Metal fusion bonding – Process – Using high frequency vibratory energy
Reexamination Certificate
1999-02-16
2001-04-10
Mills, Gregory (Department: 1725)
Metal fusion bonding
Process
Using high frequency vibratory energy
C228S180500
Reexamination Certificate
active
06213378
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to wire bonding used in packaging semiconductor devices and the like. More particularly, the present invention relates to improved capillary designs and ball bonding techniques utilizing such capillaries.
BACKGROUND OF THE INVENTION
There has been a continuing trend in the semiconductor industry toward smaller semiconductor devices with higher transistor density and an increasing number of input/output connections. This trend has led to semiconductor devices having an increased density of chip input/output connections and shrinking bond pad sizes. Semiconductor devices having small bond pad center to center distances are called fine pitch semiconductor devices. Wire bonding technology is currently being challenged by requirements of semiconductor devices having bonding pad center to center distances of less than 100 micrometers.
In semiconductor fabrication, wire bonding remains the dominant chip interconnection technology for fine pitch semiconductor devices. Gold or aluminum wire is commonly used to connect a bonding pad of a semiconductor die to a lead of the semiconductor device. Typically, ball bonding is used to connect the wire to the bond pad while wedge bonding, also called stitch bonding, is used to connect the wire to the lead. Commonly, a wire bonding apparatus including a capillary is used for both the ball bonding and the wedge bonding.
FIG. 1A
shows a vertical cross section of a prior art capillary
111
. The capillary
111
has a longitudinally extending wire feed bore
102
formed therethrough. The bore
102
typically includes a chamfer
105
that splays slightly outward towards the distal tip of the capillary
111
to an outer chamfer diameter
106
. In operation, wire is fed downward through the wire feed bore
102
, and out a bottom aperture
104
of the capillary
111
.
FIG. 1B
shows a vertical cross section of a prior art capillary
100
having a double chamfer structure. The capillary
100
has a first chamfer diameter
107
and a second chamfer diameter
109
.
FIG. 1C
shows a vertical cross section of the capillary
100
horizontally restraining a wire
110
while an electronic flame off mechanism (EFO)
112
applies energy to a distal end of the wire
110
. The application of energy by the EFO
112
creates a free air ball
114
at the distal end of the wire
110
. The wire
110
is held by a clamp (not shown) during this free air ball formation process. Size parameters of the free air ball
114
include a free air ball diameter
115
. For a wire bonding apparatus using the capillary
100
, the size of the free air ball
114
can be controlled by varying hardware and software parameters of the wire bonding apparatus. After formation of the free air ball
114
, the clamp releases the wire
110
and the capillary
100
is used to bond the distal end of the wire
110
to a bond pad surface as explained below.
FIG. 1D
shows a vertical cross section of the capillary
100
being used to form a ball bond
115
between the distal end of the wire
110
and a surface of a bond pad
116
. The bond pad
116
is located on a semiconductor die which has a center to center bond pad distance (also called the bond pad pitch of the semiconductor device). After the formation of the free air ball
114
, as explained above, the free air ball
114
(
FIG. 1B
) is forced downward to the bond pad
116
by the capillary
100
. The capillary
100
is used in conjunction with thermal and ultrasonic energy to create the ball bond
115
between the distal end of the wire
110
and the bond pad
116
. An anchoring area
103
represents the surface of the capillary
100
in contact with the ball bond
115
. Size parameters associated with the ball bond
115
include a ball bond height
120
, a standoff distance
123
and a footprint
122
.
As the center to center bond pad distance (or bond pad pitch) is decreased in a semiconductor device, the size of the bond pad
116
is typically decreased. For example, a semiconductor device having a 70 micron bond pad pitch can have a 60 micron×60 micron bond pad
116
. It is very difficult to consistently achieve a ball bond
115
small enough to fit on a bond pad
116
of this size using the capillary
100
. The footprint
122
must be limited in order to prevent flash of wire metal over to an adjacent bond pad
116
thereby creating a short between adjacent bond pads
116
. A short between adjacent bond pads
116
can result in operational failure of the semiconductor device.
With reference still to
FIG. 1C
, one problem with use of the capillary
100
is that it is difficult to precisely control the size of the ball bond
115
. For a wire bonding apparatus using the capillary
100
, the size of the ball bond
115
(including the ball bond height
120
and footprint
122
) is dependent on the size of the free air ball
114
(FIG.
1
B). Hardware and software parameters of the bonding apparatus must be adjusted to vary the size of the free air ball
114
(FIG.
1
B). For a wire bonding apparatus using the capillary
100
, the size of the ball bond
115
is also dependent on parameters such as the bonding power, capillary tip position, bonding force, component temperatures and the nature of the ultrasonic energy delivered during the formation of the ball bond
115
. For ball bonding of fine pitch semiconductor devices, the tip dimension
108
of the capillary
100
can be reduced so that the capillary
100
can form a ball bond
115
small enough to fit on the small bond pad
116
. However, reducing the outer diameter tip dimension
108
weakens the capillary
100
which is subjected to great stress particularly during wedge bonding as explained below. The most significant factors that decide the shape and strength of the ball bond
115
are the tip dimension
108
and second chamfer diameter
109
of the capillary
100
.
Conventional ball bonding techniques require a combination of ultrasonic energy provided by an ultrasonic transducer coupled to the capillary
100
and thermal energy stored in the bond pad. Typically, the bond pad and semiconductor die must be pre-heated to at least 150 degrees Celsius which may have detrimental effects on the semiconductor die. As an example, for silicon used in most conventional wafer technology surfaces, preheating the bonding surface prior to ball bonding may result in degradation of dopant regions of the silicon substrate. Current ball bonding techniques also require ultrasonic energy input at no less than 60 mW. Ultrasonic bond power is kept to a minimum for a number of reasons including shorter tool longevity and reduced capillary tip position control associated with higher bond power levels. Bond force is also kept to a minimum for the same reasons as bond power. Lower force also results in less gold contamination at capillary face dimensions that ensure reliable wedge bonding.
FIG. 1E
shows a vertical cross section of a wedge bond
124
formed by the capillary
100
. The wedge bond
124
is formed between an extended length of the wire
110
and a surface of an inner lead
126
of a lead frame. Reducing the tip dimension
108
of the capillary
100
, to reduce the size of the ball bond
115
as described above, causes degradation in strength of the wedge bond
124
. This is due to the fact that the area in which the wedge bond
124
is formed depends on the outer diameter tip dimension
108
of the capillary
100
. Therefore, a difficult problem with using the capillary
100
concerns the tradeoff between a small outer diameter tip dimension
108
for achieving small ball bonds and a larger outer diameter tip dimension
108
for achieving strong wedge bonds. In view of the foregoing, it should be apparent that improved wire bonding techniques would be desirable.
SUMMARY OF THE INVENTION
An improved capillary construction as well as a variety of improved wire bonding techniques are described. In one method aspect of the invention, a free air ball is formed that is at least partially contained within an opening in the distal tip of th
Beyer Weaver & Thomas LLP
Mills Gregory
National Semiconductor Corporation
Pittman Zidia T.
LandOfFree
Method and apparatus for ultra-fine pitch wire bonding does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for ultra-fine pitch wire bonding, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for ultra-fine pitch wire bonding will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2436488