Metal fusion bonding – Process – With shaping
Reexamination Certificate
1999-08-05
2001-07-17
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
With shaping
C228S004500, C228S180500
Reexamination Certificate
active
06260753
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the fabrication of integrated circuits, and, more particularly, to a method for prearranging external connections of an integrated circuit.
BACKGROUND OF THE INVENTION
A common technique for prearranging an external connection of an integrated circuit is a wire bonding technique. This wire bonding technique includes bonding a wire to each metal pad of the chip of the integrated circuit, and to a respective metal lead defined by die stamping a metal assembly frame of the integrated circuit. The bonding of the wire, which is typically a small diameter gold or aluminum wire, is done by friction welding the wire to the metal surface of the pad while providing vibrating energy to the tool.
Alternative encapsulating techniques have been developed to allow for less delicate and critical ways to make the necessary external connections. A recently developed technique is known as a “flip chip” technique. The flip chip technique is based on the formation of a metal bump or ball on each connecting pad. The bumps are pressure contactable, thus eliminating the need of bonding between the pads and the leads.
For low power devices containing signal processing systems, the metal bumps required on the connection metal pads of the chip may be formed by electrochemical deposition of a suitable metal. For high power devices, the only reliable method to form such bumps on the respective metal pads is to friction weld pre-formed balls of gold, or an equivalent alloy, with a technique similar to the one traditionally used to bond a gold wire.
Suitable tools have been developed to form bumps or balls of gold, and to friction weld respective metal pads to the chip. One such device is a tiny capillary tool with a dielectric tip commonly formed of aluminum, zircon or other refractory oxide. The axial conduit of the capillary tool opens to a flat end face with a tapered mouth at the tip of the tool. The machine is equipped with means to feed a thin wire of gold or alloy through the capillary, and has clamps for functionally blocking the wire intermittently advanced through the capillary tool. A bump is formed by advancing the wire so that it protrudes out a few millimeters from the end face of the tool tip, and by arcing between the wire and a counter-electrode suitably placed in proximity of the tool tip. The arcing is sufficient to melt the metal, which conforms to the concave shape of the mouth of the capillary.
The metal ball created on the tool tip is pressed to the metal pad by lowering the tool over it, and vibrating power is applied to the tool causing the bonding by friction welding the metal ball to the metal pad. Once the bonding is accomplished, the clamp blocks the movement of the wire relative to the capillary, and the tool is pulled away. This breaks off the wire from the bonded metal bump. Thereafter, the tool shifts over a new metal pad. The clamp is released and the wire is advanced a predetermined distance. The clamp closes again and a new electrical arc melts the protruding portion of wire into a new metal ball. This new metal ball is bonded to a corresponding new metal pad.
This process produces bonded metal bumps having a short stump or tiny tail of wire left during the breaking off from the wire. This is caused by retracting the tool while maintaining clamped the feed wire. Commonly, the same machine is equipped with a second tool by which, through a dedicated realigning software, each metal bump after having been bonded to the respective metal pad is stamped. This is done for flattening the short stump of wire left by the breaking-off of the wire, and to produce a plastic flattening of the top of the metal bump. Leveled areas are produced suitable for establishing good electrical contact for assembling the device in a flip chip package.
The bump forming and bonding process, as well as the successive stamping, is done with commercially available machines. One of these machines is described in the brochure for the KS1488 Turbo model, released Feb. 27, 1996, by Kulicke and Soffa Industries, Inc, located in Pennsylvania. Also described in the brochure is the realigning software for stamping the metal bumps after they have been bonded to the metal pads. The tool for forming the metal ball and for bonding it to a metal pad of a chip is a commercially available machine which has a stylus with an axial capillary passage. The tip is primarily made of a dielectric refractory ceramic material with a tapered cavity through which projects the metal wire fed through the capillary.
Commonly, the end face of the tip is a convex surface with a large radius of curvature, and has a concave junction of a relatively large radius connecting with the conical surface of the dielectric tip, as shown in the data sheet of Aprova Ltd. of Lyss, Switzerland, 1992. This is relative to Tip Style CSS, Thermocompression (T/C) or Thermosonic (T/S) Tailless Capillary Bonding tips.
The step of forming and bonding a metal bump to a metal pad using a special capillary tool, and thereafter stamping the metal bump using a different tool, is burdensome in terms of manufacturing times. A number of specific tools are required, including software for realigning a second stamping tool to the same points on which a first tool has been already aligned for forming and bonding the metal bump.
It is evident that there is a need to form and bond a metal bump onto a metal pad, break off the metal wire and stamp the top of a bonded metal bump in an uninterrupted sequence of operations. This is to be done without repeating the alignment of a specific tool to the same metal pad, and without substituting tools.
SUMMARY OF THE INVENTION
A object of the invention is to fulfill the above described requirements by using the same tool throughout the process with straightforward and noncritical modifications to the tool.
By modifying the geometry of the tip of the capillary tool used for forming and bonding the bump, and by programming a modified sequence of movements of the tool, the necessary stamping operation of the top of each metal bump bonded to a respective metal pad is performed. These movements include breaking off the wire from the bonded bump before moving to another metal pad to repeat the process.
The process is completed using as a stamping tool the same ball forming and bonding capillary tool. The dielectric and refractory tip of the capillary is modified to have a substantially flat end surface forming as a sharp edge the conical surface of the tip, and with the surface of the concave mouth of the axial passage through which the metal wire is fed. The sharp edge is formed without any beveling or blunting.
After aligning the capillary tool on a metal pad, forming the metal ball, and bonding by thermosonically friction welding the bump on the pad according to conventional operations, the process may proceed with the following steps. The tool from the bonded bump is lifted while allowing for a free movement of the feed metal wire through the axial conduit of the tool. Movement of any wire relative to the lifted tool is then blocked. The lifted tool is shifted by a distance sufficient to displace the concave mouth of the axial passage off the projection of the bonded bump, thus breaking off the wire from the bonded bump.
The method further includes stamping the bonded bump by lowering the tool onto the bump with sufficient pressure for plastically flattening the top of the bump and the residual wire stump laying radially out and being coplanar to the flattened area of the top of the bump produced by the stamping. This is preferably done by optionally applying a pulse of ultrasonic vibrations at a reduced power with respect to the power used to bond the bump.
The tool is lifted while releasing the clamp that impeded the relative movement of the wire with respect to the tool. The tool is moved above another metal pad with a length of wire emerging from the concave mouth at the center of the end face of its tip. The above process may then be repeated. The process according to t
Renard Loïc
Vitali Battista
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Dunn Tom
Galanthay Theodore E.
STMicroelectronics S.r.l.
Stoner Kiley
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