Critical tensile and compressive strains for cracking of Al_2O_3 films grown by atomic layer deposition

摘要

Al_2O_3 atomic layer deposition (ALD) is a model ALD system and Al_2O_3 ALD films are excellent gas diffusion barrier on polymers. However, little is known about the response of Al_2O_3 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 Al_2O_3 ALD films, the critical strains at which the Al_2O_3 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 Al_2O_3 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 Al_2O_3 ALD film thickness. The fracture toughness for cracking, K_(IC), of the Al_2O_3 ALD film was also determined to be K_(IC) = 2.30 MPa m~(1/2). Thinner Al_2O_3 ALD film thicknesses also had higher critical strains for cracking from compressive strains. Field-emission scanning electron microscopy (FE-SEM) images revealed that Al_2O_3 ALD films with thicknesses of 30-50 nm on Teflon fluorinated ethylene propylene (FEP) substrates cracked at a critical compressive strain of ～1.0%. The critical compressive strain increased to ～2.0% at a film thickness of ～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 ～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 Al_2O_3 ALD films should be very useful for flexible gas diffusion barriers on polymers.