Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2001-01-25
2003-05-20
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S004500
Reexamination Certificate
active
06564989
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wire bonding method known as a method for establishing an electrical connection in producing a semiconductor device, and a wire bonding apparatus used therefor. Further, it relates to a semiconductor device produced using the method and apparatus.
2. Description of the Background Art
Hitherto, in producing a semiconductor device having a semiconductor chip, the wire bonding method has been used for establishing electrical connection between a connection electrode of the semiconductor chip and an external leading electrode on the side on which the semiconductor chip is to be mounted. The material of the wire used for bonding generally contains aluminum or gold as a major component; however, those containing copper as a major component may be used in some cases. Hereafter, in the wire bonding method, the operation of connecting one end of the wire to the first site among the two sites to be connected with the wire is referred to as “first bonding”, and the operation of connecting the other end of the wire to the second site is referred to as “second bonding”.
Roughly classified, the wire bonding methods can be divided into the ball bonding method and the wedge bonding method.
With reference to
FIGS. 10A
,
10
B,
11
, and
12
, the ball bonding method will be described. Referring to
FIGS. 10A and 10B
, a step of forming a ball is performed before the first bonding. Namely, referring to
FIG. 10A
, a wire
2
is allowed to protrude from a tip end of a bonding tool
1
, and a torch electrode
35
is allowed to approach a wire tip end part
3
to generate an arc discharge. This allows the wire tip end part
3
to be instantaneously melted locally with a high temperature caused by the arc discharge, whereby a ball
31
such as shown in
FIG. 10B
is formed and resolidified in an extremely short period of time. After this ball
31
is pressed onto a site to be joined, a pressure is applied with the bonding tool
1
, and a supersonic wave is applied for vibration. Even if an oxide coating film or the like is present on a surface of the ball
31
or on a surface of the site to be joined, the oxide coating film is destroyed by friction accompanying the supersonic wave vibration, whereby the metals are brought into direct contact with each other to generate diffusion for joining. A heat may be applied besides the supersonic wave. For example, the metals are subjected to the supersonic wave vibration while being heated at about 300° C. Here, the ball bonding method is generally utilized only for the first bonding, and the second bonding is usually carried out in the same manner as the later mentioned wedge bonding method.
In the ball bonding method, a wire
2
material once melted immediately after the arc discharge resolidifies as the ball
31
, whereby a recrystallized region
34
appears at a neck part of the ball
31
, as shown in FIG.
11
. Such a recrystallized region
34
has a property of being hard and brittle, so that this part cannot be bent to a great extent. Therefore, the configuration of the wire bonding must be designed in such a manner that the wire
2
starts to bend mainly at a part above the recrystallized region
34
, as shown in FIG.
12
. For this reason, the ball bonding method cannot meet the demand of a so-called “lower loop” for restraining the loop height H, which is the maximum height of the wire
2
from the joining surface
15
.
As a countermeasure for solving this problem, an attempt is made to reduce the recrystallized region
34
by adjusting the amount of impurities mixed into the material of the wire
2
; however, another new problem occurs by adjusting the amount of the impurities. Namely, a so-called “sink mark” is generated in which holes appear in the inside of the ball
31
, or the joining property is deteriorated, or a brittle alloy layer is liable to be generated.
Further, by the ball bonding method, the ball
31
is deformed to a ball collapse diameter of 60 to 100 &mgr;m, which corresponds to about three to four times of the diameter (20 to 30 &mgr;m) of the wire
2
, for joining. This requires a large area for joining. Furthermore, the diameter of the ball
31
formed as a result of the arc discharge is varied, and it is difficult to precisely control the diameter. For this reason, the configuration must be designed on the basis of the maximum attainable diameter, thereby requiring a further large area. Therefore, the ball bonding method is extremely disadvantageous in satisfying the demand of a so-called “finer pitch” for reducing the pitch of the wire bonding and carrying out the wire bonding at a high density.
A method that compensates for this disadvantage is the wedge bonding method. Referring to
FIGS. 13
to
18
, the wedge bonding method will be described. In this method, first, a part of the wire
2
is allowed to protrude from the bonding tool
1
, as shown in FIG.
13
. The process up to this step is the same as that of the aforesaid ball bonding method. A wire bending rod
6
is displaceably placed in the vicinity of this protruding wire tip end part
3
. As shown in
FIG. 14
, the wire bending rod
6
moves to hit the wire tip end part
3
thereby to bend the wire tip end part
3
. As a result of this, the wire tip end part
3
is bent along the shape of a pressing surface
14
of the bonding tool
1
.
As shown in
FIG. 15
, the bonding tool descends together with the wire
2
, and the bent wire tip end part
3
is pressed onto an external leading electrode
7
which is an object of the first bonding. Here, in the same manner as in the ball bonding method, a supersonic wave is applied while applying pressure with the pressing surface
14
. Heat may be applied besides the supersonic wave. For example, supersonic wave vibration is applied while heating at about 300° C. As a result of this, the wire tip end part
3
is crushed into a flat shape, as shown in
FIG. 15
, and is joined to the external leading electrode
7
.
Next, as shown in
FIG. 16
, the bonding tool
1
is moved to a position of the second bond, and is pressed onto a connection electrode
53
of a semiconductor chip
8
. The wire
2
follows the movement of the bonding tool
1
from the first bond even if it is not bent by the wire bending rod
6
, so that a part of the wire
2
is always sandwiched by the pressing surface
14
and pressed. Here, in the same manner as in the first bonding, a supersonic wave is applied to the sandwiched wire
2
while the wire
2
is pressed by the pressing surface
14
. Alternatively, heat is applied besides the supersonic wave. Thus, the wire
2
is joined to the connection electrode
53
.
While a wire cutting clamp
9
is in a released state, the bonding tool rises by a length of L, as shown in
FIG. 17
, and the wire cutting clamp
9
is closed. While the wire
2
is held by the wire cutting clamp
9
, the bonding tool
1
rises. Then, the wire
2
is fractured at an end of the region where the wire is pressed into a flat shape by the second bond. As a result of this, the bonding tool
1
rises in a state in which the wire
2
is protruding by the length of L from its tip end, as shown in FIG.
18
. Then, the process proceeds to the next operation for the first bonding.
The wire tip end part
3
crushed by the wedge bonding method occupies an elongate shape having its width increased to about twice the diameter; however, the width is small as compared with the area occupied by the ball
31
in the ball bonding method, thereby providing an advantage for the finer pitch. Further, since it is not resolidified after being once melted, the recrystallized region
34
(See
FIG. 11
) does not appear, thereby providing an advantage for the lower loop.
However, in the wedge bonding method, the bending direction of the wire tip end part
3
is determined by the direction in which the wire bending rod
6
can move, so that the lying direction of the wire pressed onto the joining surface is determined. Therefore, the direction in which the wire bonding
Dunn Tom
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Stoner Kiley
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