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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for metal surface treatment, and
more particularly to an effective process for removing an oxide film, and
residues of organic matters, carbon, etc., if any, from a solder surface
or a metal surface to be treated, when, for example, a circuit wiring
substrate and a semiconductor integrated circuit (LSI) or the like are
bonded to each other by soldering, or when plating is applied to a metal
surface.
2. Related Art
Heretofore, it has been required that, when a circuit wiring substrate and
a semiconductor integrated circuit or the like are bonded to each other by
soldering, the surfaces of metal to be bonded be kept clean and there be
no solder wettability-inhibiting substances. Furthermore, it has been also
required that there be no oxide film, etc. on the surface of metal to be
plated and the surface of the metal be kept clean. When an Au wire or
ribbon is bonded to the metal surface by ultrasonic welding, the presence
of an oxide film on the surface of the metal is a serious problem, and
thus the surface of the metal must be kept clean.
Such solder wettability-inhibiting substances include, for example, oxides,
chlorides, sulfides, carbonates, and organic compounds. Particularly in
the processes for soldering, plating, or bonding of an Au wire or ribbon
by ultrasonic welding, most serious inhibiting substance is an oxide film
existing on the surface of such metal to be treated, as solder, nickel
(Ni), nickel alloys (alloys of nickel with other element or elements).
Generally, the oxide film is chemically converted to a liquid compound by a
flux, whereby the metal atoms of the surface of the metal and the metal
atoms of a solder have a chance of direct collisions to each other for
forming a metal bond by shearing of their outer electron orbits to form an
alloy.
In the case of plating, it is impossible to conduct plating if there is an
oxide film on the metal surface. For example, in the case of
electroplating, which is typical of the plating, the oxide film acts as an
insulating film and inhibits the necessary electrical conduction for
electroplating, resulting in a failure to conduct plating.
In the case of substitution plating, the oxide film also acts as an
inhibitor and there occurs no substitution reaction between the surface of
metal to be treated and a plating solution, resulting in a failure to
conduct plating.
The oxide film can be removed by treating the surface of the metal with
hydrochloric acid or the like before the plating, but hydrochloric acid or
the like remains as residues on the treated surface and such residues
serve to be a factor of lowering the bonding reliability. Thus, washing of
the surface of metal with flons (fluorocarbons) has been so far carried
out after the treatment with an acid.
Recently, it has been proposed to use a very small amount of abietic acid
(rosin) and adipic acid or the like as a flux without any after-washing of
flux residues, but the proposed process is still not satisfactory in the
bonding reliability [see "Alumit Technical Journal 19" (1992) and an
article "Working mechanism and problems of flux for unnecessitating
washing" by N. Kubota of K.K. Nihon Kogyo Gizyutsukaihatsu Kenkusho
(Industrial Technology Development Research Institute of Japan, Ltd.)].
On the other hand, a glazing process for irradiating the surfaces of a
metallic materials, steel, carbides, etc. with a laser beam, thereby
producing materials having a uniform microfine structure or amorphous
structure and good corrosion resistance and wear resistance has been
proposed and has been applied to processing of metallic materials to be
exposed to high temperature and high pressure, such as materials for
automobile turbines [see "Laser Processing" (continued part), page 164, by
A. Kobayashi, published by Kaihatsu-sha].
It has been also proposed to remove oxide films from metal surfaces by
argon sputtering without using any flux or hydrochloric acid.
Furthermore, it has been proposed to form a coating film having a good
adhesion without pinholes by roughening a metal surface by blasting,
plating the roughened metal surface with an alloy element and then
irradiating the plating layer with a laser beam, thereby melting the
plating layer (JP-A-63-97382).
Still furthermore, it has been proposed to form a surface layer having a
good corrosion resistance and a ready susceptibility to soldering by
forming an anodized oxide film on the surface of aluminum or its alloys
(JP-A-62-256961).
As a result of extensive study of the above-mentioned prior art, the
present inventors have found the following problems:
(1) When a circuit substrate and an integrated circuit or the like as to be
bonded to each other by soldering, an oxide film must be removed from
their surfaces by a flux before the soldering, and the resulting flux
residues must be washed off from their surfaces. In the case of removing
oxide films from metal surfaces by washing with an acid before the
plating, the resulting acid residues or the metal surfaces cause a later
corrosion. Furthermore, an additional drying step is imperative after the
washing.
(2) When oxide films are removed from metal surfaces by argon sputtering,
the sputtering must be carried out in vacuum. Thus, an additional
sputtering apparatus and its complicated operation control are required.
There is also such a problem that the argon sputtering has an adverse
effect on working elements of electronic parts or electronic devices.
(3) In the glazing process using a laser beam and the laser irradiation
process disclosed in the above-mentioned JP-A-63-97382, the surface metal
structure is forcedly changed by irradiation with a laser beam of higher
energy level and by the resulting melting, thereby endowing a high wear
resistance and a high compactness to the metal surface, but an oxide film
is inevitably formed on the metal surface in the course of metal surface
solidification.
(4) The surface treatment disclosed in the above-mentioned JP-A-62-256961
does not relate to an oxide film-removing technique.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for metal
surface treatment for cleaning a metal surface, that is, removing an oxide
film and residues of organic matters, carbon, etc., if any, from the metal
surface simply without using any complicated steps and without giving any
adverse effects on electronic parts or electronic devices.
Other objects of the present invention will be apparent from disclosures
which follow and the accompanying drawings.
According to the present invention, there is provided (1) a process for
metal surface treatment, which comprises irradiating a surface of metal
with a laser beam of lower energy level than energy level capable of
changing the metal surface structure, thereby cleaning the surface of
metal, wherein (2) the laser beam has a pulse span of not more than 1
.mu.s; (3) the laser beam has a wavelength of 150 nm to 400 nm; (4) the
metal is one of solder, nickel and nickel alloys, where (5) the laser beam
has an energy density of 0.5 J/cm.sup.2 to 4.0 J/cm.sup.2 ; and (6) a
reoxidation-preventing plating layer is formed on the cleaned metal
surface after the irradiation of the surface of metal by the laser beam.
In the present invention, a metal surface to be treated is irradiated with
a laser beam of lower-energy level than energy level capable of changing
the metal surface structure. More particularly, irradiation is carried out
with a laser beam of higher energy level than the bonding energy between
metal atoms and oxygen atoms on the metal surface, but of lower energy
level than the bonding energy between the metal atoms themselves, whereby
only the bonding between the metal atoms and the oxygen atoms on the metal
surface is dissociated and the oxide film is removed from the metal
surface, and at the same time residues of organic matters, carbon, etc.,
if any, is removed from the metal surface. That is, the metal surface can
be cleaned thereby.
That is, the main purpose of irradiation with a laser beam is to dissociate
bonding between the metal atoms and the oxygen atoms on the metal surface,
and it is preferable to use a pulse laser beam having a pulse span of not
more than 1 .mu.s. Since the bonding between the metal atoms and the
oxygen atoms on the metal surface is dissociated by a pulse laser beam
having a pulse span of not move than 1 .mu.m, it is preferable to use, for
example, an eximer laser having a short wavelength (that is, a high level
of photon energy) as a laser beam.
Atmosphere for irradiation with the laser beam can be any one of
atmospheric air, vacuum and a He gas atmosphere, where the oxide films can
be removed from the metal surfaces without any problems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical cross-sectional view explaining the present process
for metal surface treatment according to Example 1 of the present
invention.
FIG. 2 is a vertical cross-sectional view explaining one modification of
Example 1 of the present invention, i.e. irradiation of solder bump
surface with a laser beam reflected on a mirror and passing through a lens
in making of electronic devices such as semiconductor integrated circuits
(LSI) in place of the solder layer shown in FIG. 1.
FIG. 3 is a picture of the solder layer surface of Example 1 of the present
invention before the irradiation with a laser beam, taken by a scanning
electron microscope.
FIG. 4 is an enlarged picture of FIG. 3.
FIG. 5 is a picture of the solder layer surface of Example 1 of the present
invention after the irradiation with the laser beam, taken by the scanning
electron microscope.
FIG. 6 is an enlarged picture of FIG. 5.
FIG. 7 is a diagram showing a relationship between the weight percentage
(wt. %) of oxide film existing on an Sn--Pb solder surface after constant
6 runs of irradiation with a laser beam onto the same region of the Sn--Pb
surface as plotted on the ordinate and the energy density per pulse
(J/cm.sup.2) of the laser beam used for the irradiation as plotted on the
abscissa.
FIG. 8 is a diagram showing a relationship between the weight percentage
(wt. %) of oxide film existing on an Sn--Pb solder surface after
irradiation with a laser beam as plotted on the ordinate and number of
irradiation runs onto the same region of the solder metal surface, as
plotted on the abscissa, when the energy density is kept constant at 1.5
J/cm.sup.2.
FIG. 9 is a vertical cross-sectional view showing an embodiment of an
electronic device bonded by soldering in Example 1 of the present
invention.
FIG. 10 is a vertical cross-sectional view showing another embodiment of an
electronic device bonded by soldering in Example 1 of the present
invention.
FIG. 11 is a vertical cross-sectional view explaining the present process
for metal surface treatment according to Example 2 of the present
invention.
FIG. 12 is a diagram showing a relationship between the thickness (nm) of
oxide film existing on the surface of a nickel layer as plotted on the
ordinate and the laser beam energy density (J/cm.sup.2) as plotted on the
abscissa, while setting the number of laser beam irradiation runs onto the
same region of the nickel layer to constant 10 according to Example 2 of
the present invention.
FIG. 13 is a diagram showing a relationship between the thickness (nm) of
an oxide film existing on the surface of nickel layer as plotted on the
ordinate and the number of laser beam irradiation runs onto the same
region of the nickel layer as plotted on the abscissa, while setting the
laser beam energy density to constant 0.75 (J/cm.sup.2) according to
Example 2 of the present invention.
FIG. 14 is a vertical cross-sectional view explaining the present process
applied to making of electronic devices according to Example 3 of the
present invention.
FIG. 15 is a vertical cross-sectional view showing an electronic device to
which the reoxidation-preventing means is applied according to Example 3
of the present invention.
FIG. 16 is a vertical cross-sectional view showing electrical connection
between the electronic device and a nickel (Ni) layer or a nickel alloy
layer on a ceramic substrate, bonded directly by a solder or a soft solder
without using any input/output (I/O) pin shown in FIG. 15.
FIGS. 17A and 17B are a plan view and a vertical cross-sectional view along
the line A--A of FIG. 17A, respectively, showing the present process
applied to making of an electronic device according to Example 4 of the
present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention will be described in detail below, referring to
Examples and Drawings, where members having the same functions are
identified by the same numerals throughout the drawings explaining
Examples of the present invention to omit repeated explanation.
FIG. 1 is a vertical cross-sectional view explaining the present process
for metal surface treatment, where an oxide film 4 (and residues of
organic matters, carbon, etc., if any) is removed from the surface of a
solder layer 3a on the metallized layer 2 formed as an overlayer on a
ceramic substrate 1. The metallized layer 2 is a film of, for example,
titanium (Ti), nickel (Ni), nickel alloy or the like.
Oxide film 4 (and residues of organic matters, carbon, etc., if any) can be
removed from the surface of solder layer 3a on the metallized layer 2 by
irradiation of the surface of solder layer 3a with a laser beam 5
reflected on a mirror 7 and passing through a lens 6.
FIG. 2 shows a modification of the above-mentioned embodiment shown in FIG.
1, where the surface of solder bump 3b is irradiated with a laser beam 5
reflected on the mirror 7 and passing through the lens 6 in making of
electronic devices such as semiconductor integrated circuits (LSI) in
place of the solder layer 3a shown in FIG. 1.
Laser beam 5 used in Example 1 is a laser beam having a lower energy level
than energy level capable of changing the metal structure of solder layer
3a or solder bump 3b, and more particularly is a laser beam having an
energy level higher than the bond energy between Sn--Pb atoms and O atoms
on the surface of solder layer 3a or solder bump 3b, but lower than the
bond energy between Sn--Pb atoms themselves.
When the surface of solder layer 3a or solder bump 3b is irradiated with
laser beam 5, only bonding between Sn--Pb atoms and O atoms on the surface
is dissociated by the energy of laser beam 5 without melting the solder
layer 3a or solder bump 3b, and thus oxide film 4 can be removed from the
surface of solder layer 3a or solder bump 3b. At the same time, residues
of organic matters, carbon, etc. if any, can be removed from the metal
surface.
In this case, the main purpose of irradiation with a laser beam 5 is to
dissociate the bonding between the Sn--Pb atoms and the oxygen atoms on
the surface, and thus preferable laser beam 5 is a pulse laser beam having
a pulse span of not more than 1 .mu.s. Furthermore, since the bonding
between the Sn--Pb atoms and the O atoms on the surface is dissociated by
a pulse laser beam having a pulse span of not more than 1 .mu.s, for
example, a eximer laser having a short wavelength (high photon energy
level) is preferable as a laser beam 5.
Atmosphere for irradiation with laser beam 5 is any one of atmospheric air,
vacuum and a He gas atmosphere, where the oxide film 4 can be removed from
the surface of solder layer 3a or solder bump 3b.
FIG. 3 is a picture of the surface state of solder layer 3a or solder bump
3b before irradiation with a laser beam, taken by a scanning electron
microscope, and FIG. 4 is an enlarged picture of FIG. 3, from which oxide
films and residues of organic matters, carbon, etc. are found as black
residues on the surfaces of solder layer 3a or bump 3b.
FIG. 5 is a picture of the surface state of solder layers 3a or solder
bumps 3b after irradiation with a laser beam, taken also by the scanning
electron microscope and FIG. 6 is an enlarged picture of FIG. 5, from
which oxide films and residues of organic matters, carbon, etc. are
substantially completely removed from the surface.
FIG. 7 is a diagram showing a relationship between the weight percentage
(%) of existing oxide film existing on the Sn--Pb surface after 6 turns of
irradiation with a laser beam onto the same region of the Sn--Pb surface
on the basis of the oxide film on the surface before the irradiation as
100, as plotted on the ordinate, and the laser beam energy density
(J/cm.sup.2) per pulse as plotted on the abscissa, from which it is
evident that the weight percentage of existing oxide film after the
irradiation of the laser beam in a range of energy density of 0.5
J/cm.sup.2 to 4.0 J/m.sup.2 is smaller than that before the irradiation,
and above all a laser beam having an energy density of 1.5 J/cm.sup.2 is
preferable.
The weight percentage of oxide film on the ordinate of FIG. 7 is based on
an oxygen concentration determined by energy dispersive X-ray spectroscopy
(EDX).
FIG. 8 shows a relationship between the weight percentage (%) of oxide film
existing on the Sn--Pb surface after irradiation with a laser beam on the
basis of the oxide film existing on the surface before the irradiation as
100, as plotted on the ordinate, and the number of irradiation runs with a
laser beam having a constant energy density per pulse of 1.5 J/cm.sup.2
onto the same region of the metal surface, as plotted on the abscissa,
from which it is evident that the weight percentage of oxide film existing
on the Sn--Pb surface becomes a minimum by about 8 runs of the
irradiation. That is, the weight percentage of oxide film existing on the
Sn--Pb surface becomes a minimum by 8 runs of irradiation of a laser beam
having a constant energy density of 1.5 J/cm.sup.2 and the wettability of
solder 3a or solder bump 3b can be improved thereby.
FIG. 9 is a vertical cross-sectional view showing essential parts of
semiconductor device structure comprising an integrated circuit (LSI) 8
bonded to a ceramic substrate 1 by soldering through metallized layers 2
formed as overlayers on both surfaces of the integrated circuit and the
ceramic substrate and through solder bumps 3b, from whose surfaces an
oxide film has been removed according to the present process of Example 1,
in a flux-free state.
FIG. 10 is a vertical cross-sectional view of the essential part of seal
cap 9 bonded to the semiconductor device structure as shown in FIG. 9 by
soldering through solder bumps 3b, from whose surfaces an oxide film has
been removed according to the present process of Example 1, in a flux-free
state.
Example 2
FIG. 11 is a vertical cross-sectional view explaining the present process
for metal surface treatment according to Example 2 of the present
invention, where an oxide film 4 (and residues of organic matters, carbon,
etc., if any) is removed from the surface of a nickel (Ni) layer or nickel
alloy layer 2a formed as an overlayer on a ceramic substrate 1.
Generally, nickel (Ni) layer or nickel alloy layer 2a is liable to undergo
oxidation, and thus an oxide film 4 is easily formed on the surface of
nickel (Ni) layer or nickel alloy layer.
The oxide film 4 can be removed from the surface of nickel layer or nickel
alloy layer 2a by irradiation of the surface of the layer 2a with a laser
beam 5 reflected on a mirror 7 and passing through a lens 2.
FIG. 12 shows a relationship between the thickness (nm) of oxide film 4
existing on the surface of nickel layer 2a after 10 runs of irradiation
with a laser beam 5 as plotted on the ordinate and the energy density
(J/cm.sup.2) per pulse of laser beam 5 per unit area as plotted on the
abscissa. That is, the number of irradiation with laser beam 5 onto the
same region of the nickel layer 2a was set to constant 10 runs. It is
evident from FIG. 12 that the oxide films 4 having different initial
thicknesses (40 nm and 25 nm, as indicated by black dots and white dots,
respectively) can be removed with increasing energy density of the laser
beam 5.
FIG. 13 shows a relationship between the thickness (nm) of oxide film 4
existing on the surface of nickel layer 2a after irradiation of laser beam
5 having a constant energy density of 0.75 J/cm.sup.2 per pulse as plotted
on the ordinate and the number of laser beam irradiation run onto the same
region of nickel layer 2a as plotted on the abscissa. That is, the energy
density of laser beam 5 was set to constant 0.75 J/cm.sup.2. It is evident
from FIG. 13 that the thickness of oxide film decreases with increasing
number of laser beam irradiation runs.
Example 3
FIG. 14 is a cross-sectional view showing essential parts of electronic
device such as a semiconductor integrated circuit (LSI), etc. to which the
present invention is applied according to Example 3.
Electronic device shown in FIG. 14 is made as follows; an oxide film (and
residues of organic matters, carbon, etc., if any) is removed from the
surface of nickel (Ni) layer or nickel alloy layer 2a formed as overlayer
on a ceramic substrate 1 according to the present process for metal
surface treatment as shown in Examples 1 and 2, and then a plating layer
10 is formed on the resulting surface-treated, i.e. cleaned nickel layer
or nickel alloy layer 2a by electroplating, electroless plating or
substitution plating, where a plating material is generally gold (Au) to
prevent reoxidation of the cleaned nickel layer or nickel alloy layer.
That is, according to the embodiment of Example 3, an oxide film (and
residues of organic matters, carbon, etc., if any) is removed from the
surface of a nickel (Ni) layer or nickel alloy layer 2a as a metallized
layer and a plating layer 10 is formed on the resulting cleaned metallized
layer 2a to prevent reoxidation of the resulting cleaned metallized layer
2a.
FIG. 15 is a vertical cross-sectional view showing essential parts of the
structure of an electronic device, to which a reoxidation-preventing means
is specifically applied according to the embodiment of Example 3. Where a
nickel (Ni) layer or nickel alloy layer 2a is formed as a metallized layer
partly on the surface of a ceramic substrate 1; an organic insulating
layer 15 is formed entirely on the surfaces of the metallized layer 2a and
the ceramic substrate 1; a hole is formed through the organic insulating
layer 15 to expose the nickel (Ni) layer or nickel alloy layer 2a; an
oxide film (and residues of organic matters, carbon, etc., if any) is
removed from the exposed surface of the metallized layer 2a according to
the present process for metal surface treatment shown in Examples 1 and 2;
then a plating layer 10 for preventing reoxidation of the metallized layer
2b is formed on the resulting cleaned metallized layer 2b; and an
input/output (I/O) pin 12 to an electronic device is fixed to the plating
layer 10 by a solder or soft solder 11.
Electrical connection between the input/output (I/O) pin 12 and the ceramic
substrate 1 of electronic devices such as semiconductor integrated
circuits (LSI), etc. can be improved by removing the oxide film 4 (and the
residues of organic matters, carbon, etc. if any) from the surface of
nickel layer or nickel alloy layer 2a by irradiation with a laser beam 5,
as shown in FIGS. 1 and 11, and then forming a plating layer 10 for
preventing the reoxidation on the resulting cleaned metallized layer 2a
according to the present invention. Within about one week after the
removal of oxide film 4, (and the residues, if any) from the surface of
the metallized layer 2a by irradiation with the laser beam 5, the
input/output (I/O) pin 12 to the electronic device can be electrically
connected directly to the metallized layer 2a on the ceramic substrate 1
by the solder or soft solder 11 without using the plating (e.g. Au
plating) layer 10 for preventing the reoxidation therebetween.
FIG. 16 is a vertical cross-sectional view showing essential parts of
structure of direct electrical connection between the electronic device 8
and the metallized layer 2a on the ceramic substrate 1 by a solder or soft
solder 11 without using the input/output (I/O) pin 12 as shown in FIG. 15.
In the foregoing embodiments of the present invention, it is not necessary
to use a flux, etc.
Example 4
FIGS. 17A and 17B are a plan view and a vertically cross-sectional view
along the line A--A of FIG. 17A, respectively, of essential parts of an
electronic device such as a semiconductor integrated circuit, etc. to
which the present invention is applied according to Example 4, where a
metal film 13 having a good adhesion to an organic insulating layer 15,
for example, chromium (Cr) or titanium (Ti) layer, is formed as an
overlayer on the organic insulating layer 15; a nickel (Ni) layer or
nickel alloy layer 2a is formed as a metallized overlayer on the surface
of the metal film 13; an oxide film (and residues of organic matters,
carbon, etc. if any) is removed from the surface of the metallized layer
2a by irradiation with a laser beam according to the process for metal
surface treatment of Examples 1 and 2; and then an gold (Au) ribbon or
wire 14 is bonded to the cleaned metallized layer 2a by ultrasonic
welding.
Generally, it is difficult to conduct bonding between the gold ribbon or
wire 14 and the metallized layer 2a due to the presence of the oxide film
(and the residues of organic matters, etc., if any) on the surface of the
metallized layer 2a. By removing the oxide film (and the residues of
organic matters, etc., if any) from the surface of the metallized layer 2a
according to the process for metal surface treatment of Examples 1 and 2,
good bonding can be obtained therebetween.
In the foregoing Examples, the metals to be surface-treated according to
the present invention are exemplified by nickel (Ni) layer or nickel alloy
layer 2a and solder layer 3a or solder bumps 3b, but are not limited only
thereto. That is, the present invention is applicable to any metals from
whose surfaces it is necessary to remove oxide films (and residues of
organic matters, etc., if any) where an appropriate energy level of laser
beam must be selected in view of properties of metals to be surface
treated.
In the foregoing Examples, the laser beam is exemplified by a pulse laser
beam. With such a control means as not to melt the metal itself, a laser
beam having a long wavelength such as a CO.sub.2 laser, etc. can be used
and the similar effects to those of pulse laser beam can be obtained by
continuous irradiation.
Melting of metal on the surface sometimes occurs by laser irradiation, but
such melting is permitted so long as it occurs only for a short duration.
The present invention has been described in detail, referring to Examples,
but is not limited only to the embodiments shown in Examples and can be
modified to various degrees, if the modification is not deviated from the
spirit and scope of the present invention.
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