Positive Secondary Ion Emission Research Articles

Core ionization and ion ejection during SIMS analysis

Energy distributions of secondary ions emitted during either 10 keV O2+ or low-energy electron bombardment have been studied for different ion species implanted into silicon. Some elements such as F and Cl, which are among elements with the highest ionization potential, have been shown to exhibit close similarity between ion-induced and electron-induced energy distributions. This indicates the dominance of electron-mediated processes in ion-stimulated positive ion ejection. From threshold measurements of electron-stimulated desorption (ESD) for F+ and Cl+, an Auger-related process has been identified as a possible mechanism for ion-stimulated emission of positive secondary ions. These results are also correlated with measurements of the positive secondary ion yield dependence on ionization potential.

Pressure dependence of secondary ion emission from selected transition metals under nitrogen dioxide

For Ar+ bombarded polycrystalline surfaces of Ta, Co, Ni, Pd and Pt the emission of positive secondary ions was observed using nitrogen dioxide as reactant gas with varied partial pressure (0.001 mPa <p{spno}2 < 10 mPa) and dynamic SIMS conditions (2 keV; 32 μA/cm2). The results indicate that NO2 molecules appear to be completely destroyed in adsorption to Ta. Different behaviour was found for the other target metals. This can be explained by assuming surface species of partially molecular type. In some cases the results indicate two different modes of surface interaction with the reactant gas.

Secondary ion emission of selected transition metals under nitrogen oxides

The emission of various positive secondary ions has been investigated for polycrystalline targets of Ti, V, Cr, Nb, Ta, Co, Ni, Cu, Pd and Pt, which were bombarded by Ar+ ions under “dynamic” SIMS (DSIMS) conditions in the presence of the gaseous nitrogen oxides N2O, NO and NO2 at fixed pressure and under residual gas. Besides ions of the Me+ type several fragmentary ions (e.g. N+, O+, NO+, MeN+ and MeO+) and also cluster ions MexOy+ (x ≥ 2, 0 ≤y ≤ 2) were detected. Signals of a more molecular type with respect to the reactant gas, e.g. MeNO+, were only found for Co, Ni, Cu, Pd and Pt. From this, one may infer that for the other targets the nitrogen oxides will exist preferentially in a dissociatively adsorbed state at the metal surface. Several aspects of secondary ion emission can be explained assuming a different degree of oxidation for the metals under the influence of reactant gas.

Possibilities and limitations of the LZS model in positive secondary ion emission

Abstract A quantitative description of the breaking of a bond between two ions is given by the Landau-Zener-Stueckelberg (LZS) model. In this paper, the possible application of this model to the metallic positive emission is discussed. The example of Cu+ emission is chosen in order to show that the model alone is unable to describe the whole ionization processes, and that it has to be inserted into a more comprehensive description. The bond breaking could for instance follow the promotion of a bound electron, or it could occur between the emitted cation and a complex anion including oxygen. Both possibilites can take into acccount the “matrix effect” observed in alloy sputtering.

Positive secondary ion emission from metal surfaces

The absolute yield of positive ions, P +, ejected by 2 keV argon ion bombardment of clean metal surfaces was measured. The secondary ion current was measured for normally ejected ions, and the ion yields were calculated as a function of the secondary ion velocity. Experimental measurements included careful calibration and correction for the instrument transmission as a function of ion energy. Materials chosen for this study were polycristalline Cu, Ag, Cr and Zr, which provided a wide range in ionization potential and cohesive energy. Earlier treatment of the data showed reasonably good conformity to the expression P + ∝ exp[(− A/v) cos θ], indicating a predominance of the electron tunneling mechanism under these conditions. A simple correction of the ion kinetic energy for the binding force to the solid brings the data into much better agreement with the exponential dependence than previously observed, and further supports this model.

Enhanced positive secondary ion emission from substituted polynuclear aromatic hydrocarbon/sulfuric acid solutions

AbstractSecondary ion mass spectra of singly substituted aromatic hydrocarbon/H2SO4 solutions showed intense aromatic molecular ion and protonated aromatic molecule peaks characteristic of dissolved aromatic compounds from a number of aromatic compound classes, including acids, aldehydes, ketones, nitriles and nitrogen heterocycles. The presence of simultaneously abundant peaks for molecular ions and protonated molecules in secondary ion mass spectra of each aromatic compound/sulfuric acid solution is consistent with known or expected gas‐phase proton transfer chemistry. The ratio of intensities, M+˙:[M + H]+, appears to be determined by sulfuric acid solution chemistry of the compound. Spectra obtained from 1–2 μl samples were relatively free from chemical noise and persisted for up to 20 min. Detection limits for some substituted aromatic compounds are estimated to be 10−12.

Effect of chemical bonding on positive secondary-ion yields in sputtering.

We have studied the effect of chemical bonding on the emission of positive secondary ions from solid surfaces in static mode sputtering. In all the three systems investigated, N-Si(100), O-Si(100), and O-Ge(111), the ${\mathrm{Si}}^{+}$ and ${\mathrm{Ge}}^{+}$ yields were enhanced by at least 2 orders of magnitude. X-ray photoemission showed that the ion yields were linearly related to the amount of surface nitrides or oxides formed during the reactions. The ion yields were site specific and showed emission-velocity and angle dependences. The results are compared to the predictions of a recently proposed bond-breaking model.

Urban aerosols, analysed by secondary ion mass spectroscopy

Urban aerosols, collected on membrane filters by an eleven stage low pressure impactor have been analysed by SIMS (Secondary Ion Mass Spectroscopy). An atomic beam of argon neutrals, 3.5 mm in diameter, with an energy equivalent of 1.4 keV and a flux of 6·10 13 atoms cm −2 sec, generates the emission of positive and negative secondary ions (SI). The intensity of these SI vs. their ratio m/ e has been recorded. As a result the distributions of the relative concentrations vs. the particle size of several elements, as well as chemical compounds of these aerosols beginning with hydrogen up to a m/ e of 100, are shown.

Evaluation of a liquid metal ion source for secondary ion mass spectrometry

Positive and negative secondary ion emission of 23 pure elements have been studied for 10 keV In + and 10 keV O 2 + bombardment. In + ions were produced in a liquid metal ion source. For most of the elements investigated positive and negative secondary ion yields under In + impact are comparable to those obtained with O 2 + primary ions. Admission of oxygen into the sample chamber enhances positive and negative ion intensitities in a strongly element-specific manner. Depth profiles of a Ni/Cr multilayer (100 Å single-layer thickness) using 5 keV In + primary ions show that these ions may also be applied successfully for secondary ion mass spectrometric depth profiling.

Oxygen-concentration dependent enhancement of positive secondary ion emission from silicon

The enhancement of positive secondary ion yieds of silicon due to the presence of oxygen has been investigated quantitatively by low-energy (5 keV) oxygen implantation. Implantation and sputter profiling with 9 keV In + were performed in the same ion microprobe instrument. Depth profiles of substrate and implanted oxygen atoms were measured for fluences ranging from 5×10 15 to 4×10 16 O-atoms/cm 2. The oxygen concentration, c(O), in the sample was deduced from the implanted fluence and the range distribution in the Gaussian approximation. It was found that the oxygen-enhanced Si + intensity is proportional to c(O) (with x=1.4) in the concentration regime, 1.5⩽ c(O)⩽30%. The O + intensity shows a similar dependence for c(O)≳20 at.%.

Effect of surface chemistry and work function in secondary ion mass spectrometry

By using the electropositive Cs and Li to induce changes in the surface work function φ, the positive secondary ion emission is shown to vary exponentially with φ, in close resemblance to negative secondary ion emission. This is in contrast to the enhancement effect of adsorbed oxygen on positive secondary ion emission which shows no correlation with the work function change. This suggests that the oxygen enhancement effect originates mostly from surface chemical changes rather than from the formation of a surface electric dipole layer.

Positive secondary-ion emission from Fe-Ni alloys under O+2 bombardment at 45° incidence

Normalized ionization probabilities α+ are reported for binary Fe-Ni alloys under O+2 bombardment at a 45° incidence angle. It is shown that the dependence of α+ on the composition is much smaller than at normal incidence. Photoemission spectroscopy reveals that both nickel and iron remain essentially metallic under oblique-incidence bombardment, whereas almost complete oxidation occurs at normal incidence. This is attributed to our observation that sputter yields under oxygen bombardment increase much faster than cos α−1.

Pressure independence of secondary ion emission enhancement of Al during oxidation at low pressure

We have performed measurements of positive secondary ion emission during low pressure oxidation of aluminum. We found that the enhancement of the ion yield is independent of the pressure at which the exposure is made, in disagreement with what one would expect based on work function models of SIMS. This result partially supports “chemical” ion emission models.

Pressure independence of secondary ion emission enhancement of Al during oxidation at low pressure

We have performed measurements of positive secondary ion emission during low pressure oxidation of aluminum. We found that the enhancement of the ion yield is independent of the pressure at which the exposure is made, in disagreement with what one would expect based on work function models of SIMS. This result partially supports “chemical” ion emission models.

Molecular secondary ion mass spectrometry (SIMS)

Ion beams have been used for a number of years in material analysis. SIMS has been used recently to study the interaction of adsorbed gases on metal surfaces and the molecular structure of organic, inorganic, biologic, and polymeric compounds. This paper reviews the advancing field of molecular SIMS applied in two distinct areas: as an analytical tool for surface analysis and as a sensitive ionization source for nonvolatile and thermally-labile molecules. The present understanding of the molecular ion formation process is illustrated by results of calculation and experiment. Comparison of SIMS with other new ionization methodologies in mass spectrometry such as field, plasma, and laser desorption shows clear spectral similarities. In addition, SIMS demonstrates an extreme surface sensitivity, best detection limits in the picogram range, and high mass capability (≳5000 amu). Ionization of nonvolatile and thermally labile molecules can occur via three separate processes involving cationization/anionization, electron transfer, or direct sputter emission of positive and negative secondary ions. Over 80 papers are referenced.

Intrinsic secondary ion emission from binary alloys during Ar+ bombardment

The emission of positive atomic secondary ions from single phase binary alloys during 15 keV Ar+ bombardment was studied in an ultrahigh vacuum environment. For all the five alloy systems: Fe–Ni, Cu–Ni, Pd–Ni, Al–Ni, and Si–Ni, the ionization probabilities of the elements show little or no correlation with the bulk alloy density of states or the local density of states near the Fermi level. When the ionization probabilities are compared with the electronegativity differences of the elements, we notice that only the emission of Al+ and Ni+ from Al–Ni alloys seems to follow the direction of the anticipated charge transfer.

A quantitative model for the interpretation of secondary ion mass spectra of dilute alloys

A model is developed for the emission of singly-charged positive secondary ions of trace elements from metallic matrices under perpendicular ion bombardment. The model combines the sputtering theory of Sigmund with the adiabatic surface ionization model of Schroeer , and provides analytical expressions for the energy distribution and the yield of sputtered atomic particles (eqns. (7) and (8)) of a trace element, and of the fraction thereof with single positive charge (eqns. (16) and (17)). While the expressions for the ions still contain one fitting parameter (a modified form of Schroeer's “surface thickness”), this parameter falls away in expressions for the relative secondary ion yield of any two elements sputtered from the same matrix. Of the different solid phases that can be present at a metallic surface two important case are considered, viz. homogeneous solutions and pure precipitates of trace elements. The influence of the presence of the different phases on the relative ion yields and on an “average” analysis is discussed. It is also shown that in order to obtain meaningful analyses the band of emission energies which is accepted by the SIMS instrument has to be known exactly. Analytical expressions are developed which take into account this effect.