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Rapid Characterization of Thin Film Dissolution in Water with in Situ Monitoring of Film Thickness Using Reflectometry Alexander S. Yersak, Ryan J. Lewis, Jenny Tran, and Yung-Cheng Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03606 • Publication Date (Web): 16 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016
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ACS Applied Materials & Interfaces
Rapid Characterization of Thin Film Dissolution in Water with in Situ Monitoring of Film Thickness Using Reflectometry Alexander S. Yersak,*,# Ryan J. Lewis,* Jenny Tran,+ and Yung C. Lee* *Department of Mechanical Engineering and +Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0215, United States KEYWORDS: Atomic layer deposition, SiO2, copper, dissolution, corrosion, water, and reflectometry
ABSTRACT
Reflectometry was implemented as an in situ thickness measurement technique for rapid characterization of the dissolution dynamics of thin film protective barriers in elevated water temperatures above 100 oC. Using this technique multiple types of coatings were simultaneously evaluated in days rather than years. This technique enabled the uninterrupted characterization of dissolution rates for different coating deposition temperatures, post-deposition annealing conditions, and locations on the coating surfaces. Atomic layer deposition (ALD) SiO2 and wet
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thermally grown SiO2 (wtg-SiO2) thin films were demonstrated to be dissolution-predictable barriers for the protection of metals such as copper. A ~49% reduction in dissolution rate was achieved for ALD SiO2 films by increasing the deposition temperatures from 150 oC to 300 oC. ALD SiO2 deposited at 300 oC and followed by annealing in an inert N2 environment at 1065 oC resulted in a further ~51% reduction in dissolution rate compared with the non-annealed sample. ALD SiO2 dissolution rates were thus lowered to values that of wtg-SiO2 in water by the combination of increasing the deposition temperature and post-deposition annealing. Thin metal films, such as copper, without a SiO2 barrier corroded at an expected ~1-2 nm/day rate when immersed in room temperature water. This measurement technique can be applied to any optically transparent coating.
1. INTRODUCTION ALD coatings have been incorporated into many emerging devices requiring a thin, conformal, protective, coating, and have enabled implantable biocompatible1 nanowire nanoelectronic biological devices2, microelectromechanical system flow sensors,3 Si-based transient electronics4, diffusion barriers,5 metal encapsulation6–11, and cathode protection12. Metal films, such as copper, without a protective ceramic barrier have been reported with corrosion rates in water of 1.1 to 2.5 nm/day at room temperature.13 As a result, nm-scale copper films used in these devices will fully corrode within days in room temperature water. To solve this problem, thin film ALD ceramic coatings may be used as a corrosion barrier to protect thin metal films. However, the ALD coatings themselves are susceptible to dissolution in water over time. As a result, after years of immersion in water a thin ceramic corrosion barrier will no longer adequately protect a metal surface. An accelerated lifetime test is needed to predict slow
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dissolution rates of ALD coatings in low-temperature water that would otherwise require unacceptable experimental measurement periods of up to years. ALD films are most often deposited in expensive batch processes, but spatial ALD (S-ALD) is a concept where the ALD precursors are separated in space rather than time, and has enabled manufacturable ALD coatings on large surface areas >> m2.14,15 For example, the roll-to-roll Beneq WCS 500 can coat a 500 mm wide substrate with a 25 nm film at a rate of 400,000 m2/year.16 Other film deposition techniques such as plasma-enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), and electron beam (E-beam) evaporation have been reported as alternative deposition methods to grow SiO2 corrosion barrier films;4 However PECVD, LPCVD, and E-beam SiO2 coatings contain more defects than ALD SiO2 coatings4, and cannot provide a thin and conformal barrier on high aspect ratio structures. The intrinsic quality of SiO2 coatings is a direct result of the deposition process. For example, Ebeam deposited SiO2 coatings in room temperature water have been reported with dissolution rates two orders of magnitude higher than wtg-SiO2 coatings immersed in water at room temperature.4 Uniform, conformal, biocompatible, and thin ALD films have begun to be investigated in pure water and under physiological conditions over a range of predetermined pH values and over limited temperatures for TiO2, Al2O3, and SiO2 thin film coatings.4,17 ALD SiO2 films in 37 oC water have been reported with
