  BSE profile 
       522  BSE signal waveform 
       523 ,  524  range 
       601  binarized image 
       602  individual unit 
       701  BSE differential signal waveform 
       711  BSE signal waveform 
       801  measurement position specification unit 
       802  interface specification unit 
       803  depth specification unit 
       804  pattern display unit 
       805  cursor 
       806  tag 
       807  orientation specification unit 
       808  3D profile selection unit 
       810  wafer map 
       811  region