JP2008135336A - Vienna filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To positively generate an electrostatic hexapole field with respect to a hexapole field generated accompanying an electrostatic deflection field and a magnetostatic deflection field in a Wien filter and correct astigmatism three times.
An electrostatic field generation unit 4 is configured to switch each power source according to an instruction operated by a user using an operation unit 6 in order to focus a beam on a slit in a monochromator to which the 12-pole Wien filter is applied. The applied voltage is adjusted to generate a multipole electrostatic field such as an electrostatic deflection field, an electrostatic quadrupole field, or an electrostatic hexapole field.
[Selection] Figure 5

Description

  The present invention relates to a Wien filter used in an energy filter, a mass filter, a spin rotator, a monochromator, an energy analyzer and the like of an electron microscope.

  Conventionally, a Wien filter is used to select a charged particle beam by energy. An accelerated charged particle beam such as an electron beam is deflected in the positive pole direction with respect to the electric field, and is deflected in a direction perpendicular to the traveling direction of the charged particle beam and the magnetic field with respect to the magnetic field. The Wien filter utilizes this property and arranges an electric field and a magnetic field orthogonal to each other in a plane orthogonal to the traveling direction of the charged particle beam, thereby causing the charged particle beam to travel straight in a specific direction.

  This Wien filter is used in energy filters, mass filters, spin rotators, monochromators, energy analyzers and the like of electron microscopes. In particular, the monochromator transmits only a component having a predetermined energy with respect to an incident electron beam in order to make the electron beam monochromatic. The electron gun with a monochromator is used together with an energy selection slit directly under the electron gun for the purpose of improving the resolution of electron energy loss spectroscopy using an analytical electron microscope. The monochromator includes an optical element that disperses an electron beam according to the energy of electrons, and a slit that selects a desired component from the electron beam dispersed according to energy. A Wien filter is used as an optical element that disperses an electron beam according to energy.

  FIG. 13 is a diagram showing the direction of electrostatic field generation in the Wien filter. Moreover, FIG. 14 is a figure which shows the static magnetic field generation direction in a Wien filter. The conventional Wien filter generates an electrostatic deflection field (E1) and a magnetostatic deflection field (B1) perpendicular to each other on a plane orthogonal to the optical axis, and an astigmatic imaging type Wien filter adds to the deflection field. The electrostatic quadrupole field (E2) and the magnetostatic quadrupole field (B2) are superimposed. In FIGS. 13 and 14, it is assumed that the traveling direction of the electron gun advances from the front side to the back side of the drawing.

  A 12-pole Wien filter has been proposed as a Wien filter capable of generating the electromagnetic field. Patent Document 1 below discloses a Wien filter that has an optical axis substantially parallel to the Z axis, generates a dipole electric field in at least the X axis direction, and generates a dipole magnetic field in the Y axis direction. Wien filter having twelve poles extending substantially parallel to the optical axis and having the tip of each pole facing the optical axis in the XY plane has 12-fold rotational symmetry about the optical axis Is described. Patent Document 2 below also describes a 12-pole Wien filter.

  FIG. 15 is a schematic view of a 12-pole Wien filter capable of generating an electromagnetic field. Pole P1, P2, P3, P4, P5, P6, P7, P8, P9, P10, P11 and P12 are formed in the clockwise direction in the figure. The poles P2, P5 and P8, P11 are mechanically connected outside the Wien filter.

  In order for the 12-pole Wien filter to generate an electric field and a magnetic field as shown in FIGS. 13 and 14, the directions of the electric and magnetic fields applied to each pole are as follows.

  That is, in order to generate the electrostatic deflection field (E1) whose direction is shown in FIG. 13, the poles P8, P9, P10, and P11 are set to + poles, and P2, P3, P4, and P are set to -poles. Similarly, in order to generate the electrostatic quadrupole field (E2) shown in FIG. 13, the poles P3, P4, P9, and P10 are set to + poles, and the poles P1, P6, P7, and P12 are set to -poles. .

  In order to generate the magnetostatic deflection field (B1) whose direction is shown in FIG. 14, the poles P1, P2, P11, and P12 are set to N poles, and the poles P5, P6, P7, and P8 are set to S poles. Similarly, in order to generate the magnetostatic quadrupole field (B2) shown in FIG. 14, the poles P2 and P8 are N poles and the poles P5 and P11 are S poles.

  In FIG. 15, AA 'and BB' are coils for exciting the magnetostatic deflection field (B1), and C, D, E, and F are coils for exciting the magnetostatic quadrupole field (B2). It is. Further, G-G ′ and H-H ′ are alignment coils, and are not essentially related to the present invention.

As described above, in the conventional 12-pole Wien filter, an electrostatic deflection field and an electrostatic quadrupole field are generated by a combination of electrode power sources.
JP 2003-234077 A JP 2004-095489 A

  By the way, the Wien filter generates energy dispersion using strong electrostatic and magnetostatic deflection fields. The deflection field can be represented by a field having an odd function distribution having a first-order symmetry with the optical axis as the origin in the electrostatic or magnetostatic potential notation. On the other hand, the hexapole field is a field having an odd function distribution having third-order symmetry, and is generated along with the deflection field. That is, when a difference occurs in the intensity of the deflection field at a position away from the optical axis with respect to the deflection field on the optical axis, this difference appears as a hexapole field component. When the uniformity of the deflection field is high, the hexapole field becomes weak, and when the uniformity is low, the hexapole field appears strongly.

  The hexapole field causes astigmatism three times with respect to the electron beam. Three-fold astigmatism causes blurring of the light source image, and in a monochromator equipped with a Wien filter, an increase in the beam diameter on the energy selection surface causes a decrease in energy resolution, and a decrease in energy selection efficiency due to the energy selection slit. This causes a decrease in the amount of beam current when performing monochromatic electron beams with a monochromator. A decrease in the amount of beam current causes a decrease in the brightness of the electron beam, leading to a decrease in performance as a normal electron microscope.

  The present invention has been made in view of the above circumstances, and an electrostatic hexapole field is actively applied to the hexapole field generated in the Wien filter accompanying the electrostatic deflection field and the magnetostatic deflection field. The purpose is to correct astigmatism three times.

  By introducing the present invention, an improvement in energy resolution can be expected in a monochromator equipped with a Wien filter, and further, a reduction in beam current when energy is selected can be suppressed.

  In order to solve the above problems, the Wien filter according to the present invention has an optical axis substantially parallel to the Z axis, and when two axes orthogonal to the optical axis are taken as an X axis and a Y axis, the X axis direction A Wien filter that generates an electrostatic deflection field and generates a magnetostatic deflection field in the Y-axis direction, and has 2n (n is an integer of 4 or more) poles extending parallel to the optical axis in the Z-axis direction. In a Wien filter in which the tip of each pole facing the optical axis has rotational symmetry about the optical axis in the XY plane, five independent controls that can independently supply power to the 2n poles are possible. An electrostatic quadrupole field in addition to the electrostatic deflection field by controlling a simple power supply unit, and further an electrostatic field generating means for generating an electrostatic hexapole field, and the electrostatic hexapole field is generated by the electrostatic field generating means. Is generated, and astigmatism is corrected three times.

  The generation direction of the three-fold astigmatism of the beam due to the hexapole field generated accompanying the deflection field of the Wien filter is the same regardless of the electric field and magnetic field, as shown in FIG. On the other hand, in FIG. 15, voltages are applied with the poles P2, P5, P9, and P10 as positive poles and the poles P3, P4, P8, and P11 as negative poles. The poles 2 and 5 and the poles 8 and 11 are in mechanical contact. Of course, there is no problem even if the signs of + and-are reversed.

  As a result, a hexapole electric field can be generated without changing the mechanical and electrical configurations described above, and the three-fold astigmatism that greatly affects the performance of the monochromator can be corrected.

  In order to maintain the symmetry of the electrostatic field, the absolute values of the applied voltages applied to the poles must be the same. However, due to the symmetry problem caused by the mechanical structure of the Wien filter, it is described in Table 2. Voltage is applied. That is, 0 is applied to the poles P1, P6, P7, P12 when the hexapole field is generated, + 1.47 * V3 is applied to P2, P5, + V3 is applied to P9, P10, and -1 is applied to P8, P11. .47 * V3 and -V3 is applied to P3 and P4 to balance the potential between the electrodes. The coefficient of 1.47 shown here is a numerical value in an ideal state obtained from theoretical calculation. In implementation, errors due to processing accuracy and assembly accuracy at the time of manufacturing each electrode occur. The coefficients on the negative side and the negative side do not always coincide with each other, and the coefficient of 1.47 is not always guaranteed. For this reason, an electric circuit that can independently set these coefficients to an arbitrary value is required.

  According to the Wien filter of the present invention, an electrostatic hexapole field is positively generated with respect to an electrostatic deflection field and a hexapole field generated accompanying the magnetostatic deflection field in the Wien filter. Correct astigmatism. For example, in an electron gun with a monochromator that uses a Wien filter, a hexapole electric field is superimposed in the Wien filter to correct three astigmatisms that are one of the aberrations generated by the Wien filter. Allows you to squeeze the beam.

  By reducing astigmatism three times and making it possible to focus the beam on the slit, the energy resolution of the electron gun with a monochromator can be improved.

  By reducing the astigmatism three times and making it possible to focus the beam on the slit, the energy selection efficiency is improved, and it is possible to suppress the reduction in the amount of beam current when monochromating the electron beam with a monochromator. become.

  By reducing the amount of beam current when the electron beam is monochromatized by the monochromator, it is possible to suppress the decrease in the electron beam luminance of the electron gun with the monochromator as much as possible.

  The best mode for carrying out the present invention will be described below with reference to the drawings. This embodiment is a 12-pole Wien filter. FIG. 1 is a cross-sectional view of a 12-pole Wien filter. FIG. 2 is a cross-sectional view of the 12-pole Wien filter along the YZ plane. FIG. 3 is a diagram showing the direction of electrostatic field generation in a 12-pole Wien filter. FIG. 4 is a diagram illustrating the static magnetic field generation direction of the 12-pole Wien filter.

  As shown in FIG. 1, this 12-pole Wien filter has an optical axis o substantially parallel to the Z-axis. The 12-pole Wien filter has 2 × n = 2 × 6 = 12 poles P1 to P12 extending in parallel with the optical axis o in the Z-axis direction. The poles P1 to P12 are arranged clockwise. When the two axes orthogonal to the optical axis o are the X axis and the Y axis, the tip portions of the poles P1 to P12 facing the optical axis o are rotationally symmetric about the optical axis o in the XY plane. Have The twelve poles P1 to P12 are all made of a magnetic material. This magnetic material includes, for example, iron, nickel, permalloy and the like.

  In more detail, it is as follows. As shown in FIGS. 1 and 2, this 12-pole Wien filter has a desired space inside a cylindrical space 2 through which an electron beam as a charged particle beam passes (its center line coincides with the optical axis o). In order to generate a distributed electromagnetic field, it has 12 poles P1 to P12 extending parallel to the optical axis o.

  The 12-pole Wien filter generates an electrostatic deflection field E1 in the X-axis direction and a magnetostatic deflection field B1 in the Y-axis direction, as shown in FIGS. When the 12-pole type Wien filter is an astigmatic imaging type Wien filter, in addition to the electrostatic deflection field E1, the electrostatic quadrupole field E2 and the magnetostatic deflection field B1 as well as the magnetostatic quadrupole field. B2 is superimposed. This 12-pole Wien filter also generates an electrostatic hexapole field.

  FIG. 5 is a connection diagram of the electrostatic field generating unit 4 and the poles of the poles (P1 to P12) 3 of the 12-pole Wien filter, which are the main parts of the present invention, and five independently controllable power supply units. . The electrostatic field generation unit 4 controls the five electrostatically controllable power supply units (power supplies SA, SB, SC, SD, and SE) 5 that supply power independently to the 12 poles P1 to P12, thereby providing an electrostatic deflection field. In addition to E1, an electrostatic quadrupole field E2 and an electrostatic hexapole field are generated.

  The electrostatic field generating unit 4 applies the applied voltage of each power source in accordance with an instruction operated by the user using the operation unit 6 in order to focus the beam on the slit in a monochromator to be described later to which the 12-pole Wien filter is applied. To generate a multipole electrostatic field such as an electrostatic deflection field, an electrostatic quadrupole field, or an electrostatic hexapole field.

The electrostatic field generator 4 applies an applied voltage as shown in Table 2 to each power source (power sources SA, SB, SC, SD, and SE) and a 12-pole Wien filter connected as shown in Table 1 below. .

A power supply SA is connected to the poles P1, P6, P7 and P12. The power source SB is connected to the poles P2 and P5. The power supply SC is connected to the poles P3 and P4. The power supply SD is connected to the poles P8 and P11. The power source SE is connected to the poles P9 and P10.

  Table 2 shows voltages applied to the respective power supplies when an electrostatic deflection field, an electrostatic quadrupole field, and an electrostatic hexapole part are generated as a multipole electrostatic field. The applied voltage of the electrostatic deflection field is V1, the applied voltage of the electrostatic quadrupole field is V2, and the applied voltage of the electrostatic hexapole field is V3. In order to generate an electrostatic deflection field, 0 is applied to the power supply SA, -V1 is applied to the power supply SB, -V1 is applied to the power supply SC, + V1 is applied to the power supply SD, and + V1 is applied to the power supply SE. FIG. 6 is a diagram showing the generation of an electrostatic deflection field.

  In order to generate an electrostatic quadrupole field, -V2 is applied to the power supply SA, 0 is applied to the power supply SB, + V2 is applied to the power supply SC, 0 is applied to the power supply SD, and + V2 is applied to the power supply SE. FIG. 7 is a diagram showing the generation of an electrostatic quadrupole field.

  To generate an electrostatic hexapole field, the power supply SA is 0, the power supply SB is + 1.47 × V3, the power supply SC is −V3, the power supply SD is −1.47 × V3, and the power supply SE is + V3. Apply. FIG. 8 is a diagram showing the generation of an electrostatic hexapole field.

  Then, the electrostatic field generator 4 positively generates an electrostatic hexapole field and corrects astigmatism three times. FIG. 9 is a diagram showing a 12-pole Wien filter and the direction of three-fold astigmatism. The generation of the electrostatic hexapole field shown in FIG. 8 can correct astigmatism three times. Thereby, as shown in FIG. 10, when applied to a monochromator, the beam can be narrowed down on the energy selection slit.

  FIG. 11 is a configuration diagram of an electron optical system having a monochromator to which a 12-pole Wien filter is applied. This electron optical system has a monochromator 12 immediately below the electron source 11 and before the acceleration stage 16. The monochromator 12 includes a diaphragm 13, a Wien filter 14 and a slit 15. The Wien filter 14 is a 12-pole Wien filter and has the configuration shown in FIGS. 1 to 5. The electrostatic field generator 4 actively generates an electrostatic hexapole field and corrects astigmatism three times. To do.

  The electron beam emitted from the electron source 11 is selected for a predetermined energy component having a predetermined energy distribution width while the astigmatism is corrected three times by the monochromator 12. The electron beam composed of the component selected by the monochromator 12 is accelerated by the electrostatic gradient in the acceleration unit 16 and is irradiated onto the sample 18 by the focusing lens 17.

  In this electron optical system, the monochromator 12 can reduce astigmatism three times with the 12-pole Wien filter 14 and can focus the beam on the slit 15, thereby improving the energy resolution of the electron source 11. become able to. Further, by reducing astigmatism three times and allowing the beam to be focused on the slit, the energy selection efficiency is improved, and the reduction of the beam current amount is suppressed when the monochromator 12 makes the electron beam monochromatic. It becomes possible. In addition, when the monochromator 12 makes the electron beam monochromatic, it is possible to suppress a reduction in the amount of beam current, thereby minimizing the decrease in the electron beam brightness of the electron gun with a monochromator (electron source). become.

  Next, another embodiment of the present invention will be described. Another embodiment is an 8-pole Wien filter. FIG. 12 is a cross-sectional view of an 8-pole Wien filter. This 8-pole Wien filter has an optical axis o substantially parallel to the Z-axis. The 8-pole Wien filter has 2 × n = 2 × 4 = 8 poles P1 to P8 extending in parallel with the optical axis o in the Z-axis direction. The poles P1 to P8 are arranged clockwise. When the two axes orthogonal to the optical axis o are the X axis and the Y axis, the tip portions of the poles P1 to P8 facing the optical axis o are rotationally symmetric about the optical axis o in the XY plane. Have Each of the eight poles P1 to P8 is made of a magnetic material. This magnetic material includes, for example, iron, nickel, permalloy and the like.

  This 8-pole Wien filter also generates an electrostatic deflection field E1 in the X-axis direction and a magnetostatic deflection field B1 in the Y-axis direction, similar to those shown in FIGS. When the octupole Wien filter is an astigmatic imaging type Wien filter, in addition to the electrostatic deflection field E1, the electrostatic quadrupole field E2 and the magnetostatic deflection field B1 as well as the magnetostatic quadrupole field. B2 is superimposed. This 8-pole Wien filter further generates an electrostatic hexapole field.

  Even in this 8-pole type Wien filter, if FIG. 5 is applied mutatis mutandis, the electrostatic field generator 4 may generate an electrostatic quadrupole field E2 and further an electrostatic hexapole field in addition to the electrostatic deflection field E1. I understand.

The electrostatic field generator 4 applies an applied voltage as shown in Table 4 to each power source (power sources SA, SB, SC, SD and SE) and an 8-pole Wien filter connected as shown in Table 3 below. .

The power source SA is connected to the poles P1 and P5. The power source SB is connected to the poles P2 and P4. The power supply SC is connected to the pole P3. The power supply SD is connected to the poles P6 and P8. The power source SE is connected to the pole P7.

  Table 4 shows voltages applied to the respective power supplies when an electrostatic deflection field, an electrostatic quadrupole field, and an electrostatic hexapole part are generated as a multipole electrostatic field. The applied voltage of the electrostatic deflection field is V1, the applied voltage of the electrostatic quadrupole field is V2, and the applied voltage of the electrostatic hexapole field is V3. In order to generate an electrostatic deflection field, 0 is applied to the power supply SA, -V1 is applied to the power supply SB, -V1 is applied to the power supply SC, + V1 is applied to the power supply SD, and + V1 is applied to the power supply SE.

  In order to generate an electrostatic quadrupole field, -V2 is applied to the power supply SA, 0 is applied to the power supply SB, + V2 is applied to the power supply SC, 0 is applied to the power supply SD, and -V2 is applied to the power supply SE.

  To generate an electrostatic hexapole field, the power supply SA is 0, the power supply SB is + 1.47 × V3, the power supply SC is −V3, the power supply SD is −1.47 × V3, and the power supply SE is + V3. Apply.

  Then, the electrostatic field generator 4 positively generates an electrostatic hexapole field and corrects astigmatism three times. When this 8-pole Wien filter is applied to a monochromator, the beam can be narrowed down on the energy selection slit.

  In the electron optical system shown in FIG. 11, an 8-pole Wien filter can be applied to the monochromator. The electrostatic field generator 4 actively generates an electrostatic hexapole field and corrects astigmatism three times.

  The electron beam emitted from the electron source 11 is selected for a predetermined energy component having a predetermined energy distribution width while the astigmatism is corrected three times by the monochromator 12. The electron beam composed of the component selected by the monochromator 12 is accelerated by the electrostatic gradient in the acceleration unit 16 and is irradiated onto the sample 18 by the focusing lens 17.

  Also in this electron optical system, the monochromator 12 can reduce the astigmatism three times by the octupole Wien filter 14 and can focus the beam on the slit 15, thereby improving the energy resolution of the electron source 11. become able to. Further, by reducing astigmatism three times and allowing the beam to be focused on the slit, the energy selection efficiency is improved, and the reduction of the beam current amount is suppressed when the monochromator 12 makes the electron beam monochromatic. It becomes possible. In addition, when the monochromator 12 makes the electron beam monochromatic, it is possible to suppress a reduction in the amount of beam current, thereby minimizing the decrease in the electron beam brightness of the electron gun with a monochromator (electron source). become.

  The Wien filter of the present invention is used not only in a monochromator but also in an electron microscope energy filter, mass filter, spin rotator, energy analyzer, and the like.

It is sectional drawing which follows the XY plane orthogonal to the optical axis of a 12 pole type Wien filter. It is sectional drawing which follows the YZ plane of a 12 pole type Wien filter. It is a figure which shows the electrostatic field generation | occurrence | production direction of a 12 pole type Wien filter. It is a figure which shows the static magnetic field generation | occurrence | production direction of a 12 pole type Wien filter. It is a connection diagram of an electrostatic field generation part, each pole of a pole (P1-P12) part of a 12 pole type Wien filter, and five power supplies which can be controlled independently. It is an electrostatic deflection field generation | occurrence | production figure. It is an electrostatic quadrupole field generation | occurrence | production figure. It is an electrostatic hexapole field generation | occurrence | production figure. It is a figure which shows a 12 pole type | mold Wien filter and a 3 times astigmatism generating direction. It is a figure which shows the relationship between a slit and 3 times astigmatism. It is a block diagram of the electron optical system provided with the monochromator to which a 12 pole type Wien filter is applied. It is sectional drawing along XY plane of an octupole Wien filter. It is an electrostatic field generation | occurrence | production direction map in a Wien filter. It is a static magnetic field generation | occurrence | production direction map in a Wien filter. It is sectional drawing of a 12 pole type Wien filter. It is a figure which shows a 3 times astigmatism generation | occurrence | production direction.

Explanation of symbols

3 pole part 4 electrostatic field generation part 5 power supply part 6 operation part

Claims (1)

When the optical axis is almost parallel to the Z axis and the two axes perpendicular to the optical axis are the X axis and the Y axis, an electrostatic deflection field is generated in the X axis direction, and the magnetostatic deflection is performed in the Y axis direction. The Wien filter for generating a field has 2n (n is an integer of 4 or more) poles extending parallel to the optical axis in the Z-axis direction, and the tip of each pole facing the optical axis in the XY plane In the Wien filter whose part has rotational symmetry about the optical axis,
An electrostatic field that generates an electrostatic quadrupole field and further an electrostatic hexapole field in addition to the electrostatic deflection field by controlling five independently controllable power supply units that supply power independently to the 2n poles. With generating means,
A Wien filter characterized in that the electrostatic hexapole field is generated by the electrostatic field generating means and the astigmatism is corrected three times.