Numerical modeling of capacitively coupled hydrogen plasmas: Effects of frequency and pressure

In the field of plasma deposition of amorphous and microcrystalline silicon, the increase of the excitation frequency has often been considered as a way to enhance the deposition rate. Moreover, the increase of pressure has also been shown to enhance the deposition rate and improve the film properties. We attempt to clarify the effects of frequency in the 13.56-40.68 MHz range and to compare them to those of the pressure in the range of 0.5-1.5 Torr. For that purpose we use a numerical modeling of capacitively coupled hydrogen plasma, particularly relevant for the deposition of microcrystalline silicon. We use a one-dimensional time-dependent fluid model for the description of neutrals, positive and negative ions, and electrons, which involves a chemistry model taking into account 32 reactions in the gas phase and on the surface of the electrodes. The results of the model for the symmetrical system show that both pressure and frequency have pronounced influence on the parameters of the discharge: sheath thickness, ratio between power transferred to ions and electrons, and concentration and flux of atomic hydrogen at the electrode surface. We found that increasing the excitation frequency, while keeping constant the power dissipated in the plasma, leads to a more moderate increase of electron density as compared with the case of constant rf-voltage amplitude. The analysis of this phenomenon reveals that, with increase of frequency, the power coupling to the electrons becomes more efficient due to the decrease of the phase shift between voltage and current for both constant power and constant voltage conditions. There is, in addition, a significant drop of the sheath voltage with frequency when the power dissipated in the plasma is kept constant. This leads to the reduction in the drift loss rate for charged species. The increase of pressure mainly reduces the diffusive component of the loss rate for both charged and neutral species and, as a result, electron density enhancement is less pronounced. The increase of pressure leads to a more uniform spatial dissipation of the power coupled to the plasma, whereas the increase in frequency results in a higher amount of power dissipated on the plasma-sheath boundaries due to the decrease of the sheath width. (C) 2003 American Institute of Physics.