Secondary Neutral Mass Spectrometer Research Articles

CHILI, a Nanobeam Secondary Neutral Mass Spectrometer with Extraordinary Spatial Resolution, Sensitivity, and Selectivity: First Results

Journal Article CHILI, a Nanobeam Secondary Neutral Mass Spectrometer with Extraordinary Spatial Resolution, Sensitivity, and Selectivity: First Results Get access A M Davis, A M Davis Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USAChicago Center for Cosmochemistry.The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA Search for other works by this author on: Oxford Academic Google Scholar T Stephan, T Stephan Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USAChicago Center for Cosmochemistry.Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Search for other works by this author on: Oxford Academic Google Scholar R Trappitsch, R Trappitsch Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USAChicago Center for Cosmochemistry. Search for other works by this author on: Oxford Academic Google Scholar M J Pellin, M J Pellin Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USAChicago Center for Cosmochemistry.Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USAThe Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA Search for other works by this author on: Oxford Academic Google Scholar D, D Search for other works by this author on: Oxford Academic Google Scholar Rost, Rost Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USAChicago Center for Cosmochemistry.Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Search for other works by this author on: Oxford Academic Google Scholar M R Savina, M R Savina Chicago Center for Cosmochemistry.Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Search for other works by this author on: Oxford Academic Google Scholar N Dauphas N Dauphas Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USAChicago Center for Cosmochemistry.The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 1143–1144, https://doi.org/10.1017/S1431927615006509 Published: 23 September 2015

Formation of Cu x Au1- x phases by cold homogenization of Au/Cu nanocrystalline thin films.

It is shown, by using depth profiling with a secondary neutral mass spectrometer and structure investigations by XRD and TEM, that at low temperatures, at which the bulk diffusion is frozen, a complete homogenization can take place in the Cu/Au thin film system, which leads to formation of intermetallic phases. Different compounds can be formed depending on the initial thickness ratio. The process starts with grain boundary interdiffusion, which is followed by a formation of reaction layers at the grain boundaries that leads to the motion of the newly formed interfaces perpendicular to the grain boundary plane. Finally, the homogenization finishes when all the pure components have been consumed. The process is asymmetric: It is faster in the Au layer. In Au(25nm)/Cu(50nm) samples the final state is the ordered AuCu3 phase. Decrease of the film thicknesses, as expected, results in the acceleration of the process. It is also illustrated that changing the thickness ratio either a mixture of Cu-rich AuCu and AuCu3 phases (in Au(25nm)/Cu(25nm) sample), or a mixture of disordered Cu- as well as Au-rich solid solutions (in Au(25nm)/Cu(12nm) sample) can be produced. By using a simple model the interface velocity in both the Cu and Au layers were estimated from the linear increase of the average composition and its value is about two orders of magnitude larger in Au (ca. 10−11 m/s) than in Cu (ca. 10−13 m/s).

Sputtering yields for low-energy Ar+- and Ne+-ion bombardment

The development of fluorescent lamps requires reliable experimental data of low-energy (30–300 eV) sputtering yields of the metallic components of lamps. An experimental study was carried out to measure the sputtering yields of molybdenum, tungsten and iron using the plasma chamber of a Secondary Neutral Mass Spectrometer (SNMS). The sputtering ion beam was extracted from radio frequency noble gas (Ar/Ne)-plasmas. The number of atoms sputtered was determined from the weight loss of the target by means of a microbalance. The sputtering yields data for Mo/Ar, W/Ar and Fe/Ar target/projectile combinations and their energy dependence were in good agreement with those calculated by computer simulations and measured by other groups. The sputter yield obtained for Fe/Ne target/projectile combination shows some deviation from the simulation results.

Operation and application of a new time-of-flight e-gas secondary neutral mass spectrometer (ToF–SNMS)

The low-pressure rf plasma of a secondary neutral mass spectrometer (e-gas SNMS) was connected with a time-of-flight (ToF) mass spectrometer for the first time. As opposed to ToF-SIMS in e-gas SNMS, the primary ion pulse cannot be used for triggering the flight time measurement. Therefore, an extraction pulse is used which at a defined time loads an ion package from the beam of the post-ionised particles into the ToF spectrometer. The newly developed ToF-SNMS system is described, and first experimental results are presented.

Pattern formation in SiSb system

Thermal annealing of Si/Si 1− x Sb x /Si amorphous thin-film tri-layer samples ( x = 18 and 24 at%Sb) under 100 bar Ar pressure results in an interesting pattern formation. In pictures, taken by means of cross-sectional transmission electron microscopy, stripe-shaped contrast, with three maxima, parallel to the interfaces can be seen. Secondary neutral mass spectrometer measurements revealed that the regions with different contrasts correspond to Sb-rich and Sb-depleted regions. Furthermore, the Sb concentration peaks in the Sb-rich regions, especially at longer annealing times, are different: the peak developed at the Si/SiSb interface closer to the free surface decays faster than that of the inner one closer to the substrate. The pattern formation is interpreted by segregation-initiated spinodal-like decomposition, while the difference of the Sb concentration peaks is explained by the resultant Sb transport to and evaporation from the free surface. The possible role of formation of nanocrystalline grains, in the explanation of the fast transport under pressure as compared to vacuum is also discussed.

Detection in the ppm range and high-resolution depth profiling with the new SNMS instrument INA-X

The design and the specification of the newly developed INA-X instrument for the electron-gas version of a secondary neutral mass spectrometer (SNMS) are described. A modified plasma chamber inside a UHV vessel in combination with optimized r.f. coupling enable a high plasma density at low power input. The ionization efficiency and the current density for sample sputtering are significantly improved compared to those of previous SNMS instruments. A novel detection optics allows us to define an energy window of 1–2 eV. The secondary neutral particle flux is collected under an oblique take-off angle against the surface normal, improving the quantifiability of the obtained SNMS spectra. The variable sputtering rate can be adjusted from some nm per min up to 1–2 μm per min, depending on the operator’s need, either to obtain high depth resolution or short analysis time. Examples for depth profiling of conducting and insulating films and coatings on the nano- and micrometer scales are presented.

Low-energy BO and BO 2 emission from H 2BO 3 sputtered in a low-pressure high-frequency SNMS plasma

Background corrected secondary neutral energy spectra derived from Cu powder pellets with H 3BO 3, MgO, Al 2O 3, TiO 2, Y 2O 3 and ZrO 2, show energy distributions being more or less typical for collision cascades in the cases of the metal ions M +, of O + at energies >5 eV above the ion generation potential, and of the molecules Cu 2 +, AlO +, TiO +, YO + and ZrO +, whereas the larger parts of the energy distributions of BO +and BO 2 +exhibit a similar shape as the Ar +plasma gas ions. From this, and from the background of the low decomposition and melting temperatures of H 3BO 3 and B 2O 3, respectively, we conclude that the detected BO and BO 2 molecules have been emitted with thermal energies in processes implying lower energies than collision cascades. Evidence was found that the same holds for HBO 2 and H 2BO 2. In order to obtain relative correction factors for MO + molecular ion intensities, electron impact ionization cross sections have been calculated for the light MO molecules using the binary-encounter Bethe (BEB) formula, and for the respective metal M atoms by means of the semiempirical Lotz formula. For the heavy MO molecules YO and ZrO, cross sections have been estimated using the Thomson formula. The comparison of corrected relative MO/M and M/Cu intensities yields evidence that thermal (H)BO x emission amounts to the same order of magnitude as B emission from collision cascades, and that this situation is comparable to the high yield of MO molecules emitted in collision cascades from oxides with high M masses. Since the normal energy window of an HF-plasma secondary neutral mass spectrometer does not accept particles with originally thermal energies, it is concluded that these findings are relevant for quantification.

Energy spectra of sputtered species under sub-keV ion bombardment: experiments and computer simulations

Energy distributions of neutral atoms sputtered from elemental Cu and two binary alloys (Cu 0.53W 0.47 and Ni 0.92W 0.08) under steady-state conditions were investigated both by experiments and by computer simulations. Ar + ions with impact energies ranging from 65 to 1030 eV were used as bombarding species. The experimental data were recorded with a secondary-neutral mass spectrometer that combines electron-gas post-ionization with a high-transmission double focusing mass spectrometer. Due to the instrument's high detection sensitivity, the energy spectra could be determined over a large intensity range, covering up to five orders of magnitude. A comparable (or even) larger emission-energy range could be monitored in the computer simulations which were done with the dynamic TRIDYN code. The computed energy spectra are in qualitative agreement with the corresponding experimental distributions. Not surprisingly, the data from both approaches indicate that the inverse-square emission-energy dependence of the yield predicted by analytical sputtering theory is generally not valid for the low (near-threshold) impact energies used in this work and for emission energies approaching the maximum possible values. Rather, the energy spectra exhibit, over a considerable energy range, an exponentially decreasing yield with increasing emission energy. The large range of emission energies detected provided the opportunity to derive estimates of the maximum emission energy and its dependence on the primary-ion energy E for the various atomic species. These values amount to ∼0.55 E for Cu and Ni atoms and to ∼0.35 E for W atoms sputtered from the alloys and to ∼0.6 E for Cu atoms sputtered from the pure Cu specimen; they are compared to simple estimates of the maximum emission energies in low-energy sputtering based on collision sequences leading to the ejection events that involve only a small number of atoms.

Energy spectra of secondary neutrals obtained by means of the electrostatic energy filter of a commercial low-pressure HF-plasma secondary neutral mass spectrometer

The ion optics system of the commercially available HF-plasma secondary neutral mass spectrometry apparatus INA3 was found to enable ion energy spectroscopy with good performance: for sufficiently high (2 keV) primary ion energies, the energy distributions of postionized Cu, Al, and Mo atoms almost show the theoretically expected behavior up to 300 eV. Maxima are found at the energies expected from collision cascade emissions. Since ionization occurs on a plasma potential between +35 and 40 V, all ion energies are shifted accordingly. Tuning of the ion optics was accomplished in the secondary ion mass spectroscopy mode by using a sputtered H3BO3/Cu powder pellet as a B+ and BO2− ion source.

Application of secondary neutral mass spectrometry in low-energy sputtering yield measurements

An experimental study was initiated to measure low-energy (150 to 600 eV) sputtering yields of molybdenum with xenon ions using a Secondary Neutral Mass Spectrometer (SNMS). An ion gun was used to generate the ion beam. The ion current density at the target surface was approximately 30 μA/cm 2. The SNMS spectra obtained at 50° incident angle were converted to sputtering yields for perpendicular incidence by normalizing SNMS spectral data at 500 eV with the yield measured by Rutherford backscattering spectrometry. Sputtering yields as well as the shape of the yield-energy curve obtained in this manner are in reasonable agreement with those measured by other researchers using different techniques. Sputtering yields calculated by using two semi-empirical formulations agree reasonably well with the measured data.

Isotopic mass effects in low-energy sputtering of copper and molybdenum

Spherical and planar targets of copper and molybdenum were sputtered with Ar + ions in the energy range from 40 to 440 eV. The isotopic composition of the flux of neutral atoms under steady-state conditions was determined by means of a novel secondary-neutral mass spectrometer with electron-gas post-ionization. In combination with a retarding-field energy analyzer, the ejected particle flux was investigated with respect to an emission-energy dependence of the natural isotopic abundances. It was found that the isotopic composition in the near-threshold regime of sputtering changes drastically with the Ar + impact energy. Mass-dependent variations of the angular distributions lead to an enrichment of the lighter isotopes of several percent. Most prominent is the influence of the different emission energy distributions. The lighter isotopes are preferably ejected with higher kinetic energies; this effect exhibits a pronounced enhancement at very low bombarding energies. Depending on the retardation potential of the energy analyzer and thus the energy of the transmitted atoms, deviations from the natural isotopic composition of several 10% were observed.

Analytical performance of a secondary-neutral microprobe with electron-gas postionization and magnetic-sector mass spectrometer

A secondary-neutral mass spectrometer has been developed which combines postionization by means of an electron gas (plasma) with a high-transmission magnetic mass spectrometer and a finely focused ion beam produced in a Ga liquid-metal ion source. The instrument can be operated in a large-area direct bombardment-mode extracting the sputtering beam out of the plasma, or in a microprobe mode utilizing the focused Ga+ ion beam to effect sputtering. In the former case secondary-neutral intensities of up to 109 c/s can be realized; together with the high abundance sensitivity those translate in a detection limit in the few-ppb range. In the microprobe mode, secondary-neutral micrographs have been recorded using a focused 20 keV Ga+ beam with a spot size of ≲1 μm and a current of some nA. From these data an elemental detection sensitivity of 10−3 is inferred, necessitating image acquisition times of the order of 2500 s.

Measurements of energy distributions and yields of neutral Cu 2 and Cu 3 species sputtered from Cu by low energy Ar + ions

Polycrystalline copper was bombarded by normally incident, very low energy argon ions ( E 0 = 100–1000 eV). The translational kinetic energy distributions and relative yields of neutral Cu 2 and Cu 3 species sputtered normally from the copper surface were measured using a secondary neutral mass spectrometer. The energy distribution of a given species (Cu 2 or Cu 3) was found to be approximately constant in shape for 600 < E 0 < 1000 eV. However, for E 0 < 600 eV, the distributions narrowed as the bomba ion energy was lowered. For a given bombarding ion energy, the energy distributions were found to be narrower for higher-order sputtered species, and the asymptotic falloff was steeper for the latter. The average kinetic energies ( E ) were in the order: E(Cu 1) > E(Cu 2) > E(Cu 3) . E(Cu 2) increased with increasing bombarding ion energy. The relat depend on the total sputtering yield of copper squared and the relative yield of Cu 3 on the total yield cubed. The measured polyatomic energy distributions and relative yields indicate that a multiple-collision process is responsible for sputtered neutral metallic cluster formation in this system.

Relative elemental sensitivity factors in secondary neutral mass spectrometry

Using the first commercially available electron gas secondary neutral mass spectrometer (SNMS) the relative sensitivity factors D0Fe/D0X were determined for 29 elements from a variety of standards of known composition. The results show an average standard deviation which increases with decreasing energy of the bombarding Ar+ ions from 30% at 2 keV to 53% at 250 eV. Detailed analysis revealed that a standard deviation of 2% is due to the day‐to‐day reproducibility of the experimental system conditions, e.g., plasma and ion optics parameters. The remaining variations observed between different samples are attributed at least partly to matrix‐dependent angular distributions of the sputtered neutral particles. Particularly the observed difference between angular distributions of alloy constituents, being most pronounced if elements of very low mass like Be, B, C, N, and O, are present in the sample, leads to a characteristic dependence of the relative sensitivity factors for these elements on the bombarding e...