The present invention relates generally to passivation layers of integrated circuits and more particularly to improved structure of passivation layers for integrated circuits having positively biased substrates and aluminum interconnects.
Integrated circuits formed in silicon substrates generally use a silicon dioxide film formed by thermal oxidation on the surface of the substrate. This silicon dioxide film is used as a mask during the formation of the integrated circuit in the silicon substrate and is also used as a passivation layer of the finally formed integrated circuit. The thermal oxidation process for forming silicon dioxide film usually results in a contaminant ion such as sodium which is mobil by the electric field. In order to minimize the effect of the sodium ion, it is well known to provide a phosphorus glass or a phosphorus doped, silicon dioxide layered on the thermally grown silicon dioxide layer. The second layer is often formed by chemical vapor deposition and the contaminant sodium ions are trapped by the chemically vapor-deposited layer.
In integrated circuits having a positively biased substrate and aluminum interconnects, corrosion of the aluminum interconnects has been observed. The factors contributing to the corrosion of interconnects are the positively biased substrates and the poor moisture resistance of the phosphorus doped passivating layer. W. M. Paulson and R. W. Kirk observed that: (a) the corrosion requires externally applied bias; (b) the corrosion occurs predominantly to the negative electrode; (c) the corrosion initiates at the interface between the aluminum and the passivation glass; and (d) the extent of the corrosion depends upon the phosphorus in the passivation glass. This analysis is provided in their article entitled "The Effects of Phosphorus-doped Passivation Glass on the Corrosion of an Aluminum" in the Proceedings of the Twelfth Annual Reliability Physics Symposium, pages 172-179, 1974.
The presence of moisture in the phosphorus doped passivation layer is a critical factor in producing the corrosion. The corrosion of the aluminum is apparently a non-electrolytic corrosion and results from the presence of hydroxyl ions at the cathode. The corrosion mechanism is discussed by E. P. G. T. Van de Ven and H. Koelmans in J. Electrochem, Soc. 123, pps. 143-144, (Jan. 1976); and by B. Chatterjee in J. Electrochem, Soc. 123, pps. 1920-1921, Dec. 1976). Hydroxyl ions may be produced at the cathode of the integrated circuit through an electrolytic mechanism or simply by dissociation by water to hydrogen and hydroxyl ions. In the presence of water in the package, the phosphorus from the phosphorus doped passivation layer or carbon dioxide present from oxidation of residual laser scribe protective coating during the high temperature sealing process on CERDIP devices increases the conductivity of the passivated layer. A surface electric field then moves the hydrogen ions through or on the conductive, phosphorus doped passivation layer where they react at the aluminum cathode at the aluminum-phosphorus doped passivation layer edge by electron transfer to generate OH.sup.- ions and H.sub.2. If additional water is present, it would combine with the OH.sup.- ions to form a reaction product of AlO.sub.2.sup.-, the net reaction being Al + 2H.sub.2 O + e.sup.- AlO.sub.2.sup.- + 2H.sub.2.
Although the reaction was initiated by the original H.sup.+ ions generated in or on the phosphorus doped passivation layer, the aluminum corrosion reaction proceeds as long as water vapor is present at the corroding aluminum -- AlO.sub.2.sup.- interface and OH.sup.- ions are generated by cathodic reduction of H.sup.+ ions. Also, an ionic conduction path must be maintained to generate the OH.sup.- ions in the corroding region. R. B. Comizzoli discusses the current paths and conductivity of the phosphorus doped passivation layers in the March 1976 issue of the Journal of Electrochemical Society at pages 386-391. The three major conduction paths are: (a) between the two bonding pads; (b) across the P-N isolation junction; and (c) between the positively biased silicon substrate and the negative bonding pad and interconnects.
Prior efforts to reduce the corrosion problem have been directed mainly towards the elimination of water or moisture from the encapsulated integrated circuits. Since these techniques included process steps at elevated temperatures, cycling of temperatures, and the use of non-oxidizing gases, the reliability of the final product cannot be quaranteed. Thus there exists a need for a structural solution to the corrosion problem.