A study of the thermal behavior of different test patterns used in differential high resolution electromigration measurements

The differential high-resolution electromigration (EM) measurement technique requires special test patterns since for the implementation of this technique the presence of two metal lines is necessary, one metal line to be stressed with a high current density (`test' or `stressed' resistor) and a second line (`monitor' or `reference' resistor) to compensate for the thermal instabilities of the stressed one. It is accepted that due to Joule heating the stressed line always acquires higher temperature than the reference line and, therefore, additional resistance changes due to thermally induced phenomena, like precipitation of additional elements, will affect the measurement. In order to minimize these unwanted effects, an optimal high-resolution EM test structure should show a minimum temperature difference between the stressed and the reference lines. Based on this requirement, in this work we simulated three different test structures being used by various research groups for high-resolution measurements and subsequently compared these test structures on account of their thermal behavior. Each test structure was examined for the case of two different widths of metal lines, 4 and 0.5 μm. The results obtained from the simulation of these test patterns demonstrated that the test pattern comprised of two parallel stress and reference lines shows better thermal behavior than the `lined-up' (continuous) metal lines where the stress and reference lines are actually part of the same line. In particular, the test pattern comprised of two parallel straight lines has slightly better behavior than the one with meandered lines in terms of minimization of the temperature difference between the stressed and the reference resistors. The difference between the thermal behavior of the two structures though is very small. For that reason, the parallel lines should be preferred from the meandered ones only if layout restrictions do not require the choice of a more compact (i.e., meandered) solution. Both test structures have shown a better thermal behavior than the `lined-up' metal lines.