High aspect ratio (AR) nanostructured films of noble metals and noble metal oxides with large specific surface areas can have a wide variety of applications in many fields such as catalysis, sensing and energy storage. Atomic layer deposition (ALD) has become the method of choice for depositing thin films conformally in high AR structures.(1) To achieve high conformality, saturation of the surface reactions has to be achieved throughout the three-dimensional (3D) structure. For plasma assisted ALD, reaching conformal deposition in high AR structures is less straightforward than for thermal ALD due to surface recombination loss of plasma radicals.(2) In our contribution we demonstrate that plasma assisted ALD can be used to deposit Pt in high AR 3D structures with high conformality. We also describe the sample preparation technique that allowed the conformality to be analyzed using transmission electron microscopy (TEM). Depositions were performed on high AR trenches, pre-etched in silicon (AR = 1.3 - 34), and in anodized aluminum oxide (AAO) (pore diameter = 70 nm, depth = 20 µm). The results for coating the Si trenches demonstrate that for the plasma assisted ALD process, pulses of 3 s (MeCp)PtMe3 and 3 s O2plasma are sufficient to deposit Pt coatings with 95-100% conformality up to AR = 20. For AR = 20 the conformality was quantified using TEM. For this purpose, a cross-sectional TEM sample of a trench was prepared by first cleaving the Si substrate exactly through the trench heartline. A lift-out TEM sample was subsequently prepared using Focused Ion Beam (FIB) milling orthogonal to the cleavage plane (i.e. to the side wall of the trench). This provided a TEM sample in which the entire sidewall of the 20 µm deep trench was electron transparent, which allowed the layer thickness as a function of depth to be determined with high precision. The TEM images of individual Pt nanotubes deposited in the AAO pores were used to determine wall thickness as a function of depth, and showed the grain structure of the deposited Pt in great detail. In our contribution we will also report on the high AR conformality of other noble metal and noble metal oxide plasma assisted ALD processes (e.g., PtOx and Ir).

(1) P. Banerjee, I. Perez, L. Henn-Lecordier, S. B. Lee, and G. W. Rubloff, Nature Nanotechnology 4, 292 (2009).

(2) H. C. M. Knoops, E. Langereis, M. C. M. van de Sanden, and W. M. M. Kessels, J. Electrochem. Soc. 157, G241-G249 (2010).