Secondary electron emission from the entrance and exit surfaces of thin carbon foils under fast ion bombardment

The total secondary electron emission (SEE) yield from the entrance and exit surfaces of thin carbon foils under fast ion (¹⁶O, ¹⁹F, ³⁵Cl) bombardment has been measured as a function of the ion energy and the ion beam current intensity. Using a retarding field, the energy distribution of secondary electrons integrated over almost all angles of emission in the backward and forward directions has also been measured. It is found that total forward emission is larger than backward emission by factors of up to 2.5, 2.7 and 3.4 for ¹⁶O⁺³, ¹⁹F⁺³ and ³⁵Cl⁺⁵, respectively. It is suggested that the enhancement of forward SEE may be partly due to effects from the instantaneous charge state of the heavy ion beam in the solid in addition to the binary collisions of the projectile with individual electrons in the target. It is also shown that the total SEE yield from the entrance and exit surfaces of the target foils decreases with ion beam current intensity; this may be a beam-induced temperature effect. The total SEE yield in both the forward and backward directions is less sensitive to surface conditions for high velocity ions than for low velocity ions and the total yield from both surfaces of the foils is proportional to the ion stopping power in the target, where the constant of proportionality depends on the properties of material.     

Electron emission from carbon induced by the ions C, O, CO and

The secondary electron yield from carbon induced by the ions C⁺, O⁺, CO⁺ and has been measured as a function of ion energy in the range of 2 to 20 keV. It has been observed that electron yield from carbon increases with projectile energy. By comparing electron yields induced by equally fast atomic and molecular projectiles, a molecular effect as a yield reduction has been observed. The measured molecular effect was stronger than the predictions of sweeping-out-electron model.     

High yield antibiotic producing mutants of Streptomyces erythreus induced by low energy ion implantation

Conidia of Streptomyces erythreus, an industrial microbe, were implanted by nitrogen ions with energy of 40–60 keV and fluence from 1 × 10¹¹ to 5 × 10¹⁴ ions/cm². The logarithm value of survival fraction had good linear relationship with the logarithm value of fluence. Some mutants with a high yield of erythromycin were induced by ion implantation. The yield increment was correlated with the implantation fluence. Compared with the mutation results induced by ultraviolet rays, mutation effects of ion implantation were obvious having higher increasing erythromycin potency and wider mutation spectrum. The spores of Bacillus subtilis were implanted by arsenic ions with energy of 100 keV. The distribution of implanted ions was measured by Rutherford Backscattering Spectrometry (RBS) and calculated in theory. The mechanism of mutation induced by ion implantation was discussed.     

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar⁺ and Ar⁰ beams of energies between 5 and 15 keV, and In⁺ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.     

Cusp-electrons used for velocity measurements of heavy ion projectiles

A new method for precise velocity measurements (10⁻⁴) for heavy ions is presented. Electrons at projectile velocity have been measured at an observation angle of zero degree for 1.4 MeV/A U³³⁺ traversing thin gas and carbon solid targets. The velocity distribution of the electrons corresponds to the energy and angular distributions of the projectiles which is clearly demonstrated by energy loss measurements in thin carbon foils.     

Comparison of secondary electron emission in helium ion microscope with gallium ion and electron microscopes

To evaluate secondary electron (SE) image characteristics in helium ion microscope, Si surfaces with a rod and step structures is scanned by 30 keV He and Ga ion beams and 1 keV electron beam. The topographic sensitivity of He ions is in principle higher than that for scanning electron microscope (SEM) because of the stronger dependency of SE yield versus incident angle for He ions. As shrinking to sub nm patterns, the pseudo-images constructed from line profile of SE intensity by the electron beam lose their sharpness, however, the images for the He and Ga ion beams keep clearness due to darkening the bottom corners of the pattern. Here, the sputter erosion for Ga ions must be considered. Furthermore, trajectories of emitted SEs are simulated for a rectangular Al surface scanned by the beams to study voltage contrast, where positive and negative voltages are applied to the small area of the sample. Both less high energy component in the energy distribution of SEs and dominant contribution of direct SE excitation by a projectile He ion keep a high voltage contrast down to a sub nm sized area positively biased against the zero-potential surroundings.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

Secondary neutral and ionized particle measurements under MeV-energy ion bombardment

We have measured both secondary neutral and ionized particles from an InSb target under 3.0 MeV Si ion bombardment. Measurements of both ions and neutrals have not been carried out so far in the MeV-energy range. The mass spectra and axial emission energy distributions of secondary particles were investigated. Secondary ions were measured with a linear- and a reflective-type time-of-flight technique, whereas secondary neutral particles were photo-ionized by a UV pulsed laser (ArF: 193 nm) and measured with a reflective-type time-of-flight technique. Different results were obtained for neutral particles in comparison with ionized particles. The mean energy of neutral Sb atoms was much lower than that of neutral In atoms, whereas the mean energies of secondary In and Sb ions were nearly equal.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

Comparison of secondary ion emission yields for poly-tyrosine between cluster and heavy ion impacts

Emission yields of secondary ions necessary for the identification of poly-tyrosine were compared for incident ion impacts of energetic cluster ions (0.8 MeV , 2.4 MeV , and 4.0 MeV ) and swift heavy monoatomic molybdenum ions (4.0 MeV Mo⁺ and 14 MeV Mo⁴⁺) with similar mass to that of the cluster by time-of-flight secondary ion mass analysis combined with secondary ion electric current measurements. The comparison revealed that (1) secondary ion emission yields per impact increase with increasing incident energy within the energy range examined, (2) the 4.0 MeV impact provides higher emission yields than the impact of the monoatomic Mo ion with the same incident energy (4.0 MeV Mo⁺), and (3) the 2.4 MeV impact exhibits comparable emission yields to that for the Mo ion impact with higher incident energy (14 MeV Mo⁴⁺). Energetic cluster ion impacts effectively produce the characteristic secondary ions for poly-tyrosine, which is advantageous for highly sensitive amino acid detection in proteins using time-of-flight secondary ion mass analysis.     

High energy primary knock-on process in metal-deuterium systems initiated by bombardment with noble gas ions

An experimental study confirms the possibility of nuclear fusion reactions initiating in metal-deuterium targets by bombarding them with ions that are not the reagents of the fusion reaction, in particular, with noble gas ions. The yields of (d,d) and (d,t) reactions were measured as functions of energy (0.4-3.2 MeV) and mass of incident ions (He⁺, Ne⁺, Ar⁺, Kr⁺ and Xe⁺). Irradiation by heavy ions produced a number of energetic deuterium atoms in the deuteride and deuterium + tritium metal targets. At ion energies of ∼0.1-1 MeV the d-d reaction yields are relatively high. A model of nuclear fusion reaction cross-sections in atomic collision cascades initiated by noble gas ion beam in metal-deuterium target is developed. The method for calculation tritium or deuterium recoil fluxes and the yield of d-d fusion reaction in subsequent collisions was proposed. It was shown that D(d,p)t and D(t,n)⁴He reactions mainly occur in energy region of the recoiled D-atom from 10 keV to 250 keV. The calculated probabilities of d-d and d-t fusion reactions were found to be in a good agreement with the experimental data.     

Surface analysis with high energy time-of-flight secondary ion mass spectrometry measured in parallel with PIXE and RBS

The Surrey Ion Beam Centre (IBC) is routinely using focused MeV primary ions to generate two-dimensional molecular maps using time-of-flight secondary ion mass spectrometry (ToF-SIMS) collected simultaneously with particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS) spectra. Measurements made with the ToF-SIMS, PIXE, and RBS device with a focused and scanned MeV primary ion beam provide a more complete elemental and molecular evaluation of the target sample’s surface. In this paper, we explore the use of high electronic energy loss by MeV primary ions in the surface region of the target as a method for generating molecular images of the surface. We provide analyses of the relative secondary ion yield of leucine molecules as a function of primary ion velocity using MeV primary ions. We also demonstrate our ability to collect PIXE, RBS, and ToF-SIMS images generated using the same MeV primary ion.     

Reactive sputtering of simple condensed gases by kev ions III: Kinetic energy distributions

Condensed gas layers of H₂O, NH₃ and CO at 15–20 K have been bombarded by 6 keV H⁺₂ and 3 keV He⁺ and Ar⁺ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H₂O frozen on top of the CO-residue H₂CO was detected.     

The angular distribution of particles sputtered from Cu,Zr, and Au surfaces by ion bombardment at grazing incidence

The angular distribution of particles sputtered from polycrystalline Cu, Zr, and Au targets has been measured for bombardment with Ar⁺- and Xe⁺-ions at perpendicular and grazing ion incidence (80° and 85° with respect to the surface normal). The ion energy was varied between 100 keV and 900 keV. The measured distributions follow approximately a cosine-squared curve rather than the cosine function, they are found to be symmetric with respect to the surface normal and almost independent of the ion species, the ion energy and the angle of incidence. Values of sputtering yield of Cu, Au and Zr are also presented.     

Reactive sputtering of simple condensed gases by keV ions II: Mass spectra

Condensed gas layers of H₂O, NH₃ and CO at 15–20 K have been bombarded by 6 keV H⁺₂ and 3 keV He⁺ and Ar⁺ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H₂O frozen on top of the CO-residue H₂CO was detected.     

Electronic stopping dependence of ion beam induced modifications in GaN

We report here Swift heavy ion induced effects in GaN samples grown by metal organic chemical vapor deposition (MOCVD) technique. These samples were irradiated with 80 MeV Ni and 100 MeV Ag ions at a fixed fluence of 1 × 10¹³ ions/cm². Ion species and energies are chosen such that the difference in their electronic energy loss (S_e) would be 8 keV/nm. Effects of Ag on structural and optical properties over Ni ions have been discussed extensively. We employed different characterization techniques like High Resolution X-ray Diffraction (HRXRD) and Raman Spectroscopy for defect density calculations and for vibrational modes, respectively. Defect densities are calculated and compared using Williamson-Hall method from HRXRD. Change of strain and vibrational modes with S_e has been discussed.     

AODO: Secondary ion emission and surface modification

In order to study the sputtering of secondary ions from well characterized surfaces, we constructed a new UHV system named AODO. It consists of a detector chamber, a target preparation and analysis chamber, and a target transfer rod. We present the lay-out of this new instrument. The detector allows measuring the time-of-flight of emitted secondary ions and their position on a 2D imaging detector (XY-TOF imaging technique). The analysis chamber can be used to study surface modification by means of LEED (low energy electron diffraction). We show preliminary results of the evolution of the LEED patterns as a function of the projectile fluence during irradiation of HOPG (highly oriented pyrolytic graphite) with slow Xe¹⁴⁺ ions at ARIBE (the low energy, highly charged ion beam line of the French heavy ion accelerator GANIL).     

High-speed processing with Cl2 cluster ion beam

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl₂ cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO₂ films were irradiated with the Cl₂ cluster ions at acceleration energies of 10–30 keV and the etching ratio of Si/SiO₂ was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl₂ cluster ions was about 4400 atoms/ion at 30 keV acceleration energy. The etching ratio of Si/SiO₂ was above eight at acceleration energies in the range 10–30 keV. Thus, SiO₂ can be used as a mask for irradiation with Cl₂ cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important.     

Highly sensitive molecular detection with swift heavy ions

Various imaging techniques using microbeam have been applied in biology. Secondary ion mass spectrometry (SIMS) is one of the prominent tools for biological imaging; SIMS can provide data on molecular distribution in biological samples smaller than 1 μm. However, conventional SIMS has only low sensitivity for molecular ions; therefore there is a need for beams of more sensitive primary ions. Plasma desorption mass spectrometry (PDMS) is a method using high energy fission fragments from excitation of a ²⁵²Cf source, and it allows ionization of large molecules (typically up to 20 kDa) due to the dense electronic excitation. Although PDMS is not in use today because of the development of soft ionization methods, ionization induced by high energy ion collision still remains the only method which combines high spatial resolution and sensitive detection of large molecules. In this work, the secondary ion yield of amino acid and phospholipid was measured for 6 MeV Cu⁴⁺. The yields were compared to bismuth cluster ions, which achieve relatively high yield. It was confirmed that the swift heavy ion has a couple of hundred times higher yield for large molecules than bismuth cluster ions.     

Single electron capture by state-selected multiply charged Arq+ ions (q = 3,4)

State-selective single electron capture by multiply charged Ar^q+ ions (q = 3, 4) has been studied in rare gas targets with the aid of double translational spectroscopy. The energy gain spectra have been measured using different projectile beams, either extracted from an electron beam ion source or produced by electron capture reactions in an additional preparation cell. The influence of metastable projectile states as well as the reaction channel identification in complex systems are discussed.