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 includes 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 pas 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.