JP2008210571A - Aberration correcting lens for charged particle beams - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a correcting lens system in which alignment precision and excitation control are alleviated. <P>SOLUTION: In the correcting lens that is constituted of a quadrupole lens of 6 stages or 8 stages having the same geometrical dimension and three aperture electrodes, or the correcting lens that is composed of a dodecapole lens of 6 stages or 8 stages, and an octupole lens of 3 stages or more, by having a geometrically symmetrical structure against the center position of the correcting lens, an excitation is controlled so that excitation intensity is made equalized with the opposite polarity, and made linearly converged in the XZ plane and the YZ plane respectively within the correcting lens, and in the vicinity of the center position of the correcting lens excitation intensity of the quadrupole lens is adjusted so that beam yaw distances become to have nearly the same degree, and the aperture aberration coefficient is corrected in the vicinity of these positions. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an aperture aberration correction lens for charged particle beams such as an electron beam and an ion beam.

  In an electric field lens and magnetic field lens, which are axially symmetric charged particle beam lenses generally used in charged particle beam devices, spherical aberration (which is synonymous with axial asymmetric lens aperture aberration) and chromatic aberration are inevitable as axial aberrations. It cannot be corrected. By combining a multipole lens such as an axially asymmetric quadrupole lens, hexapole lens, octupole lens, or twelve-pole lens composed of a plurality of electrodes or magnetic poles paired on the XY plane when the optical axis is the Z-axis. Spherical aberration and chromatic aberration can be corrected.

The blurs ΔX (Zi) and ΔY (Zi) at the focal position due to the aperture aberration of the quadrupole lens used in the present invention are expressed as follows on the XZ plane and the YZ plane, respectively.
ΔX (Zix) ＝ C _A30 α ³ + C _A12 αβ ² (1)
ΔY (Ziy) ＝ C _A21 α ² β + C _A03 β ³ (2)
In the above formulas (1) and (2), C _A30 , C _A12 , C _A21 , and C _A03 are aperture aberration coefficients. When axial symmetry is obtained under the condition of Zix = Ziy, the values of the aperture aberration coefficients C _A12 and C _A21 are equal.

  FIG. 1 shows an example of how to use an aberration correction lens for an axisymmetric lens for finely focusing a charged particle beam. The condenser lens 1 at the front stage of the correction lens system 3 and the objective lens 2 at the rear stage are independently an electric field type or magnetic field type axially symmetric lens incapable of correcting spherical aberration and chromatic aberration. 11 and 12 are orbits of charged particle beams on the XZ plane and the YZ plane. Although the charged particle beam trajectory in the correction lens is omitted, the charged particle beam trajectories in FIGS. 2, 3, and 4 correspond to this.

  As a spherical aberration correction system for an axially symmetric lens, a quadrupole / octupole correction lens that has already been proposed includes a four-stage quadrupole lens and a three-stage octupole lens. A correction lens system combining a magnetic quadrupole lens and an octupole lens has been studied as a spherical aberration correction lens placed in front of a magnetic field type objective lens of a scanning transmission electron microscope (Non-patent Document 1), but has been put into practical use. The system uses a twelve-pole instead of a quadrupole lens to achieve strict alignment accuracy between quadrupole lenses and excitation control, and the dipole lens action and hexapole lens can be controlled by twelve-pole excitation control. A quadrupole / octupole correction system is constructed by adjusting the action to align the axes and correct mechanical and electromagnetic asymmetry.

  In a correction lens system composed of a four-stage quadrupole lens and three octupole lenses, after canceling the aperture aberration coefficient of the quadrupole lens before the aperture electrode excitation for aperture aberration correction, the subsequent stage is axially symmetric. In order to generate the charged particle trajectory and the negative aperture aberration coefficient necessary for correcting the spherical aberration of the lens, the excitation intensity of the individual lenses constituting the correction lens is strong, and the setting accuracy is necessarily strict. This is a practical difficulty of a correction lens that requires high-precision alignment.

  As a measure for improving the structure of a correction lens that is complicated and requires strict alignment accuracy, a correction lens (Non-Patent Document 2) that includes a quadrupole lens and an aperture electrode disclosed in Patent Document 1 can be used. FIG. 2 shows an example of simulation calculation of charged particle beam trajectories and aperture aberrations in the XZ plane and YZ plane in this case. Q1 to Q4 are electric field type quadrupole lenses, and A1 to A3 are aperture electrodes. The electrode diameter of the quadrupole lens is 8 mm, the length is 14 mm, the aperture diameter of the quadrupole lens and the aperture electrode is φ6.987 mm, the thickness of the aperture electrode is 2 mm, and the distance between the quadrupole lens and the aperture electrode is 5 mm.

In the electric field type quadrupole lens, when V _Q [V] is applied to the electrode on the XZ plane, −V _Q [V] is applied to the electrode on the YZ plane. When the acceleration voltage is V _a [V], V _Q / V _a corresponds to the excitation intensity of the quadrupole lens. The excitation intensity of the aperture electrode is expressed as V _A / V _a . The excitation intensity of the four-stage quadrupole lens is +0.05864, −0.05859, +0.05859, −0.05864 for the excitation of the quadrupole lens on the XZ plane, and −0.05864, +0 for the excitation of the quadrupole lens on the YZ plane. 05859, −0.05859, +0.05864, and the ratio of the voltage applied to the aperture electrode and the acceleration voltage is −0.2255, +0.0598, and −0.2255.

In Fig. 1, in order to correct the spherical aberration of an axially symmetric lens with C _s = 12 mm placed after the correction lens, the aperture aberration coefficient is set as C _A30 = C _A12 = C _A21 = C _A03 = −3 mm. It has been realized. The aperture aberration coefficient of the four-stage quadrupole lens system before exciting the aperture electrode is C _A30 = C _A03 = + 300 mm, C _A12 = C _A21 = + 38 mm.
Japanese Patent Publication No. 63-9340 MGR Thomson, Optik, 34, 528-534 (1972) S. Okayama and H. Kawakatsu, A new correction lens, Journal ofPhysics E, 15, 580-586 (1982)

  As an improvement measure to reduce the strict alignment accuracy and excitation intensity of the correction lens, the off-axis distance of the charged particle beam orbit caused by the concave lens action of the quadrupole lens can be reduced under the condition that the excitation intensity of the quadrupole lens system is kept low. If the aperture aberration coefficient by the quadrupole lens system before exciting the aperture electrode and octupole lens can be suppressed, the excitation intensity of the aperture electrode and octupole lens can be lowered, and the quadrupole lens can be reduced. The advantage that the alignment accuracy and excitation setting accuracy are relaxed can be obtained.

  The problem to be solved is to match the shape of the charged particle beam trajectories on the XZ plane and YZ plane by setting the number of stages of the quadrupole lens and the excitation control, as well as ensuring axial symmetry and charging in the correction lens. By suppressing the increase in the off-axis distance of the particle beam as much as possible, the aperture aberration and chromatic aberration before octupole lens excitation are reduced, and the increase in excitation intensity of the correction lens for correcting spherical aberration is suppressed.

  In the present invention, the geometric dimensions of the quadrupole lens constituting the correction lens are made the same, and the geometric dimensions of the incident side and the emission side of the charged particle beam are made equal to the correction lens center. By making the excitation intensity on the emission side opposite in polarity and making the excitation intensity equal, excitation control can be facilitated and the shapes of the charged particle beam trajectories on the XZ plane and YZ plane can be matched. Furthermore, by increasing the number of steps of the quadrupole lens and suppressing the increase of the off-axis distance of the charged particle beam in the correction lens as much as possible, the increase of the aperture aberration coefficient and chromatic aberration coefficient before the aperture aberration correction is suppressed, and the correction lens The present invention is characterized in that a necessary spherical aberration correction effect is realized under a condition in which an increase in excitation intensity is suppressed.

  In the correction lens according to the present invention, the excitation intensity of the quadrupole lens constituting the correction lens and the excitation intensity of the aperture electrode or octupole lens are configured by a conventionally proposed four-stage quadrupole lens and three octupole lenses. It can be made lower than the excitation intensity of the correction lens. As a result, the alignment accuracy that becomes more severe as the excitation intensity increases can be relaxed. In addition, since the aperture aberration coefficient of the quadrupole lens before correcting the aperture aberration can be reduced, there is an advantage that correction control requiring strict setting accuracy can be relaxed.

In the present invention, the number of quadrupole lenses is increased to six or eight in order to keep the defocusing of charged particle beam trajectories on the XZ plane and YZ plane in the correction lens low. When a symmetric structure is realized on the center plane, the charge on the XZ plane and the YZ plane is charged under the condition that the polarities of the individual lenses constituting the correction lens on the entrance side and exit side of the charged particle beam are reversed and the intensity is made equal. An effective correction action can be realized by exciting and controlling the quadrupole lens so that the particle beam trajectory is linearly focused on the incident side and the outgoing side, respectively.
The present invention relates to an aperture aberration correction lens composed of a six-stage quadrupole lens having the same geometric dimensions and three or more aperture electrodes, and the quadrupole lens is uneven on the XZ plane with the Z axis as the optical axis. Excited to exhibit lens action, convex / concave / concave / concave / concave lens action on the YZ plane. By converging linearly on the XZ plane and YZ plane in the correction lens, the correction control of the aperture aberration coefficients C _A30 and C _A03 is effectively realized, and the off-axis distance is almost the same near the center position of the correction lens. By adjusting the quadrupole lens excitation intensity as described above, aperture aberration coefficient _CA12 and _CA21 correction control is effectively realized. For this purpose, an octupole lens action is induced by excitation of the aperture electrode or octupole lens so as to correct the aperture aberration coefficient in the vicinity of these positions. If the correction of the aperture aberration coefficients C _A30 , C _A03 , C _A12 , and C _{A21 in} the four terms shown in Equations (1) and (2) cannot be adjusted sufficiently due to misalignment of geometric dimensions, three more It is also possible to adjust the addition of the above aperture electrode or octupole lens.
Further, according to the present invention, in an aperture aberration correction lens composed of an 8-stage quadrupole lens having the same geometric dimensions and three or more aperture electrodes, the quadrupole lens is uneven on the XZ plane with the Z axis as the optical axis. Excited to exhibit convex / concave / concave / concave lens action, and convex / concave / concave / concave / concave lens action on YZ plane. In the correction lens, the XZ and YZ planes are converged linearly, and the quadrupole lens excitation intensity is adjusted so that the off-axis distance is about the center position of the correction lens. Correct the coefficient.
In addition, an aperture aberration correction lens composed of a six-stage twelve-pole lens and three or more octupole lenses by replacing with a twelve-pole lens instead of a quadrupole lens and an octupole lens instead of an aperture electrode, The aperture aberration coefficient can be corrected. Further, an aperture aberration correction lens constituted by an 8-stage dodecapole lens and three or more octupole lenses by replacing with a twelve-pole lens instead of a quadrupole lens and an octupole lens instead of an aperture electrode. The aberration coefficient can be corrected.

FIG. 3 shows an embodiment of a correction lens according to the present invention. Q1 to Q6 are electric field type quadrupole lenses, and A1 to A3 are aperture electrodes. The excitation intensity of the quadrupole lenses Q1 to Q6 has the opposite polarity in the + Z342 plane, +0.03421, −0.03982, +0.03421, −0.03421, +0.03982, −0.03421, and YZ plane. These excitations indicate that a correction system can be realized with weaker excitation than the prior art of FIG. The excitation intensity of the aperture electrodes A1 to A3 for obtaining the aperture aberration coefficient C _A30 = C _A12 = C _A21 = C _A03 = −3 mm is −0.119, +0.177, and −0.119. 13 and 14 in the figure indicate the linear focusing positions on the XZ plane and the YZ plane. Further, a control power source for exciting the quadrupole lens and the aperture electrode is omitted.

FIG. 4 shows another embodiment of the correction lens according to the present invention. Q1 to Q8 are electric field type quadrupole lenses, A1 to A3 are aperture electrodes, and Q1 to Q6 quadrupole lenses have excitation intensity of +0.02387, −0.014725, −0.014725, +0.02387, −0.02387 on the XZ plane. , +0.014725, +0.014725, -0.02387, YZ plane has reverse polarity. These excitations can realize a correction system with weaker excitation than the embodiment of FIG. Aperture aberration coefficient C _A30 = C _A12 = C _A21 = C _A03 = Excitation strength of aperture electrodes A1 to A3 to obtain −3 mm is −0.0225, +0.02753, −0.0225, which is even weaker than that in FIG. Shows that a correction system can be realized.

In FIG. 4, since the electric field type quadrupole lens and the aperture electrode constituting the correction lens have the same geometric dimensions and a symmetrical structure at the center of the correction lens, the excitation intensity of the quadrupole lens is set to V _Q1 = V _Q4 = −V _Q5 = −V _Q8 , V _Q2 = V _Q3 = −V _Q6 = −V _Q7 , and the setting of the excitation intensity of the aperture electrode is V _A1 = V _A3 . If there is an error in the geometric dimension, the above relational expression does not hold, so it is obvious that the excitation intensity of the quadrupole lens and the aperture electrode needs to be adjusted.

  2, 3, and 4, the electrode length and the quadrupole interval of the electric field type quadrupole are shown by simulation calculation under the same conditions. However, since the total length of the correction lens system is different, the display is shown. In doing so, the dimensions in the horizontal direction (Z direction) are changed.

  A magnetic field type quadrupole lens can be used instead of the electric field type quadrupole lens of Q1 to Q6 in FIG. However, in the case of a magnetic field type quadrupole lens, this corresponds to the case where the magnetic pole of the quadrupole lens is arranged at a position rotated 45 degrees on the XY plane with respect to the electric field type electrode.

  A magnetic field type quadrupole lens can be used instead of the electric field type quadrupole lens of Q1 to Q8 in FIG. The positional relationship between the magnetic poles of the magnetic field type quadrupole lens and the electrodes of the electric field type quadrupole lens is the same as described above.

  In the embodiment of FIG. 3, a twelve-pole lens capable of adjusting an axis and correcting asymmetry is used instead of the electric field type quadrupole lenses Q1 to Q6, and an octupole lens is used instead of the aperture electrodes A1 to A3. Thus, a similar correction lens can be realized.

  In the embodiment of FIG. 4, a dodecapole lens capable of correcting the asymmetry and correcting the asymmetry is used instead of the electric field type quadrupole lenses Q1 to Q8, and an octupole lens is used instead of the aperture electrodes A1 to A3. Thus, a similar correction lens can be realized.

  When a twelve pole is used instead of the quadrupole lens, the combination of the twelve pole and the aperture electrode cannot induce the octupole lens action, and thus the aperture electrode cannot be used. Therefore, in a six-stage or eight-stage twelve-pole lens system, a combination with an octupole lens is indispensable.

  In the embodiment of FIG. 4, the excitation of the quadrupole lens is 2.4% or less of the acceleration voltage value and the excitation of the aperture electrode is 2.8% or less, so that it can be used for an electron microscope, an electron beam drawing apparatus or the like up to an acceleration voltage of about 200 KV. Is also possible. It can also be applied to high performance for 1 MeV level PIXE (Particle Induced X-ray Emission), etc., where an axially symmetric magnetic lens cannot be used.

An example of using an axisymmetric lens and an aberration correction lens Spherical aberration correction lens with four-stage quadrupole lens and three aperture electrodes (prior art) Spherical Aberration Correction Lens with Six-stage Quadrupole Lens and Three Aperture Electrodes (Example 1) Spherical Aberration Correction Lens (Embodiment 2) with an 8-stage quadrupole lens and three aperture electrodes

Explanation of symbols

1 Axisymmetric condenser lens
2 Axisymmetric objective lens
3 Correction lens system
11 XZ-plane charged particle beam trajectory
12 YZ-plane charged particle beam trajectory
13 Linear focusing position on XZ plane
14 Linear focusing position on YZ plane
Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8 Electric field type quadrupole lens
A1, A2, A3 Aperture electrode

Claims (4)

  Aperture aberration correction lens consisting of a 6-stage quadrupole lens having the same geometric dimensions and three or more aperture electrodes. With the Z axis as the optical axis, the quadrupole lens acts as a concave / convex concave / convex lens on the XZ plane, YZ The lens is excited so as to exhibit the convex / concave / concave / concave lens action, and linearly converges on the XZ plane and YZ plane in the correction lens, and the off-axis distance is approximately the same in the vicinity of the center position of the correction lens. An aperture aberration correction lens characterized by adjusting an excitation intensity of a pole lens and correcting an aperture aberration coefficient in the vicinity of these positions.   In an aperture aberration correction lens composed of an eight-stage quadrupole lens having the same geometric dimensions and three or more aperture electrodes, the quadrupole lens acts as a concave / convex concave / convex concave / convex lens on the XZ plane with the Z axis as the optical axis. Excited to produce the concave / convex concave / convex lens action on the YZ surface, and linearly converges on the XZ surface and YZ surface in the correction lens, respectively, so that the off-axis distance is about the center position of the correction lens. An aperture aberration correction lens characterized by adjusting the excitation intensity of the quadrupole lens and correcting the aperture aberration coefficient in the vicinity of these positions.   Aperture aberration correction lens composed of a six-stage twelve-pole lens and three or more octupole lenses by replacing with a twelve-pole lens instead of a quadrupole lens and an octupole lens instead of an aperture electrode. The aperture aberration correction lens according to claim 1, wherein the coefficient is corrected.   An aperture aberration coefficient for an aperture aberration correction lens constituted by an 8-stage dodecapole lens and three or more octupole lenses by substituting a twelve-pole lens for the quadrupole lens and an octupole lens for the aperture electrode. The aperture aberration correcting lens according to claim 2, wherein

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