Mechanisms of the structural modification of Ti films by pulsed direct current and inductively coupled plasma-assisted pulsed direct current sputtering

Abstract

Titanium (Ti) thin films have been deposited by direct current (dc), pulsed dc, and inductively coupled plasma (ICP)-assisted pulsed dc sputtering to investigate the effects of current pulsing and ICP assistance on film structure and properties. Ti thin films were deposited from a Ti target using Ar as a discharge gas. Ion energy was measured by energy-resolved mass spectrometry. The discharge plasma parameters were measured using a single probe with a time-resolved data acquisition system. The crystal structure was investigated by X-ray diffraction. Surface and cross-sectional morphologies were investigated by atomic force microscopy and scanning electron microscopy, respectively. The cross-sectional images demonstrated that the film structure changed from a coarse columnar structure with voids to a fine columnar structure. Changes in the roughness and crystallite sizes of the deposited Ti thin films indicated that the thin films were amorphized by the ICP-assisted sputtering, resulting in the smoothing of the film surface. Energy and mass analyses indicated that the energy fluxes to the substrate via Ar+ and Ti+ ions increased by more than four and two orders of magnitude, respectively, when ICP assistance was employed. This large difference in degree of enhancement generated by ICP assistance for Ar+ and Ti+ ions suggests different ionization efficiencies between gas-phase Ar and sputter-emitted energetic Ti, particularly in the ICP-assisted pulsed dc discharge. As a result of the increase in energy flux to the substrate, the mean energy transferred by both Ar+ and Ti+ ions per Ti metal particle (atom or ion) arriving at the substrate increased from approximately 40eV for dc sputtering to >104eV for ICP-assisted pulsed dc sputtering, and the mean energy transferred by Ti+ ions per Ti metal particle (atom or ion) arriving at the substrate increased from a few eV for dc sputtering to approximately 29–30eV for ICP-assisted pulsed dc sputtering. These increases in transferred energy reflect the formation of intense and broad high-energy peaks, as shown in the ion-energy spectra. It is shown that the formation of an additional high-energy peak to the single peak of the dc discharge results from the current pulsing, and that the further high-energy shift and splitting of the bimodal peaks result from the combination of the increase in plasma sheath potential to the grounded substrate and the increases in the electron density and electron temperature due to the subsequent ICP assistance to the pulsed dc discharge.