Corrosion‐Induced Contact Failures in Double‐Level Al‐Si‐Cu Metallization

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Abstract
The contact failure rate for double level Al‐Si‐Cu metallization was found to be much larger than that for double level Al‐Si metallization. The large failure rate in the first case was attributable to the loss of the first conductor in via holes due to pitting corrosion, which was caused at the chemical cleaning stage after the via holes had been opened. The corrosion pits were formed on the grain boundaries of Al alloys, each one accompanied by a particle in its center. The particles were identified as Cu containing precipitates with XMA. Corrosion was accelerated by increasing both Cu concentrations in the Al alloys and annealing temperatures. Some large precipitates with diameters of about 1.0 μm, which is equivalent to the film thickness, were found to exist in Al‐0.9%Si‐1.6%Cu alloys annealed at 450°C. When a large precipitate is situated in a via hole, corrosion occurs. The precipitate and Al on the surface form a galvanic cell, where the precipitate and the Al behave as the cathode and the anode, respectively, resulting in anodic dissolution of the Al. Fuming nitric acid solutions with concentrations of more than 94% prevent the corrosion. Argon ion implantation in Al alloys was found to be effective in preventing corrosion. Annealing after the implantation enhanced the effect drastically. It was found that the Cu concentration on the surface decreases by about one‐half and that the precipitates on the surface are scattered into very small pieces when annealing takes place after implantation.

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