Critical tensile and compressive strains for cracking of Al2O3 films grown by atomic layer deposition

Al2O3 atomic layer deposition (ALD) is a model ALD system and Al2O3 ALD films are excellent gas diffusion barrier on polymers. However, little is known about the response of Al2O3 ALD films to strain and the potential film cracking that would restrict the utility of gas diffusion barrier films. To understand the mechanical limitations of Al2O3 ALD films, the critical strains at which the Al2O3 ALD films will crack were determined for both tensile and compressive strains. The tensile strain measurements were obtained using a fluorescent tagging technique to image the cracks. The results showed that the critical tensile strain is higher for thinner thicknesses of the Al2O3 ALD film on heat-stabilized polyethylene naphthalate (HSPEN) substrates. A low critical tensile strain of 0.52% was measured for a film thickness of 80 nm. The critical tensile strain increased to 2.4% at a film thickness of 5 nm. In accordance with fracture mechanics modeling, the critical tensile strains and the saturation crack densities scaled as (1/h)(1/2) where h is the Al2O3 ALD film thickness. The fracture toughness for cracking, K-IC, of the Al2O3 ALD film was also determined to be K-IC=2.30 MPa m(1/2). Thinner Al2O3 ALD film thicknesses also had higher critical strains for cracking from compressive strains. Field-emission scanning electron microscopy (FE-SEM) images revealed that Al2O3 ALD films with thicknesses of 30-50 nm on Teflon fluorinated ethylene propylene (FEP) substrates cracked at a critical compressive strain of similar to 1.0%. The critical compressive strain increased to similar to 2.0% at a film thickness of similar to 20 nm. A comparison of the critical tensile strains on HSPEN substrates and critical compressive strains on Teflon FEP substrates revealed some similarities. The critical strain was similar to 1.0% for film thicknesses of 30-50 nm for both tensile and compressive strains. The critical compressive strain then increased more rapidly than the critical tensile strain for thinner films with thicknesses < 30 nm. The high critical tensile and compressive strains for thin Al2O3 ALD films should be very useful for flexible gas diffusion barriers on polymers. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3567912]