Optimizing Crack Onset Strain for Silicon Nitride/Fluoropolymer Nanolaminate Barrier Films

Abstract

Nanolaminates using alternating inorganic and organic layers have the potential to provide ultrabarrier with high resistance to gas permeation while also changing the crack onset strain (COS) to improve mechanical reliability. Previous modeling efforts highlighted the possibility to achieve an optimized design depending on thickness and material properties (elastic modulus, fracture energy), producing the highest possible value of COS. In this study, we experimentally show that the optimization can be achieved using SiNx/CYTOP laminates when guided by theoretical predictions. Nanolaminates using silicon nitride (SiNx) inorganic films and CYTOP organic films were fabricated. The fracture energy of the CYTOP layer was found to be 90 ± 10 J/m2. A 50% increase in COS (from 1.7 to 2.5%) was experimentally measured as a result of the thickness ratio optimization for a 3-layer structure consisting of two 30 nm thick SiNx layers and one 33 nm thick CYTOP layer. In the same way, a 70% increase in COS (from 1.7 to 2.8%) was measured for a 5-layer structure consisting of three 20 nm thick SiNx layers and two 25 nm thick CYTOP layers. The numerical results also showed that a 45%, 73%, 110%, and 160% increase in COS can be obtained in 3-, 5-, 9-, and 19-layer structures, respectively, if the total thickness ratio of CYTOP to SiNx layer is at the optimized value, i.e., ∼0.55, 0.83, 2.67, and 9, respectively. The same procedure can be applied to all inorganic/organic multilayered films to find the optimized COS, including the measurement of high fracture energy of organic layers, enabling the design of mechanically robust permeation barriers for flexible electronics.