Low Energy Ion Gun Research Articles

Development of focused ion beam system attached to floating type low energy ion gun for surface finishing

Ion beams of several tens of keV are widely used for the analysis and materials processing, but the occurrence of damage to the surface is of concern. For surface treatment of the ion beam processed sample to remove the damaged layer, a floating-type low-energy ion gun (FLIG) producing 100–500 eV ions at a high current intensity has been developed. This ion gun consists of a permanent magnet-aided electron impact-type ionization cell, an extractor, and a cylindrical retarding immersion lens. The gun produced current intensities of order 1.81 μA (current density of 7.9 μA/cm2) for 100 eV ions. The entire system has been compactly constructed with an ionization cell that has an outer diameter of less than 3.3 cm and a lens system that has a length of 24 cm. In addition, a focused ion beam (FIB) system attached to the FLIG has been developed to remove damaged layers formed by previous high energy etching. The thickness of the damaged layer induced on a GaAs (111) surface by 25 kV Ga+ ions was of order 24 nm, assessed using a transmission electron microscope (TEM). This damaged layer was reduced to ∼2.6 nm by subsequent bombardment by low energy 200 eV Ar+ ions.

Novel Floating Low-Energy Ion Gun for the storing matter instrument

A novel Floating-Low Energy Ion Gun (FLIG) delivering finely focussed low-energy ion beams for high depth-resolution analyses has been developed at SAM. The FLIG operates with Xe +, Ar + or O 2 + ion beams produced with a duoplasmatron ion source. In a first step, the ion beam is filtered by a Wien filter. The beam is then focussed and shaped in a floating column composed of three electrostatic lenses as well as several deflectors, stigmators and apertures. In a last step, the ions are decelerated to their final impact energy and positioned and/or raster-scanned on the sample area to be analysed. The gun can be operated at energies from 100 eV to 10 keV with beam currents up to several hundred nA and spot diameters in the μm range. The ion beam can be raster-scanned over areas of up to 1 mm × 1 mm on the sample surface. This paper provides a complete description of the Storing Matter FLIG (concept, ion optical calculations and experimental results).

Study of the useful yield of the Storing Matter technique using Ge(1 0 0) as a sputter target

Storing Matter is a new analytical technique for organic and inorganic materials, which tries to circumvent the well-known matrix effect in SIMS. This technique consists of separating the sputtering of the sample from the subsequent analysis steps. Thus, the sample to be analysed is sputtered with a focused ion beam produced by a floating low-energy ion gun (FLIG) and the particles emitted under the ion impacts are deposited at a sub-monolayer level on a well-known collector. The collector with the deposited material is then analysed in a second step by SIMS (dynamic and static mode). The main advantage of this new technique is to improve the sensitivity and the quantification of the SIMS analysis. All the processes, including all the sample and collector transfers, are performed in UHV conditions. This paper presents preliminary results obtained on the Storing Matter prototype instrument developed in our laboratory. The sputtering of a (1 0 0) Ge wafer by O 2 + ions was used as a model system to study the influence of using different collector surfaces (W, Ta and Al) on the Storing Matter useful yield.

Zero-phonon lines of nitrogen-cluster states in GaN x As1-x : H identified by time-resolved photoluminescence.

The incorporation of only a few percentage of nitrogen atoms into a III–V host material, e.g., GaAs or GaP, leads to dramatic changes in the electronic and optical properties of these compounds [1]. These materials show a very strong band gap bowing with increasing nitrogen concentration [2, 3]. Post-growth annealing of these dilute nitrides induces a noticeable blue-shift of the band-gap [1], while post-growth hydrogenation effectively passivates the incorporated nitrogen atoms due to the formation of various N–H complexes [4, 5]. Both post-growth effects have been studied intensively [1]. Numerous experimental results have shown that the nitrogen-induced disorder leads to the formation of various localized states related to isolated N atoms, NN-pair states, and nitrogen-clusters (NC) even in the very dilute regime (nitrogen concentrations lower than 0.3%) [6, 7]. The modification of the electronic properties, e.g., the electron effective mass and the gyromagnetic factor of electrons [8, 9], by these localized states cannot be explained in the framework of k p models taking into account only the interaction between the host conduction-band (CB) and the localized state of a single nitrogen atom [10, 11]. The theoretical description of the band-formation process in dilute nitrides must be modified to include various N cluster states [12, 13]. In former photoluminescence (PL) experiments the various optical transitions corresponding to different NC states were often attributed to LO-phonon replicas [6, 14]. In this article, we use time-resolved photoluminescence (TRPL) measurements to clarify that actually only zerophonon lines are observed in our PL spectra. Two GaNxAs1-x epitaxial layers of 0.5 lm thickness grown by metal-organic vapor-phase epitaxy are studied. The effective nitrogen concentration xeff is determined by the free exciton recombination energy (see Ref. [8]) to be 0.049% and 0.111%, respectively. The sample containing x = 0.111% of nitrogen is irradiated by a low-energy ion gun with different hydrogen doses (3.5 10–5 10 ions/cm) at 300 C. This reduces the effective nitrogen concentration xeff of the hydrogenated samples to about 0.083% and 0.002%, respectively [15, 16]. The samples are mounted onto a cold finger of a helium cryostat. A 100 fs Ti/Sapphire laser centered at 760 nm with a repetition rate of 80 MHz is used for optical excitation. The emitted light is collected in a backscattering geometry and is dispersed by a spectrometer with a spectral resolution of 0.5 nm. TRPL measurements are performed at low excitation densities (qexc * 8 W/cm ) and low temperatures (T B 70 K) since only at these experimental conditions the emission of various NC states are clearly resolved. The time resolution of the cooled S1 streak camera in these experiments is 15 ps. K. Hantke S. Horst (&) S. Chatterjee P. J. Klar K. Volz W. Stolz W. W. Ruhle Faculty of Physics and Material Sciences Center, Philipps-Universitat Marburg, Renthof 5, 35032 Marburg, Germany e-mail: swantje.horst@physik.uni-marburg.de

SIMS depth profiling and TEM imaging of the SIMS altered layer

A reduction in the scale of semiconducting devices has created many challenges in SIMS depth profiling. With fewer and shallower dopant atoms, depth resolutions have had to improve significantly especially in the first few nanometres. It is widely accepted that by using lower primary ion beam energies to minimise transient widths and beam induced mixing that the depth resolution can be optimized. The development of the Floating Low-energy Ion Gun (FLIG) has enabled SIMS to operate with beam energies in sub-keV regions enabling depth profiling of shallow features. Accurate depth profiling also relies on accurate depth calibration. Typically depth calibration is performed by measuring SIMS crater depths and assuming a constant sputter rate thus incurring an error. TEM analysis in conjunction with SIMS has successfully been used to depth calibrate a test structure with an error <0.5 nm. The top 5 nm of a surface is a problematic region for SIMS depth profiling due to depth calibrations issues. The current work is to obtain an accurate depth profile of a 5 nm thick altered layer using TEM to assist depth calibration. With the depth resolution being dependent on the altered layer, the analysis of sub-keV altered layers will give an insight into the ultimate depth resolution attainable.

Caesium sputter ion source compatible with commercial SIMS instruments

A simple design for a caesium sputter cluster ion source compatible with commercially available secondary ion mass spectrometers is reported. This source has been tested with the Cameca IMS 4f instrument using the cluster Si n − and Cu n − ions, and will shortly be retrofitted to the floating low energy ion gun (FLIG) of the type used on the Cameca 4500/4550 quadruple instruments. Our experiments with surface characterization and depth profiling conducted to date demonstrate improvements of analytical capabilities of the SIMS instrument due to the non-additive enhancement of secondary ion emission and shorter ion ranges of polyatomic projectiles compared to atomic ions with the same impact energy.

A new time-of-flight instrument for quantitative surface analysis

A new generation of time-of-flight mass spectrometers that implement ion sputtering and laser desorption for probing solid samples and can operate in regimes of laser post-ionization secondary neutral mass spectrometry and secondary ion mass spectrometry is being developed at Argonne National Laboratory. These new instruments feature novel ion optical systems for efficient extraction of ions from large laser post-ionization volumes and for lossless transport of these ions to detectors. Another feature of this design is a new in-vacuum all-reflecting optical microscope with 0.5-μm resolution. Advanced ion and light optics and three ion sources, including a liquid metal ion gun (focusable to 50 nm) and a low energy ion gun, give rise to an instrument capable of quantitative analyses of samples for the most challenging applications, such as determining elemental concentrations in shallow implants at ultra-trace levels (for example, solar wind samples delivered by NASA Genesis mission) and analyzing individual sub-micrometer particles on a sample stage (such as, interstellar dust delivered by NASA Stardust mission). Construction of a prototype instrument has been completed and testing is underway. A more advanced instrument of similar design is under construction. The overall design of the new instrument and the innovations that make it unique are outlined. Results of the first tests to characterize its analytical capabilities are presented also.

Novel floating‐type low‐energy ion gun for high‐resolution depth profiling in ultrahigh vacuum

AbstractA floating‐type low‐energy ion gun (FLIG) has been developed for high‐resolution depth profiling in ultrahigh vacuum (UHV). This UHV‐FLIG allows Ar+ ions of primary energy down to 50 eV to be provided with high current intensity. The developed UHV‐FLIG was sufficiently compact, being ∼30 cm long, to be attached to a commercial surface analytical instrument. The performance of the UHV‐FLIG was measured by attaching it to a scanning Auger electron microprobe (JAMP‐10, Jeol), the base pressure of which in the analysis chamber was ∼1 × 10−7 Pa. The vacuum condition of ∼5 × 10−6 Pa was maintained during operation of the UHV‐FLIG without a differential pumping facility. Current density ranged from 41 to 138 µA cm−2 for Ar+ ions of primary energy 100–500 eV at the working distance of 50 mm. This ensures a sputtering rate of ∼10 nm h−1 with 100 eV Ar+ ions for Si, leading to depth profiling of high resolution in practical use. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Construction and performance of a low energy ion gun

We report the conversion of a conventional high energy ion gun into a low energy ion gun. The conversion procedure described here maintains the ion current capability of the high energy gun along with its good spatial and ion energy distribution characteristics. Several low energy ion guns have been described previously in the literature [J. T. Yates, Jr., Experimental Innovations in Surface Science (Springer-Verlag-AIP, New York, 1998), pp. 307–329] but all require the construction of new ion optics, whereas this approach utilizes existing ion optics and electronic control capabilities of a commercial ion gun. This ion gun may be employed for studies where it is desired to use ions as reactive species without significant sputter damage of the target.

Floating-type Compact Low-energy Ion Gun. Basic Performance for High-resolution Depth Profiling.

A floating-type low-energy ion gun (FLIG) producing 100500 eV ions at a high current intensity has been developed. This ion gun consists of a permanent magnet-aided electron impact-type ionization cell, an extractor, and a cylindrical retarding immersion lens. The gun was assessed and produced typical performance levels of current intensities of 1.81 μA (current density of 7.9 μA/cm2) for 100 eV. The entire system has been compactly constructed with an ionization cell that has an outer diameter of less than 3.3 cm and a lens system that has a length of 24 cm. Conventional surface analytical instruments are attached via an ICF 70 Conflat flange.

Quantitative high-resolution imaging with sputter-initiated resonance ionization spectroscopy

The demand for submicron lateral analysis, as a result of decreasing material size, has been met by the development of liquid metal ion gun (LMIG) sources capable of achieving spot sizes less than 50 nm. The trade-off, however, is the reduction in ion beam current at the sample. Therefore, highly sensitive detection techniques are required. Our technique, sputter-initiated resonance ionization spectroscopy (SIRIS), incorporates resonant ionization of sputtered neutral particles with time-of-flight mass detection. The two major advantages this approach has over conventional secondary ion mass spectrometry are that analysis of neutrals generally increases the detection efficiency by at least two orders of magnitude, and that resonance ionization nearly eliminates mass interferences. Additionally, analysis of neutrals substantially removes matrix effects, which is crucial for quantitative surface analysis. Sputtering is achieved with a gallium LMIG, a mass-filtered microbeam ion gun, and a mass-filtered low-energy sputtering ion gun. Submicron lateral resolution and few nanometer depth resolution have been obtained by eroding the sample with the low-energy ion gun while analyzing with the LMIG. In our presentation, we will describe the SIRIS technique and its dynamic range for quantitative analysis and imaging capabilities as they pertain to semiconductor research. In particular, Ge and B depth profiles on near 1 μm spot size and Cu trace element images obtained from Cd precipitates in CdZnTe films will be presented.

TEM sample preparation by ion milling/amorphization

Artefacts evolved during TEM sample preparation by ion milling are discussed. Possibilities are given to minimise the amorphization/damage of the ion milled samples. A new type of low energy ion gun is applied in the ion milling device; ion beam induced artefacts are minimised, as shown for samples of GaAs and Si.

Sputtering yield of chromium by argon and xenon ions with energies from 50 to 500 eV

A radioactive tracer technique is described for the quantitative measurement of the sputtering yield of a target material electroplated on a copper substrate. Sputtering yields of chromium by argon and xenon ions with energies from 50 to 500 eV are reported. The ion beams, having a current density ranging from 0.01 to 0.1 mA/cm2 at an operating pressure of 2×10−5 Torr, were produced by a low-energy ion gun. The sputtered atoms were collected on an aluminum foil surrounding the target. 51Cr was used as the tracer isotope. The results indicate that the radioactive tracer technique is sensitive enough in measuring the extremely small amount of sputtered material at low ion currents and low ion energies.

Ultrastatic secondary ion mass spectrometry using very-low-energy reactive ions

A low-energy reactive ion gun has been developed for static secondary ion mass spectrometry (SSIMS). It consists of an ECR ion source and an acceleration-deceleration system. It has the potential to yield an ion beam of 1.3 (μA with an energy width of 2 eV at 10 eV even for reactive gases such as halogen compounds. Ultrastatic secondary ion mass spectrometry is demonstrated for three different primary ion beams at 10 eV. SSIMS yields information on the mechanism of secondary ion formation by the chemical process using BF 2 + under 100 eV as a primary ion.

Picosecond spectroscopy of hydrogenated MBE-GaAs

Picosecond-resolved and steady-state photoluminescence at LHe temperature in low-energy ion-gun hydrogenated GaAs/GaAlAs heterostructures are reported. The exciton in the GaAs layer shows an increase in lifetime — up to a factor of 3 — for moderate hydrogenation, followed by a sharp decrease below the value for the untreated sample, for higher H doses. Luminescence efficiency shows a consistent behavior. Incorporation of H generates a strong D-A band falling ̃64 meV below the gap energy. The behavior for heavy hydrogenation indicates the formation of a new type of deep defect, not ascribed to surface damage, because of the protective GaAlAs layer, plus the fact that the excitonic emission of the latter shows no variation.

Transport optics for a space-charge broadening ion beam

The design and performance of a low-energy ion gun system suitable for surface analysis is described. The ion gun system is capable of delivering up to a 0.72 μA beam of Ar+ ions at a potential of 500 eV into a spot diameter of 1 mm. This performance is accomplished by using ion optics to refocus the space-charge diverging beam along the 44-cm path to the target. The ion optics have been optimized by the use of a new simulation program, chden, which can model a series of lenses and the effects of space-charge forces.

A low-energy metal-ion source for primary ion deposition and accelerated ion doping during molecular-beam epitaxy

The design, operation, and use of a single-grid, electron impact, ultrahigh vacuum (UHV) compatible, low-energy ion gun capable of operating with a low vapor pressure solid source material such as In are described. The gun consists of a single chamber which combines the functions of an effusion cell, a vapor transport tube, and a glow discharge ionizer. It has been operated over a wide range of In pressures from 10−5 to 10−3 Torr (10−3 to 10−1 Pa) and provides total current densities of up to 300 μA cm−2, without magnetic discharge confinement, at ion energies Ei from 50 to 400 V. Accelerated In+ (Ei=100 eV) doping during Si(100) crystal growth by molecular-beam epitaxy (MBE) resulted in increases in the In incorporation probability ranging from approximately one to four orders of magnitude above values obtained for thermal In doping at film growth temperatures between 570 and 800 °C. Accelerated beam doping profiles were also much more abrupt than thermal beam profiles. Initial results of experiments designed to investigate the role of ion/surface interactions during nucleation and the early stages of crystal growth in UHV showed that for deposition on amorphous substrates, the use of a partially ionized In+ beam (Ei=150–300 eV) resulted in a progressive shift towards larger average island sizes, a decreased rate of secondary nucleation, and a more uniform island size distribution.

Production of thin films with use of a cylindrical low energy ion gun

We report on the production of protective films using a mixed technique of thermal deposition and (simultaneous) ion implantation on metallic surfaces. A cylindrical implanter working up to 30 keV and 1 mA was used with cylindrical steel or aluminium samples of 100 cm 2 area. Titanium based coatings on SS 304 were obtained through titanium evaporation and nitrogen implantation. Measurements of tribological properties indicate significant improvements especially for adhesion. Depth-profiling with Auger electron spectroscopy revealed O and C contamination of the TiN x films.

Simple low-energy sputter ion gun based on a Bayard–Alpert pressure gauge

The design and performance of a simple sputter ion gun is given. It uses a commercial Bayard–Alpert ionization gauge. A two-stage extraction and acceleration system provides a variable beam profile as well as high currents at low energies (&amp;gt;1 μA above 200 eV). At a distance of 4 cm from the end of the gun a current density of more than 150 μA/cm2 was obtained at 1.2 keV. A narrow energy distribution may also be obtained (≤1.6 eV FWHM at 1.2 keV).

A low energy ion beam system for thermal evolution measurements of damage in ion bombarded single crystals

The use of thermal evolution mass spectrometry in conjunction with the helium probe represents a powerful method of studying the gas-defect interaction processes associated with ion bombardment of single crystal targets. The energy and fluence dependence of the damage build-up processes together with the annealing characteristics of the damage are of considerable importance in view of the widespread use of low energy ion beam and plasma cleaning techniques. The apparatus described comprises a low energy ion gun based on an electron bombardment source and Wien filter and target chamber incorporating a multiple target holder assembly. The design considerations and operating characteristics of the overall system are described and the often used ion bombardment cleaning procedures discussed with reference to experimental thermal evolution measurements of low energy argon and helium trapping.