JP2005038641A - Charged particle beam device equipped with aberration control unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To overcome a problem that the adjustment accuracy of an aberration compensator 12 is deteriorated because a standard sample 5 used for adjusting the aberration compensator 12 is gradually contaminated by the irradiation of a charged particle beam, in a charged particle beam device incorporating the aberration compensator 12. <P>SOLUTION: Preparing a plurality of standard samples 5 on a sample stage, the standard sample used for the adjustment is selected based on the number of times for adjusting the aberration compensator 12 in the past. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a charged particle beam apparatus such as a scanning electron microscope, and more particularly to an aberration control apparatus for a charged particle beam apparatus incorporating an aberration corrector.
[0002]
[Prior art]
An example of the charged particle beam apparatus will be described below based on the scanning electron microscope shown in FIG.
[0003]
In FIG. 6, reference numeral 100 denotes an electron optical system constituting a scanning electron microscope, which is in a vacuum atmosphere. The electron beam 102 generated by the electron gun 101 passes through a plurality of electron lenses, such as a condenser lens 103, and is finally focused finely by the objective lens 104 and irradiated onto the sample 105. The sample 105 is placed on a sample stage 111 provided in a sample chamber 110 that is also in a vacuum atmosphere. The stage 111 can move in any direction on at least a horizontal plane, and can bring a desired location on the sample 105 under irradiation of the electron beam 102. Further, the electron beam 102 is deflected and scanned two-dimensionally by a deflector 106. Along with the irradiation of the electron beam 102, secondary electrons are generated from the sample 105, and the generated secondary electrons are detected by the detector 107. The detected electrical signal from the detector 107 is input to the CRT 120 that is swept in synchronization with the scanning, and the secondary electron intensity is displayed, whereby a scanning electron microscope image (hereinafter referred to as an enlarged image of the sample 105) is displayed. Simply referred to as a scanned image).
[0004]
In such a scanning electron microscope, if the beam diameter of the electron beam 102 on the sample 105 is focused more narrowly, a scanning image with a higher resolution can be obtained. However, the beam diameter is limited by the aberration of the electron optical system constituting the scanning electron microscope, mainly the aberration of the objective lens 104. Therefore, recently, it has become possible to improve the resolution by incorporating an aberration corrector 130 that cancels the aberration of the objective lens 104 and the like. Note that the aberration corrector 130 is normally disposed on the upstream side of the objective lens 104. In such an aberration corrector 130, since the structure of the aberration corrector itself is extremely complicated, high-accuracy adjustment is required. For this reason, even if the observation conditions of the scanning electron microscope, for example, only the focus of the objective lens is changed, the adjustment of the aberration corrector may be out of order, leading to degradation of resolution. In such a case, the aberration corrector is readjusted by a known adjustment method to restore the performance.
[0005]
The principle and operation of the aberration corrector are described in detail in Non-Patent Document 1, for example, and the adjustment method is described in detail in Non-Patent Document 2.
[0006]
[Non-Patent Document 1]
Design of a high-resolution low-voltage scanning electron microscope, Optic. 83, no. 1 (1989) p. 30-40
[Non-Patent Document 2]
Aberration correction in a low voltage SEM by a multipole collector, Nuclear Instruments and Methods in Physics research, A363 (1995) pp. 316-325
[0007]
[Problems to be solved by the invention]
As described above, when the adjustment of the aberration corrector 130 is out of order and the resolution is deteriorated, a standard sample previously mounted at a predetermined location on the sample stage is brought under the irradiation of the electron beam, After making the standard sample come to the center of the scanned image, the aberration corrector 130 is readjusted by a known adjustment method based on information obtained from the scanned image.
[0008]
However, when performing such an operation, due to the electron beam irradiation, the standard sample is gradually contaminated, so the shape of the standard sample is changed, and an accurate aberration correction value cannot be obtained. In order to recover the performance or to recover, it is necessary to search for an appropriate standard sample.
[0009]
An object of the present invention is to provide an aberration control apparatus that solves the above-described problems of the prior art, prepares an optimal standard sample, and can reliably and quickly adjust the aberration corrector.
[0010]
[Means for Solving the Problems]
The charged particle beam apparatus having the aberration control apparatus of the present invention is a charged particle beam apparatus that observes or processes a sample in a scanned image by two-dimensionally scanning a focused charged particle beam and irradiating the sample. ,
An aberration corrector for correcting the aberration of the optical system constituting the charged particle beam device;
A standard sample placed at a specific location on the stage on which the sample is placed;
An objective lens for focusing a charged particle beam on the standard sample;
A secondary electron detector for detecting secondary electrons generated from the standard sample;
An aberration calculator for calculating aberrations based on the output of the secondary electron detector;
A correction determination unit for determining whether to control the aberration corrector from the magnitude of aberration;
A lens controller for controlling the lens intensity of the aberration corrector or the objective lens;
And a stage controller for controlling the stage position based on the output of the correction judgment unit.
[0011]
Furthermore, a plurality of the standard samples are mounted on the stage, and the aberration control device selects the standard sample for performing aberration correction based on the number of times or history of aberration correction in the past. It is characterized by that.
[0012]
Still further, the plurality of attached standard samples have different heights from the upper surface of the stage, and the aberration control device has different heights according to the lens strength of the objective lens. A standard sample for correcting aberrations is selected from the above.
[0013]
In addition, the standard sample is a pattern on the substrate on which gold particles, platinum particles, aluminum particles, latex spheres, or repeated figures of circles, squares, and triangles are drawn.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing a main part of a scanning electron microscope incorporating the present invention for explaining the configuration and operation of the present invention. In FIG. 1, the electron gun, the condenser lens, and the deflector described in FIG. 6 are omitted.
[0015]
In FIG. 1, 1 is an electron beam, 12 is an aberration corrector, 2 is an objective lens, 3 is a sample stage, 4 is a sample to be observed or processed, and 5 is a standard sample for adjusting the aberration corrector. It is. The electron beam 1 is focused by the objective lens 2 and irradiated on the sample 4 or the standard sample 5. The standard sample 5 in the present invention is composed of a large number of standard samples, and it is possible to select which one to use. As described in detail in Non-Patent Document 1, the aberration corrector 12 includes a plurality of multipole elements. 6 is a secondary electron detector for detecting, for example, secondary electrons 7 generated from the sample 4 or the standard sample 5 with the irradiation of the electron beam 1, and 11 is the secondary electron detector 6. It is a signal processor for processing the output signal from. In normal observation of the sample, the sample is observed as a scanned image on the CRT shown in FIG. 6 using the electrical signal from the signal processor 11.
[0016]
On the other hand, in the operation for adjusting the aberration corrector 12, the standard sample 5 is placed under the irradiation of the electron beam 1, the secondary electrons from the standard sample 5 are detected by the secondary electron detector 6, and the signal The signal processed by the processor 11 is sent to the aberration calculator 10 to calculate the amount of aberration.
[0017]
As described in detail in Non-Patent Document 2, the calculation of the amount of aberration uses spherical particles such as latex spheres as the standard sample 5 and observes them as a scanned image, while the objective lens 2 and the aberration corrector 12 are used. The intensity of each multipole that constitutes (the excitation intensity of an electromagnetic lens or magnetic type multipole, or the applied voltage intensity of an electrostatic lens or electrostatic multipole) is adjusted, and then obtained. Based on the size and direction of the blur of the spherical particles on the scanned image to be obtained, what kind of aberration and how much is determined. In the correction of the aberration, the aberration corrector 12 generates a reverse aberration having the same magnitude as the aberration obtained by the above calculation, and cancels the aberration in the entire optical system.
[0018]
The calculation result is sent to the correction determination unit 13 to determine whether or not aberration correction adjustment should be performed. When the correction determiner 13 determines that the aberration correction adjustment should be performed, the correction determiner 13 selects an appropriate standard sample from the standard samples 5, and the stage controller 9 based on the selection result. Then, the stage 3 is driven to the position of the standard sample. The method of determination by the correction determination unit 13 will be described in detail later. Further, when the correction determination unit 13 determines that the aberration correction adjustment should be performed, the lens controller 8 controls the aberration correction unit 12 and the objective lens 2 using the calculation result by the aberration calculator 10 based on the correction determination unit 13. .
[0019]
Further, under the control of the aberration corrector 12 and the objective lens 2, secondary electrons from the standard sample 5 are detected by the secondary electron detector 6, and the signal processed by the signal processor 11 is used. Then, the aberration calculator 10 calculates the aberration amount again. If the correction determination unit 13 determines from the calculation result that it is not necessary to perform aberration correction adjustment, the correction of the aberration is completed.
[0020]
Next, the operation of the aberration calculator 10 will be described in detail with reference to FIGS. FIG. 3 (f) shows one spherical particle sample 14 in the standard sample 5. FIG. 2 (a) shows a scanning image when the objective lens 2 is observed in an overfocus state using the standard sample shown in FIG. 3 (f), and FIG. A scanning image when observed under an under-focus state is shown. FIG. 2C shows a scanned image when the objective lens 2 is observed in a just focus state.
[0021]
As described in Non-Patent Document 2, for example, after the spherical particle sample 14 shown in FIG. 3 (f) is observed on the scanning image and a just focus image as shown in FIG. 2 (c) is obtained, Suppose that the intensities of the multipole elements of the objective lens 2 and the aberration corrector 12 are adjusted to obtain an image as shown in FIGS. 2A and 2B, for example. Therefore, with reference to the just focus image as shown in FIG. 2 (c), in FIG. 2 (a) and FIG. 2 (b), the size and direction of the blur of the image of the spherical particle 14 as the standard sample are conventionally determined. It is calculated by a technique of image processing that is a technique. Then, from the result, the type of the generated aberration and the amount to be corrected are obtained. That is, the aberration calculator 10 has a just-focus image (FIG. 2C) on the image surface on the electron optics of the spherical particle 14 shown in FIG. 3F and an upper side from the image plane shown in FIG. This occurs due to the aberration that occurs in the image when the image plane is moved (over focus) and the aberration that occurs in the image when the image plane is moved downward (under focus) as shown in FIG. Calculate the aberrations.
[0022]
For example, as shown in FIGS. 2 (a) and 2 (b), focusing on the x direction, upper right aberration 15, upper left aberration 16, lower right aberration 17, and lower left aberration 18 are observed. Suppose that As shown in the figure for each distance relationship,
W ^o : full upper aberration width, W ^u : full lower aberration width,
w ^o _l : upper left aberration amount, w ^u _l : lower left aberration amount,
W ^o _{r is the} upper right aberration amount, w ^u _{r is the} lower right aberration amount, and each is calculated from the image. The type and size of aberration can be quantified by the following index.
Relativity of generated aberration: ΔW = W ^u −W ^o
Relativity of upward generated aberration: Δw ^o = w ^o _r -w ^o _l
Lower occurrence aberration of _{^{relativity: Δw u = w u r -w}} u l
In the above, for example, ΔW represents the relative amount of the generated aberration, and ΔW = 0 indicates that the aberration is corrected. Further, if Δw ^o ≠ 0 or Δw ^u ≠ 0, it indicates that an asymmetrical aberration (for example, coma aberration) has occurred. From these indices, the correction amount to be controlled using the aberration corrector 12 can be set from the following equation (1).
ΔP ^{n, x} _m = k ₁ (n, m) · ΔW + k ₂ (n, m) · Δw ^o + k ₃ (n, m) · Δw ^u (1)
n: Number of multipole, = 1 to 4 (stage)
m: type of multipole, = 1 is a dipole field, = 2 is a quadrupole field, = 3 is a hexapole field, = 4 is an octupole field ΔP ^{n, x} _m : aberration correction amount k _{1 in the} x direction (n , M), k ₂ (n, m), k ₃ (n, m): parameters depending on n and m In the above equation (1), ΔP ^{n, x} _m represents the aberration correction amount in the x direction, Similarly, the aberration correction amount [Delta] P ⁿ in the y ^direction, _{y m} can also be set.
[0023]
From the aberration correction amount calculated in this way, the correction determiner 13 determines whether or not to adjust the output of the aberration corrector 12 from the following equation (2).
| ΔP ^{n, x} _m |> ξ (n, m), or | ΔP ^{n, y} _m |> ξ (n, m) (2)
Here, ξ (n, m) is a parameter depending on n and m. When Expression (2) is satisfied, it is determined that the output of the aberration corrector 12 is adjusted, and an instruction is issued to the lens controller 8 that controls the lens output. At the same time, the correction determiner 13 outputs a stage movement instruction and x and y coordinates indicating the position to be moved to the stage controller 9 that controls the stage position. The x and y coordinates indicating the position to be moved indicate one of the positions of a plurality of samples in the standard sample 5.
[0024]
By the way, the standard sample 5 has, for example, spherical particles 14 as shown in FIG. 3 (f). To correct the aberration, the electron beam is irradiated onto the spherical particles 14 to obtain a scanned image. As the beam is irradiated, contaminants gradually adhere to the spherical particles 14. For this reason, since an accurate aberration correction value cannot be obtained, the same standard sample cannot be used repeatedly.
[0025]
Therefore, a large number of spherical particles 14 are prepared as shown in FIG. 3B, and the coordinates S (X, Y) of the individual spherical particles 14 are stored in advance in a memory in the stage controller 9, for example. The stage 3 is moved so that the spherical particles 14 used for the measurement are changed one after another as the measurement for correcting the aberration is repeated. For example, as shown in FIG. 5A, the coordinates of the spherical particles 14 used for the measurement are S (X1, Y1), S (X2, Y2),..., S (Xn, Yn), S (Xn + 1, Yn + 1),..., And is sequentially moved at every measurement for aberration correction. Here, (Xn, Yn) represents the coordinates of the standard sample used for the measurement for correcting the nth aberration.
[0026]
In the above method, the coordinates of the individual spherical particles 14 are stored in advance, but not the coordinates of the position of the spherical particles 14 but only the range on the stage where any of the spherical particles 14 will exist. In addition, a method of sequentially designating the visual field within the range may be used. However, in this method, since the spherical particle 14 is not always in the center of the designated field of view, the spherical particle 14 is brought to the center of the field of view manually or automatically. In this way, the trouble of storing the coordinates of a large number of spherical particles 14 in advance can be saved.
[0027]
Further, the above is an example in which the coordinates of the standard sample are selectively moved in a predetermined order, but random numbers are generated and the coordinates are selected probabilistically based on the result, as shown in FIG. It can also be moved randomly.
[0028]
Furthermore, a memory for storing the history or history of which standard sample was used for the measurement for aberration correction performed in the past may be provided in the stage controller 9 or the correction determination unit 13, for example. Then, which standard sample is used for each measurement (for example, coordinate position) is stored, and based on this, the standard sample to be used next is selected. In this way, since the measurement is performed with a standard sample to which no contaminant is attached or is less attached, an accurate aberration correction value can be obtained.
[0029]
Further, since the focus of the objective lens 2 changes according to the height of the sample 4 (or the distance between the objective lens 2 and the sample 4), the aberration of the objective lens 2 changes. Therefore, the aberration corrector 12 needs to set the aberration correction conditions according to the height of the sample 4. Therefore, in the aberration control device of the present invention, the lens strength of the objective lens 2 according to the height of the sample 4 (between the lens strength of the objective lens 2 at the time of just focus and the distance between the objective lens 2 and the sample 4). Or the height of the sample 4 is the highest among the standard samples 5 prepared from a plurality of preset standard samples according to the output of a sample height detector (not shown). Select a standard sample close to.
[0030]
In such a case, the standard sample 5 may be attached to a standard sample mounting plate having an appropriate level difference, or a large number of standard samples may be placed on a flat standard sample mounting plate and the standard sample mounting plate may be mounted. By attaching it to the stage 3 with a slight inclination, a large number of standard samples having a desired height can be present in a specific portion. Alternatively, the stage 3 is provided with a mechanism capable of moving up and down separately, so that the lens strength at the time of just focus of the objective lens 2 on the standard sample is equal to the lens strength at the time of just focus at the height of the sample 4. It is also conceivable to adjust the vertical movement. Further, a standard sample having a step and a mechanism for adjusting the vertical movement may be combined.
[0031]
Furthermore, Fourier transformation calculation can be used for the calculation of the aberration in the aberration calculator 10. This technique is described in detail in re-published patent WO01 / 56057 (Method for Detection Geometrical-Optical Aberrations). According to this method, using the spherical particle 14 shown in FIG. 3 (f), the upper right aberration 15 which is the aberration in the specific direction of the spherical particle 14 shown in FIG. 2 (a) and FIG. It is not necessary to detect the left aberration 16, the lower right aberration 17, and the lower left aberration 18 by an image processing technique. That is, the amount of aberration and the type of aberration in an arbitrary direction can be quantitatively calculated from the just focus image, the overfocus image, and the underfocus image.
[0032]
In the case of using the Fourier transform calculation, the standard sample 5 can usually use gold particles, platinum particles, aluminum particles, or the like on a carbon substrate as shown in FIG. Further, a latex sphere dispersed as shown in FIG. 3 (b), a circle repeating figure as shown in FIG. 3 (c), or a square repeating figure as shown in FIG. 3 (d). Further, a triangular repeating figure as shown in FIG.
[0033]
Furthermore, in the correction determination unit 13, the above-described determination as to whether or not to correct is stored in the knowledge base 20 as a rule of the “if / then” format as shown in FIG. By incorporating an inference engine 19 such as an expert system, it is possible to make the above-mentioned judgment in detail. Similarly, the above determination can be made by incorporating fuzzy inference, which is a known technique, into the inference engine 19.
[0034]
The present invention has been described above by taking the scanning electron microscope as an example. However, the aberration control device according to the present invention is not limited to the scanning electron microscope incorporating the aberration corrector, but the focused ion beam device incorporating the aberration corrector. The present invention can be applied to all charged particle beam apparatuses such as (FIB apparatus) that observe and process a sample by irradiation with a focused charged particle beam.
[0035]
【The invention's effect】
As described above, the aberration control apparatus according to the present invention automatically selects a standard sample that is free from contamination even if the standard sample is contaminated by electron beam irradiation. That is, the accuracy of adjustment of the aberration corrector 12 can be improved. The electron beam alignment accuracy in the aberration corrector can be improved by automatically detecting the height of the sample and selecting a standard sample that matches the height. Since the sample can be automatically selected, it is not necessary for the operator to select the optimum standard sample, and the electron beam alignment in the aberration corrector can be completed in a short time.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the configuration and operation of the present invention.
FIG. 2 is a diagram for explaining aberration measurement according to the present invention.
FIG. 3 is a diagram for explaining a standard sample according to the present invention.
FIG. 4 is a diagram for explaining another example of determination of a correction determination device according to the present invention.
FIG. 5 is a diagram for explaining a method of selecting a standard sample according to the present invention.
FIG. 6 is a diagram for explaining a scanning electron microscope incorporating an aberration corrector as a conventional technique.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1,102: Electron beam 2, 104: Objective lens 3, 111: Sample stage 4, 105: Sample 5: Standard sample 6, 107: Secondary electron detector 7: Secondary electron 8: Lens controller 9: Sample stage Controller 10: Aberration calculator 11: Signal processor 12, 130: Aberration corrector 13: Aberration determiner 14: Spherical particle 15: Upper right aberration 16: Upper left aberration 17: Lower right aberration 18: Lower left Diaphragm 19: Inference engine 20: Knowledge base

Claims (12)

In a charged particle beam device that scans a focused charged particle beam two-dimensionally and irradiates the sample with a scanned image,
An aberration corrector for correcting the aberration of the optical system constituting the charged particle beam device;
A standard sample placed at a specific location on the stage on which the sample is placed;
An objective lens for focusing a charged particle beam on the standard sample;
A secondary electron detector for detecting secondary electrons generated from the standard sample;
An aberration calculator for calculating aberrations based on the output of the secondary electron detector;
A correction determination unit for determining whether to control the aberration corrector from the magnitude of aberration;
A lens controller for controlling the lens intensity of the aberration corrector or the objective lens;
A charged particle beam apparatus having an aberration control apparatus, comprising: a stage controller that controls a stage position based on an output of the correction determiner. The charged particle beam apparatus having an aberration control apparatus according to claim 1, wherein a plurality of the standard samples are attached on the stage. 3. The charged particle beam apparatus having an aberration control apparatus according to claim 2, wherein the plurality of attached standard samples have different heights from the upper surface of the stage. 3. The aberration control apparatus according to claim 2, wherein the aberration control apparatus selects the standard sample for performing aberration correction based on the number or history of aberration correction performed in the past. Charged particle beam device. The said aberration control apparatus selects the area | region on the said stage regarding the said standard sample for performing aberration correction based on the frequency | count or history which performed aberration correction in the past. Charged particle beam apparatus provided with an aberration control apparatus. 6. The charged particle beam apparatus with an aberration control device according to claim 5, wherein the aberration control device selects a region at random using a random number when selecting the region on the stage. 7. The standard sample is a gold particle, platinum particle, aluminum particle, latex sphere on a carbon substrate, or a pattern on a substrate on which repeated figures of circles, squares and triangles are drawn. A charged particle beam device comprising the aberration control device according to any one of the above. 8. The particle on the carbon substrate is a mixture of a plurality of particles having different sizes, or the pattern on the drawn substrate is a mixture of patterns having different sizes. Charged particle beam apparatus provided with an aberration control apparatus. 8. The charged particle beam apparatus having an aberration control apparatus according to claim 7, wherein the drawn pattern on the substrate is a mixture of patterns having different shapes. 4. The aberration control according to claim 3, wherein the aberration control device selects a standard sample for performing aberration correction from the standard samples having different heights according to the lens intensity of the objective lens. Charged particle beam device equipped with the device. The aberration controller according to claim 1, wherein the stage controller controls the stage position so that the position of the selected standard sample is the center of the scanned image. Charged particle beam device. 12. The correction judgment unit includes a fuzzy inference engine or an inference engine based on an expert system, and judges correction judgment according to rules stored in a knowledge base. A charged particle beam device provided with an aberration control device.

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