U.S. patent application number 09/946104 was filed with the patent office on 2002-04-11 for determination of center of focus by diffraction signature analysis.
Invention is credited to Littau, Michael Eugene, Raymond, Christopher J..
Application Number | 20020041373 09/946104 |
Document ID | / |
Family ID | 26924293 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020041373 |
Kind Code |
A1 |
Littau, Michael Eugene ; et
al. |
April 11, 2002 |
DETERMINATION OF CENTER OF FOCUS BY DIFFRACTION SIGNATURE
ANALYSIS
Abstract
The present invention provides methods for determination of
parameters in lithographic devices and applications by diffraction
signature difference analysis, including determination of center of
focus in lithography devices and applications, such as for
photoresist lithographic wafer processing.
Inventors: |
Littau, Michael Eugene;
(Albuquerque, NM) ; Raymond, Christopher J.;
(Albuquerque, NM) |
Correspondence
Address: |
PEACOCK MYERS AND ADAMS P C
P O BOX 26927
ALBUQUERQUE
NM
871256927
|
Family ID: |
26924293 |
Appl. No.: |
09/946104 |
Filed: |
September 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60230491 |
Sep 6, 2000 |
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Current U.S.
Class: |
356/124 |
Current CPC
Class: |
G03F 7/70591 20130101;
G03F 7/70641 20130101 |
Class at
Publication: |
356/124 |
International
Class: |
G01M 011/02 |
Claims
What is claimed is:
1. A method of measuring parameters relating to a lithography
device comprising the steps of: providing a substrate comprising a
plurality of diffraction gratings formed on the substrate by
lithographic process utilizing the lithography device, the
diffraction gratings comprising a plurality of spaced elements;
measuring a diffraction signature for at least three of the
plurality of diffraction gratings by means of a radiation
source-based tool; and determining the differences between the
diffraction signatures to determine a desired parameter of said
lithography device.
2. The method of claim 1 wherein the substrate comprises a
wafer.
3. The method of claim 1, wherein the radiation source-based tool
comprises a light source-based tool.
4. The method of claim 3, wherein the light source-based tool
comprises an incident laser beam source, an optical system focusing
the laser beam and scanning through some range of incident angles,
and a detector for detecting the resulting diffraction signature
over the resulting measurement angles.
5. The method of claim 4, wherein the light source-based tool
comprises an angle-resolved scatterometer.
6. The method of claim 3, wherein the light source-based tool
comprises a plurality of laser beam sources.
7. The method of claim 3, wherein the light source-based tool
comprises an incident broad spectral light source, an optical
system focusing the light and illuminating through some range of
incident wavelengths, and a detector for detecting the resulting
diffraction signature over the resulting measurement
wavelengths.
8. The method of claim 3, wherein the light source-based tool
comprises an incident light source, components for varying the
amplitude and phase of the S and P polarizations, an optical system
focusing the light and illuminating over some range of incident
phases, and a detector for detecting the phase of the resulting
diffraction signature.
9. The method of claim 1, wherein measuring a diffraction signature
comprises phase measurement by means of a broad spectral radiation
source-based tool source, operating at a fixed angle, a variable
angle .crclbar. or a variable angle .crclbar..
10. The method of claim 1, wherein measuring a diffraction
signature comprises phase measurement by means of a single
wavelength radiation source-based tool source, operating at a fixed
angle, a variable angle .crclbar. or a variable angle .phi..
11. The method of claim 1, wherein measuring a diffraction
signature comprises phase measurement by means of a multiple
discrete wavelength radiation source-based tool source.
12. The method of claim 1, wherein the diffraction signature is a
reflective diffraction signature.
13. The method of claim 1, wherein the diffraction signature is a
transmissive diffraction signature.
14. The method of claim 1, wherein the diffraction signature is a
specular order diffraction signature.
15. The method of claim 1, wherein the diffraction signature is a
higher order diffraction signature.
16. The method of claim 1, the method further comprising forming
the plurality of diffraction gratings utilizing the lithography
device at known different focus settings, and determining the two
adjacent focus setting diffraction gratings wherein the difference
between the diffraction signatures is less than the difference of
the diffraction signatures between other adjacent focus setting
diffraction gratings, whereby the parameter is the center of focus
of the lithography device.
17. The method of claim 16, wherein the known different focus
settings are equal increment different focus settings.
18. The method of claim 16, wherein the known different focus
settings are non-equal increment different focus settings, and the
method further comprises use of a mathematical algorithm to
normalize the non-equal increment different focus settings.
19. The method of claim 16, wherein the difference in diffraction
signatures between diffraction gratings increases as an
approximation of a parabolic curve with a slope of zero over the
center of focus.
20. The method of claim 1, wherein determining the difference in
diffraction signatures between diffraction gratings comprises
determination of the difference using a metric.
21. The method of claim 20, wherein the metric is a root mean
square error method of data analysis.
22. The method of claim 1, wherein determining the minimal
difference comprises comparing the weighted averages of differences
between diffraction signatures between diffraction gratings.
23. The method of claim 1, the method further comprising forming
the plurality of diffraction gratings utilizing the lithography
device at the same focus setting and determining the differences as
a function of the location of the diffraction gratings on the
substrate.
24. The method of claim 1, the method further comprising forming
the plurality of diffraction gratings at known different focus
settings and known different dose settings and determining the
effect of dose on focus.
25. The method of claim 23, wherein the plurality of diffraction
gratings comprise sets of the same known different focus setting
diffraction gratings, the sets varying by different known dose
settings.
26. A method of determining the center of focus in a lithography
device, comprising the steps of providing a substrate comprising a
plurality of diffraction gratings made utilizing the lithography
device, the plurality of diffraction gratings comprising different
known focus settings; determining a diffraction signature for at
least three of the plurality of diffraction gratings by means of a
radiation source-based tool; measuring the differences between the
diffraction signatures between adjacent focus setting diffraction
gratings; and determining the center of focus as the focus setting
wherein there is a minimal difference between the diffraction
signatures of adjacent focus setting diffraction gratings.
27. The method of claim 26, wherein the difference in diffraction
signatures between adjacent focus setting diffraction gratings
increases as an approximation of a parabolic curve with a slope of
zero on the minimal difference.
28. The method of claim 26, wherein determining the difference in
diffraction signatures between adjacent focus setting diffraction
gratings comprises determination of the difference using a
metric.
29. The method of claim 28, wherein the metric is a root mean
square error method of data analysis.
30. The method of claim 26, wherein determining the minimal
difference comprises comparing the weighted averages of differences
between diffraction signatures between adjacent focus setting
diffraction gratings.
31. The method of claim 26, wherein determining the minimal
difference comprises fitting data derived from differences between
diffraction signatures between adjacent sequential focus setting
diffraction gratings to a parabolic curve, whereby the minimal
difference encompasses the minima of the parabolic curve.
32. The method of claim 26 wherein the substrate comprises a
wafer.
33. The method of claim 26, wherein the radiation source-based tool
comprises a light source-based tool.
34. The method of claim 33, wherein the light source-based tool
comprises an incident laser beam source, an optical system focusing
the laser beam and scanning through some range of incident angles,
and a detector for detecting the resulting diffraction signature
over the resulting measurement angles.
35. The method of claim 34, wherein the light source-based tool
comprises an angle-resolved scatterometer.
36. The method of claim 33, wherein the light source-based tool
comprises a plurality of laser beam sources.
37. The method of claim 33, wherein the light source-based tool
comprises an incident broad spectral light source, an optical
system focusing the light and illuminating through some range of
incident wavelengths, and a detector for detecting the resulting
diffraction signature over the resulting measurement
wavelengths.
38. The method of claim 33, wherein the light source-based tool
comprises an incident light source, components for varying the
amplitude and phase of the S and P polarizations, an optical system
focusing the light and illuminating over some range of incident
phases, and a detector for detecting the phase of the resulting
diffraction signature.
39. The method of claim 26, wherein measuring a diffraction
signature comprises phase measurement by means of a broad spectral
radiation source-based tool source, operating at a fixed angle, a
variable angle .crclbar. or a variable angle .phi..
40. The method of claim 26, wherein measuring a diffraction
signature comprises phase measurement by means of a single
wavelength radiation source-based tool source, operating at a fixed
angle, a variable angle .crclbar. or a variable angle .phi..
41. The method of claim 26, wherein measuring a diffraction
signature comprises phase measurement by means of a multiple
discrete wavelength radiation source-based tool source.
42. The method of claim 26, wherein the diffraction signature is a
reflective diffraction signature.
43. The method of claim 26, wherein the diffraction signature is a
transmissive diffraction signature.
44. The method of claim 26, wherein the diffraction signature is a
specular order diffraction signature.
45. The method of claim 26, wherein the diffraction signature is a
higher order diffraction signature.
46. The method of claim 26, wherein the different focus settings
comprise a constant difference between sequential different focus
settings.
47. The method of claim 26, wherein the different known focus
settings are non-equal increment different focus settings, and the
method further comprises use of a mathematical algorithm to
normalize the non-equal increment different focus settings.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of the filing of U.S.
Provisional Patent Application Ser. No. 60/230,491, entitled
Determination Of Center Of Focus By Diffraction Signature Analysis,
filed on Sep. 6, 2000, and the specification thereof is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention (Technical Field):
[0003] The present invention relates to methods for determination
of parameters in lithography applications by diffraction signature
analysis, including determination of center of focus in lithography
applications, such as for photoresist lithographic wafer
processing.
[0004] 2. Background Art:
[0005] Note that the following discussion refers to a number of
publications by author(s) and year of publication, and that due to
recent publication dates certain publications are not to be
considered as prior art vis-a-vis the present invention. Discussion
of such publications herein is given for more complete background
and is not to be construed as an admission that such publications
are prior art for patent ability determination purposes.
[0006] Lithography has a variety of useful applications in the
semiconductor, optics and related industries. Lithography is used
to manufacture semiconductor devices, such as integrated circuits
created on wafers, as well as flat-panel displays, disk heads and
the like. In one application, lithography is used to transmit a
pattern on a mask or reticle to a resist layer on a substrate
through spatially modulated light. The resist layer is then
developed and the exposed pattern is either etched away (positive
resist) or remains (negative resist) to form a three dimensional
image pattern in the resist layer. However, other forms of
lithography are employed in addition to photoresist
litholography.
[0007] In one form of lithography, particularly used in the
semiconductor industry, a wafer stepper is employed, which
typically includes a reduction lens and illuminator, an excimer
laser light source, a wafer stage, a reticle stage, wafer cassettes
and an operator workstation. Modern stepper devices employ both
positive and negative resist methods, and utilize either the
original step-and-repeat format or a step-and-scan format, or
both.
[0008] Exposure and focus determine the quality of the image
pattern that is developed, such as in the resist layer utilizing
photoresist lithography. Exposure determines the average energy of
the image per unit area and is set by the illumination time and
intensity. Focus determines the decrease in modulation relative to
the in-focus image. Focus is set by the position of the surface of
the resist layer relative to the focal plane of the imaging
system.
[0009] Local variations of exposure and focus can be caused by
variations in the resist layer thickness, substrate topography, as
well as stepper focus drift. Because of possible variations in
exposure and focus, image patterns generated through lithography
require monitoring to determine if the patterns are within an
acceptable tolerance range. Focus and exposure controls are
particularly important where the lithographic process is being used
to generate sub-micron lines.
[0010] A variety of methods and devices have been used to determine
focus of stepper and similar lithography devices. Scanning electron
microscopes (SEM) and similar devices are employed. However, while
SEM metrology can resolve features on the order of 0.1 microns, the
process is costly, requires a high vacuum chamber, is relatively
slow in operation and is difficult to automate. Optical microscopes
can be employed, but do not have the required resolving power for
sub-micron structures. Other methods include the development of
specialized targets and test masks, such as are disclosed in U.S.
Pat. Nos. 5,712,707, 5,953,128, and 6,088,113. Overlay error
methods are also known, as disclosed in U.S. Pat. No. 5,952,132.
However, these methods, while increasing resolution because of the
nature of the targets, still require use of SEM, optical
microscopes or similar direct measurement devices.
[0011] A variety of scatterometer and related devices and
measurements have been used for characterizing the microstructure
of microelectronic and optoelectronic semiconductor materials,
computer hard disks, optical disks, finely polished optical
components, and other materials having lateral dimensions in the
range of tens of microns to less than one-tenth micron. For
example, the CDS200 Scafterometer, made and sold by Accent Optical
Technologies, Inc. is a fully automated nondestructive critical
dimension (CD) measurement and cross-section profile analysis
system, partially disclosed in U.S. Pat. No. 5,703,692. This device
can repeatably resolve critical dimensions of less than 1 nm while
simultaneously determining the cross-sectional profile and
performing a layer thickness assessment. This device monitors the
intensity of a single diffraction order as a function of the angle
of incidence of the illuminating light beam. The intensity
variation of the 0.sup.th or specular order as well as higher
diffraction orders from the sample can be monitored in this manner,
and this provides information that is useful for determining the
properties of the sample target which is illuminated. Because the
process used to fabricate the sample target determines the
properties of a sample target, the information is also useful as an
indirect monitor of the process. This methodology is described in
the literature of semiconductor processing. A number of methods and
devices for scatterometer analysis are taught, including those set
forth in U.S. Pat. Nos. 4,710,642, 5,164,790, 5,241,369, 5,703,692,
5,867,276, 5,889,593, 5,912,741, and 6,100,985.
[0012] Scatterometers and related devices can employ a variety of
different methods of operation. In one method, a single, known
wave-length source is used, and the incident angle .crclbar. is
varied over a determined continuous range. In another method, a
number of laser beam sources are employed, optionally each at a
different incident angle .crclbar.. In yet another method, an
incident broad spectral light source is used, with the incident
light illuminated from some range of wavelengths and the incident
angle .crclbar. optionally held constant. Variable phase light
components are also known, utilizing optics and filters to produce
a range of incident phases, with a detector for detecting the
resulting diffracted phase. It is also possible to employ variable
polarization state light components, utilizing optics and filters
to vary the light polarization from the S to P components. It is
also possible to adjust the incident angle over a range .crclbar.
such that the light or other radiation source rotates about the
target area, or alternatively the target is rotated relative to the
light or other radiation source. Utilizing any of these various
devices, and combinations or permutations thereof, it is possible
and known to obtain a diffraction signature for a sample
target.
[0013] Besides scafterometer devices, there are other devices and
methods capable of determining the diffraction signatures at the
0.sup.th order or higher diffraction orders using a light-based
source that can be reflected off of or transmitted through a
diffraction grating, with the light captured by a detector. These
other devices and methods include ellipsometers and reflectometers,
in addition to scatterometers. It is further known that
non-light-based diffraction signatures may be obtained, using other
radiation sources as, for example, X-rays.
[0014] A variety of sample targets are known in the art. A simple
and commonly used target is a diffraction grating, essentially a
series of periodic lines, typically with a width to space ratio of
between about 1:1 and 1:3, though other ratios are known. A typical
diffraction grating, at for example a 1:3 ratio, would have a 100
nm line width and a 300 nm space, for a total pitch (width plus
space) of 400 nm. The width and pitch is a function of the
resolution of the lithographic process, and thus as lithographic
processes permit smaller widths and pitches, the width and pitch
may similarly be reduced. Diffraction techniques can be employed
with any feasible width and pitch, including those substantially
smaller than those now typically employed.
[0015] Diffraction gratings are typically dispersed, in a known
pattern, within dies on a wafer. It is known in the art to employ
multiple dies (or exposure fields) on a single wafer. Each
diffraction pattern may be made by lithographic means to be at a
different focus, such as by employing a different focus setting or
a different exposure setting or dose. It is also known that center
of focus may be determined using scatterometry and diffraction
gratings by comparing diffraction signatures from a variety of
different focus diffraction gratings to a theoretical model library
of diffraction grating signatures yielding information regarding
CD. The actual diffraction measures are compared to the model, from
which CD values are derived. The CD value thus obtained is plotted
against focus and the results fit to a parabolic curve. However,
this method requires significant time and computer resources to
generate the theoretical model.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
[0016] The present invention provides a method of measuring
parameters relating to a lithography device utilizing the steps of
providing a substrate comprising a plurality of diffraction
gratings formed on the substrate by lithographic process utilizing
the lithography device, the diffraction gratings comprising a
plurality of spaced elements; measuring a diffraction signature for
at least three of the plurality of diffraction gratings by means of
a radiation source-based tool; and determining the differences
between the diffraction signatures to determine a desired parameter
of said lithography device. In this method, the substrate can
include a wafer.
[0017] The method can further include forming the plurality of
diffraction gratings utilizing the lithography device at different
known focus settings, and determining the two adjacent focus
setting diffraction gratings wherein the difference between the
diffraction signatures is less than the difference of the
diffraction signatures between other adjacent focus setting
diffraction gratings, whereby the parameter is the center of focus
of the lithography device.
[0018] In a preferred embodiment, the different known focus
settings are equal increment different focus settings.
Alternatively, the different known focus settings are non-equal
increment different focus settings, and the method further includes
use of a mathematical algorithm to normalize the non-equal
increment different focus settings.
[0019] The method further includes plotting the diffraction
signature differences, wherein the difference in diffraction
signatures between diffraction gratings increases as an
approximation of a parabolic curve with a slope of zero over the
center of focus. Determination of the difference in diffraction
signatures between diffraction gratings can also include use of a
metric. One metric that may be employed is a root mean square error
method of data analysis. Determining the minimal difference can
further include comparing the weighted averages of differences of
diffraction signatures between diffraction gratings.
[0020] In one embodiment of the method, the method further includes
forming a plurality of diffraction gratings utilizing the
lithography device at the same focus setting and determining the
differences as a function of the location of the diffraction
gratings on the substrate. In another embodiment of the method, the
method further includes forming the plurality of diffraction
gratings at different known focus settings and different known dose
settings and determining the effect of dose on focus. The plurality
of diffraction gratings can include sets of the same known
different focus setting diffraction gratings, the sets varying by
different known dose settings.
[0021] The invention further provides a method of determining the
center of focus in a lithography device, the method including the
steps of providing a substrate comprising a plurality of
diffraction gratings made utilizing the lithography device, the
plurality of diffraction gratings comprising different known focus
settings; determining a diffraction signature for at least three of
the plurality of diffraction gratings by means of a radiation
source-based tool; measuring the differences between the
diffraction signatures between adjacent focus setting diffraction
gratings; and determining the center of focus as the focus setting
wherein there is a minimal difference between the diffraction
signatures of adjacent focus setting diffraction gratings.
[0022] In one embodiment of this method, the difference in
diffraction signatures between adjacent focus setting diffraction
gratings increases as an approximation of a parabolic curve with a
slope of zero on the minimal difference. Determining the difference
in diffraction signatures between adjacent focus setting
diffraction gratings can include determination of the difference
using a metric, including but not limited to a root mean square
error method of data analysis. The method also includes determining
the minimal difference by comparing the weighted averages of
differences between diffraction signatures of adjacent focus
setting diffraction gratings. In yet another embodiment of this
method, determining the minimal difference includes fitting data
derived from differences between diffraction signatures between
adjacent sequential focus setting diffraction gratings to a
parabolic curve, whereby the minimal difference encompasses the
minima of the parabolic curve.
[0023] In all of the foregoing methods, the radiation source-based
tool includes light source-based tools. In one embodiment, the
light source-based tool includes an incident laser beam source, an
optical system focusing the laser beam and scanning through some
range of incident angles, and a detector for detecting the
resulting diffraction signature over the resulting measurement
angles. The light source-based tool can further include an
angle-resolved scatterometer. In a different embodiment, the light
source-based tool includes a plurality of laser beam sources. In
yet another embodiment, the light source-based tool includes an
incident broad spectral light source, an optical system focusing
the light and illuminating through some range of incident
wavelengths, and a detector for detecting the resulting diffraction
signature over the resulting measurement wavelengths. In yet
another embodiment, the light source-based tool includes an
incident light source, components for varying the amplitude and
phase of the S and P polarizations, an optical system focusing the
light and illuminating over some range of incident phases, and a
detector for detecting the phase of the resulting diffraction
signature.
[0024] In all of the foregoing methods, measuring a diffraction
signature includes phase measurement by means of a broad spectral
radiation source-based tool source, operating at a fixed angle, a
variable angle .crclbar. or a variable angle .phi.. In the methods,
measuring a diffraction signature also includes phase measurement
by means of a single wavelength radiation source-based tool source,
operating at a fixed angle, a variable angle .crclbar. or a
variable angle .phi.. Measuring a diffraction signature can also
include phase measurement by means of a multiple discrete
wavelength radiation source-based tool source. The diffraction
signature can be a reflective diffraction signature or a
transmissive diffraction signature. The diffraction signature can
be a specular order diffraction signature or a higher order
diffraction signature, either positive or negative.
[0025] A primary object of the present invention is to provide a
method for measuring parameters relating to a lithography device
without the use of optical, SEM or similar microscopy metrology
tools.
[0026] Another object of the present invention is to provide a
method for determining center of focus of a lithography device by
analyzing the diffraction signature difference between members of a
series of different focus diffraction gratings.
[0027] Another object of the present invention is to provide a
method for determining or measuring parameters associated with a
lithography device, including center of focus, by obtaining a
diffraction signature utilizing either reflective or transmissive
diffraction.
[0028] Another object of the present invention is to provide a
method for determining or measuring parameters associated with a
lithography device, including center of focus, by obtaining a
diffraction signature utilizing any method to create a diffraction
signature, including but not limited to reflective or transmissive
angle-resolved, variable wavelength, variable phase, variable
polarization state or variable orientation diffraction, or a
combination thereof, of the 0.sup.th or specular diffraction order
or any higher orders.
[0029] Another object of the present invention is to provide a
method and device for determining or measuring parameters
associated with a lithography device, including center of focus,
without requiring direct use of either a theoretical model or
library of known parameters.
[0030] Another object of the present invention is to provide a
method for determining or measuring parameters associated with a
lithography device, including center of focus, as a function of
dose, by means of diffraction signature difference response and
analysis.
[0031] Another object of the present invention is to provide a
method for determining or measuring parameters associated with a
lithography device by means of any order of diffraction signature
of different focus diffraction gratings, including the 0.sup.th or
specular order or any higher order diffraction, either positive or
negative.
[0032] A primary advantage of the present invention is that it
permits measuring parameters relating to a lithography device
without the use of optical, SEM or similar microscopy metrology
tools.
[0033] Another advantage of the present invention is that it
permits use of a series of different focus diffraction gratings on
a conventional wafer made by means of a stepper, including
conventional photoresist lithography means, to determine center of
focus utilizing determination of diffraction signatures, and the
differences therebetween, for the diffraction gratings.
[0034] Another advantage of the present invention is that it
provides a method and device that permits obtaining results,
including center of focus, in a lithography device, such as a
stepper, in a shorter period of time and at lower cost than
conventional and known methods.
[0035] Other objects, advantages and novel features, and further
scope of applicability of the present invention will be set forth
in part in the detailed description to follow, taken in conjunction
with the accompanying drawings, and in part will become apparent to
those skilled in the art upon examination of the following, or may
be learned by practice of the invention. The objects and advantages
of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The accompanying drawings, which are incorporated into and
form a part of the specification, illustrate one or more
embodiments of the present invention and, together with the
description, serve to explain the principles of the invention. The
drawings are only for the purpose of illustrating one or more
preferred embodiments of the invention and are not to be construed
as limiting the invention.
[0037] In the drawings:
[0038] FIG. 1 is a schematic representation of a wafer with dies
thereon, the dies including diffraction gratings;
[0039] FIG. 2 is a schematic representation of various modes of
obtaining a reflective 0.sup.th order diffraction signature;
[0040] FIG. 3 depicts a three-dimensional diffraction grating;
[0041] FIG. 4 depicts a series of diffraction gratings;
[0042] FIGS. 5A-C depict a series of plotted diffraction signatures
obtained utilizing an angle-resolved scatterometer, each signature
varying by one focus step, with the S and P polarizations
concatenated;
[0043] FIG. 6 is a plot of diffraction signature difference
determined by root mean square error plotted against focus;
[0044] FIGS. 7A and B depict plots of parabolic curves encompassing
the minima for narrow and wide range centers of focus,
respectively;
[0045] FIG. 8 depicts a three-dimensional plot of the center of
focus derived from diffraction signature difference at a multitude
of positions in the field, thereby showing the center of focus as a
function of position in the field; and
[0046] FIG. 9 depicts a three-dimensional plot of the center of
focus derived from diffraction signature difference over the tilt
in the field, thereby showing the stage tilt effects of the center
of focus as a function of position in the field.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(BEST MODES FOR CARRYING OUT THE INVENTION)
[0047] The present invention provides methods and devices for
measuring parameters relating to a lithography device, and in a
preferred embodiment, for determining the center of focus of a
lithography device. In the methods, a series of diffraction
signatures of different diffraction gratings are obtained, the
diffraction gratings having been made utilizing the lithography
device, and made employing a plurality of different focus settings,
and optionally a plurality of different dose settings. The
diffraction signatures are sequentially ordered, which ordering may
be done subsequent to obtaining the diffraction signatures, such as
in order of increase or decrease of focus setting, preferably in an
equal increments, and the differences between diffraction
signatures of adjacent focus setting diffraction gratings are
determined. The differences are compared, optionally utilizing a
metric such as a root mean square error method of analysis. The
diffraction signatures will become closer together, with less
difference between equal increment adjacent focus settings, as the
center of focus is reached. Thus by employing the method of
diffraction signature difference analysis of this invention, the
center of focus and related parameters can be determined without
reference to a theoretical model or database of historical data,
and without use of direct measurement metrology devices, such as an
optical microscope or SEM.
[0048] Before proceeding to further describe the invention, the
following definitions are given.
[0049] A lithography device refers to any device that utilizes an
image, such as a mask, to transfer a pattern to and optionally into
a substrate. This thus includes conventional optical lithography,
such as photoresist lithography, but also includes other methods of
lithography. In photoresist lithography, also called
photolithography, optical methods are used to transfer circuit
patterns from master images, called masks or reticles, to wafers.
In this process, one or more specialized materials called resists
are coated on the wafers on which the circuits are to be made. A
resist coat is applied as required, and as required the wafer is
further processed, such as by a softbake. Either positive or
negative photoresist materials may be employed. Positive resists
are normally insoluble in chemicals used as resist developers, but
become soluble by exposure to light. Negative resists are normally
soluble in chemicals used as resist developers, but become
insoluble by exposure to light. By exposing the resist selectively
in some areas but not others, the pattern of the circuit or other
structure is created in the resist film. In optical lithography,
the selective exposure is accomplished by imaging of a mask,
typically by shining light onto the mask and projecting the
transmitted image onto the resist film.
[0050] The lithography devices referenced in this invention include
steppers, also known as wafer steppers, which are used to project
the image of a circuit or other structure from a photomask onto a
resist-coated wafer. A stepper typically includes reduction lens
and illuminator, excimer laser light source, wafer stage, reticle
stage, wafer cassettes and an operator workstation. Steppers employ
both positive and negative resist methods, and utilize either a
step-and-repeat format or a step-and-scan format, or combination
thereof.
[0051] There is employed in the practice of this invention a wafer
or other substrate on which is posited a series of diffraction
gratings by means of a lithographic device. In its simplest terms,
a diffraction grating is any structure or image made by
lithographic means which generates a periodic variation of the
refractive index relative to an incident illumination. This change
in refractive index can be either due to a physical difference or a
chemical difference. Physical differences include photoresist or
other lithographically generated changes, such as utilizing a
material with one refractive index coupled with air, such as
ordinary scored optical diffraction gratings, or a material coupled
with a different material. Chemical differences include wafers with
photoresist exposed diffraction gratings, where the resist has not
yet been developed. In this case all of the resist is still
present, but the portions that have been exposed have a different
refractive index than the non-exposed resist portions, thereby
creating a diffraction grating consisting of periodic variations of
refractive index in the resist. The periodic difference is obtained
by the periodicity of structural or chemical elements. This thus
includes conventional diffraction gratings consisting of a series
of parallel lines, but also includes gratings such as a
three-dimensional array of posts or holes, wherein there is
periodicity in both in the X direction and Y direction. A
diffraction grating with periodicity in both the X and Y directions
is shown in FIG. 3, and a diffraction grating with periodicity in
one direction, consisting of parallel lines 25, is shown in FIG. 1.
Diffraction gratings thus include photoresist gratings, etched film
stack gratings, metal gratings and other gratings known in the art.
A diffraction grating typically has a line width to space ratio of
between about 1:1 to 1:3, though other ratios may be employed. A
typical diffraction grating, at for example a 1:3 ratio, could have
a 100 nm line width and a pitch of 400 nm. The width and pitch can
be significantly smaller, depending in part on the resolution of
the lithographic device.
[0052] In the practice of this invention, a diffraction grating is
used to generate a diffraction signature. A diffraction signature
can be generated by any of a number of instruments, such as
scatterometers, ellipsometers or reflectometers. Any device
employing radiation to generate a diffraction signature is referred
to herein as a radiation source-based tool. Typically a visible
radiation source-based tool, such as a light source-based tool, is
employed, but the radiation source may be other than visible
radiation, such as an X-ray source. These devices generate a
diffraction pattern or signature by changing at least one
diffraction-associated parameter. In one embodiment, the
diffraction signature is created by a reflective mode, wherein the
radiation, such as light, is reflected. Thus a diffraction
signature may be generated by means of an angle-resolved
scatterometer, wherein a single known wave-length source is used,
and the incident angle .crclbar. is varied over a determined
continuous range, as shown in FIG. 2. The resulting diffraction
signature is shown in FIG. 5, wherein the intensity of light is
plotted against the incident and reflective angle e. In another
method, a number of laser beam sources are employed, optionally
each at a different incident angle .crclbar.. In yet another
method, an incident broad spectral light source is used, with the
incident light illuminated from some range of wavelengths and the
incident angle .crclbar. optionally held constant, as is shown in
FIG. 2. Variable phase light sources are also known, utilizing a
range of incident phases, with a detector for detecting the
resulting diffracted phase, as is shown in FIG. 2. Variable
polarization light sources are also known, utilizing a range of
polarization from the S to P components or the P to S components.
It is also possible to adjust the incident angle over a range
.phi., such that the light source rotates about the diffraction
grating, or alternatively the diffraction grating is rotated
relative to the light source, as shown in FIG. 2. Utilizing any of
these various devices, and combinations or permutations thereof, it
is possible and known to obtain a diffraction signature for a
sample target. In general, the detected light intensity is plotted
against the at least one variable parameter, such as angle of
incidence .crclbar., wavelength of incident light, phase of
incident light, angle of sweep .phi. or the like. The diffraction
signature may represent the 0.sup.th or specular diffraction order,
or may represent any higher diffraction order. It is also possible
and contemplated that a transmissive mode may be employed to
generate a diffraction signature, such as by use of an X-ray
radiation source as a component of the radiation source-based
tool.
[0053] In one embodiment of the invention, a wafer 10 as in FIG. 1
is provided, on which is disposed a series of dies 15. Each die
typically represents that portion of the wafer representing the
exposure field of the lithographic device, such as a stepper. In a
step-and-repeat system, the entire area of the mask or reticle to
be exposed is illuminated when the shutter is opened, thereby
simultaneously exposing the entire die exposure field. In a
step-and-scan system, only a part of the reticle or mask, and thus
only a part of the die exposure field, is exposed when the shutter
is opened. In either event, the reticle or mask may be moved such
that a diffraction grating set 20 is produced, the diffraction
grating set 20 being composed of a series of different, optionally
different focus, diffraction gratings. It is also possible that the
diffraction grating set 20 is composed of a series of the same
diffraction gratings, or is composed of a series of same focus but
different dose diffraction gratings. In a preferred embodiment, the
diffraction grating set 20 is composed of a series of different
focus diffraction gratings, preferably varying by a known and
incremental focus step, wherein all diffraction gratings are at a
fixed dose. From die to die on a wafer 10, either the dose range or
focus setting range, or both, may vary. Conventionally, the dose or
focus is varied in constant incremental steps, thereby facilitating
subsequent analysis. Thus the focus, for example, might vary in 50
to 100 nm steps over a determined range, and the dose, for example,
might vary in 1 or 2 mJ increments over a determined range. The
diffraction grating 20 may employ conventional lines 25 separated
by spaces 30, or may employ a three-dimensional pattern, such as
shown in FIG. 3.
[0054] The diffraction gratings are typically created in a resist
material by preparing masks with opaque and transparent areas
corresponding to the desired shape, size and configuration of the
desired diffraction grating. A source of radiation is then applied
on one side of the mask, thereby projecting the mask shape and
spaces onto the resist layer, the resist layer being on the
opposite side of the mask. One or more lens or other optical
systems may be interposed between the mask and the resist layer,
and also optionally between the radiation source and the mask. When
exposed to radiation or energized at sufficient levels to effect a
change in the resist, a latent image is formed in the resist. The
latent images, representing a chemical change in the resist
material, result in changes in reflectivity of the resist layer,
and thus may be employed to generate a diffraction signature as set
forth above. In one embodiment, the wafer with latent images in the
resist may be subjected to a post-exposure bake, used to drive
additional chemical reactions or to diffuse components within the
resist layer. In yet another embodiment, the resist may be
developed by a development process, optionally a chemical
development process, whereby a portion of the resist is removed,
the portion determined by whether a positive resist or negative
resist was employed. The development process is also referred to as
an etching process, resulting in etched areas or spaces of the
resist layer, and optionally the substrate material, such as other
films, on which such resist layer is posited.
[0055] In the methods and devices of this invention, the
diffraction grating may be exposed but not developed, or may
alternatively be developed. Similarly, while the foregoing
generally describes a conventional method of generating a
diffraction grating, any method may be employed, including use of
phase shift masks, any of a variety of sources of radiation,
including electron beam exposure, and the like.
[0056] Focus is a critical parameter in any lithography device,
including a stepper or similar lithography device. Focus and
depth-of-focus are functions of dose, or quanta of radiation
energy, and focus, or distance from the lens to the target. The
resulting imaging must be good for all points within a given
exposure field, thereby resulting in a definable usable
depth-of-focus. However, factors other than dose and focus affect
the focus and depth-of-focus, including astigmatism, field
curvature, lens quality, orientation of the wafer stage in the x-
and y-axes, and the like. Typical production wafer steppers have a
resolution of from about 0.15 to about 1.25 microns, and a usable
depth-of-focus of from about 0.40 to about 1.50 microns.
[0057] Determination of the center of focus for a fixed dose is
thus critical in efficient operation of a lithography device, such
as for a stepper during the photoresist exposure step in wafer
processing. Dose variations compound the difficulty in determining
this center. The lenses that are used in steppers and other
lithographic devices have a very limited depth of focus, so utmost
precision is necessary. Lenses that are in focus will yield sharply
printed photoresist images, and lack of focus will result in
non-functional photoresist features. Being at the center of focus
also significantly improves process repeatability. Once the center
of focus is known and determined, any of a variety of different
autofocus systems or schemes may be employed for determining that
the separation between the lens and the wafer is held constant.
These systems include optical methods, such as employing reflected
light, capacitance methods and pressure sensor methods, such as
employing pressurized air. However, these systems and schemes are
incapable of determining center of focus, but simply maintain the
lens-to-wafer distance at a constant. In typical operations, the
center of focus must be determined periodically, as often every six
hours or less of operation of a lithography device.
[0058] Turning to FIG. 5A, depicted therein are two diffraction
signatures generated utilizing an angle-resolved scatterometry
light radiation source-based tool, the diffraction signatures
representing the specular order from one focus step to the next
focus step (focus step n and n+1). In each of the figures of FIG.
5, a constant dose was employed in photoresist exposure of the
diffraction grating, with the focus, or distance from the lens to
the wafer, varied in incremental focus steps. The resulting
diffraction signatures are obtained either following photoresist
exposure but subsequent to development, or subsequent to
development wherein the structure is etched into the resist layer
and optionally the substrate including a portion of the wafer. A
series of diffraction gratings is measured, and the resulting
diffraction signatures recorded, such as in the memory of a
processor-associated device. The diffraction grating is any
structure employing a repeating or periodic feature capable of
diffracting light, including but not limited to the structures of
FIGS. 1 and 3. The differences in diffraction signatures of the
specular order, or any higher diffraction order, are analyzed by
measuring the differences in diffraction signatures from one focus
step to the next. The difference in diffraction signatures from one
focus step to the next will become less and less as the center of
focus is approached. Under theoretically ideal conditions, the
center of focus is the point as which variation in diffraction
signatures are at minima. Thus, as is shown in FIG. 5, the distance
separating the diffraction signatures of adjacent focus step
diffraction gratings decreases as the center of focus is
approached, such that in FIG. 5C the two resulting diffraction
signatures are virtually superimposable, with no significant
difference therebetween.
[0059] The difference in diffraction signatures from one focus step
to the next, and the determination of the center of focus, may be
ascertained by visually comparing the resulting adjacent focus step
diffraction signatures, as depicted in FIG. 5. However, this method
requires operator judgment and is not directly quantifiable, and is
also comparatively slow. Accordingly, any of a variety of metrics
or methods of analysis may be employed to measuring the differences
in diffraction signatures from one focus step to the next. Such
methods include, but are not limited to, minimizing the mean square
error (MSE) or root mean square error (RMSE), and other Euclidean
distance measures. Such methods also include averaging, weighted
averaging, sum of averages and other methods to characterize the
difference in diffraction signatures.
[0060] In one embodiment, diffraction signatures are obtained from
a series of sequential different focus setting diffraction gratings
40, 45, 50, 55, and 60 as shown in FIG. 4. The RMSE difference
between 40 and 45 is determined, and represents the diffraction
signature difference for diffraction grating 40 at its
corresponding focus setting. The average of the RMSE difference
between 40 and 45 and between 45 and 50 is determined, and
represents the diffraction signature difference for diffraction
grating 45 at its corresponding focus setting. The average of the
RMSE difference between 45 and 50 and between 50 and 55 is
determined, and represents the diffraction signature difference for
diffraction grating 50 at its corresponding focus setting.
Similarly the average of the RMSE difference between 50 and 55 and
between 55 and 60 is determined, and represents the diffraction
signature difference for diffraction grating 55 at its
corresponding focus setting. The RMSE difference between 55 and 60
is employed as the diffraction signature difference for diffraction
grating 60 at its corresponding focus setting. This thus generates
a series of diffraction signatures differences corresponding to the
difference in focus settings between ordered different focus
diffraction gratings.
[0061] Once obtained, the difference in diffraction signatures may
be used to determine center of focus by means of a weighted average
determination. In one such embodiment, the center of focus can be
determined by means of the following equation (1): 1 COF = ( Focus
Step ) ( DSD RMSE ) 2 1 ( DSD RMSE ) 2 ( 1 )
[0062] where COF is the center of focus and DSD.sub.RMSE is the
RMSE diffraction signature difference (DSD).
[0063] The numerical representation of the differences in
diffraction signatures may be also compared by other means to
determine the center of focus as the focus setting corresponding to
the region with the minima difference between adjacent focus
setting diffraction gratings. The numerical representation may be
plotted against the focus setting steps, thereby resulting in a
plot as in FIG. 6, depicting a parabolic curve centered about the
center of focus. At the center of focus, the slope of the parabolic
curve is at or approximates zero, this locus further representing
the area of minimal difference between the diffraction signatures
for adjacent different focus setting diffraction gratings.
[0064] Any of a variety of filters and related mathematical models
may be employed to discard outliers prior to determination of the
center of focus. Particularly with diffraction gratings exposed at
a focus setting substantially out of focus, the resulting focus
curve may become erratic. Individual diffraction gratings may
further yield aberrant results for reasons unrelated to the focus
setting, such as exposure errors, resist defects and the like.
[0065] The depth-of-focus or robustness of a given lithography
device, such as a stepper, may be quantitated by analysis of the
resulting parabolic curve. If the plotted function has a very tight
parabola, as shown in FIG. 7A, then the depth-of-focus is
correspondingly small, since the area encompassing the minima
corresponds to a small series of focus settings. If the plotted
function has a broad parabola, as shown in FIG. 7B, depicting a
large area corresponding to the minima, then the depth-of-focus is
larger, permitting a good focus value for a variety of
settings.
[0066] For a parabolic response to be obtained, the center of focus
must be included within the series of incremental focus settings
utilized to expose the diffraction grating. That is, no parabolic
curve with a zero slope at the center of focus can be generated
where the range does not encompass the center of focus. Further, at
points significantly out of focus, for example where sequential
focus steps are completely removing resist, then diffraction
signatures from one focus step to the next focus step may be very
close. This is a function of the difference in focus steps not
resulting in any significant difference in the resulting images.
Here too a grating model or uniform film model may be employed to
determine areas that are significantly out of focus. Typically such
points cannot be plotted to a parabolic curve.
[0067] While the example of FIG. 5 shows a comparison of different
focus diffraction signatures as a function of the angle of
incidence, with both S and P polarizations concatenated and plotted
against the diffraction intensity, it can readily be appreciated
that in other modes of diffraction the diffraction signature can
similarly be plotted. Thus for variable wavelength diffraction the
diffraction signature is generated by plotting wavelength against
intensity, for variable phase diffraction by plotting phase against
intensity, for variable polarization state diffraction by plotting
polarization state against intensity, for variable orientation
diffraction by plotting .phi. against intensity and the like.
Similarly, while the diffraction signatures of FIG. 5 result from
reflective diffraction, similar diffraction signatures may be
obtained by means of transmissive diffraction, provided only that
the radiation source-based tool utilized for diffraction can be
transmitted through at least a portion of the diffraction grating,
such as may be obtained by using an X-ray radiation source-based
tool, or for a light source-based tool, by means of a transparent
or semi-transparent diffraction grating and substrate. FIG. 5
depicts the 0.sup.th or specular order diffraction, but similar
results may be obtained by means of any higher diffraction order,
it being understood that for most embodiments the diffraction
signatures from the same diffraction order are most conveniently
compared.
[0068] In generating a plot as shown in FIG. 6, or in generating a
center of focus as shown in equation (1), it is understood that
various statistical techniques can be used to interpolate between
measured focus points to give a more precise measurement of center
of focus. These methods are known in the art, and may be
conventionally employed. Similarly, analysis means can be employed
which do not use each focus point, but rather perform an initial
analysis based on the diffraction signature difference across
multiple focus settings. It is preferred that the incremental
difference between focus setting steps be held constant, but it is
also contemplated that interpolation means can be employed where
the focus setting steps are not uniform.
[0069] Utilizing the methods of this invention, the differences in
the center of focus are typically less than 0.03 microns, which is
below a typical focus step size of 0.07 microns. This thus permits
focusing within the resolution of the lithography device, such as a
stepper.
[0070] The effect of dose on the center of focus may be analyzed in
a similar means. A series of diffraction grating sets, such as
diffraction grating sets 20, are generated over a determined
different focus range encompassing the center of focus, with the
dose varied in stepwise fashion from grating set to grating set.
The result is a series of diffraction grating sets each at a
different and known dose. A series of diffraction signatures are
then obtained for each diffraction grating set, by means of a
radiation source-based tool as set forth above. The resulting
series of diffraction signatures can be analyzed as above, such as
by diffraction signature difference analysis. The resulting center
of focus can be plotted against dose, thereby yielding the effect
of dose on the center of focus. By this means the dose setting or
settings with the most robust focus curves can be ascertained, such
that a dose setting with a minimum impact on the focus curve or
depth-of-focus can be selected.
[0071] It is further readily apparent that utilizing the
diffraction signature difference, such as shown in FIG. 6, and data
as to the location of the diffraction grating in the field, which
field may conventionally be a wafer stage, that the center of focus
as a function of position in the field may be plotted, as shown in
FIG. 8. Such plot may reveal aberrations in the lens system,
astigmatism, or other defects causing the center of focus over the
field to be non-uniform. Similarly, as shown in FIG. 9, tilt in the
field over both the x- and y-axes may be plotted, thereby showing
the stage tilt effects of the center of focus as a function of
position in the field.
[0072] Utilizing the methods and devices of this invention, it is
also possible to match the signatures at the center of focus as
determined by diffraction signature difference to a known library
of theoretical or actual diffraction signature differences. Such
match library can be significantly smaller than a conventional
theoretical library, which necessarily encompasses a wide range of
out-of-focus settings, thereby permitting more rapid library
generation, in the case of a theoretical library, smaller storage
requirements for the library, and a faster analysis time.
[0073] The methods and devices of this invention may also be used
for quality control testing, including analysis of the center of
focus determined by other means. This may be done in conjunction
with an angle-resolved scatterometer, described above, including
its associated computer system, or with other suitable devices
capable of making the described measurements.
[0074] By means of employing an angle-resolved scatterometer, the
diffraction signature is separated into distinct diffraction orders
at angular locations specified by the grating equation (2):
sin.crclbar..sub.l+sin.crclbar..sub.n=n.lambda./d (2)
[0075] where .crclbar..sub.l, is the angle of incidence, taken as
negative, .crclbar..sub.n is the angular location of the nth
diffraction order, .lambda. is the wavelength of incident light and
d is the spatial period or pitch of the diffraction grating. It can
thus be seen that for the 0.sup.th or specular diffraction order,
the angle of incidence is equal to the angular location of the
specular diffraction order. However, diffraction orders other than
the specular may be employed, and the appropriate angular location
determined as set forth above. Similar relationships govern other
modes of generating diffraction signatures, so that with any mode
of generating a diffraction signature either the specular
diffraction order or some higher diffraction order may be employed.
For example, in a wavelength resolve device, the angle
.crclbar..sub.l, may be held constant and the wavelength A varied,
and the equation solved for .crclbar..sub.n at a given n.
[0076] The methods and devices of this invention may also be used
for determination of the center of focus, whereby the center of
focus is adjusted by any suitable means, including use of
computer-based control systems, and the methods of this invention
used to determine when an acceptable or optimal focus has been
determined. The adjustment may be done by dose variations, or by
other means known in the art.
[0077] The invention may be further used for automatic or automated
determination of the center of focus, utilizing an autofocus
control system, whereby information as to the diffraction signature
analysis is used in a control system to determine the focus, such
as by dose variations.
[0078] Although the invention has been described in detail with
particular reference to these preferred embodiments, other
embodiments can achieve the same results. Variations and
modifications of the present invention will be obvious to those
skilled in the art and it is intended to cover in the appended
claims all such modifications and equivalents. The entire
disclosures of all references, applications, patents, and
publications cited above are hereby incorporated by reference.
* * * * *