Ion Beam Current Research Articles

A multibeam radio-frequency ion source with a discharge chamber inside a resonator

The design of a multibeam ion source with a discharge chamber inside a half-wavelength radio-frequency (RF) resonator with a spiral load is described. The resonator is excited by a loop insulated from the body of the resonator. The RF oscillator operating at a frequency of 38 MHz is grounded. The use of the resonator facilitates the initiation of a discharge at a low power level. The amplitude-frequency characteristics of the resonator at the operating frequencies of the ion source are presented. The effect of the RF power and the extracting voltage on the ion-beam current has been studied. The experimental results on the ion-beam extraction at a low RF power (<100 W) are presented.

High resolution energy analyzer for broad ion beam characterization

Characterization of the ion energy distribution function (IEDF) of low energy high current density ion beams by conventional retarding field and deflection type energy analyzers is limited due to finite ion beam emittance and beam space charge spreading inside the analyzer. These deficiencies are, to a large extent, overcome with the recent development of the variable-focusing retarding field energy analyzer (RFEA), which has a cylindrical focusing electrode preceding the planar retarding grid. The principal concept of this analyzer is conversion of a divergent charged particle beam into a quasiparallel beam before analyzing it by the planar retarding field. This allows analysis of the beam particle total kinetic energy distribution with greatly improved energy resolution. Whereas this concept was first applied to analyze 5-10 keV pulsed electron beams, the present authors have adapted it to analyze the energy distribution of a low energy (<or=1 KeV) broad ion beam. In this paper we describe the RFEA design, which was modified from the original, mainly as required by the specifics of broad ion beam energy analysis, and the device experimental characterization and modeling results. Among the modifications, an orifice electrode placed in front of the RFEA provides better spatial resolution of the broad ion beam ion optics emission region and reduces the beam plasma density in the vicinity of analyzer entry. An electron repeller grid placed in front of the RFEA collector was found critical for suppressing secondary electrons, both those incoming to the collector and those released from its surface, and improved energy spectrum measurement repeatability and accuracy. The use of finer mesh single- and double-grid retarding structures reduces the retarding grid lens effect and improves the analyzer energy resolution and accuracy of the measured spectrum mean energy. However, additional analyzer component and configuration improvements did not further change the analyzed IEDF shape or mean energy value. This led us to conclude that the optimized analyzer construction provides an energy resolution considerably narrower than the investigated ion beam energy spectrum full width at half maximum, and the derived energy spectrum is an objective and accurate representation of the analyzed broad ion beam energy distribution characteristics. A quantitative study of the focusing voltage and retarding grid field effects based on the experimental data and modeling results have supported this conclusion.

Indium tin oxide thin films prepared by ion beam assisted deposition technique at different ion beam currents

AbstractIndium tin oxide (ITO) films have been deposited onto glass substrates at room temperature by ion beam assisted deposition technique (IBAD) at different ion beam currents (80–120 mA). The effect of the ion beam current on films properties has been studied. The films prepared at low ion beam current (80 mA) show an almost amorphous structure. As the ion beam current is increased, films show a preferred orientation along the (222) direction. AFM measurements show that the surface RMS roughness of the film increases as the current goes up. The optical properties have been studied by measuring the transmittance. It has been found that the film prepared at 100 mA current has the highest transmittance. The optical constants of the films have been calculated by fitting the transmittance. The Hall measurements also showed that the film prepared at 100 mA current has one of the lower electrical resistivities. FTIR measurements show that the film prepared at 100 mA current has the highest infra‐red reflectance. All the measurements show the 100 mA ion beam current can produce good quality ITO films. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

De-tethering of high aspect ratio metallic and polymeric MEMS/NEMS parts for the direct pick-and-place assembly of 3D microsystem

We report our study on several de-tethering methods for various high aspect ratio metallic and polymeric MEMS and NEMS parts including 5:1 aspect ratio 50 μm thick metallic (nickel) MEMS parts, 3:1 aspect ratio 1 μm thick sub-micron (350 nm) feature size metallic NEMS actuators, and 10:1 aspect ratio 100 μm thick polymer/metal bi-layer MEMS actuators. Resistive heating was found to be effective for the de-tethering of high aspect ratio metallic MEMS parts. In order to de-tether metallic NEMS parts and polymer/metal bi-layer devices, we performed the milling of tethers using a focused ion beam. Very low current (20 pA) ion beam was found to be effective means of de-tethering the metallic NEMS parts. Relatively larger current (0.3–20 nA) ion beam was found to be good for the polymer/metal bi-layer parts. We demonstrated 3D assembly and complete packaging of the de-tethered high aspect ratio metallic and metal/polymer bilayer MEMS parts.

The surface charge effect by using boron ions implantation in p-channel MOSFETs fabricated

This article reports on the effects of surface charge on bare wafers and p-channel MOSFETs by a positive ion beam accompanied by an electron beam current for surface charge neutralization. Without the negative electron beam the films show a higher sheet resistance and the pMOSFETs exhibit a lower threshold voltage, a lower breakdown voltage and a lower gain factor. If the electron beam current is equal to or higher than the ion beam current of 6mA, the uniformity of sheet resistance, fluctuations of breakdown voltage and gain factor are significantly improved by controlling the charge neutralization in this experiment. It prevents the positive charges from penetrating the poly-gate and causing catastrophic damage in the gate oxide layer. The sheet resistance deviation for the samples with a capped photoresist is higher than that for the bare wafer because the insulating property of the photoresist enhances the wafer surface charge accumulation and thus repels the subsequently implanted ions. The thinner gate oxide layer leads to larger deviations of threshold voltage, breakdown voltage and gain factor due to the imbalance of surface charge.

Microwave power absorption coefficient of an ECR xenon ion thruster

A 20-cm diameter xenon ion thruster with electron cyclotron resonance (ECR) discharge generates 30 mN of thrust at a total electric power consumption of 1 kW for spacecraft propulsion by ejecting 1.1 keV ion beam. By optimizing the discharge chamber length, magnetic field and propellant flow injection, ion beam currents of 500 mA at a microwave power of 100 W had been obtained at a frequency of 4.25 GHz with SmCo magnets arranged on a flat discharge chamber. The performance was highly dependent on the propellant injection method that affects electron-heating process. Two-dimensional microwave E-field distributions inside the discharge chamber were experimentally investigated for the best and the worst injector layouts. Microwave power absorption coefficient was estimated using the E-field distributions with and without plasma discharge. The coefficient decreases as the microwave power decreases and electron density gets close to an ECR cutoff density in all cases. The worst injector layout showed larger reflection and smaller absorption coefficient even at small beam currents. In the best configuration, microwave reflection was sufficiently smaller than 10% and 70–90% of the microwave power launched into the discharge chamber was absorbed by plasma electrons.

Generation of charged nanoparticles in point ion sources

The conditions of formation of a dispersed phase in a point source of indium and tin ions have been studied. Charged droplets are generated in a threshold manner at a certain value of the ion beam current. This is accompanied by the excitation of ion current oscillations at a frequency of 15–20 MHz as a result of the capillary instability of the conducting liquid surface in a strong electric field. The dimensions of emitted nanoparticles determined using an electron microscope are continuously distributed in a range of 2–20 nm, and their average specific charge amounts to 5 × 104 C/kg. Such charged nanodroplets can be used to create various surface quantum structures. The possibility of obtaining nanoparticles of semiconductor materials using a modified source with a porous electrode is discussed.

A new approach for preventing charging up of soft material samples by coating with conducting polymers in SIMS analysis

Dynamic secondary ion mass spectroscopy (SIMS) analysis of soft materials such as polymer or biomaterial is one of challenging subjects due to the charge up effect brought from the irradiation of a primary ion beam, hampering the collection of secondary ions. Conventional methods against the charging up are the electron beam irradiation for charge compensation and surface coating with metal, normally gold. Those methods require a compromise analytical condition, reducing the primary ion beam current to suppress the range of the charging, which degrading the performances of the SIMS analyses. We have proposed that a thicker conductive layer, capable of delocalizing the charge onto the surface, should be put on a soft insulator sample to avoid charging up. The depth profile of the hair sample coated wholly with a polythiophen-based conducting polymer was successfully measured in longer time without any charging up even in the maximum current of the oxygen primary ion beam (O 2 +: 7.5 keV, 400 nA) or using an electron beam compensation system. Thus, the proposed method coating with a conductive organic polymer against the charging issue would be expected as a breakthrough on SIMS analysis.

Achieving sub-1.6-nm resolutions of a low-voltage microscopic focused-ion-beam system not involving aberration correction

The final, second, part is presented of an investigation into possible methods of achieving the ultimate resolution for focused-ion-beam (FIB) microscopes. Our strategy differs from the well-known FIB technologies in the following respects: (i) It employs an advanced ion source of effective radius ≈1 nm. (ii) The ion energy at the target is as low as −300 eV. (iii) Potential secondary emission of ions and electrons is used as a means of imaging at the nanometer scale. The version in part 1 relies on a chromatic-aberration correction system based on a combined electromagnetic mirror. As an alternative, we here propose reducing the ion-optical system to a scale of tens of micrometers. It is shown by computer simulation that the resolution thus obtained should be as good as the one reported in part 1 (∼1.6 nm). Under optimal imaging conditions, the ion-beam current on a target is found to depend only on the properties of the ion source and to be the same as those of macroscopic FIB systems regardless of their operating voltage.

Generation of space charge compensated low energy ion flux

The results of an experimental study of low-energy (<200 eV) ion flux generation with space charge neutralization are presented. Argon was used as a working gas. The working gas pressure in the vacuum chamber was 2-4 x 10(-2) Pa. Ion beam was extracted from the hollow cathode of main discharge plasma by a single mesh extractor with subsequent deceleration of ions to a required energy in a layer between the mesh and the beam plasma. The ion beam current was measured on the collector located on the distance of 30-60 cm from the discharge system. The penetration of electron component from the main discharge plasma through the mesh into the region of the ion beam drift space was realized by potential barrier reduction, in conditions of the optimal extractor potential with respect to the hollow cathode. The space charge neutralization of low-energy ion beam resulted in drift space plasma potential reduction and ion beam current growth. At the main discharge current of 1 A and main discharge voltage of 300 V, the ion beam current of up to 100 mA with the ion energy of 50-150 eV was obtained.

Acceleration of ampere class H− ion beam by MeV accelerator

The H(-) ion accelerator R&D to realize the international thermonuclear experimental reactor neutral beam is ongoing at Japan Atomic Energy Agency (JAEA). The required performance for the prototype MeV accelerator developed at JAEA is 1 MeV, 500 mA (current density of 200 A/m(2)) H(-) ion beam at the beamlet divergence angle of less than 7 mrad. Up to 2005, 836 keV, 146 A/m(2) H(-) ion beam was successfully accelerated as the highest record of the current density at MeV class energy beams. In the present work, high current negative ion beam acceleration test was performed by increasing the beam extraction apertures from 3 x 3 (9 apertures) to 3 x 5 (15 apertures). By fixing the air leak at the source chamber due to backstream ions as well as the improvement of voltage holding capability by a new fiber reinforced plastic insulator ring, the performance of the MeV accelerator was improved. So far, H(-) ion beam of 320 mA was successfully accelerated up to 796 keV with the beam divergence angle of 5.5 mrad. The accelerated drain current including the electron reaches close to the power supply limit for the MeV test facility. The heat flux by the backstream ion during the above beam acceleration was estimated to be 360 W/cm(2). The Cs leakage to the accelerator during the test campaign (Cs total input of 5.0 g) was 0.26 mg (7.0 microg/cm(2)). This is considered to be the allowable level from the viewpoint of voltage holding.

Direct plasma injection scheme with beam extraction in a radio frequency quadrupole linac cavity

Direct plasma injection scheme (DPIS) is a recently developed idea about beam injection into a radio frequency quadrupole (RFQ) linac to obtain high current pulsed heavy ion beam. Since a laser ion source is directly connected to an RFQ linac, a low energy beam transport is not needed. An ion beam is extracted at the edge of the cavity. We shifted the extraction position in the RFQ cavity to improve the beam loss problem due to beam diverging before arriving at the entrance of the RFQ electrode region. In this scheme, ion beam is extracted at the entrance of the RFQ electrode region and the extracted beam is immediately captured by a rf quadrupole electric field. A plasma electrode is inserted in the accelerator resonant cavity. It is required to avoid serious effect on the acceleration characteristics of the RFQ linac. The rf properties were measured using a network analyzer to verify the effect. The shifts of resonant frequency were negligibly small by inserting the electrode. The measurements of the electric field distribution along the beam axis in the RFQ linac showed that the plasma electrode had little effect on it. The result of beam acceleration experiment showed that the beam current was increased by about 20%. It was confirmed that the beam loss was reduced by this scheme.

Development of an all-permanent-magnet microwave ion source equipped with multicusp magnetic fields for high current proton beam production

An all-permanent-magnet (APM) microwave hydrogen ion source was developed to reduce the size and to simplify structure of a conventional solenoid coil microwave ion source developed for reliability improvement of high current proton linac application systems. The difficulty in developing the APM source was sensitive dependence of the source performance on axial magnetic field in the microwave discharge chamber. It was difficult to produce high current proton beam stably without precise tuning of the magnetic field using solenoid coils. We lowered the sensitivity using multicusp magnetic fields for plasma confinement at the discharge chamber sidewall of the source. This enabled stable high current proton beam production with the APM microwave ion source with no tuning coil. The water cooling and the power supply for the coils are not necessary for the APM source, which leads to better reliability and system simplification. The outer diameter of the APM source was around 300 mm, which was 20% lower than the coil source. The APM source produced a maximum hydrogen ion beam current of 65 mA (high current density of 330 mA/cm(2), proton ratio of 87%, and beam energy of 30 keV) with a 5 mm diameter extraction aperture, pulse width of 400 micros, and 20 Hz repetition rate at 1.3 kW microwave power. This performance is almost the same as the best performances of the conventional coil sources. The extracted ion beams were focused with electrostatic five-grid lens to match beam to acceptance of radio-frequency quadrupole linacs. The maximum focused beam current through the orifice (5 mm radius) and the lens was 36 mA and the 90% focused beam half-width was 1-2 mm.

Linear ion source with closed drift and extended acceleration region

Ion source with closed drift, which is caused by E x B field, and extended acceleration region is discussed. Though conventional circular-type closed drift ion source has advantages of high efficiency of gas ionization and low ion beam energy, there is a limitation in enlarging the beam size. Linear ion source with horse-track shape with 270 mm ceramic channel width is newly designed and tested. Inert gas (Ar) and reactive gas (O(2)) are discharged. Discharge is ignited with voltage of 90 V. Discharge current is proportional to discharge voltage and increases up to 16.3 A in argon and 15.6 A in oxygen at discharge voltage of 320 V. Extracted ion beam current is also proportional to discharge voltage and is saturated after 280 V for both gases. It is measured up to 0.78 mA/cm(2) in argon beam and 0.73 mA/cm(2) in oxygen beam at a distance of 100 mm from the ion source. Argon ion beam shows better space uniformity than oxygen across the beam extraction region.

Investigation of Factors That Affect the Sensitivity of Accelerator Mass Spectrometry for Cosmogenic 10Be and 26Al Isotope Analysis

We report experiments designed to improve accelerator mass spectrometry (AMS) of (10)Be and (26)Al for a wide range of geological applications. In many cases, the precision of the AMS isotope ratio measurement is restricted by counting statistics for the cosmogenic isotope, which are in turn limited by the intensity of AMS stable ion beam currents. We present data obtained at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratories (LLNL) indicating that AMS ion beam currents are impacted by certain elemental impurities. For (10)Be analysis, the AMS ion beam current is most adversely affected by the presence of titanium (which can be challenging to separate chemically during sample preparation because of its tendency toward stable refractory forms) and aluminum (which can coelute with beryllium during cation exchange chromatography). In order to minimize impurities that suppress AMS ion beam currents, we evaluate, using inductively coupled plasma atomic emission spectroscopy (ICP-AES), a widely used chemical separation protocol involving a multiacid digestion scheme, preseparation elemental analysis, anion exchange chromatography, ad hoc selective precipitation, cation exchange chromatography, and postseparation elemental analysis.

Status of high current ion source operation at the GSI accelerator facility

Vacuum arc ion sources, Penning ion sources, and filament driven multicusp ion sources are used for the production of high current ion beams of a variety of metallic and gaseous ions at the GSI accelerator facility. For accelerator operation, the ion sources have to provide a stable beam over a long period of time with an energy of 2.2 keV/u and a maximum mass over charge ratio of 65. The status of beam time operation at the high current injector is presented here giving an outline on important ion source data, such as ion beam current, ion beam spectrum, transversal emittance, life time, duty factor, and transmission along the low energy beam transport section.

High current ion beam transport using solenoids

In the framework of the future project FAIR several upgrade programs and construction of new facilities are in progress such as the U(4+) upgrade for the existing high current injector and the new 70 MeV proton injector. For both injectors solenoids in the low energy beam transport section are foreseen to inject the beam into the following rf accelerator. The paper presents beam quality measurements of high current ion beams behind a solenoid using a slit-grid emittance measurement device, viewing targets, and a pepper pot measurement device at the high current test bench at GSI.

小型イオンスラスタの推進性能における形状依存性

The effects of antenna shape and magnetic field configuration on thrust performance in a 30W class miniature microwave discharge ion thruster were investigated with the objective of improving its thrust performance. The ion beam currents were measured for three different antenna configurations: disc, star, and cross. The results showed that the optimum antenna shape depends on the desired mass flow level because of the tradeoff between microwave-plasma coupling efficiency and surface area recombination. The ion beam current with various magnitude of magnetic field was measured, since magnetic confinement and microwave-plasma coupling could be affected by the magnetic field configuration. The ion beam current was increased with increasing the number of magnets, though under too much magnitude of magnetic field, the thruster cannot be ignited. So, there is an optimum magnitude of magnetic field. The thrust performance of the miniature microwave discharge ion thruster, i.e., propellant utilization, and ion beam production cost were 0.97 and 600W/A, respectively, at mass flow rate of 0.018mg/s, and incident microwave power of 8W.

Reactive sputtering deposition of SiO2 thin films

SiO2 layers were deposited in a UHV chamber by 1 keV Ar+ ion sputtering from a high purity silicon target, using different values of the oxygen partial pressure (5?10-6-2?10-4 mbar) and of the ion beam current on the target (1.67-6.85 mA). The argon partial pressure during operation of the ion gun was 1?10-3 mbar. The substrate temperature was held at 550?C and the films were deposited to a thickness of 12.5-150 nm, at a rate from 0.0018-0.035 nm s-1. Structural characterization of the deposited thin films was performed by Rutherford backscattering spectrometry (RBS analysis). Reactive sputtering was proved to be efficient for the deposition of silica at 550?C, an oxygen partial pressure of 2?10-4 mbar (ion beam current on the target of 5 mA) or, at a lower deposition rate, ion beam current of 1.67 mA and an oxygen partial pressure of 6?10-5 mbar. One aspect of these investigations was to study the consumption of oxygen from the gas cylinder, which was found to be lower for higher deposition rates.

Effects of magnetic field gradient on ion beam current in cylindrical Hall ion source

The effects of the magnetic gradient on the ion beam current in an end Hall-type ion source with a magnetic mirror field are investigated. In a cylindrical Hall ion source in which a cylindrical magnetic ring other than a regular magnetic pole is shortened and centrally inserted, a mirror magnetic field profile can be formed around the annular anode. A positive-negative variable magnetic gradient is shown experimentally to enhance ionization; the ionization efficiency is substantially affected by the different magnetic gradient. The high ionization results in 60% efficiency in the conversion of discharge current to ion beam current. The experimental results and interpretation of the effects are presented in this paper.