JP2006114305A - Multipole field correction method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multipole field correction method and a device by the multipole field of an aberration corrector in which a regular operator can correct the distortion of a multipole electrode due to mechanical and electrical slippage without being conscious of the instrumental error of the multipole electrode with respect to an alignment automatic correction method and device on the multipole field correction method and device. <P>SOLUTION: In the device mounting an aberration corrector 6 consisting of multipole electrodes, when the surface of a sample is observed using charged particles, the device is constructed of a parasitic bipole field correction device 11a and a parasitic quadrupole field correction device 11b which are installed for correcting the parasitic bipole field and the parasitic quadrupole field generated by mechanical and electrical slippage of the multipole electrodes, a memory means 12 for storing the correction amount at the time the multipole field is shifted for a prescribed amount in X, Y direction in the stage where the aberration corrector is located, and a correction means 11 for correcting the parasitic bipole field and the parasitic quadrupole field using the correction amount stored in the memory means 12. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a multipole field correction method and apparatus.

  A scanning electron microscope (SEM) will be described as an example of a sample surface observation apparatus using charged particles. FIG. 2 is a diagram showing a configuration example of a conventional apparatus. This figure shows a scanning electron microscope equipped with an aberration corrector. An electron beam 2 is emitted from the emitter 1, and the electron beam 2 incident on the aberration corrector 6 is controlled by the lens 3 acting on the electron beam 2, and the electron beam 2 emitted from the aberration corrector 6 is used as the objective lens 4. To converge on the surface of the sample 5.

  The electron beam 2 is scanned on the sample surface, and secondary electrons 7 emitted from the sample surface are detected by the secondary electron detector 8 in synchronization with the scanning, whereby an image is displayed on the CRT 13 in synchronization with the scanning signal. Display as. Since the detection efficiency of the secondary electrons 7 is usually low, noise is removed by the normal image integrator 9. The image from which the noise has been removed is displayed on the CRT 13. As an example of such an aberration corrector, a technique using a multipole is known (for example, see Non-Patent Document 1).

  The aberration corrector 6 includes four stages of multipole elements (for example, 12 poles). FIG. 3 is a diagram showing a cross section perpendicular to the optical axis of the multipole element. In the figure, 6a is a pole, and an example composed of 12 poles from # 1 to # 12 is shown. The twelve poles are arranged with a phase difference of π / 6 around the optical axis. The 12-pole element can generate a dipole field, a quadrupole field, a hexapole field, and an octupole field depending on how the voltage or magnetic field applied to each pole 6a is distributed. Further, by shifting the phase, the X and Y directions of each multipole field can be defined and generated.

  FIG. 4 is a diagram showing an equipotential distribution of each multipole. (A) shows the equipotential distribution of the dipole, (b) shows the equipotential distribution of the quadrupole, (c) shows the equipotential distribution of the hexapole, and (d) shows the equipotential distribution of the octupole. As shown in the figure, four different multipole fields (a total of eight different multipole fields in consideration of the X and Y directions) can be generated simultaneously.

  By combining these multiple multipole fields, the trajectory of the electron beam 2 passing through the aberration corrector 6 is controlled, and the magnitude of the aberration is controlled. Usually, the dipole field is a deflector and is used for alignment of the multipole field.

Conventionally, the potentials or magnetic potentials V _x , V _y applied to the poles that generate these X-direction and Y-direction multipole fields are expanded by Fourier series as a function of the multipole field type and the pole number as Has been calculated.

V _x (t, p) = α (t) · cos {t · (2π / n) · p} (1)
V _y (t, p) = α (t) · sin {t · (2π / n) · p} (2)
Here, t indicates the type of the multipole field.

t = 1: dipole 2: quadrupole 3: hexapole 4: octupole p represents the pole number of the multipole (p = 0, 1, 2,..., n−1). n represents the number of poles. α (t) is a constant for each t.

It is to be noted that there is known a technique for correcting the deformation of the image of the third-order optical axis of an electron optical system having a hexapole to correct the deformation of the image of the first-order, second-order, and third-order optical axes (for example, patents). Reference 1).
Special table 2001-516139 gazette Aberration correction in a low voltage SEM by multipole corrector (Nuclear Instrument and Methods in Physics Research A 363 (1995) 316-325)

  In the above-described aberration corrector composed of multipoles, the centers of the plurality of multipole fields have a slight shift in mechanical and / or electrical (hereinafter referred to as mechanical / electrical). In addition, it is normal that the ideal multipole distribution is not distorted due to mechanical / electrical deviation. When the magnitude of the multipole field having such mechanical / electrical deviation is changed, the electron beam is deflected according to the degree of the deviation.

  Conventionally, in such a case, an operator controls the dipole field to correct the deflection of the electron beam. In addition, a parasitic quadrupole field is generated due to mechanical / electrical deviation, and similarly, an operator controls the quadrupole field to correct the distortion of the electron beam. However, this operation is very complicated because an aberration corrector having a complicated structure is operated, and there is a problem that it cannot be performed unless an operator who fully understands the characteristics of the aberration corrector. In addition, there is a problem that even a skilled operator takes a long time to control.

  The present invention has been made in view of such a problem, and it is possible for a normal operator to correct the distortion of the multipole due to mechanical / electrical deviation without being aware of the difference in the multipole. An object of the present invention is to provide a multipole field correction method and apparatus using a multipole field of an aberration corrector that can be used.

  (1) According to the first aspect of the present invention, in the case of observing the sample surface using charged particles in an apparatus equipped with an aberration corrector composed of a multipole element, a parasitic element 2 caused by mechanical and electrical misalignment of the multipole element. A step of providing a parasitic dipole field correction device and a parasitic quadrupole field correction device in order to correct distortion due to the pole field and the parasitic quadrupole field; and a multipole field in the X and Y directions of a certain stage of the aberration corrector. Is stored in the storage means in advance, and the correction value stored in the storage means is used to correct the parasitic dipole field and the parasitic quadrupole field. It is characterized by that.

  (2) According to the second aspect of the present invention, in the case of observing the sample surface using charged particles equipped with an aberration corrector comprising a multipole, a parasitic dipole field generated by mechanical and electrical misalignment of the multipole A parasitic dipole field correction device, a parasitic quadrupole field correction device, and a multipole field shifted by a predetermined amount in the X and Y directions of a certain stage of the aberration corrector provided to correct the parasitic quadrupole field. It is characterized by comprising storage means for storing correction values in advance and correction means for correcting distortion due to the parasitic dipole field and the parasitic quadrupole field using the correction values stored in the storage means.

(3) The invention described in claim 3 is characterized in that the parasitic dipole field and the parasitic quadrupole field are each expressed by Fourier series expansion with respect to an angle at the center of the multipole.
(4) The invention described in claim 4 is characterized in that a parasitic dipole field and a parasitic quadrupole field expressed by a Fourier series expansion term are superimposed to obtain a correction value.

(5) The invention according to claim 5 is characterized in that the parasitic dipole field is an amount for correcting image movement when the multipole field is minutely changed.
(6) The invention according to claim 6 is characterized in that the parasitic quadrupole field is an amount for correcting image distortion when the multipole field is minutely changed.

  (1) According to the first aspect of the invention, an aberration corrector capable of correcting the distortion of the multipole due to mechanical / electrical displacement without a normal operator being aware of the difference between the multipoles. It is possible to provide a multipole field correction method and apparatus using a multipole field.

  (2) According to the invention described in claim 2, an aberration corrector capable of correcting the distortion of the multipole due to the mechanical / electrical deviation without causing a normal operator to be aware of the difference between the multipoles. It is possible to provide a multipole field correction method and apparatus using a multipole field.

(3) According to the invention described in claim 3, the correction process can be appropriately performed by expressing the parasitic dipole field and the parasitic quadrupole field by Fourier series expansion.
(4) According to the invention described in claim 4, the parasitic dipole field and the quadrupole field are expressed by a Fourier series expansion term, and the parasitic dipole field and the quadrupole field are superimposed to obtain a correction value. It is possible to correct the distortion of the multipole due to the mechanical / electrical deviation.

(5) According to the invention described in claim 5, the parasitic dipole field can be set to an amount for correcting the image movement when the multipole field is minutely changed, and the image movement due to the multipole is corrected. Can do.
(6) According to the invention described in claim 6, the parasitic quadrupole field can be set to an amount for correcting image distortion when the multipole is minutely changed, and distortion of the multipole can be corrected. .

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of the present invention. The same components as those in FIG. 2 are denoted by the same reference numerals. In the figure, 1 is an emitter emitting a charged particle beam, 2 is an electron beam emitted from the emitter 1, 3 is a lens acting on the electron beam 2, and 6 is various corrections of the electron beam 2 subjected to the lens action. Aberration corrector 4 is an objective lens for converging the electron beam 2 on the sample 5, 5 is a sample, 7 is a secondary electron emitted from the sample 5, and 8 is a secondary electron for detecting the secondary electron 7. It is a detector.

  9 is an image accumulator for accumulating images in order to make the secondary electron image resistant to noise, 10 is an aberration correction controller for giving a control signal for aberration correction to the aberration corrector 6, and 11 is an aberration correction controller 10. This is a multipole field correction device that provides a control signal for multipole field correction. For example, a computer is used as the multipole field correction device 11. In the multipole field correction device 11, 11a is a parasitic dipole field correction device, and 11b is a parasitic quadrupole field correction device. That is, the multipole field correction device 11 includes a parasitic dipole field correction device 11a and a parasitic quadrupole field correction device 11b.

The parasitic dipole field correction device 11a and the parasitic quadrupole field correction device 11b correct the parasitic field of the multipole field caused by the mechanical and electrical misalignment of the multipoles constituting the aberration corrector 6.
A CRT 13 is connected to the image integrator 9 and displays various information. A correction value storage unit 12 stores in advance a correction value when the multipole field is shifted by a predetermined amount in the X and Y directions of a certain stage. That is, the electromagnetic field distribution of the multipole corrected by the multipole field correction device 11 is stored. The correction value storage unit 12 is connected to the multipole field correction device 11 and is also connected to the aberration correction controller 10. The operation of the apparatus configured as described above will be described as follows.

  An electron beam 2 is emitted from the emitter 1, the electron beam 2 incident on the aberration corrector 6 is controlled by the lens 3 acting on the electron beam 2, and the electron beam 2 emitted from the aberration corrector 6 is sampled by the objective lens 4. 5 converges on the surface. The electron beam 2 is scanned on the sample surface, and secondary electrons 7 emitted from the sample surface in synchronization with the scanning are detected by the secondary electron detector 8, so that they are displayed as an image on the CRT 13 in synchronization with the scanning signal. Let Since the detection efficiency of the secondary electrons 7 is usually low, noise is removed by the image integrator 9.

  The aberration correction controller 10 controls the lens intensity of the aberration corrector 6. The configuration and operation of the aberration corrector 6 are described in detail in Non-Patent Document 1. The multipole field correction device 11 includes a parasitic dipole field correction device 11a and a parasitic quadrupole field correction device 11b. The multipole field correction device 11b is a multipole field generated by mechanical and electrical misalignment of the multipoles constituting the aberration corrector 6. Correct the parasitic field. The correction value storage unit 12 stores a multipole electromagnetic field distribution corrected by the multipole field correction device 11. Hereinafter, the operation of the multipole field correction apparatus 11 according to the present invention will be described in detail.

  As an example of the aberration corrector 6, the non-patent document 1 details the aberration corrector 6 using a multipole, which is a known technique. The aberration corrector 6 is composed of four stages of multipoles (for example, twelve poles), and FIG. 3 shows a cross-sectional view perpendicular to the optical axis near the center of one stage. These 12-pole elements are composed of 12 pole elements 6a, and are arranged with a phase difference of π / 6 around the optical axis. These twelve poles can generate a dipole field, a quadrupole field, a hexapole field, and an octupole field depending on how the voltage or magnetic field applied to each pole 6a is distributed. Also, by shifting the phase, the X and Y directions of each multipole field can be defined and generated.

  FIG. 4 shows the equipotential distribution of each multipole field. (A) is a dipole field equipotential distribution, (b) is a quadrupole field equipotential distribution, (c) is a hexapole field equipotential distribution, and (d) is an octupole field equipotential distribution. Respectively. As shown in the figure, four different multipole fields (in addition, a total of eight different multipole fields considering the X and Y directions) can be generated simultaneously. By combining these multiple multipole fields, the trajectory of the electron beam 2 passing through the aberration corrector 6 is controlled, and the magnitude of the aberration is controlled. Usually, the dipole field is a deflection field and is used for alignment of the multipole field.

  Thus, in the aberration corrector 6 composed of multipoles, the centers of the plurality of multipole fields usually have some mechanical and electrical misalignment, and mechanical and electrical It is normal that distortion is not ideal multipole distribution due to misalignment. When the multipole field having such a mechanical / electrical deviation is changed, the electron beam is deflected according to the degree of the deviation. Further, since the mechanical / electrical deviation also appears as a parasitic quadrupole field, such a parasitic dipole field and a parasitic quadrupole field are corrected.

  Now, a method of correcting the parasitic dipole field and the parasitic quadrupole field of the hexapole will be described in detail as an example. If the X-direction hexapole field at a certain stage of the aberration corrector is shifted by a certain value ΔHx, the image is moved and distorted due to the mechanical / electrical displacement of the hexapole. This is returned to the center with the same dipole field, and the distortion of the image is corrected with the quadrupole field. The correction amount is divided by the ΔHx to be converted into a correction amount per unit intensity and stored in the correction value storage unit 12. Similarly, correction per unit intensity for correcting the quadrupole field in the X and Y directions, the hexapole field, the parasitic dipole field of the octupole field, and the parasitic quadrupole field in each stage (from the first stage to the fourth stage). A quantity is obtained, stored in the correction value storage unit 12, and a new multipole field in this apparatus is newly obtained by adding these correction quantities to a pure multipole field as an example. A specific calculation method is shown below.

In the multipole field correction device 11, Fourier series expansion is performed by using the potentials or magnetic potentials V _x , V _y applied to the poles that generate the X and Y direction multipole fields as a function of the multipole field type number t and the pole number p. Then, the following equation is calculated.

Here, t indicates the type of the multipole field.
t = 1: dipole 2: quadrupole 3: hexapole 4: octupole p represents the pole number of the multipole (p = 0, 1, 2,..., 10, 11,..., n−1).
n represents the number of poles. α (t) is a constant for each t. Ax (t) represents the magnitude of the multipole field possessed for each t in the X direction, and Ay (t) represents the magnitude of the multipole field possessed for each t in the Y direction. Further, (correction term) _x and (correction term) _{y in the} equations (3) and (4) are respectively expressed by the following equations.
(Correction term) _x = (parasitic dipole field correction term) _x + (parasitic quadrupole field correction term) _x (5)
(Correction term) _y = (parasitic dipole field correction term) _y + (parasitic quadrupole field correction term) _y (6)
In Equations (5) and (6), (parasitic dipole field correction term) _x , (parasitic dipole field correction term) _y and (parasitic quadrupole field correction term) _x , (parasitic quadrupole field correction term) _y are The parasitic dipole field correction device 11a and the parasitic quadrupole field correction device 11b perform Fourier series expansion, and are calculated by the following equation.

Here, t _x and t _y indicate multipole field types in the X direction and the Y direction, respectively. D _x and D _y are the magnitudes of the dipole field per unit intensity in the X and Y directions necessary to restore a shifted image as a result of applying a small amount of the multipole field. Q _x and Q _y are the magnitudes of the quadrupole field per unit intensity in the X and Y directions necessary to restore a distorted image as a result of applying a small amount of the multipole field.

In this way, the equations (3) to (10) are calculated for all stages of multipole elements constituting the aberration corrector 6, and a correction term added is newly added to the multipole field of the apparatus. And Further, the correction terms calculated by the equations (5) to (10) are stored in the correction value storage unit 12, and when used, the multipole field is calculated by the equations (3) and (4). By substituting (parasitic dipole field correction term) _x and (parasitic dipole field correction term) _y expressed by equations (7) and (8) into equations (3) and (4), parasitic dipoles are obtained. The potential distribution or the magnetic potential distribution V _x , V _y can be obtained by adding a correction term based on the field, and (parasitic quadrupole field correction term) _x , ( Parasitic quadrupole field correction term) By substituting _y into equations (3) and (4), the potential distribution or magnetic potential distribution V _x , V _y is obtained in the form of adding a correction term based on the parasitic quadrupole field. be able to. Further, as a correction term in the equations (3) and (4), a correction term for a parasitic dipole field and a correction term for a parasitic quadrupole field may be used.

  As described above, according to the present invention, the normal operator can correct the distortion of the multipole due to the mechanical / electrical deviation without being aware of the difference between the multipoles. Further, according to the present invention, the correction process can be appropriately performed by expressing the parasitic dipole field and the parasitic quadrupole field by Fourier series expansion. In addition, by expressing the parasitic dipole field and the quadrupole field with a Fourier series expansion term, the parasitic dipole field and the quadrupole field can be superimposed to obtain a correction value, and mechanical and electrical misalignment can be achieved. Can correct distortion of multipoles. Further, the parasitic dipole field can be set to an amount for correcting the image movement when the multipole field is slightly changed, and the distortion of the multipole can be corrected. Further, the parasitic quadrupole field can be set to an amount for correcting the distortion of the image when the multipole field is minutely changed, and the distortion of the multipole can be corrected.

The effects of the present invention are enumerated as follows.
1. Because it automatically corrects the parasitic dipole field and parasitic quadrupole field generated by the mechanical and electrical misalignment of the multipole, image movement and image distortion do not occur when the magnitude of the multipole field is changed. This eliminates the need for complicated adjustments.
2. Since image movement and image distortion do not occur when the size of the multipole field is changed, functions such as automatic focusing and automatic astigmatism, which are basic functions of the apparatus, operate normally.
3. The difference between multipoles can be eliminated.
4). Beam alignment of multipoles with complicated configurations becomes easy.

It is a block diagram which shows one embodiment of this invention. It is a figure which shows the structural example of a conventional apparatus. It is a figure which shows a cross section perpendicular | vertical to the optical axis of a multipole. It is a figure which shows equipotential distribution of each multipole.

Explanation of symbols

1 Emitter 2 Electron Beam 3 Lens 4 Objective Lens 5 Sample 6 Aberration Corrector 7 Secondary Electron 8 Secondary Electron Detector 9 Image Accumulator 10 Aberration Correction Controller 11 Multipole Field Correction Device 13 CRT

Claims (6)

To correct a parasitic dipole field and a parasitic quadrupole field caused by mechanical and electrical misalignment of a multipole when a sample surface is observed using charged particles in an apparatus equipped with an aberration corrector composed of a multipole. A step of providing a parasitic dipole field correction device and a parasitic quadrupole field correction device;
Storing a correction value in advance in the storage means when the multipole field is shifted by a predetermined amount in the X and Y directions of a certain stage of the aberration corrector;
Correcting the distortion caused by the parasitic dipole field and the parasitic quadrupole field using the correction value stored in the storage means;
A multipole field correction method comprising: To correct a parasitic dipole field and a parasitic quadrupole field caused by mechanical and electrical misalignment of a multipole when a sample surface is observed using charged particles in an apparatus equipped with an aberration corrector composed of a multipole. A parasitic dipole field correction device, a parasitic quadrupole field correction device,
Storage means for storing in advance a correction value when the multipole field is shifted by a predetermined amount in the X and Y directions of a certain stage of the aberration corrector;
Correction means for correcting distortion caused by the parasitic dipole field and the parasitic quadrupole field using the correction value stored in the storage means;
A multipole field correction device comprising:   3. The multipole field correction apparatus according to claim 2, wherein the parasitic dipole field and the parasitic quadrupole field are each expressed by Fourier series expansion with respect to an angle at the center of the multipole.   4. The multipole field correction apparatus according to claim 3, wherein a parasitic dipole field and a parasitic quadrupole field expressed by a Fourier series expansion term are superimposed to obtain a correction value.   3. The multipole field correction apparatus according to claim 2, wherein the parasitic dipole field is an amount for correcting image movement when the multipole field is minutely changed.   3. The multipole field correction device according to claim 2, wherein the parasitic quadrupole field is an amount for correcting image distortion when the multipole field is minutely changed.

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