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Description  |
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FIELD OF THE INVENTION
The present invention relates to an anisotropic conductive film having high
reliability in electrical connection and a process for producing the same.
BACKGROUND OF THE INVENTION
In the field of semi-conductors, with the recent development of electronic
equipment having multiple functions, a reduced size and a reduced weight,
a circuit has become denser, and a fine circuit pattern having many pins
at a narrow pitch has been used. In order to cope with the demand for
fineness of a circuit pattern, it has been attempted to connect a
plurality of conducting patterns formed on a substrate and a conducting
pattern or an Integrated Circuit (IC) or an Large Scale Integration (LSI)
via a anisotropic conductive film therebetween.
For example, JP-A-55-161306 (the term "JP-A" as used herein means an
"unexamined published Japanese patent application") discloses an
anisotropic conductive sheet comprising an insulating porous sheet in
which the fine through-holes of a selected area are metal-plated. On
connecting an IC, etc., since the sheet has no metallic projections on its
surface, it is necessary to form a projected electrode (bump) on the IC on
the connecting pad side, making the connection step complicated.
In an attempt to facilitate connection, as shown in FIG. 2, it has been
proposed to fill a metallic substance 3 in fine through-holes 2 of an
insulating sheet 1 formed in the thickness direction in such a manner that
the resulting anisotropic conductive film has metallic bumps 4 projected
from the film surface, as disclosed in JP-A-62-43008, JP-A-63-40218, and
JP-A-63-94504. However, adhesion between filled metallic substance 3 and
insulating film 1 is not so sufficient that the metallic substance is apt
to fall off. It follows that the fine through-holes, which ought to
exhibit conductivity, fail to exhibit conductivity and lack reliability in
electrical connection.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an anisotropic conductive
film which surely exhibits anisotropic conductivity to assure high
reliability in electrical connection.
Another object of the present invention is to provide a process for
producing the above anisotropic conductive film.
Other objects and effects of the present invention will be apparent from
the following description.
As a result of extensive investigations, the inventors have found that the
above objects of the present invention are accomplished by an anisotropic
conductive film comprising an insulating film having fine through-holes
independently piercing the film in the thickness direction of the
insulating film, each of the through-holes being filled with a metallic
substance in such a manner that at least one end of each through-hole has
a bump-like projection of the metallic substance having a bottom area
larger than the opening of the through-hole.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cross section of the anisotropic conductive film
according to one embodiment of the present invention.
FIG. 2 illustrates a cross section of a conventional anisotropic conductive
film having bumps.
FIG. 3 illustrates a cross section of another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is now explained by referring to the accompanying
drawings.
FIG. 1 shows a cross section of the anisotropic conductive film according
to one embodiment of the present invention. In FIG. 1, insulating film 1
has fine through-holes 2 which pierce the film in the thickness direction.
A conducting path filled with metallic substance 3 reaches both the
obverse and the reverse of the film. On each end of each through-hole 2
there is provided a metallic bump-like projection 4 having a larger bottom
area than the opening area of through-hole 2. The metallic substance
obstructs through-hole 2 in the form of a double-headed rivet.
The diameter of the through-hole is generally from 15 to 100 .mu.m, and
preferably from 20 to 50 .mu.m. The pitch of the through-holes is
generally from 15 to 200 .mu.m, and preferably from 40 to 100 .mu.m.
Insulating film 1 which can be used in the present invention is not
particularly limited in material as long as it possesses electrically
insulating characteristics. The material of the insulating film can be
selected according to the end use from a wide variety of resins, either
thermosetting or thermoplastic, including polyester resins, epoxy resins,
urethane resins, polystyrene resins, polyethylene resins, polyamide
resins, polyimide resins, ABS resins, polycarbonate resins, and silicone
resins. For example, elastomers, such as a silicone rubber, a urethane
rubber, and a fluorine rubber, are preferably used in cases where
flexibility is required; and heat-resistant resins, such as polyimide,
polyether sulfone, and polyphenylene sulfide, are preferably used in cases
where heat resistance is required.
The thickness of insulating film 1 is arbitrarily selected. From the
viewpoint of precision and variability of film thickness and through-hole
diameter, the film thickness is generally from 5 to 200 .mu.m, and
preferably from 10 to 100 .mu.m.
Metallic substance 3 which is filled in the fine through-hole to form a
conducting path and which forms bump-like projections 4 includes various
metals, e.g., gold, silver, copper, tin, lead, nickel, cobalt, and indium,
and various alloys of these metals. The metallic substance preferably does
not have high purity, but preferably contains a slight amount of known
organic and inorganic impurities. Alloys are preferably used as the
metallic substance.
The conducting path can be formed by various techniques, such as
sputtering, vacuum evaporation, and plating. In the case of plating, for
example, the bump-like projection having a bottom area larger than the
opening of the through-hole can be produced by prolonging the plating
time.
Fine through-holes 2 can be formed in insulating film 1 by mechanical
processes, such as punching, dry etching using a laser or plasma beam,
etc., and chemical wet etching using chemicals or solvents. Etching can be
carried out by, for example, an indirect etching process in which a mask
of a desired shape, e.g., a circle, a square, a rhombus, etc., is placed
on insulating film 1 in intimate contact and the film is treated via the
mask; a dry etching process in which a condensed laser beam is irradiated
on insulating film 1 in spots or a laser beam is irradiated on insulating
film through a mask, and a direct etching process in which a pattern of
fine through-holes is previously printed on insulating film 1 by using a
photosensitive resist and the film is then subjected to wet etching. In
order to make a finely patterned circuit, the dry etching process and the
wet etching process are preferred. In particular, a dry etching process
utilizing aggression by an ultraviolet laser beam, such as an eximar laser
beam, is preferred for obtaining a high aspect ratio.
If the through-holes are formed by using a laser beam, the diameter of the
through-hole on the side on which the laser beam is incident becomes
larger than the diameter on the opposite side, as shown in FIG. 3. It is
preferred that the through-holes are formed in such a manner that the
angle .alpha. formed by the through-holes with the surface of the
insulating film as shown in FIG. 1 and 3 falls within a range of
90.degree..+-.20.degree. and that the planar area of the through-holes is
more than the square of the product of 1.25.times.the film thickness (film
thickness.times.5/4).sup.2. Such a structure is effective for the
subsequent step of metal filling taking wettability of the hole wall by a
plating solution into consideration.
Metallic projection(s) 4 formed on the opening(s) of through-hole 2 should
have a larger bottom area than the planar area of through-hole 2,
preferably a bottom area at least 1.1 times the planar area of
through-hole 2, whereby the conducting path formed in through-hole 2 never
falls off while exhibiting sufficient strength against a shearing force
exerted in the film thickness direction and, thus, reliability of
electrical connection can be improved.
The anisotropic conductive film according to the present invention can be
produced, for example, by a process comprising:
(1) a step in which fine through-holes are provided in only an insulating
film of a laminated film comprising an insulating film and a conductive
layer (laminated either directly or via an adhesive layer), or a
conductive layer is laminated on an insulating film previously having fine
through-holes therein (the conductive layer should be laminated so that
the fine pores may pierce the insulating film or be removed after
laminating);
(2) a step in which the conductive layer positioned at the bottom of the
through-holes is etched to form a rivet-like dent;
(3) a step in which a metallic substance is filled in the fine
through-holes and the rivet-like dent, and further deposited to form
bump-like projections by plating (e.g., electroplating or electroless
plating); and
(4) a step in which the conductive layer laminated on the insulating film
is removed by chemical etching or electrolytic corrosion.
The formation of the bump-like metallic projections in step (3) above may
be conducted after step (4).
In the case where the bump-like projections are formed on one side of the
insulating film, the projections are preferably formed on the side where
the diameter of the through-hole is smaller than that of the opposite side
as shown in FIG. 3. Therefore, in the above step (1), the conductive layer
is preferably provided on the side having a smaller through-hole diameter
and a rivet-like dent is formed on the conductive layer.
In the formation of the bump-like metallic projections, it is preferred
that the metallic substance is formed as microcrystalline. Where
electroplating is performed at a high electrical current density,
arborescent crystals are formed in some cases, failing to form bumps.
Smooth and uniform projections can be formed by controlling a deposition
rate of metallic crystals or controlling the kind of a plating solution or
the temperature of a plating bath.
In order to form bump-like metallic projections having a larger bottom area
than the opening area of through-holes, it is necessary to allow a
metallic deposit to grow not only over the level of the opening, i.e., the
surface of the insulating film, but to the transverse direction from the
opening to make a rivet form. The height of the projections can be
selected arbitrarily according to the pitch of the holes or the end use,
and is generally 5 .mu.m or more, preferably from 5 to 100 .mu.m.
In cases where a conductive layer on the bottom side of the through-holes
is removed and a rivet-like bump is formed there, the bottom area of the
bump is preferably at least 1.1 times that of the through-hole. If the
bottom area of the bump is smaller than 1.1 times that of the though-hole,
the projection formed is less effective as a rivet-like bump, and desired
effects cannot be obtained in some cases.
The present invention is now illustrated in greater detail by way of the
following example, but it should be understood that the present invention
is not deemed to be limited thereto.
EXAMPLE
A polyimide precursor solution was coated on a copper foil to a dry film
thickness of 1 mil and cured to prepare a two-layer film composed of a
copper foil and a polyimide film.
A KrF exima laser beam having an oscillation wavelength of 248 nm was
irradiated on the polyimide film through a mask for dry etching to form
fine through-holes having a diameter of 60 .mu.m at a pitch of 200 .mu.m
per mm in an area of 8 cm.sup.2.
A resist was coated on the copper foil and cured for insulation. The film
having a resist layer was immersed in a chemical polishing solution at
50.degree. C. for 2 minutes, followed by washing with water. The copper
foil was connected to an electrode and soaked in a gold cyanide plating
bath at 60.degree. C., and a gold deposit was allowed to grow in the
through-holes with the copper foil as a negative electrode. Electroplating
was ceased when the gold deposit slightly projected from the polyimide
film surface (projection height: 5 .mu.m).
Finally, the resist layer was peeled off, and the copper foil was removed
by dissolving with cupric chloride to obtain an anisotropic conductive
film according to the present invention.
In the anisotropic conductive film of the present invention, the metallic
substance filled as a conducting path is sufficiently adhered to the
insulating film and undergoes no fall off. Thus, the fine through-holes
sufficiently exhibit conductivity as essentially required as conducting
paths to afford high reliability of electrical connection.
While the invention has been described in detail and with reference to
specific examples thereof, it will be apparent to one skilled in the art
that various changes and modifications can be made therein without
departing from the spirit and scope thereof.
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