ИСТИНА

Damage of low-k materials during etching could be caused by ions, photons and radical’s fluxes to films. An average ion energy can be controlled during processing. But it is very difficult to control VUV damage since different plasma species can radiate in the VUV range. These problems are discussed in more detail in [1]. The interest in the application of CF3I as a gas feedstock for plasma etching is increasing recently due to the promising features of CF3I plasmas. It was reported in [2] that this plasma does not emit in VUV range (~ 250 nm) and the etching process is an almost damage-free. At the same time the growth of VUV emission in 250 nm range is observed in [3]. No self-consistent models of RF CCP in CF3I containing gas mixtures have been reported by this time to reveal its main features. In this paper a one-dimensional self-consistent numerical model is developed for a capacitively coupled rf discharge in a Ar/CF4/CF3I. The multipurpose hybrid approach is used in which the particle-in-cell Monte Carlo method is applied to describe the behavior of electrons, whereas the behavior of ions and neutral species is treated by a fluid model, taking into account the complicated plasma chemistry in the Ar/CF4/CF3I mixture. The details of the numerical model were described in [4].There are 27 species (electrons, 5 positive ions, 3 negative ions and 18 neutral) considered in this model, and 101 reactions are taken into account. The model is applied to an RF discharge in the Ar/CF4/CF3I mixtures at a gas pressure of 150 mTorr between two parallel plate electrodes with interelectrode distance L=2.34 cm. The RF frequency is equal to 27 MHz, the power input is 0.5W/cm2. The content of Ar in the gas mixture is fixed at 50%, the relative concentrations of CF4 and CF3I were varied from 0% till 50%. The chosen discharge parameters are very close to the experimental conditions under study in [3]. As it can be seen in Fig. 1 the admixture CF3I essentially changes the discharge features: plasma becomes strongly electronegative as a result of the high electron attachment rate to CF3I molecule, in comparison with CF4. The resulted concentration of electrons in Ar/CF4/CF3I discharge is more than order lower than in Ar/CF4 at the same power input. The average electron energy in the bulk strongly increases from 0.5 eV till more than 5 eV with the CF3I admixture since the low-energy electrons attach most effectively. The EEDF in the center of the discharge calculated in different gas mixtures is presented in Fig.2. The high energy tail of EEDF (>10eV) becomes depleted noticeably because of the relatively large inelastic energy losses (dissociation and ionization) for CF3I molecule in comparison with CF4. As a result the integral Ar ionization rate in Ar/CF3I is four times lower than in Ar/CF4 although the total ionization rates changes little. This trend for Ar ionization agrees very well with experimental data [3] on Ar radiation line at 750nm at the similar discharge conditions. The total dissociation rate to neutral radicals is about an order higher in Ar/CF3I in comparison with Ar/CF4 because of the high dissociative cross section for CF3I. There is no data available on the dissociation channels for CF3I molecule, but the analysis of potential surface has shown that the most probable channel is CF3 and I production. Concentrations of all main radicals and neutral products are presented in Fig.3 as functions of CF3I percentage. As appears from Fig. 3 the concentration of F atoms becomes very low in CF3I-containing plasma because of the high loss rate in the following reaction: CF3I + F => CF3 + IF (k=1.7e-10). Also there is no known direct production channels for F and CF2 through the electron impact dissociation. As a result the F and CF2 concentration decrease noticeably with the increase in CF3I percentage. This calculated trend for CF2 concentration is in contradiction with spectroscopy data [3]. Therefore the probable additional channels for CF2 production in CF3I plasma are considered, namely the neutral products dissociation and ion-ion recombination. The sensitivity analysis has shown that the ion recombination may be the noticeable source of CF2 because of the high ions concentrations and therefore the high recombination rates, as it is illustrated in Fig. 4. The relatively high fluxes of CFxIy+ ions may also indicate the importance of the reactive ion etching especially when the F atom flux is very low. This research supported by SRC program Contract 2012-KJ-2280, the Russian Foundation of Basic Research (12-02-00536-a), Government Grant No 11.519.11.1008 and Grant MK-6009.2012.2.