Controlled plating on reactive metals

Details Controlled plating on reactive metals Controlled plating on reactive metals

2000-09-11

2003-01-07

Sheehan, John (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

C148S248000, C148S271000, C148S273000, C148S276000, C148S280000, C148S283000

Reexamination Certificate

active

06503343

ABSTRACT:

FIELD OF THE INVENTION
This invention is concerned with plating highly reactive metals, such as aluminum and its alloys, with more stable metals, such as nickel, that provide enhanced solderability, bondability and corrosion resistance.
DESCRIPTION OF THE RELATED ART
Aluminum is widely used for circuitry patterns on integrated circuit (IC) chips but is very difficult be directly solder because it is a very reactive metal that forms a thick oxide layer, which prevents direct contact between the solder and the aluminum metal. This thick oxide is recalcitrant in that it is difficult to remove and reforms almost instantaneously under ambient conditions. Consequently, electrical contact to aluminum IC pads is typically made by wire bonding, which utilizes a combination of ultrasonic vibration, pressure and thermal energy to cause a gold wire to penetrate the aluminum oxide layer and form a bond to the underlying metal. The other end of the gold wire is bonded to a pad on a substrate, or on a package (e.g., ball grid array or dual in-line package) that connects to a solderable pad or pin that is subsequently soldered to a printed wiring board (PWB) or other substrate.
Wire bonding imposes significant limitations for microelectronic applications. Since the bonds are made one at a time, the process is relatively slow and expensive compared to soldering, which can form thousands of connections almost simultaneously. The expense of the gold wire adds significantly to the costs. In addition, the fixture (head) for holding the gold wire and applying the pressure and energy needed for wire bonding has finite dimensions that limit the minimum spacing between adjacent bond sites. Another important drawback is the inductance of the gold wires themselves, which becomes appreciable at high signal frequencies and limits device switching times or clock speeds. Furthermore, because of wire length and routing issues, wire bonds are not practical for connecting to the area arrays, e.g., ball grid arrays, that are becoming commonplace as the sizes of IC chips decrease and clock speeds and the number of input/output (I/O) connections increases. Solderable IC pads are essential to the emerging flip chip technology, which provides the ultimate in performance and cost reduction. In this case, pads on the IC chip are soldered directly to pads on the substrate PWB, which eliminates the cost and limitations of a package, provides area array capability, and minimizes signal losses.
The most widely used approach for rendering aluminum IC pads solderable involves sputter cleaning/ablation to remove the aluminum surface oxide layer, and immediate sputter or vapor deposition of a layer of oxidation-resistant metal to protect the aluminum pads against re-oxidation. Such vacuum processing is inherently very expensive and also requires photoresist masking to enable lift-off of metal deposited in non-pad areas, or to confine subsequently electrodeposited metal layers to the pads. Electroplating is sometimes used to provide thicker layers of protective metal needed for reliable soldering, and/or to deposit solder that is reflowed to form solder balls for ball grid array (BGA) devices. In this case, the thin metal buss layer needed to provide electrical contact to the aluminum pads is often deposited directly on the IC surface and must be removed by etching (after photoresist removal). A significant concern for electroplating processes is non-uniformity of the plated layers, especially overplating of isolated pads and underplating of those close together.
Displacement plating from solution is attractive as a potential alternative for rendering aluminum pads solderable without the need for costly vacuum deposition, photoresist masking, and etching processes. In this case, the aluminum surface oxide would be dissolved in a solution that contains ions of a more noble metal, e.g., nickel, which would be deposited on the aluminum substrate by the electrons generated by aluminum dissolution in the solution. Since the displacement process should cease when the aluminum surface becomes completely covered with the displacement metal and is no longer exposed to the solution, the layer of deposited metal would necessarily be thin but could readily be thickened by subsequent electroless deposition. An additional thin layer of a noble metal, e.g., gold, could then be deposited by electroless or displacement plating to protect the thickened coating against oxidation and solderability loss.
Processes for direct displacement plating of copper on aluminum from aqueous alkaline or acidic fluoride baths are reported in standard handbooks (e.g., Metal Finishing Guidebook and Directory Issue, published annually by
Metal Finishing
magazine, Tarrytown, N.Y.). However, copper can migrate rapidly in aluminum and degrade both the mechanical properties of the aluminum and the electronic performance of the underlying silicon. Consequently, a barrier layer, e.g., titanium/tungsten or tantalum nitride, is required between copper and silicon on IC chips, which precludes the possibility of utilizing a copper displacement process. Furthermore, copper displacement coatings on aluminum tend to be porous and poorly adherent, and can produce rapid corrosion of the aluminum via galvanic action, so that they are only moderately effective for protecting aluminum against corrosion or serving as the basis for a corrosion-resistant overlayer. Note that copper is generally added to aluminum IC pads to improve electromigration resistance but the concentration is low (<3%) and alloying prevents migration into the silicon.
The only displacement plating process believed to be presently available for applying a solderable metal suitable for IC chip applications to aluminum involves intermediate displacement plating of zinc from a strongly alkaline solution (zincating), and subsequent displacement of the zinc by nickel in an electroless nickel bath. In this case, the aluminum surface oxide dissolves via reaction with hydroxide ion in the alkaline zincating solution (Al
2
O
3
+2OH
−
→2AlO
2
−
+H
2
O) and oxidation of the underlying aluminum drives reduction of zincate ion to produce a layer of zinc metal (2Al+3ZnO
2
2−
+2H
2
O→3Zn+2AlO
2
−
4OH
−
). In a separate electroless nickel bath, the zinc layer is displaced by a nickel layer (Zn+Ni
2+
→Zn
2+
+Ni), which is thickened by electroless nickel deposition.
This process for indirect displacement plating of nickel on aluminum via zincating has major drawbacks, especially for plating pads on IC chips. One fundamental problem is that hydroxide ion in the strongly alkaline zincating solution aggressively attacks the aluminum itself with evolution of hydrogen gas (2Al+2OH
−
+2H
2
O→2AlO
2
−
+3H
2
). Pads that are not at least 1 &mgr;m thick may exhibit bare spots or be completely consumed. In addition, the zinc deposit is porous and non-uniform since the displacement reaction must occur rapidly while dissolution and hydrogen evolution are also occurring on both the bare aluminum substrate and the zinc metal deposit. Double zincating consumes even more aluminum and only partially improves the zinc deposit quality. Furthermore, poor adhesion of the nickel deposit results if the zinc deposit is not completely removed by dissolution in the electroless nickel bath, which is difficult to ensure. Another fundamental problem with the overall process is the use of an electroless nickel bath for both the nickel/zinc displacement and electroless deposition reactions, which have dichotomous requirements. In particular, strongly complexed nickel ions should provide better quality deposits by slowing the displacement reaction to a moderate rate, but would not be reducible by the mild reducing agents needed to avoid extraneous electroless deposition. Buildup of zinc ions in the electroless nickel plating bath can also degrade the deposit quality. It is not surprising that such a dynamic process with many uncontro

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