Failure analysis and process improvement of copper diffusion

Abstract

Cu diffusion caused by stress, current or light induced migration in Cu interconnect is the main cause of circuit failure in BEOL (Back End of Line) process. The diffusion at the Cu/cap layer interface has been widely demonstrated to be the main path of copper diffusion. The copper diffusion between the copper line and the upper cap layer is studied in this work. In order to eliminate copper diffusion and improve reliability in BEOL process, several methods are used in this work. 1. Adding the count of via above Cu line to reduce stress gradient below the via. 2. Increasing the pre-clean flow rate before depositing the via glue layer to form a self-forming film to improve adhesion of glue layer and cap layer in order to improve the uniformity of sidewall interface adhesion strength. 3. The wafer is shielded from light to reduce photochemical reactions and suppress Cu migration induced by light. Based on these optimization factors, the lowest circuit failure caused by the copper diffusion is achieved.