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 °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 Al2O3 or HfO2 are shown to fully block copper oxidation after reliability stress tests.

Delamination of Plasticized Devices in Dynamic Service Environments

“…Obstructing O2 Capabilities H2O Corrosion Resistance Low-Temperature Manufacturability 15 nm AI2O3 [84] No oxide layer Rapid failure Best 12 nm HfO2 [85] No oxide layer non-corrosive Worse 13 nm TiO2 [85] 440 nm oxide layer appears Better 200 um liquid mold layers [87] No oxide layer non-corrosive Best [85] No oxide layer non-corrosive Worse 13 nm TiO 2 [85] 440 nm oxide layer appears Better 200 um liquid mold layers [87] No oxide layer non-corrosive Best…”

“…For example, Pinho [ 84 ] describes a process to integrate an oxygen diffusion barrier into the top of an RDL stack using atomic layer deposition (ALD), which exhibits a good barrier effect and can prevent Cu oxidative delamination. In addition, Chery [ 85 ] conducted an experimental comparison of several inorganic coatings based on low-temperature ALD technology—AI 2 O 3 , HfO 2 , and TiO 2 —in terms of their ability to block O 2 and H 2 O from entering the interior of the RDL layer. As shown in Figure 12 d,e, the 12 nm HfO 2 inorganic coating performs the best, and the coating not only protects against the oxidation of O 2 , but is also completely unaffected by the presence of H 2 O in the corrosion stress test and is a better inorganic coating than AI 2 O 3 in terms of performance indicators.…”

“…( b ) RDL copper wire covered with 15 nm AI 2 O 3 inorganic coating did not show oxidation when kept at 150 °C for 1000 h. ( c ) RDL copper wire covered with 13 nm TiO 2 inorganic coating showed an oxidized layer of 440 nm when kept at 150 °C for 1000 h. ( d ) RDL copper wire covered with 12 nm HfO 2 inorganic coating did not show oxidation when kept at 150 °C for 1000 h. ( e ) RDL copper wire covered with 12 nm HfO 2 inorganic coating showed no oxidation during the corrosive stress test. Reproduced with permission [ 85 ]. Copyright 2023, Springer.…”