Inductively Coupled Plasmas for Varying Antenna Aspect Ratio - Experiment*

Electronegative pulsed inductively coupled plasmas (ICPs) are used in the microelectronics industry for etching, and passivation.  Pulsing is a primary control strategy to optimize conditions.  Pulsed ICPs are prone to repetitive mode transitions from the capacitive (E mode) to the inductive (H mode) at the beginning of each power pulse due to the decrease in electron density in attaching gas mixtures during the interpulse afterglow.  The E-H transitions are sensitive to circuit and reactor configurations, in particular the geometry of the antenna.  In this work, we report on measurements of E-H transitions in pulsed ICPs (tens of mTorr) sustained in Ar/O₂ mixtures for different antenna aspect ratios (i.e., the height vs width of individual windings of a stovetop style coil) and duty cycle.  A 3D probe drive system was used to measure changing magnetic fields throughout the plasma volume.  These results are used to compute volumetric current and azimuthal electric field, from which we calculate inductive power dissipated in the plasma as a function of time. Antenna power is carefully measured, with the difference attributed to capacitive coupling. In addition, capacitive probes measure the induced voltage inside the ceramic window just underneath the antenna. Results are reported for varying ratios of Argon to Oxygen flow rate. Results are compared to the results from 2-dimensional modeling.