Low-Frequency Dependence of Plasma Characteristics in Dual-Frequency Capacitively Coupled Plasma Sources

Dual-frequency capacitively coupled plasmas (DF CCP) play an indispensable role in the dry etching process due to their advantage of independent control of ion energy and ion flux.
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As the device feature size shrinks to atomic scale, more energetic positive ions are required to penetrate deeply into the etched trenches to avoid unwanted etching profiles. Lowering the low frequency (LF) can produce increased ion energy with a narrower angular spread,
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which is crucial for achieving high selectivity, anisotropy, and critical dimension control. While the underlying physics of DF CCPs driven at the conventional 2 MHz LF source has been extensively investigated, the dependence of plasma characteristics on LF below 1 MHz has not yet been fully revealed, particularly at low pressures (<3 Pa). In this work, we reveal the effect of LF value (40 kHz-6.8 MHz) on the plasma density and ion energy/angular distribution in a low-pressure (2 Pa), DF capacitively coupled argon plasma based on a combination of experiments and kinetic particle simulations. As the low frequency (f
L
) is decreased from 6.8 MHz to 40 kHz, the plasma density undergoes a moderate decline initially, followed by an abrupt increase, reaching a maximum at ff
L
=400 kHz. The enhanced plasma density at f
L
=400 kHz is attributed to a combined effect of i) an attenuated modulation effect of the LF source on the high-frequency electron heating, and ii) enhanced emission of electroninduced secondary electrons. Besides, a lower f
L
can yield a higher ion energy with a narrower angular spread. Both features are favorable for the high-aspect-ratio etching extensively used in the semiconductor industry.