Separation of the effects of rf sources used for biasing the wafer and for sustaining the plasma is studied by measuring the space profiles of net excitation rate of Ar(3p5) for a two-frequency capacitively coupled plasma as a representation of a typical oxide etcher. Measurements were performed in Ar and in CF4/Ar mixtures. For biasing supply operating at low frequency, 700 kHz, it was shown that the effect of the voltage becomes significantly smaller as the sustaining voltage is changed from high frequency, 13.56 MHz, to very high frequency (VHF), 100 MHz, and it even disappears for pulsed operation in mixtures. This is the result of the low dc self-bias at the VHF electrode that allows the high energy secondary electrons to leave the plasma without excessive contribution to ionization and dissociation.
Electron transport coefficients and the excitation, attachment
and ionization rates in pure CF4 and in CF4-Ar mixtures have been
calculated for a set of cross-sections which was based on the work of
Nakamura (1991 Gaseous Electronics and Their Applications
ed R W Crompton et al (Tokyo: KTK Scientific) pp 178-200) but which was
modified to include more complex dissociation and ionization kinetics
required to develop an adequate plasma chemical model. The procedure for
the calculation was the direct numerical procedure (DNP) solution to the
Boltzmann equation which makes no approximations in the number of terms
used to represent the anisotropy of the distribution function in the
velocity space. However, its accuracy may be limited by the density of the
grid used. In this paper, we apply the DNP technique to establish its
adequacy for plasma modelling in pure CF4 and in 5% CF4-Ar
mixture, as well as establishing the adequacy of the cross-section set
that was used.
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