Comparison of copper electroplating, copper wet etching and linear sweep voltammetry as techniques to investigate the porosity of atomic layer deposited Al2O3

Abstract

In recent years, there has been an increased interest in soft packaging for non-rigid medical implants. A promising technique to realize reliable soft packaging involves the inclusion of thin layers of atomic layer deposited (ALD) ceramics between biocompatible polymer layers. Crucial requirements are that those ALD layers are biocompatible and pinhole free to prevent unwanted interactions between the implant and the human body. In this research, a biocompatible thermally deposited 20 nm thick Al2O3-ALD layer is investigated for porosity by using three different techniques: copper electroplating, copper wet etching and linear sweep voltammetry (LSV). The porosity values obtained with the copper electroplating and copper wet etching techniques are comparable with one another (3.14 × 10−6%–3.18 × 10−7%) but are four to five orders of magnitude smaller than the porosity estimation obtained by the LSV test (8.04 × 10−2%–3.23 × 10−3%). This divergence in porosity is explained by two different contributions leading to an overestimation of pinhole density obtained through LSV. The destructive character of the LSV technique during which additional current contributions arise from copper corrosion and pinhole enlargement is identified as a first factor contributing to an overestimation of pinhole distribution. A second contribution arises from Poole-Frenkel emission and trap-assisted tunneling through the insulating Al2O3-ALD layer. As a consequence, LSV does not only detect pinholes but also single point defects, cluster defects and sealed line defects originating from unreacted OH ligands during ALD deposition. Based on those findings it was concluded that copper wet etching is the preferred method to be used in pinhole detection.