Low Beam Currents Research Articles

High resolution channeling STIM in a thin crystal

Scanning Transmission Ion Microscopy (STIM) has been combined with Channeling Contrast Microscopy (CCM) to examine damage in a thin crystal (4 μm thick Si) with high spatial resolution (200 nm). Such measurements require very low beam currents (<1 fA), whereas the crystal was damaged only by relatively large currents (nA).

An examination of the electronic structure of SiO by soft x-ray emission spectroscopy

The electronic structure of SiO was examined by soft X-ray spectroscopy. In particular, the SiL 23 for this compound was obtained for the first time. The key to this measurement was the use of a highly efficient spectrometer which enables one to use low electron beam currents and obtain spectra in relatively short periods of time. The results indicate that SiO cannot be explained by a macroscopic mixture model of Si and SiO 2, but rather is better described by a model which assumes that SiO has a unique structure.

Application of gold plated edges for the measurement of the electron beam diameter

Gold plated edges fabricated on bulk and thin substrates are used as targets in electron beam diameter measurements and for system calibration in a Cambridge Instruments E-Beam Lithography machine. A target of 0.85 μm thick gold plated on a 2 μm thick silicon nitride membrane is shown to significantly enhance the signal to noise ratio and also improves the reliability of diameter measurements for low beam currents at all beam energies.Typically a statistical accuracy of 5 nm (1 sigma value) for a beam diameter of 38 nm is achieved. Monte Carlo simulation has also been used to quantify the electron scattering effects which take place in the target.

Automatic mark detection in electron beam nanolithography using digital image processing and correlation

A method of mark detection for a vector scan eCF1-beam nanolithography system has been developed, in particular, taking into account that fairly low beam currents are used in high resolution writing. The digitally stored image of the scanned mark region is compared to a computer generated template using an advanced correlation technique. This method does not require a threshold determination. It exhibits high immunity to noise and poor contrast in the mark signal. The achieved accuracy is better than one beam step size (LSB) even for partially damaged marks. Due to specially chosen mark shapes, the probability of detecting wrong features is low. This paper describes the method and its hardware implementation along with experimental results for accuracy and repeatability of the mark position evaluation.

Principles and Practice of On-Line Data Acquisition For Transmission Electron Microscopy

For about 15 years now TV-camera based image acquisition systems have been used in many laboratories. These TV-systems facilitate focusing and stigmating of the instrument at low beam currents and the recording of dynamic events in the microscope with a video-tape recorder. Recently, digital image processing systems have become available which make image accumulation or averaging possible, or which can correct for uneven illumination conditions. This simple image processing is the basis for further image analysis (Automatic control of TEM parameters or real analysis of the specimen).The bottle neck in on-line data acquisition for a TEM is the image pick-up system. Compared to a photographic plate, which has about 10.000 by 10.000 resolved pixels, the resolution of commercial, TV-based camera systems is very poor, in the best case about 500 by 500 pixels. The poor resolution of TV-systems restricts the use of image analysis to objects which do not need large image fields.Fig. 1 illustrates the principles of TV-based image pick-up systems.

A long-pulse millimeter-wave free electron maser

An experimental free electron maser (FEM) facility is described which operates in the millimeter-wave regime. The experiment demonstrates that high gain and beam extraction efficiency can be obtained with low-current but high-quality electron beams. In the oscillator configuration, microwave powers of 110 kW were achieved with a 300-kV, 4-A beam, corresponding to a beam extraction efficiency of 9.2%. At low beam currents, with the system operating in the linear regime, the FEM gain curve was measured. The experiment uses a Pierce-type gun with a calculated beam emittance of 10.7-mm mrad at 300 kV and 4 A. The wiggler is of the iron permanent magnet hybrid design. The experiment is intended as a first step towards the development of continuous-wave (CW) devices that use beam energy recovery to make use of low-current, high-voltage power supplies. >

Apparatus for positron annihilation-induced Auger electron spectroscopy

The apparatus used in the first direct measurement of energy spectra of positron annihilation-induced Auger electrons is described. The apparatus consists of a magnetically guided low-energy positron beam, a UHV surface preparation and characterization chamber, and an energy spectrometer. The spectrometer includes a trochoidal monochromator and a microchannel plate detector. A permanent magnet is mounted behind the sample to produce a field gradient which redirects the outgoing Auger electrons along the spectrometer axis. The combination of trochoidal monochromator and permanent magnet permits the measurement of the total kinetic energy of Auger electrons with an effective angular acceptance of ∼2π. The large angular acceptance and single particle detection capability of this spectrometer make it possible to perform Auger measurements using extremely low incident beam currents (∼10−15 A), and may make it useful in other low signal experiments. The spectrometer response function is modeled and compared to experimental results for different values of the magnetic field gradient.

N2O laser pumped by a self-sustained volume discharge

An investigation was made of the parameters of an N2O laser pumped by a self-sustained volume discharge with the aim of improving the output characteristics of this laser. The discharge was ignited as a result of a preliminary filling of the discharge gap with electrons and preionization of the gas in the gap by a low-current beam of accelerated electrons. In this way it was possible to reach an output energy up to 465 J, a specific output energy up to 16.4 J · liter − 1 · atm − 1, and an efficiency of 6.7%, all of which exceeded the earlier values reported for N2O lasers pumped by a self-sustained volume discharge.

Measurement of bunch lengthening in TERAS

The bunch length was measured by using a highly sensitive streak camera with a time resolution of 2 ps. It was found that fine structures appeared in the electron bunch shape and that the shapes of electron bunches were described by a Gaussian distribution on the average. The dependence of bunch length on beam current was measured for an electron beam of 607 MeV. The bunch length was well represented by a power function of beam current with an exponent of 0.197 at currents lower than 35 mA or 0.30 at high currents. The experimental results suggest that the longitudinal coupled-bunch beam instability takes place at low beam currents and the turbulent instability dominates at high currents. It was also found from the three-dimensional bunch shape measurements that the bunch shape tended to blow up at high currents.

Wave phenomena preceding and during a beam‐plasma discharge

The waves and associated phenomena which are generated prior to and during the beam‐plasma discharge (BPD) in the waves on magnetized beams and turbulence (WOMBAT) apparatus are described. These range in frequency from below the ion gyrofrequency of a few kilohertz to strong harmonics of the electron plasma frequency above 1 GHz. The low‐frequency ion waves appear to be driven by E × B forces arising from the nonneutrality of the electron beam‐plasma column at low beam currents (Ib) or low gas pressures (P). As Ib increases, the motion becomes chaotic after a sequence of wave period doublings resulting in the generation of subharmonies. Windows of coherent motion are observed within the chaotic regions. Further increases in Ib lead to increases in ionizing collisions and when the background plasma density becomes sufficiently high, waves around the electron plasma frequency can be generated. Initially, these waves are modulated by the gross density perturbations of the large‐amplitude ion waves, but subsequently they also increase to large amplitudes: eϕ/kT ∼ 1. This is sufficient to accelerate the background electrons to ionizing energies and produce the BPD. At low pressures this occurs for fpe/fce ∼ 0.5 (BPD1) with the process occurring in short bursts (≲100τpe). Increasing Ib produces a sudden jump in the frequency to ∼1.5fce and a further increase in the plasma density and luminosity, which we term BPD2. A sequence of BPDs then occurs as Ib is further increased with characteristic frequencies between gyroharmonics up to fpe ≤ 6fce.

Low-beam-current profile monitoring and track formation in SSNTD CR-39

Abstract The potential of nuclear track detector CR-39 as a means to characterize very low current beams of accelerator protons, deuterons and alpha particles in the energy region of 6 MeV/amu is examined. The beam radial specific density and its energy distribution are determined via semiautomatic video analysis of the recorded tracks for a Radial Ridge Cyclotron intermittent operation. Results show CR-39 to be capable of spectrometric measurements with variable resolution for the particles tested, and as a viable and useful medium for spatial isodose distribution applications at very low beam currents in biological irradiations. Scanning electron microscope analysis of the tracks and reaction rate calculations suggest Rutherford scattering as a likely cause of deviations in the energy discrimination ability for alpha particles.

High-throughput nanolithography using an oxygen-plasma resistant two-layer resist system

LaB6-based electron beam nanolithography systems suffer from low beam current, resulting in low throughput for nanometer-sized patterns. This drawback is further aggravated by the low sensitivity of the standard high-resolution resist [poly(methylmethacrylate) (PMMA)] and by the reduction of intraproximity effect. We have investigated the lithography aspects (contrast, resolution, exposure latitude, throughput, and statistics) of two-layer resist systems consisting of a sensitive positive electron beam resist poly(3-butenyltrimethylsilane sulfone) (PBTMSS) as the top layer, and a variety of materials such as diamondlike carbon film or polyimide as the bottom layer. In these systems, exposed and developed PBTMSS is first oxidized to SiO2, which masks etching of the bottom layer. The pattern is in turn transferred to the substrate either by wet or dry etching using the bottom layer as a mask. Grating patterns with 0.2- and 0.1-μm pitch can be routinely fabricated on semiconductors, and 0.05-μm pitch patterns have been demonstrated. To achieve such high resolution, a low-strength developer is used, but a throughput about 30 to 50 times higher than that of PMMA can still be obtained. Gratings now can be patterned in a few hours over square-centimeter areas. Such a task would require days for PMMA. Implications of our results on lithography system clock rate and resist resolution–sensitivity tradeoff are also discussed.

SAM analysis of fracture surfaces of ceramics

AbstractThe application of high resolution scanning Auger electron spectroscopy on non‐conductive ceramic materials has been demonstrated. Advanced ceramics, e.g. SiC, ZrO2, AIN and Si3N4, have been analysed at a low beam voltage and a low beam current of typically 3 kV and 20 nA. Ion conductive materials were analysed at elevated temperatures up to 550°C. The application on interfacial details of advanced ceramic materials was found to be restricted only in the case of very fine‐grained materials, due to the unsufficient lateral resolution when very weak primary conditions were used to reduce charging.

On the performance of a straight acceleration tube with axial gradient modulation

An earlier inclined field configuration has been replaced by a straight field configuration in the acceleration tube of the 5 MV electrostatic accelerator at the Institute of Nuclear Research, with the aim of improving lateral position stability of the beam at the tube exit. Making use of the so-called decoupling sections proposed by Herb [1], axial gradient modulation has been introduced by conic electrodes arranged periodically in shorted sections along the tube axis, in order to prevent secondary electrons from travelling long distances towards the terminal. Good beam stability has been achieved at high, medium and very low beam currents as a result of this modification. The voltage holding properties did not show any observable change. The results obtained from a series of bremsstrahlung measurements performed with thermoluminescent dose meters in the stack of the machine and around the vessel clearly indicate the main electron suppression properties of this tube geometry.

Particle simulation of a high-power gyrotron oscillator

A self consistent and time-dependent particle code has been developed to simulate the beam-wave dynamics in a gyrotron oscillator. The code is first applied to investigate the effect of the self-consistent field profile on the scaling of the cavity-filling rate with beam current. The fixed-field theory predicts that the transient-wave growth rate depends linearly on the beam current. The simulation results agree with the theoretical prediction at low beam currents. As the beam current increases, the modified field profile changes the nature of the gain scaling from linear to nonlinear. At higher beam currents, the excited wave is observed to exhibit the behavior of the modulated oscillation, due to unequal couplings to the forward- and backward-going waves by the beam. Associated with such amplitude modulation is a periodic variation of the self-consistent field profile. At still higher beam currents, the system becomes chaotic, showing the effect of periodic doubling. Simulation results are presented for the efficiency, the self-consistent field profile, and the scaling of the transient growth rate with the beam current. >

Radiation effects on the catalytic activity of β-Mn02

Abstract Effect of 40-MeV α-particle irradiation on the activity of β-MnO2 has been studied by following the decomposition of H2O2 polarographically. Catalytic activity increased with increasing α-dose and reached a plateau when low beam currents (0.8 μA) were employed. At high beam currents, the catalytic activity showed a maximum with increasing α-dose and then decreased. The decrease in activity at high beam currents could be attributed to both thermal as well as radiation annealing. In the case of γ-irradiation, it was observed that it can produce new catalytic centres as well as lead to the annealing of existing centres. A mechanism involving the intermediate valence states of Mn has been proposed to explain these results.

Water‐dispersible graft copolymer mixtures prepared by electron irradiation. I. Polymerization behavior

AbstractHighly viscous systems made up of 70 wt% epoxy resin dissolved in 30 wt% monomer mixtures of acrylic acid and styrene were irradiated with 1.5 MeV electrons to initiate graft copolymerization. The temperature measurement of the systems reveals that the polymerization reaction takes place both during and after the electron‐beam exposure of about 5‐sec duration. The reaction lasts about 50 sec for a low beam current of 0.6 mA, while for a high beam current of 6.0 mA it lasts merely about 20 sec. The gel permeation chromatography (GPC) measurement of obtained products demonstrates that the molecular weight distribution of ungrafted copolymer of acrylic acid and styrene becomes strikingly broader as the beam current is lowered. The GPC data suggest that the fraction of epoxy resin having a grafted branch increases with an increase in the beam current. All of these results are interpreted in terms of the mobility and the concentration of reactive species such as polymer radicals and low molecular weight free radicals.

Effect of electron beam temperature on the gain of a collective free-electron laser

At sufficiently low beam currents, electron beam temperature effects cause the gains of collective (Raman) regime free-electron lasers to be lower than the predictions of cold beam theory. This gain degradation has been measured as a function of the beam current, the wiggler magnetic field, and the interaction frequency. The measurements are used to estimate the electron beam temperature, and the estimated temperature is close to the temperature predicted by numeric simulations.

Examinations of low-density polymer foams with a low-voltage FESEM

The cell structure of low-density polymer foams is extremely delicate, and the application of conductive coatings for SEM examinations can obliterate much of the fine structural detail. In extreme cases, the structure can be significantly altered by the coating. This problem has been found to be particularly critical in low-density polymer foams. The thin membranes of polymer foams also are exceptionally susceptable to electron-beam damage. Consequently, both the improved low-voltage resolution and the low beam currents now available on an SEM equipped with a field-emission gun (FESEM) have proven to be highly beneficial for the examination of low-density polymer foams. This will be demonstrated for polystyrene foams; equally successful results have been obtained on other types of polymer foams.The simplest technique to examine the cell structure of polystyrene foams is to fracture specimens and examine the fracture surface. Since polystyrene is nonconductive, it must be coated to be examined in the conventional SEM.

Conditioning of the graphite bumper limiter for enhanced confinement discharges in TFTR

A strong pumping effect was observed with plasma operation on the toroidal graphite bumper limiter in TFTR. The pumping effect was induced by conditioning the limiter with a short series (10-20) of low density deuterium or helium initiated discharges. The decay constantfor gas fuelled Ohmic discharges was reduced frombefore conditioning to a minimum ofafter conditioning, which corresponds to a reduction in the global recycling coefficient from about 100% to less than 50%. Coincident with the low recycling conditions, low current neutral beam fuelled discharges showed global energy confinement times which were enhanced by a factor of two over the values obtained with an unconditioned limiter. Two models are proposed for the observed pumping effects: (1) a depletion model, based on pumping of hydrogenic species in the near-surface region of the limiter after depletion of the normally saturated surface layer by (carbon and helium) ion induced desorption; and (2) a co-deposition model, based on adsorption of hydrogenic species in carbon films formed by material sputtered from the limiter during conditioning.