Real-time, noninvasive monitoring of ion energy at a wafer surface during plasma etching

Summary form only given. Energetic ion bombardment plays a crucial role in plasma etching. A better understanding and better control of etch processes could be obtained if the energy distributions of ions striking the wafer surface were known. Although it is difficult or impossible to directly measure ion energy distributions at the wafer surface, they can be determined indirectly using noninvasive measurements of the applied RF bias current and voltage, interpreted by plasma sheath models. One such method for monitoring ion energy has previously been validated and used to monitor drift at a bare aluminum electrode, with no wafers present. This study reports validations and demonstrations of the method with wafers present. Experiments were performed in an inductively coupled plasma reactor during actual etching of oxidized and bare silicon wafers in Ar/CF₄ plasmas. At sufficiently high bias frequencies (ges1 MHz) the wafer and wafer-to-chuck contact were found to contribute a total impedance of only a few ohms, which, if unaccounted for in the model, contributes an uncertainty in ion energies of a few electron volts. In contrast, at bias frequencies les100 kHz the wafer impedance was larger, causing larger errors in the inferred ion energies. At high bias frequencies, the technique was used to monitor ion energy over the course of a normal etch and during abnormal conditions caused by simulated faults in RF power delivery, gas flow, and pressure control. Implications of this work for assuring more stable ion energies will be discussed