The Role of Secondary Electrons in Low Pressure RF Glow Discharge

We concerned on the role of secondary electrons in low pressure rf glow discharge. The Particle in Cell/Monte Carlo computer simulations (PIC/MC) was applied to simulate the discharge. The influence of secondary electron emission yield (SEY) on the plasma density was studied. Furthermore the relative density and the electron energy probability function of secondary particles, i.e., secondary electrons and the products of ionizing collisions of secondary electrons was determined. We found that the secondary particles play very important role in this type of discharge. The relative density of secondary electrons created on electrodes is 2-10%, 5-15% for the value of SEY = 0.1, 0.2 respectively. But due to the high electric intensity in electrode sheaths these electrons have relatively high energy and therefore they can ionize neutral atoms. As consequence of this the relative density of the secondary electrons (secondaries) and electrons created in ionizing collisions of the secondaries is more than 40% and the relative density of ions created in ionizing collisions of the secondaries is more than 20%, 40% for value of SEY = 0.1, 0.2, respectively. Introduction Low pressure radio frequency (rf) discharges are often used for sputtering, etching and plasma enhanced chemical vapor deposition (PECVD). It is well-known that the secondary electron emission on the electrodes affects the plasma density [Ruas, et al., 2002, Lieberman and Lichtenber, 1994]. In the present paper the role of the secondary electrons and the influence of the value of the secondary emission yield (SEY) on the plasma density are studied via computer simulation. Our simulation model was based on Particle In Cell/Monte Carlo (PIC/MC) method. The PIC/MC method was used extensively for the study of plasma and the method is well documented in [Birdsall et al.,1991] and [Vahedi and Surendra,1991]. Modified simulation program PDP1 [Verboncoeur et al., 1993] was used. Model description We studied low pressure rf capacitively coupled discharge. One dimensional electrostatic model was applied, i.e., the electric field was only in one direction perpendicular to electrodes. Our model system was composed of two planar parallel electrodes, the diameter of the electrode was chosen 10 cm, the distance between the electrodes was chosen 3 cm. One of the electrode was powered by rf voltage with amplitude U0 = 150 V and frequency f = 13.56 MHz, the second one was grounded (zero voltage). Argon (pressure 3–20 Pa) was used as working gas. For the background neutral gas we considered Maxwellian velocity distribution with temperature 300 K. Density of neutral gas was uniform and depended only on the gas pressure. So we have plasma which is composed from electrons and positive ions, they move in the electric field and collide with the neutral atoms of the working gas. Their movement can be written by the Lorentz equation. Only the binary collisions were considered, as can be seen in the Table 1. Table 1. Collision processes used in the simulation. The energy necessary to the ionization is about 15.7 eV. electrons + Ar elastic collision e + Ar → e + Ar excitation e + Ar → e + Ar(11.55 eV) ionization e + Ar → 2e + Ar, εi = 15.7 eV Ar + Ar scattering Ar + Ar → Ar + Ar charge transfer Ar + Ar → Ar + Ar WDS'05 Proceedings of Contributed Papers, Part II, 306–312, 2005. ISBN 80-86732-59-2 © MATFYZPRESS