U.S. patent application number 15/795089 was filed with the patent office on 2019-01-03 for methods and apparatus for polarizing reticle inspection.
This patent application is currently assigned to KLA-Tencor Corporation. The applicant listed for this patent is KLA-Tencor Corporation. Invention is credited to Haifeng Huang, Amrish Kelkar, Damon F. Kvamme, Rui-Fang Shi.
Application Number | 20190003960 15/795089 |
Document ID | / |
Family ID | 64736637 |
Filed Date | 2019-01-03 |
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United States Patent
Application |
20190003960 |
Kind Code |
A1 |
Huang; Haifeng ; et
al. |
January 3, 2019 |
METHODS AND APPARATUS FOR POLARIZING RETICLE INSPECTION
Abstract
Disclosed are methods and apparatus for measuring and
controlling polarization for inspection of a semiconductor sample.
The method includes (i) setting up an inspection system in a
specific mode of operation, (ii) incrementing a first waveplate of
the system through a plurality of rotations while keeping a second
waveplate of the system static, (iii) measuring an intensity signal
from non-patterned areas of the sample for each rotation of the
first waveplate, (iv) incrementing the second waveplate through a
plurality of rotations while keeping the first waveplate static (v)
measuring an intensity signal from non-patterned areas of the
sample for each rotation of the second waveplate, (vi) generating a
model of a plurality of polarization and waveplate parameters for
the system to simulate the intensity signals that were measured for
each rotation of the first and/or second waveplate, and (vii)
determining the polarization and waveplate parameters for the
system based on the model and a polarization state on photomask
plane based on the polarization and waveplate parameters.
Inventors: |
Huang; Haifeng; (Livermore,
CA) ; Shi; Rui-Fang; (Cupertino, CA) ; Kvamme;
Damon F.; (Los Gatos, CA) ; Kelkar; Amrish;
(San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KLA-Tencor Corporation |
Milpitas |
CA |
US |
|
|
Assignee: |
KLA-Tencor Corporation
Milpitas
CA
|
Family ID: |
64736637 |
Appl. No.: |
15/795089 |
Filed: |
October 26, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62528038 |
Jul 1, 2017 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2201/0683 20130101;
G01N 21/956 20130101; G01N 2021/95676 20130101; G01N 21/21
20130101 |
International
Class: |
G01N 21/21 20060101
G01N021/21; G01N 21/956 20060101 G01N021/956 |
Claims
1. A system for controlling and measuring polarization for
inspection of a sample, comprising: an illumination optics
subsystem for generating and directing illumination light towards a
sample, wherein the illumination optics subsystem includes two or
more illumination polarization components for controlling the
polarization state of the illumination light; a collection optics
subsystem for collecting output light from non-patterned areas of
the sample in response to the illumination light, wherein the
collection optics subsystem comprises at least a first and second
collection polarization component for measuring polarization state
of the illumination light at or near the sample and a sensor for
detecting the output light after the polarization state is adjusted
by the first and second collection polarization components; and a
controller that is configured to perform the following operations:
setting up the system in a specific mode of operation; incrementing
the first collection polarization component through a plurality of
rotations while keeping the second collection polarization
component static; measuring an intensity signal by the sensor for
each rotation of the first collection polarization component;
incrementing the second collection polarization component through a
plurality of rotations while keeping the first collection
polarization component static; measuring an intensity signal by the
sensor for each rotation of the second collection polarization
component; generating a model of a plurality of polarization state
and polarization component parameters for the system to simulate
the intensity signals that were measured for each rotation of the
first and/or second collection polarization component; and
determining the polarization state and polarization component
parameters for the system based on the model.
2. The system of claim 1, wherein: the sample is a photomask, the
illumination optics subsystem includes one or more light sources
for generating illumination light that is transmitted through the
sample in a transmitted light (TL) mode, the at least first and
second collection polarization components comprise a first and
second waveplate, the system is set up in the TL mode, a model is
first generated based on Jones calculus and used to determine a
retardance value for the first waveplate and the polarization state
between the first waveplate and the second waveplate based on
measuring for each rotation of such first waveplate, and a model is
next used to determine a retardance of the second waveplate and the
polarization state at or near the sample based on measuring for
each rotation of such second waveplate and the determined
retardance of the first waveplate and its static rotation
position.
3. The system of claim 2, wherein the first waveplate is a
rotatable 1/4 waveplate and the second waveplate is a rotatable 3/8
waveplate, wherein the 3/8 waveplate is positioned to receive the
output light from the sample, after which the 1/4 waveplate
receives the output light from the 3/8 waveplate.
4. The system of claim 3, wherein the illumination optics subsystem
includes one or more light sources for generating illumination
light that is reflected from the sample in a reflected light (RL)
mode, wherein the collection optics subsystem includes a polarizing
beam splitter (PBS) for receiving the output light from the 1/4
waveplate and transmitting only a y-polarization component of the
output light towards the sensor, wherein the PBS is arranged and
configured to reflect only an x-polarization component of
illumination light in a reflecting light (RL) mode towards the
sample.
5. The system of claim 4, wherein the polarization state of the
illumination light at or near the sample is determined as a
function of field position.
6. The system of claim 4, wherein the first and second waveplates
are the same as at least some of the illumination polarization
components, and wherein the controller is further configured for
performing the following operations: setting up the system in RL
mode; in the RL mode, incrementing the first waveplate through a
plurality of rotations while keeping the second waveplate static;
in the RL mode, measuring an intensity signal by the sensor for
each rotation of the first waveplate; in the RL mode, generating a
model based on Jones calculus and of a plurality of polarization
and waveplate parameters, including the retardance of the first and
second waveplates, for the system to simulate the intensity signals
that were measured in the RL mode for each rotation of the first
waveplate; and in the RL mode, determining illumination
polarization state at or near the sample based on the retardance of
the first and second waveplates that was determined in the RL mode
and based on the rotation of the first waveplate.
7. The system of claim 6, wherein the controller is further
configured for, in the RL mode, modeling a sinusoidal behavior of
the retardance of the second waveplate as a function of rotation
and separating a real retardance of the second waveplate from a
retardance of an objective of the system based on such modelled
sinusoidal behavior.
8. The system of claim 1, wherein the at least first and second
collection polarization component comprise a plurality of
waveplates having a retardance sum equal to or greater than a half
wave.
9. The system of claim 1, wherein the collection optics subsystem
includes multiple detection planes for detecting output light, and
the controller is further configured to model and determine the
polarization state and polarization component parameters for
measurements at the multiple detection planes and average the
determined polarization state and polarization component parameters
from such multiple detection planes.
10. The system of claim 1, wherein the model is based on Jones
calculus and uses a nonlinear least squares fit process to
determine the polarization state and polarization component
parameters.
11. The system of claim 1, wherein the controller is further
configured for simulating a plurality of polarization states at or
near the sample based on the determined polarization state and
polarization component parameters and selecting a rotation for the
first and second collection polarization components to result in a
particular orientation of linear polarization at the sample.
12. A method of controlling and measuring polarization for
inspection of a sample, comprising setting up an inspection system
in a specific mode of operation; incrementing a first polarization
component of the system through a plurality of rotations while
keeping a second polarization component of the system static;
measuring an intensity signal from non-patterned areas of the
sample for each rotation of the first polarization component;
incrementing the second polarization component through a plurality
of rotations while keeping the first polarization component static;
measuring an intensity signal from non-patterned areas of the
sample for each rotation of the second polarization component;
generating a model of a plurality of polarization state and
polarization component parameters for the system to simulate the
intensity signals that were measured for each rotation of the first
and/or second polarization component; and determining the
polarization state and polarization component parameters for the
system based on the model.
13. The method of claim 12, wherein: the system is set up in a
transmitted light (TL) mode, the first and second polarization
components are a first and second waveplate, the sample is a
photomask, a model is first generated and used to determine a
retardance value for the first waveplate and the polarization state
between the first and second waveplate based on measuring for each
rotation of such first waveplate, and a model is next used to
determine a retardance of the second waveplate and the illumination
polarization state at or near the sample based on measuring for
each rotation of such second waveplate and the determined
retardance of the first waveplate and its static rotation
position.
14. The method of claim 13, wherein the first waveplate is a
rotatable 1/4 waveplate and the second waveplate is a rotatable 3/8
waveplate, wherein the 3/8 waveplate is positioned to receive the
output light from the sample, after which the 1/4 waveplate
receives the output light from the 3/8 waveplate.
15. The method of claim 14, wherein only a y polarization component
of the output light is measured, wherein only an x polarization
component of illumination light in RL mode towards the sample.
16. The method of claim 15, wherein the illumination polarization
state at or near the sample is determined as a function of field
position.
17. The method of claim 15, further comprising: setting up the
system in RL mode; in the RL mode, incrementing the first waveplate
through a plurality of rotations while keeping the second waveplate
static; in the RL mode, measuring an intensity signal for each
rotation of the first waveplate; in the RL mode, generating a model
of a plurality of polarization state and waveplate parameters,
including the retardance of the first and second waveplates, based
on Jones calculus to simulate the intensity signals that were
measured in the RL mode for each rotation of the first waveplate;
and in the RL mode, determining illumination polarization state at
or near the sample based on the retardance of the first and second
waveplates that was determined in the RL mode and based on the
rotation of the first waveplate.
18. The method of claim 17, further comprising, in the RL mode,
modeling a sinusoidal behavior of the retardance of the second
waveplate as a function of rotation and separating a real
retardance of the second waveplate from a retardance of an
objective of the system based on such modelled sinusoidal
behavior.
19. The method of claim 12, wherein the model is based on Jones
calculus and uses a nonlinear least squares fit process to
determine the polarization state and polarization component
parameters.
20. The method of claim 12, further comprising simulating a
plurality of polarization states at or near the sample based on the
determined polarization state and polarization component parameters
and selecting a rotation for the first and second collection
polarization components to result in a particular orientation of
linear polarization at the sample.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Patent
Application No. 62/528,038, filed 1 Jul. 2017, which application is
incorporated herein by reference in its entirety for all
purposes.
TECHNICAL FIELD OF THE INVENTION
[0002] The invention generally relates to the field of reticle or
photomask inspection systems. More particularly the present
invention relates to techniques for management of polarization for
defect detection.
BACKGROUND
[0003] Generally, the industry of semiconductor manufacturing
involves highly complex techniques for fabricating integrated
circuits (IC) using semiconductor materials which are layered and
patterned onto a substrate, such as silicon. Due to the large scale
of circuit integration and the decreasing size of IC features in
semiconductor devices, fabricated devices have become increasingly
sensitive to defects. That is, defects which cause faults in
devices are becoming increasingly smaller. Devices need to be
generally fault free prior to shipment to the end users or
customers.
[0004] Various inspection systems are used within the semiconductor
industry to detect defects on a semiconductor reticle or wafer.
However, there is a continuing demand for improved semiconductor
reticle and wafer inspection systems and techniques.
SUMMARY
[0005] The following presents a simplified summary of the
disclosure in order to provide a basic understanding of certain
embodiments of the invention. This summary is not an extensive
overview of the disclosure and it does not identify key/critical
elements of the invention or delineate the scope of the invention.
Its sole purpose is to present some concepts disclosed herein in a
simplified form as a prelude to the more detailed description that
is presented later.
[0006] An inspection system for controlling and measuring
polarization for inspection of a photomask sample is disclosed. The
system generally includes an illumination optics subsystem for
generating and directing illumination light towards a sample,
wherein the illumination optics subsystem includes two or more
illumination polarization components for controlling the
polarization state of the illumination light. The system also
includes a collection optics subsystem for collecting output light
from non-patterned areas of the sample in response to the
illumination light. The collection optics subsystem comprises at
least a first and second collection polarization component for
measuring polarization state of the illumination light at or near
the sample and a sensor for detecting the output light after the
polarization state is adjusted by the first and second collection
polarization components. The system further comprises a controller
that is configured to perform the following operations: (i) setting
up the system in a specific mode of operation, (ii) incrementing
the first collection polarization component through a plurality of
rotations while keeping the second collection polarization
component static, (iii) measuring an intensity signal by the sensor
for each rotation of the first collection polarization component,
(iv) incrementing the second collection polarization component
through a plurality of rotations while keeping the first collection
polarization component static, (v) measuring an intensity signal by
the sensor for each rotation of the second collection polarization
component, (vi) generating a model of a plurality of polarization
state and polarization component parameters for the system to
simulate the intensity signals that were measured for each rotation
of the first and/or second collection polarization component, and
(vii) determining the polarization state and polarization component
parameters for the system based on the model. In one aspect, the
model is based on Jones calculus and uses a nonlinear least squares
fit process to determine the polarization state and polarization
component parameters.
[0007] In a specific implementation, the illumination optics
subsystem includes one or more light sources for generating
illumination light that is transmitted through the sample in a
transmitted light (TL) mode. In one implementation, the at least
first and second collection polarization components comprise a
first and second waveplate, and the system is set up in the TL
mode. In one aspect, a model is first generated based on Jones
calculus and used to determine a retardance value for the first
waveplate and the polarization state between the first waveplate
and the second waveplate based on measuring for each rotation of
the first waveplate and a model is next used to determine a
retardance of the second waveplate and the polarization state at or
near the sample based on measuring for each rotation of such second
waveplate and the determined retardance of the first waveplate and
its static rotation position. In a further aspect, the first
waveplate is a rotatable 1/4 waveplate and the second waveplate is
a rotatable 3/8 waveplate, and the 3/8 waveplate is positioned to
receive the output light from the non-patterned areas of the
sample, after which the 1/4 waveplate receives the output light
from the 3/8 waveplate. Other waveplate combinations having a
retardance sum equal to or greater than a half wave can be
used.
[0008] In yet a further implementation aspect, the illumination
optics subsystem includes one or more light sources for generating
illumination light that is reflected from the sample in a reflected
light (RL) mode, and the collection optics subsystem includes a
polarizing beam splitter (PBS) for receiving the output light from
the 1/4 waveplate and transmitting only a y-polarization component
of the output light towards the sensor, and the PBS is arranged and
configured to reflect only an x-polarization component of
illumination light in RL mode towards the sample. In a specific
aspect, the polarization state of the illumination light at or near
the sample is determined as a function of field position.
[0009] In another embodiment, the first and second waveplates are
the same as at least some of the illumination polarization
components, and the controller is further configured for setting up
the system in RL mode and performing the following operations: (i)
incrementing the first waveplate through a plurality of rotations
while keeping the second waveplate static, (ii) measuring an
intensity signal by the sensor for each rotation of the first
waveplate, (iii) generating a model based on Jones calculus and of
a plurality of polarization and waveplate parameters, including the
retardance of the first and second waveplates, for the system to
simulate the intensity signals that were measured for each rotation
of the first waveplate, and (iv) determining illumination
polarization state at or near the sample based on the determined
retardance of the first and second waveplates and the rotation of
the first waveplate. In a further aspect, the controller is
configured for, modeling a sinusoidal behavior of the retardance of
the second waveplate as a function of rotation and separating a
real retardance of the second waveplate from a retardance of an
objective of the system based on such modelled sinusoidal
behavior.
[0010] In an alternative embodiment, the at least first and second
collection polarization component comprise a plurality of
waveplates having a retardance sum equal to or greater than a half
wave. In another example, the collection optics subsystem includes
multiple detection planes for detecting output light, and the
controller is further configured to model and determine the
polarization state and polarization component parameters for
measurements at the multiple detection planes and average the
determined polarization state and polarization component parameters
from such multiple detection planes. In another aspect, the
controller is further configured for simulating a plurality of
polarization states at or near the sample based on the determined
polarization state and polarization component parameters and
selecting a rotation for the first and second collection
polarization components to result in a particular orientation of
linear polarization at the sample.
[0011] In another embodiment, the invention pertains to a method of
controlling polarization for inspection of a semiconductor sample.
The method includes (i) setting up an inspection system in a
specific mode of operation, (ii) incrementing a first polarization
component of the system through a plurality of rotations while
keeping a second polarization component of the system static, (iii)
measuring an intensity signal from non-patterned areas of the
sample for each rotation of the first polarization component, (iv)
incrementing the second polarization component through a plurality
of rotations while keeping the first polarization component static
(v) measuring an intensity signal from non-patterned areas of the
sample for each rotation of the second polarization component, (vi)
generating a model of a plurality of polarization state and
polarization component parameters for the system to simulate the
intensity signals that were measured for each rotation of the first
and/or second polarization component, and (vii) determining the
polarization state and polarization component parameters for the
system based on the model. In further aspect, the method includes
operations that are similar to one or more of the controller
operations outlined above.
[0012] These and other aspects of the invention are described
further below with reference to the figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a diagrammatic representation of a
multiple-waveplate cascade design for controlling and measuring
polarization for both reflected and transmitted illumination modes
in accordance with one embodiment of the present invention.
[0014] FIG. 2 show two graphs of TL (transmitted light) mode
detected signal levels as a function of rotation of two different
waveplates in accordance with one implementation of the present
invention.
[0015] FIG. 3 shows one example of RL (reflected light) mode
polarization measurement results from the same tool that is used
for the TL mode results in accordance with one implementation of
the present invention.
[0016] FIG. 4 displays one example of the fitted effective
waveplate retardance vs. orientation positions (in motor counts) in
accordance with one implementation of the present invention.
[0017] FIG. 5 is a flow chart illustrating a procedure for managing
polarization in accordance with one embodiment of the present
invention.
[0018] FIGS. 6A and 6B show simulated maps of ellipticity and
ellipse orientation angle, respectively, on the mask plane when
both waveplates are rotated within a range of 180.degree., with
typical values for waveplate and objective retardance, in
accordance with one embodiment of the present invention.
[0019] FIGS. 7A and 7B show both simulation and experimental
verification results of a particular category of defect on an EUV
(extreme ultra-violet) mask as a function of orientation angle and
focus in accordance with one embodiment of the present
invention.
[0020] FIG. 8 is a diagrammatic representation of a system, which
includes both TL and RL side waveplates, in accordance with one
embodiment of the present invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0021] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
present invention. The present invention may be practiced without
some or all of these specific details. In other instances, well
known component or process operations have not been described in
detail to not unnecessarily obscure the present invention. While
the invention will be described in conjunction with these specific
embodiments, it will be understood that it is not intended to limit
the invention to these embodiments.
[0022] Certain inspection system embodiments are described herein
as being configured for inspecting semiconductor structures,
specifically photolithography reticles. Other types of structures,
such as semiconductor wafers, solar panel structures, optical
disks, etc., may also be inspected or imaged using the inspection
apparatus of the present invention.
[0023] Certain imaging inspection systems include configurable
polarization control and measurements systems. For instance,
polarization state control of 193 nm illumination light is one of
the many useful tuning knobs that are available in RAPID 6xx
photomask defect inspection tools by KLA-Tencor from Milpitas,
Calif. Since the inspection sensitivity of certain types of reticle
defects strongly depend on the polarization state of 193 nm
illumination light, a polarization feature and its control has
become increasingly important for improving inspection results for
reticle defect detection and maintaining tool-to-tool matching. The
importance of a polarization management system is especially
evident for extreme ultra violet (EUV) photomasks on which the
pattern sizes are typically much smaller than the wavelengths used
in such systems, resulting in strong dependence of both the base
pattern contrast and defect signal strength on the polarization
state of the light.
[0024] Certain embodiments of the present invention provide
polarization systems and techniques for measuring and controlling,
both in situ and quantitatively, the polarization state of
illumination light (e.g., 193 nm) on the photomask plane by use of
certain inspection systems' available waveplate (WP, retarder)
cascade module, which contains multiple waveplates. The techniques
described herein include both polarization measurement and control
and do not require an additional hardware change/replacement for
systems having multiple waveplates, such as on the RAPID 6xx tool
of KLA-Tencor. Certain techniques described herein provide detailed
information of the illumination light polarization state, for
example, in both inspection simulation and inspection data
modeling, which also facilitates the quantitative assessment of
inspection tool performance.
[0025] Certain techniques described herein provide measurements of
the polarization states of illumination light (e.g., 193 nm) on the
mask plane under both transmitted light (TL) and reflected light
(RL). The polarization state is described by two parameters
ellipticity and ellipse orientation angle. As a byproduct, these
techniques can also provide accurate measurements of the phase
retardance and slow axis orientation angles of waveplates. Based on
these results, various polarization states, e.g. linear
polarization with a specific orientation angle, can be predicted
and produced by adjusting the orientation angles of the waveplates
in the cascade design, by way of example.
[0026] Certain embodiments of the present invention provide
compensation for system errors in a multiple-waveplate cascade
design so as to achieve accurate polarization states, e.g. accurate
circular polarization. In addition to compensating for
imperfections in the waveplates (e.g., have retardance errors),
compensation for other system errors can also be applied. For
instance, certain techniques described herein can also provide
measurements and compensation for the small residual birefringence
of optics and finite extinction ratio of the polarizing beam
splitter (PBS) in the image path in KLA-Tencor RAPID 6xx tools or
the like.
[0027] In certain embodiments, polarization may be measured and
controlled for both reflected and transmitted illumination light
although either mode may be utilized. FIG. 1 is a diagrammatic
representation of a multiple-waveplate cascade design for
controlling and measuring polarization for both reflected and
transmitted illumination modes in accordance with one embodiment of
the present invention. As shown, direction z corresponds to a
direction in which the TL illumination beams are traveling. In this
implementation, the illumination beams have a wavelength of about
193 nm. The illustrated xyz coordinate system is right-handed, and
all the angles are defined herein in this xyz coordinate although
other coordinate systems may be used.
[0028] In the illustrated example, the illumination light can take
two forms: reflected light (RL) input 101a or transmitted light
(TL) input) 101b. In this example, the system 100 is configured to
operate in either a TL or RL mode, but not at the same time. That
is, RL or TL is performed one at a time.
[0029] The TL input 101b passes through the reticle (or other)
sample 102, through objective 103, waveplates WP2 104a and WP1
104b, relay lens 106, and polarizing beam splitter (PBS) 108
towards a sensor (not shown). For the TL mode, the PBS is
configured to pass y-polarization of such TL. In contrast, the PBS
108 is configured to reflect the x-polarization component of such
TL light away from the sensor (not shown).
[0030] In contrast, RL input is coupled in through the PBS and
travels against the z-axis, first passing the optics in a reversed
sequence as compared to TL mode. More specifically, RL input 101a
is reflected from PBS 108 at a right angle, through the relay lens
106, WP1 104b, WP2 104a, and objective lens 103, and reflected from
the sample 102. The reflected output travels along the z-axis, in a
same sequence as the TL mode, through objective 103, waveplates WP2
104a and WP1 104b, relay lens 106, and polarizer beam splitter 108
towards a sensor (not shown). For RL mode, the PBS 108 reflects the
incident x-polarization RL towards the sample 102. Upon returning,
the PBS 108 transmits the y-polarization component of the RL output
towards the sensor (not shown), while reflecting x-polarization
away from the sensor (not shown). Thus, the PBS 108 is configured
to pass only y-polarization output beams to the sensor.
[0031] A waveplate or retarder is an optical device that alters the
polarization state of a light wave travelling through it. Two
common types of waveplates are the half-wave plate, which rotates
the polarization direction of linearly polarized light, and the
quarter-wave plate, which converts linearly polarized light into
circularly polarized light with correct relative orientation angle
between linear polarization and the waveplate slow axis, and vice
versa. A quarter-wave plate can be used to produce elliptical
polarization as well.
[0032] Waveplates are constructed out of a birefringent material
(such as quartz or mica), for which the index of refraction is
different for different orientations of light passing through it.
The behavior of a waveplate (that is, whether it is a half-wave
plate, a quarter-wave plate, etc.) depends on the thickness of the
crystal, the wavelength of light, and the variation of the index of
refraction. By appropriate choice of the relationship between these
parameters, it is possible to introduce a controlled phase shift
between the two polarization components of a light wave, thereby
altering its polarization
[0033] A waveplate works by shifting the phase between two
perpendicular polarization components of the light wave. A typical
waveplate is simply a birefringent crystal with a carefully chosen
orientation and thickness. The crystal is cut into a plate, with
the orientation of the cut chosen so that the optic axis of the
crystal is parallel to the surfaces of the plate. For a light wave
normally incident upon the plate, the polarization component
perpendicular to the optic axis travels through the crystal with a
speed v.sub.o=c/n.sub.o, while the polarization component along the
optic axis travels with a speed v.sub.e=c/n.sub.e with different
refractive index. This leads to a phase difference between the two
components as they exit the crystal. When n.sub.e<n.sub.o, as in
calcite, the optic axis is called the fast axis and the
perpendicular axis is called the slow axis. For n.sub.e>n.sub.o
the situation is reversed. Although reference is made to a "slow
axis" below, of course, these techniques can be used for the fast
axis of the waveplate if a 90.degree. shift is included.
[0034] In this example, WP1 is nominally a 1/4 waveplate, while WP2
is nominally a 3/8 waveplate. WP1 is also referred to herein as WP1
(.lamda./4), while WP2 is referred to as WP2(3.lamda./8).
WP2(3.lamda./8) is closer to the objective than WP1(.lamda./4).
Although the illustrated system 100 includes two waveplates, more
than two waveplates may be implemented. Other waveplate
combinations, with the combined waveplate phase retardance being
equal to or greater than half wave, are also feasible designs of
the waveplate cascade module.
[0035] The illustrated system 100 can have any number and type of
parameters that can affect the polarization of light as it travels
through the system, and these parameters can be measured using the
techniques described herein. Parameter .delta..sub.obj is the
effective retardance of the residual birefringence of the objective
103, which can come from CaF.sub.2 parts, residual stress, and
optical coatings of such component. .delta..sub.1 and .delta..sub.2
are the retardance parameters for WP1 and WP2, respectively. Their
slow axis angles are .theta..sub.1 and .theta..sub.2, respectively.
.delta..sub.r1 is the residual birefringence of relay lens 106
(e.g., made of fused silica), which tends to be negligible.
[0036] FIG. 2 show two graphs A and B of TL detected signal levels
as a function of rotation of WP1(.lamda./4) and WP2(3.lamda./8),
respectively. The vertical axis for graphs A and B corresponds to
light intensity, which can be any units. The horizontal axis is the
rotation angle of either WP1(.lamda./4) and WP2(3.lamda./8), in
units of degrees. To obtain these plots, one waveplate,
WP1(.lamda./4) or WP2(3.lamda./8), is held static while the other
waveplate, WP2(3.lamda./8) or WP1(.lamda./4), is rotated
step-by-step through a range of rotations angles, such as 0 to
180.degree.. Each dot on each graph A and B is a detected light
measurement.
[0037] After a number of measurements are obtained for each set of
waveplate rotations, a model may then be used to fit the
measurements to a curve, as illustrated by curves 202a and 202b,
for example. This curve fitting process can then be used to obtain
a retardance value for the different waveplates as further
described below.
[0038] As background for generally characterizing light travelling
through the system, the electric field convention is defined
as:
e.sup.i(kz-.omega.t+.phi.) Equation 1
[0039] with k being the wave vector; .omega. being the angular
frequency of the light; and .phi. being the phase.
[0040] In Jones calculus, the polarization state of incident TL can
be described by a Jones vector:
E in = E 1 [ cos ( .beta. ) sin ( .beta. ) e i .alpha. ] Equation 2
##EQU00001##
[0041] E.sub.1 is amplitude, .beta. is an angle between 0 and
.pi./2, and .alpha. is the phase difference between y- and
x-components of the electric field. When describing polarization
state, using the parameter pair (.alpha., .beta.) in above Jones
vector is equivalent to using another pair of commonly used
quantities: ellipticity and ellipse orientation angle, which are
related to .alpha. and .beta. through the following equations.
[0042] The parameter S.sub.3 of normalized Stokes vector is
S.sub.3=sin(2.beta.)sin(.alpha.), and the ellipticity .rho. is
defined as
.rho.=S.sub.3/(1+ {square root over (1-S.sub.3.sup.2)}) Equation
3
[0043] The ellipse orientation angle .psi. is defined in the range
from -90.degree. to 90.degree. and is
tan(2.psi.)=tan(2.alpha.)cos(.alpha.) Equation 4
[0044] The two waveplates can then be described by two Jones
matrices, which are functions of the waveplate retardance
parameters .delta..sub.1 and .delta..sub.2, respectively, and slow
axis orientation angles .theta..sub.1 and .theta..sub.2 which are
defined in xyz coordinates in FIG. 1, respectively.
J.sub.1(.delta..sub.1,.theta..sub.1)
and
J.sub.2(.delta..sub.2,.theta..sub.2)
[0045] After TL transmits through WP2 and WP1, the output Jones
vector is defined as the following equation:
E.sup.out=J.sub.1J.sub.2E.sup.in Equation 5
[0046] The PBS transmits y-polarization TL, which is sensed by a
sensor and can be modelled as:
| E y out | 2 = [ ( - A 2 cos ( .beta. ) + sin ( .beta. ) cos (
.alpha. ) A 0 + sin ( .beta. ) sin ( .alpha. ) A 1 ) 2 + ( A 3 cos
( .beta. ) - sin ( .beta. ) cos ( .alpha. ) A 1 + sin ( .beta. )
sin ( .alpha. ) A 0 ) 2 ] Equation 6 ##EQU00002##
[0047] with A.sub.0, A.sub.1, A.sub.2, and A.sub.3 defined as:
A 0 = cos ( .delta. 2 2 ) cos ( .delta. 1 2 ) - sin ( .delta. 2 2 )
sin ( .delta. 1 2 ) cos ( 2 ( .theta. 1 - .theta. 2 ) )
##EQU00003## A 1 = cos ( .delta. 2 2 ) sin ( .delta. 1 2 ) cos ( 2
.theta. 1 ) + sin ( .delta. 2 2 ) cos ( .delta. 1 2 ) cos ( 2
.theta. 2 ) ##EQU00003.2## A 2 = sin ( .delta. 2 2 ) sin ( .delta.
1 2 ) sin ( 2 ( .theta. 1 - .theta. 2 ) ) ##EQU00003.3## A 3 = cos
( .delta. 2 2 ) sin ( .delta. 1 2 ) sin ( 2 .theta. 1 ) + sin (
.delta. 2 2 ) cos ( .delta. 1 2 ) sin ( 2 .theta. 2 )
##EQU00003.4##
[0048] Considering the finite extinction ratio .epsilon. of the
PBS, the light intensity after the PBS is:
F= E.sub.1.sup.2+(1- )|E.sub.y.sup.out|.sup.2 Equation 7
[0049] It is this light intensity F that is captured by the sensor.
Rotating either WP1 or WP2 will cause the above detected light
intensity to change, and the changing of the light intensity as a
function of waveplate rotation can be described by the above model.
Fitting the recorded light intensity to the model F gives both the
polarization states, i.e. .beta. and .alpha. polarization
parameters, and waveplate parameters, such as slow angle
orientations, and retardance parameters.
[0050] FIG. 2 shows one example of TL measurements obtained on one
of the 6xx tools of KLA-Tencor. The fitted model function for these
TL measurements is also plotted in FIG. 2. From the modelling and
multiple measurements, .delta..sub.1 and .delta..sub.2 are
determined to have values of 88.4.degree. and 127.2.degree.,
respectively. These determined values are off by a few degrees from
the ideal retardance values 90.degree. and 135.degree. of .lamda./4
and 3.lamda./8 waveplates, respectively. It is clear that with a
single non-ideal .lamda./4 waveplate, one cannot realize accurate
circular polarization state on the photomask plane. However, with a
pair of non-ideal .lamda./4 and 3.lamda./8 waveplates, one can.
With the data of WP1 rotation (WP2 static, FIG. 2A), at the WP1
position for circular polarization, the polarization state between
WP1 and WP2 is determined to have an ellipticity of -0.346 and an
orientation angle of 68.3.degree.. With the data of WP2 rotation
(WP1 static at designed position, FIG. 2B), at the WP2 position for
circular polarization, the polarization state of Zone 2 in FIG. 1
is measured to have an ellipticity of 0.963.
[0051] A Levenberg-Marquardt (LM) nonlinear least squares fit
algorithm may be used to obtain these parameter values and then to
calculate polarization states. A curvature matrix may be generated
by calculating all the necessary partial derivatives of the model
(e.g. Equation 7) with regards to each fitted parameter such as
polarization parameters .alpha. and .beta., and waveplate
parameters (retardance and slow axis angles). The fitting programs
may be fully optimized for the problem being solved. With the
correct initial values of fitted parameters, the solution from the
nonlinear least squares fitting is unique. This uniqueness is also
true for the RL case, which is discussed below. Certain embodiments
of the present invention provide a new method of using Jones
calculus and nonlinear least squares fitting algorithms to measure
polarization state and waveplate parameters at the same time.
[0052] In the RL case, 193 nm illumination light passes the optics
twice. For the first pass, the light travels against the z-axis.
The PBS 108 also only reflects the x polarization component of the
RL input towards the sample. Thus, .alpha. and .beta. for the RL
input into the system are both zero. The coordinate x'y'z' used in
calculation for this pass is also right-handed, with x' direction
being the same as x direction, and y' and z' the reverse of y and z
respectively. Therefore, the slow axis angles of both waveplates in
x'y'z' are also the reverse of those in the xyz coordinate.
[0053] Both E.sub.x and E.sub.y fields have a phase shift of .pi.
upon reflection on a clear surface (unpatterned area) of the
photomask. Effectively, the Jones matrix K of this reflection gives
a phase shift of .pi. only to Ey field:
K = [ 1 0 0 - 1 ] Equation 8 ##EQU00004##
[0054] When the illumination light travels along z' direction, the
product of the two Jones matrices of WP1 and WP2 is:
J.sub.2(.delta..sub.2,-.theta..sub.2)J.sub.1(.delta..sub.1,-.theta..sub.-
1)
[0055] The Jones vector before the PBS is:
E.sup.out=J.sub.1(.delta..sub.1,.theta..sub.1)J.sub.2(.delta..sub.2,
.theta..sub.2)KJ.sub.2(.delta..sub.2,
-.theta..sub.2)J.sub.1(.delta..sub.1, -.theta..sub.1)E .sup.in
Equation 9
[0056] The y component intensity is:
|E.sub.y.sup.out|.sup.2=E.sub.1.sup.2[
sin(.beta.)cos(.alpha.)(A.sub.0.sup.2-A.sub.1.sup.2+A.sub.2.sup.2-A.sub.3-
.sup.2)+2
sin(.beta.)sin(.alpha.)(A.sub.0A.sub.1+A.sub.2A.sub.3].sup.2+E.s-
ub.1.sup.2[-2
sin(.beta.)cos(.alpha.)(A.sub.0A.sub.1+A.sub.2A.sub.3)+2
cos(.beta.)(A.sub.1A.sub.2-A.sub.0A.sub.3)+sin(.beta.)sin(.alpha.)(A.sub.-
0.sup.2-A.sub.1.sup.2+A.sub.2.sup.2-A.sub.3.sup.2)].sup.2 Equation
10
[0057] The definitions of A.sub.0, A.sub.1, A.sub.2, and A.sub.3
are the same as in TL case. The light intensity captured by the
sensor is:
F= E.sub.1.sup.2+(1- ) |E.sub.y.sup.out|.sup.2 Equation 11
[0058] In an RL polarization measurement for the illustrated system
100, the incident illumination light is always at x-polarization.
One only needs to take measurements for WP1(.lamda./4) rotation
since this data is sufficient to determine the retardance and
orientation of both waveplates from one single nonlinear fit, from
which the polarization state on the mask plane may be
calculated.
[0059] FIG. 3 shows one example of RL polarization measurement
results after nonlinear least squares fitting data processing from
the same tool that is used for the TL mode results. In one
implementation, the captured or detected images may be divided into
20 slices along the y-field so as to measure the y-field dependence
of the polarization state on the mask plane. The images may be
divided into any suitable number of slices. For instance, the
number of slices may be between 10 and 40.
[0060] In the example measurements obtained from the 6xx tool from
KLA-Tencor, the determined retardance parameters .delta..sub.1 and
.delta..sub.2 for the RL mode are significantly different from
those for the TL mode measurements. This difference is likely
caused by system errors, especially objective residual
birefringence since the light passes twice through the objective
for the RL mode. In fact, there may be challenges in modelling the
data of a WP2(3.lamda./8) rotation because of the existence of
residual birefringence (.delta..sub.obj) of the objective.
[0061] The objective (including all optics between mask and WP2)
residual birefringence was determined to be a few degrees, which is
much smaller than the retardance .delta..sub.2. In the RL
polarization measurements, the effect of .delta..sub.obj can be
included in an effective .delta..sub.2', e.g., objective and WP2
can be approximated by one effective waveplate. This assumption is
based on the equivalence theorem in Jones calculus, which states
that an optical system consisting of a sequence of retarders is
equivalent to a single retarder followed by a rotation matrix. For
the problem being solved, the rotation matrix has been determined
to be very close to an identity matrix and can be ignored.
[0062] For the two-retarder case of objective and WP2, the
effective retardance .delta..sub.2' is a sinusoidal function of the
relative orientation angle between the objective and WP2:
.delta..sub.2'=.delta..sub.2+.delta..sub.objsin(2.PHI.+.tau.)
[0063] .PHI. is the relative angle (between objective and WP2) and
.tau. is a phase. The factor of 2 in the above equation comes from
the fact that the effect of a waveplate is the same if it is
rotated by 180.degree.. For the RL polarization measurements, the
fitted retardance of WP2 is determined to be .delta..sub.2'. If one
performs RL polarization measurements (i.e., by rotating
WP1(.lamda./4) and recording light intensity) for multiple
WP2(3.lamda./8) positions, this sinusoidal behavior of
.delta..sub.2' can be observed and the true .delta..sub.2 and
.delta..sub.obj values can be determined directly, using the above
equation for .delta..sub.2'.
[0064] FIG. 4 displays one example of the fitted .delta..sub.2' vs.
WP2 orientation positions (in motor counts), which clearly shows a
sinusoidal dependence. Converting the motor counts into radians,
one may determine .delta..sub.obj and .delta..sub.2 from the
sinusoidal fit of FIG. 4. In this example, .delta..sub.obj is
3.9.degree. and .delta..sub.2 is 129.2.degree.. If more than one
detection plane is being used, the results may be averaged. In the
6xx tool, which has two detection planes P0 and P1, the results
from the P0 and P1 (not shown here) detection planes may be
averaged together, e.g., resulting in a .delta..sub.obj of
3.6.degree..
[0065] Any suitable technique may be used to obtain and utilize
various polarization parameters in an inspection system. FIG. 5 is
a flow chart illustrating a procedure 100 for managing polarization
in accordance with one embodiment of the present invention.
Initially, the inspection tool may be set up in a specific mode of
operation in operation 501. For example, a TL or RL mode may be
selected.
[0066] The process 500 may then include incrementing through
different rotations for a first waveplate, while keeping other one
or more waveplate(s) static in operation 502. Additionally, an
intensity signal may also be detected at each rotation. For
instance, images may be collected at each rotation. After a first
waveplate is rotated, each of the other two or more waveplates may
then be incrementally rotated, while an intensity signal is
collected for each rotation, in operation 504, e.g., from the
unpatterned areas of the photomask. If there are two waveplates as
illustrated in the system of FIG. 1, the first waveplate and then
the second waveplate are rotated while the other waveplate remains
static. If there are more than two waveplates, then each of the
other waveplates (e.g., 3.sup.rd waveplate, 4.sup.th waveplate,
etc.) are then rotated one at a time while the other waveplates are
kept static.
[0067] The entire set of waveplates may each be incrementally
rotated one at a time under the particular mode of operation. This
process may then be repeated for each waveplate under a next mode
of operation. For example, each waveplate is incrementally rotated
under the TL mode and then under the RL mode. Intensity values are
collected for each set of rotations for each waveplate and each
operation mode.
[0068] Referring back to the illustrated process 500, the
measurements may then be modelled using Jones calculus as a
function of the polarization parameters and waveplate parameters in
operation 516. The polarization and waveplate parameters may also
be determined based on a nonlinear least squares fitting method in
operation 518. For instance, the polarization parameters and the
waveplate retardance and orientation parameters can be determined
using equations 4 and 5. In a specific implementation, a least
squares fit process, such as the Levenberg-Marquardt algorithm
(LMA), may be used. Other nonlinear fitting algorithms (such as the
Gauss-Newton algorithm, QR decomposition, gradient methods, direct
search methods, etc.) may alternatively or additionally be used.
Details of LMA can be easily found in references such as the book
publication "Numerical Recipe". Below brief summary of this
algorithm has been included.
[0069] Assuming one has a set of n data points (x.sub.i, y.sub.i),
which are fitted to a physical model of m parameters F(x, p.sub.1,
p.sub.2 , . . . , p.sub.m). The index i runs from 1 to n. In this
patent application, the symbol x stands for waveplate rotation
angle and y stands for the signal measured by the sensor. The
fitted parameters include both polarization and waveplate
parameters. All the parameters are determined when the Chi-square
value reaches a minimum.
.chi..sup.2=.SIGMA..sub.i=1.sup.n(y.sub.i-F.sub.i).sup.2 Equation
12
[0070] F.sub.i is the model value with x=x.sub.i and a set of
parameter values. In order to find a solution of all parameters
from the set of n data points (x.sub.i, y.sub.i), an iteration
process is typically used because for such a complex problem, there
is no analytical solution for the parameters. The Jacobian matrix M
has a size of n.times.m. The ij.sup.th element of this matrix
is
M ij = .differential. F ( x i ) .differential. p j Equation 13
##EQU00005##
[0071] The index j runs from 1 to m. The Jacobian matrix is
calculated by taking the partial derivatives of model F with
respect to all fitted parameters. For the method developed in this
application, the Jacobian matrix can be written down analytically.
The curvature matrix size is m.times.m. It is defined as
C=M.sup..dagger.M+.lamda.diag(M.sup..dagger.M) Equation 14
[0072] .lamda. is called damping factor. It starts with an initial
value and is adjusted, depending on the Chi-square value, after
each iteration. Its purpose is to make the nonlinear least squares
fitting process numerically stable. For each iteration cycle, the
discrepancy between the data and model prediction is
.delta.y=M.sup..dagger.(y-F) Equation 15
[0073] For each iteration cycle, the correction to the parameter
vector (p.sub.1, p.sub.2, . . . , p.sub.m) is found by solving the
following matrix equation
C.delta.p=.delta.y Equation 16
[0074] The calculated parameter correction is added to the
parameter vector after each iteration cycle and the new parameter
vector will be used for next iteration. This process stops when the
parameter correction is smaller than a preset threshold, at which
the iteration process is defined as converged. In order to make the
above LMA succeed, one has to start the iteration process with a
set of initial values of fitted parameters, which are close to the
converged solution values and can be estimated from the physical
conditions of the problem to be solved.
[0075] In the example of FIG. 1, only the WP1(.lamda./4) 104b is
first rotated stepwise through a range about 180.degree., while the
WP2 (3.lamda./8) remains static in the TL mode. The intensity
measurements that were obtained for the different WP1 rotations may
then be used to determine its retardance .delta..sub.1, slow axis
angle .theta..sub.1, and the polarization state between WP1 and
WP2, by way of example. With WP1 .lamda./4) set to a fixed
.theta..sub.1 angle and rotating WP2 (3.lamda./8) in a stepwise
fashion through a range of 180.degree. and taking the retardance
parameters .delta..sub.1 and .theta..sub.1 from the first waveplate
measurements, the polarization state between WP2 and objective,
retardance .delta..sub.2 and slow axis angle .theta..sub.2 may also
be determined from the above modelling equation for the TL mode
(Equation 4 and 5). The slow axis angles .theta..sub.1 and
.theta..sub.2 correspond to the known static rotation settings of
the corresponding waveplates WP1 and WP2, respectively. If the
objective does not cause any polarization state change, then the
polarization state on the mask plane may also be determined.
[0076] In the RL mode measurement example of FIG. 1, the incident
illumination light is always at x-polarization. Rotation of the WP1
(.lamda./4) waveplate is sufficient to determine the retardance and
orientation of both waveplates from one single nonlinear fit, from
which the polarization state on the mask plane may then be easily
calculated.
[0077] One may utilize the determined polarization and waveplate
parameters in any suitable application. As shown in FIG. 5,
optimized polarization settings may be determined based on the
determined waveplate parameters in operation 520. For example, the
determined .delta..sub.1, .delta..sub.2, and .delta..sub.obj
parameters may be used to simulate arbitrary polarization states
for a plurality of waveplate rotations. For instance, rotation of
WP1(.lamda./4) and WP2(3.lamda./8) may be modeled to generate
arbitrary polarization states on the mask plane in RL mode.
[0078] FIGS. 6A and 6B show simulated maps of ellipticity and
ellipse orientation angle, respectively, on the mask plane when
both waveplates are rotated within a range of 180.degree., with
typical values of .delta..sub.1, .delta..sub.2, and .delta..sub.obj
in RL mode. Specifically, FIG. 6A illustrates a simulated
ellipticity map, and FIG. 6B illustrates an ellipse orientation
angle map. The maps include simulated, arbitrary polarization
states from left-to right-circular polarization. The waveplate
setting angles of certain important polarization states are
labelled on the maps. For instance, Y-linear ellipticity is shown
on 0 value contour lines with corresponding points on the ellipse
orientation map having +/-90.degree. orientation angle, while an
X-linear ellipticity is also shown on 0 value contour lines but
with zero degree orientation angle on the ellipse orientation map.
Right-and left-handed circular ellipticity are located at -1 and +1
value contour positions, respectively.
[0079] One specific capability of polarization state control is to
rotate the orientation angle of linear polarization from
-90.degree. to 90.degree. by tuning the rotations of the waveplates
to result in a polarization state along the zero contour lines of
the ellipticity map in FIG. 6A and a certain orientation in FIG.
6B. That is, waveplate orientation angles that result in an
arbitrary polarization state at the sample can be obtained from the
simulated ellipticity and ellipse orientation maps of FIGS. 6A and
6B. Being able to achieve linear polarization is especially
relevant to EUV photomask inspection because the signal strength
that is detected from certain types of defect strongly depends on
the linear polarization orientation of illumination light.
[0080] To give one example application, FIGS. 7A and 7B show both
PROLITH simulation and experimental verification results of a
particular category of defect on an EUV mask as a function of
orientation angle and focus in accordance with one embodiment of
the present invention. When the orientation angle of the linear
polarization illumination light is rotated from 0.degree. to
180.degree., it is equivalent to rotating from -90.degree. to
90.degree.. FIG. 7A shows simulation results, while FIG. 7B shows
experimental results. As shown in both plots, the defect signal has
a very strong dependence with the orientation angle. Near
15.degree. and 75.degree. angles, for example, the signal is near
the minimum. In contrast near 45.degree. and 135.degree., the
defect signal peaks and is enhanced multiple times as compared to
the minimum signal. The orientation angles that result in a
maximized signal for a particular defect type can then be selected.
The asymmetry between 45.degree. and 135.degree. signal peaks is
expected.
[0081] The TL polarization state can similarly be controlled by a
cascade of multiple waveplates to generate arbitrary polarization
state on the mask plane. FIG. 8 is a diagrammatic representation of
a system 800, which includes TL side waveplates, in accordance with
one embodiment of the present invention. As shown, the system 800
includes a TL light source 802b for generating an illumination
beam, a relay lens 804 for relaying the illumination beam, a first
waveplate 806a, a second waveplate 806b, and a condenser lens 808
for focusing the illumination beam towards the sample 102. In
addition to the RL illumination source 802a and sensor 810, the
other components of RL side of the system 800 may be similar to the
components of FIG. 1.
[0082] Examples of light sources for generating the RL or TL
incident beam (e.g., any suitable electromagnetic waveform) include
a laser-driven light source, a high-power plasma light source, a
transillumination light source (e.g., halogen or Xe lamp), a
filtered lamp, LED light sources, etc. The inspection system may
include any suitable number and type of additional light sources,
including broadband light sources.
[0083] The incident TL and/or RL beam from the light source may
generally pass through any number and type of lenses which serve to
relay (e.g., shape, focus or adjust focus offset, filter/select
wavelengths, filter/select polarization states, resize, magnify,
reduce distortion, etc.) the beam towards a sample. For instance,
the illumination module for both the TL and RL paths may also
include any number of linear polarizers and waveplates as described
further herein.
[0084] The collection module of the system 800 may include any
suitable number and type of optical components (not shown), such as
an aperture or field stop, collimator, aperture mask, analyzer
subsystem, splitter, and focus lens for focusing the scattered
light towards a sensor 810. A magnified image of the sample is
formed on the image sensor at back end of collection path. By way
of example, the sensor 810 may be in the form of a CCD (charge
coupled device) or TDI (time delay integration) detector,
photomultiplier tube (PMT), or other sensor.
[0085] Similar to the RL mode, arbitrary TL polarization state can
also be generated on the photomask plane, e.g., by using a pair of
waveplates .lamda./4 and 3.lamda./8. One difference between TL and
RL is that photomask substrate can have significantly residual
birefringence, which changes the TL polarization state when it
transmits through the substrate. Therefore, in order to precisely
control TL polarization state on mask plane, one either uses a
photomask substrate of very low birefringence or uses a photomask
substrate with its residual birefringence measured accurately in
advance. The effect of substrate birefringence may then be
compensated by the multiple waveplates used for TL polarization
control. In order to realize target TL polarization state (e.g.
linear or circular) on the bottom surface of the photomask
substrate (102 in FIG. 8), the TL polarization state on the top
surface of the substrate is generally a certain elliptical
polarization state because of residual birefringence of the
substrate. Based on FIGS. 6A and 6B, TL side multiple waveplates
can generate arbitrary polarization states, including the above
elliptical polarization state on the top surface of the
photomask.
[0086] Certain embodiments described above allow arbitrary
polarization state measurements and control for photomask
inspection, especially for EUV photomask inspection. Under
different inspection and specimen conditions, a specific
polarization angle can be selected to minimize noise and improve
defect SNR and, thereby, improve defect detection sensitivity.
These techniques can provide a convenient and accurate solution to
the challenge of illumination light polarization measurement and
control. Certain techniques can serve to both expand and
consolidate the capability of inspection tools. For example, next
generation EUV scanners will have anamorphic designs, resulting in
circular contacts on the wafer and requiring rectangular instead of
square contact arrays on the reticle. The rectangular contact
arrays will demand linear polarization orientation at an angle
different from 0, 45, 90, or 135 degrees in order to optimize SNR
(signal to noise ratio) for certain defect types.
[0087] Any suitable tool may be utilized, as long as variable
polarization states may be set up on the tool. In general, an
applicable inspection tool for implementation of techniques of the
present invention may include at least one light source for
generating an incident light beam at different polarization states.
Such an inspection may also include illumination optics for
directing the incident beam to the area-of-interest, collection
optics for directing scattered electromagnetic waveforms (e.g.,
scattered light, X-rays, etc.) from the area-of-interest in
response to the incident beam, a sensor for detecting this
scattered output and generating an image or signal from the
detected scattered output, and a controller or computer subsystem
for controlling the components of the inspection tool and
facilitating polarization control and defect detection in various
materials and structures as described further herein.
[0088] Regardless of form, a computer subsystem (e.g., 812) may be
connected to both illumination subsystem and analyzer subsystem for
automated control. For instance, the signals captured by each
detector can be processed by computer subsystem, which may include
a signal processing device having an analog-to-digital converter
configured to convert analog signals from each sensor into digital
signals for processing. The computer subsystem may be configured to
analyze intensity, phase, and/or other characteristics of the
sensed light beam. The computer subsystem may be configured (e.g.,
with programming instructions) to provide a user interface (e.g.,
on a computer screen) for displaying resultant images and other
inspection characteristics. The computer subsystem may also include
one or more input devices (e.g., a keyboard, mouse, joystick) for
providing user input (e.g., as changing wavelength, polarization,
mask configuration, aperture configuration, etc.), viewing
detection results data or images, setting up an inspection tool
recipe, etc.
[0089] The computer subsystem may be any suitable combination of
software and hardware and is generally configured to control
various components or other controllers of the inspection system.
The computer subsystem may control selective activation of the
illumination source, the illumination or output aperture settings,
wavelength band, focus offset setting, polarization settings,
analyzer settings, etc. The computer subsystem 624 may also be
configured to receive images or signals generated by each detector
and analyze the resulting images or signals to determine whether
defects are present on the sample, characterize defects present on
the sample, or otherwise characterize the sample. For example, the
computer subsystem may include a processor, memory, and other
computer peripherals that are programmed to implement instructions
of the method embodiments of the present invention. The computer
subsystem may also have one or more processors coupled to
input/output ports, and one or more memories via appropriate buses
or other communication mechanisms.
[0090] Because such information and program instructions may be
implemented on a specially configured computer system, such a
system includes program instructions/computer code for performing
various operations described herein that can be stored on a
computer readable media. Examples of machine-readable media
include, but are not limited to, magnetic media such as hard disks,
floppy disks, and magnetic tape; optical media such as CD-ROM
disks; magneto-optical media such as optical disks; and hardware
devices that are specially configured to store and perform program
instructions, such as read-only memory devices (ROM) and random
access memory (RAM). Examples of program instructions include both
machine code, such as produced by a compiler, and files containing
higher level code that may be executed by the computer using an
interpreter.
[0091] Regardless of the particular system embodiment, each optical
element may be optimized for the particular wavelength range of the
light in the path of such optical element. Optimization may include
minimizing wavelength-dependent aberrations, for example, by
selection of glass type, arrangement, shapes, and coatings (e.g.,
anti-reflective coatings, highly reflective coatings) for
minimizing aberrations for the corresponding wavelength range. For
example, the lenses are arranged to minimize the effects caused by
dispersion by shorter or longer wavelength ranges. In another
embodiment, all the optical elements are reflective. Examples of
reflective inspection systems and configurations are further
described in U.S. Pat. No. 7,351,980 issued 1 Apr. 2008, which is
incorporated herein by reference in its entirety.
[0092] The optical layout of the inspection tool can vary from that
described above. For example, the system microscope objective lens
can be one of many possible layouts, as long as the transmission
coatings are optimized for the particular selected wavelength band
or sub-band and the aberration over each waveband is minimized.
Different light sources can be used for each path. For instance, a
Xe source may be used for the long wavelength path and an HgXe or
Hg lamp may be used for the short wavelength path. Multiple LED or
speckle buster laser diodes are also possible sources for each
path. The zoom ratio can be modified to include different
magnification ranges either via a lens-only approach, a mostly
fixed lens with an optical trombone, or any combination
thereof.
[0093] As illustrated above, the sample may also be placed on a
stage of the inspection system, and the inspection system may also
include a positioning mechanism for moving the stage (and sample)
relative to the incident beam. By way of examples, one or more
motor mechanisms may each be formed from a screw drive and stepper
motor, linear drive with feedback position, or band actuator and
stepper motor. The one or more positioning mechanisms may also be
configured to move other components of the inspection system, such
as illumination or collection mirrors, apertures, FP relay lens,
wavelength filters, polarizers, analyzers, waveplates, etc.
[0094] It should be noted that the above description and drawings
of an inspection system are not to be construed as a limitation on
the specific components of the system and that the system may be
embodied in many other forms. For example, it is contemplated that
the inspection or measurement tool may have any suitable features
from any number of known imaging or metrology tools arranged for
detecting defects and/or resolving the critical aspects of features
of a reticle or wafer. By way of example, an inspection or
measurement tool may be adapted for bright field imaging
microscopy, dark field imaging microscopy, full sky imaging
microscopy, phase contrast microscopy, polarization contrast
microscopy, and coherence probe microscopy. It is also contemplated
that single and multiple image methods may be used in order to
capture images of the target. These methods include, for example,
single grab, double grab, single grab coherence probe microscopy
(CPM) and double grab CPM methods. Non-imaging optical methods,
such as scatterometry, may also be contemplated as forming part of
the inspection or metrology apparatus.
[0095] Any suitable lens arrangement may be used to direct the
incident beam towards the sample and direct the output beam
emanating from the sample towards a detector. The illumination and
collection optical elements of the system may be reflective or
transmissive. The output beam may be reflected or scattered from
the sample or transmitted through the sample. Likewise, any
suitable detector type or number of detection elements may be used
to receive the output beam and provide an image or a signal based
on the characteristics (e.g., intensity) of the received output
beam.
[0096] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims. It should be noted that
there are many alternative ways of implementing the processes,
systems, and apparatus of the present invention. For example, the
defect detection characteristic data may be obtained from a
transmitted, reflected, or a combination output beam. Accordingly,
the present embodiments are to be considered as illustrative and
not restrictive, and the invention is not to be limited to the
details given herein.
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