Flexibility of ALD & MLD Films
The mechanical properties of ALD and MLD films is a relatively unexplored area [1,2]. We are particularly interested in the flexibility of ALD films. We are currently asking: How flexible are ALD films? When do ALD films crack when subjected to tensile or compressive stress? How does flexibility depend on film thickness? These questions are all important for developing ALD films for flexible displays and flexible coatings.
We are able to measure the flexibility of ALD films by depositing ALD films on polymers. Tensile stress is applied by pulling uniaxially on the ALD-coated polymer. Compressive stress is applied by depositing ALD films on polymers with high thermal expansion coefficients at elevated temperatures and then cooling to room temperature. As the polymer cools, the polymer contracts and applies compressive stress to the ALD film.
Recent results for the threshold for cracking for Al2O3 ALD films exposed to tensile stress are shown in the accompanying figure [2]. The critical tensile strain for cracking increases dramatically for thinner Al2O3 ALD films. Similar are results for Al2O3 ALD films are obtained for the critical compressive strain for cracking. These results indicate that very flexible films can be prepared using ultrathin films.
We are measuring the flexibility of individual ALD and MLD films. We are also growing multilayer films composed of ALD and MLD layers. By tuning the thicknesses and properties of the individual ALD and MLD layers, we hope to design flexible multilayers that have higher critical strains for cracking than the individual constituent films in the multilayer.
1. M.K. Tripp, C. Stampfer, D.C. Miller, T. Helbling, C.F. Herrmann, C. Hierold, K. Gall, S.M. George and V.M. Bright, "The Mechanical Properties of Atomic Layer Deposited Alumina for Use in Micro- and Nano-Electromechanical Systems", Sensors and Actuators A 130-131, 419 (2006).
2. D.C. Miller, R.R. Foster, Y. Zhang, S.H. Jen, J.A. Bertrand, Z. Lu, D. Seghete, J.L. O'Patchen, R. Yang, Y.C. Lee, S.M. George and M.L. Dunn, "The Mechanical Robustness of Atomic Layer- and Molecular Layer-Deposited Coatings on Polymer Substrates", J. Appl. Phys. 105, 093527 (2009).
