U.S. patent application number 11/281820 was filed with the patent office on 2006-03-30 for periodic patterns and technique to control misalignment between two layers.
Invention is credited to Ibrahim Abdulhalim, Mike Adel, Michael Faeyrman, Michael Friedmann.
Application Number | 20060065625 11/281820 |
Document ID | / |
Family ID | 25263374 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060065625 |
Kind Code |
A1 |
Abdulhalim; Ibrahim ; et
al. |
March 30, 2006 |
Periodic patterns and technique to control misalignment between two
layers
Abstract
A method and system to measure misalignment error between two
overlying or interlaced periodic structures are proposed. The
overlying or interlaced periodic structures are illuminated by
incident radiation, and the diffracted radiation of the incident
radiation by the overlying or interlaced periodic structures are
detected to provide an output signal. The misalignment between the
overlying or interlaced periodic structures may then be determined
from the output signal.
Inventors: |
Abdulhalim; Ibrahim; (Kfar
Manda, IL) ; Adel; Mike; (Zichron Ya' akov, IL)
; Friedmann; Michael; (Nesher, IL) ; Faeyrman;
Michael; (Kiryat Motzkin, IL) |
Correspondence
Address: |
PARSONS HSUE & DE RUNTZ LLP
595 MARKET STREET
SUITE 1900
SAN FRANCISCO
CA
94105
US
|
Family ID: |
25263374 |
Appl. No.: |
11/281820 |
Filed: |
November 16, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10699153 |
Oct 30, 2003 |
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11281820 |
Nov 16, 2005 |
|
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09833084 |
Apr 10, 2001 |
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10699153 |
Oct 30, 2003 |
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Current U.S.
Class: |
216/59 ;
156/345.24; 216/60; 257/E21.53 |
Current CPC
Class: |
H01L 22/12 20130101;
H01L 2223/5446 20130101; G01B 11/14 20130101; H01L 2223/54453
20130101; G01N 21/9501 20130101; H01L 2223/54426 20130101; G03F
7/70633 20130101; H01L 2924/0002 20130101; G01B 11/26 20130101;
H01L 23/544 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
216/059 ;
216/060; 156/345.24 |
International
Class: |
H01L 21/306 20060101
H01L021/306; G01L 21/30 20060101 G01L021/30 |
Claims
1. A method of measuring alignment accuracy between two or more
patterned layers formed on a substrate comprising, forming test
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical instrument capable
of measuring any one or more of transmission, reflectance, or
ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the offset between the gratings from the measurements from the
optical instrument using an optical model, wherein the optical
model accounts for the diffraction of the electromagnetic waves by
the gratings and the interaction of the gratings with each other's
diffracted field; wherein at least one layer between the grating in
layer A and the grating in layer B is opaque in the wavelength
range of the optical instrument, and the presence of the grating in
layer A causes a grating-shaped topography on the surface of the
opaque layer.
2. A method of measuring alignment accuracy between two or more
patterned layers formed on a substrate comprising, forming test
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical instrument capable
of measuring any one or more of transmission, reflectance, or
ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the offset between the gratings from the measurements from the
optical instrument using an optical model, wherein the optical
model accounts for the diffraction of the electromagnetic waves by
the gratings and the interaction of the gratings with each other's
diffracted field; wherein the optical model represents the
electromagnetic field in the gratings and in the layers between the
gratings as a sum of more than one diffracted orders.
3. A method of measuring alignment accuracy between two or more
patterned layers formed on a substrate comprising, forming test
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical instrument capable
of measuring any one or more of transmission, reflectance, or
ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the offset between the gratings from the measurements from the
optical instrument using an optical model, wherein the optical
model accounts for the diffraction of the electromagnetic waves by
the gratings and the interaction of the gratings with each other's
diffracted field; wherein offset is determined by; calculating,
according to a model of a wafer sample, the optical response of the
sample with the said two overlapping gratings, the model of the
sample taking into account parameters of the sample including any
of the overlay misalignment of layers A and B, the profiles of the
grating structures, and asymmetries caused in the grating
structures by manufacturing processes; changing the parameters of
the sample model to minimize the difference between the calculated
and measured optical responses; and repeating the previous two
steps until the difference between the calculated and measured
optical responses is sufficiently small or cannot be significantly
decreased by further iterations.
4. The method of claim 3 wherein at least a portion of the
calculation is done at the measurement time.
5. The method of claim 3 wherein at least a portion of the
calculated optical response is retrieved from a pre-computed
database.
6. The method of claim 3 wherein the calculation involves
interpolating the optical response from pre-computed entries in a
database.
7. A method of measuring alignment accuracy between two or more
patterned layers formed on a substrate comprising, forming test
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical instrument capable
of measuring any one or more of transmission, reflectance, or
ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the offset between the gratings from the measurements from the
optical instrument using an optical model, wherein the optical
model accounts for the diffraction of the electromagnetic waves by
the gratings and the interaction of the gratings with each other's
diffracted field; wherein the first and second diffraction gratings
have different pitches.
8. A method of measuring alignment accuracy between two or more
patterned layers formed on a substrate comprising: forming test
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B, observing the
overlaid diffraction gratings using an optical instrument capable
of measuring any one or more of transmission, reflectance, or
ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the offset between the gratings from the measurements from the
optical instrument using an optical model, wherein the optical
model accounts for the diffraction of the electromagnetic waves by
the gratings and the interaction of the gratings with each other's
diffracted field; wherein at least one of the two gratings contains
more than one line per pitch, the widths of the at least two lines
in each pitch being substantially different from each other.
9. A method of determining a degree of registration between an
upper layer and a lower layer formed on a substrate, each of said
layers including a periodic structure formed thereon and arranged
to at least partially overlap, said method comprising the steps of:
illuminating the layers with a probe beam of radiation; monitoring
the zeroth order light diffracted from the layers; generating a
parameterized model representing the geometry and registration of
parameters of the model; and comparing the predicted optical
response with the monitored zeroth order light to determine the
registration of the structures.
10. A method as recited in claim 9 wherein said predicting step is
at least partially carried out in advance for a number of different
parameters and wherein the corresponding responses are stored in a
database for later comparison with the monitored response.
11. A method as recited in claim 9 wherein the predicting and
comparing steps are repeated while changing the parameters used in
the predicting step in order to cause the predicted optical
response to converge with the monitored response.
12. A method as recited in claim 9 wherein said probe beam is
generated from a broadband source and said monitoring step is
carried out as function of wavelength.
13. An apparatus for determining overlay error between two or more
patterned layers of a sample, comprising, a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the sample under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical instrument
that illuminates part or all of the metrology target and that
measures properties of light that has interacted with the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
offset of the grating pair from the measured properties; wherein
the first and second diffraction gratings have different
pitches.
14. An apparatus for determining overlay error between two or more
patterned layers of a sample comprising: a metrology target
comprising a first diffraction grating built into a patterned layer
A and second diffraction grating built into a patterned layer B,
where layers A and B are part of the sample under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical instrument
that illuminates part or all of the metrology target and that
measures properties of light that has interacted with the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
offset of the grating pair form the measured properties; wherein at
least one of the two gratings contains more than one line per
pitch, the widths of the at least two lines in each pitch being
substantially different from each other.
15. An apparatus for determining overlay error between two or more
patterned layers of a sample, comprising, a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the sample under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical instrument
that illuminates part or all of the metrology target and that
measures properties of light that has interacted with the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
offset of the grating pair from the measured properties; wherein at
least one other layer of material separates layers A and B at the
metrology target.
16. An apparatus for determining overlay error between two or more
patterned layers of a sample, comprising, a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the sample under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical instrument
that illuminates part or all of the metrology target and that
measures properties of light that has interacted with the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
offset of the grating pair from the measured properties: wherein
the processor has been programmed to iteratively (i) calculate an
optical response for a set of sample parameters, including overlay
misalignment, (ii) compare the measured properties with the
calculated optical response, and (iii) change one or more sample
parameters so as to minimize the difference between the measured
properties and the calculated optical response, wherein the
calculation of an optical response is according to an optical model
of the sample that accounts for the diffraction of electromagnetic
waves by the pair of gratings of the metrology target and the
interaction of the gratings with each other's diffracted field.
17. The apparatus of claim 16 wherein the processor has access to a
pre-computed database from which at least a portion of the
calculated optical response can be retrieved.
18. The apparatus of claim 17 wherein the calculation performed by
the programmed processor involves interpolating the optical
response from pre-computed entries in said database.
19. An apparatus for determining the overlay error comprising, a
metrology target comprising a first diffraction grating built into
a patterned layer A and a second diffraction grating is built into
a patterned layer B, where layers A and B are desired to be aligned
with respect to each other, the two gratings substantially
overlapping when viewed from a direction that is perpendicular to
the layers A and B; an ellipsometer that illuminates part or all of
the metrology target and that measures properties of light that has
interacted with the metrology target; and a processor which
estimates the offset of the grating pair from the pair's measured
optical characteristics.
20. The method of claim 19 wherein first and second diffraction
gratings have different pitches.
21. The apparatus of claim 19 wherein at least one of the two
gratings contains more than one line per pitch, the widths of the
at least two lines in each pitch (unit cell) being substantially
different from each other.
22. The apparatus of claim 19 wherein at least one other layer of
material separates layers A and B at the metrology target.
23. The apparatus of claim 19 wherein the ellipsometer measures
properties of light that has interacted with the metrology target
as a function of wavelength.
24. The apparatus of claim 19 wherein the processor has been
programmed to iteratively (i) calculate an optical response for a
set of sample parameters, including overlay misalignment, (ii)
compare the measured properties with the calculated optical
response, and (iii) change one or more sample parameters so as to
minimize the difference between the measured properties and the
calculated optical response, wherein the calculation of an optical
response is according to an optical model of the sample that
accounts for the diffraction of electromagnetic waves by the pair
of gratings of the metrology target and the interaction of the
gratings with each other's diffracted field.
25. The apparatus of claim 24 wherein the processor has access to a
pre-computed database from which at least a portion of the
calculated optical response can be retrieved.
26. The apparatus of claim 25 wherein the calculation performed by
the programmed processor involves interpolating the optical
response from pre-computed entries in said database.
27. The apparatus of claim 19, wherein the metrology target further
includes a second pattern built into layers A and B, and wherein
the optical instrument further includes a camera disposed to
observe said second pattern and obtain measurements therefrom of
any gross overlay errors, said processor connected to also receive
said measurements from said camera.
28. The apparatus of claim 27, wherein said second pattern
comprises a box-in-box pattern.
29. The apparatus of claim 27, wherein said second pattern
comprises a bar-in-bar pattern.
30. A method of measuring alignment accuracy between two or more
patterned layers formed on a substrate comprising, forming test
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical instrument capable
of measuring any one or more of transmission, reflectance, or
ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; determining the
offset between the gratings from the measurements from the optical
instrument using an optical model, wherein the optical model
accounts for the diffraction of the electromagnetic waves by the
gratings and the interaction of the gratings with each other's
diffracted field; and observing at least one second test area on
said substrate using a camera, the second test area having a
pattern built into layers A and B for measuring any gross overlay
errors, and wherein determining the offset includes using gross
overlay measurements obtained from the camera; wherein said pattern
in said second area comprises a bar-in-bar pattern.
31. An apparatus for determining overlay error between two or more
patterned layers of a sample, comprising. a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the sample under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical instrument
that illuminates part or all of the metrology target and that
measures properties of light that has interacted with the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
offset of the grating pair from the measured properties; wherein
the metrology target further includes a second pattern built into
layers A and B, and wherein the optical instrument further includes
a camera disposed to observe said second pattern and obtain
measurements therefrom of any gross overlay errors, said processor
connected to also receive said measurements from said camera; and
wherein said second pattern comprises a bar-in-bar pattern.
32. A method of measuring misalignment between two or more
patterned layers formed on a substrate comprising, forming target
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical signal processor
capable of measuring any one or more of transmission, reflectance,
or ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the misalignment between the gratings from the measurements from
the optical signal processor using a reference signal, wherein the
reference signal accounts for the diffraction of the radiation
waves by the gratings and the interaction of the gratings with each
other's diffracted field; wherein at least one layer between the
grating in layer A and the grating in layer B is opaque in the
wavelength range of the signal processor, and the presence of the
grating in layer A causes a grating-shaped pattern on the surface
of the opaque layer.
33. A method of measuring misalignment between two or more
patterned layers formed on a substrate comprising, forming target
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical signal processor
capable of measuring any one or more of transmission, reflectance,
or ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the misalignment between the gratings from the measurements from
the optical signal processor using a reference signal, wherein the
reference signal accounts for the diffraction of the radiation
waves by the gratings and the interaction of the gratings with each
other's diffracted field; wherein the reference signal represents
the radiation field in the gratings and in the layers between the
gratings as a sum of more than one diffracted orders.
34. A method of measuring misalignment between two or more
patterned layers formed on a substrate comprising, forming target
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical signal processor
capable of measuring any one or more of transmission, reflectance,
or ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the misalignment between the gratings from the measurements from
the optical signal processor using a reference signal, wherein the
reference signal accounts for the diffraction of the radiation
waves by the gratings and the interaction of the gratings with each
other's diffracted field; wherein misalignment is determined by;
calculating, according to a reference signal of a wafer sample, the
derived signal of the sample with the said two overlapping
gratings, the reference signal of the sample taking into account
parameters of the sample including any of the overlay misalignment
of layers A and B, the profiles of the grating structures, and
asymmetries caused in the grating structures by manufacturing
processes; changing the parameters of the sample reference signal
to minimize the difference between the calculated and measured
derived signals; and repeating the previous two steps until the
difference between the calculated and measured derived signals is
sufficiently small or cannot be significantly decreased by further
iterations.
35. The method of claim 34 wherein at least a portion of the
calculation is done at the measurement time.
36. The method of claim 34 wherein at least a portion of the
calculated derived signal is retrieved from a pre-computed
database.
37. The method of claim 34 wherein the calculation involves
interpolating the derived signal from pre-computed entries in a
database.
38. A method of measuring misalignment between two or more
patterned layers formed on a substrate comprising, forming target
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical signal processor
capable of measuring any one or more of transmission, reflectance,
or ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the misalignment between the gratings from the measurements from
the optical signal processor using a reference signal, wherein the
reference signal accounts for the diffraction of the radiation
waves by the gratings and the interaction of the gratings with each
other's diffracted field; wherein the first and second diffraction
gratings have different pitches.
39. A method of measuring misalignment between two or more
patterned layers formed on a substrate comprising: forming target
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B, observing the
overlaid diffraction gratings using an optical signal processor
capable of measuring any one or more of transmission, reflectance,
or ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; and determining
the misalignment between the gratings from the measurements from
the optical signal processor using a reference signal, wherein the
reference signal accounts for the diffraction of the radiation
waves by the gratings and the interaction of the gratings with each
other's diffracted field; wherein at least one of the two gratings
contains more than one line per pitch, the widths of the at least
two lines in each pitch being substantially different from each
other.
40. A method of determining a degree of registration between an
upper layer and a lower layer formed on a substrate, each of said
layers including a periodic structure formed thereon and arranged
to at least partially overlap, said method comprising the steps of:
illuminating the layers with a probe beam of radiation; monitoring
the zeroth order light diffracted from the layers; generating a
parameterized reference signal representing the geometry and
registration of parameters of the reference signal; and comparing
the predicted derived signal with the monitored zeroth order light
to determine the registration of the structures.
41. A method as recited in claim 40 wherein said predicting step is
at least partially carried out in advance for a number of different
parameters and wherein the corresponding responses are stored in a
database for later comparison with the monitored response.
42. A method as recited in claim 40 wherein the predicting and
comparing steps are repeated while changing the parameters used in
the predicting step in order to cause the predicted derived signal
to converge with the monitored response.
43. A method as recited in claim 40 wherein said probe beam is
generated from a broadband source and said monitoring step is
carried out as function of wavelength.
44. An apparatus for determining misalignment between two or more
patterned layers of a wafer, comprising, a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the wafer under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical signal
processor that illuminates part or all of the metrology target and
that measures properties of light diffracted from the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
misalignment of the grating pair from the measured properties;
wherein the first and second diffraction gratings have different
pitches.
45. An apparatus for determining misalignment between two or more
patterned layers of a wafer comprising: a metrology target
comprising a first diffraction grating built into a patterned layer
A and second diffraction grating built into a patterned layer B,
where layers A and B are part of the wafer under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical signal
processor that illuminates part or all of the metrology target and
that measures properties of light diffracted from the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
misalignment of the grating pair form the measured properties;
wherein at least one of the two gratings contains more than one
line per pitch, the widths of the at least two lines in each pitch
being substantially different from each other.
46. An apparatus for determining misalignment between two or more
patterned layers of a wafer, comprising, a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the wafer under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical signal
processor that illuminates part or all of the metrology target and
that measures properties of light diffracted from the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
misalignment of the grating pair from the measured properties;
wherein at least one other layer of material separates layers A and
B at the metrology target.
47. An apparatus for determining misalignment between two or more
patterned layers of a wafer, comprising, a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the wafer under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical signal
processor that illuminates part or all of the metrology target and
that measures properties of light diffracted from the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor which estimates the
misalignment of the grating pair from the measured properties:
wherein the processor has been programmed to repeatedly (i)
calculate a derived signal for a set of parameters, including
overlay misalignment, (ii) compare the measured properties with the
calculated derived signal, and (iii) change one or more parameters
so as to minimize the difference between the measured properties
and the calculated derived signal, wherein the calculation of a
derived signal is according to a reference signal of the wafer that
accounts for the diffraction of radiation waves by the pair of
gratings of the metrology target and the interaction of the
gratings with each other's diffracted field.
48. The apparatus of claim 47 wherein the processor has access to a
pre-computed database from which at least a portion of the
calculated derived signal can be retrieved.
49. The apparatus of claim 48 wherein the calculation performed by
the programmed processor involves interpolating the derived signal
from pre-computed entries in said database.
50. An apparatus for determining misalignment comprising, a
metrology target comprising a first diffraction grating built into
a patterned layer A and a second diffraction grating is built into
a patterned layer B, where layers A and B are desired to be aligned
with respect to each other, the two gratings substantially
overlapping when viewed from a direction that is perpendicular to
the layers A and B; an optical system that illuminates part or all
of the metrology target and that measures properties of light
diffracted from the metrology target; and a processor which
estimates the misalignment of the grating pair from the pair's
measured optical characteristics.
51. The method of claim 50 wherein first and second diffraction
gratings have different pitches.
52. The apparatus of claim 50 wherein at least one of the two
gratings contains more than one line per pitch, the widths of the
at least two lines in each pitch (unit cell) being substantially
different from each other.
53. The apparatus of claim 50 wherein at least one other layer of
material separates layers A and B at the metrology target.
54. The apparatus of claim 50 wherein the optical system measures
properties of light diffracted from the metrology target as a
function of wavelength.
55. The apparatus of claim 50 wherein the processor has been
programmed to repeatedly (i) calculate a derived signal for a set
of parameters, including overlay misalignment, (ii) compare the
measured properties with the calculated derived signal, and (iii)
change one or more parameters so as to minimize the difference
between the measured properties and the calculated derived signal,
wherein the calculation of a derived signal is according to a
reference signal of the wafer that accounts for the diffraction of
radiation waves by the pair of gratings of the metrology target and
the interaction of the gratings with each other's diffracted
field.
56. The apparatus of claim 55 wherein the processor has access to a
pre-computed database from which at least a portion of the
calculated derived signal can be retrieved.
57. The apparatus of claim 56 wherein the calculation performed by
the programmed processor involves interpolating the derived signal
from pre-computed entries in said database.
58. The apparatus of claim 50, wherein the metrology target further
includes a second pattern built into layers A and B, and wherein
the optical signal processor further includes a camera disposed to
observe said second pattern and obtain measurements therefrom of
any gross misalignments, said processor connected to also receive
said measurements from said camera.
59. The apparatus of claim 58, wherein said second pattern
comprises a box-in-box pattern.
60. The apparatus of claim 58, wherein said second pattern
comprises a bar-in-bar pattern.
61. A method of measuring misalignment between two or more
patterned layers formed on a substrate comprising, forming target
areas as part of the patterned layers, wherein a first diffraction
grating is built into a patterned layer A and a second diffraction
grating is built into a patterned layer B, where layers A and B are
desired to be aligned with respect to each other, zero or more
layers of other materials separating layers A and B, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the surfaces of A and B; observing the
overlaid diffraction gratings using an optical signal processor
capable of measuring any one or more of transmission, reflectance,
or ellipsometric parameters as a function of any one or more of
wavelength, polar angle of incidence, azimuthal angle of incidence,
or polarization of the illumination and detection; determining the
misalignment between the gratings from the measurements from the
optical signal processor using a reference signal, wherein the
reference signal accounts for the diffraction of the radiation
waves by the gratings and the interaction of the gratings with each
other's diffracted field; and observing at least one second test
area on said substrate using a camera, the second test area having
a pattern built into layers A and B for measuring any gross
misalignments, and wherein determining the misalignment includes
using gross overlay measurements obtained from the camera; wherein
said pattern in said second area comprises a bar-in-bar
pattern.
62. An apparatus for determining misalignment between two or more
patterned layers of a wafer, comprising. a metrology target
comprising a first diffraction grating built into a patterned layer
A and a second diffraction grating built into a patterned layer B,
where layers A and B are part of the wafer under test and layers A
and B are desired to be aligned with respect to each other, the two
gratings substantially overlapping when viewed from a direction
that is perpendicular to the layers A and B; an optical signal
processor that illuminates part or all of the metrology target and
that measures properties of light diffracted from the metrology
target as a function of any one or more of polar angle of
incidence, azimuthal angle of incidence, and polarization of the
illumination and detection; and a processor Which estimates the
misalignment of the grating pair from the measured properties;
wherein the metrology target further includes a second pattern
built into layers A and B, and wherein the optical signal processor
further includes a camera disposed to observe said second pattern
and obtain measurements therefrom of any gross misalignments, said
processor connected to also receive said measurements from said
camera; and wherein said second pattern comprises a bar-in-bar
pattern.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/699,153, filed Oct. 30, 2003; which application is a
continuation of application Ser. No. 09/833,084 filed Apr. 10,
2001, now abandoned; which applications are incorporated by
reference as if fully set forth herein. This application is also
related to application Ser. No. 10/682,544, filed Oct. 8, 2003.
BACKGROUND OF THE INVENTION
[0002] The invention relates in general to metrology systems for
measuring periodic structures such as overlay targets, and, in
particular, to a metrology system employing diffracted light for
detecting misalignment of such structures.
[0003] Overlay error measurement requires specially designed marks
to be strategicaily placed at various locations, normally in the
scribe line area between dies, on the wafers for each process. The
alignment of the two overlay targets from two consecutive processes
is measured for a number of locations on the wafer, and the overlay
error map across the wafer is analyzed to provide feedback for the
alignment control of lithography steppers.
[0004] A key process control parameter in the manufacturing of
integrated circuits is the measurement of overlay target alignment
between successive layers on a semiconductor wafer. If the two
overlay targets are misaligned relative to each other, then the
electronic devices fabricated will malfunction, and the
semiconductor wafer will need to be reworked or discarded.
[0005] Measurement of overlay misregistration between layers is
being performed today with optical microscopy in different
variations: brightfield, darkfield, confocal, and interference
microscopy, as described in Levinson, "Lithography Process
Control," Chapter 5, SPIE Press Vol. TT28, 1999. Overlay targets
may comprise fine structures on top of the wafer or etched into the
surface of the wafer. For example, one overlay target may be formed
by etching into the wafer, while another adjacent overlay target
may be a resist layer at a higher elevation over the wafer. The
target being used for this purpose is called box-in-box where the
outer box, usually 10 to 30 .mu.m, represents the position of the
bottom layer, while the inner box is smaller and represents the
location of the upper layer. An optical microscopic image is
grabbed for this target and analyzed with image processing
techniques. The relative location of the two boxes represents what
is called the overlay misregistration, or the overlay. The accuracy
of the optical microscope is limited by the accuracy of the line
profiles in the target, by aberrations in the illumination and
imaging optics and by the image sampling in the camera. Such
methods are complex and they require full imaging optics. Vibration
isolation is also required.
[0006] These techniques suffer from a number of drawbacks. First,
the grabbed target image is highly sensitive to the optical quality
of the system, which is never ideal. The optical quality of the
system may produce errors in the calculation of the overlay
misregistration. Second, optical imaging has a fundamental limit on
resolution, which affects the accuracy of the measurement. Third,
an optical microscope is a relatively bulky system. It is difficult
to integrate an optical microscope into another system, such as the
end of the track of a lithographic stepper system. It is desirable
to develop an improved system to overcome these drawbacks.
SUMMARY OF THE INVENTION
[0007] A target for determining misalignment between two layers of
a device has two periodic structures of lines and spaces on the two
different layers of a device. The two periodic structures overlie
or are interlaced with each other. The layers or periodic
structures may be at the same or different heights. In one
embodiment, either the first periodic structure or the second
periodic structure has at least two sets of interlaced grating
lines having different periods, line widths or duty cycles. The
invention also relates to a method of making overlying or
interlaced targets.
[0008] An advantage of the target is the use of the same
diffraction system and the same target to measure critical
dimension and overlay misregistration. Another advantage of the
measurement of misregistration of the target is that it is free
from optical asymmetries usually associated with imaging.
[0009] The invention also relates to a method of detecting
misalignment between two layers of a device. The overlying or
interlaced periodic structures are illuminated by incident
radiation. The diffracted radiation from the overlying or
interlaced periodic structures is used to provide an output signal.
In one embodiment, a signal is derived from the output signal. The
misalignment between the structures is determined from the output
signal or the derived signal. In one embodiment, the output signal
or the derived signal is compared with a reference signal. A
database that correlates the misalignment with data related to
diffracted radiation can be constructed.
[0010] An advantage of this method is the use of only one incident
radiation beam. Another advantage of this method is the high
sensitivity of zero-order and first-order diffracted light to the
overlay misregistration between the layers. In particular,
properties which exhibited high sensitivity are intensity, phase
and polarization properties of zero-order diffraction; differential
intensity between the positive and negative first-order
diffraction; differential phase between the positive and negative
first-order diffraction; and differential polarization between the
positive and negative first-order diffraction. These properties
also yielded linear graphs when plotted against the overlay
misalignment. This method can be used to determine misalignment on
the order of nanometers.
[0011] In one embodiment, a neutral polarization angle, defined as
an incident polarization angle where the differential intensity is
equal to zero for all overlay misregistrations, is determined. The
slope of differential intensity as a function of incident
polarization angle is highly linear when plotted against the
overlay misregistration. This linear behavior reduces the number of
parameters that need to be determined and decreases the
polarization scanning needed. Thus, the method of detecting
misalignment is faster when using the slope measurement
technique.
[0012] The invention also relates to an apparatus for detecting
misalignment of overlying or interlaced periodic structures. The
apparatus comprises a source, at least one analyzer, at least one
detector, and a signal processor to determine misalignment of
overlying or interlaced periodic structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIGS. 1a-1h are cross-sectional views illustrating basic
process steps in semiconductor processing.
[0014] FIG. 2a is a cross-sectional view of two overlying periodic
structures. FIGS. 2b and 2c are top views of the two overlying
periodic structures of FIG. 2a.
[0015] FIG. 3 is a top view of two overlying periodic structures
illustrating an embodiment of the invention.
[0016] FIGS. 4a and 4b are cross-sectional views of overlying or
interlaced periodic structures illustrating other embodiments of
the invention.
[0017] FIGS. 5a and 5b are cross-sectional views of two interlaced
periodic structures illustrating interlaced gratings in an
embodiment of the invention.
[0018] FIG. 6 is a cross-sectional view of two interlaced periodic
structures illustrating interlaced gratings in another embodiment
of the invention.
[0019] FIGS. 7a and 7b are schematic views illustrating negative
and positive overlay shift, respectively.
[0020] FIG. 8 is a schematic view illustrating the diffraction of
light from a grating structure.
[0021] FIG. 9a is a schematic block diagram of an optical system
that measures zero-order diffraction from overlying or interlaced
periodic structures. FIG. 9b is a schematic block diagram of an
integrated system of the optical system of FIG. 9a and a deposition
instrument.
[0022] FIGS. 10a and 11a are schematic block diagrams of an optical
system that measures first-order diffraction from a normal incident
beam on overlying or interlaced periodic structures. FIGS. 10b and
11b are schematic block diagrams of integrated systems of the
optical systems of FIGS. 10a and 11a, respectively, and a
deposition instrument.
[0023] FIGS. 12a and 12b are graphical plots of derived signals
from zero-order diffraction of incident radiation on overlying
structures.
[0024] FIGS. 13-14 and 16-17 are graphical plots of derived signals
from first-order diffraction of incident radiation on overlying
structures. FIG. 15 is a graphical plot illustrating the mean
square error.
[0025] FIGS. 18-19 and 21-22 are graphical plots of derived signals
from zero-order diffraction of incident radiation on interlaced
gratings. FIGS. 20 and 23 are graphical plots illustrating the mean
square error.
[0026] FIG. 24 is a graphical plot illustrating the determination
of misalignment from a slope near a neutral polarization angle.
[0027] For simplicity of description, identical components are
labeled by the same numerals in this application.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0028] FIG. 2a is a cross-sectional view of a target 11 comprising
two periodic structures 13, 15 on two layers 31, 33 of a device 17.
The second periodic structure 15 is overlying or interlaced with
the first periodic structure 13. The layers and the periodic
structures may be at the same or different heights. The device 17
can be any device of which the alignment between two layers,
particularly layers having small features on structures, needs to
be determined. These devices are typically semiconductor devices;
thin films for magnetic heads for data storage devices such as tape
recorders; and flat panel displays.
[0029] As shown in FIGS. 1a-1h, a device 17 is generally formed in
a basic series of steps for each layer. First, as shown in FIG. 1a,
a layer 2 is formed on a semiconductor substrate 1. The layer 2 may
be formed by oxidization, diffusion, implantation, evaporation, or
deposition. Second, as shown in FIG. 1b, resist 3 is deposited on
the layer 2. Third, as shown in FIG. 1c, the resist 3 is
selectively exposed to a form of radiation 5. This selective
exposure is accomplished with an exposure tool and mask 4, or data
tape in electron or ion beam lithography (not shown). Fourth, as
shown in FIG. 1d, the resist 3 is developed. The resist 3 protects
the regions 6 of the layer 2 that it covers. Fifth, as shown in
FIG. 1e, the exposed regions 7 of the layer 2 are etched away.
Sixth, as shown in FIG. 1f, the resist 3 is removed. Alternatively,
in another embodiment, another material 8 can be deposited in the
spaces 7, as shown in FIG. 1e, of the etched layer 2, as shown in
FIG. 1g, and the resist 3 is removed after the deposition, as shown
in FIG. 1h. This basic series of steps is repeated for each layer
until the desired device is formed.
[0030] A first layer 31 and a second layer 33 can be any layer in
the device. Unpatterned semiconductor, metal or dielectric layers
may be deposited or grown on top of, underneath, or between the
first layer 31 and the second layer 33.
[0031] The pattern for the first periodic structure 13 is in the
same mask as the pattern for a first layer 31 of the device, and
the pattern for the second periodic structure 15 is in the same
mask as the pattern for a second layer 33 of the device. In one
embodiment, the first periodic structure 13 or the second periodic
structure 15 is the etched spaces 7 of the first layer 31 or the
second layer 33, respectively, as shown in FIG. 1f. In another
embodiment, the first periodic structure 13 or the second periodic
structure 15 is the lines 2 of the first layer 31 or the second
layer 33, respectively, as shown in FIG. 1f. In another embodiment,
the first periodic structure 13 or the second periodic structure 15
is another material 8 deposited in the spaces 7 of the first layer
31 or the second layer 33, respectively, as shown in FIG. 1h. In
yet another embodiment, the second layer 33 is resist, and the
second periodic structure 15 is resist 3 gratings, as shown in FIG.
1d.
[0032] The first periodic structure 13 has the same alignment as
the first layer 31, since the same mask was used for the pattern
for the first periodic structure 13 and for the pattern for the
first layer 31. Similarly, the second periodic structure 15 has the
same alignment as the second layer 33. Thus, any overlay
misregistration error in the alignment between the first layer 31
and the second layer 33 will be reflected in the alignment between
the first periodic structure 13 and the second periodic structure
15.
[0033] FIGS. 2b and 2c are top views of target 11. In one
embodiment, as illustrated in FIG. 2a, the first periodic structure
13 has a first selected width CD1, and the second periodic
structure 15 has a second selected width CD2. The second selected
width CD2 is less than the first selected width CD1. The pitch,
also called the period or the unit cell, of a periodic structure is
the distance after which the pattern is repeated. The distance
between the left edge of the first periodic structure 13 and the
left edge of the second periodic structure 15 is d.sub.1, and the
distance between the right edge of the first periodic structure 13
and the right edge of the second periodic structure 15 is d.sub.2.
In a preferred embodiment, when layers 31, 33 are properly aligned
relative to each other, the second periodic structure 15 is
centered over the first periodic structure 13. In other words, when
the second periodic structure 15 is perfectly centered over the
first periodic structure 13, the misregistration is zero, and
d.sub.1=d.sub.2. In this embodiment, the misregistration is
indicated by d.sub.2-d.sub.1. To obtain misregistration in both the
X and Y directions of the XY coordinate system, another target 12
comprising two periodic structures 14, 16 similar to target 11 is
placed substantially perpendicular to target 11, as shown in FIG.
2c.
[0034] The target 11 is particularly desirable for use in
photolithography, where the first layer 31 is exposed to radiation
for patterning purposes of a semiconductor wafer and the second
layer 33 is resist. In one embodiment, the first layer 31 is etched
silicon, and the second layer 33 is resist.
[0035] FIGS. 4a and 4b show alternative embodiments. In one
embodiment, FIG. 4a illustrates a first periodic structure 13 of
oxide having a trapezoidal shape on a first layer 31 of silicon
substrate and a second periodic structure 15 of resist with a
second layer 33 of resist. The first layer 31 of silicon is etched,
and shallow trench isolation ("STI") oxide is deposited in the
spaces of the etched silicon. The lines of STI oxide form the first
periodic structure 13. An oxide layer 34 and a uniform polysilicon
layer 35 are deposited between the first layer 31 of silicon and
the second layer 33 of resist. The configuration in FIG. 4a shows a
line on space configuration, where the second periodic structure 15
is placed aligned with the spaces between the first periodic
structure 13. The invention also encompasses embodiments such as
the line on line configuration, where the lines in the second
periodic structure 15 are placed on top of and aligned with the
lines in the first periodic structure 13, as shown by the dotted
lines in FIG. 4a.
[0036] In another embodiment, FIG. 4b illustrates a first periodic
structure 13 of tungsten etched in a first layer 31 of oxide and a
second periodic structure 15 of resist with a second layer 33 of
resist. The first layer 31 and the second layer 33 are separated by
an aluminum blanket 37.
[0037] The invention relates to a method of making a target 11. A
first periodic structure 13 is placed over a first layer 31 of a
device 17. A second periodic structure 15 is placed over a second
layer 33 of the device 17. The second periodic structure 15 is
overlying or interlaced with the first periodic structure 13.
[0038] In one embodiment, another target 12 is placed substantially
perpendicular to target 11, as shown in FIG. 2c. A third periodic
structure 14 is placed over the first layer 31, and a fourth
periodic structure 14 is placed over the second layer 33. The third
periodic structure 14 is substantially perpendicular to the first
periodic structure 13, and the fourth periodic structure 16 is
substantially perpendicular to the second periodic structure
15.
[0039] An advantage of the target 11 is that the measurement of
misregistration of the target is free from optical asymmetries
usually associated with imaging. Another advantage of this
measurement is that it does not require scanning over the target as
it is done with other techniques, such as in Bareket, U.S. Pat. No.
6,023,338. Another advantage of the target 11 is the elimination of
a separate diffraction system and a different target to measure the
critical dimension ("CD") of a periodic structure. The critical
dimension, or a selected width of a periodic structure, is one of
many target parameters needed to calculate misregistration. Using
the same diffraction system and the same target to measure both the
overlay misregistration and the CD is more efficient. The
sensitivity associated with the CD and that with the
misregistration is distinguished by using an embodiment of a target
as shown in FIG. 3. The second periodic structure 15 extends
further to an area, the CD region 21, where the first periodic
structure 13 does not extend. The first selected width CD1 is
measured before placing the second periodic structure 15 on the
device 17. After forming the target, the second selected width CD2
alone can be measured in the CD region 21. In a separate
measurement, the misregistration is determined in an overlay region
19 where both the first 13 and second 15 periodic structures
lie.
[0040] FIGS. 5a and 5b are cross-sectional views of an embodiment
of a target having interlaced gratings. The first periodic
structure 13 or the second periodic structure 15 has at least two
interlaced grating lines having different periods, line widths or
duty cycles. The first periodic structure 13 is patterned with the
same mask as that for the first layer 31, and the second periodic
structure 15 is patterned with the same mask as that for the second
layer 33. Thus, the first periodic structure 13 has the same
alignment as the first layer 31, and the second periodic structure
15 has the same alignment as the second layer 33. Any
misregistration between the first layer 31 and the second layer 33
is reflected in the misregistration between the first periodic
structure 13 and the second periodic structure 15.
[0041] In the embodiment shown in FIGS. 5a and 5b, the first
periodic structure 13 has two interlaced grating lines 51, 53. The
first interlaced grating lines 51 have a line-width L.sub.1, and
the second interlaced grating lines 53 have a line-width L.sub.2.
The second periodic structure 15, as shown in FIG. 5b, has a
line-width L.sub.3 and is centered between the first interlaced
grating lines 51 and the second interlaced grating lines 53. The
distance between the right edge of the first interlaced grating 51
and the adjacent left edge of the second interlaced grating 53 is
represented by b, and the distance between the right edge of the
second periodic structure 15 and the adjacent left edge of the
second interlaced grating 53 is represented by c. The
misregistration between the first layer 31 and the second layer 33
is equal to the misregistration .epsilon. between the first
periodic structure 13 and the second periodic structure 15. The
misregistration .epsilon. is: = b 2 - L 3 2 - c ( 1 ) ##EQU1##
Where c=0, the resulting periodic structure has the most asymmetric
unit cell composed of a line with width of L.sub.2+L.sub.3 and a
line with width L.sub.1. Where c=b-L.sub.3, the resulting periodic
structure has the most symmetric unit cell composed of a line with
width L.sub.1+L.sub.3 and a line with width L.sub.2. For example,
if the two layers are made of the same material and
L.sub.1=L.sub.3=L.sub.2/2, then the lines are identical where c=0,
while one line is twice as wide as the other line where
c=b-L.sub.3.
[0042] FIG. 6 shows an alternative embodiment of a target having
interlaced gratings. The first periodic structure 13 is etched
silicon, and the second periodic target 15 is resist. The first
layer 31 of silicon substrate and the second layer 33 of resist are
separated by an oxide layer 39.
[0043] The invention also relates to a method of making a target
11. A first periodic structure 13 is placed over a first layer 31
of a device 17. A second periodic structure 15 is placed over a
second layer 33 of the device 17. The second periodic structure 15
is overlying or interlaced with the first periodic structure 13.
Either the first periodic structure 13 or the second periodic
structure 15 has at least two interlaced grating lines having
different periods, line widths or duty cycles.
[0044] An advantage of interlaced gratings is the ability to
determine the sign of the shift of the misregistration from the
symmetry of the interlaced gratings. FIGS. 7a and 7b are schematic
drawings illustrating negative and positive overlay shift,
respectively, in the X direction of the XY coordinate system.
Center line 61 is the center of a grating 63. When the grating 63
is aligned perfectly, the center line 61 is aligned with the Y axis
of the XY coordinate system. As shown in FIG. 7a, a negative
overlay shift is indicated by the center line 61 being in the
negative X direction. As shown in FIG. 7b, a positive overlay shift
is indicated by the center line 61 being in the positive X
direction. The negative overlay shift is indicated by a negative
number for the misregistration, and the positive overlay shift is
indicated by a positive number for the misregistration. The
misregistration can be determined using the method discussed below.
In the case of the interlaced gratings, a negative overlay shift
results in a more symmetrical unit cell, as where c=b-L.sub.3,
discussed above. A positive overlay shift results in a more
asymmetrical unit cell, as where c=0, discussed above.
[0045] The invention relates to a method to determine misalignment
using diffracted light. FIG. 8 is a schematic view showing the
diffraction of light from a grating structure 71. In one
embodiment, incident radiation 73 having an oblique angle of
incidence .theta. illuminates the grating structure 71. The grating
structure 71 diffracts radiation 75, 77, 79. Zero-order diffraction
75 is at the same oblique angle .theta. to the substrate as the
incident radiation 73. Negative first-order diffraction 77 and
positive first-order diffraction 79 are also diffracted by the
grating structure 71.
[0046] Optical systems for determining misalignment of overlying or
interlaced periodic structures are illustrated in FIGS. 9a, 10a,
and 11a. FIG. 9a shows an optical system 100 using incident
radiation beam 81 with an oblique angle of incidence and detecting
zero-order diffracted radiation 83. A source 102 provides polarized
incident radiation beam 81 to illuminate periodic structures on a
wafer 91. The incident radiation beam may be substantially
monochromatic or polychromatic. The source 102 comprises a light
source 101 and optionally a collimating/focusing/polarizing optical
module 103. The structures diffract zero-order diffracted radiation
83. A collimating/focusing/analyzing optical module 105 collects
the zero-order diffracted radiation 83, and a light detection unit
107 detects the zero-order diffracted radiation 83 collected by the
analyzer in module 105 to provide an output signal 85. A signal
processor 109 determines any misalignment between the structures
from the output signal 85. The output signal 85 is used directly to
determine misalignment from the intensity of the zero-order
diffracted radiation 83. In a preferred embodiment, the
misalignment is determined by comparing the intensity with a
reference signal, such as a reference signal from a calibration
wafer or a database, compiled as explained below. In one
embodiment, the signal processor 109 calculates a derived signal
from the output signal 85 and determines misalignment from the
derived signal. The derived signal can include polarization or
phase information. In this embodiment, the misalignment is
determined by comparing the derived signal with a reference
signal.
[0047] In one embodiment, optical system 100 provides ellipsometric
parameter values, which are used to derive polarization and phase
information. In this embodiment, the source 102 includes a light
source 101 and a polarizer in module 103. Additionally, a device
104 causes relative rotational motion between the polarizer in
module 103 and the analyzer in module 105. Device 104 is well known
in the art and is not described for this reason. The polarization
of the reflected light is measured by the analyzer in module 105,
and the signal processor 109 calculates the ellipsometric parameter
values, tan(.psi.) and cos(.DELTA.), from the polarization of the
reflected light. The signal processor 109 uses the ellipsometric
parameter values to derive polarization and phase information. The
phase is .DELTA.. The polarization angle .alpha. is related to
tan(.psi.) through the following equation: tan .times. .times.
.alpha. = 1 tan .times. .times. .PSI. ( 2 ) ##EQU2## The signal
processor 109 determines misalignment from the polarization or
phase information, as discussed above.
[0048] The imaging and focusing of the optical system 100 in one
embodiment is verified using the vision and pattern recognition
system 115. The light source 101 provides a beam for imaging and
focusing 87. The beam for imaging and focusing 87 is reflected by
beam splitter 113 and focused by lens 111 to the wafer 91. The beam
87 then is reflected back through the lens 111 and beam splitter
113 to the vision and pattern recognition system 115. The vision
and pattern recognition system 115 then sends a recognition signal
88 for keeping the wafer in focus for measurement to the signal
processor 109.
[0049] FIG. 10a illustrates an optical system 110 using normal
incident radiation beam 82 and detecting first-order diffracted
radiation 93, 95. A source 202 provides polarized incident
radiation beam 82 to illuminate periodic structures on a wafer 91.
In this embodiment, the source 202 comprises a light source 101, a
polarizer 117 and lens 111. The structures diffract positive
first-order diffracted radiation 95 and negative first-order
diffracted radiation 93. Analyzers 121, 119 collect positive
first-order diffracted radiation 95 and negative first-order
diffracted radiation 93, respectively. Light detection units 125,
123 detect the positive first-order diffracted radiation 95 and the
negative first-order diffracted radiation 93, respectively,
collected by analyzers 121, 119, respectively, to provide output
signals 85. A signal processor 109 determines any misalignment
between the structures from the output signals 85, preferably by
comparing the output signals 85 to a reference signal. In one
embodiment, the signal processor 109 calculates a derived signal
from the output signals 85. The derived signal is a differential
signal between the positive first-order diffracted radiation 95 and
the negative first-order diffracted radiation 93. The differential
signal can indicate a differential intensity, a differential
polarization angle, or a differential phase.
[0050] Optical system 110 determines differential intensity,
differential polarization angles, or differential phase. To
determine differential phase, optical system 110 in one embodiment
uses an ellipsometric arrangement comprising a light source 101, a
polarizer 117, an analyzer 119 or 121, a light detector 123 or 125,
and a device 104 that causes relative rotational motion between the
polarizer 117 and the analyzer 119 or 121. Device 104 is well known
in the art and is not described for this reason. This arrangement
provides ellipsometric parameters for positive first-order
diffracted radiation 95 and ellipsometric parameters for negative
first-order diffracted radiation 93, which are used to derive phase
for positive first-order diffracted radiation 95 and phase for
negative first-order diffracted radiation 93, respectively. As
discussed above, one of the ellipsometric parameters is
cos(.DELTA.), and the phase is .DELTA.. Differential phase is
calculated by subtracting the phase for the negative first-order
diffracted radiation 93 from the phase for the positive first-order
diffracted radiation 95.
[0051] To determine differential polarization angles, in one
embodiment, the polarizer 117 is fixed for the incident radiation
beam 82, and the analyzers 121, 119 are rotated, or vice versa. The
polarization angle for the negative first-order diffracted
radiation 93 is determined from the change in intensity as either
the polarizer 117 or analyzer 119 rotates. The polarization angle
for the positive first-order diffracted radiation 95 is determined
from the change in intensity as either the polarizer 117 or
analyzer 121 rotates. A differential polarization angle is
calculated by subtracting the polarization angle for the negative
first-order diffracted radiation 93 from the polarization angle for
the positive first-order diffracted radiation 95.
[0052] To determine differential intensity, in one embodiment, the
analyzers 119, 121 are positioned without relative rotation at the
polarization angle of the first-order diffracted radiation 93, 95.
Preferably, at the polarization angle where the intensity of the
diffracted radiation is a maximum, the intensity of the positive
first-order diffracted radiation 95 and the intensity of the
negative first-order diffracted intensity 93 is detected by the
detectors 125, 123. Differential intensity is calculated by
subtracting the intensity for the negative first-order diffracted
radiation 93 from the intensity for the positive first-order
diffracted radiation 95.
[0053] In another embodiment, the differential intensity is
measured as a function of the incident polarization angle. In this
embodiment, the polarizer 117 is rotated, and the analyzers 119,
121 are fixed. As the polarizer 117 rotates, the incident
polarization angle changes. The intensity of the positive
first-order diffracted radiation 95 and the intensity of the
negative first-order diffracted radiation 93 is determined for
different incident polarization angles. Differential intensity is
calculated by subtracting the intensity for the negative
first-order diffracted radiation 93 from the intensity for the
positive first-order diffracted radiation 95.
[0054] The imaging and focusing of the optical system 110 in one
embodiment is verified using the vision and pattern recognition
system 115. After incident radiation beam 82 illuminates the wafer
91, a light beam for imaging and focusing 87 is reflected through
the lens 111, polarizer 117, and beam splitter 113 to the vision
and pattern recognition system 115. The vision and pattern
recognition system 115 then sends a recognition signal 88 for
keeping the wafer in focus for measurement to the signal processor
109.
[0055] FIG. 11a illustrates an optical system 120 where first-order
diffracted radiation beams 93, 95 are allowed to interfere. The
light source 101, device 104, polarizer 117, lens 111, and
analyzers 119, 121 operate the same way in optical system 120 as
they do in optical system 110. Device 104 is well known in the art
and is not described for this reason. Once the negative first-order
diffracted radiation 93 and positive first-order diffracted
radiation 95 are passed through the analyzers 119, 112,
respectively, a first device causes the positive first-order
diffracted radiation 95 and the negative first-order diffracted
radiation 93 to interfere. In this embodiment, the first device
comprises a multi-aperture shutter 131 and a flat beam splitter
135. The multi-aperture shutter 131 allows both the negative
first-order diffracted radiation 93 and the positive first-order
diffracted beam 95 to pass through it. The flat beam splitter 135
combines the negative first-order diffracted radiation 93 and the
positive first-order diffracted radiation 95. In this embodiment,
the mirrors 127, 133 change the direction of the positive
first-order diffracted radiation 95. A light detection unit 107
detects the interference 89 of the two diffracted radiation signals
to provide output signals 85. A signal processor 109 determines any
misalignment between the structures from the output signals 85,
preferably by comparing the output signals 85 to a reference
signal. The output signals 85 contain information related to phase
difference.
[0056] In one embodiment, phase shift interferometry is used to
determine misalignment. The phase modulator 129 shifts the phase of
positive first-order diffracted radiation 95. This phase shift of
the positive first-order diffracted radiation 95 allows the signal
processor 109 to use a simple algorithm to calculate the phase
difference between the phase for the positive first-order
diffracted radiation 95 and the phase for the negative first-order
diffracted radiation 93.
[0057] Differential intensity and differential polarization angle
can also be determined using optical system 120. The multi-aperture
shutter 131 operates in three modes. The first mode allows both the
positive first-order diffracted radiation 95 and the negative
first-order diffracted radiation 93 to pass through. In this mode,
differential phase is determined, as discussed above. The second
mode allows only the positive first-order diffracted radiation 95
to pass through. In this mode, the intensity and polarization angle
for the positive first-order diffracted radiation 95 can be
determined, as discussed above. The third mode allows only the
negative first-order diffracted radiation 93 to pass through. In
this mode, the intensity and polarization angle for the negative
first-order diffracted radiation 93 can be determined, as discussed
above.
[0058] To determine differential intensity, the multi-aperture
shutter 131 is operated in the second mode to determine intensity
for positive first-order diffracted radiation 95 and then in the
third mode to determine intensity for negative first-order
diffracted radiation 93, or vice versa. The differential intensity
is then calculated by subtracting the intensity of the negative
first-order diffracted radiation 93 from the intensity of the
positive first-order diffracted radiation 95. The signal processor
109 determines misalignment from the differential intensity.
[0059] In one embodiment, the differential intensity is measured at
different incident polarization angles. The measurements result in
a large set of data points, which, when compared to a reference
signal, provide a high accuracy in the determined value of the
misregistration.
[0060] To determine differential polarization angle, the
multi-aperture shutter 131 is operated in the second mode to
determine polarization angle for positive first-order diffracted
radiation 95 and then in the third mode to determine polarization
angle for negative first-order diffracted radiation 93, or vice
versa. The differential polarization angle is then calculated by
subtracting the polarization angle of the negative first-order
diffracted radiation 93 from the polarization angle of the positive
first-order diffracted radiation 95. The signal processor 109
determines misalignment from the differential polarization
angle.
[0061] The imaging and focusing of the optical system 120 is
verified using the vision and pattern recognition system 115 in the
same way as the imaging and focusing of the optical system 110 is
in FIG. 10. In one embodiment, the beam splitter 113 splits off
radiation 89 to reference light detection unit 137, which detects
fluctuations of the light source 101. The reference light detection
unit 137 communicates information 86 concerning intensity
fluctuation of source 101 to the signal processing and computing
unit 109. The signal processor 109 normalizes the output signal 85
using fluctuation information 86.
[0062] Optical systems 100, 110, 120 can be integrated with a
deposition instrument 200 to provide an integrated tool, as shown
in FIGS. 9b, 10b and 11b. The deposition instrument 200 provides
the overlying or interlaced periodic structures on wafer 91 in step
301. Optical systems 100, 110, 120 obtains misalignment information
from the wafer 91 in step 302. The signal processor 109 of optical
systems 100, 110, 120 provides the misalignment to the deposition
tool 200 in step 303. The deposition tool uses the misalignment
information to correct for any misalignment before providing
another layer or periodic structure on wafer 91 in step 301.
[0063] Optical systems 100, 110, 120 are used to determine the
misalignment of overlying or interlaced periodic structures. The
source providing polarized incident radiation beam illuminates the
first periodic structure 13 and the second periodic structure 15.
Diffracted radiation from the illuminated portions of the overlying
or interlaced periodic structures are detected to provide an output
signal 85. The misalignment between the structures is determined
from the output signal 85. In a preferred embodiment, the
misalignment is determined by comparing the output signal 85 with a
reference signal, such as a reference signal from a calibration
wafer or a database, compiled as explained below.
[0064] The invention relates to a method for providing a database
to determine misalignment of overlying or interlaced periodic
structures. The misalignment of overlying or interlaced periodic
structures and structure parameters, such as thickness, refractive
index, extinction coefficient, or critical dimension, are provided
to calculate data related to radiation diffracted by the structures
in response to a beam of radiation. The data can include intensity,
polarization angle, or phase information. Calculations can be
performed using known equations or by a software package, such as
Lambda SW, available from Lambda, University of Arizona, Tucson,
Ariz., or Gsolver SW, available from Grating Solver Development
Company, P.O. Box 353, Allen, Tex. 75013. Lambda SW uses
eigenfunctions approach, described in P. Sheng, R. S. Stepleman,
and P. N. Sandra, Exact Eigenfunctions for Square Wave Gratings:
Applications to Diffraction and Surface Plasmon Calculations, Phys.
Rev. B, 2907-2916 (1982), or the modal approach, described in L.
Li, A Modal Analysis of Lamellar Diffraction Gratings in Conical
Mountings, J. Mod. Opt. 40, 553-573 (1993). Gsolver SW uses
rigorous coupled wave analysis, described in M. G. Moharam and T.
K. Gaylord, Rigorous Coupled-Wave Analysis of Planar-Grating
Diffraction, J. Opt. Soc. Am. 73, 1105-1112 (1983). The data is
used to construct a database correlating the misalignment and the
data. The overlay misregistration of a target can then be
determined by comparing the output signal 85 with the database.
[0065] FIGS. 12-24 were generated through computer simulations
using either the Lambda SW or the Gsolver SW. FIGS. 12a and 12b are
graphical plots illustrating the ellipsometric parameters obtained
using an overlying target of FIG. 2a with the optical system of
FIG. 9a. The calculations were performed using the Lambda SW. The
overlying target used in the measurement comprises first periodic
structure 13 and the second periodic structure 15 made of resist
gratings having 1 .mu.m depth on a silicon substrate. The depth of
the first periodic structure 13 and the second periodic structure
15 is 0.5 .mu.m, and the pitch is 0.8 .mu.m. The first selected
width CD1 for the first periodic structure 13 is 0.4 .mu.m, and the
second selected width CD2 for the second periodic structure 15 is
0.2 .mu.m. The incident beam in this embodiment was TE polarized.
These target parameters and the overlay misregistration were
inputted into the Lambda SW to obtain ellipsometric parameter
values. The ellipsometric parameter values were obtained for
zero-order diffracted radiation using an incident radiation beam 81
at an angle of 25.degree. to the wafer surface. The ellipsometric
parameters, Tan [.psi.] and Cos [.DELTA.], were plotted as a
function of the wavelengths in the spectral range 230 to 400
nanometers. The ellipsometric parameters are defined as: tan
.times. .times. .PSI. = r p r s ( 3 ) ##EQU3## where r.sub.p and
r.sub.s are the amplitude reflection coefficients for the p(TM) and
s(TE) polarizations, and .DELTA.=.phi..sub.p-.phi..sub.s (4) where
.phi..sub.p and .phi..sub.s are the phases for the p(TM) and s(TE)
polarizations. Results were obtained for different values of
overlay misregistration d.sub.2-d.sub.1 varying from -15 nanometers
to 15 nanometers in steps of 5 nanometers. The variations for tan
[.psi.] and cos [.DELTA.] show sensitivity to the misregistration
in the nanometer scale. To get more accurate results, first-order
diffracted radiation is detected using normal incident radiation,
as in FIGS. 13-14.
[0066] FIGS. 13 and 14 are graphical plots illustrating the
differential intensity obtained using overlying targets of FIG. 2a
and an optical system detecting first-order diffracted radiation
using normal incident radiation. The calculations were performed
using Gsolver SW. The first periodic layer 13 is etched silicon,
while the second periodic layer 15 is resist. The overlay
misregistration and target parameters were inputted into Gsolver SW
to obtain the differential intensity in FIGS. 13 and 14. FIG. 13
shows the normalized differential intensity between the positive
and negative first-order diffracted radiation as a function of
overlay misregistrations. The differential intensity is defined as:
DS = R + 1 - R - 1 R + 1 + R - 1 .times. % ( 5 ) ##EQU4## where
R.sub.+1 is the intensity of the positive first-order diffracted
radiation and R.sub.-1 is the intensity of the negative first-order
diffracted radiation. The different curves in FIG. 13 correspond to
the different incident polarization angles (0.degree., 50.degree.,
60.degree., 74.degree., 80.degree., and 90.degree.) of the incident
linearly polarized light relative to the plane of incidence. The
polarization angle .alpha. is defined as: .alpha. = arctan
.function. ( E s E p ) ( 6 ) ##EQU5## where E.sub.s is the field
component perpendicular to the plane of incidence, which for normal
incidence is the Y component in the XY coordinate system, and
E.sub.p is the field component parallel to the plane of incidence,
which for normal incidence is the X component. Polarization scans
from incident polarization angles of 0.degree. to 90.degree. were
performed to generate the graphical plots in FIGS. 13 and 14. FIG.
14 shows the differential intensity as a function of incident
polarization angle at different overlay misregistration (-50 nm,
-35 nm, -15 nm, 0 nm, 15 nm, 35 nm, and 50 nm). FIG. 14 shows that
there is a neutral polarization angle, defined as an incident
polarization angle where the differential intensity is equal to
zero for all overlay misregistration. FIGS. 13 and 14 illustrate
the high sensitivity of differential intensity to the overlay
misregistration and the linear behavior of differential intensity
with the overlay misregistration. They also show that the
differential intensity is zero at zero overlay misregistration for
any polarization angle. Similar graphical plots were obtained at
different wavelengths. FIG. 15 shows the mean square error ("MSE")
variation with the overlay misregistration. The MSE exhibits
linearity and sensitivity of approximately 0.6 per one nanometer
overlay misregistration.
[0067] FIGS. 16 and 17 are graphical plots, using the same target
with different structure parameters and the same optical system as
the ones in FIGS. 13 and 14. However, the calculations were
performed using the Lambda SW, instead of the Gsolver SW. The kinks
or the deviations from the montonicity of the curves at certain
points in FIGS. 16 and 17 are believed to be due to numerical
instabilities frequently known to occur in the use of the Lambda
SW. The overlay misregistration and the target parameters were
inputted into Lambda SW to obtain differential polarization angle
and differential phase in FIGS. 16 and 17, respectively. FIG. 16
shows the variation of the difference between the polarization
angles of the positive and negative first-order diffracted
radiation as a function of overlay misregistration for different
incident polarization angles (0.degree., 5.degree., 15.degree.,
30.degree., 45.degree., 60.degree., and 90.degree.). FIG. 17 shows
the variation of the difference between the phase angles of the
positive and negative first-order diffracted radiation. The phase
angle here represents the phase difference between the p and s
polarized components of the diffracted light.
[0068] FIGS. 16 and 17 also illustrate the high sensitivity of
differential polarization angle and differential phase,
respectively, to the overlay misregistration and the linear
behavior of differential polarization angle and differential phase,
respectively, when plotted against the overlay misregistration.
They also show that the differential polarization angle and
differential phase is zero at zero overlay misregistration for any
polarization angle. However, FIG. 17 shows that the phase
difference does not depend on incident polarization. In one
embodiment, the difference between the polarization angles, as
shown in FIG. 16, is easily measured with an analyzer at the
output, while the phase difference, as shown in FIG. 17, is
measured with interferometry. In another embodiment, the
differential polarization angle and the differential phase is
derived from ellipsometric parameters.
[0069] Similar results were obtained using the overlying targets in
FIGS. 4a and 4b. However, for the particular target in FIG. 4a,
there was no neutral polarization angle in the line on line
configuration, where the second periodic structure 15 is centered
on the first periodic structure 13. The line on space
configuration, where the second periodic structure 15 is centered
on the spaces between the first periodic structure 13, did exhibit
a neutral polarization angle. These results show that the neutral
polarization angle apparently has a complicated dependence on the
structure parameters.
[0070] FIGS. 18-19 and 21-22 are graphical plots illustrating the
intensity of the zero-order diffracted radiation 83, as shown in
FIG. 9a, for interlaced gratings, as shown in FIG. 6. Table 1
summarizes the parameters used in the calculations by the Gsolver
SW. TABLE-US-00001 TABLE 1 Structure parameters used in the
simulations Parameter Data76 Data0 h1 850 nm 850 nm h2 850 nm 850
nm h3 600 nm 600 nm Pitch (P) 1000 nm 2000 nm CD1 150 nm 200 nm CD2
300 nm 600 nm CD3 150 nm 200 nm Incidence angle (.theta.)
76.degree. 0 Azimuth angle (.phi.) 0 0 Wavelength (.lamda.) 670 nm
500 nm The incidence angle is 76.degree. in the Data76
configuration, and the incidence angle is 0.degree. (normal) in the
Data0 configuration.
[0071] FIGS. 18-20 were derived using the Data76 configuration.
FIG. 18 shows the intensity of the zero-order diffracted radiation
versus the overlay misregistration at different polarization angles
(0.degree. to 90.degree. in steps of 15.degree.). Within a range of
140 nm, the changes are monotonic with the overlay misregistration.
The point where all the curves cross is at an overlay
misregistration value of 50 nm, rather than zero. At an overlay
misregistration value of 50 nm, the structure is effectively most
symmetric. In contrast, in an overlying target as in FIG. 2a, the
structure is most symmetric at zero overlay misregistration. FIG.
19 shows the dependence of the intensity of the zero-order
diffracted radiation on the incident polarization angle at
different overlay misregistrations (-50 nm, -15 nm, 0 nm, 20 nm, 40
nm, 60 nm, 80 nm, 100 nm, and 130 nm). Unlike with the differential
intensity of the first-order diffracted radiation, there is not a
neutral polarization angle where the differential intensity is zero
for different overlay misregistration. However, there is a
quasi-neutral polarization angle where most of the curves for
different misregistration cross. FIG. 20 shows the MSE variation as
a function of overlay misregistration. FIGS. 18 and 19 show the
high sensitivity of the intensity of zero-order diffracted
radiation to the overlay sign for a configuration using incident
radiation having an oblique angle of incidence on interlaced
gratings. They also show the linear behavior of the intensity when
plotted against the overlay misregistration.
[0072] FIGS. 21-23 were derived using the Data0 configuration. FIG.
21 shows the intensity of the zero-order diffracted radiation
versus the overlay misregistration at different polarization angles
(0.degree., 40.degree., 65.degree., and 90.degree.). FIG. 22 shows
the dependence of the intensity of the zero-order diffracted
radiation on the incident polarization angle at different overlay
misregistrations (-140 nm, -100 nm, -50 nm, 0 nm, 50 nm, and 100
nm). FIG. 23 shows the MSE variation as a function of overlay
misregistration. FIGS. 21 and 22 show the high sensitivity of the
intensity of zero-order diffracted radiation to the overlay sign
for a configuration using normal incident radiation on interlaced
gratings. They also show the linear behavior of the intensity when
plotted against the overlay misregistration.
[0073] FIG. 24 is a graphical plot generated by the Gsolver SW
illustrating the determination of misalignment from the neutral
polarization angle. As shown in FIG. 14, the differential intensity
equals zero independent of the overlay misregistration at the
neutral polarization angle. However, the slope of the differential
intensity varies with overlay misregistration. FIG. 24 shows the
slope near the neutral polarization angle as a function of overlay
misregistration. FIG. 24 shows linear behavior of the slope versus
the overlay misregistration with a slope of 0.038% per 1 nm overlay
misregistration. An advantage of the slope measurement technique is
the reduction of the number of parameters that need to be
determined. Another advantage is the decreased polarization
scanning needed. In FIG. 14, a polarization scan using incident
polarization angles from 0.degree. to 90.degree. is performed. In
contrast, using the slope measurement technique in one embodiment,
the derived signal is compared with the reference signal for
polarization angles within about five degrees of the neutral
polarization angle. Thus, the method of detecting misalignment is
faster when using the slope measurement technique. Another
embodiment of the invention is the use of the slope measurement
technique for the quasi-neutral polarization angle.
[0074] Misalignment of overlying or interlaced periodic structures
can be determined using the database in a preferred embodiment. The
source providing polarized incident radiation illuminates the first
periodic structure 13 and the second periodic structure 15.
Diffracted radiation from the illuminated portions of the overlying
or interlaced periodic structures are detected to provide an output
signal 85. The output signal 85 is compared with the database to
determine the misalignment between the overlying or interlaced
periodic structures.
[0075] In another embodiment, misalignment of overlying or
interlaced periodic structures is determined using the slope
measurement technique. A neutral polarization angle or
quasi-neutral polarization angle is provided. The derived signal is
compared with the reference signal near the neutral polarization
angle or the quasi-neutral polarization angle to determine
misalignment of the overlying or interlaced periodic
structures.
[0076] While the invention has been described above by reference to
various embodiments, it will be understood that changes and
modifications may be made without departing from the scope of the
invention, which is to be defined only by the appended claims and
their equivalent. All references referred to herein are
incorporated by reference.
* * * * *