Inductively Coupled Plasmas for Varying Antenna Aspect Ratio - Modeling*

Electronegative plasmas sustained in halogen gases are used in the microelectronics industry for etching, and passivation.  Pulsed inductively coupled plasmas (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 the attaching gas mixture during the interpulse afterglow.  The E-H transitions are sensitive to circuit and reactor configurations, and in particular the geometry of the antenna.  In this work, we discuss results from a computational investigation of E-H transitions in pulsed ICPs using flat-coils for different antenna aspect ratios (height vs width of windings of the coil), duty cycle and gas mixtures.  This investigation was performed using the Hybrid Plasma Equipment Model.  Typical conditions are Ar/Cl₂ and Ar/O₂ gas mixtures at tens of mTorr and a pulse repetition frequency of a few to 10 kHz.  We will discuss the plasma densities, energy distributions of ions to the window, and the power distribution between capacitive and inductive modes.  We found that the inductive coupling is a weak function of aspect ratio of the antenna.  Capacitive coupling is a sensitive function of aspect ratio, with low aspect ratio (i.e., long dimension parallel to window) producing more intense capacitive coupling.  Comparisons are made to experimental measurements of plasma properties.