U.S. patent application number 11/729303 was filed with the patent office on 2009-03-05 for endpoint detection system for wafer polishing.
This patent application is currently assigned to Strasbaugh. Invention is credited to Stephan H. Wolf.
Application Number | 20090061734 11/729303 |
Document ID | / |
Family ID | 24362394 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090061734 |
Kind Code |
A1 |
Wolf; Stephan H. |
March 5, 2009 |
Endpoint detection system for wafer polishing
Abstract
An wafer polishing pad assembly for use in CMP includes an
optical sensor for sensing reflectivity of the wafer during
polishing, and produces a corresponding signal, and transmits the
signal from the rotating pad to a stationary portion of the
assembly. The signal is transmitting off the pad through
non-contact couplings such inductive coupling or optical couplings
after being converted into signal formats enabling non-contact
transmission.
Inventors: |
Wolf; Stephan H.; (Los Osos,
CA) |
Correspondence
Address: |
CROCKETT & CROCKETT, P.C.
26020 ACERO, SUITE 200
MISSION VIEJO
CA
92691
US
|
Assignee: |
Strasbaugh
|
Family ID: |
24362394 |
Appl. No.: |
11/729303 |
Filed: |
March 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11443788 |
May 30, 2006 |
7195541 |
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11729303 |
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10785393 |
Feb 23, 2004 |
7052366 |
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11443788 |
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10303621 |
Nov 25, 2002 |
6695681 |
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10785393 |
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09590470 |
Jun 9, 2000 |
6485354 |
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10303621 |
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Current U.S.
Class: |
451/6 |
Current CPC
Class: |
B24B 37/205 20130101;
B24B 37/013 20130101; B24B 49/12 20130101 |
Class at
Publication: |
451/6 |
International
Class: |
B24B 49/00 20060101
B24B049/00 |
Claims
1. A polishing pad assembly for polishing a wafer surface and
collecting and transmitting data relating to the condition of the
wafer surface, said polishing pad assembly comprising: a polishing
pad; means for directing light at the wafer surface, said means
disposed within the polishing pad; means for detecting light
reflected from the wafer surface and creating an electrical signal
corresponding to the light reflected, said means for detecting
light disposed within the polishing pad; means for processing the
electrical signal corresponding to the light reflected and
producing a corresponding processed signal, said means for
processing the electrical signal disposed within the pad; and a
transmitter for transmitting the processed signal, said transmitter
operably coupled to the means for processing the electrical signal.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 11/443,788 filed May 30, 2006, now U.S. Pat. No. 7,195,541,
which is a continuation of U.S. application Ser. No. 10/785,393
filed Feb. 23, 2004, now U.S. Pat. No. 7,052,366, which is a
continuation of U.S. application Ser. No. 10/303,621 filed Nov. 25,
2002, now U.S. Pat. No. 6,695,681, which is a continuation of U.S.
application Ser. No. 09/590,470, filed Jun. 9, 2000, now U.S. Pat.
No. 6,485,354.
FIELD OF THE INVENTIONS
[0002] The inventions described below relate the field of
semiconductor wafer processing, and more specifically relates to a
disposable polishing pad for use in a chemical mechanical polishing
operation performed on the semiconductor wafers wherein the
polishing pad contains an optical sensor for monitoring the
condition of the surface being polished while the polishing
operation is taking place to permit determination of the endpoint
of the process
BACKGROUND OF THE INVENTIONS
[0003] In U.S. Pat. No. 5,893,796 issued Apr. 13, 1999 and in
continuation Pat. No. 6,045,439 issued Apr. 4, 2000, Birang et al.
show a number of designs for a window installed in a polishing pad.
The wafer to be polished is on top of the polishing pad, and the
polishing pad rests upon a rigid platen so that the polishing
occurs on the lower surface of the wafer. That surface is monitored
during the polishing process by an interferometer that is located
below the rigid platen. The interferometer directs a laser beam
upward, and in order for it to reach the lower surface of the
wafer, it must pass through an aperture in the platen and then
continue upward through the polishing pad. To prevent the
accumulation of slurry above the aperture in the platen, a window
is provided in the polishing pad. Regardless of how the window is
formed, it is clear that the interferometer sensor is always
located below the platen and is never located in the polishing
pad.
[0004] In U.S. Pat. No. 5,949,927 issued Sep. 7, 1999 to Tang,
there are described a number of techniques for monitoring polished
surfaces during the polishing process. In one embodiment Tang
refers to a fiber-optic cable embedded in a polishing pad. This
cable is merely a conductor of light. The light source and the
detector that do the sensing are located outside of the pad.
Nowhere does Tang suggest including a light source and a detector
inside the polishing pad. In some of Tang's embodiments,
fiber-optic decouplers are used to transfer the light in the
optical fibers from a rotating component to a stationary component.
In other embodiments, the optical signal is detected onboard a
rotating component, and the resulting electrical signal is
transferred to a stationary component through electrical slip
rings. There is no suggestion in the Tang patent of transmitting
the electrical signal to a stationary component by means of radio
waves, acoustical waves, a modulated light beam, or by magnetic
induction.
[0005] In another optical end-point sensing system, described in
U.S. Pat. No. 5,081,796 issued Jan. 21, 1992 to Schultz there is
described a method in which, after partial polishing, the wafer is
moved to a position at which part of the wafer overhangs the edge
of the platen. The wear on this overhanging part is measured by
interferometry to determine whether the polishing process should be
continued.
[0006] In conclusion, although several techniques are known in the
art for monitoring the polished surface during the polishing
process, none of these techniques is entirely satisfactory. The
fiber optic bundles described by Tang are expensive and potentially
fragile; and the use of an interferometer located below the platen,
as used by Birang et al., requires making an aperture through the
platen that supports the polishing pad. Accordingly, the present
inventor set out to devise a monitoring system that would be
economical and robust, taking advantage of recent advances in the
miniaturization of certain components.
SUMMARY
[0007] It is an objective of the present invention to provide a
polishing pad in which an optical sensor is contained, for
monitoring an optical characteristic, such as the reflectivity, of
a wafer surface that is being polished, during the polishing
operation. The real-time data derived from the optical sensor
enables, among other things, the end point of the process to be
determined.
[0008] It is a further objective of the present invention to
provide apparatus for supplying electrical power to the optical
sensor in the polishing pad.
[0009] It is a further objective of the present invention to
provide apparatus for supplying electrical power for use in
transmitting an electrical signal representing the optical
characteristic from the rotating polishing pad to an adjacent
non-rotating receiver.
[0010] It is a further objective of the present invention to
provide a disposable polishing pad containing an optical sensor,
wherein the polishing pad is removably connectable to a
non-disposable hub that contains power and signal processing
circuitry.
[0011] In accordance with the present invention, an optical sensor
that includes a light source and a detector is disposed within a
blind hole in the polishing pad so as to face the surface that is
being polished. Light from the light source is reflected from the
surface being polished and the reflected light is detected by the
detector which produces an electrical signal related to the
intensity of the light reflected back onto the detector.
[0012] The electrical signal produced by the detector is conducted
radially inward from the location of the detector to the central
aperture of the polishing pad by a thin conductor concealed between
the layers of the polishing pad.
[0013] The disposable polishing pad is removably connected, both
mechanically and electrically, to a hub that rotates with the
polishing pad. The hub contains electronic circuitry that is
concerned with supplying power to the optical sensor and with
transmitting the electrical signal produced by the detector to
non-rotating parts of the system. Because of the expense of these
electronic circuits, the hub is not considered to be disposable.
After the polishing pad has been worn out from use, it is disposed
of, along with the optical sensor and the thin conductor.
[0014] In accordance with the present invention, electrical power
for operating the electronic circuits within the hub and for
powering the light source of the optical sensor may be provided by
several techniques. In a preferred embodiment, the secondary
winding of a transformer is included within the rotating hub and a
primary winding is located on an adjacent non-rotating part of the
polishing machine. In a first alternative embodiment, a solar cell
or photovoltaic array is mounted on the rotating hub and is
illuminated by a light source mounted on a non-rotating portion of
the machine. In another alternative embodiment, electrical power is
derived from a battery located within the hub. In yet another
embodiment, electrical conductors in the rotating polishing pad or
in the rotating hub pass through the magnetic fields of permanent
magnets mounted on adjacent non-rotating portions of the polishing
machine, to constitute a magneto.
[0015] In accordance with the present invention, the electrical
signal representing an optical characteristic of the surface being
polished is transmitted from the rotating hub to an adjacent
stationary portion of the polishing machine by any of several
techniques. In a preferred embodiment, the electrical signal to be
transmitted is used to frequency modulate a light beam that is
received by a detector located on adjacent non-rotating structure.
In alternative embodiments, the signal is transmitted by a radio
link or an acoustical link. In yet another alternative embodiment,
the signal may be applied to the primary winding of a transformer
on the rotating hub and received by a secondary winding of the
transformer located on an adjacent non-rotating portion of the
polishing machine. This transformer may be the same transformer
that is used for coupling electrical power into the hub, or it can
be a different transformer.
[0016] The novel features which are believed to be characteristic
of the invention, both as to organization and method of operation,
together with further objects and advantages thereof, will be
better understood from the following description considered in
connection with the accompanying drawings in which several
embodiments of the invention are illustrated by way of example. It
is to be expressly understood, however, that the drawings are for
the purpose of illustration and description only and are not
intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is an exploded view in perspective showing the
general arrangement of the elements of a preferred embodiment of
the invention;
[0018] FIG. 2 is a front top perspective view of the optical sensor
used in a preferred embodiment of the invention;
[0019] FIG. 3 is a side elevational diagram showing an optical
sensor in an alternative embodiment of the invention;
[0020] FIG. 4 is a diagram showing a medial cross sectional view of
a hub in accordance with a preferred embodiment of the
invention;
[0021] FIG. 5 is a diagram showing a medial cross sectional view of
a hub in a first alternative embodiment of the invention;
[0022] FIG. 6 is a diagram showing a medial cross sectional view of
a hub in a second alternative embodiment of the invention; and,
[0023] FIG. 7 is a diagram showing a medial cross sectional view of
a hub in a third alternative embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTIONS
[0024] The wafers with which the present invention is used are
composite structures that include strata of different materials.
Typically, the outermost stratum is polished away until its
interface with an underlying stratum has been reached. At that
point it is said that the end point of the polishing operation has
been reached. The polishing pad of the present invention is
applicable to detecting transitions from an oxide layer to a
silicon layer as well as to transitions from a metal to an oxide or
other material.
[0025] Clearly, stopping a polishing machine to remove a wafer to
inspect it and then replacing the wafer into the machine and
starting the machine is a highly inefficient way of determining
whether the process has been carried far enough. Ideally, with the
present invention, the polishing process can be allowed to progress
until the optical sensor of the present invention has provided
information that permits a determination that the end point has
been reached.
[0026] Although end point sensing is the main objective of the
present invention, other possibilities for using the present
invention are under consideration. These include determining how
far away the end point is, sampling various areas on a wafer, and
mapping the surface of a wafer. Although a single optical sensor is
described in the following paragraphs, it is contemplated that for
some uses of the invention a number of optical sensors may be
included in a polishing pad.
[0027] The present invention involves modifying a conventional
polishing pad by embedding within it an optical sensor and other
components. The unmodified polishing pads are widely available
commercially, and the Model IC 1000 made by the Rodel Company of
Newark, N.J., is a typical unmodified pad. Pads manufactured by the
Thomas West Company may also be used. The manner in which these
pads are modified in accordance with the present invention and used
will be clear from the discussion below.
[0028] In that discussion, it will be seen that the optical sensor
of the present invention senses an optical characteristic of the
surface that is being polished. Typically, the optical
characteristic of the surface is its reflectivity. However, other
optical characteristics of the surface can also be sensed,
including its polarization, its absorptivity, and its
photoluminescense (if any). Techniques for sensing these various
characteristics are well known in the optical arts, and typically
they involve little more than adding a polarizer or a spectral
filter to the optical system. For this reason, in the following
discussion the more general term "optical characteristic" is
used.
[0029] The words "optical" and "light" as used below include
ultraviolet, visible, and infrared types of light. The terms
"radio" and "acoustic" are used in their usual broad sense.
[0030] As shown in FIG. 1, the polishing pad 10 has a circular
shape and a central circular aperture 12. In accordance with the
present invention, a blind hole 14 is formed in the polishing pad,
and the hole 14 opens upwardly so as to face the surface that is
being polished. In accordance with the invention, an optical sensor
16 is placed in the blind hole 14 and a conductor ribbon 18, which
extends from the optical sensor 16 to the central aperture 12, is
embedded within the polishing pad.
[0031] When the polishing pad is to be used, a hub 20 is inserted
from above into the central aperture 12 and secured there by
screwing a base 22, which lies below the polishing pad, onto a
threaded portion of the hub 20, As best seen in FIG. 4, the
polishing pad 10 is thus clamped between portions of the hub and
portions of the base. During the grinding process, the polishing
pad, the hub and the base rotate together about a central vertical
axis 24.
[0032] Also seen in FIG. 1 and FIGS. 4-7 is a non-rotating portion
26 of the polishing machine. Preferably, it is located adjacent and
above the hub 20. Although it is not considered to be part of the
present invention, the non-rotating portion 26 is ancillary to the
present invention and its purpose will be described more fully
below.
[0033] FIG. 2 is a top front perspective view showing the optical
sensor 16, in a preferred embodiment, in greater detail. The
optical sensor 16 includes a light source 28, a detector 30, a
reflective surface 32, and the conductor ribbon 18. The conductor
ribbon 18 includes a number of generally parallel conductors
laminated together for the purpose of supplying electrical power to
the light source 28 and for conducting the electrical output signal
of the detector 30 to the central aperture 12. Preferably, the
light source 28 and the detector 30 are a matched pair. In general,
the light source 28 may be a light emitting diode and the detector
30 is a photodiode. The central axis of the bundle of light emitted
by the light source 28 is directed horizontally initially, but upon
reaching the reflective surface 32 the light is redirected upward
so as to strike and reflect from the surface that is being
polished. The reflected light also is redirected by the reflective
surface 32 so that the reflected light falls on the detector 30,
which produces an electrical signal in relation to the intensity of
the light falling on it. The arrangement shown in FIG. 2 was chosen
to conserve the height of the sensor.
[0034] As smaller light sources and detectors become available, it
may be possible to dispense with the reflective surface 32 and
instead to use the arrangement shown in side view in FIG. 3.
[0035] The optical components and the end of the conductor ribbon
18 are encapsulated in the form of a thin disk 34 that is sized to
fit snugly within the blind hole 14 of FIG. 1. In the arrangements
of FIGS. 2 and 3, it is understood that baffles may be used to
reduce the amount of stray light reaching the detector.
[0036] Included within the conductor ribbon 18 are at least three
conductors: a power conductor 36, a signal conductor 38, and one or
more return or ground conductors, not shown.
[0037] As best seen in FIG. 4, the power conductor 36 terminates
adjacent the central aperture 12 of the polishing pad 10 at a power
plug 40, and the signal conductor 38 likewise terminates at a
signal plug 42. When the hub 20 is inserted into the central
aperture 12, the power plug 40 makes electrical contact with the
power jack 44, and the signal plug 42 makes electrical contact with
the signal jack 46. An O-ring seal 48 prevents the liquids used in
the polishing process from reaching the plugs and jacks. Ajar lid
type of seal 50 is provided in the base 22 to further insure that
the electronic circuits within the hub remain uncontaminated.
[0038] An electrical signal produced by the detector 30 and related
to the optical characteristic is carried by the conductor 52 from
the signal jack 46 to a signal processing circuit 54, that produces
in response to the electrical signal a processed signal on the
conductor 56 representing the optical characteristic. The processed
signal on the conductor 56 is then applied to a transmitter 58.
[0039] In the embodiment shown in FIG. 4, the transmitter 58
applies a time-varying electrical current to the primary winding 60
of a transformer that produces a varying magnetic field 62
representative of the processed signal. The magnetic field 62
extends upward through the top of the hub 20 and is intercepted by
a secondary winding 64 of the transformer which is located on an
adjacent non-rotating portion 26 of the polishing machine, or on
some other non-rotating object. The varying magnetic field 62
induces a current in the secondary winding 64 that is applied to a
receiver 66 that produces on the terminal 68 a signal
representative of the optical characteristic. This signal is then
available for use by external circuitry for such purposes as
monitoring the progress of the polishing operation and/or
determining whether the end point of the polishing process has been
reached.
[0040] A similar inductive technique may be used to transfer
electrical power from the adjacent non-rotating portion 26 of the
polishing machine to the rotating hub 20. A prime power source 70
on the non-rotating portion 26 applies an electrical current to the
primary winding 72 of a transformer that produces a magnetic field
74 that extends downward through the top of the hub 20 and is
intercepted by a secondary winding 76 in which the varying magnetic
field induces an electrical current that is applied to a power
receiver circuitry 78. The power receiver 78 applies electrical
power on the conductor 80 to the power jack 44, from which it is
conducted through the power plug 40 and the power conductor 36 to
the light source 28. The power receiver 78 also supplies electrical
power to the signal processing circuit 54 through the conductor 82,
and to the transmitter 58 through the conductor 84. At present, the
magnetic induction technique is the best mode and preferred
embodiment for transferring power into the rotating hub 20. In one
embodiment the winding 60 is the same winding 76, and the winding
64 is the same winding 72. The superimposed power and signal
components are at different frequency ranges in this embodiment and
are separated by filtering.
[0041] FIGS. 5-7 show alternative embodiments in which other
techniques are used to transfer signals from the rotating hub 20 to
a non-rotating portion 26 of the polishing machine, and to transfer
electrical power from the non-rotating portion 26 into the rotating
hub 20.
[0042] In the embodiment shown in FIG. 5, the transmitter 58
further includes a modulator 86 that applies to a light emitting
diode or laser diode 88 a frequency modulated current
representative of the processed signal that represents the optical
characteristic. The light-emitting diode 88 emits light waves 90
that are focused by a lens 92 onto a photodiode detector 94. The
detector 94 converts the light waves into an electrical signal that
is demodulated in the receiver 96 to produce on the terminal 68 an
electrical signal representative of the optical characteristic. At
present, this is the best mode and preferred technique for
transferring the electrical signal from the rotating hub 20 to the
non-rotating portion 26 of the polishing machine.
[0043] Also, in the embodiment of FIG. 5, the prime source of
electrical power is a battery 98 that supplies power to a power
distribution circuit 100 that, in turn, distributes electrical
power to the power jack 44, to the signal processing circuit 54,
and to the transmitter circuit 58.
[0044] In the embodiment of FIG. 6, the transmitter 58 is a radio
transmitter having an antenna 102 that transmits radio waves 104
through the top of the hub 20. The radio waves 104 are intercepted
by the antenna 106 and demodulated by the receiver 103 to produce
an electrical signal on the terminal 68 that is representative of
the optical characteristic.
[0045] Also in the embodiment of FIG. 6, electrical power is
generated by a magneto consisting of a permanent magnet 110 located
in the non-rotating portion 26 and an inductor 112 in which the
magnetic field of the permanent magnet 110 induces a current as the
inductor 112 rotates past the permanent magnet 110. The induced
current is rectified and filtered by the power circuit 114 and then
distributed by a power distribution circuit 116.
[0046] In the embodiment of FIG. 7, the transmitter 58 further
includes a power amplifier 118 that drives a loudspeaker 120 that
produces sound waves 122. The sound waves 122 are picked up by a
microphone 124 located in the non-rotating portion 26 of the
polishing machine. The microphone 124 produces an electrical signal
that is applied to the receiver 126 which, in turn, produces an
electrical signal on the terminal 68 that is representative of the
optical characteristic.
[0047] Also in the embodiment of FIG. 7 electrical power is
generated in the rotating hub 20 by a solar cell or solar panel 128
in response to light applied to the solar panel 128 by a light
source 132 located in the non-rotating portion 26. The electrical
output of the solar panel 128 is converted to an appropriate
voltage by the converter 134, if necessary, and applied to the
power distribution circuit 116.
[0048] Thus, there has been described a polishing pad, for use in a
chemical mechanical polishing operation, containing an optical
sensor for monitoring the condition of the surface that is being
polished, during the polishing operation. The polishing pad,
including the optical system, is disposable, and is used with a
non-disposable hub that contains circuitry for receiving the signal
produced by the optical sensor, for processing the signal and for
transmitting the signal to a non-rotating station. The hub also
contains circuitry for supplying power to the optical sensor as
well as to the other electronic circuits located in the hub. In the
several embodiments described above, it is seen that the signal may
be transmitted from the rotating hub to the non-rotating station by
radio waves, sound waves, light waves, or by magnetic induction.
Also, in the various embodiments, power may be supplied by
including a battery in the hub or by coupling electrical power into
the hub through a solar panel activated by externally applied light
or by a magneto in which a stationary permanent magnet induces a
current in an inductor that is mounted on the rotating hub.
[0049] The foregoing detailed description is illustrative of
several embodiments of the invention, and it is to be understood
that additional embodiments thereof will be obvious to those
skilled in the art. The embodiments described herein together with
those additional embodiments are considered to be within the scope
of the invention.
* * * * *