Détection of négative ions in a helium-silane rf plasma

In order to detect négative ions in a helium-silane rf plasma, temporal évolutions of the densities of électrons and ions are observed by square-wave-amplitude modulation of an rf discharge voltage. Densities of électrons and ions are deduc- ed using a microwave interferometer and ion saturation current of a Langmuir probe, respectively. The experiments show that négative ions are formed in the plasma even for a low concentration of 0.5% silane and their density is esti- mated to be about 10⁹ cm^-3 which is comparable to the électron density. The energy of électrons which form négative ions due to attachment is estimated to be below 8.9 eV, that is, below the résonance peak energy of dissociative électron attachment to SiH₄.