The correct evaluation of the test temperature is an extremely important task when performing electromigration (EM) experiments on metal lines. In fact, reliable values of activation energy for the EM mechanism can be obtained from experimental data if and only if the true value of the temperature of each test structure during the tests is known. On the other hand true temperature measurements involve more than one practical and theoretical difficulties, ranging from heating due to Joule effect to bad thermal coupling between the test devices and the temperature sensors.
In this work a method is presented for true temperature evaluation of the test stripes during electromigration median-time-to-failure tests performed at the wafer level. In particular the problem of the thermal coupling between two or more samples tested at the same time is examined.
The experimental setup for simultaneous wafer-level testing of more than one stripe, requiring a probe-card and a switching matrix, is described, along with the development of a procedure for the calculation of the true temperature of each stripe, which takes into account Joule self-heating and mutual heating of the samples.
Results obtained on Al-Si and Al-Si/TiN/Ti metallizations are shown and design guidelines to avoid thermal coupling of the samples, thus allowing more reliable temperature measurements, are provided.