SEMICONDUCTOR DAMAGE FROM INERT AND MOLECULAR GAS PLASMAS

The change in sheet conductance of a thin, highly doped layer of GaAs is measured after exposure to inert gas plasmas (He, Ar, and Xe) and to molecular gas plasmas (CCl2F2, CF4, SF6, and O2) in a parallel-plate rf discharge. In order to compare these data, the change in sheet conductance is converted to a damage depth scale. A different linear relationship is found for the damage dependence on the rf-induced dc bias for each plasma. An inverse-mass relationship is derived from the data for He, Ar, and Xe plasma exposures. Using this, two models are tested for their ability to predict the damage from the molecular gas plasmas. For one model, the assumption is that damage is caused by molecular ions that remain intact upon impact. For the other model, which is quite successful at predicting the measured damage, the assumption is that molecular ions fragment completely upon impact. This interpretation indicates that Cl+ and/or F from CF+ in CCl2F2 plasma, F from CF+ in CF4 plasma, and S from SF+ in SF6 plasma are responsible for the measured damage effect. Neither model adequately predicts the low level of damage from O2 plasma. Helium ions caused the greatest amount of damage; when mixed with CCl2F2, the measured damage was a factor of three lower. Using optical emission spectroscopy, quenching of He ions was observed when molecular gases were introduced into a He plasma. Quenching was also observed in other mixed gas plasmas and this indicates that ions with ionization potentials substantially higher than those of other ions in the plasma will not be formed in typical rf glow discharges.