Microstructural Control of Internal Electromigration Failure in Narrow Al-Cu-Si Lines
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shown schematically in Fig. 1. The failure occurs at the upstream end of a polygranular segment of the line, and usually takes the form of an erosion void or slit failure across a bamboo grain. A second characteristic feature is the depletion of Cu from the region of the failure site. As evidenced by the disappearance of Cu-containing precipitates, Cu is swept from both the polygranular segment and the bamboo grain that terminates the segment on the upstream side. A large, 0-phase (Al2Cu) precipitate appears at the downstream end of the polygranular segment. It is, presumably, formed from the Cu that is swept from the segment. The observation of Cu depletion prior to failure strongly suggests that this is a necessary element of the failure sequence in Al-Cu lines. The first failure of a quasi-bamboo line almost occurs at one of the longest polygranular segments it contains, and is usually found at the longest segment. To a good approximation, the time to failure decreases exponentially with the length of the polygranular segment that leads to failure [2].
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e Schematic diagram of a typical failure site in an Al-Cu line, showing a void-precipitate pair at the ends of a polygranular segment, and the sweeping of Cu-rich precipitates from the segment and the upstream bamboo grain.
While the detailed mechanisms of failure are still under investigation, the microstructural features of the failure site suggest two metallurgical approaches to inhibit failure. First, since the failure time decreases with the polygranular segment length, the failure time should be increased by modifying the microstructure of the line to shorten the longest polygranular segment that appears within it. Second, since the failure happens after Cu is swept from the polygranular segment, the failure time should be increased by adjusting the chemical state or the distribution of Cu to maximize the time required to sweep it away. These simple guidelines lead to the three methods described below. INHIBITING FAILURE THROUGH MICROSTRUCTURE CONTROL Minimizing the Maximum Length of Polygranular Segments The simplest way to shorten the longest polygranular segment within a line is to decrease the effective line width, w* = W/G, where W is the line width and G is the mean grain size. For given line width, w* can be decreased by changing the deposition conditions of the metal film to maximize grain size or by annealing the film to coarsen the grain size prior to patterning. However, this method is only partly successful. The reliability of a device that contains many conducting lines is ordinarily determined by the time (t1) at which the first of them fails. While the mean time to failure (MTF) increases dramatically as w* is decreased below unity, the line-to-line scatter in failure time increases as well, with the consequence that there is a relatively small drop in the value of ti for an array of many lines. The reason is statistical. Even when W
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