Ian S. Gilmore

Surface Analysis– The Principal Techniques

List of Contributors. Preface. 1 Introduction ( John C. Vickerman). 1.1 How do we Define the Surface? 1.2 How Many Atoms in a Surface? 1.3 Information Required. 1.4 Surface Sensitivity. 1.5 Radiation Effects - Surface Damage. 1.6 Complexity of the Data. 2 Auger Electron Spectroscopy ( Hans Jorg Mathieu). 2.1 Introduction. 2.2 Principle of the Auger Process. 2.3 Instrumentation. 2.4 Quantitative Analysis. 2.5 Depth Profile Analysis. 2.6 Summary. References. Problems. 3 Electron Spectroscopy for Chemical Analysis ( Buddy D. Ratner and David G. Castner). 3.1 Overview. 3.2 X-ray Interaction with Matter, the Photoelectron Effect and Photoemission from Solids. 3.3 Binding Energy and the Chemical Shift. 3.4 Inelastic Mean Free Path and Sampling Depth. 3.5 Quantification. 3.6 Spectral Features. 3.7 Instrumentation. 3.8 Spectral Quality. 3.9 Depth Profiling. 3.10 X-Y Mapping and Imaging. 3.11 Chemical Derivatization. 3.12 Valence Band. 3.13 Perspectives. 3.14 Conclusions. Acknowledgements. References. Problems. 4 Molecular Surface Mass Spectrometry by SIMS ( John C. Vickerman). 4.1 Introduction. 4.2 Basic Concepts. 4.3 Experimental Requirements. 4.4 Secondary Ion Formation. 4.5 Modes of Analysis. 4.6 Ionization of the Sputtered Neutrals. 4.7 Ambient Methods of Desorption Mass Spectrometry. References. Problems. 5 Dynamic SIMS ( David McPhail and Mark Dowsett). 5.1 Fundamentals and Attributes. 5.2 Areas and Methods of Application. 5.3 Quantification of Data. 5.4 Novel Approaches. 5.5 Instrumentation. 5.6 Conclusions. References. Problems. 6 Low-Energy Ion Scattering and Rutherford Backscattering ( Edmund Taglauer). 6.1 Introduction. 6.2 Physical Basis. 6.3 Rutherford Backscattering. 6.4 Low-Energy Ion Scattering. Acknowledgement. References. Problems. Key Facts. 7 Vibrational Spectroscopy from Surfaces ( Martyn E. Pemble and Peter Gardner). 7.1 Introduction. 7.2 Infrared Spectroscopy from Surfaces. 7.3 Electron Energy Loss Spectroscopy (EELS). 7.4 The Group Theory of Surface Vibrations. 7.5 Laser Raman Spectroscopy from Surfaces. 7.6 Inelastic Neutron Scattering (INS). 7.7 Sum-Frequency Generation Methods. References. Problems. 8 Surface Structure Determination by Interference Techniques ( Christopher A. Lucas). 8.1 Introduction. 8.2 Electron Diffraction Techniques. 8.3 X-ray Techniques. 8.4 Photoelectron Diffraction. References. 9 Scanning Probe Microscopy ( Graham J. Leggett). 9.1 Introduction. 9.2 Scanning Tunnelling Microscopy. 9.3 Atomic Force Microscopy. 9.4 Scanning Near-Field Optical Microscopy. 9.5 Other Scanning Probe Microscopy Techniques. 9.6 Lithography Using Probe Microscopy Methods. 9.7 Conclusions. References. Problems. 10 The Application of Multivariate Data Analysis Techniques in Surface Analysis ( Joanna L.S. Lee and Ian S. Gilmore). 10.1 Introduction. 10.2 Basic Concepts. 10.3 Factor Analysis for Identification. 10.4 Regression Methods for Quantification. 10.5 Methods for Classification. 10.6 Summary and Conclusion. Acknowledgements. References. Problems. Appendix 1 Vacuum Technology for Applied Surface Science ( Rod Wilson). A1.1 Introduction: Gases and Vapours. A1.2 The Pressure Regions of Vacuum Technology and their Characteristics. A1.3 Production of a Vacuum. A1.4 Measurement of Low Pressures. Acknowledgement. References. Appendix 2 Units, Fundamental Physical Constants and Conversions. A2.1 Base Units of the SI. A2.2 Fundamental Physical Constants. A2.3 Other Units and Conversions to SI. References. Index.

XPS: binding energy calibration of electron spectrometers 5—re-evaluation of the reference energies

The binding energies of the calibration peaks for x-ray photoelectron spectroscopy—Cu 2p3/2, Ag 3d5/2 and Au 4f7/2—have been reassessed based on the traceable data recorded in 1984 using unmonochromated x-rays and an analyser resolution of 0.3 eV. The changes in those energies, for different x-ray sources and analyser resolutions, have been calculated and the results compared with further data. This includes work with monochromatic Al x-rays recorded at high energy resolution, allowing the binding energies to be referred to a new zero value set at the Fermi edge measured for Ag. A consistent set of data is presented for the calibration and assessment of photoelectron spectrometers with energy resolutions in the range 0.2–1.5 eV, when used with unmonochromated Al or Mg x-rays or monochromated Al x-rays.

Quantitative XPS: I. Analysis of X-ray photoelectron intensities from elemental data in a digital photoelectron database

Abstract An analysis of the correlation of theoretical predictions for photoelectron intensities is made with experimental data from an XPS digital database for 46 solid elements measured using a spectrometer with calibrated intensity and energy scales. This analysis covers single element samples measured for Al and Mg Kα X-rays. The spectral data are for widescans at 1 eV energy intervals with kinetic energies from 200 to 1506 eV using Al X-rays and to 1273 eV using Mg X-rays. In addition are narrow scans around the photoelectron peaks at 0.1 eV energy intervals. All spectra have the instrument intensity/energy response function removed so that the peak areas are proportional to the number of electrons emitted per steradian per incident Kα photon. Correlations are made for the ionisation cross sections of Scofield and the inelastic mean free paths given by the TPP-2M formula. The correlations are excellent, apart from a factor which may be associated with the background removal arising from the use of the Tougaard Universal cross section. These correlations lead directly to pure element relative sensitivity factors suitable for quantitative analysis. General equations are also provided to extract values for a new form of relative sensitivity factor for an average matrix. These average matrix relative sensitivity factors lead to simpler equations involving matrix factors that are effectively unity instead of the traditional values in the range 0.3 to 3.0.

Argon Cluster Ion Beams for Organic Depth Profiling: Results from a VAMAS Interlaboratory Study

The depth profiling of organic materials with argon cluster ion sputtering has recently become widely available with several manufacturers of surface analytical instrumentation producing sources suitable for surface analysis. In this work, we assess the performance of argon cluster sources in an interlaboratory study under the auspices of VAMAS (Versailles Project on Advanced Materials and Standards). The results are compared to a previous study that focused on C(60)(q+) cluster sources using similar reference materials. Four laboratories participated using time-of-flight secondary-ion mass spectrometry for analysis, three of them using argon cluster sputtering sources and one using a C(60)(+) cluster source. The samples used for the study were organic multilayer reference materials consisting of a ∼400-nm-thick Irganox 1010 matrix with ∼1 nm marker layers of Irganox 3114 at depths of ∼50, 100, 200, and 300 nm. In accordance with a previous report, argon cluster sputtering is shown to provide effectively constant sputtering yields through these reference materials. The work additionally demonstrates that molecular secondary ions may be used to monitor the depth profile and depth resolutions approaching a full width at half maximum (fwhm) of 5 nm can be achieved. The participants employed energies of 2.5 and 5 keV for the argon clusters, and both the sputtering yields and depth resolutions are similar to those extrapolated from C(60)(+) cluster sputtering data. In contrast to C(60)(+) cluster sputtering, however, a negligible variation in sputtering yield with depth was observed and the repeatability of the sputtering yields obtained by two participants was better than 1%. We observe that, with argon cluster sputtering, the position of the marker layers may change by up to 3 nm, depending on which secondary ion is used to monitor the material in these layers, which is an effect not previously visible with C(60)(+) cluster sputtering. We also note that electron irradiation, used for charge compensation, can induce molecular damage to areas of the reference samples well beyond the analyzed region that significantly affects molecular secondary-ion intensities in the initial stages of a depth profile in these materials.

Ion detection efficiency in SIMS:: Dependencies on energy, mass and composition for microchannel plates used in mass spectrometry

Abstract The effects of ion energy, mass and composition on the detection efficiency of a microchannel plate (MCP) have been studied in detail, using a time-of-flight (TOF) mass spectrometer. This spectrometer is used for static secondary ion mass spectrometry (static SIMS) although the data are relevant to any ion-detection system. A model is developed that shows how the efficiency falls with increased mass and decreased ion impact energy at the front of the MCP. At an impact energy of 20 keV, the efficiency for the detection of cationised PS oligomers of mass 10,000 amu is approximately 80%, whereas at 5 keV it has fallen to ∼5%. The model is extended to estimate the effect of ion composition on the detection efficiency. It was found that ions with a high hydrogen content have a lower efficiency than those that consist of a cluster of high atomic number atoms. The spread of detection efficiencies arising from both composition and mass may be reduced by increasing the ion impact energy. Therefore, up to a mass of 4000 amu, the spread for ions of 100% observed for 5-keV ion impact energy is reduced to a negligible spread for ions of 20-keV impact energy, where the efficiency is approximately unity, independent of the composition. A simple method is provided to determine the correct voltage to operate the MCP for a given efficiency. This operating voltage should be determined for the highest mass ions in the required range. . Published by Elsevier Science B.V.

Static SIMS: towards unfragmented mass spectra — the G-SIMS procedure

Abstract A study is presented of the effects of the different positive ion beam species: Ar+, Ga+, Xe+, Cs+ and SF5+ and of their energies from 4 to 25 keV, on the fragmentation behaviour in static Secondary Ion Mass Spectrometry (SIMS) spectra for samples of the polymers: polytetrafluoroethylene (PTFE), polystyrene (PS) and polycarbonate (PC). The overall effect of energy is found to be weak over the entire mass spectrum. However, large differences are observed in restricted mass ranges amongst fragmentation groups. The fragmentation is quantified in terms of the partition functions of the fragments from a plasma with effective temperature, Tp. It is found that fragmentation is least for high mass projectiles at low energies, but that the trend is different for polyatomic ions. A methodology is developed, which unifies all of the fragmentation behaviour to a single plot — the Unified Cascade Gradient plot. An equivalence of mass and energy is shown and that the chemistry of the bombarding ion is unimportant. By extrapolation of the data to low Tp, a new spectroscopy, known as gentle-SIMS or G-SIMS is formed. The G-SIMS spectrum is in the static regime. Significant peaks in the G-SIMS spectra are those peaks, which would be emitted from a surface plasma of very low temperature and thus have little post-emission rearrangement or fragmentation. Those peaks are, thus, directly characteristic of the material without rearrangement and provide a direct interpretation and identification. In the tests of the method described, this is supported and indicates that the G-SIMS analysis will be significantly less ambiguous than static SIMS so that interpretation will be possible in the absence of a relevant reference spectrum.

Single-Cell Analysis: Visualizing Pharmaceutical and Metabolite Uptake in Cells with Label-Free 3D Mass Spectrometry Imaging.

Detecting metabolites and parent compound within a cell type is now a priority for pharmaceutical development. In this context, three-dimensional secondary ion mass spectrometry (SIMS) imaging was used to investigate the cellular uptake of the antiarrhythmic agent amiodarone, a phospholipidosis-inducing pharmaceutical compound. The high lateral resolution and 3D imaging capabilities of SIMS combined with the multiplex capabilities of ToF mass spectrometric detection allows for the visualization of pharmaceutical compound and metabolites in single cells. The intact, unlabeled drug compound was successfully detected at therapeutic dosages in macrophages (cell line: NR8383). Chemical information from endogenous biomolecules was used to correlate drug distributions with morphological features. From this spatial analysis, amiodarone was detected throughout the cell, with the majority of the compound found in the membrane and subsurface regions and absent in the nuclear regions. Similar results were obtained when the macrophages were doped with amiodarone metabolite, desethylamiodarone. The fwhm lateral resolution measured across an intracellular interface in high lateral resolution ion images was approximately 550 nm. Overall, this approach provides the basis for studying cellular uptake of pharmaceutical compounds and their metabolites on the single cell level.

Argon Cluster Ion Source Evaluation on Lipid Standards and Rat Brain Tissue Samples

Argon cluster ion sources for sputtering and secondary ion mass spectrometry use projectiles consisting of several hundreds of atoms, accelerated to 10-20 keV, and deposit their kinetic energy within the top few nanometers of the surface. For organic materials, the sputtering yield is high removing material to similar depth. Consequently, the exposed new surface is relatively damage free. It has thus been demonstrated on model samples that it is now really possible to perform dual beam depth profiling experiments in organic materials with this new kind of ion source. Here, this possibility has been tested directly on tissue samples, 14 μm thick rat brain sections, allowing primary ion doses much larger than the so-called static secondary ion mass spectrometry (SIMS) limit and demonstrating the possibility to enhance the sensitivity of time-of-flight (TOF)-SIMS biological imaging. However, the depth analyses have also shown some variations of the chemical composition as a function of depth, particularly for cholesterol, as well as some possible matrix effects due to the presence or absence of this compound.

TOF-SIMS: Accurate mass scale calibration

A study is presented of the factors affecting the calibration of the mass scale in time-of-flight secondary ion mass spectrometry (TOF-SIMS). At the present time, TOF-SIMS analysts using local calibration procedures achieve a rather poor relative mass accuracy of only 150 ppm for large molecules (647 u) whereas for smaller fragments of <200 u this figure only improves to 60 ppm. The instrumental stability is 1 ppm and better than 10 ppm is necessary for unique identification of species. The above experimental uncertainty can lead to unnecessary confusion where peaks are wrongly identified or peaks are ambiguously assigned. Here we study, in detail, the instrumental parameters of a popular single stage reflection TOF-SIMS instrument with ion trajectory calculations using SIMION. 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. These data provide the basis for a coherent procedure for optimizing the settings for accurate mass calibration and rules by which calibrations for inorganics and organics may be incorporated. This leads to a new generic set of ions for mass calibration that improves the mass accuracy in our interlaboratory study by a factor of 5. A calibration protocol is developed, which gives a relative 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. Additionally, we can alternatively operate our instrument in a regime with good energy discrimination (i.e., poor energy compensation) to study the fragmented energies of molecules. This leads to data that support previous concepts developed in G-SIMS.

Static SIMS: A Study of Damage Using Polymers

As a result of work to establish the surface potential of insulators accurately in a quadrupole static SIMS system of high sensitivity, we have been able to study the effects of increasing dose-related damage on the intensities of the mass spectral peaks in the two archetypal bulk polymers PET and PTFE, as well as thin hydrocarbon contamination layers, with high accuracy. It is shown that the intensities follow very well-defined functions which give damage cross-sections whose values reflect the fragmentation behaviour of the polymers. The effects reflect the number of bonds to be broken to liberate the fragment, the internal complexity of that fragment and the typical damage zone of the ion impact. These concepts show that a static SIMS limit of below 3 x 10 15 ions m -2 exists for changes of <10% in the intensities of the most easily damaged species but that some larger fragments may require a dose of 2 x 10 17 ions m -2 to maximize their intensity This work has three main conclusions. Firstly, by defining a figure of merit factor, F, equal to the ratio of the absolute intensity of a peak to the fractional rate of change of that peak with the ion dose, it is possible to define the optimal beam parameters for static SIMS measurements. The higher the value of F, the more the intensity per unit of onset of damage. The highest F values occur at higher beam energies and, in general, xenon gives greater efficiency than argon. Secondly, the development of damage may be described by simple bond breaking. Thirdly, a study of the damage process gives quite detailed structural bonding information not directly available from the traditional static SIMS spectrum.