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

Abstract

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.