Inorganic Capping Layers in RDL Technologies: Process Advantages and Reliability

Abstract

Atomic layer-deposited (ALD) inorganic films were processed on top of copper metal lines in a polymer-based redistribution layer (RDL). The primary objective was to develop capping layers thinner than 15 nm to prevent copper oxidation. Due to their uniformity and high density, ALD layers are established permeation barriers. Nonetheless, owing to the presence of polymers in the final product, a low deposition temperature is required, resulting in an increased defect density and a greater susceptibility to degradation by moisture. In this study, various inorganic cappings, based on Al2O3, HfO2 and TiO2, deposited at 100 degrees C were integrated in an RDL stack. It is demonstrated that they impede the reaction of the polymer photo acid generator with copper, improving the lithography process, and ultimately allowing to print smaller critical dimensions. Additionally, capping layers built upon Al(2)O(3 )or HfO2 are shown to fully block copper oxidation after reliability stress tests.