6.3 Precursors for chemical vapor deposition of copper  (Page 5/5)

A number of researchers have demonstrated the potential of a series of β-diketonate Cu(I) compounds, (β-diketonate)CuL _n , where L is Lewis base and n = 1 or 2, that fulfill most of the criteria outlined for precursor design before. These species were chosen as copper precursors for the following reasons:

• They contain the β-diketonate ligand which generally imparts volatility to metal-organic complexes, particularly when fluorinated, as a result of a reduction in hydrogen-bonding in the solid-state.
• They are capable of systematic substitution through both the β-diketonate and Lewis base ligands to tailor volatility and reactivity.
• Lewis bases such as phosphines, olefins and alkynes are unlikely to thermally decompose at temperatures where copper deposition occurs.
• These precursors can deposit copper via thermally induced disproportionation reactions and no ligand decomposition is required since the volatile Lewis base the Cu(II) disproportionation products are transported out of the reactor intact at the disproportionation temperature.

Reaction mechanism

A general feature of the reactions of Cu(I) precursors is that they thermally disproportionate, a mechanism likely to be responsible for the high purity of the copper films observed since ligand decomposition does not occur. The disproportionation mechanism is shown in [link] for (β-diketonate)CuL. The unique capabilities of this class of compounds result from this reaction mechanism by which they deposit copper. This mechanism is based on the dissociative adsorption of the precursor to form Cu(hfac) and L, disproportionation to form Cu(hfac) ₂ and Cu and desorption of Cu(hfac) ₂ and L.

Thus, the starting material acts as its own reducing agent and no external reducing agent such as H ₂ is required. Another advantage of the Cu(I) β-diketonates over the Cu(II) β-diketonates is that in the former the ligand L can be varied systematically, allowing the synthesis of a whole series of different but closely related compounds.

Selectivity

Selectivity deposition has been studied in both hot- and cold-wall CVD reactors as a function of the nature of the substrate, the temperature of the substrate and the nature of the copper substituents. Selectivity has usually been evaluated by using Si substrates on which SiO ₂ has been grown and patterned with various metals by either electron-beam deposition, CVD or sputtering. Research has suggested that selectivity on metallic surfaces is attributable to the biomolecular disproportionation reaction involved in precursor decomposition.

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