Electromigration Induced Drift and Noise in a Single Aluminum Via
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ABSTRACT We have examined small changes in the resistance of a single aluminum via test structure with a resolution of 10-8 ohms. The via is roughly one cubic micron and contains a well defined TiN diffusion barrier. The high resolution of these resistance measurements allows the observation of atomic drift rates corresponding to roughly 100 atoms/second into and out of the well defined via structure. Resistance changes are observed with moderate current densities in the temperature range of 100'C to 2000 C. We observe reversible increases and decreases in the resistance caused by the accumulation of atoms at the diffusion barrier within the via. Comparisons can be made to recent models for the transient electromigration induced vacancy drift. We are able to extract microscopic electromigration parameters as well as differentiate between copper and aluminum as the initial diffusing element. INTRODUCTION High sensitivity resistance measurements are increasingly being used to study electromigration in aluminum alloys.[1-4] Attempts have been made to correlate electromigration lifetime to resistance noise or initial resistance changes under current stress. The test structures used for these experiments are typically long lines with many grains and grain boundaries. Interpretation of the results can be difficult in these samples because the precise location on line of the resistance changes and the relevant length scales are not known. In this experiment, we have monitored small changes in the resistance of a single aluminum via interconnect under current stress. The small size of the sample and the sensitivity of the measurements allows us to observe the time dependent accumulation of defects at a well defined diffusion barrier within the structure. We observe reversible increases and decreases in the resistance as defects are swept into and out of the test structure. A quantitative analysis can be made for the initial formation of a defect concentration gradient due to electromigration. EXPERIMENT The test structures used a Kelvin probe geometry with a 1.5 gam diameter via interconnect through 0.8 gtm of PETEOS passivation as shown in fig. I and fig 2b.[51 The lower metallization (MI) was 0.45 itm of Al-0.75wt% Si-0.5wt% Cu with a 0.06 gtm Ti-Si-N underlayer. The upper metal (M2) was 1.0 jtm of the same alloy deposited on 0. 14m of reactively sputtered TiN. The line widths for Ml and M2 were 2.75 4tm and 3.0 rtm with median grain sizes of 1.6 and 1.2 utm, re501
Mat. Res. Soc. Symp. Proc. Vol. 391 ©1995 Materials Research Society
Temperature Regulation
Temperature Controlled Figure 1. Experimental setup for high resolution resistance measurements.
spectively. Similar test structures have been used in previous studies of mean time to failure and drift velocity.[5,6] The experimental setup for the high resolution resistance measurements is shown in figure 1. Temperature stability of the sample was a major concern for these measurements due to possible resistance changes from temperature fluctuations. A double oven
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