U.S. patent application number 12/192351 was filed with the patent office on 2009-02-26 for method of recycling abrasive slurry.
This patent application is currently assigned to SUMCO TECHXIV CORPORATION. Invention is credited to Isamu Gotou, Kazuaki Kozasa.
Application Number | 20090053981 12/192351 |
Document ID | / |
Family ID | 40382618 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090053981 |
Kind Code |
A1 |
Kozasa; Kazuaki ; et
al. |
February 26, 2009 |
METHOD OF RECYCLING ABRASIVE SLURRY
Abstract
A method of recycling an abrasive slurry for recycling a slurry
that: contains colloidal silica; and has been used in polishing
semiconductor wafer(s) is provided. The method includes: adding a
dispersant to the used slurry having been collected so as to
prevent the used slurry from being gelled; irradiating ultrasound
to the used slurry having been added with the dispersant so as to
disperse a gelled portion and aggregated silica in the used slurry;
and, by using a filter, removing a foreign substance contained in
the used slurry having been irradiated with the ultrasound.
Inventors: |
Kozasa; Kazuaki; (Omura-shi,
JP) ; Gotou; Isamu; (Omura-shi, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue, 16TH Floor
NEW YORK
NY
10001-7708
US
|
Assignee: |
SUMCO TECHXIV CORPORATION
Omura-shi
JP
|
Family ID: |
40382618 |
Appl. No.: |
12/192351 |
Filed: |
August 15, 2008 |
Current U.S.
Class: |
451/447 |
Current CPC
Class: |
B24B 57/00 20130101;
B24B 37/0056 20130101 |
Class at
Publication: |
451/447 |
International
Class: |
B24B 57/00 20060101
B24B057/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 2007 |
JP |
2007-217118 |
Claims
1. A method of recycling an abrasive slurry for recycling a slurry
containing colloidal silica, the slurry being a used slurry having
been used in polishing semiconductor wafer(s), the method
comprising: adding a dispersant to the used slurry having been
collected so as to prevent the used slurry from being gelled;
irradiating ultrasound to the used slurry having been added with
the dispersant so as to disperse a gelled portion and aggregated
silica in the used slurry; and removing a foreign substance
contained in the used slurry having been irradiated with the
ultrasound, the foreign substance being removed by a filter.
2. The method of recycling an abrasive slurry according to claim 1,
wherein a pH value of the used slurry is measured and the pH value
is adjusted by adding an alkali solution before the dispersant is
added to the used slurry.
3. The method of recycling an abrasive slurry according to claim 1,
wherein viscosity of the used slurry is measured and the viscosity
is adjusted by adding a water-soluble polymer before the dispersant
is added to the used slurry.
4. The method of recycling an abrasive slurry according to claim 1,
wherein a temperature of the used slurry is measured and the
temperature is adjusted by using a heat exchanger before the
dispersant is added to the used slurry.
5. The method of recycling an abrasive slurry according to claim 1,
wherein metal ion contained in the used slurry is removed after the
ultrasound is irradiated to the used slurry.
6. The method of recycling an abrasive slurry according to claim 5,
wherein the metal ion contained in the used slurry is removed by
adding a chelate agent to the used slurry.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of recycling an
abrasive slurry for recycling a used slurry having been used in
polishing semiconductor wafer(s).
[0003] 2. Description of Related Art
[0004] Polishing of semiconductor wafer(s) is generally classified
into two major categories of rough polishing and finish polishing
according to surface roughness to be made on the semiconductor
wafer(s).
[0005] In finish polishing, where ultra-fine surface roughness is
required to be made, the semiconductor wafer(s) is usually polished
with an ammonia-base colloidal silica slurry having been added with
a water-soluble polymer such as ethylcellulose.
[0006] The colloidal silica slurry having been used in finish
polishing has conventionally been discarded because the slurry may
contain metal contamination originating from components of a
polishing apparatus, a giant silica solid formed by aggregation of
silica in the slurry, and the like.
[0007] However, in view of environment protection, it is preferable
to recycle such a wasted slurry. For this purpose, the following
recycling methods of an abrasive slurry have been suggested.
[0008] For instance, a Document 1 (JP-A-2002-170793) proposes a
method of reproducing a slurry, according to which coarse particles
in a CMP (chemical mechanical polishing) slurry are filtrated by a
filter and the filtrated slurry is condensed by such a method as
centrifugation.
[0009] Alternatively, a Document 2 (JP-A-2004-63858) proposes a
method of retrieving a slurry, according to which aggregated
abrasive grains contained in a used slurry are crushed by
ultrasound, a temperature of the slurry is adjusted and the
aggregated abrasive grains are separated from non-aggregated
abrasive grains.
[0010] However, the methods disclosed in the above Documents are
not sufficiently applicable in retrieving and recycling the
above-described colloidal silica slurry, and the following problems
are pertinent to the methods.
[0011] Specifically, since ammonia contained in the colloidal
silica slurry is a volatile substance, a pH value of the colloidal
silica slurry may be varied by the time when the slurry is
retrieved, so that the retrieved slurry may not be directly
recycled.
[0012] In addition, the water-soluble polymer contained in the
slurry tends to be aggregated to form a gel. Thus, even when the
slurry experiences filtration by a filter, such gel-like
aggregation can easily clog the filter.
[0013] Further, in finish polishing, which is the last process in
manufacturing processes of semiconductor wafer(s), greater cautions
are required to be paid to metal contamination.
[0014] The methods disclosed in the above Documents cannot solve
all the problems although the methods may be able to solve one of
the problems.
SUMMARY OF THE INVENTION
[0015] An object of the invention is to provide a method of
recycling an abrasive slurry, by which a used slurry having been
used in polishing semiconductor wafer(s), particularly a slurry
having been used in finish polishing, can be recycled, so that
considerable reduction in slurry usage can contribute to reduction
in manufacturing cost of the semiconductor wafer(s).
[0016] A method of recycling an abrasive slurry according to an
aspect of the invention is for recycling a slurry containing
colloidal silica, the slurry being a used slurry having been used
in polishing semiconductor wafer(s), the method including:
[0017] adding a dispersant to the used slurry having been collected
so as to prevent the used slurry from being gelled;
[0018] irradiating ultrasound to the used slurry having been added
with the dispersant and dispersing a gelled portion and aggregated
silica in the used slurry; and
[0019] removing a foreign substance contained in the used slurry
having been irradiated with the ultrasound, the foreign substance
being removed by a filter.
[0020] The dispersant may be any one of (1) salt, (2) polarizable
molecule and (3) pH stabilizer. [0021] (1) Salt: Salts formed by
combining a cation selected from Li.sup.+, Na.sup.+, K.sup.+,
Mg.sup.2+, Ca.sup.2+ and NH.sub.4.sup.+ with an anion selected from
CO.sub.3.sup.2-, Cl.sup.-, SO.sub.4.sup.2-, F.sup.-, NO.sup.3-,
PO.sub.4.sup.3-, CH.sub.3COO.sup.- and OH.sup.- are all usable.
[0022] (2) Polarizable Molecule: Any material containing ammonia
water, alcohols, sugars or ethers is usable. [0023] (3) pH
stabilizer: Ammonia water, KOH and NaOH are usable.
[0024] According to the aspect of the invention, while the used
slurry is prevented from being gelled by the addition to the
dispersant to the used slurry, the gelled portion of the slurry and
the aggregated silica are dispersed by the irradiation of
ultrasound, and foreign substance(s) contained therein is removed
by the filter. Thus, foreign substance(s) can be efficiently
removed by the filter while an amount of silica contained in the
slurry can be prevented from being reduced due to a capture of
gelled and aggregated silica by the filter. With this arrangement,
the used slurry can be favorably recycled.
[0025] In the above aspect of the invention, it is preferable that
a pH value of the used slurry is measured and the pH value is
adjusted by adding an alkali solution before the dispersant is
added to the used slurry.
[0026] An example of the alkali solution for adjusting the pH value
is ammonia water.
[0027] According to the aspect of the invention, by adjusting the
pH value in advance, the slurry and silica can be further prevented
from being aggregated.
[0028] In the above aspect of the invention, viscosity of the used
slurry is preferably measured and adjusted with a supplement of a
water-soluble polymer before the dispersant is added to the used
slurry.
[0029] Examples of the water-soluble polymer for adjusting the
viscosity are ethylcellulose and ethylene glycol.
[0030] According to the aspect of the invention, by supplementing
the water-soluble polymer, the viscosity of the used slurry can be
suitably adjusted. In this manner, an abrasive slurry suitable for
recycling can be obtained.
[0031] In the above aspect of the invention, a temperature of the
used slurry is preferably measured and adjusted with a use of a
heat exchanger before the dispersant is added to the used
slurry.
[0032] According to the aspect of the invention, by adjusting the
temperature of the used slurry before the dispersant is added,
gelled substance(s) contained in the used slurry can be dispersed
therein at the optimal temperature condition.
[0033] In the above aspect of the invention, metal ion contained in
the used slurry is preferably removed after the ultrasound is
irradiated to the used slurry.
[0034] An exemplary method of removing the metal ion is to add a
chelate agent to the used slurry.
[0035] The chelate agent may be an organic-base agent formed of
aminocarboxylate. Examples of the chelate agent are EDTA
(ethylenediaminetetraacetic acid), DTPA
(diethylenetriaminepentaacetic acid) and NTA (nitrilotriacetic
acid).
[0036] According to the aspect of the invention, the metal ion
having been mixed into the slurry during polishing can be removed.
Thus, when semiconductor wafer is polished with the used slurry, it
is possible to prevent the semiconductor wafer(s) from being
contaminated by metal ion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 schematically shows an arrangement of a recycling
apparatus according to an exemplary embodiment of the
invention.
[0038] FIG. 2 is a flow chart showing steps of a recycling method
according to the exemplary embodiment.
[0039] FIG. 3 is a graph showing differences in dispersion effects
between dispersants.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0040] An exemplary embodiment of the invention will be described
with reference to the attached drawings below.
[0041] FIG. 1 shows a recycling apparatus 1 for recycling an
abrasive slurry according to the exemplary embodiment of the
invention. The recycling apparatus 1 collects a slurry having been
used in a finish-polishing machine 2, adjusts a pH value, a
relative density and viscosity of the collected abrasive slurry and
then removes foreign metal substance(s) therefrom so as to
reproduce a slurry, thereby recycling the used slurry as an
abrasive slurry to be used again in the finish-polishing machine
2.
[0042] The recycling apparatus 1 includes a multiple-stage cascade
tank 3, a heat exchanger 4, a storage tank 5 and a
foreign-substance filtrating filter 6.
[0043] The multi-stage cascade tank 3 serves as a precipitator that
removes giant silica solids contained in the collected slurry by
sedimentation separation. The inside of the tank 3 is partitioned
into plural processing tanks by plural shuttering boards 31 each of
which has a different height.
[0044] The plural shuttering boards 31 are arranged such that the
plural shuttering boards 31 as a whole reduces its height as
extending to a lateral lace 32 (i.e., where an outlet is provided)
from a side to which the collected slurry is supplied. Then, when
the used slurry is supplied in a first processing tank and
subsequently overflows therein, an amount of the overflowing slurry
flows into a processing tank contiguous to the first processing
tank. This operation is sequentially repeated, and then the used
slurry is finally ejected from the outlet.
[0045] In each of the processing tanks, giant silica solids, which
have a higher specific gravity than the used slurry, are settled
out while supernatant fluid of the used slurry, which has the lower
specific gravity than the giant silica solids, flows into a
processing tank contiguous thereto. With this operation being
repeated, giant silica solids are separated and removed by
sedimentation from the used slurry.
[0046] The heat exchanger 4, which is connected to a rear portion
of the multiple-stage cascade tank 3 via a piping, is for adjusting
a temperature of the used slurry by cooling the used slurry ejected
from the multiple-stage cascade tank 3. The heat exchanger 4 is so
arranged that a channel for the used slurry ejected from the outlet
provided on a lower portion of the multiple-stage cascade tank 3
and a channel for circulation of cooling water are partitioned by a
highly thermally-conductive material, thereby exchanging heat
between the used slurry and the cooling water for temperature
adjustment of the used slurry. It should be noted that the heat
exchanger 4 may be selected from various heat exchangers, as long
as the heat exchanger can exchange heat between liquid and liquid,
examples of which are a plate-type heat exchanger, a double
pipe-type heat exchanger, and a multitubular cylinder-type heat
exchanger.
[0047] The storage tank 5, which is connected to a rear portion of
the heat exchanger 4 via a piping, stores the used slurry having
experienced temperature adjustment by the heat exchanger 4 and
adjusts conditions of the used slurry. The storage tank 5 is
provided with a thermometer 51, a viscometer 52, a hydrometer 53
and a pH meter 54 respectively for measuring temperature,
viscosity, relative density and pH values of the used slurry stored
in the storage tank 5.
[0048] A bottom of the storage tank 5 is additionally provided with
a ultrasonic oscillator.
[0049] The ultrasonic oscillator irradiates ultrasound to the used
slurry in the storage tank 5 so as to disperse gelled portions and
aggregated silica in the used slurry. The ultrasound to be
irradiated to the used slurry is preferably kHz-frequency
ultrasound because MHz-frequency ultrasound may not be able to
disperse aggregated silica due to influence of the water-soluble
polymer agent.
[0050] The foreign-substance filtrating filter 6, which is
connected to a rear portion of the storage tank 5 via a piping,
filters the used slurry to capture foreign substances such as giant
silica solids present in the used slurry. The foreign-substance
filtrating filter 6 includes filters such as a depth filter and a
membrane filter disposed in the channel in which the used slurry
flows.
[0051] Next, operations of the above-described recycling apparatus
1 will be described based on the flow chart shown in FIG. 2.
[0052] After the used slurry is collected from the finish-polishing
machine 2 (step S1), the collected used slurry is supplied to the
multiple-stage cascade tank 3 of the recycling apparatus 1, and the
supplied used slurry experiences sedimentation separation in each
of the processing tanks thereof, so that giant silica solids are
removed by sedimentation (step S2).
[0053] Then, a temperature of the used slurry from which giant
silica solids have been removed is adjusted to a suitable
temperature by circulating cooling water in the heat exchanger 4
(step S3), and the used slurry having experienced temperature
adjustment is subsequently supplied to the storage tank 5. The
temperature of the used slurry is adjusted to be in a range of
approximately 20 to 30 degrees C. based on the measurement of the
thermometer 51 in the storage tank 5.
[0054] Relative density, viscosity and a pH value of the used
slurry supplied to the storage tank 5 are sequentially
adjusted.
[0055] Relative density of the used slurry is adjusted by
supplementing a stock solution of colloidal silica slurry to the
storage tank 5 based on the measurement of the hydrometer 53 (step
S4). Relative density of the used slurry is adjusted within a range
of 1.010 to 1.020.
[0056] Viscosity of the used slurry is adjusted by supplementing a
water-soluble polymer such as ethylcellulose to the storage tank 5
based on the measurement of the viscometer 52 (step S5). Viscosity
of the used slurry is adjusted within a range of 0.004 to 0.01 Pas
(values obtained by converting 4 to 10 cP).
[0057] A pH value of the used slurry is adjusted by supplementing
ammonia water to the storage tank 5 based on the measurement of the
pH meter 54 (step S6). A targeted pH value is roughly in a range of
pH 10 to pH 11. An excessively low value of pH may reduce a
polishing rate while an excessively high value of pH may dissolve
silica.
[0058] When the above adjustments are finished, one of salt,
polarizable molecule and pH stabilizer is added thereto as a
dispersant so as to prevent aggregation of the used slurry (step
S7). In addition, ultrasound is irradiated to the used slurry with
the ultrasonic oscillator driven, so that gelled portions of the
slurry and aggregated silica are dispersed (step S8). Examples of
the dispersant are KCl, NH.sub.4HCO.sub.3 (examples of salt).
[0059] Finally, the used slurry having been irradiated with
ultrasound is filtrated by the foreign-substance filtrating filter
6, so that foreign substances are removed (step S9). Then, a
reproduced abrasive slurry is collected through a branch piping to
be supplied to the finish-polishing machine 2 for use again.
EXAMPLE(S)
[0060] Next, examples of the invention will be described. It should
be noted that the invention is not limited to the examples.
Experiment Example 1
[0061] After a slurry prepared by diluting a stock solution with
water 20 times was used for polishing for 500 minutes, the used
slurry was irradiated with ultrasound and subsequently filtrated by
the foreign-substance filtrating filter 6 for removal of foreign
substances. Then, a reproduced slurry was obtained.
Experiment Example 2
[0062] The same used slurry as in the experiment example 1 was
added with ammonia water as a dispersant, irradiated with
ultrasound and filtrated by the foreign-substance filtrating filter
6 for removal of foreign substances. Then, a reproduced slurry was
obtained.
Experiment Example 3
[0063] The same used slurry as in the experiment example 1 was
added with KCl water as a dispersant, irradiated with ultrasound
and filtrated by the foreign-substance filtrating filter 6 for
removal of foreign substances. Then, a reproduced slurry was
obtained.
Experiment Example 4
[0064] A slurry was obtained by diluting a stock solution of
colloidal silica slurry with water 20 times.
Experiment Example 5
[0065] The same used slurry as in the experiment example 1 was
directly used as a reproduced slurry.
1. Quality of Polished Semiconductor Wafer
[0066] A semiconductor wafer having a diameter of 150 mm was
polished using the slurry according to each of the experiment
examples 1 to 5. Then, the quality of a surface of the polished
semiconductor wafer was evaluated for each. The semiconductor wafer
was polished under the following conditions: Politex.TM. was used
as an abrasive pad; a table rotation number was 50 rpm; a head
rotation number was 70 rpm; pressure of 80 g/cm.sup.2 was applied;
and the wafer was polished for 30 minutes.
[0067] Items of quality evaluation were a polishing rate,
microroughness and the number of defects.
[0068] The polishing rate corresponds to a value after the wafer
having a diameter of 150 mm was polished for 30 minutes.
[0069] Microroughness for short wavelengths is represented by an
index number that is calculated in relation to a value indicated by
a particle counter (SP1 manufactured by KLA-Tencor Corporation)
after 30 minutes of polishing in the experiment example 4 (the
value is set as 100). On the other hand, microroughness for long
wavelengths is represented by an index number that is calculated in
relation to a value obtained by averaging rms values of three
points in the vicinity of the wafer center after 30 minutes of
polishing in the experiment example 4 (the value is set as
100).
[0070] The number of defects is obtained by counting the number of
defects on a polished surface of a single wafer after 30 minutes of
polishing.
[0071] The results are shown in Table 1.
TABLE-US-00001 TABLE 1 Experiment Experiment Experiment Experiment
Experiment Example 1 Example 2 Example 3 Example 4 Example 5
Polishing Rate 0.031 0.033 0.044 0.026 0.027 (.mu.m/min)
Microroughness: Haze 88 80 76 100 204 (Short wavelength)
Microroughness: rms 107 100 102 100 207 (Long wavelength) Number of
Defects 58 26 25 32 112 (pieces)
[0072] With respect to the polishing rate, the slurry irradiated
with ultrasound as a whole exhibited enhanced polishing rates.
Particularly, the slurry according to the experiment example 3,
which was added with a dispersant made of KCl water, exhibited a
greatly-enhanced polishing rate. On the other hand, in the slurry
according to the experiment example 4, which was prepared by merely
diluting the stock solution with water, the water-soluble polymer
is considered to have had such a low dispersivity as to be
partially aggregated on the to-be-polished surface at the time of
polishing, and to have served like a protective film thereon. It is
presumed that, as the consequence, the polishing rate of the slurry
according to the experiment example 4 was low.
[0073] With respect to the microroughness for short wavelengths, as
is understood from the above, any one of the experiment examples 1
to 3 exhibited a level of haze that is approximate to that of the
experiment example 4, in which the slurry was prepared by diluting
the stock solution with water. On the other hand, as is also
understood from the above, the experiment example 5, in which the
slurry experienced neither ultrasound irradiation nor filtering,
exhibited a greatly-reduced value of haze.
[0074] With respect to the microroughness for long wavelengths, any
one of the experiment examples 1 to 3 is also observed to have
exhibited a level of roughness that is approximate to that of the
experiment example 4. On the other hand, as is understood from the
above, the experiment example 5 likewise exhibited an increased
value of roughness.
[0075] With respect to the number of defects, any one of the
experiment examples 1 to 3 is also observed to have exhibited the
number of defects that is approximate to that of the experiment
example 4. On the other hand, as is understood from the above,
defects in the experiment example 5 were greatly increased.
[0076] It has been found from the above that, as shown in the
experiment example 5, a polishing level (i.e., polishing rate,
microroughness and the number of defects) exerted by directly
applying the collected used slurry to polishing for recycling is
not comparable to a polishing level exerted by a slurry prepared by
diluting the stock solution as in the experiment example 4. It has
been also found that ultrasound irradiation on the used slurry and
addition of a dispersant to the used slurry can so greatly improve
the values of the polishing level exerted by the slurry as to make
the used slurry applicable as a reproduced slurry.
2. Filtration by Foreign-Substance Filtering Filter 6
[0077] The slurry according to each of the experiment examples 1 to
5 was filtrated by foreign-substance filtrating filters 6 each of
which had a different filter size so as to verify to what degree of
fineness in filter the slurry could pass through the filter. The
results are shown in Table 2.
TABLE-US-00002 TABLE 2 Experiment Experiment Experiment Experiment
Experiment Example 1 Example 2 Example 3 Example 4 Example 5 Filter
Size: 20 .mu.m A A A C C Filter Size: 10 .mu.m C A A C C Filter
Size: 5 .mu.m C B A C C In Table 2: "A" means that the slurry could
pass through the filter; "B" means that the slurry could partially
pass through the filter; and "C" means that the slurry could not
pass through the filter.
[0078] As is understood from Table 2, the slurry according to the
experiment example 4 (slurry prepared by diluting the stock
solution) and the slurry according to the experiment example 5
(used slurry having experienced no treatment) so considerably
clogged the filters that the slurries could not pass through the
filters.
[0079] The slurry according to the experiment example 1 (i.e.,
slurry irradiated with ultrasound) could pass through a filter
having a filter size of 20 .mu.m. However, the slurry clogged
filters respectively having finer filter sizes of 10 .mu.m and 5
.mu.m and could not pass through the filters.
[0080] On the other hand, the slurry according to the experiment
example 2 (slurry added with ammonia water and irradiated with
ultrasound) and the slurry according to the experiment example 3
(slurry added with KCl water and irradiated with ultrasound) could
pass through even a filter having a filter size of 5 .mu.m. It has
been found that silica solids (dried silica) of 1 to 10 .mu.m order
can be captured by filters.
[0081] Next, experiments were conducted on the slurry according to
each of the experiment examples 1 to 3 in order to see how much
silica solids could be captured by the foreign-substance filtrating
filter 6. In the experiments, each slurry was circulated to
experience filtering continuously for 300 minutes, and the number
of dried silica having a size of 3 .mu.m after the filtering was
measured for evaluation. The results are shown in Table 3. In
filtering each slurry, a filter having such a filter size as to be
capable of favorably continuing the filtering was used.
Specifically: a filter having a filter size of 20 .mu.m was used
for the slurry according to the experiment example 1; a filter
having a filter size of 10 .mu.m was used for the slurry according
to the experiment example 2; and a filter having a filter size of 5
.mu.m was used for the slurry according to the experiment example
3.
TABLE-US-00003 TABLE 3 Experiment Experiment Experiment Experiment
Experiment Example 1 Example 2 Example 3 Example 4 Example 5 Number
of 80 to 100 10 to 20 1 to 10 1000 to 6000 to Remaining Dried
Silica 2000 8000 (After 300 minutes)
[0082] As is understood from Table 3, when the slurry is filtrated
by the foreign-substance filtrating filter 6 after the addition of
the dispersant and after the ultrasound irradiation, dried silica
in the slurry can be dramatically removed. For comparison, when the
number of dried silica in the slurry according to the experiment
example 5 was measured, dried silica of approximately 6000 to 8000
was found present therein. It can be understood therefrom that the
number of dried silica can be considerably decreased.
3. Observation of Dispersive Effects
[0083] Dispersive effect brought about by ultrasound irradiation
and addition of a dispersant was checked by measuring an average
particle diameter of fine silica particles in the slurry and zeta
potential of the slurry. The above checking was conducted on the
slurries according to the experiment examples 1, 3 to 5. In
addition, in order to see a difference between dispersants, the
following experiment example 6 was prepared. The zeta potential
represents electrification of a surface of silica. The larger a
value of the zeta potential becomes, the more favorable the
dispersion is.
Experiment Example 6
[0084] The same used slurry as in the experiment example 1 was
added with methanol as a dispersant and irradiated with ultrasound,
so that a reproduced slurry was obtained.
[0085] The results are shown in Table 4.
TABLE-US-00004 TABLE 4 Experiment Experiment Experiment Experiment
Experiment Example 1 Example 3 Example 4 Example 5 Example 6
Average Particle Diameter 137.5 77.2 134.5 129.2 148.3 (nm) Zeta
Potential 6.82 24.24 5.86 0.93 7.02 (mV)
[0086] As compared with the slurry according to the experiment
example 4 (slurry prepared by diluting the stock solution), the
slurry according to the experiment example 5 (used slurry with no
treatment) exhibited a greatly reduced value of the zeta potential
and deteriorated slurry dispersivity. In contrast, the slurry
according to the experiment example 1 (slurry irradiated with
ultrasound) has been found to have restored a value of the zeta
potential up to a zeta potential level of the slurry according to
the experiment example 4. From the above, slurry dispersivity
thereof has been found considerably enhanced.
[0087] Further, the slurry according to the experiment example 6
(slurry added with methanol and irradiated with ultrasound) has
been found to have exhibited slightly better dispersivity than the
slurry according to the experiment example 1.
[0088] The slurry according to the experiment example 3 (slurry
added with KCl water and irradiated with ultrasound) exhibited much
more increased value of the zeta potential than the slurries
according the other experiment examples. From the above, it has
been found that dispersivity can be considerably enhanced by adding
the slurry with KCl water as a dispersant. In addition, the slurry
reproduced by the method of the experiment example 3 exhibited a
smaller value of the average particle diameter than the slurries
according to the other experiment examples. In this respect as
well, it has been found that the slurry according to the experiment
example 3 is favorably applicable to polishing.
4. Difference in Dispersive Effect Due to Difference in
Dispersant
[0089] Next, comparison was made to see a difference in an
aggregation degree between used slurries respectively prepared by
adding a different dispersant to the slurry according to the
example 1. The results are shown in FIG. 3. In FIG. 3, "Ref" refers
to a slurry with no addition, "KCl" refers to a slurry prepared by
adding KCl thereto as a dispersant, and "NH.sub.4HCO.sub.3" refers
to a slurry prepared by adding NH.sub.4HCO.sub.3 thereto as a
dispersant. When a dispersant made of such salt is added to a
slurry prepared by diluting with water 20 times, an optimal
additive amount of the dispersant is in a range of 0.01 mol/L to
0.001 mol/L.
[0090] As is understood from FIG. 3, it has been found that, while
KCl is also sufficiently effective in reducing the aggregation
degree within a typical usage region of pH9.8 to 10.1,
NH.sub.4HCO.sub.3 is effective in reducing the aggregation degree
within a wider pH region beyond the typical usage region.
Accordingly, NH.sub.4HCO.sub.3 has been found to be a considerably
favorable dispersant.
[0091] The priority application Number JP 2007-217118 upon which
this patent application is based is hereby incorporated by
reference.
* * * * *