Monitoring sheath voltages and ion energies in high-density plasmas using noninvasive radio-frequency current and voltage measurements

To obtain optimal results from plasma processing, the energy of ions incident on substrate wafers must be carefully controlled. Such control has been difficult to achieve, however, because no practical method exists for monitoring the energy distributions of ions at a wafer surface during processing. To solve this problem, we have developed a noninvasive, model-based method for determining ion energy distributions that is suitable for use during actual processing in commercial plasma reactors. The method relies solely on measurements of the rf current and voltage applied to the reactor. The method was validated by tests performed in argon and CF4 discharges at 1.3 Pa (10 mTorr) in an inductively coupled, high-density plasma reactor, with rf substrate bias at frequencies of 100 kHz to 20 MHz. Plasma potential and sheath voltage wave forms obtained from the noninvasive rf technique agreed well with independent measurements made using a capacitive probe. Ion energy distributions from the rf technique were also in good agreement with distributions measured by ion energy analyzers. To further test the technique a sensitivity analysis was performed which quantifies the sensitivity of the noninvasive results to any uncertainties in the electrical measurements or the other input parameters required by the technique