U.S. patent application number 11/437915 was filed with the patent office on 2011-04-28 for optical detection of planarization, breakthrough and end-point in membrane-mediated electropolishing of metal layers.
Invention is credited to Stephen Mazur.
Application Number | 20110094895 11/437915 |
Document ID | / |
Family ID | 43897472 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110094895 |
Kind Code |
A1 |
Mazur; Stephen |
April 28, 2011 |
Optical detection of planarization, breakthrough and end-point in
membrane-mediated electropolishing of metal layers
Abstract
Methods and apparatus for monitoring in situ the progress of
planarization and/or removal of a thin metal layer from a substrate
during membrane-mediated electropolishing processes are
provided.
Inventors: |
Mazur; Stephen; (Wilmington,
DE) |
Family ID: |
43897472 |
Appl. No.: |
11/437915 |
Filed: |
May 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60683256 |
May 20, 2005 |
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Current U.S.
Class: |
205/655 ;
204/252 |
Current CPC
Class: |
C25F 7/00 20130101; C25F
3/16 20130101 |
Class at
Publication: |
205/655 ;
204/252 |
International
Class: |
C25F 3/16 20060101
C25F003/16 |
Claims
1. An apparatus for use in membrane-mediated electropolishing a
metal-coated workpiece comprising: A. a cathode half-cell
comprising: 1) a fully or partially enclosed volume, cavity or
vessel; 2) a charge-selective ion-conducting membrane having an
internal surface and an external surface, wherein the membrane
forms a surface of the enclosed volume, cavity or vessel; 3) an
electrolyte composition which at least partially fills the enclosed
volume, cavity or vessel, wherein at least a portion of the
internal surface of the membrane contacts the electrolyte
composition and at least a portion of the external surface of the
membrane contacts the work-piece during electropolishing; and 4) an
electrode in contact with the electrolyte composition; and B. an
optical detection system comprising: 1) a light source for
illuminating the workpiece; and 2) a means for detecting light that
is reflected from the workpiece.
2. The apparatus of claim 1, wherein the means for detecting light
that is reflected from the workpiece is located within the enclosed
volume of the cathode half-cell and the membrane and electrolyte
are optically transparent.
3. The apparatus of claim 1, wherein the means for detecting light
that is reflected from the workpiece comprises an optical fiber and
a photovoltaic transducer.
4. The apparatus of claim 3, wherein the optical fiber is
bifurcated.
5. The apparatus of claim 1, wherein the membrane comprises a
perfluorosulfonate ionomer.
6. A process comprising: A. providing a light source for
illuminating a workpiece being electropolished by MMEP; B.
illuminating the workpiece with light from the light source; C.
detecting light reflected from the workpiece; and D. analyzing the
reflected light to determine the topography of the workpiece.
Description
FIELD OF THE INVENTION
[0001] The invention is directed to methods and apparatuses for
monitoring in situ the progress of planarization and/or removal of
a thin metal layer from a substrate during membrane-mediated
electropolishing processes.
BACKGROUND
[0002] Interconnections on integrated circuits are fabricated by
the Cu damascene process in which the interconnect circuit pattern
is lithographically etched into the surface of a dielectric layer
on the surface of the wafer. The pattern is then coated with thin
conformal layers of a barrier metal, such as Ta, followed by Cu.
Additional Cu in then electroplated over the entire surface of a
wafer to fill completely the recessed circuit features with Cu. It
is then necessary to planarize topographic features on the surface
of the electroplated Cu and remove all excess Cu from the plateau
areas between the circuit features so that the underlying barrier
metal is exposed. Planarization and removal of excess Cu is
customarily achieved by a chemical mechanical polishing (CMP)
process. In CMP, the wafer is mechanically polished with a soft
polymeric pad in the presence of an aqueous slurry containing
oxidizing agents and abrasive colloid particles.
[0003] Because of the critical tolerances required for IC
interconnects, it is desirable during Cu CMP to detect and/or
monitor changes in the surface of the wafer during the course of
polishing. In the first stage of CMP, topographic features are
planarized prior to removal of Cu from any of the barrier surface.
In the second stage of CMP, excess Cu is removed to the point of
breakthrough, where portions of the barrier layer first become
exposed, and then the Cu is cleared entirely from the surface of
plateau areas separating the circuit feature(s). The process is
stopped just beyond this end-point, before significant loss of Cu
occurs from within the circuit features.
[0004] In CMP the surface of the wafer is continuously flooded by a
polishing slurry containing suspended colloidal particles that
scatter light and complicate optical measurements. Moreover, the
CMP polishing pad comprises an optically opaque material.
Consequently, optical measurements can be difficult to carry out
without significant modifications of the polishing pad and the
platen that supports the pad.
[0005] Various methods have been developed to monitor in situ the
progress of the CMP process. These methods vary in sensitivity and
reliability and in the cost and complexity they contribute to the
equipment. For example, Thomas Tucker (p. 106 in
"Chemical-Mechanical Planarization of Semiconductor Materials",
Edited by M.R. Oliver, Springer-Verlag, Berlin, 2003) states:
"Optical reflectometry is the preferred method for endpointing CMP
processes. However, the method is highly intrusive, requiring
process compromises or extensive hardware modifications." Such
modifications are described in U.S. Pat. No. 5,893,796; U.S. Pat.
No. 5,964,643; U.S. Pat. No. 6,045,439; U.S. Pat. No. 6,280,290;
and U.S. Pat. No. 6,454,630.
[0006] Alternate approaches to planarization and removal of excess
Cu have been disclosed by S. Mazur et al., (co-pending applications
U.S. Ser. No. 10/976,897; U.S. Ser. No. 10/986,048; and U.S. Ser.
No. 11/291,697, which are hereby incorporated by reference). The
disclosed approaches involve a membrane-mediated electropolishing
(MMEP) process. Because all metal removed from the work-piece is
re-deposited onto the cathode, the MMEP process occurs with
substantially no change in the composition or physical properties
of the low-conductivity solvent, the membrane or the electrolyte
solution.
[0007] Since MMEP consumes no reagents and generates no waste it
provides advantages over CMP. However, there remains a need for
methods to monitor the progress of planarization and/or to detect
breakthrough and end-point in apparatus designed for the MMEP
process. In particular, there is a need for an in situ method to
monitor the planarization of topographic features, and also to
detect breakthrough and end-point in electropolishing Cu damascene
wafers by the MMEP process.
SUMMARY OF THE INVENTION
[0008] One aspect of this invention is an apparatus for use in
membrane-mediated electropolishing a metal-coated workpiece
comprising:
[0009] A. a cathode half-cell comprising: [0010] 1) a fully or
partially enclosed volume, cavity or vessel; [0011] 2) a
charge-selective ion-conducting membrane having an internal surface
and an external surface, wherein the membrane forms a surface of
the enclosed volume, cavity or vessel; [0012] 3) an electrolyte
composition which at least partially fills the enclosed volume,
cavity or vessel, wherein at least a portion of the internal
surface of the membrane contacts the electrolyte composition and at
least a portion of the external surface of the membrane contacts
the work-piece during electropolishing; and [0013] 4) an electrode
in contact with the electrolyte composition; and
[0014] B. an optical detection system comprising: [0015] 1) a light
source for irradiating the workpiece; and [0016] 2) a means for
detecting light that is reflected from the workpiece.
[0017] Another aspect of this invention is a process comprising:
providing a source of light to illuminate a workpiece; illuminating
the workpiece with light from the light source; detecting light
reflected from the workpiece; and analyzing the reflected light to
determine the topography of the workpiece.
BRIEF DESCRIPTION OF THE FIGURES
[0018] FIG. 1 is a schematic cross-section of MMEP apparatus with a
bifurcated optic fiber incorporated in the cathode half-cell to
detect changes in reflectivity of the work-piece.
[0019] FIG. 2 is a schematic cross-section of MMEP apparatus with a
bifurcated optic fiber incorporated outside the cathode half-cell
to detect changes in reflectivity of the work-piece.
DETAILED DESCRIPTION
[0020] This invention is directed to optical methods and
apparatuses for detecting in situ changes in the topography of a
work-piece during the progress of MMEP. The optical apparatuses and
methods can be used to detect in situ the point of breakthrough,
where a thin metal coating is first cleared from certain areas of a
substrate. The apparatuses and methods can also be used to detect
in situ the end-point, where a thin metal coating has been entirely
removed from raised, plateau areas on the substrate surface, prior
to removal by MMEP of the metal coating from recessed areas of the
substrate.
[0021] In an MMEP process, the workpiece is physically separated
from the electrolyte and cathode by a charge-selective
ion-conducting membrane, wherein the membrane is essentially
impermeable to the electrolyte but permeable to the ions produced
by anodic oxidation of the work piece. MMEP is described in
co-pending applications (U.S. Ser. No. 10/976,897; U.S. Ser. No.
10/986,048; and U.S. Ser. No. 11/291,697).
[0022] In MMEP, the low-conductivity solvent that covers the wafer,
the membrane, and the electrolyte can be selected so as to be
optically transparent. As a consequence, one can easily detect
light that is reflected off the surface of the workpiece and
transmitted through the membrane. In one embodiment of this
invention, an optical detector is placed inside the half-cell, near
the portion of the membrane in contact with the workpiece, to
monitor the change in reflectivity of the surface during the MMEP
process. Changes in reflectivity can be correlated with changes in
topography of the workpiece surface, e.g., its roughness, as well
as changes in composition of the surface, e.g., as the barrier
layer beneath a Cu layer is exposed. The changes in topography and
composition can be used to determine the extent of planarization of
the surface.
[0023] While optical detection systems with various designs and
principles of operation can be employed in the processes of this
invention, optical reflectivity measurements are especially useful
for this purpose. In one embodiment, the optical detection system
employs bifurcated optical fibers to detect reflectivity in situ
during MMEP. Such an embodiment is illustrated in FIG. 1, which
shows a schematic cross-section of an MMEP apparatus in which a
bifurcated pair of optical fibers 9, 10 has been incorporated
within the electrolyte cavity 8 of the cathode half-cell 4. The
ends of the optical fibers are positioned near the inside surface
of the membrane 7 opposite the plated Cu layer 5 on the surface of
the wafer 6 (the workpiece). The wafer is flooded by deionized
water 1. Under the influence of the interfacial velocity v, a thin
layer of deionized water is maintained at the interface between the
Cu layer and the membrane by hydrodynamic lubrication forces within
the contour area A.sub.c. The electrolyte 8 is optically
transparent, and in one embodiment comprises a homogeneous aqueous
solution of sulfuric acid and a Cu salt. The membrane 7 comprises
an optically transparent charge-selective ion-conducting membrane.
Light is supplied to the transmission fiber 9 from an external
light source (not shown), and any light that is reflected back into
the receiving fiber 10 is detected by an external photovoltaic
transducer (not shown). When the intensity of light in fiber 9 is
constant, variations in the optical reflectivity of the Cu surface
5 are manifest by corresponding changes in the intensity of light
detected by the photovoltaic transducer. Since the composition and
optical properties of the water, membrane and electrolyte are
unaffected by the MMEP process, they do not interfere with the
measurements and have little effect on the photovoltaic signal.
Scratches or embossed patterns on the membrane surface may
attenuate the light beams, but this effect will not change with
time, and can therefore be ignored in comparison with the
time-dependent effects of the electropolishing process.
[0024] Variations in reflectivity detected by the optical detection
system can derive from two different types of physical effects.
When the surface is rough, or contains topographic features
corresponding to the circuit features, the intensity of reflected
light captured by the receiving fiber 10 is attenuated by
deflection, diffraction and/or scatting of the light. Therefore, as
topographic features are removed in the course of the MMEP process,
the time-averaged intensity of reflected light that is captured by
the receiving fiber increases. This is one way in which
reflectivity measurements can be used to monitor the progress of
planarization and electropolishing.
[0025] The time-dependent fluctuations in the reflected light can
also be monitored as the surface of the work-piece passes by the
bifurcated optical fiber detector. The reflected intensity
fluctuates at a frequency proportional to the interfacial velocity
v due to the variations in deflection, diffraction and scattering
of the light beam by the topographic features on the work-piece.
During electropolishing, as the amplitude of the topographic
features decreases the amplitude of fluctuations in the reflected
intensity will also decrease. Therefore, monitoring the amplitude
of the time-dependent fluctuations in the reflected intensity
provides a way to monitor the planarization of the work-piece.
[0026] Changes in the surface composition can also be monitored.
For example, the optical reflectivity of a Ta barrier layer differs
from that of an equivalently smooth Cu surface. Depending upon the
wavelength of light, and depending upon the thickness of Ta oxides
which develop on exposed surfaces of the barrier layer, the
intensity of light reflected from the exposed barrier surface may
be either higher or lower than that reflected from an equivalently
smooth Cu surface. This effect can be used to detect the point of
breakthrough, in which the barrier layer (e.g., Ta) is first
exposed on certain areas of the wafer, and also the end-point, in
which Cu has been cleared from all plateau areas between the
circuit elements. The optical contrast between Cu and the barrier
layer can be enhanced by a suitable choice of optical band-pass
filter located either after the light source or before the
detector.
[0027] FIG. 1 shows a preferred embodiment in which the optical
detection system is incorporated inside the cathode half-cell so
that the light passes through the electrolyte solution and membrane
before and after reflecting from the surface of the work-piece.
This configuration is useful because the membrane and electrolyte
solution can be chosen to be optically transparent, but other
configurations can also be employed. In other embodiments, the
components of the optical detection system can be mounted remotely
from the cathode half-cell or on a portion of the cathode half-cell
situated opposite areas of the work-piece, which are momentarily
not within the contour area. In that case the light passes through
the layer of water on the surface of the work-piece and it is not
necessary that either the membrane or the electrolyte be optically
transparent.
[0028] By incorporating the detector inside the half-cell, the
contour area can be made arbitrarily large, for example, up to the
entire area of the work piece. By locating the area of optical
inspection within the contour area, the signal from the detector
reflects the process at the actual time and location at which it
occurs. Also, the thickness of the layer of low-conductivity
solvent within the contour area is much thinner, more uniform and
more stable than it is on free surfaces outside the contour area.
The thickness of the water layer on free surfaces outside the
contour area is non-uniform and fluctuates with time, which may
produce noisy signals for reflectance measurements at those
locations.
[0029] While the embodiment illustrated in FIG. 1 employs a
bifurcated optical fiber for the detector, other types of optical
detectors may also be used to provide a stable source of
illumination and to capture and measure the intensity of reflected
light. Optical detectors for use in the invention are commercially
available. An example of an optical system designed to detect the
intensity of light reflected from a surface by bifurcated optical
fibers is the Fotonic Sensor (MTI Instruments Inc., Albany, N.Y.).
FIG. 1 illustrates an embodiment of the invention using a Fotonic
Sensor or an optical system similar to the Fontonic Sensor. The
Fotonic Sensor is designed so that the intensity of the light
collected by a return fiber 10 is governed both by the reflectivity
of the surface and also by the average distance between the surface
and the ends of the delivery and return fibers 9 and 10
respectively. The sensitivity to distance is caused by divergence
of the light source as it emerges from the delivery fiber 9. By
this principle, distance changes less than 100 nm can be detected.
Thus the signal from the Fotonic Sensor changes with changes in
reflectivity and/or changes in distance from the surface of the
work-piece.
[0030] The light source should provide a stable source of light,
both with respect to intensity and wavelength.
[0031] To avoid damaging the detector (sensor) and/or the membrane,
it is preferred to position 9 and 10 so as not to touch the
internal surface of the membrane.
[0032] One embodiment of the present invention provides a way to
follow the progress of the MMEP process by monitoring changes in
the optical reflectance of the work-piece due to changes in the
surface topography and/or changes in the average composition of the
surface as Cu is removed to expose the underlying barrier layer. A
cathode half-cell is provided, which comprises a fully or partially
enclosed volume, cavity or vessel. The half-cell also contains an
electrode in contact with an electrolyte solution or gel, and is
sealed on at least one surface with a charge-selective
ion-conducting membrane. Preferably, the electrolyte in the
half-cell is maintained at a hydrostatic pressure greater than
ambient atmospheric pressure, and the membrane is sufficiently
flexible to expand under the influence of this pressure to
establish a convex external surface (a "bulge" or "blister") to
contact the work piece. A source of DC electrical power is
connected between the work piece (which functions as the anode) and
the electrode in the cell (which functions as the cathode).
Electropolishing is accomplished when a portion of the external
surface of the ion-conducting membrane is pressed against a portion
of the work piece otherwise covered by the low-conductivity fluid,
and this interfacial area is moved across the surface of the work
piece. The workpiece can be held stationary and the membrane moved
across its surface, or the half-cell can be held stationary and the
work-piece moved, or both the work piece and the half-cell can be
in motion, provided that the area of contact is not static. As used
herein, "contact" (of the membrane and the workpiece) means that
preferably the workpiece and the membrane are within close
proximity, e.g., between 1 nm and 1 micron. When a suitable voltage
is applied between the anode and cathode under such conditions,
some of the metal becomes oxidized to form solvated metal ions that
migrate across the membrane into the half-cell.
[0033] In the MMEP process, the external surface of the membrane
used in the cathode half-cell can be topographically patterned to
create a plurality of "lands", locally projecting substantially
flat areas, surrounded by locally recessed channels, which extend
to the edges of the contour area A.sub.c. Suitable topographic
patterns comprise substantially flat projecting lands less than 10
mm wider, preferably less than 2 mm wide, surrounded by recessed
channels greater than 1 .mu.m deep, preferably greater than 5 .mu.m
deep, which extend to the edges of the contour area A.sub.c.
[0034] A convenient way to prepare a topographically patterned
membrane for use in this invention is to permanently emboss a
desired pattern into the surface of a pre-existing smooth membrane.
The embossing tool can consist of a screen, grid, mesh, pierced,
porous, or woven sheet or expanded lattice composed of metal,
glass, polymeric or natural fibers or any other material
sufficiently hard to permanently deform the surface of the membrane
when the two surfaces are pressed together under a suitable
combination of temperature and pressure. The topographic pattern
embossed into the membrane surface in this way corresponds to a
cast of the embossing tool. In this, or any other operation it is
desired that no holes or tears occur, which would compromise the
ability of the membrane to serve as a barrier between the
electrolyte solution and low-conductivity fluid.
[0035] For MMEP, suitable membranes are substantially impermeable
to the electrolyte, but permeable by the solvated or complexed
metal ions produced in anodic oxidation of the work piece. If these
ions are positively charged, then the membrane is a
cation-conducting membrane. Similarly, under conditions where
anodic oxidation produces a complex metal anion, the membrane is an
anion-conducting membrane.
[0036] Charge-selective ion-conducting membranes are generally
solid, film-forming organic polymers that bear covalently bound
ionic functional groups. The bound ions constitute fixed charges
that are balanced by unbound, mobile counter-ions of the opposite
charge. The latter may diffuse within the membrane or migrate under
the influence of an electric field to carry electric current. Small
ions in adjacent solutions, having the same sign as the mobile
counter-ions, exchange readily with those in the membrane. By
contrast, ions in adjacent solution with the same charge as the
fixed ions in the membrane tend to be excluded from such membranes
due to electrostatic repulsion. Thus, all charge-selective
ion-conducting membranes are to some extent impermeable to
electrolyte solutions due to exclusion of ions that share the same
sign as the fixed charges.
[0037] Suitable charge-selective ion-conducting membranes include
film-forming ionic polymers that are stable under the conditions of
the electropolishing process. For some embodiments of this
invention, suitable membranes are also optically transparent. Ionic
polymers that are useful as fuel-cell membranes may also be useful
as membranes in this invention.
[0038] A preferred class of membranes is cation-conducting
membranes, especially those formed from polymeric ionomers
functionalized with strong acid groups, i.e., acid groups with a
pKa of less than 3. Sulfonic acid groups are preferred strong acid
groups. Preferred polymeric ionomers are copolymers of fluorinated
and/or perfluorinated olefins and monomers containing strong acid
groups.
[0039] Perfluorosulfonate ionomer membranes (e.g., Nafion.RTM.
membranes, E.I. du Pont de Nemours, Inc., Wilmington, Del.) contain
fluorocarbon chains bearing highly acidic sulfonic acid groups. On
exposure to water, the acid groups ionize, leaving fixed sulfonate
anions and mobile hydrated protons. The protons may be readily
exchanged with various metal cations. Perfluorosulfonate ionomer
membranes are particularly well-suited for use in MMEP and in the
apparatuses and processes of this invention due to their strong
common-ion exclusion, high conductivity, strong acidity, chemical
stability and robust mechanical properties. Perfluorosulfonate
ionomer membranes are also well-suited to the present invention
because they are optically transparent and colorless. Also,
perfluorosulfonate ionomer membranes are easily patterned by common
thermoforming techniques.
[0040] The low-conductivity solvent or solution used in MMEP
contacts both the anode and the external surface of the membrane.
To limit the anodic dissolution reaction, the electrical
conductivity of the solvent or solution is relatively low,
preferably less than 500 .mu.S/cm. The solvent or some component of
the solution should be capable of solvating or otherwise
coordinating the metal ions produced at the anode to create an
ionic complex that is soluble and mobile within the membrane. In
some instances, it is preferable to use aqueous acetonitrile or a
very dilute aqueous acid solution as the low conductivity solution
rather than deionized water. For use in the apparatuses and
processes disclosed herein, the low-conductivity fluid is optically
transparent to the wavelengths of light used for the reflectivity
measurements.
[0041] The electrolyte is chosen to provide high solubility for the
metal ions or coordinated metal ions produced by oxidation of the
anode, and to have sufficiently high conductivity to carry current
densities up to several hundred mA/cm.sup.2 without introducing
significant voltage drop or heating. Conductivities of at least 100
mS/cm are preferred. The most common EP electrolytes are
concentrated aqueous solutions of strong acids. For many typical
anode compositions suitable for use in MMEP processes,
corresponding suitable choices for cathode and electrolyte
compositions can be found in Electroplating Engineering Handbook,
4.sup.th Edition, pp. 100-120, by D. E. Ward, L. J. Durney, Ed.,
Van Nostrand Co., NY, 1984. Chloride ion has been found to be
effective in maintaining the solubility of Sn.sup.+2 and Al.sup.+3.
A source of strong base or cyanide ion can be used in the
electrolyte or in the low-conductivity solution in an anodic
generation of complex metal anions such as Al(OH).sub.6.sup.-3 or
Fe(CN).sub.6.sup.-3. Under these conditions, an anion-conducting
membrane is used instead of a cation-conducting membrane.
[0042] The cathode can be made from any electrically conductive
material that is chemically stable in the electrolyte. During
electropolishing, one or more reduction reactions may occur at the
cathode. In order to minimize hydrogen evolution relative to
plating, metal salts, e.g., CuSO.sub.4, can be included in the
electrolyte. An electrolyte solution useful in one embodiment of
the processes and apparatuses of this invention is 0.5 M CuSO.sub.4
in 1.0 M aqueous H.sub.2SO.sub.4. For use in some embodiments of
the processes and apparatuses of the present invention, the
electrolyte is optically transparent and transmits most of the
light at the wavelengths used for the reflectivity measurements.
Over the wavelengths used for the measurements the optical
transmittance of the electrolyte solution is typically greater than
10%, preferably greater than 50%, more preferably greater than
90%.
[0043] Membrane-mediated electropolishing processes can be used to
polish and/or planarize a wide variety of metals and metal alloys.
The apparatuses and processes of this invention can be used to
monitor the planarization or electropolishing of a corresponding
number of metals and metal alloys. Such metals and alloys include
silver, nickel, cobalt, tin, aluminum, copper, lead, tantalum,
titanium, iron, chromium, vanadium, manganese, zinc, zirconium,
niobium, molybdenum, ruthenium, rhodium, hafnium, tungsten,
rhenium, osmium, iridium, and alloys such as brass, lead/tin alloys
and steel. Preferred metals include silver, nickel, cobalt, tin,
aluminum, copper, lead, and alloys such as brass, steel and
lead/tin.
* * * * *