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Thin films are generally thought of as being the product of a reaction between the surface and atoms and/or molecules in the gas phase. However, a relatively new theory, the theory of charged clusters (TCC), suggests that charged clusters nucleate in the gas phase and become the growth unit for a thin film. The aim of this study was to determine whether or not TiO2 thin film deposition by DC reactive sputtering occurs via this mechanism. TiO2 was deposited on unheated transmission electron microscopy grids to observe TiO2 clusters, as well as glass and silicon substrates to observe the resulting thin films. The results showed that TiO2 clusters were indeed produced in the chamber of a direct current reactive sputtering system. Furthermore, these clusters were observed as close as 50 mm away from the target. Clusters 3 nm and

In the thin-film formation process, it is generally accepted that thin film growth occurs via a reaction between the surface and atoms and/or molecules in the gas phase. The theory of charged clusters proposes instead that a thin film is a self-assembly of charged clusters that nucleate in the gas phase. It was demonstrated that similar-sized anatase clusters have quite different deposition behaviors depending on the RF power in the reaction chamber. At 180 W a highly crystalline nanostructured film was produced. However, at 90 W, a nanopowder was deposited instead. This can be explained by the theory of charged clusters, in that the clusters become charged at higher RF power, i.e. the charging efficiency of clusters increases with increasing RF power. Lower power (90 W) did not efficiently charge the anatase clusters. Cluster charging at 180 W resulted in Coulombic repulsion, which prevented the agglomeration observed at 90 W. The self-assembly characteristics of charged clusters is highlighted by the formation of a nanostructured film.

Recently, charged copper clusters of a few nanometers were shown to be generated spontaneously during the thermal evaporation of copper at 1573 K, and were shown to be the major flux for the film growth. In order to identify the charging mechanism at low temperatures, the activation energy for positive charging was estimated based on the temperature dependence of the current generated during thermal evaporation. The activation energy was ∼ 2.81 eV at 1573 K. This low activation energy could be explained by a Saha–Langmuir equation if the copper clusters are formed and undergo surface ionization on the oxidized tungsten. According to this mechanism, the activation energy for charging decreases with increasing cluster size and with increasing work function of the related surface. Based on this charging mechanism, some puzzling phenomena in the thermal evaporation of metals such as the irreproducibility of the process and the degraded film quality with decreasing evaporation rate could be explained.

Investigating the generation of charged clusters during thermal evaporation of gold at 1523 K has created a new understanding of film growth. In order to verify the presence of charged clusters in the gas phase and their polarity biases of +200, 0 and −200 V were applied to the substrate. The charge on the majority of clusters was shown to be positive by observing their deposition behaviors under the applied electric field. These charged clusters were shown to be the major flux for gold film formation. The charge carried by clusters could be detected on a Faraday cup as an electric current. From the temperature dependence of the current, the activation energy for positive charging was estimated to be 1.56 eV. Such a low activation energy suggests that the charging mechanism of clusters is the surface ionization described by the Saha–Langmuir equation.