Low Bias Frequencies for High Aspect Ratio Plasma Etching

Semiconductor processing employs inductively plasmas (ICPs) and capacitively coupled plasmas (CCPs) with large substrate biases to fabricate features with high aspect ratios (HAR) for the production of high-density memory. To maintain critical dimensions of these HAR structures, ion energy and angular distributions (IEADs) must have increasingly high energies and narrow angular distributions. To achieve these goals, substrate biases with progressively lower frequencies are being used. Since lower frequencies are not efficient at heating electrons and plasma production, these systems are typically multi-frequency CCPs (a higher frequency sustains the plasma) or an ICP (where the wave heating sustains the plasma). The trend towards lower frequencies is intended to extend the maximum ion energy to fully that of the sum of the RF amplitude and dc bias, while narrowing the IEAD. However, these lower frequencies also produce nearly quasi-steady state conditions, which affects sheath thickness and flux uniformity, and charging of adjacent surfaces.

In this paper results from a computational investigation of ICPs and CCPs using very low bias frequencies will be discussed. The simulations, conducted with the Hybrid Plasma Equipment Model (HPEM), investigated two test systems – an ICP sustained in Ar/Cl₂ and a 2-frequency CCP sustained in Ar/C₄F₈/O₂. IEADs, uniformity of fluxes to the wafer and sheath structure for these systems for substrate biases as low as 100 kHz will be discussed.