U.S. patent application number 15/900415 was filed with the patent office on 2019-08-22 for apparatus and method for inspecting a surface of a sample, using a multi-beam charged particle column.
The applicant listed for this patent is Applied Materials Israel, Ltd., Technische Universiteit Delft. Invention is credited to Pieter KRUIT, Ron NAFTALI.
Application Number | 20190259570 15/900415 |
Document ID | / |
Family ID | 66349609 |
Filed Date | 2019-08-22 |
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United States Patent
Application |
20190259570 |
Kind Code |
A1 |
KRUIT; Pieter ; et
al. |
August 22, 2019 |
Apparatus and method for inspecting a surface of a sample, using a
multi-beam charged particle column
Abstract
Apparatus and method for inspecting a surface of a sample. The
apparatus includes a multi-beam charged particle column comprising
a source for creating multiple primary beams directed towards the
sample, an objective lens for focusing the primary beams on the
sample, an electron-photon converter unit having an array of
electron to photon converter sections, each section is located next
to a primary beam within a distance equal to a pitch of the primary
beams at the electro-photon converter unit, a photon transport unit
for transporting light from the electron to photon converter
sections to a photo detector, and an electron collection unit for
guiding secondary electrons created in the sample, towards the
electron-photon converter unit. The electron collection unit is
arranged to project secondary electrons created in the sample by
one of said primary beams to at least one of the electron to photon
converter sections.
Inventors: |
KRUIT; Pieter; (DELFT,
NL) ; NAFTALI; Ron; (DELFT, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Technische Universiteit Delft
Applied Materials Israel, Ltd. |
Delft
Rehovot |
|
NL
IL |
|
|
Family ID: |
66349609 |
Appl. No.: |
15/900415 |
Filed: |
February 20, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 2237/152 20130101;
H01J 2237/1508 20130101; H01J 2237/25 20130101; H01J 2237/24592
20130101; H01J 37/224 20130101; H01J 37/226 20130101; H01J
2237/2445 20130101; H01J 37/1475 20130101; H01J 37/244 20130101;
H01J 37/28 20130101; H01J 37/147 20130101; H01J 2237/2448
20130101 |
International
Class: |
H01J 37/22 20060101
H01J037/22; H01J 37/147 20060101 H01J037/147 |
Claims
1. A multi-beam charged particle column for inspecting a surface of
a sample, which multi-beam charged particle column comprising: one
or more emitters which are arranged for creating multiple primary
charged particle beams directed along trajectories towards the
surface of the sample, an objective lens unit for focusing said
multiple primary charged particle beams on said sample, an
electron-photon converter unit comprising a plurality of electron
to photon converter sections, wherein at least one electron to
photon converter section of said plurality of electron to photon
converter sections is located next to a trajectory of a primary
charged particle beam and within a distance equal to a pitch of
trajectories of the primary charged particle beams at the
electron-photon converter unit, a photon transport unit for
transporting light from said electron to photon converter sections
to a light detector, and an electron collection unit comprising
multi aperture plates for guiding secondary electrons created in
the sample upon incidence of the primary charged particle beams,
towards the electron to photon converter sections of the
electron-photon converter unit, wherein the electron collection
unit is configured for projecting secondary electrons created in
the sample by one of said primary charged particle beams to at
least one of said electron to photon converter sections arranged at
one side with respect to the trajectory of said one of the primary
charged particle beams.
2. The multi-beam charged particle column according to claim 1,
wherein at least one electron to photon converter section of said
plurality of electron to photon converter sections is arranged
between the trajectories of two adjacent primary charged particle
beams of said multiple primary charged particle beams.
3. The multi-beam charged particle column according to claim 1,
wherein said photon transport unit comprises a plurality of optical
fibers.
4. The multi-beam charged particle column according to claim 3,
wherein at least one optical fiber of said plurality of optical
fibers has a first end, wherein the first end is arranged adjacent
to or attached to one of said electron to photon converter sections
for coupling light from said electron to photon converter section
into the optical fiber.
5. The multi-beam charged particle column according to claim 4,
wherein the at least one optical fiber of said plurality of optical
fibers has a second end, wherein the second end is configured to
project light from said optical fiber onto the photo detector.
6. The multi-beam charged particle column according to claim 3,
wherein at least one optical fiber of said plurality of optical
fibers has a first end, wherein the first end is arranged between
the trajectories of two adjacent primary charged particle beams of
said multiple primary charged particle beams.
7. The multi-beam charged particle column according to claim 3,
wherein at least one of the plurality of optical fibers is at least
partially coated with a photo-reflecting layer.
8. The multi-beam charged particle column according to claim 1,
wherein at least one of the plurality of optical fibers is tapered
towards the first end.
9. The multi-beam charged particle column according to claim 8,
wherein the at least one optical fiber is cut at said first end at
an angle between 10.degree. and 90.degree. with respect to an
central axis of said at least one optical fiber.
10. The multi-beam charged particle column according to claim 1,
wherein said plurality of electron to photon converter sections
comprises a plurality of strips of luminescent material, wherein at
least one strip of said plurality of strips is located next to the
trajectory of a primary charged particle beam and within a distance
equal to the pitch of the trajectories of the primary charged
particle beams at the electron-photon converter unit.
11. The multi-beam charged particle column according to claim 1,
wherein said plurality of electron to photon converter sections
comprises a plate or a layer of luminescent material.
12. The multi-beam charged particle column according to claim 11,
wherein the plate or the layer of luminescent material is provided
with passage openings for the primary charged particle beams.
13. The multi-beam charged particle column according to claim 1,
wherein said plurality of electron to photon converter sections are
at least partially coated with a photo-reflecting layer.
14. The multi-beam charged particle column according to claim 1,
wherein the multi-beam charged particle column comprises an optical
axis, wherein the trajectories of the multiple primary charged
particle beams are arranged in multiple rows, wherein each row
extends in a first direction substantially perpendicular to the
optical axis, wherein the rows are arranged next to each other in a
second direction substantially perpendicular to said first
direction and said optical axis.
15. The multi-beam charged particle column according to claim 14,
wherein said photon transport unit comprises a plurality of optical
fibers, wherein at least one of the plurality of optical fibers is
at least partially arranged in between two adjacent rows.
16. The multi-beam charged particle column according to claim 1,
wherein the one or more emitters comprises a single thermal field
emission source for emitting a diverging charged particle beam
towards a beam splitter, wherein the beam splitter comprises a
plate with multiple apertures which are arranged for creating
multiple primary charged particle beams, one primary charged
particle beam per aperture.
17. The multi-beam charged particle column according to claim 1,
wherein the electron collection unit comprises a Wien deflector
unit for providing a magnetic field to disentangle the primary
charged particle beams from the secondary electron beams coming
from the surface of the sample upon incidence of the primary
charged particle beams.
18. Method for inspecting a surface of a sample using a multi-beam
charged particle column, wherein said method comprising the steps
of: operating one or more emitters for creating multiple primary
charged particle beams directed along trajectories towards the
surface of the sample, focusing said multiple primary charged
particle beams on said sample, guiding secondary electrons created
in the sample upon incidence of the primary charged particle beams
towards an electron-photon converter unit using an electron
collection unit, converting at least part of the secondary
electrons into photons using the electron-photon converter unit,
wherein the electron-photon converter unit comprising a plurality
of electron to photon converter sections, wherein at least one
electron to photon converter section of said plurality of electron
to photon converter sections is located next to a trajectory of a
primary charged particle beam and within a distance equal to a
pitch of trajectories of the primary charged particle beams at the
electron-photon converter unit, wherein the electron collection
unit is configured for projecting secondary electrons created in
the sample by one of said primary charged particle beams to at
least one of said electron to photon converter sections arranged at
one side with respect to the trajectory of said one of the primary
charged particle beams, and transporting light from said electron
to photon converter sections to a photo detector.
19. The method according to claim 18, wherein said multi-beam
charged particle column comprises at least one optical fiber
wherein said at least one optical fiber has a first end, wherein
the first end is arranged adjacent to or attached to one of said
electron to photon converter sections, wherein the method comprises
the step of: coupling light from said electron to photon converter
section into the optical fiber.
20. The method according to claim 19, wherein the at least one
optical fiber has a second end, wherein the method comprises the
step of: projecting light from the second end of said at least one
optical fiber onto the photo detector.
Description
[0001] Embodiments of the invention relate to an apparatus and
method for inspecting a surface of a sample, using a multi-beam
charged particle column.
BACKGROUND
[0002] One of the routine steps in the production process of
integrated circuits is the inspection of patterned surfaces,
especially when starting up a new design. A substantial part of the
whole 300 mm wafer is imaged to check for defects in the pattern
and for particles imbedded in the pattern or on top of the wafer.
This kind of inspection is presently performed by high-throughput
optical microscopy in dedicated instruments.
[0003] With the progress in lithography, it is desirable that
instruments detect smaller and smaller defects and particles. A
problem is that the light scattering from particles rapidly
decreases when the particle's size decreases, so the
signal-to-background (and noise) ratio is decreasing.
[0004] In order to solve this problem, electron beam inspection
machines have been used and for some purposes are still in use.
Electron beam inspection machines can have a much higher resolution
than optical system. However, electron beam inspection machines are
limited in the speed at which the electron beam inspection machines
can inspect a wafer. In order to increase the speed, multi-beam
electron beam systems have been proposed.
[0005] US 2007/0272856, described a method and an apparatus for
inspecting a specimen surface. The method comprises the steps of
generating a plurality of primary beams directed towards the
specimen surface, focusing the plurality of primary beams onto
respective loci on the specimen surface, collecting a plurality of
secondary beams of charged particles originating from the specimen
surface upon incidence of the primary beams, converting at least
one of the collected secondary beams into an optical beam, and
detecting the optical beam. The apparatus described in this Patent
Publication comprises a screen with fluorescent material, which
screen is disposed between an emitter for generating a plurality of
primary beams and the specimen surface, the primary beams are
focused at the level of the screen and which screen is constructed
such that the primary beams can travers through holes in the
screen. The secondary beams from the specimen surface are directed
from the specimen surface towards the screen and are defocused on
the screen to provide a spot which covers an area around the holes
in the screen. At these spots, the secondary beams are converted
into optical beams, which optical beams are then imaged through
free space onto a plane in which the optical detector is
located.
SUMMARY
[0006] A disadvantage of the system described in US 2007/0272856
is, that the spots of secondary beams on the screen are relatively
large and spots of adjacent secondary beams are partially
overlapping. In addition, the relatively large spots are imaged
onto the optical detector, which results in relatively large spots
on the detector. Accordingly, the pitch of the primary beams on the
specimen surface is relatively large.
[0007] A further disadvantage of this system is, that the optical
beams are imaged through free space onto the optical detector via
an optical lens. Typically the optical lens is arranged next to the
array of primary beams so that said optical lens does not obstruct
the trajectory of the primary beams. The optical lens is therefore
arranged remote from the screen of fluorescent material where the
secondary electrons are converted into light, and the numerical
aperture and thus the light-gathering ability of the lens is
relatively small, which makes the detection of the secondary
electrons inefficient.
[0008] It is an object to least partially solve at least one of the
above identified disadvantages and/or to provide an alternative
inspection apparatus, which allows for high throughput inspection
of samples, in particular semiconductor wafers.
[0009] According to a first aspect, a multi-beam charged particle
column for inspecting a surface of a sample is provided, which
multi-beam charged particle column comprising:
[0010] one or more emitters which are arranged for creating
multiple primary charged particle beams directed along trajectories
towards the surface of the sample,
[0011] an objective lens unit for focusing said multiple primary
charged particle beams on said sample,
[0012] an electron-photon converter unit comprising a plurality of
electron to photon converter sections, wherein at least one
electron to photon converter section of said plurality of electron
to photon converter sections is located next to a trajectory of a
primary charged particle beam and within a distance equal to a
pitch of trajectories of the primary charged particle beams at the
electron-photon converter unit,
[0013] a photon transport unit for transporting light from said
electron to photon converter sections to a light detector, and
[0014] an electron collection unit comprising multi aperture plates
for guiding secondary electrons created in the sample upon
incidence of the primary charged particle beams, towards the
electron to photon converter sections of the electron-photon
converter unit, wherein the electron collection unit is configured
for projecting secondary electrons created in the sample by one of
said primary charged particle beams to at least one of said
electron to photon converter sections arranged at one side with
respect to the trajectory of said one of the primary charged
particle beams.
[0015] According to embodiments of the present invention, the
secondary electrons created in the sample by one of the primary
charged particle beams are directed to a position at one side with
respect to said one of the primary charged particle beam. By
configuring the electron collection unit to project the secondary
electrons to a position at one side with respect to the primary
charged particle beam which produced the secondary electrons, an
overlap of the secondary electrons created by multiple primary
charged particle beams can at least substantially be prevented, in
particular when the electron collection unit is configured for
projecting secondary electrons all at a same side with respect to
the corresponding primary charged particle beams that created the
secondary electrons in the sample. In a preferred embodiment, the
secondary electrons created in the sample by one of the primary
charged particle beams are directed onto the electron to photon
converter section arranged adjacent to or neighboring the
trajectory of said one of said primary charged particle beams.
[0016] It is noted that in the apparatus described in US
2007/0272856, the secondary beams from the sample are directed
towards a screen and are defocused on the screen to provide a spot
which covers an area around the holes in the screen. Contrarily to
the arrangement in the prior art, the electron collection unit
according to embodiments of the present invention, is arranged for
projecting the secondary electrons to one side next to the primary
charged particle beam. In the apparatus of embodiments of the
present invention, an overlap of the spots of secondary electrons
from adjacent primary charged particle beams can be prevented,
which makes it easier to detect and distinguish the secondary
electrons coming from adjacent primary charged particle beams.
Accordingly, the detection and evaluation of the secondary
electrons from the surface of the sample can be faster, and thus
the surface of the sample can be inspected more quickly, which
increases the throughput for the inspection of samples.
[0017] It is further noted that in the apparatus described in WO
2016/036246, the multi-sensor detector system is arranged at a
position which is spaced apart from the array primary electron
beams, in a direction perpendicular to an optical axis of the
primary electron beams. According to embodiments of the present
invention, the electron-photon converter unit comprises an array of
electron to photon converter sections, wherein each electron to
photon converter section is located next to a primary charged
particle beam and within a distance equal to a pitch of the primary
charged particle beams at the electro-photon converter unit. In the
apparatus of embodiments of the present invention, the electron to
photon converter sections are arranged close to the primary charged
particle beams. Preferably, at least one electron to photon
converter section of said array of electron to photon converter
sections is arranged between two adjacent primary charged particle
beams. By arranged the each electron to photon converter section
close to a primary beam or even between two adjacent primary beams,
the width of the multi-beam charged particle column can be reduces,
which makes it easier to arranged multiple multi-beam charged
particle column close to each other and to arrange more multi-beam
charged particle columns within a certain area above the sample.
Accordingly, the surface of the sample can be inspected more
quickly, which increase the throughput for the inspection of
samples.
[0018] It is possible to use a free space optical imaging for
transporting light from said electron to photon converter section
to a light detector, as for example described in US 2007/0272856,
or to arrange the light detector directly above or on top of the
electron to photon converter section. When the light detector is
arranged directly above or on top of the electron to photon
converter section, the photon transport unit does not need to have
extra or separate component and is essentially established by the
configuration of the array of electron to photon converter sections
and the photo detectors.
[0019] In an embodiment, the photon transport unit comprises a
plurality of optical fibers. In an embodiment, at least one optical
fiber of said plurality of optical fibers has a first end, wherein
the first end is arranged adjacent or attached to one of said
electron to photon converter sections for coupling light from said
electron to photon converter section into the optical fiber. In an
embodiment, the at least one optical fiber of said plurality of
optical fibers has a second end, wherein the second end is
configured to project light from said optical fiber onto the photon
detector or light detector. In an embodiment, at least the first
end of said at least one optical fiber of said array of optical
fibers is arranged between the trajectories of two adjacent primary
charged particle beams of said multiple primary charged particle
beams.
[0020] In an embodiment, at least one of the plurality of optical
fibers is at least partially coated with a photo-reflecting layer.
When using optical fibers which are coated with a photo-reflecting
layer, the acceptance cone of the optical fibers is not limited by
the conditions of total internal reflection, and the acceptance
cone can be much larger. The acceptance cone represents all angles
with respect to the longitudinal axis of the optical fiber at which
photons may enter the optical fiber such that the photons will
convey along the optical fiber. When using optical fibers with a
photo-reflecting coating, the maximum angle at which light may
enter the optical fiber so that the light will propagate along the
optical fiber is much larger. An example of such a photo-reflecting
layer is a mirror coating, for example using an aluminum coating
which may be enhanced by dielectric coatings.
[0021] In an embodiment, at least one of the plurality of optical
fibers is tapered towards the first end. In an embodiment, the at
least one optical fiber is cut at said first end at an angle
between 10.degree. and 90.degree. with respect to an central axis
of said at least one optical fiber. When using a tapered optical
fiber, the optical fiber can be more easily inserted in between and
arranged next to the trajectories of the primary charged particle
beams of the multi-beam charged particle column.
[0022] In an embodiment, at least one electron to photon converter
section of said plurality of electron to photon converter sections
is arranged between the trajectories of two adjacent primary
charged particle beams of said multiple primary charged particle
beams. Accordingly this embodiment advantageously utilizes the area
between the trajectories of two adjacent primary charged particle
beams to arrange the electron to photon converter section inside
the plurality of primary charged particle beams. This provides a
very compact multi-beam charged particle column and/or allows to
arrange at least two multi-beam charged particle columns close to
each other for inspecting adjacent parts of the surface of a
sample.
[0023] In an embodiment, said plurality of electron to photon
converter sections comprises a plurality of strips of luminescent
material, wherein at least one strip of said plurality of strips is
located next to the trajectory of a primary charged particle beam
and within a distance equal to the pitch of the trajectories of the
primary charged particle beams at the electron-photon converter
unit. Preferably said plurality of strips are arranged in a plane,
wherein said plane is arranged such that the trajectories of the
primary charged particle beams traverse said plane in a direction
substantially perpendicular to said plane.
[0024] In an embodiment, the plurality of electron to photon
converter sections comprises or are a part of a plate or a layer of
luminescent material. In an embodiment, the plate or layer of
luminescent material is provided with passage openings for the
primary charged particle beams. Preferably said plate or layer of
luminescent material is arranged in a plane, wherein the plane is
arranged such that the trajectories of the primary charged particle
beams traverse said plane in a direction substantially
perpendicular to said plane.
[0025] In an embodiment, the layer of luminescent material is
arranged on top of an optically transparent carrier, such as a
glass carrier. In an embodiment, the layer of luminescent material
is preferably arranged at a side of said optically transparent
carrier which is facing the electron collection unit. The electron
to photon converter sections comprises any converting material
having the property that converts an incident electrons into one or
more photons. Examples are scintillating materials, for instance
crystal scintillators such as YAG, YAP, NaI, etc . . . , or plastic
scintillators, or a fluorescent or phosphorescent materials. Within
the meaning of this application such materials are also called
luminescent materials.
[0026] In an embodiment, said electron to photon converter
sections, in particular said strips of luminescent material, are
coated with a photo-reflecting layer. Preferably the
photo-reflecting layer is arranged at a side of said electron to
photon converter sections which is facing away from the photon
transport unit. Preferably the photo-reflecting layer is at least
partially transparent for secondary electrons. This allows at least
part of the secondary electrons to pass through the
photo-reflecting layer. The secondary electrons that pass through
the photo-reflecting layer are converted into photons by the
converting material of the electron to photon converter sections.
Only a part of the generated photons will be traveling in a
direction towards the photon transport unit, which part may be
transported by said photon transport unit to the photo detector.
The part of the generated photons that is traveling opposite to the
direction towards the photon transport unit, is reflected by the
photo-reflecting layer and is substantially re-directed toward the
photon transport unit after said reflection. Accordingly, the part
of the generated photons that can be collected by the photon
transport unit for transporting towards the photo detector can be
increased.
[0027] In an embodiment, the multi-beam charged particle column
comprises an optical axis, wherein the trajectories of the multiple
primary charged particle beams are arranged in multiple rows,
wherein each row extends in a first direction substantially
perpendicular to the optical axis, wherein the rows are arranged
next to each other in a second direction substantially
perpendicular to said first direction and said optical axis.
Accordingly the multi-beam charged particle column according to
this embodiment comprises an array of charged particle beam
trajectories which are arranged in multiple rows, which rows are
arranged next to each other in a direction substantially
perpendicular to the direction of the rows. Preferably the pitch
between adjacent trajectories of the primary charged particle beams
is substantially constant. Preferably the pitch between the rows of
trajectories of the primary charged particle beams is substantially
equal to the pits between adjacent trajectories of the primary
charged particle beams in the rows.
[0028] In an embodiment, at least one of the plurality optical
fibers of said photon transport unit is arranged at least partially
in between two adjacent rows. Accordingly the space between the
rows of the multiple primary charged particle beams is used to
accommodate at least part of the optical fibers, in particular to
arrange the first end of said optical fibers adjacent or attached
to one of said electron to photon converter sections, in particular
adjacent or attached to one of said strips of luminescent
material.
[0029] In an embodiment, the emitter comprises a single thermal
field emission source, preferably of the Schottky type, for
emitting a diverging electron beam towards a beam splitter, wherein
the beam splitter comprises a plate with multiple apertures which
are arranged for creating multiple primary beams, one primary beam
per aperture. In this embodiment, the multiple primary charged
particle beams are primary electron beams.
[0030] In an embodiment, the multi-beam charged particle column
comprises a collimator lens for substantially collimating the
primary charged particle beams from the emitter. In an embodiment,
the electron-photon converter is preferably arranged between the
collimator lens and the sample holder, more preferably arranged
between the collimator and the objective lens unit.
[0031] In an embodiment, the electron collection unit comprises a
Wien deflector unit for providing a magnetic field to disentangle
the primary charged particle beams from the secondary electron
beams coming from the surface of the sample upon incidence of the
primary charged particle beams. In an embodiment, such a Wien
deflector unit in use comprise perpendicular electric and magnetic
fields which are configured so that a deflection of the primary
charged particle beams by the electric field is substantially equal
to but in an opposite direction of a deflection of the primary
charged particle beams by the magnetic field. In addition the
electric and magnetic fields are configured so that the deflection
of the secondary electrons by the electric field is substantially
in the same direction as the deflection of the secondary electrons
by the magnetic field.
[0032] According to a second aspect, a method for inspecting a
surface of a sample using a multi-beam charged particle column is
provided, wherein said method comprises the steps of:
[0033] operating one or more emitters for creating multiple primary
charged particle beams directed along trajectories towards the
surface of the sample,
[0034] collimating the primary charged particle beams,
[0035] focusing said multiple primary charged particle beams on
said sample,
[0036] guiding secondary electrons created in the sample upon
incidence of the primary charged particle beams towards an
electron-photon converter unit by means of an electron collection
unit,
[0037] converting at least part of the secondary electrons into
photons by means of the electron-photon converter unit, wherein the
electron-photon converter unit comprising a plurality of electron
to photon converter sections, wherein at least one electron to
photon converter section of said plurality of electron to photon
converter sections is located next to a trajectory of a primary
charged particle beam and within a distance equal to a pitch of
trajectories of the primary charged particle beams at the
electron-photon converter unit, wherein the electron collection
unit is configured for projecting secondary electrons created in
the sample by one of said primary charged particle beams to at
least one of said electron to photon converter sections arranged at
one side with respect to the trajectory of said one of the primary
charged particle beams, and
[0038] transporting light from said electron to photon converter
sections to a photo detector.
[0039] In an embodiment, the method is performed by means of a
multi-beam charged particle column or an embodiment thereof, as
described above.
[0040] In an embodiment, the multi-beam charged particle column
comprises at least one optical fiber wherein said at least one
optical fiber has a first end, wherein the first end is arranged
adjacent to or attached to one of said electron to photon converter
sections, wherein the method comprises the step of:
[0041] coupling light from said electron to photon converter
section into the optical fiber.
[0042] In an embodiment, the at least one optical fiber has a
second end, wherein the method comprises the step of:
[0043] projecting light from the second end of said at least one
optical fiber onto the photo detector.
[0044] The various aspects and features described and shown in the
specification can be applied, individually, wherever possible.
These individual aspects, in particular the aspects and features
described in the attached dependent claims, can be made subject of
divisional patent applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] Embodiments of the invention will be elucidated on the basis
of an exemplary embodiment shown in the attached drawings, in
which:
[0046] FIG. 1 schematically shows an example of a multi-beam
charged particle column;
[0047] FIG. 2 schematically shows a first detailed view in the XZ
plane of a part of the multi-beam charged particle column according
to embodiments of the invention;
[0048] FIG. 3 schematically shows a first detailed view in the YZ
plane of a part of the multi-beam charged particle column of FIG.
2;
[0049] FIG. 4 schematically shows a first detailed view in the XY
plane of a part of the multi-beam charged particle column of FIGS.
2 and 3;
[0050] FIG. 5 schematically shows detailed view of a part of a
second example of a photon transport unit, and
[0051] FIG. 6 schematically shows a detailed view in the XZ plane
of a part of the multi-beam charged particle column according to a
second example of a multi-beam charged particle column according to
embodiments of the invention.
DETAILED DESCRIPTION
[0052] FIG. 1 shows a schematic representation of a multi-beam
charged particle column 1 comprising an emitter 2, which is
arranged substantially on an optical axis OA, for generating a
diverging charged particle beam 3 which extends along said optical
axis OA. Preferably, said emitter 2 comprises a Schottky
source.
[0053] Downstream from said emitter 2, a lens array 4 is provided,
which lens array 4 is provided with an aperture array for splitting
the diverging charged particle beam 3 in multiple primary charged
particle beams 5; each aperture of said aperture array provides one
primary charged particle beam 5. In addition the lenses of the lens
array 4 focusses each individual primary charged particle beam 5 at
or near a collimator lens 6, which is arranged at a side of the
lens array 4 facing away from the emitter 2.
[0054] Accordingly, the emitter 2 and the lens array 4 constitutes
an arrangement for creating multiple primary charged particle beams
5, which multiple primary charged particle beams 5 are directed
towards the surface of a sample 11.
[0055] The collimator lens 6 is arranged for substantially
collimating the primary charged particle beams 5 from the emitter
2, in particular to direct each primary charged particle beam 5
substantially parallel to the optical axis OA. In embodiment, the
collimator lens 6 comprises a deflector array which is arranged for
deflecting the individual primary charged particle beams 5, in
order to produce an array of primary charged particle beams 7 which
are arranged substantially parallel to the optical axis OA as
schematically shown in FIG. 1. Although a collimator is not
strictly necessary, it makes the positioning of the components at
the side of the collimator 6 facing away from the emitter 2, such
as the objective lens unit, much less critical, at least in a
direction along the optical axis OA.
[0056] Subsequently, a detector array 8 is arranged at the optical
axis OA, which detector array 8 will be described in more detail
with reference to FIGS. 2, 3 and 4 below. As schematically shown in
FIG. 1, the detector array 8 is arranged to allow the primary
charged particle beams 7 to travel through the detector array 8
towards an objective lens unit 10.
[0057] The objective lens unit 10 is arranged for focusing each one
of said multiple primary charged particle beams 7 on said sample
11.
[0058] In between the objective lens unit 10 and the detector array
8, a Wien filter 9, e.g. a Wien deflector array, is arranged. In
use, the Wien deflector array provides at least a magnetic field to
disentangle the primary charged particle beams 7 and secondary
electron beams coming from the surface of the sample 11 upon
incidence of the primary charged particle beams 7, as will be
explained in more detail below.
[0059] FIGS. 2, 3 and 4 show different views of a part of an
example of a multi-beam charged particle column according to
embodiments of the invention. The part shown in FIGS. 2, 3 and 4
corresponds to the particular the part of the multi-beam charged
particle column 1 of FIG. 1, below the collimator lens 6. The
example shown in FIGS. 2, 3 and 4, comprises the same upper part of
the charged particle column of FIG. 1, in particular the part above
the detector array 8 for producing an array of primary charged
particle beams 7 which are arranged substantially parallel to the
optical axis OA.
[0060] According to the example shown in the FIGS. 2, 3 and 4, the
multi-beam charged particle column 1' comprises an electron-photon
converter unit 81 comprising an array electron to photon converter
sections, in particular an array of fluorescent strips 82. Each
fluorescent strip 82 is located in the plane of the electron-photon
converter unit 81, next to a primary beam 7 and within a distance d
equal to a pitch of the primary beams 7 at the electron-photon
converter unit 81. As shown in the FIGS. 2 and 3 in particular, the
multiple primary charged particle beams 7 traverses the plane of
the electron-photon converter unit 81 which plane extends in a XY
direction, substantially perpendicular to the optical axis OA.
[0061] It is noted that at least one strip of said array of
fluorescent strips 82 is arranged between two adjacent primary
charged particle beams 7 of said multiple primary charged particle
beams.
[0062] It is further noted that in the example shown in FIGS. 2, 3
and 4, the electron-photon converter unit 81 comprises a series of
parallel arranged fluorescent strips 82 which extend substantially
in the Y-direction. However, the electron-photon converter unit may
alternatively also a plate of a fluorescent material with through
holes or openings 83 for the primary charged particle beams 7,
which plate of fluorescent material extends in the XY direction.
The parts of such a plate which extend in the X or Y direction in
between the through holes for the primary charged particle beams 7,
are also considered to be fluorescent strips in accordance with
embodiments of the present invention.
[0063] As schematically indicated in FIG. 2, the primary charged
particle beams 7 travel through the plane of the electron-photon
converter unit 81, via the openings between the fluorescent strips
82, towards the Wien filter 9. The Wien filter comprises a
combination of a magnetic deflector 91 and an electrostatic
deflector 92. In use, the electrostatic deflector 92 is arranged to
at least substantially counteract the deflection of the magnetic
deflector 91 for the primary charged particle beams 7. Accordingly,
the primary charged particle beams 7' which have traversed the Wien
filter, are shifted to a small extend in the X-direction, but are
still arranged substantially parallel to the optical axis OA, and
thus substantially parallel to the primary charged particle beams 7
above the Wien filter 9.
[0064] Subsequently, the primary charged particle beams 7' are
focused onto a sample 11 via an objective lens unit 10.
[0065] The objective lens unit 10 comprises an electron collection
unit comprising multi aperture plates for, in use, guiding
secondary electrons 12 created in the sample 11 upon incidence of
the primary charged particle beams 7', towards the Wien filter 9.
For the secondary electrons 12, which travel in opposite direction
with respect to the primary charged particle beams 7, 7', the
electrostatic deflector 92 does not counteract the deflection of
the magnetic deflector 91, but now the deflections of the secondary
electrons 12 by the electrostatic deflector 92 and the magnetic
deflector 91 add up. Accordingly, the secondary electrons 12' which
have passed the Wien filter are no longer traveling substantially
parallel to the optical axis OA, but are deflected to travel at an
angle with respect to the optical axis OA in order to project the
secondary electrons 12' onto the fluorescent strips 82 of the
electron-photon converter unit 81, as schematically shown in FIG.
2.
[0066] At the fluorescent strips 82 of the electron-photon
converter unit 81, photons are created upon incidence of the
secondary electrons 12'. At least a part of said photons are
transported from the fluorescent strip 82 to a photo detector 13
via a photon transport unit. In the example as shown in FIGS. 2 and
3 and according to embodiments of the present invention, said
photon transport unit comprises an array of optical fibers 14. Each
optical fiber 14 comprises a first end 15 which is arranged
adjacent or attached to one of said fluorescent strips 82 for
coupling light (photons) from said fluorescent strip 82 into the
optical fiber 14, and a second end 16 which is arranged to project
light from said optical fiber 14 onto the photo detector 13.
[0067] As schematically shown in FIG. 2, the first end 15 of the
optical fibers 14 of said array of optical fibers is arranged
between two adjacent primary charged particle beams 7 of said
multiple primary charged particle beams.
[0068] FIG. 4 shows a schematic top view at the plane of the
electron-photon converter unit 81, in particular in the XY plane
which extends substantially perpendicular to the optical axis OA,
as indicated in FIG. 2 by the reference IV-IV. As shown in FIG. 4,
the multiple primary charged particle beams 7 are arranged in
multiple rows 71, 72, wherein each row 71, 72 extends in a first
direction, in this example in the Y-direction, substantially
perpendicular to the optical axis OA as schematically shown in FIG.
3. The rows 71, 72 of primary charged particle beams 7 are arranged
next to each other in a second direction, in this example in the
X-direction, substantially perpendicular to said first direction
and said optical axis OA. The fluorescent strips 82 of the
electron-photon converter unit 81, are arranged next to a row 71,
72 of primary charged particle beams 7 and within a distance equal
to a pitch d of the rows 71, 72 of the primary charged particle
beams 7 at the electron-photon converter unit 81. As schematically
indicated in FIG. 4, the openings 83 between the fluorescent strips
82 are arranged to allow passage of the primary charged particle
beams 7 through the plane of the electron-photon converter unit
81.
[0069] In use, secondary electrons 12' created in the sample 11
upon incidence of the primary charged particle beams 7, are
deflected by the Wien filter 9 in the X-direction and are projected
onto the fluorescent strips 82 of the electron-photon converter
unit 81, as schematically shown in FIG. 4. The secondary electrons
12' incident on the fluorescent strips 82 at a side facing the Wien
filter 9, are converted by fluorescent material of the fluorescent
strips 82 into photons (light). At a side of the fluorescent strips
82 facing away from the Wien filter, in particular at or near the
position where the secondary electrons 12' are deflected to,
optical fibers 14 are arranged to collect at least part of the
created photons and to guide the collected photons a photo detector
13, as schematically shown in FIG. 3. All the optical fibers 14
which are arranged to collect the photons from the various spots of
secondary electrons 12' on a specific fluorescent strip 82, are
arranged above said fluorescent strip 82, in particular in a ZY
plane, at least the parts of the optical fibers 14 which is
arranged next to or in between the rows 71, 72 of primary charged
particle beam 7. As schematically shown in FIG. 3, the optical
fibers 14 are bent or curved in the YZ plane in order to arranged
the second end 16 of the fibers at least out of the area of the
primary charged particle beams 7 to project light from said optical
fiber 14 onto the photo detector 13. The bended or curved optical
fibers 14 are substantially confined to the area above the
fluorescent strips 82 in order to circumvent that the optical
fibers 14 get in the way of the primary charged particle beams 7.
The assembly of optical fibers 14 constitute a photon transport
unit according to embodiments of the present invention.
[0070] The photons created by the conversion of the secondary
electrons 12' in the fluorescent strips 82 may also be emitted in a
direction away from the first end 15 of the optical fibers 14. In
order to redirect these photons back towards the first end 15 of
the optical fibers 14, the fluorescent strips 82 may be coated with
a photo-reflecting layer 21 at a side of said fluorescent strips 82
facing away from the first end 15 of the optical fibers 14, as
schematically indicated in FIG. 3. Preferably, the photo-reflecting
layer 21 is substantially transparent for the secondary electrons
12', so that at least a substantial amount of the secondary
electrons 12' reach the fluorescent material of the fluorescent
strips 82 and is converted into photons.
[0071] An alternative of the curved or bent optical fibers 14, is
shown in FIG. 5. In this alternative example, the optical fibers
14' are tapered towards the first end 15'. The first end 15' of the
optical fibers 14' are cut at an angle .alpha. between 10.degree.
and 90.degree. with respect to an central axis CA of said optical
fibers 14'. At the tapered first end 15' of the optical fibers 14',
a fluorescent plate or a fluorescent layer 82' is arranged, which
in use may be arranged instead of and at the position of the
fluorescent strips 82 in the example of FIGS. 2, 3 and 4. The
secondary electrons 12' which are projected onto the fluorescent
plate or fluorescent layer 82' are converted into photons 20. At
least part of the generated photons 20 are coupled into the first
end 15' of the optical fibers 14' and are conveyed or directed
through said optical fiber 14' towards a photo-detector. The
photons 20 are confined inside optical fiber 14' due to total
internal reflection at the side surface of the optical fibers 14'.
Alternatively, the optical fibers 14' may be at least partially
coated with a photo-reflecting layer 22, as schematically indicated
at one of the fibers 14' in FIG. 5.
[0072] As discussed above, it is also possible to arrange the photo
detector 130 directly above or on top of the electron to photon
converter sections (e.g. fluorescent strip 82), as schematically
shown in a second example shown in FIG. 6. According to this
example, the photo detector 130 comprises an array of through holes
131 for the primary charged particle beams 7. The photo detector
130 preferably comprises an array of photo detector sections 132,
each of said photo detector section 132 is arranged directly above
or on top of one of the electron to photon converter sections (e.g.
fluorescent strip 82). Accordingly, the photon transport unit does
not need to have extra or separate components, such as optical
fibers, and is essentially established by the configuration of the
array of electron to photon converter sections and the array of
photo detector sections 132.
[0073] It is to be understood that the above description is
included to illustrate the operation of the preferred embodiments
and is not meant to limit the scope of the invention. From the
above discussion, many variations will be apparent to one skilled
in the art that would yet be encompassed by the spirit and scope of
the present invention.
[0074] It is noted that in use the charged particle beams, in
particular the primary charged particle beams, travel along the
corresponding trajectories of the charged particle beams, and the
representation of the charged particle beams in the enclosed
figures also depict the corresponding trajectories of the charged
particle beams. When not in use, the charged particle beams are not
present in the multi-beam charged particle column. However, the
trajectories, which represent the path the charged particle beams
follow when the multi-beam charged particle column is in use, can
be defined even if the multi-beam charged particle column is not in
use.
[0075] In summary, embodiments of the present invention relates to
an apparatus and method for inspecting a surface of a sample. Said
apparatus comprises a multi-beam charged particle column comprising
arranged source for creating multiple primary beams directed
towards the sample, an objective lens for focusing said primary
beams on said sample, an electron-photon converter unit comprising
an array of electron to photon converter sections, each section is
located next to a primary beam within a distance equal to a pitch
of the primary beams at the electro-photon converter unit, a photon
transport unit for transporting light from said electron to photon
converter sections to a photo detector, and an electron collection
unit for guiding secondary electrons created in the sample, towards
the electron-photon converter unit. The electron collection unit is
arranged to project secondary electrons created in the sample by
one of said primary beams to at least one of said electron to
photon converter sections.
* * * * *