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

W/Al 2 O 3 nanolaminates were fabricated using atomic layer deposition (ALD) techniques. ALD of tungsten was performed using alternating exposures of WF 6 and Si 2 H 6 . ALD of Al 2 O 3 was performed using alternating exposures of Al(CH 3 ) 3 and H 2 O. The fabrication of optimum nanolaminates was very dependent on nucleation during the ALD of each layer. The nucleation of Al 2 O 3 ALD on W surfaces and W ALD on Al 2 O 3 surfaces was examined using quartz crystal microbalance investigations. Al 2 O 3 ALD on W nucleated easily during the first Al(CH 3 ) 3 /H 2 O reaction cycle. In contrast, W ALD nucleation on Al 2 O 3 required multiple WF 6 /Si 2 H 6 reaction cycles and was very sensitive to the Si 2 H 6 exposure. The W ALD nucleation required at least six or seven WF 6 /Si 2 H 6 reaction cycles. The optimum reaction parameters for nucleation were utilized to fabricate W/Al 2 O 3 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/Al 2 O 3 nanolaminates grown at 177 °C. Depth-profile secondary ion mass spectrometry (SIMS) measurements confirmed the composition of the W/Al 2 O 3 nanolaminates. Both the TEM and SIMS results showed that sharper interfaces were observed when W ALD nucleates on Al 2 O 3, and rougher interfaces were observed when Al 2 O 3 ALD nucleates on W. X-ray reflectivity (XRR) investigations indicated that the W/Al 2 O 3 nanolaminates were much thicker than the ∼1000 Å target for the total thickness when the W/Al 2 O 3 nanolaminates were composed of a larger number of bilayers. These results were consistent with much lower than expected film densities for the thin Al 2 O 3 ALD nanolayers. Atomic force microscope studies revealed that the surface roughness of W/Al 2 O 3 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/Al 2 O 3 nanolaminates grown at 177 °C. The X-ray reflectivity of the 64-bilayer W/Al 2 O 3 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/Al 2 O 3 nanolaminates as thermal barrier coatings and X-ray mirrors.