Insights from Modeling Low-Pressure High-Voltage Dual-Frequency Capacitively Coupled Plasmas*

Dual-frequency capacitively coupled plasmas are often utilized to improve control in etching processes in the semiconductor industry. Etching of high aspect ratio (HAR) features, with depths as much as 100 times their width, are particularly prone to distortions during the etch process such as tapering and twisting. To overcome this, higher ion energies are required, so HAR etching is generally performed with low pressure (mTorr) and high voltage (kV) dual-frequency capacitively coupled plasmas.

This low-pressure high-voltage regime is challenging for computational modeling. At these low pressures, kinetic effects become important and fluid or fluid-based hybrid models are no longer valid. The relatively high plasma densities used in industrial processes result in small Debye lengths, which must be spatially resolved. The high voltages produce energetic electrons, whose high velocities limit the timestep (through Courant-Friedrichs-Lewy constraints). In this work, Aleph, a particle-in-cell direct simulation Monte Carlo (PIC-DSMC) model, was used to investigate dual-frequency capacitively coupled Ar plasmas in conditions relevant to HAR etching. The upper electrode is powered with 1 kV at 40 MHz, and the lower electrode (i.e., the wafer) is powered at 3-6 kV at 2 MHz. The effect of pressure (4-8 mTorr) and gap size (3-7 cm) were explored. The resulting ion energy and angular distributions, electron energy and angular distributions, and the role of secondary electron emission in the plasma dynamics are discussed. The 40 MHz was shown to have some influence on the ion energies, and the fact that 40 MHz is a harmonic of 2 MHz was also shown to produce additional structures in the ion energy distribution.