Patent Publication Number: US-2020292298-A1

Title: Height Measuring Device and Beam Irradiation Device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese patent application JP2019-043297 filed on Mar. 11, 2019, the content of which is hereby incorporated by reference into this application. 
     TECHNICAL FIELD 
     The present disclosure relates to a height measurement device, and more particularly, to a height measurement device and a beam irradiation device which measure the height based on detection of a reflected light ray obtained when a measurement target is irradiated with a light ray. 
     BACKGROUND ART 
     Japanese Patent No. 4426519 (corresponding U.S. Pat. No. 7,599,076) discloses a height measuring device mounted on a measuring device such as a Critical Dimension-Scanning Electron Microscope (CD-SEM). Japanese Patent No. 4426519 (corresponding U.S. Pat. No. 7,599,076) discloses a height measuring device which detects the height of a measurement target object by projecting a two-dimensional slit light ray from obliquely above the measurement target object and detecting a reflected light ray of the measurement target object. 
     SUMMARY OF INVENTION 
     Technical Problem 
     According to a height detecting method disclosed in Japanese Patent No. 4426519 (corresponding U.S. Pat. No. 7,599,076), it is possible to capture the change in the height of a sample and adjust the device conditions so as to compensate for the change. However, due to the configuration in which a light ray is emitted from an inclined direction, when there is a large difference in height between a reference height and a sample surface, a position different from a position where the height measurement should be originally performed is irradiated with the light ray. Further, the size of a detection surface of a detector which receives a reflected light ray is limited, and thus the range in which the height can be measured with high accuracy is limited. 
     The following proposes a height measuring device of which an object is to measure the height with high accuracy at each height with a relatively simple configuration even when a sample surface height changes greatly. 
     Solution to Problem 
     As an aspect for achieving the object described above, a height measuring device which includes a projection optical system configured to project a light ray onto an object to be measured, a detection optical system including a detection element configured to detect a reflected light ray from the object to be measured, and a processing device configured to measure a height of the object to be measured based on an output of the detection element, where the projection optical system includes a light splitting element which splits a trajectory of the light ray with which the object to be measured is irradiated into a plurality of parts, or a beam irradiation device equipped with the height measuring device is proposed. 
     Further, as another aspect for achieving the object described above, a height measuring device which includes a sample stage configured to place an object to be measured, a drive mechanism for moving the sample stage in a normal line direction of a surface of the object to be measured, a projection optical system configured to project a light ray onto the object to be measured, a detection optical system including a detection element configured to detect a reflected light ray from the object to be measured, and a processing device configured to measure a height of the object to be measured based on an output of the detection element, where the projection optical system includes a switching element configured to switch a trajectory of the light ray with which the object to be measured is irradiated, and the switching element is configured to switch the trajectory of the light ray so as to make a position of an irradiation point of a light ray when the object to be measured is positioned at a first height coincide with a position of an irradiation point of a light ray with respect to the object to be measured when the object to be measured is positioned at a second height different from the first height with the drive mechanism, or a beam irradiation device equipped with the height measuring device is proposed. 
     Advantageous Effects of Invention 
     According to the configuration described above, even when the sample height changes greatly, the height can be measured with high accuracy at each height. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating an example of a height measuring device including a light splitting element which splits a light ray into a plurality of light paths. 
         FIG. 2  is a view illustrating an example of the height measuring device which measures the height of an object to be measured placed on a Z stage. 
         FIG. 3  is a view illustrating an example of the light splitting element. 
         FIG. 4  is a view illustrating an example of a height measuring device including a light splitting element which splits a light ray into a plurality of optical paths and an adjustment element which matches a focusing condition of one divided optical path with another optical path. 
         FIG. 5  is a view illustrating an example of a height measuring device including a light splitting element which splits a light ray into a plurality of optical paths and including an element which focuses a plurality of reflected light trajectories at one position of a detection element. 
         FIG. 6  is a view illustrating an example of a height measuring device including a light splitting element which splits a light ray into a plurality of optical paths and including a plurality of detection elements. 
         FIG. 7  is a view illustrating an example of a height measuring device including a switching element which switches a light trajectory. 
         FIG. 8  is a view illustrating an example of a height measuring device including a switching element which switches a light trajectory. 
         FIG. 9  is a view illustrating an example of a two-dimensional slit used in the height measuring device. 
         FIG. 10  is a view illustrating an example of a scanning electron microscope provided with the height measuring device. 
         FIG. 11  is a flowchart illustrating a process of adjusting the height of an object to be measured in accordance with the acceleration voltage and measuring the height of the object to be measured after the adjustment with the height measuring device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In CD-SEM or SEM type inspection equipment used to measure fine pattern widths and hole diameters in a semiconductor manufacturing process, when the focus of the electron beam is not on the sample, the apparent pattern width and hole diameter change, and thus correct length measurement will not be possible. Further, in defect detection by pattern comparison, there is a risk that a correct pattern is erroneously detected as a defect. Therefore, focusing of the electron beam is important. However, in automatic focus control which automates general focusing, the focal position of an objective lens is changed and an electron beam image is detected at each point, and then the in-focus position is detected from the electron beam image. As a result, auto focusing takes time. 
     Throughput is also an important performance index for the CD-SEM or SEM type inspection equipment used in semiconductor manufacturing processes. When focus adjustment is performed only by automatic focusing using image processing or the like, a corresponding time is required, which is not preferable. Therefore, by mounting an optical height measurement device on a CDSEM or the like, the approximate height of a sample is measured and a lens is controlled so that the height is in focus, in such a manner that it is desirable to minimize the amount of focus variation during automatic focus adjustment. 
     The optical height measuring instrument is inferior in accuracy to the automatic focusing of the electron beam using image processing or the like, but is excellent in terms of measurement time. That is, when the measurement accuracy of the optical height measuring instrument can be improved, it can contribute to the improvement of the throughput of the CD-SEM device or the SEM type inspection device. 
     When an oblique projection and oblique detection type optical height measuring device is used as the height measuring device, the irradiation position on the sample changes when the height fluctuation amount of the sample (wafer) increases. For example, in a case of a device which measures the height of the electron beam irradiation position with high accuracy by making the electron beam irradiation position the same as the light projection position of the optical height measuring device, when the sample height changes significantly, the light irradiation position deviates from the electron beam irradiation position and the height measurement for adjusting the focus of the electron beam will not be performed properly. In addition, in a case of a detection optical system which displaces and expands the light ray reflected by a sample surface with an expansion optical system, when the variation in height is large, the arrival position of the light ray on a sensor surface also changes greatly. As a result, a large sensor is required. 
     The following describes a height measuring device which realizes a wafer movement amount in millimeters while maintaining a measurement accuracy of hundreds of nanometers with a simple optical system, having low cost, high accuracy, and practical detection speed. 
     the examples below describe a height measuring device in which a light ray (for example, a two-dimensional slit image) is projected from a direction (in a case of a scanning electron microscope, a direction inclined from an electron beam optical axis) inclined with respect to an object (sample surface) and the reflected light obtained as a result of reflection of the projected light from the object is detected, and further the projected image of the detected light ray is converted into an electric signal by a two-dimensional area sensor or the like, and then the height is detected by calculation from the electric signal. 
     Furthermore, the following example describes a height measuring device in which a projection optical system includes a light splitting element (for example, an opening forming member provided with a plurality of openings including a plurality of slits) which splits a light ray into a plurality of optical paths and which of the reflected light rays of the divided lights is selected to be used for height measurement according to the sample height. 
     Also, the light ray split by the splitting element provided in the projection optical system is split on the premise that different sample heights are irradiated with the split light ray, and thus the optical path length (distance from a light source to the sample) changes. Therefore, a height measuring device provided with an adjustment element for matching the condition of the other optical path with one optical path to make the condition of the divided light ray uniform will be described. 
     Furthermore, a height measuring device provided with a switching element which switches the trajectory of light rays so that an irradiation point of the light ray to the sample is maintained between when the sample is located at a first height and when the sample is located at a second height different from the first height will be described. 
     A height measuring device which includes a slit forming member in which a plurality of two-dimensional slits in which a plurality of slits are periodically arranged are formed and splits a light ray into a plurality of parts by selectively allowing the light ray to pass through the plurality of two-dimensional slits by projecting the light ray onto the plurality of two-dimensional slits will be described. 
     According to the configuration described above, height detection can be performed with a resolution of several tens of nanometers in a measurement range of several hundreds of micrometers with respect to a plurality of sample reference heights separated by millimeters. 
     By arranging a plurality of periodic slits according to a plurality of sample reference heights separated by millimeters, with a simple structure, an optical height detector capable of measuring heights with a resolution of several tens of nanometers at a plurality of sample reference heights separated by millimeters can be configured. 
     In the examples described below, an example in which the reference height is two will be described. However, the reference height may be three or more and the optical path may be divided into three or more. 
     EXAMPLE 1 
       FIGS. 1 to 3  are views for illustrating a height measuring device provided with a dividing element which divides a light ray emitted from a light source into a projection optical system so as to correspond to different sample heights. The height measuring device is provided with a light source  101 , and a light ray emitted from the light source  101  passes through a lens  102  and is condensed at a position of a diaphragm  104  through a double multi-slit  103  (a plurality of two-dimensional slits) described below. 
     The double multi-slit  103  is provided to divide the light trajectory into a plurality (two in this embodiment) of parts.  FIG. 3  is a view illustrating an example of the double multi-slit. In this example, two multi slits  118  in which a plurality of single slits are arranged are formed on a slit forming member  301  having a circular shape. By selectively allowing the light ray irradiated to the slit forming member  301  to pass through the two multi-slits  118  (two openings), the optical path is divided into a plurality of parts. 
     A plurality of light rays which have passed through the diaphragm  104  are collected by a projection lens  105  and their trajectories are changed so that the trajectories of the two light rays are parallel to each other and irradiated on the sample. The projection lens  105  not only collects the light rays on the sample, but also unifies the directions of the divided light rays, in such a manner the projection lens  105  adjusts the trajectory so that the same position is irradiated with the light rays even when the samples have different heights. For example, when the height measuring device illustrated in  FIG. 1  is mounted on a scanning electron microscope, whether the object to be measured is at either a first reference height  106  or a second reference height  107 , it is configured to irradiate a point that intersects an ideal optical axis  111  of the electron beam. That is, even when the heights are different, the optical path is divided so that the irradiation positions of the light rays to the object to be measured match. 
     The light rays whose trajectory is adjusted by the projection lens  105  is incident obliquely on the object to be measured at the first reference height  106  or the object to be measured at the second reference height  107 , and then the incident light ray forms an image of one multi-slit of the double multi-slits near a surface of the object to be measured at the first reference height  106  or the object to be measured at the second reference height  107 . 
     The two-dimensional slit image reflected from the surface of the object to be measured at the first reference height  106  or the object to be measured at the second reference height  107  is re-imaged on the surface of a two-dimensional image sensor element (detection element)  109  by a detection lens  108  included in a detection optical system. 
       FIG. 2  is a view illustrating an example of a system including a stage mechanism for moving the object to be measured and a height measuring device for the object to be measured.  FIG. 2  illustrates, for example, a part of the configuration mounted on a scanning electron microscope. The object to be measured at the first reference height  106  is mounted on a stage mechanism including a sample suction mechanism  112  such as an electrostatic chuck, an X-Y stage  113 , and a Z stage  114 . The Z stage  113  incorporates a drive mechanism for moving the object to be measured in the direction of the ideal optical axis  111  of the electron beam. 
     A control device  117  controls the light source  101  to emit a light ray from the light source  101  toward the sample and controls a stage drive power supply  115  so as to move the X-Y stage  113  and the Z stage  114  based on preset conditions. 
     An optical path  110  illustrated in  FIG. 2  is set so as to intersect with the ideal optical axis  111  of the electron beam. Also, the light ray passing through the optical path  110  is divided into two by the double multi-slit  104  and the trajectory of the light ray is bent by the projection lens  105 , and then the light ray passes through the trajectory parallel to the optical path  110  and is projected onto the object to be measured. It is configured such that one of the two divided light rays intersects with the ideal optical axis  111  of the electron beam on the surface of the object to be measured when the object to be measured is located at the first reference height  106  and the other light ray intersects with the ideal optical axis  111  of the electron beam on the surface of the object to be measured when the object to be measured is located at the second reference height  107 . 
     The two-dimensional imaging sensor  109  is installed at a position where the reflected light from the object to be measured is received and detects a change in the position where the reflected light reaches. The output (electric signal) of the two-dimensional imaging sensor  109  is transmitted to a height arithmetic processing unit  116 , and based on an arithmetic expression described below, the lens condition (excitation current, applied voltage, DAC value, and the like) of an object lens of an electron microscope is calculated based on a parameter indicating the relative change from the height of the object to be measured or the reference height. Equation 1 is stored in advance in a predetermined storage medium. The output of the height arithmetic processing unit  116  is transmitted to the control device  117  and the control device  117  controls the lens based on the obtained height information or the lens condition. 
     The height arithmetic processing unit  116  calculates a relative height ΔZ with respect to the reference height based on the arithmetic expression illustrated in Equation 1. 
     
       
         
           
             
               
                 
                   
                     Δ 
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                     Z 
                   
                   = 
                   
                     
                       Δ 
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                        
                       S 
                       × 
                       p 
                     
                     
                       2 
                        
                       m 
                       × 
                       sin 
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                       θ 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
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     In the equation, θ is the relative angle (incident angle) of the optical path  110  with respect to an electron beam optical axis  111  (normal line to the surface of the object to be measured) and ΔS is the amount of movement of the center of gravity coordinate of the slit image reaching the detection surface of the two-dimensional imaging sensor  109  with respect to the reference position. Further, p is the pixel size of the pixels included in a two-dimensional area sensor  109  and m is the imaging magnification of the imaging optical system after being reflected by the object to be measured. In this example, an example using a two-dimensional area sensor in which a plurality of pixels are arranged in a matrix has been described. However, a plurality of one-dimensional line sensors may be arranged to form the two-dimensional sensor. 
     In this example, the height is measured at each of the first reference height and the second reference height. Therefore, at least two reference heights (reference positions on the sensor (detection element)) are set in advance and the relative height is obtained from the difference (movement amount). 
     In a length measurement or inspection device using an electron beam, high accuracy in height measurement is required in order to obtain a more stable high-quality image. Also, there is a need to observe the internal structure of the sample by highly accelerating the electron beam. For example, as the energy (acceleration energy) of the electron beam reaching the sample is higher, the electron beam reaches the inside of the sample, and thus information that cannot be seen from the sample surface can be visualized. In addition, high resolution is required for a device which performs length measurement and inspection using an electron beam. In order to increase the resolution of the electron beam device, it is desirable to shorten the distance (working distance: WD) between the sample surface and the object lens. 
     However, the object lens converges the electron toward the ideal optical axis and focuses the beam, so that the higher the beam acceleration, the stronger the convergence action is required. Also, when the working distance is short, it is necessary to converge the electrons at a short distance, so that a strong convergence action is still necessary, and it is difficult to achieve both shortening of the working distance and high energy of the beam. 
     Therefore, this example describes an electron beam device which controls the position of the stage in the height direction so that the working distance is shortened by giving priority to resolution during measurement and inspection using a low-energy beam and the working distance is lengthened by giving priority to the action of converging electrons during measurement and inspection using a high-energy beam. 
       FIG. 10  is a schematic configuration diagram of a scanning electron microscope which is an example of a charged particle beam apparatus. The scanning electron microscope is mainly constituted of an electron beam column which is abeam irradiation optical system, a vacuum sample chamber  1007  for maintaining the atmosphere of the object to be measured in a vacuum state, and the control device  117  for controlling optical elements included in the electron beam column to be described below. An electron beam extracted from an electron source  1001  by an extraction electrode  1002  and accelerated by an acceleration electrode (not illustrated) is incident on a condenser lens  1004 , which is a form of a focusing lens, along the ideal optical axis  1003  of the beam and focused, and then the electron beam is scanned one-dimensionally or two-dimensionally on a sample  1009  by a scanning deflector  1005 . The electron beam is decelerated by the negative voltage applied to the electrode built in a sample stage  1008  and is focused by the lens action of an object lens  1006  and irradiated onto the sample  1009 . 
     When the sample  1009  is irradiated with the electron beam, secondary electrons and electrons  1010  such as backscattered electrons are emitted from the irradiated portion. The emitted electrons  1010  are accelerated in a direction of the electron source by an acceleration action based on a negative voltage applied to the sample and collide with a conversion electrode  1012  to generate secondary electrons  1011 . The secondary electrons  1011  emitted from the conversion electrode  1012  are captured by the detector  1013  and an output I of the detector  1013  changes depending on the amount of captured secondary electrons. Depending on the output I, the brightness of a display device (not illustrated) changes. For example, when a two-dimensional image is formed, an image of the scanning region is formed by synchronizing the deflection signal to the scanning deflector  1005  with the output I of the detector  1013 . The scanning electron microscope illustrated in  FIG. 10  includes a deflector (not illustrated) which moves the scanning region of the electron beam. 
     The example of  FIG. 10  demonstrates an example which converts the electron discharged from the sample once by the conversion electrode and detects it. However, needless to say, it is not limited to such a configuration. For example, it is also possible to adopt a configuration in which a detection surface of an electron multiplier tube or a detector is arranged on an accelerated electron trajectory. The control device  117  controls each component of the scanning electron microscope and includes a function of forming an image based on detected electrons and a function of measuring a pattern width of a pattern formed on the sample based on a detected electron intensity distribution called a line profile. 
     Further, the electron beam apparatus illustrated in  FIG. 10  includes a height measuring device including the light source  1015  and a light receiving element  1016 . This height measuring device is the same as that illustrated in  FIG. 2 , for example. A sample chamber  1007  incorporates an x-y drive mechanism for moving the sample stage  1008  on which a sample (object to be measured) is placed in an x-y direction and a z drive mechanism for moving the sample stage  1008  in a z direction. Those drive mechanisms are controlled by a control signal supplied from the control device  117 . 
     An optical element constituting the height measuring device is installed such that the beam trajectory of the projection optical system of the height measuring device and the beam trajectory of the detection optical system are mirror-symmetric with respect to a virtual plane including the ideal optical axis  1003 . 
     Furthermore, the control device  117  controls the z drive mechanism based on the output of the height measuring device and also controls the excitation current of the object lens  1006  based on the output of the height measuring device (Z sensor). The control device  117  controls the lens condition (excitation current) of the object lens  1006  based on the relationship information between the output of the height measuring device stored in advance and the control signal of the object lens  1006 . 
       FIG. 11  is a flowchart illustrating a process of controlling the scanning electron microscope so as to perform stage control and height measurement according to set beam conditions (acceleration voltage or energy reaching the sample of the beam). First, the control device  117  reads a condition such as an acceleration voltage set in advance by an operation program (recipe) or the like. Then, when low acceleration is set, the z drive mechanism is controlled so that WD 1 (&lt;WD 2 ) is satisfied, and when high acceleration is set, the z drive mechanism is controlled so that WD 2 (&gt;WD 1 ) is obtained. Here, the determination of high acceleration or low acceleration may be determined by a preset threshold value. Alternatively, it may be performed such that identification information indicating whether the acceleration is high or low is be given in advance to a specific acceleration voltage and the determination may be made based on the identification information. Further, the working distance or the sample height (for example, a difference in height between a predetermined reference position and the sample surface) may be directly set and the z drive mechanism may be controlled based on the setting. 
     Next, height measurement using a Z sensor is performed. In this case, when WD 1  is set in the recipe, ΔZ 1 =(ΔS 1 ×p)/(2×m×sin θ) is calculated as a value related to the sample height, and when WD 2  is set in the recipe, the difference between the sample surface height and the reference height is obtained by calculating ΔZ 2 =(ΔS 2 ×p)/(2×m×sin θ). S 1  is the difference (difference in the center of gravity of the slit image) between the reference position on the two-dimensional imaging sensor and the actual light arrival position when the object to be measured is positioned at the first reference height  106  and S 2  is the difference between the reference position on the two-dimensional imaging sensor and the actual light arrival position when the object to be measured is positioned at the second reference height  107 . As illustrated in  FIG. 1 , when the object to be measured is positioned at the first reference height  106  and when the object to be measured is positioned at the second reference height  107 , the arrival position of the reflected light ray with respect to the two-dimensional imaging sensor  109  is different. Therefore, a reference position is set for each height. 
     In this example, although an example of performing the calculation using the arithmetic expression described above is described, a table indicating the relationship between ΔZ and ΔS may be created in advance and ΔZ may be output by referring to the table. The control device  117  specifies the lens condition using a relational expression between ΔZ and the lens condition of the object lens, or a table, and controls the object lens  1006 . 
     In addition, the arithmetic expression or table which shows the relationship between, instead of ΔZ, ΔObj (difference in excitation current (beam focusing condition) of the object lens) and ΔS is prepared and the excitation current of the object lens may be adjusted based on the input of ΔS. 
     In a case of an apparatus which adjusts the focus by controlling the z drive mechanism, the z drive mechanism may be controlled to cancel the ΔZ described above. Alternatively, an arithmetic expression or table showing the relationship between AS and the stage movement amount may be prepared and the z drive mechanism may be controlled using these information. 
     In this example, a scanning electron microscope is described as an example of a type of beam irradiation apparatus, but application to an optical inspection apparatus or the like is also possible. In particular, the invention can be applied to a case where a sample having a large difference in height is measured, or to various apparatuses including a z stage which moves an object to be measured in the z direction as necessary as described above. 
       FIG. 4  is a view illustrating an example in which correction elements for unifying optical conditions between an optical system (first irradiation optical system) for irradiating a light ray to the object to be measured positioned at first reference height  106  and a detection optical system (first detection optical system) for detecting a reflected light ray, and a correction element for unifying optical conditions are respectively installed in the optical system (second irradiation optical system) for irradiating a light ray to the object to be measured positioned at the second reference height  107  and the detection optical system (second detection optical system) for detecting the reflected light ray. 
     In a case of the height measuring device illustrated in  FIG. 1 , it is possible to divide the optical path into two and irradiate the same part (electron beam irradiation point) in the same direction even at different heights. However, the optical path length between the projection lens  105  and the object to be measured and the optical path length between the object to be measured and the detection lens  108  are different. That is, when the projection lens  105  is focused on the surface of the object to be measured positioned at the first reference height  106 , the surface of the object to be measured positioned at the second reference height is not focused. 
       FIG. 4  is a view illustrating an example in which an adjustment element for adjusting the focal position is installed in the optical path of the second irradiation optical system in order to enable the focal point of the projection lens  105  to be appropriately positioned also on the object to be measured at the second reference height  107 . The adjustment element is, for example, a glass member whose light incident surface and light emission surface are both flat. By inserting the glass member, the focal position can be positioned farther than when the glass member is not inserted. 
     By inserting the glass member having flat ends (incident surface and emission surface), the position where the light ray is focused can be changed by the refractive effect of the glass. 
     In the configuration illustrated in  FIG. 4 , by inserting a glass  119  on the light source side with respect to the object to be measured at the second reference height  107 , the position where the image of the double multi-slit  103  is formed and the position of the surface of the object to be measured at the second reference height  107  can be matched. More specifically, by inserting an element (element which extends the optical path) which extends the focal length by the amount of Opt_ext 1  into the optical path with respect to the first irradiation optical system, the focusing conditions of the two irradiation optical systems can be matched. As a result, highly accurate height measurement can be performed regardless of the set height of the object to be measured. The Opt_ext 1  can be obtained by Equation 2. |WD 1 −WD 2 | 0  is a difference (WD) between the first height and the second height. 
     
       
         
           
             
               
                 
                   
                     Opt 
                     
                       ext 
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     Similarly to the irradiation optical system, the distance between the object to be measured and the detection lens  108  is different between the first detection optical system and the second detection optical system. Therefore, regardless of the set height of the object to be measured, in order to make the reflected light ray enter the detection lens  108  under the same conditions, as a correction element, a glass  120  is disposed between the object to be measured of the second detection optical system and the detection lens  108 . Further, since the second detection optical system has a longer distance between the object to be measured and the detection lens  108  than the first detection optical system by the length obtained by Equation 2, a correction element which makes the imaging state of the second detection optical match the imaging state of the first detection optical system is employed. 
     After being reflected by the object to be measured at the second reference height  107 , by inserting a glass  120  in relation to the detection lens  108 , both the light ray reflected by the object to be measured at the first reference height  106  and the light ray reflected by the object to be measured at the second reference height  107  can be imaged on the surface of the two-dimensional image sensor element  109  in focus at the same time. 
     Here, the glass  119  and the glass  120  may have smooth flat surfaces with both ends (light incident surface and light emission surface) being parallel and may have any shape such as a plate, a column, or a block. Further, the lengths of the glass  119  and the glass  120  in the optical path direction are not necessarily the same. Since the length corresponds to the correction amount of the imaging position, the glass  120  is shorter than the glass  119 . 
       FIG. 5  is a view illustrating an example of a height measuring device including an element for focusing the light trajectories of the first detection optical system and the second detection optical system on one place on the two-dimensional image sensor element  109 . In a case of the height measuring device illustrated in  FIG. 1  or the like, since the position of the light ray reaching the two-dimensional image sensor element  109  is different between the case where the object to be measured exists at the first reference height  106  and the case where the object to be measured exists at the second reference height  106 , a detection surface having a size that covers the arrival positions of the two light rays is required. On the contrary, according to the configuration illustrated in  FIG. 5 , by providing an element for condensing two light trajectories at one point on the detection surface or close to it, between the object to be measured and the two-dimensional image sensor element  109 , even a two-dimensional image sensor element having a small detection surface can detect light rays reflected at different set heights. 
     Specifically, a double-sided wedge substrate  121  formed with incident surfaces (a plurality of inclined surfaces) which refract a plurality of light rays toward a position on the detection surface of the two-dimensional image sensor element  109  that intersects the optical path  110  is disposed between the detection lens  108  and the two-dimensional image sensor element  109  as a light collecting element. 
     In this way, by inserting the double-sided wedge substrate  121  having a flat surface on one side and two angles on the other side into the optical path, the light is refracted. As a result, the image can be formed at the same position or close positions on the surface of the two-dimensional image sensor element  109 . This makes it possible to perform height measurement at a plurality of set heights even with a two-dimensional image sensor element having a detection surface of a limited size. 
       FIG. 6  is a view illustrating an example in which two two-dimensional image sensor elements are provided. In the example of  FIG. 6 , by arranging a mirror  122  on one optical path (in  FIG. 6 , on the trajectory of the reflected light reflected from the object to be measured at the second reference height  107 ), the reflected light ray from the object to be measured is further reflected, and thus the reflected light ray reaches the two-dimensional image sensor element  123  which is spaced apart from the original optical path. With this configuration, it is possible to perform highly accurate height measurement at a plurality of set heights without increasing the detection surface of the two-dimensional image sensor element  109 . 
     EXAMPLE 2 
     Next, a height measuring device having an irradiation optical system and an incident optical system for switching the light trajectory between when the object to be measured is positioned at the first reference height  106  and when the object to be measured is positioned at the second reference height will be described. In this embodiment, instead of the double multi-slit illustrated in  FIG. 1  or the like, a slit forming member  302  in which one multi-slit  118  as illustrated in  FIG. 9  is formed is installed in the irradiation optical system as a multi-slit  122  illustrated in  FIGS. 7 and 8 . 
     The light rays emitted from the light source  101  are converged by the lens  102  and enters the multi-slit  122 . The light ray that has passed through the multi-slit  122  is collected at the passage aperture of the diaphragm  104  and further converged by the lens  105  to reach the object to be measured at the first reference height  106  or the object to be measured at the second reference height  107 . 
     A thick glass plate  124  (light trajectory adjusting element) which is one of switching element that switches the light trajectory and changes the light trajectory from the optical path  110  is provided between the object to be measured and the lens  105 . By providing the optical trajectory adjusting element with a rotation mechanism (not illustrated) and controlling the rotation mechanism in accordance with the reference height setting by the control device  117 , the light trajectory is switched so that an appropriate position is irradiated with the light ray (for example, the point of the object to be measured irradiated with the electron beam) when ( FIG. 8 ) the object to be measured is positioned at the first reference height  106  and when ( FIG. 7 ) the object to be measured is positioned at the second reference height  107  . By using a thick glass plate or the like, it is possible to adjust the light trajectory using the light refraction action. 
     Further, by arranging an optical trajectory adjusting element which returns the light trajectory to the original optical path  110  between the object to be measured and the detection lens  108 , even with the two-dimensional image sensor element  109  having a small detection surface, it is possible to measure the height of an object to be measured having a plurality of set heights. In the configuration illustrated in  FIGS. 7 and 8 , a thick glass plate  125  supported by a rotation mechanism is also arranged in the detection optical system as well as the irradiation optical system and the control device  117  adjusts the angle of the thick glass plate  125  in conjunction with the glass plate  124  so as to change the light trajectory so that the light trajectory changed by the thick glass plate  124  is returned to the original state. Thereby, the shift of the light beam due to the change in the height of the object to be measured is canceled and an image is formed at the center of the surface of the two-dimensional image sensor element  109 . Accordingly, the height can be measured using one two-dimensional image sensor element regardless of the height of the object to be measured. 
     In this embodiment, the thick glass plate  124  and the thick glass plate  125  may have the same thickness or different thicknesses. However, in order to correct the deviation of the irradiation position of the light ray reflected from the object to be measured at the second reference height  107 , it is desirable that the thicknesses of the thick glass plate  124  and the thick glass plate  125  are the same. 
     Further, the detection optical system may be configured as illustrated in  FIGS. 5 and 6 . 
     REFERENCE SIGNS LIST 
       101 : light source 
       102 : lens 
       103 : double multi-slit 
       104 : diaphragm 
       105 : projection lens 
       106 : first reference height 
       107 : second reference height 
       108 : detection lens 
       109 : two-dimensional image sensor element 
       110 : optical path 
       111 : ideal optical axis of beam (normal line) 
       112 : sample suction mechanism 
       113 : X-Y stage 
       114 : Z stage 
       115 : stage drive power supply 
       116 : height arithmetic processing unit 
       117 : control device 
       118 : multi-slit 
       119 : glass member 
       120 : glass member 
       121 : double-sided wedge substrate 
       122 : mirror 
       123 : second two-dimensional image sensor element 
       124 : thick glass plate 
       125 : thick glass plate