Electrostatic (Langmuir) probe measurements in RF driven helium, nitrogen, boron chloride, and boron chloride/nitrogen plasmas

A Langmuir probe diagnostic system was developed to measure the electron density distribution in BCl3/N2 mixtures to determine if an average electron energy increase might be partially responsible for the previously observed etch rate enhancement of GaAs with nitrogen addition. The system was validated in both helium and nitrogen plasmas. Probe measurements showed that as the nitrogen concentration in BCl3/N2 plasmas increased, the average electron energy actually decreased (5.52 eV 0% N2, 4.14 eV 30% N2, and 3.69 eV 60% N 2). However, an increase in negative ion density was observed with nitrogen addition reaching a maximum at 40% N2. Although negative ions play no role in etching, this trend seems to correlate with the observed etch rate enhancement. A plausible explanation for both the increase in etch species and the increase in negative ion density is an increase in dissociation due to energy transfer from N2 metastables.