Albert R. Ellingboe

Comparison of measurements and particle-in-cell simulations of ion energy distribution functions in a capacitively coupled radio-frequency discharge

The ion dynamics in the high-voltage sheath of a capacitively coupled radio-frequency plasma has been investigated using mass-resolved ion energy analysis in combination with a two-dimensional particle-in-cell (PIC) code. A symmetric confined discharge is designed allowing highly accurate comparisons of measured ion energy distribution functions in high-voltage sheaths with simulation results. Under the conditions investigated, the sheaths are not only collisional, but also chemically complex. This situation is common in applications but rare in laboratory experiments. Excellent agreement has been found for a hydrogen discharge benchmarking the code. Hydrogen is of particular interest since its light mass gives detailed insight into sheath dynamics, and an extensive database of collisional cross sections is available. The H3+ ion was found to be the dominant ion in the sheaths and the plasma bulk under most conditions investigated. H3+ exhibits the typical saddle-shaped ion energy distribution function in...

Analysis of uncompensated Langmuir probe characteristics in radio-frequency discharges revisited

Measurements of the electron temperature, plasma density, and floating and plasma potentials with Langmuir probes in radio-frequency discharges often represent a challenge due to rf oscillations of the plasma potential. These oscillations distort the probe characteristic, resulting in wrong estimates of the plasma parameters. Both active and passive rf compensation methods have previously been used to eliminate rf fluctuation effects on the electron current drawn by an electrostatic probe. These effects on an uncompensated probe have been theoretically and experimentally studied by Garscadden and Emeleus [Proc. Phys. Soc. London 79, 535 (1962)], Boschi and Magistrelli [Nuovo Cimento 29, 487 (1963)], and Crawford [J. Appl. Phys. 34, 1897 (1963)]. They have shown theoretically that, assuming a Maxwellian distribution and sinusoidal plasma-potential oscillation, the electron temperature can be deduced directly from an uncompensated Langmuir probe trace, by taking the natural logarithm of the electron current...

A floating hairpin resonance probe technique for measuring time-resolved electron density in pulse discharge

We present an automated hairpin resonance probe for obtaining time-varying plasma electron density in a pulsed-magnetron discharge, operated with a 13.56 MHz radio-frequency source. When the resonator is placed in plasma, its characteristic resonance frequency in vacuum shifts to a higher value. From the frequency shifts, electron density is easily determined. By applying a fixed microwave frequency, the probe immersed in plasma resonates only at a specific time of the pulse waveform. At a different time of the pulse, the probe resonates at a different frequency. The procedure is automated using a Labview™ program, which increments the applied microwave frequency in small steps of the prescribed value and reads the corresponding resonance peak from an oscilloscope. The spatial and temporal electron density measured using this technique shows a sharp drop in density during the first few microseconds in the on-phase, followed by an increase in density as the discharge develops in the steady-state on-phase. The off-phase shows that decay in electron density at different rates is faster in the region where the magnetic field lines intersect the target. A quantitative model is described to explain different features observed in the experiment.

Characterization of the pulse plasma source

Characterization of the pulse plasma source through the determination of the local thermodynamic equilibrium (LTE) threshold is described. The maximum electron density measured at the peak in discharge current is determined by the width of the He II Paschen alpha spectral line, and the electron temperature is determined from the ratios of the relative intensities of spectral lines emitted from successive ionized stages of atoms. The electron density and temperature maximum values are measured to be 1.3 ? 1017?cm?3 and 19?000?K, respectively. These are typical characteristics for low-pressure, pulsed plasma sources for input energy of 15.8?J at 130?Pa pressure in helium?argon mixture.The use of LTE-based analysis of the emission spectra is justified by measurement of the local plasma electron density at four positions in the discharge tube using a floating hairpin resonance probe. The hairpin resonance probe data are collected during the creation and decay phases of the pulse. From the spatio-temporal profile of the plasma density a 60??s time-window during which LTE exists throughout the entire plasma source is determined.

H- to W-mode transitions and properties of a multimode helicon plasma reactor

Magnetically enhanced inductively coupled plasmas (MEICPs) typically display mode jumps as power or the magnetic field is increased. Inductive (H-mode) to helicon (W-mode) transitions in research reactors having uniform axial magnetic fields are often accompanied by a change in the spatial model structure and rapid increase in plasma density. In industrial plasma sources, the magnetic field structure is often non-uniform, producing less pure modal structure and transitions. In this paper, the characteristics of research and industrial MEICPs are computationally investigated using results from a three-dimensional plasma equipment model. When excluding the electrostatic term in the solution of Maxwells equations, experimental observations for mode structure and transitions from H- to W-mode in the research reactor are reproduced and occur coincident with the formation of shorter axial wavelength modes, radial modes and the onset of rotation of the electric fields. Similar scaling laws were observed in the industrial reactor having a flaring magnetic field. Simultaneous H- and W-mode behaviour occurred (as characterized by radial modes and rotation of the electric field) in different regions of the plasma source depending on the local value of the helicon wavelength. When including the electrostatic terms in the solution of Maxwells equations, more power deposition occurs in the periphery of the reactor and the H- to W-mode transition occurs at lower powers.

Real-time plasma control in a dual-frequency, confined plasma etcher

The physics issues of developing model-based control of plasma etching are presented. A novel methodology for incorporating real-time model-based control of plasma processing systems is developed. The methodology is developed for control of two dependent variables (ion flux and chemical densities) by two independent controls (27 MHz power and O2 flow). A phenomenological physics model of the nonlinear coupling between the independent controls and the dependent variables of the plasma is presented. By using a design of experiment, the functional dependencies of the response surface are determined. In conjunction with the physical model, the dependencies are used to deconvolve the sensor signals onto the control inputs, allowing compensation of the interaction between control paths. The compensated sensor signals and compensated set–points are then used as inputs to proportional-integral-derivative controllers to adjust radio frequency power and oxygen flow to yield the desired ion flux and chemical density...

Numerical experiment to estimate the validity of negative ion diagnostic using photo-detachment combined with Langmuir probing

This paper presents a critical assessment of the theory of photo-detachment diagnostic method used to probe the negative ion density and electronegativity α = n-/ne. In this method, a laser pulse is used to photo-detach all negative ions located within the electropositive channel (laser spot region). The negative ion density is estimated based on the assumption that the increase of the current collected by an electrostatic probe biased positively to the plasma is a result of only the creation of photo-detached electrons. In parallel, the background electron density and temperature are considered as constants during this diagnostics. While the numerical experiments performed here show that the background electron density and temperature increase due to the formation of an electrostatic potential barrier around the electropositive channel. The time scale of potential barrier rise is about 2 ns, which is comparable to the time required to completely photo-detach the negative ions in the electropositive chann...

Characteristics of silicon nitride deposited by VHF (162 MHz)-plasma enhanced chemical vapor deposition using a multi-tile push–pull plasma source

To prevent moisture and oxygen permeation into flexible organic electronic devices formed on substrates, the deposition of an inorganic diffusion barrier material such as SiN x is important for thin film encapsulation. In this study, by a very high frequency (162 MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) using a multi-tile push–pull plasma source, SiN x layers were deposited with a gas mixture of NH3/SiH4 with/without N2 and the characteristics of the plasma and the deposited SiN x film as the thin film barrier were investigated. Compared to a lower frequency (60 MHz) plasma, the VHF (162 MHz) multi-tile push–pull plasma showed a lower electron temperature, a higher vibrational temperature, and higher N2 dissociation for an N2 plasma. When a SiN x layer was deposited with a mixture of NH3/SiH4 with N2 at a low temperature of 100 °C, a stoichiometric amorphous Si3N4 layer with very low Si–H bonding could be deposited. The 300 nm thick SiN x film exhibited a low water vapor transmission rate of 1.18 × 10−4 g (m2 d)−1, in addition to an optical transmittance of higher than 90%.

Method for Estimation of Electron Density in a Pulse Plasma Source

The time-dependent spatial electron density distribution in a constricted, pulsed plasma source is measured using a floating hairpin resonance probe, and an extrapolation method is described for determining peak electron density from experimental data. Using these techniques, a detailed characterization of the spatio-temporal evolution of electron density, outside the constricted region above the anode of the pulsed plasma source is presented. Electron density increases sharply during the creation phase, and the rate of increase is found to decrease with the distance from the axis of the constricted channel. By modeling the plasma creation characteristics vs position, the electron density along the axis of the constricted pulsed plasma sources can be determined.

Measurement of electron density in a laser produced plasma using a hairpin resonance probe

A floating hairpin resonance probe has been used for the first time to measure the spatial and time evolution of local electron density in a laser produced plasma expanding in vacuum. The measured variation in electron density agrees closely with the variation of ion charge density as measured with a time-of-flight planar Langmuir ion probe confirming the reliability of Langmuir probe in the laser produced plasma.