Seung-Ju Oh

Experimental observation of the skin effect on plasma uniformity in inductively coupled plasmas with a radio frequency bias

Electromagnetic effects such as the skin effect, standing wave effect, and edge effect have been mainly studied in large-area, high-frequency gas discharges. In this study, we show that the skin effect can be a major factor of plasma uniformity in a high-density plasma, even if the discharge regime is in a small electrode area and under moderate excitation frequency. When a radio frequency (RF) bias power was applied to an inductively coupled plasma (ICP), the plasma density near the radial edge was largely increased and the plasma uniformity was significantly enhanced in the high-density plasma regime. This change could be understood by considering the enhanced electric field in the radial edge, the so-called skin effect. The time-dependent behavior of the plasma density and electron temperature by the bias power was also studied. When the RF bias power was applied to the ICP, the electron temperature abruptly increased due to electron heating, while variations of plasma density depending on the radial position were observed with the RF bias power in the high-density plasma of the ICP.

Evolution of electron temperature in inductively coupled plasma

It is generally recognized that the electron temperature T_e either remains constant or decreases slightly with plasma power (plasma density). This trend can be simply verified using a single-step or multi-step fluid global model. In this work, however, we experimentally observed that T_e evolved with plasma power in radio frequency (RF) inductively coupled plasmas. In this experiment, the measured electron energy distributions were nearly Maxwellian distribution. In the low RF power regime, T_e decreased with increasing plasma power, while it increased with plasma power in the high RF power regime. This evolution of T_e could be understood by considering the coupling effect between neutral gas heating and stepwise ionization. Measurement of gas temperature via laser Rayleigh scattering and calculation of T_e using the kinetic model, considering both multi-step ionization and gas heating, were in good agreement with the measured value of T_e. This result shows that T_e is in a stronger dependence on the plasma power.

Radio frequency-compensated Langmuir probe with auxiliary double probes

A radio frequency (rf) compensation design using auxiliary double probes connected in parallel with a main measurement probe was developed for Langmuir probe diagnostics. This probe structure can reduce the sheath impedance of the main probe. In our probe design, the sheath capacitance of the probe can be increased and its sheath resistance can be decreased with increasing dc bias differential voltage between the auxiliary double probes. The I-V characteristic curve and electron energy distribution functions measured by our probe system had sufficient rf compensation performance in inductively coupled plasmas.

A study on plasma parameters in Ar/SF6 inductively coupled plasma

Sulfur hexafluoride (SF6) gas or Ar/SF6 mixing gas is widely used in plasma processes. However, there are a little experimental studies with various external parameters such as gas pressure and mixing ratio. In this work, a study of the plasma parameters by changing the gas mixing ratio was done in an Ar/SF6 inductively coupled plasma from the measurement of the electron energy distribution function. At a low gas pressure, as the mixing ratio of SF6 gas increased at a fixed inductively coupled plasma (ICP) power, the electron density decreased and the electron temperature increased, while they were not changed drastically. At a high gas pressure, a remarkable increase in the electron temperature was observed with the decrease in the electron density. These variations are due to the electron loss reactions such as the electron attachment. It was also found that at a fixed ICP power, the negative ion creation with the diluted SF6 gas can change the discharge mode transition from an inductive mode to a capac...

Global model including multistep ionizations in helium plasmas

Particle and power balance equations including stepwise ionizations are derived and solved in helium plasmas. In the balance equations, two metastable states (21S1 in singlet and 23S1 triplet) are considered and the followings are obtained. The plasma density linearly increases and the electron temperature is relatively in a constant value against the absorbed power. It is also found that the contribution to multi-step ionization with respect to the single-step ionization is in the range of 8%–23%, as the gas pressure increases from 10 mTorr to 100 mTorr. Compared to the results in the argon plasma, there is little variation in the collisional energy loss per electron-ion pair created (ec) with absorbed power and gas pressure due to the small collision cross section and higher inelastic collision threshold energy.

Control of Spatial Power Deposition by Wireless Power Transfer Method Applicable to Inductively Coupled Plasma

In this paper, numerical calculations of the surface coil current and the spatial electric field in the vicinity of the antenna were investigated using wireless power transfer method. It is observed that the wirelessly transferred power to a resonant coil is dramatically changed with a variation of the excitation frequency. This result indicates that the spatial plasma distribution can be controlled through wireless resonance power transfer. This method could be applied to the large-area plasma source.

Discharge dynamics and plasma density recovery by on/off switches of additional gas

Measurement of the plasma density is investigated to study plasma dynamics by adding reactive gas (O2) or rare gas (He) in Ar plasmas. When the O2 or He gas is added, plasma density is suddenly decreased, while the plasma density recovers slowly with gas off. It is found that the recovery time is strongly dependent on the gas flow rate, and it can be explained by effect of gas residence time. When the He gas is off in the Ar plasma, the plasma density is overshot compared to the case of the O2 gas pulsing due to enhanced ionizations by metastable atoms. Analysis and calculation for correlation between the plasma density dynamics and the gas pulsing are also presented in detail.

Inductively coupled RF heating of nano-particle for non-invasive and selective cancer cell destruction

We developed a new inductively coupled RF field (ICRF) heating source applicable for non-invasive cancer therapy. Carbon nanotube and optimized solutions were used to confirm energy transfer by the ICRF heating source. Remarkable increases in the temperature of the nano-particle solutions were observed indicating efficient non-invasive nano-particle heating.