Electromigration resistance and long term stability of textured silver thin films on LTCC

Abstract

Silver (Ag) is regarded as advanced material for metallization purposes in microelectronic devices because of its high conductivity and its enhanced electromigration resistance. Besides the typical use of silicon based substrate materials for device fabrication, thin film metallization on ceramic and glass–ceramic LTCC (low temperature co-fired ceramics) substrates gets more and more into focus as only thin film technology can provide the required lateral resolutions of structures in the μm-range needed for e.g. high frequency applications. Therefore, the reliability of Ag thin films is investigated under accelerated aging conditions, utilizing test structure which consists of 5 parallel lines stressed with current densities up to 1.5 × 10 7 A cm −2 at temperatures ranging from room-temperature up to 300 °C. To detect the degradation via the temporal characteristics of the current signal a constant voltage is applied taking the overall resistance of the test structure into account. The mean time to failure of the Ag metallization substantially depends on the degree of (1 1 1)-orientation which, in turn, is strongly affected by the plasma power P P during deposition. Therefore, Ag thin films deposited at P P = 1000 W feature a 7 times higher reliability than those deposited at P P = 100 W. Due to the enhanced stability of grains being (1 1 1)-oriented in textured thin films the material transport predominantly occurs along grain boundaries, whereas in Ag films without a (1 1 1)-orientation volume-related diffusion effects dominate due to the lower stability of these grains.