U.S. patent application number 15/218643 was filed with the patent office on 2017-08-03 for charged-particle microscope with astigmatism compensation and energy-selection.
This patent application is currently assigned to FEI Company. The applicant listed for this patent is FEI Company. Invention is credited to Alexander Henstra, Bohuslav Sed'a, Lubomir Tuma.
Application Number | 20170221673 15/218643 |
Document ID | / |
Family ID | 55299335 |
Filed Date | 2017-08-03 |
United States Patent
Application |
20170221673 |
Kind Code |
A1 |
Sed'a; Bohuslav ; et
al. |
August 3, 2017 |
CHARGED-PARTICLE MICROSCOPE WITH ASTIGMATISM COMPENSATION AND
ENERGY-SELECTION
Abstract
A method of producing a corrected beam of charged particles for
use in a charged-particle microscope, comprising the following
steps: Providing a non-monoenergetic input beam of charged
particles; Passing said input beam through an optical module
comprising a series arrangement of: A stigmator, thereby producing
an astigmatism-compensated, energy-dispersed intermediate beam with
a particular monoenergetic line focus direction; A beam selector,
comprising a slit that is rotationally oriented so as to match a
direction of the slit to said line focus direction, thereby
producing an output beam comprising an energy-discriminated portion
of said intermediate beam.
Inventors: |
Sed'a; Bohuslav; (Blansko,
CZ) ; Tuma; Lubomir; (Brno, CZ) ; Henstra;
Alexander; (Utrecht, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FEI Company |
Hillsboro |
OR |
US |
|
|
Assignee: |
FEI Company
Hillsboro
OR
|
Family ID: |
55299335 |
Appl. No.: |
15/218643 |
Filed: |
July 25, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/26 20130101;
H01J 37/153 20130101; H01J 37/09 20130101; H01J 37/28 20130101;
H01J 2237/2826 20130101; H01J 2237/1532 20130101; H01J 2237/0451
20130101; H01J 37/05 20130101; H01J 2237/0453 20130101 |
International
Class: |
H01J 37/153 20060101
H01J037/153; H01J 37/09 20060101 H01J037/09; H01J 37/26 20060101
H01J037/26 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 2, 2016 |
EP |
16153875.6 |
Claims
1. A method of producing a corrected beam of charged particles for
use in a charged-particle microscope, comprising: providing a
non-monoenergetic input beam of charged particles; and passing said
input beam through an optical module comprising a series
arrangement of: a stigmator, thereby producing an
astigmatism-compensated, energy-dispersed intermediate beam with a
particular monoenergetic line focus direction; and a beam selector,
comprising a slit that is rotationally oriented so as to match a
direction of the slit to said line focus direction, thereby
producing an output beam comprising an energy-discriminated portion
of said intermediate beam.
2. A method according to claim 1, wherein: stigmator is used to
mitigate a first, systematic astigmatism effect; and beam selector
is used to address a second, parasitic astigmatism effect.
3. A method according to claim 2, wherein; said first, systematic
astigmatism effect is associated with eccentric lens traversal by
said input beam; and second, parasitic astigmatism effect is
associated with positioning errors in optical components upstream
of the beam selector.
4. A method according to claim 1, wherein: beam selector comprises
an opaque plate containing a plurality of slits of different
orientations; and particular slit is selected by effecting
appropriate relative motion of said plate and said intermediate
beam.
5. A method according to claim 1, wherein said beam selector
comprises an opaque plate having a slit of adjustable
orientation.
6. A corrector device for use in a charged-particle microscope,
characterized in that it comprises: input for a non-monoenergetic
input beam of charged particles; and optical module comprising a
series arrangement of: stigmator, for producing an
astigmatism-compensated, energy-dispersed intermediate beam with a
particular monoenergetic line focus direction; and beam selector,
comprising a slit that can be rotationally oriented so as to match
a direction of the slit to said line focus direction, thereby to
produce an output beam comprising an energy-discriminated portion
of said intermediate beam.
7. A method of calibrating a corrector device as claimed in claim
6, comprising the following steps: providing an aperture plate
containing a test aperture with a cross-section that is
substantially smaller than the cross-section of the intermediate
beam in the plane of the aperture plate; producing relative
scanning motion of the test aperture and the intermediate beam
cross-section and measuring the beam intensity transmitted through
the test aperture as a function of scan position, thereby producing
an intensity profile for the beam cross-section; using image
recognition software to analyze said intensity profile and derive
therefrom an associated line focus direction; and choosing a slit
orientation of said beam selector that is most closely matched to
said line focus direction.
8. A Charged Particle Microscope, comprising: a specimen holder,
for holding a specimen; a source, for producing an irradiating beam
of charged particles; an illuminator, for directing said beam so as
to irradiate the specimen; and an detector, for detecting a flux of
radiation emanating from the specimen in response to said
irradiation, wherein the illuminator comprises a corrector device
as claimed in claim 6.
9. A method according to claim 2, wherein: said beam selector
comprises an opaque plate containing a plurality of slits of
different orientations; and a particular slit is selected by
effecting appropriate relative motion of said plate and said
intermediate beam.
10. A method according to claim 3, wherein: said beam selector
comprises an opaque plate containing a plurality of slits of
different orientations; and a particular slit is selected by
effecting appropriate relative motion of said plate and said
intermediate beam.
11. A method according to claim 2, wherein said beam selector
comprises an opaque plate having a slit of adjustable
orientation.
12. A method according to claim 3, wherein said beam selector
comprises an opaque plate having a slit of adjustable
orientation.
13. A method according to claim 10, wherein said beam selector
comprises an opaque plate having a slit of adjustable
orientation.
14. A method of producing a corrected beam of charged particles for
use in a charged-particle microscope, comprising: providing a
non-monoenergetic beam of charged particles; compensating for
astigmatism in the beam of charged particles; dispersing the
charged particles in the beam to focus monoenergetic charged
particles in a line; and filtering the dispersed beam to produce an
energy-discriminated output beam of the charged particles focused
in the line.
15. A method according to claim 14, in filtering the dispersed beam
to produce an energy-discriminated output beam of the charged
particles focused in the line comprises passing the beam through a
slit rotationally oriented so as to match a direction of said line
focus direction.
16. A method according to claim 14, in which: compensating for
astigmatism comprises mitigating a first, systematic astigmatism
effect; and filtering the dispersed beam to produce an
energy-discriminated output beam addresses a second, parasitic
astigmatism
Description
[0001] The invention relates to a method of producing a corrected
beam of charged particles for use in a charged-particle
microscope.
[0002] The invention also relates to a corrector device that makes
use of such a method.
[0003] The invention further relates to a Charged Particle
Microscope comprising such a corrector device.
[0004] The invention additionally relates to a method of
calibrating/adjusting such a corrector device.
[0005] Charged particle microscopy is a well-known and increasingly
important technique for imaging microscopic objects, particularly
in the form of electron microscopy. Historically, the basic genus
of electron microscope has undergone evolution into a number of
well-known apparatus species, such as the Transmission Electron
Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning
Transmission Electron Microscope (STEM), and also into various
sub-species, such as so-called "dual-beam" tools (e.g. a FIB-SEM),
which additionally employ a "machining" Focused Ion Beam (FIB),
allowing supportive activities such as ion-beam milling or
Ion-Beam-Induced Deposition (IBID), for example. More specifically:
[0006] In a SEM, irradiation of a specimen by a scanning electron
beam precipitates emanation of "auxiliary" radiation from the
specimen, in the form of secondary electrons, backscattered
electrons, X-rays and photoluminescence (infrared, visible and/or
ultraviolet photons), for example; one or more components of this
emanating radiation is/are then detected and used for image
accumulation purposes. [0007] In a TEM, the electron beam used to
irradiate the specimen is chosen to be of a high-enough energy to
penetrate the specimen (which, to this end, will generally be
thinner than in the case of a SEM specimen); the transmitted
electrons emanating from the specimen can then be used to create an
image. When such a TEM is operated in scanning mode (thus becoming
a STEM), the image in question will be accumulated during a
scanning motion of the irradiating electron beam.
[0008] More information on some of the topics elucidated here can,
for example, be gleaned from the following Wikipedia links: [0009]
http://en.wikipedia.org/wiki/Electron_microscope [0010]
http://en.wikipedia.org/wiki/Scanning_electron_microscope [0011]
http://en.wikipedia.org/wiki/Transmission_electron microscopy
[0012]
http://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopy
[0013] As an alternative to the use of electrons as irradiating
beam, charged particle microscopy can also be performed using other
species of charged particle. In this respect, the phrase "charged
particle" should be broadly interpreted as encompassing electrons,
positive ions (e.g. Ga or He ions), negative ions, protons and
positrons, for instance. As regards non-electron-based charged
particle microscopy, some further information can, for example, be
gleaned from references such as the following: [0014]
https://en.wikipedia.org/wiki/Focused_ion_beam [0015]
http://en.wikipedia.org/wiki/Scanning_Helium_Ion_Microscope [0016]
W. H. Escovitz, T. R. Fox and R. Levi-Setti, Scanning Transmission
Ion Microscope with a Field Ion Source, Proc. Nat. Acad. Sci. USA
72(5), pp 1826-1828 (1975). [0017]
http://www.ncbi.nlm.nih.gov/pubmed/22472444
[0018] It should be noted that, in addition to imaging and
performing (localized) surface modification (e.g. milling, etching,
deposition, etc.), a charged particle microscope may also have
other functionalities, such as performing spectroscopy, examining
diffractograms, etc. [0019] In all cases, a non-transmission
Charged Particle Microscope (CPM) will comprise at least the
following components: [0020] A radiation source, such as a Schottky
electron source or ion gun. [0021] An illuminator, which serves to
manipulate a "raw" radiation beam from the source and perform upon
it certain operations such as focusing, aberration mitigation,
cropping (with an aperture), filtering, etc. It will generally
comprise one or more (charged-particle) lenses, and may comprise
other types of (particle-)optical component also. If desired, the
illuminator can be provided with a deflector system that can be
invoked to cause its exit beam to perform a scanning motion across
the specimen being investigated. [0022] A specimen holder, on which
a specimen under investigation can be held and positioned (e.g.
tilted, rotated). If desired, this holder can be moved so as to
effect scanning motion of the beam w.r.t. the specimen. In general,
such a specimen holder will be connected to a positioning system
such as a mechanical stage. [0023] A detector (for detecting
radiation emanating from an irradiated specimen), which may be
unitary or compound/distributed in nature, and which can take many
different forms, depending on the radiation being detected.
Examples include photodiodes, CMOS detectors, CCD detectors,
photovoltaic cells, X-ray detectors (such as Silicon Drift
Detectors and Si(Li) detectors), etc. In general, a CPM may
comprise several different types of detector, selections of which
can be invoked in different situations.
[0024] In the case of a transmission-type microscope (such as a
(S)TEM, for example), the CPM will also comprise: [0025] An imaging
system, which essentially takes charged particles that are
transmitted through a sample (plane) and directs (focuses) them
onto analysis apparatus, such as a detection/imaging device,
spectroscopic apparatus (such as an EELS device), etc. As with the
illuminator referred to above, the imaging system may also perform
other functions, such as aberration mitigation, cropping,
filtering, etc., and it will generally comprise one or more
charged-particle lenses and/or other types of particle-optical
components.
[0026] In what follows, the invention may--by way of
example--sometimes be set forth in the specific context of electron
microscopy; however, such simplification is intended solely for
clarity/illustrative purposes, and should not be interpreted as
limiting.
[0027] A method as set forth in the opening paragraph above is, for
example, known from U.S. Pat. Nos. 7,034,315 and 8,461,525
(incorporated herein by reference), which have an inventor in
common with the present invention. In said patents, a
charged-particle beam is directed eccentrically (off-axis) through
a particle-optical lens, as a result of which it becomes
(energy-)dispersed, i.e. fanned out into a "spectrum" of different
"colors" (particle energies). A diaphragm is then used to select
from this spectrum a narrow window in which there is a relatively
narrow range .DELTA. of particle energies--thereby converting the
dispersed beam into a substantially monoenergetic
(confined-energy-spread) sub-beam.
[0028] A problem with this known method is that it suffers from
detrimental effects of (twofold) astigmatism, particularly
associated with the intentionally eccentric passage of the beam
through the lens. As a result of such astigmatism, a selected
energy range .DELTA. will typically be "polluted" by the presence
of charged particles with energies above and/or below .DELTA..
[0029] It is an object of the invention to address this problem. In
particular, it is an object of the invention to provide a
method/apparatus for generating an improved charged-particle beam
for use in a charged-particle microscope. More specifically, it is
an object of the invention that such a method/apparatus should
provide a means of compensating for (twofold) astigmatism effects.
Additionally, it is an object of the invention that said
method/apparatus should be able to produce a substantially
monoenergetic output beam.
[0030] These and other objects are achieved in a method as set
forth in the opening paragraph above, characterized by the
following steps: [0031] Providing a non-monoenergetic input beam of
charged particles; [0032] Passing said input beam through an
optical module comprising a series arrangement of: [0033] A
stigmator, thereby producing an astigmatism-compensated,
energy-dispersed intermediate beam with a particular monoenergetic
line focus direction; [0034] A beam selector, comprising a slit
that is rotationally oriented so as to match a direction of the
slit to said line focus direction, thereby producing an output beam
comprising an energy-discriminated portion of said intermediate
beam. The terminology used here will receive further elucidation
below.
[0035] Astigmatism is traditionally corrected using a pair of
co-operating stigmators (e.g. quadrupole optical elements) that are
mutually rotated (e.g. through 45.degree.). The present invention
produces the surprising insight that, in the current context
(production of an acceptable incipient beam for use in a CPM),
astigmatism effects can also be satisfactorily compensated using
just one stigmator and an associated slit with a selectable
rotational stance--which is a great advantage in the cramped
confines of a (high-voltage) CPM source/illuminator, since it saves
space and reduces the required number of (high-voltage) electrical
feedthroughs (which are bulky, and tend to act as undesirable
mechanical bridges for environmental vibrations). The invention
works as follows: [0036] Astigmatism will cause a point focus to be
aberrated into a pair of line foci, which are spaced from one
another along an axial direction (beam propagation direction). This
effect will occur for each particle energy in the input beam.
[0037] In the inventive method, the (suitably configured/excited)
stigmator is used to reduce the axial separation of the line foci
in the intermediate beam to a (near) minimum (see, for example,
Embodiment 3 below). [0038] In a focal plane of a selected one of
the line foci (e.g. the proximal line focus, located closest to the
stigmator), the direction of the line focus will be unpredictable
(see below). To deal with this, the beam selector will have to
comprise a rotational degree of freedom, so as to be able to match
the slit direction to the particular line focus direction involved.
[0039] Dispersion in the intermediate beam will produce a
train/series of parallel line foci on the slit plane, each
corresponding to a different energy. A particular one of these can
be selected by producing appropriate translation of the beam
relative to the slit (or vice versa). Because of this
anomalous/non-systematic nature of the line focus orientation in
the intermediate beam, passing it through a fixed-direction slit
(as in the prior art) will generally not result in selection of a
well-defined energy range; however, providing a choice between a
variety of slit directions allows selection of an optimum-choice
slit orientation that is best matched to a given line focus
direction (in that the two are perfectly parallel, approximately
parallel, or as parallel as possible in the case of a limited
choice of discrete slit orientations--see below). A "nominal"
rotational stance of the beam selector can be predetermined based
on the chosen rotational stance of the (poles of the) stigmator
(about the optical axis).
[0040] Another way of (functionally) understanding the invention is
to consider the astigmatism of the intermediate beam to be
composite in nature, whereby: [0041] The stigmator is used to
mitigate a first, systematic astigmatism effect/component; [0042]
The beam selector is used to address a second, parasitic
astigmatism effect/component. Somewhat of a loose analogy can be
made in this scenario to the situation of a signal that is composed
of an "AC" (variable) fluctuation seated upon a "DC" (constant)
base: when the DC base is subtracted (analogous to what the
stigmator does), the AC component is laid bare.
[0043] In a particular aspect of the scenario set forth in the
previous paragraph: [0044] Said first, systematic astigmatism
effect is associated with eccentric lens traversal by said input
beam; [0045] Said second, parasitic astigmatism effect is
associated with positioning errors in optical components upstream
of the beam selector. The "lens" traversed here may be the
stigmator, or it may be a lens located upstream of the stigmator
(similar to the set-up in the aforementioned U.S. Pat. No.
7,034,315/U.S. Pat. No. 8,461,525), or both; it should be
remembered in this regard that the stigmator will act as a lens if
its differential excitation (multipole effect) is superimposed upon
an underlying non-differential excitation (lens effect). The term
"upstream" here refers to (portions of) the optical column
(including the source) preceding the beam selector. The
"positioning errors" referred to here may, for example, include
effects such as positional shift/drift, mechanical deformation,
form inaccuracy, etc.; a typical example of such an effect is
source tip positional shift.
[0046] The invention has a number of pronounced advantages relative
to the prior art. For example: [0047] It achieves more
accurate/defined energy selection than, for example, the technique
in aforementioned U.S. Pat. No. 7,034,315/U.S. Pat. No. 8,461,525;
[0048] It is capable of delivering a much higher energy-elected
output beam current, e.g. increasing a relatively low, typical
prior-art value of ca. 25 pA to a much higher value of 200 pA, with
an attendant significant improvement in attainable resolution
(particularly for beam energies below .about.5 keV). [0049] It
effectively increases the usable lifetime of upstream components,
such as the source. In the past, these had to be discarded once
they had become unusable due to (cumulative) drift effects.
However, the present invention provides a functionality whereby
such drift effects can be (continually) compensated for, allowing
prolonged use, with associated cost reduction and uptime
improvement. [0050] It mitigates an annoying phenomenon called
"banding", in which interference fringes are produced when a beam
skims along imperfections (such as burrs) that occur along edges of
an energy-selection diaphragm as used in U.S. Pat. No.
7,034,315/U.S. Pat. No. 8,461,525. Because the energy-selection
slit of the present invention can be rotationally matched to the
orientation of (a monoenergetic line focus of) the
astigmatism-compensated intermediate beam emerging from the
stigmator, a smaller portion of the slit tends to be illuminated,
with an attendant reduction in interference effects. [0051] It
reduces cost and complexity, and increases available volume, by
removing the need to use a second stigmator for astigmatism
compensation.
[0052] It should be explicitly noted that the slit of the inventive
beam selector does not necessarily have to be located on the
optical axis of the stigmator. Instead, if desired, it may be
located (slightly) off-axis, so as not to get in the way of an
on-axis, uncorrected, high-current beam that may be needed for some
applications. This is somewhat analogous to the situation
illustrated in U.S. Pat. No. 7,034,315/U.S. Pat. No. 8,461,525, in
which the employed diaphragm has an on-axis opening (for a
non-eccentric beam) and an off-axis opening (for eccentric beam
passage). See, also, FIG. 2 below.
[0053] In a particular embodiment of the invention: [0054] Said
beam selector comprises an opaque plate containing a plurality of
slits of different orientations; [0055] A particular slit is
selected by effecting appropriate relative motion of said plate and
said intermediate beam. Such a plate offers a discrete collection
of slit orientations to the intermediate beam emerging from the
stigmator, e.g. a collection of slits that subtend angles of
n.theta. with a reference direction (in the plane of the plate),
where n is an integer and .theta. is an incremental angle (such as
15.degree., for instance). A plate of this type may be mounted on a
slider (or rotating carrousel, for example) that can be used to
displace it relative to the stigmator's optical axis (allowing
different slits to be placed in the intermediate beam path);
alternatively/supplementally, a deflector unit can be used to
displace the intermediate beam across the plate (steering the beam
onto different slits). An example of an embodiment of this type is
illustrated in FIGS. 2 and 3, for example--in which the illustrated
plate is arranged perpendicular to the optical axis of the
stigmator.
[0056] In an alternative embodiment, the beam selector comprises an
opaque plate having a slit of adjustable orientation. Such a plate
may, for instance, be rotatable about the optical axis of the
stigmator (or an off-axis direction parallel thereto), e.g. by
mounting it in a ring-shaped bearing chase and rotating it to a
given (roll) stance using a mechanism employing a cog/screw drive,
for instance. If desired, the slit in such a rotatable plate may
also be adjustable in width, e.g. by displacing a movable knife
edge back/forth across the slit opening, as desired.
[0057] In yet another alternative to the two previous embodiments,
a library (e.g. rack/cassette) of different slit plates is stored
in situ, and a retriever device (such as a robot arm) is used to
fetch a particular plate (from the library) and insert it into the
beam selector position, as required; after use, the plate in
question can be returned to the library by said retriever arm.
[0058] The current invention further relates to a method of
calibrating/adjusting the inventive corrector device. One way to do
this would be to use a relatively basic procedure such as the
following: [0059] Selecting a first slit orientation in said beam
selector, and measuring an energy distribution of the output beam
emerging from it; [0060] Repeating this procedure for at least one,
second slit orientation; [0061] From the set of (at least two) slit
orientations thus selected, choosing a member with minimal measured
energy distribution. However, the inventors have developed a more
efficient/sophisticated alternative, which comprises the following
steps: [0062] Providing an aperture plate containing a test
aperture with a cross-section that is substantially smaller than
the cross-section of the intermediate beam in the plane of the
aperture plate; [0063] Producing relative scanning motion of the
test aperture and the intermediate beam cross-section and measuring
the beam intensity transmitted through the test aperture as a
function of scan position, thereby producing an intensity profile
for the beam cross-section; [0064] Using image recognition software
to analyze said intensity profile and derive therefrom an
associated line focus direction; [0065] Choosing a slit orientation
of said beam selector that is most closely matched to said line
focus direction. Such a procedure can be performed at desired
intervals, so as to ensure/maintain optimal performance of the
corrector device. The scanning motion alluded to may be produced by
scanning the intermediate beam across the test aperture and/or by
laterally displacing the aperture plate relative to the beam. A
test aperture as referred to here might, for example, be a square
or round hole with a width of the order of about 100 nm.
[0066] The invention will now be elucidated in more detail on the
basis of exemplary embodiments and the accompanying schematic
drawings, in which:
[0067] FIG. 1 renders a longitudinal cross-sectional view of a CPM
in which the present invention is implemented.
[0068] FIG. 2 renders an illustration of the structure and
operating principle of an embodiment of the invention.
[0069] FIG. 3 renders an elevational view of a particular
embodiment of a beam selector as used in the present invention.
[0070] In the Figures, where pertinent, corresponding parts may be
indicated using corresponding reference symbols.
Embodiment 1
[0071] FIG. 1 is a highly schematic depiction of an embodiment of a
CPM in which the present invention is implemented; more
specifically, it shows an embodiment of a microscope M, which, in
this case, is a SEM (though, in the context of the current
invention, it could just as validly be a (S)TEM, or an ion-based
microscope, for example). The microscope M comprises an illuminator
(particle-optical column) 1, which produces a beam 3 of input
charged particles (in this case, an electron beam) that propagates
along a particle-optical axis 3'. The illuminator 1 is mounted on a
vacuum chamber 5, which comprises a specimen holder 7 and
associated stage/actuator 7' for holding/positioning a specimen S.
The vacuum chamber 5 is evacuated using vacuum pumps (not
depicted). With the aid of voltage supply 17, the specimen holder
7, or at least the specimen S, may, if desired, be biased (floated)
to an electrical potential with respect to ground.
[0072] The illuminator 1 (in the present case) comprises an
electron source 9 (such as a Schottky gun, for example), lenses 11,
13 to focus the electron beam 3 onto the specimen S, and a
deflection unit 15 (to perform beam steering/scanning of the beam
3). The apparatus M further comprises a controller/computer
processing apparatus 25 for controlling inter alia the deflection
unit 15, lenses 11, 13 and detectors 19, 21, and displaying
information gathered from the detectors 19, 21 on a display unit
27.
[0073] The detectors 19, 21 are chosen from a variety of possible
detector types that can be used to examine different types of
emergent radiation E emanating from the specimen S in response to
irradiation by the input beam 3. In the apparatus depicted here,
the following (non-limiting) detector choices have been made:
[0074] Detector 19 is a solid state detector (such as a photodiode)
that is used to detect photoluminescence emanating from the
specimen S. It could alternatively be an X-ray detector, such as
Silicon Drift Detector (SDD) or Silicon Lithium (Si(Li)) detector,
for example. [0075] Detector 21 is a segmented silicon electron
detector, comprising a plurality of independent detection segments
(e.g. quadrants) disposed in annular configuration about a central
aperture 23 (allowing passage of the primary beam 3). Such a
detector can, for example, be used to investigate the angular
dependence of a flux of emergent backscattered electrons emanating
from the specimen S. It will typically be biased to a positive
potential, so as to attract electrons emitted from the specimen S.
The skilled artisan will understand that many different types of
detector can be chosen in a set-up such as that depicted.
[0076] By scanning the input beam 3 over the specimen S, emergent
radiation--comprising, for example, X-rays,
infrared/visible/ultraviolet light, secondary electrons (SEs)
and/or backscattered electrons (BSEs)--emanates from the specimen
S. Since such emergent radiation is position-sensitive (due to said
scanning motion), the information obtained from the detectors 19,
21 will also be position-dependent. This fact allows (for instance)
the signal from detector 21 to be used to produce a BSE image of
(part of) the specimen S, which image is basically a map of said
signal as a function of scan-path position on the specimen S.
[0077] The signals from the detectors 19, 21 pass along control
lines (buses) 25', are processed by the controller 25, and
displayed on display unit 27. Such processing may include
operations such as combining, integrating, subtracting, false
colouring, edge enhancing, and other processing known to the
skilled artisan. In addition, automated recognition processes (e.g.
as used for particle analysis) may be included in such
processing.
[0078] It should be noted that many refinements and alternatives of
such a set-up will be known to the skilled artisan, including, but
not limited to: [0079] The use of dual beams--for example an
electron beam 3 for imaging and an ion beam for machining (or, in
some cases, imaging) the specimen S; [0080] The use of a controlled
environment at the specimen S--for example, maintaining a pressure
of several mbar (as used in a so-called Environmental SEM) or by
admitting gases, such as etching or precursor gases, etc.
[0081] In the specific context of the current invention, the
illuminator 1 comprises a corrector device C comprising a series
arrangement of a stigmator and a beam selector, as set forth above
and as illustrated in more detail in FIGS. 2 and 3 below. This
device C serves to perform astigmatism compensation and energy
selection, thus producing an output beam 3 that is of superior
quality--e.g. as regards (higher) beam current and (reduced)
banding errors--and that allows mechanical misalignments upstream
of item C, e.g. in the source 9, to be compensated for. See
Embodiment 2 below.
Embodiment 2
[0082] FIG. 2 schematically illustrates the structure and operation
of an embodiment of a method/corrector device according to the
present invention (refer also to FIG. 1). In FIG. 2, a source 9
emits charged particles (e.g. electrons) in a multitude of
directions, here depicted by a cone emanating from a tip of source
9. A spatial filter 31 (extractor aperture plate) contains an
off-axis aperture 31a and an on-axis aperture 31b, considered
relative to optical axis 3'. An input beam 3a propagating from
source 9 through aperture 31a passes eccentrically through
stigmator (quadrupole lens) 33 (centered on axis 3'), and emerges
from stigmator 33 as intermediate beam 3b; on the other hand, an
(axial) beam 3a' passes non-eccentrically through the center of
stigmator 33. As already set forth above, intermediate beam 3b will
demonstrate (twofold) astigmatism and energy dispersion.
[0083] Downstream of stigmator 33 is a set of deflectors 35a, 35b,
which can be used to change the direction of beams 3b, 3a' emerging
from stigmator 33. More specifically, as a result of appropriate
electrical excitations applied to deflectors 35a, 35b: [0084]
Intermediate beam 3b can have its course changed, becoming movable
beam segment 3c that can be steered onto different regions of beam
selector 37, which is located in a focal plane (slit plane) of one
of the line foci of beam 3b/3c. More details regarding beam
selector 37 will be given below. [0085] Axial beam 3a' can be
diverted off course when it is not needed downstream, so as to
impinge on screen (beam block) 37'. Intermediate beam 3b/3c emerges
from beam selector 37 as output beam 3d--which, as set forth above,
is less polyenergetic (more energy-discriminated) than intermediate
beam 3b/3c. It then passes through deflector pair 39a, 39b, which
can be used to deflect it "on-axis", so that it propagates
along/substantially parallel to optical axis 3'.
[0086] To give a specific, non-binding example, the following
approximate axial separations (along axis 3') may be employed:
[0087] Source 9 to spatial filter 31: 2.6 mm. [0088] Source 9 to
(median plane of) stigmator 33: 5.5 mm. [0089] Source 9 to beam
selector 37 (slit plane): 12.5 mm.
[0090] Turning now to FIG. 3, this shows the beam selector 37 in
more detail, viewed in a direction parallel to axis 3' (Z axis).
The particular embodiment shown here comprises a plate 41 in which
a plurality of slit-like (elongate) openings have been provided. A
particular (central) one (43) of these slits is oriented along axis
Y, but others are canted by varying amounts clockwise and
anticlockwise relative to this reference: for example, slits 43a,
43a' are respectively canted through +5.degree. and -5.degree.
relative to Y, slits 43b, 43b' are respectively canted through
+10.degree. and -10.degree. relative to Y, etc. By using the
deflector pair 35a, 35b (FIG. 2) and/or by suitably displacing the
plate 41 within the XY plane, the cross-section of beam 3c (FIG.
2)--which will have a particular (monoenergetic) line focus
direction after emergence from stigmator 33--can be matched
(aligned) with a "best fit" from the slits in plate 41; as a
result, only a laterally-confined (and directionally matched)
portion of the beam cross-section will be allowed to traverse the
beam selector 37. As a non-limiting example, the depicted slits in
plate 41 each have a length of ca. 2 .mu.m and a width of ca. 0.15
.mu.m, for instance.
Embodiment 3
[0091] What follows is an example of a straightforward stigmator
adjustment (calibration) routine that can be used in the present
invention: [0092] (i) For a first one of the line foci (e.g. the
distal line focus, furthest from the stigmator), one selects a
particular differential excitation (E.sub.D) of the stigmator and
adjusts the non-differential excitation (E.sub.C) of the stigmator
to achieve best focus at the slit plane. One then repeats this
procedure for at least one other value of E.sub.D, allowing a first
plot to be made of E.sub.C versus E.sub.D. [0093] (ii) The
procedure in (i) is repeated for the second one of the line foci
(the proximal line focus, nearest to the stigmator), resulting in a
second plot of E.sub.C versus E.sub.D. [0094] (iii) The value of
E.sub.D at the point of intersection of said first and second plots
is the value that will minimize the axial separation of the first
and second line foci. Such a procedure is well within the ambit of
the skilled artisan, who can, for example, abbreviate it so that he
can obtain the required information from just three data points in
total (two for one line focus, and one for the other line
focus).
* * * * *
References