U.S. patent application number 12/098092 was filed with the patent office on 2008-08-07 for method and apparatus for real-time measurement of trace metal concentration in chemical mechanical polishing (cmp) slurry.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Theodore G. Van Kessel.
Application Number | 20080185102 12/098092 |
Document ID | / |
Family ID | 36125546 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080185102 |
Kind Code |
A1 |
Van Kessel; Theodore G. |
August 7, 2008 |
METHOD AND APPARATUS FOR REAL-TIME MEASUREMENT OF TRACE METAL
CONCENTRATION IN CHEMICAL MECHANICAL POLISHING (CMP) SLURRY
Abstract
A system (and method) for real-time measurement of trace metal
concentration in a chemical mechanical polishing (CMP) slurry,
includes an electromagnetic radiation flow cell carrying a CMP
slurry, a slurry pickup head coupled to the flow cell, and an
analyzer for measuring properties of the slurry flowing through the
flow cell.
Inventors: |
Van Kessel; Theodore G.;
(Millbrook, NY) |
Correspondence
Address: |
MCGINN INTELLECTUAL PROPERTY LAW GROUP, PLLC
8321 OLD COURTHOUSE ROAD, SUITE 200
VIENNA
VA
22182-3817
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
36125546 |
Appl. No.: |
12/098092 |
Filed: |
April 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11875987 |
Oct 22, 2007 |
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12098092 |
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10953380 |
Sep 30, 2004 |
7333188 |
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11875987 |
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Current U.S.
Class: |
156/345.13 ;
250/364 |
Current CPC
Class: |
G01N 2223/076 20130101;
G01N 23/223 20130101; G01N 23/06 20130101 |
Class at
Publication: |
156/345.13 ;
250/364 |
International
Class: |
C23F 1/08 20060101
C23F001/08; G01T 1/20 20060101 G01T001/20 |
Claims
1. A system for real-time measurement of trace predetermined
material concentration in a chemical mechanical polishing (CMP)
slurry, comprising: a CMP slurry flow cell analyzer for determining
at least one of transmission and fluorescent properties of a slurry
flowing through the cell; and a slurry pickup pumping head coupled
to said flow cell.
2. The system of claim 1, further comprising: a radiation source
for illuminating said slurry.
3. The system of claim 1, wherein said analyzer includes a bent
crystal analyzer.
4. The system of claim 3, wherein said analyzer further includes a
detector for receiving and measuring a quantity of photons from
said bent crystal analyzer.
5. The system of claim 4, wherein said detector comprises a SiLi
detector.
6. The system of claim 4, wherein said processor receives an output
from said detector for providing any of a slurry concentration
signal and an endpoint signal.
7. The system of claim 1, further comprising: a pump coupled to
said flow cell.
8. The system of claim 1, wherein said slurry is removed one of
from a polishing pad using said pickup head, and a direct
connection through said flow cell to a polishing pad drain.
9. The system of claim 1, wherein the slurry is piped through said
flow cell, said flow cell comprising a plastic tube transparent to
said electromagnetic radiation.
10. The system of claim 4, further comprising: an electromagnetic
radiation source for illuminating the flow cell, wherein photons
traveling through the tube produce fluorescent rays in a
predetermined material atoms contained in the slurry.
11. The system of claim 10, wherein the detector of the flow cell
analyzer receives said fluorescent rays.
12. The system of claim 4, further comprising: a detector shield
positioned adjacent the flow cell for shielding the detector from
any of stray and scattered photons from other than the desired
locations.
13. The system of claim 4, wherein adjustment to an angle of a
crystal of the bent crystal analyzer is provided such that only
photons with an energy of interest are directed to the
detector.
14. The system of claim 13, further comprising: a baffle for
allowing only predetermined ones of said photons to be passed
therethrough to the detector.
15. The system of claim 1, said slurry being usable by a polishing
tool, wherein data representing an interpretation of the measured
characteristic emissions of the slurry is output from the processor
to the polishing tool.
16. The system of claim 1, wherein the system determines a process
endpoint based on detected trace materials.
17. The system of claim 1, wherein the trace predetermined material
concentration is measured without regard for slurry particles.
18. A material concentration measurement system, comprising: a
slurry flow cell for having a slurry flow therethrough; a radiation
source for illuminating said slurry, such that radiation traveling
through the flow cell produces fluorescence in predetermined
material atoms contained in the slurry; and an analyzer for
measuring, in real-time, a concentration of trace predetermined
material atoms in the slurry in response to the fluorescence.
19. A system for real-time measurement of trace metal concentration
in a chemical mechanical polishing (CMP) slurry, comprising: an
optical flow cell for having a CMP slurry flow therethrough; and a
slurry pickup head operatively coupled to said optical flow cell.
Description
[0001] The present application is a Continuation application of
U.S. patent application Ser. No. 11/875,987 filed on Oct. 22, 2007,
which is a Continuation application of Ser. No. 10/953,380 filed on
Sep. 30, 2004, now U.S. Pat. No. 7,333,188.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to a method and
apparatus for measuring a material in a slurry during a polish
process, and more particularly to a method and apparatus for
accurately monitoring polish processes by measuring a removed metal
concentration in the slurry.
[0004] 2. Description of the Related Art
[0005] Copper and Tantalum liner polish operations typical of
current copper back end of line (BEOL) (and aluminum BEOL in some
cases) interconnect layers involve the bulk removal of metal by
chemical mechanical polishing (CMP) on processed semiconductor
wafers.
[0006] In operation, there is some level of interest to which the
designer wishes to make connections. To do so, an oxide is
deposited over the level, lines and holes (vias) are made in the
oxide, patterned therein and extending down to the level of
interest (e.g., the layer to which the connections are to be made).
Then, a liner (e.g., formed of tungsten, tantalum, etc.) is formed
over the wafer surface and in each of the holes, and then a copper
layer is electroplated over the structure. Thereafter, the copper
layer is polished back until the liner is exposed everywhere except
where there are trenches or holes. Thus, all that is left is the
interconnect wires, and the bulk copper is removed.
[0007] A condition arises in that, as the final stage of polishing
is reached, the copper concentration in the slurry drops, as the
relatively thin liner (which has a possibly different selectivity
to the polish) is reached, and thus the liner becomes present in
the slurry (e.g., because the liner is now being ground). The liner
components in the slurry typically exist in fairly low
concentrations (e.g., about 30 ppm to about 50 ppm). Thus,
detection is somewhat difficult.
[0008] It is desirable at this point to stop the copper polish
before a large amount of liner has been removed. Prior to the
invention, no method has existed which could precisely stop the
polishing process to avoid polishing the liner.
[0009] Thus, the correct performance of these process operations
depend on each polishing step being stopped precisely as the
material of each layer is depleted.
[0010] Prior to the present invention, no conventional method has
been developed which can achieve such precise stopping.
[0011] Further, it is noted that thermal endpoint methods exist
which take advantage of the evolved heat due to friction. As the
liner is reached, the friction between the polish pad and wafer in
the presence of slurry changes. This results in a sensible change
in temperature in some polish operations. Due to the thermal masses
involved and the thermal time constant of the system, it takes a
certain amount of time to sense the change (typically several
seconds or more). Thus, the thermal endpoint method does not work
for all processes.
SUMMARY OF THE INVENTION
[0012] In view of the foregoing and other exemplary problems,
drawbacks, and disadvantages of the conventional methods and
structures, an exemplary feature of the present invention is to
provide a method and structure in which real-time measurement of
predetermined trace material (e.g., metal) concentration in a
chemical mechanical polishing (CMP) slurry can be performed.
[0013] In a first aspect of the present invention, a system (and
method) for real-time measurement of trace metal concentration in a
chemical mechanical polishing (CMP) slurry, includes an
electromagnetic radiation flow cell for carrying a CMP slurry, a
slurry pickup head coupled to the flow cell, and an analyzer for
measuring the concentration of a material of interest in the slurry
flowing through the flow cell.
[0014] With the unique and unobvious method and structure of the
invention, the waste slurry is pumped from a pickup head on a
platen (or directly from a platen drain) through a flow cell where
a concentration of a desired metal is measured in real-time using,
in a preferred example, x-ray fluorescence methods. The flow cell
is sensitive to concentrations in the .about.30 ppm range, and
responds on the order of seconds or tens of seconds.
[0015] Thus, with the invention, as a given layer of surface metal
is depleted or encountered by CMP polishing, the concentration of
metal monitored will change. Sensing this change enables the
process endpoint to be detected and the process monitored.
[0016] Additionally, process monitoring advantages include the
feedback of the actual top surface metal thickness to prior level
deposition processes and the uniformity of thickness across the
wafer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing and other exemplary purposes, aspects and
advantages will be better understood from the following detailed
description of an exemplary embodiment of the invention with
reference to the drawings, in which:
[0018] FIG. 1 illustrates a flow diagram of a system 100 according
to the present invention;
[0019] FIG. 2 illustrates a system block diagram of a slurry trace
element analyzer 200 according to the present invention;
[0020] FIG. 3 illustrates a slurry spectra 300 of the slurry trace
element analyzer according to the present invention;
[0021] FIGS. 4A-4C illustrate wafer surface films during CMP copper
and liner polishing operations according to the present invention,
and more specifically:
[0022] FIG. 4A illustrates a side view of a wafer surface before
copper polishing;
[0023] FIG. 4B illustrates a side view of the wafer surface after
copper polishing and before liner polishing; and
[0024] FIG. 4C illustrates a side view of the wafer surface after
liner polishing; and
[0025] FIG. 5 illustrates a flowchart of a method 500 according to
the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0026] Referring now to the drawings, and more particularly to
FIGS. 1-5, there are shown exemplary embodiments of the method and
structures according to the present invention.
[0027] Generally, as discussed briefly above, the present invention
allows sampling of the slurry immediately (and directly) coming off
of the polishing pad. Then, the sample is pumped through a flow
cell in which the invention has constructed a very sensitive X-ray
detector. The X-ray detector is for detecting the fluorescent X
rays from a material of interest (e.g., copper, tantalum, titanium,
aluminum, tungsten, etc.; any metal of interest). Each of these
elements has a unique, characteristic fluorescent X-ray
emission.
[0028] Thus, if these elements are bombarded with X rays, then the
elements will fluoresce, and will emit a characteristic X ray at a
certain wavelength that is unique. Such a unique wavelength can be
used to identify the particular material in the presence of other
different materials. Ideally, a system should be provided which
provides a fairly rapid response. That is, polishing operations
typically occur in tens of seconds, or fractions of minutes. Thus,
a system is desirable which provides a rapid, crisp response in
seconds or less.
[0029] With X-ray operations, detection essentially requires
counting. Conventionally, X-ray detectors in general are typically
limited, due to electronic considerations, etc., to a rate of about
50,000 counts/second. Typically, an "accurate" count (e.g., a count
made with certainty that one knows the number) is proportional
approximately to the square root of the total number of counts.
[0030] Thus, if one wishes to be accurate to within 1%, then 10,000
counts must be obtained. Hence, in the optimal case, the invention
excludes all photons but those in the energy range of interest for
that element.
[0031] For example, assuming a 10 keV emission line of interest,
only the photons at 10 keV are desired to be counted. Hence, the
sample is being illuminated with this broad sample of X rays. As a
result, all of the elements in the sample are fluorescing, not just
the ones of interest. If one were to simply position a counter near
the sample, then all of the counts from all of the elements would
be present. This is problematic since only a fraction of a percent
of the counts is from the element of interest. Hence, if all that
is done is to count the X rays coming directly off of the sample
after illuminating it, then to obtain an accurate count (given
50,000 counts/second) would take a prohibitively long time.
[0032] An exemplary embodiment of the present invention solves the
above and other problems by integrating a bent crystal diffraction
grating into the design. The diffraction grating spatially spreads
a partially collimated beam of photons according to their energy
(similarly to a prism spreading a beam of light). By partially
collimating the fluorescent emissions coming from the flow cell,
directing these emissions into the diffraction grating and into an
X-ray detector placed to receive the photos of the desired energy,
all but the desired characteristic fluorescent emissions are
detected.
[0033] Generally, a SiLi detector is used in combination with a
multichannel analyzer. The multichannel analyzer is understood to
include a computer and electronics for pulse shaping, analog
digital conversion, channel counting and binning etc., and the
control and analysis software necessary to reduce the raw detector
signal to a usable output signal (material concentration and
endpoint). This configuration combines wavelength dispersive and
energy dispersive methods of X-ray spectroscopy to achieve the
desired result.
[0034] In the case where speed is less critical, the bent crystal
can be excluded and a detector with multichannel analysis
capability used by itself.
[0035] That is, invention pumps waste slurry from a pickup head on
a platen or directly from a platen drain through a flow cell where
a concentration of a desired metal is measured in real time by
measuring its X-ray fluorescence. The flow cell is sensitive to
concentrations in the .about.30 ppm range, and responds on the
order of tens of seconds or seconds.
[0036] Thus, with the invention, as a given layer of surface metal
is being depleted by CMP polishing, the concentration of metal
monitored will drop. Sensing this allows precisely stopping the
polishing operations.
Exemplary Embodiment
[0037] FIG. 1 illustrates a flow diagram of an apparatus 100
according to the present invention.
[0038] Specifically, slurry is removed from a polishing platen 110
(having a polishing pad 115) using a pickup head 120 or a direct
connection to the platen drain. A carrier 125 is adjacent the
slurry pickup head 120. Then, the slurry is piped through a flow
cell contained in the trace element analyzer 140 shown in FIG. 1
containing a thin-walled plastic tube that is transparent to X rays
230.
[0039] A radiation source 225 (e.g., X ray source 225 not shown in
FIG. 1, but illustrated in FIG. 2) illuminates the tube (slurry
line 230). The X-ray source is set to produce X rays up to 30 keV.
The X-ray photons traveling through the tube produce fluorescence
in the metal atoms contained in the slurry.
[0040] The fluorescent X rays, produced in the flow tube 230,
illuminate a bent crystal illuminate diffraction grating (shown in
greater detail in FIG. 2 at reference numeral 235), and are
diffracted from there to an energy dispersive X-ray detector (shown
in greater detail in FIG. 2 at reference numeral 240). Counts from
the detector are monitored and interpreted by a computer
(microprocessor) 250 which also connects to the polisher to provide
the signal to notify it when endpoint is reached. In the exemplary,
non-limiting embodiment, a SiLi detector is used. Other detectors
could be used including scintillation/photomultiplier detectors,
etc. Then, the slurry exits to a drain or a recirculation port.
[0041] The pickup head provides pumping action under normal
circumstances of platen rotation and where the sensor is located
below the level of the platen due to the Bernoulli effect.
Optionally, the system may include a pump 130 between the pickup
head 120 and the trace element analyzer 140 to ensure precise flow
or more convenient placement of the analyzer.
[0042] It is important to the function of the device that it be
able to respond with a time constant on the order of seconds (or
preferably tenths of seconds) As mentioned above, typical energy
dispersive X-ray detector will count a maximum of 50,000 counts per
second. To achieve a desired response time, approximately 100 to
1000 counts per second should be measured in the region of interest
for an accurate count.
[0043] FIG. 2 illustrates a system diagram of a slurry trace
element analyzer 200 (e.g., similar to that of analyzer 140 in FIG.
1) according to the present invention.
[0044] Again, slurry is removed from a polishing platen (having a
polishing pad) using a pickup head or a direct connection to the
platen drain. Then, the slurry is piped through a flow cell 230
(e.g., a slurry flow tube) containing a thin-walled plastic tube
that is transparent to X rays.
[0045] An X-ray source 225 illuminates the tube 230. Again, the
X-ray source 225 is set to produce X rays up to 30 keV. The X-ray
photons traveling through the tube 230 produce fluorescence in the
metal atoms contained in the slurry.
[0046] A detector shield 231 is positioned adjacent the flow tube
230 for shielding a detector 240. The shield 231 may be comprised
of sheet metal or like material having suitable stopping power for
the energy range of the source and is shaped to exclude X-ray
scatter for all but the desired path. It is noted that this is
understood to include apertures and slits at multiple points in the
beam path.
[0047] The fluorescent X rays so produced in the flow tube 230 are
routed to a bent crystal diffraction grating 235. Many gratings can
be used. The particular grating is chosen to diffract photons at
the particular X-ray line energy at the correct angle to the
detector and exclude adjacent and unwanted X-ray lines.
[0048] From the grating 235, X rays are diffracted to the detector
240 (e.g., an energy dispersive X-ray detector such as a SiLi
detector or the like) which is understood to contain an entrance
aperture or slit.
[0049] The detector 240 is operatively coupled to a microprocessor
250 with the appropriate electronics for pulse shaping,
analog-to-digital conversion, multichannel analysis, and control
and signal interpretation software necessary to output slurry
concentration data 251 and/or an endpoint signal 252. In a
different embodiment, the source 225, grating 235, and detector 240
are replaced by visible wavelength optics to detect a change in
color, and thus could function similarly to a spectrometer or the
like. Indeed, the slurry containing some of the trace elements
(e.g., metals) may have a completely different color (e.g., be
visibly different) from a slurry which does not, even if the
concentration is very low.
[0050] Fluorescent X rays at many energies are produced in the flow
tube 230. FIG. 3 shows such a spectrum with, for example, the
tantalum lines shown.
[0051] That is, FIG. 3 shows the counts in relation to the
multichannel analyzer (MCA) channel. In FIG. 3, a raw X-ray
fluorescence spectra for slurry with and without a Ta liner. The
X-ray source was at about 20.0 kV at a current of about 67
.mu.A.
[0052] The Ta peaks (eV) (for L1/L2 spectroscopic levels) are
11681.5 and 11136.1, respectively, and are shown at reference
numerals 310 and 315. The Ta peak for L3 was 9881.1 and is shown at
reference numeral 320. The waveform for slurry with the Ta liner is
shown at reference numeral 330, whereas the waveform for slurry
only is shown at reference numeral 340.
[0053] In this example, assuming the invention is interested in K
line and L line emissions characteristic of the element of
interest, the invention employs the bent crystal analyzer 235 to
achieve this selection.
[0054] That is, by adjusting the angle of the crystal 235, only
photons with the energy of interest are directed to the detector
240, while others are blocked by an optional baffle 260.
[0055] The baffle 260 could be a slit or an aperture allowing
certain photons to be passed therethrough to the detector, but
which blocks all other photons. This allows the invention to
utilize most of the detector counting bandwidth to count photons of
interest. Again, the baffle/slit is optional, as it is noted that
the tube size also effectively defines the spot size.
[0056] The counts per unit time at the fluorescent energy is
proportional to the concentration of the element. By counting the
number of photons at the energy of interest per unit time, the
invention can monitor the concentration of the desired material
sufficiently for endpoint detection purposes. This data may be
constructed by a computer system (e.g., microprocessor 250)
attached to the X-ray detector 240.
[0057] Thus, the invention only counts the photons in the
wavelength range of interest, and thus if 10,000 counts are needed,
then, in principle, one can obtain five measurements per second
(assuming that there are enough photons to do it), which is purely
a function of how bright the source is. Hence, a brighter source
could be provided. Thus, the invention provides an efficient
instrument which counts only the photons in the wavelength of
interest, and such can be detected.
[0058] The effect of the CMP polishing process on the surface films
of a wafer is exemplarily shown in FIGS. 4A-4C for the copper 420
and copper liner 415 CMP polish process. A prior level material 410
is provided, having the liner 415 formed thereon, followed by the
copper layer 420 deposition.
[0059] It is noted that as each layer of copper 420 or liner (e.g.,
Ta, etc.) 415 is removed, there will be an abrupt change in the
amount of surface metal flowing into the slurry as the polish
reaches the interface between the material being polished and the
next material. The resultant change in concentration of this
surface metal in the slurry is what the inventive sensor
monitors.
[0060] FIG. 5 illustrates a method 500 according to the present
invention for real-time measurement or trace metal concentration in
a CMP slurry.
[0061] In step 510, slurry is removed from the polishing platen
using the pickup head or a direct connection to the platen
drain.
[0062] In step 520, the slurry is piped through the flow cell
containing a thin walled plastic tube that is transparent to
x-rays
[0063] In step 530, the slurry in the tube is illuminated by a
radiation source (e.g., an X-ray source will be assumed in the
present exemplary embodiment). The x-ray photons traveling through
the tube produce fluorescence in the metal atoms contained in the
slurry.
[0064] In step 540, the fluorescent X rays produced in the flow
tube are routed to a bent crystal diffraction grating, and from the
grating to the energy dispersive X-ray detector.
[0065] In step 550, the microprocessor outputs a slurry
concentration data and/or an endpoint signal.
[0066] As described above, the method of the invention allows the
user to know the progress of the CMP process, and allows the user
to control the process. Thus, for example, the invention allows
stopping the CMP (e.g., grinding) process immediately (e.g.,
through feedback), or could allow some over-polish to occur.
[0067] Additionally, the invention is advantageous in that it
allows the equipment of the system to be monitored. For example, if
the slurry failed (e.g., wrong slurry in the tool, the slurry was
not being delivered in the correct rate, a defective pad, etc.;
each of which may be a "silent" failure), then the invention would
detect the same. Thus, it would become obvious that the tool
failed, thereby allowing the system to be shut down. As such, the
invention provides a tool performance monitor. Hence, if one tool
is finishing (or starting) at a different time than the other tools
in the system (e.g., one tool is operable 5 seconds longer than
others), the system can be shut down and the failure
investigated/remedied.
[0068] As described above, with the unique and unobvious method and
structure of the invention, the waste slurry is pumped from a
pickup head on a platen or directly from a platen drain through a
flow cell where a concentration of a desired metal is measured in
real-time using x-ray fluorescence methods. The flow cell is
sensitive to concentrations in the .about.30 ppm range, and can
respond on the order of seconds (or tenths of seconds).
[0069] Hence, with the invention, as a given layer of surface metal
is depleted by polishing (e.g., CMP polishing), the concentration
of metal monitored will drop. Sensing this drop will enable the
process endpoint to be detected.
[0070] While the invention has been described in terms of several
exemplary embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the appended claims.
[0071] For example, the invention is not limited to X rays, but
other radiation could be used. Thus, for example, a system for
real-time measurement of trace metal concentration in a chemical
mechanical polishing (CMP) slurry could be provided in which an
optical source would be provided instead of the X-ray source, an
optical flow cell for carrying a CMP slurry would be provided
instead of an X-ray flow cell, a diffraction grating would be
provided instead of the bent crystal analyzer, and a photodetector
would be provided instead of a SiLi detector. Thus, in the above
system, an optical spectrometer would be provided in place of the
X-ray spectrometer.
[0072] Further, it is noted that Applicant's intent is to encompass
equivalents of all claim elements, even if amended later during
prosecution.
* * * * *