JP5588944B2 - Scanning electron microscope - Google Patents

Description

  The present invention relates to a scanning electron microscope, and more particularly, to a method and apparatus suitable for observing and measuring a sample by controlling field of view movement by beam shift.

  A scanning electron microscope (hereinafter referred to as SEM) irradiates a sample with an electron beam emitted from an electron source and detects secondary signal electrons such as secondary electrons or reflected electrons generated from the sample. Thus, a two-dimensional signal (image) reflecting the surface shape of the sample is acquired.

  In recent years, with the miniaturization of semiconductor devices, SEM has been used for inspections and measurements using these as target samples instead of optical microscopes. Typical apparatuses include a length measurement SEM (Critical dimension SEM: CD-SEM) for measuring a pattern of a substrate (wafer) having a fine circuit pattern, and a defect review SEM for inspecting defects and foreign matters on the wafer. (Defect Review SEM: DR-SEM). A semiconductor device is manufactured by repeating a lithography process and an etching process in which a resist is applied on a silicon substrate and a specific pattern is transferred using a photomask. In such a manufacturing process, stable operation of the manufacturing process is indispensable for improving the yield, and it is necessary to detect defects early by in-line inspection and to solve the defects. In CD-SEM, high precision and stability of measurement are required along with miniaturization of devices. In an inspection apparatus such as DR-SEM, high-resolution observation is important for inspecting fine defects. Yes. In such a semiconductor device inspection / measurement apparatus, improvement in position accuracy in movement for irradiating an electron beam to a position of a specific pattern to be measured, and speeding up of a series of operations (throughput) Improvement) is required.

  One means for improving the position accuracy is to move the stage and improve the stop accuracy. However, in a device having a fine structure in which the size of the inspection / measurement object is several tens of nanometers, It is difficult to realize the movement only by moving the stage. In addition, there are problems such as a rise in cost due to mounting a highly accurate stage on the apparatus and a decrease in throughput due to mechanical movement of the stage for moving the visual field to the measurement point. Therefore, as a moving method that compensates for such a problem of the stage, there is control (hereinafter, image shift) in which the field of view is moved by electrically shifting the electron beam.

  Patent Document 1 discloses that a beam diameter is based on a look-up table that associates a pre-registered image shift position with a change in beam diameter, as a means for correcting an increase in the beam diameter of an electron beam, particularly when measuring a pattern line. A dimension measurement method is shown in which image processing is performed so as to compensate for the change in the above, so that a measurement error does not occur effectively even when an image shift is used.

  In Patent Document 2, in order to suppress the influence on magnification error and dimension measurement error when using image shift, a plurality of deflectors are used to adjust the charged particle beam to be incident on the sample along the optical axis of the objective lens. An axis adjustment method is disclosed.

  In Patent Document 3, for example, an error amount is acquired for each known pitch with respect to a wafer in which patterns are arranged at a constant pitch, so that accurate scanning alignment is performed regardless of the image shift deflection position. Is disclosed.

JP 2010-175318 A JP 2011-9127 A JP 2008-84626 A

  However, in the case where the field of view is greatly moved by image shift for speeding up the processing, as will be described later with reference to FIG. 3, the influence of the increase in the beam diameter of the electron beam due to the increase in the generated aberration causes the measurement accuracy to deteriorate. As a problem.

  In addition, along with the miniaturization of devices, the objective lens has become shorter in focus as a technique for improving the image resolution of the SEM for inspecting and measuring these devices. It is known to increase. For this reason, when image shift is simply used within a physically controllable range, an accurate measurement result may not be obtained.

  Therefore, in order to improve throughput and achieve high measurement accuracy, it is important to control the movement of the electron beam irradiation position relative to the measurement position on the sample in an optimal combination of movement by the stage and visual field movement by image shift. It is. In other words, among the total amount of movement required for measurement, the relative ratio of the visual field movement amount by the image shift to the movement amount by the stage is increased, and the effective image resolution and length measurement accuracy are maintained during the image shift. It is important to control under possible conditions.

  According to the method disclosed in Patent Document 1, although the error of the length measurement value obtained as a result can be corrected by image processing, there is a specific disclosure regarding the combined control of visual field movement by image shift and stage movement. Absent.

  Further, according to the method disclosed in Patent Document 2, the charged particle beam can be appropriately incident on the sample along the optical axis of the objective lens by the action of the deflector. Not thinking.

  According to the method disclosed in Patent Document 3, the stage drift amount can be measured and corrected while the error amount is obtained for each known pitch by performing deflection by image shift. There is no mention of the relationship between stage movement with respect to quantity and visual field movement by image shift.

  An object of the present invention is to control the movement of the electron beam with respect to the measurement position on the sample in an optimal combination of movement by the stage and visual field movement by image shift.

  As one aspect for solving the above-described problem, when the target irradiation position of the electron beam on the sample is within the first range of the sample, a deflection signal is given to the deflector, and when the position exceeds the first range. In the method and apparatus for controlling the irradiation position of the electron beam by moving the sample stage, the target irradiation position of the electron beam on the sample when the electron beam scans an area smaller than a predetermined scanning width. Provides a deflection signal to the deflector when it is within a second range smaller than the first range of the sample, and realizes a method for moving the sample stage when the second range is exceeded and the control thereof We propose a device to do this.

  According to the above configuration, the image shift can be executed under the condition that the influence of the increase in the beam diameter of the electron beam is small, and it is possible to observe and measure the sample at high speed with high measurement accuracy.

The figure which shows schematic structure of SEM. The figure which shows the relationship of deflection | deviation of the electron beam by the control of a deflection coil, the scanning area | region of an electron beam, and image shift. The figure which shows the relationship between the resolution of SEM, various aberrations (diffraction aberration, spherical aberration, chromatic aberration), and a light source diameter. The figure explaining an example of the movement method of the irradiation position of the electron beam on the sample based on an image shift control range. The flowchart which shows an example of the movement method of the irradiation position of the electron beam on the sample based on the image shift control range. The figure which shows an example of the relationship between the pixel resolution for every scanning width (observation magnification) of an electron beam, and a beam diameter. The flowchart which shows an example of the movement method of the irradiation position of the electron beam on the sample based on the image shift effective control range. The figure which shows the relationship between the image shift amount for every optical condition, and a beam diameter. The figure which shows the relationship between the image shift amount for every optical condition, and an image sharpness value. The figure which shows the relationship between the image shift amount for every optical condition, and a line length measurement value. The flowchart which shows an example of the adjustment method of the reference data (control table) based on the variation | change_quantity of the value regarding an image shift amount and a beam diameter. The figure which shows an example of the control range of an image shift, an effective control range, and a control table. The figure which shows GUI which sets an image shift control condition. The figure which shows an example in case the position where a beam diameter becomes the minimum is not a position where the amount of image shift becomes zero. The figure which shows an example of arrangement | positioning of the deflection coil for suppressing an increase in a beam diameter.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiment described below is an example, and the present invention is not limited to this. For example, in the following description, the SEM will be described as an example. However, the present invention is not limited to this, and a general-purpose arithmetic device that executes processing by a computer program may be used to perform processing as described later. good.

  In the first embodiment, an embodiment in which an image shift can be executed under a condition where the influence of an increase in the beam diameter of an electron beam is small will be described.

  FIG. 1 is a diagram showing a schematic configuration of an SEM. A voltage is applied between the cathode 1 and the first anode 2 by a high voltage control power source 30 controlled by a control arithmetic device 40 (control processor), and a predetermined emission current is drawn from the cathode 1. An acceleration voltage is applied between the cathode 1 and the second anode 3 by a high voltage control power source 30 controlled by the control arithmetic unit 40, so that the electron beam 4 emitted from the cathode 1 is accelerated to the rear lens system. proceed. The electron beam 4 is focused by the focusing lens 5, and unnecessary regions of the electron beam 4 are removed by the diaphragm plate 8, and then controlled by the focusing lens 6 and the objective lens control power source 36 controlled by the focusing lens control power source 32. The objective lens 7 is focused as a fine spot on the sample 10 placed on the sample stage 11. The booster system using the acceleration electrode 19 in which a positive voltage is applied by the acceleration electrode power supply 38 in the magnetic field of the objective lens 7 shown in the figure is provided. The sample stage 11 is a high-accuracy XY stage capable of observing a semiconductor wafer with an SEM, and has a stop accuracy of several hundred nm to several μm. Further, a negative voltage can be applied to the sample stage 11 by a sample application power source 37 as a retarding method for increasing the resolution by irradiating the electron beam 4 with low energy.

  The electron beam 4 is two-dimensionally scanned on the sample 10 by a deflection magnetic field generated by scanning upper and lower deflection coils 13 and 14 to which a signal is given by a deflection coil control power source 33 controlled by a control arithmetic unit 40. The The deflection coil can also be replaced as a deflection electrode. In this case, the electron beam 4 is deflected by the electric field. The signal given to the deflection coil or the deflection electrode at this time is a current or a voltage.

  Secondary signal electrons 12, such as secondary electrons and reflected electrons generated from the sample 10, are lifted by a pulling magnetic field generated by the objective lens 7, a retarding potential applied to the sample 10, or a boosting potential applied to the acceleration electrode 19. After traveling above the objective lens 7, it is converted into converted secondary electrons 15 by the reflector 9. The converted secondary electrons 15 travel in the direction of the signal detector 17 by the electromagnetic field orthogonal deflector 16 (ExB) and are detected and amplified by the signal amplifier 18, then transferred to the image memory 41, and transferred to the image display device 42. Displayed as an SEM image. Various conditions and information can be input from the outside through the input device 44.

  Here, the ratio of the upper and lower stages of the current flowing through the deflection coil (upper stage) 13 and the deflection coil (lower stage) 14 is controlled so that the electron beam 4 passes through the axis center (deflection fulcrum) of the objective lens 7. By controlling in this way, the deflection amplitude can be changed while the deflection fulcrum is controlled to be constant, and the observation magnification can be changed with a small aberration through the axial center of the objective lens. Further, the deflection coil (upper stage) 13 and the deflection coil (lower stage) 14 function as an image shift for moving the observation position of the sample while viewing the sample.

A specific control image of the deflection coil will be described with reference to FIG. FIG. 2A is a diagram showing the relationship between the deflection coil control and the electron beam scanning region. When changing the observation region, as shown in FIG. 2A, the deflection coil control power supply 33 changes the amplitude of the current passed through the deflection coil (upper stage) 13 and the deflection coil (lower stage) 14 to change the observation area. The scanning area is changed like L ₁ or L ₂ . On the other hand, FIG. 2B is a diagram showing a relationship between deflection coil control and electron beam deflection by image shift. When used as an image shift, the field of view is moved (L ₃ ) as shown in FIG. 2B by applying an offset current to the current flowing through the deflection coil (upper stage) 13 and the deflection coil (lower stage) 14. it can. If the deflection amplitude (scanning width) for scanning is changed in the image shifted state, the scanning region, that is, the observation magnification (L ₁ , L ₂ ) can be changed at the position where the visual field is moved.

  FIG. 3 is a diagram showing the relationship between the resolution of the SEM, various aberrations (diffraction aberration, spherical aberration, chromatic aberration), and the size of the light source diameter. Here, the horizontal axis is an opening angle α in which the spread of the electron beam irradiated to the sample is indicated by a half angle, and the vertical axis indicates the beam diameter of the electron beam. As shown in FIG. 3, the beam diameter of the electron beam focused on the sample is generally determined by diffraction aberration, spherical aberration, chromatic aberration, and the reduced light source diameter on the sample. By moving the field of view, curvature of field, astigmatism, coma aberration, deflection chromatic aberration, and the like are further generated, and the beam diameter is increased.

  Next, the relationship between stage movement and visual field movement by image shift will be described with reference to FIG. FIG. 4 is a diagram showing an example of a method for moving the irradiation position of the electron beam on the sample based on the image shift control range.

  When a specific pattern on the sample is set as an electron beam irradiation target position (measurement point), the movement of the electron beam irradiation position to the measurement point recognizes the position of the specific pattern displayed in the SEM image by image processing (hereinafter referred to as the image processing). , Pattern matching), and necessary visual field movement amount (distance) is calculated. The visual field movement amount ΔL is calculated according to the relational expression ΔL = M × Δl from the magnification M at the time of observation with respect to the pixel shift amount Δl on the screen obtained by pattern matching. Usually, the image shift is controlled so that the visual field movement of several μm to several tens of μm is possible for the purpose of supplementing the stage stop accuracy. The range in which the visual field can be moved by this image shift is called an image shift control range. In the present embodiment, a method for high-magnification observation of two specific patterns of a rhombus and a triangle illustrated in FIG. 4 will be described. FIG. 4A shows a case where a diamond pattern is a target, and FIG. 4B shows a case where a triangular pattern is a target.

  In FIG. 4A, when high-magnification observation of the rhombus pattern is desired, the position of the rhombus pattern is within the image shift control range in low-magnification observation, and the visual field movement amount calculated by pattern matching is moved by visual shift. Later, high magnification observation is possible by increasing the observation magnification. In FIG. 4B, when high-magnification observation of a triangular pattern is desired, the position of the triangular pattern is outside the image shift control range in low-magnification observation. Move the stage so that is centered on the screen. However, since the stage stopping accuracy varies with a certain accuracy, it may not move to a desired position in the center of the screen. Here, pattern matching is performed again to determine whether there is a triangular pattern within the image shift control range. When there is a triangular pattern within the image shift control range, it is possible to observe the triangular pattern at a high magnification by increasing the observation magnification after moving the field of view by the image shift.

  FIG. 5 is a flowchart showing an example of a method of moving the irradiation position of the electron beam based on the above-described image shift control range. When a recipe in which measurement conditions are registered in advance is executed (step 501), after the semiconductor wafer is transferred to the sample stage 11, the stage is moved to the measurement point based on the coordinate registration information of the pattern (step 502). A low-magnification observation image is acquired at the measurement point (step 503), and the amount of positional deviation from the center of the SEM screen is calculated by pattern matching by image processing for the specific pattern to be measured (step 504). At this time, the SEM image may not be in focus, but in this case, automatic focus correction can be set in the recipe. Alternatively, a measuring device such as a Z sensor capable of measuring the height (focal position) of the wafer may be provided, and focus adjustment may be performed by feeding back the measured height information to the focus current of the objective lens. It is determined whether the amount of positional deviation (= the amount of visual field movement) from the image center (reference position) acquired with the low-magnification observation image is within the image shift control range (step 505). If the specific pattern is outside the image shift control range, the stage is slightly moved so as to move to the center of the SEM screen (step 506). As the stage moves, the specific pattern moves to the vicinity of the center of the screen, but the amount of positional deviation from the center at that time varies uniformly within the stopping accuracy of the stage. The procedures of (Step 504), (Step 505), and (Step 506) are repeated until the image shift control range is entered. If the specific pattern falls within the image shift control range, the visual field movement amount is set as the image shift deflection amount, and a high-magnification observation image is acquired (step 508). At the time of image shift, defocusing and astigmatism increase due to field curvature occur, so automatic focus correction and automatic astigmatism correction are performed on a high-magnification image (step 509), and an observation image is acquired. Alternatively, the specific pattern is measured using the acquired image (step 510). The above processing is performed for the number of measurement points registered in the measurement recipe (step 511).

  Next, the setting of the scanning width (observation magnification) of the electron beam when executing visual field movement and stage movement control by image shift in the present embodiment will be described with reference to FIG. In general, the image resolution in an SEM image is determined by the balance between the beam diameter and the pixel resolution corresponding to the observation magnification (size per pixel when observing at a certain magnification). FIG. 6 is a diagram illustrating an example of the relationship between pixel resolution and beam diameter for each scanning width (observation magnification) of an electron beam. The beam diameter does not change even when the observation magnification changes, but the pixel resolution decreases as the observation magnification increases. Therefore, when the pixel resolution is smaller than the beam diameter, the influence of the beam diameter on the image resolution increases. FIG. 6 shows an example in which the pixel resolution becomes smaller than the beam diameter at 100 k times as a boundary. When used at a high magnification of 100 k times or more, a change in the beam diameter affects the image resolution. That is, the influence of the increase in beam diameter due to the image shift appears remarkably at an observation magnification of 100 k times or more. On the other hand, if the observation magnification is 100 k times or less, the influence of the beam diameter increase due to the image shift does not greatly affect the image resolution. Therefore, when this embodiment is applied, it is effective to control the movement of the irradiation position of the electron beam according to the observation of the specific pattern and the measurement magnification. When the observation / measurement magnification of the specific pattern set by the user in the recipe does not affect the image resolution, the image shift amplitude can be increased within the image shift control range described in FIG. Therefore, it is possible to reduce the amount and frequency of movement of the stage when moving the visual field, and it is possible to speed up the measurement process. Here, the magnification for enabling the control of the movement of the irradiation position of the electron beam can be set by the user inputting in an input field (1105) shown in FIG.

  A method of moving the irradiation position of the electron beam on the sample in consideration of the above observation magnification will be described with reference to FIG. FIG. 7 is a flowchart showing an example of a method of moving the irradiation position of the electron beam on the sample based on the image shift effective control range. When a recipe in which measurement conditions are registered in advance is executed (step 701), after the semiconductor wafer is transferred to the sample stage 11, the stage is moved to the measurement point based on the coordinate registration information of the pattern (step 702). Here, when the observation magnification is lower than a predetermined threshold, that is, a magnification at which the influence of the beam diameter on the image resolution becomes large, the process proceeds to Steps 503 to 507 in FIG. 5 to move the visual field based on the image shift control range. Perform (step 705). On the other hand, when the observation magnification is higher than the threshold, an image is acquired at the measurement point (step 704), and the amount of positional deviation from the center of the SEM screen is calculated by pattern matching by image processing for the specific pattern to be measured (step 704). Step 706). Next, it is determined whether or not the acquired position shift amount (= field-of-view movement amount) from the image center (reference position) is within the image shift effective control range (step 707). Here, the image shift effective control range refers to an image shift range that can be controlled under a condition in which effective measurement accuracy can be maintained. This range may be set based on data relating an image shift amount and a change amount of a value related to the beam diameter, which will be described later with reference to FIGS. 8 to 14, or may be arbitrarily set by the user. It can also be automatically executed by registering it in a recipe. However, the image shift effective control range described here is an area smaller than the image shift control range in which the visual field movement by the image shift shown in FIGS. Needless to say. If the specific pattern is outside the image shift effective control range, the stage is slightly moved so as to move to the center of the SEM screen (step 708). The procedures of (Step 706), (Step 707), and (Step 708) are repeated until the image shift effective control range is entered. If the specific pattern falls within the image shift effective control range, the visual field movement amount is set and moved as the image shift deflection amount (step 709), and imaging is performed (step 710). Automatic focus correction and automatic astigmatism correction are performed (step 711), and an observation image is acquired. Or the dimension measurement of a specific pattern is implemented using the image acquired after that (step 712). The above processing is performed for the number of measurement points registered in the measurement recipe (step 713).

  Next, an example of setting the effective control range of the image shift in consideration of the influence of the increase amount of the beam diameter for each optical condition of the SEM will be described with reference to FIG. Although an example of changing the acceleration voltage condition is shown here, the present embodiment is not limited to this, and other optical conditions such as currents or voltages applied to various electrodes and coils in the SEM, for example. This can also be applied when changing.

  In the first embodiment, the embodiment in which the image shift is executed under the condition where the influence of the increase in the beam diameter of the electron beam is small has been described. In the second embodiment, the effective image shift control range is set for each optical condition. Thus, an example of performing more reliable control will be described.

  FIG. 8 is a diagram showing the relationship between the image shift amount and the beam diameter for each optical condition.

  FIG. 8A shows the relationship between the image shift and the beam diameter when the acceleration voltage is changed to 500V, 1000V and 1500V. Further, FIG. 8B shows the relationship between the image shift and the beam diameter increase amount obtained by calculating the beam diameter increase amount Δ from the minimum beam diameter value based on FIG. 8A. Generally, the beam diameter increases due to the influence of deflection chromatic aberration, coma aberration, and the like according to the image shift amount, but the influence is larger as the acceleration voltage is lower. Here, as described later, data obtained by measuring the beam diameter increase amount in advance is recorded, and an allowable value of the beam diameter increase amount is set, so that the image shift effective control range can be set for each acceleration voltage. In the example of FIG. 6B, the allowable value of the beam diameter increase amount from the image shift zero position is set to 5%. However, the image shift effective control range that satisfies this is different for each acceleration voltage, Δr ( 300), Δr (1000), and Δr (1500). By controlling in this way, it is possible to acquire an SEM image in which the influence of the increase in beam diameter is suppressed. For example, in the case of an apparatus that performs dimension measurement such as a length measurement SEM, the increase in the beam diameter affects the length measurement value as a dimensional change, but for example, when the 0.1 nm beam diameter changes, 0.1 nm. Appears as a change in measured value. Therefore, in semiconductor device measurement where length measurement reproducibility is required to be controlled at 0.2 nm or less, visual field movement control by image shift that suppresses the increase in beam diameter is an effective means for improving measurement accuracy. Become. Since the beam diameter increase varies depending on the acceleration voltage conditions provided by the device, optical conditions such as high resolution mode and depth of focus mode, the image shift control range can be set according to each condition. The measurement accuracy can be improved even under various conditions. Further, the degree of influence of the increase amount of the beam diameter is a parameter that should be determined according to the observation magnification condition for observing the pattern size to be measured and the device management status that differs for each user. Therefore, by allowing the user to set the allowable value of the beam diameter increase amount, highly robust measurement can be realized.

  In the third embodiment, a method for acquiring in advance the relationship between the image shift amount and the change amount of the value related to the beam diameter as reference data will be described. By using the reference data acquired in advance in the present embodiment, it is possible to set the image shift effective range more easily.

  FIG. 9 is a diagram illustrating the relationship between the image shift amount and the image sharpness value for each optical condition.

  An example of measuring the increase in beam diameter will be described with reference to FIG. FIG. 9A shows an SEM image of a sample in which gold particles are vapor-deposited on carbon that is generally used for evaluating image resolution. As a means for measuring the amount of increase in beam diameter using such a sample, the Contrast-to-Gradient method (CG method) or Derivative-Method method for evaluating the sharpness of the SEM image by image processing can be applied. In this case, the allowable value of the beam diameter increase amount can be set as the Δ image sharpness value. FIG. 9B shows the relationship between the image shift and the image sharpness increase when the acceleration voltage is changed to 500V, 1000V, and 1500V. By acquiring an SEM image in a state of being deflected by image shift and obtaining an image sharpness value, a difference value from the minimum value of the image sharpness value, that is, an increase amount is obtained as shown in FIG. 9B. By using such a method, it becomes possible to quantitatively measure the influence of the increase in beam diameter from the viewpoint of degradation of the resolution of the SEM image, and depending on the allowable value of the image sharpness value set by the user, A shift effective control range can be set.

  Another example of measuring the beam diameter increase will be described with reference to FIG. FIG. 10 is a diagram illustrating the relationship between the image shift amount and the line length measurement value for each optical condition. In the case of an apparatus such as a length measurement SEM, the amount of increase in beam diameter affects the length measurement result, and therefore the influence of the amount of increase in beam diameter can be measured by measuring the dimensional change with respect to the image shift deflection amount. Measure the measured value change when using image shift on a dedicated wafer or sample with lines and spaces in-plane. Line measurement mainly includes X measurement and Y measurement, and it is also possible to acquire a change in line measurement value in each. In this case, the user can set the image shift control range by specifying the size as a management standard to be used. Of course, it is possible to cope with the case of measuring a hole diameter such as a contact hole by taking the same method.

  Here, FIG. 11 is a flowchart showing an example of a method for adjusting the reference data (image shift control table) based on the image shift amount measured by the above-described method and the change amount of the value related to the beam diameter. Here, the case where the image sharpness value is used as a value related to the beam diameter will be described as an example, but the present invention is not limited to this, and the above-described pattern dimension measurement value and other values related to the beam diameter may be used depending on conditions. good.

  After setting the image shift deflection amount to zero (step 1102), the adjustment magnification is set, automatic focus correction and automatic astigmatism correction are performed (step 1103), and an SEM image is acquired (step 1104). An image sharpness value is calculated by image processing (step 1105), and the value is stored in the storage device 43. The image shift is changed by the range and step width determined by the adjustment (here, range: ± 10 μm, step width: 5 μm) while moving the field of view, (Step 1103), (Step 1104), (Step 1105) )repeat. After all the measurement points are completed, a difference value (Δ image sharpness value) between the minimum value of the image sharpness value and the value acquired at each point is obtained and stored as reference data in the image shift control table. As will be described later, the image shift effective control range can be set based on this control table.

  In the fourth embodiment, an example in which the above-described reference data is displayed will be described.

  FIG. 12 is a diagram illustrating an example of an image shift control range, an effective control range, and an image shift control table. FIG. 12A shows the adjustment result of the image shift control range and the image shift effective control range to which the present embodiment is applied. Here, an image shift control table in the case where beam deflection is performed by image shift at a matrix position of 5 × 5 = 25 points and an image sharpness value is obtained at each point is shown. The Δ image sharpness values obtained in Nos. 1 to 25 shown in FIG. 12A are stored in the example of the image shift control table shown in FIG. 12B, and this Δ image sharpness value is controlled by the image shift effective control. Used to set the range. In practice, if N × N data is acquired by finely deflecting the image shift, adjustment with higher accuracy becomes possible. Further, for example, after acquiring data of a plurality of points, it is an effective means to reduce the number of measurement points and increase the adjustment accuracy by performing quadratic curved surface approximation as shown in the control table example of FIG. is there. This adjustment can be handled by having the image shift control table shown in FIG. 12B under each condition so as to correspond to various acceleration voltages and optical modes. According to the display method according to the present embodiment, the image shift control range and the image shift effective control range can be recognized more clearly.

  An example using a user interface (Graphical User Interface: hereinafter referred to as GUI) that allows the user to set the allowable value of the beam diameter increase amount when the present invention is applied will be described with reference to FIG. FIG. 13 is a diagram illustrating a GUI for setting image shift control conditions. In this case, the user can input an allowable value, that is, an image shift effective control range, as the resolution management value in the input field (1304). The image shift effective control range is set so as to be equal to or smaller than the input value with reference to the image shift control table measured in advance. By displaying the result in the image shift effective control range (1304), it is possible for the user to confirm, and to input and modify information from the screen using a mouse or other input device. Further, this allowable value can be set together with an effective observation magnification (1305) for executing the control in the present embodiment for each optical condition such as the acceleration voltage (1301) and the optical mode (1302).

FIG. 14 shows the relationship between the image shift and the beam diameter. Ideally, the beam diameter is minimized when the image shift deflection amount is zero. However, the SEM that is actually used is composed of a large number of focusing lens groups, deflectors, and correctors, and has mechanical tolerances. The electron beam does not necessarily pass through an ideal orbit due to an adjustment error during the optical axis adjustment. In such a case, beam tilt, asymmetrical aberration, etc. occur, and the beam diameter may not be minimized when the image shift deflection amount is zero. FIG. 14 is a diagram illustrating an example where the position where the beam diameter is minimum is not the position where the amount of image shift is zero. As a specific example, a two-dimensional map showing the relationship between the deflection amount due to image shift and the minimum beam diameter position is shown in FIG. 14A, and the deflection amount due to image shift and the beam diameter increase amount. FIG. 14 (b) shows a relationship between the two and the three-dimensional map. In this case, the value related to the beam diameter, that is, the place where the resolution is minimum is not the position of the image shift deflection amount zero (X ₀ , Y ₀ ) but the position of (X ₁ , Y ₁ ) deflected by the image shift. . Such a case can be dealt with by acquiring a map of the change in the beam diameter increase amount with respect to the image shift deflection amount as shown in FIG. The image shift deflection position that minimizes the beam diameter is set to (X ₁ , Y ₁ ), and the image shift effective control range is set with respect to the allowable increase in beam diameter centered on (X ₁ , Y ₁ ). The shift amount r is set as an image shift effective control range. If the specific pattern is within the radius r that is the image shift effective control range, the visual field is moved by image shift. If the specific pattern is not within the image shift effective control range, the specific pattern is (X ₁ , Y ₁ ), after moving the stage slightly, it is determined again whether the specific pattern is within the image shift effective control range. Assuming that the position of the specific pattern obtained by pattern matching is (X _i , Y _i ), it is possible to determine whether or not the visual field movement by image shift is performed by whether or not the following is satisfied.
(X ₁ −X ₀ ) ² + (Y ₁ −Y ₀ ) ² ≦ r ² (1)

  The control sequence in this case can be handled by the flow shown in FIG. Specifically, the determination based on the image shift effective control range shown in step 707 of FIG. 7 can be performed based on the above-described method. According to the present embodiment, regardless of the image shift deflection amount, the image shift effective control range can be set centering on the position where the resolution is actually minimized, and more accurate measurement is possible.

  In the fifth embodiment, another method for suppressing the increase in beam diameter will be described.

  FIG. 15 is a diagram illustrating an example of the arrangement of deflection coils for suppressing an increase in beam diameter. As described above, in the SEM in the present embodiment, the focusing condition 6 is used to control the crossover position of the electron beam, so that the optical condition for realizing high resolution with an optimum opening angle is set, or the opening angle is set. Can be set to an optical condition that realizes a high depth of focus. In general, the field of view is moved by image shift using the deflection coil (upper stage) 13 and the deflection coil (lower stage) 14 which are paired in two stages regardless of the optical conditions. It is possible to suppress an increase in beam diameter at the time of image shift by providing a device. FIG. 15 shows an example in which the deflection coil 23 is arranged at the crossover position of the focusing lens 6 and the visual field is moved by image shift using the deflection coil 23 and the deflection coil (lower stage) 14. When the image shift using the deflection coil 23 and the deflection coil (lower stage) 14 is performed as shown in the electron beam trajectory 22 at the time of image shift using the deflection coil 23 and the deflection coil (lower stage) 14, the deflection is performed at the crossover position. Therefore, the deflection chromatic aberration due to the deflection coil is smaller than that of the electron beam trajectory 21 at the time of image shift using the deflection coil (upper stage) 13 and the deflection coil (lower stage) 14. This is because, by deflecting the electron beam by the deflection coil at the crossover position where the beam diameter of the electron beam is the thinnest, the deflection angle accuracy by the deflection coil becomes higher than when deflecting at the position where the beam diameter is widened. It is. Therefore, when the deflection is performed using the deflection coil 23 and the deflection coil (lower stage) 14, the influence of the beam diameter increase due to the image shift is reduced. By providing a plurality of deflection coils, it is possible to determine an optimum deflection coil that is less affected by an increase in beam diameter due to image shift, corresponding to the crossover position of any optical condition. The means for determining the deflection coil can be determined by executing the adjustment sequence shown in FIG. In addition, this configuration can be applied with a deflection electrode instead of the deflection coil. By suppressing the increase in the beam diameter by adjusting the deflection coil at an observation magnification at which the increase in the beam diameter greatly affects the image resolution, an effective effect can be achieved without reducing the throughput. It becomes possible to obtain.

DESCRIPTION OF SYMBOLS 1 Cathode 2 1st anode 3 2nd anode 4 Electron beams 5 and 6 Focusing lens 7 Objective lens 8 Diaphragm plate 9 Reflecting plate 10 Sample 11 Sample stage 12 Secondary signal electron 13 Deflection coil (upper stage)
14 Deflection coil (lower)
15 Conversion secondary electron 16 Electromagnetic field orthogonal type deflector (ExB deflector)
Reference Signs List 17 Signal Detector 18 Signal Amplifier 19 Accelerating Electrodes 21, 22 Electron Beam Trajectory 23 During Image Shift Deflection Coil 30 High Voltage Control Power Supply 31, 32 Focusing Lens Control Power Supply 33 Deflection Coil Control Power Supply 36 Objective Lens Control Power Supply 37 Sample Application Power Supply 38 Accelerating electrode power supply 40 Control arithmetic device 41 Image memory 42 Image display device 43 Storage device 44 Input device

Claims (18)

An electron source,
A power source to which an acceleration voltage for accelerating an electron beam emitted from the electron source is applied;
A deflector for deflecting and scanning the electron beam;
A sample stage on which the sample is mounted;
A detector for detecting a signal obtained from electrons generated from a sample irradiated with the electron beam;
When the irradiation target position of the electron beam on the sample is within the first range of the sample, a deflection signal for moving the field of view is given to the deflector, and when the position exceeds the first range, the sample stage is moved. In a scanning electron microscope provided with a control computer that moves the irradiation position of the electron beam closer to the irradiation target position,
The control computer is
When scanning an area where the electron beam is larger than a predetermined scanning width, a deflection signal for moving the field of view to the deflector in the first range is given,
When scanning an area where the electron beam is smaller than a predetermined scanning width,
When the irradiation target position of the electron beam on the sample is within a second range smaller than the first range of the sample, a deflection signal for moving the field of view is given to the deflector, and the second range is A scanning electron microscope characterized by moving the sample stage when exceeding. In claim 1,
The control computer is
A storage unit that stores data associating the deflection position of the electron beam and the amount of change in the value related to the beam diameter of the electron beam;
Said 2nd range is set based on the data memorize | stored in the said memory | storage part, The scanning electron microscope characterized by the above-mentioned. In claim 2,
The scanning electron microscope according to claim 2, wherein the second range is set with a deflection position on the sample when a change amount of a value related to a beam diameter of the electron beam is minimized as a center position. In claim 1,
The predetermined scan width is
A scanning electron microscope characterized in that a scanning width when a size of a pixel in an image formed by the control computer based on a signal detected by the detector is smaller than a beam diameter of the electron beam . In claim 2,
The value regarding the beam diameter of the electron beam is
A scanning electron microscope, which is a feature amount obtained by a change in contrast of an image formed based on a signal detected by the detector. In claim 5,
The feature amount is
A scanning electron microscope characterized by an image sharpness value. In claim 2,
The value regarding the beam diameter of the electron beam is
A scanning electron microscope characterized in that the dimension value is obtained based on a line profile formed based on a signal detected by the detector. In claim 2,
The storage unit is for each optical condition controlled by the control computer,
A scanning electron microscope storing data in which a deflection position of the electron beam and an amount of change in a value related to a beam diameter of the electron beam are associated with each other. In claim 8,
The optical conditions are:
A scanning electron microscope comprising at least one of a voltage applied to the power source, a signal applied to an objective lens, and a beam opening angle of an electron beam. In claim 2,
The control computer is
A scanning electron microscope comprising an input unit for inputting information relating to data stored in the storage unit. In claim 10,
The control computer is
A scanning electron microscope comprising a user interface for displaying information to be input to the input unit and an input result on a screen. An electron source,
A focusing lens for focusing the electron beam emitted from the electron source;
A plurality of deflectors for deflecting the electron beam;
A sample stage on which the sample is mounted;
A detector for detecting a signal obtained from electrons generated from a sample irradiated with the electron beam;
When the irradiation target position of the electron beam on the sample is within a predetermined range of the sample, a deflection signal for moving the field of view is given to the deflector, and when the position exceeds the predetermined range, the sample stage is moved to move the sample stage. In a scanning electron microscope comprising a control computer for controlling the irradiation position of the electron beam,
The control computer is
When scanning an area where the electron beam is smaller than a predetermined scanning width,
A scanning electron microscope characterized in that a deflection signal for selectively moving the visual field is given to at least one of the plurality of deflectors. In claim 1 2,
The predetermined scan width is
A scanning electron microscope characterized in that a scanning width when a size of a pixel in an image formed by the control computer based on a signal detected by the detector is smaller than a beam diameter of the electron beam . In claim 1 2,
The control computer is
In order to minimize the amount of change in the value related to the beam diameter of the electron beam,
Among the plurality of deflectors,
A scanning electron microscope characterized in that a signal is selectively given to at least one deflector. In claim 13,
For the deflector to which the signal is selectively given,
A scanning electron microscope comprising a deflector disposed at a position where a crossover of the focusing lens exists. When the irradiation target position of the electron beam on the sample is within the first range of the sample, the electron beam is deflected by controlling a deflector that moves the field of view. In the electron beam irradiation method of moving the sample beam mounting position and moving the electron beam irradiation position,
When scanning an area where the electron beam is larger than a predetermined scanning width, a deflection signal for moving the field of view to the deflector in the first range is given,
When scanning an area where the electron beam is smaller than a predetermined scanning width,
When the irradiation target position of the electron beam on the sample is within a second range smaller than the first range of the sample, a deflection signal for moving the field of view is given to the deflector, and the second range is An electron beam moving method characterized by moving the sample stage when exceeding. In claim 16,
Storing data associating the deflection position of the electron beam and the amount of change in the value related to the beam diameter of the electron beam;
The method of moving an electron beam, wherein the second range is set based on the stored data. In claim 16,
The predetermined scan width is
Electron beam movement characterized by having a scanning width when a pixel size in an image formed by the control computer based on a signal detected by the detector is smaller than a beam diameter of the electron beam Method.