Inductively coupled plasma driven by asymmetric triangular current waveform

Two-dimensional particle-in-cell simulations of an inductively coupled plasma (ICP) are used to investigate the influence of radio-frequency (RF) current waveform and frequency on plasma characteristics, collision processes, and the electron velocity distribution function (EVDF). Plasmas driven by sinusoidal coil current over the frequency range of 1–30 MHz and by asymmetric triangular waveform current at 1 MHz are examined at 25 mTorr. For sinusoidal excitation, significantly lower coil current is required at higher frequencies to achieve comparable electron density, consistent with the scaling of inductive electric fields with frequency. Plasma production shifts toward the plasma—dielectric interface with increasing frequency due to reduced skin depth. Despite differences in power deposition localization, strong ambipolar diffusion in the small chamber results in broadly similar plasma density profiles across frequencies. For asymmetric triangular waveform current, the inductive electric field is strongly enhanced during the faster current ramp-down phase, leading to pronounced temporal modulation of electron heating. Increasing waveform duty cycle (DC) enhances the contribution of higher current harmonics, which results in higher steady-state electron density. Excitation and ionization processes also shift closer to the plasma—dielectric interface. The EVDF becomes increasingly asymmetric with increasing DC, reflecting preferential electron acceleration during the current ramp-down phase. Time-resolved analysis shows that excitation and ionization rates peak sharply during this phase, with ionization exhibiting stronger relative enhancement. The intense plasma current produced during the current ramp-down phase persists beyond the reversal of the source electric field due to electron inertia and collisional effects. Consequently, inductive electron power deposition continues in the plasma for several tens of nanoseconds after the coil current reverses direction. These results demonstrate that RF current waveform asymmetry provides a viable mechanism for controlling electron power deposition, EVDF, and electron-impact collisional processes in ICPs.