Structured discharges in high frequency plasmas

Experiments are described in which structured discharges, including stationary and moving striations, were observed in 4 MHz rf plasmas generated using two solenoidal coil configurations. The structured plasmas which formed in the vicinity of the solenoidal coils were not quenched when elaborate precautions were taken to shield the capacitive electric fields. Measurements of the rf power input to the plasma and the resistance reflected by the plasma into the drive circuit showed no discontinuous change during the formation or disappearance of the structured discharges surrounding the solenoidal coils. Observations of striated rf discharges reported by other investigators are reviewed. Arguments are given to support the conclusion that the striation-like structures observed in rf plasmas and the moving striations observed in dc plasmas are part of the same general family.Much of the work described in this paper was supported by the Air Force Office of Scientific Research. One of us (RCW) received partial support from Contract DAAA 13-68-C-0033 and Grant NSF GK-24150.     

RF discharges at atmospheric pressure

The described plasma source is based on the RF torch discharge. The powered RF electrode of the torch discharge plasma source is made from the thin metal pipe with an inner diameter of 1–2 mm and with a length of several cm. The working gas (argon pure or with an admixture of reactive components) flows through the RF electrode as the nozzle. The electrode is connected through the matching unit to the RF generator of the frequency of 13.56 MHz.The advantage of this described plasma source consists in the fact that the torch discharge remains stable up to the atmospherical pressure of the working gas even in the liquid environments. Up to now the torch discharge has been used only for treatment in liquid environment only for archaeological artefacts. For further applications it is necessary to additional study of this phenomenon. While in previous papers we presented a measurement of different temperatures from spectra emitted by plasma, there is drawn attention to equiintensity maps inside the nozzle.     

Numerical modeling of low-pressure RF plasmas

A particle-in-cell simulation is used to model the plasma generated in a parallel plate RF reactor at low pressure. Nonperiodic boundary conditions are used, and the electric field and particle motion are obtained by finite-difference methods leading to the self-consistent creation of sheaths on the boundaries. Model cross sections are used to describe collisions between particles. Ionization is included, and the plasma is maintained by fast electrons generated in the RF sheaths. Most of the power dissipation is due to the acceleration of ions in the time-average sheath fields. At high applied voltage, the power dissipation is described well by the power law P∝V^5/2. Simple scaling laws for the density and plasma potential are obtained. The effect of ion mass and charge-exchange colisions on the ion energy spectrum collected by the electrodes is examined. The ion loss rate drops in the presence of charge-exchange collisions, and this leads to an increase in the density. The collisions also markedly alter the ion energy distribution function     

Atomic populations and temperatures in mixed plasmas

Atomic populations and temperatures have been measured in a mixed plasma formed by a mercury-lead-iodide arc in a quartz lamp. The excited levels observed for the mercury atom show a Boltzmann distribution when the electron density is above 4 × 10¹⁵ cm^-3. Departures from this distribution occur when the temperature falls below 0.25 eV with an electron density around 2 × 10¹⁵ cm^-3.     

Electrical characteristics of parallel-plate RF discharges in argon

Electrical characteristics were measured in a parallel-plate, capacitively coupled (E-type), low-pressure, symmetrical RF discharge driven at 13.56 MHz. The discharge voltage, current, and phase shift between them were measured over a very wide range of discharge parameters (gas pressures between 3 mtorr and 3 torr with discharge power between 20 mW and 100 W). From these measurements the discharge impedance components, the power dissipated in the plasma and in the sheaths, the sheath width, and the ion current to the RF electrodes were found over a wide range of discharge conditions. Some of the general relationships between the various measured and determined parameters are discussed. The experimental results can be used as a database for straightforward comparison with existing RF discharge models and numerical simulations     

Model of magnetically enhanced, capacitive RF discharges

Magnetically enhanced, capacitive RF discharges (called RF magnetrons or MERIE discharges) are playing an increasing role in thin film etching for integrated circuit processing. In these discharges, a weak DC magnetic field is imposed, lying parallel to the powered electrode surface. The authors determine the RF power transferred to the discharge electrons by the oscillating electron sheath in the presence of the magnetic field. Using this, along with particle and energy conservation, they obtain discharge parameters such as the ion flux and ion bombarding energy at the powered electrode as functions of pressure, RF power, and the magnetic field. Some results of the model show good agreement with experiments done on a commercial MERIE system     

Kinetic theory of stochastically heated RF capacitive discharges

Stochastic sheath heating is the dominant heating mechanism at low pressures for radio frequency (RF) capacitive discharges. It produces an electron energy probability distribution function (EEPF) that approximates a two-temperature Maxwellian, as seen in both experiments and numerical simulations. We have used the fundamental kinetic equation to obtain a space- and time-averaged kinetic equation. We assume that electrons with the x component kinetic energy lower than a certain threshold Φ are prevented from interacting with the sheath heating fields. With these approximations and either a knowledge of the central density or an ansatz on Φ, we obtain a self-consistent solution for the quasiequilibrium discharge parameters valid for low pressures in argon. The results are compared to those found in experiments, yielding reasonable agreement     

Effect of finite charging time on particulates in RF discharges

The effect of the finite charging time on spherical and cylindrical particulates in an RF discharge is considered. Using probe theory, analytical expressions for the charging rate under various conditions are derived and compared with numerical solutions. Scaling of the charging time with discharge parameters, in particular the electron temperature, is demonstrated. Using a one-dimensional fluid model for an rf discharge, the equilibrium electric and ion drag forces are compared for spherical and cylindrical particulates. The effect of the finite charging time on the dynamics of particulates of various sizes in the model discharge are discussed. Overall, a long cylindrical particle with the same mass as a spherical particle charges up more slowly to a larger net (negative) charge and has a significantly larger polarization. As a consequence, small cylindrical particles introduced near the electrodes are more likely to escape from the sheath region before becoming fully charged     

Level shifts and inelastic electron scattering in dense plasmas

A completely quantum mechanical formalism has been developed to describe the high density plasma effects on fundamental atomic parameters. Both the bound and free electrons are treated by a method which in principle is similar to Hartree's self-consistent field method. The free plasma electrons' wavefunction is obtained from the Schrödinger equation with the effective pottential representing the spherically averaged Coulomb interaction with bound and free electrons. Results are given for level shifts, coefficients of transition probabilities, and electron collision cross sections of Ne⁺⁹ for temperatures of 200 and 500 eV for an electron density range of 1–6 × 10²⁴ cm^?3.     

Inversion of hydrogen level populations in stationary plasmas

Large inversions in the dense two-temperature hydrogen plasma are obtained by solving balance equations with inelastic collisions between heavy particles taken into account.     

A study of methods for moving particles in RF processing plasmas

Methods for moving charged particles in RF processing plasmas are investigated. These methods include varying RF power, varying chamber pressure, attraction and repulsion by an electrostatic probe, and movement with magnetic fields. Varying RF power changes the depth of the potential wells where particles are trapped. The RF power affects shape and location of the traps and the bulk plasma potential. Increasing the chamber pressure moves the sheath edge closer to the wafer being processed. Since particle traps are found at the plasma sheath edge increasing the chamber pressure will move the particle traps (and any trapped particles) closer to the wafer being processed. The Langmuir probe can repel particles when under negative bias and attract them when positively biased. This probe can also distort the sheath edge when the tip resides within the sheath. Applying a magnetic field can change the characteristics of the particle traps and produce a force on the charged dust particles     

板条射频放电产生单重态氧实验研究

 通过考察各种放电状态及气流条件下发生器内外物种的自发辐射谱,发现光谱峰值强度与对应物种浓度成正比。分析了主要的等离子体动力学过程,了解了单重态氧及其它物种的浓度变化规律。考察了α放电和γ放电两种不同的放电方式。发现在α放电状态下,体系中有较少氧原子等淬灭性粒子,更有利于O₂(¹Δ)产生。加入He,有效地降低了气体体系的离子化阈能和约化场强,约化场强最小时,产生的O₂(¹Δ)浓度最大,相较于纯氧放电,O₂(¹Δ)浓度提高一倍以上。考察了腔外各物种浓度的变化,O₂(¹Δ)离开放电腔后浓度稳定,沿距离减少较慢,有益于出光。优化了本系统的放电极间距,极间距太大或太小,都不利于单重态氧的产生。     

Correlated fluctuations of electron populations in high temperature plasmas

This paper presents a model for a simple treatment of the fluctuations of electron bound states populations in equilibrium plasmas at high temperature. Fluctuations in the various states are correlated by matrix elements of the Coulomb interaction. The model imposes to recover the average occupation numbers given by usual Average Atom (AA) calculations. The additional numerical work consists only in a matrix inversion. Examples of application are shown (calculation of correlation factors, fluctuations of the total bound charges of the ions), and comparison with other published results is made.     

Scaling characteristics of plasma parameters for low-pressureoxygen RF discharge plasmas

For a low-pressure (1-100 mtorr) oxygen RF discharge plasma, the scaling laws for the densities of charged species such as positive ion, negative ion, and electron are estimated in terms of external and internal plasma parameters for the ion-flux-loss-dominated region based on the global balance equations. The scaling formulas are compared with Langmuir probe measurement results performed on a planar inductively coupled oxygen plasma. The transition point from the ion-flux-loss-dominated region to the recombination-loss-dominated region moves to a lower pressure region as the absorbed power increases     

Gyrocenter shift of low-temperature plasmas and the retrograde motion of cathode spots in arc discharges

The gyrocenter shift phenomenon explained the mechanism of radial electric field formation at the high confinement mode transition in fusion devices. This Letter reports that the theory of gyrocenter shift is also applicable to low temperature high collisional plasmas such as arc discharges by the generalization of the theory resulting from a short mean free path compared with the gyroradius. The retrograde motion of cathode spots in the arc discharge is investigated through a model with the expanded formula of gyrocenter shift. It is found that a reversed electric field is formed in front of the cathode spots when they are under a magnetic field, and this reversed electric field generates a rotation of cathode spots opposite to the Amperian direction. The ion drift velocity profiles calculated from the model are in agreement with the experimental results as functions of magnetic flux density and gas pressure.     

Ionization state and bound level populations in hot,dense plasmas

The ionization state and bound level populations in hot, dense plasmas are studied in the average atom approximation. Bound level energies are fixed self-consistently with the complete set of population numbers, using pre-existent Hartree-Fock calculations. We present a pressure- ionization scheme that gradually merges bound electrons into the continuum. To solve the nonlinear algebraic system of equations of the model, we have derived an efficient iterative algorithm. Results are shown for aluminum and iron.     

Theoretical treatment of RF discharges in coaxial He–Ne laser

Based on the ambi-polar diffusion, a model to simulate the coaxial RF-excited He–Ne laser plasmas is set up, and the microprocesses in laser plasmas are studied. Some parameters such as the spatial distribution of electron density, RF electric field, RF power density and excitation efficiency for upper laser level are calculated and discussed, which are suitable to describe the electrical properties of the discharges. The theoretical results can explain some experimental phenomena and guide the experimental study.     

RF electric field penetration and power deposition into nonequilibrium planar-type inductively coupled plasmas

The RF electric field penetration and the power deposition into planar-type inductively coupled plasmas in low-pressure discharges have been studied by means of a self-consistent model which consists of Maxwell equations combined with the kinetic equation of electrons. The Maxwell equations are solved based on the expansion of the Fourier--Bessel series for determining the RF electric field. Numerical results show the influence of a non-Maxwellian electron energy distribution on the RF electric field penetration and the power deposition for different coil currents. Moreover, the two-dimensional spatial profiles of RF electric field and power density are also shown for different numbers of RF coil turns.