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One of the main concerns when working with negative ion beams is collimation during propagation. The beam is composed of like charged particles; therefore they tend to repel each other causing the beam to widen after acceleration. In environments where the beam needs to travel long distances before reaching its target, the compensation of its space charge is necessary; the most straightforward way of achieving this is the creation of a plasma around the beam, created by ionization of the background gas by the beam itself. This effect, called space charge compensation, plays therefore a key role in the beam propagation and a deep understanding of this phenomenon is fundamental when operating negative ion sources. In order to perform measurements of the particles forming such beam-generated plasma, a retarding field energy analyzer (RFEA) can be adopted, which is an indirect non perturbing instrument, analysing the particles emitted radially from the beam plasma, and allowing to estimate the plasma parameters from the energy distribution function of ions and electrons. The paper presents the design and the layout of the system for voltage control and data acquisition of the RFEA, developed at Consorzio RFX and used for the experimental campaigns in NIO1 to study the phenomena that cause space charge compensation.

Plasma-enhanced atomic layer deposition (PEALD) has obtained a prominent position in the synthesis of nanoscale films with precise growth control. Apart from the well-established contribution of highly reactive neutral radicals towards film growth in PEALD, the ions generated by the plasma can also play a significant role. In this work, we report on the measurements of ion energy and flux characteristics on grounded and biased substrates during plasma exposure to investigate their role in tailoring material properties. Insights from such measurements are essential toward understanding how a given PEALD process at different operating conditions can be influenced by energetic ions. Ion flux-energy distribution functions (IFEDFs) of reactive plasmas typically used for PEALD (O2, H2, N2) were measured in a commercial 200-mm remote inductively-coupled-plasma ALD system equipped with RF substrate biasing. IFEDFs were obtained using a gridded retarding field energy analyzer and the effects of varying ICP power, pressure and bias conditions on the ion energy and flux characteristics of the three reactive plasmas were investigated. The properties of three material examples – TiOx, HfNx and SiNx – deposited using these plasmas were investigated on the basis of the energy and flux parameters derived from IEDFs. Material properties were analyzed in terms of the total ion energy dose delivered to a growing film in every ALD cycle, which is a product of the mean ion energy, total ion flux and plasma exposure time. The properties responded differently to the ion energy dose depending on whether it was controlled with RF substrate biasing where ion energy was enhanced, or without any biasing where plasma exposure time was increased. This indicated that material properties were influenced by whether or not ion energies exceeded energy barriers related to physical atom displacement or activation of ion-induced chemical reactions during PEALD. Furthermore, once ion energies were enhanced beyond these threshold barriers with RF substrate biasing, material properties became a function of both the enhanced ion energy and the duration for which the ion energy was enhanced during plasma exposure. These results have led to a better insight into the relation between energetic ions and the ensuing material properties, e.g., by providing energy maps of material properties in terms of the ion energy dose during PEALD. It serves to demonstrate how the measurement and control of ion energy and flux characteristics during PEALD can provide a platform for synthesizing nanoscale films with the desired material properties.

A Retarding Field Energy Analyzer - RFEA, based on the analysis of the energy spectrum of particles, was developed to study the plasma potential in the drift region of a travelling beam and, specifically, to analyze the energy spectrum of the secondary particles. This diagnostic has four grids biased with variable voltage s to select the particles and to measure the collected currents. To this purpose a polarization and acquisitions ystem was designed and developed suitable to the RFEA.