Optimization and Structural Characterization of W/Al2O3 Nanolaminates Grown Using Atomic Layer Deposition Techniques

Abstract

W/Al2O3 nanolaminates were fabricated using atomic layer deposition (ALD) techniques. ALD of tungsten was performed using alternating exposures of WF6 and Si2H6. ALD of Al2O3 was performed using alternating exposures of Al(CH3)3 and H2O. The fabrication of optimum nanolaminates was very dependent on nucleation during the ALD of each layer. The nucleation of Al2O3 ALD on W surfaces and W ALD on Al2O3 surfaces was examined using quartz crystal microbalance investigations. Al2O3 ALD on W nucleated easily during the first Al(CH3)3/H2O reaction cycle. In contrast, W ALD nucleation on Al2O3 required multiple WF6/Si2H6 reaction cycles and was very sensitive to the Si2H6 exposure. The W ALD nucleation required at least six or seven WF6/Si2H6 reaction cycles. The optimum reaction parameters for nucleation were utilized to fabricate W/Al2O3 nanolaminates with a total targeted thickness of ∼1000 Å that were composed of various numbers of bilayers. Transmission electron microscopy (TEM) images revealed very well defined two-bilayer and four-bilayer W/Al2O3 nanolaminates grown at 177 °C. Depth-profile secondary ion mass spectrometry (SIMS) measurements confirmed the composition of the W/Al2O3 nanolaminates. Both the TEM and SIMS results showed that sharper interfaces were observed when W ALD nucleates on Al2O3, and rougher interfaces were observed when Al2O3 ALD nucleates on W. X-ray reflectivity (XRR) investigations indicated that the W/Al2O3 nanolaminates were much thicker than the ∼1000 Å target for the total thickness when the W/Al2O3 nanolaminates were composed of a larger number of bilayers. These results were consistent with much lower than expected film densities for the thin Al2O3 ALD nanolayers. Atomic force microscope studies revealed that the surface roughness of W/Al2O3 nanolaminates was lower at lower growth temperatures and lower at higher interfacial densities. XRR investigations also revealed a pronounced Bragg peak with a high X-ray reflectivity from 64-bilayer W/Al2O3 nanolaminates grown at 177 °C. The X-ray reflectivity of the 64-bilayer W/Al2O3 nanolaminates grown at 177 °C was constant versus annealing up to 500 °C. The reflectivity then dropped rapidly for annealing at ≥600 °C. This thermal instability will place some limits on the application of W/Al2O3 nanolaminates as thermal barrier coatings and X-ray mirrors.