Min-Hyong Lee

Floating probe for electron temperature and ion density measurement applicable to processing plasmas

A floating-type probe and its driving circuit using the nonlinear characteristics of the probe sheath was developed and the electron temperature and the plasma density which is found from the ion part of the probe characteristic (ion density) were measured in inductively coupled plasmas. The floating-type probe was compared with a single Langmuir probe and it turned out that the floating-type probe agrees closely with the single probe at various rf powers and pressures. The ion density and electron temperature by the floating-type probe were measured with a film on the probe tip coated in CF4 plasma. It is found that the ion density and electron temperature by the floating-type probe were almost the same regardless of the coating on the probe tip while a single Langmuir probe does not work. Because the floating-type probe is hardly affected by the deposition on the probe tip, it is expected to be applied to plasma diagnostics for plasma processing such as deposition or etching.

Experimental observation of the transition from nonlocal to local electron kinetics in inductively coupled plasmas

The transition from nonlocal to local kinetics was observed through the spatially resolved measurements of electron energy distribution functions in inductively coupled plasmas. As gas pressures increase, the spatial profiles of the effective electron temperatures (Teff) from the electron energy distribution functions changed dramatically from hollow shapes to flat shapes. With further increases in gas pressures, the Teff had saddle-shaped profiles with the highest Teff in the vicinity of an antenna coil. These changes in the radial profiles of the Teff show a transition of the electron kinetics from nonlocal to local regimes. This transition occurred when the electron energy relaxation lengths became smaller than the antenna half size.

Self-consistent global model with multi-step ionizations in inductively coupled plasmas

Modified particle and power balance equations including multi-step ionizations are derived and solved in a self-consistent manner. According to the modified balance equations, the electron temperature (Te) decreases with the electron density (ne) to balance between the generation and the loss of an electron-ion pair. The multi-step ionizations give rise to the decrease in the collisional energy loss per electron-ion pair created (ec) so that the electron density is rapidly increased compared to when only single step ionization is considered. The effect of neutral pressure on the multi-step ionizations is presented and discussed.

On the E to H and H to E transition mechanisms in inductively coupled plasma

Inductively coupled plasmas (ICP) exhibit two mode operations of capacitive coupling (E mode) and inductive coupling (H mode), and the density jump and hysteresis have been reported during the transition between these modes. In this study, the total power transferred to the plasma by capacitive and inductive coupling is calculated from Maxwell’s equations and global model, and from this, conditions required for stable E and H mode operations are obtained. The E to H and the H to E transitions occur when the system reaches critical electron densities. Analytical criterion for stable H mode operation that the skin depth should be smaller than 23R at low pressure, and 23(ω∕ν)R at high pressure is derived from the calculation. The dependence of transition electron densities and powers of E to H and H to E transitions on the pressure and discharge dimension is also discussed.

Effects of rf-bias power on plasma parameters in a low gas pressure inductively coupled plasma

Remarkable changes of the electron temperature and the plasma density by increasing bias power were observed in low gas pressure inductively coupled plasma (ICP) by the measurement of electron energy distribution function (EEDF). As the bias power increases, the electron temperature increased with accompanying the evolution of the EEDF from a bi-Maxwellian to a Maxwellian distribution. However, a different trend of the plasma density was observed with a dependence on the ICP powers. When the ICP power was relatively small or the discharge is in capacitive mode (E mode), the plasma density increased considerably with the bias power, while decrease of the plasma density was observed when the discharge is in inductive mode (H mode). The change of the plasma density can be explained by the balance between total power absorption and power dissipation.

On the hysteresis in E to H and H to E transitions and the multistep ionization in inductively coupled plasma

Plasma densities, E to H and H to E transition coil currents, and electron energy distribution functions (EEDFs) are measured at various argon pressures in an inductively coupled plasma. The measured plasma density versus coil current shows that the hysteresis during the E-H transition is clearly observed only when the pressure is sufficiently high. At low gas pressures the hysteresis is not obvious. The measured EEDFs show that when the hysteresis is obvious (high pressures), electrons whose energy is not sufficient for excitation or ionization of the ground state atom are strongly depleted in the H mode. This depletion may be caused by multistep ionization. However, for the case where the hysteresis is not obvious (low pressures), the depletion due to multistep ionization is also not present. These experimental results show that the multistep ionization is a dominant factor in the E-H transition hysteresis.

Low energy electron heating and evolution of the electron energy distribution by diluted O2 in an inductive Ar/O2 mixture discharge

A remarkable increase in electron temperature with diluted O2 gas was observed in a low pressure Ar/O2 mixture inductive discharge from the measurement of the electron energy distribution function (EEDF). At a pure Ar gas discharge of 3 mTorr and 100 W, the measured EEDF had a bi-Maxwellian distribution with two electron temperature groups. However, as the O2 flow rate increased with fixing total gas pressure, a significant increase in the low energy electron temperature was observed. Finally, the EEDF evolved from a bi-Maxwellian to a Maxwellian distribution. These results can be understood by an efficient low energy electron heating from both an enhanced collisionless and a collisional heating mechanism because of increases of both skin depth and the elastic collision with the non-Ramsauer gas, O2. These experiments were also studied with different ICP power and Ar/He mixture.

On the multistep ionizations in an argon inductively coupled plasma

The effect of the multistep ionizations on the plasma parameters in the inductively coupled plasma (ICP) has been investigated by experiments and theory. To obtain electron density and electron temperature precisely at various powers and pressures in the ICP, the electron energy distribution functions (EEDFs) are measured. It is found that at high pressures, the electron temperature from the EEDFs decreases and the electron density increases rapidly with the absorbed power while, at low pressures, the electron temperature is hardly changed and the electron density is almost linearly proportional to the absorbed power. The comparison between the experiment and our model including the multistep ionizations [M. H. Lee and C. W. Chung, Phys. Plasmas 12, 73501 (2005)] was done and the experiment was in close agreement with the model. This shows that the changes in the electron density and the electron temperature in the ICP are mainly due to the multistep ionizations.

Effect of multistep ionizations on the electron temperature in an argon inductively coupled plasma

Electron energy distribution functions (EEDFs) are measured in an argon inductively coupled plasma. It is observed that the measured EEDFs are nearly Maxwellian distributions, and the electron temperature derived from the EEDFs decreases with increasing input radio-frequency (rf) power. It appears that additional ionization processes exist. To investigate this decrease in the electron temperature, multistep ionizations are taken into account. An analytical particle balance equation, including the multistep ionizations, are derived. According to the new balance equation, to balance between the total volume ionization and the total surface particle loss, the electron temperature should be decreased with increasing rf power. The comparison between the model and the experiment are presented, and the model agrees well with the experiment.

Observation of collisionless heating of low energy electrons in low pressure inductively coupled argon plasmas

Collisionless heating of low energy electrons was observed in low pressure argon rf-biased inductively coupled plasmas (ICPs) by measurement of the electron energy distribution function (EEDF). When only capacitive power (bias) was supplied, the EEDF in the discharge was a bi-Maxwellian distribution with two electron groups. It was found that the low energy electrons were heated up significantly even with a little inductive power (<20 W) even when the discharge was in E mode. Due to the low gas pressure and low temperature of low energy electrons (close to the energy of the Ramsauer minimum), the collisional heating of low energy electrons appears to be negligible. Therefore, this effective heating of the low energy electrons showed a direct experimental evidence of the collisionless heating by inductive field. The significant heating of low energy electrons in E mode indicates that collisionless heating in the skin layer is an important electron heating mechanism of low pressure ICP even when the dischar...