Experimental Characterization of a new Plasma Source for Controlled Deposition of Thin Films

Thin films find several applications depending on both their morphology and chemistry. We have recently implemented a double step deposition process in which film chemistry can be controlled separately from film growth. The molecules of the substance chosen for the deposit are obtained by dissociating an appropriate precursor into an inductively coupled plasma (ICP) obtained from an argon-oxygen mixture. Molecules formed during this first phase are then extracted through a nozzle into a lower pressure deposition chamber. A supersonic freely expanding jet is thus produced, until recompression occurs through a shock wave. Before and after the latter, the flow properties and the degree of molecule clusterization are different. Consequently, film growth can be controlled by changing the substrate location. We will present the design and properties of this new versatile plasma source for thin film deposition. It is well know that ICP discharges work in the inductive regime (H) only after a transition from the capacitive regime (E), which is obtained if the applied voltage is high enough [1]. This transition has been studied using optical and electrical probes. Optical emission spectroscopy has been chosen as an easy and non-intrusive tool for studying other plasma properties. We have selected and modified a simplified radiative model for argon proposed in reference [2], with the aim of evaluating electron temperature. When plasma is obtain from both argon and oxygen the actinometry technique [3] allows to monitor the abundance of oxygen atoms, which will play a role in precursor dissociation. When the precursor is injected, the shape of light emission spectra is useful for monitoring the deposition process. Flow dynamics and the supersonic expansion influence on film growth will be described. Eventually, a few examples of titanium dioxide deposits will be presented in order to demonstrate this new source potentialities.