Electromigration of Bias-Sputtered Al and Comparison with Others

The electromigration of d.c. bias-sputtered aluminum has been studied and compared with other metals and alloys frequently used for VLSI interconnection. Step coverage of the bias-sputtered aluminum is excellent; it completely planarizes 1.0 micron deep bias of 1.3 micron diameter using only ¿120 V bias. However, the electromigration resistance of the samples studied was much inferior even to pure aluminum sputtered without bias. The meantime to failure is only 153 hours with a current density of lE5 A/cm sq. The bias-sputtered samples failed as voids formed along grain boundaries under the current bias. An extremely small grain size as well as a broad distributiorn of sizes was involved in the failure and will be described with photo micrographic illustrations. The grain characteristics of bias sputtered aluminum alloys are a function of deposition pararmeters which are being investigated further. The electromigration properties will be compared with those of Al-Si Al-Cu, Al-Cu-Si, Al-Ti-Si, Al/TiW/Al and Mo/TiW. The structural effect of Al-Si conductor will also be discussed.