Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1998-10-16
2001-05-15
Picardat, Kevin M. (Department: 2823)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S106000, C438S613000, C257S734000, C257S737000
Reexamination Certificate
active
06232211
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a two-staged projecting bump of a semiconductor element and a method for forming the same by bonding a metallic ball formed at a leading end of a metallic wire to an electrode of the semiconductor element.
BACKGROUND ART
A conventional two-staged projecting bump and its formation method will be described with reference to
FIGS. 12A-20
.
FIGS. 12A-12F
represent a conventional example of a method for forming a two-staged projecting bump.
In the conventional example, as shown in
FIG. 12A
, a metallic wire
1
of gold, copper, aluminum or solder is passed through a ceramic or ruby capillary
3
. A discharging action is brought about between a leading end of the passed metallic wire
1
and an electrode
6
, namely, torch, whereby a metallic ball
2
is formed at the leading end of the metallic wire
1
.
In
FIG. 12B
, the metallic ball
2
is pressed onto a preheated electrode
4
of a semiconductor element
8
, to which an ultrasonic vibration is impressed. The metallic ball
2
is bonded to the electrode
4
because of the temperature, pressure and the ultrasonic vibration. A reference numeral
5
is a passivation film for protecting an active face of the semiconductor element
8
.
In
FIG. 12C
, the capillary
3
is raised in a vertical direction. Then, the capillary
3
is shifted sideways and descended as in
FIG. 12D
to bring the metallic wire
1
into contact with the metallic ball
2
. The metallic wire
1
is bonded to the metallic ball
2
by temperature and pressure or, temperature, pressure and ultrasonic vibration. The capillary
3
is moved upward as shown in
FIG. 12E
, and the metallic wire
1
is pulled and broken as in
FIG. 12F
, whereby a two-staged projecting bump
7
is formed.
According to the above-described prior art, when the metallic ball
2
is formed by the metallic wire
1
through the discharging action, crystal grains of the metallic wire
1
at a part B immediately above the metallic ball
2
(which is denoted as a recrystallized area) are influenced by the heat and become coarse, as shown in
FIG. 13. A
breaking load of the part B is consequently reduced to nearly half in comparison with a part A not influenced by the heat.
In a case where the two-staged projecting bump
7
is formed through the processes of
FIGS. 12A-12F
, if a length of the part B which is decreased in breaking load to nearly half due to the influences of the heat as described above is elongated as shown in
FIG. 14
with respect to a length of a part C where the metallic wire
1
and metallic ball
2
come in touch with each other, that is, a length C from immediately above the metallic ball
2
to a point D where the metallic wire
1
is to be pulled and broken, a breaking load of the point D where the metallic wire
1
is to be pulled and broken becomes approximately equal to the breaking load of the part B. Therefore, the exact part to be broken cannot be controlled, resulting in the generation of a defective two-staged projecting bump
7
a
including an excess portion of the metallic wire
1
as illustrated in
FIG. 16
, in contrast to the two-staged projecting bump
7
of
FIG. 15
in a normal shape. The projections of bumps in two stages are thus irregularly shaped as shown in FIG.
17
A.
When a conductive paste film
9
is transferred to the defective two-staged projecting bump
7
a
in a transfer method in the next step as shown in
FIG. 17B
, the amount of a conductive paste
10
transferred is too much as indicated in FIG.
17
C.
In the event that the thus-constituted semiconductor element including the defective two-staged projecting bump
7
a
is bonded to electrodes
13
of a circuit board
12
in the above state, referring to
FIG. 18
, there is a likelihood that the excessive amount of conductive paste
10
will cause shortcircuits at portions
11
among the electrodes
13
.
The above defective two-staged projecting bump
7
a
which may cause the shortcircuit accident is also formed by the following reason.
In the process of bonding the metallic wire
1
with the metallic ball
2
, depending on a diameter of the used metallic wire
1
or a size of the-metallic ball
2
formed, etc., the metallic wire
1
is caught by the capillary
3
and pressed at E as shown in
FIG. 19
, which leads to a narrow part (neck)
14
as shown in FIG.
20
. The metallic wire
1
is eventually broken starting from the narrow part
14
when pulled and broken. In consequence, there is a likelihood that the defective two-staged projecting bump
7
a
with the excessive metallic wire
1
is generated as shown in
FIG. 16
, and too much conductive paste is transferred in the next process, thereby shortcircuiting the electrodes when the semiconductor element is mounted to the circuit board, as mentioned above.
Accordingly, the object of the present invention is to provide a good two-staged projecting bump of a semiconductor element without any variation in shape and a method for forming the bump.
SUMMARY OF THE INVENTION
In order to achieve the aforementioned objective, according to a first aspect of the present invention, there is provided a two-staged projecting bump which is formed on an electrode of a semiconductor element in a method of melting a leading end of a metallic wire passed through a capillary thereby forming a metallic ball. The metallic ball is bonded to the electrode of the semiconductor element and the capillary is moved sideways and down. The metallic wire is then bonded onto the metallic ball bonded to the electrode, and the metallic wire is pulled and broken.
A metallic wire part immediately above the metallic ball is thermally influenced to turn crystal grains coarse during the process of melting the leading end of the metallic wire thereby forming the metallic ball. The wire art is located inside a boundary where the bonding between the metallic ball and the metallic wire terminates.
In the arrangement of the first aspect, a crystal grain structure of the metallic wire on or above the position where the metallic wire is to be pulled and broken is made uniform, thereby making a breaking load at the part uniform. The metallic wire is accordingly broken starting from the position having a reduced sectional area due to the pressure of the capillary at which the metallic wire is to be pulled and broken. In other words, the metallic wire is always broken at the position to be pulled and broken, so that the two-staged projecting bump of a uniform shape can be obtained. Thus, the two-staged projecting bumps can be formed with no variations in shape.
According to a second aspect of the present invention, there is provided a method of forming a two-staged projecting bump on a semiconductor element, which includes melting a leading end of a metallic wire passed through a capillary thereby forming a metallic ball. The metallic ball is bonded to an electrode of a semiconductor element, and the capillary is moved sideways and down. The metallic wire is bonded onto the metallic ball bonded to the electrode, and the metallic wire is pulled and broken, thereby forming a two-staged projecting bump on the electrode.
A parameter of melting in the process where the leading end of the metallic wire passed through the capillary is melted thereby forming the metallic ball is controlled. As a result, a metallic wire area immediately above the metallic ball where crystal grains are thermally influenced by the melting and thus turned coarse is accommodated in length within a position where the metallic wire is to be pulled and broken.
According to a third aspect of the present invention, there is provided a method of forming a two-staged projecting bump on a semiconductor element defined in the second aspect, wherein the parameter of the melting process is a discharging time for impressing a voltage between the leading end of the metallic wire and a metallic ball formation electrode when the leading end of the metallic wire is melted thereby forming the metallic ball.
In general, in a case where the melting process is carried out by meltin
Collins D. M.
Matsushita Electric - Industrial Co., Ltd.
Picardat Kevin M.
Wenderoth , Lind & Ponack, L.L.P.
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