JP2020119665A - Scanning electron microscope - Google Patents

Abstract

To provide a scanning electron microscope comprising a surface height measurement system capable of improving resolution of height measurement, and an objective lens control unit for correcting a focal position of an electron beam.SOLUTION: A scanning electron microscope 100 comprises: an electron beam irradiation system 101; and a surface height measurement system 140 that measures a surface height of a sample 124 on a sample table 121 arranged in a vacuum chamber 120. The surface height measurement system 140 comprises: a spectral interference displacement meter 144 having a sensor head 142 arranged to face a table surface 121a of the sample table 121; a beam splitter 146 arranged between the sensor head 142 and the table surface 121a; and a reflector 148 arranged on an optical path branched by the beam splitter 146. The surface height measurement system 140 is connected to an objective lens control unit 135 for correcting a focal position of an electron beam.SELECTED DRAWING: Figure 2

Description

The present invention relates to a scanning electron microscope, and more particularly to a technique for improving the resolution of height measurement of a sample surface on a sample table and correcting the focal position of an electron beam.

In order to clearly image the pattern of the semiconductor integrated circuit device, it is important to measure the surface height of the wafer with high accuracy. Therefore, the scanning electron microscope is equipped with an optical height measuring system that measures the height of the sample surface in a non-contact manner. Generally, the distance between the objective lens of a scanning electron microscope and the sample surface is very short, such as several millimeters. Therefore, a method has been adopted in which a laser beam is obliquely incident on the sample surface, the reflected light is detected by an optical sensor such as CCD or PSD, and the height from the objective lens to the sample surface is calculated.

FIG. 14 is a schematic diagram illustrating a conventional height measuring system. The light source 202 and the light projecting lens 203 are arranged on one side of the objective lens 200, and the optical sensor 210 and the light receiving lens 211 are arranged on the opposite side of the objective lens 200. The sample 220 is placed on the sample table 230. The height Z of the surface of the sample 220 can be obtained from the following equation given by the principle of triangulation.
Z=2L・(sinα・sinβ)/sin(α+β)
Here, 2L represents the distance between the light projecting lens 203 and the light receiving lens 211.

When α=β, the height Z is L·tan β. Here, the amount of change in height (ΔZ) corresponding to the change in angle (Δβ) when the position of the image on the detection surface of the optical sensor 210 slightly changes (ΔX) is
ΔZ=(∂Z/∂β)・Δβ=(∂Z/∂β)・(∂β/∂X) ΔX=(L・sec ² β)γΔX
Δβ=(∂β/∂X)・ΔX=γΔX
(Γ: angle change amount per pixel on the detection surface of the optical sensor 210, rad/pixel)
Is given.

When the incident angle (α=β) becomes small like 1 degree, it is necessary to lengthen the reference distance L in order to secure a mounting space for the light source 202 and the optical sensor 210. Therefore, the reference distance L is considerably longer than the distance f between the optical sensor 210 and the light receiving lens 211. When the reference distance L becomes long, the change amount of the surface height of the sample 220 according to the angle change (Δβ=γΔX) corresponding to the position resolution (minimum change amount that can be detected) ΔX on the detection surface of the optical sensor 210, That is, the height resolution ΔZ becomes coarse (decreases).

Japanese Patent Laid-Open No. 1-120749

Since there is such a relationship between the incident angle of light and the height measurement resolution, there is only a gap of several millimeters between the tip of the objective lens 200 and the surface of the sample 220, and the incident angle of light is 1 degree. If it becomes too small, there is a problem that the required height measurement resolution cannot be obtained.

Furthermore, since the optical path length between the light source 202 and the optical sensor 210 is as long as several tens of centimeters to 1 meter, the optical path length variation and the incident angle variation are likely to occur due to environmental changes. In particular, since the scanning electron microscope has a vacuum chamber as a constituent element, the deformation of the wall portion caused by the pressure change easily affects the optical path. From such a background, the resolution of height measurement is as low as about 1 μm, and accurate height measurement cannot be performed. Further, since the optical system such as the light source 202 and the optical sensor 210 is installed in the vacuum chamber in which the sample is placed, there is a problem that maintenance work is not easy.

Therefore, the present invention provides a scanning electron microscope including a surface height measuring system capable of improving the resolution of height measurement and an objective lens control unit for correcting the focal position of an electron beam.

In one aspect, an electron beam irradiation system, a vacuum chamber, a sample table arranged in the vacuum chamber, a table stage for moving the sample table, and a surface for measuring the surface height of the sample on the sample table. A height measuring system; and an objective lens control unit for correcting the focal position of the electron beam, wherein the surface height measuring system has a spectral interference displacement meter having a sensor head arranged facing the table surface of the sample table. A beam splitter arranged between the sensor head and the table surface, a reflector arranged on an optical path branched by the beam splitter, a reference structure fixed to the sample table, and the sample Calculation for calculating the distance between the lower end of the electron beam irradiation system and the surface of the sample by subtracting the offset value from the measured value of the surface height and adding a predetermined correction value to the measured value. And a correction value, wherein the correction value is the sum of the distance between the lower end of the electron beam irradiation system and the reference structure, the predetermined reference height and the distance between the lower end of the electron beam irradiation system, and the surface height. There is provided a scanning electron microscope, which is a numerical value obtained by subtracting the measurement value of the height of the reference structure obtained by the size measurement system.

In one aspect, the sensor head is perpendicular to the table surface.
In one aspect, the surface height measurement system is located outside the vacuum chamber.
In one aspect, the sensor head is arranged in a housing fixed to an upper wall forming the vacuum chamber.
In one aspect, the surface height measurement system further comprises a transparent block to which the beam splitter and the reflector are fixed.
In one aspect, the surface height measurement system further comprises a concave lens disposed between the sensor head and the beam splitter.
In one aspect, the surface height measurement system further comprises a circular polarizer disposed between the beam splitter and the table surface.

According to the surface height measuring system, the resolution of height measurement can be improved to about 0.1 μm. When the height measurement using the surface height measurement system is performed before the electron beam focus search and the electron beam focus position is corrected by the objective lens control unit connected to the surface height measurement system, the electron beam is sampled. The surface of the can be focused substantially. Therefore, the scanning electron microscope can significantly reduce the number of images generated in the focus search, and can perform the focus search at high speed. As a result, scanning electron microscopes can quickly produce clear pattern images.

It is a schematic diagram which shows one Embodiment of a scanning electron microscope. It is a figure which shows the detailed structure of a surface height measurement system. It is a graph which shows the result of the verification experiment which measured the difference of the distance from the sensor head to the reflector via the beam splitter, and the distance from the sensor head to the surface of the wafer with the surface height measurement system. It is a figure which shows other embodiment of the surface height measurement system. It is a figure which shows other embodiment of a surface height measurement system. It is a figure which shows other embodiment of a surface height measurement system. It is a figure which shows other embodiment of a surface height measurement system. It is a figure which shows other embodiment of a surface height measurement system. It is a figure which shows other embodiment of a surface height measurement system. It is a figure which shows the state when a reference|standard structure is located below the objective lens. It is a figure which shows the height map which shows the relationship of the position of the measurement point on the surface of a wafer, and the distance from the lower end of an objective lens to the measurement point on the surface of a wafer. It is a flowchart which performs the height measurement of a wafer. 7 is a flowchart illustrating an autofocus operation of the scanning electron microscope. It is a schematic diagram explaining the conventional height measuring system.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing an embodiment of a scanning electron microscope. As shown in FIG. 1, the scanning electron microscope 100 includes an electron beam irradiation system 101, a vacuum chamber 120, a sample table 121 arranged in the vacuum chamber 120, a table stage 123 for moving the sample table 121, and a sample. A surface height measuring system 140 that measures the surface height of the sample on the table 121, and an objective lens control unit 135 that adjusts the focus value based on the surface height of the sample are provided. The vacuum chamber 120 is connected to a vacuum pump (not shown) so that a vacuum is formed in the vacuum chamber 120.

The electron beam irradiation system 101 includes an electron gun 111 that emits an electron beam composed of primary electrons (charged particles), a focusing lens 112 that focuses the electron beam emitted from the electron gun 111, and an X deflection that deflects the electron beam in the X direction. It has a device 113, a Y deflector 114 that deflects the electron beam in the Y direction, and an objective lens 115 that focuses the electron beam on a wafer 124 that is a sample. The objective lens 115 is connected to the objective lens control unit 135, and the focus value of the objective lens 115 is controlled by the objective lens control unit 135. The surface height measuring system 140 is connected to the objective lens control unit 135.

The position and height of the sample table 121 in the vacuum chamber 120 is adjusted by the table stage 123. The table stage 123 can move the sample table 121 in the X and Y directions parallel to the table surface 121a of the sample table 121, and further can move the sample table 121 in the Z direction perpendicular to the table surface 121a. It is a configured XYZ stage. The wafer 124, which is a sample, is placed on the table surface 121 a of the sample table 121. The sample table 121 is configured to hold the lower surface of the wafer 124. An example of the sample table 121 is an electrostatic chuck table.

The scanning electron microscope 100 further includes a secondary electron detector 130, a backscattered electron detector 131, and an image acquisition device 118. The secondary electron detector 130 and the backscattered electron detector 131 are connected to the image acquisition device 118. The image acquisition device 118 is configured to convert the output signals of the secondary electron detector 130 and the backscattered electron detector 131 into an image.

The operation of the scanning electron microscope 100 is as follows. The electron beam emitted from the electron gun 111 is focused by the focusing lens 112, is then deflected by the X deflector 113 and the Y deflector 114, is focused by the objective lens 115, and is irradiated on the surface of the wafer 124. When the wafer 124 is irradiated with primary electrons of the electron beam, secondary electrons and reflected electrons are emitted from the wafer 124. Secondary electrons are detected by the secondary electron detector 130, and reflected electrons are detected by the reflected electron detector 131. The detection signal of the secondary electrons and the detection signal of the reflected electrons are input to the image acquisition device 118 and converted into an image. In this embodiment, the wafer 124 is used as an example of the sample, but the present invention is not limited to this embodiment. For example, the sample may be a glass substrate.

The surface height measuring system 140 is arranged outside the vacuum chamber 120. The surface height measuring system 140 measures the surface height of the wafer 124 on the sample table 121 before the scanning electron microscope 100 executes the autofocus operation. The surface height measuring system 140 will be described below.

FIG. 2 is a diagram showing a detailed structure of the surface height measuring system 140. The surface height measurement system 140 includes a spectral interference displacement meter 144 having a sensor head 142 arranged facing the table surface 121a of the sample table 121, and a beam splitter 146 arranged between the sensor head 142 and the table surface 121a. And a reflector 148 arranged on the optical path branched by the beam splitter 146. The angle of the beam splitter 146 with respect to the direction of the light emitted from the sensor head 142 is 45 degrees. The reflective surface of the reflector 148 is parallel to the direction of light emitted from the sensor head 142.

The spectral interference displacement meter 144 generates a light source 150 that emits light, a spectroscope 151 that decomposes light according to a wavelength, and measures the intensity of the light for each wavelength, and a spectral waveform from the light intensity data obtained by the spectroscope 151. The apparatus further includes a waveform analysis device 155 that determines the height of the surface of the wafer 124 by generating and analyzing the spectral waveform. The spectroscope 151 includes an optical sensor (not shown) such as a CCD. The light source 150 and the spectroscope 151 are connected to the sensor head 142.

The sensor head 142 has a lens and an optical fiber (not shown) for guiding the light emitted by the light source 150 to the beam splitter 146 and receiving the reflected light from the wafer 124 and the reflector 148. The reflected light from the surface of the wafer 124 and the reflected light from the reflector 148 enter the sensor head 142 while interfering with each other. The two reflected lights that interfere with each other are transmitted to the spectroscope 151 through the sensor head 142. The spectroscope 151 decomposes the two reflected lights that interfere with each other according to the wavelength, and measures the intensity of the light for each wavelength. The waveform analyzer 155 generates a spectral waveform from the light intensity data obtained by the spectroscope 151.

The manner in which the reflected light from the surface of the wafer 124 and the reflected light from the reflector 148 interfere with each other depends on the distance L1 from the sensor head 142 to the reflector 148 via the beam splitter 146 and the surface of the wafer 124 from the sensor head 142. It changes depending on the difference from the distance L2 to The relative positions of the sensor head 142, the beam splitter 146, and the reflector 148 are fixed. Therefore, the distance L1 is always constant. On the other hand, the distance L2 changes depending on the surface height of the wafer 124. Therefore, the manner of interference between the reflected light from the surface of the wafer 124 and the reflected light from the reflector 148 changes depending on the surface height of the wafer 124. The spectral waveform generated by the waveform analyzer 155 changes according to the manner of interference of the two reflected light waves. Therefore, the waveform analyzer 155 can determine the height of the surface of the wafer 124 by analyzing the spectral waveform.

The reflector 148 is arranged at a position such that the difference between the distance from the sensor head 142 to the table surface 121a and the distance L1 is within the set range. The setting range is predetermined based on the effective measurement range of the spectral interference displacement meter 144. The set range may include zero.

The surface height measurement system 140 further includes a concave lens 156 disposed between the sensor head 142 and the beam splitter 146. The concave lens 156 is provided to position the focus of the light (focused light) emitted from the sensor head 142 on the surface of the wafer 124. Even if the concave lens 156 is not provided, the concave lens 156 may not be provided as long as the focus of light can be located on the surface of the wafer 124.

The sensor head 142, the beam splitter 146, the concave lens 156, and the reflector 148 are housed in the housing 158 and fixed to the housing 158. The housing 158 is fixed to an upper wall 137 that constitutes the vacuum chamber 120. The sensor head 142 is perpendicular to the table surface 121a. The sensor head 142 arranged in this way can guide light perpendicularly to the surface of the wafer 124 and receive reflected light from the surface of the wafer 124. The sensor head 142 is arranged beside the objective lens 115. The distance between the sensor head 142 and the objective lens 115 is preferably as short as possible.

In the present embodiment, the distance L2 between the sensor head 142 and the surface of the wafer 124 is 50 mm to 100 mm. This distance is much shorter than the optical path length (tens of centimeters to 1 meter) of the conventional surface height measuring system shown in FIG. Therefore, even when the upper wall 137 is deformed due to the pressure fluctuation in the vacuum chamber 120, the surface height measuring system 140 is hardly affected by such an environmental change, and the surface height of the wafer 124 can be accurately measured. Can be measured.

FIG. 3 is a verification in which the difference between the distance L1 from the sensor head 142 to the reflector 148 via the beam splitter 146 and the distance L2 from the sensor head 142 to the surface of the wafer 124 is measured by the surface height measuring system 140. It is a graph which shows the result of an experiment. In this verification experiment, the difference between the distance L1 and the distance L2 was measured 1000 times under the same conditions. The distance L2 from the sensor head 142 to the surface of the wafer 124 was 70 mm. 3σ of the obtained measurement data was 0.05 μm. This indicates that the variation in measured values is within the range of 0.1 μm. Therefore, it can be seen from the result of the verification experiment that the surface height measuring system 140 of the present embodiment can obtain a resolution of less than 0.1 μm.

The surface height of the wafer 124 measured by the surface height measuring system 140 is the distance from a certain reference height to the surface of the wafer 124. In this embodiment, the reference height is the height of the lower surface of the upper wall 137 of the vacuum chamber 120. The reference height is preset in the surface height measuring system 140. As shown in FIG. 2, the measurement value of the surface height measuring system 140 is represented by Mns. The measured value Mns is represented as the distance from the reference height to the surface of the wafer 124. The distance Zno between the objective lens 115 and the surface of the wafer 124 can be obtained from the following equation (1).
Zno=Mns-Zoff (1)
However, Zoff is an offset value that represents the distance from the lower surface of the upper wall 137 of the vacuum chamber 120, which is the reference height, to the lower end of the objective lens 115. The offset value Zoff is a fixed value unique to the scanning electron microscope 100.

The surface height measuring system 140 further includes an arithmetic unit 160 connected to the waveform analyzer 155 of the spectral interference displacement meter 144. The arithmetic device 160 includes a processing device 160A such as a processing device 160A (for example, a CPU), a storage device 160B, and a display device 160C that displays the measurement result. The equation (1) and the offset value Zoff are stored in advance in the storage device 160B. The measured value Mns of the surface height of the wafer 124 is sent from the waveform analyzer 155 to the arithmetic unit 160, and the arithmetic unit 160 calculates the distance Zno between the objective lens 115 and the surface of the wafer 124 using the above equation (1). .. The arithmetic unit 160 may be composed of a general-purpose computer or a dedicated computer. The arithmetic device 160 is connected to the objective lens control unit 135.

4 and 5 are views showing another embodiment of the surface height measuring system 140. In the embodiment shown in FIG. 4, the angle of the beam splitter 146 with respect to the direction of the light emitted from the sensor head 142 is an angle other than 45 degrees. The reflector 148 is generally higher than the beam splitter 146. The embodiment shown in FIG. 5 is the same as the embodiment shown in FIG. 2 in that the angle of the beam splitter 146 with respect to the direction of the light emitted from the sensor head 142 is 45 degrees, but it has two reflectors, namely The difference is that the first reflector 148A and the second reflector 148B are provided. The second reflector 148B is arranged above the first reflector 148A. The embodiment shown in FIG. 5 has the advantage that the distance between the first reflector 148A and the beam splitter 146 can be shortened, and the width of the required installation space can be reduced.

FIG. 6 is a diagram showing still another embodiment of the surface height measuring system 140. The surface height measuring system 140 according to the embodiment shown in FIG. 6 includes a transparent block 170 to which a beam splitter 146 and a reflector 148 are fixed. The transparent block 170 is made of a transparent material such as glass or crystal. The beam splitter 146 is located within the transparent block 170. Such a configuration can be manufactured by forming a film-shaped beam splitter 146 on the side surface of the triangular prism and sandwiching the beam splitter 146 between this triangular prism and another triangular prism. The reflector 148 is fixed to the end surface of the transparent block 170. In one example, the reflector 148 can be formed by depositing a metal such as aluminum or a dielectric multilayer film on the end surface of the transparent block 170.

The relative position and relative angle between the beam splitter 146 and the reflector 148 are fixed by the transparent block 170. Therefore, it is not necessary to adjust the angles of the beam splitter 146 and the reflector 148, and the measurement accuracy is stable. Further, the number of members for fixing the beam splitter 146 and the reflector 148 is reduced.

FIG. 7 is a diagram showing still another embodiment of the surface height measuring system 140. The configuration of this embodiment that is not particularly described is the same as that of the embodiment shown in FIG. 6, and thus the duplicate description thereof will be omitted. The embodiment shown in FIG. 7 includes a first reflector 148A and a second reflector 148B. These two reflectors 148 are fixed to the end surface of the transparent block 170. In addition to the advantages obtained in the embodiment shown in FIG. 6, this embodiment has an advantage that the width of the installation space required can be reduced.

FIG. 8 is a view showing still another embodiment of the surface height measuring system 140. The configuration of this embodiment that is not particularly described is the same as that of the embodiment shown in FIG. 2, and thus the duplicate description thereof will be omitted. The surface height measuring system 140 further includes a circularly polarizing plate 175 arranged between the beam splitter 146 and the table surface 121a. The light emitted from the sensor head 142 passes through the circularly polarizing plate 175 and travels toward the wafer 124 on the table surface 121a. The circularly polarizing plate 175 has a function of converting linearly polarized light into circularly polarized light. When a laser is used as the light source 150, the light emitted from the sensor head 142 is linearly polarized light. The linearly polarized light emitted from the sensor head 142 is converted into circularly polarized light when passing through the circularly polarizing plate 175.

When the pattern formed on the surface of the wafer 124 is a regular pattern extending in one direction, the reflectance of light on the surface of the wafer 124 may be influenced by the direction of the pattern. Therefore, in the present embodiment, the surface of the wafer 124 is irradiated with circularly polarized light. The reflectance of circularly polarized light hardly depends on the direction of the pattern, and as a result, the measurement accuracy of the surface height becomes stable.

FIG. 9 is a view showing still another embodiment of the surface height measuring system 140. The configuration of this embodiment that is not particularly described is the same as that of the embodiment shown in FIG. 2, and thus the duplicate description thereof will be omitted. The surface height measuring system 140 includes a reference structure 180 fixed to the sample table 121. Although the reference structure 180 is fixed to the side surface of the sample table 121 in this embodiment, the reference structure 180 may be fixed to the upper surface of the sample table 121.

The reference structure 180 has a flat upper surface parallel to the table surface 121a. The material of the reference structure 180 is not particularly limited, but is made of a material (for example, metal) that easily reflects light. In the present embodiment, the height of the upper surface of the reference structure 180 is the same as the surface of the wafer 124 on the table surface 121a, but may be different from the height of the surface of the wafer 124.

The reason why the reference structure 180 is provided is to correct the distance Zno between the objective lens 115 and the surface of the wafer 124. That is, as shown in FIG. 9, when the entire sample table 121 is tilted or the position of the sensor head 142 in the height direction is incorrect, the distance Zno obtained by the above equation (1) is the objective lens. It does not show the correct distance between 115 and the surface of the wafer 124. Therefore, in the present embodiment, the surface height measurement system 140 calculates the distance Zno using the following equation (2).
Zno=Mns-Zoff+correction value (2)

The correction value is obtained by the following equation (3).
Correction value=ΔZns+ΔZnt=Zoff+Zre−Mre (3)
However, as shown in FIG. 10, ΔZns represents the difference between the reference height set in the surface height measuring system 140 and the actual reference height (height of the lower surface of the upper wall 137), and ΔZint is The difference in height of the reference structure 180 due to the inclination of the sample table 121, that is, the height of the reference structure 180 when the reference structure 180 is located directly below the objective lens 115, and the reference structure 180 is a sensor. A difference between the height of the reference structure 180 and the height of the reference structure 180 when the head is directly below the head 142, and Zre is the lower end of the objective lens 115 and the reference when the reference structure 180 is directly below the objective lens 115. The distance from the structure 180 is represented, and Mre represents the measured value of the height of the reference structure 180 when the reference structure 180 is located directly below the sensor head 142, as indicated by the dotted line in FIG. ing.

The distance Zre is measured as follows. Scanning electron microscope 100 produces multiple images of reference structure 180 while changing focus. The focus changes according to the value of the current flowing through the objective lens 115. The scanning electron microscope 100 selects one of the obtained images that is the clearest, and determines the focus (that is, the current value that has flown through the objective lens 115) corresponding to the selected image. The distance Zre between the lower end of the objective lens 115 and the reference structure 180 can be calculated from the determined focus.

The arithmetic device 160 calculates the correction value by the above equation (3), and stores the obtained correction value in the storage device 160B. After measuring the surface height of the wafer 124, the arithmetic device 160 calculates the distance Zno between the objective lens 115 and the surface of the wafer 124 using the above equation (2). According to this embodiment, the distance Zno is obtained in which the influence of the inclination of the sample table 121 and the positional deviation of the sensor head 142 in the height direction is eliminated.

The information on the distance Zno obtained by the above equation (2) is sent from the arithmetic device 160 to the objective lens control unit 135. The objective lens control unit 135 operates the objective lens 115 to correct the focal position of the electron beam based on the distance Zno.

FIG. 11 is a diagram showing a height map showing the relationship between the position of the measurement point on the surface of the wafer 124 and the distance Zno from the lower end of the objective lens 115 to the measurement point on the surface of the wafer 124. In FIG. 11, the X axis represents the position on the surface of the wafer 124 along the X direction, the Y axis represents the position on the surface of the wafer 124 along the Y direction, and the Z axis represents the distance Zno. .. The height map is displayed on the display device 160C (see FIG. 2) of the arithmetic device 160. The user can know the height distribution of the surface of the wafer 124 from the height map appearing on the display device 160C.

The distance Zno at an arbitrary target measurement point on this height map can be obtained by using a known value around it. Hereinafter, one example of obtaining the distance Zno at the target measurement point will be described.
The arithmetic unit 160 has four measured data strings (x0, y0, z0), (x1, y1, z1), (x2, y2) around the target measurement point represented by the coordinates (X′, Y′). , Z2), (x3, y3, z3) are determined, and the plane Zno=ax+by+c closest to these four points is obtained. Further, the arithmetic device 160 substitutes X′ for x and Y′ for y in the above equation representing the plane to obtain the distance Zno at the target measurement point as Zno=aX′+bY′+c. be able to.

Here, the parameters a, b, and c of the equation expressing the plane are values that minimize the sum of squares Q=Σ(axi+byi+ci−zi) ² of the distance between all the point sequences and the plane, and are calculated by the following equation. ..
a=[C(GH-nE)+F(EG-BH)+D(nB-G ² )]/[A(G ² -nB)-2CFG+BF ² +nC ² ]
b=[A(GH-nE)+F(-CH-DG)+EF ² +nCD]/[A(G ² -nB)-2CFG+BF ² +nC ² ]
c=[A(EG-BH)+C ² H-CDG+(BD-CD)F]/[A(G ² -nB)-2CFG+BF ² +nC ² ]
However, A=Σxi ² , B=Σyi ² , C=Σxiyi, D=Σxizi, E=Σyizi, F=Σxi, G=Σyi, H=Σzi

FIG. 12 is a flowchart for performing the height measurement of the wafer 124. In step 1-1, the wafer 124 is placed on the table surface 121 a of the sample table 121. In step 1-2, the table stage 123 moves the sample table 121 and the wafer 124 until the measurement point on the surface of the wafer 124 is located directly below the sensor head 142. In step 1-3, the surface height measuring system 140 measures the surface height of the wafer 124. In step 1-4, the arithmetic device 160 calculates the distance Zno between the lower end of the objective lens 115 and the surface of the wafer 124 at the above measurement point using the above equation (2) or equation (3). The operations of steps 1-2 to 1-4 are repeated until the distance Zno is calculated at all of the previously designated measurement points. In step 1-5, the arithmetic device 160 creates a height map showing the relationship between the value of the distance Zno and the corresponding measurement point, and displays it on the display device 160C.

FIG. 13 is a flowchart illustrating the autofocus operation of the scanning electron microscope 100. In step 2-1, the table stage 123 moves the sample table 121 and the wafer 124 until the target measurement point on the surface of the wafer 124 is located directly below the objective lens 115. In step 2-2, the arithmetic device 160 calculates the distance Zno at the target measurement point from the height map. In step 2-3, the scanning electron microscope 100 sets the focus of the objective lens 115 to the calculated distance Zno. In step 2-4, the scanning electron microscope 100 executes the focus search of the electron beam. In this focus search, the scanning electron microscope 100 generates a plurality of images of the surface of the wafer 124 at the target measurement point, determines the sharpest image among the generated images, and determines the determined image. The focus (that is, the value of the current flowing through the objective lens 115) at the time of generating is determined. In step 2-5, scanning electron microscope 100 produces an image of the surface of wafer 124 at the determined focus.

The embodiment described above can be modified as appropriate. For example, two sets of sensor heads 142, beam splitters 146, and reflectors 148 may be placed on either side of objective lens 115, or multiple sets of sensor heads 142, beam splitters 146, and reflectors 148 may be placed on the objective. They may be arranged at equal intervals around the lens 115.

The respective embodiments described above can be combined as appropriate. For example, the transparent block 170 shown in FIGS. 6 and 7 can be applied to the embodiments shown in FIGS. 8 and 9.

The above-described embodiments are described for the purpose of enabling a person having ordinary knowledge in the technical field to which the present invention belongs to implement the present invention. Various modifications of the above-described embodiment can be naturally made by those skilled in the art, and the technical idea of the present invention can be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be construed in the broadest scope according to the technical idea defined by the claims.

100 Scanning electron microscope 101 Electron beam irradiation system 111 Electron gun 112 Focusing lens 113 X deflector 114 Y deflector 115 Objective lens 118 Image acquisition device 120 Vacuum chamber 121 Sample table 121a Table surface 123 Table stage 130 Secondary electron detector 131 Reflection Electron detector 135 Objective lens control unit 137 Upper wall 140 Surface height measuring system 142 Sensor head 144 Spectral interference displacement meter 146 Beam splitter 148 Reflector 148A First reflector 148B Second reflector 150 Light source 151 Spectrometer 155 Waveform analyzer 156 concave lens 158 housing 160 arithmetic device 160A processing device 160B storage device 160C display device 170 transparent block 175 circularly polarizing plate 180 reference structure

Claims (7)

An electron beam irradiation system,
A vacuum chamber,
A sample table arranged in the vacuum chamber,
A table stage for moving the sample table,
A surface height measuring system for measuring the surface height of the sample on the sample table;
Equipped with an objective lens control unit that corrects the focal position of the electron beam,
The surface height measuring system,
A spectral interference displacement meter having a sensor head arranged facing the table surface of the sample table;
A beam splitter disposed between the sensor head and the table surface,
A reflector arranged on the optical path branched by the beam splitter,
A reference structure fixed to the sample table,
The offset value is subtracted from the measured value of the surface height of the sample, and a predetermined correction value is added to the measured value to calculate the distance between the lower end of the electron beam irradiation system and the surface of the sample. Equipped with a computing device that
The correction value is a distance between the lower end of the electron beam irradiation system and the reference structure, a predetermined reference height and a distance between the lower end of the electron beam irradiation system and the surface height measurement system. A scanning electron microscope, which is a numerical value obtained by subtracting the obtained measurement value of the height of the reference structure. The scanning electron microscope according to claim 1, wherein the sensor head is perpendicular to the table surface. The scanning electron microscope according to claim 1, wherein the surface height measuring system is arranged outside the vacuum chamber. The scanning electron microscope according to claim 1, wherein the sensor head is arranged in a housing fixed to an upper wall of the vacuum chamber. The scanning electron microscope according to claim 1, wherein the surface height measuring system further includes a transparent block to which the beam splitter and the reflector are fixed. The scanning electron microscope according to claim 1, wherein the surface height measuring system further includes a concave lens arranged between the sensor head and the beam splitter. The scanning electron microscope according to claim 1, wherein the surface height measuring system further includes a circularly polarizing plate arranged between the beam splitter and the table surface.

Images