Samit Paul

Ion energy distribution near a plasma meniscus with beam extraction for multi element focused ion beams

An earlier study of the axial ion energy distribution in the extraction region (plasma meniscus) of a compact microwave plasma ion source showed that the axial ion energy spread near the meniscus is small (∼5 eV) and comparable to that of a liquid metal ion source, making it a promising candidate for focused ion beam (FIB) applications [J. V. Mathew and S. Bhattacharjee, J. Appl. Phys. 105, 96101 (2009)]. In the present work we have investigated the radial ion energy distribution (IED) under the influence of beam extraction. Initially a single Einzel lens system has been used for beam extraction with potentials up to −6 kV for obtaining parallel beams. In situ measurements of IED with extraction voltages upto −5 kV indicates that beam extraction has a weak influence on the energy spread (±0.5 eV) which is of significance from the point of view of FIB applications. It is found that by reducing the geometrical acceptance angle at the ion energy analyzer probe, close to unidirectional distribution can be obt...

Random walk of electrons in a gas in the presence of polarized electromagnetic waves: Genesis of a wave induced discharge

The average number of collisions N of seed electrons with neutral gas atoms during random walk in escaping from a given volume, in the presence of polarized electromagnetic waves, is found to vary as N=B(Λ/λ)2/[1+C(Λ/λ)]2, indicating a modification to the conventional field free square law N=A(Λ/λ)2, where Λ is the characteristic diffusion length and λ the mean free path. It is found that for the field free case A=1.5 if all the electrons originate at the center and is 1.25 if they are allowed to originate at any random point in the given volume. The B and C coefficients depend on the wave electric field and frequency. Predictions of true discharge initiation time τc can be made from the temporal evolution of seed electrons over a wide range of collision frequencies. For linearly polarized waves of 2.45 GHz and electric field in the range (0.6–1.0)×105 V/m, τc=5.5–1.6 ns for an unmagnetized microwave driven discharge at 1 Torr argon.

Effect of beam limiting aperture and collector potential on multi-element focused ion beams.

A compact microwave driven plasma based multi-element focused ion beam system has been developed. In the present work, the effect of reduced beam limiter (BL) aperture on the focused ion beam parameters, such as current and spot size, and a method of controlling beam energy independently by introducing a biased collector at focal point (FP) are investigated. It is found that the location of FP does not change due to the reduction of BL aperture. The location of FP and beam size are found to be weakly dependent on the collector potential in the range from -8 kV to -18 kV.

Observation of threshold energy and hysteresis in high current ion beam guiding and transmission through a micro-glass-capillary

The energy dependent guiding of high current density (0.1–3 A/m2) argon ion beams through a micro-glass-capillary is studied. It is observed that ion transmission through the capillary takes place only if its energy is greater than the retarding potential barrier, which depends upon the amount of charge deposited on the capillary inner wall. Foremost evidence of the observation that the transmitted current exhibits hysteresis with ion energy is presented. Particle in cell simulations carried out by solving Poissons and Newtons force equation self-consistently, agree reasonably well with the experimental results.

Electron random walk and collisional crossover in a gas in presence of electromagnetic waves and magnetostatic fields

This paper deals with random walk of electrons and collisional crossover in a gas evolving toward a plasma, in presence of electromagnetic (EM) waves and magnetostatic (B) fields, a fundamental subject of importance in areas requiring generation and confinement of wave assisted plasmas. In presence of EM waves and B fields, the number of collisions N suffered by an electron with neutral gas atoms while diffusing out of the volume during the walk is significantly modified when compared to the conventional field free square law diffusion; N=1.5(Λ/λ)2, where Λ is the characteristic diffusion length and λ is the mean free path. There is a distinct crossover and a time scale associated with the transition from the elastic to inelastic collisions dominated regime, which can accurately predict the breakdown time (τc) and the threshold electric field (EBD) for plasma initiation. The essential features of cyclotron resonance manifested as a sharp drop in τc, lowering of EBD and enhanced electron energy gain is wel...

Investigation of hysteresis in high current ion beam guiding through micro-glass capillary: time and dimension dependence

Guiding of high current density (0.1–3 A m−2) argon ion beams through a straight and tapered micro-glass-capillary is studied. It is observed that the ion beam requires a minimum threshold energy to pass through the capillary. The variation of beam current with applied voltage exhibits hysteresis due to the charges accumulated on the inner wall of the capillary. The dependence of hysteresis and beam transmission on the time interval between successive experiments including capillary dimensions and geometry are investigated. The guiding capability of a tapered capillary is demonstrated where beam size reduction without compromising total current is achieved. A particle-in-cell simulation code, developed by solving the Poissons equation and the equation of motion simultaneously, shows reasonable agreement with the experimental results.

Characteristics of nonlinear dust acoustic waves in a Lorentzian dusty plasma with effect of adiabatic and nonadiabatic grain charge fluctuation

In this paper, characteristics of small amplitude nonlinear dust acoustic wave have been investigated in a unmagnetized, collisionless, Lorentzian dusty plasma where electrons and ions are inertialess and modeled by generalized Lorentzian Kappa distribution. Dust grains are inertial and equilibrium dust charge is negative. Both adiabatic and nonadiabatic fluctuation of charges on dust grains have been taken under consideration. For adiabatic dust charge variation reductive perturbation analysis gives rise to a KdV equation that governs the nonlinear propagation of dust acoustic waves having soliton solutions. For nonadiabatic dust charge variation nonlinear propagation of dust acoustic wave obeys KdV-Burger equation and gives rise to dust acoustic shock waves. Numerical estimation for adiabatic grain charge variation shows the existence of rarefied soliton whose amplitude and width varies with grain charges. Amplitude and width of the soliton have been plotted for different electron Kappa indices keeping ...

Bump-on-tail instability in space plasmas

Bump-on-tail instability of Langmuir waves propagating in unmagnetized Lorentzian plasma modeled by a Kappa velocity distribution with spectral index κ has been investigated in this paper. Growth rate of this microinstability has been determined analytically from the Langmuir wave dispersion relation. Change in the maximum growth rate with increasing κ has been numerically estimated for different number density and temperature ratios using solar wind data.

Momentum transfer using variable gaseous plasma ion beams and creation of high aspect ratio microstructures

Intense gaseous ion beams are created from compact microwave plasmas confined in a multicusp magnetic field. The wave frequency (ω) is comparable to the electron plasma frequency (ωpe) and ⪢ the ion plasma frequency (ωpi); therefore, the heavier plasma (ions) are least disturbed by the high frequency electromagnetic waves. By changing the experimental gas, ion beams of different species are obtained, which expands the applicability of the ion beams. For the same applied accelerating potential, the controllability of the beam current owing to different velocities for different ionic species adds to the enhanced functionality. The ion beams are utilized to create a variety of microstructures by direct writing on metallic substrates, and microstructures of a high aspect ratio (ar = line width/depth) in the range of 100–1000 are created by varying the ion species and writing speed. For fixed species (Ga) and low current (1 pA) focused ion beam systems, typically ar ∼ 2.0 to 9.3 may be realized in a single bea...

Rapid measurement of charged particle beam profiles using a current flux grating

The principle and physics issues of charged particle beam diagnostics using a current flux grating are presented. Unidirectional array of conducting channels with interstitial insulating layers of spacing d is placed in the beam path to capture flux of charge and electronically reproduce an exact beam current profile with density variation. The role of secondary electrons due to the impinging particle beam (both electron and ion) on the probe is addressed and a correction factor is introduced. A 2-dimensional profile of the electron beam is obtained by rotating the probe about the beam axis. Finally, a comparison of measured beam profile with a Gaussian is presented.