Flexible displays require various thin films that must withstand stress without cracking. Very little is known about the critical stress for cracking for ALD films for either tensile or compressive stress. The critical stress for cracking is particularly important for the design of gas diffusion barrier films on polymer substrates. Flexible organic light emitting diodes (OLEDs) require barriers with extremely low water vapor transmission rates of < 1 x 10-6 g/m2/day. Film cracking will severely impair the gas diffusion barrier.

 

Compressive stress can be applied by depositing films at elevated temperature on a high thermal expansion substrate and then letting the sample cool to room temperature. The critical compressive stress for cracking of Al2O3 ALD films was determined using Teflon FEP which is a polymer with a high thermal expansion coefficient. Different compressive stresses were defined using different deposition temperatures. Crack densities were visualized using scanning electron microscopy (SEM) images. The critical stress for cracking was determined for various Al2O3 ALD film thicknesses.

  

Al2O3 ALD film thicknesses were examined from 19-48 nm. The SEM images showed that the films buckled and then cracked with increasing compressive stress. The critical stress for cracking was constant at ~2 GPa for the thicker films with thicknesses >30 nm. The critical stress for cracking increased dramatically to >4 GPa for film thicknesses < 20 nm. These results indicate that thinner Al2O3 ALD films are more flexible and able to withstand higher compressive stresses without cracking. Enhanced multilayer gas diffusion barriers should employ thin Al2O3 ALD layers for optimum flexibility.