JP2002075263A - Electron beam device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron beam device which can obtain a maximum enlargement effect of focal depth at minimum number of image uptake by acquiring a plurality of images with an optimum focal shifting amount in interlocking with image- forming conditions. SOLUTION: With the electron beam device equipped with beam-focusing means 5, 6 for stopping down a first electron beam 4 emitted from an electron beam source with an object lens, a beam scanning means for scanning the first electron beam on a sample 10, a detecting means 13 for detecting a second signal 12 generated from the sample by the beam scanning, and an image-forming means forming a sample image from the second signal 12 to obtain a scanning image of the sample, the device is further equipped with a focus control volume determining means 15 for determining a changing width of a beam-focusing position in interlocking with the image-forming conditions, a focus controlling means for controlling focus conditions of the beam in correspondence with the focus control volume, image number determining means for determining the number of image sheets, an image uptaking means for continuously uptaking a plurality of images of different focus conditions controlled by the focus controlling means, and a memory means for memorizing the plurality of images.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam apparatus,
In particular, the present invention relates to an electron beam apparatus (electron microscope) suitable for obtaining a sample image with a large depth of focus while minimizing sample damage.

[0002]

2. Description of the Related Art An electron beam apparatus, such as a scanning electron microscope, which obtains an enlarged image of a sample by scanning a converging electron beam on the sample, has a shorter electron wavelength than light. This is an apparatus that can obtain a magnified image of the sample with high resolution and a large depth of focus.

However, the beam convergence angle has increased with the recent increase in the resolution of the apparatus, and as a result, the depth of focus of the enlarged image has decreased.

[0004] On the other hand, originally with respect to an optical microscope image having a shallow depth of focus, image processing software for composing a plurality of digital images having different focal positions to construct an image having a large depth of focus is commercially available.

Japanese Patent Application Laid-Open No. 5-299048 discloses a method in which a plurality of scan images having different focus conditions are taken in.
A technique for forming a three-dimensional image of a sample has been disclosed.

[0006]

In general, the magnification of a scanning electron microscope can be varied in the range of several tens to one million times, and has a dynamic range that is orders of magnitude higher than that of an optical microscope. are doing. In such an observation device, it is important to optimally control the amount of defocus between images to be synthesized in order to synthesize an image having a deep depth of focus from a plurality of images having different focus positions.

SUMMARY OF THE INVENTION It is an object of the present invention to acquire a plurality of images with an optimum defocus amount in conjunction with image forming conditions such as apparatus parameters and observation conditions, thereby maximizing the depth of focus with a minimum number of image captures. An object of the present invention is to provide an electron beam device that can obtain an effect.

[0008]

Means for Solving the Problems In order to clarify the above problems, first, the relationship between the image forming conditions and the depth of focus will be described with reference to FIGS.

FIGS. 6 and 7 both show the relationship between the observation magnification and the focused range (depth of focus) when a scanned image is formed under a constant focus condition.

FIG. 6 shows the depth of focus when the resolution of the apparatus is different under the same accelerating voltage condition, as A when the resolution is low and B when the resolution is high. On the other hand, FIG. 7 shows the depth of focus when the acceleration voltage is different for the same beam resolution, as A when the acceleration voltage is high and B when the acceleration voltage is low.

In each image forming condition, when an image having a focal depth deeper than the focal depth shown in these graphs is required, it is necessary to take in a plurality of images having different focal positions (focus conditions) and synthesize them. become.

As shown in FIGS. 6 and 7, the depth of focus largely depends not only on the observation magnification but also on beam conditions such as beam resolution and acceleration voltage. Further, the depth of focus also depends on the number of pixels when forming an image or the pixel size. That is, as the number of pixels increases, the pixel size decreases, so that the allowable value of the blur amount of the beam that can be regarded as being in focus decreases, and the depth of focus decreases.

Therefore, it is important to capture the scanned image while calculating the optimum defocus amount in conjunction with these image forming conditions including the observation magnification. For example, in a high-magnification image, it is necessary to acquire and synthesize an image with a slight defocus amount, but in a low-magnification image, the defocus amount is too fine.

That is, even if a plurality of images are captured with a certain defocus amount and reconstructed, a sufficient depth of focus effect cannot be obtained in the entire magnification range of the scanning electron microscope.

In the prior art, no consideration was given to this point, and it was necessary to capture more images than necessary to obtain an image with a predetermined depth of focus. On the other hand, when acquiring a plurality of images in order to increase the depth of focus, the same location on the sample is repeatedly scanned with a beam. Therefore, when the number of images used for synthesis increases, the sample receives beam damage,
A problem arises in that a composite image accurately reflecting the characteristics of the sample cannot be obtained. For this reason, it is particularly important to minimize the number of images used to increase the depth of focus.

To solve the above problems, the present invention provides:
Means for determining an optimal defocus amount from image forming conditions;
Means are provided for setting the depth of focus necessary for the composite image, and means for determining the minimum number of captured images that satisfy the set value of focal depth from these means.

When the observation magnification is low, the depth of focus fd of one scan image with a constant focus condition is expressed by the following equation [1].

[0018]

Fd = A1 × (dpix / M) × R × √Vacc (1) where A1 is a constant, dpix is a pixel size, M is an observation magnification, and R is a beam resolution (resolution determined by a beam diameter). , V
acc represents an acceleration voltage.

As the observation magnification increases, the resolution of the scanned image is limited to the beam resolution R, and the depth of focus at this time is expressed by the following equation [2].

[0020]

Fd = A2 × R ² × √Vacc / √ (1 + 0.73 × (Ip / B0) × 10 ¹⁴ ) (2) where A2 is a constant, Ip is a probe current, and B0 is 1V.
It represents the brightness of the electron gun converted per hit.

In the high magnification range, many conditions contributing to image formation affect the depth of focus as shown in equation [2]. In the case of a field emission type electron source having a very high brightness B0, the term (Ip / B0) in equation [2] becomes very small.

[0022]

Equation 3 can be represented with ^{fd = A2 × R 2 × √Vacc} ... [3].

In equations [1] to [3], the beam resolution R is given by the following equation [4].

[0024]

R = 0.61λ / α = 0.75 / (α × √Vacc) (4) Since the beam resolution R of the equations [1] to [3] can be expressed by the equation [4]. 4] can be replaced with the second term or the third term. Here, λ is the wavelength of the electron, α
Represents the convergence angle (half angle) of the primary beam.

If a plurality of images having different focuses are acquired and appropriately synthesized, an image having a depth of focus deeper than the depth of focus expressed by Expressions [1] to [3] can be obtained. By making the defocus amount between them equal to or slightly smaller than the values represented by the equations [1] to [3], the maximum depth of focus effect can be obtained with the minimum number of images.

The defocus amount determining means determines an optimum value based on calculations of equations [1] to [3] based on image formation conditions such as acceleration voltage, electron source luminance, probe current, number of pixels, magnification and beam resolution. Calculate the appropriate defocus amount.

In the defocus amount determining means, the results of these calculations can be described in a table in advance, and the corresponding defocus amount can be determined from the table based on the image forming conditions.

By providing means for setting the upper limit and the lower limit (the range of the depth of focus) to be focused or means for directly inputting the numerical value of the depth of focus range, the formulas [1] to [3] can be obtained. The results can be used to determine the minimum and optimal number of image captures.

Further, based on the set value of the defocus amount,
There are provided a focus control means for changing the focus every time one image is taken, a continuous image taking means for continuously taking an image with the focus control means, and an image saving means for saving a continuous image.

The focus control means performs various controls, such as controlling the focus with the current focus condition at the center, controlling the focus with the current focus as an end point, or controlling the focus within a preset focus range. Type is possible.

Further, there are provided means for calculating the depth of focus obtained by synthesizing a plurality of captured images from the focus change width between images, the number of images, and the depth of focus of the electron optical system, and display means for displaying this value. It is provided so that an observer can easily understand the depth of focus of the obtained image.

When the image synthesis for increasing the depth of focus (focus synthesis) is performed independently of the function of the scanning electron microscope, a plurality of images stored in the image storage means are combined with other image synthesis means (for example, commercially available). Software) to synthesize an image with a deep depth of focus.

In the present invention, an image construction means for constructing a composite image having a large depth of focus from a continuous image is provided in order to collectively perform the process from capturing a continuous image to displaying and storing a composite image having a large depth of focus.

Since a plurality of images used to increase the depth of focus have focal position information of each objective lens, the focal position information of the objective lens corresponding to the image position designated in the composite image is determined. can do. Therefore, an image position designating means for designating any two points on the composite image and a focus position extracting means for extracting the designated two focal points of the objective lens are provided, and designated from the extracted focal position information. Display means for calculating the height difference between the two points and displaying the result is provided.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the electron beam apparatus according to the present invention will be described below.

FIG. 1 is a schematic configuration diagram of a scanning electron microscope (SEM) as an example of the present invention. Cathode 1 and first anode 2
During this period, a voltage is applied by the high-voltage control power supply 20 controlled by the microprocessor (CPU) 40, and the primary electron beam 4 is extracted from the cathode 1 with a predetermined emission current. Since an accelerating voltage is applied between the cathode 1 and the second anode 3 by the high-voltage control power supply 20 controlled by the CPU 40, the primary electron beam 4 emitted from the cathode 1 is accelerated to the subsequent lens system. proceed.

The primary electron beam 4 is converged by a converging lens 5 (beam converging means) controlled by a lens control power supply 21.
After the unnecessary area of the primary electron beam is removed by the aperture plate 8, the converging lens 6 controlled by the second converging lens control power supply 22
(Beam converging means), and objective lens control power supply 23
Is converged as a minute spot on the sample 10 by the objective lens 7 controlled by The objective lens 7 can take various forms such as an in-lens system, an out-lens system, and a snorkel system (semi-in-lens system).

The primary electron beam 4 is applied to the sample 10 by the scanning coil 9.
The top is scanned two-dimensionally. Secondary signal (sample signal) 1 of secondary electrons and the like generated from sample 10 by irradiation of primary electron beam 1
After advancing to the upper part of the objective lens 7, 2a and 12b
The quadrature electromagnetic field generator 11 for separating the secondary signals separates the secondary signal detectors 1 according to the difference in energy.
Proceed in the direction of 3a and 13b.

These secondary signals 12a and 12b are thereafter detected by secondary signal detectors 13a and 13b.

The signals of the secondary signal detectors 13a and 13b are
The signals are stored as image signals in the display image memory 25 via the signal amplifiers 14a and 14b, respectively. The image information stored in the display image memory 25 is displayed on the image display device 26 as needed.

The signal of the scanning coil 9 is controlled by a scanning coil control power supply 24 according to the observation magnification.

For a plurality of images having different focuses, focus control conditions are calculated by the CPU 40, and the images are continuously taken in.
It is stored in the image memory 32 (image construction means). The image data stored in the image memory 32 can be taken out from the SEM.

The image stored in the image memory 32 is stored in the CPU 4.
The image is processed at 0 and synthesized as an image with an increased depth of focus, stored in the image memory 25 and displayed on the image display device 26.

The composite image can also be stored in the image memory 32, and the composite image data can be taken out of the SEM. The image processing can be performed by a program stored in the CPU 40, but can also be performed at high speed by dedicated hardware. Furthermore, since image processing can be performed at high speed with dedicated hardware,
It is also possible to combine images with different depths of focus while performing sequential image processing while capturing continuous images with different focal points.

FIG. 2 shows a C for hardware control of the SEM.
FIG. 2 is a schematic configuration diagram of a scanning electron microscope (SEM) as an example of the present invention in which another computer having data processing and a man-machine interface function is incorporated and connected separately from a PU.

In this example, after a continuous image is temporarily stored in the image memory 32 incorporated in the control CPU 40, the data is transferred to the data processing PC (computer) 42. The image data transferred to the PC 42 is
To synthesize an image with a deep depth of focus. This composite image is displayed on a display monitor 43 connected to the data processing PC 42.

FIG. 3 is a schematic configuration diagram of a scanning electron microscope in which focus control at the time of continuously taking in a plurality of images is performed by electrodes arranged in an objective lens portion.

An axially symmetric focus control electrode 15 (focus control amount determining means) is arranged in the objective lens 7. The potential distribution of this electrode is arranged so that at least a part thereof is superimposed on the magnetic field of the objective lens 7, and the voltage is controlled by a control power supply 17 for focus control,
The focus position of the primary electron changes.

FIG. 4 is a schematic configuration diagram of a scanning electron microscope in which focus control at the time of continuously taking in a plurality of images is performed by another magnetic field generating coil arranged in the objective lens portion.

Another focus control magnetic field generating coil 16 is arranged in the vicinity of the objective lens 7, and the excitation current is changed by the control power supply 18 for the focus control coil to change the focus position of the primary electron. .

FIG. 5 is a schematic configuration diagram of a scanning electron microscope in which focus control at the time of continuously capturing a plurality of images is performed by controlling the voltage applied to the sample.

A focus control magnetic field generating coil 16 for accelerating primary electrons and a sample application voltage control power supply 19 for applying a voltage to the sample are arranged in the objective lens section 7, and the voltage of the sample is controlled by the control power supply 19. As a result, the focus position of the primary electron changes.

FIG. 8 is a flowchart showing an example of a control flow for capturing a continuous image by designating the number of images. The operator can input the number of images directly on the number-of-images setting screen (number-of-images setting means) or can input the number of images by selecting the number of images from a predetermined selection range (number-of-images determining means).

On the other hand, the control CPU calculates the focal depth of the beam from the current image forming conditions (magnification, acceleration voltage, beam resolution, number of pixels, etc.), and determines the optimal focus change from the result and the designated number of images. Determine the amount.

The operator further controls the focus control at the time of acquiring the image in the Under focus direction and Ove.
r The focus direction or the bidirectional focus control centered on the current value can be set on the setting screen. The control CPU continuously captures a specified number of images while performing focus control corresponding to the specified conditions of the control method. These images are stored in an image memory and used for subsequent processing (image transfer, image synthesis, etc.).

FIG. 9 is a flowchart showing an example of a control flow for taking in a continuous image by designating the value of the depth of focus. In this case, the operator directly specifies the required value of the depth of focus. The control CPU determines the number of images and the amount of defocus that satisfy the designated depth of focus from the focal depth of the beam, and captures a continuous image in the same procedure as in FIG.
save.

FIG. 10 is a flowchart showing an example of a control flow for taking in a continuous image by designating a focused range. In this case, the operator designates a lower limit and an upper limit to be focused in the observation image (designating means for designating arbitrary two points). For that purpose, first, the focus is adjusted to the portion of the sample which is the lower limit of the focus, and this focus condition is registered in the CPU as a first condition.

Next, the focus is adjusted on the part of the sample which is the upper limit of the focus, and is registered as the second focus condition in the CPU.

The control CPU calculates a focus control range based on these registration conditions, and determines an appropriate number of images and an appropriate defocus amount from the depth of focus of the beam.

[0060]

According to the present invention, it is possible to construct an image having a depth of focus intended by an operator by synthesizing a plurality of images captured in the above-mentioned various forms.

Since the beam irradiation amount (in proportion to the number of images) required for capturing an image to a sample can be reduced to a theoretical minimum value, it is possible to suppress beam damage and reduce image acquisition time and processing time. Can be shortened.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a scanning electron microscope which is an example of the present invention.

FIG. 2 is a schematic configuration diagram of a scanning electron microscope to which another computer having a data processing and a man-machine interface function is connected.

FIG. 3 is a schematic configuration diagram of a scanning electron microscope that performs focus control during continuous capture of a plurality of images using electrodes arranged in an objective lens unit.

FIG. 4 is a schematic configuration diagram of a scanning electron microscope that performs focus control at the time of continuously capturing a plurality of images by using another magnetic field generating coil disposed in an objective lens unit.

FIG. 5 is a schematic configuration diagram of a scanning electron microscope that performs focus control at the time of continuously capturing a plurality of images by controlling a voltage applied to a sample.

FIG. 6 is a graph showing the relationship between the observation magnification and the depth of focus when the acceleration voltage is the same and the beam resolution is different.

FIG. 7 is a graph showing the relationship between the observation magnification and the depth of focus when the beam resolution is the same and the acceleration voltage is different.

FIG. 8 is a flowchart illustrating an example of a control flow for acquiring continuous images with different focuses by specifying the number of images.

FIG. 9 is a flowchart illustrating an example of a control flow for acquiring a continuous image with different focus by designating a value of the depth of focus.

FIG. 10 is a flowchart illustrating an example of a control flow for capturing a continuous image with different focus by designating an in-focus range.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... 1st anode, 3 ... 2nd anode, 4 ... Primary electron beam, 5 ... 1st converging lens, 6 ... 2nd converging lens, 7 ... Objective lens, 8 ... Aperture plate, 9 ... Scanning Coil, 10 ... sample,
11 ... quadrature electromagnetic field (EXB) generator for secondary signal separation, 1
2a: low energy secondary signal, 12b: high energy secondary signal, 13a: detector for low energy secondary signal,
13b: High energy secondary signal detector, 14a: Low energy secondary signal amplifier, 14b: High energy secondary signal amplifier, 15: Focus control electrode, 16: Focus control magnetic field generating coil, 17: Focus Control power supply for control electrode, 18 ... Control power supply for focus control coil, 19 ... Sample applied voltage control power supply, 20 ... High voltage control power supply, 21 ... First convergent lens control power supply, 22 ... Second convergent lens control power supply, 23 ... Objective Lens control power supply, 24: scanning coil control power supply, 25: display image memory, 26: image display device, 32: image memory, 40: control CPU, 42 ...
Data processing PC, 43 ... Display monitor.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01J 37/28 H01J 37/28 B (72) Inventor Hideo Todori 882 Address Ichige, Ichihiro, Hitachinaka-shi, Ibaraki Pref. Within the Hitachi, Ltd.Measurement Instruments Group (72) Inventor Makoto Esumi 882, Oji-shi, Oita, Hitachinaka-shi, Ibaraki Co., Ltd. Within the Hitachi, Ltd. Instrumentation Group (72) Inventor 882 address F-term in the Hitachi Measuring Instruments Group, Ltd. (reference) 5C033 MM02 MM05 UU05 UU06

Claims (23)

[Claims] 1. A beam converging means for narrowing down a primary electron beam emitted from an electron source with an objective lens, a beam scanning means for scanning primary electrons on a sample, and a secondary beam generated from the sample by the beam scanning. Detecting means for detecting a signal;
An electron beam apparatus for obtaining a scanned image of a sample, comprising: an image forming means for forming a sample image from a next signal; a focus control amount determining means for determining a beam convergence position change width in conjunction with an image forming condition; Focus control means for controlling a beam focus condition in response to the above, image number determination means for determining the number of images, and image acquisition for continuously capturing images under a plurality of different focus conditions controlled by the focus control means Means for storing the plurality of images. 2. A beam converging means for narrowing down a primary electron beam emitted from an electron source with an objective lens, a beam scanning means for scanning the primary electrons on a sample, and a beam generated from the sample by the beam scanning. Detecting means for detecting a next signal;
An electron beam apparatus that obtains a scanned image of a sample by providing an image forming unit that forms a sample image from the secondary signal; a focus control amount determining unit that determines a beam convergence position change width in conjunction with an image forming condition; Focus control means for controlling the beam focus condition in accordance with the focus control amount; image number determination means for determining the number of images; and images of a plurality of different focus conditions controlled by the focus control means. An electron beam apparatus comprising: an image acquisition unit to be captured; a storage unit that stores the plurality of images; and an image construction unit that constructs a new image from the plurality of storage images. 3. The focus control amount determining means determines an image determined by an acceleration voltage (Vacc), a working distance (WD) of an objective lens, a beam convergence angle (α), an image magnification (M), and a beam diameter of a primary beam. The focus control amount is determined by any one of resolution (R), the number of pixels of an image (Npix), the pixel size of an image (dpix), and a probe current (Ip), or a combination thereof. An electron beam apparatus according to claim 1. 4. Coefficients A1 and A2, observation magnification (M)
Is smaller than a predetermined value, the focus control amount =
4. The electron beam apparatus according to claim 3, wherein A1 × dpix / M is set, and when the observation magnification is larger than a predetermined magnification, the value is determined based on a relationship of focus control amount = A2 (constant). 5. 5. The electron beam apparatus according to claim 4, wherein the value of the coefficient A1 is determined in a substantially inversely proportional relationship with the beam convergence angle α. 6. The value of the coefficient A1 is determined by a relationship that is substantially proportional to the product of the square root of the acceleration voltage (Vacc) and the resolution (R) of the primary electron beam under the beam conditions. Item 5. An electron beam device according to item 4. 7. The electron beam apparatus according to claim 4, wherein the value of the coefficient A2 is determined by a relationship substantially proportional to a product of a square root of an acceleration voltage (Vacc) and a square of a resolution. 8. The coefficient A1 and / or A2 is determined by a relationship that is substantially proportional to a function value determined by a ratio between the brightness (Bs) of the electron source and the probe current (Ip). An electron beam apparatus according to any one of claims 1 to 7. 9. The electron beam apparatus according to claim 1, wherein the value of the focus control amount is determined from a table value predetermined for each acceleration voltage. 10. The electronic device according to claim 1, wherein the focus control means controls a beam convergence condition by changing a magnetic field intensity of an objective lens that finally converges the primary beam on the sample. Line equipment. 11. The electron beam apparatus according to claim 1, wherein the focus control means is constituted by another magnetic field type lens provided separately from the objective lens or an electrostatic type lens. 12. The electron beam apparatus according to claim 1, further comprising sample voltage applying means for applying a voltage to the sample, and wherein said focus control is performed by controlling the voltage of said sample voltage applying means. 13. The electron beam apparatus according to claim 1, wherein the number-of-images determining means is a means for setting the number of images to be continuously captured by inputting a numerical value or selecting from a list. An electron beam apparatus according to claim 1. 14. The electron beam apparatus according to claim 13, wherein the image number setting means limits the set maximum number according to the number of pixels of the image. 15. The image number determining means includes a focus range setting means for setting an upper limit and a lower limit of a focus range, and an image number calculating means for calculating the number of images from the set value of the focus range setting means. Claim 1
13. The electron beam device according to any one of 12 above. 16. The image number deciding means comprises focus adjustment value registering means for respectively registering two focus adjustment values, and image number calculating means for calculating the number of images from the registered values of the focus value registering means. The electron beam apparatus according to claim 15, wherein the electron beam apparatus is used. 17. The image number determination means, wherein a focus depth setting means for setting a difference (focus depth) in a focus control range, and a calculation means for calculating the number of images based on a set value of the focus depth setting means The electron beam apparatus according to any one of claims 1 to 12, comprising: 18. The apparatus according to claim 1, wherein said focus control means continuously takes in a predetermined number of images while controlling focus in an underfocus direction based on a current focus state. Electron beam equipment. 19. The apparatus according to claim 1, wherein said focus control means continuously takes in a predetermined number of images while controlling focus in an overfocus direction based on a current focus state. Electron beam equipment. 20. The apparatus according to claim 1, wherein the focus control means takes in a predetermined number of images continuously while controlling a focus in a range from overfocus to overfocus with a current focus state as a center state. An electron beam device according to any one of the above. 21. The image constructing means for constructing a new image from a plurality of images, comprising: a position correcting step for correcting a positional shift between the images; and a brightness correcting step for correcting a difference in brightness between the images. A focus image extracting process for extracting an image element having the best focus from corresponding image elements between images, and a focused image constructing step of constructing a new image by combining the extracted image elements. Two
21. The electron beam apparatus according to any one of 20. 22. The electron beam apparatus according to claim 1, further comprising display means for displaying a value of a focal depth of an image obtained by synthesizing a plurality of images obtained continuously. 23. Designating means for designating any two points on an image obtained by combining continuous acquired images, and calculating a height difference between the two points designated by the designating means. The electron beam apparatus according to any one of claims 2 to 22, further comprising a unit and a display unit for displaying a calculation result of the calculation unit.