Systematic characterization of low-pressure capacitively coupled hydrogen discharges

Abstract

This paper presents a systematic characterization of pure hydrogen capacitively coupled discharges, produced in a parallel plate cylindrical setup. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions H ϩ , H 2 ϩ , and H 3 ϩ , at different frequencies ͑13.56-60 MHz͒, pressures ͑0.2-8 Torr͒, rf applied voltages ͑50-450 V͒ and geometric dimensions ͑1.6-12.8 cm radii and 1.6-6.4 cm interelectrode distances͒. A good agreement is found between calculation results and experimental measurements for the coupled electrical power, the plasma potential, and the self-bias potential, at various frequencies and rf applied voltages. However, the model generally underestimates the electron density with respect to its measured values. The paper discusses different space-time events, such as the development of double-ionization structures or the occurrence of field inversion and field reversal phenomena. The dependencies on pressure and frequency of the time-average electric field distribution are analyzed and related to the electron displacement within space-charge sheaths. This study is later used to understand the variations of the hydrogen dissociation rate, with changes in discharge operating conditions. The influence of reactor dimensions on the spatial profiles of the plasma potential, the rf electric field, the electron density, and the electron mean energy are analyzed in terms of discharge symmetry. An investigation of the space-time averaged rf electric field variations, with changes in the applied voltage, pressure, and geometric dimensions is carried out. These variations are shown to follow a universal similarity curve, if an adequate normalization is used when plotting the rf electric field as a function of pressure. This innovative representation of rf discharges allows a univocal definition of a reactor working point, for given operating conditions.

Dominant ion species in VHF SiH4/H2 plasma

“…There are some reports [3,9] that dominant ion species in H 2 plasma is H 3 + at high pressures, where QMS was used to measure ion species. However, Fig.…”

Capacitively Coupled Hydrogen Discharges: Modeling vs. Experiment

“…either the rf applied potential or the excitation frequency. In fact, both of these changes lead to higher electric fields within discharge sheaths (hence, higher effective electrical powers coupled to the plasma), in order to limit the electron displacement and thus the electron wall losses [4]. Note, in Fig.2, the effect of the superelastic vibrational pumping, which results in an enhancement of the vdf's tail.…”

“…However, smv predictions for the plasma density and the self-bias voltage showed only a qualitatively agreement with respect to measurements. In fact, the values of these quantities, calculated with the smv, were systematically underestimated with respect to measurements [4,5], which was attributed to both experimental uncertainties and the simplified hydrogen kinetics considered.…”

“…The simulations use a two-dimensional (2D) fluid model, which has first adopted a simplified hydrogen kinetics to describe the dynamics of electrons and positive ions H the reactor under study [3,4]. This simplified model version (smv) yielded a good quantitative agreement between calculation and experiment for the coupled electrical power and the plasma potential.…”

Numerical investigation on electrical characterization of a capacitive coupled radio-frequency plasma

“…A wide variety of methods have been employed in this effort. These include full numerical solutions of the Boltzmann Equation [1] , particle methods [2,3] and fluid models [4][5][6][7][8] . In this paper, a model combines the fluid theory and kinetic theory [9][10][11] was adopted.…”