Investigation of Standing Wave Effects of Harmonics Induced by Plasma Series Resonance in Narrow Gap VHF-CCP

Capacitively coupled plasma (CCP) systems operating in the very high frequency (VHF) range (>30 MHz) with narrow electrode gap are critical for advanced semiconductor etch process due to their high plasma density, and directionality of ions. However, standing wave effects (SWE) cause spatial plasma density nonuniformity when electrode dimensions approach the RF wavelength, leading to central "hot spots" that degrade etching uniformity. This study identifies the mechanism behind these localized density peaks.

Recent findings indicate SWE originates from harmonics propagating within the sheath rather than bulk plasma. These harmonics are generated by plasma series resonance (PSR), a nonlinear interaction between the sheath’s capacitance and bulk plasma inductance under high-frequency asymmetric conditions. The harmonics propagate through the sheath at reduced phase velocities, forming standing wave patterns that intensify electron heating near the reactor center.

Experiments in an asymmetric CCP reactor (300-mm lower electrode, 400-mm upper electrode, 25-cm gap) revealed a central plasma density peak attributed to PSR-SWE coupling. Measurements demonstrate that harmonics induce standing waves within the sheath, directly correlating with localized electron heating—a departure from conventional SWE models focused on bulk plasma dynamics.

This work provides experimental confirmation that sheath-propagated harmonics dominate SWE in narrow-gap VHF-CCP systems. The results establish a quantitative relationship between harmonic generation and central heating intensity, offering new insights for plasma uniformity control. The results establish a foundational framework for optimizing plasma uniformity and advancing narrow gap VHF-CCP processing tools.