U.S. patent application number 10/980788 was filed with the patent office on 2005-03-24 for substrate with terminal pads having respective single solder bumps formed thereon.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Akamatsu, Toshiya, Karasawa, Kazuaki, Nakanishi, Teru.
Application Number | 20050062157 10/980788 |
Document ID | / |
Family ID | 17084543 |
Filed Date | 2005-03-24 |
United States Patent
Application |
20050062157 |
Kind Code |
A1 |
Karasawa, Kazuaki ; et
al. |
March 24, 2005 |
Substrate with terminal pads having respective single solder bumps
formed thereon
Abstract
Methods and apparatus for forming solder bumps on terminal pads
of a semiconductor substrate for an integrated circuit device
employ a solder bump transfer plate and a mask to form solder
deposits on the plate. One embodiment of the invention employs a
metal mask having a plurality of through holes for forming solder
deposits on the solder bump transfer plate by vapor phase
deposition through the through holes each area of which increases
in step wise from the first surface of the mask to the second
surface opposite to the first surface, thereby preventing solder
deposits in the through holes from being removed when the mask is
separated from the plate. Another embodiment of the invention is a
solder bump transfer plate having a plurality of solder deposits on
the surface non-wettable to molten solder both diameter and spacing
of which are both smaller than diameter and spacing of the terminal
pads on the semiconductor substrate, whereby a single solder bump
is accurately formed on each of the terminal pads without a fine
alignment technique.
Inventors: |
Karasawa, Kazuaki;
(Kawasaki-shi, JP) ; Nakanishi, Teru;
(Kawasaki-shi, JP) ; Akamatsu, Toshiya;
(Kawasaki-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
17084543 |
Appl. No.: |
10/980788 |
Filed: |
November 4, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10980788 |
Nov 4, 2004 |
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09650633 |
Aug 28, 2000 |
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09650633 |
Aug 28, 2000 |
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09222316 |
Dec 29, 1998 |
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6136047 |
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09222316 |
Dec 29, 1998 |
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08649837 |
Apr 30, 1996 |
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6008071 |
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Current U.S.
Class: |
257/738 ;
257/E21.508 |
Current CPC
Class: |
H01L 2224/05144
20130101; H01L 2224/05171 20130101; H01L 2924/351 20130101; H01L
2224/05144 20130101; H01L 2224/05644 20130101; H01L 2924/10253
20130101; H01L 2924/00013 20130101; H01L 2924/00013 20130101; H01L
2924/014 20130101; H01L 2224/05155 20130101; H01L 2224/05155
20130101; H01L 2924/01049 20130101; H05K 3/3457 20130101; H01L
2224/05147 20130101; H01L 24/11 20130101; H01L 2224/1143 20130101;
H01L 24/17 20130101; H01L 2924/01013 20130101; H01L 2924/01051
20130101; H01L 2924/01079 20130101; H01L 2224/11334 20130101; H01L
2924/00012 20130101; H01L 2924/00 20130101; H01L 2224/05147
20130101; H01L 2924/00014 20130101; H01L 2224/1701 20130101; H01L
2224/13027 20130101; H05K 2203/0338 20130101; H01L 2224/05155
20130101; H01L 2924/01029 20130101; H01L 2224/13099 20130101; H01L
2224/05644 20130101; H01L 2224/05166 20130101; H01L 2924/01078
20130101; H01L 2924/01024 20130101; H01L 2224/05171 20130101; H01L
2224/11849 20130101; H01L 2924/14 20130101; H01L 24/12 20130101;
H01L 2224/1701 20130101; H05K 2201/0317 20130101; H01L 2924/09701
20130101; H01L 2924/01082 20130101; H01L 2924/00014 20130101; H01L
2924/351 20130101; H01L 2924/00014 20130101; H01L 2924/013
20130101; H01L 2924/01024 20130101; H01L 2224/29099 20130101; H01L
2924/00014 20130101; H01L 2924/00014 20130101; H01L 2924/00014
20130101; H01L 2924/00 20130101; H01L 2924/01074 20130101; H01L
2224/05166 20130101; H05K 3/048 20130101; H01L 2924/01072 20130101;
H01L 2924/10253 20130101; H01L 2224/11003 20130101; H01L 2924/01033
20130101; H01L 2924/01006 20130101; H01L 2924/01032 20130101 |
Class at
Publication: |
257/738 |
International
Class: |
H01L 023/48 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 20, 1995 |
JP |
7-242116 |
Claims
1. (Cancelled)
2. (Cancelled)
3. (Cancelled)
4. (Cancelled)
5. (Cancelled)
6. (Cancelled)
7. (Cancelled)
8. (Cancelled)
9. (Cancelled)
10. (Cancelled)
11. (Cancelled)
12. (Cancelled)
13. (Cancelled)
14. (Cancelled)
15. (Cancelled)
16. A mask having a first surface and a second surface, opposite to
the first surface, for forming solder deposits onto a surface of a
substrate against which the second surface of the mask is pressed,
comprising: a mask sheet; and a plurality of through holes
extending through the mask sheet, a cross-sectional area of each
throughole increasing in steps from the first surface of the mask
to the second surface while maintaining a similar cross-sectional
shape and centered about a common axis.
17. The mask according to claim 16, wherein the mask sheet
comprises: plural laminated layers having circular through-holes
extending therethrough, each of which through-holes consists of
concentric holes having successively increasing diameters, layer by
layer, for respective, successive layers from the first surface of
the mask to the second surface of the mask.
18. The mask according to claim 16, wherein the substrate is a
semiconductor substrate having a plurality of metalized terminal
pads.
19. The mask according to claim 16, wherein the substrate is a
substrate for a solder bump transfer plate for transferring bumps
onto terminal pads on an integrated circuit device.
20. (Cancelled)
21. (Cancelled)
22. (Cancelled)
23. (Cancelled)
24. (Cancelled)
25. (Cancelled)
26. (Cancelled)
27. (Cancelled)
28. The mask according to claim 16, further comprising a
mask-holder for pressing the second surface of the mask against the
substrate.
29. The mask according to claim 16, wherein the mask sheet is a
laminated mask of nickel alloy sheets.
30. (Cancelled)
31. (Cancelled)
32. (Cancelled)
33. (Cancelled)
34. (Cancelled)
35. (Cancelled)
36. (Cancelled)
37. (Cancelled)
38. (Cancelled)
39. (Cancelled)
40. (Cancelled)
41. (Cancelled)
42. (Cancelled)
Description
FIELD OF INVENTION
[0001] The present invention relates to an integrated circuit
device, more particularly to methods of transferring solder bumps
onto an integrated circuit device, such as a flip chip
semiconductor device, and to an apparatus for transferring the
solder bumps, such as a solder bump transfer plate or a metal mask
for forming solder deposits on the plate.
BACKGROUND OF THE INVENTION
[0002] It is known that a semiconductor chip having an array of
terminal pads on a chip surface is mounted on a printed circuit
board or another semiconductor chip also having an array of
terminal pads by flip-chip method, wherein the arrays of terminal
pads on a chip are connected with each other by vertical solder
bump interconnections between a chip and a printed circuit board or
another semiconductor chip. For typical processing, solder bumps
are transferred from a solder bump transfer plate to each of
terminal pads on a chip surface. Solder deposits on a solder bump
transfer plate are usually formed on a glass substrate by vapor
phase deposition with a metal mask or by selective electroplating
method. Generally, as packing-density of integrated circuits
increases, both size and space of terminal pads are needed to
decrease, from which various technical problems arise, such as
solder bridges connecting between adjacent terminal pads causing
short circuit between the terminal pads, or non-uniformity of
solder amount application per pads causing electrical disconnection
between a vertical solder bump interconnection. Solder deposits,
which are predecessors of solder bumps, on a solder bump transfer
plate formed by vapor phase deposition through through-holes of a
metal mask are often detached from deposited sites when the metal
mask is separated from the solder-bump transfer plate, because the
solder deposits are often adhering to inside walls of the
through-holes. In Japanese Laid-open Patent Application No.
5-235003, a method is described that an inside wall of a
through-hole of a metal mask is lined with material having
non-wettable tendency to molten solder. In this method, however,
repelled solder is solidified around a solder bump in cooling as
solder bridges or solder balls which often cause short circuit
between terminal pads adjacent to each other. A solder ball is
usually produced on a surface of a semiconductor chip between
terminal pads from an excessive solder extending to the outskirts
of a solder deposit deposited on a solder bump transfer plate using
a metal mask when the solder deposit is melt to transfer onto the
terminal pad. As an attempt to remove the solder ball described in
Japanese Laid-open Patent Application No. 63-261857, photosensitive
polyimide film is formed on the whole surface of a semiconductor
chip except terminal regions and solved by organic solvent later.
However, this method is incompatible to semiconductor chips having
polyimide as an insulating film.
SUMMARY OF INVENTION
[0003] It is an object of the present invention to provide a solder
bump transfer device for transferring solder bumps onto terminal
pads of a semiconductor device without a severe aligning
requirement.
[0004] It is another object of the present invention to provide a
solder bump transfer device for transferring solder bumps onto
terminal pads having a fine size and a narrow spacing on a
semiconductor device without leaving solder bridges or solder balls
between the terminal pads.
[0005] It is a further object of the present invention to provide a
method for transferring solder bumps having a uniformity in height
and strength onto terminal pads of a semiconductor device.
[0006] It is a still further object of the present invention to
provide a mask for forming solder deposits on a surface of a
substrate or a plate by vapor phase deposition through
through-holes of the mask and for being removed easily without
detaching the solder deposits in the through-holes.
[0007] In one aspect of the present invention, both diameter and
spacing of solder deposits on a solder bump transfer plate are
smaller than diameter and spacing as well of terminal pads on a
semiconductor device, whereby a single solder bump is formed on
each of the terminal pads without a severe aligning requirement.
Needless to say, an cross-sectional area of the solder deposits and
an area of the terminal pads are not necessarily limited to a
circle. In another aspect of the present invention, the whole
surface except terminal pads of a semiconductor device is coated
with material non-wettable to molten solder which is removed later
together with solder balls remaining thereon. In further aspect of
the present invention, a mask has through-holes each diameter of
which increases in step wise from the first surface of the mask to
the second surface opposite to the first surface, thereby, after
solder deposits are deposited through the through-holes on a
surface of a solder bump transfer plate against which the second
surface of the mask is pressed, the mask is easily removed without
detaching a solder deposit in a through-hole.
[0008] The techniques according to the present invention may be
applicable to any planar surface of a substrate to form a plurality
of solder bumps thereon, and to stacked flat plates interconnected
by solder bumps therebetween.
BRIEF DESCRIPTION OF DRAWINGS
[0009] Preferred embodiments of the invention are described with
reference to the accompanying drawings, in which:
[0010] FIGS. 1A through 1D are diagrammatic section views of a pair
of a solder bump transfer plate and a semiconductor substrate in
various processing steps related to the first embodiment in
accordance with the present invention.
[0011] FIGS. 2A through 2F are diagrammatic section views of a pair
of a solder bump transfer plate and a semiconductor substrate in
various processing steps related to the second embodiment in
accordance with the present invention.
[0012] FIGS. 3A through 3C are diagrammatic section views of a
solder bump on a semiconductor substrate in various processing
steps related to the third embodiment in accordance with the
present invention.
[0013] FIGS. 4A through 4D are diagrammatic section views of a pair
of a solder bump transfer plate and a semiconductor substrate in
various processing steps related to the fourth embodiment in
accordance with the present invention.
[0014] FIGS. 5A through 5D are diagrammatic section views of a pair
of a metal mask and a solder bump transfer plate in various
processing steps related to the fifth embodiment in accordance with
the present invention.
[0015] FIG. 6 is a diagrammatic section view of a pair of a metal
mask and a solder bump transfer plate related to the sixth
embodiment in accordance with the present invention.
[0016] FIGS. 7A through 7D are diagrammatic section views of a
solder bump transfer plate, a metal mask, and a semiconductor
substrate in various processing steps related to the seventh
embodiment in accordance with the present invention.
[0017] FIG. 8 is a graph showing the height distribution of solder
bumps across a semiconductor chip fabricated by a single
transferring operation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Referring to FIG. 1A, a solder bump transfer plate 1 is a
glass plate a surface on which a plurality of solder deposits 11
made of Indium (In) alloy with 48 wt % Tin (Sn) are formed in a
matrix having a spacing of 200 .mu.m and a diameter of 100 .mu.m by
screen printing method. A substrate 2 is an alumina ceramic circuit
board for a hybrid IC a surface on which a plurality of terminal
pads 21 made of 0.1 .mu.m thick electro-plated gold on 0.5 .mu.m
thick nichrome (hereafter noted by 0.1 .mu.m thick Au/0.5 .mu.m
thick Ni) are arranged in a matrix having a spacing of 800 .mu.m
and a diameter of 400 .mu.m.
[0019] Referring to FIG. 1B, after solder flux 4 is applied, the
solder bump transfer plate 1 is positioned on the substrate 2
without a fine alignment such that the same number of the solder
deposits 11 rest on each of the terminal pads 21.
[0020] Referring to FIG. 1C, an assembly of the plate 1 and the
substrate 2 is heated to 150.degree. C. for 2 minutes in a furnace
such that the solder deposits resting on a terminal pad melt to be
transferred onto the terminal pad to form a single solder bump 3,
and solder deposits not resting on a terminal pad melt to form
solder balls 31 in solder flax 4.
[0021] Referring to FIG. 1D, after the assembly is cooled, the
substrate 2 is separated from the plate 1 and washed to removed the
solder flux together with the solder balls therein, then an alumina
ceramic circuit board having a single solder bump on each of the
terminal pads is obtained.
[0022] The above example can be modified as follows:
[0023] A solder bump transfer plate 1 is a polyimide film, solder
deposits 11 made of Indium alloy are formed in a matrix having a
spacing of 100 .mu.m, a diameter of 50 .mu.m, and height of 50
.mu.m by employing a metal mask (not shown). Terminal pads 21 made
of 0.1 .mu.m thick Au/0.5 .mu.m thick Ni are arranged in a matrix
having a spacing of 500 .mu.m and a diameter of 200 .mu.m. After
solder flux 4 is applied, the solder bump transfer plate 1 is
positioned on the substrate 2 without a fine alignment, and pressed
at 150.degree. C. with 5 kgf such that the solder deposits 11
resting on the terminal pads 21 are thermally bonded to the
terminal pads. The assembly is heated to 220.degree. C. such that
the solder deposits resting on a terminal pads are transferred onto
the terminal pad to form a single solder bump 3, the rest of
process is substantially the same as that of the first example,
wherein a substrate may be Si chip, solder alloy may be Bi, Ga, Ge,
Sb, or Pb-63% Sn other than or In-48% Sn.
[0024] It should be noticed that since spacing and diameter of the
solder deposits 11 are both smaller than those of the terminal pads
21 in these examples, no fine alignment of solder deposits to
terminal pads is required.
[0025] Referring to FIG. 2A, a solder bump transfer plate consists
of a Si substrate 1 and Pb-5% Sn solder deposits thereon. The
solder deposits are deposited through a metal mask (not shown)
having through-holes of 130 .mu.m in diameter at the first surface
and 170 .mu.m in diameter at the second surface opposite to the
first surface by pressing the second surface against the Si
substrate 1, where an inside-wall of the through-holes is sloped by
an angle of 100.degree. from the first surface. A deposit 111 on
the most right site is intentionally drawn smaller than the others
11 having a height of 30 .mu.m. A Si substrate 2 for integrated
circuits has a plurality of terminal pads 21 which has the same
diameter and spacing as those of the solder deposits, respectively
in this embodiment.
[0026] Referring to FIG. 2B, after solder flux is applied, the
solder bump transfer plate 1 is positioned on the Si substrate 2
such that each of the terminal pads is aligned to a corresponding
one of the solder deposits and that all of the aligned solder
deposits including the smaller solder deposit 111 are in contact
with the corresponding terminal pads. The solder flux is applied to
the surface of the solder bump transfer plate 1 as same as
before.
[0027] Referring to FIG. 2C, an assembly of the plate and the Si
substrate is heated at 360.degree. C. such that each of the solder
deposits is transferred onto the corresponding terminal pad to form
a single solder bump on each of the terminal pads after separating
the plate from the Si substrate and washing away the solder
flux.
[0028] When the assembly is heated, the solder deposit is melt to
transform itself into a droplet of molten solder. Transferring the
droplet of molten solder onto the corresponding terminal pad
probably arises from collective effects of a gravity, a wettable
tendency of the terminal pad to molten solder, a non-wettable
tendency of the glass plate to molten solder, and a surface tension
of the droplet. Therefore, the solid single solder bump maintains a
spherical shape covering the entire wettable surface of the
metalized terminal pad. A specific single solder bump transferred
from the shorter deposit 111 inevitably has a height lower than
those the others have. This would cause disconnection of a vertical
interconnection if the semiconductor substrate would be mounted on
a printed circuit board by flip-chip method as it is. The main
reason for non-uniformity of a bump height is result from
non-uniformity of a diameter of through-holes in a metal mask which
is originated from.
[0029] FIG. 8 is a graph of height distribution of solder bumps
across a chip made by a single transferring operation, which
indicates that solder bumps having heights deviated largely denoted
by a solid arrow for a taller bump and an empty arrow for a shorter
bump, for example, by more than 10% of the average value, are quite
few. Since a taller bump can be deformed at flip-chip connection,
it is not needed to be considered as a detective bump. For a
practical use, no disconnection is found out for solder deposits
having height not less than 90% of the average value. Thus, the
disconnection problem caused by a shorter bump can be avoided by
repeating the entire processing steps from forming solder deposits
on a glass plate to transferring solder bumps onto metalized
terminal pads of a semiconductor chip. For example, if it is
repeated twice, the solder deposits for a single deposition may
have half a volume of the finally required volume.
[0030] Although a certain precaution is needed to prevent a shorter
solder deposit on a bump transfer plate from being systematically
aligned to another shorter solder bump at an identical specific
site on a semiconductor substrate, if a through-hole of a metal
mask has a diameter smaller by 20% than the average value at the
rate of {fraction (1/10,000)}, the probability that two smaller
bumps will meet with each other is less than ({fraction
(1/10,000)}).sup.2 which is practically a negligible small value.
Thus, according to the twice repetition method, for instance, if a
semiconductor device has 3,000 terminals on a chip, the
disconnection will occur at the rate of less than one out of 30,000
units, while it will occur at the rate of one out of three units by
the conventional method. The repetition numbers increase, the
defective rate decreases sharply.
[0031] The finally required bump height is obtained by
twice-repetition of solder deposition, wherein height of a solder
deposit is one-half of the solder deposit by a single deposition
which will give the finally required bump height. Since relative
volumes of two cylindrical solder deposits deposited through a
circular through-hole having a normal diameter and another circular
through-hole having a diameter smaller by 20% than the normal one
are 0.5 and 0.5.times.(1-0.2).sup.2, namely 0.32, respectively, a
spherical solder bump made by the above two solder deposits will
have a bump height of (0.5+0.32).sup.1/3, namely 0.94 which is
within .+-.10% tolerance.
[0032] Referring to FIG. 2D, for the reason discussed above, a
metal mask having a through-hole whose diameter is less than 80% of
the average value is removed as a defective unit in mask
inspection.
[0033] Referring to FIG. 2E, subsequently, the steps are repeated
so as to transfer another solder bump onto each of the single
solder bumps already made on the terminal pads by the previous
steps.
[0034] Referring to FIG. 2F, after separating the plate, and
washing away the solder flux, the Si substrate 2 is obtained which
has a single solder bump on each of the terminal pads whose height
is higher than that of the first single solder bump and that an
error of the height will be less than 10% of the average value.
[0035] An application of the above repetition method to a Si
substrate showed an average height of the solder bumps 84.3 .mu.m
high, the minimum height 87.9 .mu.m high, and the maximum height
87.9 .mu.m high. By employing this Si substrate, a CPU module is
assembled with a nitric aluminium circuit board by flip-chip
bonding method without flux wherein no defective unit is found out
at electric testing in vertical interconnections. Similar
experimental data are summarized in Table 1 and 2, where Table 1
shows heating temperatures for various bump solders and Table 2
shows bump heights for the various bump solders.
1 TABLE 1 Transfer Deoxidized Bonding Bump solders temperature
temperature temperature Pb-63 wt % Sn 220.degree. C. 210.degree. C.
260.degree. C. In 200.degree. C. 180.degree. C. 260.degree. C.
In-34 wt % Bi 200.degree. C. 150.degree. C. 260.degree. C.
[0036]
2 TABLE 2 Average Minimum Maximum Bump solders height height height
Pb-63 wt % Sn 83.8 .mu.m 79.1 .mu.m 87.5 .mu.m In 84.1 .mu.m 79.6
.mu.m 88.5 .mu.m In-34 wt % Bi 84.0 .mu.m 78.0 .mu.m 87.4 .mu.m
[0037] As an example, by employing a metal mask having
through-holes of a nominal diameter 150 .mu.m on the first surface
and a nominal diameter 180 .mu.m on the second surface with an
inside wall of a tapering angle 100.degree. from the first surface
which actually has an average diameter 150 .mu.m, and the minimum
diameter larger than 125 .mu.m on the first surface, cylindrical
solder deposits of Pb-5 wt % Sn were formed on a solder bump
transfer glass plate to get spherical solder bumps of an average
height 85 .mu.m and the minimum height 75 .mu.m, and then
transferred onto Ni-metalized terminal pads on a Si substrate at
360.degree. C. This transfer processing step was repeated twice.
The following measurement of bump heights revealed that an average
height of 84.9 .mu.m, the minimum height of 78.3 .mu.m, and the
maximum height of 90.1 .mu.m. The Si substrate with these bumps was
mounted on an AlN substrate by flip-chip method to complete a CPU
module. Electrical reliability tests on these devices gave a result
that no defective units were found out. Particularly, it was found
out from various reliability tests that a hourglass shaped solder
bump connecting both substrates at terminal pads is more desirable
than a barrel shaped solder bump, because a thermal stress may
easily concentrate on an interface between a solder bump and the
connecting terminal pad, while easy inelastic deformation of a
solder bump near the middle point would absorb the thermal
stress.
[0038] From these experiments, it has been assured that a
combination of selecting a metal mask by inspecting through-holes
such that the minimum diameter of the through-hole is determined in
advance and of repeating transfer of solder bumps is effective to
equalizing the final height of the solder bumps.
[0039] A method for fabricating a flip-chip device comprising two
major processing steps is effective to achieve a remarkable result
in higher reliability in electric interconnections, wherein the two
major processing steps are firstly to select a metal mask by
inspecting through-holes such that an acceptable mask has
through-holes whose opening area has a predetermined minimum limit
in unavoidable deviation from the average value, and secondly to
repeat the solder bump transfer process until every bumps reach a
finally required height on metalized terminal pads of a substrate.
The method effectively eliminates defective units having
disconnection failure.
[0040] Referring to FIG. 3A, a basic part of the method for forming
solder bumps on terminal pads on a semiconductor substrate for this
embodiment was the same as that as shown in FIGS. 2A through 2C.
However, the semiconductor substrate 2 had an insulating layer 22
on an entire surface except the terminal pads 21, and also had a
metal pattern 6 partly on the terminal pads 21 and partly on the
insulating layer 22 such that a surface of the corresponding
terminal pad was partly exposed. The terminal pads 21 was metalized
by nickel which may be replaced by other metals wettable to molten
solder such as Au, Ti, Cu, Cr or any combination of these. The
insulating layer 22, which was polyimide, was non-wettable to
molten solder. The metal pattern 6 was 0.5 .mu.m thick gold layer
and had essentially the same diameter as that of the solder
deposit. The metal pattern 6 was wettable to molten solder and
easily melts into the molten solder. In the step of positioning the
solder bump transfer plate on the semiconductor substrate 2, each
of the solder deposits 11 was aligned to, and in contact with the
metal pattern 6 which was already aligned eccentrically from the
corresponding terminal pad 21.
[0041] Referring to FIG. 3B, by heating, the solder deposit was
melt into a solder droplet 11 resting on the metal pattern 6 in an
early stage, and subsequently the metal pattern 6 was also melt
into the solder droplet. Melt-down of the metal pattern 6 made the
solder droplet contact with both the insulating layer 22 and the
terminal pad 21 which resulted in a repulsive force to push the
solder droplet to the terminal pad 21, and simultaneously an
attractive force to pull the solder droplet into the exposed
surface of the terminal pad 21 as indicated by an arrow. A fact
that a surface of the insulating layer was higher than that of the
terminal pad also assisted the solder droplet to move to a center
of the terminal pad by gravitational force.
[0042] Referring to FIG. 3C, at the final stage, a surface tension
made the solder droplet spherical resting on the entire surface of
the terminal pad 21 within an opening of the insulating layer, and
by cooling, the solder droplet was solidified into a solder bump 3
which maintained a spherical shape concentric with the terminal pad
21.
[0043] Referring to FIG. 4A, a solder bump transfer plate 1 had
solder deposits 11 each of which usually had a tapering part 32
around the solder deposit. The tapering part 32 was, more or less,
concomitantly formed by depositing Pb-5 w % Sn solder deposits of
30 .mu.m high through a metal mask (not shown) by vapor phase
deposition. The metal mask had through-holes of 130 .mu.m in
diameter at the first surface and 170 .mu.m in diameter at the
second surface opposite to the first surface. The deposition was
carried out by pressing the second surface against the Si substrate
1, where an inside wall of the through-holes was tapered by an
angle of 100.degree. from the first surface. A semiconductor
substrate 2 had terminal pads 7 on the surface and a 0.1 .mu.m
thick aluminium layer 21 therebetween. The terminal pads 7 was
metalized by gold. The aluminium layer 21 was non-wettable to
molten solder.
[0044] Referring to FIG. 4B, after applying solder flux, the solder
bump transfer plate was positioned on the substrate 2 by aligning
the solder deposits to the gold metalized terminal pads, and then
the assembled substrate was heated at 360.degree. C. until each of
the solder deposits were melt into a single solder droplet on the
corresponding terminal pad.
[0045] Referring to FIG. 4C, after cooling the assembled substrate,
and then separating the glass plate 1 from the substrate 2, a
solidified spherical single solder bump 3 was formed on each of the
terminal pads and solder balls 31 were left on the aluminium layer
21 with remainder of the solder flux 11.
[0046] Referring to FIG. 4D, by washing away the solder flux and
then immersing the substrate into an etchant consisting of 90 ml
water, 15 ml HCL, and 10 ml HF to remove the aluminium layer 21 and
solder balls 31 together, the substrate 2 having the single solder
bumps on the terminal pads 7 was finally obtained.
[0047] As a non-wettable layer to molten solder formed on an entire
surface except terminal pads, heat resistant polymer like polyimide
is easily removed by basic solution, but for a substrate already
employing polyimide for a component, a metal layer like aluminium
is preferred in selective etching without etching solder bumps and
metalized terminal pads.
[0048] Referring to FIG. 5A, Pb-5 wt % Sn solder deposits 11 of 30
.mu.m high were formed on a solder bump transfer glass plate 1 by
using a metal mask 5, wherein in advance to depositing the solder
deposits 11, an aluminium layer 7 of 0.1 .mu.m thick was deposited
on the entire surface of the glass plate 1 except areas for the
solder deposits 11 to be deposited, and the metal mask had
through-holes of 130 .mu.m in diameter on the first surface and 170
.mu.m in diameter on the second surface with an inside wall of a
tapering angle 100.degree. from the first surface. The second
surface of the metal mask was pressed against the surface of the
glass plate 1 when the solder deposits 11 were formed.
[0049] Referring to FIG. 5B, after separating the metal mask 5 from
the solder bump transfer glass plate 1, a tapering part 32 was
unavoidably left on the aluminium layer 7 around each of the solder
deposits 11.
[0050] Referring to FIG. 5C, by heating the solder bump transfer
glass plate 1 at 320.degree. C. in an atmosphere of
N.sub.2--H.sub.2 (4:1 in volume), each of the solder deposits 11
changed into a spherical solder bump on the solder bump transfer
glass plate 1 while the tapering part 32 changed into solder balls
31 on the aluminium layer 7.
[0051] Referring to FIG. 5D, by immersing the solder bump transfer
glass plate 1 into an etchant consisting of 90 ml distiled water,
15 ml HCl, and 10 ml HF, the aluminium layer 7 around each of the
solder deposits 11 were removed together with the solder balls 31
completely. Thus, a solder bump transfer glass plate 1 was obtained
which had spherical solder bumps 3 without a tapering part 32 or a
solder ball 31 around the spherical solder bumps.
[0052] Referring to FIG. 6, a metal mask 5 was a laminated mask
consisting of the first mask 51 of 50 .mu.m thick 42 -Nickel (Ni)
alloy and the second mask 52 of 50 .mu.m thick 42-Ni alloy. The
first and second masks had concentric holes of 170 .mu.m and 140
.mu.m in diameters, respectively. A substrate 2 for printed
circuits had terminal pads 21 of 100 .mu.m in diameters metalized
by a triple layer of Au(top)/Ni/Ti(bottom). To form a plurality of
solder bumps on the substrate 2, the hole of the mask 5 was aligned
to the terminal pad pressing the second mask against the substrate
2 by a magnetic mask-holder (not shown). Subsequently a solder
deposit of 30 .mu.m in height was deposited on the substrate by
vapor phase deposition of Pb-5 wt % Sn solder through each of the
concentric holes over the first mask. Since the solder deposits
formed on the substrate in concentric holes of the metal mask were
not in contact with side walls of the concentric holes, none of the
solder deposits was found to be defective after the metal mask 5
was separated from the substrate 2. It made mask-separation without
detaching solder deposits possible that a hole of the first metal
mask was smaller than that of the second metal mask. For
deoxidizing and shaping the solder bumps 3, the substrate 2 was
coated by solder flux and then heated at a temperature higher than
314.degree. C. to melt the solder bumps. After cooling the
substrate 2 and washing the solder flux away, the substrate 2
having a plurality of spherical solder bumps was completed without
a defective bump.
[0053] Thus, a flip-chip bonded device will be easily constructed
by positioning the above-completed substrate 2 having a plurality
of spherical solder bumps on a Si chip having Au/Ni/Ti metalized
terminal pads and subsequently reflowing the solder bumps in
N.sub.2 atmospheric furnace at a temperature of 350.degree. C.
[0054] Referring to FIG. 6, Pb-5 wt % Sn solder in the above
example was replaced by Indium (In) solder, which changed the bump
transferring temperature from 314.degree. C. to 215.degree. C., and
the flip-chip bonding temperature from 350.degree. C. to
260.degree. C., respectively. Indium solder bumps was formed on the
metalized terminal pads of the Si chip in advance to flip-chip
bonding which was actually carried out by bonding the In solder
bumps to each other between the Si chip and the substrate for
printed circuit board, wherein the In solder bumps were bonded to
each other at a temperature of 260.degree. C. in a vapor of
fluorocarbon without solder flux.
[0055] Referring to FIG. 7A, a metal mask 5 was a laminated 42
Ni-alloy mask consisting of the first mask 51 having holes of 170
.mu.m in diameter and 50 .mu.m in thickness and the second mask 52
having holes of 140 .mu.m in diameter and 50 .mu.m in thickness as
referred to FIG. 6. The metal mask was pressed against a Si
substrate 1 for a solder bump transfer plate such that each of the
holes of the first mask was concentrically aligned to the
corresponding hole of the second mask by employing a mask holder
(not shown) in a solder deposition chamber (also not shown). Pb-63%
Sn solder of 30 .mu.m thick was deposited over the metal mask 5 and
subsequently the mask was separated from the Si substrate to form
solder deposits 11 on the Si substrate. Thus, the completed solder
bump transfer plate was obtained without any defective bumps.
[0056] Referring to FIG. 7B, another Si substrate 2 was provided to
transfer solder bumps from the solder bump transfer plate. The Si
substrate 2 already had integrated circuits therein and a plurality
of Ni/Ti metalized terminal pads 21 on a surface of the Si
substrate.
[0057] Referring to FIG. 7C, the solder bump transfer plate was
positioned on the Si substrate 2 with solder flux 4 such that each
of the deposits 11 was aligned to the corresponding Ni/Ti metalized
terminal pads 21, and then the whole substrate was heated at a
temperature of 250.degree. C. in an N.sub.2 atmospheric furnace
(also not shown) such that the solder bumps were reflowed to be
transferred to the Ni/Ti metalized terminal pads 21.
[0058] Referring to FIG. 7D, after the solder bump transfer plate 1
was separated from the Si substrate 2, to deoxidize and reshape the
solder bumps 3, the substrate 2 was coated by solder flux and again
heated at a temperature of 250.degree. C. to melt the solder bumps.
After cooling the substrate 2 and washing the solder flux away, the
substrate 2 having a plurality of spherical solder bumps was
completed without a defective bump.
[0059] Referring to FIGS. 7A through 7D, Pb-5 wt % Sn solder in the
above example can be replaced by other solders containing In, Bi,
Ga, or Sb. The bump transfer plate can be chosen from ceramics and
heat resistant polymer like polyimide instead of Si and glass. The
terminal pads can be metalized by various combination of metal
layers such as Au/Ni/Ti or Cu/Cr depending upon bump materials.
[0060] While the invention has been described having references in
particular preferred embodiments and modifications thereto, various
changes in form and detail may be made without departing the spirit
and scope of the invention as claimed.
* * * * *