Introduction of ice protective film for 3D microscale analysis of biological sample

It is urgently necessary for secondary ion mass spectrometry (SIMS) analysis to overcome influence on the compositional distribution of the sample in vacuum chamber. In this study, we investigated the handling of the ice protective film in techniques such as the gallium focused ion beam (Ga FIB) etching. Here we demonstrate the technique with frozen Hymenochirus boettgeri red blood cell. The red blood cells covered with an ice protective film were cross-sectioned by using Ga FIB, and the two-dimensional SIMS mapping over the cross-section was carried out. The distributions of Na and K were observed on the cross-section and surface of red blood cell with ice protective film. This result agrees qualitatively with physiological intracellular and extracellular concentrations of vital cells. The technique used for SIMS was proved to be a reliable method, preserving the cells in their living state.     

SIMS analysis of residual gas elements with a Cameca IMS-6f ion microprobe

In this paper, we present experimental data for SIMS analysis of residual gas elements (RGEs) with a Cameca IMS-6f ion microprobe. We considered a simple experimental technique, which provides an effective separation of the secondary ions, sputtered from the bulk of a target, and from the molecules, adsorbed on the analyzed surface from the residual atmosphere. The technique needs the sputtering yield of one monolayer (ML) per second to be applied. The method improves (in more than one order of magnitude) the detection limit for RGEs in SIMS analysis, and simultaneously, provides information about the residual atmosphere at the sample surface and in the main chamber of the experimental instrument. The method provides a calibration method for an ion gauge, and can be used for SIMS analysis with a gas (O₂) flooding.     

Depth profiling using C60 SIMS—Deposition and topography development during bombardment of silicon

A C₆₀⁺ primary ion source has been coupled to an ion microscope secondary ion mass spectrometry (SIMS) instrument to examine sputtering of silicon with an emphasis on possible application of C₆₀⁺ depth profiling for high depth resolution SIMS analysis of silicon semiconductor materials. Unexpectedly, C₆₀⁺ SIMS depth profiling of silicon was found to be complicated by the deposition of an amorphous carbon layer which buries the silicon substrate. Sputtering of the silicon was observed only at the highest accessible beam energies (14.5 keV impact) or by using oxygen backfilling. C₆₀⁺ SIMS depth profiling of As delta-doped test samples at 14.5 keV demonstrated a substantial (factor of 5) degradation in depth resolution compared to Cs⁺ SIMS depth profiling. This degradation is thought to result from the formation of an unusual platelet-like grain structure on the SIMS crater bottoms. Other unusual topographical features were also observed on silicon substrates after high primary ion dose C₆₀⁺ bombardment.     

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.     

The application of correlated SIMS and RBS techniques to the measurement of ion implanted range profiles

A simple, inexpensive secondary ion mass spectrometer (SIMS) instrument is de. scribed and its application to the determination of the range profiles of 20–30 keV Cs⁺ ions implanted into silicon and aluminium targets is demonstrated. The results are compared with those obtained by the alternative method of Rutherford backscattering (RBS) analysis and it is shown that provided the SIMS sputtering yield, S, (which is required for the calculation of the depth scale of the SIMS data) is chosen as a fitting constant, the agreement between the two techniques is excellent. On this basis, therefore, it is proposed that, provided a trace implant of Cs⁺ is included to provide an in-built calibration of S, the SIMS apparatus offers a universal technique for the determination of the profiles of impurities present at concentrations of 1–100 ppm.     

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.     

Cations in mammalian cells and chromosomes: Sample preparation protocols affect elemental abundances by SIMS

The focus of our current research aims at detailing and quantifying the presence of cations, primarily Ca and Mg, in mammalian cells and chromosomes throughout the different stages of the cell cycle, using our high resolution scanning ion microprobe, the UC-SIM. The 45 keV Ga⁺ probe of this instrument, typically ∼40 nm in diameter, carries a current of 30-40 pA, appropriate for surface SIMS studies, but limited in sample erosion rate for dynamic SIMS mapping over cell-size areas, of order 100 μm × 100 μm. Practical and reliable use of this probe toward the above SIMS goals requires a careful matching of the latter factors with the physical and chemical consequences of sample preparation protocols. We examine here how the preferred sample cryo-preservation methodologies such as freeze-fracture and lyophilization affect high resolution SIMS analysis, and, from this standpoint, develop and evaluate the advantages and disadvantages of fast alternate approaches to drying frozen samples. The latter include the use of methanol, ethanol, and methanol/acetic acid fixative. Methanol-dried freeze-fractured samples preserve histological morphology and yield Ca and Mg distributions containing reliable differential dynamical information, when compared with those following lyophilization.     

Molecular SIMS – A journey from single crystal to biological surface studies

The development of Static or Molecular secondary ion mass spectrometry (SIMS) is reviewed with particular reference to the journey made by the Manchester group and its collaborators. The earliest studies focussed on the application of static SIMS to single crystal surface studies. These studies successfully demonstrated that static SIMS delivered information on the delicate adsorbate state that mirrored that obtained by other surface science techniques. Subsequent application of the technique to studying the state and reactivity of bimetallic surfaces stimulated by collaboration with the Ertl group, demonstrated that static SIMS could be applied to the investigation of quite complex surface chemistry. This success stimulated the application of the technique to surface chemistry studies of much more complex systems such as polymers, ice mimics of polar stratospheric clouds, aerosols, culminating in biological systems. The need to enhance ion yields of the larger biological molecules led to the development and introduction of polyatomic primary ion beams, most notably based on C₆₀ buckminsterfullerene. This type of ion beam has transformed molecular analysis by SIMS. Not only have the yields of larger molecular ions been greatly increased, the bombardment induced damage that necessitated the static limit has been dramatically reduced such that for many materials the static limit requirement can be abandoned. A completely new analytical regime has opened up so that molecular depth profiling and 3D chemical imaging is possible. To fully realise the new capabilities for biological analysis a new generation of ToF-SIMS instrument is being developed that overcomes the compromises of pulsed beam instruments and that enables high mass resolution, high spatial resolution and high duty cycle to be attained simultaneously.     

TOF-SIMS 5 instrument sensitivity to matrix elements in GeSi Layers: Analysis based on recording of complex secondary ions

Ge _x Si_{1 − x} layers are investigated by means of secondary ion mass spectrometry (SIMS). Experimental results obtained with the use of a TOF-SIMS 5 instrument are presented. To surmount the so-called matrix effect, SIMS analysis is performed by using complex secondary ions: Ge₂⁻, CsGe⁺, and Cs₂Ge⁺.     

3D molecular imaging SIMS

Thin monolayer and bilayer films of spin cast poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(lactic) acid (PLA) and PLA doped with several pharmaceuticals have been analyzed by dynamic SIMS using SF₅⁺ polyatomic primary ion bombardment. Each of these systems exhibited minimal primary beam-induced degradation under cluster ion bombardment allowing molecular depth profiles to be obtained through the film. By combing secondary ion imaging with depth profiling, three-dimensional molecular image depth profiles have been obtained from these systems. In another approach, bevel cross-sections are cut in the samples with the SF₅⁺ primary ion beam to produce a laterally magnified cross-section of the sample that does not contain the beam-induced damage that would be induced by conventional focussed ion beam (FIB) cross-sectioning. The bevel surface can then be examined using cluster SIMS imaging or other appropriate microanalysis technique.     

Analysis of solids by secondary ion and sputtered neutral mass spectrometry

A mass spectrometer is described, which allows the analysis of sputtered neutral and charged particles as well as of residual gas composition. This combined SIMS, SNMS, and RGA instrument consists of a scanning primary ion beam column, an electron impact ionizer, an electrostatic energy filter and an rf quadrupole mass analyzer.Various examples of surface and bulk analysis are presented which demonstrate the beneficial complementary features of these techniques. These are, in particular: a substantial reduction of the matrix effect and fewer complications with samples of low electrical conductivity in SNMS, and the possibility of measuring the depth distribution of gases included in small cavities in the solid in the SNMS/RGA mode. SIMS, on the other hand, allows in many cases higher detection sensitivities.EURATOM Association     

Annealing study of Sb and Al ion-implanted ZnO

In this work we have studied diffusion and electrical activation in Al⁺ and Sb⁺ implanted ZnO samples using secondary ion mass spectrometry (SIMS), scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). The samples were hydrothermally grown and post-implant annealing was performed at 800, 900 and 1000 C in pure oxygen atmosphere. After each annealing step the samples were characterized with SSRM/SCM and SIMS. The thermal treatments did not induce any significant impurity redistribution as measured by SIMS, while electrical compensation is observed by SSRM/SCM for the Sb-implanted sample yielding less n-doping than in the as-grown samples. In the Al-implanted samples, an increase in carrier concentration is observed; we ascribe this to Al-related donors and possibly interstitial lithium, a common residual impurity in the samples that have been shown to be very mobile by SIMS.     

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.     

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.     

Photoexcitation of electron-hole pairs during SIMS

During analysis with SIMS (secondary ion mass spectroscopy) a HeNe laser beam was focussed on the sample surface. While sputtering Si with oxygen ions, the laser irradiation produced a strong increase of the target current and the SIMS intensities as well. This was found for lightly p-doped Si only, whereas no effect was observed for highly p-doped or n-doped Si. To explain this we assume that a depletion layer exists under the surface oxide layer and free charged carriers are created therein by laser excitation. The laser induced effects observed in the SIMS intensity or the target current can be used for measuring the profile of an ion beam or for measuring the alignment of an ion beam at a laser marked target. In addition, laser irradiation combined with SIMS allows one to measure qualitatively both the profile of the doping impurity and its electrically active part.     

Depth profiling of organic materials using improved ion beam conditions

We used the so-called dual beam mode of depth profiling to start a systematic investigation of organic depth profiling with a time of flight secondary ion mass spectrometer (TOF-SIMS) instrument. Similar to inorganic profiling, we found the dual beam mode beneficial because sample erosion and sample analysis are decoupled and can be optimised independently. We applied different primary projectiles such as C₆₀, O₂ and Cs for sputtering to a variety of organic specimens, using a wide range of impact energies. Results are discussed with respect to the feasibility of the different approaches to organic depth profiling in SIMS.     

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.     

Surface Analysis of Pulverised Fuel Ash by Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy

Samples of pulverised fuel ash (PFA) obtained both from power stations and from laboratory combustion experiments have been examined by SIMS (secondary ion mass spectrometry) and XPS (x-ray photo-electron spectroscopy). The elemental analysis is at present semi-quantitative and indicates presence of sulphur mainly as sulphate ion in the outer surface layer (5–10 nm) of some samples. Other elements, notably Mg, Fe, K and Ti appear at higher concentrations, up to ～ 10% once the outer 10–20 nm surface layers have been removed by ion etching.     

Relative ion sputtering yield measurements by integration of secondary ion energy distribution using a retarding-dispersive Ion energy analyzer

The application of a retarding-dispersive energy analyzer as the pre-filter of a quadrupole mass analyzer has made it possible to combine a standard high-speed Secondary Ion Mass Spectrometer (SIMS) and a high-resolution secondary-ion energy analyzer into one instrument. Data taken with this instrument indicate the presence of very significant high-energy tails in the energy distribution of all observed secondary ions, even with relatively low (2 keV) primary ion energies. The shape of the energy distribution varies widely from element to element, for atomic compared to molecular species sputtered from a clean metal surface, and depends, for a given species sputtered from a metal surface, on the degree of surface oxidation. The variations established in the present work are large enough to introduce in many cases substantial discrepancies between published values of both relative ion sputtering yields and surface elemental concentrations and values obtained by considering the complete energy distribution. Methods of obtaining accurate secondary ion yields by integrating the energy distribution are discussed. Work performed under the auspices of the Division of Physical Research of the U.S. Energy Research and Development Administration.     

A new approach for preventing charging up of soft material samples by coating with conducting polymers in SIMS analysis

Dynamic secondary ion mass spectroscopy (SIMS) analysis of soft materials such as polymer or biomaterial is one of challenging subjects due to the charge up effect brought from the irradiation of a primary ion beam, hampering the collection of secondary ions. Conventional methods against the charging up are the electron beam irradiation for charge compensation and surface coating with metal, normally gold. Those methods require a compromise analytical condition, reducing the primary ion beam current to suppress the range of the charging, which degrading the performances of the SIMS analyses. We have proposed that a thicker conductive layer, capable of delocalizing the charge onto the surface, should be put on a soft insulator sample to avoid charging up. The depth profile of the hair sample coated wholly with a polythiophen-based conducting polymer was successfully measured in longer time without any charging up even in the maximum current of the oxygen primary ion beam (O₂⁺: 7.5 keV, 400 nA) or using an electron beam compensation system. Thus, the proposed method coating with a conductive organic polymer against the charging issue would be expected as a breakthrough on SIMS analysis.