On the Mechanism of the Atomic Layer Deposition of Cu Films on Silicon Oxide Surfaces: Activation Using Atomic Hydrogen and Three-Dimensional Growth

The molecular details of the atomic layer deposition (ALD) of metallic copper on silicon oxide substrates using metallorganic precursors, iminopyrrolidinate complexes in particular, were characterized under ultrahigh vacuum conditions by temperature-programmed desorption (TPD), in its temperature ramping and isothermal modes, and by X-ray photoelectron spectroscopy (XPS). Two specific issues were addressed. First, it was demonstrated that H₂ is quite inefficient as a reducing agent and cannot be effectively used to remove the organic ligands in the adsorbed species during the ALD cycles; insufficient ligand removal leads to the incorporation of impurities in the growing films. On the other hand, atomic hydrogen can accomplish both metal reducing and ligand removal functions and can be incorporated in ALD processes to deposit incremental amounts of Cu in sequential cycles. The second aspect discussed here is the fact that the deposited metallic Cu grows in the form of three-dimensional (3D) nanoparticles rather than as conformal two-dimensional films. The TPD and XPS evidence collected in our studies points to a mechanism where individual Cu atoms, once cleaned from their organic ligands and reduced to their metallic state, become quite mobile and diffuse on the SiO₂ surface to form the 3D nanostructures. Similar Cu atom mobility and sintering were seen even in Cu physical vapor deposition processes carried out at room temperature and may therefore be an intrinsic feature of these ALDs regardless of the source of the metal atoms.