U.S. patent application number 12/313875 was filed with the patent office on 2009-05-28 for solution for processing of metal replacement with metal aluminum or aluminum alloy and method for surface processing using such solution.
This patent application is currently assigned to C.Uyemura & Co., Ltd.. Invention is credited to Toshiaki Shibata, Hiroki Uchida, Kazuki Yoshikawa.
Application Number | 20090133782 12/313875 |
Document ID | / |
Family ID | 40668706 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090133782 |
Kind Code |
A1 |
Uchida; Hiroki ; et
al. |
May 28, 2009 |
Solution for processing of metal replacement with metal aluminum or
aluminum alloy and method for surface processing using such
solution
Abstract
There is disclosed a processing solution for metal replacement
for metal aluminum or an aluminum alloy. The processing solution is
used for surface processing of an underlying aluminum material. The
processing solution for metal replacement removes an oxide film on
the underlying aluminum material and suppresses corrosive attack to
it to allow a plating film having high smoothness and good plating
appearance to be deposited on the underlying aluminum material. The
processing solution for metal replacement at least includes a metal
salt capable of being replaced with aluminum, and an alkaline
compound. A quaternary ammonium hydroxide is contained in the
processing solution for metal replacement as the alkaline
compound.
Inventors: |
Uchida; Hiroki; (Osaka,
JP) ; Yoshikawa; Kazuki; (Osaka, JP) ;
Shibata; Toshiaki; (Osaka, JP) |
Correspondence
Address: |
Jonathan O. Owens;HAVERSTOCK & OWENS LLP
162 North Wolfe Road
Sunnyvale
CA
94086
US
|
Assignee: |
C.Uyemura & Co., Ltd.
|
Family ID: |
40668706 |
Appl. No.: |
12/313875 |
Filed: |
November 24, 2008 |
Current U.S.
Class: |
148/275 ;
148/22 |
Current CPC
Class: |
H01L 21/321 20130101;
C23C 18/54 20130101; C23C 18/1601 20130101; C23C 22/83 20130101;
H01L 21/32134 20130101; C23C 22/66 20130101 |
Class at
Publication: |
148/275 ;
148/22 |
International
Class: |
C23C 22/56 20060101
C23C022/56 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 26, 2007 |
JP |
P2007-304514 |
Claims
1. A processing solution for metal replacement, for use on metal
aluminum or an aluminum alloy, the processing solution at least
containing a metal salt capable of being replaced for aluminum, and
an alkaline compound, wherein the alkaline compound is a quaternary
ammonium hydroxide.
2. The processing solution for metal replacement according to claim
1 wherein the quaternary ammonium hydroxide contains an alkyl group
and/or a hydroxyalkyl group.
3. The processing solution for metal replacement according to claim
1 wherein the quaternary ammonium hydroxide is tetramethylammonium
hydroxide and/or trimethyl (2-hydroxyethyl) ammonium hydroxide.
4. The processing solution for metal replacement according to claim
1 wherein the metal is zinc.
5. A method for surface processing of metal aluminum or an aluminum
alloy comprising the steps of: contacting a material for
processing, containing metal aluminum or an aluminum alloy on a
surface thereof, with the processing solution for metal replacement
according to claim 1; removing an oxide film on the metal aluminum
or aluminum alloy; and performing the processing of metal
replacement of replacing the aluminum with the metal contained in
the processing solution for metal replacement to form a film of the
metal.
6. The method for surface processing of metal aluminum or an
aluminum alloy according to claim 5 in which, after forming the
film of the metal, a plating film is deposited on the surface of
the film of the metal.
7. The method for surface processing of metal aluminum or an
aluminum alloy according to claim 5 in which, after forming the
film of the metal, the material for processing is immersed in an
acidic solution having an oxidative effect to remove the film of
the metal.
8. The method for surface processing of metal aluminum or an
aluminum alloy according to claim 7 wherein, after removing the
film of the metal by the acidic solution having the oxidative
effect, a further processing of metal replacement is carried out to
deposit another film of the metal on the same surface of the
material for processing.
9. The method for surface processing of metal aluminum or an
aluminum alloy according to claim 5 wherein the metal is zinc.
10. A method for surface processing of metal aluminum or an
aluminum alloy comprising the steps of: contacting a material for
processing, containing metal aluminum or an aluminum alloy on a
surface thereof, with the processing solution for metal replacement
according to claim 2; removing an oxide film on the metal aluminum
or aluminum alloy; and performing the processing fo metal
replacement of replacing the aluminum with the metal contained in
the processing solution for metal replacement to form a film of the
metal.
11. A method for surface processing of metal aluminum or an
aluminum alloy comprising the steps of: contacting a material for
processing, containing metal aluminum or an aluminum alloy on a
surface thereof, with the processing solution for metal replacement
according to claim 3; removing an oxide film on the metal aluminum
or aluminum alloy; and performing the processing fo metal
replacement of replacing the aluminum with the metal contained in
the processing solution for metal replacement to form a film of the
metal.
12. A method for surface processing of metal aluminum or an
aluminum alloy comprising the steps of: contacting a material for
processing, containing metal aluminum or an aluminum alloy on a
surface thereof, with the processing solution for metal replacement
according to claim 4; removing an oxide film on the metal aluminum
or aluminum alloy; and performing the processing fo metal
replacement of replacing the aluminum with the metal contained in
the processing solution for metal replacement to form a film of the
metal.
Description
BACKUNDERLYING MATERIAL OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a solution for metal replacement,
for use on metal aluminum or an aluminum alloy, which solution may
be effective in particular for pre-processing in forming a bump on
a wafer, for instance. This invention also relates to a method for
processing the surface of metal aluminum or the aluminum alloy
using the solution for metal replacement.
[0003] 2. Description of Related Art
[0004] Various methods have so far been used for forming an
under-bump metallization or a bump on a silicon wafer. Among these,
there is known a method consisting in subjecting a aluminum thin
film electrode formed by patterning on the silicon wafer, to a
processing of zinc replacement for thereby forming a zinc film, and
subsequently forming a bump on the so formed zinc film by
electroless nickel plating. There is also known a method consisting
in subjecting the aluminum thin film electrode to palladium
processing, in place of the processing of zinc replacement, and
subsequently forming a bump by electroless nickel plating. There is
further known a method consisting in subjecting the surface of the
aluminum thin film electrode replace with nickel directly and
subsequently forming a bump by self-catalyzed electroless nickel
plating.
[0005] No matter which of these methods is used to form an
under-bump metallization or a bump, the processing of degreasing
the aluminum thin film electrode or the processing of removing an
aluminum oxide film on the aluminum thin film electrode is
generally carried out by way of pre-processing. In such case, if
the aluminum oxide film is an oxide film of an extremely thin film
thickness, such as is generated by, for example, immersion in
nitric acid, it may directly be subjected to plating by way of
post-processing, without raising any particular problems. However,
in case of a rigid aluminum oxide film produced in a certain
production process, such as polishing or annealing, is left on the
surface, it may sometimes occur that a plating film, generated in a
subsequent process, is insufficient in tight adhesion performance,
or the plating film may be perforated, with the plating film being
insufficient in adhesion performance. It is therefore strongly
desired that the rigid aluminum oxide film is completely removed
beforehand, and it is also strongly desired that the plating film
from the surface processing process is not liable to be perforated
and exhibits high smoothness.
[0006] An example of such processing solution for the
above-described surface processing is a processing solution as
disclosed in, for example, the Japanese Laid-Open Patent
Publication 2001-316831. This processing solution contains a zinc
compound, an alkali hydroxide, an iron salt, and a chelating agent,
such as gluconic acid, for complexing iron ions. In this processing
solution, sodium hydroxide, potassium hydroxide or lithium
hydroxide is used, either singly or in combination, as the
aforementioned alkali hydroxide. Surface processing methods,
exemplified by a double zincate method, are carried out, using this
processing solution, to remove the oxide film to prevent pitting
corrosion to provide a plating film having high adhesion
performance. [0007] [Patent Publication 1] Japanese Laid-Open
Patent Publication 2001-316831
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0008] The conventional processing solution, described above,
contains an alkali hydroxide exhibiting strong alkalinity. Hence,
the underlying aluminum material is excessively etched due to
strong attack of metal aluminum or the aluminum alloy by this
alkaline compound, with the result that numerous wedge-shaped
recesses are formed in the wafer surface. In the subsequent plating
film forming process, the nickel plating, for example, is intruded
into these recesses to form a plating film which is insufficient in
surface smoothness, thus detracting from electrical conductivity
and appearance.
[0009] In light of the foregoing, it is an object of the present
invention to provide a solution for metal replacement, for use on
metal aluminum or an aluminum alloy, and a method for processing
the surface of metal aluminum or the aluminum alloy using the
solution for metal replacement. The solution for metal replacement
is to be usable for removing an oxide film on metal aluminum or an
aluminum alloy to improve the adhesion performance of the plating
film. In addition, the solution for metal replacement is to be low
in corrosive attack to the underlying aluminum material, and also
is to form a plating film exhibiting high smoothness and good
plating appearance.
[Means for Solving the Problem]
[0010] The present inventors have conducted eager searches to solve
the above-described problem, and found that, by using a quaternary
ammonium hydroxide as the alkaline compound, contained in the
solution for metal replacement in addition to the salt of metal
capable to replace aluminum, it is possible to suppress corrosive
attack to metal aluminum or to an aluminum alloy.
[0011] The present inventors have also found that, by contacting a
material for processing, containing metal aluminum or an aluminum
alloy on its surface, with the processing solution for metal
replacement containing a quaternary ammonium hydroxide as an
alkaline compound, removing an oxide film on metal aluminum or the
aluminum alloy and performing the processing of metal replacement
of replacing the aluminum with the metal, contained in the
processing solution, for aluminum, thereby depositing a film of the
metal, it is possible to remove an oxide film adhered to the
underlying aluminum material as well as to process the surface of
the underlying aluminum material in readiness for depositing a
plating film having good plating appearance.
[0012] The present invention provides a processing solution for
metal replacement, for use on metal aluminum or an aluminum alloy,
containing at least a salt of metal capable to replace aluminum,
and an alkaline compound, in which, according to the present
invention, the alkaline compound is a quaternary ammonium
hydroxide.
[0013] The present invention also provides a method for surface
processing of metal aluminum or an aluminum alloy. The method
comprises the step of contacting a material for processing,
containing metal aluminum or an aluminum alloy on its surface, with
a processing solution for metal replacement containing a quaternary
ammonium hydroxide as an alkaline compound, removing an oxide film
on metal aluminum or the aluminum alloy, and performing the
processing of metal replacement of replacing a aluminum with a
metal contained in the processing solution for metal replacement to
form a film of the metal.
[0014] With the processing solution for metal replacement for metal
aluminum or the aluminum alloy, according to the present invention,
the alkaline compound contained in the solution is the quaternary
ammonium hydroxide. It is thereby possible to suppress corrosive
attack to the underlying aluminum material to suppress
cracking.
[0015] Furthermore, with the method for surface processing of metal
aluminum or an aluminum alloy with the use of the processing
solution for metal replacement according to the present invention,
it is possible to perform pre-plating surface processing of
removing an oxide film adhered to the underlying aluminum material
and for depositing a plating film having good appearance and high
smoothness.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] The solution for metal replacement for metal aluminum or an
aluminum alloy, and the method for processing the surface of metal
aluminum or the aluminum alloy using the solution for metal
replacement, according to the present invention, will now be
described in detail.
[0017] The processing solution for metal replacement according to a
preferred embodiment of the present invention at least contains a
metal salt that may be replaced with aluminum, and an alkaline
compound. The processing solution contains a quaternary ammonium
hydroxide as an alkaline compound
[0018] The metal that forms the metal salt contained in the
processing solution for metal replacement according to the present
embodiment is a metal that may be replaced for aluminum. Such metal
exhibits the ionization tendency lower than aluminum, and may be
enumerated by zinc, palladium, nickel, iron, cobalt, tin, zinc,
lead, copper, silver, gold and platinum. The metal salts used may
be oxides, sulfates, chlorides or gluconates of these metals.
Specifically, zinc oxide, zinc sulfate, zinc chloride or zinc
gluconate may be used. One or more of these metal salts may be used
in desired proportions for the processing solution for metal
replacement of the present embodiment.
[0019] Although there is no particular limitation to the
concentration of the metal salt, the metal amount is normally not
less than 1 ppm and preferably not less than 10 ppm. An upper limit
of the concentration of the metal salt is normally not higher than
100,000 ppm and preferably not higher than 20,000 ppm. If the metal
salt concentration is too low, the metal may not be sufficiently
replaced with the underlying aluminum material, or it may become
necessary to supply additional amounts of metal salts. On the other
hand, if the metal salt concentration is too high, with metal
aluminum or the aluminum alloy being an electrode patterned on a
wafer, there are cases where a part other than underlying metal
aluminum or aluminum alloy material is eroded, or where the metal
salt seeps to a site other than underlying aluminum or aluminum
alloy material so as to be precipitated thereon.
[0020] The processing solution for metal replacement of the present
embodiment contains quaternary ammonium hydroxides as an alkaline
compound. Although there is no limitation to the quaternary
ammonium hydroxides used, these may, for example, be quaternary
ammonium hydroxides containing alkyl and/or hydroxyalkyl groups
with 1 to 4 carbon atoms, such as tetramethylammonium hydroxide
(TMAH), tetraethylammonium hydroxide, tetrapropylammonium
hydroxide, tetrabutylammonium hydroxide, trimethyl (2-hydroxyethyl)
ammonium hydroxide (choline) or triethyl (2-hydroxyethyl) ammonium
hydroxide. From the perspective of the oxide film removing effect,
stability and cost, tetramethylammonium hydroxide (TMAH) and
trimethyl (2-hydroxyethyl) ammonium hydroxide (choline) are most
preferred. These alkaline compounds may be used either alone or in
combination. If the alkaline compounds are used in combination,
these may be used in desired optional proportions.
[0021] Although there is no particular limitation to the
concentration of the quaternary ammonium hydroxides, as the
alkaline compounds, it is on the order of 100 g/L to 1,000 g/L. It
may optionally be desirable to change the concentration depending
on whether only one type of the quaternary ammonium hydroxides is
used or at least two types of the quaternary ammonium hydroxides
are used in combination. With the processing solution for metal
replacement for use on metal aluminum or an aluminum alloy of the
present embodiment, containing the quaternary ammonium hydroxide
instead of an alkali hydroxide, as alkaline compound, it is
possible to prevent corrosive attack to the underlying aluminum
material to render it possible to carry out optimum pre-processing
in a manner free from cracking.
[0022] The processing solution for metal replacement may further
contain an iron salt or salts. By the processing solution for metal
replacement containing an iron salt or salts, it is possible to
form a dense film by metal replacement, such as a dense zinc film,
on metal aluminum or on an aluminum alloy. Examples of the iron
salts include iron chloride, iron sulfate, iron nitrate and iron
gluconate, only by way of illustration. These iron salts may be
used either alone or in combination. If these iron salts are used
in combination, these may be used in desired optional proportions.
The concentration of the iron salts may be in a range from 0.1 to
100 mmol/L and preferably in a range from 0.5 to 50 mmol/L.
[0023] The processing solution for metal replacement may further
contain a complexing agent. The complexing agent, contained in the
processing solution for metal replacement, may form a complex with
iron ions in case the aforementioned iron salt is contained in the
solution. With the complexing agent thus forming a complex with
iron ions, it is possible to suppress pitting corrosion of the
underlying aluminum material by the iron ions. The complexing agent
may be general complexing agents or chelating agents, and may, for
example, be hydroxycarboxylic acid, such as glycolic acid, lactic
acid, malic acid, tartaric acid, citric acid, gluconic acid or
glucoheptonic acid, and salts thereof, aminocarboxylic acids, such
as glycine, aminodicarboxylic acid, nitrilotriacetic acid, EDTA,
hydroxyethyl ethyleneamine triacetic acid,
diethylenetriaminepentaacetic acid or polyaminopolycarboxylic acid,
and salts thereof, phosphorous acid-based chelating agents, such as
HEDP, aminotrimethylphosphonic acid or ethylenediamine tetramethyl
phosphonic acid, and salts thereof, amine-based chelating agents,
such as ethylenediamine, diethylenetriamine or
triethylenetetramine, and salts thereof The concentration of the
complexing agent may, for example, be in a range from 0.5 to 100
g/L and preferably in a range from 1 to 50 g /L in case of using
tartaric acid as the complexing agent.
[0024] The processing solution for metal replacement may further
contain sodium nitrate, as necessary. Sodium nitrate contained in
the processing solution for metal replacement, acts in conjunction
with the iron ions to improve characteristics of the film of the
metal. Specifically, the concentration of the complexing agent may
be in a range from approximately 0.01 to 10 g/L and preferably in a
range from approximately 1 to 5 g/L.
[0025] In addition, surface active agents may be contained in the
processing solution for metal replacement of the present embodiment
with a view to improving the capacity of removing oxide films and
providing for water wettability. The surface active agents may be
enumerated by, for example, nonionic surfactants, such as
polyethylene glycol or polyoxyethylene oxypropylene block
copolymers, anionic surfactants, such as dodecylbenzene sodium
sulfonate, polyoxyethylene laurylether sodium sulfate or
polyoxyethylene nonyl phenylether sodium sulfonate, and cationic
surfactants. These surfactants may be used either alone or in
combination. If these surfactants are used in combination, these
may be used in desired optional proportions. The concentration of
the surfactants may be in a range from 1 to 10,000 ppm, preferably
in a range from 5 to 5,000 ppm and more preferably in a range from
10 to 2,000 ppm.
[0026] Preferably, the processing solution for metal replacement of
the embodiment described above is prepared as an aqueous solution
from the perspective of operational safety. However, it is also
possible to use other solvents, such as methanol, ethanol, ethylene
glycol, diethylene glycol, triethylene glycol, glycerin, IPA or
mixed solvent with the water. These solvents may be used either
alone or in combination. If the solutions are used in combination,
these may be used in desired optional proportions.
[0027] The method for processing the surface of metal aluminum or
the aluminum alloy using the processing solution for metal
replacement of the embodiment of the present invention will now be
described in detail.
[0028] With the present surface processing method, a material for
processing, at least including metal aluminum or an aluminum alloy
on its surface, is brought into contact with the aforementioned
processing solution for metal replacement. This removes an oxide
film adhered to metal aluminum or to the aluminum alloy. Aluminum
is replaced with a metal contained in the processing solution for
metal replacement is replaced with aluminum to form a film of the
metal on the surface of the material for processing. This surface
processing method is a pre-processing method to be carried out in
advance of processing of forming a plating film, such as a nickel
plating film or a palladium plating film, on the material for
processing. More specifically, the processing solution for metal
replacement is brought into contact with the material for
processing, having at least metal aluminum or the aluminum alloy on
its surface, to remove the oxide film adhered to the surface of the
material to increase the adhering performance of, for example, a
nickel plating film in the course of the subsequent process.
[0029] The alkaline compound, contained in the processing solution
for metal replacement of the present embodiment, is the quaternary
ammonium hydroxide, as described above. It is thus possible with
the present processing solution for metal replacement to prevent
excessive attack to the underlying aluminum material by an alkali
hydroxide, as compared to the case of using the conventional
processing solution for metal replacement containing the alkali
hydroxide, such as sodium hydroxide or potassium hydroxide. In this
manner, it is possible to generate a plating film of high
smoothness and good surface appearance. In addition, from the
perspective of safety, the present processing solution for metal
replacement may be handled more easily than the conventional
processing solution for metal replacement which needed meticulous
attention in handling. Hence, disposal of drainage water may be
done in a manner that makes much of environment protection.
[0030] A method for surface processing of metal aluminum or an
aluminum alloy, more specifically, a method for surface processing
of metal aluminum or an aluminum alloy by a processing solution for
metal replacement that contains zinc as a metal species capable of
being replaced with aluminum, that is, a zincate processing
solution, is now described in detail. The zincate processing
solution, used here, is an alkaline solution containing zinc ions
and also containing a quaternary ammonium hydroxide as an alkaline
agent. With the zincate processing solution, an oxide film adhered
to the material for processing, including metal aluminum or an
aluminum alloy at least on its surface, is removed, and particles
of zinc are precipitated on the surface of the material for
processing as a result of the replacement reaction brought forth
due to the difference between the electrode potential of zinc and
that of aluminum. The material for processing is sometimes referred
to below as an aluminum substrate. This method for surface
processing, a processing carried out prior to plating on the
aluminum substrate with the use of the zincate processing solution,
is carried out in general as a double zincate process. This double
zincate process includes (1) first zinc replacement processing of
the aluminum substrate, (2) acid cleaning and (3) second zinc
replacement processing. The double zincate process is followed by
(4) plating processing, such as electroless nickel plating. This
double zincate processing is also explained in the following
detailed explanation of the surface processing method. It should be
noted that the surface processing method that uses the
above-described processing solution for metal replacement is not
limited to this double zincate processing, and single or triple
zincate processing may also be used within the scope of the present
invention.
(1) First zinc Replacement Processing
[0031] The aluminum substrate, as the material of plating
processing, includes metal aluminum or an aluminum alloy at least
on its surface, and may be prepared by coating an aluminum layer on
a non-aluminum material, such as a silicon plate, by any suitable
method, such as sputtering. The aluminum layer may be coated on all
or part of the non-aluminum material, and may be of a thickness
which is ordinarily not less than 0.5 .mu.m and preferably not less
than 1 .mu.m. In preparing the aluminum substrate, a method of
vacuum evaporation or an ion plating method may also be used in
place of sputtering. It should be noted that metal aluminum or an
aluminum alloy, existing at least on the surface of the aluminum
substrate, used herein, may not only be pure industrial aluminum
A1100 according to JIS standard, or a highly corrosion resistant
alloy, but may also be a highly corrosive alloy. For example, blank
sheets, a rolled material or a casting may be used satisfactorily.
There is no limitation to the shape of metal aluminum or an
aluminum alloy, such that materials of variable shapes, such as
those of a plate-like or rectangular shape, may be used. There is
also no limitation to the composition of metal aluminum or an
aluminum alloy. That is, the method for surface processing,
employing the processing solution for metal replacement of the
present embodiment, may be used for an underlying aluminum material
of, for example, the Al--Si or Al--Cu composition.
[0032] Initially, this aluminum substrate is subjected to cleaning,
such as degreasing, by any suitable conventional method. It is then
washed with water and subjected to etching, as conventionally,
using an alkali or an acid. Specifically, the degreasing processing
is carried out by immersion in a degreasing solution for aluminum,
or by electrolytic degreasing. On the other hand, the etching
processing is carried out by immersing the aluminum substrate in an
alkaline solution of approximately 1 to 10% or an acidic solution
of approximately 1 to 20%, at a liquid temperature of approximately
40 to 70.degree. C. for about 1 to 15 minutes.
[0033] The so processing aluminum substrate is immersed in an
acidic solution for a preset time for removing etching residues
(smuts) by an alkali or an acid. More specifically, the aluminum
substrate, etched as described above, is immersed in an aqueous
solution of nitric acid, with a concentration of nitric acid of
approximately 200 to 700 ml/L and preferably approximately 450 to
550 ml/L, for approximately 30 seconds to 2 minutes, thereby
removing the smuts.
[0034] The aluminum substrate, deputed as described above, is then
washed with water and immersed in a zincate processing solution
(processing solution for metal replacement) by way of performing
the first processing of zinc replacement. This zincate processing
solution is an alkaline zincic acid solution containing a
quaternary ammonium hydroxide. Specifically, the aluminum substrate
is immersed in the zincate processing solution including the above
composition with a liquid temperature of 10 to 50.degree. C. and
preferably 15 to 30.degree. C. If the temperature of the zincate
processing solution is not less than 10.degree. C., the replacement
reaction is not excessively retarded such that the zinc film free
from surface irregularities may be formed. If the temperature of
the zincate processing solution is not higher than 50.degree. C.,
the replacement reaction is not excessively promoted such that it
is possible to prevent the zinc film from becoming roughed. The
aforementioned temperature range is therefore desirable.
[0035] There is further no limitation to the time of immersion such
that it may be optionally set by taking into account the thickness
of the film of aluminum oxide to be removed. For example, the time
of immersion may ordinarily be not less than five seconds and
preferably not less than ten seconds, with the upper limit being
five minutes or less. If the time of immersion is too short, the
process of replacement is retarded and the oxide film may be
removed only insufficiently, whereas, if it is too long, there is
the risk that the processing solution is intruded via small holes
in the replaced metal layer to permit metal aluminum or an aluminum
alloy to become dissolved and discharged. These points need to be
taken into account in setting the time of immersion.
[0036] By immersing the aluminum substrate in the zincate
processing solution, in this manner, it is possible to remove the
oxide film adhered to the substrate.
(2) Processing of Acid Cleaning
[0037] The aluminum substrate, thus immersed in the zincate
processing solution, is rinsed with cold water, and is then
immersed in an acidic solution having an oxidative effect, such as
an aqueous solution of nitric acid, an acidic solution not having
an oxidative effect, such as hydrochloric acid or sulfuric acid, or
in an aqueous solution prepared by adding hydrogen peroxide or
sodium persulfate, having an oxidative effect, to the acidic
solution, such as hydrochloric acid or sulfuric acid. This peels
off and removes the film of the zinc. If an aqueous solution of
nitric acid is used as the acidic solution, such solution having
the concentration of nitric acid of approximately 350 to 600 ml/L
and preferably approximately 450 to 550 ml/L may be used. The
aqueous solution of nitric acid may contain iron ions, as also
disclosed in the U.S. Pat. No. 5,141,778. The aqueous solution of
nitric acid, having a liquid temperature of approximately 15 to
30.degree. C., for example, may be used, and the aluminum substrate
may be immersed therein for approximately 30 to 60 seconds to
remove the film of zinc. During this processing, the aluminum
substrate may be standstill or the solution may be stirred.
(3) Second Zinc Replacement Processing
[0038] After the film of the zinc by the first zinc replacement is
removed by immersion in the acidic solution, this aluminum
substrate is washed with water, and the second zinc replacement
processing is carried out. For the second processing of zinc
replacement, the zincate processing solution of the same
composition as that of the first processing of zinc replacement may
be used. Although the conditions for processing, such as processing
time or processing temperature, may be the same as those of the
first processing of zinc replacement, the processing time for the
second processing of zinc replacement may be longer by about 1 to
60 seconds than that for the first processing of zinc replacement.
Of course, the composition of the processing solution for the
second processing of zinc replacement may be different from that of
the processing solution for the first processing of zinc
replacement. If it is desired to reduce the film thickness of the
film of the zinc by the second processing of zinc replacement, the
composition of the processing solution may desirably be changed
such as by decreasing the zinc ion concentration.
[0039] Thus, the aluminum substrate is subjected to the first
processing of zinc replacement, using the zincate processing
solution, and then immersed in the acidic solution, such as a
nitric acid solution to remove the film of the zinc. The second
processing of zinc replacement is then carried out to remove the
oxide film adhered to the surface of the substrate. A film of the
zinc is further deposited to activate the surface of the substrate
to enable an optimum plating film to be deposited on the material
for processing.
[0040] Although the processing by the double zincate method has
been described above, it is also possible to form plating film such
as electroless nickel plating after having performed the first
processing of zinc replacement, or to form plating film after
having removed the film of the zinc. The latter alternative is
preferred from the perspective of positively removing the oxide
film and from the perspective of improving the density of the
plating film.
(4) Processing of Plating
[0041] This processing of plating is carried out by subjecting the
zincate-processed aluminum substrate to electroless plating or
electrolytic plating. For example, plating is performed to an
ultimate film thickness, using a proper metal plating solution,
such as a plating bath of electroless nickel plating, electroless
palladium plating or a copper plating.
[0042] Specifically, electroless nickel plating is now described as
an example. With the electroless nickel plating solution, nickel
ions are afforded by the use of water-soluble nickel salts, such as
nickel sulfate, nickel chloride or nickel acetate, with the
concentration of nickel ions being approximately 1 to 10 g/L. The
electroless nickel plating solution contains a complexing agent for
nickel, such as ammonium salts or amine salts, or organic acid
salts, such as acetates, succinates or citrates, in a concentration
range of approximately 20 to 80 g/L. The electroless nickel plating
solution also contains hypophosphous acid or hypophosphites, such
as sodium hypophosphite, with the concentration range from
approximately 20 to 40 g/L, as a reducing agent. With the plating
solution containing e.g. hypophosphites, for example, as a reducing
agent, it is possible to elevate the stability of the plating
solution to render it possible to deposit an inexpensive
nickel-phosphorus alloy film. The plating solution composed of
these compounds is prepared so that the pH value will be
approximately 4 to 7. Plating with the plating solution is carried
out by immersing the aluminum substrate in the plating solution for
approximately 15 seconds to 120 minutes, as the plating solution is
adjusted to a temperature of 80 to 95.degree. C. The thickness of
the plating film may be changed by properly changing the plating
time duration.
[0043] The plating is not limited to the electroless plating and
may also be electrolytic plating. Plating metals may also be Cu or
Au, in place of those shown above. Plating may also be carried out
in such a way that two or more layers will be formed by, for
example, immersion plating method.
[0044] It should be noted that the processing conditions or
concentration setting in the zincate processing or plating
processing are not limited to those shown above, and may properly
be changed depending on e.g. the thickness of the film being
formed.
[0045] If, in carrying out the surface processing, described above,
the processing solution for metal replacement of the present
embodiment, in which the quaternary ammonium hydroxide is contained
as an alkaline compound in place of an alkali hydroxide, is used,
it is possible to remove the aluminum oxide film, adhered to the
underlying aluminum material, as well as to reduce corrosive attack
to the surface of the aluminum substrate as the material of
processing. Hence, a plating film with high smoothness and good
appearance may be generated by subsequent plating processing
without excessive etching.
[0046] It should be noted that the conventional processing solution
for metal replacement, containing an alkali hydroxide, such as
sodium hydroxide, needs to be handled only with meticulous
attention. Conversely, the processing solution for metal
replacement of the present embodiment, in which the alkali
hydroxide is not contained and the quaternary ammonium hydroxide is
contained as the alkaline compound, can be handled with ease.
Furthermore, the processing solution for metal replacement of the
present embodiment can be discharged with ease as compared to the
conventional processing solution for metal replacement which can be
discharged extremely onerously. Thus, with the processing solution
for metal replacement of the present embodiment, it may be said
that the aspect of environmental protection has duly been taken
into account.
[0047] It should also be noted that, in surface processing with the
conventional processing solution for metal replacement, containing
an alkali hydroxide, changes in temperature severely affect the
process of removal of the oxide film or deposition of the film.
With the use of the present processing solution for metal
replacement, containing the quaternary ammonium hydroxide as the
alkaline agent, the processing of metal replacement may be carried
out at an ambient temperature, that is, without the necessity of
using a cooling device or the like, thereby reducing equipment cost
and shortening the processing time.
[0048] The present invention in not limited to the above-described
embodiment such that design changes which do not depart from the
purport of the present invention may be made within the scope of
the invention.
[0049] Certain specified Examples of the present invention and a
Comparative Example are now described hereinbelow.
EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLE
[0050] A silicon plate, coated with an aluminum layer to a
thickness of 5 .mu.m by a sputtering method, was immersed with
cleaning and etched, as conventionally, for use as a material for
plating processing.
[0051] This material for plating processing was immersed for one
minute in an aqueous solution of nitric acid, with a concentration
of 500 ml/L, for smut removal. The so processed material for
plating was then immersed in an alkaline zincic acid solution,
having the composition shown in Table 1, shown below, in order to
carry out alkaline zinc replacement processing. The so processed
material was then immersed in an aqueous solution of nitric acid,
with the concentration of 500 ml/L, at 25.degree. C. for one
minute, in order to peel off and remove the film of the zinc. The
so processed material was then again immersed in an alkaline zincic
acid solution, prepared in accordance with the composition, shown
in Table 1 below, in order to carry out the processing of alkali
replacement in similar manner (double zincate method).
[0052] The resulting material was then subjected to nickel plating
by the electroless plating method to a film thickness of 0.5 .mu.m,
and then to gold plating on the so formed nickel plating by the
immersion plating method to a thickness of 0.05 .mu.m.
[0053] The resulting plated material was visually checked as to
appearance, and evaluation was made of the state of the plating
film. Since the electroless nickel plating film was formed to a
thin thickness and the gold plating film was formed thereon,
neither nickel nor gold was not precipitated and a hole (colored in
white) was left if the oxide film was not removed but left. Thus,
the state of the plating film could be evaluated by evaluating the
state of non-adhesion (the left-over state of the oxide film) as
contrasted to the gold color. Further, a cross-section of the
plated material was formed by a focused ion beam (FIB) method to
visually check the etched state of the underlying aluminum
material. The etched state was visually checked by taking advantage
of the fact that, if the underlying aluminum material is etched to
a recessed shape, nickel is intruded into the recess and may thus
be observed as a shaped of spike. The results are shown in the
following Table 1.
TABLE-US-00001 TABLE 1 Compar- concentration of the ative
ingredients in 1 L of Examples Example water 1 2 3 1 zincate alkali
TMAH 900 450 process- (base) (25%) ing (g/L) solution Choline 600
300 (50%) (g/L) NaOH 100 (g/L) zinc oxide ZnO (g/L) 12 12 12 12
gluconic (g/L) 20 20 20 20 acid (50%) iron (g/L) 5 5 5 5 chloride
solution .degree. C. 25 25 25 20 tempera- ture Time sec. 60 60 60
60 presence or absence of absent absent absent present alkali metal
appearance as visually checked good good good good FIB check; Ni
spikes small small small many
[0054] It is seen from the results of the above Table 1, in case
surface processing is carried out using the zincate processing
solution of the present embodiment, the oxide film, adhered to the
underlying aluminum material, has been removed satisfactorily,
while Ni spikes are scarcely noticed, and the state of etching of
the underlying aluminum material is good, thus indicating that the
corrosive attack has been suppressed sufficiently.
[0055] Conversely, with surface processing with the use of a
zincate processing solution, containing sodium hydroxide as an
alkaline agent, there were noticed numerous nickel spikes on the
underlying aluminum material, even though the oxide film adhered to
the underlying aluminum material, could be removed. It was thus
seen that the underlying aluminum material was excessively etched
due to the strong corrosive attack from the alkaline agent.
[0056] It is seen from these results that the zincate processing
solution of the present embodiment has a high oxide film removing
performance and exhibits only low corrosive attack to the
underlying aluminum material.
* * * * *