Environmentally Assisted Cracking in Silicon Nitride Barrier Films on Poly(ethylene terephthalate) Substrates

A singular critical onset strain value has been used to characterize the strain limits of barrier films used in flexible electronics. However, such metrics do not account for time-dependent or environmentally assisted cracking, which can be critical in determining the overall reliability of these thin-film coatings. In this work, the time-dependent channel crack growth behavior of silicon nitride barrier films on poly(ethylene terephthalate) (PET) substrates is investigated in dry and humid environments by tensile tests with in situ optical microscopy and numerical models. The results reveal the occurrence of environmentally assisted crack growth at strains well below the critical onset crack strain and in the absence of polymer-relaxation-assisted, time-dependent crack growth. The crack growth rates in laboratory air are about 1 order of magnitude larger than those tested in dry environments (dry air or dry nitrogen). In laboratory air, crack growth rates increase from ∼200 nm/s to 60 μm/s for applied stress intensity factors, K, ranging from 1.0 to 1.4 MPa·m^1/2, below the measured fracture toughness K_c of 1.8 MPa·m^1/2. The crack growth rates in dry environments were also strongly dependent on the prior storage of the specimens, with larger rates for specimens exposed to laboratory air (and therefore moisture) prior to testing compared to specimens stored in a dry environment. This behavior is attributed to moisture-assisted cracking, with a measured power law exponent of ∼22 in laboratory air. This study also reveals that much larger densities of channel cracks develop in the humid environment, suggesting an easier initiation of channel cracks in the presence of water vapor. The results obtained in this work are critical to address the time-dependent and environmental reliability issues of thin brittle barriers on PET substrates for flexible electronics applications.