U.S. patent application number 09/758077 was filed with the patent office on 2002-07-11 for apparatus and method of determining an endpoint during a chemical-mechanical polishing process.
Invention is credited to Crevasse, Annette M., Easter, William G., Maze, John A., Miceli, Frank.
Application Number | 20020090889 09/758077 |
Document ID | / |
Family ID | 25050402 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020090889 |
Kind Code |
A1 |
Crevasse, Annette M. ; et
al. |
July 11, 2002 |
Apparatus and method of determining an endpoint during a
chemical-mechanical polishing process
Abstract
The present invention provides a polishing apparatus for use in
polishing a substrate, including: (1) A polishing platen, and (2) a
rotational strain sensor coupled to the polishing platen configured
to detect a change in a rotational strain of the polishing platen
during a polishing process. In addition, the present invention
provides an accompanying method of detecting an endpoint during the
polishing process by detecting a change between a first rotational
strain and a second rotational strain with the rotational strain
sensor.
Inventors: |
Crevasse, Annette M.;
(Apopka, FL) ; Easter, William G.; (Orlando,
FL) ; Maze, John A.; (Clermont, FL) ; Miceli,
Frank; (Orlando, FL) |
Correspondence
Address: |
Charles W. Gaines
Hitt Gaines & Boisbrun, P.C.
P.O. Box 832570
Richardson
TX
75083
US
|
Family ID: |
25050402 |
Appl. No.: |
09/758077 |
Filed: |
January 10, 2001 |
Current U.S.
Class: |
451/8 ;
451/41 |
Current CPC
Class: |
B24B 49/02 20130101;
B24B 37/013 20130101 |
Class at
Publication: |
451/8 ;
451/41 |
International
Class: |
B24B 049/00; B24B
051/00; B24B 001/00 |
Claims
What is claimed is:
1. A polishing apparatus, comprising: a polishing platen; and a
rotational strain sensor coupled to the polishing platen and
configured to detect a change in a rotational strain of the
polishing platen during a polishing process.
2. The polishing apparatus as recited in claim 1 wherein the
polishing apparatus further includes a polishing pad coupled to an
outer surface of the polishing platen and the rotational strain
sensor is a piezoelectric sensor located between the outer surface
and the polishing pad.
3. The polishing apparatus as recited in claim 1 wherein the
polishing platen includes a base plate having a backing block
projecting therefrom and an upper plate having a cavity formed
therein and configured to receive the backing block therein, the
rotational strain sensor being located between a face of the
backing block and a face of the cavity.
4. The polishing apparatus as recited in claim 1 wherein the
polishing platen includes a backing block projecting from a surface
thereof and the polishing apparatus further includes a polishing
pad having a cavity formed therein and configured to receive the
backing block therein, the rotational strain sensor being located
between a face of the backing block and a face of the cavity.
5. The polishing apparatus as recited in claim 1 wherein the
polishing platen includes a base plate and the rotational strain
sensor extends from the base plate and the polishing platen further
includes an upper plate having a cavity formed therein and
configured to receive the rotational strain sensor therein.
6. The polishing apparatus as recited in claim 5 wherein the
rotational strain sensor includes a flexible stud member having
adjacent, spaced-apart conductors located within the flexible stud
member.
7. The polishing apparatus as recited in claim 5 wherein the
rotational strain sensor includes a rigid stud member supported on
a flexible base having adjacent, spaced-apart conductors.
8. The polishing apparatus as recited in claim 1 wherein the
rotational strain sensor extends from a surface of the polishing
platen and the polishing platen further includes a polishing pad
having a cavity formed therein and configured to receive the
rotational strain sensor therein.
9. The polishing apparatus as recited in claim 8 wherein the
rotational strain sensor includes a flexible stud member having
adjacent, spaced-apart conductors located within the flexible stud
member.
10. The polishing apparatus as recited in claim 8 wherein the
rotational strain sensor includes a rigid stud member supported on
a flexible base having adjacent, spaced-apart conductors.
11. The polishing apparatus as recited in claim 1 further including
a motor driven shaft coupled to the polishing platen, the
rotational strain sensor coupled to the motor driven shaft.
12. A method of detecting an endpoint during polishing of a
substrate, comprising: pressing a substrate having a first layer
composed of a first material and a second layer composed of a
second material against a polishing pad coupled to a polishing
platen; producing a first rotational strain of the polishing platen
by polishing the first layer of the substrate with the polishing
pad; producing a second rotational strain of the polishing platen
by polishing the second layer of the substrate with the polishing
pad; and detecting a change between the first rotational strain and
the second rotational strain with a rotational strain sensor
coupled to the polishing platen.
13. The method as recited in claim 12 wherein detecting the change
includes detecting a change with a piezoelectric sensor located
between the polishing platen and the polishing pad.
14. The method as recited in claim 12 wherein detecting the change
includes detecting a change with a rotational strain sensor located
between a face of a backing block projecting from a base plate of
the polishing platen and a face of a cavity formed in an upper
plate of the polishing platen.
15. The method as recited in claim 12 wherein detecting the change
includes detecting a change with a rotational strain sensor located
between a face of a backing block projecting from the polishing
platen and a face of a cavity formed in the polishing pad.
16. The method as recited in claim 12 wherein detecting the change
includes detecting a change with a rotational strain sensor that
extends from a base plate of the polishing platen and is received
in a cavity formed in an upper plate of the polishing platen.
17. The method as recited in claim 16 wherein detecting the change
includes detecting a change with a rotational strain sensor that
includes a flexible stud member having adjacent, spaced-apart
conductors located within the flexible stud member.
18. The method as recited in claim 16 wherein detecting the change
includes detecting a change with a rotational strain sensor that
includes a rigid stud member supported on a flexible base having
adjacent, spaced-apart conductors.
19. The method as recited in claim 12 wherein detecting the change
includes detecting a change with a rotational strain sensor that
extends from the polishing platen and is received in a cavity
formed in the polishing pad.
20. The method as recited in claim 19 wherein detecting the change
includes detecting a change with a rotational strain sensor that
includes a flexible stud member having adjacent, spaced-apart
conductors located within the flexible stud member.
21. The method as recited in claim 19 wherein detecting the change
includes detecting a change with a rotational strain sensor that
includes a rigid stud member supported on a flexible base having
adjacent, spaced-apart conductors.
22. The method as recited in claim 12 wherein detecting the change
includes detecting a change with a rotational strain sensor coupled
to a motor driven shaft that is coupled to the polishing
platen.
23. The method as recited in claim 12 wherein pressing a substrate
includes pressing a substrate located over a semiconductor
wafer.
24. The method as recited in claim 23 wherein pressing a substrate
includes pressing a first layer comprising a dielectric and
pressing a second layer comprising a metal.
25. The method as recited in claim 24 further including forming
transistors on the semiconductor wafer, forming a plurality of
alternating first and second layers over the transistors and
interconnecting the transistors to form an operative integrated
circuit.
26. A method of polishing a substrate comprising: detecting a
change in a rotational strain during a polishing process of the
substrate.
27. The method as recited in claim 26 further comprising: changing
the polishing process when a change in the rotational strain is
detected.
28. The method as recited in claim 26 further comprising: providing
a strain gauge to measure the rotational strain.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention is directed, in general, to
chemical-mechanical polishing (CMP) of a semiconductor wafer and,
more specifically, to a method of determining an endpoint by
monitoring rotational strain during the CMP process and a polishing
apparatus including the same.
BACKGROUND OF THE INVENTION
[0002] In the fabrication of semiconductor components, metal
conductor lines are formed over a substrate containing device
circuitry. The metal conductor lines serve to interconnect discrete
devices, and thus form integrated circuits (ICs). The metal
conductor lines are further insulated from the next interconnection
level by thin films of insulating material deposited by, for
example, Chemical Vapor Deposition (CVD) of oxide or application of
Spin On Glass (SOG) layers followed by fellow processes. Holes, or
vias, formed through the insulating layers provide electrical
connectivity between successive conductive interconnection layers.
In such wiring processes, it is desirable that the insulating
layers have a smooth surface topography, since it is difficult to
lithographically image and pattern layers applied to rough
surfaces.
[0003] Also, deep (greater than 3 .mu.m) and narrow (less than 2
.mu.m) trench structures have been used in advanced semiconductor
design for three major purposes: (1) to prevent latch-up and to
isolate n-channel from p-channel devices in CMOS circuits; (2) to
isolate the transistors of bipolar circuits; and (3) to serve as
storage-capacitor structures in DRAMS. However, in this technology
it is even more crucial to precisely determine the endpoint of
differing materials to prevent unnecessary dishing out of the
connector metal.
[0004] Chemical-mechanical polishing (CMP) has been developed for
providing smooth insulator topographies. Briefly, the CMP processes
involve holding and rotating a thin, reasonably flat semiconductor
wafer against a wetted polishing surface under controlled chemical,
pressure, and temperature conditions. A chemical slurry containing
a polishing agent, such as alumina or silica, is used as the
abrasive material. Additionally, the chemical slurry contains
selected chemicals which etch or oxidize various surfaces of the
wafer during processing. The combination of mechanical and chemical
removal of material during polishing results in superior planarity
of the polished surface.
[0005] CMP is also used to remove different layers of material from
the surface of a semiconductor wafer. For example, following via
formation in a dielectric material layer, a metallization layer is
blanket-deposited, and then CMP is used to produce planar metal
studs. When used for this purpose, it is important to remove a
sufficient amount of material to provide a smooth surface, without
removing an excessive amount of underlying materials. The accurate
removal of material is particularly important in today's submicron
technologies where the layers between device and metal levels are
constantly getting thinner. To better determine endpoints between
removed and remaining layers of a semiconductor wafer, an accurate
polishing endpoint detection technique is invaluable.
[0006] In the past, endpoints have been detected by interrupting
the CMP process, removing the wafer from the polishing apparatus,
and physically examining the wafer surface by techniques which
ascertain film thickness and/or surface topography. However, with
such prior art processes if the wafer did not meet specifications,
it was loaded back into the polishing apparatus for further
polishing to achieve the desired planarity. This would have to be
repeated until a sufficient amount of material was removed.
Unfortunately, in addition to the excess time required by this
technique, if too much material was removed, the wafer was likely
found to be substandard to the required specifications, and often
discarded altogether. By experience, an elapsed CMP time for a
given CMP process has been developed with some accuracy. However,
like the prior art technique described above, this endpoint
detection technique is time consuming, unreliable, and costly.
[0007] Various active processes have been developed to circumvent
the problems associated with prior art endpoint detection
techniques. However, these active processes suffer from their own
disadvantages and inaccuracies. One of the better known of these
prior art techniques involves the continuous monitoring of the
motor current of the CMP apparatus. Specifically, the drive motor
used to rotate the platen holding the polishing pad is continuously
monitored during the polishing process for changes in load current.
As each layer of a semiconductor wafer is polished, a certain
amount of friction develops between the polishing pad and wafer
layer. The amount of friction that develops is dependent, at least
in part, on the coefficient of friction present at the interface
between the layer being polished and the polishing pad. When the
CMP process finishes the removal of one layer of the wafer and
begins on the next, a change in the amount of friction between the
polishing pad and wafer layer affects the amount of work required
by the drive motor. As the work required by the drive motor changes
with each different layer, the load current of the motor changes as
well. These changes in load current may be monitored to determine
when the polishing process has begun on a new wafer layer.
[0008] Unfortunately, the techniques for monitoring the load
current of the drive motor also suffer from deficiencies.
Specifically, only monitoring changes in the load current of the
motor does not obtain layer information directly from the polishing
platen. As a result, a change in load current caused by layers
composed of materials having similar composition may be too small
to detect. Conversely, changes in the load current of the drive
motor, such as a power surge, caused by other means may incorrectly
inform the operator that an endpoint of a particular layer of the
wafer has been reached. With the high cost of semiconductor
materials in the industry, a more direct technique for determining
a polishing endpoint, with less risk than those found in the prior
art, is desirable. Accordingly, what is needed in the art is an
improved technique for accurately determining the endpoint of one
semiconductor wafer layer and the beginning of the next during a
polishing process.
SUMMARY OF THE INVENTION
[0009] To address the above-discussed deficiencies of the prior
art, the present invention provides a polishing apparatus for use
in polishing a substrate. In one advantageous embodiment, the
polishing apparatus provides a polishing platen and a rotational
strain sensor coupled to the polishing platen configured to detect
a change in a rotational strain of the polishing platen during the
polishing process. In addition, the present invention provides a
method of detecting an endpoint during polishing of the substrate
by detecting a change between a first rotational strain and a
second rotational strain with the rotational strain sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0011] FIG. 1 illustrates a polishing apparatus using the motor
current detection technique of the prior art;
[0012] FIG. 2A illustrates a polishing pad assembly having a
rotational strain detection system according to the principles of
the present invention;
[0013] FIG. 2B illustrates a related embodiment of the polishing
pad assembly of FIG. 2A;
[0014] FIG. 3A illustrates a polishing pad assembly having an
alternative embodiment of the rotational strain detection system of
the present invention;
[0015] FIG. 3B illustrates a related embodiment of the polishing
pad assembly of FIG. 3A;
[0016] FIG. 4 illustrates a polishing pad assembly having a
piezoelectric embodiment of the rotational strain detection system
of the present invention;
[0017] FIG. 5 illustrates a polishing pad assembly having yet
another embodiment of the detection system of the present
invention;
[0018] FIG. 6 illustrates a polishing apparatus using the
rotational strain detection system of FIG. 2A;
[0019] FIG. 7A illustrates a close-up view of a first rotational
strain detected by the detection system of FIG. 2A;
[0020] FIG. 7B illustrates a close-up view of a second rotational
strain detected by the detection system of FIG. 2A; and
[0021] FIG. 8 illustrates a sectional view of a conventional
integrated circuit (IC) that may be manufactured according to the
principles of the present invention.
DETAILED DESCRIPTION
[0022] Referring initially to FIG. 1, illustrated is a polishing
apparatus 100 using the motor current detection technique of the
prior art. The polishing apparatus 100 includes a polishing pad 120
for polishing a semiconductor substrate 160 and a platen 110 on
which the polishing pad 120 is securely mounted.
[0023] The polishing apparatus 100 further includes a drive motor
130 coupled to a drive shaft 140. The drive shaft 140, in turn, is
coupled to the platen 110. During a polishing operation, such as a
CMP process, the drive motor 130 is used to turn the drive shaft
140, thereby rotating the platen 110 and polishing pad 120 about a
central axis A.sub.1.
[0024] The polishing apparatus 100 still further includes a carrier
head 150. Mounted to the carrier head 150 is the substrate 160,
such as a semiconductor wafer, that has been selected for
polishing. During the polishing process, a downward force 170 is
applied to the carrier head 150, causing the carrier head 150 to
press the substrate 160 against the polishing pad 120, as the
polishing pad 120 is rotated on the platen 110 by the drive motor
130. Optionally, the carrier head 150 may also be rotated during
polishing about a second axis A.sub.2.
[0025] In this embodiment of the prior art, the load current of the
drive motor 130 used to rotate the platen 110 holding the polishing
pad 120 is continuously monitored during the polishing process. As
each layer of the substrate 160 is polished, a certain amount of
friction develops between the polishing pad 120 and the substrate
160 layer being polished. The amount of friction that develops is
dependent, at least in part, on the coefficient of friction present
at the interface between the layer being polished and polishing pad
120.
[0026] For example, a dielectric material in a first layer will
likely generate less friction than a metal in a second layer. Thus,
when the polishing process finishes the removal of the first layer
of the substrate 160 and begins polishing the second layer, a
change in the amount of friction between the polishing pad 120 and
substrate 160 layers affects the amount of work required by the
drive motor 130. As the work required by the drive motor 130
changes with each different layer, the load current of the motor
130 changes as well. These changes in load current are monitored by
a current meter 180 to determine when the polishing process has
begun on a new substrate 160 layer.
[0027] Turning now to FIG. 2A, illustrated is a polishing pad
assembly 200 having a rotational strain detection system according
to the principles of the present invention. The polishing pad
assembly 200 includes a polishing platen 210A with a polishing pad
220 mounted thereon.
[0028] The polishing platen 210A is composed of two pieces, a base
plate 260 and an upper plate 270, with the polishing pad 220
fixedly mounted on an upper, outer surface of the upper plate 270.
In this embodiment of the present invention, the two plates 260,
270 of the platen 210A are slidably coupled together so that the
upper plate 260 will slightly slide against the base plate 270 when
a there is an increase in friction between a layer of a substrate
(not illustrated) being polished and the polishing pad 220.
Alternatively, the upper plate 260 may be composed of a material
having more flexibility than the base plate 270. However, in such
an embodiment neither plate 260, 270 of the platen 210A is composed
of a material so flexible that durability is compromised. Those
skilled in the art understand that even extremely hard materials
still maintain a certain amount of flexibility. As such, the upper
plate 260 need only be composed of a material having some increased
flexibility over the material of the base plate 270, even if only a
slight amount. Moreover, the present invention is not limited to
any particular means by which to couple the upper 260 and base 270
plates.
[0029] The polishing pad assembly 200 further includes a drive
shaft 250 coupled to the base plate 270 of the polishing platen
210A. As with the polishing apparatus 100 of FIG. 1, the drive
shaft 250 is rotated by a drive motor (not illustrated) about a
central axis A.sub.1 during a polishing process. This, in turn,
also rotates the platen 210A and the polishing pad 220 about the
central axis A.sub.1.
[0030] In this particularly advantageous embodiment, the rotational
strain detection system of the present invention includes a backing
block 240 projecting from the base plate 270 of the platen 210A.
The backing block 240 projects towards the polishing pad 220 and
into a cavity 280 in the upper plate 260 configured to receive the
backing block 240. In addition, the polishing pad assembly 200
includes a rotational strain sensor 230. The rotational strain
sensor 230 is located between an outer face of the backing block
240 and a face of the cavity 280 in the upper plate 260. These
faces cooperate to compress the rotational strain sensor 230 to
determine the endpoint between layers of the substrate being
polishing.
[0031] Specifically, during a polishing process the platen 210A and
polishing pad 220 are rotated at a preset velocity by the drive
shaft 250. A substrate selected for polishing is then pressed
downward, against the polishing pad 220. Since the polishing
surface of the polishing pad 220 has a predetermined texture, a
friction force develops between the layer being polished and the
polishing pad 220. While overcoming this friction and maintaining
rotation of the platen 210A at the original velocity, the interface
between the upper plate 260 and the base plate 270 experiences some
degree of strain just before the upper plate 260 begins to slide
against the base plate 270. However, since the upper plate 260 does
not yet begin to slide against the base plate 270, the strain is
not enough to trigger the rotational strain sensor 230. More
specifically, the strain occurs as the shaft 250 attempts to
continue the platen's 210A rotation at the original velocity while
the friction between the polishing pad 220 and the first layer of
the substrate works to slow or stop the upper plate's 260
rotation.
[0032] Then, when the first layer of the substrate has been
sufficiently removed, and polishing begins on a second layer having
a higher coefficient of friction, the rotational strain experienced
by the two plates 260, 270 of the platen 210A changes.
Specifically, as the drive shaft 250 continues to rotate the platen
210A at the original velocity and the friction between the second
layer of the substrate and the polishing pad 220 increases, the
platen 210A undergoes an increase in rotational strain. Since the
rotational strain sensor 230 is positioned between the backing
block 240 affixed to the base plate 270 and the wall of the cavity
280 in the upper plate 260, the increase in rotational strain
causes the upper plate 260 to momentarily slow its rotation
compared to the base plate 270. This, in turn causes the upper
plate 260 to slightly slide against the base plate 270, and the
cavity wall to compress the sensor 230 against the backing block
240. This compression triggers the rotational strain sensor 230,
indicating that the endpoint of the first layer and the beginning
of the second layer has been reached.
[0033] Those skilled in the art will understand that any number of
sensors may be used for the rotational strain sensor 230. For
example, a sensor simply indicating the difference between two
degrees of rotational strain may be employed in the assembly 200.
However, a sensor having multiple trigger points, configured to
send different signals in response to various increases or
decreases in friction between the polishing pad 220 and multiple
layers of the substrate may also be employed. Thus, in its broadest
form, a rotational strain detection system according to the present
invention encompasses a number of different types of sensors
capable of detecting a broad range of rotational strains for use as
a rotational strain sensor 230.
[0034] Moreover, the rotational strain sensor 230 is not limited to
detecting an increase in frictional forces. For example, the
rotational strain sensor 230 may be compressed during the polishing
of the first layer of substrate, and then signal a decrease in
friction when polishing of a second layer, having a lower
coefficient of friction, begins. In such an embodiment, it is not
the compression of the rotational strain sensor 230 between the
cavity 280 wall when the upper plate 260 slightly slides against
the backing block 240 of the base plate 270 that indicates an
endpoint has been reached, but rather the lack of compression that
indicates the endpoint. As before, in this embodiment the
rotational strain sensor 230 may be configured to detect multiple
changes in friction between the substrate layers and the polishing
pad 220.
[0035] Turning briefly to FIG. 2B, illustrated is a related
embodiment of the polishing pad assembly 200 of FIG. 2A.
Specifically, FIG. 2B illustrates a polishing platen 210B having
only a single plate. In this embodiment, the backing block 240
extends from the platen 210B and is received by a cavity 280 formed
in the polishing pad 220 rather than in an upper plate. The
rotational strain sensor 230 is still located between an outer face
of the backing block 240 and the wall of the cavity 280.
[0036] To determined the endpoint between two substrate layers in
this embodiment, the rotational strain sensor 230 is still
compressed between the cavity 280 wall and the backing block 240 as
the rotational strain of the platen 210B is increased. As
illustrated, the only difference is that the cavity 280 wall is now
part of the polishing pad 220 rather than an upper plate of the
polishing platen 210B. As a result, the increase in rotational
strain evidenced by the polishing pad 220 is transmitted to the
rotational strain sensor 230, as described with respect to FIG. 2A,
indicating an endpoint between the first and second layers. Of
course, as with the previous embodiments discussed above, the
strain sensor 230 may alternatively be triggered by a decrease in
rotational strain rather than an increase, and may be configured to
detect various changes in friction caused by multiple substrate
layers.
[0037] By monitoring and detecting changes directly from the source
of those changes, rather than merely monitoring the drive motor or
other components of a polishing apparatus, more accurate and
precise information regarding the polishing process may be
obtained. Those skilled in the art understand that reliance on
indirect information may be costly if that information is
unknowingly altered or otherwise corrupted by factors external to
the critical item or items being monitored. In short, using
information gathered directly from the source, such as the contact
point between a semiconductor substrate and a polishing pad, allows
the operator or other authorized personnel to proceed with the
polishing operation, confident that the endpoints of the various
layers of the substrate are accurately determined throughout the
entire polishing process. As discussed above, such accurate
determination of endpoints substantially reduces the risk of damage
to the substrate undergoing the polishing process.
[0038] Referring now concurrently to FIGS. 3A and 3B, illustrated
is a polishing pad assembly 300 having an alternative embodiment of
the rotational strain detection system of the present invention. As
before, the polishing pad assembly 300 includes a polishing platen
310A with a polishing pad 320 mounted thereon. FIG. 3A illustrates
the platen 310A as having an upper plate 360 and a base plate 370,
while FIG. 3B illustrates the platen 310B as a single plate. As
before, the two plates 360, 370 of the platen 310A are slidably
coupled together so that the upper plate 360 will slightly slide
against the base plate 370 when a there is an increase in friction
between a layer of a substrate (not illustrated) being polished and
the polishing pad 320. Alternatively, the upper plate 360 may be
composed of a material having more flexibility than the base plate
370.
[0039] The polishing pad assembly 300 includes a rotational strain
sensor 330 having a stud member 340 extending therefrom. In FIG.
3A, the rotational strain sensor 330 is located in the base plate
370 of the polishing platen 310A. Additionally, the stud member 340
extends from the base plate 370 of the platen 310 and is received
by a cavity 380 formed in the upper plate 360 of the platen 310A.
In FIG. 3B, the rotational strain sensor 330 is located in the
platen 310B, but is positioned nearest the polishing pad 320.
Therefore, the stud member 340 extends from the polishing platen
310B and is received by a cavity 380 formed in the polishing pad
320 rather than remaining entirely in the platen 310B.
[0040] As discussed above, as the endpoint of a first layer of
substrate is reached a change in rotational strain occurs when
polishing of the second layer begins. In FIG. 3A, as a drive shaft
350 attempts to maintain the platen's 310A rotational velocity, an
increase (or decrease) in rotational strain will occur in the upper
and base plates 360, 370 of the polishing platen 310A, based on the
friction between the second layer and the polishing pad 320. The
change in friction will cause the upper plate 360 to slightly slide
against the base plate 370, with the direction of the slight slide
depending on whether friction is increasing or decreasing. In FIG.
3B, the change in rotational strain occurs primarily in the
polishing pad 320, which is comprised of a more flexible material
than the platen 310B. The rotational strain caused by the change in
friction forces the polishing pad 320 to slightly distort, twisting
slightly at the middle while its base is firmly adhered to the
platen 310B.
[0041] In either embodiment, however, the stud member 340 and
rotational strain sensor 330 react in a similar manner. In FIG. 3A,
the wall of the cavity 380 formed in the upper plate 360 of the
platen 310A eventually contacts and moves the stud member 340 as
the rotational strain increases. Alternatively, the stud member 340
has already been contacted and is no longer contacted as the
rotational strain decreases. Similarly, in FIG. 3B the cavity 380,
thus it is the polishing pad 320 that eventually contacts and moves
the stud member 340 in response to an increase in rotational
strain, or removes any contact from the stud member 340 in response
to a decrease in rotational strain.
[0042] In one embodiment of the present invention, the stud member
340 is a flexible stud member 340 having electrical conductors
placed within the body of the member 340. In this embodiment, the
conductors are adjacent, but spaced-apart so as not to contact one
another until the flexible member 340 is bent. In an alternative
embodiment, the stud member 340 is rigid but affixed to a flexible
base, designated 390 in FIG. 3B, located in the rotational strain
sensor 330. The flexible base 390 would then include the adjacent
conductors, spaced so as not to contact one another until the
entire rigid stud member 340 is tilted so as to flex the base 390.
It should be noted that although the flexible base 390 is only
illustrated in FIG. 3B, either type of stud member 340 may be
employed with the embodiment illustrated in FIGS. 3A and 3B.
[0043] With a flexible stud member 340, when rotational strain
increases (or decreases) when an endpoint of a substrate is
reached, the wall of the cavity 380 receiving the stud member 340
contacts a side of the stud member 340 and causes it to flex along
its shaft. As the shaft of the flexible stud member 340 bends, the
conductors within the stud member 340 are also bent so as to
contact one another. By contacting, the conductors cause a signal
to be sent to the rotational strain sensor 330 to indicate the
increase or decrease in rotational strain experienced when a change
in friction indicates an endpoint has been reached. The sensor 330
may then transmit a signal to a computer or other type of system
(not illustrated) coupled to the sensor 330 to notify an operator
of the endpoint. In the embodiment of FIG. 3A, it is the upper
plate 360 of the platen 310A that has the cavity 380 and slightly
slides against the base plate 370 to bend the flexible stud member
340. In the embodiment of FIG. 3B, the cavity 380 is formed in the
polishing pad 320 and it is the polishing pad 320 that slightly
twists to contact and bend the stud member 340. Of course, if a
reduction in rotational strain is detected, due to a decrease in
friction, the upper plate 360 or polishing pad 320 remove pressure
from the stud member 340, allowing it to straighten and cause the
conductors to cease contacting one another.
[0044] With a rigid stud member 340, when rotational strain
increases the wall of the cavity 380 receiving the stud member 340
still contacts a side of the stud member 340. However, in such an
embodiment the stud member 340 is rigid and does not flex along its
shaft. Instead, the stud member 340 is forced to tilt at the
flexible base 390, causing the conductors within the base 390 to
bend until they contact one another. As with the flexible stud
member 340, when the conductors contact one another a signal is
sent to the rotational strain sensor 330 to indicate the increase
in rotational strain experienced when an endpoint is reached. The
sensor 330 may then transmit a signal to the computer or other
system used with the sensor 330. As before, if a reduction in
rotational strain is detected, due to a decrease in friction, the
upper plate 360 or polishing pad 320 remove pressure from the
tilting stud member 340, allowing it to straighten and cause the
conductors in the flexible base 390 to cease contacting one
another.
[0045] Although two rotational strain sensors 330 are shown in both
FIG. 3A and 3B, the present invention is not limited to any
particular number of sensors 330 or stud members 340. In its
broadest form, the polishing pad assembly 300 can encompass a
single sensor 330 or any number of multiple sensors 330.
Additionally, a rotational strain detection system using stud
members 340 along with rotational strain sensors 330 still provides
the same advantages over the prior art described above.
[0046] Looking now at FIG. 4, illustrated is a polishing pad
assembly 400 having a piezoelectric embodiment of the rotational
strain detection system of the present invention. Those skilled in
the art are familiar with the properties of piezoelectric material
and its ability to generate an output voltage based on the amount
of stress applied to the material.
[0047] In this advantageous embodiment, the polishing pad assembly
400 includes a polishing platen 410 affixed to a drive shaft 450.
The drive shaft 450 applies a rotational force to the platen 410,
rotating the platen 410 about a central axis A.sub.1. Affixed to an
upper surface of the platen 410 is a rotational strain sensor 430
composed of piezoelectric material. Affixed to the opposite face of
the piezoelectric rotational strain sensor 430 is a polishing pad
420 for use in polishing layers on a substrate (not illustrated).
Since the piezoelectric sensor 430 is mounted on the platen 410,
and the polishing pad 420 is mounted to the sensor 430, both the
sensor 430 and the pad 420 rotate about the central axis
A.sub.1with the polishing platen 410 during the polishing
process.
[0048] As the endpoint of one layer of the substrate is reached and
polishing begins on a second layer, the rotational strain on the
polishing assembly 400 changes in response to the change in
friction between the polishing pad 420 and the new layer. As
mentioned above, rotational strain is increased in the assembly 400
because the drive shaft 450 attempts to continue the platen's 410
rotation while the friction force between the substrate and the
polishing pad 420 attempts to slow or stop that rotation. This
increase in rotational strain exerts a stress onto the
piezoelectric sensor 430 because it is located between the pad 420
and the platen 410. When this change in stress is applied to the
piezoelectric sensor 430, an output voltage 440 of the
piezoelectric sensor 430 changes as well. Thus, the change in the
output voltage 440 of the sensor 430 may be used to determine when
each endpoint of a substrate's layers is reached and polishing
begins on a new layer.
[0049] Moreover, because the output voltage 440 of the
piezoelectric sensor 430 simply changes as a result of changes in
rotational strain, the polishing pad assembly 400 may be used to
determine increases or decreases in rotational strain, in
accordance with the principles of the present invention. Also,
since the output voltage of a piezoelectric material varies across
a given range in response to a range of stresses applied to the
material, the piezoelectric sensor 430 may be used to determine the
endpoints of numerous layers composed of differing materials while
still providing the advantages over the prior art discussed
above.
[0050] Turning to FIG. 5 there is illustrated a polishing pad
assembly 500 having still a further embodiment of the detection
system of the present invention. This advantageous embodiment uses
a polishing platen 510 and polishing pad 520 combination exactly as
found in the prior art. More specifically, the platen 510 is a
single plate with an unmodified polishing pad 520 affixed to its
upper surface.
[0051] However, in this embodiment a rotational strain sensor 530
is located between a lower surface of the platen 510 and a drive
shaft 550 coupled to and used to rotate the platen 510 about a
central axis A.sub.1. In the illustrated embodiment, the rotational
strain sensor is a torque sensor, and is affixed to the lower
surface of the platen 510 and to an upper end of the drive shaft
550 via a collar 540. The rotational strain sensor is coupled
between the drive shaft 550 and the platen 510 to determine the
amount of torque produced by the drive shaft 550 when turning the
platen 510 during various stages of the polishing process. The
collar 540 includes a cavity formed therein to house the rotational
strain sensor 530, as well as offer structural support between the
drive shaft 550 and the polishing platen 510.
[0052] As the drive shaft 550 rotates the platen 510, a change in
the friction between the differing substrate layers and the
polishing pad 520 causes a corresponding change in rotational
strain of the assembly 500, for example the torque between the
drive shaft 550 and the platen 510. The drive shaft 550 attempts to
continue rotating the platen 510 at the original velocity while the
friction acting against the platen's 510 rotation causes a
rotational strain to appear at or near the rotational strain sensor
530, i.e., the point where the drive shaft 550 and the platen 510
meet. Thus, the rotational strain sensor 530 detects the changes in
rotational strain at this junction point and indicates the changes
to a computer or other system (not illustrated) attached thereto.
In accordance with present invention, these detected changes
indicate to an operator that an endpoint of a substrate layer has
been reached and that polishing on a new layer has begun. Of
course, the rotational strain sensor 530 illustrated in FIG. 5 is
not limited to a torque sensor and may encompass any sensor capable
of detecting a rotational strain between the platen 510 and the
drive shaft 550.
[0053] Referring now to FIG. 6, illustrated is a polishing
apparatus 600 using the rotational strain detection system and
polishing pad assembly 200 illustrated in FIG. 2A. The polishing
apparatus 600 includes the polishing pad 220 for polishing a
semiconductor substrate 630 and the platen 210A on which the
polishing pad 220 is securely mounted.
[0054] As discussed with respect to FIG. 2A, the polishing platen
210A includes a base plate 260 and an upper plate 270. In this
exemplary embodiment, the base plate 270 is coupled to the drive
shaft 250 while the upper plate 260 is used to support the
polishing pad 220. As noted before, the upper plate 260 of the
platen 210A includes a cavity 280 formed therein and configured to
receive the rotational strain sensor 230 and backing block 240 of
the rotational strain detection system of the present invention.
The backing block 240 is mounted to the base plate 270 of the
platen 210A, projecting towards the polishing pad 220 of the
polishing apparatus 600 and into the cavity 280.
[0055] The polishing apparatus 600 further includes a drive motor
610 coupled to the drive shaft 250. The drive shaft 250, in turn,
is coupled to the base plate 270 and used to rotate the platen
210A, and consequently the polishing pad 220, about a central axis
A.sub.1. Further included is a carrier head 620 having the
substrate 630, such as a semiconductor wafer that has been selected
for polishing, mounted thereon. During the polishing process, a
downward force 640 is applied to the carrier head 620, causing the
carrier head 620 to press the substrate 630 against the polishing
pad 220, while the polishing pad 220 is rotated on the platen 210A
by the drive motor 610. Optionally, the carrier head 620 may also
be rotated during polishing about a second axis A.sub.2.
[0056] The polishing pad assembly 600 now also includes a computer
system 650. The computer system 650 includes signal lines 660
coupled between the rotational strain sensor 230 and a signal input
of the computer system 650. As before, as the platen 210A is
rotated by the drive motor 610 and a first layer of the substrate
630 is polished, a certain amount of rotational strain is present
at the interface between the upper plate 260 and the base plate
270. As discussed above, this first rotational strain is caused by
the friction between the first layer of the substrate 630 and the
polishing pad 220 working against the rotation upper and base
plates 260, 270 of the platen 210A. Then, as an endpoint of the
first layer is reached and polishing on a second layer of the
substrate 630 begins, an increase or decrease in friction between
the second layer and polishing pad 220 directly causes an increase
or decrease in the rotational strain, respectively.
[0057] The rotational strain sensor 230 detects this second
rotational strain as different than the first rotational strain and
signals the computer system 650 via the signal lines 660 that a
change in rotational strain has occurred. If an increase in
rotational strain is detected the rotational strain sensor 230 is
compressed, while if a decrease in rotational strain is detected
compression of the rotational strain sensor 230 ceases. Finally,
the computer system 650 is programmed to determine whether such a
change in rotational strain is indicative of reaching an endpoint
for the particular layer being polished. If the computer system 650
has determined that an endpoint has been reached, it can further be
programmed to determine which specific endpoint, and thus which
particular layer, has been reached by determining the degree of
change in rotational strain. Thus, in accordance with the
principles of the present invention, the detection system of the
polishing apparatus 600 more directly determines the endpoint of a
layer being polished by sensing a change in rotational strain based
on the friction between the substrate 630 being polished and the
polishing pad 220, rather than by merely monitoring the load
current of the drive motor 610.
[0058] Turning to FIGS. 7A and 7B, FIG. 7A illustrates a close-up
view of a first rotational strain, and FIG. 7B illustrates a
close-up view of a second rotational strain, detected by the
rotational strain detection system illustrated in FIG. 2A.
[0059] Both FIGS. include the polishing platen 210A and polishing
pad 220 of FIG. 2A, with the polishing pad 220 mounted on the
platen 210A to perform a polishing operation. Also as before, the
polishing platen 210 has an upper plate 260 and a base plate 270.
The backing block 240 projects from the base plate, into the cavity
280 in the upper plate 260 configured to receive the backing block
240 and rotational strain sensor 230. The rotational strain sensor
230 is also located in the cavity 280, positioned between a face of
the backing block 240 and a wall of the cavity 280.
[0060] FIGS. 7A and 7B further include a substrate 710 having a
first layer 720 and a second layer 730. The first layer 720 has a
predetermined coefficient of friction for its composition that
helps generate a certain amount of friction when polished by the
polishing pad 220. This friction, in turn, generates a first
rotational strain at the interface between the upper and base
plates 260, 270 of the platen 210A. In the illustrated embodiment,
the first layer 720 has a relatively low coefficient of friction,
such as a dielectric material used to insulate metal components in
the substrate 710. In contrast, the second layer 730 has a
coefficient of friction higher than that of the first layer 720. In
this embodiment, the second layer may be a metal material used in
forming transistors or interconnections in the substrate 710. With
a higher coefficient of friction, the second layer 730 will help
generate a second rotational strain, greater in magnitude than the
first rotational strain.
[0061] In FIG. 7A, as the first layer 720 is being polished by the
rotating polishing pad 220 the first rotational strain is detected
by the rotational strain sensor 230. That information is
transmitted to a system (not illustrated) coupled to the sensor 230
for informing an operator when the endpoint of the first layer 720
has been reached. In FIG. 7B, the portion of the first layer 720
being polished in FIG. 7A has been removed. As a result, the second
layer 730 is now being polished along with remaining portions of
the first layer 720 located at the same depth as the second layer
730. Since polishing the second layer 730 produces more rotational
strain, as discussed above, the rotational strain sensor 230
detects the increase in rotational strain when the upper plate 260
slightly slides against the base plate 270, causing the rotational
strain sensor 230 to be compressed between the wall of the cavity
280 and the backing block 240. The sensor 230 then transmits this
information to the attached system, and the system indicates to an
operator that the endpoint of the first layer 720 has been reached
and polishing has begun on the second layer 730.
[0062] In an alternative embodiment, the coefficient of friction of
the first layer 720 may be greater than the coefficient of friction
for the second layer 730. In this embodiment, the resulting first
rotational strain is greater than the second rotational strain
experienced by the polishing platen 210A. As such, the rotational
strain sensor 230 detects a decrease in rotational strain rather
than an increase. Of course, in such an embodiment the system to
which the strain sensor 230 is attached would be programmed to
determine that the endpoint of the first layer 720 has been reached
by the decrease in rotational strain. In yet another embodiment of
the present invention, the system would be programmed with
information regarding degrees of increase and decrease in
rotational strain, caused by corresponding increases and decreases
in friction between the polishing pad 220 and substrate 630, in
order to determine any of a number of corresponding endpoints
reached throughout the polishing process. In such an embodiment,
the operator of the polishing apparatus would be informed when the
endpoint of each layer of the substrate 630 was reached, regardless
of the composition of that layer.
[0063] Those skilled in the art are familiar with the advantages of
employing sensors having high precision and accuracy. In addition,
those advantages are further increased when using sensors that
provide a high degree of sensitivity. In sum, employing an accurate
and precise sensor, with appreciable sensitivity, as a rotational
strain sensor in accordance with the principles of the present
invention results in a direct detection system that assists a
polishing apparatus in providing superior polishing for a
semiconductor substrate. Moreover, such superior polishing is
achieved while still protecting against over-polishing the layers
of a substrate and overcoming deficiencies associated with the
techniques found in the prior art.
[0064] Turning finally to FIG. 8, illustrated is a sectional view
of a conventional integrated circuit (IC) 800 that may be
manufactured according to the principles of the present invention.
The IC may be derived from the edge portion of a wafer after
completing a CMP process with the rotational strain detection
system of the present invention.
[0065] The integrated circuit 800 may include a CMOS device, a
BiCMOS device, a Bipolar device, or other type of IC device. Those
skilled in the art are familiar with the various types of devices
which may be located in the IC 800. Illustrated in FIG. 8 are
components of the conventional IC 800, including transistors 810, a
gate oxide layer 860, and dielectric layers 820, in which
interconnect structures 830 are formed (together forming
interconnect layers). In the embodiment shown in FIG. 8, the
interconnect structures 830 connect the transistors 810 to other
areas of the IC 800. Also shown in FIG. 8, are conventionally
formed tubs 840, 845, source regions 850, and drain regions
855.
[0066] Of course, use of the detection system of the present
invention, or a method employing the system, is not limited to the
manufacture of a particular IC device. In fact, the present
invention is broad enough to encompass the manufacture of any IC
device derived from a substrate that requires at least some degree
of polishing along the way. Additionally, although the present
invention has been described in detail, referring to several
embodiments, those skilled in the art should understand that they
can make various changes, substitutions and alterations herein
without departing from the spirit and scope of the invention in its
broadest form.
* * * * *