1. Field of the Invention
The present invention relates to a projecting electrode structure in semiconductor devices such as a bump electrode structure and a process for forming the projecting electrode structure.
2. Description of the Related Art
A prior-art process for forming a projecting electrode (i.e. a bump electrode) on a semiconductor substrate comprises, for example, the steps of forming a plating resist layer on a projecting-electrode forming surface of the substrate, forming an opening in the plating resist layer by etching the plating resist layer, causing a pad to appear in the opening, and plating the pad with a metal such as gold so that the metal forms the projecting electrode.
FIGS. 3(A) to 3(E) illustrate a prior-art process for forming a projecting electrode on the substrate. As shown in FIG. 3(A), an aluminum electrode 33 is first formed on a semiconductor substrate 31 via an oxide layer 32 of silicon oxide. A passivation layer 34 covers the edge of the aluminum electrode 33. The passivation layer 34 provides an insulating layer of silicon oxide or silicon nitride.
As shown in FIG. 3(B), a metal substrate layer 35 for passing a plating current in an electroplating step described below is subsequently formed by a thin-film formation method such as vacuum deposition or sputtering on the aluminum electrode 33 and the passivation layer 34. The metal substrate layer 35 has a trilayered structure, not shown.
In the trilayered structure of the metal substrate layer 35, a first layer adjoining an aluminum electrode 33 serves to bond the aluminum electrode 33 to the passivation layer 34. The first layer is made of chromium (Cr), titanium (Ti) or a titanium-tungsten (W) alloy. A second layer (i.e. a barrier layer) serves to prevent or retard a mutual diffusion between aluminum and the metal material for the projecting electrode. The second layer is made of platinum (Pt), palladium (Pd), copper (Cu) or nickel (Ni). A third layer (if necessary) may serve to prevent the oxidation of the surface of the barrier layer and/or keep the shear strength of the barrier layer good in a bump plating step of the prior-art process for forming a projecting electrode. The third layer is made of gold (Au) or the like and has a thickness of about 500-2000 .ANG..
A plating resist is subsequently applied by spin-coating or the like to the surface of the metal substrate layer 35. As well known, the applied plating resist is exposed through a glass mask (not shown) and developed to be patterned. Thus, the plating resist layer 36 and a predetermined opening 37 are formed as shown in FIG. 3(C).
As shown in FIG. 3(D), the substrate 31 is subsequently faced down. As well known, the face of the substrate 31 is placed in a bump-plating cup (not shown) and plated. The plating precipitates a projecting electrode metal 38 (gold or a solder) in the opening 37 in the plating resist layer 36. The projecting electrode metal 38 has a thickness of about 15-25 .mu.m.
As well known, the plating resist layer 36 and an unnecessary part of the metal substrate layer 35 are eliminated by etching or the like. Thus, the projecting electrode 39 is formed as shown in FIG. 3(E).
As described above, the prior-art process for forming the projecting electrode 39 is an electroplating process using a plating solution (a wet process). In the electroplating process, it is very difficult to control the plating solution. In particular, since the substrate 31 is faced down and the face of the substrate 31 is placed in the plating cup, air bubbles from the plating solution stay on the face of the substrate 31 and in the opening 37 to disable a stable bump plating. In addition, a gas caused by an electrochemical reaction in the precipitation of a plating metal also stays on the face of the substrate 31 and in the opening 37 to hinder the deposition of a bump-plating.
Thus, the prior-art bump-plating process causes defective appearances (a depression, a hole etc.) in the projecting electrode due to air bubbles from the plating solution, and degrades the yield of products, the stability of the surface state of the projecting electrode, the hardness of the projecting electrode. On the other hand, a dispersion in the height of the projecting electrode is increased. That is, it is technically difficult to form a uniform projecting electrode by an electroplating process.