ALD Coatings on Polymers & Gas Diffusion Barriers
ALD can be performed at sufficiently low temperatures to deposit inorganic or metallic coatings on polymers [1]. One of the key applications of ALD on polymers is the fabrication of high quality gas diffusion barriers. The continuous and pinhole-free nature of ALD films leads to excellent gas diffusion barriers. The water vapor transmission rate (WVTR) through the polymer is the most relevant measure of the barrier performance. Applications such as organic light emitting diodes (OLEDs) require barriers with WVTR < 1 x 10-6 g/(m2 day).
ALD gas diffusion barriers achieve WVTR values that are below the lower limit of commercial measurement techniques such as the MOCON instrument. As a result, we have used alternative methods based on radioactivity using the HTO test [2] and the Ca test [3]. The Ca test has measured WVTR values as low as 1.7 x 10-5 g/(m2 day) at 38°C/85% RH for single Al2O3 ALD films [3]. Improvements have also been observed using Al2O3/SiO2 multilayer barriers [4].
Our current work is utilizing the Ca test and depositing ALD barriers directly on Ca films. The resistance of the Ca films then can monitor the Ca oxidation and determine the WVTR. We have measured WVTR values of ~2 x 10-5 g/(m2 day) at accelerated conditions of 70°C/28%RH. The barriers deposited on the Ca film also allow the direct visualization of the defects in the ALD films. The accompanying figure shows several pinhole defects in the Al2O3 ALD films on the Ca film at early and later time. The pinhole defects lead to progressive radial growth of an oxidized Ca region centered on the defect. A few pinhole defects per cm2 is equivalent to a WVTR of ~2 x 10-5 g/(m2 day).
We are currently working to lower the WVTR for the ALD barriers to ~ 1 x 10-6 g/m2 day. This low WVTR value should be obtained by optimizing the individual ALD barrier layers and using the ALD barrier layers together with flexible MLD interlayers to fabricate a multilayer barrier. We are also using ALD coatings to protect polymers from corrosion from atomic oxygen and VUV radiation in the space environment [5].
1. C.A. Wilson, R.K. Grubbs and S.M. George, "Nucleation and Growth during Al2O3 Atomic Layer Deposition on Polymers", Chem. Mater. 17, 5625 (2005).
2. M.D. Groner, S.M. George, R.S. McLean and P.F. Carcia, "Gas Diffusion Barriers on Polymers Using Al2O3 Atomic Layer Deposition", Appl. Phys. Lett. 88, 051907 (2006).
3. P.F. Carcia, R.S. McLean, M.H. Reilly, M.D. Groner and S.M. George, "Ca-Tests of Al2O3 Gas Diffusion Barriers Grown by Atomic Layer Deposition on Polymers", Appl. Phys. Lett. 89, 031915 (2006).
4. A.A. Dameron, S.D. Davidson, B.B. Burton, P.F. Carcia, R.S. McLean and S.M. George, "Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by Al2O3 and Rapid SiO2 Atomic Layer Deposition", J. Phys. Chem. C 112, 4573 (2008).
5. R. Cooper, H.P. Upadhyaya, T.K. Minton, M.R. Berman, X. Du and S.M. George, "Protection of Polymer from Atomic-Oxygen Erosion Using Al2O3 Atomic Layer Deposition Coatings", Thin Solid Films 516, 4036 (2008).
