Diffusion Barriers for Copper Metallization: Predicting Phase Stability and Reactivity using Equilibrium Thermodynamics
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LTPCM-ENSEEG - BP 75 - 38402 St. Martin d'H~res, (France) *LMGP-ENSPG - BP 46 - 38402 St. Martin d'H~res, (France) ABSTRACT The guidelines for designing a conductive, amorphous material, capable of thermodynamic equilibrium with copper, are defined using readily available thermodynamic
information. The tradeoff between desired properties - equilibrium at the interfaces, amorphous microstructure, and electronic conductivity - are described, along with trends in relevant binary systems that result in these properties. These guidelines defined systems for experimental study, for which preliminary results are presented. INTRODUCTION An implicit requirement for continuously decreasing device sizes is the increased performance of the materials from which these devices are fabricated. Technology has reached the level at which materials properties (e.g. microstructure, dielectric function, etch rate) become design parameters. As described in review articles,"' 2 the transition from aluminum to copper metallization exemplifies this need for robust materials design, in that the required properties of Cucompatible diffusion barriers are in conflict. Essentially, these properties are: i) electronic conductivity, with a resistivity less than 1 m•.cm, ii) nonreactivity with any of the other materials (Cu, Si, or dielectric), iii) amorphous microstructure.2
These requirements are typically in conflict with each other - finding a material that is (conductive and amorphous and nonreactive) is difficult. As discussed below, stabilization of an amorphous microstructure typically requires several elements in the system, plus a certain amount of directional bond character. As more elements are incorporated into the system, the chemical potentials (the thermodynamic description of each element's reactivity) of the other elements change, and the requirement of interfacial stability is often lost. As bond character becomes more directional (indeed, covalent), the solid tends to form a band gap, and electronic conductivity is lost. In addition to these solid-state materials requirements, fabrication method also becomes a critical issue. Physical Vapor Deposition (PVD) has been used to successfully make amorphous, conductive diffusion barriers for Cu- metallization, 3' 4 but with increasing device aspect ratios, these line-of-sight deposition techniques are inadequate. Chemical Vapor Deposition (CVD) techniques can uniformly coat high aspect ratio devices, but tend to produce equilibrium (i.e. crystalline) phases. Thus, CVD of amorphous, conductive, diffusion barriers requires systems with the strongest thermodynamic propensity for amorphization. The objective of this work was to use equilibrium thermodynamic calculations to address the following barrier requirements: Interracial Stability and Amorphous Microstructure. The Inteffacial Stability section describes the use of thermodynamic calculations to define the conditions of nonreactivity at the interfaces. Specifically, for each element that goes into the diffusion barrier, what are th
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