DETECTION OF NEGATIVE-IONS IN A HELIUM-SILANE RF PLASMA

In order to detect negative ions in a helium-silane rf plasma, temporal evolutions of the densities of electrons and ions are observed by square-wave-amplitude modulation of an rf discharge voltage. Densities of electrons and ions are deducted using a microwave interferometer and ion saturation current of a Langmuir probe, respectively. The experiments show that negative ions are formed in the plasma even for a low concentration of 0.5% silane and their density is estimated mated to be about 10(9) cm-3 which is comparable to the electron density. The energy of electrons which form negative due to attachment is estimated to be below 8.9 eV, that is, below the resonance peak energy of dissociative electron attachment to SiH4.