Secondary Ion Mass Spectrometry Imaging

Secondary ion mass spectrometry (SIMS) is a chemical analysis technique that employs mass spectrometry to analyze solid and low volatility liquid samples [1]. Although there are numerous configurations of SIMS instrumentation, the fundamental basis of SIMS analyses is the measurement of the mass and intensity of secondary ions produced in a vacuum by sputtering the surface of the sample with energetic ion or neutral beams. The sputtering beam is referred to as the primary beam and typically has a kinetic energy of several thousand electronvolts (keV). The primary beam removes atomic or molecular layers at a rate determined principally by the intensity, mass, and energy of the primary species and the chemical and physical characteristics of the sample [2]. Particle sputtering at the kiloelectronvolt level produces a variety of products including electrons, photons, atoms, atomic clusters, intact molecules, and distinctive molecular fragments. A small fraction of these sputter products are ionized, and these ions are the secondary ions in secondary ion mass spectrometry.     

Atoms, clusters and photons: Energetic probes for mass spectrometry

The physical mechanisms underlying the surface based mass spectrometry techniques of atomic SIMS, MALDI and cluster SIMS are discussed along with the relation of the physics to the measured quantities. In particular, there are at least two types of motion resulting from cluster bombardment in SIMS. One scenario involves the individual atoms in the cluster initiating collision cascades similar to atomic bombardment. The second mechanism involves a mesoscale motion of the cluster as a whole. This mesoscale motion can induce an organized flow of the ejected material in a plume.     

Mass accuracy—TOF-SIMS

A study is presented of the factors affecting the calibration of the mass scale for time-of-flight SIMS (TOF-SIMS). The effect of the ion kinetic energy, emission angle and other instrumental operating parameters on the measured peak position are determined. This shows clearly why molecular and atomic ions have different relative peak positions and the need for an aperture to restrict ions at large emission angles. A calibration protocol is developed which gives a fractional mass accuracy of better than 10 ppm for masses up to 140 u. The effects of extrapolation beyond the calibration range are discussed and a recommended procedure is given to ensure that accurate mass is achieved within a selectable uncertainty for large molecules.     

Secondary ion mass spectrometry for quantitative surface and in-depth analysis of materials

Secondary ion mass spectrometry (SIMS) is a technique based on the sputtering of material surfaces under primary ion bombardment. A fraction of the sputtered ions which largely originate from the top one or two atomic layers of the solid is extracted and passed into a mass spectrometer where they are separated according to their mass-to-charge ratios and subsequently detected. Because the sputter-yields of the individual species, coupled with their ionization probabilities, can be quite high and the mass spectrometers can be built with high efficiencies, the SIMS technique can provide an extremely high degree of surface sensitivity. Using a particular mode like static SIMS where a primary ion current is as low as 10^?11 amp, the erosion rate of the surface can be kept as low as 1 Å per hour and one can obtain the chemical information of the uppermost atomic layer of the target. The other mode like dynamic SIMS where the primary ion current is much higher can be employed for depth profiling of any chemical species within the target matrix, providing a very sensitive tool (～ 1 ppm down to ppb) for quantitative characterization of surfaces, thin films, superlattices, etc. The presence of molecular ions amongst the sputtered species makes this method particularly valuable in the study of molecular surfaces and molecular adsorbates. The range of peak-intensities in a typical SIMS spectrum spans about seven to eight orders of magnitude, showing its enormously high dynamic range; an advantage in addition to high sensitivity and high depth-resolution. Furthermore, the high sensitivity of SIMS to a very small amount of material implies that this technique is adaptable to microscopy, offering its imaging possibilities. By using this possibility in static SIMS or dynamic SIMS mode of analysis, one can obtain a two-dimensional (2D) surface mapping or a three-dimensional (3D) reconstruction of the elemental distribution, respectively within the target matrix. Secondary ion yields for elements can differ from matrix to matrix. These sensitivity variations pose serious limitations in quantifying SIMS data. Various methods like calibration curve approach, implantation standard method, use of relative sensitivity factor, etc. are presently employed for making quantitative SIMS analysis. The formation of secondary ions by ion bombardment of solids is relatively a complex process and theoretical research in this direction continues in understanding this process in general. The present paper briefly reviews the perspective of this subject in the field of materials analysis.     

The use of secondary ion mass spectrometry in surface analysis

Secondary ion mass spectrometry (SIMS) is based on the bombardment of solids by ions and subsequent mass analysis of the sputtered ions or of the post-ionized neutrals. Various models have been proposed for the emission of secondary ions from the target. These models can roughly be grouped into two categories: (a) lonization takes place outside the target; the sputtered particles are assumed to leave the target in an excited or super-excited state; ionization can then result from such processes as Auger de-excitation or resonance ionization. (b) Ions are generated inside the target by collision cascades initiated by a primary impinging ion. Experimental data of the element- and matrix-dependent ion yield and estimates of the minimum detectable concentrations are shown as well as the rate of consumption of the target. Methods for efficient use of the sputtered material will be discussed. Examples of the wide range of applications of SIMS for determination of the chemical composition and structure of the surface layer and for imaging of the distribution of elements in the surface are given. The SIMS results are compared with those of Auger and Ion Scattering Spectrometry. The essentially non-destructive method of static SIMS is shown to be a powerful tool for the investigation of the outermost atomic layers of a solid.     

Improved quantitative analysis of Cu(In,Ga)Se2 thin films using MCs+-SIMS depth profiling

The chalcopyrite semiconductor, Cu(InGa)Se₂ (CIGS), is popular as an absorber material for incorporation in high-efficiency photovoltaic devices because it has an appropriate band gap and a high absorption coefficient. To improve the efficiency of solar cells, many research groups have studied the quantitative characterization of the CIGS absorber layers. In this study, a compositional analysis of a CIGS thin film was performed by depth profiling in secondary ion mass spectrometry (SIMS) with MCs⁺ (where M denotes an element from the CIGS sample) cluster ion detection, and the relative sensitivity factor of the cluster ion was calculated. The emission of MCs⁺ ions from CIGS absorber elements, such as Cu, In, Ga, and Se, under Cs⁺ ion bombardment was investigated using time-of-flight SIMS (TOF-SIMS) and magnetic sector SIMS. The detection of MCs⁺ ions suppressed the matrix effects of varying concentrations of constituent elements of the CIGS thin films. The atomic concentrations of the CIGS absorber layers from the MCs⁺-SIMS exhibited more accurate quantification compared to those of elemental SIMS and agreed with those of inductively coupled plasma atomic emission spectrometry. Both TOF-SIMS and magnetic sector SIMS depth profiles showed a similar MCs⁺ distribution for the CIGS thin films.     

High resolution quantitative SIMS analysis of shallow boron implants in silicon using a bevel and image approach

Secondary ion mass spectrometry (SIMS) is frequently used as the preferred tool for dopant profiling due to its sensitivity and depth resolution. However, as dopant profiles become shallower most, if not all of the implant profile lies in the pre-equilibrium or transient region of an SIMS depth profile. In this region sputter yield and ionisation rate vary making accurate quantification of the implant profile very difficult. These problems can be reduced through the use of much lower beam energies or oxygen flooding of the sample. However, most SIMS instruments do not have these capabilities. In this paper an alternative technique for producing an accurate depth profile of a shallow implant, using existing SIMS technology is presented.Through the fabrication of bevels with very small slope angles on a shallow boron implanted silicon via a chemical etch, SIMS ion imaging is performed on the exposed surface. Ion image data is then summed, and in conjunction with accurate measurement of the bevel morphology, a shallow boron implant profile produced. The ‘bevel-image’ profile compares very well with a profile obtained using a 1 keV oxygen beam. To ensure a good dynamic range on the ‘bevel-image’ profile it is important to clean the bevel with a HF etch, prior to imaging.     

Plasma irradiation effects in phthalocyanine films

Plasma irradiation effects in copper phthalocyanine films were examined. The film surface and chemical composition were characterized by means of atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS), including SIMS profiling.     

Dopant-pair structures segregated on a hydrogen-terminated Si(100) surface

Novel atomic structures on a H-terminated Si(100)-(2x1)-H surface were found using scanning tunneling microscopy (STM). The structures are distinguishable only from Si dimers in empty-state STM images. They were observed on arsenic- and phosphorus-doped substrates, but not on boron-doped substrates. Surface density of these structures was found to be proportional to the dopant density in the substrate. First-principles calculations clarify that they are consisting of dopant pairs that are segregated from the bulk material. Hydrogen atoms attached to the dopant pair are found to flip between two positions on the surface due to a quantum effect.     

Functionality of novel black silicon based nanostructured surfaces studied by TOF SIMS

A functionality of the novel black silicon based nanostructured surfaces (BS 2) with different metal surface modifications was tested by time-of-flight secondary ion mass spectrometry (TOF SIMS). Mainly two surface functions were studied: analytical signal enhancement and analyte pre-ionization effect in SIMS due to nanostructure type and the assistance of the noble metal surface coating (Ag or Au) for secondary ion formation. As a testing analyte a Rhodamine 6G was applied. Bi⁺ has been used as SIMS primary ions. It was found out that SIMS signal enhancement of the analyte significantly depends on Ag layer thickness and measured ion mode (negative, positive). The best SIMS signal enhancement was obtained at BS2 surface coated with 400 nm of Ag layer. SIMS fragmentation schemes were developed for a model analyte deposited onto a silver and gold surface. Significant differences in pre-ionization effects can play an important role in the SIMS analysis due to identification and spectra interpretation.     

硅纳米结构晶体管中与杂质量子点相关的量子输运

在小于10 nm的沟道空间中,杂质数目和杂质波动范围变得十分有限,这对器件性能有很大的影响.局域纳米空间中的电离杂质还能够展现出量子点特性,为电荷输运提供两个分立的杂质能级.利用杂质原子作为量子输运构件的硅纳米结构晶体管有望成为未来量子计算电路的基本组成器件.本文结合安德森定域化理论和Hubbard带模型对单个、分立和耦合杂质原子系统中的量子输运特性进行了综述,系统介绍了提升杂质原子晶体管工作温度的方法.     

Metal-ion implantation in glasses: Physical and chemical aspects

A review of the state-of the-art of the research in the field of chemical interactions in silica and silicate glasses implanted with metal ions (e.g., Si, Ti, W, Ag, Cu, Cr) and N is presented in terms of new compounds formation. Moreover, under certain circumstances, the formation of nanometer-radius metal colloidal particles in a thin surface layer is observed. The chemical state of the implanted atoms is determined by X-ray photoelectron (XPS) and X-ray excited Auger-electron spectroscopies (XE-AES). Rutherford backscattering spectrometry (RBS) and secondary-ion mass spectrometry (SIMS) are used to determine the in-depth elemental distributions. Optical absorption measurements and transmission electron microscopy (TEM) are used to detect the presence of metallic clusters, as well as to determine their mean size and size distribution. A thermodynamics approach is used to explain the interaction between the implanted ion and the separate atomic species of the target glass and/or between the implanted ion and the target molecular species.     

Compositional analysis in the nano-regime:A SIMS perspective

A serious problem in secondary ion mass spectrometry (SIMS) analysis is its "matrix effect" that hinders the quantification of a certain species in a sample and consequently, appropriate corrective measures are taken to calibrate the secondary ion currents into respective concentrations for accurate compositional analysis. Use of "calibration standards" is necessary for this purpose. Detection of molecular MCsn+ ions (M-element to be analyzed, n=1, 2, 3,....) under Cs+ ion bombardment is a possible mean to minimize such matrix effect, enabling one to quantify without the need of calibration standards. Our recent studies on MCsn+ molecular ions aim towards the understanding of their formation mechanisms, which are important to know their effects on SIMS quantification.In-depth quantitative analysis is a major strength of SIMS for which 'depth resolution' is of significant relevance. The optimal choice of the impact parameters during SIMS analyses can play an effective role in obtaining data with ultra-high depth resolution. SIMS is possible at depth resolution in the nm or even sub-nm range, with quantifiable data obtained from the top monolayer onwards into the material. With optimized experimental conditions, like extremely low beam current (down to ～10 nA), and low bombarding energy (below 1 keV), ultra-high depth resolution SIMS has enabled interfacial composition analysis of ultra-thin films, quantum wells, heterostructures, etc. and complex low-dimensional structures with high precision and repeatability.     

New developments in the surface analysis of solids

In both fundamental and applied surface physics, it is essential to know as much as possible about the chemical composition of the outer atomic layers of solids. Rapid progress has recently been made in the development of analytical methods which could be used in surface analysis. All utilize some type of emission (photons, electrons, atoms, molecules, ions), caused by excitation of the surface states. Both the “excitation” and emission processes must meet certain basic requirements as regards information depth, form in which the information is obtained, sensitivity, changes in the surface layer during analysis, etc. The more important of the methods that qualify, namely Auger-Electron Spectroscopy (AES), photo-Electron Spectroscopy for Chemical Analysis (ESCA) and the static method of Secondary-Ion Mass Spectrometry (SIMS), are discussed and their potentialities and limitations illustrated by characteristic examples.     

Quantitative secondary ion mass spectrometry of fluorocarbon polymers

Secondary ion mass spectrometry (SIMS) is used to measure quantitatively the thickness of thin (6–160 Å) polyperfluoroether films on silicon and gold surfaces. Linear relationship between ellipsometrically measured thicknesses and integrated SIMS signals is demonstrated. Time dependence of SIMS signals indicates that the polymeric films have a uniform thickness down to the thinnest layers studied. In the lower limit, the fluorocarbon polymers have extended, flat conformation due to polymer-substrate interactions. Sputtering yield and effective sputtering depth of oxygen ions are determined for these liquid polymers. It is also shown that organic adsorbates reside between the solid surface and the low surface tension fluorocarbon films.     

Comparative SNMS and SIMS studies of oxidized Ce and Gd

Polycrystalline Ce and Gd surfaces of various oxidation states have been investigated in situ and alternately by Sputtered Neutral Mass Spectrometry SNMS and by SIMS. For the bombardment with 4 keV Ar⁺ ions the dominating peaks in the mass spectra of postionized sputtered neutrals and positive secondary ions refer to metal atoms Me and monoxide molecules MeO. At oxygen concentrations of several atomic percent the sensitivity of SNMS and SIMS are of the same order. Characteristic differences are found between the behaviour of corresponding SNMS and SIMS signals: For SNMS the intensitiesI(MeO⁰) and I(Me⁰) in general vary complementarily when the oxygen is removed from the surface. The intensity ratios $\text{I(MeO}{}^0\text{)}\text{I(Me}{}^0\text{)}$ decrease monotonously by 1 to 2 orders of magnitude. With SIMS both I(MeO⁺) and I(Me⁺) are found to decrease with the oxygen content. The SIMS ratios $\text{I(MeO}{}^{\mathrm{+}}\text{I(Me}{}^{\mathrm{+}}\text{)}$ display an oscillatory behaviour with only little variation of their absolute values. As an example, quantitative results for the behaviour of the ionisation probability α⁺_Me for the secondary ions Ce⁺ and Gd⁺ are derived for low oxygen concentrations.     

Band-structure engineering of gold atomic wires on silicon by controlled doping

We report on the systematic tuning of the electronic band structure of atomic wires by controlling the density of impurity atoms. The atomic wires are self-assembled on Si(111) by substitutional gold adsorbates and extra silicon atoms are deposited as the impurity dopants. The one-dimensional electronic band of gold atomic wires, measured by angle-resolved photoemission, changes from a fully metallic to semiconducting one with its band gap increasing above 0.3 eV along with an energy shift as a linear function of the Si dopant density. The gap opening mechanism is suggested to be related to the ordering of the impurities.     

Spontaneous 2D accumulation of charged Be dopants in GaAs p-n superlattices

In a classical view, abrupt dopant profiles in semiconductors tend to be smoothed out by diffusion due to concentration gradients and repulsive screened Coulomb interactions between the charged dopants. We demonstrate, however, using cross-sectional scanning tunneling microscopy and secondary ion mass spectroscopy, that charged Be dopant atoms in GaAs p-n superlattices spontaneously accumulate and form two-dimensional dopant layers. These are stabilized by reduced repulsive screened Coulomb interactions between the charged dopants arising from the two-dimensional quantum mechanical confinement of charge carriers.     

Investigation of surface layers by SIMS and SIIMS

In the Secondary Ion Mass Spectrometry (SIMS) the sample to be analysed is bombarded with a beam of primary ions. The secondary ions sputtered away from the sample, characteristic for its composition near the surface at any time, are mass selected and detected in a mass spectrometer. The yields of several elements in a Fe-matrix and in technically pure samples bombarded with positive oxygen and argon ions have been determined to study the influence of the matrix and the primary ions on the ion yields. The properties of SIMS and of two of its special modes viz. Static Secondary Ion Mass Spectrometry (SSIMS) and Secondary Ion Imaging Mass Spectrometry (SIIMS) with respect to the analysis of surface layers are discussed.     

Secondary ion mass spectrometry and optical characterization of Ti:LiNbO3 optical waveguides

Secondary ion mass spectrometry (SIMS) and m-lines spectroscopy have been applied to study Ti:LiNbO₃ slab optical waveguides with high titanium surface concentration. By combining the two techniques, a saturation in the dependence of the refractive index change on the dopant concentration has been found. By the use of SIMS in image mode, the lateral diffusion of titanium in Ti:LiNbO₃ channel waveguides has been observed and analyzed.