Polyimide Passivation Reliability Study

Special n-channel FET memory chips with non-random, purposely etched passivation defects are used to ascertain the effect of incompletely cured polyimide on the product\´s functional reliability. Polyimide Is applied to the chip\´s surface and cured at 300°C for five minutes. This cure results in approximately 95% linkage formation as ascertained from IR spectrum analysis. Subsequently, the product Is broken Into four groups and further "cured" at 225°C for 0 (controls), 5, 25, and 50 hours, respectively. These samples are then put on life stress at 40, 85, and 130°C. For the control case, cumulative percent fail versus time data for steps in stress temperature is shown to have an Arrhenius model dependency with a ¿H of 1.1 eV. However, recovery of the failed cells under stress conditions (with bias applied) indicates the failure mechanism to be other than classical surface inversion. Data is presented showing the inverse dependency of the additional cure on cumulative percent fail. Explanation of this phenomena is attributed to dipoles that are associated with Incompletely cured polyimide (potential linkages), aligning in the preferential E-field direction, thus Inducing an image charge (an electron) at the silicon surface (quasi inversion). Degree of cure is ascertained from dissipation factor measurements performed on metallized silicon test wafers which have 2 - 4 ¿m polyimide films on their upper surfaces. Aluminum dot and mercury probe techniques are compared.