One of the major concerns of material scientists and technologists nowadays is to understand the fundamental processes of thin film nanostructuration. Plasma-assisted thin film deposition is of specific importance due to the strong interaction between plasma-generated species and the film during film growth. This leads to the formation of metastable structures, resulting in the optimization or initiation of singular properties for a wide range of applications.
As a part of the PLASMATER (P10-FQM-6900), funded by the Junta de Andalucía, researchers have investigated these key processes and their impact on the film nanostructure. This work has been carried out by combining film characterization methods including optical response measurements, atomic force microscopy, scanning electron microscopy etc., with plasma diagnosis techniques, like Langmuir probe measurements, mass spectrometry or optical emission spectroscopy.
Furthermore, the researchers have analyzed the role of ion impingement on the growth of SiO2 thin films that are deposited by DC pulsed magnetron sputtering, using an energy-resolved mass spectrometer.
DC Pulsed Magnetron Sputtering
The DC pulsed magnetron sputtering discharges involve production of positively charged ions in the plasma bulk. The positive ions are then accelerated towards the film with kinetic energies of only a few electronvolts under non-biased substrate conditions.
While these ions have a relevant role in certain cases of thin film nanostructuration, their energy is based on the displacement energy threshold of the adatoms in the material. What most likely occurs is these ions heat up the film surface and weakly develop thermally activated relaxation processes.
By contrast, the negative ions that are made available on introducing an electronegative gas into the reactor, can be produced at the cathode surface. These ions are stimulated towards the film with kinetic energies of a few hundred electronvolts (in the order of cathode potential fall).
These ions are provided with sufficient energy to initiate film adatoms mobilization via collisions, resulting in major changes in the film nanostructure. While the negative ions seem to be relevant from an energetic point of view, the amount of flux required to induce changes in the film was unclear until these findings were published.
In this work, researchers have shown that microstructural changes on magnetron sputtered SiO2 thin films can be induced through negative oxygen ions. To accomplish this, researchers have deposited different coatings under various conditions using several microstructures.
Based on the analysis of the role of relevant processes which influence the film growth, including positive and negative ion impingement, thermally activated processes on the surface and surface shadowing, it was inferred that the microstructural changes are the result of the impingement of intermediate energy O- ions. In the discharge, these ions seem to be a result of the dissociation of high-energy molecular oxygen negative ions generated near the cathode surface and stimulated towards the plasma.
Four cross-sectional scanning microscopy images of the films deposited increasing O- ion flux bombardment are shown in Figure 1. The void percentage for increasing negative oxygen fluxes and the elimination of vertical geometrical patterns are evident from the images.
Figure 1. Cross-sectional electron microscopy images of the films for increasing O- flux
The work has demonstrated the presence of two thin film nanostructuration regimes at low temperatures due to the competition between the surface shadowing mechanism and the negative ion-induced adatom surface mobility processes. The quantity of O2 present in the deposition reactor controls these regimes.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
For more information on this source, please visit Hiden Analytical.