Negative Secondary Ion Yields Research Articles

Quantitative ToF‐SIMS depth profiling of a multi‐phased III–V semiconductor matrix via the analysis of secondary cluster ions

An approach for quantification of time‐of‐flight (ToF)‐SIMS depth profiling data is demonstrated using an independently characterised multilayer, an AlGaAs/GaAs stack from certified reference material BAM‐L200. The sample was studied with a Bi cluster ion source as the analysis projectile and a Cs+ sputter beam. Under these conditions, the negative secondary ion yield is significantly enhanced, especially for cluster ions comprising all possible elemental combinations. Each cluster ion carries a number of sputtered neutrals so if all cluster ions containing an element could be collected and identified, a large population of secondary neutrals will be included in the analysis, improving the analytical signal and approaching a ‘matrix‐effect‐free’ measurement. By measuring the intensities of a range of secondary cluster ions of AliGajAsk− where i, j, k are integers such 0, 1, 2, and so on, the atomic counts for each major element were accumulated based on the elemental contents of those cluster ions. The summed atomic counts were then normalised without using the more common approach of applying relative sensitivity factors. The resulting elemental depth profiles show an excellent match to the structure and stoichiometry of the sample. The quantitative relationship of various elements and the interface appearance derived by this cluster ion approach are strongly competitive to the results available from XPS. Copyright © 2016 John Wiley & Sons, Ltd.

Significant enhancement of negative secondary ion yields by cluster ion bombardment combined with cesium flooding.

In secondary ion mass spectrometry (SIMS), the beneficial effect of cesium implantation or flooding on the enhancement of negative secondary ion yields has been investigated in detail for various semiconductor and metal samples. All results have been obtained for monatomic ion bombardment. Recent progress in SIMS is based to a large extent on the development and use of cluster primary ions. In this work we show that the enhancement of negative secondary ions induced by the combination of ion bombardment with simultaneous cesium flooding is valid not only for monatomic ion bombardment but also for cluster primary ions. Experiments carried out using C60+ and Ar4000+ bombardment on silicon show that yields of negative secondary silicon ions can be optimized in the same way as by Ga+ and Cs+ bombardment. Both for monatomic and cluster ion bombardment, the optimization does not depend on the primary ion species. Hence, it can be assumed that the silicon results are also valid for other cluster primary ions and that results obtained for monatomic ion bombardment on other semiconductor and metal samples are also valid for cluster ion bombardment. In SIMS, cluster primary ions are also largely used for the analysis of organic matter. For polycarbonate, our results show that Ar4000+ bombardment combined with cesium flooding enhances secondary ion signals by a factor of 6. This can be attributed to the removal of charging effects and/or reduced fragmentation, but no major influence on ionization processes can be observed. The use of cesium flooding for the imaging of cells was also investigated and a significant enhancement of secondary ion yields was observed. Hence, cesium flooding has also a vast potential for SIMS analyses with cluster ion bombardment.

NanoSIMS50 imaging of thin samples coupled with neutral cesium deposition

AbstractThanks to the 50 nm of lateral resolution of the NanoSIMS50, new fields of application have been opened to Secondary Ion Mass Spectrometry. The NanoSIMS50 imaging capabilities are however often hampered by the fact that, due to a high impact energy (16 keV) and normal incidence of the primary ion beam, the steady state is only reached after sputtering several nanometers (10–20 nm) of the sample. As a consequence, the secondary ion emission is very low during the sputtering of these first nanometers. This phenomenon, which is most pronounced for organic and biologic samples, induces artifacts of measurement. In a previous study, we have shown that negative secondary ion yields can be enhanced in Secondary Ion Mass Spectrometry (SIMS) by coupling the primary ion bombardment with a simultaneous neutral cesium deposition. As this method cannot be used directly on the NanoSIMS50, we propose here to perform SIMS analysis after a prior deposition of Cs0 in a standalone UHV system. With this new method of analysis, organic thin films and proteins have been 3D monitored. Copyright © 2010 John Wiley & Sons, Ltd.

A new approach to measuring D/H ratios with the Cameca IMS‐7F

AbstractSIMS offers spatial resolution on the scale of micrometers and consumes an exceedingly small amount of sample. In addition, easy sample preparation, rapid analysis and high sensitivity allow SIMS to be applied to hydrous minerals. Hydrogen isotopes normally can be measured as negative or positive secondary ions using either a Cs+ or an O− primary ion beam, respectively. The hydrogen negative secondary ion yields are only marginally higher than hydrogen positive secondary ion yields. There are advantages and disadvantages to using either method to analyze hydrogen isotopes by SIMS. For insulating samples, O− is commonly used to avoid using the electron gun. The main disadvantage in using an O− beam is hydride interference (H2+) associated with deuterium (D) that must be resolved using a mass resolving power of ∼1000–2000. We have developed a method where we combine a lower mass resolution and a small voltage offset to obtain rapid (12 min), high‐precision (∼2‰, 2σ) D/H ratios in turquoise using a Cameca IMS‐7F ion microprobe. This technique can be applied to a variety of hydrous minerals in geological materials. Copyright © 2010 John Wiley & Sons, Ltd.

Neutral cesium deposition prior to SIMS analysis of inorganic and organic samples

AbstractThe enhancement of negative secondary ions yields in SIMS by the use of electropositive primary ions is well known. In previous papers, the authors of this article have reported on the simultaneous use of primary ion bombardment coupled with neutral cesium deposition to optimize the useful yields of negative secondary ions in the steady‐state regime. For electronegative elements, total ionization was achieved while the gain for the other elements attained two orders of magnitude. In this paper, we study the enhancement of negative secondary ion yields in the pre‐equilibrium regime by depositing neutral cesium onto the sample surface prior to the SIMS analysis. The main areas of application of this technique lie in the field of secondary ion imaging of sample surfaces. Of particular interest is the analysis of organic and biological samples on the Cameca NanoSIMS50 instrument. Both inorganic and organic samples will be investigated in this paper. Copyright © 2010 John Wiley & Sons, Ltd.

Influence of changes in the resistivity of the sample surface on ultra-shallow SIMS profiles for arsenic

High concentration dopant distributions in silicon like those required to form ultra shallow junctions can affect SIMS analyses introducing matrix effects on secondary ions, otherwise not observed in more dilute regimes. In this work the effect of high arsenic concentration on negative secondary ion yield has been investigated when sputtering with a 0.5 keV Cs + primary beam in a magnetic sector instrument. Two effects have been distinguished: the first one is a variation of matrix signals observed on as implanted samples in correspondence with the projected range, probably due to the damage induced by the high dose implant; the second is a step observed for several species and related to the arsenic distribution. The latter seems due to the effect of the active dopant on secondary ion energy distributions. A wide and well-centred energy slit can limit this effect allowing a more accurate quantification of the activated high As concentrations.

SIMS analysis with neutral cesium deposition: Negative secondary ion sensitivity increase and quantification aspects

To overcome the SIMS matrix effect in the negative secondary ion mode, analyses can be performed on the Cation Mass Spectrometer using neutral cesium deposition with simultaneous primary ion bombardment. This paper discusses the advantages of this technique by applying it on several samples. The useful yields of various elements detected as negative secondary ions are calculated and discussed in terms of work function and electron affinity. The additional Cs deposition allows a significant increase of the useful yields of negative secondary ions and thus of the analysis sensitivity compared to traditional Cs + primary ion bombardment. At maximal cesium surface concentrations, quantitative analyses become possible for elements with high electron affinities. For other elements a significant increase of the analysis sensitivity is achieved.

Quantitative study of oxygen enhancement of sputtered ion yields. I. Argon ion bombardment of a silicon surface with O 2 flood

A novel quantification approach is applied to determine in situ the amount of surface oxygen within the sputtered particle escape depth during steady-state sputter depth profiling of silicon under simultaneous oxygenation with an oxygen flood gas or with an oxygen primary ion beam. Quantification is achieved by comparing the secondary ion intensities of 16O that is adsorbed or implanted at the Si surface with the measured peak intensities of a calibrated 18O ion implant used as a reference standard. Sputtered ion yields can thereby be related to surface oxygen levels. In the present work the dependences of the partial silicon sputter yield Y and of the positive and negative secondary ion useful yields UY(X ±) (X = B, O, Al, Si, P) on the oxygen/silicon ratio, O/Si, in the sputtered flux are studied for 40Ar + bombardment of Si with simultaneous O 2 flooding. The silicon sputter yield is found to decrease with increasing flood pressure and O/Si ratio by up to a factor of 3. Both positive and negative secondary ion yields are enhanced by the presence of oxygen at the silicon surface. The useful ion yield of Si + scales non-linearly with the atom fraction of surface oxygen; this behavior is shown to invalidate models that suggest that Si + ion yield enhancement is dominated either by isolated oxygen atoms or by formation of SiO 2 precipitates. In contrast a microscopic statistical model that assumes that local Si + ion formation depends only on the number of oxygen atoms coordinated to the Si atom to be ejected fits the ion yield data quantitatively.

Transient effects induced through ripple topography growth during Cs + depth profile analysis of Si at high incidence angles

Cs + depth profiling of Si at high incidence angles introduces additional transient effects and significantly longer transient widths (up to 30 nm in depth). Analysis of a well-defined delta layered Si structure with a 250 eV Cs + ion beam incident at 75°, reveals that the enhancement of these transient effects are accompanied by: (a) a sputter rate reduction, (b) ripple topography formation and (c) a continually increasing implanted Cs concentration over the first ∼30 nm. Steady state sputtering ensues after this depth with no further roughening. These trends indicate that that the enhanced transient effects are initiated through surface roughening, which reduces sputter rates. A reduction in the sputter rate allows for more Cs to be retained within the near surface region, which in turn enhances negative secondary ion yields.

Secondary ion yields of C, Si, and Ge in InP, and Cs surface density and concentration, in SIMS

In secondary ion mass spectrometry (SIMS), the relationship between negative secondary ion yield ( γ −) of C, Si, and Ge in an InP substrate and sputtering yield ( Y), and that between the secondary ion yield and the surface density of reactive primary ion Cs ( n Cs(0); zero refers to substrate surface) were examined by changing the incident angle of the primary ion to the surface. The γ − value was derived by dividing the corresponding useful yield by the instrumental transmission factor, which was obtained by measuring the useful yield of an element with high electron affinity, such as fluorine. Monte Carlo simulations using TRIM code and theoretical analysis were performed to obtain the Cs density (atoms/unit volume) depth distribution, n Cs( x), in the sputtered substrate. The density depth distribution was measured directly using SIMS, under conditions where the energy of the O 2 + primary ion was sufficiently reduced. n Cs(0) was obtained by setting x of n Cs( x) to zero. The Cs ions used for sputtering were distributed mostly according to a Gaussian function. The n Cs( x) profile estimated on the basis of the Gaussian distribution showed good accordance with the measured one. At the sputtered surface, n Cs(0) was closely proportional to 1/ Y. The values of γ − for C, Si and Ge changed proportionally to the power of n Cs(0), or 1/ Y. For C, Si, and Ge, the lower the γ −, the more sensitively γ − changed in response to n Cs(0). Furthermore, the composition at the Cs-sputtered surface was examined using Auger electron and wavelength dispersive X-ray spectrometry. These measurements showed that n In(0), n P(0) and n Cs(0) changed almost linearly with 1/ Y InP, suggesting that the Cs surface concentration (at.%), C Cs(0), is proportional not to 1/ Y itself, but to a function of 1/ Y. However, under certain special conditions, C Cs(0) is also proportional to 1/ Y itself, as is n Cs(0).

Secondary ion emission from keV energy atomic and polyatomic projectile impacts on sodium iodate

Sodium iodate and sodium iodide are inorganic solids in which iodine exists in different chemical environments. In sodium iodate, the iodine atom is bonded to oxygen to make the trigonal pyramidal IO 3 anion, which in turn is incorporated with sodium into an ionic crystal. In sodium iodide, however, the iodide anion and sodium cation are ionically bound in a crystal lattice. Nearly half of the negative secondary ion yield generated from keV energy polyatomic ion impacts on a sodium iodate surface is characteristic of ion emission expected from sodium iodide (i.e. (NaI) n I −), yet x-ray photoelectron spectroscopy data indicate that the solid material resulting from aliquots of aqueous sodium iodate dried on stainless steel contains no more than 2% sodium iodide. To determine how the number of atoms in the primary ion influences the amount of iodide type ion formation from sodium iodate, secondary ion yield measurements were performed using Cs, (CsI)Cs, and (CsI) 2Cs projectiles incident at energies ranging from 10 to 25 keV. The experiments were run on an event-by-event basis at the level of single ion impacts. The yields of iodate (composed of Na, I, and O) and iodide type secondary ions increase with the energy of the projectile and the number of constituent atoms. When compared on a per-incident atom basis, however, we found that the yields of secondary ions characteristic of iodate saturate at three total projectile atoms, but continue to increase nonlinearly for iodide species (i.e. the yield per impacting atom increases).

Secondary ion yields produced by keV atomic and polyatomic ion impacts on a self-assembled monolayer surface

A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd.

Emission of secondary particles from metals and insulators at impact of slow highly charged ions

The emission of secondary electrons and ions from clean Au, C x H y Au and SiO 2 surfaces at impact of slow ( v ≈ 0.3 v Bohr) ions has been measured as a function of incident ion charge for 1 + ≤ q ≤ 75 +. Electron yields from thermal silicon dioxide films (150 nm on Si) are found to be lower than those from Au and C x H y Au for q > 3 +. Yields of negative secondary ions from SiO 2 and C x H y Au were recorded in parallel with electron emission data and exhibit a q n , n ≈ 4, dependency on incident ion charge. A direct comparison of collisional and electronic contributions to secondary ion production from SiO 2 films using a beam of charge state equilibrated Xe q= qeq (at 2.75 keV/u) shows positive and negative secondary ion yield increases with incident ion charge of > 400. Results are discussed in relation to key signatures of electronic sputtering by Coulomb explosions.

A systematic study of the characteristic velocities of the negative secondary ion emissions of H, O and matrix species from Sc, Ti, V, Co, Ni and Cu substrates

The negative H, O and matrix secondary ion emissions resulting from Ar +, Cs + and O + 2 bombardment of Sc, Ti, V, Co, Ni and Cu foils were found to exhibit a wide range of energy distribution shapes and intensities. These energy distributions could be reconstructed using two velocity dependent functions. Characteristic velocity values extracted from these velocity dependent functions were found to fall into groups according to the primary and secondary ions studied. Characteristic velocity values from within a particular group were found to correlate with work function and electron affinity arguments consistent with the Auger and resonance charge transfer processes (the Auger process predominating at lower emission energies (< 30 eV) while the resonance process predominates at higher energies (> 40 eV)). All secondary ion groups (H, O and the matrix secondary ion) resulting from a particular primary ion species could then be related on introducing a function relating the covalent radius of the secondary ion-surface atom pair. This, in turn, allowed for an empirical description of the negative secondary ion yield from metal surfaces to be defined.

A coincidence counting study of spontaneous desorption

The “spontaneous” emission of ions and photons from surfaces was studied with coincidence counting time-of-flight mass spectrometry. Photons were produced by the spontaneous desorption process with an experimental yield of about 0.01%. Both positive and negative secondary ion yields were measured under various conditions. Results imply classes of desorption events. This may indicate that several types of primary ions are being produced during a single experiment. There was no detectable change in the types of primary ions involved in the spontaneous desorption process as a function of sample potential within the range examined. Positive and negative valine molecular ion yields were found to be similar.

Surface secondary electron and secondary ion emission induced by large molecular ion impacts

This paper presents results on secondary ion and secondary electron yields under impact of large ions at 18 keV and with 100 < m/z < 70 000 on a CsI-coated surface. Ratios of secondary electron emission to secondary ion emission have been measured and it is shown that the negative secondary ion yield is much larger than the electron yield. Electrons are still emitted below an energy of impact per mass unit of 1 eV Da −1 (10 6 cm s −1). Effects of grids in the acceleration of secondary ions and secondary electrons are important.

Correction Method for Ion Yield Transients in the Near Surface Region of Sims Depth Profiles

As solid state device features continue to decrease in size, it has become more important to characterize dopant concentrations within the first several hundred angstroms of the surface. Secondary ion mass spectrometry (SIMS) is the technique of choice for dopant depth profiling due to its high sensitivity and good depth resolution. In order to increase the sensitivity of SIMS, electropositive elements (e.g. oxygen) or electronegative elements (e.g. cesium) are used as primary ion species to enhance positive or negative secondary ion yields, respectively. This has the disadvantage, however, of causing secondary ion yields to vary by up to several orders of magnitude over the first few hundred angstroms of a depth profile as the implanted primary ion concentration increases [1,2]. Secondary ion yields stabilize once the primary ion reaches a steady state concentration, which occurs at a depth proportional to the range of the primary ions in the solid. This ion yield transient artifact hinders quantification of dopant concentrations until the primary ion concentration reaches steady state.