U.S. patent application number 16/813572 was filed with the patent office on 2020-08-06 for charged particle beam apparatus.
The applicant listed for this patent is ASML Netherlands B.V.. Invention is credited to Zhongwei CHEN, Jack JAU, Weiming REN.
Application Number | 20200251305 16/813572 |
Document ID | / |
Family ID | 1000004769534 |
Filed Date | 2020-08-06 |
![](/patent/app/20200251305/US20200251305A1-20200806-D00000.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00001.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00002.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00003.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00004.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00005.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00006.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00007.png)
![](/patent/app/20200251305/US20200251305A1-20200806-D00008.png)
![](/patent/app/20200251305/US20200251305A1-20200806-M00001.png)
United States Patent
Application |
20200251305 |
Kind Code |
A1 |
CHEN; Zhongwei ; et
al. |
August 6, 2020 |
CHARGED PARTICLE BEAM APPARATUS
Abstract
The present invention provides apparatuses to inspect small
particles on the surface of a sample such as wafer and mask. The
apparatuses provide both high detection efficiency and high
throughput by forming Dark-field BSE images. The apparatuses can
additionally inspect physical and electrical defects on the sample
surface by form SE images and Bright-field BSE images
simultaneously. The apparatuses can be designed to do single-beam
or even multiple single-beam inspection for achieving a high
throughput.
Inventors: |
CHEN; Zhongwei; (San Jose,
CA) ; JAU; Jack; (Los Altos Hills, CA) ; REN;
Weiming; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML Netherlands B.V. |
Veldhoven |
|
NL |
|
|
Family ID: |
1000004769534 |
Appl. No.: |
16/813572 |
Filed: |
March 9, 2020 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14830062 |
Aug 19, 2015 |
10586681 |
|
|
16813572 |
|
|
|
|
14220358 |
Mar 20, 2014 |
9190241 |
|
|
14830062 |
|
|
|
|
61804794 |
Mar 25, 2013 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/261 20130101;
H01J 37/026 20130101; H01J 2237/24528 20130101; H01J 2237/2814
20130101; H01J 2237/2804 20130101; H01J 2237/1405 20130101; H01J
2237/24475 20130101; H01J 37/28 20130101; H01J 37/20 20130101; H01J
37/292 20130101; H01J 37/10 20130101; H01J 2237/2806 20130101; H01J
2237/24495 20130101; H01J 2237/04926 20130101; H01J 37/22 20130101;
H01J 2237/2801 20130101; H01J 37/285 20130101; H01J 37/244
20130101; H01J 37/14 20130101 |
International
Class: |
H01J 37/28 20060101
H01J037/28; H01J 37/29 20060101 H01J037/29; H01J 37/285 20060101
H01J037/285; H01J 37/02 20060101 H01J037/02; H01J 37/22 20060101
H01J037/22; H01J 37/20 20060101 H01J037/20; H01J 37/14 20060101
H01J037/14; H01J 37/10 20060101 H01J037/10; H01J 37/244 20060101
H01J037/244; H01J 37/26 20060101 H01J037/26 |
Claims
1-8. (canceled)
9. A charged particle beam apparatus, comprising: an incidence side
detector provided with a through hole; a deflector configured to
scan a surface of a sample at an oblique angle of incidence with a
primary electron beam passing through the through hole; wherein the
incidence side detector is configured to detect backscattered
electrons generated from scanning and traveling towards an
incidence side of the primary electron beam; and an electrode
adjacent to a path of the primary electron beam, the electrode
positioned along a path of the backscattered electrons,
intersecting a first virtual axis, wherein the first virtual axis
is perpendicular to a sample plane and intersecting an illuminated
area of the surface, and closer than the incidence side detector to
an illuminated area of the surface; wherein the electrode is
configured to attract secondary electrons generated from the
surface to prevent the secondary electrons from otherwise hitting
the incidence side detector, and the charged particle beam
apparatus is configured to generate a dark-field backscattered
electron image from the backscattered electrons detected by the
incidence side detector.
10. The charged particle beam apparatus according to claim 9,
further comprising a reflection side detector configured to detect
backscattered electrons generated from the scanning and traveling
towards a reflection side from the primary electron beam.
11. The charged particle beam apparatus according to claim 9,
further comprising a secondary electron detector, wherein the
electrode is a grid electrode, and the secondary electron detector
is configured to detect the secondary electrons passing through the
grid electrode.
12. The charged particle beam apparatus according to claim 11,
further comprising a reflection side detector configured to detect
backscattered electrons generated from the scanning and traveling
towards a reflection side from the primary electron beam.
13. The charged particle beam apparatus of claim 9, wherein the
incidence side detector is disposed between an objective lens of
the primary electron beam and the surface.
14. The charged particle beam apparatus according to claim 13,
further comprising a reflection side detector configured to detect
backscattered electrons generated from the scanning and traveling
towards a reflection side from the primary electron beam.
15. The charged particle beam apparatus according to claim 11
wherein the incidence side detector is disposed between an
objective lens of the primary electron beam and the surface.
16. The charged particle beam apparatus according to claim 15,
further comprising a reflection side detector configured to detect
backscattered electrons generated from the scanning and traveling
towards a reflection side from the primary electron beam.
17. The charged particle beam apparatus according to claim 9,
wherein the sample is disposed on the sample plane, and the
incidence side detector is disposed on an electron detection plane
greater than or equal to 45 degrees to the sample plane.
18. The charged particle beam apparatus of claim 9, wherein the
charged particle beam apparatus includes a single-beam
apparatus.
19. The charged particle beam apparatus of claim 9, wherein the
charged particle beam apparatus includes a multi-beam
apparatus.
20. An apparatus configured to inspect a surface of a sample,
comprising: an inspection tool configured to use a charged particle
beam to scan the surface at an oblique angle of incidence; a first
detector located between the inspection tool and the sample and
provided with a through hole for passing the charged particle beam,
wherein the first detector is configured to detect backscattered
charged particles bouncing back from the surface and along a
direction of the oblique angle; and an electrode positioned closer
to the surface than the first detector and configured to attract
secondary electrons emanating from the surface.
21. The apparatus according to claim 20, wherein the first detector
is configured such that the detected backscattered charged
particles are close to zero when the surface is flat, and the
backscattered charged particles are detected when the surface is
not flat, thereby obtaining a dark field image.
22. The apparatus of claim 20, wherein the inspection tool includes
a single-beam apparatus.
23. The apparatus of claim 20, wherein the inspection tool includes
a multi-beam apparatus.
24. A detecting device comprising: a first detector configured to
detect charged particles received from a sample, the first detector
including an opening configured to allow a primary charged particle
beam to pass through the first detector so as to irradiate an
illuminated area on a surface of the sample at an oblique angle;
and an electrode disposed closer to the illuminated area than the
first detector, the electrode configured to attract secondary
charged particles emanating from the sample.
25. The detecting device according to claim 24, further comprising:
a grid configured to be biased relative to the sample, wherein the
grid is provided on the first detector or on the electrode.
26. The detecting device according to claim 24, further comprising:
a second detector disposed at a side of the sample opposite from
the first detector.
27. The detecting device according to claim 26, wherein the first
detector includes a dark-field backscattered electron detector and
the second detector includes a bright-field backscattered electron
detector.
28. The detecting device according to claim 24, wherein the
detecting device comprises a plurality of detecting units
configured to inspect respective regions of the sample, a first
unit of the detecting units including the first detector and the
electrode.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation application and claims
priority to U.S. application Ser. 14/220,358 filed Mar. 20, 2014,
which claims the benefit of priority to U.S. Provisional
application No. 61/804,794 entitled to Zhongwei Chen et al. filed
Mar. 25, 2013 and entitled "Charged Particle Beam Apparatus", the
entire disclosures of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to a charged particle beam
apparatus which employs a scanning electron microscope (SEM) to
inspect particles and/or defects on a sample surface. More
particularly, it relates to a low-voltage scanning electron
microscope (LVSEM) for inspecting particles and/or defects on
surfaces of wafers or masks in semiconductor manufacturing
industry.
2. Description of the Prior Art
[0003] In semiconductor manufacturing industry, sometime particles
appear and remain on surfaces of masks and/or wafers during
semiconductor fabrication process for some reasons, which impact
the yield to a great degree. To monitor and therefore ensure the
yield, optical apparatuses or called optical tools which are
typically based on microscopy, have been employed to inspect
particles after some fabrication processes because of their high
inspection throughput and a good detection efficiency. As
integrations of IC chips are required higher and higher, critical
dimensions of patterns on wafer and mask are shrunk, and
consequently smaller and smaller particles become killers in the
yield. On development trends, optical tools are losing their
abilities to detect killer particles due to their longer
wavelengths relative to particle dimensions.
[0004] Theoretically, an electron beam (e-beam) has a relatively
shorter wavelength (such as 0.027 nm/2 keV) relative to particle
dimensions (down to several nm), and therefore can provide higher
detection sensitivity for small particles than an optical beam.
Higher detection efficiency comes from higher detection
sensitivity. Conventional e-beam apparatuses or called e-beam tools
for inspecting defects on wafer/mask, which are based on
Low-voltage Scanning Electron Microscopy (LVSEM), can directly
perform particle inspection. However they are always criticized for
low throughput. A large beam current is necessary to get a high
throughput, but incurs strong Coulomb Effect which impacts the
image resolution and thereby reducing the sensitivity.
[0005] Accordingly, a new-type e-beam tool especially for small
particle inspection, which can provide high detection efficiency
and high inspection throughput, is needed. In addition, it will be
advantageous if the tool can perform defect inspection as well.
SUMMARY OF THE INVENTION
[0006] The object of this invention is to provide an electron beam
apparatus employing LVSEM technology to inspect particles in a
sample surface. Firstly, this invention employs the difference
between the irregular scattering on particles and regular
scattering on sample surface due to an illumination of a primary
electron (PE) beam. By specifically arranging illumination of PE
beam and Collection of Backscattered Electrons (BSEs) and Secondary
Electrons (SEs) in a column of the apparatus, the apparatus can
provide high detection sensitivity and high inspection throughput.
Secondly, in the column, the objective lens and/or the condenser
lens are compacted by using permanent magnets. The compact
objective not only enables a favorable illumination of the PE beam,
but also reduces the size of the column. Then, this invention
configures the apparatus to accommodate multiple foregoing compact
columns so as to inspect multiple areas of a sample simultaneously.
So, this invention will especially benefit the particle inspection
in semiconductor yield management.
[0007] Accordingly, the invention therefore provides a method for
inspecting a surface of a sample, which comprises steps of
providing a primary electron beam (PE beam) to illuminate and scan
the surface of the sample by oblique incidence, providing a first
detector to detect backscattered electrons generated from the
surface of the sample and traveling towards an incidence side of
the PE beam, and providing an electrode to collect secondary
electrons generated from the surface of the sample so as not to hit
the first detector. The first detector has a through hole for the
PE beam passing through. The electrode is placed close to the
illuminated area on the surface of the sample. The inspecting
method may further comprise a step of providing a second detector
to detect backscattered electrons generated from the surface of the
sample and traveling towards a reflection side of the PE beam.
[0008] The present invention further provides a method for
inspecting a surface of a sample, which comprises steps of
providing a primary electron beam (PE beam) to illuminate and scan
the surface of the sample by oblique incidence, providing a first
detector to detect backscattered electrons generated from the
surface of the sample and traveling towards an incidence side of
the PE beam, providing a grid electrode to attract and make
secondary electrons generated from the surface of the sample pass
through, and providing a second detector to detect the secondary
electrons passing through the grid electrode. The first detector
has a through hole for the PE beam passing through. The grid
electrode is placed close to the illuminated area on the surface of
the sample. The inspecting method may further comprise a step of
providing a third detector to detect backscattered electrons
generated from the surface of the sample and traveling towards a
reflection side of the PE beam.
[0009] The present invention therefore provides a device of
detecting electrons generated from a surface of a sample, which
comprises a first detector having a through hole on an electron
detection plane thereof, and a first electrode beside the first
detector and close to the sample surface so as to attract and
prevent secondary electrons generated therefrom (called as SEs)
from hitting the first detector. The through hole is for a primary
electron beam (PE beam) passing through and illuminating the sample
surface by oblique incidence, and the electron detection plane is
inclined towards the sample surface so as to collect backscattered
electrons generated therefrom by the PE beam and traveling towards
an incidence side thereof (called as Dark-field BSEs).
[0010] The first electrode can have a grid structure so as to
attract and make the SEs pass through. The detecting device may
further comprise a second detector behind the first electrode so as
to detect the SEs therethrough. The detecting device may further
comprise a third detector on an reflection side of the PE beam,
wherein the third detector is inclined towards the sample surface
so as to collect backscattered electrons generated therefrom by the
PE beam and traveling towards the reflection side (called as
Bright-field BSEs). The detecting device may further comprise a
second electrode in front of the electron detection plane of the
first detector, wherein the second electrode has a grid structure
so as to repel and prevent the SEs from passing through and make
the Bright-field BSEs pass through.
[0011] The present invention therefore provides a single-beam
apparatus which comprises an electron source emitting primary
electrons along an optical axis of the single-beam apparatus, a gun
aperture plate below the electron source and having at least one
first opening, a condenser lens below the gun aperture plate and
aligned with the optical axis, a beam-limit aperture plate below
the condenser lens and having at least one second opening, a
magnetic objective lens below the beam-limit aperture plate and
aligned with the optical axis, a first scanning deflector inside a
bore of the magnetic objective lens, a first detector below the
magnetic objective lens and having a through hole, an attraction
electrode beside the first detector, and a sample stage below the
first detector and attraction electrode and for supporting a
sample. The primary electrons form a primary electron beam (PE
beam). One of the first openings is aligned with the optical axis
and limits a current of the PE beam to a first current value. One
of the second openings is aligned with the optical axis and limits
the current of the PE beam to a second current value. The through
hole is on an electron detection plane of the first detector and
aligned with the optical axis so that the PE can pass through. A
being-inspected surface of the sample is upturned. An illumination
angle formed between the optical axis and a normal of the
being-inspected surface is larger than 0.degree.. The condenser
lens and magnetic objective lens together focus the PE beam onto
that being-inspected surface, thereby forming illumination thereon.
The electron detection plane of the first detector faces aslant
that being-inspected surface so as to collect backscattered
electrons which are generated therefrom by the PE beam and travel
towards an incidence side thereof (called as Dark-field BSEs). The
attraction electrode attracts secondary electrons generated from
that being-inspected surface by the PE beam (called as SEs) so as
to prevent said SEs from hitting the first detector. The first
scanning deflector deflects the PE beam so as to scan the
being-inspected surface, thereby obtaining a Dark-field BSEs image
by said first detector.
[0012] The single-beam apparatus may further comprise a second
scanning deflector above the first scanning deflector, wherein the
first and second scanning deflectors together deflect the PE beam
so as to scan that being-inspected surface with smaller deflection
aberrations.
[0013] The illumination angle is preferred equal to or larger than
45.degree.. The attraction electrode can have a grid structure so
as to attract and make the SEs pass therethrough. The single-beam
apparatus may further comprise a second detector above the
attraction electrode so as to detect the SEs therethrough, thereby
obtaining a SEs image. The single-beam apparatus may further
comprise a third detector above that being-inspected surface and on
a reflection side of the PE beam so as to collect backscattered
electrons generated by the PE beam and travelling towards the
reflection side (called as Bright-field BSEs), thereby obtaining a
Bright-field BSEs image.
[0014] The magnetic objective lens may have an upper pole-piece and
a lower pole-piece both forming an axial magnetic-circuit gap. The
first scanning deflector can be inside the axial magnetic-circuit
gap so as to generate small deflection aberrations. The magnetic
objective lens may comprise a permanent magnet and an excitation
coil. The condenser lens may be a magnetic lens which has an inner
pole-piece and an outer pole-piece both forming a radial
magnetic-circuit gap. The single-beam apparatus may further
comprise an acceleration tube to establish a high-energy region
which is around the optical axis, starting from or below the gun
aperture plate and stop above the first scanning deflector.
[0015] The first detector is preferred positively biased with
respect to the sample so as to detect the Dark-field BSEs with high
signal gain. The single-beam apparatus may further comprise a
shielding box with a grid front plane to prevent an electric field
generated by the first detector from influencing the PE beam,
wherein the grid front plane directly faces the electron detection
plane of the first detector so that the Dark-field BSEs can pass
through and be detected.
[0016] The present invention therefore provides a multi-beam
apparatus which comprises a sample stage for supporting a sample
and making a being-inspected surface thereof upturned, and a
plurality of single-beam units above that being-inspected surface.
Each of said single-beam units comprises an electron source
emitting primary electrons along an optical axis of the single-beam
unit, a gun aperture plate below the electron source and having at
least one first opening, a condenser lens below the gun aperture
plate and aligned with the optical axis, a beam-limit aperture
plate below the condenser lens and having at least one second
opening, a magnetic objective lens below the beam-limit aperture
plate and aligned with the optical axis, a first scanning deflector
inside a bore of the magnetic objective lens, a first detector
below the magnetic objective lens and having a through hole, and an
attraction electrode beside the first detector and above that
being-inspected surface. The primary electrons form a primary
electron beam (PE beam). One of the first openings is aligned with
the optical axis and limits a current of the PE beam to a first
current value. One of the second openings is aligned with the
optical axis and limits the current of the PE beam to a second
current value. The magnetic objective lens comprises an excitation
coil and a permanent magnet. The through hole of the first detector
is on an electron detection plane of the first detector and aligned
with the optical axis so that the PE can pass through. An
illumination angle formed between the optical axis and a normal of
that being-inspected surface is larger than 0.degree.. The
condenser lens and magnetic objective lens together focus the PE
beam onto that being-inspected surface, thereby forming
illumination thereon. The electron detection plane of the first
detector faces aslant that being-inspected surface so as to collect
backscattered electrons which are generated therefrom by the PE
beam and travel towards an incidence side thereof (called as
Dark-field BSEs). The attraction electrode attracts secondary
electrons generated from that being-inspected surface by the PE
beam (called as SEs) to prevent the SEs from hitting the first
detector. The first scanning deflector deflects the PE beam so as
to scan that being-inspected surface, thereby obtaining a
Dark-field BSEs image by the first detector. Therefore each of the
single-beam units inspects an area on that being-inspected surface,
and consequently the plurality of single-beam units inspects a
plurality of corresponding areas on that being-inspected surface
simultaneously or on a schedule.
[0017] Each single-beam unit may further comprise a second scanning
deflector above the first scanning deflector, wherein the first and
second scanning deflectors together deflect the PE beam so as to
scan that being-inspected surface with smaller deflection
aberrations. The illumination angle of each single-beam unit is
preferred equal to or larger than 45.degree..
[0018] The attraction electrode of each single-beam unit may have a
grid structure so as to attract and make the SEs pass therethrough.
Each single-beam unit may further comprise a second detector above
the attraction electrode so as to detect said SEs therethrough,
thereby obtaining an SEs image. Each single-beam unit can further
comprises a third detector above that being-inspected surface and
on a reflection side of the PE beam so as to collect backscattered
electrons generated by the PE beam and traveling towards that
reflection side (called as Bright-field BSEs), thereby obtaining a
Bright-field BSEs image.
[0019] The magnetic objective lens of each single-beam unit may
have an upper pole-piece and a lower pole-piece both forming an
axial magnetic-circuit gap. The first scanning deflector of each
single-beam unit can be inside the axial magnetic-circuit gap to
generate small deflection aberrations.
[0020] The condenser lens of single-beam unit can be a magnetic
lens with a permanent magnet and an excitation coil. Each
single-beam unit may further comprise an acceleration tube to
establish a high-energy region which is around the optical axis,
staring from or below the gun aperture plate and stop above the
first scanning deflector.
[0021] The first detector of each single-beam unit may be
positively biased with respect to the sample so as to detect the
Dark-field BSEs with high signal gain. Each single-beam unit may
further comprise a shielding box with a grid front plane to prevent
an electric field generated by the first detector from influencing
the PE beam, wherein the grid front plane directly faces the
electron detection plane of the first detector so that the
Dark-field BSEs can pass through and be detected.
[0022] Other advantages of the present invention will become
apparent from the following description taken in conjunction with
the accompanying drawings wherein are set forth, by way of
illustration and example, certain embodiments of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The present invention will be readily understood by the
following detailed description in conjunction with the accompanying
drawings, wherein the same or like reference numerals designate the
same or like structural elements, and in which:
[0024] FIGS. 1A-1C are schematic illustrations of a fundamental
configuration for particle inspection of a sample in accordance
with one embodiment of the present invention.
[0025] FIGS. 2A and 2B show a simulation result of an example of
the configuration in accordance with the embodiment of the present
invention shown in FIG. 1A.
[0026] FIG. 3 is a schematic illustration of a configuration for
inspecting particles on a sample surface in accordance with another
embodiment of the present invention.
[0027] FIG. 4A is a schematic illustration of a configuration for
multifunctional inspection of a sample in accordance another
embodiment of the present invention.
[0028] FIG. 4B is a schematic illustration of a configuration for
multifunctional inspection of a sample in accordance another
embodiment of the present invention.
[0029] FIG. 4C is a schematic illustration of a configuration for
multifunctional inspection of a sample in accordance another
embodiment of the present invention.
[0030] FIG. 5A is a schematic illustration of an apparatus for
particle inspection of a sample in accordance with another
embodiment of the present invention.
[0031] FIG. 5B is a schematic illustration of an apparatus for
particle inspection of a sample in accordance with another
embodiment of the present invention.
[0032] FIG. 5C is a schematic illustration of an apparatus for
particle inspection of a sample in accordance with another
embodiment of the present invention.
[0033] FIG. 6 is a schematic illustration of a configuration of a
detector for particle inspection of a sample in accordance with
another embodiment of the present invention.
[0034] FIGS. 7A and 7B are schematic illustrations of a
multi-column apparatus for particle or multifunctional inspection
of a sample in accordance with another embodiment of the present
invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0035] Various example embodiments of the present invention will
now be described more fully with reference to the accompanying
drawings in which some example embodiments of the invention are
shown. Without limiting the scope of the protection of the present
invention, all the description and drawings of the embodiments will
exemplarily be referred to an electron beam. However, the
embodiments are not be used to limit the present invention to
specific charged particles.
[0036] In the drawings, relative dimensions of each component and
among every component may be exaggerated for clarity. Within the
following description of the drawings the same or like reference
numbers refer to the same or like components or entities, and only
the differences with respect to the individual embodiments are
described.
[0037] Accordingly, while example embodiments of the invention are
capable of various modifications and alternative forms, embodiments
thereof are shown by way of example in the drawings and will herein
be described in detail. It should be understood, however, that
there is no intent to limit example embodiments of the invention to
the particular forms disclosed, but on the contrary, example
embodiments of the invention are to cover all modifications,
equivalents, and alternatives falling within the scope of the
invention.
[0038] In this invention, particle means any kind of uninvited
protrusions.
[0039] In this invention, X, Y and Z axes form Cartesian
coordinate, and a sample surface is on XOY plane.
[0040] In this invention, "axial" means "in the optical axis
direction of an apparatus, column or a lens", while "radial" means
"in a direction perpendicular to the optical axis".
[0041] As it is well known that when a PE beam impinges on a
sample, secondary electrons (energy 50 eV) and backscattered
electrons (50 eV<energy.ltoreq.PE landing energy) will be
generated and emitted from the sample surface. For Secondary
electron (SE) emission, the angular distribution conforms Lambert's
law (proportional to cos .PHI., where .PHI. is emission angle
relative to the surface normal) irrespective of energy and
incidence angle a (relative to the surface normal) of PE beam and
sample material. The SE yield .delta. increases with increasing
incidence angle, and this characteristic provides possibility to
image topography of the sample surface.
[0042] For Backscattered electron (BSE) emission, the backscattered
coefficient .eta. increase with increasing incidence angle .alpha.
and atomic number, and this enable the imaging of both topography
and material contrast of the sample surface. The angular
distribution depends on energy and incidence angle a of PE beam and
sample material, which generally consists of two parts, a diffusely
scattered part with Lambert' distribution and a reflection-like
part with emission maximum. In the range
.alpha.=0.degree..about.60.degree., the diffusely scattered part
remain approximately constant, which is an advantage for
observation of material contrast. The reflection-like emission
maximum is getting obvious with increasing incidence angle, i.e. in
the case of oblique incidence (.alpha..gtoreq.45.degree.).
[0043] If there is a particle on a sample surface, the values of
incidence angle of a PE beam will be different when respectively
hitting the sample surface and the particle due to the differences
in normal direction. This difference provides possibilities for
detecting the particle by material contrast and topography
contrast. High contrast of a detection signal results in high
detection sensitivity for the particle. To get high contrast, it is
advantage to reduce the background component (due to the collection
of the electrons emitted from the sample surface) and/or increase
the feature component (due to the collection of the electrons
emitted from the particle). In terms of the foregoing
characteristics, this invention provides a method to detect
particles on a sample surface by detecting Dark-field BSE image
with high contrast, and one of its basic configurations is shown in
FIGS. 1A, 1B, 3 4A, 4B and 4C.
[0044] In FIG. 1A, a PE beam 1 passes through a hole of a detector
3 and illuminates a surface of a sample 2 by oblique incidence.
Consequently, BSEs 11 and SEs 12 are generated from the illuminated
area of the sample surface. Most of BSEs 11 travel to the
reflection side, and a very small part of BSEs 11 will travel to
the incidence side and collected by the detector 3. Most of SEs 12
will travel upwards. In order to make the detector 3 not collecting
SEs 12, an electrode 4 is placed above the illuminated area of the
sample surface and slightly positively biased with respect to the
sample 2 and the detector 3 to attract SEs 12 away from hitting the
detector 3.
[0045] For the sample surface having particles thereon, the PE beam
1 may hit a particle as shown in FIG. 1B if the sample surface is
scanned by some ways such as scanning the PE beam 1 and/or moving
the sample in the directions parallel to the sample surface. In
this case, most of BSEs 11 generated from the illuminated area of
the particle travel to the incidence side of the PE beam 1 and
consequently are collected by the detector 3, while SEs 12 will be
attracted by the electrode 4 and consequently not hit the detector
3.
[0046] FIG. 1C shows the detection signal S3 of the detector 3 when
the sample surface is scanned. The abscissa shows the geometric
location on the sample surface and the ordinate shows the detection
signal S3 of the detector 3 in arbitrary unit. As the PE beam 1
approaches the particle position P1, the detection signal S3 is
getting strong from the lower value S3_0 to the higher value S3_1.
Because the detector 3 only detects a small part of the diffusely
scattered BSEs from the sample surface and a large part of both the
diffusely scattered BSEs and the reflection-like scattered BSEs
from the particle, the value S3_0 is very small with respect to the
value S3_1. Therefore the particle will be shown in the scanning
image with a dark background and a high contrast which is shown in
the equation (1). Accordingly, the scanning image is a Dark-field
BSE image.
Contrast = S 3 - 1 - S 3 - 0 S 3 - 1 + S 3 - 0 ( 1 )
##EQU00001##
[0047] FIG. 2A and 2B show the trajectories of PE, SE and BSE and
the equipotential lines of an example of the forgoing
configuration, respectively in the meridional plane XOZ and the
sagittal plane YOZ. The incidence angle a of the PE beam 1, in this
embodiment, is 60.degree., and the distance is 20 mm from the
detector 3 to the sample surface along the incidence direction of
the PE beam 1. When the potentials V2 and V3 of the sample 2 and
the detector 3 are equal and the potential V4 of the electrode 4 is
40V higher than V2 and V3, more than 90% SEs are attracted to hit
the electrode 4.
[0048] FIG. 3 shows another way to prevent SEs 12 from hitting the
detector 3. A gird 5 is placed between the detector 3 and the
sample and preferred to be close to the detector. The grid 5 is
made of materials of electrical conductor and its potential V5 is
slightly negatively biased with respect to the potential V2 of the
sample 2. In this case, the potential V4 of the electrode 4 can be
equal to slightly higher than V2.
[0049] In the foregoing basic configurations, the SEs 12 and BSEs
11 on the reflection side can also be detected to form SE image and
Bright-field BSE image. Although these two images are not
advantageous for particle inspection because the particle will be
shown in a bright background and a lower contrast, the Bright-field
BSE image can show the material contrast on the sample surface and
the SE image can show the topography of the sample surface.
Therefore, the separation of the Dark-field BSE image, the
Bright-field BSE image and the SE image provides possibilities for
multifunctional inspections as well as particle inspection. FIGS.
4A-4C show how to detect the SE image and the Bright-field image in
the basic configuration shown in FIG. 1A.
[0050] In FIG. 4A, the electrode 4 has a grid structure, and a SE
detector 6 is placed above the electrode 4. The potential V6 of the
SE detector 6 is much higher than the potential V2 of the sample 2
to attract and accelerate the SEs passing through the grid 4 so as
to obtain a high gain on the SE detection. The grid 4 and the
electrically-shielding cover 7 enclose the SE detector 6 so as to
avoid the electrical field thereof leaking out. The potential V7 of
the electrically-shielding cover 7 can be equal to V4 or V2. The
combination of the grid 4, the detector 6 and the cover 7 is called
as SE detection unit. In FIG. 4B, a Bright-field BSE detector 8 is
further added and placed on the reflection side so as to detect
BSEs 11 on the reflection side. Compared with FIG. 1A, in FIG. 4C
only a Bright-field BSE detector 8 is added and placed on the
reflection side so as to detect BSEs 11 on the reflection side. As
shown in FIG. 3, a grid can be placed between the Bright-field
detector 8 and the sample 2 so as to prevent SEs 12 from hitting
the detector 8 as well.
[0051] Besides the collection efficiency and purity of the signal
electrons, the size, current and landing energy of the probe spot
of the PE beam 1 on the sample surface are the other factors
significantly determining the contrast of each image mentioned
above. A larger probe current within a smaller spot size will be
advantageous for obtaining a higher contrast. The backscattered
coefficient .eta. is approximately independent of the landing
energy in the range 5-100 keV. Below 5 keV, .eta. decreases for
heavy elements and increases for light elements with decrease in
the landing energy. Because most of materials used in semiconductor
manufacturing are light elements, it is preferred to use 5 keV
landing energy, i.e. in LVSEM mode. For the applications which need
the SE image while inspecting particles, a lower landing energy
such as <3 keV will be better to get a high SE yield 6. This
invention provides three embodiments of LVSEM in terms of the
foregoing considerations, as shown in FIGS. 5A-5C. FIGS. 5A-5C only
show the basic detection for particle inspection. They can further
comprise the SE detection unit and/or the Bright-field BSE detector
if necessary.
[0052] FIG. 5A shows an elementary embodiment which comprises the
column 100, the dark-field BSE detector 112 and the SE-attraction
electrode 113. The PE beam 121 is emitted from an electron source
101 and travel downward along the optical axis 150 of the column
100. To realize an oblique incidence of the PE beam 121 on the
surface of the sample 114, the optical axis 150 is set to form an
angle 60.degree. with respect to the surface normal of the sample.
The electron source 101 is set at a negative potential V.sub.c with
respect to the ground potential, while the anode 102 and the gun
aperture 103 are at a potential Va higher than V.sub.c and the
ground potential respectively. Therefore the PE beam 121 is
accelerated to a kinetic energy e(Vc-Va) after passing through the
gun aperture 103. Then the PE beam 121 is slightly focused by the
condenser lens 104 and partially limited by the beam-limit aperture
105 to get a desired probe current on the sample 114. The condenser
lens 104 can be a magnetic lens although it is shown as an
electrostatic lens here. Next the PE beam 121 passes through the
objective lens 111 and a scanning deflector 110 and finally
impinges on the surface of the sample 114. The objective lens 111
finely focuses the PE beam 121 to form a small probe spot, while
the deflector 110 deflects the PE beam 121 to scan the sample
surface with the finely focused probe spot, thereby obtaining a
field of view (FOV). The SE-attraction electrode 113 attracts SEs
122 so as to make the detector 112 only detect the dark-field BSEs
123.
[0053] Except the electron source 101, the anode 102 and the
electrode 113, all the other parts of the column 100 are set at the
ground potential for the sake of convenience in column
manufacturing. In this case the landing energy of the PE beam 121
is equal to eVc. If the detector 112 needs to be set at a potential
much higher than the ground potential for obtaining a higher signal
gain, it can be covered by a shielding box as shown in FIG. 6. In
FIG. 6, the shielding box 117 is set at the ground potential and
has a front grid 118 for the BSEs passing through. The potential V5
of the grid 118 can be equal to the ground potential or a light
lower than the ground potential for repelling SE from hitting the
detector 112. Either or both of the gun aperture 103 and beam-limit
aperture 105 can be a plate with one opening or a movable plate
with several different-size openings for obtaining various desired
values of the probe current and corresponding small spot sizes.
[0054] The objective lens 111 can be either an electrostatic lens
or a magnetic lens. However, a magnetic objective lens is preferred
due to its small aberrations. For a magnetic objective lens, its
aberrations decreases with decrease in its working distance (WD,
the axial distance between the lower surface of the objective lens
and the sample surface). Because of the oblique incidence, a short
WD requires the objective lens 111 has a small volume and a conical
front end. Accordingly, the objective lens 111 is proposed to have
a permanent magnet 107 as well as an excitation coil 108. The
strong permanent magnet 107 provides a fixed magnetic excitation
which takes the fixed and large part for the required magnetic
excitation range, and the coil 108 provides an adjustable magnetic
excitation to cover the rest small part. Because the coil 108 only
takes space much less than a conventional magnetic lens without
permanent magnet, the objective lens 111 can be constructed small
in volume. The upper pole-piece 109_1 and the lower pole-piece
109_2 sandwiches the permanent magnet 107 and forms an axial
magnetic-circuit gap close to the optical axis 150. The magnetic
field leaked out through this magnetic-circuit gap focuses the PE
beam 121 onto the sample surface. The scanning deflector 110 can be
either electrostatic or magnetic. However, an electrostatic one is
preferred due to its ability to deflect the PE beam 121 with high
speed. The scanning deflector 110 is placed inside the
magnetic-circuit gap between the upper and lower pole-pieces so as
to reduce the aberrations generated by the deflection.
[0055] During scanning, defocus of the PE beam 121 on the sample
surface will appear because the sample surface is not perpendicular
to the optical axis 150 in oblique incidence. The defocus can be
dynamically compensated by adjusting the excitation current of the
coil 108. Besides, the scanning deflector 110 can be a multiple
lens which can generate a dipole field for deflection scanning and
a round lens field for the compensation of the defocus. In
addition, the SE-attraction electrode 113 will slightly divert the
PE beam 121 as well as attracting SEs 122. The deviation will incur
a position shift of the PE beam 121 on the sample surface. The
shift can also be compensated by the scanning deflector 110.
[0056] FIG. 5B shows an advanced embodiment which can provide a
larger probe current within a smaller spot size and over a larger
FOV, in comparison with the foregoing elementary embodiment. For
the sake of the clarity, the column is denoted as 200. In the
column 200, at first, the magnetic condenser lens 104 is designed
to have a radial magnetic-circuit gap so that the electron source
101 can be deeply immersed in the magnetic field of the magnetic
condenser lens 104. The deep immersion greatly reduces the
aberrations of the condenser lens 104, especially when the
condenser lens 104 works with a strong focus power to make a large
PE current passing through the beam-limit aperture 105. In
addition, similar to the objective lens 111, the condenser lens 104
can have a permanent magnet 131 as well as an excitation coil 132.
The strong permanent magnet 131 provides a fixed magnetic
excitation which takes the fixed and large part for the required
magnetic excitation range, and the excitation coil 132 provides an
adjustable magnetic excitation to cover the rest small part.
Because the excitation coil 132 only takes space much less than a
conventional magnetic lens without permanent magnet, the condenser
lens 104 can be constructed small in volume. The inner pole-piece
133 and the outer pole-piece 134 sandwiches the permanent magnet
131 and forms the radial magnetic-circuit gap facing the electron
source 101. Secondly, one more scanning deflector 106 is added and
placed above the scanning deflector 110. These two scanning
deflectors together realize swing deflection (proposed in U.S. Pat.
No. 6,392,231) to further reduce off-axis aberrations due to the
deflection scanning The effect can greatly increase the effective
FOV.
[0057] FIG. 5C shows a more advanced embodiment which can provide a
larger probe current within a smaller spot size, in comparison with
the foregoing embodiments. For the sake of the clarity, the column
is denoted as 300. In FIGS. 5A and 5B, from the gun aperture 103 to
the sample 114, the PE beam 121 travels with the energy same as the
landing energy which is equal to eVc and 5 keV. If the PE beam 121
has a large current, the Coulomb effect occurring on the way will
obviously increase the final spot size of the PE beam 121. In the
column 300, to reduce the Coulomb effect, a high-energy region is
established around the optical axis 150 and between the gun
aperture and the scanning deflector 110 by placing an acceleration
tube 116 over there. The acceleration tube 116 is set at a
potential Ve much higher than the sample potential V2 which is
equal to the ground potential. Consequently, the PE beam 121 passes
through the high-energy region with energy e(Vc-Ve) much higher
than the landing energy eVc. The higher the energy of the electron
beam is, the weaker the Coulomb effect is. Although the upper end
of the acceleration tube 116 is shown close to the gun aperture
here, it can be connected with the gun aperture, and thereby
extending the high-energy region upwards to the gun aperture.
[0058] In FIG. 5C, one more detector 115 is placed below the
beam-limit aperture 105 to detect the BSEs passing through the hole
of the Dark-field BSEs detector 112 so as to increase the
collection efficiency of the signal electrons. The detector 115 has
a hole for the PE beam 121 passing through. The acceleration tube
116 has one or more side openings designed for installing and
replacing the beam limit aperture 105 and the detector 115 when
operating routine maintenance.
[0059] Although the methods used in FIG. 5B and 5C enable the
columns to generate a smaller spot size with a large probe current,
the inspection throughput by a single beam may be not sufficient
for some applications in mass production. In those cases, using
multiple beams to do inspection is a powerful solution. For
structuring a multi-beam apparatus with multiple single-beam
columns, the number of electron beams available for a wafer or a
mask is limited by the spatial interval required to physically
accommodate two adjacent single-beam columns in parallel. The
column embodiments shown in FIGS. 5A-5C are small in volume due to
the employment of permanent magnet in the magnetic objective lens.
This advantage can greatly reduce the spatial interval, thereby
increasing the number of electron beams available for a wafer or a
mask. FIG. 7A and FIG. 7B are respectively top and side views of a
multi-beam apparatus 1000 with twelve units 11-16 and 21-26 for
inspecting particles on a wafer/mask respectively and
simultaneously. Each of the twelve units comprises a column as
shown in FIGS. 5A-5C and a detection configuration as shown in
FIGS. 1A, 1B, 3 4A,4B and 4C. The units 11-16 are placed on the
left side and form a left-side group, while the units 21-26 are
placed on the right side and form a right-side group. Each unit of
the left-side group illuminates the surface of the sample 10 in
oblique incidence from the left side, such as the PE beam 16-1 of
the unit 16 shown in FIG. 7B. Accordingly, each unit of the
right-side group illuminates the surface of the sample 10 in
oblique incidence from the right side, such as the PE beam 26-1 of
the unit 26 shown in FIG. 7B.
[0060] In summary this invention provides methods to construct
e-beam apparatuses for inspecting small particles on the surface of
a sample such as wafer and mask. The apparatuses fundamentally
provide both high detection efficiency and high throughput by
forming Dark-field BSE image. Besides, they are able to form SE
image and Bright-field BSE image simultaneously to realize
additional inspection functions such as inspecting physical and
electrical defects on the sample surface as well. The e-beam
apparatus can comprise only one single-beam unit to do single-beam
inspection or multiple single-beam units to form multi-beam
inspection for achieving high throughput. In the column of a
single-beam unit, the objective lens or both of the objective lens
and the condenser lens is/are compacted by using permanent magnet
inside. The compact structure not only enables the favorable
oblique incidence of the PE beam, but also increases the number of
single-beams available for a wafer or a mask. This invention will
especially benefit the particle inspection in semiconductor yield
management.
[0061] Although the present invention has been described in
accordance with the embodiments shown, one of ordinary skill in the
art will readily recognize that there could be variations to the
embodiments and those variations would be within the spirit and
scope of the present invention. Accordingly, many modifications may
be made by one of ordinary skill in the art without departing from
the spirit and scope of the invention as limited only by the
appended claims.
* * * * *