Bump electrode, semiconductor integrated circuit device using the same, multi-chip module having the semiconductor integrated circuit devices and method for producing semicondutcor device having the bump electrode

A bump electrode includes a core portion provided on an intermediate electrode layer formed on an electrode pad formed on a surface of an element. The core portion contains a material having a Young's modulus less than that of soldering. An electrically conductive film covers the core portion.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention generally relates to semiconductor integrated circuit 
devices, and more particularly to a bump electrode used in such devices. 
More particularly, the present invention is concerned with a bump 
electrode for flip chip mounting. 
High-density mounting of semiconductor elements has been required to cope 
with recent demands of speeding-up and down-sizing of computer systems. A 
flip chip mounting method utilizing bump electrodes is known as a method 
for achieving high-density mounting of semiconductor elements. In the 
above flip chip mounting, it has been desired to lengthen the 
fatigue-based duration of life in regard to a temperature cycle. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a bump electrode having a 
longer fatigue-based duration of life. 
The above object of the present invention is achieved by a bump electrode 
comprising: a core portion provided on an intermediate electrode layer 
formed on an electrode pad formed on a surface of an element, the core 
portion containing a material having a Young's modulus less than that of 
soldering; and an electrically conductive film covering the core portion. 
For example, the core portion comprises silicone resin, and the core 
portion has a dome shape. It is possible to further provide a protection 
film which covers the electrically conductive film except for a portion of 
the electrically conductive film. The portion of the electrically 
conductive film is to be in contact with an electrode formed on a member 
on which the element should be mounted. The protection film comprises an 
insulating material. 
Another object of the present invention is to provide a semiconductor 
device equipped with such a bump electrode. 
This object of the present invention is achieved by a semiconductor device 
comprising: a chip; and a plurality of bump electrodes, each of the bump 
electrodes comprising: a core portion provided on an intermediate 
electrode layer formed on an electrode pad formed on a surface of the 
chip, the core portion containing a material having a Young's modulus less 
than that of soldering; and an electrically conductive film covering the 
core portion. 
The above object is also achieved by a semiconductor device comprising: a 
chip; a plurality of bump electrodes; a circuit board having pads which 
are in contact with the plurality of bumps; a package base supporting the 
circuit board and having terminals connected to the pads; and leads 
electrically connecting the circuit board to the terminals of the package 
base. Each of the bump electrodes comprises: a core portion provided on an 
intermediate electrode layer formed on an electrode pad formed on a 
surface of the chip, the core portion containing a material having a 
Young's modulus less than that of solder; and an electrically conductive 
film covering the core portion. 
Yet another object of the present invention is to provide a method for 
producing the above-mentioned bump electrode. 
This object of the present invention is achieved by a method for producing 
a semiconductor device having a prior art bump electrode as described 
above, the method comprising the steps of: (a) forming a core portion on 
an intermediate electrode layer formed on an electrode pad formed on a 
surface of an element, the core portion containing a material having a 
Young's modulus less than that of soldering; and (b) forming an 
electrically conductive film covering the core portion. For example, the 
step (a) comprises steps of: (a-1) forming an ultraviolet-hardening 
silicone resin on the surface of the element; and (a-2) patterning the 
ultraviolet-hardening silicon resin so that a projection of the 
ultraviolet-hardening silicon resin is formed, the projection 
corresponding to the core portion of the bump electrode. For example, the 
step (a-2) further comprises the step of removing edges of the projection. 
For example, the method further comprises the step (c) of forming a 
protection film which covers the electrically conductive film except for a 
portion of the electrically conductive film, the portion of the 
electrically conductive film to be in contact with an electrode formed on 
a member on which the element should be mounted, the protection film 
comprising an insulating material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To facilitate a better understanding of the present invention, a 
description will now be given with reference to an art related to the 
present invention. 
FIG. 1 is a cross-sectional view of a prior art bump electrode 10 related 
to the present invention. An electrode pad 12 and a protection film 13 are 
formed on a lower surface 11a of a semiconductor element 11. The bump 
electrode 10 is formed so that a soldering bump 14 is provided on the 
electrode pad 12 via a metallic intermediate layer 15. 
FIG. 2 shows a semiconductor device 17 in which the semiconductor element 
11 equipped with the bump electrode 10 is flip-chip mounted on a mounting 
circuit board 16. The semiconductor device 17 radiates heat during 
operation. When the semiconductor device 17 stops operating, it is cooled 
to the room temperature and subjected to a temperature cycle. 
The soldering bump 14 is an alloy of Pb/Sn (9:1), and the Young's modulus 
is as high as 1.9.times.10.sup.3 (kgf/mm.sup.2), so that the soldering 
bump 14 is not likely to deform. Generally, the semiconductor element 11 
is made of silicon, and the coefficient of thermal expansion thereof is 
3.5.times.10.sup.-6 (1/.degree.C.). The mounting circuit board 16 is made 
of AlN or glass epoxy resin. The coefficient of thermal expansion of AlN 
is 4.2.times.10.sup.-6 (1/.degree.C.), and the coefficient of thermal 
expansion of glass epoxy resin is 12.0.times.10.sup.-6 (1/.degree.C.). 
There is a difference between the coefficient of thermal expansion of the 
mounting circuit board 16 and that of the semiconductor element 11. Each 
time the semiconductor device 17 is subjected to the temperature cycle, 
stress based on the above difference is exerted on the soldering bump 14. 
The inventors obtained, by a computer simulation, the fatigue-based 
duration of life of bump electrode 10 (soldering bump 14) in accordance 
with the temperature cycle. Even when the AlN mounting circuit board 16 is 
used in such a way that the difference in the coefficient of thermal 
expansion is reduced, the fatigue-based duration of life of the bump 
electrode 10 (soldering bump 14) was approximately 2600 times. 
As shown in FIG. 3, after a number of times that the temperature cycle is 
repeatedly carried out, cracks 18 occur in the soldering bump 14. When the 
temperature cycle has been carried out approximately 2600 times, soldering 
bump 14 is broken as indicated by reference number 19, and accordingly, 
electric conduction is broken in soldering bump 14. The duration of life 
of 2600 temperature cycles is not a sufficiently long period of time. 
If the mounting circuit board 16 is made of glass epoxy resin in order to 
reduce the production cost of the semiconductor device 17, there is a 
large difference between the coefficient of thermal expansion of the 
semiconductor element 11 and that of the mounting circuit board 16. Hence, 
the duration of life will be greatly shorter than the duration of life of 
2600 temperature cycles. 
The present invention is intended to lengthen the duration of life of bump 
electrodes and thus semiconductor devices equipped with such bump 
electrodes. 
FIG. 4 shows a prior art bump electrode 30 according to an embodiment of 
the present invention. 
A description will first be given of a structure related to bump electrode 
30. 
A semiconductor element 31 such as a chip to be mounted, is made of 
silicon, and has a lower surface 31a on which an electrode pad 32, made of 
aluminum, is formed. The electrode pad 32 has a diameter a of 200 to 300 
.mu.m. Further, the lower surface 31a of the semiconductor element 31 is 
covered by a protection film 33 made of C-PSG except for the electrode pad 
32. The electrode pad 32 is covered by a metallic intermediate layer 34 
serving as a metal barrier layer. 
The bump electrode 30 will be described below. The bump electrode 30 is 
made up of a core portion 35 and an electrically conductive film 36. The 
core part 35 of the bump electrode 30 is made of UV 
(Ultraviolet)-hardening silicone resin marketed by SHINETSU SILICONE 
COMPANY in Japan. The core portion 35 has a reverse dome shape and is 
formed on the metallic intermediate layer 34. The core portion 35 has a 
diameter b of 150 to 200 .mu.m, and a height of 50 to 150 .mu.m. The 
Young's modulus of the UV-hardening silicone resin is 1.4.times.10.sup.1 
(kgf/cm.sup.2), and is approximately equal to 1/100 of the Young's modulus 
of soldering. Hence, the core portion 35 has a small stiffness and is 
flexible. 
The conductive film 36 is made of gold (Au) and covers the whole surface of 
the core portion 35. The conductive film 36 is electrically connected to 
the surface of the metallic intermediate layer 34. The conductive film 36 
is 5 to 20 .mu.m thick. 
The core portion 35 has a small stiffness and is flexible, and is easily 
deformed by a small magnitude of external force. Also, the conductive film 
36 is easily deformed in response to a deformation of the core portion 35. 
Hence, the bump electrode 30 is easily deformed by an external force, as 
indicated by the two-dot chained lines shown in FIG. 4. 
The bump electrode 30 is covered by the protection film 37 except for the 
lowermost (apex) portion 30a, the protection film 37 being made of 
thermoplastic silicone resin. The protection film 37 is provided taking 
into account the fact that the adhesion between the conductive film 36 and 
the core portion 35 is weak. The protection film 37 presses down the 
conductive film 36 in order to prevent the conductive film 36 from coming 
off from the core portion 35. 
The protection film 37 covers the periphery of the bump electrode 30. The 
protection film 37 has a small Young's modulus of 1.4.times.10.sup.1 
(kgf/mm.sup.2), and is flexible. Hence, the protection film 37 prevents 
deformations of the bump electrode 30. 
A description will now be given, with reference to FIG. 5, of the function 
of the bump electrode 30 in the state in which the semiconductor element 
31 is mounted. 
FIG. 5 is a multi-chip module having bump electrodes having the structure 
shown in FIG. 4. A plurality (two in FIG. 5) of semiconductor elements 31 
are mounted on an AlN mounting circuit board 41 by the flip-chip method 
utilizing bump electrodes 30. The bump electrodes 30 have lowermost 
portions, which are electrically connected to electrode pads 42 provided 
on the mounting circuit board 41. The semiconductor elements 31 and the 
mounting circuit board 41 are electrically connected via the conductive 
films 36 of the bump electrodes 30. 
The mounting circuit board 41 is fastened to the upper surface of a package 
base 43. Electrodes 51 formed on the mounting circuit board 41 are 
electrically connected to pads 49 formed on the upper surface of the 
package base 43 by means of wires 45. The pads formed on the upper surface 
of the package base 43 are electrically connected to pin terminals 44, 
which project from the lower surface of the package base 43. A heat sink 
46 to which the semiconductor elements 31 are attached is supported by a 
seal ring 47 placed on the upper surface of the package base 43, so that 
the semiconductor elements 31 and the mounting circuit base 41 are sealed. 
While the multi-chip module 40 is operating, it radiates heats and the 
semiconductor elements 31 and mounting circuit board 41 are thermally 
expanded. A mismatch between the coefficient of thermal expansion of the 
semiconductor elements 31 and that of the mounting circuit board 41 can be 
easily absorbed due to the mechanism by which the bump electrodes 30 
yields in the shearing direction of the bump electrodes 30, as shown in 
FIG. 6. 
The inventors studied, by computer simulation, the duration of life of the 
bump electrodes 30 due to the temperature cycle. The obtained duration of 
life of the bump electrodes 30 was approximately equal to 5000 times, 
which is approximately twice the duration of life of the related art bump 
electrodes. It is thus concluded that the bump electrode 30 has a greatly 
longer duration of life than the related art bump electrodes. 
Further, the bump electrodes 30 have the following advantages in addition 
to the above advantage. First, even if there are deviations in the heights 
of the bump electrodes 30, higher bump electrodes are deformed in the 
direction in which the higher bump electrodes are depressed so that the 
bump electrodes can be leveled. Hence, even if there are deviations in the 
heights of the bump electrodes 30, abnormal stress is not applied to the 
semiconductor elements 31 and the mounting surface board 41, and the 
multi-chip module 40 has a high reliability. 
Second, cleaning on the mounting circuit board 41 is not needed when a 
defective semiconductor element is removed and a new one is mounted on the 
mounting circuit board 41. 
A description will now be given, with reference to FIGS. 7A through 7F, of 
the method of producing the bump electrode 30 shown in FIG. 4. 
FIG. 7A shows step 50 of forming an UV-hardening silicone resin layer 60. 
In step 50, UV-hardening silicone resin is coated to a thickness t.sub.1 
of 50 to 150 .mu.m on the surface of the semiconductor element 31 
including the protection layer 33. The electrode pad 32, the protection 
film 33 and the metallic intermediate layer 34 have been formed on the 
semiconductor element 31 before step 50. 
FIG. 7B shows an UV exposure step 51. In this step, a mask 61 is used and 
ultraviolet rays are selectively projected onto portions of the 
UV-hardening silicone resin layer 60 corresponding to the position of the 
electrode pad 32. The above portion onto which the ultraviolet rays are 
projected is hardened. 
FIG. 7C shows an etching step 52. In step 52, etching is performed using an 
organic solvent, and the masked unexposed portion of the UV-hardening 
silicone resin layer 60 is removed. A projection 64 made of silicone resin 
remains on the metallic intermediate layer 34. 
FIG. 7D shows an etching step 53, in which etching is performed by using an 
organic solvent so that edges of the projection 64 are removed and the 
projection 64 has a dome shape. The dome-shaped projection 64 corresponds 
to the core portion 35. 
FIG. 7E shows step 54 of forming the conductive film 36. In step S54, 
evaporating or plating of Au is performed so that the conductive film 36 
having a thickness t.sub.2 of 5 to 20 .mu.m is formed on the surface of 
the core portion 35. Thereby, the bump electrode 30 is formed. 
FIG. 7F shows step 55 of forming the protection film 37. In this step, 
thermoplasticity silicone resin is coated so that the apex portion 30a of 
the bump electrode 30 is exposed. 
The bump electrode 30 thus formed can be applied to another element to be 
mounted on a circuit board. For example, an InSb infrared image sensor 
equipped with the bump electrode 30 can be mounted on a circuit board. 
The present invention is not limited to the specifically disclosed 
embodiments, and variations and modifications may be made without 
departing from the scope of the present invention.