Secondary Ion Emission Process Research Articles

Alkali-metal-based molecular-ion secondary ion mass spectrometry for precise quantitative analysis of low-dimensional materials without standards

Advancement in the understanding of fundamental aspects of Secondary Ion Mass Spectrometry (SIMS) has made this technique to be extremely powerful for quantitative materials analysis. The secondary ion emission process is a complex phenomenon involving various mechanisms to understand the mechanism. Among the existing mechanisms, the “electron-tunneling model” is the most accepted concept in the understanding of ionization probability for positive and negative ions. This model essentially concerns the survival probability of an ion emitted from the surface. As the intensity of secondary ions of a certain element depends strongly on the efficiency of ionization of a sputtered atom or molecule, the “local surface chemistry” plays a significant role in the secondary ion emission. This is purported as the “Matrix Effect” in SIMS, making the technique extremely challenging for quantification of materials in spite of the fact that it has the highest detection sensitivity (<ppb) and exceptional depth-resolution (<1[Formula: see text]nm). Therefore, it is required to correct the “matrix effect” for quantitative analysis. If the probing element (M) in its close proximity sees an alkali element (such as Li, Rb, K, Na and Cs, referred to as “A”) on the sample surface, the sputtered neutral atom (M0) of the probing element can attach an alkali-ion forming a quasi-molecular (MA)[Formula: see text] ion. Such a phenomenon can also occur if an alkali ion beam is chosen as the primary beam for sputtering. The MA[Formula: see text] molecular ion formation is directly related to the atomic polarizability of the element M. Since the sputtering of neutral atom (M0) is unconnected with the formation of MA[Formula: see text] ion, the “matrix effect” decreases dramatically in the analysis using MA[Formula: see text] ions. The phenomenon is very similar to the mechanism of “secondary neutral mass spectrometry” (SNMS). Although these (MA)[Formula: see text] molecular ions are potentially applicable in the direct quantification of materials without the need for “calibration standards”, it generally has a weak intensity. In such a case, the (MA)[Formula: see text] molecular ions are preferred in the analysis because of their higher detection sensitivity (by a few orders of magnitude in some cases), compared to that of (MA)[Formula: see text] molecular ions. Monitoring of the molecular ions is routinely preferred in conventional SIMS experiments to discriminate the sputtered ion species which are affected by mass interference or have poor dynamic ranges. For example, while making the SIMS analysis of GaAs, carbon as an impurity element is detected by monitoring (AsC)[Formula: see text] molecular ions instead of C− ions, because the C− ions have high background arising from the carbon-containing residual-gas species in the analysis chamber. Caesium is decidedly chosen for MCs[Formula: see text] or MCs[Formula: see text] molecular ions in SIMS because of the strongest reactivity and electropositive nature of caesium. This paper deals with a comprehensive discussion on the ion-emission phenomena in sputtering, the “matrix effect” and its compensation, and the potential application of the “MCs[Formula: see text]-SIMS” ([Formula: see text]) in the effective quantitative chemical analysis of materials. A special prominence has been given to the quantification of low-dimensional materials, superlattices and quantum-confined structures using this state-of-the-art “MCs[Formula: see text]-SIMS” approach in all complexities.

Incident Energy Dependence of the Molecular Orientation Effect of MeV \(\text{C}_{2}^{ + }\) Projectiles in Secondary-Ion Emission Processes

Incident Energy Dependence of the Molecular Orientation Effect of MeV \(\text{C}_{2}^{ + }\) Projectiles in Secondary-Ion Emission Processes

Fast-ion-induced secondary ion emission from submicron droplet surfaces studied using a new coincidence technique with forward-scattered projectiles.

A mass spectrometric study of secondary ions emitted from droplet surfaces by MeV-energy heavy ion impact was performed to investigate fast-ion-induced molecular reaction processes on liquid surfaces. Herein, a new coincidence technique was developed between secondary ions and scattered projectile ions at a small forward angle. The advantages of this technique were demonstrated by measurement of the collision between 4-MeV C3+ and ethanol droplets. Secondary ion emission probabilities were obtained directly from the coincidence data. Notably, this technique enabled positive fragment ions that had not been identified in previous measurements to be observed by suppressing the strong background originating from gas-phase molecules more than 104-fold. H+, H3O+, C2H5 +, and C2H5O+ were found to be produced as major positive fragment ions, in addition to minor fragments H2 +, C2H3 +, and CH2OH+. Production of these ions suggests that competition between rapid hydrogen ion emission from multiply ionized states and intermolecular proton transfer accompanied by fragmentation through protonated ethanol occurs after fast heavy-ion collisions. Clarification of the positive fragment ions also revealed the characteristic features of negative ions. Negative ions were realized to exhibit higher degrees of fragmentation and reactivity compared with positive ions. Furthermore, the energy loss by forward-scattered ions during droplet penetration was used to evaluate the target thickness at a submicron level. Variations in secondary ion yield, mass distribution, and kinetic energies depending on the penetration length were observed below 1 µm. These results highlight the unknown mechanism of these "submicron effects" observed in secondary ion emission processes as a new phenomenon.

Effects of molecular axis orientation of MeV diatomic projectiles on secondary ion emission from biomolecular targets

We report first experimental results on the orientation effect of fast diatomic molecular projectiles on secondary ion emission processes for 3.6-MeV C2+ traversing a self-supporting target of phenylalanine (Phe) film evaporated on a carbon foil. Coincidence measurements were performed on the image of fragments resulting from the Coulomb explosion of C2+ projectiles and the time-of-flight mass spectrometry of secondary ions emitted from the Phe film. We compared the secondary ion yield for C2+ projectiles with parallel and perpendicular orientations to the beam direction. The parallel orientation was found to enhance the secondary ion yield by a factor of approximately 1.1 compared with that for the perpendicular orientation. This enhancement corresponds to an increase in the average charge of Coulomb-exploded fragments. This finding implies that there is a linear correlation between the electronic energy deposition causing secondary ion production and the effective charge number of the incident molecular ions which interact with biomolecular targets.

Characterization of secondary ion emission processes of sub-MeV C60 ion impacts via analysis of statistical distributions of the emitted ion number

We report probability distributions of the number of secondary ions (SIs) emitted by sub-MeV C60 ion impacts on an organic polymer target and the characterization of their emission processes through the analysis of the distributions. The probability distributions were obtained by analyzing experimental SI counting data obtained by a time-of-flight SI mass spectrometer combined with pulsed primary ion beams, using an analytical model developed to derive the distributions from the experimental data. A series of probability distribution functions was investigated for ion impacts of C60 with sub-MeV energies (0.12-0.54 MeV), which can provide sufficient SIs per impact to determine the functions. Their complicated and undefined SI emission processes were characterized based on the determined functions.

Insights into the yield enhancement and ion emission process in metal‐assisted SIMS

Although nowadays the use of cluster ion sources seems to enhance the secondary ion emission for nearly all materials, the technique of metal‐assisted SIMS can still give further insights in the secondary ion emission process. In this study, the metallization for static SIMS analysis was performed in situ. The combination of a detailed morphology study by SEM, TEM and the secondary yield enhancements in time‐of‐flight SIMS allows to develop a model explaining the secondary yield enhancement in metal‐assisted SIMS. Copyright © 2012 John Wiley & Sons, Ltd.

Secondary ion emission processes of sputtered alkali ions from alkali/Si(100) and Si(111)

The secondary alkali ion yield vs. the work function change (Δ φ) of Na, K and Cs/Si(100) and Si(111) was measured to discuss the details of secondary ion emission processes. In the case of alkali/metal systems, the secondary ion emission is explained by the electron tunneling model. In this model, the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal, and the secondary ion yield depends on exponentially the work function change of metal surface. For alkali/Si(100) systems, the secondary ion emission processes are explained in terms of the electron tunneling model since the secondary alkali ion yield vs. the work function change (Δ φ) follows the exponential manner. However, it is not easy to apply the simple electron tunneling model to our experimental results for alkali/Si(111) systems. There is the essential difference in surface structures between Si(100) and Si(111). Therefore, it is suggested that the local electronic environment around the adsorbates might be taken into consideration for alkali/Si(111) systems.

Chemisorption of poly(methylhydrogensiloxane) on oxide surfaces: a quantitative investigation using static SIMS

Monolayers of poly(methylhydrogensiloxane) (PMHS) were prepared on oxide powder surfaces consisting mostly of SiO 2. The polymer is covalently bonded via SiOSi bonds formed by reaction of SiH groups with surface SiOH groups. The large powder surface area allows the determination of the average number of surface bonds by classical chemical titration. Depending on the coverage of PMHS the average number of surface bonds ranges from 15 to 25 for a linear polymer chain with an average length of 30 repeat units. Static SIMS spectra of the samples show peak patterns similar to those obtained from PMHS prepared on silver targets. However, the fragment ion intensity distributions strongly depend on the average number of surface bonds per polymer chain. These intensity distributions are quantitatively analyzed using a simple statistical fragmentation model. The model assumes that fragments having the structure of the polymer backbone with different lengths are released from nonbonded polymer sections only. The probability of bond cleavage in the backbone is assumed to be constant. The model predicts the experimentally obtained intensity distributions very well. The average numbers of surface bonds calculated from fragment ion intensities are in very good agreement with the independent data obtained by chemical titration. Some information about the population distribution of surface bonds can also be obtained. This example shows that appropriate physical models of the secondary ion emission process in static SIMS can be used to obtain very detailed quantitative information about the molecular structure of polymer surfaces.

A heavy negative ion sputter source: Production mechanism of negative ions and their applications

Heavy negative ions can be produced by sputtering a metal surface at high efficiency by means of a secondary ion emission process. The most important factor for obtaining high negative ion production efficiencies is to maintain approximately one-half monolayer of Cs on the sputter target which produces a minimum in the surface work function. In a dynamic situation in which the surface is being bombarded by heavy ions, the cesium coverage is determined by a balance between the incoming and outgoing cesium fluxes. The supply of additional cesium to the surface and control of the target temperature are then essential for high negative ion production efficiency. The Neutral and Ionized Alkaline metal Bombardment type heavy Negative Ion Source (NIABNIS) in which a continuous negative ion current of a few hundred μA for several elements can be extracted, can be directly used for applications in materials science, such as negative ion beam deposition and negative ion implantation.

Chemical enhancement effects in SIMS analysis

This paper summarizes some of the recent advances in the understanding of the secondary ion emission process, and relates them to the important cesium and oxygen enhancement effects in secondary ion mass spectrometry (SIMS). The oxygen enhancement of ionization results from local breaking of oxide bonds and its magnitude is related to the microscopic distribution of oxygen atoms in the target. The Cs enhancement effect is found to be connected with the work function change induced by the Cs on the sample surface, and the ionization process is best described by an electron tunneling model. These differences affect the concentration dependence, alloying effect and the optimum operating conditions in SIMS analysis.

Molecular rearrangement and cluster formation in the secondary ion mass spectra (SIMS) of fluoride salts

Secondary ion clusters with mass greater than 700 amu, e.g., K(KF) 12 + and up to 27 atoms, e.g., Na(NaF) 13 +, have been observed in the static SIMS spectra of MF (M = Li, Na, K), NaBF 4, and KPF 6. The long series of detected cluster ions of the type M(MF) n + indicates that there is a high degree of stability associated with these clusters. The observation of such clusters in the NaBF 4 and KPF 6 spectra suggest that there is significant molecular rearrangement occurring in the secondary ion emission process from such salts. The secondary ion Intensities provide a crude fit to the Saha-Eggert equation, yielding an electron temperature of ~12,000 K. The data are consistent with the plasma model of surface ionization in which rearrangement and cluster formation occur in the plasma.