Performance of New Copper-Based Metallization Systems in an 85°C, 78% RH, SO2Contaminated Environment

The performance of Ti-Pd-Cu-Ni-Au (TPCNA), Ti-CuNi-Au (TCNA), and Ti-Cu metallization systems in an 85°C, 78% RH environment contaminated with SO2was compared with that of the standard Ti-Pd-Au (TPA) metallization. Electrolytic and galvanic corrosion failure modes were identified for encapsulated (RTV silicone tubber) and unencapsulated triple track conductor specimens fabricated using each of the above thin-film technologies. Electrolytic corrosion was studied by biasing TPCNA, TCNA, Ti-Cu, and TPA triple track specimens in the corrosive environment and measuring in situ leakage currents as a function of time. Median leakage currents for the unencapsulated Ti-Cu specimens were significantly lower than median currents for any of the other unencapsulated groups. Failure of the TPA was due to Pd migration and the formation of dendrites between oppositely biased conductors. Both TPCNA and TCNA showed extensive Cu dendritic growth. Significantly fewer Cu dendrites were observed on the Ti-Cu specimens. TPCNA and TCNA failures were also due to chemical and galvanic corrosion which filled the space between the tracks with hygroscopic salts. The salts were identified by X-ray diffraction to be NiSO3º6H20 and NiSO4º6H20 after 237 hours. After 1302 hours the initial salts had become amorphous and a basic nickel sulfate was found. Ti-Cu had only a light tarnish, ~1000Å thick, after 1302 hours. This consisted of Cu20 and an unidentified minor phase containing S. Galvanic attack at the Cu-Ni-Au interfaces occurred on TPCNA, TCNA, and the Cu-Ni-Au external leads. Oxidation of Ni in the galvanic cell caused delamination of Au on the triple tracks and separation of the bulk of the lead from the Au layer in the bond areas. Galvanic attack was also observed at the Ti-Cu interface for TCNA with sputtered or normally evaporated Ti-Cu layers. TCNA with a graded evaporated Ti-Cu interface showed very little galvanic attack and delamination. No delamination was observed for the Ti-Cu samples or at Ti-Pd-Cu interfaces of TPCNA. The RTV encapsulant prevented high leakage and dendritic growth on all coated specimens and retarded galvanic and chemical corrosion.