Title: Conductive material and method for filling via-hole
Abstract: The conductive material comprises a first metal material having a melting point of not more than 250° C. and a second metal material having a melting point of not less than 500° C., and is paste at a temperature not more than 250° C. Whereby the conductive material can have much higher conductivity than the resin paste. The conductive material can be used in paste, whereby the conductive material can be buried in the via-hole in the same way as the resin paste.
Patent Number: 6,886,248 Issued on 05/03/2005 to Watanabe, et al.
| Inventors:
|
Watanabe; Isao (Kawasaki, JP);
Hashimoto; Kaoru (Kawasaki, JP);
Taniguchi; Osamu (Kawasaki, JP)
|
| Assignee:
|
Fujitsu Limited (Kawasaki, JP)
|
| Appl. No.:
|
348948 |
| Filed:
|
January 23, 2003 |
Foreign Application Priority Data
| Jan 23, 2002[JP] | 2002-014470 |
| Current U.S. Class: |
29/852; 29/825; 29/854; 427/97.7 |
| Intern'l Class: |
H01K 003//10 |
| Field of Search: |
29/825,852,854,832
427/977,97
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Arbes; Carl J.
Attorney, Agent or Firm: Armstrong, Kratz, Quintos, Hanson & Brooks, LLP
Claims
1. A method for filling a via-hole comprising the steps of:
in an equipment having a first pressure, supplying a conductive material to a
substrate with a via-hole formed in to bury the conductive material in an upper
end part of the via-hole; and
changing the pressure in the equipment to a second pressure which is higher than
the first pressure to thereby draw the conductive material buried in the upper
end part of the via-hole into the via-hole.
2. A method for filling a via-hole according to claim 1, wherein
a temperature of the substrate is raised to a temperature where the conductive
material become paste.
3. A method for filling a via-hole according to claim 1, wherein
the step of burying the conductive material and the step of drawing the conductive
material are repeated to fill the conductive material in the via-hole.
4. A method for filling a via-hole according to claim 1, further comprising the
step of:
performing heat processing at a temperature higher than a melting point of the
conductive material after the conductive material has been filled in the via-hole.
5. A method for filling a via-hole according to claim 1, wherein
in the step of burying the conductive material, the conductive material is rubbed
into the substrate with a squeegee.
6. A method for filling a via-hole according to claim 1, wherein
in the step of burying the conductive material, the conductive material is poured
into the substrate with a dispenser.
7. A method for filling a via-hole according to claim 1, wherein
the conductive material comprises a first metal material having a melting point
of not more than 250° C. and a second metal material having a melting point
of not less than 500° C., and is paste at a temperature of not more than 250° C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior
Japanese Patent Application No. 2002-014470, filed in Jan. 23, 2002, the entire
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a conductive material and a method for filling
a via-hole, more specifically a conductive material of high conductivity which
can fill micronized via-hole without generating void, and method for filling a
via-hole suitable for such the conductive material.
In electronic circuit boards and semiconductor devices, conductors are buried
in via-holes formed in insulating films, for connecting interconnection layers
on different levels.
As methods for filling conductors in such via-holes, conventionally method for
growing conductor in a via-hole by electroplating and method for filling in a via-hole
a conductive resin paste containing metal powders dispersed in an organic component
have been used.
However, with the recent micronization of electronic circuit boards and
semiconductor devices, diameters of via-holes are so micronized that the conventional
methods for filling the via-hole are insufficient to fill such via-holes.
Specifically, in the method for filling the via-hole by electroplating,
as shown in FIG. 3A, usually a plated film 106 starts to grow on the sidewall
and the bottom surface of the via-hole 104. At the upper end edge, where
an electric field is concentrated, and a current density is higher, a growth rate
is increased in this region. Accordingly, before the inside of the via-hole 104
is completely filled with the plated film 106, the upper part of the via-hole
104 is closed, leaving a void 108 at the center of the via-hole 104
(FIG. 3B). The generation of the void 108 becomes significant as
the via-hole 104 has a larger aspect ratio. Resultantly, defective conduction
and resistance increases often take place, and the voids are often expanded by
thermal treatment to thereby break device structures.
As an improvement of this method, a method for suppressing the growth of a plated
film from the sidewall of the via-hole to fill the via-hole only by the growth
of the plated film from the bottom surfaces is being studied. However, this method
makes no void in the via-hole, but the plating takes much time, which results in
very high costs.
The filling method using the resin paste is advantageous in costs to the filling
method by the plating. However, the resin paste, which is mixtures of the organic
component and the metal powders, has problems of high conduction resistances, residual
gas components, low usable limit temperatures, etc.
As described above, it cannot be said that the conventional methods for filling
the via-hole have been sufficient to fill micronized via-holes. A technique for
filling micronized via-holes with a material of high conductivity at low costs
without generating voids has been required.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a conductive material of high
conductivity which can fill micronized via-holes at low costs without generating voids.
Another object of the present invention is to provide a method for filling
a via-hole which is suitable for such the conductive material.
According to one aspect of the present invention, there is provided a conductive
material comprising a first metal material having a melting point of not more than
250° C. and a second metal material having a melting point of not less than
500° C., and being paste at a temperature of not more than 250° C.
According to another aspect of the present invention, there is provided
a method for filling a via-hole comprising the steps of: in an equipment having
a first pressure, supplying a conductive material to a substrate with a via-hole
formed in to bury the conductive material in an upper end part of the via-hole;
and changing the pressure in the equipment to a second pressure which is higher
than the first pressure to thereby draw the conductive material buried in the upper
end part of the via-hole into the via-hole.
According to the present invention, the conductive material contains the
first metal material of a melting point of not more than 250° C. melting point
and the second metal material of a melting point of not less than 500° C.,
and is paste at a temperature of not more than 250° C., whereby the conductive
material can have much higher conductivity than the resin paste. The conductive
material can be used in paste, whereby the conductive material can be buried in
the via-hole in the same way as the resin paste.
In an equipment having a first pressure, the conductive material is fed into a
via-hole formed in a substrate to be buried in upper end part of the via-hole,
the pressure in the equipment is changed a second pressure which is higher than
the first pressure, whereby the conductive material buried in the upper end part
of the via-hole is drawn into the via-hole to thereby fill the conductive material
in the via-hole. Even micronized via-hole can be filled without generating voids.
BRIEF DESCRIPTION FO THE DRAWINGS
FIGS. 1A-1C and 2A-2D are sectional views explaining the method
for filling the via-hole according to one embodiment of the present invention.
FIGS. 3A and 3B are sectional views explaining the problems of the conventional
method for filling the via-hole.
DETAILED DESCRIPTION OF THE INVENTION
The conductive material according to the present invention contains a metal material
A of a melting point of not more than 250° C. and a metal material B of a
melting point of not less than 500° C., and is a paste at a temperature of
not more than 250° C.
Here, the conductive material according to the present invention contains the
metal material A of a melting point of not more than 250° C. so that the conductive
material becomes paste when the conductive material is filled in the via-hole.
The conductive material according to the present invention is filled in the via-hole
by the method which will be described later at about 250° C. at highest. For
the conductive material to be paste at this temperature, the metal material A is
liquid, i.e., a melting point of the metal material A must be not more than 250° C.
Preferably a melting point of the metal A is suitably selected in accordance
with a substrate temperature, etc. in a process using the conductive material.
In consideration of workability it is preferable to use a metal material of a single
element or an alloy material which is liquid at the room temperature to about 100°
C. For example, an alloy of low melting point, such as In-76% Ga alloy, which is
liquid at the room temperature, 12% Sn-18% Pb-49% Bi-21% In, whose melting point
is 58° C., 16% Sn-33% Bi-51% In, whose melting point is 61° C., 47% Sn-57%
Bi-26% In, whose melting point is 79° C., 22% Sn-28% Pb-50% Bi, whose melting
point is 96-110° C., etc.
As the metal material A, it is preferable to use an alloy containing bismuth.
The usual metal has a property of expanding the volume with temperature increases,
but bismuth has a property of shrinking with temperature increase. The metal material
A contains bismuth so that the volume expansion of the other metal due to temperature
increase can be absorbed by bismuth, whereby the volume expansion of the conductive
material as a whole can be suppressed.
The conductive material according to the present invention contains the metal
material B whose melting point is not less than 500° C. so that the metal
material B is dispersed in the liquid metal material A to form the paste conductive
material. To this end it is necessary to disperse the metal material B in a state
of powder into the metal material A.
The metal material B can be a metal which can be pulverized and does not melt
at a melting point of the metal material A. For example, a single metal powder
as of Cu, Ag, Zn, Ni, Fe, Pd, Au, Pt or others, an ally powder as of Au—Sn,
Cu—Sn, Fe—Ni or others, or a composite metal powder as of Ag plated
Cu powder, Au plated Ni powder or others can be used. The surface oxide film degrades
wetting property with respect to the metal material A. It is preferable that all
the metal powders are subjected to anti-oxidation treatment with fatty acid, etc.
When the melting point of the metal material B is near that of the metal material
A, the metal material B melts into the metal material A to increase a melting point
of the conductive material itself, which causes a risk of making difficult to fill
the via-hole. From this viewpoint, it is preferable that the melting point of the
metal material B is not less than 500° C.
The conductive material according to the present invention is made paste for
the following reason. That is, in using the conductive material according to the
present invention as a conductive material for filling the via-hole, the conductive
material formed of the molten metal material A alone cannot fill micronized via-hole
due to high surface tension, and even when the conductive material can be filled
in the via-hole, it is difficult to retain the conductive material in the via-hole.
The metal material B is added to the metal material A and sufficiently kneaded.
The surfaces of the metal material B is wetted with respect to the metal material
A, the metal material A plays the role of vehicles of the conductive paste and
solder paste, and the composite material becomes paste. Thus, the paste can be
filled in even micronized via-hole in the same way as the resin paste. The metal
material A even in the molten state is retained owing to the wetting with respect
to the metal material B, thereby settling stable without flowing out after filled
in the via-hole.
An addition ratio of the metal material B for making the conductive material
paste
varies depending on a kind and a particle size of the metal material B but is in
the range of about 5-40 wt %.
On the other hand, in the resin conductive paste, the electric conduction is
made
by the contacts among the particles of a metal powder, and a total of resistances
of the contacts is a conduction resistance of the via-hole. A value is large. On
the other hand, the conductive material according to the present invention, which
is paste containing the metal material B dispersed in the metal material A, becomes
a single metal in via-hole, and has low conduction resistance and can have conductivity
equal to that of bulk metal.
Next, the method suitable to fill the conductive material according to the
present invention in the via-hole will be explained with reference to FIGS. 1A-1C
and
2A-
2D. FIGS. 1A-1C and
2A-
2D are sectional views
explaining the method for filling the via-hole according to the present embodiment.
In the following description, the conductive material according to the present
invention is filled in the via-hole which are passed through a silicon substrate,
but the present invention is not limited to the present embodiment.
First, the conductive material according to the present invention, containing
powder of the metal material B dispersed in the metal material A is prepared. The
conductive material is prepared by heating the metal material A up to above a melting
point thereof, dispersing, and mixing and kneading a prescribed amount of the powder
of the metal material B in the molten metal material A to thereby prepare the paste
conductive material.
Then, a silicon substrate
10 with via-holes
12 for the conductive
material to be filled in is mounted on a stage
10 of, e.g., a vacuum printing
machine (FIG.
1A). The via-holes
12 are formed through the silicon
substrate
10. A silicon oxide film
14 is formed on the surfaces of
the silicon substrate including the inside walls of the via-holes
12.
A conducting layer
22 of, e.g., a 0.5 μm-thick Pt/Ti film is formed
on the stage
20. The conducting layer
22 is used as an electrode.
Next, the stage
20 is heated to a temperature which is higher by 5-10°
C. than a melting point of the conductive material.
Then, the conductive material
30 which has become paste by heating is
set on the silicon substrate
10. The conductive material is set on the silicon
substrate
10 at the edge where the via-holes
12 are not formed.
Then, the interior of the printing machine is depressurized down to a prescribed pressure.
Next, the conductive material
30 is rubbed into the via-holes
12
by means of, e.g., urethane or stainless squeegee
40. Thus, the conductive
material
30 is buried in the upper end part of the via-holes
12 (FIG.
1B-FIG.
1C).
Then, the interior of the printing machine is returned to the atmospheric pressure.
At this time, the voids
16 formed in the via-holes
12 on the side
nearer the stage
20 have a lower pressure than the atmospheric pressure,
and the conductive material
30 buried in the via-holes
12 on the
side of the surfaces thereof is drawn into the via-holes
12 (FIG.
2A).
A pressure in the printing machine is made higher than an internal pressure in
the printing machine at the time of burying the conductive material in the upper
end part of the via-holes, whereby the conductive material can be drawn into the
via-holes. Accordingly, an internal pressure of the printing machine at the time
of rubbing in the conductive material is not essentially the atmospheric pressure.
Similarly therewith, an internal pressure of the printing machine at the time of
burying the conductive material in the upper end parts of the via-holes is not
essentially a decreased pressure.
Then, in the same way as described above, the application of the conductive
material
30, the depressurized processing, the squeezing (FIG.
2B),
and the atmospheric pressure processing (FIG. 2C) are repeated, whereby the conductive
material
30 buried in the via-holes
12 is sequentially drawn into
the via-holes
12, and finally the via-holes
12 are filled with the
conductive material
30 (FIG.
2D).
Next, after the conductive material
30 is buried in the via-holes
12,
heat processing is performed as required. This heat processing has the effect of
reacting the metal material B with the metal material A to thereby making a melting
point of the conductive material high. This heat processing permits the device
to be used stable at temperatures higher than an application temperature of the
conductive material.
The conductive material is thus filled in the via-holes, whereby the conductive
material of high conductivity can be filled in the via-holes without leaving voids.
It is possible to use, e.g., a dispenser in place of the squeegee to feed the
conductive material sequentially into the respective via-holes. In this case as
well, the application of the conductive material, the depressurized processing,
and the atmospheric pressure processing are repeated to effectively fill the conductive
material into the via-holes. In the case where the dispenser is used, the dispenser
having a heating mechanism is used to thereby apply the conductive material according
to the present invention in paste.
After the conductive material
30 is filled in the via-holes
12,
a cap plating may be formed on the surface of the via-holes
12.
The conductive material
30 according to the present invention can be filled
in the via-holes in the same way as the conventional resin conductive paste by
making a temperature for the filling higher than a melting point of the conductive material.
EXAMPLE 1
16 g of Sn, 33 g of Bi and 51 g of In were loaded in a porcelain crucible, and
heated and melted at a temperature of 70-80° C. Then, 10 g of 2-3 μm-diameter
Cu powder was added and mixed, and the conductive material was prepared.
Then, a silicon substrate having 50 μm-diameter via-holes formed at a
150 μm-pitch and having the surface covered with a silicon oxide film is
set on the stage of a vacuum printing machine.
Then, the stage of the vacuum printing machine is heated to 70° C.
Next, the conductive material heated in advance to 70° C. is set on the
upper side of the substrate (the edge part where the via-holes are not formed),
and the printing machine was drawn the vacuum.
Then, when a vacuum level arrived at 0.1 kPa, the conductive material was rubbed
in by urethane rubber squeegee. Furthermore, the conductive material was oppositely
squeezed, and then the vacuum level was returned to the atmospheric pressure at once.
Then, the drawing the vacuum was again performed, and the same operation described
above was repeated twice.
The thus-prepared specimen was cleaved, and the cleavage plane was polished and
observed with an SEM. The conductive material was filled down to the bottoms of
the via-holes, and neither voids nor gaps were observed.
A conduction resistance of the via was measured and was 25 mΩ/via. The
conduction
resistance which is much lower than about 300-400 mΩ/via of the typical conduction
resistance of the via using the conventional resin paste could be realized.
EXAMPLE 2
20 g of Ag-coated Cu powder of Φ6-7 μm is added to 100 g of molten
metal of 18.8 wt % of Sn, 31.3 wt % of Pb and 50 wt % of Bi having a melting point
of 95° C., and agitated and mixed into paste. The conductive material was
thus prepared.
The conductive material was filled into the via-holes in the same way as in Example
1. In Example 2, the filling operation was made with a stainless squeegee with
the stage heated to 105° C.
The thus-prepared specimen was cleaved, and the cleavage plane was polished and
observed with an SEM. The conductive material was filled down to the bottoms of
the via-holes, and neither voids nor gaps were observed. The conduction resistance
of the via was 20 mΩ/via.
EXAMPLE 3
20 g of Cu powder of Φ5 μm is added to 100 g of molten metal of 48
wt % of Sn and 52 wt % of In having a melting point of 117° C., and agitated
and mixed into paste. The conductive material was thus prepared.
The conductive material was filled into via-holes in the same way as in Example
1. In Example 2, the filling operation was made with a stainless squeegee with
the stage heated to 125° C.
The thus-prepared specimen was cleaved, and the cleavage plane was polished and
observed with an SEM. The conductive material was filled down to the bottoms of
the via-holes, and neither voids nor gaps were observed. The conduction resistance
of the via was 20 mΩ/via.
*
