Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2000-11-15
2002-04-09
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S001100
Reexamination Certificate
active
06367685
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of semiconductor package wire bonding, and, more particularly, to a novel apparatus and method for producing an ultrasonic vibration mode to improve the bond between a wire and a die or lead.
2. Description of the Related Art
Wire bonding today is used throughout the microelectronics industry as a means of interconnecting chips, substrates, and output pins. Automatic ultrasonic gold ball bonding is a high yield interconnect process that uses heat and ultrasonic energy to form a metallurgical bond. Typically, high purity gold wire is used with a ball bond formed at one end and a stitch bond at the other.
FIGS. 1
a
through
1
g
show the typical sequence of steps involved in forming a gold ball bond.
FIG. 1
a
shows a capillary
10
which is targeted on the bond pad and positioned above a die
12
with a ball
14
formed on the distal end of a wire
16
and pressed against the face of capillary
10
. Capillary
10
descends as shown in
FIG. 1
b
, bringing ball
14
into contact with die
12
. The inside radius of capillary
10
grips ball
14
in forming the bond. Ultrasonic vibration energy is then applied. The ultrasonic vibration energy is typically produced by piezoelectric transducers. Piezoelectric transducers are well known in the industry and comprise a piezoelectric material, i.e. a material that converts mechanical energy into electrical energy and vice versa. In the case of producing ultrasonic vibration energy, an electric field is applied to a piezoelectric ceramic to stimulate vibration. After ball
14
is bonded to die
12
with the aid of the ultrasonic vibration energy, capillary
10
raises to the loop height position as shown in
FIG. 1
c
. A clamp
18
is then opened and wire
16
is free to feed out of the end of capillary
10
. Next, a lead
20
of the device is positioned under capillary
10
and capillary
10
is lowered to the lead. Wire
16
is fed out the end of capillary
10
, forming a loop as shown in
FIG. 1
d
. The capillary continues downward and deforms wire
16
against lead
20
, producing a wedge-shape bond which has a gradual transition into the wire as shown in
FIG. 1
e
. Ultrasonic vibration energy is once again applied to enhance the bond strength. Capillary
10
then raises off lead
20
as shown in
FIG. 1
f
, leaving a stitch bond. At a pre-set height, clamp
18
is closed while capillary
10
is still rising with the bonding lead. This prevents wire
16
from feeding out capillary
10
and produces an upward force on the bond. The force builds until wire
16
breaks, which it does at the smallest cross section of the bond. Finally, a new ball
14
is formed on the new distal end of wire
16
by employing a hydrogen flame or an electronic spark as shown in
FIG. 1
g
. The process can then be repeated.
Ultrasonic aluminum wire bonding is also a widely used high speed, high throughput interconnect process. In this process, stitch bonds such as described above with reference to
FIG. 1
f
are formed at both ends of the interconnect by a combination of pressure and ultrasonic energy. As the wire softens, freshly exposed metal in the wire comes in contact with the freshly exposed metal on the pad and a metallurgical bond is formed. Aluminum wire is typically doped with silicon (e.g., 1%) to more closely match the hardness of the wire with the bond pad material. Both gold and aluminum wire are used extensively today in packaging, with gold ball to aluminum bond pads being the most common interconnect system.
In conventional wire bonding processes, it is well known by those in the art that bonding strength is enhanced by employing ultrasonic vibration and heat during the bonding procedure and this is typically done. The strength of the bond is only enhanced, however, in the same direction as the ultrasonic vibration being applied. Current processes typically apply only unidirectional vibrations during wire bonding, whereas it would be desirable to enhance the bond strength in all directions. In addition, in order to ensure that integrated circuits are not degraded during the attachment of the bonding wires, it is desirable to conduct the ultrasonic wire bonding at relatively low temperatures. However, the lower the temperature, the more difficult it may be to form a sufficient bond. Therefore there is a continuing need to create better bonds at lower temperatures and at faster rates to increase productivity.
It has been proposed that the application of ultrasonic waves that are circular or elliptical can enhance the bond strength at lower temperatures and with a shorter dwell time, in each of the vibration directions. See e.g., Tsujino, “Ultrasonic wire bonding using high frequency 330, 600 kHz and complex vibration 190 kHz welding systems” (Ultrasonics 34 (1996) 223-228). This strengthening phenomenon has purportedly been achieved by producing the circular or elliptical vibration modes using multiple piezoelectric transducers. It is particularly desirable to generate a circular or elliptical vibration mode for better bond strength in all directions. However, current known methods for producing complex ultrasonic waves using multiple transducers typically employ separate, non-synchronous controls for each transducer, such that error or other difficulties may be introduced by the two separate controls that do not work together and result in a less than ideal higher order wave. In addition, a single transducer with a single control apparatus would be less expensive than two transducers with separate controls. There is a need for production of circular or elliptical ultrasonic vibrations with a mechanism that ensures that the two perpendicular modes needed for circular or elliptical modes are always vibrating synchronously.
SUMMARY OF INVENTION
In accordance with one aspect of the present invention, a novel ultrasonic vibration mode for wire bonding is provided. A second vibration direction is added to the conventional ultrasonic apparatus. The combined elliptical or circular vibration enhances wire bond strength in all directions and decreases bonding time and temperature. The second vibration direction is created with a single piezoelectric transducer and control mechanism in combination with a reflecting arm apparatus. The present invention enables two perpendicular wave modes to always vibrate synchronously. The invention can advantageously be applied to any ultrasonic bonding process to improve yield strength and reliability.
REFERENCES:
patent: 5207370 (1993-05-01), Mochida et al.
patent: 5699950 (1997-12-01), Jang
patent: 5816476 (1998-10-01), Buice et al.
patent: 5894983 (1999-04-01), Beck et al.
patent: 5931372 (1999-08-01), Miller
patent: 5976314 (1999-11-01), Sans
patent: 5996877 (1999-12-01), Koduri
Jiang Tongbi
Wu Zhiqiang
Dunn Tom
Johnson Jonathan
Kress Hugh R.
Winstead Sechrest & Minick P.C.
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