Shih-Hui Jen

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 densiti...

The mechanical robustness of atomic-layer- and molecular-layer-deposited coatings on polymer substrates

films was determined to be KIC= 1.89 0.10 and 0.17 0.02 MPa m 0.5 , respectively. From measurements of the saturated crack density, the critical interfacial shear stress was estimated to be c = 39.5 8.3 and 66.6 6.1 MPa, respectively. The toughness of nanometer-scale alumina was comparable to that of alumina thin films grown using other techniques, whereas alucone was quite brittle. The use of alucone as a spacer layer between alumina films was not found to increase the critical strain at fracture for the composite films. This performance is attributed to the low toughness of alucone. The experimental results were supported by companion simulations using fracture mechanics formalism for multilayer films. To aid future development, the modeling method was used to study the increase in the toughness and elastic modulus of the spacer layer required to render improved critical strain at fracture. These results may be applied to a broad variety of multilayer material systems composed of ceramic and spacer layers to yield robust coatings for use in chemical barrier and other applications.

Critical tensile strain and water vapor transmission rate for nanolaminate films grown using Al2O3 atomic layer deposition and alucone molecular layer deposition

Critical tensile strains (CTSs) and water vapor transmission rates (WVTRs) were measured for nanolaminate films grown on polyimide substrates using Al2O3 atomic layer deposition (ALD) and alucone molecular layer deposition (MLD). Nanolaminate composition was controlled by varying the ratio of ALD:MLD cycles during film growth. For ∼100 nm film thicknesses, the CTS obtained its highest value of ∼1.0% for the 3:1 nanolaminate. The WVTR decreased dramatically versus nanolaminate composition and reached the measurement sensitivity limit at WVTR ∼1 × 10−4 g/(m2day) for the 7:2, 5:1, and 6:1 nanolaminates. The ALD:MLD nanolaminates may be useful as flexible gas/vapor diffusion barriers on polymers.

Alucone Interlayers to Minimize Stress Caused by Thermal Expansion Mismatch between Al2O3 Films and Teflon Substrates

Alucone films were employed as interlayers to minimize stress caused by thermal expansion mismatch between Al(2)O(3) films grown by atomic layer deposition (ALD) and Teflon fluorinated ethylene propylene (FEP) substrates. The alucone films were grown by molecular layer deposition (MLD) using trimethylaluminum (TMA), ethylene glycol (EG), and H(2)O. Without the alucone interlayer, the Al(2)O(3) films were susceptible to cracking resulting from the high coefficient of thermal expansion (CTE) mismatch between the Al(2)O(3) film and the Teflon FEP substrate. Cracking was observed by field emission scanning electron microscopy (FE-SEM) images of Al(2)O(3) films grown directly on Teflon FEP substrates at temperatures from 100 to 160 °C and then cooled to room temperature. With an alucone interlayer, the Al(2)O(3) film had a crack density that was reduced progressively versus alucone interlayer thickness. For Al(2)O(3) film thicknesses of 48 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >60 nm on 50 μm thick Teflon FEP substrates. For thinner Al(2)O(3) film thicknesses of 21 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >40 nm on 50 μm thick Teflon FEP substrates. Slightly higher alucone interlayer thicknesses were required to prevent cracking on thicker Teflon FEP substrates with a thickness of 125 μm. The alucone interlayer linearly reduced the compressive stress on the Al(2)O(3) film caused by the thermal expansion mismatch between the Al(2)O(3) coating and the Teflon FEP substrate. The average compressive stress reduction per thickness of the alucone interlayer was determined to be 8.5 ± 2.3 MPa/nm. Comparison of critical tensile strains for alucone films on Teflon FEP and HSPEN substrates revealed that residual compressive stress in the alucone film on Teflon FEP could help offset applied tensile stress and lead to the attainment of much higher critical tensile strains.

P-106: Real-Time Inspection of a Moisture Barrier Film Buried by a Protective Layer for Flexible Displays

Stress-induced cracks in moisture barriers may lead to device degradation in flexible displays. We developed an optical technique that can inspect the cracking of stressed barrier films buried by protective layers. This technique allows rapid and large-area evaluation of mechanical reliability of moisture barriers and functional layers for flexible displays.

12.3: Defect Visualization of Atomic Layer Deposition Enabled Polymer Barriers Using Fluorescent Tags

Nanometer thick alumina barrier coatings deposited using atomic layer deposition (ALD) can be used to enable flexible packaging to protect organic light emitting diodes (OLED) and other electronic/optoelectronic components from moisture-and oxygen-aided deterioration. Individual defects and cracks generated in the coating during processing, subsequent handling and application, allow the leakage of reactive species that may lead to device degradation. Therefore, novel fluorescent tags have been developed to visualize defects and damage, enabling rapid quality inspection of barrier layers. for ALD alumina coatings deposited on polyethylene naphthalate (PEN) substrates, the fluorescent taggant identified cracks ∼20nm in width and individual defects as small as ∼200nm in diameter. This novel approach, which is non-destructive and fully compatible with the optical inspection methods that are widely used in manufacturing, is expected to have a major impact on the quality control of flexible polymer packaging of OLED and other organic components or systems.