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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of removing a metal impurity
contained in a hydrofluoric-acid-containing chemical solution used in the
steps in manufacturing a semiconductor device.
2. Description of Related Arts
With an increase in integration density of a VLSI, demand for cleaning a
silicon wafer surface becomes severer. As contaminants on the silicon
wafer surface, fine particles, a metal, an organic material, an oxide
film, and the like are known.
An oxide film is removed by etching using a hydrofluoric-acid-containing
solution. However, although the oxide film can be removed by this process,
when a metal impurity (copper, gold, or the like) is contained in the
hydrofluoric-acid-containing chemical solution, this metal impurity is
attached to a wafer to adversely affect the electrical characteristics of
the wafer. For this reason, the hydrofluoric-acid-containing solution in
which a metal impurity is accumulated by dipping wafers in the
hydrofluoric-acid-containing solution must be regenerated and used by
purifying the hydrofluoric-acid-containing solution, or the
hydrofluoric-acid-containing solution must be replaced with a new one.
As a conventional method of removing a metal impurity, a distillation
method, an ion-exchange resin method, and a silicon granule adsorption
method (Japanese Patent Laid-Open Nos. 3-102827 and 4-286328) are used.
The distillation method is not suitably applied to purification of a
solution mixture such as a hydrofluoricacid-containing chemical solution
because the composition ratio of the solution changes after distillation.
An ion-exchange resin method can be suitably applied to only a diluted
hydrofluoric acid solution, and cannot be suitably applied to a chemical
solution which contains high-concentration hydrofluoric acid or ammonium
fluoride.
A method of removing a metal impurity contained in a
hydrofluoric-acid-containing chemical solution performed by a silicon
granule adsorption method is as follows. That is, the
hydrofluoric-acid-containing chemical solution is brought into contact
with silicon granules, the metal impurity in the
hydrofluoric-acid-containing chemical solution is removed by adsorbing the
metal impurity on the silicon granules.
In removing the metal impurity contained in the
hydrofluoric-acid-containing chemical solution using the above
conventional silicon granule adsorption method, when this method is used
for a long time, the metal impurity adsorbed on silicon granule surfaces
is oxidized by oxygen dissolved in the hydrofluoric-acid-containing
chemical solution, and the metal impurity is disadvantageously dissolved
in the hydrofluoric-acid-containing chemical solution again.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems in the
prior art, and has as its object to provide a method of stably removing a
metal impurity contained in a hydrofluoric-acid-containing chemical
solution for a long time to an extent corresponding to a purity which
makes it possible to perform the steps in manufacturing a semiconductor
device.
In order to achieve the above object, according to the first aspect of the
present invention, there is provided a method of removing a metal
impurity, comprising the steps of removing oxygen dissolved in a
hydrofluoric-acid-containing chemical solution, and, in order to remove a
metal impurity contained in the hydrofluoric-acid-containing chemical
solution free from the dissolved oxygen, bringing the
hydrofluoric-acid-containing chemical solution into contact with silicon
granules to adsorb the metal impurity on the silicon granules.
According to the second aspect of the present invention, there is provided
a method of removing a metal impurity, comprising the steps of removing
oxygen dissolved in a hydrofluoric-acid-containing chemical solution, and,
in order to remove a metal impurity contained in the
hydrofluoric-acid-containing chemical solution free from the dissolved
oxygen, circulating the hydrofluoric-acid-containing chemical solution in
a column filled with silicon granules to adsorb the metal impurity on the
silicon granules.
According to the third aspect of the present invention, there is provided a
method of removing a metal impurity wherein the silicon granules according
to the first and second aspects are metal-precipitated silicon granules
each having a silicon Granule surface on which a metal is precipitated.
According to the present invention, since the dissolved oxygen contained in
a hydrofluoric-acid-containing chemical solution is removed, the metal
impurity adsorbed on the silicon granule surfaces can be prevented from
being dissolved in the hydrofluoric-acid-containing chemical solution
again.
When metal-precipitated silicon granules each having the silicon granule
surface on which the metal is precipitated are used as silicon granules to
improve metal impurity adsorption performance, the adsorbed metal impurity
is prevented by the same effect as described above from being dissolved in
the hydrofluoric-acid-containing chemical solution again.
Therefore, the metal impurity in the hydrofluoric-acid-containing chemical
solution can be stably removed (concentration level of 0.01 ppb or less)
for a long time.
The above and many other advantages, features and additional objects of the
present invention will become manifest to those versed in the art upon
making reference to the following detailed description and accompanying
drawings in which preferred structural embodiments incorporating the
principles of the present invention are shown by way of illustrative
example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing an apparatus for explaining Example 1 of
the present invention and a comparative example thereof;
FIG. 2 is a graph showing a change in copper concentration in a
hydrofluoric acid solution as a function of time in Example 1;
FIG. 3 is a sectional view showing an apparatus for explaining Example 2 of
the present invention;
FIG. 4 is a graph showing a change in copper concentration in a solution
from a column as a function of a column circulation time in Example 2;
FIG. 5 is a sectional view showing an apparatus for explaining Example 3 of
the present invention; and
FIG. 6 is a graph showing a change in copper concentration in a solution
from a column as a function of a column circulation time in Example 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described below on the basis of several
preferred embodiments. According to the embodiments, a diluted
hydrofluoric acid solution is used as a hydrofluoric-acid-containing
chemical solution, copper is used as a metal impurity, and
gold-precipitated silicon granules each having a silicon granule surface
on which gold is precipitated are used as silicon granules.
FIG. 1 is a sectional view showing an apparatus for explaining Example 1 of
the present invention and a comparative example thereof.
A copper-containing (1 ppm) 5% diluted hydrofluoric acid solution 4 (100
ml) was poured into each of reaction vessels 2 and 3 placed on a magnetic
stirrer 1, and the reaction vessel 3 was defined as a reaction vessel used
for explaining Example 1 of the present invention.
In the copper-containing (1 ppm) 5% diluted hydrofluoric acid solution in
the reaction vessel 3, nitrogen bubbling was performed by a nitrogen bomb
5 to remove dissolved oxygen from the solution. In the copper-containing
(1 ppm) 5% diluted hydrofluoric acid solution in the reaction vessel 2,
nitrogen bubbling was not performed to compare this comparative example
with Example 1. Gold-precipitated silicon granules 6 (10 g) were added in
each of the solutions in the reaction vessels 2 and 3, and these solutions
were stirred by stirrers 7.
FIG. 2 shows changes in copper concentration in the hydrofluoric acid
solutions with respect to a stirring time. As is apparent from FIG. 2, it
was found that, when oxygen dissolved in the hydrofluoric acid solution
was removed, copper could be stably adsorbed and removed for a long time
until the copper concentration became 0.01 ppb or less. On the other hand,
it was found that, when nitrogen bubbling was not performed, copper
temporarily adsorbed on the gold-precipitated silicon granules 6 was
dissolved in the hydrofluoric acid solution again so as to increase the
copper concentration.
FIG. 3 is a sectional view showing an apparatus used in Example 2 of the
present invention. This apparatus is constituted by a solution tank in
which a copper-containing (1 ppm) 5% diluted hydrofluoric acid solution 8
is poured, a pump 9, a nitrogen bomb 10, and a column 12 filled with
gold-precipitated silicon granules 11 (20 g). Nitrogen bubbling was
performed in the copper-containing (1 ppm) 5% diluted hydrofluoric acid
solution 8 by the nitrogen bomb 10 to remove the dissolved oxygen from the
solution, and the solution was caused to pass through the column 12 at a
flow rate of 12 ml/min so as to adsorb and remove copper from the solution
by the gold-precipitated silicon granules 11.
FIG. 4 shows a change in copper concentration in a solution 13 from the
column 12 as a function of a column circulation time. In comparison, FIG.
4 also shows a result obtained when nitrogen bubbling is not performed. It
was found that, when the nitrogen bubbling was not performed, the copper
concentration in the solution from the column increased to elute copper
adsorbed on the gold-precipitated silicon granules 11. On the other hand,
and it was found that, when the nitrogen bubbling was performed, no copper
was detected in the solution from the column, and copper could be stably
adsorbed and removed for a long time until the copper concentration became
a concentration of 0.01 pph or less.
As Example 3, a case wherein a metal impurity removing apparatus is
incorporated in an oxide film etching apparatus will be described below
with reference to FIG. 5.
The oxide film etching apparatus incorporating with the metal impurity
removing apparatus is constituted by a chemical tank 14 in which a 5%
diluted hydrofluoric acid solution 20 is stored, a pump 15, a nitrogen
bubbling apparatus 16, a column 18 filled with gold-precipitated silicon
granules 17, and a filter 19 for removing fine particles from the
hydrofluoric acid solution and silicon granules produced by the
silicon-granule-filled column 18.
The nitrogen bubbling apparatus 16 is constituted by a nitrogen bomb 26, a
bubbling tank 27, and a pump 28. Oxygen dissolved in the hydrofluoric acid
solution 20 is removed from the hydrofluoric acid solution 20 by
performing nitrogen bubbling.
The chemical tank 14 has a copper concentration monitor 21 and a pump 22
and receives a copper-containing (1,000 ppm) 5% diluted hydrofluoric acid
solution 23 and a 5% diluted hydrofluoric acid solution 24 through the
pump 22. While a copper concentration in the chemical tank 14 is
monitored, the copper-containing (1,000 ppm) 5% diluted hydrofluoric acid
solution 23 or the 5% diluted hydrofluoric acid solution 24 is added to
the 5% diluted hydrofluoric acid solution 20 in the chemical tank 14. In
this case, copper contamination having an arbitrary concentration can be
experimentally simulated, and the copper concentration in the chemical
tank 14 can be kept constant. Therefore, the performance of a metal
impurity removing apparatus 25 according to the present invention can be
evaluated and confirmed.
According to the above method, the 5% diluted hydrofluoric acid solution 20
having a copper concentration adjusted to 1 ppm was supplied from the
chemical tank 14 at a flow rate of 21 ml/min, and circulated through the
bubbling tank 27, a column 18 filled with the gold-precipitated silicon
granules 17 (400 g), the filter 19, and the chemical tank 14 in this
order. A small amount of solution flowing from the column was extracted
from a valve 29, and the copper concentration of this solution was
quantitatively analyzed. FIG. 6 shows a change in copper concentration in
the solution from the column as a function of a circulation time. In
comparison, FIG. 6 also shows a result obtained when nitrogen bubbling is
not performed. It was found that, when the nitrogen bubbling was not
performed, the copper concentration in the solution from the column
increased, and copper was eluted from the column.
It was found that, when nitrogen bubbling was performed, no copper was
detected, and copper could be stably adsorbed and removed until the copper
concentration became a concentration of 0.01 ppb or less.
As a method of removing dissolved oxygen, in addition to the nitrogen
bubbling, a method such as a vacuum deaeration method, a film deaeration
method, or a reduction method using a catalytic resin is known. The same
effect as the effect of suppressing elution of a metal impurity adsorbed
on silicon granules when nitrogen bubbling is performed can be expected.
In addition, when the metal impurity removing apparatus used in the present
invention is used to be connected to a chemical circulation line for
washing a silicon wafer or etching an oxide film, a
hydrofluoric-acid-containing chemical solution can be circulated and
regenerated, and the service life-of the chemical solution can be
considerably prolonged.
Note that in Examples 2 and 3, although gold is used as a metal
precipitated on silicon granule surfaces such that a metal impurity in a
hydrofluoric-acid-containing chemical solution is highly efficiently
adsorbed and removed, a metal which is rarely oxidized may be used in
place of gold.
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