The Enhancement of Electromigration Lifetime under High Frequency Pulsed Conditions (Special Section on Reliability)
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概要
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Experimental evidence of a two-step enhancement in electromigration lifetime is presented through pulsed testing that extends over a wide frequency range from 7 mHz to MHz. It is also found, through an accompanying failure analysis, that the failure mechanism is not affected by current pulsing. Test samples were the lower metal lines and the through-holes in double-level interconnects. The same results were obtained for both samples. The testing temperature of the test conductor was determined considering the Joule heating to eliminate errors in lifetime estimation due to temperature errors. A two-step enhancement in lifetime is extracted by normalizing the pulsed electromigration lifetime by the continuous one. The first step occurs in the frequency range from 0.1 to 10 kHz where the lifetime increases with (duty ratio)^<-2> and the second step occurs above 100 kHz with (duty ratio)^<-3>. The transition frequency in the first-step enhancement shifts to the higher frequency region with a decrease in stress temperature or an increase in current density, whereas the transition frequency in the second step is not affected by these stress conditions. The lifetime enhancement is analyzed in relation to the relaxation process during the current pulsing. According to the two-step behavior, two distinct relaxation times are assumed as opposed to the single relaxation time in other proposed models. The results of the analysis agree with the experimental results for the dependence on the frequency and duty ratio of pulses. The two experimentally derived relaxation times are about 5s and 1 μs.
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- 1994-01-25