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

Jen, Shih-Hui; Bertrand, Jacob A.; George, Steven M.

doi:doi:10.1063/1.3567912

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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 ∼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 Al₂O₃ ALD films should be very useful for flexible gas diffusion barriers on polymers.

Article Outline

INTRODUCTION
EXPERIMENTAL
1. Al ₂ O ₃ ALD film growth
2. Tensile strain measurements
3. Compressive stress measurements
RESULTS AND DISCUSSION
1. Tensile strain for cracking
2. Compressive strain for cracking
3. Comparison between critical tensile and compressive strains for cracking
CONCLUSIONS

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KEYWORDS and PACS

Keywords

alumina, atomic layer deposition, compressive strength, cracks, field emission electron microscopy, fracture mechanics, fracture toughness, scanning electron microscopy, tensile strength, thin films

PACS

68.60.Bs

Mechanical and acoustical properties
81.40.Np

Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mt

Cracks
81.40.Lm

Deformation, plasticity, and creep
68.55.A-

Nucleation and growth
68.55.jd

Thickness

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3567912

PUBLICATION DATA

ISSN

0021-8979 (print)

1089-7550 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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