U.S. patent application number 17/386222 was filed with the patent office on 2022-02-03 for electroless palladium plating bath.
This patent application is currently assigned to C. Uyemura & Co., Ltd.. The applicant listed for this patent is C. Uyemura & Co., Ltd.. Invention is credited to Takuma MAEKAWA, Toshiaki SHIBATA, Katsuhisa TANABE.
Application Number | 20220033973 17/386222 |
Document ID | / |
Family ID | 77358097 |
Filed Date | 2022-02-03 |
United States Patent
Application |
20220033973 |
Kind Code |
A1 |
MAEKAWA; Takuma ; et
al. |
February 3, 2022 |
ELECTROLESS PALLADIUM PLATING BATH
Abstract
A plating bath at least contains a palladium compound, a
reducing agent, a complexing agent, and a stabilizer. The
stabilizer is an organic compound in which a divalent sulfur
compound is bonded to a compound with a heterocyclic structure, and
the organic compound contains neither a thiol group nor a disulfide
bond.
Inventors: |
MAEKAWA; Takuma; (Osaka,
JP) ; TANABE; Katsuhisa; (Osaka, JP) ;
SHIBATA; Toshiaki; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
C. Uyemura & Co., Ltd. |
Osaka |
|
JP |
|
|
Assignee: |
C. Uyemura & Co., Ltd.
|
Family ID: |
77358097 |
Appl. No.: |
17/386222 |
Filed: |
July 27, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 18/1651 20130101;
C23C 18/44 20130101 |
International
Class: |
C23C 18/44 20060101
C23C018/44; C23C 18/16 20060101 C23C018/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2020 |
JP |
2020-127473 |
Claims
1. An electroless palladium plating bath at least comprising: a
palladium compound; a reducing agent; a complexing agent; and a
stabilizer, the stabilizer being an organic compound in which a
divalent sulfur compound is bonded to a compound with a
heterocyclic structure, the organic compound containing neither a
thiol group nor a disulfide bond.
2. The bath of claim 1, wherein the stabilizer has a concentration
ranging from 0.01 mg/L to 10 mg/L.
3. The bath of claim 1, wherein the heterocyclic structure is a
nitrogen-containing heterocyclic structure or a sulfur-containing
heterocyclic structure.
4. The bath of claim 3, wherein the compound with the heterocyclic
structure is at least one selected from the group consisting of
imidazole, imidazolidine, imidazoline, oxadiazole, oxazine,
thiadiazole, thiazole, thiazolidine, tetrazole, triazine, triazole,
piperazine, piperidine, pyrazine, pyrazole, pyrazolidine, pyridine,
pyridazine, pyrimidine, pyrrole, pyrrolidine, benzothiazole,
benzimidazole, isoquinoline, thiophene, tetrahydrothiophene,
pentamethylene sulfide, and a derivative thereof.
5. The bath of claim 1, wherein the divalent sulfur compound is at
least one selected from the group consisting of: thiadiazole,
thiazole, thiazolidine, benzothiazole, thiophene,
tetrahydrothiophene, methanethiol, benzenethiol, pentamethylene
sulfide, dimethyl sulfide, methyl mercaptan, ethyl mercaptan, allyl
mercaptan, thiopropionic acid, thioacetic acid, ethyl methyl
sulfide, 1-propanethiol, 2-propanethiol, 2-aminoethanethiol,
2-mercaptoethanol, 4-mercaptopyridine, dimethyl sulfoxide,
thiazolidine, S-methyl thioacetate, ethyl sulfide, methylpropyl
sulfide, 1-butanethiol, thioglycolic acid, 2-(methylthio)ethanol,
3-mercapto-1-propanol, 2-methylthiazoline, cyclopentanethiol,
2-methyltetrahydrothiophene, pentamethylene sulfide,
thiomorpholine, S-methyl thiopropionate, 3-mercaptopropionic acid,
and a derivative thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Japanese Patent
Application No. 2020-127473 filed on Jul. 28, 2020, the entire
disclosure of which is incorporated by reference herein.
BACKGROUND
[0002] The present disclosure relates to an electroless palladium
plating bath.
[0003] In the field of electronic industry, for example, an
electroless nickel (Ni)/electroless palladium (Pd)/immersion gold
(Au) (ENEPIG) has been used as a surface treatment for mounts or
terminals of printed circuit boards or IC packages. Using this
ENEPIG process, a plating film sequentially including an
electroless nickel plating film, an electroless palladium plating
film, and an immersion gold plating film is obtained.
[0004] The palladium film exhibits a good electrical conductivity
and a high corrosion resistance, and moreover has the function of
avoiding the diffusion of underlying nickel onto the gold surface
due to thermal history. Thus, the palladium film plays an important
role in the ENEPIG process described above.
[0005] In general, the plating bath is required to be highly
stable. In a typical electroless palladium plating bath, an
ethylenediaminetetraacetic acid or a salt thereof, for example, is
used as a stabilizer. However, the plating bath is easily
biodegradable and is therefore not sufficiently stable.
[0006] To address the problem, an electroless palladium plating
bath having an organic compound containing divalent sulfur blended
therein has been proposed. It is described (see, e.g., Japanese
Patent No. 3972158) that the stability of a plating bath improves
by using an organic compound containing divalent sulfur.
SUMMARY
[0007] Here, the typical plating bath described above has improved
stability by blending the organic compound containing divalent
sulfur, but has a problem of lower deposition properties of
palladium onto a nickel plating film.
[0008] There are devices that address the problem by using nickel
plating films containing phosphorus (P) (nickel plating films
having a concentration of phosphorus in the film ranging from 4% to
8%). In recent years, however, with an increase in the guaranteed
operating temperatures of the devices, there is an increasing
demand for a nickel plating film with a lower phosphorus content (a
nickel plating film having a concentration of phosphorus in the
film being less than 4%) that is applicable to a device with a high
guaranteed operating temperature. The typical plating bath
described above has the problem in particular that the deposition
properties of palladium onto a nickel plating film having a lower
phosphorus content is reduced significantly. Development of an
electroless palladium plating bath applicable to nickel plating
films having a lower phosphorus content is therefore desired.
[0009] In view of the above problems, it is an objective of the
present disclosure to provide an electroless palladium plating bath
enabling improvement in stability of a plating bath, while reducing
the deterioration in the deposition properties of palladium onto a
nickel plating film.
[0010] In order to achieve the above objective, an electroless
palladium plating bath according to the present disclosure at least
contains: a palladium compound; a reducing agent; a complexing
agent; and a stabilizer. The stabilizer is an organic compound in
which a divalent sulfur compound is bonded to a compound with a
heterocyclic structure, and the organic compounds contains neither
a thiol group nor a disulfide bond.
[0011] The present disclosure provides a plating bath with improved
stability, while reducing the deterioration in the deposition
properties of palladium onto a nickel plating film.
DETAILED DESCRIPTION
[0012] Now, an electroless palladium plating bath according to the
present disclosure will be described.
<Electroless Palladium Plating Bath>
[0013] The electroless palladium plating bath according to the
present disclosure contains a palladium compound, a reducing agent,
a complexing agent, and a stabilizer.
(Palladium Compound)
[0014] The palladium compound is a palladium ion source for
palladium plating. The palladium compound may be at least water
soluble. Examples of the palladium compound include: water-soluble
inorganic palladium salts such as palladium chloride, palladium
sulfate, and palladium acetate; and water-soluble organic palladium
salts such as tetraamminepalladium hydrochloride,
tetraamminepalladium sulfate, tetraamminepalladium acetate,
tetraamminepalladium nitrate, and
bis(ethylenediamine)palladium(II)chloride. Note that these
palladium compounds may be used alone or two or more kinds may be
used in a mixture.
[0015] The concentration of the palladium ions in the electroless
palladium plating bath is not particularly limited. However, a too
low concentration of the palladium ions may significantly lower the
rate of deposition of the plating film. The concentration is thus
preferably 0.1 g/L or more, more preferably 0.3 g/L or more, and
still more preferably 0.5 g/L or more. On the other hand, a too
high concentration of the palladium ions may cause abnormal
deposition which deteriorates the physical properties of the film.
The concentration is thus preferably 10 g/L or less, more
preferably 5 g/L or less, and still more preferably 3 g/L or
less.
[0016] The concentration of the palladium ions can be measured by
atomic absorption spectrometry (AAS) using an atomic absorption
spectrophotometer.
(Reducing Agent)
[0017] The reducing agent has the function of depositing palladium
in an electroless palladium plating bath. As the reducing agent,
various known kinds of reducing agent can be used. Examples of the
reducing agent include formic acid and salts thereof, hydrazines,
hypophosphorous acid and salts thereof, phosphorous acid and salts
thereof, amine borane compounds, hydroboration compounds, formalin,
and ascorbic acid and salts thereof.
[0018] Examples of the above mentioned salts include: alkali metal
salts such as potassium and sodium salts; alkaline earth metal
salts such as magnesium and calcium salts; ammonium salts;
quaternary ammonium salts; and amine salts containing primary to
tertiary amines.
[0019] Examples of the amine borane compounds include dimethylamine
borane (DMAB) and trimethylamine borane (TMAB). Examples of the
hydroboration compounds include alkali metal borohydride salts such
as sodium borohydride (SBH) and potassium borohydride (KBH).
[0020] Among these reducing agents, formic acid or a salt thereof
(e.g., sodium formate) is used in one preferred embodiment in view
of achieving both the stability of the plating bath and the
deposition properties of the plating film. These reducing agents
may be used alone or two or more kinds may be used in a
mixture.
[0021] The content of the reducing agent in the electroless
palladium plating bath (i.e., the single amount where the agent is
used alone, and the total amount where two or more kinds are used
in a mixture) may be adjusted as appropriate in consideration of
the rate of deposition in the plating processing and the stability
of the plating bath. The lower limit is preferably 1 g/L or more,
more preferably 3 g/L or more, still more preferably 5 g/L or more,
and particularly preferably 10 g/L or more. The upper limit of the
content of the reducing agent is preferably 100 g/L or less, more
preferably 80 g/L or less, and still more preferably 50 g/L or
less.
(Complexing Agent)
[0022] The complexing agent has the main function of stabilizing
the solubility of palladium in the electroless palladium plating
bath. As the complexing agent, various known kinds of complexing
agent can be used. Examples of the complexing agent include at
least one selected from the group consisting of ammonia and amine
compounds, among which an amine compound is selected in a more
preferred embodiment. Examples of the amine compounds include
methylamine, dimethylamine, trimethylamine, benzylamine,
methylenediamine, ethylenediamine, ethylenediamine derivatives,
tetramethylenediamine, diethylenetriamine,
ethylenediaminetetraacetic acid (EDTA) or alkali metal salts
thereof, EDTA derivatives, and glycine. These complexing agents may
be used alone or two or more kinds may be used in a mixture.
[0023] The content of the complexing agent in the electroless
palladium plating bath (i.e., the single amount where the agent is
used alone, and the total amount where two or more types are used
in a mixture) may be adjusted as appropriate in consideration of
the stable solubility of palladium. The lower limit is preferably
0.1 g/L or more, more preferably 1 g/L or more, and still more
preferably 3 g/L or more. The upper limit of the content of the
complexing agent is preferably 15 g/L or less and more preferably
10 g/L or less.
(Stabilizer)
[0024] The stabilizer is added for the purpose of stabilizing the
plating bath, improving appearance after the plating, adjusting the
rate of forming a plating film, and other purposes. In the
electroless palladium plating bath according to the present
disclosure, an organic compound represented by the following
Formula (1) can be used in which a divalent sulfur compound (i.e.,
a compound containing divalent sulfur) is bonded to a compound with
a heterocyclic structure.
[Chemical 1]
R.sub.1-R.sub.2 (1)
where R.sub.1 is the compound with the heterocyclic structure, and
R.sub.2 is the divalent sulfur compound. R.sub.1-R.sub.2 represents
an organic compound with neither a thiol group nor a disulfide
bond.
[0025] Examples of the compound R.sub.1 with a heterocyclic
structure include compounds with a nitrogen-containing heterocyclic
structure or a sulfur-containing heterocyclic structure such as
imidazole, imidazolidine, imidazoline, oxadiazole, oxazine,
thiadiazole, thiazole, thiazolidine, tetrazole, triazine, triazole,
piperazine, piperidine, pyrazine, pyrazole, pyrazolidine, pyridine,
pyridazine, pyrimidine, pyrrole, pyrrolidine, benzothiazole,
benzimidazole, isoquinoline, thiophene, tetrahydrothiophene,
pentamethylene sulfide, and derivatives thereof.
[0026] Examples of the divalent sulfur compound R.sub.2 include
thiadiazole, thiazole, thiazolidine, benzothiazole, thiophene,
tetrahydrothiophene, methanethiol, benzenethiol, pentamethylene
sulfide, dimethyl sulfide, methyl mercaptan, ethyl mercaptan, allyl
mercaptan, thiopropionic acid, thioacetic acid, ethyl methyl
sulfide, 1-propanethiol, 2-propanethiol, 2-aminoethanethiol,
2-mercaptoethanol, 4-mercaptopyridine, dimethyl sulfoxide, S-methyl
thioacetate, ethyl sulfide, methylpropyl sulfide, 1-butanethiol,
thioglycolic acid, 2-(methylthio)ethanol, 3-mercapto-1-propanol,
2-methylthiazoline, cyclopentanethiol, 2-methyltetrahydrothiophene,
pentamethylene sulfide, thiomorpholine, S-methyl thiopropionate,
3-mercaptopropionic acid, and derivatives thereof.
[0027] Examples of the stabilizer represented by Formula (1)
include 2-(4-thiazolyl)benzimidazole, 2-(methylthio)benzimidazole,
2-(methylthio)benzothiazole, (2-benzothiazolylthio)acetic acid,
3-(2-benzothiazolylthio)propionic acid, 2-(methylthio)pyridine,
(4-pyridylthio)acetic acid, 4,4'-dipyridyl sulfide,
2-methylthio-4-pyrimidinol, S-methylthiobarbituric acid,
4-amino-6-chloro-2-(methylthio)pyrimidine,
5-(methylthio)-1H-tetrazole, 5-(ethylthio)-1H-tetrazole,
N-(phenylthio)phthalimide, and
5-(methylthio)thiophene-2-carboxaldehyde. Note that these
stabilizers may be used alone or two or more kinds may be used in a
mixture. Chemical formulas of these stabilizers are as follows.
##STR00001## ##STR00002##
[0028] The organic compound (R.sub.1-R.sub.2) used as the
stabilizer in the electroless palladium plating bath according to
the present disclosure includes an organic compound in which the
divalent sulfur compound R.sub.2 bonded to the compound R.sub.1
with the heterocyclic structure derives from a thiol (--SH)
group-containing compound.
[0029] More specifically, for example, the
2-(methylthio)benzimidazole is an organic compound
(R.sub.1-R.sub.2) in which benzimidazole as R.sub.1 and
methanethiol as R.sub.2 are bonded. The compound contains no thiol
(--SH) group in the state of R.sub.1-R.sub.2 as shown in the
chemical formula. However, since R.sub.2 (i.e., methanethiol)
before being bonded to R.sub.1 contains the thiol (--SH) group,
R.sub.2 bonded to R.sub.1 derives from the compound (i.e.,
methanethiol) containing the thiol (--SH) group. The same applies
to 2-(methylthio)benzothiazole (where R.sub.1 is benzothiazole and
R.sub.2 is methanethiol) and 2-(methylthio)pyridine, (where R.sub.1
is pyridine and R.sub.2 is methanethiol).
[0030] For example, (2-benzothiazolylthio)acetic acid contains no
thiol (--SH) group in the state of R.sub.1-R.sub.2 (where R.sub.1
is benzothiazole and R.sub.2 is thioacetic acid) as shown in the
chemical formula. However, since R.sub.2 (i.e., thioacetic acid)
before being bonded to R.sub.1 contains the thiol (--SH) group,
R.sub.2 bonded to R.sub.1 derives from the compound (i.e.,
thioacetic acid) containing the thiol (--SH) group. The same
applies to (4-pyridylthio)acetic acid (where R.sub.1 is pyridine
and R.sub.2 is thioacetic acid).
[0031] For example, 3-(2-benzothiazolylthio)propionic acid contains
no thiol (--SH) group in the state of R.sub.1-R.sub.2 (where
R.sub.1 is benzothiazole and R.sub.2 is thiopropionic acid) as
shown in the chemical formula. However, since R.sub.2 (i.e.,
thiopropionic acid) before being bonded to R.sub.1 contains the
thiol (--SH) group, R.sub.2 bonded to R.sub.1 derives from the
compound (i.e., thiopropionic acid) containing the thiol (--SH)
group.
[0032] For example, 4,4'-dipyridyl sulfide contains no thiol (--SH)
in the state of R.sub.1-R.sub.2 (where R.sub.1 is pyridine and
R.sub.2 is 4-mercaptopyridine) as shown in the chemical formula.
However, since R.sub.2 (i.e., 4-mercaptopyridine) before being
bonded to R.sub.1 contains the thiol (--SH) group, R.sub.2 bonded
to R.sub.1 derives from the compound (i.e., 4-mercaptopyridine)
containing the thiol (--SH) group.
[0033] Here, as described above, the plating bath has improved
stability by blending the organic compound containing divalent
sulfur, but has the problem of lower deposition properties of
palladium onto the nickel plating film. In particular, there is a
problem of significantly lower deposition properties of palladium
onto a nickel plating film having a low phosphorus content.
[0034] The present inventors have studied the above problem and
found the following: Using a stabilizer made of an organic compound
in which a divalent sulfur compound is bonded to a compound with a
heterocyclic structure (i. e., R.sub.1-R.sub.2 described above)
reduces a reduction in the deposition properties of palladium on
the nickel plating film and improves the stability of the plating
bath.
[0035] The present inventors also have found the following: Using
an organic compound having a thiol group or a disulfide bond, among
organic compounds in which a divalent sulfur compound is bonded to
a compound with a heterocyclic structure, alters the thiol group or
the disulfide bond due to an oxidation-reduction reaction in the
plating bath (i.e., a disulfide bond is generated by an oxidation
reaction of the thiol group, and a thiol group is generated by a
reduction reaction of the disulfide bond), which changes the
deposition properties of palladium and reduces the stability of the
plating bath.
[0036] That is, it is possible to achieve both of the deposition
properties of palladium and the stability of the plating bath by
using, as the stabilizer, an organic compound in which a divalent
sulfur compound is bonded to a compound with a heterocyclic
structure and which contains neither a thiol group nor a disulfide
bond.
[0037] It is also possible to deposit palladium even on a tiny
region on the nickel plating film with a low phosphorus
content.
[0038] The content of the stabilizer in the electroless palladium
plating bath (i.e., the single amount where the agent is used
alone, and the total amount where two or more kinds are used in a
mixture) may be adjusted as appropriate in consideration of the
deposition properties of palladium in the plating processing and
the stability of the plating bath. The lower limit is preferably
0.01 mg/L or more, more preferably 0.03 mg/L or more, and still
more preferably 0.05 mg/L or more. The upper limit of the content
of the stabilizer is preferably 10 mg/L or less, more preferably 5
mg/L or less, and still more preferably 1 mg/L or less.
(Other Components)
[0039] In the electroless palladium plating bath according to the
present disclosure, various additives usually used in the field of
plating bath can be added in addition to the components described
above. Examples of such additives include a pH adjuster, a buffer,
and a surfactant. The pH adjuster is an additive that having the
function of adjusting the pH of the plating bath. Examples of the
pH adjuster include: acids such as hydrochloric acid, sulfuric
acid, nitric acid, citric acid, malonic acid, malic acid, tartaric
acid, and phosphoric acid; and alkalis such as sodium hydroxide,
potassium hydroxide, and ammonia water. Note that these pH
adjusters may be used alone or two or more kinds may be used in a
mixture.
[0040] A too low pH tends to lower the rate of depositing
palladium, whereas a too high pH may reduce the stability of the
electroless palladium plating bath. The electroless palladium
plating bath according to the present disclosure may have a pH
preferably ranging from 4 to 10, and more preferably from 5 to
8.
[0041] A buffer with a buffering function may be added. Examples of
the buffer include: carboxylic acids such as citric acids (e.g.,
trisodium citrate dihydrate), tartaric acid, malic acid, and
phthalic acid; phosphoric acids such as orthophosphoric acid,
phosphorous acid, hypophosphorous acid, and pyrophosphoric acid;
phosphates thereof such as potassium salts, sodium salts (e.g.,
trisodium phosphate dodecahydrate), and ammonium salts; boric acid;
and tetraboric acid. These buffers may be used alone or two or more
kinds may be used in a mixture. A surfactant is added as necessary
to improve the stability, avoid pits, improve the appearance of the
plate, and for other purposes. The surfactant is not particularly
limited. Various kinds such as nonionic, cationic, anionic, and
amphoteric surfactants can be used.
(Applications)
[0042] The electroless palladium plating bath according to the
present disclosure is applicable to, for example, a multilayer
plating film including a palladium plating film and a gold plating
film. The underlayer on which the palladium plating film is formed
is not particularly limited. Examples of the underlayer include:
various known base materials such as aluminum (Al), aluminum-based
alloys, copper (Cu), and copper-based alloys; and plating films
obtained by coating base materials with metal, such as iron (Fe),
cobalt (Co), nickel (Ni), copper, zinc (Zn), silver (Ag), gold,
platinum (Pt), and alloys thereof, having catalytic properties for
reductive deposition of a palladium plating film. Even metal with
no catalytic properties can be used as an object to be plated by
various methods.
[0043] In addition, the electroless palladium plating bath
according to the present disclosure is applicable to an ENEPIG
process. In the ENEPIG process, for example, a multilayer plating
film (i.e., the electroless nickel/palladium/gold plating film) is
obtained which includes a nickel plating film, the palladium
plating film described above, and a gold plating film, in this
order on aluminum, an aluminum-based alloy, copper, or a
copper-based alloy constituting electrodes. Each of the plating
films may be formed by a commonly used method.
[0044] Next, a method of producing a multilayer plating film
including the palladium plating film formed in the electroless
palladium plating bath according to the present disclosure will be
described based on the ENEPIG process described above. The
conditions for forming the palladium plating film are not limited
to those described in the following description and changeable as
appropriate based on known techniques.
[0045] The plating conditions and plating equipment for electroless
nickel plating using an electroless nickel plating bath are not
particularly limited. Various known methods can be selected as
appropriate. For example, an object to be plated may be brought
into contact with an electroless nickel plating bath at a
temperature ranging from 50.degree. C. to 95.degree. C. for about
15 to 60 minutes. The thickness of the nickel plating film may be
set as appropriate in accordance with required characteristics, and
usually ranges from about 3 .mu.m to about 7 .mu.m. Various known
compositions such as the nickel-phosphorus alloy and the
nickel-boron (B) alloy can be used for the electroless nickel
plating bath.
[0046] The plating conditions and plating equipment for electroless
palladium plating using an electroless palladium plating bath
according to the present disclosure are not particularly limited.
Various known methods can be selected as appropriate. For example,
an object to be plated including a nickel plating film may be
brought into contact with an electroless palladium plating bath at
a temperature ranging from 50.degree. C. to 95.degree. C. for about
15 to 60 minutes. The thickness of the palladium plating film may
be set as appropriate in accordance with required characteristics,
and usually ranges from about 0.001 .mu.m to about 1.0 .mu.m.
[0047] The plating conditions and plating equipment for electroless
gold plating using an electroless gold plating bath are not
particularly limited. Various known methods can be selected as
appropriate. For example, an object to be plated including a
palladium plating film may be brought into contact with an
electroless gold plating bath at a temperature ranging from
40.degree. C. to 90.degree. C. for about 3 to 20 minutes. The
thickness of the gold plating film may be set as appropriate in
accordance with required characteristics, and usually ranges from
about 0.001 .mu.m to about 2 .mu.m.
[0048] The electroless palladium plating bath according to the
present disclosure is also useful for electronic device components
having a plating film. Examples of the electronic device components
include components, such as a chip component, a crystal oscillator,
a bump, a connector, a lead frame, a hoop material, a semiconductor
package, and a printed board, that constitute an electronic
device.
<Palladium Plating Film>
[0049] The palladium plating film according to the present
disclosure is obtained using the electroless palladium plating bath
according to the present disclosure described above. The palladium
plating film includes both of a pure palladium film and a palladium
alloy plating film containing an alloy component. This is because
elements other than palladium may be contained in the palladium
plating film depending on the type of the reducing agent to be
used. In some cases, components derived from the various additives
may also be contained. The rest of the palladium plating film
includes palladium and inevitable impurities.
[0050] For example, if formic acid or a salt thereof or hydrazine
or a salt thereof is used as the reducing agent, a pure palladium
film is obtained. On the other hand, if a phosphoric acid compound
such as hypophosphite or phosphite is used as the reducing agent
other than the formic acid or the salt thereof, a palladium plating
film containing phosphorus is obtained. If a boron compound such as
an amine borane compound or a hydroboration compound is used, a
palladium plating film containing boron is obtained. If both of a
phosphoric acid compound and a boron compound are used, a palladium
plating film containing both of phosphorus and boron is
obtained.
EXAMPLES
[0051] The following describes the invention related to the present
application more specifically based on examples and comparative
examples. However, the present disclosure is not limited to the
following examples at all.
Examples 1 to 18, Comparative Examples 1 to 8, Reference Example
1
(Preparation of Plating Bath)
[0052] The plating bath of each of Examples 1 to 18, Comparative
Examples 1 to 8, and Reference Example 1 (i.e., an example
containing no stabilizer) was prepared by mixing and stirring a
palladium compound (i.e., palladium salt), ethylenediamine as the
complexing agent, trisodium citrate dihydrate as the buffer, sodium
formate as the reducing agent, and a stabilizer at concentrations
shown in Tables 2 to 4. The plating bath was set to a temperature
(i.e., temperature of the plating processing) of 60.degree. C., and
the pH was set to 6.0.
[0053] Chemical formulas of the stabilizers used in Comparative
Examples 1 to 8 are as follows.
##STR00003##
(Pretreatment)
[0054] Before forming the electroless plating film, the substrate
was subjected sequentially to Pretreatment Steps 1 to 5 shown in
Table 1.
[0055] Step 1: Using MCL-16 (trade name: EPITHAS MCL-16
manufactured by C. Uyemura & Co., Ltd.), the substrate (a Si or
TEG wafer) was subjected to cleaner treatment.
[0056] Step 2: Then, acid rinse treatment was performed using 30
mass % of a nitric acid solution to form an oxide film on the
surface of the substrate.
[0057] Step 3: After that, using MCT-51 (trade name: EPITHAS MCT-51
manufactured by C. Uyemura & Co., Ltd.), the substrate was
subjected to primary zincate treatment.
[0058] Step 4: Next, the substrate was subjected to acid rinse
treatment using 30 mass % of a nitric acid solution to remove a Zn
substitution film, thereby forming an oxide film on the surface of
the substrate.
[0059] Step 5: Then, using MCT-51 (trade name: EPITHAS MCT-51
manufactured by C. Uyemura & Co., Ltd.), the substrate was
subjected to secondary zincate treatment.
(Plating Processing)
[0060] Next, the substrate subjected to the pretreatment described
above was subjected to Plating Processing Step 6 shown in Table 1
to form an electroless nickel plating film on the substrate. More
specifically, using a nickel plating bath (trade name: NIMUDEN
NPR-18 manufactured by C. Uyemura & Co., Ltd.), electroless
plating processing was performed to form an electroless nickel
plating film containing phosphorus (a nickel plating film having a
concentration of phosphorus in the film ranging from 4% to 8%) on
the substrate. Similarly, using a nickel plating bath (trade name:
NIMUDEN NLL-1 manufactured by C. Uyemura & Co., Ltd.),
electroless plating processing was performed to form a nickel
plating film with a low phosphorus content (a nickel plating film
having a concentration of phosphorus in the film being less than
4%) on the substrate.
[0061] Next, the substrate having the above-described nickel
plating film thereon was subjected to Plating Processing Step 7
shown in Table 1 (i.e., the electroless plating processing using
the palladium plating baths of Examples 1 to 18, Comparative
Examples 1 to 8, and Reference Example 1) to form a palladium
plating film on the surface of the nickel plating film (a pad of
100 .mu.m.times.100 .mu.m and a pad of 2 mm.times.3 mm) on the
substrate.
TABLE-US-00001 TABLE 1 Processing Processing Processing Solution
Temperature Time (sec) Pretreatment 1 Cleaner MCL-16 50.degree. C.
300 Step 2 Acid rinse 30 mass % 21.degree. C. 60 Nitric Acid 3
Primary MCT-51 21.degree. C. 20 Zincate Treatment 4 Acid rinse 30
mass % 21.degree. C. 60 Nitric Acid 5 Secondary MCT-51 21.degree.
C. 40 Zincate Treatment Plating 6 Electroless NPR-18 80.degree. C.
1200 Processing Ni (MP-Ni) Step Electroless NLL-1 90.degree. C. Ni
(LP-Ni) 7 Electroless Examples 1-18 240 Pd Comparative Examples 1-8
Reference Example 1
(Measurement of Thickness of Palladium Plating Film)
[0062] Next, using an X-ray fluorescence spectrometer (trade name:
XDV-.mu. manufactured by FISCHER INSTRUMENTS K.K.), the thickness
of the palladium plating film formed on each of the pads was
measured. The results are shown in Tables 2 to 4.
(Evaluation on Bath Stability)
[0063] Whether or not palladium particles are deposited in the
palladium plating bath after the electroless palladium plating
processing was visually observed and evaluated under the following
criteria. The results are shown in Tables 2 to 4.
[0064] .largecircle.: Even after one week from the plating
processing, no deposition of palladium particles was observed.
[0065] x: Deposition of palladium particles was observed within one
week after the plating processing.
TABLE-US-00002 TABLE 2 Plating Bath Composition Example 1 2 3 4 5 6
7 8 9 Pd Salt Tetraamminepalladium g/L 1.5 1.5 1.5 1.5 1.5 Sulfate
(as Pd) Tetraamminepalladium g/L 1.5 1.5 Hydrochloride (as Pd)
Palladium Sulfate (as Pd) g/L 1.5 Palladium Chloride (as Pd) g/L
1.5 Complexing Ethylenediamine g/L 4 4 4 4 4 4 4 4 4 Agent Buffer
Trisodium Citrate Dihydrate g/L 10 10 10 10 10 10 10 10 10 Reducing
Formic Acid Na g/L 25 25 25 25 25 25 25 25 25 Agent Stabilizer
2-(4-thiazolyl)benzimidazole mg/L 0.1 1 2-(methylthio)benzimidazole
mg/L 0.1 0.3 2-(methylthio)benzothiazole mg/L 0.2
(2-benzothiazolylthio)acetic acid mg/L 0.2
3-(2-benzothiazolylthio)propionic acid mg/L 0.2
2-(methylthio)pyridine mg/L 0.2 (4-pyridylthio)acetic acid mg/L 0.2
pH 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 Processing Temperature 60 60
60 60 60 60 60 60 60 Palladium Underlayer: Nickel Plating Film with
Pad of 2 mm .times. 3 mm 0.13 0.11 0.13 0.12 0.13 0.14 0.15 0.14
0.12 Plating Phosphorus Concentration in Film of Pad of 100 .mu.m
.times. 0.11 0.11 0.12 0.11 0.12 0.12 0.13 0.13 0.09 Film 4% to 8%
100 .mu.m Thickness Underlayer: Nickel Plating Film with Pad of 2
mm .times. 3 mm 0.13 0.10 0.11 0.10 0.14 0.13 0.15 0.13 0.11
(.mu.m) Phosphorus Concentration in Film of Pad of 100 .mu.m
.times. 0.10 0.09 0.09 0.09 0.12 0.11 0.12 0.12 0.07 Less Than 4%
100 .mu.m Stability of Plating Solution .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle.
TABLE-US-00003 TABLE 3 Plating Bath Composition Example 10 11 12 13
14 15 16 17 18 Pd Salt Tetraamminepalladium g/L 1.5 1.5 1.5 1.5 1.5
1.5 1.5 1.5 1.5 Sulfate (as Pd) Tetraamminepalladium g/L
Hydrochloride (as Pd) Palladium Sulfate (as Pd) g/L Palladium
Chloride (as Pd) g/L Complexing Ethylenediamine g/L 4 4 4 4 4 4 4 4
4 Agent Buffer Trisodium Citrate Dihydrate g/L 10 10 10 10 10 10 10
10 10 Reducing Formic Acid Na g/L 25 25 25 25 25 25 25 25 25 Agent
Stabilizer 4,4'-dipyridyl sulfide mg/L 0.4
2-methylthio-4-pyrimidinol mg/L 0.2 S-methylthiobarbituric acid
mg/L 0.2 4-amino-6-chloro-2-(methylthio)pyrimidine mg/L 0.2
5-(methylthio)-1H-tetrazole mg/L 0.2 5-(ethylthio)-1H-tetrazole
mg/L 0.2 5-(benzylthio)-1H-tetrazole mg/L 0.2
N-(phenylthio)phthalimide mg/L 0.3
5-(methylthio)thiophene-2-carboxaldehyde mg/L 0.4 pH 6.0 6.0 6.0
6.0 6.0 6.0 6.0 6.0 6.0 Processing Temperature 60 60 60 60 60 60 60
60 60 Palladium Underlayer: Nickel Plating Film with Pad of 2 mm
.times. 3 mm 0.11 0.13 0.12 0.14 0.16 0.12 0.13 0.12 0.12 Plating
Phosphorus Concentration in Film of Pad of 100 .mu.m .times. 0.10
0.12 0.11 0.13 0.14 0.12 0.12 0.11 0.08 Film 4% to 8% 100 .mu.m
Thickness Underlayer: Nickel Plating Film with Pad of 2 mm .times.
3 mm 0.11 0.12 0.10 0.13 0.16 0.13 0.11 0.13 0.11 (.mu.m)
Phosphorus Concentration in Film of Pad of 100 .mu.m .times. 0.10
0.12 0.08 0.11 0.14 0.11 0.10 0.12 0.08 Less Than 4% 100 .mu.m
Stability of Plating Solution .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle.
TABLE-US-00004 TABLE 4 Plating Bath Composition Reference
Comparative Example Example 1 2 3 4 5 6 7 8 1 Tetraamminepalladium
g/L 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Sulfate (as Pd) Pd Salt
Tetraamminepalladium g/L Hydrochloride (as Pd) Palladium Sulfate
(as Pd) g/L Palladium Chloride (as Pd) g/L Complexing
Ethylenediamine g/L 4 4 4 4 4 4 4 4 4 Agent Buffer Trisodium
Citrate Dihydrate g/L 10 10 10 10 10 10 10 10 10 Reducing Formic
Acid Na g/L 25 25 25 25 25 25 25 25 25 Agent Stabilizer
Pyridine-2-Carboxylic Acid mg/L 10 Benzothiazole mg/L 1.0
Piperazine mg/L 300 2-mercaptobenzothiazole mg/L 0.5
2,2'-dithiodipyridine mg/L 0.5 4-mercaptotoluene mg/L 0.5
2,2'-dibenzothiazolyl disulfide mg/L 0.2 2-mercaptobenzimidazole
mg/L 0.2 pH 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 Processing
Temperature 60 60 60 60 60 60 60 60 60 Palladium Underlayer: Nickel
Plating Film with Pad of 2 mm .times. 3 mm 0.03 0.14 0.10 0.15 0.10
0.13 0.11 0.12 0.16 Plating Phosphorus Concentration in Film of Pad
of 100 .mu.m .times. 0.02 0.12 0.06 0.13 0 0.12 0.12 0.11 0.14 Film
4% to 8% 100 .mu.m Thickness Underlayer: Nickel Plating Film with
Pad of 2 mm .times. 3 mm 0.03 0.11 0.10 0.12 0.09 0.11 0.12 0.12
0.15 (.mu.m) Phosphorus Concentration in Film of Pad of 100 .mu.m
.times. 0 0 0 0 0 0 0 0 0.13 Less Than 4% 100 .mu.m Stability of
Plating Solution x x x x x x x x x
[0066] The following is found from Tables 2 and 3: In each of
Examples 1 to 18 using, as the stabilizer, an organic compound in
which a divalent sulfur compound was bonded to a compound with a
heterocyclic structure and which contained neither a thiol group
nor a disulfide bond, the thickness of the palladium plating film
on the nickel plating film (i.e., the pad of 100 .mu.m.times.100
.mu.m and the pad of 2 mm.times.3 mm) is maintained as in Reference
Example 1 containing no stabilizer. This means that even if a
stabilizer is used, a reduction in the deposition properties of
palladium is reduced In particular, it is found that even on a
nickel plating film having a low phosphorus content (a nickel
plating film having a concentration of phosphorus in the film being
less than 4%), palladium is sufficiently deposited as on an
electroless nickel plating film containing phosphorus (a nickel
plating film having a concentration of phosphorus in the film
ranging from 4% to 8%).
[0067] In addition, even after one week from the plating
processing, no deposition of palladium particles was observed in
the plating bath, which shows excellent stability of the plating
bath.
[0068] On the other hand, as shown in Table 4, no palladium was
deposited at all on the nickel plating film (the pad of 100
.mu.m.times.100 .mu.m) having a lower phosphorus content in each of
Comparative Examples 1 to 3 using, as a stabilizer, a compound with
a heterocyclic structure to which no divalent sulfur compound was
bonded, Comparative Examples 4, 6, and 8 using, as a stabilizer, an
organic compound in which a divalent sulfur compound was bonded to
a compound with a heterocyclic structure and which contained a
thiol group, and Comparative Examples 5 and 7 using, as a
stabilizer, an organic compound in which a divalent sulfur compound
was bonded to a compound with a heterocyclic structure and which
contained a disulfide bond. It is also found that deposition of
palladium particles was observed in the plating bath within one
week after the plating processing, which shows poor stability of
the plating bath.
[0069] The present disclosure is particularly advantageously used
for a multilayer plating film including a palladium plating film
and a gold plating film, and an electroless palladium plating bath
used in an ENEPIG process or other processing.
* * * * *