A Novel Reliability Failure Mode of Cu Single Damascus Process

Abstract

Ultra thick top metal has been identified as the most promising technology enhancement for RF IC applications. As the top metal's ultra thickness, top via etching will have a high aspect ratio and will easily over etch the via bottom and damage the under layer metal in conventional dual Damascus Cu process. Single Damascus instead of traditional dual Damascus Cu process was selected for thick top metal fabrication to improve the etch window. But a novel failure mode, void formed in via top at anode side is observed during the electromigration (EM) reliability evaluation, which leads a bad reliability performance. A physical model is proposed in this paper to explain the strange failure mode. Based on our model, an optimized pre-clean (to increase the adhesion between the cooper and barrier) and barrier/seed thickness (to reduce the stress of interface) split is developed and shows better EM performance.Index Terms--Ultra thick top metal, single Damascus process, eletromigration, interface mechanical stress I. IntroductionAdvanced CMOS technology has attracted tremendous interest of the field of radiofrequency (RF) industry. In order to improve the RF chip with a low power, low cost, and high integration density performance, many passive devices such as varactors, inductors, and capacitors are integrated with CMOS devices in a single chip by a standard logic process without extra masks. High performance inductors are the key passive components used in most RF applications such as Voltage Control Oscillators (VCO), Low Noise Amplifiers (LNA) and Power Amplifiers (PA). The ultra thick top metal stacked with the aluminum capping layer is often used to fabricate the inductor with a good quality factor Q and current capability Imax. [1] In this paper electromigration (EM) performance is evaluated for ultra thick top metel via terminated structure at the early process development stage. A novel reliability failure mode is found by physical failure analysis on single Damascus process which is selected to fabricate the ultra thicker top metal and top via. A physical model is proposed to explain the strange failure mode. Then base on our understanding, an optimized process is developed and it shows good electromigration performance.