JP2011014299A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning electron microscope which realizes highly precise focus adjustment even under an observation condition with a great depth of focus.SOLUTION: A plurality of aperture holes of different diameters are formed on an aperture plate that eliminates an unnecessary area of a primary electron beam, and the aperture holes are switched to control an aperture angle of the primary electron beam that passes through the aperture holes without changing lens conditions of a converging lens and an objective lens. A condition is set to determine a resolution to be high and a depth of focus to be small. In this manner, focus adjustment precision is improved.

Description

  An object of the present invention is to control the opening angle of a primary electron beam passing through a diaphragm hole by the diameter of the diaphragm hole, thereby realizing improvement in focus accuracy.

  A scanning electron microscope (hereinafter referred to as “SEM”) accelerates a primary electron beam emitted from a field emission electron source, and a spot beam focused on a sample using an electrostatic or magnetic lens. , Scans two-dimensionally on the sample, detects secondary signals such as secondary electrons or reflected electrons generated secondary from the sample, and scans the detected signal intensity in synchronization with the scanning of the primary electron beam A two-dimensional scanning image (SEM image) is obtained by using the luminance modulation input of the monitor.

  In recent years, as the miniaturization of semiconductor devices has progressed, scanning electron microscopes have been used for inspection of semiconductor devices (for example, dimension measurement using an electron beam and inspection of electrical operation) instead of optical microscopes. In particular, a CD-SEM used for pattern dimension measurement is used for device dimension management, and high precision and stable magnification accuracy and length measurement performance are required for the purpose of semiconductor manufacturing process management. In addition, as device miniaturization has progressed, higher resolution is also demanded for devices that inspect and measure the device. At the same time, for three-dimensional device structures and high aspect contact holes, etc. Therefore, high precision and stable magnification accuracy and length measurement performance are required even under SEM conditions with a high focal depth.

  For example, Patent Document 1 discloses a technique for optimizing resolution at a high focal depth, and Patent Document 2 discloses probe current control under SEM conditions at a high focal depth.

JP 2006-294301 A JP-A-5-62629

FIG. 1 is a diagram showing a relationship between resolution and depth of focus with respect to a beam opening angle of a primary electron beam focused on a sample in a scanning electron microscope. The resolution R includes a light source size Rss that is reduced on the sample surface by a focusing lens (optical magnification of each lens: M ₁ , M ₂ , M ₃ ) having a plurality of light source diameters rss of the beam emitted from the cathode, and a spherical aberration Rs. And chromatic aberration Rc and diffraction aberration Rd. That is, the resolution R is theoretically expressed by the following equation.

On the other hand, as shown in FIG. 1, the opening angle and the focal depth are in a relationship such that if the opening angle incident on the sample is increased, the focal depth becomes shallower, and if the opening angle is decreased, the focal depth becomes deeper. In the electron optical system having such characteristics, the optimum opening angle that provides the highest resolution is the angle indicated by the opening angle α _i1 in FIG. If the opening angle is larger than α _i1 , the resolution is lowered due to the influence of aberration or the like.

Further, as shown in FIG. 1, the resolution is deteriorated by setting the opening angle of the primary electron beam incident on the sample to α _i2 , but the lens condition with a deep focal depth can be obtained. When the primary electron beam is focused on the sample, the focal position of the primary electron beam is changed by the objective lens, and the focal position where the image becomes sharpest is the position where the primary electron beam is focused on the sample. This has variations depending on the resolution and the depth of focus. The better the resolution and the shallower the depth of focus, the better the focusing accuracy. Accordingly, when the lens conditions of the opening angle α _i1 and the opening angle α _i2 in FIG. 1 are compared, at the opening angle α _i2 , the focusing accuracy deteriorates.

FIG. 2 schematically shows the observation magnification of the scanning electron microscope. The observation magnification is such that the primary electron beam 4 is scanned on the sample surface of the sample 10 by the deflection coil 9 (L _s1 , L _s2 , L _s3 in the figure) and a screen displayed on the image display device 42 (L in the figure). _d1 and L _d2 ). In FIG. 2A, in the case of the lens condition 401 in which the primary electron beam 4 is controlled so that the object point of the objective lens 7 becomes the crossover point 22, the primary electron beam 4 is scanned using the deflection coil 9. It schematically represents the beam trajectory at the time. The scanning region L _s1 of the deflection coil 9 is calibrated in accordance with the observation magnification using a pattern having a known dimension in advance. The optical magnification at the time of calibration is expressed as M ₁ = b3 / a3 using the height b3 when the primary electron beam 4 is focused on the sample 10 by the objective lens 7.

However, as shown in FIG. 2B, when the height b3 is changed to the height b3 ′ due to the height accuracy of the sample 10 and the sample stage having different heights, the scanning of the primary electron beam 4 in the sample 10 is performed. The region changes from the scanning region L _s1 to the scanning region L _s2 , and the observation magnification changes. Therefore, by feeding back the fluctuation of the optical magnification M ₂ = b3 ′ / a3 obtained from the control value of the objective lens 7 under the lens condition 402 in which the primary electron beam 4 is focused on the sample 10, the observation magnification becomes constant. Control is performed so that the scanning region of the deflection coil 9 is set to the same scanning region as the scanning region L _s1 . A focus control value for the height of the sample 10 is calculated using simulation or the like, and is stored in the CPU or storage device in advance as a control table, thereby quickly controlling the scanning region of the scanning coil 9 when an optical magnification change occurs. be able to.

  On the other hand, even in the lens condition 403 in which the object point of the objective lens is controlled to the crossover point 23, the scanning area can be controlled to be constant from the change in the optical magnification M3 = b3 / a3 ′ as described above. Is possible. The observation magnification of the SEM controlled in this way largely depends on the focusing accuracy of the objective lens that determines the optical magnification.

  Therefore, in order to perform magnification control with high accuracy, it is important to control focusing accuracy with the objective lens 7 with high accuracy. However, particularly in a CD-SEM or the like that requires high accuracy and stability in dimension measurement, this deterioration in focusing accuracy causes variations in optical magnification, resulting in fluctuations in device dimension measurement values. It will be.

  Further, Patent Documents 1 and 2 describe means for changing resolution and depth of focus by changing the opening angle of the optical system by controlling the focusing lens group, or control for making the probe current constant. The deterioration of focusing accuracy in a deep optical system is not considered.

  The present invention has been made in view of such a situation, and provides a technique that enables high-precision focus control even under lens conditions with a deep focal depth.

  In order to solve the above-mentioned problems, in the present invention, a primary electron that passes through a throttle hole by arranging a throttle hole having a plurality of hole diameters on the diaphragm plate that removes an unnecessary region of the primary electron beam and changing the throttle hole. By controlling the opening angle of the line and changing the focusing lens and objective lens conditions without changing the lens conditions, high resolution and shallow depth of focus can be set to improve focusing accuracy and achieve high-precision focus adjustment. It is characterized by being able to do it. After adjusting the focus, return to the original size aperture (observation aperture) and observe.

  Further, in the present invention, coarse focus adjustment is performed by setting a condition with a deeper focal depth than during observation by changing the aperture diameter. Then, after coarse focus adjustment, the focus depth is controlled to be shallower than the focus depth at the time of observation, and fine focus adjustment is executed.

  Furthermore, the probe current that increases with the area ratio of the aperture diameter can be used to precharge the sample by changing the aperture.

  Further features of the present invention will become apparent from the best mode for carrying out the present invention and the accompanying drawings.

  According to the present invention, focus adjustment can be performed with high accuracy even under lens conditions with a deep focal depth, and higher magnification can be achieved.

It is a figure which shows the relationship between the resolution | decomposability with respect to an opening angle, and a focal depth. It is explanatory drawing of the observation magnification in a scanning electron microscope. It is a figure which shows schematic structure of the scanning electron microscope by a comparative example. It is a figure which shows schematic structure of the scanning electron microscope by embodiment of this invention. It is a figure which shows the performance characteristic in the opening angle control by change of an aperture diameter. It is a figure which shows the auto-focus characteristic at the time of opening angle control implementation by change of an aperture diameter. It is a flowchart for demonstrating the autofocus control by this embodiment. It is a figure which shows the change method of the aperture hole diameter using the aperture | diaphragm movable unit which has XY stage. It is a figure which shows the change method of the opening angle using the aperture | diaphragm movable unit which has Z stage. It is a figure which shows the change method of the aperture hole diameter which used the deflector.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be noted that this embodiment is merely an example for realizing the present invention and does not limit the technical scope of the present invention. In each drawing, the same reference numerals are assigned to common configurations.

<Configuration of Scanning Electron Microscope as Comparative Example>
Before describing the configuration of the scanning electron microscope of the present embodiment, a configuration example of a conventional scanning electron microscope will be described as a comparative example first to assist in understanding the present invention.

  FIG. 3 is a diagram illustrating a configuration example of a conventional scanning electron microscope. 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. Since 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, the primary electron beam 4 emitted from the cathode 1 is accelerated and the latter lens system. Proceed to.

  The primary electron beam 4 is focused by the focusing lens 5 controlled by the focusing lens control power source 31, and after the unnecessary area of the primary electron beam 4 is removed by the fixed aperture plate 50, the primary electron beam 4 is controlled by the focusing lens control power source 32. The sample 10 is focused as a minute spot by the focusing lens 6 and the objective lens 7 controlled by the objective lens control power source 36. The focusing lens 6 can control the object point of the objective lens 7 to an arbitrary position, and can control the incident angle of the objective lens 7.

  A retarding method is also possible in which a negative voltage is applied to the sample 10 by the sample application power source 37 via the sample stage 11 to decelerate the primary electron beam 4. Further, a boosting method in which an acceleration electrode for reducing aberration is arranged directly or in the vicinity of the objective lens 7 is also possible.

  The primary electron beam 4 is scanned two-dimensionally on the sample 10 by the scanning coil 9 controlled by the scanning coil control power source 33. Secondary signals 12 such as secondary electrons and reflected electrons generated from the specimen 10 by irradiation of the primary electron beam 4 travel to the upper part of the objective lens 7 by the action of a pulling magnetic field or pulling electric field by the objective lens 7, and the secondary signal. The signal is deflected toward the signal detector 17 by the deflection electric field formed by the separating orthogonal electromagnetic field generator (EXB) 16. The signal detected by the signal detector 17 is amplified by the signal amplifier 18, transferred to the image memory 41, and displayed as an SEM image on the image display device 42.

In FIG. 3, it is assumed that the opening angle of the primary electron beam 4 incident on the sample 10 is controlled to the optimum opening angle α _i1 by controlling the focal position of the focusing lens 6 to the crossover point 22 (FIG. 3). 3 solid line). In this electron optical system, when setting a lens condition with a deep focal depth, the angle is set to an angle indicated by an opening angle α _i2 in FIG. 1 so that the beam opening angle is reduced by the primary electron beam 4 incident on the sample 10. It ’s fine. This is realized by controlling the focal position of the focusing lens 6 to the crossover point 23 in FIG. 3 (dotted line in FIG. 3).

  However, if the opening angle is adjusted by changing the crossover point as in the prior art (FIG. 3), the condition (magnetic field or the like) of the focusing lens 6 is changed, and it is affected by the hysteresis of the coil. This makes it difficult to achieve highly accurate focus adjustment and highly accurate magnification adjustment.

  Therefore, in the present embodiment, the following configuration is adopted.

<Configuration of Scanning Electron Microscope of the Present Invention>
FIG. 4 is a diagram illustrating a configuration example of the scanning electron microscope according to the embodiment of the present invention. The scanning electron microscope according to the embodiment of the present invention is characterized in that means for controlling the opening angle of the primary electron beam 4 by the aperture hole of the aperture plate 8 and improving the focus accuracy is employed. The aperture plate 8 has a plurality of aperture holes with different hole diameters, and can be changed to any aperture hole by the aperture movable unit 38.

First, it is assumed that the focal position of the focusing lens 6 is controlled at the crossover point 23 so that the depth of focus becomes deeper at the aperture diameter shown in FIG. At this time, the beam trajectory passes through the trajectory indicated by the solid line 24, and the opening angle of the primary electron beam 4 on the sample 10 is controlled to α _i2 (solid line in FIG. 4). From this state, at the time of focus adjustment, the diaphragm plate 8 is moved by the diaphragm movable unit 38 to change to a diaphragm having a large hole diameter. The beam trajectory becomes a dotted line 25, and the beam opening angle in the sample 10 can be set to α _i1 without changing the crossover point 23 of the focusing lens 6 (dotted line in FIG. 4).

  Therefore, as shown in FIG. 1, it is possible to perform focus adjustment under a condition of high resolution and shallow depth of focus. When changing to a diaphragm with a different hole diameter, the position (coordinate of the diaphragm 8) adjusted in advance is registered in the storage device 43, and a control signal is sent to the diaphragm movable unit 38 to move to the registered coordinates. By moving it, it is possible to quickly change the aperture.

<Characteristics obtained by opening angle control>
FIG. 5 is a diagram showing details of probe current characteristics, resolution characteristics, depth of focus characteristics, and focus characteristics when opening angle control is performed. FIG. 5A shows the characteristics when the opening angle is controlled by the diameter of the throttle hole (the present invention). FIG. 5B shows characteristics when the opening angle is controlled by changing the crossover by the focusing lens 6 (comparative example).

As shown in FIG. 5A, when the opening angle of the primary electron beam 4 in the sample 10 is changed from α _i2 to α _i1 by changing the aperture diameter, the resolution is good and the depth of focus is shallow. . Ideally, since the optical magnification does not change, the focus control value is constant. However, since the focus accuracy depends on the resolution and the depth of focus, there is an error depending on the lens condition. However, the focus control value error Δf ₂ when controlled by the opening angle α _i2 is controlled by the opening angle α _i1 . In this case, the error of the focus control value can be reduced to Δf ₁ and the focus accuracy is improved. That is, as the depth of focus becomes shallower, variations in focus are suppressed (see FIG. 6). In order to obtain the best focus accuracy, it is desirable to select the aperture diameter so that the opening angle of the primary electron beam 4 becomes the optimum opening angle α _i1. However, the opening angle is not limited to the optimum opening angle condition. The present embodiment can be applied to any diameter of the aperture hole whose opening angle is larger than α _i2 . On the other hand, if the diameter of the aperture hole is too large, the resolution is deteriorated due to an increase in chromatic aberration and spherical aberration, and the focus accuracy is lowered. For this reason, it is necessary to set the diameter of the aperture hole within a range where the focusing accuracy is improved.

  In the present embodiment, when the diameter of the throttle hole is increased, the probe current increases with the area ratio of the throttle hole, and the probe current increases. This can be expected to improve the image S / N of the scanning electron microscope. Accordingly, it is possible to increase the aperture diameter and perform focus adjustment with a high image S / N. Naturally, as a method of increasing the probe current, it is possible to set the voltage value of the first anode 2 drawn out from the cathode 1 high. However, as the characteristic of the cathode 1, if the extraction electric field becomes strong, the energy width ΔE is increased. There is a concern about resolution degradation due to an increase in chromatic aberration. Although it is possible to increase the probe current by bringing the crossover of the converging lens 5 closer to the diaphragm plate 8, the optical magnification of the converging lens 5 tends to increase. This leads to an increase in the aberration of each lens, and there is a concern about resolution degradation. In the present invention, the influence of lens aberration due to an increase in current can be ignored, so that focus adjustment with the best resolution can be realized.

On the other hand, in the comparative example shown in FIG. 5B, when the opening angle is changed, the focus control value also changes. That is, the focus control value obtained by adjusting the focus at the opening angle α _i2 cannot be used as the focus control value at the opening angle α _i1 . Therefore, in the comparative example, the quality of the resolution directly affects the focus adjustment.

  In the present invention, as described above, since the focus control value obtained when the opening angle is large can be applied to the focus control value when the opening angle is small, focus adjustment can be performed with good resolution. . Therefore, highly accurate focus adjustment can be realized.

<Accuracy improvement in autofocus function>
FIG. 6 is a diagram for explaining the improvement in accuracy of the automatic focus adjustment, that is, the so-called autofocus function to which the present invention is applied.

  The autofocus function controls the objective lens 7 by the objective lens control power source 36, acquires an SEM image while shifting the focal position of the primary electron beam 4 within a predetermined control value range, and performs image processing on the SEM image at each focal position. After extracting the image sharpness 602, the approximate function 603 is obtained, and the maximum value of the approximate function 603 is assumed to be the focal position 601. When acquiring the image sharpness 602 at each focal position, if the image S / N is low, the image sharpness 602 varies, resulting in a deviation between the focal position obtained from the approximate function and the actual focal position (see FIG. 6A). ). On the other hand, when the image S / N is high, variations in the image sharpness 602 can be reduced, and the evaluation function 603 can be obtained with high accuracy (see FIG. 6B).

Therefore, since the acquisition time of autofocus can be shortened by acquiring a high S / N image, the autofocus process can be speeded up. Further, the focusing accuracy determined from the resolution and the depth of focus can be improved from Δf ₂ to Δf ₁ , and the optimum focus position can be easily found, so that smooth and highly accurate focus adjustment can be realized. .

In the present embodiment, by setting a hole smaller than a hole diameter that is normally used, a deep focus state can be set and rough adjustment of autofocus can be controlled. For example, when the height of the sample 10 is greatly different under the lens conditions with the optimum opening angle α _i1 and the shallow depth of focus, if the focal position is not within a predetermined control value range, the focal position is estimated. Can not be. Alternatively, it is necessary to shift the focus control value again by a predetermined offset amount and perform autofocus retry, which requires twice or more time. At this time, it is possible to facilitate the search for the focal position by changing the aperture diameter, reducing the opening angle, and setting the focal depth deep. In this case, the resolution is degraded, but there is no problem as long as the degradation does not affect the autofocus function.

  Further, the probe current decreases with the area ratio of the aperture hole. However, since the adjustment is performed as a rough adjustment, there is no problem if the image S / N level allows image processing.

  Furthermore, since high-precision focus accuracy is not required, the probe current may be increased by setting the voltage value of the first anode 2 drawn from the cathode 1 high so that the influence of chromatic aberration can be ignored. In this case, in order to obtain an image S / N that does not affect the autofocus function, the aperture diameter and the voltage value of the first anode 2 are registered in the storage device 43 in advance, and a command for changing the aperture diameter is issued. At the time of execution, it can be realized by setting the voltage value registered by the high voltage control power supply 30 and controlling the probe current to increase.

  If the method of the present embodiment is used for probe current control for preliminary charging, an increase in current can be realized by changing the aperture diameter without changing the lens conditions. For example, in a pattern such as a high aspect constant hole, shape observation and dimension measurement of the contact hole hole bottom are required, but in a normal low probe SEM image, a secondary signal 12 generated at the sample hole bottom is required. Cannot escape from the bottom of the hole and cannot be detected as a signal. For this reason, as a pretreatment for acquiring the SEM image, the sample is irradiated with the primary electron beam 4 to charge the sample surface or the side wall of the contact hole, and the secondary signal 12 is pulled up. At this time, since irradiation is performed with a larger current than the probe current under normal SEM use conditions, if this embodiment is applied, the probe current can be increased without changing the lens conditions. There is no need to worry about the reproducibility of the optical magnification.

<Contents of focus control processing>
FIG. 7 is a flowchart for explaining the details of the focus control process (example) based on the principle described above. The control program according to the flowchart is stored in, for example, the storage device 43, and the processing of each step is executed in cooperation with the control arithmetic device 40. Here, the aperture plate 8 is provided with a plurality of holes (Φ ₁ , Φ ₂ , Φ ₃ , Φ ₄ ), and the aperture hole that is normally used when acquiring the SEM image is adjusted to Φ ₁ . To do. Φ ₂ is a large aperture diameter for preliminary charging, Φ ₃ is a smaller diameter than Φ ₁ for coarse focusing, and Φ ₄ is a larger diameter than Φ ₁ for fine autofocusing. It has become. It is assumed that Φ ₂ is optimized according to the precharge current conditions, and Φ ₃ and Φ ₄ are optimized according to the conditions of resolution and depth of focus necessary for the required focusing accuracy. In addition, each throttle hole is adjusted in advance, and the adjusted position is registered in the storage device 34 as coordinates. When the throttle hole is changed, a control command is sent to the movable diaphragm unit 38 so that the position can be reproduced quickly. It has become.

First, the control arithmetic unit 40 moves the irradiation position of the beam to the observation point (S001), and then performs preliminary charging by increasing the current (for example, whether or not preliminary charging is performed according to a recipe (S002). ) the), change the hole aperture by the movable unit 38 stop the throttle hole [Phi _2, and a large current condition (S003). At this time, the irradiation time can be set according to the required charge amount, and the operator can arbitrarily set the waiting time (S004).

After pre-charging process ends, if the coarse focus adjustment is necessary, the control calculation unit 40, the diaphragm transmits a control signal to the movable unit 38 is changed to the throttle hole Φ ₃ (S006). The aperture hole Φ ₃ is set to have a smaller opening angle than Φ ₁ , and autofocus is executed under observation conditions with a deep focal depth (S 007).

Next, the control arithmetic unit 40 changes the throttle hole Φ ₄ (S008), executes the fine auto focus (S009). At this time, since the focal position where the primary electron beam 4 truly focuses on the sample 10 is within the range of focus accuracy (variation) during coarse focus, the amount of focus shift during fine autofocus is about the range of fine autofocus variation. Good.

After spinning autofocus completed, the control arithmetic unit 40 returns the throttle hole [Phi ₁ normally used (S010), acquires the SEM image, or performs processing such as dimension measured using a SEM image (S011) .

<Means for changing the aperture hole>
FIG. 8 is a diagram showing a schematic configuration of the diaphragm plate 8 and the diaphragm movable unit 38 that can be used in the present embodiment. The diaphragm plate 8 is attached to a movable mechanism of the XY stage, and can move on a plane to a position where the primary electron beam 4 passes. When changing the aperture diameter of the diaphragm, the coordinate information of the XY stage that has been adjusted in advance is read from the storage device 43, and the control arithmetic device 40 sends a control signal to the movable unit 38 to control the position of the XY stage. Can be implemented. The diaphragm plate 8 may have any shape as long as it has different hole diameters. For example, a rectangular diaphragm as shown in FIG. 8, a circular, trapezoidal, rectangular, triangular, etc. diaphragm You may arrange | position the shape from which a diameter of an aperture hole differs. Regarding the hole diameter, it is desirable to prepare a plurality of hole diameters in consideration of the opening angle control and the probe current increase amount according to the lens conditions to be used, but it is not particularly specified.

FIG. 9 is a diagram showing a configuration in which the aperture plate 8 is moved and controlled by an elevating mechanism to change the opening angle. In the case of this configuration, the opening angle can be arbitrarily controlled without changing the hole diameter of the diaphragm. For example, as shown in FIG. 9, it is assumed that the incident opening angle of the focus lens 6 is adjusted to α _c2 by the aperture hole of the aperture plate 8 at a position L ₂ from the object point 26 of the focusing lens 6. In order to change the opening angle to α _C1 , the diaphragm position is moved to the position L _{1 by} the diaphragm movable unit 38. The hole diameter of the aperture 50 [μm], when the _{L 2} to 50 [mm], α _c2 is because it is 0.5 [mrad], the L2 if you want to change the opening angle doubling 25 [mm] The diaphragm movable unit 38 may be moved so as to set. The range that can be physically moved up and down is determined by the structure such as the lens and coil, so if you want to change the opening angle further, it can be realized by installing a diaphragm movable unit controlled by the XYZ axes in combination with the configuration of FIG. .

  Further, by using the Z-axis diaphragm movable unit shown in FIG. 9, the opening angle can be adjusted according to variations in the diameter of the diaphragm hole. The diameter of the aperture hole is generally processed by etching. However, in the case of an inexpensive aperture, the manufacturing variation is large, and the hole diameter varies about 5 to 10%. In addition, the diaphragm is contaminated by irradiating the electron beam, and the charged up causes performance degradation such as resolution, so that the diaphragm plate 8 is generally replaced periodically. If the aperture diameter varies, as shown in FIG. 1, the opening angle of the primary electron beam 4 in the sample 10 changes due to variations in aperture diameter, which affects the resolution and depth of focus. Even if an accurate throttle hole can be manufactured, there is a variation in the hole diameter, so it is necessary to adjust the opening angle in accordance with the variation in the diameter of the throttle hole. Whether the primary electron beam is emitted from the aperture hole at a predetermined opening angle is measured by measuring the probe current value of the emitted electron beam, and the current value registered in the storage device 43 in advance (a plurality of opening angles). It is possible to control by adjusting the height of the diaphragm plate 8 so that a predetermined current value is obtained. Further, when there are a plurality of apparatuses, the variation in the optical performance such as the resolution is caused by the variation in the diameter of the aperture hole. In such a case, by applying the configuration shown in FIG. 9, the height of the aperture position can be adjusted so that the opening angle becomes a constant angle, and variations in optical performance due to the aperture plate 8 can be suppressed. Become.

  FIG. 10 is a diagram illustrating a configuration in which the opening angle is controlled by deflecting the primary electron beam 4 and passing the aperture holes having different hole diameters without moving the aperture. The diaphragm plate 8 has a plurality of diaphragms having different hole diameters, and four stages of deflectors 19 controlled by a deflector control power source 34 are arranged before and after the diaphragm plate 8. In the normal use state, the primary electron beam 4 is adjusted so as to pass through the aperture hole arranged on the optical axis. However, when adjusting the focus, the aperture hole having a different hole diameter in the plane of the aperture plate 8 is allowed to pass. To do. Since this electron beam control can be realized electrically, the configuration shown in FIG. 10 is desirable when speeding up.

Deflection aberration occurs because the primary electron beam 4 is deflected by the deflector. As shown in FIG. 10, the deflection angle deflected by the former stage deflector and the latter stage deflector is, for example, θ ₁ = θ ₄ = θ. by configuring such that the _{2 = θ 3,} the influence of the aberration can be neglected. Further, even if aberrations occur, the influence of the aberrations can be reduced by reducing the objective lens 8 if the diaphragm plate 8 is arranged on the upper stage of the focusing lens 6. With such a configuration, since there is no mechanical movement and processing can be performed electrically at high speed, high accuracy and high speed of autofocus can be realized.

<Summary>
In the present embodiment, the aperture angle of the charged particle beam is changed without changing the lens condition of the charged particle optical system by using a diaphragm having a hole for removing an unnecessary region of the charged particle beam. The control arithmetic device controls the resolution and depth of the charged particle beam while changing the opening angle of the charged particle beam while keeping the focal position of the focusing lens closest to the objective lens at the same position. The focus is being adjusted. Thereby, it is possible to realize highly accurate focus adjustment without changing the lens conditions of the charged particle optical system.

  In this embodiment, it is possible to select that the control arithmetic device performs control so that the depth of focus is deeper than the depth of focus at the time of observation and performs coarse focus adjustment. Further, the control arithmetic unit performs fine focus adjustment by controlling the depth of focus to be shallower than the focus depth at the time of observation after coarse focus adjustment or without coarse focus control. Thereby, highly accurate focus adjustment can be realized according to the situation at that time.

  Further, the control arithmetic unit controls the opening angle at the time of preliminary charging, and can perform selection of performing preliminary charging of the sample under a larger current condition than at the time of observation. Thereby, highly accurate focus adjustment can be realized in a state where preliminary charging is executed according to the type of sample.

  The opening angle of the charged particle beam can be controlled by changing the size of the aperture hole. For example, a diaphragm plate having a plurality of sizes of diaphragm holes and a diaphragm plate movable unit for moving the diaphragm plate on the XY stage are provided. At this time, the control arithmetic unit controls the diaphragm plate movable unit, moves the diaphragm plate to the coordinates registered in advance on the XY plane of the XY stage, and selects one of the plurality of sizes of the diaphragm holes. Change the opening angle with. As another aspect of changing the size of the aperture hole, a fixed aperture plate having a plurality of types of aperture holes and deflectors arranged at the front and rear stages of the aperture plate are provided. At this time, the control arithmetic device changes the opening angle by controlling the deflector to allow the charged particle beam to pass through any of a plurality of types of aperture holes.

  Further, the opening angle of the charged particle beam can be changed by moving the diaphragm plate having one diaphragm hole up and down. In this case, the control arithmetic device controls the diaphragm plate movable unit, and adjusts the current value of the charged particle beam emitted from the diaphragm plate to a value registered in advance, thereby adjusting to a predetermined opening angle. Since the opening angle is controlled by setting the current value to a predetermined value, the opening angle can be appropriately adjusted when there are variations in aperture holes in a plurality of scanning electron microscopes.

DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... 1st anode, 3 ... 2nd anode, 4 ... Primary electron beam, 5 ... Condensing lens, 6 ... Condensing lens, 7 ... Objective lens, 8 ... Diaphragm plate, 9 ... Scanning coil, 10 ... Sample , 11 ... Sample stage, 12 ... Secondary signal, 16 ... Orthogonal electromagnetic field generator (EXB) for secondary signal separation, 17 ... Signal detector, 18 ... Signal amplifier, 19 ... Deflector, 22 ... Crossover point, 23 ... crossover point, 24 ... beam trajectory, 25 ... beam trajectory, 26 ... crossover point, 30 ... high voltage control power supply, 31 ... focusing lens control power supply, 32 ... focusing lens control power supply, 33 ... scanning coil control power supply, 34 ... Deflector control power supply, 36 ... Objective lens control power supply, 37 ... Sample application power supply, 40 ... Control arithmetic device, 41 ... Image memory, 42 ... Image display device, 43 ... Storage device, 50 ... Fixed diaphragm (single Diameter)

Claims (9)

A charged particle source;
A charged particle optical system that scans a sample by focusing a charged particle beam emitted from the charged particle source by a plurality of focusing lens groups;
An opening angle changing means for changing an opening angle of the charged particle beam without changing a lens condition of the charged particle optical system, using a diaphragm having a hole for removing an unnecessary region of the charged particle beam; ,
By changing the opening angle of the charged particle beam using the opening angle changing means, the resolution and the focal depth of the charged particle beam are controlled while maintaining the focal position of the focusing lens closest to the objective lens at the same position, A focus control unit for performing focus adjustment of the charged particle beam;
A detector for detecting secondary signal particles generated from the sample by scanning the charged particle beam,
A scanning electron microscope, wherein a sample image is formed using the secondary signal particles output from the detector. In claim 1,
The scanning electron microscope, wherein the focus control unit controls the depth of focus to be deeper than the depth of focus at the time of observation by using the opening angle changing unit, and performs coarse focus adjustment. In claim 2,
The focus control unit performs fine focus adjustment by using the opening angle changing unit again after the coarse focus adjustment to control the focus depth to be shallower than the focus depth at the time of observation. Scanning electron microscope. In claim 1,
The scanning electron microscope, wherein the focus control unit performs fine focus adjustment by controlling the depth of focus to be shallower than the depth of focus at the time of observation using the opening angle changing unit. In claim 1,
The scanning control microscope, wherein the focus control unit controls the opening angle at the time of preliminary charging using the opening angle changing unit, and executes the preliminary charging of the sample under a larger current condition than at the time of observation. In claim 1,
The opening angle changing means includes a diaphragm plate having a plurality of sizes of diaphragm holes, and a diaphragm plate movable unit for moving the diaphragm plate on the XY stage,
The focus control unit controls the diaphragm plate movable unit to move the diaphragm plate to pre-registered coordinates on the XY plane of the XY stage, and selects one from the plurality of types of diaphragm holes. A scanning electron microscope characterized in that the opening angle is changed. In claim 1,
The opening angle changing means includes a diaphragm plate having a diaphragm hole of one type of size, and a diaphragm plate movable unit for moving the diaphragm plate up and down along the Z axis.
The focus control unit controls the diaphragm plate movable unit to change the opening angle by moving the diaphragm plate up and down on the Z axis. In claim 7,
The focus control unit controls the diaphragm plate movable unit and adjusts a current value of a charged particle beam emitted from the diaphragm plate to a pre-registered value, thereby adjusting to a predetermined opening angle. Scanning electron microscope. In claim 1,
The opening angle changing means includes a fixed diaphragm plate having a plurality of sizes of aperture holes, and deflectors arranged at the front and rear stages of the diaphragm plate,
The focus control unit controls the deflector to change the opening angle by allowing the charged particle beam to pass through any of the plurality of sizes of aperture holes.

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