Effect of Surface Chemistry on the diffusion of Copper in nanoporous dielectrics
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Effect of Surface Chemistry on the diffusion of Copper in nanoporous dielectrics Oscar Rodriguez, Woojin Cho, Ravi Saxena, Ravi Achanta, William N. Gill and Joel L Plawsky Department of Chemical and Biological Engineering Rensselaer Polytechnic Institute, Troy NY 12180 USA ABSTRACT This work is aimed at understanding the nature of the interactions between metal interconnects and nanoporous dielectrics in integrated circuits. Electrical testing of MIS capacitors is used to assess Cu diffusion and charge injection in the dielectric in the presence of an electric field. We have found that surface modification of nanoporous silica reveals the importance of chemically bound or adsorbed water species in the dielectric and how they trigger metal diffusion. We propose that a combination of moisture-related species in the dielectric and interfacial oxygen oxidize Cu. The copper oxide acts as a source for Cu ions available for diffusion. A quantitative analysis of Cu drift in nanoporous dielectrics that shows the importance of surface chemistry is presented and the mechanism of metal diffusion and charge injection in nanoporous dielectrics is discussed. INTRODUCTION In an interconnect structure, the dielectric is in contact with other materials, such as a metal (copper), a diffusion barrier (for example Ta or Ta nitride), or a hard mask (Si nitride). It is important to minimize the interactions between these different materials to insure long-term device stability though some minimal interaction is necessary to provide bonding and good adhesion between layers. The increased speed in semiconductor devices requires copper and low-k ILD solutions to minimize RC delay. Porous materials have been proposed to replace SiO 2 as the interlayer dielectric. Copper is known to be a fast diffuser through SiO 2 , several porous low-k dielectrics and Si [1-3]. However, a mechanism of how Cu diffuses has not been established. In this work, we show that surface chemistry plays an important role in stopping Cu diffusion, which may lead to an understanding of Cu drift in nanoporous dielectrics. EXPERIMENTAL DETAILS Bias-temperature stressing (BTS) measurements in Metal-Insulator-Silicon (MIS) capacitors were used to study Cu drift in nanoporous dielectrics [4]. Epitaxial n-type silicon wafers with an epi layer resistivity of 3-5 ohm-cm were used. The insulator films used include nanoporous silica (xerogel), Methyl-Silsesquioxane (MSQ) and sintered nanoporous silica. The nanoporous silica films were spin-coated on Si wafers using the two-solvent sol-gel method described in a prior publication [5]. A surface modi?cation step was done using 4% vol. solution of trimethyl chlorosilane (TMCS) in hexane. Sintering of nanoporous silica was done at 900 C in air for 2 hours. After sintering, the films were treated with TMCS to remove any moisture in the film.
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Copper dots of 1 mm in diameter and 300 nm thick were e-beam deposited on top of the dielectric. To complete the MIS structure, the backside of the Si wafer was carefully etched an
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