IEDs have been measured at the anode of a reactive ion etcher using a retarding field analyser (RFA) that has been developed from a similar instrument used in fusion plasma studies. SIMION studies show that even at the highest sheath voltages the analyser is only expected to induce 6% error in the measurement of ion energies. Measurements were performed for Ar, He, Xe, CO2, N2, CF4/8%O2, SF6, CF3Cl and CCl4 discharges for a variety of process conditions. Results show that all the IEDs exhibit some degree of RF modulation as characterized by a double peaked distribution where the peak splitting, Delta E is typically 10 eV. The degree of this RF modulation is found to depend strongly upon discharge conditions. As the voltage is increased in electropositive gas discharges the degree of RF modulation decreases as evidenced by a decrease of Delta E of ~2-5 eV; this is attributed to a sheath thickness that increases with applied RF voltage. For these same discharges the degree of RF modulation is found to increase when the discharge pressure is increased as similarly indicated by an increase of Delta E of ~2-5 eV; this is attributed to a decrease in the sheath thickness. Electronegative discharges exhibit less change in the degree of RF modulation of the IED as process conditions are altered. It is proposed that this is due to the discharge being better confined between the electrodes leading to a more stable sheath.
IEDs have also been measured for Ar where the anode to cathode area ratio was increased. Values of Delta E up to 25 eV were observed. The previously observed changes in the IED due to changes in the degree of RF modulation were progressively removed with increasingly large area ratios. This is attributed to the presence of a sheath whose thickness does not change significantly due to the discharge being better confined between the electrodes.
Simulation of Ar IEDs were performed using a Monte Carlo computer program. Where the plasma potential was described as a rectified sinusoid a full fit to the experimental data was not possible. It is demonstrated that improved agreement of simulation and experiment, using the sheath thickness as a fitting parameter, can be achieved when the Bohm criterion is included and the plasma potential expression altered to describe a sinusoid. This suggests that the plasma potential in the reactor is sinusoidal. Cathode IEDs have also been calculated using the fitting parameters derived from anode IEDs; these are directly relevant for the process engineer.