Roderick Boswell

Very efficient plasma generation by whistler waves near the lower hybrid frequency

Experimental results are presented which show that r.f. power at frequencies near the lower hybrid frequency couples resonantly into a standing whistler wave. For an input power flux of less than 5 W cm-2 densities above 1012 cm-3 with close to 100% ionization have been achieved. Measured density, temperatures and wave fields are presented and are used as input parameters for a theoretical model.

Helicons-the past decade

First observed in gaseous plasmas in the early 1960s, helicon discharges lay like a sleeping giant until they emerged in the 1980s, when their usefulness as efficient plasma sources for processing microelectronic circuits for the burgeoning semiconductor industry became recognized. Research on helicons spread to many countries; new, challenging, unexpected problems arose, and these have spawned solutions and novel insights into the physical mechanisms in magnetized radio-frequency discharges. Among the most baffling puzzles were the reason for the high ionization efficiency of helicon discharges and the dominance of the right-hand polarized mode over the left-hand one. The most recent results indicate that a nonobvious resolution of these problems is at hand.

Helicons-the early years

Helicon waves are right-hand polarized (RHP) waves which propagate in radially confined magnetized plasmas for frequencies /spl omega//sub ci//spl Lt//spl omega//spl Lt//spl omega//sub ce/ where /spl omega//sub ci/ is the ion cyclotron frequency and /spl omega//sub ce/ is the electron cyclotron frequency. They are part of a much larger family of waves which can propagate down to zero frequency and constitute a very rich field for studying complex propagation characteristics and wave-particle interactions. This paper gives a historical perspective of the waves and their relationship to plasma source development up to the mid 1980s, presents a simple description of their propagation characteristics in free and bounded plasmas, and finishes with their first reported use in plasma processing experiments.

Current-free double-layer formation in a high-density helicon discharge

A strong, current-free, electric double-layer with eΦ/kTe∼3 and a thickness of less than 50 debye lengths has been experimentally observed in an expanding, high-density helicon sustained rf (13.56-MHz) discharge. The rapid potential decrease is associated with the “neck” of the vacuum vessel, where the glass source tube joins the aluminum diffusion chamber, and is only observed when the argon gas pressure is less than about 0.5 mTorr. The upstream electron temperature Te appears 25% greater than the downstream Te, and there is a density hole on the downstream edge. This experiment differs from others in that the potentials are self-consistently generated by the plasma itself, and there is no current flowing through an external circuit. The plasma electrons are heated by the rf fields in the source, provide the power to maintain the double-layer, and hence accelerate ions created in the source out into the diffusion chamber.

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 varies as V/sup 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. >

Laboratory evidence of a supersonic ion beam generated by a current-free “helicon” double-layer

An electric double-layer is generated near the open end of a high-density low pressure helicon sustained radio frequency (13.56 MHz) plasma source which expands into a diffusion chamber. Ion energy distribution functions measured with a retarding field energy analyzer placed in the diffusion chamber with its aperture facing the double-layer show the presence of a low energy peak (∼29 V) around the local plasma potential and a high energy peak (∼47 V) corresponding to a supersonic ion beam (∼2.1cs). At an axial distance 12 cm downstream of the double-layer, the beam density is 14% of the local density at that position and the ion energy gain is approximately 70% of the potential drop of the double-layer. The ion beam is observed from the center out to a radius corresponding to that of the plasma source tube (−6.8 cm⩽r⩽+6.8 cm) and is not greatly affected by the expanding magnetic field. A depression in the total ion flux just downstream of the double-layer—previously measured on the main z-axis of the reactor—is also present across the chamber diameter. Evidence of an electron beam near the closed end of the source tube, generated via “backwards” acceleration through the double-layer, has been observed on a Langmuir probe trace.An electric double-layer is generated near the open end of a high-density low pressure helicon sustained radio frequency (13.56 MHz) plasma source which expands into a diffusion chamber. Ion energy distribution functions measured with a retarding field energy analyzer placed in the diffusion chamber with its aperture facing the double-layer show the presence of a low energy peak (∼29 V) around the local plasma potential and a high energy peak (∼47 V) corresponding to a supersonic ion beam (∼2.1cs). At an axial distance 12 cm downstream of the double-layer, the beam density is 14% of the local density at that position and the ion energy gain is approximately 70% of the potential drop of the double-layer. The ion beam is observed from the center out to a radius corresponding to that of the plasma source tube (−6.8 cm⩽r⩽+6.8 cm) and is not greatly affected by the expanding magnetic field. A depression in the total ion flux just downstream of the double-layer—previously measured on the main z-axis of the reac...

Capacitive, inductive and helicon‐wave modes of operation of a helicon plasma source

Vector‐rf‐B‐field measurements in the near‐field of a helicon plasma source taken throughout the volume of the source are reported. Three distinct modes of operation of the helicon plasma source, capacitive, inductive, and helicon‐wave, are identified by the structure of the plasma‐wave‐fields. Results are reported for a double‐half‐turn antenna, which is believed to be the first reporting for such an antenna structure in application to helicon‐wave plasma sources. Comparison is made to a double‐saddle‐coil antenna which also demonstrates the distinct inductive and helicon‐wave modes.

Pulsed RF discharges, glow and filamentary mode at atmospheric pressure in argon

The properties of a pulsed radio frequency capacitive discharge are investigated at atmospheric pressure in argon. The discharge can operate in two different modes: a homogeneous glow discharge or turn into filaments. By pulsing the 13.56 MHz generator both the filamentary and the glow modes can be selected depending on the pulse width and period. For a 5 µs pulse width (∼70 RF cycles in the pulse), short pulse periods (less than 100 µs) result in a filamentary discharge while long pulse periods (greater than 1 ms) result in a glow discharge. Optical emission spectroscopy and power measurements were performed to estimate the plasma temperature and density. Water vapour was introduced to the discharge as a source of hydrogen and the Stark broadening of the Balmer Hβ line was measured to allow the plasma density to be estimated as 10 15 cm −3 in the filamentary mode. The estimation of the glow mode density was based on power balance and yielded a density of 5 × 10 11 cm −3 . Emission line ratio measurements coupled with the Saha equation resulted in an estimate of electron temperature of approximatively 1.3 eV for the glow mode and 1.7 eV for the filaments. Using the glow mode at a duty cycle of 10% is effective in decreasing the hydrophobicity of polymer films while keeping the temperature low. (Some figures in this article are in colour only in the electronic version)

Direct thrust measurement of a permanent magnet helicon double layer thruster

Direct thrust measurements of a permanent magnet helicon double layer thruster have been made using a pendulum thrust balance and a high sensitivity laser displacement sensor. At the low pressures used (0.08 Pa) an ion beam is detected downstream of the thruster exit, and a maximum thrust force of about 3 mN is measured for argon with an rf input power of about 700 W. The measured thrust is proportional to the upstream plasma density and is in good agreement with the theoretical thrust based on the maximum upstream electron pressure.

Pulsed high rate plasma etching with variable Si/SiO2 selectivity and variable Si etch profiles

Very high etch rates of Si in SF6 have been obtained in a low‐pressure resonant rf discharge. By pulsing the discharge the etch selectivity between Si and SiO2 can be varied from greater than 100 at high repetition rates to around 6 at low repetition rates or continuous operation. A ‘‘lifetime’’ of atomic fluorine has been derived which is in agreement with previous measurements. With a biased substrate the etch profile can be varied from isotropic at high repetition rates to anisotropic with continuous operation.