Chemical Vapor Deposition of Copper for IC Metallization: Precursor Chemistry and Molecular Structure
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MRS BULLETIN/AUGUST 1994
by-products, nucleation of metal adatoms, coalescence of the nuclei, and steady-state growth of the film.2 The majority of the film thickness is deposited during the steadystate growth phase. An understanding of the physical processes and the chemical reactions occurring during heterogeneous decomposition of the precursor on the substrate (i.e., barrier layer) and on copper metal is required for a complete description of the CVD process. Metal CVD precursors must possess thermal stability and adequate volatility, yet decompose to highpurity metal at temperatures below 300°C. The physicochemical properties of the precursor play a critical role in the CVD process because they determine the vapor pressure, the adsorption/desorption behavior, the decomposition temperature, the chemical reaction pathways, the purity of the deposited metal, and in many cases, the rate of thin-film formation. These combined chemical and physical properties are a direct result of the precursor molecular structure. The mechanism for pyrolytic decomposition of the precursor will be directly responsible for the quality of deposited metal. The formation of gaseous and/or chemically inert decomposition by-products, which are readily removed from the growing film, are critical to achieving pure metallic deposits. Thus, a basic understanding of the decomposition mechanisms for a metal precursor can stimulate
the synthesis of reactants which are tailored to produce high-purity metallic films. In some cases, new precursors may be synthesized to enable the deposition of a specific metal or dielectric, to increase the vapor pressure of the precursor, to improve the handling and stability of the precursor, to lower the decomposition temperature, or to lower the overall cost of the precursor. For practical applications and device manufacturing, the precursor must be stable, easy to handle, relatively nontoxic, and cost competitive. To this end, the synthetic chemist can contribute heavily toward precursor development. Lastly, the ideal CVD precursor will consistently produce pure metal films with near-bulk properties under identical growth and reactor conditions. The elemental purity of the deposited metal is the most stringent requirement for microelectronics applications where near-bulk properties, such as electrical resistivity, must be rigorously maintained. In this article, we will describe the chemical structures and physical properties of both copper(I) and copper(II) precursors used for CVD. In general, the best results have been obtained from copper(I) and copper(II) /3-diketonate complexes, when compared to copper precursors of differing molecular structures. The formation of device quality copper with conformal filling of submicron vias has been demonstrated in several cases. Although scientific interest in selective copper deposition is widespread, blanket metallization in tandem with chemical mechanical polishing (CMP)3 can provide uniformly planar, multilevel structures. Critical issues regarding the utility of co
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