Test Structures for Electromigration Evaluation in Submicron Technology

Electromigration tests have been performed on a number of test structures with different geometries and microstructures, highlighting the limits of each test structure in relation with the technologies employed in submicron interconnections. A new set of test structures based on the single level stripe, the multilevel Kelvin contact and via chain structures, are then proposed to investigate electromigration in submicron technology devices. The new test structures are envisaged to overcome the problems which have previously been associated with conventional ASTM test structures. The use of a multifinger Babel Tower structure ensures that the microstructure of the end segment is consistent with the test stripe and the use of tungsten via plugs in multilevel systems eliminates the reservoir effect. These modifications will allow the reliability of the interconnections to be evaluated in a manner which is representative of actual ULSI device structures. As submicron lines are not the critical element of an IC metallisation, an additional set of test structures using a larger linewidth was designed to enable a comparison between bamboo and multigrain structures.