Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2002-10-25
2004-04-20
Lee, John R. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML, C250S305000, C250S306000, C250S310000
Reexamination Certificate
active
06723997
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aberration corrector for use in an instrument (e.g., an instrument using an electron beam or ion beam, such as a scanning electron microscope or ion microprobe) utilizing a charged-particle beam to correct chromatic and spherical aberrations in such an instrument.
2. Description of Related Art
In a scanning electron microscope or transmission electron microscope, an aberration corrector is incorporated in the optics in order to provide high-resolution imaging or enhance the probe current density. One proposed example of this aberration corrector uses a combination of electrostatic quadrupole elements and magnetic quadrupole elements to correct chromatic aberration. The corrector also uses four stages of octupole elements to correct spherical aberration. The principle is introduced in detail in various literature: [1] H. Rose,
Optik
33, Heft 1, pp. 1-24 (1971); [2] J. Zach,
Optik
83, No. 1, pp. 30-40 (1989); and [3] J. Zach and M. Haider,
Nucl. Instru. and Meth. in Phys. Res
. A 363, pp. 316-325 (1995).
The principle of the above-described aberration corrector is described briefly now by referring to
FIG. 1
, where an aberration corrector C is placed ahead of an objective lens
7
. The aberration corrector C comprises four stages of electrostatic quadrupole elements
1
,
2
,
3
,
4
, two stages of magnetic quadrupole elements
5
,
6
, and four stages of electrostatic octupole elements
11
,
12
,
13
,
14
. The two stages of magnetic quadrupole elements
5
,
6
create a magnetic potential distribution analogous to the electric potential distribution created by the second and third stages of the electrostatic quadrupole elements to produce a magnetic field superimposed on the electric field. The four stages of electrostatic octupole elements
11
,
12
,
13
,
14
create an electric field superimposed on the electric field created by the four stages of electrostatic quadrupole elements
1
-
4
.
In an actual instrument, four stages of dipole elements and four stages of hexapole elements are also mounted to produce fields superimposed on the fields created by the aforementioned quadrupole and octupole elements. The dipole elements act as deflecting devices for axial alignment. The hexapole elements act to correct the second-order aperture aberration. Since these dipole and hexapole elements are not closely related to the present invention, they will not be described in detail below.
In this configuration, a beam of charged particles is entered from the left side as viewed in the figure. The four stages of electrostatic quadrupole elements
1
-
4
and the objective lens
7
together act to form a reference orbit for the beam. As a result, the beam is focused onto a specimen surface
20
. In
FIG. 1
, both orbit R
x
of the particle beam in the X-direction and orbit R
y
in the Y-direction are schematically drawn on the same plane.
The reference orbit can be regarded as a paraxial orbit, that is, an orbit assumed where there is no aberration. The quadrupole element
1
causes the Y-direction orbit R
y
to pass through the center of the quadrupole element
2
. The quadrupole element
2
causes the X-direction orbit R
x
to pass through the center of the quadrupole element
3
. Finally, the quadrupole elements
3
,
4
and objective lens
7
together focus the beam onto the specimen surface. In practice, these components need to be adjusted mutually for complete focusing. At this time, the four stages of dipole elements are used for axial alignment.
Referring more particularly to
FIG. 1
, the charged-particle beam in the X-direction orbit R
x
is diverged by the quadrupole element
1
acting like a concave lens. Then, the beam is converged by the quadrupole element
2
acting like a convex lens. The beam is thus made to pass through the center of the quadrupole element
3
. Then, the beam is converged by the quadrupole element
4
and travels toward the objective lens
7
. On the other hand, the charged-particle beam in the Y-direction orbit R
y
is converged by the quadrupole element
1
and made to pass through the center of the quadrupole element
2
. Then, the beam is converged by the quadrupole element
3
. Finally, the beam is diverged by the quadrupole element
4
and moves toward the objective lens
7
. In this way, the function of a single concave lens is created by combining the divergent action of the quadrupole element
1
acting on the X-direction orbit R
x
and the divergent action of the quadrupole element
4
acting on the Y-direction orbit R
y
.
Correction of chromatic aberration using the aberration corrector C is described. To correct chromatic aberration by the system shown in
FIG. 1
, the potential &phgr;
q2
volts at the electrostatic quadrupole element
2
and the magnetic excitation J
2
amp turns (or magnetic potential) of the magnetic quadrupole element
5
are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the X-direction chromatic aberration to zero. Similarly, the potential &phgr;
q3
volts at the electrostatic quadrupole element
3
and the magnetic excitation J
3
amp turns of the magnetic quadrupole element
6
are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the Y-direction chromatic aberration to zero.
Correction of spherical aberration (correction of the third-order aperture aberration) is next described. Before spherical aberration is corrected, X- and Y-direction chromatic aberrations are corrected. Then, the X-direction spherical aberration in the whole lens system is corrected to zero by the potential &phgr;
02
volts at the electrostatic octupole element
12
. The Y-direction spherical aberration is corrected to zero by the potential &phgr;
03
volts at the electrostatic octupole element
13
.
Then, the spherical aberration in the resultant direction of the X- and Y-directions is corrected to zero by the electrostatic octupole elements
11
and
14
. In practice, repeated mutual adjustments are necessary. Superimposition of the potentials and magnetic excitations at the quadrupole and octupole elements has been put into practical use by varying the potential or excitation applied to each pole of a single twelve-pole element by using this twelve-pole element to synthesize dipoles, quadrupoles, hexapoles, octupoles, etc. This method has been introduced, for example, in [4] M. Haider et al.,
Optik
63, No. 1, pp. 9-23 (1982).
In particular, in an electrostatic design, a final stage of power supplies A
n
(n=1, 2, . . . , 12) capable of supplying a voltage to 12 electrodes U
n
(n=1, 2, . . . , 12) independently is connected as shown in
FIG. 9
of this patent application. Where a quadrupole field is produced, output voltages from a quadrupole power supply
10
are supplied to the final-stage power supplies A
n
to obtain a quadrupole field close to an ideal quadrupole field. If it is assumed that the output voltages from the final-stage power supplies A
n
are proportional to the output voltages from the quadrupole power supply
10
, the ratio of the output voltages from the power supply
10
assumes a value as given in the reference [4] above. Where an octupole field is created to be superimposed on this quadrupole field, output voltages from an octupole power supply
18
are added to the output voltages from the quadrupole power supply
10
and supplied to the final-stage power supplies A
n
to obtain a field close to an ideal octupole field. Similarly, a field on which a multipole field produced by a 2n-pole element (n=1, 2, . . . , 6) is superimposed is obtained using the single twelve-pole element.
In a magnetic design, a final stage of power supplies B
n
(n=1, 2, . . . , 12) capable of supplying excitation currents to the coils on 12 magnets W
n
(n=1, 2, . . . , 12) independently is connected as shown in
FIG. 10
of this patent application. Where a quadrupole
Honda Kazuhiro
Matsuya Miyuki
Jeol Ltd.
Lee John R.
Leybourne James J.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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