Spatial distribution of the electrical potential and ion concentration in the downstream area of atmospheric pressure remote plasma

Abstract

This paper presents the results from an experimental study of the ion flux characteristics behind the remote plasma zone in a vertical tube reaction chamber for atmospheric pressure plasma enhanced chemical vapor deposition. Capacitively coupled radio frequency plasma was generated in pure He and gas mixtures: He–Ar, He–O2, He–TEOS. We previously used the reaction system He–TEOS for the synthesis of self-assembled structures of silicon dioxide nanoparticles. It is likely that the electrical parameters of the area, where nanoparticles have been transported from the synthesis zone to the substrate, play a significant role in the self-organization processes both in the vapor phase and on the substrate surface. The results from the spatial distribution of the electrical potential and ion concentration in the discharge downstream area measured by means of the external probe of original design and the special data processing method are demonstrated in this work. Positive and negatives ions with maximum concentrations of 106–107 cm−3 have been found at 10–80 mm distance behind the plasma zone. On the basis of the revealed distributions for different gas mixtures, the physical model of the observed phenomena is proposed. The model illustrates the capability of the virtual ion emitter formation behind the discharge gap and the presence of an extremum of the electrical potential at the distance of approximately 10−2–10−1 mm from the grounded electrode.

A computational view on vapour phase coagulation of nanoparticles synthesized by atmospheric pressure PECVD

“…The presence of spherical micron-sized particles covered with trenches among nanoparticle deposit allows us to assume that microparticle formation is caused by nanoparticle coagulation in the downstream area. Based on our previous findings on the electric potential distribution and gas ion concentration measurements in the downstream area [25] we conclude that levitation of charged particles is the nonuniformities of the electric field is a crucial requirement for microparticles formation. Though the proposed model does not take into account such processes as gas-phase formation of fractal structures and possible particle sintering, it allows for better understanding of processes leading to nanoparticles size variations in AP-PECVD equipment regardless of particles chemical composition and electrical conductivity.…”

“…A particle nucleus of 10 nm in diameter may acquire a charge via chemical or photonic process, or via interactions with a gas ion. From our previous studies [25] we know that the concentration of positive ions in the plasma downstream area is 4-6 times higher than the concentration of negatively charged species, hence acquiring a positive charge is a more probable process. For simplicity, we consider a particle nucleus to pick a positive charge q bas = +1.6×10 -19 C. However, all the considerations remain relevant for a negatively charged particle and may be done similarly.…”

“…In order to study the particles coagulation process in plasma downstream the particles were collected on a substrate placed outside of discharge volume. In other words, a remote RF plasma layout was used throughout the experiments, similar to our previous works [25][26][27][28][29][30]. The setup (depicted in Fig.…”

“…The first area [A; B] is the plasma zone between the mesh electrodes; the second area [B; D] is the plasma downstream region spanning from the grounded mesh electrode to the substrate. These areas are characterized by a non-uniform axial distribution of the electric potential -details on its measurement and results may be found in our previous report [25].…”

“…Particle motion takes place in the area with inhomogeneous electrical potential (Fig. 3, [B; D]), which was specified analytically [25]. By analogy to the previous case, we assume that some particles are initially electrically neutral; however a particle may pick up an electrical charge from positive or negative ions in the downstream region.…”