Hyo-Chang Lee

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.

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.

Evolution of the electron energy distribution and E-H mode transition in inductively coupled nitrogen plasma

Electron energy distribution function (EEDF) measurements were conducted in nitrogen gas inductively coupled plasma (ICP). At a low ICP power (capacitive mode) and a high gas pressure, the measured EEDF had an unusual distribution with a hole near this electron energy of 3 eV. This distribution is primarily due to vibrational excitation collisions because the vibrational cross section has a sharp peak at the electron energy in nitrogen gas. However, the EEDF evolved into a Maxwellian distribution and the hole disappeared, when the discharge mode transition from E mode to H mode occurred. This evolution of the EEPF can be understood by the electron-electron collision effect, and it occurs when the electron-electron collision time become shorter than the electron residence time.

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.

Experimental measurements of spatial plasma potentials and electron energy distributions in inductively coupled plasma under weakly collisional and nonlocal electron kinetic regimes

Spatial profiles of the plasma potential and electron energy distribution function (EEDF) were measured in inductively coupled plasma (ICP) under weakly collisional and electron nonlocal kinetic regimes. The measured EEDF at the discharge center was a bi-Maxwellain distribution with low (T1) and high (T2) electron temperature groups, while the EEDF at the radial boundary was closely Maxwellian distribution due to cutting of the low energy electrons by relatively large ambipolar potential in this discharge regime. The ambipolar potential in the entire radial region was in the scale of Teff − 1.5 Teff, where Teff is the effective electron temperature. At the boundary region with the ion mean free path scale, the ambipolar potential increased abruptly and was about Teff,edge/2, where the Teff,edge is the effective electron temperature at the boundary, which corresponds to the presheath scale. These results of the ICP, which are contrary to the ambipolar potential of capacitively coupled plasma in a nearly fr...

Collisionless electron heating by radio frequency bias in low gas pressure inductive discharge

We show experimental observations of collisionless electron heating by the combinations of the capacitive radio frequency (RF) bias power and the inductive power in low argon gas pressure RF biased inductively coupled plasma (ICP). With small RF bias powers in the ICP, the electron energy distribution (EED) evolved from bi-Maxwellian distribution to Maxwellian distribution by enhanced plasma bulk heating and the collisionless sheath heating was weak. In the capacitive RF bias dominant regime, however, high energy electrons by the RF bias were heated on the EEDs in the presence of the ICP. The collisionless heating mechanism of the high energy electrons transited from collisionless inductive heating to capacitive coupled collisionless heating by the electron bounce resonance in the RF biased ICP.

Adsorption kinetics of alkanethiols studied by quartz crystal microbalance

Abstract The quartz crystal microbalance (QCM) has been employed extensively for measurement of mass changes at metal surface while the metal coated electrode is immersed in a liquid. We report the real time in-situ adsorption kinetic trends of alkanethiol CH3(CH2)n–1SH (n=8, 12, 16) monolayers with different chain lengths and solvents using QCM. Regarding 1-octanethiol monolayer formation on gold surface, we could identify the two distinct monolayer formation steps in hexane and ethanol solutions. The adsorption process of alkanethiols is characterized by two distinct steps. In the kinetically controlled initial monolayer formation step, the adsorption rate of short chain alkanethiols is faster than that of long chain alkanethiols. The second step is a thermodynamically controlled ordering and packing process. In case of short chain alkanethiols, these two separated adsorption steps were clearly observed, but in the case of long chain alkanethiols (over 10 carbon numbers) only one step of monolayer formation was observed.

Experimental verification of the Boltzmann relation in confined plasmas: Comparison of noble and molecule gases

Experimental verification of the Boltzmann relation is performed in argon and oxygen gas inductively coupled plasmas from the measurements of both the spatial electron currents (as a fluid approach) and the electron energy probability functions (EEPFs, as a kinetic approach). At a low gas pressure of 10 mTorr, the measured electron currents are spatially uniform, and the EEPFs in the total electron energy scale are identical, which indicate that the Boltzmann relation is valid at both the argon and oxygen gases. As the gas pressure increases to 30–40 mTorr, however, the Boltzmann relation is broken in the oxygen gas discharge, while the Boltzmann relation is still valid in the argon gas discharge. This different variation in the oxygen gas discharge is mainly due to the presence of various inelastic collisions in the entire electron energy region, which causes the transition of the electron kinetics from a non-local to a local regime.

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...

Discharge mode transition and hysteresis in inductively coupled plasma

Experimental verification of the discharge mode transition and the hysteresis by considering matching circuit is investigated in inductively coupled plasma using measurements of the plasma density and the power absorption to the plasma. At an argon gas pressure of 100 mTorr where the hysteresis loop of the plasma density had been observed in some previous experiments, there is no hysteresis loop against either the input power or the absorbed power delivered via an automatic impedance matching network. At a higher gas pressure of 350 mTorr, however, the hysteresis loop is clearly seen as functions of both the absorbed power and the input power. This result suggests that the observed hysteresis is due to not only the matching effect but also the nonlinearity of the plasma during capacitive (E) to inductive (H) and H to E heating mode transitions.