Bias Humidity Performance and Failure Mechanisms of Nonhermetic Aluminum SIC's in an Environment Contaminated with SO2

The aging of aluminum-metallized silicon-integrated circuit test vehicles (bare, SiN CAPS only, room temperature vulcanizing silicone rubber (RTV) only, SiN CAPS + RTV) at 85°C, 85 percent relative humidity (RH), and 1 ppm SO2 for 4968 hours was carried out with either a + 10 V, or - 10 V, or 0 Vdc bias imposed on the samples. The leakage currents for the unencapsulated samples increased several orders of magnitude after the addition of SO2. After an induction period of several hundred hours, the leakage currents for the encapsulated samples also increased by several orders of magnitude. Most of the failures were observed in the unencapsuiated groups. The anodic corrosion rate is the highest. Subsequent to environmental stressing, the specimens were examined with a scanning electron microscope (SEM) with X-ray capability. No corrosion was observed on the encapsulated samples. The main failure mode of the unencapsulated samples showed corrosive growths containing Al and S near the Au-Al interconnect of the wire bond. This localized corrosion occured on both anodicaily and cathodicaily biased postions. There were more failures at the anodic electrode. The application of bias is not necessary for corrosion. Dow Corning QC 3-6550 RTV silicone rubber is an effective encapsulant in a moist SO2 environment inhibiting corrosive attack. It retards the occurrence of high leakage currents. A mechanism for corrosion is discussed. It involves the oxidation of sulfur dioxide in aqueous solution to sulfate ion and the acceleration of corrosion by the galvanic couple formed at the Au-Al interconnect. An estimate of the median life of the encapsulated samples under worst case use conditions was calculated to be better than 1.5 x 106 h. The results show that the failure rate is less than 1 FIT (failure in 109 device hours) after 20 years. This was obtained by assuming the same acceleration factors as obtained in previous work on encapsulated aluminum.