Impact of plasma operating conditions on the ion energy and angular distributions in dual-frequency capacitively coupled plasma reactors using CF4 chemistry

Abstract

A two-dimensional hybrid model is used to simulate an industrial dual-frequency capacitively coupled plasma reactor working at closely spaced frequencies (13.56–40.68 MHz) in pure CF4 chemistry. The goal is to understand how plasma operating conditions (pressure, low-frequency and high-frequency RF powers, and chamber wall conditions) influence critical etching parameters such as the ion energy and angular distribution (IEAD) and the ion flux at the wafer. In base case conditions, the ionic and radical composition at the center of the plasma is analyzed, revealing CF3+ and F− as the primary ions, and F, CF, CF3, CF2, and F2 as the predominant radicals (by decreasing density). The impact of the surface recombination coefficient of F radicals into F2 at the reactor walls, γ(rec,F > F2), is then explored; it is found that increasing γ(rec,F > F2) has a strong impact on the final plasma composition, decreasing CF and F densities while increasing CF3, CF2, and F densities, which highlights the importance of properly considering wall conditions in CF-based plasmas simulation. The IEAD at the wafer is then characterized, showing that the total IEAD shape is affected by the plasma ion composition: heavy ions such as CF3+ (69 amu) form the core of the distribution while lighter species such as F+ (19 amu) form the wing of the distribution due to their lower mass. The low frequency (LF) power (100–900 W) is shown to substantially modify the ion energy distribution function (IEDF) owing to sheath voltage changes, but to also marginally increase the ion flux at the wafer. Conversely, the high-frequency (HF) power (100–1500 W) strongly impacts the ion flux at the wafer due to HF voltage fluctuations, while the IEDF remains mostly unaffected. This study also reveals some coupling between the effects of the LF (13.56 MHz) and HF (40.68 MHz) power, a phenomenon attributable to their proximity in frequency which should not be neglected. Finally, increasing the pressure from 30 to 200 mTorr is found to increase the electronegativity by a factor 4 and to strongly impact the plasma structure, primarily due to variations in ion mobility; it also widens the ion angular spread, potentially influencing etch uniformity. Notably, higher pressures exceeding 100 mTorr result in a decrease in the average ion density and the emergence of a low-energy peak in the ion energy distribution, attributed to charge exchange collisions.

Role of gas composition in weakened nonlinear standing wave excitation and improved plasma radial uniformity in very-high-frequency asymmetric capacitive Ar/CF4 discharges

“…S +i,z = 2n +i,s u B / d represents the loss for positive ions at the plasma-sheath edge, where u B is the Bohm velocity, and n +i,s = h +i n +i is the positive ion density there, with h +i the axial edge-to-center density ratio [1,31]. Since negative ions are confined by the ambipolar electric field and barely reach the sheath edge when electronegativity is low, the loss of negative ions at the plasma-sheath interface is negligible [1,46], and the electron density at the sheath edge is given by n es = ∑ +i n +i,s . According to the quasineutral assumption in the bulk region, the electron density is determined by…”