U.S. patent application number 13/683911 was filed with the patent office on 2014-05-22 for in-sequence spectrographic sensor.
This patent application is currently assigned to Applied Materials, Inc.. The applicant listed for this patent is Applied Materials, Inc.. Invention is credited to William H. McClintock, Wen-Chiang Tu, Zhihong Wang, Jimin Zhang.
Application Number | 20140141694 13/683911 |
Document ID | / |
Family ID | 50728360 |
Filed Date | 2014-05-22 |
United States Patent
Application |
20140141694 |
Kind Code |
A1 |
Zhang; Jimin ; et
al. |
May 22, 2014 |
In-Sequence Spectrographic Sensor
Abstract
A method of controlling a polishing system includes polishing a
substrate at a first polishing station, transporting the substrate
to an in-line optical metrology system positioned between the first
polishing station and a second polishing station, at the in-line
optical metrology system measuring a spectrum reflected from the
substrate, and generating a characterizing value from the spectrum,
determining that the substrate needs rework based on the
characterizing value, returning the substrate to the first
polishing station and performing rework of the substrate at the
first polishing station; and transporting the substrate to the
second polishing station and polishing the substrate at the second
polishing station.
Inventors: |
Zhang; Jimin; (San Jose,
CA) ; Wang; Zhihong; (Santa Clara, CA) ; Tu;
Wen-Chiang; (Mountain View, CA) ; McClintock; William
H.; (Los Altos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Materials, Inc. |
Santa Clara |
CA |
US |
|
|
Assignee: |
Applied Materials, Inc.
Santa Clara
CA
|
Family ID: |
50728360 |
Appl. No.: |
13/683911 |
Filed: |
November 21, 2012 |
Current U.S.
Class: |
451/5 |
Current CPC
Class: |
B24B 37/005 20130101;
B24B 49/12 20130101 |
Class at
Publication: |
451/5 |
International
Class: |
B24B 49/12 20060101
B24B049/12 |
Claims
1. A method of controlling a polishing system, comprising:
polishing a substrate at a first polishing station; transporting
the substrate to an in-line optical metrology system positioned
between the first polishing station and a second polishing station;
at the in-line optical metrology system measuring a spectrum
reflected from the substrate, and generating a characterizing value
from the spectrum; determining that the substrate needs rework
based on the characterizing value; returning the substrate to the
first polishing station and performing rework of the substrate at
the first polishing station; and transporting the substrate to the
second polishing station and polishing the substrate at the second
polishing station.
2. The method of claim 1, wherein the substrate is held by a
carrier head and the carrier head is suspended from a track, and
the transporting the substrate is performed by moving the carrier
head along the track.
3. The method of claim 1, wherein generating the characterizing
value comprises obtaining a plurality of measured spectra with the
optical metrology system from a plurality of different measurement
spots within an area on the substrate, generating a plurality of
values based on the plurality of measured spectra, and combining
the values to generate the characterizing value.
4. The method of claim 3, wherein generating a plurality values
comprises comparing each of the plurality of measured spectra to a
reference spectrum to generate a similarity value.
5. The method of claim 3, wherein the substrate comprises a
plurality of dies, and the area is substantially equal to an area
of one of the dies.
6. The method of claim 1, wherein generating the characterizing
value comprises at least one of identifying a matching reference
spectrum from a library of reference spectra and determining the
characterizing value associated with the matching reference
spectrum, determining a wavelength or width of a peak or valley in
the spectrum, or fitting an optical model to the spectrum.
7. The method of claim 1, comprising measuring another spectrum
reflected from the substrate at the in-line optical metrology
system positioned between the first polishing station and a second
polishing station after performing rework and before transporting
the substrate to the second polishing station.
8. The method of claim 1, wherein the polishing the substrate at
the first polishing station comprises a filler layer clearing
recipe, and polishing the substrate at the second polishing station
comprises an underlying layer polishing recipe.
9. The method of claim 1, wherein polishing the substrate at the
first polishing station comprises a bulk polishing step of a copper
damascene process.
10. The method of claim 9, wherein polishing the substrate at the
second polishing station comprises a dielectric exposure step of a
copper damascene process.
11. The method of claim 1, comprising transporting the substrate to
a cassette and performing polishing of at least one other substrate
at the first polishing station before returning the substrate to
the first polishing station and performing rework of the substrate
at the first polishing station.
12. A polishing system, comprising: a first polishing station
including a first support for a first polishing pad; a second
polishing station including a second support for a second polishing
pad; a carrier head to hold a substrate, the carrier head supported
by a support structure and movable between the first polishing
station and the second polishing station; an in-line optical
metrology system positioned between the first polishing station and
the second polishing station, the optical metrology system
configured to measure a spectrum reflected from the substrate and
generate a characterizing value from the spectrum; and a controller
configured to cause the carrier head to move to the first polishing
station, to cause the substrate to be polished at the first
polishing station, to cause the carrier head to move to the in-line
metrology system, to determining whether the substrate needs rework
based on the characterizing value, to cause the carrier head to
return to the first polishing station and performing rework of the
substrate at the first polishing station, to cause the carrier head
to move to the second polishing station, and to cause the substrate
to be polished at the second polishing station.
13. The system of claim 12, wherein the support structure comprises
a track and the carrier head is movable along the track.
14. The system of claim 13, wherein the carrier head is suspended
from a carriage on the track.
15. The system of claim 12, wherein the controller is configured to
cause the carrier head to move to the in-line optical metrology
system and to receive another spectrum reflected from the substrate
at the in-line optical metrology system after performing rework and
before transporting the substrate to the second polishing
station.
16. The system of claim 12, comprising a cassette and a robot to
transfer the substrate from the carrier head to the cassette, and
wherein the controller is configured to cause the robot to
transport the substrate from the carrier head to the cassette
before the substrate is returned to the first polishing station and
rework of the substrate is performed at the first polishing
station.
17. The system of claim 16, wherein the controller is configured to
cause polishing of at least one other substrate at the first
polishing station before the substrate is returned to the first
polishing station and rework of the substrate is performed at the
first polishing station.
Description
TECHNICAL FIELD
[0001] This disclosure relates to optical metrology and control of
a polishing apparatus.
BACKGROUND
[0002] An integrated circuit is typically formed on a substrate by
the sequential deposition of conductive, semiconductive, or
insulative layers on a silicon wafer. One fabrication step involves
depositing a filler layer over a non-planar surface and planarizing
the filler layer. For certain applications, the filler layer is
planarized until the top surface of a patterned layer is exposed. A
conductive filler layer, for example, can be deposited on a
patterned insulative layer to fill the trenches or holes in the
insulative layer. After planarization, the portions of the metallic
layer remaining between the raised pattern of the insulative layer
form vias, plugs, and lines that provide conductive paths between
thin film circuits on the substrate. For other applications, such
as oxide polishing, the filler layer is planarized until a
predetermined thickness is left over the non planar surface. In
addition, planarization of the substrate surface is usually
required for photolithography.
[0003] Chemical mechanical polishing (CMP) is one accepted method
of planarization. This planarization method typically requires that
the substrate be mounted on a carrier or polishing head. The
exposed surface of the substrate is typically placed against a
rotating polishing pad. The carrier head provides a controllable
load on the substrate to push it against the polishing pad. An
abrasive polishing slurry is typically supplied to the surface of
the polishing pad.
[0004] Variations in the slurry distribution, the polishing pad
condition, the relative speed between the polishing pad and the
substrate, and the load on the substrate can cause variations in
the material removal rate. These variations, as well as variations
in the initial thickness of the substrate layer, cause variations
in the time needed to reach the polishing endpoint. Therefore,
determining the polishing endpoint merely as a function of
polishing time can lead to overpolishing or underpolishing of the
substrate. Various in-situ monitoring techniques, such as optical
or eddy current monitoring, can be used to detect a polishing
endpoint.
SUMMARY
[0005] In some systems, the substrate is monitored in-situ during
polishing, e.g., by optically or eddy current techniques. However,
existing monitoring techniques may not reliably halt polishing at
the desired point. A spectrum from the substrate can be measured by
an in-sequence metrology station. That is, the spectrum can be
measured while the substrate is still held by the carrier head, but
at a metrology station positioned between the polishing stations. A
value can be calculated from the spectrum which can be used in
controlling a polishing operation at one or more of the polishing
stations.
[0006] In one aspect, a method of controlling a polishing system
includes polishing a substrate at a first polishing station,
transporting the substrate to an in-line optical metrology system
positioned between the first polishing station and a second
polishing station, at the in-line optical metrology system
measuring a spectrum reflected from the substrate, and generating a
characterizing value from the spectrum, determining that the
substrate needs rework based on the characterizing value, returning
the substrate to the first polishing station and performing rework
of the substrate at the first polishing station; and transporting
the substrate to the second polishing station and polishing the
substrate at the second polishing station.
[0007] Implementations may include one or more of the following
features. The substrate may be held by a carrier head and the
carrier head is suspended from a track, and transporting the
substrate may be performed by moving the carrier head along the
track. Generating the characterizing value may include obtaining a
plurality of measured spectra with the optical metrology system
from a plurality of different measurement spots within an area on
the substrate, generating a plurality of values based on the
plurality of measured spectra, and combining the values to generate
the characterizing value. Generating the plurality of values may
include comparing each of the plurality of measured spectra to a
reference spectrum to generate a similarity value. The substrate
may have a plurality of dies, and the area may be substantially
equal to an area of one of the dies. Generating the characterizing
value may include at least one of identifying a matching reference
spectrum from a library of reference spectra and determining the
characterizing value associated with the matching reference
spectrum, determining a wavelength or width of a peak or valley in
the spectrum, or fitting an optical model to the spectrum. Another
spectrum reflected from the substrate may be measured at the
in-line optical metrology system positioned between the first
polishing station and a second polishing station after performing
rework and before transporting the substrate to the second
polishing station. Polishing the substrate at the first polishing
station may include a filler layer clearing recipe, and polishing
the substrate at the second polishing station may include an
underlying layer polishing recipe. Polishing the substrate at the
first polishing station may be a bulk polishing step of a copper
damascene process. Polishing the substrate at the second polishing
station may be a dielectric exposure step of a copper damascene
process. The substrate may be transported to a cassette before
returning the substrate to the first polishing station and
performing rework of the substrate at the first polishing station.
At least one other substrate may be polished at the first polishing
station before returning the substrate to the first polishing
station and performing rework of the substrate at the first
polishing station.
[0008] In another aspect, a polishing system includes a first
polishing station including a first support for a first polishing
pad, a second polishing station including a second support for a
second polishing pad, a carrier head to hold a substrate, the
carrier head supported by a support structure and movable between
the first polishing station and the second polishing station, an
in-line optical metrology system positioned between the first
polishing station and the second polishing station, the optical
metrology system configured to measure a spectrum reflected from
the substrate and generate a characterizing value from the
spectrum, and a controller configured to cause the carrier head to
move to the first polishing station, to cause the substrate to be
polished at the first polishing station, to cause the carrier head
to move to the in-line metrology system, to determining whether the
substrate needs rework based on the characterizing value, to cause
the carrier head to return to the first polishing station and
performing rework of the substrate at the first polishing station,
to cause the carrier head to move to the second polishing station,
and to cause the substrate to be polished at the second polishing
station.
[0009] Implementations may include one or more of the following
features. The support structure may be a track and the carrier head
may be movable along the track. The carrier head may be suspended
from a carriage on the track. The controller may be configured to
cause the carrier head to move to the in-line optical metrology
system and to receive another spectrum reflected from the substrate
at the in-line optical metrology system after performing rework and
before transporting the substrate to the second polishing station.
The system may include a cassette and a robot to transfer the
substrate from the carrier head to the cassette. The controller may
be configured to cause the robot to transport the substrate from
the carrier head to the cassette before the substrate is returned
to the first polishing station and rework of the substrate is
performed at the first polishing station. The controller may be
configured to cause polishing of at least one other substrate at
the first polishing station before the substrate is returned to the
first polishing station and rework of the substrate is performed at
the first polishing station.
[0010] Implementations can include one or more of the following
potential advantages. Polishing endpoints can be determined more
reliably, and within-wafer non-uniformity (WIWNU) and
wafer-to-wafer non-uniformity (WTWNU) can be reduced.
[0011] The details of one or more implementations are set forth in
the accompanying drawings and the description below. Other aspects,
features and advantages will be apparent from the description and
drawings, and from the claims.
DESCRIPTION OF DRAWINGS
[0012] FIG. 1 is a schematic plan view of an example of a polishing
apparatus.
[0013] FIG. 2 is a schematic cross-sectional view of an example of
a polishing apparatus.
[0014] FIG. 3 is a schematic cross-sectional view of an example of
an in-sequence optical metrology system.
[0015] FIG. 4 illustrates an example spectrum.
[0016] FIG. 5 is a schematic cross-sectional view of a wet-process
optical metrology system.
[0017] FIG. 6 is a schematic cross-sectional view of another
implementation of a wet-process optical metrology system.
[0018] FIG. 7 is a schematic top view of a substrate.
[0019] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0020] As integrated circuits continue to develop, line widths
continue to shrink and layers in the integrated circuit continue to
accumulate, requiring ever more stringent thickness control. Thus,
polishing process control techniques, whether utilizing in-situ
monitoring or run-to run process control, face challenges to
maintain keep the post-polishing thickness within
specification.
[0021] For example, when performing in-situ spectrographic
monitoring of a multi-layer product substrate, an incident optical
beam from the spectrographic monitoring system can penetrate a
several dielectric layers before being reflected by metal lines.
The reflected beam can thus be a result of the thickness and
critical dimensions of multiple layers. A spectrum resulting from
such a complex layer stack often presents a significant challenge
in determining the thickness of the outermost layer that is being
polishing. In addition, the outermost layer thickness is an
indirect parameter for process control. This is because in many
applications the metal line thickness--a parameter that may be more
critical to yield--can vary even if the outermost layer thickness
is on target, if other dimensions such as etch depth or critical
dimension vary.
[0022] A control scheme for determining a polishing endpoint
incorporates wet metrology between CMP steps and feedforward or
feedback control. The dimensional variations in the substrate are
captured after each polishing step at an in-sequence metrology
station and used either to determine whether there is a need to
rework the substrate, or fed forward or fed back to control the
polishing operation or endpoint at a previous or subsequent
polishing station.
[0023] The polishing apparatus is configured such that a carrier
head holds a substrate during polishing at the first and second
polishing stations and moves the substrate from the first polishing
station to the second polishing station. The in-sequence metrology
station is situated to measure the substrate when the carrier head
is holding the substrate and when the substrate is not in contact
with a polishing pad of either the first polishing station or the
second polishing station.
[0024] FIG. 1 is a plan view of a chemical mechanical polishing
apparatus 100 for processing one or more substrates. The polishing
apparatus 100 includes a polishing platform 106 that at least
partially supports and houses one or more polishing stations 124.
The polishing apparatus 100 also includes a multiplicity of carrier
heads 126, each of which is configured to carry a substrate. Each
polishing station 124 is adapted to polish a substrate that is
retained in a carrier head 126.
[0025] The polishing apparatus 100 also includes a loading and
unloading station. The loading and unloading station can include
one or more load cups 122 adapted to facilitate transfer of a
substrate between the carrier heads 126 and a factory interface
(not shown) or other device (not shown) by a transfer robot 110.
The load cups 122 generally facilitate transfer between the robot
110 and each of the carrier heads 126.
[0026] Each polishing station 124 includes a polishing pad 130
supported on a platen 120 (see FIG. 2). The polishing pad 110 can
be a two-layer polishing pad with an outer polishing layer 130a and
a softer backing layer 130b (see FIG. 2).
[0027] In some implementations, at least one of the polishing
stations 124 is sized such that a plurality of carrier heads 126
can be positioned simultaneously over the polishing pad 130 so that
polishing of a plurality of substrates can occur at the same time
in the polishing station 124. Thus, a plurality of substrates,
e.g., one per carrier head, can be polished simultaneously with the
same polishing pad. Alternatively, in some implementations there is
just one carrier head 126 per polishing pad 130. In addition,
although six carrier heads 126 are shown, more or fewer carrier
heads can be depending on the needs of the polishing process and so
that the surface area of polishing pad 130 may be used
efficiently.
[0028] In some implementations, the carrier heads 126 are coupled
to a carriage 108 that is mounted to an overhead track 128. The
overhead track 128 allows each carriage 108 to be selectively
positioned over the polishing stations 124 and the load cups 122.
In the implementation depicted in FIG. 1, the overhead track 128
has a circular configuration (shown in phantom) which allows the
carriages 108 retaining the carrier heads 126 to be selectively
orbited over and/or clear of the load cups 122 and the polishing
stations 124. The overhead track 128 may have other configurations
including elliptical, oval, linear or other suitable orientation.
Alternatively, in some implementations the carrier heads 126 are
suspended from a carousel, and rotation of the carousel moves all
of the carrier heads simultaneously along a circular path.
[0029] Each polishing station 124 of the polishing apparatus 100
can include a port, e.g., at the end of an arm 134, to dispense
polishing liquid 136 (see FIG. 2), such as abrasive slurry, onto
the polishing pad 130. Each polishing station 124 of the polishing
apparatus 100 can also include pad conditioning apparatus 132 to
abrade the polishing pad 130 to maintain the polishing pad 130 in a
consistent abrasive state.
[0030] As shown in FIG. 2, the platen 120 at each polishing station
124 is operable to rotate about an axis 121. For example, a motor
150 can turn a drive shaft 152 to rotate the platen 120.
[0031] Each carrier head 126 is operable to hold a substrate 10
against the polishing pad 130. Each carrier head 126 can have
independent control of the polishing parameters, for example
pressure, associated with each respective substrate. In particular,
each carrier head 126 can include a retaining ring 142 to retain
the substrate 10 below a flexible membrane 144. Each carrier head
126 also includes a plurality of independently controllable
pressurizable chambers defined by the membrane, e.g., three
chambers 146a-146c, which can apply independently controllable
pressurizes to associated zones on the flexible membrane 144 and
thus on the substrate 10. Although only three chambers are
illustrated in FIG. 2 for ease of illustration, there could be one
or two chambers, or four or more chambers, e.g., five chambers.
[0032] Each carrier head 126 is suspended from the track 128, and
is connected by a drive shaft 154 to a carrier head rotation motor
156 so that the carrier head can rotate about an axis 127.
Optionally each carrier head 140 can oscillate laterally, e.g., by
driving the carriage 108 on the track 128, or by rotational
oscillation of the carousel itself. In operation, the platen is
rotated about its central axis 121, and each carrier head is
rotated about its central axis 127 and translated laterally across
the top surface of the polishing pad. The lateral sweep is in a
direction parallel to the polishing surface 212. The lateral sweep
can be a linear or arcuate motion.
[0033] A controller 190, such as a programmable computer, is
connected to each motor 152, 156 to independently control the
rotation rate of the platen 120 and the carrier heads 126. For
example, each motor can include an encoder that measures the
angular position or rotation rate of the associated drive shaft.
Similarly, the controller 190 is connected to an actuator in each
carriage 108 to independently control the lateral motion of each
carrier head 126. For example, each actuator can include a linear
encoder that measures the position of the carriage 108 along the
track 128.
[0034] The controller 190 can include a central processing unit
(CPU) 192, a memory 194, and support circuits 196, e.g.,
input/output circuitry, power supplies, clock circuits, cache, and
the like. The memory is connected to the CPU 192. The memory is a
non-transitory computable readable medium, and can be one or more
readily available memory such as random access memory (RAM), read
only memory (ROM), floppy disk, hard disk, or other form of digital
storage. In addition, although illustrated as a single computer,
the controller 190 could be a distributed system, e.g., including
multiple independently operating processors and memories.
[0035] Referring to FIGS. 1 and 3, the polishing apparatus 100 also
includes an in-sequence (also referred to as in-line) optical
metrology system 160, e.g., a spectrographic metrology system. An
in-sequence metrology system is positioned within the polishing
apparatus 100, but does not performs measurements during the
polishing operation; rather measurements are collected between
polishing operations, e.g., while the substrate is being moved from
one polishing station to another.
[0036] The in-line optical metrology system 160 includes a probe
180 supported on the platform 106 at a position between two of the
polishing stations 124, e.g., between two platens 120. In
particular, the probe 180 is located at a position such that a
carrier head 126 supported by the track 128 can position the
substrate 10 over the probe 170. In implementations in which the
polishing apparatus 100 include three polishing stations and
carries the substrates sequentially from the first polishing
station to the second polishing station to the third polishing
station, the probe 180 can be positioned between the second and
third polishing stations. However, in some implementations the
probe 180 is positioned between the first and second polishing
stations.
[0037] In some implementations there are multiple in-line optical
metrology systems 160, e.g., a first in-line optical metrology
system has a first probe 180 positioned between the first and
second polishing stations, and a second in-line optical metrology
system has a second probe 180 positioned between the first and
second polishing stations.
[0038] In some implementations, the probe 180 should be positioned
after a station at which the filler layer is expected to be
cleared. For example, where the controller 190 is configured with a
recipe to perform bulk polishing (but not clearance) of the filler
layer at a first polishing station, clearance of the filler layer
at a second polishing station, and removal or clearing of an
underlying layer at a third polishing station, the probe 180 can be
positioned between the second and third polishing stations. In
particular, for a copper polishing process that has bulk copper
polishing at the first polishing station, clearance of copper at
the second polishing station, and clearing of a barrier layer and a
cap layer at the third polishing station, the probe 180 can be
positioned between the second and third polishing stations.
[0039] The optical metrology system 160 can include a light source
162, a light detector 164, and circuitry 166 for sending and
receiving signals between the controller 190 and the light source
162 and light detector 164.
[0040] One or more optical fibers can be used to transmit the light
from the light source 162 to the optical access in the polishing
pad, and to transmit light reflected from the substrate 10 to the
detector 164. For example, a bifurcated optical fiber 170 can be
used to transmit the light from the light source 162 to the
substrate 10 and back to the detector 164. The bifurcated optical
fiber can include a trunk 172 having an end in the probe 180 to
measure the substrate 10, and two branches 174 and 176 connected to
the light source 162 and detector 164, respectively. In some
implementations, rather than a bifurcated fiber, two adjacent
optical fibers can be used.
[0041] In some implementations, the probe 180 holds an end of the
trunk 172 of the bifurcated fiber. In operation, the carrier head
126 positions a substrate 10 over the probe 180. Light from the
light source 162 is emitted from the end of the trunk 172,
reflected by the substrate 10 back into the trunk 172, and the
reflected light is received by the detector 164. In some
implementations, one or more other optical elements, e.g., a
focusing lens, are positioned over the end of the trunk 172, but
these may not be necessary.
[0042] The probe 180 can include a mechanism to adjust the vertical
height of the end the trunk 172, e.g., the vertical distance
between the end of the trunk 172 and the top surface of the
platform 106. In some implementations, the probe 180 is supported
on an actuator system 182 that is configured to move the probe 180
laterally in a plane parallel to the plane of the track 128. The
actuator system 182 can be an XY actuator system that includes two
independent linear actuators to move probe 180 independently along
two orthogonal axes.
[0043] The output of the circuitry 166 can be a digital electronic
signal that passes to the controller 190 for the optical metrology
system. Similarly, the light source 162 can be turned on or off in
response to control commands in digital electronic signals that
pass from the controller 190 to the optical metrology system 160.
Alternatively, the circuitry 166 could communicate with the
controller 190 by a wireless signal.
[0044] The light source 162 can be operable to emit white light. In
one implementation, the white light emitted includes light having
wavelengths of 200-800 nanometers. A suitable light source is a
xenon lamp or a xenon mercury lamp.
[0045] The light detector 164 can be a spectrometer. A spectrometer
is an optical instrument for measuring intensity of light over a
portion of the electromagnetic spectrum. A suitable spectrometer is
a grating spectrometer. Typical output for a spectrometer is the
intensity of the light as a function of wavelength (or frequency).
FIG. 4 illustrates an example of a measured spectrum 300.
[0046] As noted above, the light source 162 and light detector 164
can be connected to a computing device, e.g., the controller 190,
operable to control their operation and receive their signals. The
computing device can include a microprocessor situated near the
polishing apparatus, e.g., a programmable computer. With respect to
control, the computing device can, for example, synchronize
activation of the light source with the motion of the carrier head
126.
[0047] Optionally, the in-sequence metrology system 160 can be a
wet metrology system. In a wet-metrology system, measurement of the
surface of the substrate is conducted while a layer of liquid
covers the portion of the surface being measured. An advantage of
wet metrology is that the liquid can have a similar index of
refraction as the optical fiber 170. The liquid can provide a
homogeneous medium through which light can travel to and from the
surface of the film that is to be or that has been polished. The
wet metrology system 169 can be configured such that the liquid is
flowing during the measurement. A flowing liquid can flush away
polishing residue, e.g., slurry, from the surface of the substrate
being measured.
[0048] FIG. 5 shows an implementation of a wet in-sequence
metrology system 160. In this implementation, the trunk 172 of the
optical fiber 170 is situated inside a tube 186. A liquid 188,
e.g., de-ionized water, can be pumped from a liquid source 189 into
and through the tube 186. During the measurement, the substrate 10
can positioned over the end of the optical fiber 170. The height of
the substrate 10 relative to the top of the tube 186 and the flow
rate of the liquid 188 is selected such that as the liquid 188
overflows the tube 186, the liquid 188 fills the space between the
end of the optical fiber 170 and the substrate 10.
[0049] Alternatively, as shown in FIG. 6, the carrier head 126 can
be lowered into a reservoir defined by a housing 189. Thus, the
substrate 10 and a portion of the carrier head 126 can be submerged
in a liquid 188, e.g., de-ionized water, in the reservoir. The end
of the optical fiber 170 can be submerged in the liquid 188 below
the substrate 10.
[0050] In either case, in operation, light travels from the light
source 162, travels through the liquid 188 to the surface of the
substrate 10, is reflected from the surface of the substrate 10,
enters the end of the optical fiber, and returns to the detector
164.
[0051] Referring to FIG. 7, a typical substrate 10 includes
multiple dies 12. In some implementations, the controller 190
causes the substrate 10 and the probe 180 to undergo relative
motion so that the optical metrology system 160 can make multiple
measurements within an area 18 on the substrate 10. In particular,
the optical metrology system 160 can take multiple measurements at
spots 184 (only one spot is shown on FIG. 5 for clarity) that are
spread out with a substantially uniform density over the area 18.
The area 18 can be equivalent to the area of a die 12. In some
implementations, the die 12 (and the area 18) can be considered to
include half of any adjacent scribe line. In some implementations,
at least one-hundred measurements are made within the area 18. For
example, if a die is 1 cm on a side, then the measurements can be
made at 1 mm intervals across the area. The edges of the area 18
need not be aligned with the edges of a particular die 12 on the
substrate.
[0052] In some implementations, the XY actuator system 182 causes
the measurement spot 184 of the probe 180 to traverse a path across
the area 18 on the substrate 10 while the carrier head 126 holds
the substrate 10 in a fixed position (relative to the platform
106). For example, the XY actuator system 182 can cause the
measurement spot 184 to traverse a path which traverses the area 18
on a plurality of evenly spaced parallel line segments. This
permits the optical metrology system 160 to take measurements that
are evenly spaced over the area 18.
[0053] In some implementations, there is no actuator system 182,
and the probe 180 remains stationary (relative to the platform 106)
while the carrier head 126 moves to cause the measurement spot 184
to traverse the area 18. For example, the carrier head could
undergo a combination of rotation (from motor 156) translation
(from carriage 108 moving along track 128) to cause the measurement
spot 184 to traverse the area 18. For example, the carrier head 126
can rotate while carriage 108 causes the center of the substrate to
move outwardly from the probe 180, which causes the measurement
spot 184 to traverse a spiral path on the substrate 10. By making
measurements while the spot 184 is over the area 18, measurements
can be made at a substantially uniform density over the area
18.
[0054] In some implementations, the relative motion is caused by a
combination of motion of the carrier head 126 and motion of the
probe 180, e.g., rotation of the carrier head 126 and linear
translation of the probe 180.
[0055] The controller 190 receives a signal from the optical
metrology system 160 that carries information describing a spectrum
of the light received by the light detector for each flash of the
light source or time frame of the detector. For each measured
spectrum, a characterizing value can be calculated from the
measured spectrum. The characterizing value can be used in
controlling a polishing operation at one or more of the polishing
stations.
[0056] One technique to calculate a characterizing value is, for
each measured spectrum, to identify a matching reference spectrum
from a library of reference spectra. Each reference spectrum in the
library can have an associated characterizing value, e.g., a
thickness value or an index value indicating the time or number of
platen rotations at which the reference spectrum is expected to
occur. By determining the associated characterizing value for the
matching reference spectrum, a characterizing value can be
generated. This technique is described in U.S. Patent Publication
No. 2010-0217430, which is incorporated by reference. Another
technique is to analyze a characteristic of a spectral feature from
the measured spectrum, e.g., a wavelength or width of a peak or
valley in the measured spectrum. The wavelength or width value of
the feature from the measured spectrum provides the characterizing
value. This technique is described in U.S. Patent Publication No.
2011-0256805, which is incorporated by reference. Another technique
is to fit an optical model to the measured spectrum. In particular,
a parameter of the optical model is optimized to provide the best
fit of the model to the measured spectrum. The parameter value
generated for the measured spectrum generates the characterizing
value. This technique is described in U.S. Patent Application No.
61/608,284, filed Mar. 8, 2012, which is incorporated by reference.
Another technique is to perform a Fourier transform of the measured
spectrum. A position of one of the peaks from the transformed
spectrum is measured. The position value generated for measured
spectrum generates the characterizing value. This technique is
described in U.S. patent application Ser. No. 13/454,002, filed
Apr. 23, 2012, which is incorporated by reference.
[0057] As noted above, the characterizing value can be used in
controlling a polishing operation at one or more of the polishing
stations. The controller can, for example, calculate the
characterizing value and adjust the polishing time, polishing
pressure, or polishing endpoint of: (i) the previous polishing
step, i.e., for a subsequent substrate at the polishing station
that the substrate being measured just left, (ii) the subsequent
polishing step, i.e., at the polishing station to which the
substrate being measured will be transferred, or (iii) both of
items (i) and (ii), based on the characterizing value.
[0058] In some implementations, prior to the first CMP step,
substrate dimension information (layer thickness, critical
dimensions) from upstream non-polishing steps, if available, is fed
forward to the controller 190.
[0059] After a CMP step, the substrate is measured using wet
metrology at the in-sequence metrology station 180 located between
the polishing station at which the substrate was polishing and the
next polishing station. A characterizing value, e.g., layer
thickness or copper line critical dimension, is captured and sent
to the controller.
[0060] In some implementations, the controller 190 uses the
characterizing value to adjust the polishing operation for the
substrate at the next polishing station. For example, if the
characterizing value indicates that the etch trench depth is
greater, the post thickness target for the subsequent polishing
station can be adjusted with more removal amount to keep the
remaining metal line thickness constant. If the characterizing
value indicates that the underlying layer thickness has changed,
the reference spectrum for in-situ endpoint detection at the
subsequent polishing station can be modified so that endpoint
occurs closer to the target metal line thickness.
[0061] In some implementations, the controller 190 uses the
characterizing value to adjust the polishing operation for a
subsequent substrate at the previous polishing station. For
example, if the characterizing value indicates that the etch trench
depth is greater, the post thickness target for the previous
polishing station can be adjusted with more removal amount to keep
the remaining metal line thickness constant. If the characterizing
value indicates that the underlying layer thickness has changed,
the reference spectrum for in-situ endpoint detection at the
previous polishing station can be modified so that endpoint occurs
closer to the target metal line thickness.
[0062] In some implementations, the controller 190 analyzes the
measured spectra and determines the proper substrate route. For
example, the controller 190 can compare the characterizing value to
a threshold, or determine whether the characterizing value falls
within a predetermined range. If the characterizing value indicates
that polishing is incomplete, e.g., if it falls within the
predetermined range indicating an underpolished substrate or does
not exceed a threshold indicating a satisfactorily polished
substrate, then the substrate can be routed back to previous
polishing station for rework. Once the rework is completed, the
substrate can be measured again at the metrology station, or
transported to the next polishing station. If the characterizing
value does not indicate that polishing is incomplete, the substrate
can be transported to the next polishing station.
[0063] For example, a parameter such as metal residue can be
measured using wet metrology at the in-sequence metrology station
180. If metal residue detected, the substrate can be routed back to
previous polishing station for rework. Otherwise, the substrate can
be transported to the next polishing station.
[0064] In order to detect metal residue, the controller 190 can
evaluate the percentage of the area that is covered by the filler
material, each measured spectrum 300 is compared to a reference
spectrum. The reference spectrum can be the spectrum from a thick
layer of the filler material, e.g., a spectrum from a metal, e.g.,
a copper or tungsten reference spectrum. The comparison generates a
similarity value for each measured spectrum 300. A single scalar
value representing the amount of filler material within the area 18
can be calculated from the similarity values, e.g., by averaging
the similarity values. The scalar value can then be compared to a
threshold to determine the presence and/or amount of residue in the
area.
[0065] In some implementations, the similarity value is calculated
from a sum of squared differences between the measured spectrum and
the reference spectrum. In some implementations, the similarity
value is calculated from a cross-correlation between the measured
spectrum and the reference spectrum.
[0066] For example, in some implementation a sum of squared
differences (SSD) between each measured spectrum and the reference
spectrum is calculated to generate an SSD value for each
measurement spot. The SSD values can then be normalized by dividing
all SSD values by the highest SSD value obtained in the scan to
generate normalized SSD values (so that the highest SSD value is
equal to 1). The normalized SSD values are then subtracted from 1
to generate the similarity value. The spectrum that had the highest
SSD value, and thus the smallest copper contribution, is now equal
to 0.
[0067] Then the average of all similarity values generated in the
prior step is calculated to generate the scalar value. This scalar
value will be higher if residue is present.
[0068] As another example, in some implementation a sum of squared
differences (SSD) between each measured spectrum and the reference
spectrum is calculated to generate an SSD value for each
measurement spot. The SSD values can then be normalized by dividing
all SSD values by the highest SSD value obtained in the scan to
generate normalized SSD values (so that the highest SSD value is
equal to 1). The normalized SSD values are then subtracted from 1
to generate inverted normalized SSD values. For a given spectrum,
if the inverted normalized SSD value generated in the previous step
is less than a user-defined threshold, then it is set to 0. The
user-defined threshold can be 0.5 to 0.8, e.g., 0.7. Then the
average of all values generated in the prior step is calculated to
generate the scalar value. Again, this similarity value will be
higher if residue is present.
[0069] If the calculated scalar value is greater than a threshold
value, then the controller 190 can designate the substrate as
having residue. On the other hand, if the scalar value is equal or
less than the threshold value, then the controller 190 can
designate the substrate as not having residue.
[0070] If the controller 190 does not designate the substrate as
having residue, then the controller can cause the substrate to be
processed at the next polishing station normally. On the other
hand, controller 190 designates the substrate as having residue,
then the controller can take a variety of actions. In some
implementations, the substrate can be returned immediately to the
previous polishing station for rework. In some implementations, the
substrate is returned to the cassette (without being processed at a
subsequent polishing station) and designated for rework once other
substrates in the queue have completed polishing. In some
implementations, the substrate is returned to the cassette (without
being processed at a subsequent polishing station), and an entry
for the substrate in a tracking database is generated to indicate
that the substrate has residue. In some implementations, the scalar
value can be used to adjust a subsequent polishing operation to
ensure complete removal of the residue. In some implementations,
the scalar value can be used to flag the operator that something
has gone wrong in the polishing process, and that the operator's
attention is required. The tool can enter into a number of
error/alarm states, e.g. return all substrates to a cassette and
await operator intervention.
[0071] In another implementation, the calculated similarity value
for each measurement value is compared to a threshold value. Based
on the comparison, each measurement spot is designated as either
filler material or not filler material. For example, if an inverted
normalized SSD value is generated for each measurement spot as
discussed above, then the user-defined threshold can be 0.5 to 0.8,
e.g., 0.7.
[0072] The percentage of measurement spots within the area 18 that
are designated as filler material can be calculated. For example,
the number of measurement spots designated as filler material can
be divided by the total number of measurement spots.
[0073] This calculated percentage can be compared to a threshold
percentage. The threshold percentage can be calculated either from
knowledge of pattern of the die on the substrate, or empirically by
measuring (using the measurement process described above) for a
sample substrate that is known to not have residue. The sample
substrate could be verified as not having residue by a dedicated
metrology station.
[0074] If the calculated percentage is greater than the threshold
percentage, then the substrate can be designated as having residue.
On the other hand, if the percentage is equal or less than the
threshold percentage, then the substrate can be designated as not
having residue. The controller 190 can then take action as
discussed above.
[0075] In some implementations a probe 180' of an optical metrology
system 160 is positioned between the loading and unloading station
and one of the polishing stations. If the probe 180' is positioned
between the loading station and the first polishing station, then a
characterizing value can be measured by the metrology system and
fed forward to adjust polishing of the substrate at first polishing
station. If the probe 180' is positioned between the last polishing
station and the unloading station, then a characterizing value can
be measured by the metrology system and fed back to adjust
polishing of a subsequent substrate at the last polishing station,
or if residue is detected then the substrate can be sent back to
the last polishing station for rework.
[0076] The control schemes described above can more reliably
maintain product substrates within manufacture specification, and
can reduce rework, and can provide rerouting of the substrate to
provide rework with less disruption of throughput. This can provide
an improvement in both productivity and yield performance.
[0077] The above described polishing apparatus and methods can be
applied in a variety of polishing systems. For example, rather than
be suspended from a track, multiple carrier heads can be suspended
from a carousel, and lateral motion of the carrier heads can be
provided by a carriage that is suspend from and can move relative
to the carousel. The platen may orbit rather than rotate. The
polishing pad can be a circular (or some other shape) pad secured
to the platen. Some aspects of the endpoint detection system may be
applicable to linear polishing systems (e.g., where the polishing
pad is a continuous or a reel-to-reel belt that moves linearly).
The polishing layer can be a standard (for example, polyurethane
with or without fillers) polishing material, a soft material, or a
fixed-abrasive material. Terms of relative positioning are used; it
should be understood that the polishing surface and substrate can
be held in a vertical orientation or some other orientations.
[0078] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
* * * * *