Helicon Plasma Source Research Articles

Comparing Experiments with Modeling to Optimize Light Ion Helicon Plasma Sources for VASIMR

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, we compare results from a coupled radio frequency (RF) and transport model with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility (Mini-RFTF). Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution RF calculation including the RF parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the RF coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. These findings can be used to optimize helicon discharges for use in Variable Specific Impulse Magnetoplasma Rocket (VASIMR) designs, and estimates for the helicon power required to perform ion cyclotron heating experiments in the Mini-RFTF are given.

Comparing experiments with modeling for light ion helicon plasma sources

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, results from a coupled rf and transport model are compared with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility [Goulding et al., Proceedings of the International Conference on Electromagnetics in Advanced Applications (ICEAA 99), Torino, Italy, 1999 (Litografia Geda, Torino, 1999), p. 107] (Mini-RFTF). Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution rf calculation including the rf parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the rf coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. The effects of absorption near the lower hybrid resonance in the near-field region of the antenna are discussed. Ponderomotive effects are also examined and found to be significant only in very low density and edge regions of the Mini-RFTF discharge.

Measurements of helicon wave propagation and Ar II emission

Wave magnetic field, optical and Langmuir probe measurements are carried out to examine fast and thermal electron contributions to plasma ionization in a helicon plasma source. For the optical measurements, an Ar II line (443 nm) with a particularly short lifetime (7 ns) is chosen to resolve the excitation rate within a radio frequency period (73.7 ns). Information is then obtained regarding the acceleration processes and their action on the electron distribution that caused the excitation. Spatio-temporal measurements of 443 nm peak emission show that the emission is modulated at the source frequency. The peak count phase of the modulation propagates along the plasma at a comparable speed as the local helicon wave phase velocity. Computer modeling utilizing lab data is carried out to examine wave field effects on electron acceleration and ionization contributions arising from non-Maxwellian fast electrons.

Etching characteristics of magnetic materials (Co, Fe, Ni) using CO/NH 3 gas plasma for hardening mask etching

Etching of magnetic materials, NiFe, CoFe, and Fe, has been studied with a high-density helicon plasma source using non-corrosive CO/NH 3 gas. High etch rates, ranging from 45 to 130 nm/min, were achieved. The etch rate strongly depended on both plasma density and ion energy. The main etching mechanism is interpreted as physical sputtering rather than as chemical reaction. A high selectivity of 6–16 was achieved by using hard masks of Ti or Ta. This is attributed to the hardening surface layer of the masks by nitradation, carbonization, or oxidation in CO/NH 3 gas plasma. This etching process is named as hardening mask etching. This etching process was also applied for tunneling magneto-resistive (TMR) multi-layers that uses magnetic random access memory with a Ta mask. A high anisotropy of about 80° was obtained without residues, sidewall deposition, and corrosion. This process is potentially applicable to the etching of thin magnetic materials in TMR stacks, patterned media, and read/write head.

Carbon Nanotube Deposition using Helicon Plasma CVD at Low Temperature

AbstractWe developed a novel growth method of aligned carbon nanotubes. Aligned carbon nanotubes are grown on a metal catalyst on a glass substrate using biased Helicon plasma chemical vapor deposition (HPECVD) of CH4/H2 gases from 400 C to 500 C. The Helicon plasma source is one of the high-density plasma sources and is promising for low temperature carbon deposition. A Ni film was used as a catalyst to reduce the activation energy of the nanotubes' growth. The carbon nanotubes were deposited on the nickel catalysis layer selectively.

Helicon Plasma Source

A spiral-antenna helicon plasma source is characterized experimentally at applied magnetic flux densities below 60 G and applied RF powers below 800 W. It is shown that, for a given RF power, a magnetostatic field applied to an inductively coupled plasma increases the electron concentration fourfold.

Different operational regimes in a helicon plasma source

The characterization of a helicon plasma source has been done in argon plasmas using a rf compensated Langmuir probe. It is now well known that helicon discharges exhibit three modes of operation: a capacitive mode (known as the E mode), which is a low-density mode (around 109 cm−3), an inductive mode (known as the H mode) with higher densities (1010 cm−3), and a helicon-wave sustained mode (known as the W mode) with very high densities (1011–1012 cm−3). By increasing the injected power, the transitions between these modes can be observed and correspond to sudden increases of the plasma density and of the glow brightness. In this article the performances of the helicon source in terms of density and plasma potential are presented and discussed. Transitions between the different coupling modes are observed and explained thanks to two phenomenological models and a good agreement is found between the experimental values and the model results. It has been shown that the main parameter, which determines the coupling mode, is the plasma density, which depends on the rf power and the gas pressure. So the transition between E and H modes is observed when the density reaches a critical value: for this value the skin depth δ is equal to the radius of the Pyrex tube. A similar result has been shown for the H–W transition: a minimum density, the value of which depends on the magnetic field amplitude, is necessary to observe the first longitudinal helicon-wave mode. This value is also directly linked with the geometry of the reactor and of the antenna.

Noncollisional heating and electron energy distributions in magnetically enhanced inductively coupled and helicon plasma sources

The ability to deposit power in the volume of plasma reactors at locations deeper than the conventional skin depth makes magnetically enhanced inductively coupled plasma (MEICP) and helicon sources appealing for use in materials processing. Mechanisms for power deposition and electron energy transport in MEICPs have been computationally investigated using a two-dimensional (2D) plasma equipment model. Using a tensor conductivity in the solution of Maxwell’s equations, three-dimensional components of the inductively coupled electric field are produced from an m=0 antenna and 2D applied magnetic fields. These fields are then used in a Monte Carlo simulation to generate electron energy distributions (EEDs), transport coefficients, and electron impact source functions. The electrostatic component of the wave is resolved by estimating the charge density using an oscillatory perturbed electron density. For MEICPs operating at pressures less than 10 mTorr in Ar, significant power deposition occurs downstream when the radial and axial components of the electric field are commensurate with the azimuthal component. For magnetic fields above 100 G, the tail of the EED (>20–30 eV) is enhanced in the downstream region. This enhancement results from noncollisional heating by the axial electric field for electrons in the tail of the EED which have long mean free paths, while lower energy electrons are still somewhat collisional.

Design and performance of distributed helicon sources

Helicon plasma sources are known to be efficient generators ofhigh-density plasma for materials processing applications. By using multiplesources, arbitrarily large substrates can in principle be covered uniformlywith dense plasma. To test this idea, a seven-tube helicon source wasconstructed, and the plasma density and uniformity measured. Densities up to1012 cm-3 over a 40 cm diameter area with ±3% uniformity were achieved.

Normal modes of bounded whistler produced toroidal plasmas

In tight aspect ratio toroidal devices, characteristics of bounded whistler sustained plasma filled resonant cavities are shown to have significant differences as compared to characteristics of cylindrical cavities in helicon plasma sources. In the present article the analytical-numerical treatment of the problem in electronmagnetohydrodynamic regime is based on wave-field computation and modified dispersive properties of wave for toroidal devices. In the limit of tight aspect ratio, the parallel wavelength of the excited whistler becomes comparable to the system dimension and, thus, strong poloidal mode coupling introduces new features to radially bounded whistlers. Smooth transition of toroidal bounded whistler modes towards conventional helicon modes is observed as the major radius increases.

Wave propagation and power deposition in magnetically enhanced inductively coupled and helicon plasma sources

Magnetically enhanced inductively coupled plasma (MEICP) and helicon sources for materials processing are of interest because of their ability to deposit power within the volume of the plasma beyond the classical skin depth. The location and manner of power deposition can vary substantially depending on the mode of operation and reactor conditions. The coupling of electromagnetic fields to the plasma typically occurs through two channels; a weakly damped heliconlike wave that penetrates into the bulk plasma and an electrostatic wave. The electrostatic wave can often be suppressed resulting in the helicon component being responsible for the majority of the power deposition. A computational investigation was conducted to quantify this heating and determine the conditions for which power can be deposited in the downstream region of MEICP devices. For typical process conditions (10 mTorr, 1 kW ICP) and magnetic fields above 40 G, radial and axial electric fields exhibit nodal structure consistent with helicon behavior. As the magnetic fields are increased, axial standing wave patterns occur with substantial power deposition downstream. The ability to deposit power downstream with increasing B field is ultimately limited by the increasing wavelength. For example, if the plasma is significantly electronegative in the low power–high magnetic field regime, power deposition resembles conventional ICP due to the helicon wavelength exceeding the reactor.

Characterization of m=−1 helicons

Copropagating helicons with azimuthal mode numbers m=+1 and m=−1 have been observed in a large-diameter, weakly-magnetized helicon plasma source (13.5 cm diameter, 55 G). The amplitude and parallel wave number of the m=+1 helicon are generally larger than those of the m=−1 helicon, in agreement with theory.

Ion-beam characteristics of novel helicon ion sources for different plasma parameters

A high-current ion source requires a high-density plasma source to provide sufficient ions to be accelerated. For a continuous high-power ion source, a new concept ion source using a helicon plasma source has been developed. A compact high-density helicon plasma is generated with very high-power efficiency, and ion beams are extracted from the plasma source. With various plasma parameters, extracted ion-beam characteristics are studied in a helicon ion source for the first time. Plasma parameters, especially plasma density, are shown to be strongly correlated with the extracted beam characteristics.

Low-temperature deposition of Si and SiO 2 thin-film layers in an ultrahigh vacuum system

Low-temperature chemical beam epitaxy of Si using hydride sources was investigated for fabricating thin-film layers. In the case of a Si 2H 6 source, the temperature dependence of the growth rate is relatively small in the region of 300–600°C where the growth rate is dominated by desorption and/or decomposition at the surface. A SiO 2 thin film was also fabricated by a simultaneous supply of Si 2H 6 and activated oxygen from a helicon plasma source. The temperature dependence of the deposition rate is relatively small, which may correspond to the relatively small temperature dependence in the case of chemical beam epitaxy of Si in the same temperature region. Relatively low deposition rates, 5 nm/h for Si and 0.2–0.3 nm/min for SiO 2, were achieved. Atomic and molecular level controllability is attained.

A Helicon Plasma Source

The parameters of a helicon plasma source was studied for an rf power to 500 W and a magnetic field ranging from 0 to 200 G under an argon pressure of 3 mtorr. The electron density in the plasma was found to reach 1012 cm–3. Axial and radial electron distributions were studied. Helicon waves were excited by antennas of three types, a loop antenna being the most efficient. It is shown that the electron density in the plasma can significantly be increased if the system is under a nonuniform magnetic field such that its lower-value (relative to the rest of the system) part is applied to the antenna.

Discharge modes and wave structures using loop antennae in a helicon plasma source

Discharge modes separated by density jumps in a helicon plasma source were controlled successfully by changing the antenna wavenumber spectrum with a two-loop antenna, in addition to varying the filling pressure P and the magnetic field B. For the case of wave excitation with the anti-parallel antenna current direction, the threshold power Pth for a density jump, regardless of B and P, was larger than in the case of a parallel direction, and Pth became larger with the increase in B in both cases. From the excited wave structures, it was found that the helicon wave plasma was observed after the jump with the high field, while with the low field regardless of the density region or with the high field before the jump, an inductively coupled plasma was found. No indication of a major role of the lower hybrid wave and Trivelpiece-Gould wave on the plasma production was observed.

Ion heating in the HELIX helicon plasma source

Efficient ion heating in a steady-state helicon plasma source is observed with two external loop antennae just above the ion cyclotron frequency. The ion velocity space distribution is measured by laser induced fluorescence in an argon plasma. The measured bulk ion heating is highly anisotropic (the perpendicular temperature increase is ten times the parallel temperature increase) even though the plasma is moderately collisional. Measurements of the perturbed distribution function with laser induced fluorescence suggest that an electrostatic ion cyclotron wave is launched.

Neutral depletion and transport mechanisms in large-area high density plasma sources

Plasma uniformity has been recognized as a significant parameter in large-sized high density plasma processing tools, but neutral uniformity issues have received less attention. In this article we show experimental and modeling results which indicate that significant neutral uniformity variations can occur in high density plasma processing tools. The experiments are carried out in both inductively coupled plasma and helicon plasma sources. A movable static pressure gauge is used to obtain the static radial neutral pressure distribution both with and without a discharge present. Without a wafer present in the reactor, significant (∼20%–40%) reductions in neutral pressure are observed in these sources during steady-state plasma operations. This spatially averaged neutral depletion is accompanied by hollow neutral pressure profiles. The degree of on-axis neutral depletion depends upon both plasma density and neutral fill pressure. We show that the “plasma pumping” effect, wherein electron impact ionization of neutral particles is followed by their rapid removal from the plasma by the pre-sheath electric field, can reproduce the experimental results. This effect has the potential to result in large (∼50%) neutral density variation across 300 mm wafers in high density plasma sources.

Helicon experiments and simulations in nonuniform magnetic field configurations

Wave, antenna impedance, plasma density, and temperature anisotropy measurements are carried out for a helicon plasma source in nonuniform and uniform static magnetic fields. Strong axial density gradients associated with the nonuniform magnetic fields are observed to affect wave fields, absorption, and source efficiency. The wave field and antenna input impedance measurements are compared with a new simulation code which also calculates Poynting power flow and wave absorption profiles. Wave amplitude measurements are shown to decay more rapidly and the phase velocity varies over a wide range for the nonuniform static magnetic field case.

Formation and characterization of the fluorocarbonated-SiO 2 films by O 2/FTES-helicon plasma chemical vapor deposition

Fluorocarbonated-SiO 2 films were prepared on a p-type Si(100) substrate using FSi(OC 2H 5) 3 (FTES) and O 2 mixture gases by a helicon plasma source. High density O 2/FTES/Ar plasma of ∼10 12 cm −3 is obtained at low pressure (< 3 mTorr) with RF power above 900 W in the helicon plasma source where the FTES and O 2 gases are greatly dissociated. The fluorocarbonated-SiO 2 film deposited in the helicon plasma chemical vapor deposition contains C–F bonds which are not found in the fluorocarbonated-SiO 2 film made by thermal chemical vapor deposition, where the FTES and O 2 react chemically on the substrate. Fourier transform infrared and X-ray photoelectron spectroscopy spectra show that the film has Si–F, Si–O, and C–F bonds. The Si–F and C–F bonds may lower the dielectric constant greatly. The relative dielectric constant, leakage current density, and dielectric breakdown voltage are about 2.8, 8×10 −9 A/cm 2, and > 12 MV/cm, respectively. However the dielectric constant and leakage current density of the film annealed at 500°C are about 3.2 and 9×10 −11 A/cm 2, respectively. The step-coverage of the deposited fluorocarbonated-SiO 2 film in 0.3 μm metal pattern is about 91%.