Plasma Potential Distribution Research Articles

Characterization of a high-density, direct-current reflex discharge plasma source operating in Ar and N2

The characterization of a direct current, low-pressure, and high-density reflex discharge plasma source operating in argon and in nitrogen, over a range of pressures 1.0–10−2 mbar, discharge currents 20–200 mA, and magnetic fields 0–120 G, and its parametric characterization is presented. Both external parameters, such as the breakdown potential and the discharge voltage–current characteristic, and internal parameters, like the charge carrier’s temperature and density, plasma potential, floating potential, and electron energy distribution function, were measured. The electron energy distribution functions are bi-Maxwellian, but some structure is observed in these functions in nitrogen plasmas. There is experimental evidence for the existence of three groups of electrons within this reflex discharge plasma. Due to the enhanced hollow cathode effect by the magnetic trapping of electrons, the density of the cold group of electrons is as high as 1018 m−3, and the temperature is as low as a few tenths of an electron volt. The bulk plasma density scales with the dissipated power. Another important feature of this reflex plasma source is its high degree of uniformity, while the discharge bulk region is free of electric field.

Advanced Fluid Information. Plasma Molding over Surface Topography.

A two-dimensional fluid/Monte Carlo simulation model was developed to study plasma molding over surface topography. The plasma sheath evolution and potential distribution over the surface were predicted with a self consistent fluid simulation. The trajectories of ions and energetic neutrals were then followed with a Monte Carlo simulation. Energy and angular distributions of ions and energetic neutrals bombarding an otherwise planar target with a step are reported. As one approaches the step, the ion flux decreases and the ion impact angle increases drastically. For a time invariant sheath (DC case), the ion energy distributions (IED) remain relatively unaffected. When the plasma sheath oscillates at radio frequencies, the IED narrows, while the ion angular distribution becomes broader as one approaches the step

The role of energetic electrons in self-oscillations of a discharge plasma

The role of energetic electrons in periodic self-oscillations of a discharge plasma has been studied by measuring the spatiotemporal evolution of plasma potential, electron density, and electron velocity distribution function. It is found that the self-oscillation involves the instabilities of sheaths, propagation of a double layer and competition between the ionization, thermalization, and diffusion. The energetic electrons are the key factor which links these processes to form the oscillation cycle. The time interval of each phase in the cycle is estimated according to the physical process and the calculations are in agreement with experimental measurements. The study of the probe perturbation effect on the oscillations indicates that the length of the oscillation period is related to the amount of energetic electrons; the more energetic electrons, the shorter the period.

Electrostatic probe diagnostics of a planar-type radio-frequency inductively coupled oxygen plasma

An inductively coupled oxygen radio-frequency (13.56 MHz) discharge is investigated based on modeling and experiment. Experimental measurement is done at a range of gas pressure of 1–30 mTorr, and rf power of 100–1000 W. We measure most of the important plasma parameters such as the densities of charged species, electron temperature, plasma potential, and electron energy distribution function. The measured values are compared with the results of the spatially averaged global model. We observe a generally good agreement between the modeling and the experiment. The scaling features, the transition of the operating region, and the radial distributions of charged species are also discussed.

Theoretical and experimental characterization of a low-pressure rf plasma and its optimization in electron-gas secondary-neutral mass spectrometry

The charge-density distribution and the respective plasma potential distribution in an electrodeless low-pressure rf plasma as it is employed in electron-gas secondary-neutral mass spectrometry (SNMS) were determined theoretically by a comparatively simple plasma model. A second-order finite element program combines a Maxwell–Boltzmann like electron density distribution with a collective drift motion of plasma ions directed from the center of the discharge to the wall of the plasma chamber and calculates a self-consistent solution of the space-charge problem by iteratively solving the appropriate Poisson equation. A comparison with the experimentally determined spatial distributions of the electron density and the plasma potential for two different geometries of the plasma chamber shows good agreement with respect to the theoretical calculations. A plasma chamber optimized via this plasma modeling was incorporated into a SNMS instrument. The new chamber’s performance was studied with respect to previously used geometries. A distinct enhancement of the intensities of postionized sputtered species was observed and ascribed to a plasma-optical focusing effect. Full exploitation of this beneficial feature appears to be partially limited by a concurrent increase of space-charge accumulation in the vicinity close to the plasma chamber’s exit aperture.

Structure of plasmas generated by the interaction between metallic ions and neutral gas particles in a low-pressure arc

A numerical solution for the metallic-plasma-neutral-gas structure generated in a low-pressure arc is presented. The equations correspond to a spherically symmetric fluid-like steady model, valid for the outer region of the arc, and describe the ion slowing down by elastic scattering with the neutral particles. Technically, the obtention of the profiles of different magnitudes is complicated due to the existence of a critical point in the steady-state system of equations. The proposed approach to overcome this difficulty is to solve instead a pseudotransient system of equations which rapidly and efficiently relax to the stationary state. By employing this numerical method of second-order accuracy in space, the plasma and neutral gas density, the electron and ion drift velocities, the electron and neutral temperatures, and the electrostatic potential profiles are obtained from the border of the arc channel up to the discharge chamber wall. It is found that the value of the neutral gas filling pressure strongly influences the plasma density and plasma potential distributions. An important result is that metallic ions emitted from the arc channel deliver their kinetic energy to the filling gas in a gradual manner, up to a pressure-dependent point beyond which they move to the walls sustained against collisions with the gas by a self-consistent electric field. Near the mentioned point, the metallic ion density presents a peculiar behavior, showing an increase that is more pronounced at high pressures; a pattern also evident in the electrostatic potential.

Co-axial multicusp source for low axial energy spread ion beam production

A co-axial multicusp ion source has been designed and tested. This source uses a new magnetic filter configuration. This magnetic filter is efficient in modifying the plasma potential distribution which can reduce the axial energy spread of the extracted ion beam . Energy spreads as low as 0.6 eV have been obtained. The electron temperature in this source has also been found to be about 0.1 eV. Furthermore, the new source configuration is capable of adjusting the radial plasma potential distribution which can improve the transverse ion energy, which results in a low beam emittance . The co-axial source can be used for a number of different applications such as ion projection lithography and radioactive ion beam projets.

Langmuir probe measurements in a low pressure inductively coupled plasma used for diamond deposition

The characterization of 13.56 MHz low pressure inductively coupled plasmas used for diamond deposition has been performed with a Langmuir probe. The plasma potential (Vp), electron temperature (Te), electron density (Ne), ion density (Ni), and electron energy distribution function (EEDF) were measured in a CH4/H2 plasma with 10–50 mTorr of the gas pressure at 1 kW of the plasma power, and were compared with those of an Ar plasma. We found that the Vp, Ne, and Ni have a similar radial distribution, which has a peak at the center axis and decreases outward in the radial direction, while the Te is almost constant within the radius of 20 mm and slightly decreases toward the chamber wall. It was also found that with increasing pressure the Vp and the Te decrease, whereas the Ne increases, except for a CH4/H2 plasma at 50 mTorr. The transition from a Maxwellian distribution to a Druyveysten distribution was observed at 10 mTorr in the EEDFs of Ar plasmas, while it occurred at 20 mTorr in CH4/H2 plasmas. The EEDF of a CH4/H2 plasma at 50 mTorr has a hump at ∼6 eV corresponding to the resonant peak of the vibrational excitation cross section of CH4 molecule.

Enhancement of Mask Selectivity in SiO2 Etching with a Phase-Controlled Pulsed Inductively Coupled Plasma

We developed a new method to enhace the photoresist selectivity in SiO2 etching by modulating both the source and bias powers and by controlling the phase difference between the modulation functions. Enhancement of mask selectivity was observed in the pulse plasma, especially in the out-phase condition. To understand the heavy polymerization in the out-phase pulse plasma, we analyzed the ion energy distributions of CFx+(x=1, 2, 3) ions using the energy-spectroscopic quadrupole mass spectrometer (QMS) and measured the waveforms of the bias power with a high-voltage probe which was connected directly to the wafer. Two distinct plasma potential distributions were obtained in the pulse plasma and the dc bias voltage (VDC) was maximum in the out-phase condition. The heavy polymerization in the out-phase condition was explained as a result of high VDC. We also investigated the emission intensity of the C2 (516.5 nm) line, and found that C2 species were precursors of the polymerization and contributed to the heavy polymerization in the out-phase condition.

Multicusp sources for ion beam projection lithography

Multicusp ion sources are capable of producing positive and negative ions with good beam quality and low energy spread. The ion energy spread of multicusp sources has been measured by three different techniques. The axial ion energy spread has been reduced by introducing a magnetic filter inside the multicusp source chamber which adjusts the plasma potential distribution. The axial energy spread is further reduced by optimizing the source configuration. Values as low as 0.8 eV have been achieved.

Velocity distributions in magnetron sputter

Results of the particle simulation of magnetron sputter are presented. Using a kinetic code, we obtain the spatial profiles of plasma density, potential, and velocity distribution function, along with the electron temperature, the ion density, the current density, and the deposition profiles at the anode surface. The result of simulation is compared with the Child-Langmuir law applied to the magnetron discharge and the global model. The velocity distribution function of electrons is Maxwellian, but that of ions is non-Maxwellian near the cathode with the majority in the energy range below 50 eV.

Plasma-wall transition: The influence of the electron to ion current ratio on the magnetic presheath structure

The effect of magnetic field on the plasma-wall transition layer is investigated using the two-fluid formulation. The quasi-neutral, near-sheath plasma (presheath) is examined, with the presence of neutral particles, and a magnetic field parallel to the confining wall. A general approach is used which takes into account the electron momentum equation, including the electric field force, the magnetic force, the pressure gradient, and the drag force. The influence of the electron to ion current ratio on the potential and velocity distribution in the near-sheath plasma is investigated. It is shown that the electron density distribution in the presheath may deviate from the Boltzmann distribution normally used in previous presheath models. Even when the plasma density dependence on the potential corresponds to the Boltzmann distribution, the presheath thickness deviates from that calculated with a model based on this distribution. The potential in the presheath with respect to the plasma–presheath interface can be negative or positive depending on the electron to the ion flux ratio η and Hall parameter βi. In the case of magnetized ions (βi>1) the potential distribution has a positive maximum and is always negative at the wall edge of the presheath. The value and position of the maximum depend on the parameter η. In the case of unmagnetized ions (βi≪1) the potential is positive for large η and is negative for η<100. With large βi the influence of the electrons is significant so that the presheath thickness decreases to the electron Larmor radius and has a strong dependence on the parameter η.

Essential points for precise etching processes in pulse-time-modulated ultrahigh-frequency plasma

Since ultrahigh-frequency (UHF) plasma has a low electron temperature (less than 2 eV) and uniform density, the effects of electron confinement due to the sheath potential barrier or plasma potential distributions are less prominent than with electron cyclotron resonance plasma or inductive coupled plasma. Consequently, multiple cusp magnetic fields are needed to maintain pulsed UHF plasma even at pulse intervals of a few tens of μs and they can significantly improve the etching rate and etching selectivity. Moreover, the characteristics of pulse-time-modulated plasma strongly depend on the substrate position from the generation region to the downstream region. Near the plasma generation region, pulsed plasma can be used to produce a higher rate and higher selectivity for polycrystalline silicon etching, compared with a continuous discharge. By transporting the plasma from the generation region to the downstream region, however, improvements in the etching characteristics of the pulsed plasma are gradually reduced.

Kaufman-type xenon ion thruster coupling plasma: Langmuir probe measurements

This paper details an investigation into the physics of the external plasma in a Kaufman-type electrostatic ion thruster operating with xenon gas. The external plasma, which is located outside the hollow cathode between the keeper and the anode electrodes, was investigated with a single Langmuir probe. Detailed scans give a peak temperature of eV at the orifice of the keeper decreasing to eV at the anode. The temperature is essentially constant over the external plasma volume. The radial electron density distribution peaks on the axis at decreasing radially to at the boundaries of the plasma; these values have not been corrected for any perturbation of the plasma caused by the presence of the probe. The velocity distribution of the electrons was studied using the second derivatives of the probe characteristics assuming a collisionless sheath. It was shown that the electrons have a Maxwellian distribution. This allowed essentially independent calculations of the electron temperature and plasma potential distribution to be made. These measurements were taken in the stable `spot mode' discharge. Observations in the less stable `plume mode' were also made. However, electrical noise due to plasma instabilities masked the probe signal rendering the analysis inaccurate.

Convergence, electrostatic potential, and density measurements in a spherically convergent ion focus

Unique measurements of the basic plasma-flow characteristics in a low pressure (⩽53 mPa H2) spherically convergent ion focus are obtained using high-voltage (⩽5 kV) emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4–5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density (nic∼1015 m−3) are consistent with simple flow convergence models.

Effect of a multiple-cusp magnetic field on electron confinement in a pulse-time-modulated plasma

Since ulrahigh frequency (UHF) plasma has a low electron temperature (less than 2 eV) and uniform density, the effects of electron confinement due to the sheath potential barrier or the plasma potential distributions are less prominent than with electron cyclotron resonance plasma or inductive coupled plasma. Consequently, with no magnetic fields, the pulsed UHF plasma discharge cannot be maintained. To confine electrons at the afterglow in the pulsed UHF plasma, multiple-cusp magnetic fields are required on the chamber wall. Under this condition, the pulsed UHF plasma can be maintained even at a pulse interval of 40 μs and can significantly improve the etching rate and etching selectivity. That is, electron confinement with magnetic fields plays a very important role for the generation of negative ions at the afterglow in a pulse-time-modulated plasma.

Pulse–time-modulated electron cyclotron resonance plasma discharge for highly selective, highly anisotropic, and charge-free etching

Highly selective, highly anisotropic, notch-free, and charge-buildup damage-free silicon etching is performed using electron cyclotron resonance (ECR) Cl2 plasma modulated at a pulse timing of a few tens of microseconds. A large quantity of negative ions are produced in the afterglow of the pulse-time-modulated plasma. The decay times of electron density, electron temperature, and sheath potential are considerably reduced. This is attributable to negative-ion generation. Furthermore, the pulse-time-modulated plasma reduces the time averaged sheath potential. As a result of these effects, charged particles in the sheath are drastically modified from the continuous discharge, and they should improve the selective etching in the pulsed ECR plasma and eliminate charge accumulation on the substrate. Additionally, negative-ion generation dramatically improves the plasma potential distributions in the nonuniform ECR plasma. This technique is also suitable for large scaled etching processes.

Charge accumulation effects on profile distortion in ECR plasma etching

Recent studies of electron cyclotron resonance (ECR) plasma etching for fabricating the gate electrode of MOS LSIs indicate a serious problem in the etched profiles. The problem is a local pattern distortion which is commonly called notch. The notch has a characteristic dependence on the pattern layout and plasma conditions. For determining the validity of the model which suggests that the cause of notch is charge accumulation of fine patterns, plasma parameters are measured by electrostatic probes and relationships between the notch and the plasma properties are investigated. Lowering the electron temperature parallel to the wafer surface is one of the most effective techniques for eliminating the notch. A lower electron temperature is achieved by moving the wafer farther from the ECR region. Increasing rf bias is effective for reducing the notch. However, elimination of notch with rf bias is due to an increased ion energy. Thus high selectivity, which requires low ion energy, cannot be achieved while simultaneously preventing the notch. Modifying the distribution of plasma potential which decreases toward the wafer reduces the notch in spite of the higher ion current density.

Production of low energy spread ion beams with multicusp sources

The use of multicusp sources to generate ion beams with narrow energy spread has been investigated. It is found that the presence of a magnetic filter can reduce the longitudinal energy spread significantly. This is achieved by creating a uniform plasma potential distribution in the discharge chamber region, eliminating ion production in the extraction chamber and in the sheath of the exit aperture and by minimizing the probability of charge exchange processes in the extraction chamber. An energy spread as low as 1 eV has been measured.

Temporally and spatially resolved plasma parameters and EEDF measurements in a low-frequency discharge

A time-resolved Langmuir probe technique is used to measure the dependence of the electron density, electron temperature, plasma potential and electron energy distribution function (EEDF) on the phase of the driving voltage in a RF driven parallel plate discharge. The measurements were made in a low-frequency (100-500 kHz), symmetrically driven, radio frequency discharge operating in H2, D2 and Ar at gas pressures of a few hundred millitorr. The EEDFs could not be represented by a single Maxwellian distribution and resembled the time averaged EEDFs reported in 13.56 MHz discharges. The measured parameters showed structure in their spatial and temporal dependence, generally consistent with a simple oscillating sheath model. Electron temperatures of less than 0.1 eV were measured during the phase of the RF cycle when both electrodes are negative with respect to the plasma.