U.S. patent application number 12/750488 was filed with the patent office on 2013-06-20 for optical imaging system with catoptric objective; broadband objective with mirror; and refractive lenses and broadband optical imaging system having two or more imaging paths.
This patent application is currently assigned to KLA-Tencor Corporation. The applicant listed for this patent is Russell Hudyma, Shiow-Hwei Hwang, Hwan J. Jeong, Gregory L. Kirk, David Shafer. Invention is credited to Russell Hudyma, Shiow-Hwei Hwang, Hwan J. Jeong, Gregory L. Kirk, David Shafer.
Application Number | 20130155399 12/750488 |
Document ID | / |
Family ID | 44709317 |
Filed Date | 2013-06-20 |
United States Patent
Application |
20130155399 |
Kind Code |
A9 |
Hwang; Shiow-Hwei ; et
al. |
June 20, 2013 |
OPTICAL IMAGING SYSTEM WITH CATOPTRIC OBJECTIVE; BROADBAND
OBJECTIVE WITH MIRROR; AND REFRACTIVE LENSES AND BROADBAND OPTICAL
IMAGING SYSTEM HAVING TWO OR MORE IMAGING PATHS
Abstract
An optical system may include an objective having at least four
mirrors including an outermost mirror with aspect ratio <20:1
and focusing optics including a refractive optical element. The
objective provides imaging at numerical aperture >0.7, central
obscuration <35% in pupil. An objective may have two or more
mirrors, one with a refractive module that seals off an outermost
mirror's central opening. A broad band imaging system may include
one objective and two or more imaging paths that provide imaging at
numerical aperture >0.7 and field of view >0.8 mm. An optical
imaging system may comprise an objective and two or more imaging
paths. The imaging paths may provide two or more simultaneous
broadband images of a sample in two or more modes. The modes may
have different illumination and/or collection pupil apertures or
different pixel sizes at the sample.
Inventors: |
Hwang; Shiow-Hwei; (San
Ramon, CA) ; Kirk; Gregory L.; (Pleasanton, CA)
; Jeong; Hwan J.; (Los Altos, CA) ; Shafer;
David; (Fairfield, CT) ; Hudyma; Russell; (San
Ramon, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hwang; Shiow-Hwei
Kirk; Gregory L.
Jeong; Hwan J.
Shafer; David
Hudyma; Russell |
San Ramon
Pleasanton
Los Altos
Fairfield
San Ramon |
CA
CA
CA
CT
CA |
US
US
US
US
US |
|
|
Assignee: |
KLA-Tencor Corporation
Milpitas
CA
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20110242528 A1 |
October 6, 2011 |
|
|
Family ID: |
44709317 |
Appl. No.: |
12/750488 |
Filed: |
March 30, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US08/78493 |
Oct 1, 2008 |
|
|
|
12750488 |
|
|
|
|
60997306 |
Oct 2, 2007 |
|
|
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61000254 |
Oct 24, 2007 |
|
|
|
Current U.S.
Class: |
356/237.2 ;
359/365; 359/676; 359/730 |
Current CPC
Class: |
G01N 21/9501 20130101;
G03F 1/84 20130101; G02B 17/084 20130101 |
Class at
Publication: |
356/237.2 ;
359/730; 359/365; 359/676 |
International
Class: |
G01N 21/88 20060101
G01N021/88; G02B 17/08 20060101 G02B017/08 |
Claims
1. An optical system for sample inspection comprising: an objective
comprising at least four mirrors including an outermost mirror; and
focusing optics optically coupled to the objective, wherein the
focusing optics include one or more refractive optical elements,
wherein the objective is configured to provide imaging at a
numerical aperture greater than 0.7, central obscuration less than
35% in pupil, and wherein an aspect ratio of outermost mirror at a
sample side is no more than 20:1.
2. The optical system of claim 1 wherein the focusing optics
include a relay module configured to relay an aperture stop inside
the objective to a pupil plan located outside the objective.
3. The optical system of claim 1 wherein the focusing optics are
configured to provide variable magnification.
4. The optical system of claim 1, further comprising a refractive
element disposed in a central opening in an outermost mirror of the
objective to seal off a central obscuration at the outermost
mirror.
5. The optical system of claim 4, further comprising one or more
additional refractive elements inside the objective, wherein the
one or more additional refractive elements are configured to
provide color correction for the refractive element disposed in the
central opening in the outermost mirror.
6. The optical system of claim 1 wherein objective is characterized
by a negative field curvature.
7. The optical system of claim 6, wherein the focusing optics are
characterized by a Petzval curvature that compensates for the field
curvature of the objective.
8. The optical system of claim 1 wherein the objective is
characterized by a sample field size that is greater than 0.5
mm.
9. The optical system of claim 1 wherein the objective includes six
mirrors and is configured to provide imaging of infinite
conjugate.
10. The optical system of claim 9 wherein objective includes an
outermost mirror module that generates an intermediate field
characterized by negative spherical aberration.
11. The optical system of claim 10 wherein the objective includes
more than one convex mirror.
12. The optical system of claim 1, further comprising one or more
optical elements configured to compensate for optical aberrations
between the objective and the focusing optics.
13. An imaging objective, comprising: two or more mirrors, at least
one of which contains a refractive module that seals off a central
opening of an outermost mirror of the two or more mirrors in order
to substantially isolate an atmosphere inside the objective from a
sample atmosphere.
14. The imaging objective of claim 13 wherein the refractive module
and one or more mirrors are configured such that light passing
through the objective is incident on an optical surface of the
refractive module at an angle of incidence that is less than about
25 degrees.
15. The imaging objective of claim 13 wherein the refractive module
includes an optical element that is flat or nearly flat.
16. The imaging objective of claim 13 wherein the refractive module
is configured to provide a field curvature that is opposite to the
field curvature generated by the two or more mirrors.
17. The imaging objective of claim 13 wherein the refractive
optical element is configured to compensate for an optical
aberration of one or more of the one or more mirrors.
18. The imaging objective of claim 13 wherein the two or more
mirrors includes one or more mirrors, other than the outermost
mirror, having a body with a central opening and a refractive
optical element disposed in the central opening.
19. The imaging objective of claim 18 wherein the refractive
optical element is configured to provide color correction.
20. The imaging objective of claim 13 wherein the two or more
mirrors include at least four mirrors.
21. The imaging objective of claim 13 wherein the two or more
mirrors include at most six mirrors.
22. The imaging objective of claim 21, including six mirrors,
wherein said objective is infinite conjugate; and said objective
has a flat Petzval field.
23. The imaging objective of claim 22, wherein said six mirrors
include a two-mirror relay group adjacent a convex mirror, the
two-mirror relay group including the outermost mirror and another
mirror disposed between the convex mirror and the outermost mirror,
wherein said two-mirror relay group has a magnification close to
one, and wherein said two-mirror relay group has a negative
spherical aberration.
24. The imaging objective of claim 23, wherein said slight negative
spherical aberration less than two times a distance between a
reflective surfaces of the relay mirror and a reflective surface of
the convex mirror.
25. A optical imaging system comprising a single objective and two
or more broadband imaging paths optically coupled to the objective,
wherein the objective and imaging paths are configured to provide
broadband imaging at a numerical aperture greater than 0.7 and a
field of view greater than 0.8 mm
26. The system of claim 25 wherein each of said two or more imaging
paths contains a separate independently accessible pupil.
27. The system of claim 26 wherein the two or more imaging paths
are configured to enable simultaneous inspection of a surface of a
sample proximate the objective with a plurality of inspection
modes.
28. The system of claim 27 wherein said plurality of inspection
modes comprises a bright field mode and a dark field mode.
29. The system of claim 28 wherein said plurality of inspection
modes comprises two different wavelength bands each wavelength band
having a field on the sample and resulting in an image transmitted
to a sensor.
30. The system of claim 29, further comprising different sensors
coupled to each of said two or more imaging paths for detecting
said different wavelength bands.
31. The system of claim 29 wherein said two different wavelength
bands include a first band characterized by vacuum wavelengths less
than about 250 nm; and a second band characterized by vacuum
wavelengths between about 250 nm and about 450 nm.
32. The system of claim 29 wherein said two different wavelength
bands include a first band that is at least 10 nm wide and includes
vacuum wavelengths less than 250 nm.
33. The system of claim 32 wherein said two different wavelength
bands further include a second band that is greater than 100 nm
wide.
34. The system of claim 32, wherein said pupil relay is comprised
of refractive elements.
35. The system of claim 34, wherein a single pupil relay is shared
between a plurality of said imaging paths.
36. The system of claim 34, wherein each of said two or more
imaging paths has a separate pupil relay.
37. The system of claim 29 wherein said two different wavelength
bands include a first band that is at least 10 nm wide and includes
vacuum wavelengths less than 270 nm.
38. The system of claim 37 wherein the optical components of the
imaging paths include one or more refractive optical elements
downstream of a split between the two imaging paths.
39. The system of claim 25 wherein the objective comprises at least
four mirrors including an outermost mirror, wherein the objective
is configured to provide imaging at a numerical aperture greater
than 0.7, central obscuration less than 35% in pupil, and wherein
an aspect ratio of outermost mirror at a sample side is no more
than 20:1.
40. The system of claim 25 wherein the objective comprises two or
more mirrors, at least one of which contains a refractive module
that seals off a central opening of an outermost mirror of the two
or more mirrors in order to substantially isolate an atmosphere
inside the objective from a sample atmosphere.
41. The system of claim 25, wherein each of said two or more
imaging paths includes: a pupil relay; a pupil; and a zoom/variable
magnification module.
42. The system of claim 25, wherein each of said two or more
imaging paths includes a variable magnification module operable to
provide a plurality of magnifications that map into a plurality of
pixel sizes.
43. The system of claim 25, wherein said two or more imaging paths
have independent alignment and magnification adjustment.
44. The system of claim 25, wherein each of said imaging paths
includes a sensor having a data rate, and wherein said data rate of
each said sensor can be set independently.
45. The system of claim 25, further including a plurality of
illumination paths.
46. The system of claim 45, wherein an optical spectrum, output
power, and illumination configuration are independently chosen and
adjusted for each of said illumination paths.
47. The system of claim 46, wherein said illumination configuration
includes: numerical aperture, illumination aperture, polarization,
light power, and illumination location on the sample.
48. The system of claim 25 wherein the at least one of said imaging
paths includes a long range optical trombone.
49. An optical imaging system, comprising an objective configured
to collect light from a sample located proximate the objective and
two or more imaging paths optically coupled to the objective,
wherein the two or more imaging paths are configured to provide a
corresponding two or more simultaneous images of the sample in a
corresponding two or more modes, wherein each mode of the two or
more modes is characterized by an illumination pupil aperture
and/or a collection pupil aperture and wherein each of the two or
more simultaneous images is a broad band image.
50. An optical imaging system, comprising an objective configured
to collect light from a sample located proximate the objective and
two or more imaging paths optically coupled to the objective,
wherein the two or more imaging paths are configured to provide a
corresponding two or more simultaneous images of the sample in a
corresponding two or more modes, wherein each mode of the two or
more modes is characterized by a different pixel size at the
sample.
51. A optical imaging system comprising a single objective and two
or more imaging paths optically coupled to the objective, wherein
the imaging paths are configured to transmit two different
wavelength bands, wherein at least one wavelength band is a
broadband wavelength band have a wavelength bandwidth greater than
10 nm in width, wherein illumination for the two different
wavelength bands is coupled to a sample through the objective,
wherein the objective and imaging paths are configured to provide
imaging at a numerical aperture greater than 0.7 and a field of
view greater than 0.8 mm.
Description
CLAIM OF PRIORITY
[0001] This application claims the priority benefit of U.S.
Provisional Patent Application No. 60/997,306, filed Oct. 2, 2007,
the entire disclosures of which are incorporated by reference
herein. This application also claims the priority benefit of U.S.
Provisional Patent Application No. 61/000,254, filed Oct. 24, 2007,
the entire disclosures of which are incorporated by reference
herein.
[0002] This application claims the priority benefit of
International Patent Application No. PCT/US2008/078493, which was
filed on Oct. 1, 2008, the entire disclosures of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention relates to imaging optics for sample
inspection, and in particular to the use of catoptric objectives,
catadioptric objectives, and the use of multiple wavelength bands
and/or inspection modes simultaneously.
BACKGROUND OF THE INVENTION
[0004] Defect inspection for the DUV range is currently being
performed with all-refractive objectives or with catadioptric
(combination of refractive and reflective) objectives. For
wavelengths shorter than DUV, US Patent publication 2006/0219930
proposes the use of an all-reflective, or catoptric, optical
system. US patent publication 2006/0219930 (now U.S. Pat. No.
7,351,980) is hereby incorporated by reference in its entirety.
[0005] As design rules shrink in integrated circuit technology, one
way to improve the detection of smaller defects is to increase the
resolution of the optical inspection system by utilizing shorter
wavelengths. However, as the wavelength goes below 250 nm, the
dispersion characteristics of the available optical material such
as fused silica and CaF.sub.2 increases significantly. Furthermore,
the availability of manufacturable Anti-Reflective (AR) coating
materials effective over the spectrum from sub-200 nm to above 400
nm wavelength is limited. (Good AR coating for a reflective
microscope is essential to minimize flare, stray light, etc). These
conditions make it extremely difficult to design and manufacture an
all-refractive or catadioptric optical system that supports
broadband illumination and detection including wavelengths shorter
than 250 nm.
[0006] The all-reflective optical system proposed in US patent
publication 2006/0219930 (now U.S. Pat. No. 7,351,980) provides
substantial improvements in manufacturability and design of an
optical system for use in wafer inspection with broadband
illumination including wavelengths below 250 nm. Furthermore, the
design of the objective in U.S. Pat. No. 7,351,980 includes an
opening in the mirror adjacent to the wafer that is being
inspected. The presence of the opening presents a potential risk of
deposition of contaminants onto the wafer and/or diffusion of
contaminations into the objective. Such contamination risks are
undesirable in wafer inspection systems. One could reduce
contamination risk by providing a large flow through the opening
that is presented by an all reflective optical design. However,
this creates mechanical instabilities at or near the opening. U.S.
Pat. No. 7,138,640 describes a method for protecting optical
components using a gas purge system that blocks contaminants from
reaching the optical surfaces of optical components and transports
contaminants away from those surfaces. However, it may be difficult
to implement this method in a broadband system that includes
wavelengths below 200 nm due to color correction, AR performance
issues and or mechanical instability if the window is made too
thin.
[0007] The prior art describes a number of sample inspection
systems that have various other disadvantages. For example, U.S.
Pat. No. 6,867,424 describes an optical system in which there is
only a single imaging path preventing simultaneous usage of two
light sources with different modes.
[0008] U.S. Pat. No. 7,359,044 describes the use of laser-based
illumination (as opposed to broadband illumination) for bright
field and dark field imaging in a sample inspection system.
Multiple lasers are used to provide illumination, but there is only
a single imaging path.
[0009] Lange 20050052643 describes an inspection system in which
there are dual illumination paths but only a single imaging
path.
[0010] U.S. Pat. No. 6,404,498 also describes a system in which
there are dual illumination paths but only a single imaging
path.
[0011] U.S. Pat. No. 6,078,386 teaches an inspection system having
dual imaging paths but uses narrow band, e.g., laser illumination
and imaging. In addition, the laser beam is introduced to the
sample from outside the objective. When two light sources are used,
the narrow band illumination is introduced to the sample from
outside the objective in a spatially coherent mode.
[0012] U.S. Pat. No. 6,762,831 teaches a sample inspection system
that uses narrow band illumination with DUV and VUV radiation
introduced to a sample through the objective. Illumination through
the objective enables spatially incoherent illumination modes.
Illumination and imaging are done with light of two different
wavelengths generated by two lasers.
[0013] It is within this context that embodiments of the present
invention arise.
SUMMARY OF THE INVENTION
[0014] According to an embodiment of the invention, an optical
system for sample inspection may comprise an objective having at
least four mirrors including an outermost mirror and focusing
optics optically coupled to the objective. The focusing optics
include one or more refractive optical elements. The objective is
configured to provide imaging at a numerical aperture greater than
0.7, central obscuration less than 35% in pupil. An aspect ratio of
outermost mirror at a sample side is no more than 20:1.
[0015] In certain embodiments of the present invention, an
outermost mirror of an objective for an imaging system such as a
wafer inspection system may have improved manufacturability by
decreasing its aspect ratio at a sample side to between about 10:1
and about 20:1. This allows inspection with a more manufacturable
objective of high Numerical Aperture (NA), greater than 0.7, with
large Field of View (FOV), greater than 0.5 mm, low central
obscuration (e.g., less than 35%, less than 30% or less than 25%),
broadband spectrum below 250 nm, and requiring no more than 6
mirrors. Such a system may further utilize one or more refractive
elements in the pupil relay and imaging optics external to the
objective, in order to improve the system flexibility and
efficiency. The refractive elements in the pupil relay and imaging
optics minimize the constraints on the packaging, and create the
opportunity for aberration compensation to the objective, thereby
lowering the tolerance requirements. According to one embodiment,
the objective may be a 4 mirror objective. Another embodiment of
the invention may utilize a 6 mirror objective. Both embodiments
have more manufacturable outermost elements, with two
configurations thereof A refractive pupil relay that can be
modified to be used with either of the two embodiments of the
inventive objective is also disclosed.
[0016] Additional embodiments of the present invention improve upon
the manufacturability, photo-contamination control, optical
performance and wafer edge inspection of the front element of a
reflective broadband objective by providing a refractive element
that closes the opening closest to the sample, thereby improving
photo-contamination control (PCC). Some embodiments of the present
invention utilize a refractive optical module in a central opening
of a reflective element closest to a sample. In some embodiments of
the present invention the refractive optical element is a curved
refractive shell-like lens element. According to an embodiment of
the present invention, an imaging objective may comprise two or
more mirrors, at least one of which contains a refractive module
that seals off a central opening of an outermost mirror of the two
or more mirrors in order to substantially isolate an atmosphere
inside the objective from a sample atmosphere.
[0017] In some embodiments, the refractive element can be less
curved, and even closer to being flat. The refractive module can
also consist of more than one refractive element. Embodiments of
the present invention may utilize refractive optical elements to
redistribute field curvature contributions from the reflective
elements of the objective, thereby improving mirror
manufacturability. The reflective and refractive elements in the
objective may be configured such that the angle of incidence (AOI)
on the refractive elements is less than about 25 degrees to reduce
the complexity of the broadband anti-reflective (AR) coating and
improve the achievable AR performance.
[0018] Further embodiments of this invention may provide an
apparatus and method for providing simultaneous defect inspection
data from multiple wavelength bands, Bright Field (BF) and Dark
Field (DF), differing magnifications, and independent alignment and
magnification adjustment. These features provide more efficient
utilization of imaging resources, allow for simultaneous high
sensitivity BF inspection and better light-budget DF
inspection.
[0019] According to another embodiment, an optical imaging system
may comprise a single objective and two or more broadband imaging
paths optically coupled to the objective. The objective and imaging
paths may be configured to provide broadband imaging at a numerical
aperture greater than 0.7 and a field of view greater than 0.8
mm.
[0020] According to an alternative embodiment, an optical imaging
system may comprise an objective configured to collect light from a
sample located proximate the objective and two or more imaging
paths optically coupled to the objective. The two or more imaging
paths may be configured to provide a corresponding two or more
simultaneous images of the sample in a corresponding two or more
modes. Each mode of the two or more modes may be characterized by
an illumination pupil aperture and/or a collection pupil aperture
and wherein each of the two or more simultaneous images is a broad
band image.
[0021] According to another alternative embodiment, an optical
imaging system may comprise an objective configured to collect
light from a sample located proximate the objective and two or more
imaging paths optically coupled to the objective. The two or more
imaging paths may be configured to provide a corresponding two or
more simultaneous images of the sample in a corresponding two or
more modes. Each mode may be characterized by a different pixel
size at the sample.
[0022] According to yet another alternative embodiment, an optical
imaging system may comprise a single objective and two or more
imaging paths optically coupled to the objective. The imaging paths
may be configured to transmit two different wavelength bands. At
least one wavelength band is a broadband wavelength band having a
wavelength bandwidth greater than 10 nm in width. Illumination for
the two different wavelength bands is coupled to a sample through
the objective wherein the objective and imaging paths are
configured to provide imaging at a numerical aperture greater than
0.7 and a field of view greater than 0.8 mm.
BRIEF DESCRIPTION OF THE FIGURES
[0023] FIG. 1a depicts an optical system according to the prior
art.
[0024] FIG. 1b depicts a first example of a four-mirror objective
portion of the inventive optical system according to an embodiment
of the present invention.
[0025] FIG. 1c sets forth an exemplary set of dimensions and
specifications for the objective portion depicted in FIG. 1b.
[0026] FIG. 1d shows an example of inventive refractive focusing
optics that may be used with the objective of FIG. 1b in a system
of the type shown in FIG. 1a.
[0027] FIG. 2a illustrates a second example of a six-mirror
objective portion of the optical system according to an embodiment
of the present invention.
[0028] FIG. 2b shows an enlargement of a region of negative
spherical aberration in the objective portion depicted in FIG.
2a.
[0029] FIG. 2c sets forth an example of dimensions and
specifications for an objective of the type depicted in FIG.
2a.
[0030] FIG. 2d shows an example of focusing optics that may be used
with an objective with flat Petzval curvature, as in FIG. 2a.
[0031] FIG. 2e illustrates a third example of an embodiment of an
objective portion, which allows a curved Petzval field.
[0032] FIG. 2f depicts tables setting forth an example of
dimensions and specifications for an objective of the type depicted
in FIG. 2e.
[0033] FIG. 3 illustrates an example of an embodiment of the
invention in which the objective includes a curved front refractive
element to seal off the central opening for the outermost mirror
closer to the sample and another refractive module to provide
corrections to color aberration caused by the front refractive
element.
[0034] FIG. 4 illustrates an example of another embodiment of the
invention in which the objective includes a flatter front
refractive element to seal off the central opening for the
outermost mirror closer to the sample and another refractive module
to provide the color correction.
[0035] FIG. 5 illustrates an example of an outermost objective
mirror element according to an embodiment of the present
invention.
[0036] FIG. 6 illustrates an exemplary embodiment of the portion of
an optical system external to the objective portion, which is
configured for simultaneous inspection over two wavelength
bands.
[0037] FIG. 7 illustrates a sub-portion of FIG. 5, including the
mirrors used to generate a lateral shift between the two wavelength
bands.
[0038] FIG. 8 illustrates an example of a configuration of the
inventive optical system employing dual illumination paths.
[0039] FIG. 9 illustrates some examples of sensor utilization
configurations, and how they might be utilized with different
wavelength spectra and modes.
DETAILED DESCRIPTION OF THE INVENTION
[0040] FIG. 1a depicts an example of an optical imaging system 100
that uses a 4-mirror objective design based on the one disclosed in
US Patent publication 2006/0219930. The optical system depicted in
FIG. 1a includes a pupil relay portion 102 and an objective portion
142. The objective portion 142 includes mirrors 110 (M1), 112 (M2),
115 (M3) and 120 (M4). An incident light beam 101 from the pupil
relay portion 102 impinges first onto mirror 110 (M1), then mirror
112 (M2), then an outermost mirror 115 (M3), then mirror 120 (M4),
then onto sample 105. The sample is generally located close to the
outermost mirror 115 in a sample inspection system, such as a wafer
inspection system or biological specimen inspection system or mask
inspection system. The nomenclature as utilized herein may be
construed as a definition of the terms denoting the relative
positions of the mirrors within the example being illustrated,
i.e., the numbering of the mirrors is defined by the order in which
the incident light beam from the pupil relay portion 102 (and/or
from an illuminator) impinges on the mirrors. For a 4-mirror
objective design, the M3 mirror (the outermost mirror 115) is
closest to the sample when the objective portion 142 is used in the
imaging system 100. Emitted rays emerging from the sample 105
impinge on the mirrors in reverse order, i.e., impinging first on
M4, then, M3, then M2 then M1. The numerical aperture (NA) for the
objective design shown in FIG. 1a is defined by a collection angle
130. For a given diameter of entrance opening 135, the NA is
increased as the distance 140 between the outermost mirror 115 and
sample 105 is decreased. A first intermediate image plane 160,
interior to the objective portion 142, and a second intermediate
image plane 165, external to the objective portion, are shown.
[0041] According to some, but not all embodiments of the present
invention, a refractive optical element such as a lens may be
inserted into the opening 135 through M3 near the sample, as a
physical barrier to photocontamination of the sample. The objective
would in this case be catadioptric, i.e., containing both
reflective and refractive elements.
[0042] The objective design depicted in FIG. 1a appears from the
curvature of the four mirrors to follow the generally utilized
system constraint that the total Petzval curvature is close to zero
for best full-field imaging quality. This constraint results in the
requirement of a convex or substantially flat outermost mirror
element 115. However, to achieve low central obscuration, the
outermost mirror 115, i.e., the first mirror element adjacent to
the external field plane at the high NA side, needs to be very
close to the sample and very thin, so that the opening required to
transmit the large angle cone of light can be limited in size.
These two conditions, i.e., the convex or flat outermost mirror
element 115 and its closeness to the sample, result in an outermost
mirror element with a high aspect ratio. As used herein, the aspect
ratio refers to a ratio of the diameter of the mirror to its edge
thickness. A trade-off therefore results, since a very thin mirror
with high aspect ratio is difficult to manufacture. By way of
example, in embodiments of the present invention, the aspect ratio
of the outermost mirror 115' may be less than about 20:1, e.g.,
between about 10:1 and about 20:1. FIG. 1b depicts the objective
portion 142 of according to an alternative embodiment of the
inventive optical system, which comprises a 4 mirror objective. An
aspect of this first embodiment is improved manufacturability of
the outermost mirror 115', which is denoted M3 viewed from the long
conjugate side. The outermost mirror 115' has a curved inner
surface 145, resulting in a thicker outer region 150, although the
central region 155 remains thin. NA therefore need not be
sacrificed. An exemplary set of dimensions and specifications for
this first embodiment is listed in FIG. 1c. First intermediate
image plane 160' is within objective portion 142. A second
intermediate image plane (similar to image plane 165 in FIG. 1a) is
external to objective portion 142.
[0043] The thicker, curved outermost mirror 115' results in several
effects: [0044] 1. Improved manufacturability of the mirror and
lowered cost compared to a flat, high aspect ratio mirror; [0045]
2. The Petzval curvature of the system becomes negative. (the
exemplary design disclosed herein has Petzval curvature of about
-80 mm) Again, a system such as the present system which allows the
Petzval curvature to float to a negative value has some advantages
relative to the system constrained to a near-zero Petzval
curvature. Though custom design of pupil relay components may be
required, the loosening of the general constraint that the Petzval
curvature be near zero enables better correction of aberration, or
if aberration is held constant, allows for increased FOV. Petzval
curvature and radius and its relation to reflective and refractive
elements is described in Warren J. Smith, "Modern Lens Design"
McGraw-Hill, Inc., 1992, Chapter 16: Mirror and Catadioptric
Systems (page 271). [0046] 3. A curvature of second intermediate
image plane results; further optical correction, i.e., by relay
optics, is required to map the curved intermediate image plane onto
a flat final image plane. These additional relay optics required to
correct for the field curvature result in a smaller accessible
pupil. There are tradeoffs regarding flat vs. curved intermediate
image plane (IIP): a flat IIP makes it easier to produce a larger
accessible pupil, which can be advantageous if components are to be
placed there. However, the smaller accessible pupil associated with
a curved intermediate image plane can be advantageous in that it
can be easier to shield, and also can result in a more compact
system.
[0047] In some embodiments, the size of the outermost mirror 115'
and the central obscuration may be further optimized by allowing
for a curved field with a refractive pupil relay. In general, the
optical power of the pupil relay needs to be sufficiently large to
accommodate the Petzval curvature from the objective. FIG. 1d shows
an example of a possible configuration for the pupil relay portion
102 that may be used with an objective of the type shown in FIG. 1b
that has a curved Petzval curvature. The pupil relay portion 102 is
external to objective portion 142. An aspect of one embodiment of
the inventive optical system is the use of refractive lenses to
form a pupil relay/variable magnification system, such as 170 and
variable magnification imaging systems, which maps the curved
intermediate image plane resulting from the negative Petzval
curvature onto a flat final image plane. In addition to refractive
lens elements 175, the magnification system may comprise folding
mirrors and/or beam splitters. This is in contrast to the
all-reflective pupil relay optics disclosed in US Patent
publication 2006/0219930 (now U.S. Pat. No. 7,351,980).
[0048] The use of refractive pupil relay optics provides a
straightforward, easy correction of the curved second intermediate
image plane, allowing mapping onto a flat final image plane. It is
known that use of mirrors as optical elements causes Petzval
numbers to tend negatively, whereas use of refractive lenses as
optical elements causes Petzval numbers to tend positively, as
described in the cited reference by Smith. Combining mirrors and
lenses as described herein tends to easily cancel out the negative
and positive Petzval numbers, to yield a flat final image
plane.
[0049] An additional advantage to using refractive pupil relay
optics is that the pupil relay can be centered on the optical axis
180, as shown in FIG. 1d. In contrast, pupil relay optics composed
of mirrors must be moved off-axis to prevent obscuration. Off-axis
mirror designs are very difficult to manufacture and to align.
Though mirrors may be used for any wavelength light, we have found
that lenses have sufficient bandwidth for our purposes, especially
if the collection light is split out into two bands.
[0050] Other variations on the optical systems described above may
be implemented. For example, if a flat intermediate image plane is
desired, as for any of the reasons described above, a flat (or
nearly flat) M3 mirror 115 may be necessary. In order to improve
manufacturability of this mirror, the aspect ratio may be
decreased. To implement this, an inventive configuration
illustrated in FIG. 2a and FIG. 2b adds a 2-mirror relay group to
an objective of the type shown in FIG. 1b. As seen in FIG. 2a and
FIG. 2b an example of another alternative embodiment of the
inventive objective portion of an optical system that utilizes
convex mirrors 210 (M1), 215 (M3) and four concave mirrors 212
(M2), 218 (M4), 222 (M5), 224 (M6). An outermost mirror 222 may
have an optional refractive element that covers a central opening,
as discussed above. Mirrors 222 and 224 form a relay group 220. The
relay group 220 may have a magnification, which is approximately
equal to 1. In addition, the relay group 220 may be designed to
introduce a slight negative spherical aberration. In one
implementation, the relay group 220 may have a slight negative
spherical aberration that is be less than twice the spacing between
the top (reflective) surfaces of adjacent mirrors 215 (M3) and 224
(M6). With the resulting increase in the possible substrate
thickness of the convex mirror 215, its aspect ratio can be
decreased to a more manufacturable level. An example of dimensions
for the elements of the objective shown in FIG. 2a and FIG. 2b is
presented in FIG. 2c.
[0051] In the example illustrated in FIG. 2a the imaging is of
infinite conjugate and has a flat Petzval field. As stated earlier,
the aspect ratio of the M3 element may be large due to the fact
that it needs to be placed very close to the external field plane,
i.e., the plane of the sample 205, and thus the mirror element
can't have a thick substrate. One remedy to improve the aspect
ratio is to relay the external field to an intermediate image plane
that has significant negative spherical aberration. An enlargement
of the region of negative spherical aberration is shown (reversed
from FIG. 2a), in FIG. 2b. This negative spherical aberration will
essentially push the zonal image plane 203 from the high angular
cone 207 closer to the reflective surface of convex mirror element
215 (M3). Since the plane of the sample is no longer constrained to
be close to the mirror element due to the added relay, this will
allow us to increase the thickness of the mirror without the need
of a larger opening to accommodate the light from the large NA.
Referring again to FIG. 2a, the addition of an extra two mirror
relay unit 220 having outermost mirror 222 and mirror 224 within
the objective portion of the system provides a way to improve
manufacturability of M3 mirror 215 by decreasing its diameter,
while at the same time allowing adjustment of the Petzval curvature
of the intermediate image plane as follows.
[0052] FIG. 2d shows an example of pupil relay optics 202 that may
be used with an objective with flat Petzval curvature, as in FIG.
2a. The advantages over reflective pupil relay/variable
magnification optics include the avoiding of off-axis and
hard-to-manufacture elements. However, the spectrum bandwidth may
be limited by what is practical for the anti-reflective (AR)
coating. Anti-reflective coatings used with refractive elements are
generally higher efficiency than high-reflective coatings used with
reflective elements.
[0053] FIG. 2e illustrates a second example of an embodiment of the
inventive system, which allows a curved Petzval field. FIG. 2f sets
forth a possible example of dimensions and specifications for an
objective of the type depicted in FIG. 2e. By allowing the curved
Petzval field and using the objective with the refractive pupil
relay as illustrated in FIG. 1c, the size of mirror 210 (M1 and the
central obscuration can be further optimized.
[0054] The inventive optical systems, by utilizing multiple mirrors
for proper aberration distribution, and utilizing aspheric surfaces
to correct spherical aberrations, may be able to produce
diffraction-limited imaging quality with NA greater than 0.7 and a
field of view (FOV) greater than 0.5 mm, more preferably greater
than 0.8 mm, still more preferably greater than 1 mm, while
limiting central obscuration to about 35%. As used herein, the
field of view refers to the size of the part of the sample that is
being imaged. This may be defined as the size of the field for
which the Streh1 ratio is greater than 0.9.
[0055] The manufacturability of the objective, specifically the
aspheric mirrors, can be further improved by shifting some of the
spherical aberration to the refractive optics group and
compensating by making one or more of the refractive lens surfaces
into aspheric surfaces.
[0056] It is not intended that the invention be restricted to the
exact embodiments described herein. Those skilled in the art will
recognize that changes and modifications can be made without
departing from the inventive concept. For example, the objective
may include a front refractive element to seal off a central
opening of the outermost mirror and another refractive module to
provide the color correction.
[0057] By way of example, an optical system of the type depicted in
FIG. 1a may utilize an alternative objective 300 as shown in FIG.
3. The objective 300 includes curved mirrors 310 (M1), 312 (M2),
320 (M4) and a flat outermost mirror 315 (M3). Light enters or
exits the objective via a first refractive lens 302 and an aperture
in mirror 312 (M2), which lies between the first lens 302 and
mirror 310 (M1). Taking the example of incident light traveling
toward a sample 305, after passing through the first lens 302, the
light is reflected from mirror M1 and then reflected by mirror 312
(M2) back through a central opening in mirror 310 (M1). The light
then passes through a second lens 304 that is disposed in a central
opening of mirror M4. After passing through the second lens 304,
the light reflects from the outermost mirror 315 (M3). In this
example, the reflecting surface of mirror 315 (M3) is a flat
reflecting surface that faces away from the sample 305. The light
reflected from mirror 315 (M3) is reflected by mirror 320 (M4)
through a third lens 306 disposed in a central opening of mirror
M3.
[0058] The mirrors 310 (M1), (312) M2, (315) M3, and 320 (M4) may
be enclosed within a housing 309 to keep atmospheric contaminants
out of the optical paths through the objective 300. Contaminants
may be purged from beam paths by a purge gas delivered from a purge
gas source 311. Typically, the purge gas is a dry inert gas with
high purity, such as nitrogen or a noble gas. To protect a sample
from contaminants that might be blown through the opening in the
mirror 315 (M3) by the purge gas, the opening may be closed with a
refractive optical module, such as lens 306. By way of example, but
not bay way of limitation, the lens 306 may be a refractive
spherical shell lens.
[0059] To reduce unwanted reflections, the surfaces of the
refractive optical elements 302, 304, 306 the lenses 302, 304, 306
may be coated with anti-reflection coatings. As discussed above,
the mirrors 310 (M1), 312 (M2), 315 (M3), 320 (M4)and lenses 302,
304, 306 may be configured in such a way that light passing through
the lenses 302, 304, 306 is incident on the surfaces of the lenses
at angles of incidence less than 25 degrees, more preferably, less
than 10 degrees. The mirrors and lenses may be appropriately
configured by a suitable choice of the radii of curvature diameters
and positions of mirrors 310 (M1), 312 (M2), 315 (M3), 320 (M4),
and lenses 302, 304, 306, the size of the central openings in
mirrors 310 (M1), 312 (M2), 315 (M3) and 320 (M4), and the spacing
between mirror 315 (M3) and the sample 305. By way of example, and
not by way of limitation, the refractive element 306 at the
outermost mirror 315 may be curved such that an angle of incidence
of light on an optical surface of the refractive element 306 is
less than about 25 degrees, more preferably less than about 10
degrees if ghost images are not a concern. As used herein, an
optical surface refers to a surface at which light is refracted as
it enters or leaves a lens. Such a low angle of incidence results
in better anti-reflection performance from the optical surfaces of
the refractive module. However, if ghost reflections are a concern,
it may be desirable to increase the radius of curvature of the
front element so ghost images (from the front element back to the
sample) are highly defocused. Furthermore, the front refractive
element 306 may be very close to flat to reduce the sensitivity to
alignment error. Furthermore, a graded coating may be deposited on
a surface of the refractive element to improve AR performance.
[0060] In addition, in some implementations, the second and third
lenses 302 and 304 may implement color aberration correction to
correct color aberration introduced by the first lens 306. For a
broadband imaging system, and/or for an imaging system with
spectrum below 270 nm or below 250 nm, or below 220 nm, the
refractive module 306 may introduce excessive color aberration. To
compensate for the color aberration, one or more additional
refractive modules may be included inside the objective 300.
Alternatively, downstream refractive optics, such as pupil a relay
and zoom as discussed above, may be used to provide the color
correction.
[0061] In some embodiments, the refractive module 306 may be
configured to provide a field curvature that is opposite to the
field curvature generated by the mirrors 310, 312, 315, 320. For
example, if the mirrors produce a negative field curvature, the
refractive optical element 306 may produce a positive field
curvature and vice versa. The opposite field curvature of the
refractive element 306 relaxes the aspect ratio requirements on the
mirrors, particularly outermost mirror 315.
[0062] Furthermore, the refractive element 306 may be configured to
provide an aberration balance for the mirrors 310, 312, 315, 320 to
improve their manufacturability in terms of reduced aspheric
departure and reduced aspect ratio. Specifically, a perfectly
spherical reflecting surface typically exhibits optical
aberrations, such as spherical aberration. Spherical aberration
generally refers to a situation where a lens or mirror has
different focal lengths for light rays incident at different height
from the optical axis. A negative spherical aberration brings the
marginal ray focus closer. To compensate for spherical aberration,
a curved mirror is typically manufactured with a curvature that
departs from being spherical and is more close to being
paraboloidal. By way of example, and not by way of limitation, to
reduce the amount of aspheric departure that is necessary, the
refractive element 306 may have a refracting surface that departs
slightly from spherical curvature in a way that compensates for at
least part of the spherical aberration in one or more of the
mirrors 310, 312, 315, 320.
[0063] In addition, for some implementations it may be desirable
that the refractive elements be relatively flat. Generally
speaking, a refractive element may be said to be flat if it has
front and back refractive surfaces that characterized by an
infinite radius of curvature. As used herein, a refractive element
is said to be "nearly flat" if its front and back refractive
surfaces are characterized by a radius of curvature greater than
500 mm. Flatter elements tend to be easier to manufacture and align
and can be field replaced.
[0064] In some implementations, it may be desirable to make the
refractive module 306 in the outermost mirror 315 field
replaceable. By replaceable, it is meant that the refractive module
306 is designed to be removed from the outermost mirror 315 and
easily installed. To make front refractive element field
replaceable, the alignment sensitivity should be relatively loose
in terms of axial and lateral alignment tolerances. By way of
example, lateral and axial alignment tolerances in the range of a
few microns are suitable for a replaceable refractive module
306.
[0065] Although refractive modules based on curved refractive
elements are shown and described above, embodiments of the present
invention include implementations for which the refractive module
includes a flat or nearly flat refractive element. FIG. 4 depicts
an example of an objective 400 having a flat or nearly flat
refractive element in an outermost mirror. The objective 400
generally includes mirrors 410 (M1), 412 (M2), 415 (M3), and 420
(M4). In this example, mirror 415 (M3) is the outermost mirror,
i.e., the mirror closest to a target 405 when the objective is used
in an imaging system. A flat or nearly flat refractive element 456
covers a central opening in the outermost mirror 415 (M3).
[0066] The objective 400 may include one or more additional
refractive modules other than the refractive element 456. For
example, as depicted in FIG. 4, a second refractive element 406 may
cover a central opening in another of the mirrors, such as mirror
420 (M4).
[0067] There are a number of different configurations for the
outermost mirror 115' in the objective. As seen in the second
example depicted in FIG. 5, an outermost objective mirror element
500 may include a mirror body 502, which may be made of an
optically transparent or an optically opaque material. A surface
501 of the mirror body 502 is coated with a reflective coating 504,
except for an opening through a central portion of the mirror body.
By way of example, the surface 501 may have a curved shape. The
reflective coating 504 is on a surface of the mirror body that
faces away from a sample 505 when the mirror element 500 is
incorporated into an objective. A refractive element 508 is
disposed in the aperture in such a way that the aperture is sealed
against transmission of contaminants. The refractive optical
element 508 may have a different curvature than the portion of the
mirror body surface that is coated with the reflective coating. The
refractive optical element 508 may include one or more concave or
convex refracting surfaces to provide desired optical focusing
properties. Alternatively, the refractive optical element 508 can
include flatter optical elements that have graded optical coating
to improve the AR efficiency.
[0068] In the example depicted in FIG. 5, the mirror body is 502 in
the shape of a disk having a central aperture. The mirror body 502
may have a thickness T at its outer edge. The refractive optical
element 508 may have a thickness t that is less than the thickness
T of the mirror body 502. This allows a thicker mirror body to be
used, thereby simplifying manufacture of the optical element 500.
By way of example, and not by way of limitation, mirror body and
refractive element may be characterized by a thickness ratio T/t a
between 2:1 and 4:1.
[0069] As discussed above, an optical imaging system, e.g., an
imaging system used for defect inspection, may use an objective
coupled with a pupil relay/variable magnification system that uses
refractive optics. Further embodiments of the present invention are
directed to extending the flexibility and adaptability of such
optical systems are described below with respect to FIG. 6 to FIG.
9. The flexibility and adaptability of such an optical system may
be extended by configuring the system so as to provide two or more
imaging paths in the portion of the optical system external to the
objective. This can enable simultaneous inspection, using multiple
wavelength bands, and different modes (i.e., BF vs. DF), and/or
using different pixel sizes. Additionally, two or more illumination
paths may also be provided. One of the common components to such
inventive configurations are independent pupils in the different
imaging paths.
[0070] US Patent Publication No. US 2005/0052643 by Lange et al,
which is hereby incorporated by reference, discloses a surface
inspection method involving illuminating the surface in two optical
regimes including a first wavelength range selected so that the
surface is opaque to the light in the first wavelength range and a
second wavelength range selected so that the surface is at least
partially transmissive to light in the second wavelength range.
Lange's method mentions separate illumination sources and multiple
detection subsystems including multiple magnifications and
brightfield vs. darkfield detection. Certain embodiments of the
present invention provide improvements to Lange's method that
enable simultaneous inspection with multiple illumination paths,
multiple imaging paths, multiple wavelength bands, multiple
magnifications, multiple modes, and combinations thereof using a
common imaging objective.
[0071] Another advantage of splitting image collection from a
single, ultra-broad-band objective into two image collection bands
is that it enables the use of a refractive pupil relay and variable
magnification optics. Advantages of using a refractive pupil relay
in preference to a pupil relay composed of reflective elements are
described above and in U.S. Provisional Application No. 60/997,306.
These advantages include the higher efficiency of AR coatings used
for refractive elements as compared to the HR coatings used for
reflective elements. However, use of AR coatings for a broad
wavelength spectrum such as 193 nm-450 nm wavelength, i.e., ranging
from VDUV through DUV through UV, presents some serious challenges.
There is a fairly wide range of AR materials which work well, i.e.,
at high efficiency and with low absorption, at the higher
wavelength end of this range, i.e., approximately 250-450 nm
wavelength, but do not function well at smaller wavelengths of the
DUV range, i.e., they begin to have high absorption at shorter
wavelengths. There is only a limited number of DUV AR materials
that function well at the lower wavelength end of the range, i.e.,
approximately 193-250 nm wavelengths. A description of various
coatings and their properties can be found in Materials for Optical
Coatings in the Ultraviolet, F. Rainer, W. Howard Lowdermilk, D.
Milam, C. K. Carniglia, T. T. Hart, and T. L. Lichtenstein, Applied
Optics, Vol. 24, No. 4, 15 Feb. 1985, pg. 496 ff, the contents of
which are incorporated herein by reference. Though these materials
can function at the higher wavelengths as well as at the lower
wavelengths, they are not the preferred materials to use for higher
wavelengths for the following reasons: AR coatings are generally
formed as a mixture of low-index and high-index layers, as
described in Warren J. Smith "Modern Optical Engineering", 3rd
Edition, McGraw-Hill, Inc., Chapter 7: Optical Materials and
interference Coatings. Among the materials used for the short
wavelength DUV coatings, the high index materials, such as
Al.sub.2O.sub.3 do not have as high an index as desired, i.e., the
index spread is not as great between the low and high index
materials. This requires more layers for the AR coating, and is
harder to design. Additionally, the coating process for the DUV AR
materials suitable for the short wavelength end of the DUV range is
not as robust as the coating process for the materials used at the
higher wavelength end of the range.
[0072] For the above reasons, limiting DUV AR materials to a
narrower bandwidth than the entire 193-450 nm range can decrease
the associated design difficulties.
[0073] An aspect of the following embodiments of the present
invention is enabling simultaneous inspection of two wavelength
bands. By way of example, and not by way of limitation, these two
wavelength bands may include a first band containing wavelengths
less than 250 nm and a second wavelength band from about 250 nm to
approximately 450 nm, up to the visible range. In this way, the
optimal AR materials may be used for each wavelength sub-range,
while maintaining high throughput by means of the simultaneous
inspection. This division of wavelength bands has a fundamentally
different motivation than the division of wavelength bands
described by Lange in Publication No. US 2005/0052643. The division
detailed by Lange is at approximately 350 nm, in the region where
the division between opacity and transmissivity tends to occur for
commonly used materials in semiconductor fabrication such as
polysilicon and high-K dielectrics.
[0074] FIG. 6 illustrates an exemplary embodiment of the portion of
an optical system external to the objective portion, which is
configured for simultaneous inspection over two wavelength bands.
It is noted that the particular details of the configuration may be
modified. Objective 600 may be a catoptric objective as in any of
the embodiments described above, e.g., with respect to FIG. 1b,
FIG. 2b, FIG. 2e, FIG. 3, and FIG. 4 and previously incorporated
U.S. provisional patent application No. 60/997,306. Outgoing light
605 from objective 600 enters an imaging system 610 that includes
two or more pupil relay/variable magnification modules. A 50/50
beam splitter 615 reflects light with wavelength shorter than a
predetermined value, 250 nm for example, and transmits light with
wavelength longer than the predetermined value. The beam splitter
615 acts as a long-pass filter, i.e. HT (High Transmission) for DUV
and visible light, HR (high reflectance) for VDUV. The outgoing
light 605 collected by the objective 600 is therefore routed into
two paths; path 620 for the shorter wavelength light, path 625 for
the longer wavelength light. Each path includes: a) pupil relays
(630 for path 620, 630' for path 625), which may be comprised of
refractive elements and be configured as described above and as
disclosed in U.S. patent application No. 60/997,306. The refractive
elements used in pupil relay 630 employ VDUV AR coatings suitable
for the 190-250 nm (or 190-270 nm) wavelength band, the refractive
elements used in pupil relay 630' employ DUV/UV AR coatings
suitable for the 250-450 nm (or 270-450 nm) wavelength band; b)
pupils 632 and 632'; c) zoom/variable magnification modules (635
for path 620, 635' for path 625). The AR coatings for the
zoom/variable magnification refractive elements are employed for
paths 620 and 625 similarly to the pupil relay elements for the two
paths. The two paths re-converge at point 640 and may continue to a
shared module 642, which may be a long range optical trombone
module to provide optical path adjustment for a large magnification
range. As used herein, an optical trombone (or sometimes simply a
trombone) refers to an arrangement of two or more optically
reflecting surfaces in which displacement of one or more of the
reflecting surfaces along a direction parallel to an optical path
varies a length of the optical path. An example of a long range
trombone is described in U.S. Pat. No. 6,801,357, issued Oct. 5,
2004, the entire contents of which are incorporated herein by
reference. An additional trombone module 645 may be inserted, using
HR mirrors 650, into one of the paths to account for the path
length difference between the two paths. In the example depicted in
FIG. 6, mirror 652 is two-sided HR: one side for VUV, the other
side for DUV/VIS. However, this should not be construed as a
limitation on all embodiments of the invention. Independent zoom
modules 635 and 635' provide fine zoom control. Long-pass filter
655 provides for the re-convergence of the two paths. The
reconverged paths continue through the shared module 642 and are
directed onto sensors 660. In a preferred embodiment, sensors 660
are TDI (Time Delayed Integration) sensors. A description of TDI
can be found at www.learn.hamamatsu.com/tutorials/tdiscan.
[0075] When two distinct wavelength bands are used, the field on
the sample can be slightly different for the two wavelengths for
more efficient usage of the objective FOV. This results in a slight
offset between the images of the two wavelengths as the signal is
transmitted to the sensors, which can enable the use of different
sensors for the different wavelength bands. (FIG. 7 illustrates how
the offset can be adjusted, by translating mirrors 700 and/or 705
along the optical axis to generate a lateral shift in y. This
allows the sensors to be chosen to optimize the efficiency,
according to the specific configurations of the imaging paths. This
will be illustrated hereinafter in FIG. 9 which lists some
exemplary sensor utilization options.
[0076] An aspect of these particular embodiments of the present
invention is enabling the collection of simultaneous inspection
with bright field (BF) and dark field (DF) images in different
spectrums. BF modes collect specularly reflected light, whereas DF
modes detect scattered light, but not specularly reflected light.
In DF detection, therefore, specularly reflected light is blocked
out. In many cases for DF modes, diffraction lobes from light
scattered by regular patterns on the sample are also blocked out
using opaque structures (called pupil filters) placed in the image
pupil. By way of example and not by way of limitation, the pupil
filters may include one or more Fourier filters.
[0077] Consequently, for DF inspection, the accessible pupil, where
the blocking takes place, cannot be in a common path with
illumination, in order to avoid obscuration of the illuminating
light by the pupil filtering. This is not the case for BF
inspection, where specularly reflected light is not blocked at the
pupil. If the pupil relay elements are refractive rather than
reflective, as described above and in U.S. provisional patent
application No. 60/997,306, the pupil relay can be in a common path
with the illumination, in contrast to the pupil itself in the DF
case. Due to the required blocking present in the accessible pupil
for DF, in order to have two simultaneous different imaging modes
such as BF and DF, two independent pupils are required. The
associated pupil relays may be shared or independent, depending on
the other considerations and constraints for the system. For
example, in order to split the output of a single pupil relay into
multiple paths containing multiple pupils, there would need to be
sufficient distance between the last optical element of the pupil
relay and the pupils for the mechanical packaging of the folding
mirror, B.S., pupil mechanism, etc.
[0078] Another aspect of the embodiments of the present invention
described with respect to FIG. 6 to FIG. 9 is the incorporation of
dual variable magnification modules so as to enable use of both
high resolution wafer pixels, i.e., about 40 nm, and high
throughput wafer pixels, i.e., about 250 nm, so as to image onto
sensor pixels. By way of example and not by way of limitation, the
sensor pixels may be may be about 18 microns (.mu.m) in
dimension.
[0079] A further aspect of the embodiments of the present invention
described with respect to FIG. 6 to FIG. 9 is the capability of
incorporating a plurality of illumination paths in addition to the
plurality of imaging paths. This may be beneficial in that the
spectrum, output power, and configuration can be independently
chosen and adjusted for the different bands. Different illumination
settings could also be used. The illumination configuration
includes the numerical aperture, the illumination aperture, the
polarization, the light power, and the illumination location on the
sample.
[0080] An example of a configuration employing dual illumination
paths is illustrated in FIG. 8.
[0081] The configuration in FIG. 8 is similar to that depicted in
FIG. 6, except that separate illuminator modules 800 and 805 can be
employed in the two paths, and separate illuminators could be
utilized in place of single source 810. The rest of the components
of the configuration depicted in FIG. 8 operate as described above
with respect to FIG. 6.
[0082] An aspect of the present invention is the flexibility of
designing an illumination/imaging configuration which enables
simultaneous inspection with choices of several of the
above-mentioned parameters. FIG. 9 illustrates some examples of
sensor utilization configurations, and how they might be utilized
with different wavelength spectra and modes. Top row 905
illustrates configurations for the DUV/Vis spectrum; bottom row 910
illustrates configurations for the VDUV band. The magnification
setting and range of the zoom can be different between these two
paths.
[0083] The embodiments of the present invention described above
with respect to FIG. 6 to FIG. 9 extend the flexibility and
adaptability of current optical systems by providing two or more
imaging paths having separate, independent pupils, in the portion
of the optical system external to the objective. This can enable
inspection, which may be simultaneous inspection, using multiple
wavelength bands, using different modes (e.g., BF vs. DF), and/or
using different magnifications to map into different pixel sizes.
In general, modes could be independently set in both imaging paths
by constructing apertures in both illumination and collection
pupils of each imaging path (band). These apertures control the
collection of angles used to illuminate and collect image data in
such a way as to optimize or vary the defect contrast, patter
contrast and scattered light "noise" from the sample. Additionally,
two or more illumination paths may also be provided. Furthermore,
we propose that the system can be configured so that the pixel size
of the two imaging paths can be adjusted independently and the line
data (rate at which TDI array lines are digitized) of two TDI
sensors can be set differently, so that
(pixel size.times.line rate).sub.path1=(pixel size.times.line
rate).sub.path2=stage speed
[0084] Larger pixel size (at the sample) can be achieved by either
changing the magnification of one of the imaging path and using
identical sensors or by keeping magnification constant for both
paths and employing a sensor with larger pixel area in one of the
two paths. Thus, the system allows simultaneous two-mode inspection
with different pixel sizes. For example, to optimize inspection
sensitivity, light-budget and throughput BF inspection may be done
with smaller pixel size, while DF inspection may be done with
larger pixel size. Furthermore, this strategy can be applied to
other types of imaging systems such as spot scanning systems.
[0085] Those skilled in the art will recognize that modifications
can be made to the exact embodiments disclosed herein without
departing from the inventive concept. For example, other types of
objectives, and other types of sensors may be utilized. The scope
of the invention should be construed in view of the claims.
[0086] While the above is a complete description of the preferred
embodiment of the present invention, it is possible to use various
alternatives, modifications, and equivalents. Therefore, the scope
of the present invention should be determined not with reference to
the above description but should, instead, be determined with
reference to the appended claims, along with their full scope of
equivalents. Any feature, whether preferred or not, may be combined
with any other feature, whether preferred or not. In the claims
that follow, the indefinite article "A", or "An" refers to a
quantity of one or more of the item following the article, except
where expressly stated otherwise. The appended claims are not to be
interpreted as including means-plus-function limitations, unless
such a limitation is explicitly recited in a given claim using the
phrase "means for."
* * * * *
References