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Description  |
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This invention relates to solid state electronic devices and more
specifically to a method of planarizing or enhancing step coverage of
aluminum or aluminum alloy into via and trench features of a wafer in
order to manufacture such a device.
BACKGROUND OF THE INVENTION
The manufacture of solid state electronic devices involves the layering of
various materials in a prescribed pattern upon a wafer. Oftentimes the
topography of the prescribed pattern is not flat, and manufacture of these
devices requires the planarization of windows with a particular material
prior to formation of the next layer. These windows are referred to as
"via" or "trench" features. Vias are basically square or circularly shaped
holes of a particular depth in the top surface of the wafer. Trenches are
generally elongated and rectangularly shaped.
In order for these types of final solid state electrical devices to
function properly, it is critical that the via and trench features be
planarized, or in some applications, that the sidewall coverage be
enhanced beyond that obtained from currently available geometric coating.
That is, the features must be filled or nearly filled with the desired
material, such as aluminum or aluminum alloy, in a manner which leaves no
voids within the feature. This task becomes increasingly difficult as the
depth of the feature increases with respect to the width of the feature.
The ratio of the depth of a feature with respect to its width is referred
to as its "aspect ratio."
One conventional manner of metalizing a wafer involves sputtering the
material from a target of material mounted opposite the wafer in a sealed
enclosure. Sputtering methods and apparatus are well known, and
Applicants' commonly assigned currently pending patent application Ser.
No. 07/222,328 is expressly incorporated by reference herein in its
entirety for the purpose of providing background information on such an
apparatus. During sputtering, the target is biased negatively with respect
to the chamber. An inert gas passes through the enclosure and is ionized
to form a plasma. The potential difference between the target and the
plasma causes ions from the gas to bombard the target, thereby sputtering
the target material. Some of the sputtered material is deposited onto the
wafer.
Sputtering of a material such as aluminum or aluminum alloy from a target
onto an oppositely mounted, flat wafer surface will result in a deposition
of substantially uniform thickness. However, if there are via or trench
features to be metalized, and particularly if the features have a
relatively high aspect ratio, i.e., about 0.9 or higher, the build up of
material deposited upon the sides and the ledges of the feature tends to
block or prevent the unobstructed path of subsequently deposited material
into the bottom of the feature. This effect is referred to as "shadowing,"
and results in the reduction of metal thickness on feature sidewalls, or
the formation of voids or incomplete filling of the feature.
Various attempts have been made to prevent or to reduce the effects of
shadowing. Such attempts include, among others, movement of the wafer with
respect to the target, movement of the target with respect to the wafer,
variations in the shape of the target and variations in the shape and
strength of the magnetic field located near the target surface. Other
attempts involve variation in deposition parameters at the wafer surface.
These latter methods attempt to produce thorough planarization by movement
or diffusion of the already deposited material into the features through
the application of heat or bias to the wafer.
It is well known that heat affects the mobility of a material that has been
deposited upon a wafer. For example, Mintz U.S. Pat. No. 4,661,228
discloses an apparatus and method for producing planarized aluminum films
on a semiconductor wafer wherein the wafer is heated to temperatures above
400.degree. C. during sputtering. However, as reported in an article
entitled "Sputtering . . . Plus.TM." by the technical staff of the Machine
Technology, Inc. Thin Film Equipment Division, use of heat alone to
planarize aluminum results in an undesired increase in grain size. Thus,
while heating the wafer may produce the enhanced aluminum mobility that is
required to adequately planarize a feature, it does so in a manner which
produces undesired grain growth
Another method of improving mobility during planarization involves the use
of intermittent or continuous terms of resputtering, in which the
electrical bias of the wafer with respect to the plasma is reversed so
that some of the aluminum deposited upon the wafer will subsequently be
sputtered from the wafer. In effect, resputtering produces a migration or
rearrangement of the deposited aluminum into the feature. However,
resputtering reduces the overall rate of aluminum deposition onto the
wafer, thereby slowing down the entire planarization process and the total
throughput time for wafer treatment. Moreover, for features with
relatively high aspect ratios, resputtering alone does not produce enough
surface migration to fill the feature in an acceptable manner.
An article entitled "Planarization of Metal Using Bias Sputtering" by J.
Hems and Abe McGeown of ElectroTech in Bristol, England cites the use of
thermal pulses and metal lift off processes to achieve planarization of a
wafer, but the article also states that these methods also require extra
processing steps.
Thus, there is a recognized need to provide improvements in planarization
techniques, particularly with respect to aluminum or aluminum alloy
planarization of via and trench features having a relatively high aspect
ratio.
SUMMARY OF THE INVENTION
This invention contemplates a method for planarizing a feature that
involves first, high rate deposition at a temperature below 200.degree. C.
in order to achieve a continuous coating on the internal surfaces of the
feature, followed by low rate deposition while supplying heat to the wafer
in order to enhance diffusion into the feature, and finally, high rate
deposition with continued heat supplied to the wafer to reach the desired
total thickness of metallization. The deposition is optimally continuous
and uninterrupted throughout all three steps, in order to prevent the
formation of unwanted oxide layers that would otherwise occur during
discontinuities in the process. A bias voltage may be applied to the wafer
during this deposition.
The invention further contemplates a planarization technique whereby the
duration of the intermediate, low rate, deposition step is selected in
accordance with a maximum diffusion distance of the feature to be
planarized and the temperature of the wafer.
To these ends, in accordance with a preferred embodiment of the invention,
a planarization method includes an uninterrupted three step process. The
first step involves the application of high power to a sputtering target
to produce high rate deposition, e.g., about 220 Angstroms per second, of
aluminum or aluminum alloy onto a wafer until a layer having a thickness
of about 400 Angstroms to 1,000 Angstroms is obtained on the flat top
surface of the wafer. Under a theoretical model to be described in more
detail later, it is believed that this first step produces a continuous
layer of about 200 Angstroms thick on the interior surfaces i.e.,
"geometric" coverage, of the side and bottom walls of the feature. The
wafer temperature remains below 200.degree. C. during this first step.
During the second step, power to the sputtering target is reduced to
produce a low rate of deposition onto the wafer, e.g., about 44 Angstroms
per second. Concurrently with the low rate deposition of the second step,
an inert gas or fluid thermally connects a heated back plane to the rear
of the wafer in order to raise the temperature of the deposition surface
to a desired temperature on the order of 300.degree. C. or greater. The
duration of this second step and the wafer temperature during deposition
are determined by a characteristic size of the feature to be planarized
and the degree of planarization required. During the third step, power to
the sputtering target is increased to produce high rate deposition, again
on the order of 220 Angstroms per second, with continued application of
heat to the wafer via fluid contact with the heated back plane. This high
rate step is used to increase throughput for the process.
The first step of this inventive technique produces a thin continuous layer
of aluminum or aluminum alloy on the wafer which enhances mobility during
subsequent steps. During the second step, with an inert gas thermally
connecting the back of the wafer to a heated backplane, the applied heat
enhances the mobility of the material that has already been deposited
during the first step, while also steadily providing additional deposition
material to migrate into the feature. The slower deposition rate enables
deposited atoms to move into the feature before being buried by
subsequently deposited atoms.
The net movement of deposited material into the feature also occurs because
filling of the feature results in a reduction of the surface area of the
metal-to-vacuum interface, and therefore a reduction in the surface energy
of the wafer. As substantial planarization occurs, the distance any given
atom must diffuse in order to contribute to further reduction in surface
area decreases. Thus, step coverage and fill factor increase as the
thickness of the deposited material increases. During the third and final
step, it is no longer necessary to rely wholly upon surface diffusion of
atoms in order to continue the planarization, so high deposition rates can
be used.
Using this technique, with a back plane temperature of about 450.degree. C.
during the second and third steps, via features of 3 microns wide and 1
micron deep, simultaneously with trench features of 1.5 microns wide and 1
micron deep, have been planarized on a TiN barrier using a target material
of aluminum 2% copper with a thickness of only one micron.
These and other features of the invention will be more readily understood
in view of the following detailed description and the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of a sputter/deposition apparatus used to
carry out the inventive method;
FIG. 2 is a cross sectional view which schematically shows a typical
feature of a wafer after the first step in accordance with a preferred
embodiment of the inventive method;
FIG. 3 is a cross sectional view which schematically shows a typical
feature of a wafer after a second step in accordance with a preferred
embodiment of the inventive method;
FIG. 4 is a cross sectional view which schematically shows a typical
feature of a wafer after a third step in accordance with a preferred
embodiment of the inventive method;
FIG. 5 shows the nucleation of 250 Angstroms of pure aluminum deposited on
a Ti/TiN/Ti barrier at 200.degree. C.;
FIG. 6 shows the nucleation of 250 Angstroms of pure aluminum deposited on
a Ti/TiN/Ti barrier at 300.degree. C.;
FIG. 7 shows the nucleation of 250 Angstroms of pure aluminum deposited on
a Ti/TiN/Ti barrier at 400.degree. C.;
FIG. 8 shows the nucleation of 250 Angstroms of pure aluminum deposited on
a Ti/TiN/Ti barrier at 500.degree. C.;
FIG. 9 is a graph which depicts theoretical maximum step coverage versus
via width during deposition without mobility, for a square feature having
width w, depth h and vertical walls;
FIG. 10 is a graph which depicts theoretical maximum step coverage versus
aspect ratio for a feature of with vertical walls;
FIGS. 11 and 12 are schematics showing a cross sectional view of a feature
to be metallized and which demonstrate a theoretical model used to
calculate coverage thickness achieved during a second step of the
inventive process, each of the Figures having a different aspect ratio;
and
FIGS. 13-18 are photographs which show cross sectional views of different
feature sizes after the preferred embodiment of this inventive method has
been applied under varying second step thicknesses. The sizes are labelled
on the Figs.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a cross sectional view of a magnetron sputtering apparatus 10
that can be used to deposit aluminum or aluminum alloy upon exposed
surfaces of a wafer 12. The deposition material, i.e., the aluminum or the
aluminum alloy, comprises a target 13 mounted opposite the wafer 12, both
enclosed within a vacuum chamber 14. As mentioned previously, during
sputtering an inert gas is passed through chamber 14.
The apparatus 10 also includes a backplane member 15 and means 16 for
heating the backplane member 15. Backplane member 15 is movable toward
wafer 12 sufficiently close to form a seal tight enough to allow a
thermally conducting gas layer to exist in the space 17 between wafer 12
and the backplane 15. By regulating the heat of the backplane member 15,
through the withholding from or introducing into the space 17 an inert gas
such as Argon, the temperature of the wafer 12 can be precisely
controlled. The gas presence serves as a thermal conduit. The timing of
the heat application can also be precisely controlled. This feature is of
significant importance in the three step planarization method in
accordance with a preferred embodiment of the invention because it enables
precise control of the timing of heat applied to the wafer 12.
FIGS. 2, 3 and 4 schematically show the progressive planarization of a
feature 20 during the first, second and third steps, respectively, in
accordance with the invention. The feature 20 has a bottom surface 21 and
side surfaces 22. During the first step, shown in FIG. 2, aluminum or
aluminum alloy is deposited in a substantially uniform manner upon the top
flat surface 24 of the wafer 12 and also upon the inside surfaces 21 and
22 of the feature 20 to form an initial layer 27. As will be explained in
more detail later, coverage of the inside surfaces during this first step
is "geometric." During the second step, as shown in FIG. 3, some of the
aluminum that is being deposited will move into the via 20 to form the
second layer 28. During the third step, as shown in FIG. 4, the aluminum
moves into the feature 20 until substantial planarization has occurred, to
form the complete deposition layer 29.
Studies of the initial deposition of aluminum showed that the metal tends
to redistribute on a flat surface into "islands" during the first few
hundred Angstroms of deposition. As shown in FIGS. 5 through 8, the island
density increases with decreasing temperature. Conversely, as the
temperature increases, the size of the islands increases and the spacing
between adjacent islands also increases, or, in other words, the islands
become less dense. In order to initially cover the interior surfaces of
the features 20 with a relatively uniform, continuous film, the first step
of this planarization method should be performed at relatively low
temperature e.g., less than 200.degree. C. Otherwise, the relatively
larger islands generally cause shadowing in the lower parts of the
feature, thereby impeding deposition below and tending to result in void
formation during coating of the inside walls of the feature 20. It has
been estimated that the average spacing of the islands is only about 100 A
when deposited at room temperature. Therefore, a deposition thickness of
100 A-200 A will be composed of islands which have coalesced into a
continuous film. This continuous film formed early in the deposition
provides an optimum surface for the diffusing of material in the
subsequent high temperature steps.
A model is utilized for estimating the maximum step coverage obtainable
without redistribution of the deposition into the feature from the area
surrounding it. This model assumes no shadowing and no redistribution, so
it may be applied only for thin films at low temperatures. Hence, it may
be used to determine the nominal thickness necessary for the first step of
this process. In this model, it is assumed that all the deposited material
which enters a feature becomes uniformly distributed over the inside
surfaces of the feature, thus giving the maximum step coverage possible.
The calculations involve a comparison of the cross sectional area of the
feature 20 at the surface to the total surface area of the side and bottom
walls of the feature. It is assumed that:
##EQU1##
with step coverage defined as the ratio of the thickness of metal
deposited in the feature to the nominal deposition thickness on the flat
wafer surface (i.e., t/T).
According to the assumption, if these two expressions are equated, a value
for step coverage can be obtained. For a square feature, i.e., A=w.sup.2,
with vertical walls, the step coverage, or t/T, becomes w.sup.2 /(w.sup.2
+4wh). Using this equation, the step coverage of a square feature can be
plotted against via width for a variety of feature widths and depths, as
shown in FIG. 9. Step coverage can also be calculated for different values
of the aspect ratio, the ratio of the feature depth h to the feature width
w, as in FIG. 10. From this model, a feature with an aspect ratio of unity
will have a maximum step coverage of 20% for very thin, low temperature
films.
Therefore, in order to obtain a continuous film of about 200 Angstroms
thick on the inner surface of the feature during the first step, about
1000 Angstroms must be deposited onto the flat surface 24 of the wafer 12.
Note that for more favorable aspect ratios or favorable slopes, the 200
Angstrom film thickness may be achieved with much less than 1000 Angstroms
of deposition, even as little as 400 Angstroms. In features with
irregularities on the sidewalls, a continuous film can only be formed at
greater thicknesses than 200 Angstroms. In such cases the deposition
thickness may be as much as 2000 Angstroms.
During the first step, every attempt is made to use the highest deposition
rate possible, since this improves throughput, resistivity, reflectivity,
and grain size. This rate is preferably about 220 Angstroms per second,
and is achieved by controlling the power to the target 13. Depending upon
the past life of the target, this power is preferably in the range of
about 20 KW to 29 KW. If the target is relatively new and unused, the
power necessary to achieve the desired deposition rate will probably be in
the lower part of the range, i.e., closer to 20 KW. However, as the target
is consumed, higher power must be applied to achieve the desired rate.
On the other hand, the deposition rate during the second step is set at
rate preferably about one fifth of the rate used during the first step, or
about 44 Angstroms per second, and can be achieved by reducing power to
the target 13 to about one fifth of its original value. The duration and
deposition rate of the second step is determined according to a
theoretical maximum diffusion length and the amount of heat, i.e., the
temperature, applied to the wafer.
At room temperature, mobility is so low that step coverage cannot be
improved by any surface diffusion mechanism. Thus, while a continuous film
can be formed at room temperature, the step coverage during the first step
will be governed primarily by geometry. If the entire deposition is
performed under these conditions, the growing layer will soon cause
shadowing. In order to planarize, or nearly completely fill the feature,
mobility through the application of heat and/or bias must be added during
subsequent steps.
To utilize surface diffusion during the second step of this process, the
deposition rate must be reduced. The deposited atoms move about the
surface at a rate determined by the temperature, for a duration, or
lifetime, that is limited to the length of time it takes for these atoms
to be buried by subsequently deposited atoms. This lifetime can be
calculated as:
.UPSILON.=na/r III
where n is the number of monolayers required to bury the mobile atoms, a is
the monolayer thickness, and r is the deposition rate. During the
lifetime, the mobile atoms will move a characteristic distance, L, defined
by:
##EQU2##
and where D is the surface diffusion coefficient. The distance L is
related, in this process, to the dimensions of the feature, since
planarization requires the motion of the mobile atoms into the feature.
Assuming that the atom must move a distance equal to the depth and the
half-width of the feature to fill it up, the characteristic distance L can
be expressed as:
L=h+w/2 V
By substitution, now the deposition rate can be expressed as follows:
##EQU3##
The diffusion coefficient D has the form
D=D.sub.o exp (-E.sub.a /kT) VII
where D.sub.o is a constant, k is Boltzmann's constant, E.sub.a is the
activation energy for surface diffusion (about 0.5 eV for Aluminum), and T
is the absolute temperature. Now
##EQU4##
Our experimental work has shown for h=1 micron, w=1 micron, at 450.degree.
C., a successful second step occurs with a deposition rate of 45 A/sec.
Thus the lumped constants can be estimated as:
D.sub.o na=31 micron.sup.3 sec IX
Using this constant, the deposition rate can be calculated for other
feature dimensions and temperatures.
The thickness, t.sub.2, that is required for the second step, can be
estimated from a consideration of the volume of deposited material that
must be redistributed from the area surrounding the feature to establish a
favorable surface configuration inside the feature. This configuration may
be any one of a number of hypothetical cases. Two such hypothetical cases
are shown in FIG. 11 and FIG. 12, and these cases are used to derive an
expression for thickness t.sub.2 used in the second step of this inventive
process. The choice of which of these two cases to use depends on the
aspect ratio of the feature.
FIG. 11 shows a feature 30 which has an aspect ratio greater than unity. In
FIG. 11, the feature 30 has been filled to the extent the deposited
material defines a hollow cone defined by walls 32 and 33 and with a
90.degree. apex angle that extends down into the feature 30 from the
original side surfaces 35 of the feature 30. Although the details of the
motion of material during the redistribution are unknown, this
configuration can be used to estimate the distance the material moves
during the redistribution. The material that moves into the feature 30 can
be assumed to come from a void 36 formed in the layer of material
deposited during the second step. The void 36 has the shape of a conical
annulus (with triangular cross section) that extends from the periphery 37
of the feature 30 a distance equal to the distance that an atom will move,
which was defined earlier as L. In other words, the characteristic
distance L forms one leg of the triangularly shaped void 36.
For simplification, this case can be reduced to a two-dimensional problem,
which would be strictly applicable to the planarization of trench features
with this particular aspect ratio. It is assumed that there is a balance
between the cross sectional area 38 that is filled in the feature 30 and
the cross sectional area of the void 36 from where the redistributed
material originated. These cross sectional areas are shaded in FIGS. 11
and 12. This assumption that the area of void 36 equals the area 38 in the
feature results in the following equation:
1/2 Lt.sub.2 =w/2(h-t.sub.2 -w/2)+w.sup.2 /8 X
From which an expression for t.sub.2 can be obtained, as follows:
##EQU5##
Since the redistribution of material is by surface diffusion, the
expression for L in Equation IV can be substituted into Equation XI. By
first combining Equations III and IV to obtain the following expression:
##EQU6##
with c being a constant of proportionality, and then substituting Equation
XII into Equation XI, the following expression is obtained:
##EQU7##
When D is replaced by its equivalent expression in temperature, and the
earlier obtained expression for r is substituted, the following expression
is obtained:
##EQU8##
In other words, temperature dependence drops out, and the thickness
t.sub.2 for second step deposition is a factor of the geometry of the
feature to be planarized. From the same experimental data that was used
earlier to calculate the appropriate deposition rate, a good planarization
results were achieved with t.sub.2 =3000 A.
Thus, the constant can be estimated as:
c=1.0 (dimensionless)
Now both the rate and the thickness for the second step can be estimated,
given the temperature of the process, and the height and width of the
feature. Using these expressions for rate and thickness, the required
duration of the second step deposition can also be calculated.
The derived expressions for rate and thickness t.sub.2 apply for the
feature shown in FIG. 11, which has an aspect ratio greater than unity.
Features with different cross sectional shapes will generate different
material balance equations. For instance, FIG. 12 shows a feature with an
aspect ratio that is much less than unity. With the same assumptions made
for the derivation of Equation XIV, an expression for thickness, t.sub.2,
can be arrived at for the feature in FIG. 12. Omitting all the details,
the final expression turns out to be:
##EQU9##
The third step involves deposition at a normally high rate, preferably in
the same range used for the first step, i.e., approximately 200-220
A/second, to achieve the final desired thickness. The duration of the
third step deposition will depend upon the final desired thickness and the
amount of material already deposited during Steps 1 and 2.
In summary, the invention process involves:
Step 1: Deposit 1000 A of material (more or less according to aspect ratio)
at a high deposition rate with low temperature to suppress nucleation.
Step 2: Deposit to a thickness t.sub.2 at a reduced deposition rate and
with deposition occurring at a high temperature to enhance surface
diffusion. The thickness, rate and, ultimately, the duration of deposition
during this step are calculated from the height and width of the feature,
and indirectly, from the temperature of the heat applied to the wafer.
Step 3: Deposit additional material to a thickness according to the final
desired thickness, with deposition occurring at a high deposition rate and
with high temperature.
Note, finally, that practical considerations will limit the application of
this invention to a certain range of feature sizes and aspect ratios. The
step coverage at low temperatures is limited by the aspect ratio.
Moreover, the volume required to fill very large features may require
unusually low deposition rates and second step thicknesses beyond that
desired for the total deposition.
FIGS. 13 through 18 show the dramatic effects of changing the duration of
the second step deposition. The Figures on the left, i.e., 13-15, show the
results of a "minimum" second step, requiring about 15 seconds to achieve
a 660 A thickness (or about 44 Angstroms/sec). This duration was
calculated from a thermal model of the time required to bring the wafer 12
up to the temperature of the back plane member 15. The Figures on the
right, i.e., FIGS. 16-18, show a second step of longer duration, with a 60
second duration used to achieve a 3000 A thickness (at a rate of about 50
A/sec). The effects produced by deposition steps of intermediate duration
can be interpolated. Thus, it appears the sixty second deposition time is
much more than is required to heat the wafer to its maximum temperature,
so it can be concluded that the longer deposition thickness is responsible
for better planarization, regardless of whether the process is viewed as
metal-volume delivery or diffusion-distance.
It has been found that the diffusion processes are active at temperatures
about 300.degree. C. No wafer bias is used during the first step because
it is of too short a duration to establish a stable sputtering process.
Bias is also not usually used in the second step because it may raise the
electrical resistivity. Bias on the third step has been used to modify
film characteristics such as grain size and structure. For instance, bias
during the third step helps to raise the temperature as much as 30.degree.
C. more than the unbiased deposition alone, and at 150-250 v it improves
the surface texture of the film. There are no indications that it improves
planarization through resputtering.
To summarize the sequence of steps used to carry out the inventive method
with the Eclipse machine, during a first step of high rate deposition,
about 400-1000 Angstroms of aluminum are deposited onto the wafer 12,
thereby producing a continuous film that serves as the base material for
subsequent planarization. The duration of this first step is about 0.077
minutes, or about 41/2 to 5 seconds, and results in a substantially
uniform step coverage of about 200 Angstroms on the inside surfaces of the
features of the wafer. It is achieved by applying high power, i.e., 25 KW,
to the target 13.
Immediately following the first step, back plane gas is used to thermally
connect the heated back plane member 15 to the rear of the wafer 13. With
heat applied, power to the target 13 is reduced to about 3 kilowatts in
order to obtain reduced rate deposition, preferably about one fifth of the
deposition rate used in the first step. The depth and the half-width of
the feature to be planarized are used to determine a characteristic
distance that the deposited atoms must move in order to fill the feature.
The characteristic distance, along with the temperature of the applied
heat, are used to determine first the rate of deposition and ultimately,
the duration of the second step. The thickness of the second step
deposition is determined solely by the feature geometry, i.e., the depth
and width.
During the third step, the wafer 12 remains thermally connected to the
heated back plane while high power, high rate deposition is resumed until
the final desired thickness is achieved. The duration of the third step
depends upon the final desired thickness of material, and the amount of
material already deposited during the first and second steps. Preferably,
depositing is continuous and uninterrupted from commencement of the first
step until completion of the third step.
While a preferred embodiment of the invention has been described, applicant
does not wish to be limited thereby, and it is to be understood that
various modifications could be made without departing from the spirit of
the invention. Accordingly, it is to be understood that changes may be
made without departing from the scope of the invention as particularly set
out and claimed.
* * * * *
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