Temperature-controlled depth profiling in polymeric materials using cluster secondary ion mass spectrometry (SIMS)

Secondary ion mass spectrometry (SIMS) employing an SF₅⁺ polyatomic primary ion source was used to depth profile through poly(methylmethacrylate) (PMMA), poly(lactic acid) (PLA) and polystyrene (PS) thin films at a series of temperatures from −125 °C to 150 °C. It was found that for PMMA, reduced temperature analysis produced depth profiles with increased secondary ion stability and reduced interfacial widths as compared to analysis at ambient temperature. Atomic force microscopy (AFM) images indicated that this improvement in interfacial width may be related to a decrease in sputter-induced topography. Depth profiling at higher temperatures was typically correlated with increased sputter rates. However, the improvements in interfacial widths and overall secondary ion stability were not as prevalent as was observed at low temperature. For PLA, improvements in signal intensities were observed at low temperatures, yet there was no significant change in secondary ion stability, interface widths or sputter rates. High temperatures yielded a significant decrease in secondary ion stability of the resulting profiles. PS films showed rapid degradation of characteristic secondary ion signals under all temperatures examined.     

On the abundance of molecular ions in secondary ion mass spectrometry

Changes in molecular secondary ion intensities brought about by working in an environment of oxygen can be rationalised in simple statistical terms.     

Sputter depth profiling by secondary ion mass spectrometry coupled with sample current measurements

Ion-induced secondary electron emission determined via sample current measurements (SCM) was employed as a useful adjunct to conventional secondary ion mass spectrometry (SIMS). This paper reports on the results of 3-6 keV O₂⁺ SIMS-SCM sputter depth profiling through CrN/AlN multilayer coatings on nickel alloy, titanium dioxide films deposited on stainless steel, and corrosion layers formed onto surface of magnesium alloy after long-term interaction with an ionic liquid. For Au/AlNO/Ta films on silicon, in addition to SIMS-SCM profiles, the signal of mass-energy separated backscattered Ne⁺ ions was monitored as a function of the depth sputtered as well. The results presented here indicate that secondary electron yields are less affected by “matrix effect” than secondary ion yields, and at the same time, more sensitive to work function variations and surface charging effects. SCM depth profiling, with suppression of “the crater effect” by electronic gating of the registration system is capable of monitoring interfaces in the multilayer structure, particularly, metal-dielectric boundaries. In contrast to SIMS, SCM data are not influenced by the angle and energy windows of an analyser. However, the sample current measurements provide lower dynamic range of the signal registration than SIMS, and SCM is applicable only to the structures with different secondary electron emission properties and/or different conductivity of the layers. To increase the efficiency, SCM should be accompanied by SIMS measurements or predetermined by proper calibration using other elemental-sensitive techniques.     

In-depth concentration profiling of garnet epilayers using secondary ion mass spectrometry

Rapid and reliable SIMS profiling of insulators can be achieved by using negative primary ions and a diaphragm placed on the specimen for charge compensation. Results with garnet LPE layers demonstrate the compositional variations arising from non-steady state growth conditions.     

Hydrogen depth profiling using18O ions

Nuclear resonant reaction analysis techniques for hydrogen depth profiling in solid materials typically have used¹⁵N ion beams at 6.40 MeV and¹⁹F ion beams at 6.42 MeV, which require a tandem accelerator. We report a new technique using an¹⁸O ion beam at a resonance energy of 2.70 MeV, which requires only a single stage accelerator. Improved values of the nuclear parameters for the 2.70 MeV (¹⁸O) and 6.40 MeV (¹⁵N) resonances are reported. The beam energy spread was investigated for different ions and ion charge states and found to scale with the charge state. Data obtained using atomic and molecular gas targets reveal the research potential of Doppler spectroscopy. Examples of hydrogen depth profiling in solid materials using¹⁵N and¹⁸O ion beams are presented.     

Depth profiling using secondary ion mass spectrometry and sample current measurements

The features of the method for measuring the total current in the sample circuit in combination with secondary ion mass spectrometry (SIMS) depth profiling of various thin-film structures are discussed in detail. The results on depth profiling of multilayer CrN/AlN coatings magnetron-deposited on the surface of a nickel alloy, titanium dioxide films on stainless steel, and corrosion layers formed on the magnesium alloy surface during the interaction with an ionic liquid are considered. These results give grounds to consider the proposed method for measuring the sample current as an efficient and informative addition to the conventional SIMS.     

Effect of residual oxygen on the formation of molecular ions in secondary ion mass spectrometry

The intensity of molecular ions X_nO _m ^± in secondary ion mass spectra originating from surface oxidation by primary oxygen ions is reduced by a residual oxygen pressure. This fact can be used to increase sensibility.     

Application of the surface ionization for the detection of secondary particles in the secondary-ion mass spectrometry (SIMS)

A method for postionization in the course of ion sputtering that is based on surface ionization of the sputtered particles is developed. The estimations show that the method allows a significant increase in the sensitivity of the secondary-ion mass spectrometry for several elements. Nonadditive increase in the sputtering coefficient of indium is experimentally studied using the surface-ionization method of postionization when the number of atoms in projectile clusters Bi _m ⁺ (m = 1–7) increases at energies 2–10 keV. Such a scheme for the detection of neutral particles can be used in alternative methods for the surface analysis, in particular, laser evaporation of surface and electron-stimulated desorption.     

Analysis of C60 and Cs sputtering ions for depth profiling gold/silicon and GaAs multilayer samples by time of flight secondary ion mass spectrometry

Time of flight secondary ion mass spectrometry (ToF-SIMS) depth profiles of several inorganic layered samples using Cs⁺ and C₆₀⁺ primary sputtering ions of different energies are compared to evaluate sputter yield and depth resolution. A gold/silicon model system is employed to study interfaces between metals and semiconductors, and multilayers of AlGaAs, Al, and InAs in GaAs are analyzed to explore the ability of C₆₀⁺ to analyze semiconductor interfaces in GaAs. Roughness measurements are reported to differentiate between different factors affecting depth resolution. The best depth resolution from all samples analyzed is achieved using 1 keV Cs⁺. However, C₆₀⁺ sputtering has advantages for analyzing conductor/insulator interfaces because of its high sputter yield, and for analyzing deeper heterolayers in GaAs due to lower sputter-induced roughness.     

Raster scanning depth profiling of layer structures

Implications in the use of the electronic gating scheme in depth profiling studies of layer structures by means of raster scanning secondary ion mass spectrometry are investigated. The profile of the sputtering crater and the intensity variation after break-through are calculated with the scan width and the gate width as parameters. Thick (5.6 μm) magnetic garnet layers grown an a non-magnetic garnet substrate were used for depth profiling measurements. A relative depth resolution of 1% could be obtained. Comparison of experimental results with calculated data shows excellent agreement.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Improved method for depth profiling of multilayer structures

A modified, improved method has been developed for the dissolution of semiconductor materials containing a GaAlAs layer. This modified method involves the alternate application of anodic and cathodic processes in order to produce an arsenic-free surface for the carrier concentration measurement.

The advantageous features of this method are demonstrated by comparison of the depth profiles of various semiconductor structures obtained by conventional and modified techniques.     

SIMS depth profiling of Pd/B4C, Ni/C, and Cr/Sc multilayer metal structures using registration of cluster secondary ions: The problem of depth resolution enhancement

The possibility of minimizing matrix effects during the SIMS depth profiling of multilayer metal structures is investigated, based on the use of cluster secondary ions that include a combination of the element to be analyzed and sputtering cesium or oxygen ions. For Pd/B₄C, Ni/C, and Cr/Sc structures, the use of cluster secondary ions enabled us to substantially enhance depth resolution up to 1–2 nm.     

Accurate mass spectrometry of trapped ions

The Penning trap Ion Cyclotron Resonance (ICR) method we use to weigh atomic masses is reviewed, and our plans for future measurements, new methods, and apparatus improvements are discussed. Our ultimate goal is to develop a new technique for measuring atomic masses with an accuracy of a few parts in 10¹². We will do this by comparing the cyclotron frequencies of two simultaneously trapped ions. In order to successfully implement this new method we are developing a quieter, more sensitive DC SQUID-based detector and a new more harmonic trap, and we plan to use our classical squeezing techniques to reduce the effects of thermal noise. With our improved apparatus we will weigh Cs and Rb to help determine the fine structure constant α, weigh ²⁹Si and ³⁰Si as part of the current effort to replace the artifact kilogram standard with a Si crystal containing a known number of atoms, and measure the ³H-³He mass difference to help set a limit on the mass of the electron neutrino. Our higher accuracy will also enable us to ``weigh' the neutron capture gamma rays of <sup>28</sup>Si, <sup>32</sup>S, and <sup>48</sup>Ti to help determine the molar Planck constant N<sub>A</sub>h and the fine structure constant α. Finally, with a mass measurement accuracy \sim 10<sup>-12</sup> we will be able to ``weigh' chemical bonds. This revised version was published online in August 2006 with corrections to the Cover Date.     

Molecular depth profiling of polymers with very low energy ions

The need for a molecular depth profiling technique to study organic layers has become a strong incentive in the SIMS community in the last few years, especially with the recent successes obtained with cluster ion beam depth profiling. In this work, we have investigated a thoroughly different approach by using very low energy (down to 200 eV) monoatomic or diatomic ions to sputter organic matter. Quite surprisingly, we were able to retain specific molecular information on various polymers even at very high fluence.Polymethylmethacrylate (PMMA) and polyethylene terephthalate (PET) films were depth-profiled with 200 eV Cs⁺ and 500 eV O₂⁺ ions. With 200 eV Cs ions, the best profiles were obtained in the negative mode, due to a strong negative ionisation yield enhancement related to Cs retention in the polymer. A relatively high and stable signal from the most characteristic ions was measured all over the layer.With 500 eV O₂⁺, real molecular depth-profiles were also obtained in both the positive and the negative modes. Once again, the main characteristic fragments of PET or PMMA remain detectable with stable yields all over the profile.     

Imaging of aerosols using time of flight secondary ion mass spectrometry

Interest in environmental aerosol chemistry has grown over the last decade as a result of its role in both climate change and troposheric pollution. In this work, the combination of ToF-SIMS and SEM/EDX was employed to explore the surface chemistry of aerosols. The capabilities and limitations of ToF-SIMS were investigated using particles of known composition and size produced by a vibrating orifice aerosol generator (VOAG). Principal component analysis (PCA) proved to help in the distinction of particles of different types by consolidating the information generated by ToF-SIMS.     

Three-dimensional molecular imaging using mass spectrometry and atomic force microscopy

We combine imaging ToF-SIMS depth profiling and wide area atomic force microscopy to analyze a test structure consisting of a 300 nm trehalose film deposited on a Si substrate and pre-structured by means of a focused 15-keV Ga⁺ ion beam. Depth profiling is performed using a 40-keV C₆₀⁺ cluster ion beam for erosion and mass spectral data acquisition. A generic protocol for depth axis calibration is described which takes into account both lateral and in-depth variations of the erosion rate. By extrapolation towards zero analyzed lateral area, an “intrinsic” depth resolution of about 8 nm is found which appears to be characteristic of the cluster-surface interaction process.     

Sputtered neutral mass spectrometry (SNMS) as a tool for chemical surface analysis and depth profiling

An experimental system for mass spectrometry of supttered neutral particles involving a hf plasma operated in Ar at several 10⁻⁴ Torr is described. The potentialities of the method for quantitative surface analysis are reasoned. Depth profiling by sputtered neutral mass spectrometry is demonstrated for anodic oxide layers on Nb and Ta.