Evaluation of the interfacial resistance of thin film interconnexions

Abstract

An experimental procedure is given for the evaluation of thin-film interconnexions. The isolation of the distributed d.c. interfacial resistance from the resistance of the upper and lower conductors of an interconnexion is important if independent conclusions are to be made concerning the effect of environmental stress on the interface. A procedure for determining the interfacial resistance has been developed. The required geometrical pattern, the four-point measuring technique and the interpretation of the measurements are presented. Eight thin-film interconnexion combinations have been studied in detail: tantalum-aluminum over chromium-gold, chromium over chromium-gold, chromium-gold over chromium-gold, aluminum over chromium-gold, chromium-gold over tantalum-aluminum, aluminum over tantalum-aluminum, chromium-gold over chromium and aluminum over chromium. The adhesion of the individual thin films has been quantitatively measured and the results are presented. The change in d.c. resistance of chromium, aluminum, chromium-gold and tantalum-chromium-aluminum films is presented as a function of various environmental stresses. The d.c. interfacial resistance is given as well as the effect of environmental stresses on these values. Initial interfacial resistances ranged from 0·0001 to 0·1 Ω for a 0·254 mm (0·010 in.) by 0·254 mm (0·010 in.) area. The environmental stresses were: storage at −55°C, 125°C and 200°C for 1000 hr; 100 temperature cycles from −65°C to 125°C; and an accelerated ageing at 240°C for 100 hr. Determination of the interfacial resistance was made at regular intervals. Percentage change of interfacial resistance vs. decades of time appeared typically to be linear over the range of investigation. The current-carrying capability and the effects of a reduced power operation life test on interfacial resistance are presented. A merit rating for thin-film interfacial interconnexions is presented based upon: (1) the electrical characteristics of the interface; (2) the d.c. resistance of individual films; (3) the adhesion of the various films; (4) the physical and electrical degradation of various film combinations.