Near-surface generation of negative ions in low-pressure discharges

Formation processes of negative ions in low-pressure plasmas are not yet fu lly understood: as a rule experiments reveal higher negative ion density th an predicted by the models. In this work we report near-surface generation of negative ions. This hitherto neglected formation mechanism appears to be important in low-pressure discharges and can have large impacts on the bul k plasma chemistry. We monitor energy-resolved positive and negative ion fl uxes arriving at the electrodes in an oxygen parallel-plate radio-frequency (rf, 13.56 MHz) and dc glow plasmas by means of a quadrupole mass spectrom eter. Negative ions formed in the plasma volume are observed by extracting them through an orifice in the anode of a dc glow discharge. Unexpectedly, we record large negative ion signals at the cathode of a dc discharge and a t the grounded electrode of an rf discharge. These ions are formed in the p lasma sheath, in collision processes involving high-energy species. We prop ose an efficient mechanism of negative ion generation due to ion pair forma tion in the sheath. (C) 2001 American Vacuum Society.