Jae-Myung Choe

Dynamic sheath expansion in a non-uniform plasma with ion drift

Dynamic sheath expansion in front of a target biased with a negative, high-voltage pulse is investigated in a non-uniform plasma, taking into account the influence of ion drift, which is inevitable in diffusive plasmas with a non-uniform density profile. Temporal evolutions of a sheath edge and a rarefaction wave measured in a low-pressure argon plasma diffusing towards the target agree well with numerical predictions of their transient behaviors as obtained using a dynamic sheath model for non-uniform plasmas with ion drift. It is found that the thickness of the expanding sheath edge is reduced considerably by the ion drift velocity when compared with the thickness without ion drift. Moreover, because the ion drift prevents the propagation of the rarefaction wave significantly, the phase velocity of the wave is observed to be much less than the Bohm speed. The propagating characteristics of the rarefaction wave confirm that the ion drift velocity plays an important role in the dynamic sheath expansion in non-uniform plasmas with ion drift. The results are expected to be useful in analyzing the dose and energy of implanted ions as well as understanding the sheath dynamics in a real plasma source ion implantation system in which the plasma sheath commonly evolves in a non-uniform plasma with ion drift.

Self-consistent circuit model for plasma source ion implantation.

A self-consistent circuit model which can describe the dynamic behavior of the entire pulsed system for plasma source ion implantation has been developed and verified with experiments. In the circuit model, one-dimensional fluid equations of plasma sheath have been numerically solved with self-consistent boundary conditions from the external circuit model including the pulsed power system. Experiments have been conducted by applying negative, high-voltage pulses up to -10 kV with a capacitor-based pulse modulator to the planar target in contact with low-pressure argon plasma produced by radio-frequency power at 13.56 MHz. The measured pulse voltage and current waveforms as well as the sheath motion have shown good agreements with the simulation results.

Analysis of Repetitive Pulse Discharge System for Plasma Source Ion Implantation

The analysis of the repetitive pulse discharge system for the plasma source ion implantation is investigated with both circuit simulation and experiment. In the circuit model, the ion and electron currents on a target are self-consistently varied with the applied voltage because the waveforms of repetitive pulse are affected by the internal properties of plasma, as well as the external circuit parameters. The circuit simulation reveals that not only the plasma properties, but also the circuit components, are important for pulse system to operate at high repetition-rate. The experiments are conducted with a plane electrode immersed in rf-driven argon plasmas. When negative high-voltage pulses are applied to the electrode, the current and voltage waveforms are measured and compared with the simulation results. Control parameters for high repetition-rate operation are discussed, based on the self-consistent circuit analysis of the pulse system

Investigation of Current on the Conducting Target Biased with a Large Negative Potential in the Non-Uniform Plasma

It was investigated the current on a biased target in the non-uniform plasma. The argon plasma was generated at the low pressure of 0.5–5 mTorr and the stainless steel target was biased with 0.2–6 kV negatively. The target current increases with increasing the target voltage due to the non-uniform plasma distributed near the target, which follows the Child law. The secondary emission current, following the square root of target voltage, is superposed to the ion current. For the higher pressures of larger than 1.5 mTorr, the target current is larger than the expected current of Bohm current plus the secondary emission current. The deviation increases drastically with increasing the operating pressure, which may due to the local ionization near the target. This phenomenon is important to understand more accurately the high voltage sheath formation in practices.

Influence of the Density Gradient on the Current of the Electrode Immersed in the Non-uniform Plasma

Department of Nuclear Engineering, Seoul National University, Seoul 151-724, Korea(Received January 3, 2011; Revised February 3, 2011; Accepted May 5, 2011)Abstract: The conducting current of non-uniform plasma immersed electrode consists of ion current and secondary electron emission current caused by the impinging ion current. The ion current is determined by the ion dose passing through the sheath in front of electrode and the ion distribution in front of the electrode plays an important role in the secondary electron emission. The investigation of the distributed plasma and secondary electron effect on electrode ion current was carried out as the stainless steel electrode plugged with quartz tube was immersed in the inductively coupled Ar plasma using the antenna powered by 1 kw and the density profile was measured. After that, the negative voltage was applied by 1 kV∼6 kV to measure the conduction current for the analysis of ion current. Keywords: Secondary electron emission coefficient, Conducting current, Ion current density, Plasma density distribution, Non-uniform plasma

Investigation of Plasma Recovery during Fall Time in Plasma Source Ion Implantation

To investigate the plasma recovery during the fall time of a high voltage pulse, a numerical model is developed by applying fluid equations for ions while assuming thermal equilibrium for electrons. In the model, effects of the circuit impedance of pulse system are taken into account. The numerical analysis reveals that the plasma recovery is affected by both the properties of plasma and internal impedance of pulse system. The experiments are conducted with a plane electrode immersed in RF-driven argon plasmas. When negative, high voltage pulses are applied to the electrode, the current and voltage waveforms are measured and compared with the simulation results. Effects of internal circuit impedance as well as plasma properties on plasma recovery during pulse fall time are discussed based on the experimental and numerical results

Characteristics of plasma near the target biased with a high negative pulse

Summary form only given. In PSII, the conducting target is immersed in the plasma and the dose of implanted ions is controlled by the plasma density at the sheath edge. When evaluating the ion current or sheath formation, the stationary-bulk plasma density has been adapted. However the dynamic sheath is expanded with the applied pulse voltage on the target and the field in the sheath is enough to break down the gas. The emitted secondary electrons also employs on the perturbing the bulk plasma. Thus the sheath formation near the pulsed target should be influenced by the temporal plasma property with applying the voltage. The measurements are carried out in the target plasmas which are generated by the pulsed bias only and by the pulsed bias and the external RF power. Also the temporal property is obtained for the plasma generated by the pulsed RF plasma. The temporal plasma properties near the target and in the bulk are monitored by a RF compensated Langmuir probe. The current-voltage probe data is analyzed with the newly developed algorithm based on the biorthogonal wavelet transform. The properties, EEDF, electron temperature and density, are obtained with various pulse conditions and gases. Effects of the temporal and stationary plasmas on the Child-Langmuir sheath formation in front of the target will be discussed

Measurement of ion current on target biased with a high negative voltage

Summary form only given. Analysis of the conduction current on the negatively biased target is important to develop the pulse switch system and to improve the material surface treatment using the plasma source ion implantation. In case of highly negative biased target immersed in the plasma, the current on the target is composed of the incident ion current and the electron emission current. The virtual area of collecting current on the target is proportional to the sheath area formed over the target electrode which is expanded considerably compared with the size of electrode with increasing the high voltage. The sheath developed near the edge of the target should be considered in the analysis of the target ion current. The spatial distribution of plasma in front of the target also affects the sheath formation because the density distribution is not uniform in real situation which may shrink the sheath expansion. In this study, the effects of sheath geometry and plasma distribution with the secondary electron emission are investigated. Experiments were carried out with the planar stainless steel and aluminum targets having a diameter 100 mm, negatively bias potential ranging in 2 kV-11 kV, and various plasma gases of Ar and He, respectively. Also the target is enshrouded by the quartz tube of 100 mm long which may reduce the effect of the sheath formation at the edge of the target. Spatial plasma density was measured by Langmuir probe and compared with the values obtained from the target current which were analyzed with the current model considered with the Bohm current, the edge effect of sheath formation on the target, and spatial plasma distribution with secondary electron emission coefficient. Results will be discussed