P. A. Miller

Dynamics of collisionless rf plasma sheaths

The behavior of rf plasma sheaths has been the subject of much scientific study and also is technologically important for plasma etching and deposition in the manufacture of integrated circuits. This paper presents a semianalytic model of rf sheaths and describes an experiment that tested the model. An approximation to the first integral of the Poisson equation allows solving for the response of plasma sheaths to an imposed rf bias voltage. This approximation enables the plasma sheaths to be included within an electrical model of the plasma and external rf circuit components, and affords a prediction of the ion energy distributions impacting the electrodes, which are in contact with the plasma. The model is a significant advance beyond previous sheath models because it has no restriction on the ratio of the rf period to the ion transit time across the sheath. The model is applicable to those high-density, low-pressure plasmas in which the Debye length is a small fraction of the ion mean-free path, which i...

Ion energy and angular distributions in inductively driven radio frequency discharges in chlorine

In this article, we report values of ion energy and angular distributions measured at the grounded electrode of an inductively coupled discharge in chlorine gas. The inductive rf drive in our cell produced high plasma densities (1011/cm3 electron densities) and stable plasma potentials. As a result, ion energy distributions typically consisted of a single peak well separated from zero energy. Mean ion energy varied inversely with pressure, decreasing from 13 to 9 eV as the discharge pressure increased from 20 to 60 mTorr. Half-widths of the ion angular distributions in these experiments varied from 6° to 7.5°, corresponding to transverse energies from 0.13 to 0.21 eV. During the course of the experiment, ion energies gradually decreased, probably due to the buildup of contaminants on the chamber walls. Cell wall temperature also was an important variable, with ion fluxes to the lower electrode increasing and the ion angular distribution narrowing as the cell temperature increased.

Analytic model of Applied‐B ion diode impedance behavior

An empirical analysis of impedance data from Applied‐B ion diodes used in seven inertial confinement fusion research experiments was published recently. The diodes all operated with impedance values well below the Child’s‐law value. The analysis uncovered an unusual unifying relationship among data from the different experiments. The analysis suggested that closure of the anode‐cathode gap by electrode plasma was not a dominant factor in the experiments, but was not able to elaborate the underlying physics. Here we present a new analytic model of Applied‐B ion diodes coupled to accelerators. A critical feature of the diode model is based on magnetic insulation theory. The model successfully describes impedance behavior of these diodes and supports stimulating new viewpoints of the physics of Applied‐B ion diode operation.

Frequency dependent plasma characteristics in a capacitively coupled 300 mm wafer plasma processing chamber

Argon plasma characteristics in a dual-frequency, capacitively coupled, 300 mm-wafer plasma processing system were investigated for rf drive frequencies between 10 and 190 MHz. We report spatial and frequency dependent changes in plasma parameters such as line-integrated electron density, ion saturation current, optical emission and argon metastable density. For the conditions investigated, the line-integrated electron density was a nonlinear function of drive frequency at constant rf power. In addition, the spatial distribution of the positive ions changed from uniform to peaked in the centre as the frequency was increased. Spatially resolved optical emission increased with frequency and the relative optical emission at several spectral lines depended on frequency. Argon metastable density and spatial distribution were not a strong function of drive frequency. Metastable temperature was approximately 400 K.

Applied‐B ion diode experiments on the Particle Beam Fusion Accelerator‐I

A series of experiments was performed with an Applied‐B ion diode on the Particle Beam Fusion Accelerator‐I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied‐B diode operation from the sub‐TW level on single module accelerators to the multi‐TW level on a low impedance, self‐magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100‐TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of...

Impedance scaling of Applied-B ion diodes

Applied‐B ion diodes are used in inertial confinement fusion research to generate multiterrawatt ion beams. Impedance characteristics of these ion diodes deviate strongly from simple space‐charge‐limited flow, particularly in the higher power experiments. An examination of recent data has yielded an empirical voltage‐current relationship with surprising implications, including a ‘‘critical voltage’’ limit.

Ion distribution functions in inductively coupled radio frequency discharges in argon–chlorine mixtures

We report on measurements of positive ion energies, current densities, and angular distributions at the grounded electrode of inductively coupled discharges in mixtures of argon and chlorine. We also report on ion species and Langmuir probe measurements for these discharges. The inductive drive in our gaseous electronics conference reference cell produced high plasma densities (1011–1012/cm3) and stable plasma potentials. As a result, ion energy distributions consisted of a single peak well separated from zero energy. At pressures of 2.5–20 mTorr and constant rf power, the addition of Cl2 to an Ar discharge lowered ion current densities, reduced ion energies, and reduced the width of the ion energy distributions. Half-widths of the ion angular distributions ranged from 4.5° to 8.5° with the distributions broadening with increases in pressure or rf power. The addition of Cl2 to Ar discharges made the angular distributions less sensitive to total pressure. Cl+ replaced Ar+ as the dominant ionized species wh...

Electrical isolation of radio‐frequency plasma discharges

Plasmas used for processing of microelectronics materials are often powered by radio‐frequency (rf) electrical sources. Such plasmas have nonlinear impedance characteristics which cause the plasma state to depend on the nature of the circuitry that supplies electrical power to the plasma. This dependency occurs because the harmonics of the fundamental drive frequency, which are generated by the plasma nonlinearity, interact with the impedance of the external circuitry at the harmonic frequencies. This report describes the successful use of an rf filter in the power feed to the plasma which isolates the plasma electrically and eliminates its sensitivity to changes in the rf generator, cable plant, and matching network.

Relativistic Electron Beam Propagation in Low‐Pressure Gases

Experimental results are reported for the transport characteristics of relativistic electron beams in high‐pressure atomic and molecular gases. Experimental results obtained with two different accelerators (ν/γ ∼ 0.05 and 1) are reported. It was found that the highest pressure at which efficient transport occurred was determined by two different physical processes for the two different intensity electron beams. The lower‐intensity beam propagated efficiently throughout a large pressure range, and was attenuated at high pressure by Coulomb scattering of the beam electrons in the screened Coulomb field of the nuclei of the background gas atoms. The more intense electron beam was attenuated at a gas pressure which varied with different gases approximately inversely with the high‐energy ionization cross section. A radial breakdown mechanism is proposed which may explain the loss process of the intense electron beam.

Ion energy distributions at rf-biased wafer surfaces

We report the measurement of ion energy distributions at a radio frequency (rf)-biased electrode in inductively driven discharges in argon. We compare measurements made with a gridded energy analyzer and a commercial analyzer that contains a mass spectrometer and energy analyzer in tandem. The inductive drive and the rf bias in our Gaseous Electronics Conference reference cell were both at 13.56 MHz. By varying the plasma density, we were able to examine the transition region between the “low frequency limit” for rf bias and the intermediate frequency region where, at fixed bias frequency, the ion energy distribution width varies with the plasma density. We find that the experimental ion energy distributions become narrower as the time for ion transit through the sheath approaches the rf period, but that the ion distributions still have widths which are ∼90% of their low frequency limit when the ion transit time is 40% of the rf period. Space-charge-induced beam broadening inside our analyzers appears to ...