J. H. Whealton

Controlled alignment of carbon nanofibers in a large-scale synthesis process

Controlled alignment of catalytically grown carbon nanofibers (CNFs) at a variable angle to the substrate during a plasma-enhanced chemical vapor deposition process is achieved. The CNF alignment is controlled by the direction of the electric field lines during the synthesis process. Off normal CNF orientations are achieved by positioning the sample in the vicinity of geometrical features of the sample holder, where bending of the electric field lines occurs. The controlled growth of kinked CNFs that consist of two parts aligned at different angles to the substrate normal also is demonstrated.

Field emission from isolated individual vertically aligned carbon nanocones

Field emission from isolated individual vertically aligned carbon nanocones (VACNCs) has been measured using a small-diameter moveable probe. The probe was scanned parallel to the sample plane to locate the VACNCs, and perpendicular to the sample plane to measure the emission turn-on electric field of each VACNC. Individual VACNCs can be good field emitters. The emission threshold field depends on the geometric aspect ratio (height/tip radius) of the VACNC and is lowest when a sharp tip is present. VACNCs exposed to a reactive ion etch process demonstrate a lowered emission threshold field while maintaining a similar aspect ratio. Individual VACNCs can have low emission thresholds, carry high current densities, and have long emission lifetime. This makes them very promising for various field emission applications for which deterministic placement of the emitter with submicron accuracy is needed.

Digital electrostatic electron-beam array lithography

A concept for maskless digital electrostatically focused e-beam array direct-write lithography (DEAL) has been developed at Oak Ridge National Laboratory. This concept incorporates a digitally addressable field-emission array (DAFEA) integrated into a logic and control circuit implemented as an integrated circuit. The design goal is for 3 000 000 individually addressable field-emission cathodes with a 4 μm by 8 μm pitch on a single ∼1 cm2 integrated circuit. The DAFEA design includes built-in electrostatic focusing for each emitter with feedback dose-control circuits to drive each emitter for tightly controlled electron delivery. With the electrostatic focusing, an array of ∼460 of these integrated circuits (up to 30 across by ∼23 rows deep) are suspended on a back plane ∼100 μm above a 300 mm semiconductor wafer. This arrangement could lithographically expose an entire 300 mm wafer, with 30 nm pixels, in less than 45 s, with every wafer pixel redundantly illuminated eight times allowing gray-scale edge p...

Optics of ion beams of arbitrary perveance extracted from a plasma

A new iteration scheme is proposed for the solution of the ion optics of high current ion beams extracted from a plasma. The scheme (a) requires far less computational effort than other schemes; (b) converges for arbitrary perveance; (c) allows the solution of the problem far back into the extraction plasma and (d) is not geometry dependent, so it should be usable for a wide variety of situations.

Effect of emission aperture shape upon ion optics

Several different axisymmetric aperture shapes have been studied, experimentally and theoretically, for use in the plasma electrode employed in extracting ions from a plasma. Compared to a cylindrical bore aperture, a class of shapes opening away from the source plasma resulted in smaller beam divergence (due to reduced aberration fields), while a class opening toward the source plasma resulted in higher beam power density at optimum divergence. The minimum half‐width‐half‐maximum divergence obtained for the former class was 0.57° at a beam energy of 27 kV, as compared to 1° obtained for a conventional cylindrical bore aperture.

Calculations involving ion beam source

Abstract A convergent numerical model for the calculation of the optical properties of an ion beam is given. The ion beam is formed by extracting ions from a plasma and subsequently accelerating these ions with an electrode system. The model includes the effects of space-charge of the ions and an equilibrium distribution of electrons. Methods are given for determining the existence of the solution to the nonlinear difference equations, and a convergent iterative numerical procedure is described. Comparisons are made with a procedure that previously has been used to solve such a model.

Optics of single‐stage accelerated ion beams extracted from a plasma

A new iteration scheme for numerical solution of the Poisson–Vlasov system of equations is used to investigate parametric dependence of optics in single‐stage ion sources with fixed or similar geometry. Ion optics is found always to be aberration dominated for any reasonable low emittance. Ion temperatures strongly affect the transparency and the optics. The electron temperature has little effect, meaning that out of six physical parameters governing the ion optics only two combinations are important—an ion temperature parameter and the perveance.

A finite difference 3-D Poisson-Vlasov algorithm for ions extracted from a plasma

Abstract A Poisson-Vlasov algorithm has been constructed that solves three-dimensional (3-D) sheath problems. It can be used for accelerator design for intense ion beams extracted from a plasma. It has an advantage over an existing finite element algorithm in that it is much more accurate per unit time spent.

Effect of preacceleration voltage upon ion‐beam divergence

The effect of an insulator coating on the plasma electrode of a duoPIGatron ion source on ion‐beam optics was examined theoretically and experimentally. The effect of a preacceleration potential, applied between the insulated electrode and the target cathode of the source plasma, on ion optics was also examined. A principal result is that this electrode arrangement with precel gives a much lower beam divergence than the same arrangement without precel. Detailed comparisons between the data and calculations are presented.

Computer modeling of negative ion beam formation

Sources of root‐mean‐square (RMS) emittance growth are described for negative ion extractors in both volume and surface negative ion sources and plasma low‐energy beam transport systems. For surface negative ion sources of the Lawrence Berkeley Laboratory or Los Alamos National Laboratory [on Los Alamos Meson Particle Facility (LAMPF)], attention is paid to the nonlinear transverse emittance growth mechanism of the beam/warm plasma interaction. In some cases this is a large effect. In addition, non‐normal sheath fields at a convertor are examined as a source of emittance growth. For volume sources, attention is paid to aberration production due to field penetration from the extractor, RMS emittance growth enhancement by a negatively biased plasma extraction electrode, and emittance growth caused by transverse extraction across a magnetic field. For both volume and surface sources, RMS emittance growth due to nonlinear aberration at the plasma extraction electrode is analyzed. Time‐dependent contributions ...