We developed a high efficiency plasma source in an inductively coupled discharge using a passive resonant antenna, which has the advantage that it could be retro-fitted to existing reactors with minimal change to the reactor. At the resonance, the source has a larger total equivalent resistance that is 3-18 times larger than that at the non-resonance. As the resistance increases at the fixed RF power, the RF current decreases accordingly, which indicates that the power loss in the powered antenna including the impedance matching circuits is significantly reduced. The experimental result shows that the power transfer efficiency is improved by about 30%-70% and the plasma density at the resonance increases 2-8 times higher than that at the non-resonance. For analysis, three-winding transformer model is developed. The experimental results are consistent with the model.

A method for improving plasma uniformity in capacitively coupled plasma (CCP) is developed. Plasma contains harmonics due to the nonlinear characteristics of the sheath. Because high-frequency harmonic components in the plasma have short wavelengths, high-frequency harmonics components have a bad influence on plasma uniformity. This is because the electromagnetic standing wave effect (SWE) is severe at high frequencies. To improve plasma uniformity, a CCP using a parallel capacitor that is connected to the powered electrode is developed. By adjusting the capacitance of the parallel capacitor, a parallel resonance between the parallel capacitor and the reactor, which is net inductive, is generated. As the parallel loop approaches the parallel resonance, the currents of other harmonic frequency components in the plasma are greatly reduced. Therefore, the waveform of the plasma bulk current also becomes sinusoidal, and the amplitude of the plasma current becomes almost constant, regardless of the radial position at the parallel resonance condition. Moreover, the voltage applied across the plasma is greatly increased. Consequently, plasma uniformity and the voltage applied to the plasma along the electrode are significantly improved under the parallel resonance condition due to a greatly reduced SWE.

CO2 inductively coupled plasmas (ICPs) were generated using a radio frequency power of 13.56 MHz at 100 mTorr. Electron energy distribution functions (EEDFs) were measured using a single Langmuir probe, and various plasma parameters such as the electron density and electron temperature were obtained from the measured EEDFs. EEDFs with multislope structures are obtained. However, changes in the gas composition in the ICP were observed via optical emission spectroscopy. The electron density barely increases when a sudden change in the gas composition occurs. The E to H mode transition occurs at a stationary gas composition as the absorbed power increases. The EEDFs of CO2 plasma, CO plasma, and O2 plasma were calculated using BOLSIG+, which is a two term Boltzmann solver [G. Hagelaar and L. Pitchford, Plasma Sources Sci. Technol. 14(4), 722 (2005)]. The measured EEDF is closest to the EEDF of the CO plasma.

Total energy loss per ion–electron pair lost (εT) is investigated to optimize the plasma generation at various RF powers and gas pressures in an argon inductively coupled plasma (ICP). The ion densities and electron temperatures are measured to obtain εT at the plasma–sheath edge. At a fixed RF power, the obtained εT has a minimum at a certain electron temperature, and at this condition, an optimal plasma generation is achieved according to a global model. Since the electron temperature is a function of the gas pressure, at that certain gas pressure the energy loss in the plasma is minimized and plasma is generated most efficiently. Interestingly, the electron temperature at which εT becomes the minimum decreases as the RF power increases. This is explained by multistep ionization and the electron density dependence of the density of the excited states. Measured εT is compared with the calculated result from the global model that includes multistep ionization, and these are consistent with each other.

Hysteresis, one of the interesting characteristics of inductively coupled plasmas, takes place due to the nonlinearities of absorbed power or dissipated power [M. M. Turner et al., Plasma Sources Sci. Technol. 8, 313 (1999)]. In this work, the bias power effect on discharge mode transition and its hysteresis is investigated by measuring the antenna coil currents, time-average substrate voltages, electron densities, and electron energy probability functions (EEPFs). The behavior of hysteresis is comparatively analyzed by introducing a global model that considers the evolution of EEPFs and excitation state species (multi-step ionization). In the absence of bias power, a typical hysteresis appears at a gas pressure of 300 mTorr. The measured EEPF evolves from a Druyvesteyn distribution in the E mode to a Maxwellian distribution in the H mode with growing the antenna power from 25 W to 60 W. Interestingly, when a bias power of 30 W is applied to the substrate, the hysteresis vanishes, and the shape of the EEPF is maintained in each mode. The possible factors are considered the diminished changes in total energy loss (reduced nonlinearity of dissipated power) and in power transfer efficiency between E mode and H mode during the mode transition.