Influence of heat treatments on the electrical resistance of thin film copper/electroless nickel microcircuit interconnections

The effects of heat treatments on the electrical resistance of thin film microstrip structures are reported. The microstrips are formed photolithographically with widths of 10 micro m from electroplated copper films ranging in thickness from 0.5 micro m to 4 micro m that have been deposited on Si/SiO 2 substrates. The resulting strips are clad with a 0.5 micro m layer of electroless nickel, and then are embedded in polyimide dielectric, the curing of which typically requires a temperature of 350 °C for 1 h. The electrical resistance of these strips is observed to be increased somewhat by such thermal cycling (with most of the increase occuring on the first cycle), but the total increase after three cycles is only about 5% for strips with a copper thickness of 2 micro m or greater, implying that the electrical resistance of these strips is sufficiently stable for use as microcircuit interconnections. An empirical model is used to describe the electrical resistance of the strips and to interpret the experimental data. In this model, the small increase in strip resistance caused by heat treatment is assumed to arise from the diffusion of nickel into copper or other similar effects. The fits to this model show that the resistivity of the copper portion of the strip is consistent with a single value, 2.00 ± 0.08 micro ohm cm, independent of the heat treatments. In addition, the model results suggest that two distinct diffusion mechanisms may be involved: a rapid initial process leading to an effective diffusion depth of about 0.16 micro m after the first thermal cycle, and a slower process that increases the effective diffusion depth during subsequent thermal cycles by only about 0.015 micro m per cycle.