U.S. patent application number 15/056143 was filed with the patent office on 2016-06-23 for objective lens arrangement usable in particle-optical systems.
The applicant listed for this patent is APPLIED MATERIALS ISRAEL, LTD., CARL ZEISS MICROSCOPY GMBH. Invention is credited to Rainer KNIPPELMEYER, Stefan SCHUBERT.
Application Number | 20160181054 15/056143 |
Document ID | / |
Family ID | 37728176 |
Filed Date | 2016-06-23 |
United States Patent
Application |
20160181054 |
Kind Code |
A1 |
KNIPPELMEYER; Rainer ; et
al. |
June 23, 2016 |
OBJECTIVE LENS ARRANGEMENT USABLE IN PARTICLE-OPTICAL SYSTEMS
Abstract
An objective lens arrangement includes a first, second and third
pole pieces, each being substantially rotationally symmetric. The
first, second and third pole pieces are disposed on a same side of
an object plane. An end of the first pole piece is separated from
an end of the second pole piece to form a first gap, and an end of
the third pole piece is separated from an end of the second pole
piece to form a second gap. A first excitation coil generates a
focusing magnetic field in the first gap, and a second excitation
coil generates a compensating magnetic field in the second gap.
First and second power supplies supply current to the first and
second excitation coils, respectively. A magnetic flux generated in
the second pole piece is oriented in a same direction as a magnetic
flux generated in the second pole piece.
Inventors: |
KNIPPELMEYER; Rainer;
(Utting am Ammersee, DE) ; SCHUBERT; Stefan;
(Oberkochen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CARL ZEISS MICROSCOPY GMBH
APPLIED MATERIALS ISRAEL, LTD. |
Jena
Rehovot |
|
DE
IL |
|
|
Family ID: |
37728176 |
Appl. No.: |
15/056143 |
Filed: |
February 29, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12095198 |
Nov 6, 2008 |
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PCT/EP2006/011413 |
Nov 28, 2006 |
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15056143 |
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60740581 |
Nov 28, 2005 |
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Current U.S.
Class: |
250/505.1 |
Current CPC
Class: |
H01J 2237/14 20130101;
H01J 2237/04926 20130101; H01J 37/24 20130101; H01J 2237/12
20130101; H01J 37/21 20130101; H01J 2237/002 20130101; H01J
2237/1405 20130101; H01J 37/20 20130101; H01J 2237/2817 20130101;
H01J 2237/1215 20130101; H01J 37/28 20130101; H01J 37/145 20130101;
H01J 2237/04922 20130101; H01J 2237/141 20130101; H01J 2237/1415
20130101; H01J 37/141 20130101 |
International
Class: |
H01J 37/145 20060101
H01J037/145 |
Claims
1-51. (canceled)
52. A particle optical inspection system, comprising an objective
lens arrangement comprising: a first pole piece and a second pole
piece, wherein the first and second pole pieces are substantially
rotationally symmetric with respect to an axis of symmetry, wherein
a radial inner end of the first pole piece is disposed at a
distance from a radial inner end of the second pole piece to form a
first gap, wherein the first pole piece has an inner portion
extending at an angle towards the axis of symmetry and wherein the
first and second pole pieces are electrically insulated from each
other; an first excitation coil for generating a focusing magnetic
field in a region of the first gap; a beam tube extending through a
bore formed by the radial inner end of the first pole piece; a
first voltage source for supplying a voltage to the beam tube; the
particle-optical inspection system further comprising a beam path
splitting arrangement comprising at least one magnetic field
arrangement, wherein a lower end of the at least one magnetic field
arrangement of the beam path splitting arrangement is disposed at a
first distance from the object plane and wherein an upper end of
the first excitation coil is disposed at a second distance from the
object plane and wherein the first distance is shorter than the
second distance.
53. The particle optical inspection system according to claim 52,
wherein the inner portion of the first pole piece extends towards
the axis of symmetry such that the radial inner end of the first
pole piece is disposed closer to the object plane than a radial
outer end of the inner portion of the first pole piece and wherein
the lower end of the at least one magnetic lens is disposed within
a space defined by the inner portion of the first pole piece.
54. The particle-optical inspection system according to claim 52,
wherein the inner portion of the first pole piece has a
substantially conical shape with the radial inner end being
disposed closer to the object plane than a radial outer end and
wherein the lower end of the at least one magnetic lens is disposed
within the conus formed by the inner portion of the first pole
piece.
55. The particle optical inspection system according to claim 54,
the conus formed by the inner portion of the first pole piece
having a conus opening angle in a range of from about 20.degree. to
about 70.degree..
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an objective lens
arrangement for use in particle-optical systems. In addition, the
invention relates to a particle-optical beam system as well as a
particle-optical inspection system.
[0003] The invention may be applied to charged particles of any
type, such as electrons, positrons, muons, ions (charged atoms or
molecules) and others.
[0004] 2. Brief Description of Related Art
[0005] The increasing demand for ever smaller and more complex
microstructured devices and the continuing demand for an increase
of a throughput in the manufacturing and inspection processes
thereof have been an incentive for the development of
particle-optical systems that use multiple charged particle
beamlets in place of a single charged particle beam, thus
significantly improving the throughput of such systems. The
multiple charged particle beamlets may be provided by a single
column using a multi-aperture array, for instance, or by multiple
individual columns, or a combination of both, as will be described
in more detail below. The use of multiple beamlets is associated
with a whole range of new challenges to the design of
particle-optical components, arrangements and systems, such as
microscopes and lithography systems.
[0006] A conventional particle-optical system is known from U.S.
Pat. No. 6,252,412 B1. The electron microscopy apparatus disclosed
therein is used for inspecting an object, such as a semiconductor
wafer. A plurality of primary electron beams is focused in parallel
to each other on the object to form a plurality of primary electron
spots thereon. Secondary electrons generated by the primary
electrons and emanating from respective primary electron spots are
detected. For each primary electron beam a separate electron beam
column is provided. The plurality of separate electron beam columns
is closely packed. A density of the primary electron beam spots
formed on the object is limited by a remaining footstep size of the
electron beam columns forming the electron microscopy apparatus.
Thus, the number of primary electron beam spots, which may be
formed simultaneously on the object, is also limited in practice,
resulting in a limited throughput of the apparatus when inspecting
semiconductor wafers of a high surface area at a high
resolution.
[0007] From U.S. Pat. No. 5,892,224, US 2002/0148961 A1, US
2002/0142496 A1, US 2002/0130262 A1, US 2002/0109090 A1, US
2002/0033449 A1, US 2002/0028399 A1, electron microscopy apparatus
are known which use a plurality of primary electron beamlets
focused onto the surface of the object to be inspected. The
beamlets are generated by a multi-aperture plate having a plurality
of apertures formed therein, wherein an electron source generating
a single electron beam is provided upstream of the multi-aperture
plate for illuminating the apertures formed therein. Downstream of
the multiple-aperture plate a plurality of electron beamlets is
formed by those electrons of the electron beam that pass the
apertures. The plurality of primary electron beamlets is focused on
the object by an objective lens having an aperture, which is passed
by all primary electron beamlets. An array of primary electron
spots is then formed on the object. Secondary electrons emanating
from each primary electron spot form a respective secondary
electron beamlet, such that a plurality of secondary electron
beamlets corresponding to the plurality of primary electron beam
spots is generated. The plurality of secondary electron beamlets
also pass the objective lens, and the apparatus provides a
secondary electron beam path such that each of the secondary
electron beamlets is supplied to a respective one of a plurality of
detector pixels of a CCD electron detector. A Wien-filter is used
for separating the secondary electron beam path from a beam path of
the primary electron beamlets.
[0008] Since one common primary electron beam path comprising the
plurality of primary electron beamlets and one common secondary
electron beam path comprising the plurality of secondary electron
beamlets is used, one single electron-optical column may be
employed, and the density of primary electron beam spots formed on
the object is not limited by a foot step size of the single
electron-optical column.
[0009] The number of primary electron beam spots disclosed in the
embodiments of the above-mentioned documents is in the order of
some ten spots. Since the number of primary electron beam spots
formed on the object at the same time limits the throughput, it is
desirable to increase the number of primary electron beam spots in
order to achieve a higher throughput. It has been found, however,
that it is difficult to increase the number of primary electron
beam spots formed at the same time, or to increase a primary
electron beam spot density, employing the technology disclosed in
those documents while maintaining a desired imaging resolution of
the electron microscopy apparatus.
[0010] What has been described above with reference to electrons
applies in a similar manner to other charged particles.
[0011] It is therefore an object of the present invention to
provide an objective lens arrangement as well as a particle-optical
system having improved particle-optical properties.
SUMMARY OF THE INVENTION
[0012] The present invention is applicable to particle-optical
systems using multiple beamlets of charged particles; the present
invention is, however, not limited in the application to systems
using multiple beamlets, but is equally applicable to
particle-optical systems using only one single beam of charged
particles.
[0013] According to a first aspect, the present invention provides
an objective lens arrangement having an object plane and an axis of
symmetry and comprising first, second and third pole pieces which
are substantially rotationally symmetric with respect to an axis of
symmetry and which are disposed on a same side of the object plane.
The first, second and third pole pieces extend towards the axis of
symmetry such that radial inner ends of the first, second and third
pole pieces each define a bore which is to be traversed by a beam
path of one or more beams of charged particles. A radial inner end
of the first pole piece is disposed at a distance from a radial
inner end of the second pole piece to form a first gap between
them, and a radial inner end of the third pole piece is disposed at
a distance from the radial inner end of the second pole piece to
form a second gap between them.
[0014] The axis of symmetry referred to above generally coincides
with the optical axis of a particle-optical system the objective
lens arrangement is comprised in, such that the two terms are used
to the same effect herein. The objective lens arrangement may also
be described as having a central axis which may or may not also be
an axis of symmetry, which central axis generally coincides with
the optical axis of a system the objective lens arrangement is
comprised in and thus also be used synonymously to the term optical
axis.
[0015] A first excitation coil is provided for generating a
magnetic field in a region of the first gap, and a second
excitation coil is provided for generating a magnetic field in a
region of the second gap. A first power supply is provided for
supplying an excitation current to the first excitation coil, and a
second power supply is provided for supplying an excitation current
to the second excitation coil. The first and second power supplies
may be two portions of a same power supply. The first and second
power supplies are configured to supply currents to the first and
second excitation coils and thus generate excitation currents such
that a magnetic flux generated by the first excitation coil in the
second pole piece is oriented in the same or a different direction
as a magnetic flux generated by the second excitation coil in the
second pole piece.
[0016] Generally, the first excitation coil is disposed between the
first and second pole pieces and the second excitation coil
disposed between the second and third pole pieces.
[0017] Depending on a shape, configuration and position of the pole
pieces, a position and configuration of the excitation coil and an
excitation current, the magnetic field generated in a region of the
gap may have different magnetic field strengths and different
dimensions. For instance, the magnetic field may extend only over a
region close to the gap or may extend as far as the object plane.
Since magnetic lenses are usually employed in inspection optical
systems to provide a focusing effect, a magnetic focusing field
generally extends as far as the object plane in order to achieve a
good focusing effect, avoid defocusing before the object plane and
avoid particle-optical aberrations.
[0018] The objective lens arrangement according to the first aspect
of the present invention allows adjusting the magnetic fields in
the first and second gaps such that the magnetic field in the first
gap provides a focusing effect on the one or more beams of charged
particles traversing the focusing magnetic field whilst the
magnetic field generated in the second gap is configured to
compensate for the focusing magnetic field extending from the first
gap to locations on or at least close to the object plane.
[0019] The first and second gaps may be disposed at an angle to one
another, for instance. The angle formed between the first and
second radial gaps may be in a range of from 10 to about 170
degrees, for instance, and may be in a range of from 45 to 135
degrees or from 60 to 120 degrees, by way of example. In other
words, in this exemplary embodiment, the first gap is disposed at
an angle to the axis of symmetry that is different from an angle
formed between the second gap and the axis of symmetry. For
purposes of determining an angle between gaps, a straight line
connecting the radial inner ends of the respective pole pieces
forming the gap may be employed to represent the gap.
[0020] In an exemplary embodiment, the first gap is oriented
substantially in an axial direction, i.e. substantially parallel or
at a relatively small angle to the axis of symmetry, and thus forms
an axial gap. An axial gap does not necessarily imply that radial
innermost ends of the pole pieces forming the gap need to have the
same distance from the axis of symmetry, but also encompasses those
embodiments wherein the innermost ends have different distances
from the axis of symmetry and wherein the gap formed between points
on radial inner ends of the pole pieces that are disposed closest
to one another form an angle of less than 45.degree., for instance
less than 30.degree. or less than 15.degree. to the axis of
symmetry. The second gap may be oriented substantially in a radial
direction with respect to the objective lens arrangement, i.e.
orthogonal to the axis of symmetry and thus form a radial gap.
Radial gaps also encompass those embodiments wherein the gap
defined by a closest distance (straight line along the closest
distance) between the inner radial ends of the pole pieces is
disposed at an angle of from about 50.degree. to 90.degree. to the
axis of symmetry, such as from about 80.degree. to 90.degree. to
the axis of symmetry.
[0021] According to an exemplary embodiment, the focusing magnetic
field (generated in the first gap) is compensated for by the
magnetic field generated in the second gap to such an extent that a
total magnetic field in a region on the object plane and about the
optical axis is substantially zero, in other words, the
compensating magnetic field substantially cancels the focusing
magnetic field in a region on the object plane.
[0022] Such a configuration allows obtaining advantageous imaging
properties with the objective lens arrangement. In particular, an
image rotation caused by the focusing magnetic field may be
eliminated in the vicinity of the object plane. This allows
achieving improved overall performance of the system, in particular
with regard to structured objects, which are inspected and/or
processed using the objective lens arrangement.
[0023] The configuration of, in particular, the embodiment with the
first gap being an axial gap and the second gap being a radial gap
is particularly advantageous, since the pole pieces can be arranged
such that the radial inner end of the second pole piece is disposed
close to the object plane such that the focusing magnetic field is
also generated and disposed close to the object plane. On the other
hand, having a radial gap formed between the inner radial end of
the second pole piece, which also defines the lower end of the
first gap, and the third pole piece allows the radial gap to be
disposed close to the first gap and the focusing magnetic field to
be generated close to the object plane, thus providing the magnetic
field compensating effect in very close vicinity to the object
plane. The radial orientation of the second gap also allows
generating the compensating magnetic field downstream of the
objective lens arrangement thus not interfering with the focusing
magnetic field inside the objective lens arrangement and thus not
impairing a focusing effect provided thereby. This embodiment thus
advantageously allows the major portion of the focusing magnetic
field to remain unaffected and enables the compensating field to
take effect on/close to the object plane.
[0024] Therefore, in exemplary embodiments, the radial inner end of
the third pole piece and the radial inner end of the second pole
piece are disposed in substantially a same plane, which plane is
disposed substantially parallel to the object plane.
[0025] In a further exemplary embodiment, the objective lens
arrangement may additionally comprise a fourth pole piece which is
substantially rotationally symmetric with respect to the axis of
symmetry, wherein a third gap is formed between the fourth pole
piece and the first pole piece, and wherein the third gap is
disposed at a greater distance from the object plane than the first
gap; and a third excitation coil for generating an adjusting
magnetic field in the third gap.
[0026] The adjusting magnetic field may be used to adjust the
focusing magnetic field in terms of its strength, location,
dimension and other parameters. The adjusting magnetic field may be
used to increase or decrease the focusing magnetic field strength,
for instance. The fourth gap may be an axial gap, for example. The
inner radial end of the fourth pole piece may be disposed at a
distance from the axis of symmetry that is greater, equal to or
smaller than a distance between the radial inner end of the first
pole piece and the axis of symmetry, for instance.
[0027] The pole pieces may be disposed and configured such that,
for instance, the second and third pole pieces are electrically
connected to each other and the first pole piece is electrically
insulated from the second and third pole pieces, such as by an
insulating layer.
[0028] In those exemplary embodiments, the insulating layer may be
provided between an outer cylindrical portion of or integrally
formed with the first pole piece and a substantially cylindrical
yoke formed by and connecting the second and third pole pieces.
[0029] The outer cylindrical portion may extend around and
substantially parallel to the yoke as well as the axis of symmetry,
for example, such that the insulating layer also extends
substantially in an axial direction. In addition or alternatively,
the first pole piece may comprise an annular, substantially
disc-shaped or disc-like portion or have an annular disc-shaped or
disc-like portion integrally formed with it. In those exemplary
embodiments, the insulating layer may be provided between the outer
annular disc shaped portion integrally formed with the first pole
piece, and an outer portion of the second pole piece. In those
exemplary embodiments, the annular disc-shape or disc-like portion
and the outer portion are arranged to have parallel surfaces over
at least a portion thereof.
[0030] In further exemplary embodiments, the first pole piece
comprises an inner member and an outer member, that is comprises
two distinct parts, with the inner and outer members being
electrically insulated from one another by an insulating layer.
Inner and outer, as used herein, refer to a radial distance from
the axis of symmetry, i.e. a distance from the axis of symmetry in
a plane orthogonal to the axis of symmetry. In those exemplary
embodiments, an additional insulating layer may be provided between
the outer member of the first pole piece and an outer portion of
the second pole piece. The outer member of the first pole piece in
this exemplary embodiment of the objective lens arrangement
according to the present invention may be configured to accommodate
the first excitation coil. The inner member of the first pole piece
may comprise or consist of a substantially conical portion
extending towards the axis of symmetry. The outer member may have
or comprise a substantially annular shape, for instance.
[0031] According to a second aspect, the invention provides an
objective lens arrangement having an object plane and an axis of
symmetry and comprising first and second pole pieces which are
rotationally symmetric with respect to the axis of symmetry, with
inner ends of the first and second pole pieces defining respective
bores which are configured to be traversed by a beam path of one or
more beams of charged particles. The radial inner end of the first
pole piece is disposed at a distance from the radial inner end of
the second pole piece to form a (first) gap between them, with the
second pole piece being disposed closer to the object plane than
the first pole piece. The first and second pole pieces are
electrically insulated from each other. A first excitation coil is
provided for generating a focusing magnetic field in the first gap,
and a beam tube extends through the bore formed by the radial inner
end of the first pole piece.
[0032] The objective lens arrangement according to the second
aspect further comprises an object mount for mounting an object to
be processed such that the object is disposed in the object plane.
The object mount includes an electrical connector for supplying an
electrical voltage to the object to be processed.
[0033] As used herein, "object to be processed" is to be understood
as encompassing objects that are inspected, imaged and/or
manipulated by a charged particle beam or a plurality of charged
particle beamlets.
[0034] The objective lens arrangement according to the second
aspect of the present invention further comprises a first voltage
source configured for supplying a voltage to the beam tube such
that the beam tube is more than about 15 kV above ground potential.
A second voltage source is provided and configured for supplying a
voltage to the electrical connector such that the electrical
connector is grounded or below ground potential. In exemplary
embodiments, the second voltage source may be configured to supply
a voltage such that the electrical connector is more than about 15
kV below ground potential.
[0035] In exemplary embodiments, the objective lens arrangement
according to the second aspect of the present invention further
comprises a third voltage source configured for supplying a voltage
to the second pole piece such that a potential of the second pole
piece is from about 0.1 kV to about 10 kV above a potential of the
electrical connector. The first through third voltage sources may
be individual voltage sources or portions of a same voltage
source.
[0036] Such an arrangement allows to obtain advantageous optical
properties of an electron microscopy system, for instance, using
the objective lens arrangement since a primary electron beam having
a particularly high kinetic energy may be generated and formed by
beam shaping components of the electron microscopy system, and the
primary electrons of the beam are decelerated to desired kinetic
energies only shortly above the object plane, thus greatly reducing
Coulomb interactions between the primary electrons. Further, an
electrical field generated between the object disposed in the
object plane and the second pole piece will accelerate secondary
electrons emanating from the object.
[0037] In exemplary embodiments, voltages supplied by the first or
second voltage sources include voltages which may be equal to or
higher than 20 kV, 25 kV, 30 kV and may be equal to or higher than
45 kV, for instance.
[0038] The voltage supplied by the third voltage source may be an
adjustable voltage, for instance, which allows precise adjustment
of an electrical field immediately above the object plane to a
desired value. Likewise, in exemplary embodiments, the first and/or
second voltage sources may be adjustable voltage sources.
[0039] In an exemplary embodiment, the beam tube is electrically
insulated from the first pole piece.
[0040] In further exemplary embodiments, the first pole piece is
substantially at ground potential.
[0041] According to a further exemplary embodiment, the third
voltage source has one of its connectors connected to the second
pole piece and another of its connectors connected to the
electrical connector of the object mount, i.e. is connected to both
the second pole piece and the electrical connector.
[0042] In a further exemplary embodiment, the first pole piece is
electrically insulated from the second and third pole pieces by a
thin insulating layer. In advantageous embodiments thereof, a large
area of overlap is provided between the first pole piece on one
side and the second or third pole pieces on the other side, in
other words a large area is provided in which opposite surfaces of
the respective pole pieces are arranged in the vicinity of one
another and preferably parallel or nearly parallel to one another.
This allows for a sufficient electrical insulation between the
first pole piece and the second and third pole pieces whilst
maintaining a sufficiently low magnetic resistance for generating
the focusing magnetic field in the first gap.
[0043] Generally, in those exemplary embodiments, the insulating
layer is preferably provided between an outer portion of the first
pole piece and an outer portion of the second pole piece.
[0044] In those exemplary embodiments wherein the first pole piece
has an integrally formed outer cylindrical portion, the insulating
layer is preferably provided between the cylindrical portion of the
first pole piece and an outer portion of the second pole piece.
[0045] In further exemplary embodiments of the objective lens
arrangement according to the second aspect, the objective lens
arrangement may further comprise a third pole piece having a radial
inner end which is disposed at a distance from the radial inner end
of the second pole piece to form a second gap, wherein the first
pole piece is electrically insulated from both the second and third
pole pieces by an insulating layer.
[0046] Embodiments and features of the objective lens arrangement
according to the first aspect are equally applicable to the
objective lens arrangement according to the second aspect.
[0047] In further exemplary embodiments, the first pole piece
comprises an inner member and an outer member, that is two separate
parts, with the inner and outer members being electrically
insulated from each other by an insulating layer. The outer member
would then also comprise the outer portion of the pole piece that
is generally disposed to face the outer portion of the second pole
piece. In exemplary embodiments, the outer member of the first pole
piece is configured to accommodate the first excitation coil and
the inner member of the first pole piece comprises a substantially
conical portion extending towards the axis of symmetry. These
embodiments are particularly advantageous when the inner member of
the first pole piece is disposed adjacent to the beam tube and
electrically connected thereto.
[0048] Dividing the first pole piece into an inner member and an
outer member and electrically connecting the inner member and the
beam tube extending through the bore formed by the inner member has
the advantage that provision of electrical power to the beam tube
is facilitated. Rather than having to provide electrical wiring to
the beam tube itself, the electrical power is provided via the
inner member of the pole piece, which is more easily accessible for
electrical connections. In addition, dividing the first pole piece
into two members and electrically insulating the two members from
one another saves the provision of an electrically insulating layer
between the beam tube and the first pole piece, which tends to
require a complex layout to avoid creep currents and the like.
[0049] In a third aspect, the invention provides an objective lens
arrangement, comprising a second pole piece and a third pole piece,
wherein the second and third pole pieces are substantially
rotationally symmetric with respect to an axis of symmetry, wherein
the second and third pole pieces are disposed on a same side of an
object plane of the objective lens arrangement, wherein a radial
inner end of the third pole piece is disposed at a distance from a
radial inner end of the second pole piece to form a second gap, and
wherein the second and third pole pieces are electrically connected
to each other; a second excitation coil for generating a magnetic
field in the second gap; and a second power supply configured for
supplying an excitation current to the second excitation coil,
wherein the second power supply is substantially at ground
potential; and a third voltage source configured for supplying a
voltage to the second pole piece such that the second pole piece is
at a potential differing from a potential of the second excitation
coil by more than about 15 kV, in particular more than 20 kV, in
particular more than 25 kV, and in particular more than 30 kV.
[0050] The pole pieces of the objective lens arrangement according
to this aspect of the invention are denoted "second" and "third"
pole pieces (rather than "first" and "second") simply for sake of
clarity and easier reference to other embodiments and aspects of
the present invention as described herein, the same applies to the
numbering of the power supplies.
[0051] In this configuration, the second pole piece may be
advantageously used for shaping the electrical field in a region
close to the object plane whilst, at the same time, avoiding to
operate the power supply for supplying the excitation current to
the second excitation coil at a high electrical potential.
[0052] In exemplary embodiments, the objective lens arrangement
according to the third aspect of the present invention further
comprises: a first pole piece, wherein the first pole piece is
substantially rotationally symmetric with respect to the axis of
symmetry, wherein the first pole piece is disposed on the same side
of the object plane of the objective lens arrangement as the second
and third pole pieces, wherein a radial inner end of the first pole
piece is disposed at a distance from the radial inner end of the
second pole piece to form a first gap, and wherein the first pole
piece is electrically insulated from the second and third pole
pieces, wherein the third voltage source is further configured to
supply the voltage to the second pole piece such that the second
pole piece is at a potential differing from a potential of the
first pole piece by more than about 15 kV, in particular more than
20 kV, in particular more than 25 kV, and in particular more than
30 kV; and a first excitation coil for generating a magnetic field
in the first gap.
[0053] According to a further exemplary embodiment, a cooling
system is provided which includes a cooling medium supply for
supplying a cooling medium to the second excitation coil.
Advantageously, the cooling medium supply may be set to ground
potential or near ground potential. The cooling medium may be
water, for instance.
[0054] According to a fourth aspect of the present invention, an
objective lens arrangement is provided comprising a second pole
piece and a third pole piece, wherein the second and third pole
pieces are substantially rotationally symmetric with respect to an
axis of symmetry, wherein the second and third pole pieces are
disposed on a same side of an object plane of the objective lens
arrangement, wherein a radial inner end of the third pole piece is
disposed at a distance from a radial inner end of the second pole
piece to form a second gap, wherein the second and third pole
pieces are electrically connected with each other. The objective
lens arrangement according to the fourth aspect further comprises a
second excitation coil for generating a magnetic field in the
second gap; and a third voltage source configured for supplying a
voltage to the second pole piece such that the second pole piece is
at a potential differing from a potential of the compensating coil
by more than about 15 kV, in particular more than 20 kV, in
particular more than 25 kV, in particular more than 30 kV, and in
particular more than 45 kV.
[0055] The second excitation coil of the objective lens according
to this aspect of the present invention comprises a plurality of
windings of an insulated wire, and at least one further insulating
layer is provided for supporting the second excitation coil with
respect to at least one of the second and third pole pieces.
[0056] Such further insulating layer, which is different from an
insulating layer surrounding a wire forming the individual
windings, allows to efficiently insulate the whole body of the
second excitation coil from the second and third pole pieces and
thus allows a power supply for supplying the second excitation coil
with a suitable current to be maintained at a potential different
from the potential of the second and third pole pieces.
[0057] The insulating layer may be made from ceramic material or
cast resin, for instance.
[0058] The pole pieces of the objective lens arrangement according
to this aspect of the invention are, in analogy to the third
aspect, denoted "second" and "third" pole pieces (rather than
"first" and "second") simply for sake of clarity and easier
reference to other embodiments and aspects of the present invention
as described herein. This applies likewise to the voltage
source(s).
[0059] In an exemplary embodiment, the objective lens arrangement
according to the fourth aspect further comprises a first pole
piece, wherein the first pole piece is substantially rotationally
symmetric with respect to the axis of symmetry, wherein the first
pole piece is disposed on the same side of the object plane of the
objective lens arrangement as the second and third pole pieces,
wherein a radial inner end of the first pole piece is disposed at a
distance from the radial inner end of the second pole piece to form
a first gap, and wherein the first pole piece is electrically
insulated from the second and third pole pieces, wherein the third
voltage source is further configured to supply the voltage to the
second pole piece such that the second pole piece is at a potential
differing from a potential of the first pole piece by more than
about 15 kV, in particular more than 20 kV, in particular more than
25 kV, and in particular more than 30 kV; and a first excitation
coil for generating a magnetic field in the first gap.
[0060] In a fifth aspect, the present invention provides an
objective lens arrangement comprising an object mount for mounting
an object to be processed such that the object is disposed in an
object plane of the objective lens arrangement, wherein the object
mount includes an electrical connector for delivering an electrical
voltage to the object. The objective lens arrangement further
comprises a pole piece, which is referred to as the third pole
piece in line with the terminology of the other aspects of the
present invention, which third pole piece is substantially
rotationally symmetric with respect to an axis of symmetry of the
objective lens arrangement and which extends substantially
transversely to the axis of symmetry. A voltage source, referred to
in the following as the third voltage source, is provided and
configured for supplying a voltage to the third pole piece such
that the third pole piece is at a potential differing from a
potential of the electrical connector by from about 0.1 kV to about
10 kV. The objective lens arrangement further comprises a shielding
electrode which is disposed between the third pole piece and the
object plane, and which is electrically insulated from the third
pole piece.
[0061] The voltage applied to the third pole piece serves to
generate an electrical field on a surface of the object in a region
where it is being processed, whereas the provision of the shielding
electrode allows to shield regions of the object outside of the
region being processed from said electrical field. This shielding
effect thus allows avoiding or reducing charging effects on the
object.
[0062] According to an exemplary embodiment, the shielding
electrode is electrically connected to the electrical connector of
the object mount such that substantially no electrical field is
present in the space between the object plane and the shielding
electrode. According to a further exemplary embodiment, the
shielding electrode substantially has a ring-shape with an inner
aperture, which is substantially concentric with the optical axis
of the system or the axis of symmetry of the objective lens
arrangement, respectively.
[0063] The third pole piece has a surface facing in a direction of
the object mount. In exemplary embodiments, the third pole piece
has a radial inner annular portion in which said surface extends
substantially parallel to the object plane at a first distance from
the object plane, and a radial outer annular portion in which the
surface extends substantially parallel to the object plane at a
second distance from the object plane, wherein the second distance
is greater than the first distance. The radial inner annular
portion has a radial inner end which may coincide with the inner
peripheral edge of the third pole piece, and a radial outer end
which may coincide with the surface of the third pole piece which
is disposed at a different angle to the object plane than the inner
angular portion. Hence, the inner annular portion is disposed
closer to the object plane than the outer annular portion.
Furthermore, in this exemplary embodiment, the shielding electrode
has an inner aperture, which may be concentric about the axis of
symmetry of the third pole piece, as described above, wherein the
inner annular portion of the third pole piece is disposed and
configured such that its radial outer end is disposed within the
inner aperture of the shielding electrode. In other words, in this
exemplary embodiment, a diameter of the inner aperture is greater
than a diameter of the inner annular portion of the third pole
piece such that the inner annular portion may be contained entirely
within the inner aperture of the shielding electrode, and the inner
annular portion may be disposed in a same plane as the shielding
electrode. Thus, the shielding electrode would also be disposed at
about the first distance from the object plane. In those exemplary
embodiments, the second distance of the outer annular portion of
the third pole piece would need to be chosen such that it permits
the shielding electrode to be arranged in the same plane and a gap
to be kept in between the third pole piece, in particular in a
region of the annular outer portion, and the shielding
electrode
[0064] In those exemplary embodiments, the inner annular portion
and the outer annular portion may be disposed immediately adjacent
one another, such that the surface of the third pole piece (facing
the object plane) would have a step to accommodate the transition
from the first to the second distance, or the inner and outer
annular portions may be joined to one another by a middle annular
portion disposed at an angle to the object plane to accommodate the
transition from the first to the second distance. The middle
annular portion could be relatively small compared to the other
annular portions, for instance, such that most of the surface of
the third pole piece opposite the object would be disposed
substantially parallel to the object plane, such that in those
embodiments, the third pole piece would comprise a small bend in
the middle annular portion.
[0065] Substantially parallel, as used in this context, shall also
comprise those embodiments wherein the surface of the radial outer
annular portion is disposed at an angle of up to 30.degree., for
instance, or up to 20.degree. in a further example, with respect to
the object plane and/or wherein the radial inner annular portion is
disposed at an angle of up to 20.degree. with respect to the object
plane, or up to 10.degree. in another example.
[0066] The objective lens arrangement according to this aspect of
the present invention may also comprise additional components and
features as described herein in connection with the other aspects
of the present invention. In particular, in exemplary embodiments,
the objective lens arrangement further comprises a second pole
piece, wherein a radial inner end of the third pole piece and a
radial inner end of the second pole piece form a gap between them.
In further exemplary embodiments, the second pole piece has an
inner angular portion with a surface facing the third pole piece
and the third pole piece has an angular portion with a surface
facing the second pole piece, wherein the surfaces of the third and
second pole pieces facing each other form an angle of less than
40.degree., for instance less than 35.degree. between them. The gap
may be a substantially radial gap. In yet further exemplary
embodiments, the second pole piece has a inner annular portion
wherein the surface facing the third pole piece is disposed at an
angle of from between about 3.degree. to about 35.degree. with
respect to the surface of the inner annular portion of the third
pole piece facing the second pole piece.
[0067] According to a sixth aspect of the present invention, an
objective lens arrangement is provided which comprises an object
mount for mounting an object to be processed in an object plane,
and first and second pole pieces which are substantially
rotationally symmetric with respect to an axis of symmetry of the
objective lens arrangement. The first and second pole pieces extend
towards the axis of symmetry such that radial inner ends of the
first and second pole pieces define bores which are configured to
be traversed by one or more beams of charged particles. A first
excitation coil is provided for generating a focusing magnetic
field in a first gap formed between the radial inner end of the
first pole piece and the radial inner end of the second pole piece.
A beam tube configured for guiding the one ore more beams of
charged particles extends through the bore formed by the radial
inner end of the first pole piece. In embodiments according to this
aspect, the bore of the first pole piece generally extends from a
first plane where a diameter of the bore is a minimum diameter to a
second plane in which a front surface portion of the first pole
piece is disposed. The diameter of the bore increases from its
minimum diameter in the first plane to a front diameter in the
second plane by more than about 10 mm, wherein a distance between
the first and second planes is more than about 5 mm, thus resulting
in a tapering shape.
[0068] Such a tapering or conical shape of the first pole piece
allows shaping a distribution of a magnetic field strength on the
axis of symmetry, which generally coincides with an optical axis of
a particle-optical system, such that desired optical properties of
the objective lens may be achieved.
[0069] According to a seventh aspect of the present invention, a
charged particle beam system is provided which comprises a charged
particle source for generating a beam of charged particles, at
least one beam shaping lens configured to be traversed by the
charged particles and an objective lens configured to be traversed
by the charged particles, wherein the objective lens has an axis of
symmetry and an object plane associated therewith.
[0070] The at least one beam shaping lens and the objective lens
are configured such that an average direction of incidence of
charged particles, which average direction of incidence may be
defined as an average over all directions from which charged
particles are incident at a respective location of the object
plane, is oriented away from the optical axis in a ring-shaped
inner portion of the object surrounding the optical axis, and such
that the average directions of incidence at locations within a
ring-shaped outer portion of the portion of the object plane
surrounding the ring-shaped inner portion are oriented towards the
optical axis.
[0071] Such a configuration allows to reduce a third order
telecentricity error in the object plane by a substantial
amount.
[0072] According to an exemplary embodiment, a maximum average
angle .theta..sub.i of the average angles of incidence at the
location within the ring-shaped inner portion relates to a maximum
average angle .theta..sub.o of the average angles of incidence at
the locations within the ring-shaped outer portion as defined by
the following equation:
0.5 < .theta. i .theta. 0 < 2. ##EQU00001##
[0073] In an exemplary embodiment, the maximum average angle
.theta..sub.i differs from the maximum average angle .theta..sub.o
by at most 30% of the absolute value of the maximum average angle
.theta..sub.o, for instance at most 20%. It may even differ by as
little as 15% or less or even 10% or less.
[0074] According to a further exemplary embodiment, the maximum
average angle of the average angles of incidence at the locations
within the ring-shaped outer portion is more than about 1 mrad.
[0075] Such a configuration may be advantageously put into practice
using an objective lens arrangement that includes a pole piece
having a tapering shape, as described above. A further advantage of
such a configuration is that it allows reducing a field curvature
associated with the objective lens arrangement.
[0076] According to an eighth aspect of the present invention, an
objective lens arrangement is provided, which comprises an object
mount for mounting an object to be processed in an object plane, a
first electrode disposed at a distance from the object plane and
having an aperture of a first diameter which is concentric with an
axis of symmetry of the objective lens arrangement, and a second
electrode disposed at a second distance from the object plane and
in between the first electrode and the object plane, and having an
aperture of a second diameter, which aperture is concentric to the
axis of symmetry.
[0077] A first voltage supply is connected to the first electrode
and may be configured and operated such that the first electrode is
set to a first potential relative to the object to be processed,
and a second voltage supply is connected to the second electrode
and may be configured and operated such that the second electrode
is set to a second potential relative to the object to be
processed.
[0078] The first and second distances, the first and second
diameters and the first and second voltages are adjusted such that
a contribution of the first electrode to an electrical field
generated immediately above the object plane is of a same order of
magnitude as a contribution of the second electrode to said
electrical field. The contributions of the first or second
electrodes to the generated electrical field may be assessed and
tested by comparing two settings, a first and a second setting. In
the first setting, the first electrode is at the first potential
relative to the electrical connector and thus the object and the
second electrode is at the same potential as the electrical
connector. In the second setting, the first electrode is at the
first potential relative to the electrical connector and the second
electrode is at the same potential as the first electrode.
[0079] According to an exemplary embodiment, the following relation
is fulfilled:
( E 1 - E 2 ) 2 ( E 1 + E 2 ) < 0.3 , ##EQU00002##
wherein [0080] E.sub.1 is the electrical field at the object plane
in the first setting, and [0081] E.sub.2 is the electrical field at
the object plane in the second setting.
[0082] In exemplary embodiments, the above defined ratio
(E.sub.1-E.sub.2)/2(E.sub.1-E.sub.2) may be equal to or smaller
than 0.2, or be equal to or smaller than 0.1, or be equal to or
smaller than 0.05.
[0083] This aspect applies in particular to electron beam systems.
A configuration wherein this relation is fulfilled is particularly
advantageous when a large aperture of the electrode adjacent to the
object plane generating the electrical field in the object plane
and a correspondingly large electrical field in the object plane
are applied. A homogenous electrical field in the region of the
object plane provides a homogeneous extraction field for secondary
electrons, which is likely to result in improved secondary electron
yield and/or good aberration coefficients for the secondary
electrons.
[0084] According to a ninth aspect, the present invention provides
a particle-optical inspection system comprising: an objective lens
arrangement comprising a first pole piece and a second pole piece,
wherein the first and second pole pieces are substantially
rotationally symmetric with respect to an axis of symmetry, wherein
a radial inner end of the first pole piece is disposed at a
distance from a radial inner end of the second pole piece to form a
first gap between them, wherein the first pole piece has an inner
portion extending at an angle towards the axis of symmetry and
wherein the first and second pole pieces are electrically insulated
from each other; a first excitation coil for generating a focusing
magnetic field in a region of the first gap; a beam tube extending
through a bore formed by the radial inner end of the first pole
piece; and a first voltage source configured for supplying a
voltage to the beam tube; with the particle-optical inspection
system further comprising a beam path splitting arrangement
comprising at least one magnetic field arrangement, wherein a lower
end of the at least one magnetic field arrangement of the beam path
splitting arrangement is disposed at a first distance from the
object plane and wherein an upper end of the first excitation coil
is disposed at a second distance from the object plane and wherein
the first distance is shorter than the second distance. In other
words, the beam path splitting arrangement is at least partially
inserted into the objective lens arrangement.
[0085] Lower as used above indicates a direction with respect to
the object plane, i.e. lower indicates a closer distance to the
object plane than upper.
[0086] Beam splitting arrangements are advantageously used in
multi-beam inspection systems, such as described for instance in WO
2005/024881 A2 (U.S. provisional application Ser. No. 60/500,256)
to the same Assignee, the entire content of which is incorporated
by reference herein. A beam splitting arrangement will be described
in detail with reference to the drawings.
[0087] It has been found to be favorable for the beam path
splitting arrangement to be disposed close to the object plane. In
conventional systems employing a beam splitting arrangement, this
arrangement is typically disposed upstream of the objective lens
arrangement without any overlap between these two components. In
contrast thereto, according to this aspect of the present
invention, a lower portion of the beam splitting arrangement, i.e.
the portion of the beam splitting arrangement disposed closest to
and facing the object plane, is practically inserted into the
objective lens arrangement. This is particularly advantageous in
inspection systems using electrons as charged particles since an
image of secondary electrons generated by electrons impinging on
the object to be inspected is usually formed closely above the
object plane. Insertion of the beam splitting arrangement into the
objective lens arrangement thus allows shortening a path between
the image of secondary electrons and a nearest focusing optical
element of the inspection system and thus enables enhanced
inspection performance.
[0088] In exemplary embodiments of the inspection system according
to this aspect of the present invention, the inner portion of the
first pole piece extends towards the axis of symmetry such that the
radial inner end of the first pole piece is disposed closer to the
object plane than a radial outer end of the inner portion of the
first pole piece and thus allows the lower end of the at least one
magnetic field arrangement to be disposed within a bore or space
defined by the inner portion of the first pole piece.
[0089] In this embodiment, the particle-optical inspection system
may further comprise a mounting structure, that may be attached to
the first pole piece, for mounting the magnetic field arrangement
of the beam path splitting arrangement or, more generally, a lower
portion of the beam path splitting arrangement. The mounting
structure may also allow adjusting a position of the magnetic field
arrangement of the beam splitting arrangement relative to at least
the first pole piece.
[0090] For instance, the inner portion of the first pole piece may
have a substantially conical shape with the radial inner end of the
first pole piece being disposed closer to the object plane than a
radial outer end thereof, and with the lower end of the magnetic
field arrangement being disposed inside the conus formed by the
inner portion of the first pole piece. In those embodiments, the
conus formed by the inner portion of the first pole piece may
having a conus opening angle .alpha. in a range of from 20.degree.
to about 70.degree., for example.
[0091] In other exemplary embodiments, the inner portion may
comprise two substantially cylindrical shapes with a lower of the
two cylinders forming a bore having a smaller diameter than an
upper cylinder. In those embodiments, the lower portion of the beam
path splitting arrangement may be disposed at least partially
within the bore formed by the upper cylinder. However, it is not
necessary for the bore of the lower cylinder to be smaller, it may
also be greater or may be the same as the bore of the upper
cylinder. Other configurations are also possible and will be
readily apparent to the person skilled in the art.
[0092] Even without explicit mentioning, it will be apparent to the
skilled person that individual features or combinations of features
of embodiments of the objective lens arrangements and systems
described herein in connection with a particular aspect of the
present invention may also be applied in embodiments of the
objective lens arrangements and systems of the other aspects of the
invention.
[0093] In exemplary embodiments, the objective lens arrangements
according to the present invention may further comprise a heating
system disposed within at least one excitation coil, the heating
system comprising a heating coil disposed in the vicinity of the at
least one excitation coil and a control portion for controlling and
adjusting a current passing through the heating coil. The at least
one excitation coil may be the first, second and/or third
excitation coil of the embodiments described above. For instance,
the heating coil may be disposed within the excitation coil, i.e.
in a cavity within the excitation coil, or be interlaced therewith.
In particular, the control portion may be configured to adjust the
current passing through the heating coil in dependence of at least
one of a current passing through the at least one excitation coil
(excitation current), for instance the at least one of the first,
second and third excitation coils, a temperature of at least one of
the first, second and third pole pieces, as desired and applicable.
Furthermore, a temperature sensor may be provided in those
embodiments for sensing the temperatures of at least one of the
first, second and third pole pieces. The sensed temperature may
then be transmitted to the control portion to control a current
provided by the power supply to the heating coil. This embodiment
has the advantage that a temperature of one or more pole pieces may
be kept substantially constant. Thus, disturbances caused by
heating of the pole pieces, which may lead to an expansion of the
pole piece material and thus an undesirable change of the
dimensions and geometry of the objective lens arrangement, may be
avoided. Heating of the pole pieces may result from prolonged
operation of the objective lens system and may also result from a
change of application and thus a change of focusing power and
associated change of excitation current. This embodiment allows
keeping a magnetic field and thus focusing characteristics of the
objective lens arrangements constant and well controllable. In an
alternative embodiment, the heating coil may take a shape of an
only nearly closed ring about the axis of symmetry, i.e. an
incomplete circle wherein ends thereof do not touch. This
embodiment is advantageous in that undesired magnetic fields that
might be generated by the heating coil can be avoided.
[0094] Use of a cooling system for cooling in particular the second
excitation coil has already been described above in connection with
the third aspect of the present invention. Rather than use of a
fluid cooling medium, other embodiments of the objective lens
arrangements according to any of the aspects of the present
invention may make use of a cooling system based exclusively on
solid-state materials for conducting away heat generated in
particular by the excitation coils.
[0095] In an exemplary embodiment, which is explained in the
following with reference to the second excitation coil but may also
be applied to any other excitation coil, the second and third pole
pieces are substantially integrally formed and connected by a yoke,
and accommodate the second excitation coil between them in a region
of their outer annular regions. The excitation coil generally
comprises a number of windings of an insulated wire, which is
connected to a power supply. In this embodiment, at least an outer
side of a body of the excitation coil formed by the wire windings
is at least partially encapsulated by one ore more layers of
thermally well conducting and electrically insulating ceramic
material. This ceramic encapsulation is connected to or integrally
formed with connecting members made of the same or a similar
material that extends through portions of the yoke connecting the
second and third pole pieces. Those connecting portions may be
distributed at regular intervals around a circumference of the yoke
to establish a thermally conducting contact to a ring of thermally
well conducting solid material disposed around and adjacent to the
radial outer end of the yoke. The ring of thermally well conducting
material may be made from ceramic material, copper or comprise both
a ring of ceramic material and a ring of copper that are in contact
with one another. Those rings are connected, preferably via copper
wire, to a cooling system further remote from the second and third
pole pieces, which may be a cooling system based on liquid cooling,
for instance. The solid-state cooling system has the advantage that
insulating the cooling system from the high voltage parts comprised
in the objective lens arrangements is easier to achieve than in the
case of liquid cooling. This embodiment has therefore the advantage
that no electrically conductive material is introduced into the
vicinity of the excitation coil inside the pole piece. It will be
apparent to the person skilled in the art that other suitable
thermally well conductive materials may be used and that this kind
of cooling system may also be used for the first and second pole
pieces or any other parts of the system that may require
cooling.
[0096] In a further exemplary embodiment of the present invention,
in particular in connection with the above-described type of
solid-state cooling system, the objective lens arrangement
according to the present invention may further comprise an
adjustable mounting structure for mounting the second and third
pole pieces. The mounting structure allows adjusting a position of
the second and third pole pieces in particular relative to the
first pole piece. The adjustable mounting structure may comprise,
for instance, a mounting ring disposed around the yoke connecting
the first and second pole pieces and fixedly attached thereto. In
an exemplary embodiment, the mounting ring is held in place by
three or more wires, or flexible elements, more generally. Lower
ends of the wires are fixed to the mounting ring, for instance,
preferably at points spaced equally about a perimeter of the
mounting ring, and upper ends of the wires are advantageously
attached to one or more components upstream of the second and third
pole pieces, such as the first pole piece. This mounting structure
allows holding the second and third pole pieces in place without
the need for any bulky holding components. Thus, the second and
third pole pieces may be held entirely in a vacuum environment. A
position of the second and third pole pieces, in particular
relative the first pole piece, may be adjusted by suitable
shortening or lengthening one or more of the wires, as
required.
[0097] In further exemplary embodiments, the adjustable mounting
structure further comprises a fine adjustment arrangement. The fine
adjusting arrangement may comprise, for instance, a mechanism for
adjusting an axial position of the mounting ring, or the second and
third pole pieces, more generally, and a mechanism for adjusting a
radial position of the mounting ring, or the second and third pole
pieces, more generally, in the objective lens arrangement or a
combination of the two.
[0098] The mechanism for adjusting an axial position of the
mounting ring and thus the second and third pole pieces may
comprise a screw having one end attached to a component of the
objective lens arrangement which has a fixed or fixable position,
with the screw having a winding which is connected to the mounting
ring such that turning of the screw results in a change of an axial
position thereof. For instance, turning the screw may lift or lower
the mounting ring relative to other components of the objective
lens arrangement, such as the first pole piece, for instance.
[0099] An adjustment mechanism for adjustment of a radial position
may be provided by an arrangement comprising a combination of a
wedge-shaped member, a bearing comprising a chamber with two balls
inside, and a screw. The chamber and the balls are configured such
that the balls touch each of four side walls of the chamber, the
chamber being open to one side such that a pointed side of the
wedge can be arranged in between and in contact with the two balls.
One end of the screw extends into the top of the chamber such that
turning of the screw drives a lower end of the screw further into
or out of the chamber and optionally moves the chamber in an
upwards or downwards direction. Turning of the screw thus effects a
change in the distance between the two balls, which upon
approaching one another push the pointed side of the wedge
outwards. The wedge which is directly or indirectly connected to
the pole pieces transfers this movement onto the pole pieces, and
thus changes their radial position in the objective lens
arrangement, for instance relative to the first pole piece. Other
adjustment mechanisms known in the art may also be used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0100] The foregoing as well as other advantageous features of the
invention will be more apparent from the following detailed
description of exemplary embodiments of the invention with
reference to the accompanying drawings. It is noted that not all
possible embodiments of the present invention necessarily exhibit
each and every, or any, of the advantages identified herein.
[0101] FIG. 1 schematically illustrates basic features and
functions of an electron microscopy system according to an
embodiment of the present invention;
[0102] FIG. 2 is a schematic illustration of an embodiment of an
objective lens arrangement, which may be used in the electron
microscopy system depicted in FIG. 1;
[0103] FIG. 3 shows an electrode configuration for illustrating a
function of field generating components shown in FIG. 2;
[0104] FIG. 4 shows an enlarged view of a lower part of a beam tube
of the objective lens arrangement shown in FIG. 2;
[0105] FIG. 5 shows plural physical properties provided by the
embodiment of the objective lens arrangement shown in FIG. 2 along
the optical axis;
[0106] FIG. 6a, FIG. 6b show graphs for illustrating radial
dependencies of an average angle of incidence in an object plane of
the electron microscopy system shown in FIG. 1;
[0107] FIG. 7 schematically shows a further embodiment of an
objective lens arrangement according to the present invention;
[0108] FIG. 8 shows a further, alternative embodiment of an
objective lens arrangement according to the present invention;
[0109] FIG. 9 shows an exemplary embodiment of a beam path
splitting arrangement;
[0110] FIG. 10 shows a cooling structure used in the embodiment
depicted in FIG. 8;
[0111] FIG. 11 shows an adjusting mechanism used in the mounting
structure for holding the second and third pole pieces in the
embodiment illustrated in FIG. 8;
[0112] FIG. 12 shows a heating system incorporated into the
embodiment shown in FIG. 8; and
[0113] FIG. 13 shows a detail of the embodiment depicted in FIG.
8.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0114] In the exemplary embodiments described below, components
that are alike in function and structure are designated by alike
reference numerals, as far as possible. Therefore, in order to
understand the features of the individual components of one
specific embodiment, the descriptions of other embodiments and of
the summary of the invention may also be considered and referred
to.
[0115] FIG. 1 is a schematic diagram symbolically illustrating
basic functions and components of an electron microscopy system 1.
The electron microscopy system 1 is of a scanning electron
microscope type (SEM) using a plurality of primary electron
beamlets 3' for generating primary electron beam spots on a surface
of an object to be inspected, which surface is arranged in an
object plane 101 of an objective lens arrangement 100.
[0116] The primary electrons incident on the object at the beam
spots generate secondary electrons emanating from the surface of
the object. The secondary electrons form secondary electron
beamlets entering the objective lens arrangement 100. The electron
microscopy system 1 provides a secondary electron beam path 4' for
supplying the plurality of secondary electron beamlets to a
detecting arrangement 200. Detecting arrangement 200 comprises a
projecting lens arrangement 201, 202 for projecting the secondary
electron beamlets 4' onto a surface plane of an electron sensitive
detector 203. The detector 203 can be one or more selected from a
solid state CCD or CMOS, a scintillator arrangement, a micro
channel plate, an array of PIN diodes and others.
[0117] The primary electron beamlets 3' are generated by a beamlet
generating arrangement 300 comprising an electron source 301, a
beam liner tube 302, a collimating lens 303, a multi-aperture plate
arrangement 304 and a field lens 305.
[0118] In the embodiment depicted in FIG. 1, an electron source 301
is arranged on an optical axis of the system in a beam liner tube
302 and is additionally immersed in a magnetic field generated by
collimating lens 303. The electrons are extracted from the electron
source 301 and form a diverging electron beam, which is collimated
by collimating lens 303 to form a beam 3 for illuminating
multi-aperture arrangement 304. Multi-aperture arrangement 304
comprises a multi-aperture plate 304A, which is mounted in a center
of a cup-shaped electrode 304B. An electrical field may be
generated between the cup-shaped electrode 304B and a flange at the
end of beam liner tube 302, which may be a decelerating or
retarding field, for instance. The multi-aperture arrangement forms
a plurality of primary electron beamlets 3' from the single
illuminating beam 3 impinging on the multi-aperture plate 304A.
Details of multi-aperture arrangements may be found in the
references cited in the introduction or WO 2005/024881 A2 (U.S.
provisional application Ser. No. 60/500,256) to the same Assignee,
for instance.
[0119] A field lens 305 and objective lens arrangement 100 are
provided in the beam path 3' of the plurality of primary electron
beamlets to project an image of a focus plane of the multi-aperture
arrangement 304 onto object plane 101 to form an array of primary
electron beam spots on the object.
[0120] A beam path splitting/combining arrangement 400 is also
provided in the primary electron beam path 3' in between the
beamlet generating arrangement 300 and objective lens arrangement
100 and in the secondary electron beam path 4' in between the
objective lens arrangement 100 and the detecting arrangement
200.
[0121] Beam splitting arrangement 400 allows the beam paths of the
primary electron beamlets 3' and the secondary electron beamlets 4'
which both pass through the objective lens arrangement 100 to be
separated such that the secondary electron beamlets are directed
towards the detecting arrangement 200. An exemplary beam splitting
arrangement will be described in more detail with reference to FIG.
9.
[0122] FIG. 2 shows a schematic cross section of a side view of an
exemplary embodiment of an objective lens arrangement 100 which may
be used in the electron microscopy system shown in FIG. 1. The
objective lens arrangement 100 comprises an object mount 121 for
mounting an object 7 to be inspected such that a surface of the
object 7 is disposed within the object plane 101 of the electron
microscopy system 1. The object 7 may be a semiconductor wafer, for
instance, which is to be inspected for defects.
[0123] The objective lens 102 comprises a first pole piece 123,
which is concentric about the optical axis 120 (or axis of
symmetry) of the objective lens 102 and has a radial inner end 124.
A second magnetic pole piece 125 which is also rotationally
symmetric and concentric about the optical axis 120 has a radial
inner end 126 and is disposed at a distance from radial inner end
124 of the first pole piece 123 such that a substantially axial gap
is formed between the radial inner ends 124 and 126.
[0124] An excitation coil 129 is disposed radially outwardly of
(i.e. at a greater distance from) the gap formed between inner ends
124, 126 in between the first and second pole pieces 123, 125. A
yoke 130 forms part of the first pole piece and extends radially
outwardly there from and is disposed opposite a yoke 131 formed by
and extending radially outwardly from second pole piece 125. An
electrically insulating resin 133 is provided in a gap between yoke
130, or excitation coil 129, respectively, which is disposed to be
at least partially surrounded by yoke 130, and yoke 131 in that
region where yokes 130, 131 are disposed adjacent to one another.
Yoke 130 comprises a cylindrical portion 135, which is separated by
insulating resin 133 from a corresponding adjacent cylindrical
portion 136 of yoke 131, with the cylindrical portion 135 of the
yoke 130 of the first pole piece 123 partially surrounding the
cylindrical portion 136 of the yoke 131 of the second pole piece
125. The first yoke 130 further comprises an annular disc-shaped
portion 137, which is separated by the insulating resin 133 from an
adjacent corresponding annular disc-shaped portion 138 of yoke 131.
Thus, the first and second yokes 130, 131 are configured and
arranged such that a region between yokes 130 and 131, or more
precisely cylindrical and annular disc-shaped portions thereof,
provides a considerable surface area such that a magnetic
resistance between yokes 130 and 131 extending from and forming
part of pole pieces 123 and 125, respectively, is low whilst both
pole pieces 123, 125 are kept electrically insulated from each
other.
[0125] A power supply 141 is connected to first excitation coil 129
for supplying an excitation current to the first excitation coil
129 for generation of a magnetic field in the gap between radial
inner ends 124, 126 of first and second pole pieces 123, 125. The
electric field generated by the first excitation coil 129 induces a
magnetic flux, indicated by arrows 142, in a magnetic circuit
formed by magnetic pole pieces 123, 125 and yokes 130 and 131 such
that the magnetic circuit is closed via the first gap formed
between radial inner ends 124 and 126 of the first and second pole
pieces 123 and 125, respectively. The magnetic field generated by
the first excitation coil 129 has a focusing effect on the
electrons of primary electron beamlets exiting from a beam tube 152
arranged coaxially with the optical axis 120.
[0126] A lower end of the beam tube 152 is disposed in a region of
the first gap between radial inner ends 124, 126 of the first and
second pole pieces 123, 125. A high voltage supply 153 is provided
to maintain the beam tube 152 at a potential of about +30 kV, in
this embodiment. A voltage supply 155 is connected to the object
mount 121 via a connector 156 to supply an adjustable high voltage
of about -29.7 to -28 kV to the object mount 121. The object 7 to
be inspected is arranged to be in electrical contact with object
mount 121 such that object 7, too, is maintained at the adjustable
potential of about -29.7 to 28.0 kV.
[0127] A cathode of an electrode arrangement (upstream, not
depicted) is maintained at a voltage of from about -30 kV to about
-45 kV such that the primary electrons have a kinetic energy of
from about 60 to 90 keV when they travel through the beam tube 152.
A lower end of beam tube 152 is disposed at a distance from the
object plane 101 such that the primary electrons experience a
decelerating electric field in a space between the lower end of the
beam tube 152 and the object plane 101. The primary electrons will
then be incident on the object 7 with a landing energy of from
about 50 eV to about 3000 eV.
[0128] In addition, a radial inner portion of the first pole piece
123, i.e. a portion comprising a radial inner part of yoke 130 and
radial inner end 124 of the first pole piece 123, includes a cavity
124'' in which an excitation coil 127 is disposed. Excitation coil
127 is connected to a non-depicted further power supply in a manner
similar to power supply 141 and electrically insulated from the
first pole piece 123 including yoke 130. A further gap 124' is
formed within the radial inner end 124 of the first pole piece 123
which gap 124' is joined with cavity 124''. Thus, the first pole
piece is functionally divided and configured to form a fourth pole
piece and a third gap 124'. When excitation coil 127 is excited by
the respective power supply, a magnetic field is generated in a
region of the gap 124', which magnetic field serves to finely
adjust a strength and position of the focusing magnetic field
generated by excitation coil 129 in the first gap between the first
pole piece 123 and the second pole piece 125.
[0129] The electrical field generated between the lower end of beam
tube 152 and the object 7 is not only defined by their positions
and voltages applied thereto, but is in the depicted embodiment
also influenced by a voltage applied to the second pole piece 125.
The radial inner end 126 of the second pole piece 125, in
particular, may, for instance, be maintained at a voltage of +3.9
kV relative to the electrical connector 156 of object mount 121, by
a high voltage source 159 which is coupled to both the electrical
connector 156 and the second pole piece 125. An effect thereof is
described in more detail with reference to FIG. 5 below. In
addition, in the embodiment depicted in FIG. 2, shielding electrode
154 is shown to which the same voltage as to electrical connector
156 or object mount 121, respectively, is applied so as to shield
the object from an electrical field in an area of the shielding
electrode 154, thus preventing undesired charging of the object.
The shielding electrode has an annular shape with an inner
aperture, and is symmetric with respect to the optical axis 120 and
further disposed such that charged particles may pass through the
inner aperture to reach the object.
[0130] As illustrated in FIG. 3, a lower edge of second pole piece
125 is disposed, at its radial inner end 126, at a distance d.sub.1
from the surface of object 7 which surface coincides with object
plane 101. The lower end of beam tube 152 is disposed at a distance
d.sub.2 from the object plane 101. A diameter of the bore defined
by radial inner end 126 of pole piece 125 is denoted D.sub.1 and a
diameter of the beam tube 152 at its lower end is denoted
D.sub.2.
[0131] Distances d.sub.1 and d.sub.2, diameters D.sub.1 and D.sub.2
and the voltages applied to pole piece 125 and beam tube 152
relative to the object 7 are adjusted such that the electrical
field generated immediately above object plane 101 in a region
close to the optical axis 120 is a substantially homogeneous
electrical field. FIG. 3 shows several field lines or equipotential
lines representing the electrostatic field between the lower end of
beam tube 152 and pole piece 125, and between pole piece 125 and
the object 7. As illustrated in FIG. 3, a field line 161 closest to
the object plane 101 is a substantially straight line indicating a
substantially homogeneous electrical field in a region around
optical axis 120. Such a substantially homogeneous electrical field
is generated for the purpose of decelerating each of the primary
electron beamlets 3 to a desired landing energy. The substantially
homogenous electrical field may also provide a extraction field for
the secondary electrons emanating from the object 7 such that each
of the secondary electron beamlets 4' has a substantially same
kinetic energy when entering the objective lens 102.
[0132] In the configuration of the objective lens arrangement as
illustrated in FIG. 2, the electrical field at the object plane 101
may be divided into two components: A first component E.sub.1 of
the electrical field is generated by the potential difference
between pole piece 125 and object 7, and a second component E.sub.2
is generated by the potential difference between beam tube 152 and
object 7. Both components have a substantially same effect on the
electrical field at the object plane 101 in a region around the
optical axis 120. This may be illustrated by changing the voltages
applied to the beam tube 152 and to the pole piece 125 according to
the following two settings: in a first setting, beam tube 152 is
set to a potential of 59 kV relative to the object 7, and pole
piece 125 is at the same potential as the object 7. The resulting
electrical field at the object plane 101 and on the optical axis
120 is 1.8 kV/mm. In a second setting, pole piece 125 is at a
potential of 3.9 kV relative to the object 7 and the beam tube 152,
and the resulting electrical field at the object plane 101 is 1.2
kV/mm.
[0133] The requirement
( E 1 - E 2 ) 2 ( E 1 + E 2 ) = 0.1 < 0.3 ##EQU00003##
is thus fulfilled.
[0134] In the embodiment illustrated in FIG. 2, a third pole piece
163 extends almost parallel to the object plane and has a radial
inner end 164. The radial inner end 164 of the third pole piece 163
is disposed at a greater distance from the optical axis 120 than
the radial inner end 126 of the second pole piece 125, and both
radial inner ends are disposed in a same plane orthogonal to the
optical axis 120. A radial gap is thus formed between radial inner
end 164 of third pole piece 163 and radial inner end 126 of second
pole piece 125. Pole piece 163 is integrally formed with yoke 131
such that a magnetic circuit is formed by pole piece 125, yoke 131
and pole piece 163, with this magnetic circuit being closed via the
gap formed between inner ends 126 and 164 of pole pieces 125 and
163, respectively. A magnetic flux, indicated by arrows 166, in
this magnetic circuit is generated by an excitation coil 167 to
which current is supplied by a power supply 169. A space formed in
the gap between pole pieces 125 and 163 is filled with an
insulating resin 170 which serves to form a layer of insulating
material between excitation coil 167 and pole pieces 125 and 163
and yoke 131. Thus, the excitation coil 167 is electrically
insulated from pole pieces 125 and 163 such that it may be operated
at ground potential.
[0135] In FIG. 2, it is also indicated that the third pole piece
163 has a radial inner annular portion 163IP where a surface of the
third pole piece facing the object 7 extends substantially parallel
to the object 7 disposed in the object plane at a first distance
from the object 7. In addition, the third pole piece 163 has a
radial outer annular portion 163OP where the surface of the third
pole piece 163 facing the object 7 extends substantially parallel
to the object plane 101 at a second distance from the object 7. The
second distance is greater than the first distance, that is the
outer annular portion 163OP is disposed further away from the
object 7 than the inner annular portion 163IP. Since the inner and
outer annular portions 163IP, 163OP may be disposed at a small
angle relative to the object 7, the first and second distances may
refer to average first and second distances. Inner and outer
annular portions 163IP, 163OP are joined by middle portion 163MP,
which is disposed at a greater angle relative to the object 7 than
both the inner and outer annular portions 163IP, 163OP of the third
pole piece 163. It can also be seen from FIG. 2 that a radial outer
end of the inner annular portion of the third pole piece is
disposed radially within the inner aperture of the shielding
electrode.
[0136] FIG. 2 further schematically indicates a supply line 171 of
cooling water to provide cooling for excitation coil 167. The line
171 is supplied with cooling water by a cooling water supply 172,
which is also set to ground potential. Thus, the cooling water
supply 172 and the power supply 169 may be conveniently operated at
ground potential as a result of electrical insulation being
provided between excitation coil 167 and pole pieces 163 and
125.
[0137] The power supply 169 is adjusted to supply an excitation
current such that the magnetic field generated in the gap between
inner ends 126 and 164 of pole pieces 125 and 163 compensates the
focusing magnetic field, generated in the gap between inner ends
124 and 126 of pole pieces 123 and 125, in the object plane 101 and
on the optical axis 120. By means of said compensating magnetic
field the focusing field may be advantageously compensated to zero,
which results in the electrons of the primary electron beamlets,
which are incident on the object 7, experiencing substantially no
magnetic field immediately above the object 7. This absence of
magnetic field in said region allows improving telecentricity as
well as errors resulting from an image rotation, which would be
induced by the focusing magnetic field.
[0138] FIG. 5 shows graphs of magnetic flux density or magnetic
field strength B and electrical field strength E along the optical
axis 120. Starting from the object plane 101, the magnetic field
strength B steeply rises to a maximum at a position 181 on the
optical axis 120 of the embodiment depicted in FIG. 2. Compared to
the steep rise of the magnetic field B starting at the object plane
101 to the position 181 of the maximum, the magnetic field B then
shows only a slow decrease with increasing further distance from
object plane 101. Such a moderate decrease of B at an increasing
distance from the object plane 101 may be achieved by a tapering
shape of a bore formed by radial inner end 124 (the inner portion)
of pole piece 123. In a first plane 183 disposed at a distance of
about 28.4 mm from the object plane 101, the bore has a minimum
diameter of about 20 mm. A front surface portion of pole piece 123,
which is closest to the object plane 101, is disposed at a distance
of about 20 mm in a second plane 184, and a diameter of the bore at
this portion is about 41 mm (front diameter). Thus, the diameter of
the bore formed by the radial inner end 124 of pole piece 123
radially increases with decreasing distance from the object plane
101 from a minimum value of about 22 mm to a maximum value of about
41 mm (front diameter) in plane 184.
[0139] This particular geometry of the radial inner end 124 (or
inner portion) of pole piece 123 allows to achieve the relatively
moderate decrease of focusing magnetic field strength B with
increasing distance from the object plane 101.
[0140] FIG. 5 also indicates a .gamma.-ray, which represents a ray
starting off at a distance from the optical axis 120 and parallel
to the optical axis 120 in the focus plane of the objective lens.
This ray .gamma. crosses the optical axis 120 at a position close
to position 181, which is the location of the maximum of the
focusing magnetic field strength B. This results in a low value of
the field curvature, for instance.
[0141] FIG. 5 further illustrates that ray .gamma. intersects the
object plane 101 at an angle with respect to the optical axis. This
indicates that a linear telecentric error may be present in the
optical system. However, the small linear telecentric error is not
only tolerated but purposely chosen such that a third order
telecentric error is reduced, as illustrated with reference to
FIGS. 6a and 6b below.
[0142] FIG. 6a illustrates a dependency of the third order
telecentric error which objective lens 102 would provide if no
first order telecentric error was present. An average angle of
incidence .theta. and thus the third order telecentric error
increases with increasing distance r from the central axis or
optical axis 120, respectively, according to r.sup.3. In FIG. 6a,
cones 191 indicate focused beams of primary electrons incident on
object plane 101 at locations 192. Directions 193 indicate average
directions of incidence of the primary electrons of these focused
beams at the respective locations 192. These average directions 193
are oriented under average angles of incidence .theta. with respect
to the optical axis. A maximum average angle .theta. at a maximum
distance of a primary electron beam 191 from the optical axis 120
may be as much as 40 mrad.
[0143] The field lens 305 shown in FIG. 1, for instance, may be
designed such that it introduces a linear telecentric error such
that the beam path entering objective lens arrangement 100 is not a
telecentric beam path. This results in a dependency of the third
order telecentric error as shown in FIG. 6b: starting from r=0, the
average angle of incidence .theta. will first pass through a
minimum of -10 mrad and then reach a maximum of +10 mrad at the
maximum value of r. Thus, compared to the situation shown in FIG.
6a, a maximum value of the third order telecentric error has been
successfully reduced.
[0144] FIG. 6b may be also interpreted as follows: in an inner ring
portion 195 where the negative maximum .theta. (r) is located, the
electron beams incident on the object plane 101 are diverging from
the optical axis (negative average angle of incidence, negative
maximum .theta..sub.i), and in an outer ring portion 196
surrounding inner ring portion 195, the primary electron beams
incident on the object plane 101 are converging with respect to the
optical axis (positive angle of incidence, maximum average angle of
incidence .theta..sub.0). This scenario may be suitably expressed
by the ratio
0.5 < .theta. i .theta. 0 < 2. ##EQU00004##
[0145] Line C.sub.S in FIG. 5 further indicates the dependency of
the field curvature, and line E' a dependency of the derivative of
the electrical field strength E along the optical axis 120.
[0146] It appears that, when starting off at a great distance from
the object plane 101, the field curvature C.sub.S gradually
increases except for a region, where E' is negative and where the
focusing magnetic field B increases. This reduction of the field
curvature C.sub.S in the region of negative E' and increasing B is
advantageous for reducing the value of C.sub.S at the object plane
101.
[0147] In FIG. 4, an exemplary embodiment of a shape of beam tube
152 and insulation 132 between the beam tube 152 and the first pole
piece 123 is depicted. Beam tube 152 is, for the largest part, a
straight pipe having a substantially constant wall thickness down
to a lower end. Lower end of beam tube 152 is constituted by a part
of beam tube 152, which is turned by about 180.degree. in a
direction of a radial outer side of beam tube 152. The lower end is
formed into a rounded rim, leaving a gap 152'' between a radially
outer side of the straight section of beam tube 152 and rounded rim
152'. Gap 152'' has a substantially rectangular shape and extends
parallel to the wall of the straight part of beam tube 152. A width
t.sub.4, i.e. a dimension in radial direction, of gap 152'' is
about 2 mm. A cross-section of rim 152' may be suitably described
by means of radii of circles fitted to an outer surface of rim
152', i.e. a surface facing away from the straight part of beam
tube 152. On an uppermost part 152'a of rim 152', i.e. a part
farthest away from the object plane 101, the surface profile of rim
152' may be described by a radius of circle C1, which radius is
about 1.2 mm, an adjacent part 152'b described by a radius of
circle C4, which is about 11 mm, further adjacent part 152'c by a
radius of circle C3, which radius is about 3 mm, a radially outer
lower end 152'd of rim 152' by a radius of circle C5, which radius
is about 6 mm, and a radially inner lower end 152'e of rim 152' by
a radius of circle C2, which radius is about 1.2 mm. The uppermost
part 152'a of rim 152' is spaced a distance t.sub.2 of about 5 mm
apart from a radial inner end 124 of the first pole piece 123. The
radially lower end 152'd of rim 152' is spaced a distance t.sub.3
of about 10 mm apart from electrode 144 disposed on the second pole
piece 125.
[0148] This shape of the beam tube 152, in particular the design of
rim 152' enables the realization of an advantageous shape of
electrical field. In particular, a slanted and/or tapered area of
inner end 124 of first pole piece 123 is efficiently separated from
the optical axis 120.
[0149] An insulating member 132 is disposed in a spacing formed in
between a part of radial inner end 124 that extends in parallel to
an outer side of beam tube 152 and the outer side of beam tube 152
and has a thickness or width t.sub.1 of about 4 mm in that area. In
an area where a diameter of radial inner end 124 of the first pole
piece 123 starts to increase (beginning of slanted or tapered
portion of the first pole piece 123, see also plane 183 in FIG. 2),
insulating member 132 is split into two portions 132' and 132'',
with portion 132' extending further along a radial outer side of
beam tube 152 until a lower end of gap 152'' and portion 132''
extending a short way along the slanted portion of the first pole
piece 123. This shape and arrangement of insulating member 132
allows efficient electrical insulation of the first pole piece 123
as well as rim 152' from the pipe-shaped portion of beam tube. The
region inside gap 152'' is void of any electrical fields thus being
advantageous for avoiding occurrences of creeping currents and
surface leakages. A portion of the slanted (or tapered) portion of
the first pole piece 123 is covered by electrode material 140.
[0150] FIG. 7 illustrates a further embodiment of the objective
lens arrangement according to the present invention. The numbering
of components of the objective lens arrangement of FIG. 2 is
adhered to. A shape of the first pole piece 123 is different from
the embodiment depicted in FIG. 2 in that is does not provide a
cavity and thus no fourth pole piece, and thus no additional
adjusting magnetic field is provided. Instead, alignment elements
(not depicted) are disposed in a space 149 between upper radial
inner end 124''' of the first pole piece 123 and insulation 132 of
beam tube 152. Insulation 132 of beam tube 152 comprises several
subsections 132', 132'' that have been described in detail with
reference to FIG. 4. Other than the lack of cavity 124'', an
arrangement of the first, second and third pole pieces as well as
an arrangement of excitation coils and power supplies is quite
similar to the one of the embodiment depicted in FIG. 2. A surface
of the second magnetic pole piece 125 facing away from the object
plane 101 is, in a radially inner area, covered by electrode
material 144, which is connected to electrode material 144'
disposed on a radially inner portion of a surface of the third pole
piece 163 facing towards the object plane 101. An insulation
between first pole piece 123 and second pole piece 125 is provided
by insulating resin 133, which insulating resin 133 extends
radially inwards up to a radially outer edge of electrode material
144. A space in which excitation coil 129 is disposed inside the
first pole piece 123 is separated from an inside of the objective
lens arrangement via insulating member 143, one end of which is
attached to the first pole piece 123 by a screw 145, wherein a
gasket 146 is provided in a gap adjacent to the screw in between
the one end of the insulating member 143 and first pole piece 123.
An additional gasket 146' is provided at the other end of
insulating member 143, which end of the insulating member 143 is
interposed between insulating resin 133 and the annular disc-shaped
portion of the first pole piece 123. Water-cooling system 173 is
disposed immediately adjacent to a side of excitation coil 129,
which faces away from the object plane 101, which water cooling
system 173 is attached via an electrically insulated screw 175 to
yoke 130 of the first pole piece 123. Water-cooling system 173 is
connected to a cooling water supply 174 disposed outside of
objective lens arrangement 102. The water-cooling system is thus
provided conveniently in an environment of about atmospheric
pressure.
[0151] Excitation coil 167 as well as a line of cooling water are
embedded in cast resin 170 in a spacing formed inside second and
third pole pieces 125, 163 and yoke 131 to provide electrical
insulation from the second and third pole pieces 125, 163 as well
as allowing cooling water supply 172 to be provided in an
environment of about atmospheric pressure. A gasket 147 is provided
adjacent to a radial inner end of cast resin 170, which is also
pressed against a surface of the second pole piece 125 facing
towards the object plane 101 and a surface of the third pole piece
163 facing away from the object plane 101, thus providing a
pressure seal.
[0152] Apart from allowing to have water-cooling arrangements 173,
174, 171, 172 in an environment of about atmospheric pressure, the
above-described insulating arrangements are advantageous in that
they dispose of the need to evacuate large spacings inside the
objective lens arrangement.
[0153] A ceramic/cast resin member 134 is provided between
shielding electrode 154 and a surface of the third pole piece 163
facing towards the object plane 101 in order to provide both
electrical insulation between the third pole piece 163 and the
shielding electrode 154 as well as to provide a pressure seal. A
radially inner end of cast resin/ceramic member 134 has a portion
of decreased thickness to accommodate a gasket 148 in between the
thin portion of resin/ceramic member 134 and the object-facing
surface of the third pole piece 163. Cast resin/ceramic member 134
and shielding electrode 154 are attached to connecting ring 180',
which connects the shielding electrode to a further ring 180,
disposed in alignment with shielding electrode 154. The further
ring 180 has a ring 139 of ceramic/resin material disposed thereon,
which, in turn, is connected to cast resin/ceramic member 134 and
yoke 130 via screw 179, connecting member 178, and connecting
member 177, which is attached to yoke 130 via screw 176.
[0154] In FIG. 8, a further embodiment of the objective lens
arrangement according to the present invention is illustrated. The
principal layout of the main components, such as pole pieces and
beam tube, corresponds largely to that of the previously described
embodiments. A main difference between the embodiments of FIGS. 2
and 7 on the one hand and FIG. 8 on the other hand is given by the
arrangement of the first pole piece relative to the beam tube.
Whilst in the previous two embodiments, the first pole piece 123
was electrically insulated from the beam tube 152 and at ground
potential, in the embodiment depicted in FIG. 8, an inner portion
of the first pole piece 1501 is electrically connected to beam tube
1152. In particular, the beam tube 1152 is attached to a radial
innermost portion of the first pole piece 1501. This configuration
has an advantage in that provision of voltage to the beam tube 1152
is facilitated as compared to the previously described embodiments.
The first pole piece is hence, in this embodiment, set to the same
potential as the beam tube. This has no detrimental effect on an
electrostatic field or magnetic field in the region of the beam
tube 1152. The first pole piece being set to a voltage necessitates
the division of the first pole piece into an inner portion 1501
which is connected to the beam tube 1152 and electrically insulated
from a second portion 1502 of the first pole piece by an insulating
layer 1503. In order to allow magnetic flux to pass from the inner
portion 1501 of the first pole piece to the outer portion 1502 of
the first pole piece, the inner portion 1501 comprises a
cylindrical portion 1501A which is arranged to face and be arranged
in parallel to a cylindrical portion 1502A of the outer portion
1502 of the first pole piece. Additionally, the inner portion 1501
of the first pole piece comprises a flat, annular section 1501B
joined in a radially outwards direction to the cylindrical portion
1501A and being arranged parallel and opposite to an annular
section 1502B of the outer portion of the second pole piece, such
as to enable a closed magnetic circuit. Insulating layer 1503
extends along a section of tapering inner portion of the first pole
piece 1501, and fills a gap formed between cylindrical portions
1501A, 1502B as well as annular portions 1501B, 1502B.
[0155] In FIG. 8, it is also indicated that the inner portion 1501
of the first pole piece also comprises a conus-shaped section
having a conus opening angle .alpha. with respect to the optical
axis 1120.
[0156] In addition, water-cooling lines 1173 disposed around
excitation coil 1129 are also illustrated in FIG. 8.
[0157] A further difference to the previously described embodiments
lies in the mounting of the second and third pole pieces, the
cooling of the excitation coil arranged in between the second and
third pole pieces, and the sealing of spaces inside the various
components.
[0158] Excitation coil 1167 is encased on three sides in ceramic
insulting material 1510, with both the excitation coil 1167 as well
as the ceramic insulating material 1510 being fixed in the space
between the second and third pole pieces by cast resin 1511. The
ceramic insulating material 1510 is connected to an outer ring of
thermally conductive material, which in turn, is connected to the
first pole piece via copper wiring. This arrangement is not
depicted in FIG. 8, for simplicity's sake, but described in detail
with reference to FIG. 10. FIG. 8 shows a part of the mounting
structure for holding the second and third pole pieces 1163, 1125.
The mounting structure comprises a holding bracket 1512 disposed on
a radially outer side of the second and third pole pieces 1163,
1125, which bracket 1512 spans an upper side of the second pole
piece 1125 and a lower side of the third pole piece 1163. In
between the bracket 1512 and the pole pieces 1163, 1125, a further
insulating layer 1513 is provided, which extends further along the
pole pieces 1163, 1125. The bracket 1512 is fixed to a mounting
ring 1514. Mounting ring 1514 is held in position by three metal
wires 1515 which are fixed to a lower portion of the first pole
piece via connecting member 1516. This mounting structure allows
adjusting a position of the pole pieces 1125, 1163 with respect to
the first pole piece and the object plane. In addition, this
mounting structure enables the second and third pole pieces 1125,
1163 to be disposed entirely in a vacuum environment thus
eliminating the need to evacuate a space inside the two pole pieces
1125, 1163 and eliminating a plurality of seals thus increasing an
ease of operation and installation.
[0159] In a further aspect, the shape chosen for the inner portion
1501 of the first pole piece allows to integrate a component
disposed upstream of the objective lens arrangement within a space
or bore formed by the inner portion 1501, thus decreasing an
overall space requirement of an inspection system and improving
optical properties of the system. In the embodiment depicted in
FIG. 8, a lower part of a beam path splitting arrangement 1400 is
depicted, with an outside thereof being shown in a schematic and
simplified manner as outline 1400'. In addition, a lower portion of
a magnetic field arrangement 1407 is shown. A step-shaped
protrusion 1501C is formed on an inside of the inner portion of the
first pole piece 1501, which inside faces away from the object
plane. A holding element 1575 having an upper side with an
insulating layer 1576 attached thereto is held by and fixed to
protrusion 1501C of the inner portion 1501 of the first pole piece.
The outside 1400' of the beam path splitting arrangement may be
advantageously formed such that its outline corresponds to that
formed by the holding elements 1575 and the insulating layer 1576
thereon. The lower portion of the beam path splitting arrangement
1400 may then be arranged such that it remains spaced apart from
the holding element 1575 or alternatively such that it rests on
holding element 1575 (or insulating layer 1576, respectively).
Thus, the lower portion of beam path splitting arrangement 1400 is
inserted into a space formed by the cylindrical portion 1501A and
annular portion 1501B of the first pole piece. Thus, a lower end of
the beam path splitting arrangement, and in particular a lower end
P1 of the magnetic field arrangement 1407, is disposed at a first
distance D.sub.1 from the object plane 1101 which first distance
D.sub.1 is smaller than a second distance D.sub.2 between an upper
side P2 of the excitation coil 1129 and the object plane 1101. A
conical shaped inner lower portion and a cylindrical upper portion
of the inner portion of the first pole piece thus accommodate a
portion of the beam splitting arrangement.
[0160] In FIG. 9, a schematic illustration of an exemplary beam
path splitting arrangement 400 and a simplified embodiment of
objective lens arrangement 100 is given. Primary electron beam path
3' comprising a plurality of primary electron beamlets enters a
first magnetic field portion 403 of beam path splitting arrangement
400. Field portion 403 provides a homogeneous magnetic field
deflecting the primary electron beam path by an angle .alpha. to
one side, in particular to the left in a direction of travel of the
electrons, as viewed in FIG. 9. The primary electron beam path 3'
subsequently passes a drift region 405 which is substantially free
of magnetic fields such that the primary electron beam path 3'
follows a straight line in drift region 405. Then the primary
electron beam path 3' enters a field region 407 in which a
homogeneous magnetic field is provided for deflecting the primary
electron beam path 13 at an angle .beta. to the right.
Subsequently, primary electron beam path 3' enters the objective
lens arrangement 100 which serves to focus the primary electron
beamlets onto the surface of object 7 positioned in object plane
101. The axis 120 of the objective lens arrangement 100 coincides
with optical axis z of the entire system.
[0161] The objective lens arrangement 100 comprises a magnetic lens
group having a magnetic focusing function and an electrostatic lens
group having an electrostatic focusing function on the primary
electron beamlets. Possible configurations of this electrostatic
lens group according to the present invention have been described
before, with reference to FIGS. 2 and 8, for instance. Further, the
electrostatic lens may be configured to exert a decelerating effect
on the primary electrons by an electrical field for decelerating
the primary electrons before impinging on object surface 7. The
electrostatic lens arrangement referred to in the context of the
description of this Figure may be chosen from any suitable
embodiments as described above.
[0162] A controller 420 is provided for changing the voltage
supplied to the electrostatic lens arrangement such that the
kinetic energy with which the primary electrons impinge onto the
object, the landing energy, may be adjusted, for instance in a
range of about 0.3 keV to 2.0 keV. The kinetic energy with which
the primary electrons pass the beam path splitting arrangement 400
is generally constant and independent of the landing energy of the
primary electrons on the object surface.
[0163] Further details of the depicted beam path splitting
arrangement may be found in WO 2005/024881 A2 (U.S. provisional
application Ser. No. 60/500,256) to the same Assignee. A person
skilled in the art will be familiar with the technology for
designing and constructing the beam splitter comprising plural
magnetic field regions as illustrated above. Reference may be made
to U.S. Pat. No. 6,040,576 or "SMART: A Planned
Ultrahigh-Resolution Spectromicroscope For BESSY II" by R. Fink et
al, Journal of Electron Spectroscopy and Related Phenomena 84,
1987, pages 231 to 250 or "A Beam Separator With Small Aberrations"
by H. Muller et al, Journal of Electron Microscopy 48(3), 1999,
pages 191 to 204.
[0164] The absolute values of the field strengths in field portions
403 and 407 are about equal, and lengths of field portions 403 and
407 are chosen such that a spatial dispersion induced by the
deflection by the angle .alpha. to the left and the subsequent
deflection by the angle .beta. to the right is substantially zero.
Further, the field portions 403 and 407 and the drift region 405
are chosen such that the deflections induced by the beam path
splitting arrangement 400 on the primary electron beam path 3' are
in first order substantially stigmatic and in first order
substantially distortion free. Thus, a pattern may be imaged in
high quality onto the surface of object 7. This imaging quality is
maintained substantially independent of the landing energy of the
primary electrons onto the object 7.
[0165] The secondary electron beam path 4' comprising a plurality
of secondary electron beamlets is separated from the primary
electron beam path 3' by field region 407 which deflects the
secondary electron beam path 4' by an angle .gamma. to the
right.
[0166] The secondary electrons emanating from the object 7 with a
kinetic energy range of about 0 eV to 100 eV, for instance, will be
accelerated by the electrical field generated by electrostatic lens
arrangement of the objective lens arrangement 100 to a kinetic
energy which is dependent on a setting provided by controller 420
for adjusting the landing energy of the primary electrons. Thus,
the kinetic energy of the secondary electrons entering field region
407 will change in dependence of the landing energy of the primary
electrons.
[0167] Deflection angle .gamma. for the secondary electron beam
path 4' provided by field region 407 will change, accordingly.
After leaving field region 407, the secondary electron beam path
passes a drift region 409 which is substantially free of magnetic
fields before entering a further magnetic field region 411
providing a homogeneous magnetic field deflecting the secondary
electron beam path 4' further to the right. Field strength of field
region 411 may be adjusted by a controller 413. After leaving the
field region 411, the secondary electron beam path immediately
enters a further field region 415 providing a homogeneous magnetic
field, a field strength of which may be also adjusted by controller
413. Controller 413 operates in dependence of a setting of the
landing energy of primary electron beams and adjusts the magnetic
field strength in field regions 411 and 415 such that the primary
electron beam path leaves field region 415 at a pre-defined
position and in a pre-defined direction which are independent of
the landing energy of the primary electrons and the deflection
angle .gamma., respectively. Thus, the two field regions 411, 415
perform a function of two subsequent beam deflectors which allows
to adjust the secondary electron beam to coincide with the
pre-defined secondary electron beam path 4' when the same leaves
magnetic field region 415.
[0168] The changes in the magnetic field strengths of field regions
411, 415 caused by controller 413 result in changes of a quadrupole
effect, which these electron optical elements 411, 415 have on the
secondary electrons. In order to compensate for such changes of the
quadrupole effect a further magnetic field region 419 is provided
immediately downstream of field region 415. In magnetic field
region 419 a homogeneous magnetic field is provided, a field
strength of which is controlled by controller 413. Further,
downstream of magnetic field region 419 a quadrupole lens 421 is
provided which is controlled by controller 413 to compensate, in
cooperation with magnetic field region 419, the remaining
quadrupole effect induced by field portions 411, 415 when
compensating the beam path for different landing energies of the
primary electrons.
[0169] The electron-optical components 407, 409, 411, 415, 419 and
421 provided in the secondary electron beam path are configured
such that, for one particular setting of the landing energy of the
primary electrons, the secondary electron beam path through the
beam path splitting arrangement 400 is in first order substantially
stigmatic, in first order distortion free, and in first order
dispersion corrected. For other settings of the landing energy than
2 kV this imaging quality may be maintained, a reduction of the
dispersion correction to a limited amount occurs, however.
[0170] It is to be noted that an intermediate image of object plane
101 is formed in a region of field portions 407, 411, 415 and 419.
A position of the intermediate image will change along the beam
axis in dependence of the setting of the landing energy of the
primary electrons and the kinetic energy of the secondary
electrons, accordingly.
[0171] In FIG. 10, an embodiment of a cooling arrangement based
entirely on cooling by means of solid materials suitable for use in
particular with the embodiment shown in FIG. 8 is schematically
illustrated. Like numerals refer to like components. Excitation
coil 1167 is, in this exemplary embodiment, surrounded on
practically all sides by a ceramic, electrically insulating layer
1510. Both the excitation coil 1167 and the insulating layer 1510
extend substantially continuously in a full circle around the
optical axis (with the exception of electrical connections of the
excitation coil penetrating through the ceramic insulation which
are connected to an external power supply, not shown). A further
layer 1511 of electrically insulating material, in this instance
cast resin, is provided on three sides of the arrangement of the
excitation coil 1167 and ceramic insulation 1510. The ceramic
insulation 1510 is connected via connecting members 1510A to an
outer ring comprising both a ring of ceramic material 1512 and a
ring comprising an outer ceramic sheath 1510B encasing an inner
core 1510C made of copper. Both rings are fixed to mounting ring
1514, which is fixedly attached to the second and third pole pieces
1125, 1163 and has been described with reference to FIG. 8. The
ceramic connecting member 1510A provides a thermally conductive
contact between the ceramic insulation 1510 around the excitation
coil 1167 and the copper core ring 1510C and the ceramic ring 1512
for removing heat generated by the excitation coil 1167. In
contrast to the core ring 1510C made of copper and the ceramic
insulation 1510 surrounding the excitation coil 1167, the
connecting member 1510A is not formed as a continuous ring, but is
formed of small ring sections disposed around a circumference of
the yoke integrally formed with and connecting the second and third
pole pieces 1125, 1163, which small sections penetrate through a
radial outer side of the second and third pole pieces 1125, 1163 of
said yoke. The copper and ceramic rings, 1510 A through C are
connected via a (non-depicted) copper wire to a cooling system
outside the evacuated inside of the objective lens arrangement. The
connection may be configured, for instance, in analogy to wire 1515
and connecting piece 1516 shown in FIG. 8 in connection with the
mounting structure and further extend through the first pole piece
to connect to a cooling system of the excitation coil accommodated
within the first pole piece. Thus, a cooling system is provided
which facilitates electrical insulation of the cooling system from
the excitation coil and also allows for a flexible mounting
structure for the second and third pole pieces.
[0172] In FIG. 11, an adjusting arrangement for adjusting a radial
position of the second and third pole pieces, which are held by
mounting ring 1514, as shown in FIG. 8, is illustrated in a
schematic and simplified manner. An adjustment screw 1594 is
accommodated in a bore 1594' of the mounting ring 1514. The lower
end of the bore 1594' and thus the lower end of the screw 1594 are
operably linked to a top of a chamber 1595 which contains two balls
1597 on top of one another, i.e. an upper and a lower ball, and a
wedge-shaped member 1596, with a pointed edge 1596' of the
wedge-shaped member 1596 being disposed in between the two balls
1597. This arrangement may further comprise a counter-bearing,
which is only indicated in terms of its effect as arrows 1598 in
FIG. 11. The top of chamber 1595 and the screw 1594 are further
connected such that turning of the screw 1594 does not only drive
the screw 1594 further into the mounting ring 1514 and into the
chamber 1595 but also lifts up the mounting ring 1514 together with
the bottom of the chamber such that, upon turning of the screw
1594, not only the upper ball is pushed downwards, but also the
lower ball pushed upwards. When the two balls are pushed further
together, they both exert a force onto the wedge-shaped member 1596
such that the wedge-shaped member 1596 is moved in a radial
direction. Since the wedge-shaped member is operably connected to
the second and third pole pieces, this radial movement is
translated into radial movement of the second and third pole
pieces. The same principle applies to turning the screw 1594 in the
other direction, with the balls 1597 moving further apart and the
wedge-shaped member 1596 moving further into the chamber, again
effecting radial movement of the pole pieces.
[0173] The embodiment schematically shown in FIG. 12 largely
corresponds to that shown in FIG. 8, the difference being that the
embodiment shown in FIG. 12 comprises a heating system. The
depicted heating system comprises a heating coil 1199 which is
provided inside the second excitation coil 1167. The heating coil
1199 comprises several windings of a wire, which is made from the
same material as the wire of the second excitation coil 1167 and is
disposed adjacent to the wire forming the secondary excitation coil
1167. The heating coil 1199 is connected to a power supply PS and
controlled by a control unit C1 which adjusts a current supplied by
the power supply PS to the heating coil 1199 in dependence of a
temperature of the second and third pole pieces 1163, 1125 and an
excitation current supplied to the second excitation coil 1167. The
temperature of the second and third pole pieces 1163, 1125 is
measured by temperature sensors T1 and T2, which supply the data of
the measured temperatures to a control unit C2. The excitation
current supplied to the second excitation coil 1167 is controlled
by control unit C3. Control units C2 and C3 supply the data of the
temperatures of the second and third pole pieces 1163, 1125 and of
the excitation current supplied to the second excitation coil 1167
to control unit C1 of the heating system, which calculates an
excitation current to be provided to the heating coil 1199 on the
basis of the supplied data. Control units C1, C2, C3 may also be
portions of a single control unit. Thus, the pole pieces and an
environment on the inside of the objective lens arrangement may be
kept at a constant temperature and maintain a constant
environment.
[0174] In FIG. 13, a detail of the embodiment shown in FIG. 8 is
shown to illustrate angles formed between inside surfaces of the
second and third pole pieces 1163, 1125. The second pole piece 1125
has a surface 1125S facing the third pole piece 1163 and the third
pole piece 1163 has a surface 1163S facing the second pole piece
1125. In a first annular portion about the optical axis 1120
denoted IPR1 in FIG. 13, the surfaces 1125S, 1163S of the second
and third pole pieces 1125, 1163 form an angle .beta..sub.1 between
them which is about 9.degree.. In a second annular portion about
the optical axis 1120 denoted IPR2 in FIG. 13, the surfaces 1125S,
1163S of the second and third pole pieces 1125, 1163 form an angle
.beta..sub.2 between them which is about 10.degree.. In a third
annular portion about the optical axis 1120 denoted IPR3 in FIG.
13, the surfaces 1125S, 1163S of the second and third pole pieces
1125, 1163 form an angle .beta..sub.3 between them which is about
15.degree.. Thus, in connection with the small angles of the second
and third pole pieces 1125, 1163 with respect to object 7, a
relatively wide and flat arrangement of the pole pieces and thus
the entire objective lens arrangement is realized.
[0175] While the invention has been described also with respect to
certain specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, the exemplary embodiments of
the invention set forth herein are intended to be illustrative and
not limiting in any way. Various changes may be made without
departing from the spirit and scope of the present invention as
defined in the following claims.
* * * * *