U.S. patent application number 11/016148 was filed with the patent office on 2005-05-12 for system for measuring periodic structures.
Invention is credited to Gross, Kenneth P., Nikoonahad, Mehrdad, Smedt, Rodney, Zhao, Guoheng.
Application Number | 20050099627 11/016148 |
Document ID | / |
Family ID | 24983210 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050099627 |
Kind Code |
A1 |
Zhao, Guoheng ; et
al. |
May 12, 2005 |
System for measuring periodic structures
Abstract
A periodic structure is illuminated by polychromatic
electromagnetic radiation. Radiation from the structure is
collected and divided into two rays having different polarization
states. The two rays are detected from which one or more parameters
of the periodic structure may be derived. In another embodiment,
when the periodic structure is illuminated by a poly chromatic
electromagnetic radiation, the collected radiation from the
structure is passed through a polarization element having a
polarization plane. The element and the polychromatic beam are
controlled so that the polarization plane of the element are at two
or more different orientations with respect to the plane of
incidence of the polychromatic beam. Radiation that has passed
through the element is detected when the plane of polarization is
at the two or more positions so that one or more parameters of the
periodic structure may be derived from the detected signals. At
least one of the orientations of the plane of polarization is
substantially stationary when the detection takes place. To have as
small a footprint as possible, one employs an optical device that
includes a first element directing a polychromatic beam of
electromagnetic radiation to the structure and a second optical
element collecting radiation from the structure where the two
elements form an integral unit or are attached together to form an
integrated unit. To reduce the footprint, the measurement
instrument and the wafer are both moved. In one embodiment, both
the apparatus and the wafer undergo translational motion transverse
to each other. In a different arrangement, one of the two motions
is translational and the other is rotational. Any one of the
above-described embodiments may be included in an integrated
processing and detection apparatus which also includes a processing
system processing the sample, where the processing system is
responsive to the output of any one of the above embodiments for
adjusting a processing parameter.
Inventors: |
Zhao, Guoheng; (Milpitas,
CA) ; Gross, Kenneth P.; (San Carlos, CA) ;
Smedt, Rodney; (Los Gatos, CA) ; Nikoonahad,
Mehrdad; (Menlo Park, CA) |
Correspondence
Address: |
PARSONS HSUE & DE RUNTZ LLP
655 MONTGOMERY STREET
SUITE 1800
SAN FRANCISCO
CA
94111
US
|
Family ID: |
24983210 |
Appl. No.: |
11/016148 |
Filed: |
December 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11016148 |
Dec 17, 2004 |
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10748975 |
Dec 29, 2003 |
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10748975 |
Dec 29, 2003 |
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09742029 |
Dec 20, 2000 |
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6721052 |
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Current U.S.
Class: |
356/369 |
Current CPC
Class: |
G01B 11/00 20130101;
G01N 21/4788 20130101; G01N 21/21 20130101; G01N 21/956
20130101 |
Class at
Publication: |
356/369 |
International
Class: |
G01J 004/00 |
Claims
1. A method for measuring one or more parameters of a periodic
structure, comprising: directing a polychromatic beam of
electromagnetic radiation to the structure; collecting radiation
from the beam after it has been modified by the structure; dividing
the collected radiation into two collected rays having different
polarization states; detecting the two rays to provide two outputs;
and deriving the one or more parameters from the two outputs.
2-55. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates in general to optical techniques for
measuring periodic structures, and in particular to an improved
spectroscopic diffraction-based metrology system for measuring
periodic structures, such as grating-type targets on semiconductor
wafers.
[0002] In conventional techniques, optical microscopes have been
used for measuring the critical dimension ("CD") for semiconductor
lithographic processes. However, as the CD becomes smaller and
smaller, it cannot be resolved at any practical available optical
wavelengths in optical microscopy. Scanning electron microscope
technology has extremely high resolution. However, this technology
inherently requires large capital expenditures and heavy accessory
equipment such as vacuum equipment, which makes it impractical for
integration with lithographic processes. Two-theta-scatterometers
face similar practical challenges for integration, as they require
mechanical scanning over a wide range of angles. For this reason,
they are slow and difficult to integrate with process
equipment.
[0003] Ellipsometric techniques have also been used for CD
measurements. In U.S. Pat. No. 5,739,909, for example, Blayo et al.
describe a method of spectroscopic ellipsometry adapted to measure
the width of features in periodic structures. While ellipsometric
methods may be useful for some applications, such methods have not
been able to measure reflectances from the periodic structures.
[0004] None of the above-described techniques is entirely
satisfactory. It is therefore desirable to provide an improved
technique for measuring periodic structures where the
above-described difficulties are alleviated.
SUMMARY OF THE INVENTION
[0005] In many of the diffraction-based metrology or ellipsometric
techniques for measuring the critical dimension, theoretical models
employing libraries or non-linear regression are employed to find
the critical dimension or other parameters of the periodic
structure. Where the periodic structure measured is topographically
complex, it may be necessary to measure more than one radiation
parameter to get adequate information for the modeling process.
Where the periodic structure is simple, however, the measurement of
a single radiation parameter may be adequate. The use of fewer
radiation parameters in the modeling process reduces the
calculation time required so that the critical dimension and other
parameters of the structure can be found quickly. The embodiments
of this invention are simple in construction and flexible and may
be used for measuring one or more radiation parameters from the
radiation diffracted by the periodic structure.
[0006] In one embodiment of the invention, when the structure is
illuminated by polychromatic electromagnetic radiation, radiation
from the structure is collected and divided into two rays having
different polarization states. The two rays are detected, from
which one or more parameters of the periodic structure may be
derived. In another embodiment, when the periodic structure is
illuminated by polychromatic electromagnetic radiation, the
collected radiation from the structure is passed through a
polarization element having a polarization plane. The element and
the polychromatic beam are controlled so that the polarization
planes of the element are at two or more different orientations
with respect to the plane of incidence of the polychromatic beam.
Radiation that has passed through the element is detected when the
planes of polarization are at two or more positions so that one or
more parameters of the periodic structure may be derived from the
detected signals. At least one of the orientations of the plane of
polarization is substantially stationary when the detection takes
place.
[0007] When a device for measuring periodic structure parameters is
employed in a production environment, such as when it is integrated
with wafer processing equipment, it is desirable for the device to
have as small a footprint as possible. One embodiment of the
invention adapted for such environments employs an optical device
that includes a first element directing a polychromatic beam of
electromagnetic radiation to the structure and a second optical
element collecting radiation from the structure where the two
elements form an integral unit or are attached together to form an
integrated unit.
[0008] One way to reduce the footprint of the apparatus for
measuring the periodic structure in a wafer processing environment
is to move the apparatus relative to the wafer without moving the
wafer itself. However, since the apparatus includes a number of
components, it also has a significant size and footprint compared
to that of the wafer. Furthermore, it may be cumbersome to control
the sizable apparatus so that it can move in a two dimensional
plane without moving the wafer. Thus, it is envisioned that both
the apparatus for measuring the periodic structure and the sample
(e.g. wafer) are caused to move. In one embodiment, translational
motion of the apparatus and the wafer is caused where the two
motions are transverse to each other. In a different arrangement,
one of the two motions is translational and the other is
rotational. This facilitates the handling of the motion of the
apparatus while at the same time reduces the overall footprint.
[0009] Any one of the above-described embodiments may be included
in an integrated processing and detection apparatus which also
includes a processing system processing the sample, where the
processing system is responsive to the output of any one of the
above embodiments for adjusting a processing parameter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A is a schematic view of an apparatus for measuring
one or more parameters of the periodic structure to illustrate one
embodiment of the invention.
[0011] FIG. 1B is a schematic diagram of the plane of incidence and
the planes of polarization of the polarizer and analyzer of FIG. 1A
to illustrate the operation of the apparatus of FIG. 1A.
[0012] FIG. 1C is a schematic diagram of another apparatus for
measuring one or more parameters of the periodic structure to
illustrate an alternative embodiment of the invention particularly
suitable in an integrated processing and detection tool to
illustrate another embodiment of the invention.
[0013] FIG. 2 is a schematic diagram of yet another measurement
device for measuring one or more parameters of the periodic
structure to illustrate yet another embodiment of the
invention.
[0014] FIG. 3 is a schematic diagram of illumination and collecting
optics employed in the different embodiments of this invention to
illustrate an asymmetric numerical aperture for illumination and
collection.
[0015] FIG. 4A is the perspective view of a semiconductor wafer and
an apparatus for measuring periodic structures on the wafer and
instruments for moving the apparatus and the wafer to illustrate
yet another embodiment of the invention.
[0016] FIG. 4B is a perspective view similar to that of FIG. 4A
except that the apparatus of FIG. 4B is self-contained in the
optical head and includes a polychromatic electromagnetic radiation
source.
[0017] FIG. 4C is a schematic view of a semiconductor wafer and of
optics arranged so that the optical path length remains
substantially constant irrespective of the positions of the moving
optics head relative to the wafer and illustrates yet another
embodiment of the invention.
[0018] FIG. 5 is a schematic view of a system for moving the optics
head along a straight line and for rotating the wafer.
[0019] FIG. 6 is a schematic diagram of an apparatus illustrating
in more detail the components in the optics head in FIGS. 4A,
4C.
[0020] FIG. 7 is a schematic view of an apparatus for measuring
periodic structures illustrating in more detail an alternative
construction of the optics head in FIGS. 4A, 4C.
[0021] FIG. 8 is a schematic view of the different components in
the optics head to illustrate in more detail the optics head of
FIGS. 4B, 5.
[0022] FIG. 9 is a flowchart illustrating the operation of the
apparatuses of FIGS. 1A, 1C and 2.
[0023] FIG. 10 is a schematic block diagram illustrating a wafer
processing apparatus including a track/stepper and an etcher and a
spectroscopic measurement device where information from a periodic
structure and/or associated structures from the device as used to
control the manufacturing process and the track, stepper and/or
etcher to illustrate the invention.
[0024] FIG. 11 is a schematic block diagram and flowchart
illustrating in more detail the track/stepper of FIG. 7.
[0025] For simplicity in description, identical components are
labeled by the same numerals in this application.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0026] As shown in FIG. 1, a broadband source of electromagnetic
radiation 20 provides a polychromatic beam with wavelength
components preferably from 180 to 1000 nm. In one embodiment,
source 20 may be a xenon lamp. The beam 22 from source 20 is shaped
by beam shaping optics 24. A portion of the beam 22 is diverted by
a beam divider 26 to spectrometer 28 for monitoring intensity
variations in beam 22 in order to normalize the results of
detection. Beam 22 is polarized by a polarizer 30 and the polarized
beam is directed to sample 32 to illuminate a periodic structure
thereon. Preferably the plane of incidence 34 (see FIG. 1B) of beam
22 is perpendicular to the grating lines in the periodic structure
of sample 32. Radiation originating from beam 22 that has been
modified by sample 32, such as by reflection or transmission, is
collected by collection optics 42 as a collected beam 40 and passed
through an analyzer 44 which splits the collected radiation into
two rays: an extraordinary ray or e-ray 46 and an ordinary ray or
o-ray 48. Rays 46 and 48 are detected respectively by two
spectrometers 52, 54 with outputs delivered to computer 60 along
with the output of spectrometer 28 for determining one or more
parameters of a periodic structure on sample 32.
[0027] Depending on the orientation of the polarizer 30 and
analyzer 44, the e-ray and o-ray contain information concerning two
different radiation parameters. This is illustrated more clearly in
reference to FIG. 1B. As shown in FIG. 1B, R.sub.s is the
reflectance of the S component of beam 40 in the direction normal
to the plane of incidence and R.sub.p is the reflectance of the P
component of beam 40 within the plane of incidence 34 of beam 22.
Assuming that polarizer 30 is oriented at an angle to the plane of
incidence 34, where the angle is other than 0.degree., 90.degree.,
180.degree. or 270.degree., then beam 40 will contain both non-zero
P components and S components. When a plane of polarization is said
to be at an angle to the plane of incidence in this application,
this angle can be of any value, including 0 and 180 degrees, unless
stated otherwise. For example, beam 33 may have the input
polarization along the plane 62 at an angle between 0 and 90
degrees to plane 34 in FIG. 1B. In such event, depending on the
orientation of the plane of polarization of analyzer 44, different
radiation parameters are measured. For example, if the plane of
polarization of analyzer 44 is aligned with the plane of incidence
34 or perpendicular to the plane of incidence, then the components
R.sub.p, R.sub.s are measured, and the o-ray and e-ray would yield
these two intensities. On the other hand, if the plane of
polarization of the analyzer 44 is aligned with the input
polarization plane 62 of beam 33, or is arranged to be
perpendicular to plane 62, then one of the two rays 46, 48 would
contain information concerning a signal X and the other ray would
contain information concerning a signal Y (a vector perpendicular
to signal X), where X and Y are defined as follows:
X=.vertline.r.sub.s-r.sub.p.vertline..sup.2
Y=.vertline.r.sub.s+r.sub.p.vertline..sup.2
[0028] where r.sub.s and r.sub.p denote the complex amplitude
reflection coefficients for S and P polarizations, respectively,
while R.sub.s and R.sub.p are the intensity reflection
coefficients.
[0029] If the plane of polarization of polarizer 30 is in the plane
of incidence 34 or perpendicular to it, it is not possible to
measure both R.sub.p and R.sub.s simultaneously or measure X and Y
simultaneously. This is true when the grating is perpendicular or
parallel to the plane of incidence 34. In other words, where the
grating is not perpendicular or parallel to the plane of incidence
34, it is possible to measure both R.sub.p and R.sub.s
simultaneously or measure X and Y simultaneously irrespective of
the plane of polarization of beam 33. It is also possible to remove
polarizer 30 altogether so that the beam 22 illuminating sample 32
is unpolarized. In such event, in order to measure R.sub.s and
R.sub.p, the plane of polarization of analyzer 44 is preferably
aligned with the plane of incidence 34 or perpendicular to it. Even
where the plane of polarization of analyzer 44 is not parallel to
the plane of incidence 34 or perpendicular to it, it is possible to
measure quantities related to R.sub.s and R.sub.p, which would be
useful for measuring the structure.
[0030] Preferably the plane of polarization of the analyzer 44 is
at an angle of 0, 45, 90, 135, 180, 225, 270 or 315 degrees from
the plane of incidence 34, since this will optimize the amplitudes
of the quantities measured. Where the plane of polarization of the
analyzer 44 is at an angle of 0, 90, 180 or 270 degrees to plane
34, R.sub.p, R.sub.s may be measured. Where the plane of
polarization of the analyzer 44 is at an angle of 45, 135, 225 or
315 degrees to plane 34, X, Y may be measured. Where the
polarizer-30 is oriented at one of the angles of 0, 45, 90, 135,
180, 225, 270 or 315 degrees from the plane of incidence 34, again
the amplitudes of the quantities measured will be optimized.
[0031] After R.sub.p, R.sub.s, X, Y have been measured, it is then
possible to derive other radiation parameters therefrom in computer
60, so that a more complete list of quantities that can be obtained
from the measurement are the following: 1 R s , R p , R s - R p , (
R s - R p ) / ( R s + R p ) X = r s - r p 2 , Y = r s + r p 2 X / Y
, X - Y - 4 r s ; r p cos ( X - Y ) / ( X + Y ) sin 2 cos cos , tan
= r p r s
[0032] where r.sub.s and r.sub.p denote the complex amplitude
reflection coefficients for S and P polarizations respectively,
while R.sub.s and R.sub.p are the intensity reflection
coefficients: R.sub.s=.vertline.r.sub.s.vertline..sup.2,
R.sub.p=.vertline.r.sub.p.vert- line..sup.2. .DELTA.denotes the
phase difference between reflected s- and p-polarized fields. The
quantities tan .PSI. and cos .DELTA. are ellipsometric parameters
known to those in the art.
[0033] The radiation parameter (Rs-Rp)/(Rs+Rp) has better
sensitivity than Rs or Rp alone, since the reflectance difference
(Rs-Rp) would tend to cancel out unwanted reflectance from
isotropic film structures surrounding, over or underneath the
grating targets when the illumination beam is at normal or near
normal incidence. The ratio also reduces effects of intensity
variations of the illumination beam on the measurement. By
canceling out or reducing unwanted reflectance from isotropic film
structures surrounding, over or underneath the grating targets,
this feature renders the detection system more robust against stray
reflectance around the grating target, and reduces the influence of
thickness variations of films over or under the grating. Since the
area available for the grating target is limited, where the optical
system is not exactly aligned properly, some of the radiation from
the illumination beam may be reflected from an area outside of the
target, so that by using the computer 60 to calculate the
reflectance difference, the measurement is much more robust despite
optical misalignment errors, and reduces the influence of thickness
variations of films over or under the grating. These variations are
caused by process variations that introduce error in CD
measurement. By measuring difference in reflectance, these unwanted
effects tend to cancel to provide a more accurate CD measurement.
As noted above, the output of spectrometer 28 may be used to
normalize the output of spectrometers 52, 54 in order to eliminate
the effects of intensity variation from source 20.
[0034] Where it is desirable to employ a compact system for
detecting periodic structures, such as where the device is to be
integrated with processing equipment, the configuration shown in
FIG. 1C may be particularly useful. As shown in FIG. 1C, the
incident beam 22 is passed through an element 30' which can be a
polarizer, focused by optics 24' to sample 32. The zeroth order
diffraction from the periodic structure on sample 32 is collected
by focusing optics 24`and passed through an ` analyzer 44'. The
functions of the polarizer 30' and analyzer 44' are similar to
those of polarizer 30 and analyzer 44 of FIG. 1A described above.
For a compact design, polarizer 30' and analyzer 44' may be a
single integral unit or are separate elements attached together to
form an integrated unit 62 so that the configuration of FIG. 1C is
particularly compact. This allows the apparatus of FIG. 1C to have
a particularly small footprint.
[0035] FIG. 2 is a schematic view of an apparatus 100 for measuring
a periodic structure to illustrate another embodiment of the
invention. For simplicity, the computer that processes the output
of the two spectrometers 28, 54 has been omitted to simplify the
drawing. System 100 differs from system 10 of FIG. 1A in that in
system 100, either one or both of the polarizer 30 and analyzer 44
may be rotated whereas the polarizer and analyzer in system 10 of
FIG. 1A are stationary and do not move. Furthermore, system 100
includes a single spectrometer for detecting the collected
radiation instead of two spectrometers as in FIG. 1A. However, by
rotating the polarizer 30 or analyzer 44, two or more radiation
parameters may be detected. The operation of system 100 is
different from an ellipsometer in that, for at least one
measurement, the polarizer 30 and analyzer 44 are stationary and do
not move. This permits system 100 to measure quantities such as
R.sub.s, R.sub.p whereas it would be extremely difficult for an
ellipsometer to measure R.sub.s, R.sub.p accurately.
[0036] System 100 operates under principles similar to that of FIG.
1A. Thus, if the plane of polarization of polarizer 30 is at an
angle different from 0, 90, 180 and 270 degrees to the plane of
incidence of beam 22, the plane of polarization of analyzer 44 may
be oriented in the same manner as that described above in reference
to FIG. 1B to measure at least one of the parameter values R.sub.s,
R.sub.p, X, Y. Since only one spectrometer is used instead of two
as in FIG. 1A, the quantities are measured sequentially rather than
simultaneously as in the embodiment of FIG. 1A. Thus, in reference
to FIG. 1B, if the plane of polarization of analyzer 44 is aligned
with the plane of incidence 34 of beam 22, then the spectrometer
would detect the radiation parameter R.sub.p. Then analyzer 44 is
rotated so that its plane of polarization is substantially normal
to plane 34; at this stationary position of analyzer 44, the
spectrometer 52 measures the radiation parameter R.sub.s. The same
can be said for the measuring of X, Y, where analyzer 44 is rotated
so that its plane of polarization is substantially parallel and
normal to plane 62, where the measurements are conducted
sequentially rather than simultaneously. In other words, analyzer
44 may be sequentially oriented so that its plane of polarization
is substantially at some of the angles of about 0, 45, 90, 135,
180, 225, 270 or 315 degrees to the plane of incidence 34, in order
to measure one or more of the parameter values R.sub.s, R.sub.p, X,
Y.
[0037] FIG. 3 is a schematic view of illumination optics 24" and
collection optics 42" to illustrate another aspect of the
invention. CD measurements require a dedicated grating-type target
that is printed on the scribe line between dies on a semiconductor
wafer. The area available for such target is typically limited so
that the target size is typically kept to a minimum, such as around
25 microns square. Even a small percentage of light reflected from
areas outside of the grating target can affect the measurement
accuracy. For this reason, a relatively large focusing numerical
aperture is employed to keep the energy of the illumination beam
focused on the small target area. Thus, as shown in FIG. 3, the
illumination numerical aperture 72 is relatively large. A large
numerical aperture in the collection optics increases the computing
time for modeling since the spectrum at more incident angles would
need to be calculated. For this reason, it may be desirable to
employ a small collection numerical aperture, such as aperture 74
shown in FIG. 3, for the collection optics. While FIG. 3
illustrates the collection optics in transmission mode, the same
can be applied to reflection mode, such as those in FIGS. 1A, 1C
and 2.
[0038] In another embodiment of the invention, both the apparatus
and the wafer are moved relative to each other. Where the apparatus
is moved in a translational motion along a straight line, such
motion is easy to control despite the size and the number of
components in the apparatus. Where the wafer is moved in the
direction perpendicular to the motion of the apparatus, the
footprint of the entire system can be reduced by making the
dimension of the apparatus elongated along the direction of motion
of the wafer, in order to minimize the dimension of the apparatus
along the direction of its motion, in order to minimize footprint.
This is illustrated in FIG. 4A. As shown in FIG. 4A, an optics head
102 is mounted onto a translation track 104 and a motor (not shown)
causes head 102 to move along the track 104 along direction 106.
Track 104 is attached onto supporting base 112, having tracks 115.
The semiconductor wafer 32' is placed onto a chuck 114 which is
caused by another motor (not shown) to move along track 115 along
direction 116. Where directions 106, 116 are transverse to each
other, such as perpendicular to each other, the movement of the
optics head 102 and wafer 32' as described above would enable the
optics head 102 to inspect the entire surface area of sample or
wafer 32'. As will be evident from FIG. 4A, the optics head 102 is
elongated in shape, with the elongated dimension substantially
along direction 116, the direction of motion of wafer 32'. This
reduces the dimension of optics head 102 in the direction of motion
106 of the head, thereby reducing the footprint required. Thus, as
shown in FIG. 4A, the footprint of the entire assembly shown in
FIG. 4A has the dimension along direction 106 substantially equal
to the sum of the dimension of wafer 32' and optics head 102. The
footprint of the assembly along direction 116 is substantially
twice the dimension of wafer 32' along such direction.
[0039] FIG. 4B illustrates in perspective view an assembly 120 that
is similar to assembly 110 of FIG. 4A, except that assembly 120,
the optics head contains a radiation source and all collection
optics and sensor components, so that it has no provision for an
optical fiber leading to the optics head. In FIG. 4A, on the other
hand, the optics head 102 does not include a source of radiation,
so that an optical channel, such as an optical fiber 118, is
employed to supply radiation to optics head 102. Track 104 has a
passage therein to provide a conduit for the optical channel 118 to
the optics head. In FIG. 4B, the optics head 102' contains a
radiation source, so that track 104' does not need to provide the
passage for an optical channel.
[0040] FIG. 4C is a schematic diagram of a semiconductor wafer and
of optics arranged so that the optical path lengths to all
positions of the optics head remain substantially constant
irrespective of the position of the optics head relative to the
wafer to illustrate yet another embodiment of the invention. In
FIG. 4C, radiation used for inspection is supplied along an optical
channel 118. When the optics head is at position 102(1) or D,
radiation along channel 118 would reach the optics head 102 by
reflection by means of folding mirror 120 (at position 120(1) or C)
to the optics head. In other words, the optical path length would
include the paths BC and CD. In the embodiment shown in FIG. 4C,
the wafer is moved along direction 116 or one parallel to the x
axis, and the optics head is moved along direction 122
substantially at 45 degrees to the x axis. The folding mirror is
moved along the y axis at a speed such that the mirror and the
optics head are maintained to be along a line parallel to the x
axis or direction 116. Thus, after the optics head and the wafer
are both moved so that the optics head is now at position 102(2) or
A, and the mirror at position 120(2) or B, the optics head will
receive radiation from channel 118 along a different optical path
AB than it received when it was at position 102(1). Since the line
AD is at 45 degrees to direction 116, the optical path length AB is
equal to the sum of the path lengths BC+DC. If the optical beam
along channel 118 is not collimated, any optical path difference
depending on the positions of the optics head may cause
complications in the measurement process. At a minimum, this may
call for a calibration process which may be cumbersome. For this
reason, in the embodiment of FIG. 4C, the optics for directing
radiation from channel 118 to different positions of the optics
head is designed so that the total optical path length remains
substantially the same irrespective of the position of the optics
head.
[0041] In yet another embodiment, to further reduce the footprint
of the system, the wafer is rotated while the apparatus for
measuring the periodic structure is moved along substantially a
straight line, so that the illumination beam from the apparatus
scans a spiral path on the wafer. This is shown in FIG. 5. Thus,
the optics head 102' is mounted onto a translation track 104' as in
FIG. 4B and the optics head is moved along the track along
direction 106 by a motor (not shown) as before. The wafer (not
shown), however, is situated on top of a support 124 which is
rotated along direction 126 by a motor (not shown). In this manner,
the optics head 102' would have access to any location on the wafer
for taking measurement.
[0042] FIG. 6 is a block diagram of an optical arrangement 140
illustrating a practical implementation for the optics head 102 of
FIGS. 4A, 4C. As shown in FIG. 6, the optics head 102a contains an
optical arrangement similar to that in FIG. 1A, where the analyzer
44 splits the collected beam 40 into an o-ray directed to
spectrometer 54 and an e-ray directed to spectrometer 52 through a
prism 141, which compensates for the dispersion caused by the
analyzer 44 for the deviated beam. The light from light source is
conveyed through an optical fiber 20' and reflected by a
collimating mirror 142 to polarizer 30 and apodizer 144 in a
collimated light piston to optics head 102a. In optics head 102a,
the light is reflected by mirror 146 to the beam shaping optics 24,
42 comprising two focus/collimating mirrors to sample 32. The same
focus/collimating mirrors 24/42 collect light reflected by sample
32 to analyzer 44. As will be noted from FIG. 6, collimating or
focusing mirrors are used for directing and focusing radiation from
the light source cross the sample 32 and for collecting light
reflected by the sample towards the analyzer. By using mirrors for
such purposes, chromatic aberration is reduced, especially where
the detection employs radiation over a wide range of wavelengths,
such as 150 nanometers to 1,000 nanometers.
[0043] FIG. 7 is a schematic view of another alternative
construction 102b of the optics head in FIGS. 4A, 4C. As shown on
FIG. 7, optics head 102b includes an optical fiber 20' conveying
radiation from a radiation source (not shown) through polarizer 30
and apodizer 144 and focused by mirror 152 to sample 32. Sample 32
is moved by a stage 148 along one direction (x) or along two
transverse (x/y) directions. Radiation reflected by sample 32 is
collected by collection mirror 154 and focused to an analyzer 44.
The radiation that passes the analyzer is conveyed to a
spectrometer (not shown) by fiber 156. Analyzer 44 is rotated to
desirable positions as described above in reference to FIG. 2.
[0044] FIG. 8 is a schematic view of the different components in
the optics head 102' to illustrate in more detail the optics head
of FIGS. 4B, 5. As shown in FIG. 8, optics head 102' includes
radiation source 20 which supplies a beam of radiation that passes
polarizer 30 and is reflected by a beam splitter 162 and focused by
a broadband objective 164 to wafer 32. The reflected radiation from
the wafer is collected by objective 164, passed through beam
splitter 162 and reflected by mirror 166 to analyzer 44 to
spectrometer 54. Preferably the analyzer 44 is rotated so that its
plane of polarization and the plane of polarization of the
polarizer 30 are as described above in reference to FIG. 2.
[0045] The process for deriving one or more parameters of the
periodic structure using a computer such as computer 60 will now be
described in reference to FIG. 9 which is a flowchart illustrating
the operation of apparatuses of FIGS. 1A, 1B and 2. First, one
obtains the possible range of values of the one or more parameters
of the periodic structure, such as critical dimension (CD), height
(H), sidewall angle (SWA) and other parameters (block 202). A
theoretical model is then constructed (block 204) and a
multi-dimensional library is constructed (block 206).
[0046] One of the apparatuses of FIGS. 1A, 1C and 2 is then
utilized to obtain a measurement of the zeroth order diffraction of
the periodic structure in the manner described above (block 208),
where such diffraction is detected by a single or dual channel
spectrometer as described above (block 210). As described above,
the quantities R.sub.s, R.sub.p and X, Y may be measured directly
using the apparatuses of FIGS. 1A, 1C and 2. From these four
quantities, other radiation parameters may be derived in the manner
described above (block 212). The multi-dimensional library 206
constructed is then utilized and comparison is made between spectra
in the library and the measured data from block 212. The parameters
of the model are varied (block 206) in order to minimize the error
between the spectra corresponding to the predicted values of the
parameters of the periodic structure according to the model, and
the measured spectra from block 212 in order to arrive at the
actual values of the parameters of the periodic structure, such as
CD, height and sidewall angle (diamond 214).
[0047] Alternatively, instead of using a multi-dimensional library,
a non-linear optimization (e.g. non-linear regression) algorithm
may be employed instead. For this purpose, the initial or seed
values of the parameters of the periodic structure are obtained
(block 222). An optimization model is constructed (block 224) and a
non-linear regression algorithm (block 226) is employed with
feedback (diamond 214) to again minimize the error between the
spectra of radiation parameters corresponding to the predicted
values of the parameters of the periodic structure and the measured
spectra from block 212 in order to arrive at the actual values of
parameters such as CD, height and sidewall angle (block 216).
[0048] FIG. 10 is a block diagram of an integrated
diffraction-based metrology system, a photolithographic
track/stepper and an etcher to illustrate another aspect of the
invention. A layer of material such as photoresist is formed on the
surface of a semiconductor wafer by means of track/stepper 350,
where the photoresist forms a grating structure on the wafer. One
or more of the CD, H, SWA and/or other parameters of the grating
structure are then measured using systems 10, 80, 100 of FIGS. 1A,
1C and 2, and one or more of the above-described techniques may be
employed if desired to find the value(s) of the one or more
parameters of the photoresist pattern. Such value(s) from the
computer 60 are then fed back to the track/stepper 350, where such
information may be used to alter the lithographic process in
track/stepper 350 to correct any errors. In semiconductor
processing, after a layer of photoresist has been formed on the
wafer, an etching process may be performed, such as by means of
etcher 360. The layer of photoresist is then removed in a manner
known in the art and the resulting grating structure made of
semiconductor material on the wafer may again be measured if
desired using system 10, 80 or 100. The value(s) measured using any
one or more of the above-described techniques may be supplied to
the etcher for altering any one of the etching parameters in order
to correct any errors that have been found using system 10, 80 or
100. Of course the results obtained by one or more of the above
described techniques in system 10, 80 or 100 may be used in both
the track/stepper and the etcher, or in either the track/stepper or
the etcher but not both. The track/stepper 350 and/or etcher 360
may form an integrated single tool with the system 10, 80 or 100
for finding the one or more parameters of a periodic structure, or
may be separate instruments from it.
[0049] FIG. 11 is a schematic view of the track/stepper 350 and an
associated flowchart illustrating a process for semiconductor wafer
processing to illustrate in more detail the points of integration
of the processing process with the detection of profiles of
periodic structures and associated films to illustrate in more
detail a part of the process in FIG. 10. As shown in FIG. 11, a
semiconductor wafer 352 may be loaded from a cassette loader 354 to
several stations labeled "prime," "coat," "soft bake," "EBR" etc.
Then the wafer 352 is delivered by a stepper interface 356 to
exposure tool 358. The different processes at the four locations
mentioned above are set forth below:
[0050] At the location "Prime", the wafer undergoes chemical
treatment before a layer of photoresist is spun on it, so that the
photoresist layer can stick to wafer. At the location "Coat", a
layer of photoresist coating is spun onto the wafer. At "Soft
bake", the layer of resist is baked to remove chemical solvent from
the resist. At "EBR" which stands for "edge-bead removal", a
solvent nozzle or laser is used to remove excess photoresist from
the edge of wafer.
[0051] After the wafer has been exposed to radiation by tool 358,
the wafer then undergoes four additional processes: "PEB," "PEB
chill," "Develop," and "Hard bake." At "PEB or post exposure bake",
the wafer is baked to reduce standing-wave effect from the exposure
tool. Then it is cooled at "PEB chill". The wafer is then washed
with reagent to develop the photoresist, so that unexposed
(negative) or exposed (positive) photoresist is removed. The wafer
then is baked at "Hard bake" to stabilize the photoresist pattern.
It will be noted that all of the components of device 350 of FIG. 8
except for the "exposure tool" 358 is known as the "Track" (also
called cluster).
[0052] After these latter four processes have been completed, the
wafer 352 is then returned to the cassette loader 354 and this
completes the processing involving the stepper 350. The detection
system 10, 80 or 100 may be applied at arrow 362 to measure the
parameters of the periodic structure and associated film(s). Thus,
such parameters may be measured after "hard bake."
[0053] 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 equivalents. All references referred to herein are
incorporated by reference in their entirety.
* * * * *