Self-Assembled Monolayers as Model Substrates for Atomic Layer Deposition
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Self-Assembled Monolayers as Model Substrates for Atomic Layer Deposition Caroline M. Whelan1, Anne-Cécile Demas1, Jörg Schuhmacher1, Laureen Carbonell1 and Karen Maex1,2 1 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. 2 Department of Electrical Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 1, B-3001 Heverlee, Belgium.
ABSTRACT Our understanding of the role of the initial surface on atomic layer deposition (ALD) of Cu diffusion barrier materials is limited by the complexity of the sequential reactions and the heterogeneous nature of typical dielectric substrates. The atomically controlled surface chemistry of self-assembled monolayers (SAMs) provides a means of creating model substrates for ALD. Here we report on ALD of WCxNy films on SAMs derived from bromoundecyltrichlorosilane adsorbed on silicon dioxide. The as-prepared SAM is macroscopically ordered with the expected Br-termination and has a well-defined chemical composition as determined by contact angle measurements and X-ray photoelectron spectroscopy, respectively. Temperature programmed desorption spectroscopy confirms that the SAM is stable to 550°C. It survives multiple cycles of ALD at 300°C as evidenced by the detection of mass fragments characteristic of the alkyl chain and supported by the persistence of a Br 2p peak at 71 eV. X-ray fluorescence, ellipsometry and atomic force microscopy reveal that the underlying SAM influences WCxNy film coverage, thickness, and morphology.
INTRODUCTION The trend towards increasing functional density in integrated circuits cannot be supported by traditional materials [1]. Device performance dictates a transition from SiO2 to an insulator with lower dielectric constant (k) and Cu instead of Al for lower resistance wiring. Such interconnect metallization requires the introduction of a barrier layer to prevent Cu diffusion under electrical bias. It is, however, difficult to obtain conformal barriers of the thicknesses (< 5 nm) foreseen in future device architectures without resorting to unconventional methods. To this end, the unique self-limiting and inherently conformal method of atomic layer deposition (ALD) of films from W, Ti, and Ta compounds is being investigated [2]. Based on sequential saturated gas phasesurface reactions, the growth rate in ALD is in theory layer-by-layer, controlled by the number of deposition cycles. In practice, the early stages of film formation may be non-linear, involving three-dimensional growth depending on substrate reactivity [3]. Due to the heterogeneous nature of dielectric materials, information on the role of the initial surface on ALD-mediated growth mechanisms is limited. In particular, our understanding of the surface reactions relevant to ALD of WCxNy diffusion barrier is in its infancy. One approach is to simplify the substrate. Self-assembled monolayers (SAMs), molecular assemblies that are formed spontaneously by the exposure of an appropriate substrate to an active surfactant in an organic solvent or in the gas phase, have been extensively ex
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