Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Configuration or pattern of bonds
Reexamination Certificate
1999-07-01
2001-12-11
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Configuration or pattern of bonds
C257S784000, C257S762000, C257S690000, C257S778000, C257S783000, C257S737000
Reexamination Certificate
active
06329722
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of semiconductor devices, and more particularly to the interconnection and packaging of a integrated circuit having copper bond pads.
BRIEF DESCRIPTION OF PRIOR ART
As the demand for cheaper, faster and more power efficient integrated circuits increases, so must the packing density. Techniques have continually evolved to meet the demands by minimizing the dimensions of the transistors and of the electrical interconnections which integrate the semiconductor devices together in order to form a complete circuit.
Aluminum alloys have been the most commonly used conductive materials for electrical interconnections. While aluminum has very attractive features for use as electrical interconnection in integrated circuits, such as low electrical resistivity and strong adhesion to silicon dioxide, as the conductor line dimensions have become longer and more narrow, deficiencies in aluminum have become limiting factors. The resistance of aluminum begins to contribute significantly to the resistance-capacitance (RC) time delay of the circuit. Additionally, with decreasing dimensions, reliability concerns such as electromigration and current density limitations have increased. For these reasons, the industry has recently begun to shift toward the use of more robust conductive interconnection metals, such as copper. Copper is approximately 40% lower in resistivity and is much more resistant to electromigration. However, with a major change in the interconnect metallization on integrated circuits, there comes a host of new manufacturing and yield issues. Among those issues is the exposed copper bond pads and the difficulty in making interconnections using existing packaging and assembly infrastructure. Copper is readily oxidized, but the oxide formation is not self limiting, as is the case with aluminum. The copper interconnection within the circuit is processed largely under vacuum and the finished product is protected by a passivating layer of silicon oxide or nitride. On the other hand, the bond pads which provide an interface for interconnecting the chip to the next level of interconnection are necessarily exposed metal and the oxidation of copper creates issues for oxide removal and for wire bonding at elevated temperatures. Further, the copper hardness changes with heat exposure which can result in a need for higher bonding forces, and this in turn can cause damage to the underlying dielectric on the integrated circuit, and to uncertainties in the wire bonding process.
One approach to circumventing the copper bond pad issues is to deposit a layer of aluminum over the bonding pads. This is accomplished by vapor deposition of an aluminum layer onto the semiconductor wafer after etching of the passivation layer or overcoat (PO) from the bonding pads. Another set of lithography steps is then required to pattern and etch the aluminum, and thereby form bond pads which can be wire bonded. Such additional wafer processing steps add significantly to the cost of the devices. Alternately, ultrasonic wire bonding at near ambient temperature has been used to avoid oxidation of copper, but it too has significant drawbacks. Not only is the productivity poor, but also it requires a large bonding tool for scrubbing and creating the bond. This in turn requires large bonding pads and is contrary to the overall direction for smaller circuits.
SUMMARY OF THE INVENTION
This invention comprises an integrated circuit device having a plurality of bonding pads on the surface of the die which are extensions from the copper metallization used to interconnect the transistors of the circuit, wherein the bonding pads include a metal coating which makes them amenable to wire bonding. It further comprises such an integrated circuit device, with bond wires attached to the coated bonding pads.
The primary object of this invention is to provide a bonding surface which is compatible with wire bonding on a semiconductor device having copper bond pads, and which does not suffer from the high cost shortcomings of existing technology.
Another object of the invention is to provide a bond pad surface which is compatible with either gold or copper wire bonding to connect the integrated circuit device to the leads of a package or substrate.
Still another object of the invention is to provide a bond pad surface whose resistivity is insignificant in comparison to that of the copper interconnections on the integrated circuit and/or to that of the wire bonds which connect the circuit to the package leads.
A further object of this invention is to provide a bonding surface which adheres strongly to copper metallization on bond pad and does not delaminate under mechanical or thermal stresses.
Another object of the invention is to provide a bonding pad with self limiting surface oxidation, and that oxidation can be readily removed.
A further object of this invention is to provide a coated bond pad surface with attached bond wires comprising either copper or gold.
Still another object of this invention is to provide a bonding pad surface which forms intermetallics at a relatively low temperature and is capable of being bonded at about 200 deg C. or less.
Still a further object of the invention is to provide a bonding pad surface capable of allowing reliable bonds to be formed at a very low mechanical force.
A further object of the invention is to provide an integrated circuit having copper bond pads which are coated with a material which is solderable, and in turn provides the mechanism for a alternate interconnect techniques, such as wire bonding, flip chip or TAB (Tape Automated) bonding.
It is also an object of the invention to provide a low cost method for forming a wire bond compatible coating on the copper bond pads.
An additional object is to provide a low cost process for removing oxides from the copper surface during in the same operation as that used to coat the bond pad.
Still a further object of the invention is to provide an alternate process for forming said bondable surfaces.
Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein by way of illustration and example, an embodiment of the present invention is disclosed.
The present invention, accordingly, provides a device having a thin layer of self passivating metal, on the copper bond pads of an integrated circuit, and a mechanically strong alloy between the copper and the metal. The exposed surface of said metal coated bond pads self-limits the amount of oxidation which can form under ambient or processing conditions, and the underlying material remains malleable enough to allow the native oxide to be removed by the ultrasonic scrub step in wire bonding. The coated bond pad forms intermetallics with copper or gold, and thus is readily compatible with wire bonding to either gold and copper.
According to one aspect of this invention, there is provided a thin layer of tin on copper bond pads to form a bronze alloy between the metals, and subsequently the environment is limited to no more than about 150 deg C. for a maximum of 30 minutes. The exposed tin surface self passivates and limits further oxidation. The resulting bond pad is compatible with either gold or copper wire bonding.
Alternate metals are suitable for coating copper bond pads to provide bondable surfaces, such as zinc, indium and lead.
A low cost, reliable method for coating copper bond pad surfaces is provided by electroless immersion plating of tin onto the exposed copper pads of an integrated circuit wafer.
Alternately, a tin layer is provided by vapor deposition onto the surface of an integrated circuit. The excess tin over the protective overcoat layer is then removed by either a lift off process or by chemical mechanical polishing, leaving only the tin attached to the copper bonding pads.
An advantage of tin coated copper bond pads is that integrated circuit wafers having such metal coated copper bonding pads will be assemble
Shih Wei-Yan
Subido Willmar
Wilson Arthur
Honeycutt Gary C.
Navarro Arthur L.
Telecky Fred
Texas Instruments Incorporated
Williams Alexander O.
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