ALD Capping Layers for Superconducting Radio Frequency Accelerator Cavities M. J. Pellin 1 , J. W. Elam 1 , and J. F. Moore 2 , 1 Argonne National Laboratory, Argonne, Illinois 60439 2 MassThink, Naperville, Illinois 60565-3123 Superconducting radio frequency (SCRF) materials are becoming increasingly important for a variety of applications. Applications range from cellular phone networks to high energy physics particle accelerators such as the International Linear Collider (ILC). Improvement of these devices requires coherent, high purity surface films of nanometer thickness. Here Atomic Layer Deposition (ALD) methods are described for producing well defined dielectric (Al 2 O 3 ) films on Nb, the substrate of choice for SCRF applications. Introduction Atomic layer deposition (ALD) is a thin film growth method using alternating, self limiting reactions between gaseous precursors and a solid surface to deposit materials in an atomic layer-by-layer fashion[1]. These attributes allow highly conformal and uniform films with atomically abrupt interfaces to be deposited on complex, 3- dimensional substrates such as aerogels[2], powders[3], and anodic aluminum oxide (AAO) membranes[4]. These attributes make ALD an ideal synthesis method for modifying SCRF cavities for particle accelerator applications. The layout of the proposed International Linear Collider (ILC) for instance includes many kilometers of superconducting Nb cavities held at >2 K in ultrahigh vacuum (UHV). The purity of the interior surfaces of these Nb cavities are crucial[5] because for SCRF applications these surfaces carry all of the considerable electrical current needed to drive ions with acceleration gradients exceeding 35 MV/m. The cavities themselves are complex in shape and require the unique 3-dimensional coating capability offered by the ALD method. Nb is generally accepted as the best material for SCRF cavities. It has been chosen because it has the highest lower critical field temperature of any known superconductor. For type 2 superconductors, the lower critical field temperature is the phase transition temperature below which the material becomes truly resistance free. Because cooling these cavities is both difficult and expensive, even the slight resistance found above this temperature is unacceptable. Unfortunately, Nb is chemically labile, forming significant oxide layers even in UHV conditions. Moreover, the oxide is compositionally complex, forming identifiable phases ranging from Nb 2 O 5 to Nb 2 O with properties that range from superconducting oxides to strict dielectrics. Although there are no reports of Nb ALD, thin films of a related superconducting material, NbN, have been prepared by ALD[6-8]. The study of the surfaces and morphology of ultrahigh purity Nb for use in SCRF cavities has been a recent active area of investigation.[9-14] These recent studies demonstrate that surface and interface segregation are crucial properties of these materials for SCRF applications since dissipative behavior is limited in this case to the first few 100 nm’s of the material. Moreover, preferential diffusion/segregation at grain boundary or in the near surface are phenomena leading to large inhomogeneity in the material selvedge. Vacuum annealing of the Nb surface leads to a surface equilibrium with surface oxidation from residual vacuum gases being balanced by diffusion of the niobium oxide into the bulk material. ECS Transactions, 11 (7) 23-28 (2007) 10.1149/1.2779065 ©The Electrochemical Society 23 Downloaded 04 Jun 2009 to 146.139.77.80. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp