A method for characterizing electrical stability of organic coatings in microelectronic applications

With the surface test vehicle structure used for the experiments described in this report, significant changes in potential over device surfaces have been measured at distances of 10 to 40 mils from the driving electrodes. This potential propagation has been studied for cases involving moisture sensitivity and instabilities at high temperatures due to organic conformal coatings. Several factors should be kept in mind in applying the reported results to commercial types of integrated circuits. First, the 40 V bias used to induce drift in the test structures is considerably higher than normally used in integrated circuits. All other things being equal, the rate of surface potential drift should scale with the drift bias. A second factor involves distance from the driving electrode. All results given in this report are for an electrode-toprobe FET separation of 10 mils unless noted otherwise. On the basis of the simplified Shockley model of surface instability one would expect the rate of surface potential change to be inversely proportional to the square of the electrode to probe spacing. In integrated circuits the small separation of biased leads and active regions might be expected to aggravate surface ion drift effects. On the other hand, proximity of other interconnects near ground potential might act to limit surface potential changes to acceptable values.