Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-08-31
2003-01-21
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S251000, C148S253000, C148S264000, C148S272000, C148S274000, C148S275000, C148S276000, C148S283000, C148S285000
Reexamination Certificate
active
06508890
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to electrical connections in semiconductor chip technology. More particularly, the present invention relates to formation of substantially dielectric-free bonding pads for semiconductor chips. In particular, the present invention relates to a method of removal of oxide from a metallic bonding pad through the use of an oxidizing compound that catalyzes the reductive polymerization of an electrically-conductive monomer.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which include active or operable portions of semiconductor devices. In the context of this document, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in-assemblies comprising other materials. The term “substrate” refers to any supporting structure including but not limited to the semiconductive substrates described above. The term “semiconductor substrate” is contemplated to include such structures as silicon-on-insulator and silicon-on-sapphire.
Electrical interconnections, such as for flip chip or printed circuit board interconnections that are to connect a semiconductor die pad circuit to a supporting substrate, have historically been made with plating methods and reflow solder attachment methods. Electrically conductive epoxy has also been used to make the electrical interconnections. The use of electrically conductive epoxy interconnections typically have problems with high contact resistance. For example, when used to contact an aluminum bonding pad, which is a semiconductor industry standard, a nonconductive oxide-coated surface forms on the aluminum bonding pad under ambient conditions. Because of the nonconductive oxide, electrically conductive epoxies on otherwise bare aluminum form an interconnect having an unacceptably high electrical resistance at the contact. The contact electrical resistance may be in a range from about 100 ohms to several millions of ohms.
Efforts to reduce contact electrical resistance include using a precious metal such as gold. Gold, however, due to its cost makes mass production of gold bonding pads uneconomical. Additionally, the plating of gold as a semiconductor chip or printed circuit board bonding pad is a difficult process that in and of itself is expensive and time consuming. For example, electroless plating processes are difficult to achieve. Hence, the semiconductor substrate or printed circuit board must be electrically connected to a power supply in order to achieve gold plating.
Other methods to reduce the contact electrical resistance caused by the oxide formed upon the bonding pad include chemical and/or mechanical removal, typically by abrasive means. Although removal of oxides is achievable, the contact will immediately re-oxidize to form a native oxide film unless the contact is in a protected environment.
Another method that has been employed to resist oxidation of metal bonding pads, for example bonding pads made of aluminum, includes the deposition of a strong oxidizer directly upon the bonding pad. When a strong oxidizer is placed directly upon the bonding pad, it forms an oxide husk upon the bonding pad. The oxide husk must be subsequently consumed completely because it otherwise acts as an electrical insulator. Where a stoichiometric excess is needed to get a reaction to provide a sufficient product, formation of excess oxide husk causes additional challenges for its ultimate removal. Removal of any native oxide from the bonding pad will be undermined by the residual presence of the oxide husk formed from the strong oxidizer due to its function as an electrical resistor.
In one prior art attempt to solve the problem of removing the native oxide film, a strong oxidizer is contacted to the bonding pad and an oxide husk thereof deposits upon the native oxide film.
FIG. 1
illustrates a cross-sectional view of a portion of a bonding pad structure
10
that includes an aluminum bonding pad
12
with a native oxide film
14
upon a free surface
16
. A manganese oxide husk
18
is next formed upon native oxide film
14
by contact with a strong oxidizer such as potassium permanganate. Next, an electrically conductive polymer
20
is formed upon the manganese oxide husk. This method presents several problems. First, the proper amount of the strong oxidizer making contact with the native oxide film
14
is critical. Formation of an excessive amount of manganese oxide husk
18
upon native oxide film
14
will make its removal difficult during formation of electrically conductive polymer
20
. Thus, a dielectric interlayer between aluminum bonding pad
12
and any external wiring such as a conductive bump for flip chips or a soldering wire will substantially hinder electrical conductivity. Second, due to the nature of the strong oxidizer, additional oxidation of aluminum bonding pad
12
may occur to thicken native oxide film
14
. Third, if an insufficient amount of electrically conductive polymer
20
is formed above manganese oxide husk
18
, an insufficient quantity of the metal oxide will be reduced, including any of the native oxide film of the bonding pad. Finally, forming too much of electrically conductive polymer
20
upon manganese oxide husk
18
will prevent complete formation of the polymer such that electrical conductivity through an uncompleted polymer layer will be substantially reduced.
What is needed in the art is a method of forming a substantially oxide-free bonding pad without the problems of the prior art. What also is needed is a method of forming a substantially oxide-free bonding pad that contains no residual substance that was used to remove native oxide and that contains no such substance that might simultaneously interfere with the electrical conductivity of the bonding pad.
SUMMARY OF THE INVENTION
The present invention is drawn to a method of lowering the net resistivity of an interconnect by depositing a monomer layer upon a metal bonding pad, the treatment thereof to cross-link the monomer to form an electrically conductive polymer, and simultaneously, the substantial reduction of metal oxide to zero valent metal.
In the method of the present invention, deposition of a monomer layer in a solvent, volatilization of the solvent, and contact with a strong oxidizer such as a potassium permanganate allows for the use of the strong oxidizer without the hindrance of having to deal with a manganese oxide husk on the surface of the aluminum bonding pad.
The present invention includes cleaning at least one bonding pad of a chip package or printed circuit board by any appropriate chemical rinse so as to remove harmful particulates and other pollutants whether organic or inorganic. Following cleaning of the bonding pad, a monomer is selected and contacted with the bonding pad. The monomer may be solubilized with any preferred type of solvent such as water or an organic solvent. Care should be taken not to prematurely cause cross-linking of the monomer during the process of volatilizing at least some of the solvent.
Preferably, the chemical qualities of the monomer will include the tendency to be a reducing agent to the native oxide film of the bonding pad. By selecting a monomer that tends to reduce rather than to oxidize, the problem of thickening the native oxide film is avoided. Another preferred chemical quality of the monomer and its polymer after cross-linking, is that it will act as a protective coating to the chip package for substantially all further processing. In particular, the monomer or its cross-linked polymer will act as a protective coating to the chip package during an acid dip in an oxidizer solution such as acidic KMnO
4
.
Following the formation of the monomer into a monomer layer, a strong oxi
Jiang Tongbi
Li Li
Micro)n Technology, Inc.
Oltmans Andrew L.
Sheehan John
Workman & Nydegger & Seeley
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