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A key issue in the reliability of integrated circuits has been failures in the interconnect wiring, which has in the past been made from aluminum, and more recently, copper. Line widths are less than 1 micron, and in this size range, properties such as resistivity are strongly dependent on microstructure: grain size and defects.
Electromigration refers to the movement of the metal atoms in the wire, under the influence of current. The ‘electron wind' carries atoms along with it, typically depositing them at a grain boundary to form a ‘hillock', and leaving behind a void. Both of these features are visible in the figure. The physical mechanism underlying this process is not very well understood.
Thermal stress, which can also cause line failures, results from the fact that the metal lines are deposited onto the silicon ‘chip' at high temperatures (~400 o C). As the chip is cooled to room temperature, the metal, which has a high coefficient of thermal expansion, shrinks. The silicon has a much smaller expansion coefficient, and so shrinks very little when cooled. The result is tensile stress in the metal line, which may then be slowly pulled apart.
The figure illustrates a test structure in which the 1 micron wide line has failed by electromigration. Both voids and hillocks are visible.
Positron techniques play a key role in investigating these failure modes, because they are sensitive to very early stages in the development of the defects that will eventually cause the line to fail.
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