U.S. patent application number 13/012692 was filed with the patent office on 2011-08-04 for high sensitivity real time profile control eddy current monitoring system.
Invention is credited to Hassan G. Iravani, Boguslaw A. Swedek, Wen-Chiang Tu, Yuchun Wang, Kun Xu.
Application Number | 20110189925 13/012692 |
Document ID | / |
Family ID | 44320063 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110189925 |
Kind Code |
A1 |
Iravani; Hassan G. ; et
al. |
August 4, 2011 |
High Sensitivity Real Time Profile Control Eddy Current Monitoring
System
Abstract
An apparatus for chemical mechanical polishing includes a platen
having a surface to support a polishing pad, and an eddy current
monitoring system to generate an eddy current signal. The eddy
current monitoring system includes a core and a coil wound around a
portion of the core. The core includes a back portion, a first
prong extending from the back portion in a first direction normal
to the surface of the platen and having a width in a second
direction parallel to the surface of the platen, and second and
third prongs extending from the back portion in parallel with the
first protrusion, the second and third prongs positioned on
opposite sides of and equidistant from the first prong. A spacing
between each of the second and third prongs and the first prong is
approximately equal to twice the width of the first prong.
Inventors: |
Iravani; Hassan G.; (San
Jose, CA) ; Xu; Kun; (Sunol, CA) ; Swedek;
Boguslaw A.; (Cupertino, CA) ; Wang; Yuchun;
(Santa Clara, CA) ; Tu; Wen-Chiang; (Mountain
View, CA) |
Family ID: |
44320063 |
Appl. No.: |
13/012692 |
Filed: |
January 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61299883 |
Jan 29, 2010 |
|
|
|
Current U.S.
Class: |
451/5 |
Current CPC
Class: |
B24B 49/105 20130101;
B24B 37/013 20130101; H01L 22/14 20130101; H01L 22/26 20130101;
B24B 37/205 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101; H01L 21/3212 20130101; H01L 2924/0002 20130101 |
Class at
Publication: |
451/5 |
International
Class: |
B24B 49/00 20060101
B24B049/00 |
Claims
1. An apparatus for chemical mechanical polishing, comprising: a
platen having a surface to support a polishing pad; and an eddy
current monitoring system to generate an eddy current signal, the
eddy current monitoring system comprising a core positioned at
least partially in the platen and a coil wound around a portion of
the core, the core including a back portion, a first prong
extending from the back portion in a first direction normal to the
surface of the platen and having a width in a second direction
parallel to the surface of the platen, second and third prongs
extending from the back portion in parallel with the first
protrusion, the second and third prongs positioned on opposite
sides of and equidistant from the first prong, a spacing in the
second direction between each of the second and third prongs and
the first prong being approximately equal to twice the width of the
first prong.
2. The apparatus of claim 1, wherein the second and third prongs
have a width approximately equal to the width of the first
prong.
3. The apparatus of claim 1, wherein the width of the first prong
is about 1 mm and the spacing between each of the second and third
prongs and the first prong is about 2 mm.
4. The apparatus of claim 1, wherein the first, second and third
prongs each have a height in the first direction, the height being
greater than the width of the first prong.
5. The apparatus of claim 1, wherein the height is about four times
the width.
6. The apparatus of claim 1, wherein the first, second and third
prongs each have a length along a third direction parallel to the
surface of the platen and perpendicular to the second direction,
the length being greater than the width of the first prong.
7. The apparatus of claim 6, wherein the length is at least 10
times the width.
8. The apparatus of claim 7, wherein the length is about 20 times
the width.
9. The apparatus of claim 6, wherein the platen is rotatable about
an axis of rotation, and the length of the first, second and third
prongs is perpendicular to a radius of the platen that extends from
the axis of rotation through the core.
10. The apparatus of claim 1, wherein the core has a generally
E-shaped cross section in a plane parallel to the first direction
and the second direction.
11. The apparatus of claim 1, wherein the coil is wound only around
the first prong.
12. The apparatus of claim 1, further comprising the polishing pad,
the polishing pad having a backing layer and a polishing layer,
wherein the backing layer has an aperture therein, and wherein the
prongs of the core extend into the aperture in the backing layer
but not past a bottom surface of the polishing layer.
13. The apparatus of claim 1, wherein the eddy current monitoring
system has a resonant frequency of about 1.5 to 2 MHz.
14. An apparatus for chemical mechanical polishing, comprising: a
platen having a surface to support a polishing pad; and an eddy
current monitoring system to generate an eddy current signal, the
eddy current monitoring system comprising a core positioned at
least partially in the platen and a coil, the core including a back
portion, a first prong extending from the back portion in a first
direction normal to the surface of the platen and having a height
in the first direction, second and third prongs extending from the
back portion in parallel with the first protrusion, the second and
third prongs positioned on opposite sides of the first prong;
wherein the coil is wound around an outer portion of the first
prong and is not wound around an inner portion of the first prong
that is closer to the back portion than the outer portion, the
inner portion extending at least about half the height of the
prong.
15. The apparatus of claim 14, wherein the outer portion extends
about half the height of the prong.
16. The apparatus of claim 14, further comprising at least one
spacer positioned in a gap between the first prong and the second
and third prongs to support the coil.
17. The apparatus of claim 14, wherein the first prong has a width
in a second direction parallel to the surface of the platen, the
second and third prongs are positioned equidistant from the first
prong, and a spacing in the second direction between each of the
second and third prongs and the first prong is approximately equal
to twice the width of the first prong.
18. The apparatus of claim 14, wherein the first, second and third
prongs have the same height.
19. The apparatus of claim 18, wherein the first prong has a width
in a second direction parallel to the surface of the platen, and
the height is greater than the width.
20. The apparatus of claim 14, wherein the first prong has a width
in a second direction parallel to the surface of the platen, and
wherein the first, second and third prongs each have a length along
a third direction parallel to the surface of the platen and
perpendicular to the second direction, the length being greater
than the width of the first prong.
21. The apparatus of claim 14, wherein the coil is wound only
around the first prong.
22. The apparatus of claim 14, further comprising the polishing
pad, the polishing pad having a backing layer and a polishing
layer, wherein the backing layer has an aperture therein, and
wherein the prongs of the core extend into the aperture in the
backing layer but not past a bottom surface of the polishing
layer.
23. The apparatus of claim 14, wherein the eddy current monitoring
system has a resonant frequency of about 1.5 to 2 MHz.
24. The apparatus of claim 23, wherein the coil is wound about 12
times around the first prong.
25. The apparatus of claim 24, wherein the eddy current monitoring
system includes a capacitor in parallel with the coil, the
capacitor having a capacitance of about 1000 pF.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application Ser. No. 61/299,883, filed on Jan. 29, 2010, the
disclosure of which is incorporated by reference.
TECHNICAL FIELD
[0002] The present disclosure relates to eddy current monitoring
during chemical mechanical polishing of substrates.
BACKGROUND
[0003] An integrated circuit is typically formed on a substrate
(e.g. a semiconductor wafer) by the sequential deposition of
conductive, semiconductive or insulative layers on a silicon wafer,
and by the subsequent processing of the layers.
[0004] One fabrication step involves depositing a filler layer over
a non-planar surface, and planarizing the filler layer until the
non-planar surface is exposed. For example, a conductive filler
layer can be deposited on a patterned insulative layer to fill the
trenches or holes in the insulative layer. The filler layer is then
polished until the raised pattern of the insulative layer is
exposed. After planarization, the portions of the conductive 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. In addition, planarization may be
used to planarize the substrate surface for lithography.
[0005] Chemical mechanical polishing (CMP) is one accepted method
of planarization. This planarization method typically requires that
the substrate be mounted on a carrier head. The exposed surface of
the substrate is placed against a rotating polishing pad. The
carrier head provides a controllable load on the substrate to push
it against the polishing pad. A polishing liquid, such as slurry
with abrasive particles, is supplied to the surface of the
polishing pad.
[0006] During semiconductor processing, it may be important to
determine one or more characteristics of the substrate or layers on
the substrate. For example, it may be important to know the
thickness of a conductive layer during a CMP process, so that the
process may be terminated at the correct time. A number of methods
may be used to determine substrate characteristics. For example,
optical sensors may be used for in-situ monitoring of a substrate
during chemical mechanical polishing. Alternately (or in addition),
an eddy current sensing system may be used to induce eddy currents
in a conductive region on the substrate to determine parameters
such as the local thickness of the conductive region.
SUMMARY
[0007] In one aspect, an apparatus for chemical mechanical
polishing includes a platen having a surface to support a polishing
pad, and an eddy current monitoring system to generate an eddy
current signal. The eddy current monitoring system includes a core
positioned at least partially in the platen and a coil wound around
a portion of the core. The core includes a back portion, a first
prong extending from the back portion in a first direction normal
to the surface of the platen and having a width in a second
direction parallel to the surface of the platen, and second and
third prongs extending from the back portion in parallel with the
first protrusion, the second and third prongs positioned on
opposite sides of and equidistant from the first prong. A spacing
in the second direction between each of the second and third prongs
and the first prong is approximately equal to twice the width of
the first prong.
[0008] Implementations can include one or more of the following
features. The second and third prongs may have a width
approximately equal to the width of the first prong. The width of
the first prong may be about 1 mm and the spacing between each of
the second and third prongs and the first prong may be about 2 mm.
The first, second and third prongs may each have a height in the
first direction, the height being greater than the width of the
first prong, e.g., about four times the width. The first, second
and third prongs may each have a length along a third direction
parallel to the surface of the platen and perpendicular to the
second direction, the length being greater than the width of the
first prong. The length may be at least 10 times the width, e.g.,
length may be about 20 times the width. The platen may be rotatable
about an axis of rotation, and the length of the first, second and
third prongs may be perpendicular to a radius of the platen that
extends from the axis of rotation through the core. The core may
have a generally E-shaped cross section in a plane parallel to the
first direction and the second direction. The coil may be wound
only around the first prong. The polishing pad may have a backing
layer and a polishing layer, the backing layer may have an aperture
therein, and the prongs of the core may extend into the aperture in
the backing layer but not past a bottom surface of the polishing
layer. The eddy current monitoring system may have a resonant
frequency of about 1.5 to 2 MHz.
[0009] In another aspect an apparatus for chemical mechanical
polishing includes a platen having a surface to support a polishing
pad, and an eddy current monitoring system to generate an eddy
current signal. The eddy current monitoring system includes a core
positioned at least partially in the platen and a coil. The core
includes a back portion, a first prong extending from the back
portion in a first direction normal to the surface of the platen
and having a height in the first direction, and second and third
prongs extending from the back portion in parallel with the first
protrusion, the second and third prongs positioned on opposite
sides of the first prong. The coil is wound around an outer portion
of the first prong and is not wound around an inner portion of the
first prong that is closer to the back portion than the outer
portion. The inner portion extends at least about half the height
of the prong.
[0010] Implementations may include one or more of the following
features. The outer portion may extend about half the height of the
prong. At least one spacer may be positioned in a gap between the
first prong and the second and third prongs to support the coil.
The first prong may have a width in a second direction parallel to
the surface of the platen, the second and third prongs may be
positioned equidistant from the first prong, and a spacing in the
second direction between each of the second and third prongs and
the first prong may be approximately equal to twice the width of
the first prong. The first, second and third prongs may have the
same height. The first prong may have a width in a second direction
parallel to the surface of the platen, and the height may be
greater than the width. The first prong may have a width in a
second direction parallel to the surface of the platen, and the
first, second and third prongs may each have a length along a third
direction parallel to the surface of the platen and perpendicular
to the second direction, the length being greater than the width of
the first prong. The coil may be wound only around the first prong.
The polishing pad may have a backing layer and a polishing layer,
the backing layer may have an aperture therein, and the prongs of
the core may extend into the aperture in the backing layer but not
past a bottom surface of the polishing layer. The eddy current
monitoring system may have a resonant frequency of about 1.5 to 2
MHz. The coil may be wound about 12 times around the first prong. A
capacitor having a capacitance of about 1000 pF may be in parallel
with the coil.
[0011] Potential advantages of some implementations can include the
following. Eddy current monitoring may be performed with the sensor
positioned farther from the substrate, e.g., with the sensor that
does not project into a recess in the polishing layer. By removing
the recess from the polishing layer, polishing uniformity and pad
lifetime may be improved. Accuracy of thickness measurements may be
improved for thin layers, which may improve real time profile
control for thinner layers, and thus improve within-wafer and
wafer-to-wafer uniformity. In addition, accuracy of thickness
measurement may be improved for polishing metals of lower
conductivity than copper, e.g., for polishing of aluminum and
tungsten. This may improve real time profile control and thus
improve within-wafer and wafer-to-wafer uniformity for such low
conductivity metals.
[0012] 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.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is a schematic exploded perspective view of a
chemical mechanical polishing apparatus.
[0014] FIG. 2 is a schematic side view, partially cross-sectional,
of a chemical mechanical polishing station that includes an eddy
current monitoring system and an optical monitoring system.
[0015] FIG. 3 is a schematic cross-sectional view of a carrier
head.
[0016] FIGS. 4A-4B show a schematic diagram of an eddy current
monitoring system.
[0017] FIGS. 5A and 5B show side and perspective views of an eddy
current monitoring system with three prongs.
[0018] FIGS. 6A and 6B show top and side views of a chemical
mechanical polishing apparatus using an elongated core.
[0019] FIG. 7 shows a top view of a platen with a substrate on the
surface of the platen.
[0020] FIGS. 8A-8D schematically illustrate a method of detecting a
polishing endpoint using an eddy current sensor.
[0021] FIG. 9 is a graph illustrating the thickness of a metal
layer on a substrate after chemical mechanical polishing.
[0022] FIG. 10 is a flowchart illustrating a method of polishing a
metal layer.
[0023] FIG. 11 is a flowchart illustrating an alternative method of
polishing a metal layer.
[0024] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0025] CMP systems can use eddy current monitoring systems to
detect thickness of a top metal layer on a substrate. During
polishing of the top metal layer, the eddy current monitoring
system can determine the thickness of different regions of the
metal layer on the substrate. The thickness measurements can be
used to adjust processing parameters of the polishing process in
real time. For example, a substrate carrier head can adjust the
pressure on the backside of the substrate to increase or decrease
the polishing rate of the regions of the metal layer. The polishing
rate can be adjusted so that the regions of the metal layer are
substantially the same thickness after polishing. The CMP system
can adjust the polishing rate so that polishing of the regions of
the metal layer completes at about the same time. Such profile
control can be referred to as real time profile control (RTPC).
[0026] One problem with eddy current monitoring is an insufficient
signal for accurate thickness determination, which can result in
lack of accuracy in endpoint determination and profile control.
Without being limited to any particular theory, factors that
contribute to an insufficient signal can include (a) placement of
the sensor farther from the substrate, such that the magnetic field
reaching the substrate is weaker, (b) polishing of thinner layers,
e.g., copper less than 2000 Angstroms, which have a higher
resistance, and (c) polishing of lower conductivity metals, e.g.,
aluminum or tungsten.
[0027] Signal strength can be dramatically improved by proper
configuration of the sensor. In particular, for a core with three
prongs, signal strength can be improved by spacing the prongs
slightly farther apart, and by concentrating the windings of the
coil around the outer portion of the center prong. In addition, the
resonant frequency of the eddy current sensor can be tuned for the
layer that will be polished. Overall, signal strength can be
increased sufficiently for reliable profile control even if the
sensor is farther from the substrate, a thinner layer is being
polished, and/or a lower conductivity metal is being polished. For
example, profile control can be performed reliably even for copper
layers less than 1000 Angstroms thick, and for aluminum layers.
[0028] Another technique is to use different eddy current
monitoring systems at different polishing stations. For example, a
first polishing station can include an eddy current monitoring
system with a resonant frequency selected for an initial thickness
range of the metal layer, e.g., down to about 1000 Angstroms, and a
second polishing station can include an eddy current monitoring
system with a resonant frequency selected for a subsequent
thickness range that is lower than the initial thickness range,
e.g., down to about 200 Angstroms.
[0029] FIG. 1 shows a CMP apparatus 20 for polishing one or more
substrates 10. A description of a similar polishing apparatus can
be found in U.S. Pat. No. 5,738,574. Polishing apparatus 20
includes a series of polishing stations 22a, 22b and 22c, and a
transfer station 23. Transfer station 23 transfers the substrates
between the carrier heads and a loading apparatus.
[0030] Each polishing station includes a rotatable platen 24 having
a top surface 25 on which is placed a polishing pad 30. The first
and second stations 22a and 22b can include a two-layer polishing
pad with a hard durable outer surface or a fixed-abrasive pad with
embedded abrasive particles. The final polishing station 22c can
include a relatively soft pad or a two-layer pad. Each polishing
station can also include a pad conditioner apparatus 28 to maintain
the condition of the polishing pad so that it will effectively
polish substrates.
[0031] Referring to FIG. 2, a two-layer polishing pad 30 typically
has a backing layer 32 which abuts the surface of platen 24 and a
covering layer 34 which is used to polish substrate 10. Covering
layer 34 is typically harder than backing layer 32. However, some
pads have only a covering layer and no backing layer. Covering
layer 34 can be composed of foamed or cast polyurethane, possibly
with fillers, e.g., hollow microspheres, and/or a grooved surface.
Backing layer 32 can be composed of compressed felt fibers leached
with urethane. A two-layer polishing pad, with the covering layer
composed of IC-1000 and the backing layer composed of SUBA-4, is
available from Rodel, Inc., of Newark, Del. (IC-1000 and SUBA-4 are
product names of Rodel, Inc.).
[0032] During a polishing step, a slurry 38 can be supplied to the
surface of polishing pad 30 by a slurry supply port or combined
slurry/rinse arm 39. If polishing pad 30 is a standard pad, slurry
38 can also include abrasive particles (e.g., silicon dioxide for
oxide polishing).
[0033] Returning to FIG. 1, a rotatable multi-head carousel 60
supports four carrier heads 70. The carousel is rotated by a
central post 62 about a carousel axis 64 by a carousel motor
assembly (not shown) to orbit the carrier head systems and the
substrates attached thereto between polishing stations 22 and
transfer station 23. Three of the carrier head systems receive and
hold substrates, and polish them by pressing them against the
polishing pads. Meanwhile, one of the carrier head systems receives
a substrate from and delivers a substrate to transfer station
23.
[0034] Each carrier head 70 is connected by a carrier drive shaft
74 to a carrier head rotation motor 76 (shown by the removal of one
quarter of cover 68) so that each carrier head can independently
rotate about it own axis. In addition, each carrier head 70
independently laterally oscillates in a radial slot 72 formed in
carousel support plate 66. A description of a suitable carrier head
70 can be found in U.S. Pat. No. 7,654,888, the entire disclosure
of which is incorporated by reference. In operation, the platen is
rotated about its central axis 25, and the carrier head is rotated
about its central axis 71 and translated laterally across the
surface of the polishing pad.
[0035] FIG. 3 shows one of the carrier heads 70. Each of the
carrier heads 70 includes a housing 102, a base assembly 104, a
gimbal mechanism 106 (which can be considered part of the base
assembly 104), a loading chamber 108, a retaining ring 200, and a
substrate backing assembly 110 which includes a flexible membrane
116 that defines multiple independently pressurizable chambers,
such as an inner chamber 230, a middle chambers 232, 234, 236, and
an outer chamber 238. These chambers control the pressure on
concentric regions of the flexible membrane, thus providing
independent pressure control on concentric portions of the
substrate. In some implementations, each of the carrier heads 70
includes five chambers and a pressure regulator for each of the
chambers.
[0036] Returning to FIG. 2, the eddy current monitoring system 40
includes a drive system to induce eddy currents in a metal layer on
the substrate and a sensing system to detect eddy currents induced
in the metal layer by the drive system. The monitoring system 40
includes a core 42 positioned in recess 26 to rotate with the
platen, a drive coil 49 wound around one part of core 42, and a
sense coil 46 wound around second part of core 42. For the drive
system, monitoring system 40 includes an oscillator 50 connected to
drive coil 49. For the sense system, monitoring system 40 includes
a capacitor 52 connected in parallel with sense coil 46, an RF
amplifier 54 connected to sense coil 46, and a diode 56. The
oscillator 50, capacitor 52, RF amplifier 54, and diode 56 can be
located apart from platen 24, and can be coupled to the components
in the platen through a rotary electrical union 29.
[0037] In some implementations, the backing layer 32 includes an
aperture above the recess 26. The aperture can have the same width
and depth as the recess 26. Alternatively, the aperture can be
smaller than the recess 26. A portion 36 of the covering layer 34
can be above the aperture in the backing layer. The portion 36 of
the covering layer 34 can prevent the slurry 38 from entering the
recess 26. Part of the core 42 can be located in the aperture. For
example, the core 42 can include prongs that extent into the
aperture. In some implementations, the top of the core 42 does not
extend past the bottom surface of the covering layer 34.
[0038] In operation the oscillator 50 drives drive coil 49 to
generate an oscillating magnetic field that extends through the
body of core 42 and into the gap between the prongs of the core. At
least a portion of magnetic field extends through thin portion 36
of polishing pad 30 and into substrate 10. If a metal layer is
present on substrate 10, oscillating magnetic field generates eddy
currents in the metal layer. The eddy currents cause the metal
layer to act as an impedance source in parallel with sense coil 46
and capacitor 52. As the thickness of the metal layer changes, the
impedance changes, resulting in a change in the Q-factor of sensing
mechanism. By detecting the change in the Q-factor of the sensing
mechanism, the eddy current sensor can sense the change in the
strength of the eddy currents, and thus the change in thickness of
metal layer.
[0039] An optical monitoring system 140, which can function as a
reflectometer or interferometer, can be secured to platen 24 in
recess 26, e.g., adjacent the eddy current monitoring system 40.
Thus, the optical monitoring system 140 can measure the
reflectivity of substantially the same location on the substrate as
is being monitored by the eddy current monitoring system 40.
Specifically, the optical monitoring system 140 can be positioned
to measure a portion of the substrate at the same radial distance
from the axis of rotation of the platen 24 as the eddy current
monitoring system 40. Thus, the optical monitoring system 140 can
sweep across the substrate in the same path as the eddy current
monitoring system 40.
[0040] The optical monitoring system 140 includes a light source
144 and a detector 146. The light source generates a light beam 142
which propagates through transparent window section 36 and slurry
to impinge upon the exposed surface of the substrate 10. For
example, the light source 144 may be a laser and the light beam 142
may be a collimated laser beam. The light laser beam 142 can be
projected from the laser 144 at an angle .alpha. from an axis
normal to the surface of the substrate 10. In addition, if the
recess 26 and the window 36 are elongated, a beam expander (not
illustrated) may be positioned in the path of the light beam to
expand the light beam along the elongated axis of the window. In
general, the optical monitoring system functions as described in
U.S. Pat. Nos. 6,159,073, and 6,280,289, the entire disclosures of
which are incorporated herein by references.
[0041] The CMP apparatus 20 can also include a position sensor 80,
such as an optical interrupter, to sense when core 42 and light
source 44 are beneath substrate 10. For example, the optical
interrupter could be mounted at a fixed point opposite carrier head
70. A flag 82 is attached to the periphery of the platen. The point
of attachment and length of flag 82 is selected so that it
interrupts the optical signal of sensor 80 while transparent
section 36 sweeps beneath substrate 10. Alternately, the CMP
apparatus can include an encoder to determine the angular position
of platen.
[0042] A general purpose programmable digital computer 90 receives
the intensity signals from the eddy current sensing system, and the
intensity signals from the optical monitoring system. Since the
monitoring systems sweep beneath the substrate with each rotation
of the platen, information on the metal layer thickness and
exposure of the underlying layer is accumulated in-situ and on a
continuous real-time basis (once per platen rotation). The computer
90 can be programmed to sample measurements from the monitoring
system when the substrate generally overlies the transparent
section 36 (as determined by the position sensor). As polishing
progresses, the reflectivity or thickness of the metal layer
changes, and the sampled signals vary with time. The time varying
sampled signals may be referred to as traces. The measurements from
the monitoring systems can be displayed on an output device 92
during polishing to permit the operator of the device to visually
monitor the progress of the polishing operation.
[0043] In operation, the CMP apparatus 20 uses eddy current
monitoring system 40 and optical monitoring system 140 to determine
when the bulk of the filler layer has been removed and to determine
when the underlying stop layer has been substantially exposed. The
computer 90 applies process control and endpoint detection logic to
the sampled signals to determine when to change process parameter
and to detect the polishing endpoint. Possible process control and
endpoint criteria for the detector logic include local minima or
maxima, changes in slope, threshold values in amplitude or slope,
or combinations thereof.
[0044] In addition, the computer 90 can be programmed to divide the
measurements from both the eddy current monitoring system 40 and
the optical monitoring system 140 from each sweep beneath the
substrate into a plurality of sampling zones, to calculate the
radial position of each sampling zone, to sort the amplitude
measurements into radial ranges, to determine minimum, maximum and
average measurements for each sampling zone, and to use multiple
radial ranges to determine the polishing endpoint, as discussed in
U.S. Pat. No. 6,399,501, the entirety of which is incorporated
herein by reference.
[0045] Computer 90 may also be connected to the pressure mechanisms
that control the pressure applied by carrier head 70, to carrier
head rotation motor 76 to control the carrier head rotation rate,
to the platen rotation motor (not shown) to control the platen
rotation rate, or to slurry distribution system 39 to control the
slurry composition supplied to the polishing pad. Specifically,
after sorting the measurements into radial ranges, information on
the metal film thickness can be fed in real-time into a closed-loop
controller to periodically or continuously modify the polishing
pressure profile applied by a carrier head, as discussed further
below.
[0046] FIG. 4A shows an example of an eddy current monitoring
system 400 for measuring profile information. The eddy current
monitoring system 400 can be used as the eddy current monitoring
system 40. With eddy current sensing, an oscillating magnetic field
induces eddy currents in a conductive region on the wafer. The eddy
currents are induced in a region that is coupled with magnetic flux
lines generated by the eddy current sensing system. The eddy
current monitoring system 400 includes a core 408 with an E-shaped
body. The core 408 can include a back portion 410 and three prongs
412a-c extending from the back portion 410.
[0047] The back portion 410 of the core 408 can be a generally
plate-shape or rectangular box-shaped body, and can have a top face
parallel to the top surface of the platen, e.g., parallel to the
substrate and the polishing pad during the polishing operation. In
some implementations, the long axis of the back portion 410 is
perpendicular to a radius of the platen that extends from the axis
of rotation of the platen. The long axis of the back portion 410
can be normal to the front face of the back portion 410. The back
portion 410 can have a height that is measured normal to the top
surface of the platen.
[0048] The prongs 412a-c extend from the back portion 410 in a
direction normal to a top surface of the back portion 410 and are
substantially linear and extend in parallel with each other. Each
of the prongs 412a-c can have a long axis along a direction
parallel to the top surface of the platen, e.g., parallel to the
faces of the substrate and polishing pad during the polishing
operation, and are substantially linear and extend in parallel to
each other. The long axes of the prongs 412a-c can be normal to the
front face of the prongs 412a-c. The long axis of the back portion
410 can extend in the same direction as the long axes of the prongs
412a-c. In some implementations, the long axes of the prongs 412a-c
are perpendicular to a radius of the polishing pad that extends
from the axis of rotation of the polishing pad. The two outer
prongs 412a, 412c are on opposite sides of the middle prong 412a.
The space between the each of the outer prongs (e.g., 412a and
412c) and the center prong (e.g., 412b) can be the same, i.e., the
outer prongs 412a, 412c can be equidistant from the middle prong
412a.
[0049] The eddy current sensing system 400 includes a coil 422 and
a capacitor 424 in parallel. The coil 422 can be coupled with the
core 408 (e.g., the coil 422 can be wrapped around the center coil
412b). Together the coil 422 and the capacitor 424 can form an LC
resonant tank. In operation, a current generator 426 (e.g., a
current generator based on a marginal oscillator circuit) drives
the system at the resonant frequency of the LC tank circuit formed
by the coil 422 (with inductance L) and the capacitor 424 (with
capacitance C). The current generator 426 can be designed to
maintain the peak to peak amplitude of the sinusoidal oscillation
at a constant value. A time-dependent voltage with amplitude
V.sub.0 is rectified using a rectifier 428 and provided to a
feedback circuit 430. The feedback circuit 430 determines a drive
current for current generator 426 to keep the amplitude of the
voltage V.sub.0 constant. For such a system, the magnitude of the
drive current can be proportional to the conducting film thickness.
Marginal oscillator circuits and feedback circuits are further
described in U.S. Pat. Nos. 4,000,458, and 7,112,960 which are
incorporated by reference.
[0050] The current generator 426 can feed current to the LC
resonant tank in order for the frequency to remain the same. The
coil 422 can generate an oscillating magnetic field 432, which may
couple with a conductive region 406 of the substrate (e.g., the
substrate 10). When the conductive region 406 is present, the
energy dissipated as eddy currents in the substrate can bring down
the amplitude of the oscillation. The current generator 426 can
feed more current to the LC resonant tank to keep the amplitude
constant. The amount of additional current fed by the current
generator 426 can be sensed and can be translated into a thickness
measurement of the conductive region 406.
[0051] FIG. 4B shows another implementation of a eddy current
monitoring system 400. The eddy current monitoring system 400 can
include a drive coil 402 for generating an oscillating magnetic
field 404, which may couple with the conductive region 406 of
interest (e.g., a portion of a metal layer on a semiconductor
wafer). Drive coil 402 can be wound around the back portion 410.
The oscillating magnetic field 404 generates eddy currents locally
in conductive region 406. The eddy currents cause conductive region
406 to act as an impedance source in parallel with a sense coil 414
and a capacitor 414. The sense coil 414 can be wrapped around the
center prong 412b. The sense coil 414 can be wrapped around an
outer portion of the center prong 412b to increase the sensitivity
of the eddy current monitoring system 400. As the thickness of
conductive region 406 changes, the impedance changes, resulting in
a change in the Q-factor of the system. By detecting the change in
the Q-factor, the eddy current monitoring system 400 can sense the
change in the strength of the eddy currents, and thus the change in
thickness of the conductive region. Therefore, the eddy current
monitoring system 400 can be used to determine parameters of the
conductive region, such as a thickness of the conductive region, or
may be used to determine related parameters, such as a polishing
endpoint. Note that although the thickness of a particular
conductive region is discussed above, the relative position of core
408 and the conductive layer may change, so that thickness
information for a number of different conductive regions is
obtained.
[0052] In some implementations, a change in Q-factor may be
determined by measuring an amplitude of current in the sense coil
as a function of time, for a fixed drive frequency and drive
amplitude. An eddy current signal may be rectified using a
rectifier 418, and the amplitude monitored via an output 420.
Alternately, a change in Q-factor may be determined by measuring an
phase difference between the drive signal and the sense signal as a
function of time.
[0053] The eddy current monitoring system 400 can be used to
measure the thickness of a conductive layer on a substrate. In some
implementations, an eddy current monitoring system with a higher
signal strength, a higher signal to noise ratio and/or improved
spatial resolution and linearity may be desired. For example, in
RTPC applications, obtaining desired cross-wafer uniformity may
require an improved eddy current sensing system.
[0054] The eddy current monitoring system 400 can provide enhanced
signal strength, signal to noise ratio, enhanced linearity, and
enhanced stability. Additional benefits may be obtained by
providing an eddy current sensing system with improved signal
strength. Improved signal strength may be particularly beneficial
for RTPC. Obtaining high resolution wafer profile information
allows for more accurate adjustment of processing parameters, and
thus may enable fabrication of devices with smaller critical
dimensions (CDs).
[0055] In general, the in-situ eddy current monitoring system 400
is constructed with a resonant frequency of about 50 kHz to 10 MHz,
e.g., between about 1.5 and 2.0 MHz, e.g., between about 1.6 and
1.7 MHz. For example, the sense coil 414 can have an inductance of
about 0.3 to 30 microH and the capacitor 416 can have a capacitance
of about 470 pF to about 0.022 uF, e.g., 1000 pF. The driving coil
can be designed to match the driving signal from an oscillator. For
example, if the oscillator has a low voltage and a low impedance,
the drive coil can include fewer turns to provide a small
inductance. On the other hand, if the oscillator has a high voltage
and a high impedance, the drive coil can include more turns to
provide a large inductance. In one implementation, the sense coil
414 includes twelve turns around the center prong 412b, and the
drive coil 402 includes four turns around the base portion 410, and
the oscillator drives the drive coil 402 with an amplitude of about
0.1 V to 5.0 V
[0056] FIG. 5A shows another example of a core 500. The core 500
can have an E-shaped body formed of a non-conductive material with
a relatively high magnetic permeability (e.g., .mu. of about 2500
or more). Specifically, core 500 can be ferrite. The core 500 can
be coated. For example, the core 500 can be coated with a material
such as parylene to prevent water from entering pores in the core
500, and to prevent coil shorting. The core 500 can be the same as
the core 408 included in the eddy current monitoring system 400.
The core 500 can include a back portion 502 and three prongs 504a-c
extending from the back portion 502.
[0057] The first prong 504b has a width W1, the second prong 504a
has a width W2, and the third prong 504c has a width W3. Each of
the widths W1, W2, and W3 can be the same. For example, each of the
prongs 504a-c can have a width of 1 mm. The first prong 504b and
the second prong 504a are a separated by a distance 51, and the
first prong 504b and the third prong 504c are a distance S2 apart.
In some implementations, the distances 51 and S2 are the same and
the second prong 504a and the third prong 504c are the same
distance from the center prong 504b. For example, both the
distances S1 and S2 can be about 2 mm.
[0058] Each of the prongs 504a-c have a height Hp, which is the
distance that the prongs 504a-c extend from the back portion 502 of
the core 500. The height Hp can be greater than the widths W1, W2,
and W3. In some implementations, the higher Hp is longer than the
distances S1 and S2 separating the prongs 504a-c. In particular,
the height Hp can be 4 mm. The back portion 502 has a height Hb.
The height Hb can be the same as the distance S1 or the distance
S2, e.g., 2 mm.
[0059] A coil 506 can be wound around the center prong 504b. The
coil can be coupled with a capacitor, such as the capacitor 416. In
implementations of eddy current monitoring systems such as the
system 400, separate sense and drive coils can be used. In some
implementations, a coil such as the coil 506 may be litz wire
(woven wire constructed of individual film insulated wires bunched
or braided together in a uniform pattern of twists and length of
lay), which may be less lossy than solid wire for the frequencies
commonly used in eddy current sensing.
[0060] In some implementations, the coil 506 can be wrapped around
a portion of the center prong 504b and not the entire prong 504b.
For example, the coil 506 can be wrapped around an outer portion of
the center prong 504b. The outer portion can have a height Ho. The
coil 506 may not touch an inner portion of the center prong 504b
that has a height Hi. The inner portion can be closer to the back
portion 502 than the outer portion. In some implementations, the
heights Ho and Hi are about half the height Hp of the center prong
504b. Alternatively, the height Hi of the inner portion can be
greater than the height Ho of the outer portion. The height Ho of
the outer portion can be greater than the height Hi of the inner
portion.
[0061] In some implementations, a spacer 508 can support the coil
506 and prevent the coil 506 from contacting the inner portion of
the center prong 504b. The spacer 508 can be made from an
insulator. The spacer 508 can be soft in order to prevent damage to
the core 500. For example, the spacer 508 can be plastic, rubber,
or wood. The spacer 508 can be attached to the core 500 to prevent
the spacer 508 from moving during CMP processes.
[0062] FIG. 5B shows a perspective view of the core 500. The core
500 can have a width Wt that is the sum of the widths W1, W2, and
W3 of the prongs 504a-c and the distances S1 and S2 separating the
prongs 504a-c. The core 500 has a height Ht that is the sum of the
height Hp of the prongs 504a-c and the height Hb of the base
portion 502. In some implementations, the width Wt is greater than
the height Ht. The core 500 has a length Lt that is greater than
the width W1 of the center prong 504b, and preferably greater than
the width Wt of the core. The length Lt can be between about 10 and
20 mm. The length Lt can be greater than the width Wt of the core
500.
[0063] FIGS. 6A and 6B show top and side views of the relative
position of a substrate 600 with respect to a core 602 (which may
be similar to core 408 of FIG. 4 or core 500 of FIG. 5). For a scan
through a slice A-A' through the center of the wafer 600 having a
radius R, the core 602 is oriented so that its long axis is
perpendicular to a radius of the wafer 600. The core 602 is
translated relative to the diameter of the wafer as shown. Note
that the magnetic field produced by a coil wound around the core
602 induces eddy currents in a conductive region that is elongated
in shape as well, with a length greater than a width. However, the
length and the width are generally not the same as the length and
width of the core 602, and the aspect ratio and cross section of
the conductive region is generally different than that of the core
602 as well.
[0064] Although the configuration of FIGS. 6A and 6B may provide
improved resolution for most of slide A-A' of the wafer 600, as the
core 602 translates along a first and last segments 604 of the
radius, a portion of the core 602 is not proximate to the
substrate. Therefore the measurement for the segments 604 is less
accurate and may place a limit on the maximum desirable length L,
such as the length Lt, of the core 602. Additionally, as the core
602 approaches the center of the wafer 600, the core 602 is
sampling a larger radial range. Therefore, the spatial resolution
for a particular radial distance r.apprxeq.R is significantly
better than the spatial resolution of r.apprxeq.0.
[0065] As explained above, the length L of the core 602 is greater
than its width W. That is, the aspect ration L/W is greater than
one. Different values for L, W, and L/W may be used for different
implementations. For example, W may range from a fraction of a
millimeter to more than a centimeter, while L may range from about
a millimeter (for smaller values of W) to ten centimeters or
greater.
[0066] In a particular implementation, W is between about a
millimeter and about ten millimeters, while L is between about one
centimeter to about five centimeters. More particularly, the core
602 may be about seven millimeters wide, with each protrusion being
about a millimeter in width and with each space between adjacent
protrusions being about two millimeters. The length may be about
twenty millimeters. The height may be about six millimeters and may
be increased if desired to allow for more coil turns. Of course,
the values given here are exemplary; many other configurations are
possible.
[0067] In some implementations, the long axis of a core may not be
exactly perpendicular to a radius of a substrate. However, a core
may still provide improved resolution over available core
geometries, particularly near the wafer edge. FIG. 7 shows a CMP
system 700 in which an elongated core 702 is positioned underneath
a platen 704. Prior to sweeping underneath a substrate 706, the
core 702 is at a position 708. At the position 708, the core 702 is
positioned approximately perpendicular to a radius R of substrate
706. Therefore, for r.apprxeq.R, the portion of a conductive layer
that couples with the magnetic field produced by the coil wound
around the core 702 is generally at the same radial distance from
the center of the wafer. Note that both the platen 704 and the
substrate 706 are rotating as the core 702 sweeps beneath the
substrate 706. The substrate 706 can also sweep with respect to the
platen 704, as indicated. Additionally, a flag 710 and a flag
sensor 712 may be used to sense the rotational position of the
platen 704.
[0068] Initially, referring to FIGS. 4 and 8A, before conducting
polishing, the oscillator 50 is tuned to the resonant frequency of
the LC circuit, without any substrate present. This resonant
frequency results in the maximum amplitude of the output signal
from RF amplifier 54.
[0069] As shown in FIG. 8B, for a polishing operation, the
substrate 10 is placed in contact with the polishing pad 30. The
substrate 10 can include a silicon wafer 12 and a conductive layer
16, e.g., a metal such as copper or aluminum, disposed over one or
more patterned underlying layers 14, which can be semiconductor,
conductor or insulator layers. A barrier layer 18, such as tantalum
or tantalum nitride, may separate the metal layer from the
underlying dielectric. The patterned underlying layers 14 can
include metal features, e.g., vias, pads and interconnects. Since,
prior to polishing, the bulk of the conductive layer 16 is
initially relatively thick and continuous, it has a low
resistivity, and relatively strong eddy currents can be generated
in the conductive layer. The eddy currents cause the metal layer to
function as an impedance source in parallel with the sense coil 46
and the capacitor 52. Consequently, the presence of the conductive
film 16 reduces the Q-factor of the sensor circuit, thereby
significantly reducing the amplitude of the signal from the RF
amplifier 56.
[0070] Referring to FIG. 8C, as the substrate 10 is polished the
bulk portion of the conductive layer 16 is thinned. As the
conductive layer 16 thins, its sheet resistivity increases, and the
eddy currents in the metal layer become dampened. Consequently, the
coupling between the conductive layer 16 and sensor circuitry is
reduced (i.e., increasing the resistivity of the virtual impedance
source). As the coupling declines, the Q-factor of the sensor
circuit increases toward its original value, causing the amplitude
of the signal from the RF amplifier 56 to rise.
[0071] Referring to FIG. 8D, eventually the bulk portion of the
conductive layer 16 is removed, leaving conductive interconnects
16' in the trenches between the patterned insulative layer 14. At
this point, the coupling between the conductive portions in the
substrate, which are generally small and generally non-continuous,
and sensor circuit reaches a minimum. Consequently, the Q-factor of
the sensor circuit reaches a maximum value (although not as large
as the Q-factor when the substrate is entirely absent). This causes
the amplitude of the output signal from the sensor circuit to
plateau.
[0072] FIG. 9 shows a graph 900 of the thickness of a conductive
layer after polishing the conductive layer. A line 902 on the graph
900 indicates the thickness (in Angstroms) of the conductive layer
measured at varying distances from the center of the wafer. For
example, a CMP system can polish an aluminum layer using the core
500 to monitor variations in the thickness of the aluminum layer in
different regions of a substrate. The CMP system can use an optical
monitoring system to determine when the aluminum layer is about 200
Angstroms thick and end polishing. In some implementations, using
the core 500 and adjusting pressure on the backside of the
substrate during polishing result in the aluminum layer that has a
within substrate thickness variability of at most 50 Angstroms. In
some implementations, using the core 500 or the core 408 reduce
wafer to wafer variability in addition to within wafer
variability.
[0073] FIG. 10 shows an example flowchart of a process 1000 for
polishing a metal layer on a substrate, such as copper or aluminum.
The substrate is polished at the first polishing station 22a to
remove the bulk of the metal layer until a first eddy current
monitoring system indicates a predetermined thickness of the metal
layer remains (1002). For example, an 8000 Angstrom copper layer
can be polished until the eddy current monitoring system indicates
that the copper layer is about 2000 Angstroms thick. As another
example, a 4000 Angstrom aluminum layer can be polished until the
eddy current monitoring system indicates that the aluminum layer is
about 1000 Angstroms thick. The polishing process can be monitored
by the eddy current monitoring system 40. When a predetermined
thickness, e.g., 2000 Angstroms of the copper layer 14, remains
over the underlying barrier layer 16, the polishing process is
halted and the substrate is transferred to the second polishing
station 22b. This first polishing endpoint can be triggered when
the amplitude signal exceeds an experimentally determined threshold
value.
[0074] As polishing progresses at the first polishing station 22a,
the radial thickness information from the eddy current monitoring
system 40 can be fed into a closed-loop feedback system to control
the pressure of the different chambers of the carrier head 70 on
the substrate. The pressure of the retaining ring on the polishing
pad may also be adjusted to adjust the polishing rate. This permits
the carrier head to compensate for the non-uniformity in the
polishing rate or for non-uniformity in the thickness of the metal
layer of the incoming substrate. As a result, after polishing at
the first polishing station, a significant amount of the metal
layer has been removed and the surface of the metal layer remaining
on the substrate is substantially planarized.
[0075] The carrier head 70 transfers the substrate to a second
platen at the second polishing station 22b (1004). The substrate
can be briefly polished at a high pressure when polishing begins at
the second platen (1006). This initial polishing, which can be
termed an "initiation" step, may be needed to remove native oxides
formed on the metal layer or to compensate for ramp-up of the
platen rotation rate and carrier head pressure so as to maintain
the expected throughput.
[0076] Optionally, at the second polishing station 22b, the
substrate is polished at a lower polishing rate than at the first
polishing station and a second eddy current monitoring system
measures the thickness of the metal layer (1008). For example, the
polishing rate is reduced by about a factor of 2 to 4, e.g., by
about 50% to 75%, from the polishing rate at the first polishing
station 22a. To reduce the polishing rate, the carrier head
pressure can be reduced, the carrier head rotation rate can be
reduced, the composition of the slurry can be changed to introduce
a slower polishing slurry, and/or the platen rotation rate could be
reduced. For example, the pressure on the substrate from the
carrier head may be reduced by about 33% to 50%, and the platen
rotation rate and carrier head rotation rate may both be reduced by
about 50%.
[0077] The second eddy current monitoring system measures the
thickness of the metal layer during polishing. The measurements can
be fed into a closed-loop feedback system in order to control the
pressure of the different chambers of the carrier head 70 on the
substrate in order to polish the metal layer evenly. In some
implementations, e.g., for polishing of a copper layer, the second
eddy current monitoring system can be different from the first eddy
current monitoring system, e.g., have a different resonant
frequency. For example, the first eddy current monitoring system
can have a resonant frequency tuned to detect the thickness of a
thicker metal layer than the second eddy current monitoring system.
For example, first eddy current monitoring system can have a
resonant frequency of about 320 kHz to 400 kHz, e.g., 400 kHz and
the second eddy current monitoring system have a resonant frequency
between about 1.5 and 2.0 MHz, e.g., between about 1.6 and 1.7 MHz.
For polishing of some metal layers, e.g., copper, this can permit
accurate measurement of the layer thickness above 2000 Angstroms
the first polishing station, and can permit accurate measurement of
the layer thickness below 2000 Angstroms, e.g., down to about 200
Angstroms, at the second polishing station. Thus, feedback control
of the pressure can be performed down until the metal layer has a
thickness of 200 to 300 Angstroms, at which point the feedback
control can be deactivated.
[0078] In some implementations, e.g., for polishing of an aluminum
layer, the first eddy current monitoring system and the second eddy
current monitoring system are the same type, e.g., both eddy
current monitoring systems use the same resonant frequency, e.g., a
resonant frequency between about 1.5 and 2.0 MHz, e.g., between
about 1.6 and 1.7 MHz.
[0079] With the improved sensitivity of the eddy current sensor, it
may be possible to perform closed-loop control of the pressure
applied by the different chambers of the carrier head with greater
reliability at thinner metal layer (e.g., copper) thicknesses,
e.g., at thicknesses below 1000 Angstroms, e.g., below 500
Angstroms, e.g., down to about 200 or 300 Angstroms. In addition,
with the improved sensitivity of the eddy current sensor, it may be
possible to perform closed-loop control of the pressure applied by
the different chambers of the carrier head with greater reliability
for metal layers of lower conductivity (compared to copper), e.g.,
aluminum layers. With the improved sensitivity of the eddy current
sensor, it may be possible to perform closed-loop control of the
pressure applied by the different chambers of the carrier head with
greater reliability with the sensor spaced farther from the
substrate, e.g., with a system in which the core does not project
above the top of the backing layer.
[0080] The polishing process can be monitored at the second
polishing station 22b by an optical monitoring system. Polishing
proceeds at the second polishing station 22b until the metal layer
is removed and the underlying barrier layer is exposed (1010). Of
course, small portions of the metal layer can remain on the
substrate, but the metal layer is substantially entirely removed.
The optical monitoring system is useful for determining this
endpoint, since it can detect the change in reflectivity as the
barrier layer is exposed. Specifically, the endpoint for the second
polishing station 22b can be triggered when the amplitude or slope
of the optical monitoring signal falls below an experimentally
determined threshold value across all the radial ranges monitored
by the computer. This indicates that the barrier metal layer has
been removed across substantially all of the substrate. Of course,
as polishing progresses at the second polishing station 22b, the
reflectivity information from the optical monitoring system 40 can
be fed into a closed-loop feedback system to control the pressure
applied by the different chambers of the of the carrier head 70 on
the substrate to prevent the regions of the barrier layer that are
exposed earliest from becoming overpolished.
[0081] By reducing the polishing rate before the barrier layer is
exposed, dishing and erosion effects can be reduced. In addition,
the relative reaction time of the polishing machine is improved,
enabling the polishing machine to halt polishing and transfer to
the third polishing station with less material removed after the
final endpoint criterion is detected. Moreover, more intensity
measurements can be collected near the expected polishing end time,
thereby potentially improving the accuracy of the polishing
endpoint calculation. However, by maintaining a high polishing rate
throughout most of the polishing operation at the first polishing
station, high throughput is achieved.
[0082] Once the metal layer has been removed at the second
polishing station 22b, the substrate is transferred to the third
polishing station 22c (1012). Optionally, the substrate may be
briefly polished with an initiation step, e.g., for about 5
seconds, at a somewhat higher pressure. The polishing process is
monitored at the third polishing station 22c by an optical
monitoring system, and proceeds until the exposed layers on the
substrate are buffed (1014). In some implementations, the barrier
layer is substantially removed and the underlying dielectric layer
is substantially exposed at the third polishing station 22c. The
same slurry solution may be used at the first and second polishing
stations, whereas another slurry solution may be used at the third
polishing station.
[0083] An alternative method 1100 of polishing a metal layer, such
as a copper layer or an aluminum layer, is shown in flowchart form
in FIG. 11. Both the fast polishing step and the slow polishing
step are performed at the first polishing station 22a (1102, 1104).
Buffing of the substrate and/or removal of the barrier layer can be
performed at the second polishing station 22b. Alternatively, the
barrier layer can be removed at the second polishing station 22b,
and a buffing step can be performed at the final polishing station
22c.
[0084] While the substrate is polished at the first polishing
station 22a, the first eddy current monitoring system measures the
thickness of the metal layer, and the measurements can be fed into
a closed-loop feedback system in order to control the pressure
and/or loading area of the different chambers of the carrier head
70 on the substrate in order to polish the metal layer evenly
(1102, 1105). Feedback control of the pressure can be performed
until the metal layer has a thickness of 200 to 300 Angstroms, at
which point the feedback control can be deactivated.
[0085] Optionally, when the eddy current monitoring system
indicates that a predetermined thickness of a metal layer remains
on the substrate, less than 1000 Angstroms for an aluminum layer,
e.g., the substrate is polished at a reduced polishing speed, e.g.,
by reducing the pressure on the backside of the substrate (1104).
After the polishing rate is reduced, the polishing system can
continue to use eddy current monitoring system to measure the
thickness of the metal layer adjust the pressure in the carrier
head 70 on the backside of the substrate in order to uniformly
polish the different regions of the metal layer (1105).
[0086] The optical monitoring system determines that an underlying
layer is at least partially exposed and polishing is stopped
(1106). For example, the optical monitoring system can determine
that the underlying barrier layer 16 is partially exposed. The
carrier head 70 transfers the substrate to a second platen (1108).
The substrate is buffed at the second platen (1110).
[0087] The eddy current and optical monitoring systems can be used
in a variety of polishing systems. Either the polishing pad, or the
carrier head, or both can move to provide relative motion between
the polishing surface and the substrate. The polishing pad can be a
circular (or some other shape) pad secured to the platen, a tape
extending between supply and take-up rollers, or a continuous belt.
The polishing pad can be affixed on a platen, incrementally
advanced over a platen between polishing operations, or driven
continuously over the platen during polishing. The pad can be
secured to the platen during polishing, or there can be a fluid
bearing between the platen and polishing pad during polishing. The
polishing pad can be a standard (e.g., polyurethane with or without
fillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather
than tuning when the substrate is absent, the drive frequency of
the oscillator can be tuned to a resonant frequency with a polished
or unpolished substrate present (with or without the carrier head),
or to some other reference.
[0088] Although illustrated as positioned in the same hole, the
optical monitoring system 140 can be positioned at a different
location on the platen than the eddy current monitoring system 40.
For example, the optical monitoring system 140 and eddy current
monitoring system 40 could be positioned on opposite sides of the
platen, so that they alternately scan the substrate surface.
[0089] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *