Papers

AVS1996 Session AS-MoM: Surface Chemical Imaging and Small Volume Analysis

Monday, October 14, 1996 8:20 AM in Room 105B

Monday Morning

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8:20 AM		AS-MoM-1 Chemical Imaging and Small Volume Analysis by Secondary Ion Mass Spectrometry G. Gillen (National Institute of Standards & Technology) One of the most powerful analytical capabilities of Secondary Ion Mass Spectrometry (SIMS) is the ability to form images of sputtered secondary ions (ion microscopy). In this presentation, I will discuss three of the more unique capabilities of SIMS for spatially resolved analysis. These include: Chemical Imaging in 3 Dimensions- In this mode of operation, a series of elemental SIMS images are acquired as a function of time. Because the sample surface is continually eroded during analysis, each image will correspond to a slightly greater depth below the original sample surface. The resulting image "stacks" can be reconstructed to produce three dimensional (3D) elemental images with a spatial resolution (depending on the sample) of 0.2-1 micrometers laterally and 1-10 nm in depth. Using appropriate ion implant standards it is possible to quantify the analyte concentration for each individual pixel in the 3D image. The capabilities of this technique will be demonstrated with examples from our work on quantitative 3D imaging of semiconductor devices and focused ion beam implants and the characterization of impurities in the grain boundaries of steels and superconducting thin films. Imaging of Molecules- Ion bombardment can produce intact parent ions from organic and biological surfaces making it possible to use SIMS as a true "chemical microscope". I will describe our current efforts in this area with examples of molecular imaging for characterization of patterned self assembled monolayer films and immobilized DNA probe arrays on surfaces, and for molecular analysis of biological tissues. Isotope Ratio Imaging - One of the strengths of a mass spectrometric based surface analysis technique is the ability to characterize the isotopic composition of a material. We are exploiting this capability to determine precisely the composition of isotopically heterogeneous materials such as stable isotope tracers in biological tissues and isotpoically perturbed grains in meteorites.
9:00 AM		AS-MoM-3 An Approach for 3D Quantification in Secondary-Ion Mass Spectrometry Analysis H. Gnaser (Universit\um a\t Kaiserslautern, Germany) The 3-dimensional characterization of solids by means of secondary-ion mass spectrometry monitoring MCs\super +\ ions is investigated. MCs\super +\ molecular ions (M stands for an element of the sample) emitted from surfaces under Cs\super +\ bombardment are found to be suited for a quantitative data evaluation via relative sensitivity factors (RSFs). Using RSFs, laterally-resolved ion images recorded with acquisition times of typically a few seconds can be transformed into elemental distribution maps; these exhibit a dynamic sensitivity range in excess of 10\super 2\ and a spatial resolution of 2-3 microns. Also, from the applied RSFs local (i.e. erosion-time dependent) sputtering yields can be derived; together with atomic densities (e.g. interpolated from pure-element values) a local depth scale (relative to some reference level) is assigned to each pixel of the 3D data volume recorded during the analysis. In conjunction with the elemental concentration values, this provides the possibility of a complete reconstruction of the 3D sample volume removed by sputtering. This approach is exemplified and its validity and limitations are assessed employing a laterally inhomogeneous semiconductor test structure and various thin-film multilayer specimens.
9:20 AM		AS-MoM-4 Chemical Imaging with a Laser Desorption/Laser Postionization Mass Spectrometer M. Savina, K. Lykke, M. Pellin (Argonne National Laboratory) We present molecular and atomic images of organic and inorganic surfaces made with a relatively simple laser desorption/laser postionization time-of-flight mass spectrometer. A portion of the sample is vaporized by a pulsed laser focused on the sample with a reflective Schwarzschild objective, and the desorbed material is then ionized with a pulse from a second laser. The desorption laser is rastered across the surface to produce the image, with a spatial resolution of about one micron. Judicious choice of the two lasers allows one to tailor the imaging to bring out features of interest. For example, varying the power and wavelength of the desorption laser changes the spot size and, in some cases, the chemical selectivity of the system, while varying the irradiance and wavelength of the ionization laser changes the sensitivity, chemical selectivity, and fragmentation patterns. Images of photopatterned thin films of organic dyes and polymers are presented. In addition, we compare photoionization by conventional Q-switched lasers, with pulse durations of nanoseconds and peak irradiances on the order of terawatts per square cm, with photoionization by an ultrafast Ti-sapphire laser with a pulse duration of approximately 100 femtoseconds and peak irradiance of pitawatts per square cm. This work was supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38. The submitted manuscript has been created by the University of Chicago as operator of Argonne National Laboratory ("Argonne") under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
9:40 AM		AS-MoM-5 Laser Postionization of Sputtered Organics: A TOF-MS Study of Secondary Neutral Emission of Organic Overlayers R. M\um o\llers, A. Schnieders, A. Benninghoven (Physikalisches Institut der Universit\um a\t, Germany) Secondary neutral mass spectrometry (SNMS) in combination with multiphoton ionization is generally regarded as a sensitive and quantitative tool for the analysis of inorganic surface species. Many efforts have been made to extend the technique and its pros on the field of organic surface species. Up to now most of the research has concentrated on the optimization of resonantly enhanced multiphoton ionization which permits high photoion yields in favorable cases. In our work we focuse on the influence of sample preparation on the yields of sputtered organic neutrals. Molecular overlayers of adenine (135 u) and the amino acid alanine (89 u) were produced by evaporation of the respective molecule from a Knudsen cell under UHV condition on liquid nitrogen cooled substrates (Au, Ag, Cu, Ni, Si). Starting with a sputter cleaned substrate the flux of sputtered secondary neutrals and secondary ions was monitored online under static sputtering conditions until the substrate was covered by a multilayer of the molecule. We determined secondary neutral and secondary ion yields, the kinetic energy distributions of sputtered neutral molecular species and the dissappearance cross section as a function of coverage and substrate. The investigated molecules show neutral yields as well as ion yields which are up to one order of magnitude higher when sputtered from a submonolayer coverage compared to a multilayer coverage. Highest neutral and ion yields are obtained for noble metal substrates. The kinetic energy of sputtered neutral molecules is about 0.2 eV when sputtered from a multilayer and increases for submonolayer sputtered molecules up to 1 eV.
10:00 AM		AS-MoM-6 Temperature-Programmed Static SIMS of Silicon Wafers Modified with Photosensitive Silanes P. Cobben (Philips Research Laboratories, The Netherlands); M. Deimel (Physikalisches Institut der Universit\um a\t M\um u\nster, Germany); B. Hagenhoff (Philips Centre for Manufacturing Technology, The Netherlands) Photosensitive organic compounds are frequently used in resists for photolithographic processes [1] and UV-curable adhesion promoters. These compounds are often subjected to temperature treatments during processing. Recently, temperature programmed static-secondary ion mass spectrometry was used to investigate in situ the temperature behavior of polymer monolayers on metal surfaces [2]. In the present study surface layers of photosensitive organosilanes are used as models for the photosensitive organic compounds. Organosilanes are chosen because they bond covalently to silicon wafers. This chemical bond prohibits thermal desorption and makes it possible to study the temperature sensitivity of the photosensitive functionality. Chemical surface analysis with (sub)mono-molecular sensitivity is performed in situ on these wafers with static-SIMS at various controlled temperatures. The static-SIMS analyses of the heat treated surfaces have given important information on the temperature sensitivity of the photosensitive organosilane modified wafers. Typical results of temperature programmed static-SIMS of silicon wafers modified with silanes containing, e.g., naphtoquinone diazo [3] or methacryloxypropyl functionalities are presented.References: 1. E. Reichmanis and L. Thompson, in: Polymers in microlithography, ed. by E. Reichmanis, S.A. MacDonald and T. Iwayanagi, ACS Symposium Series 412, Chapt. 1, p. 1. American Chemical Society, Washington, DC (1989). 2. M. Deimel, B. Hagenhoff, and A. Benninghoven, in: Secondary Ion Mass Spectrometry (SIMS X), ed. by A. Benninghoven, B. Hagenhoff, H.W. Werner, John Wiley & Sons, Chichester, in press. 3. H. van der Wel, E. van der Sluis-van der Voort and N.P. Willard, Surf. Interface Anal. 21, 455 (1994)
10:40 AM		AS-MoM-8 Composition of Chemical Domains at Surfaces of Geological Materials by X-ray Absorption Microscopy T. Droubay, B. Tonner (University of Wisconsin, Milwaukee) X-ray absorption microscopy is a high spectral and spatial resolution technique for imaging chemical composition of surfaces. We have begun evaluating this technique for the study of the surfaces of geological samples and other insulating materials, through a study of the distribution of chemical domains in the mineral ilmenite, which consists of Fe-Ti oxides. We use an x-ray photoelectron microscope (XPEEM), and high resolution soft x-ray near-edge absorption spectroscopy (microXANES), to determine the spatial distribution of Fe in the +2 and +3 valence. We show that, with high spectral resolution, it is possible to quantitatively fit the near-edge spectra and get accurate values for the stoichiometry of Fe and Ti in the various chemically distinct regions seen at the sample surface. These results will be discussed in comparison to other related analytical microscopies, including electron-loss microscopy (PEELS) and photoemission microscopy.
11:00 AM		AS-MoM-9 Comparative Magnetic-Field Imaging, Electric-Field Imaging, and Scanning Auger Microscopy Study of Metal-Matrix Composites R. Rosenberg, Q. Ma (Argonne National Laboratory); C. Kim, J. Grepstad, P. Pianetta (Stanford Synchrotron Radiation Laboratory); T. Droubay, D. Dunham, B. Tonner (University of Wisconsin, Milwaukee) Metal-matrix composites (MMCs) are finding numerous applications in areas such as aerospace where their high strength and low density can represent significant savings. We have initiated a research program aimed at understanding the interfacial interactions that occur between the metal matrix and the ceramic reinforcement. The reinforcements can range in size from 10-40 \mu\m (particulates) to 140 \mu\m (fibers) so microscopic analysis is essential for these studies. Our initial experiments have examined the structure of Ti/SiC(fiber) MMCs and SiC fibers which were obtained from Textron Specialty Materials. Polished samples were Ar\super +\ ion sputter cleaned and studied using an electric-field imaging photoelectron microscope located at the Advanced Light Source (ALS), while the use of a magnetic-field imaging microscope located at the Stanford Synchrotron Radiation Laboratory (SSRL) made possible the study of in situ fractured samples. In addition, scanning Auger microscopy measurements were performed on both polished and fractured samples. The data taken at ALS shows good chemical contrast and excellent resolution; however, the sample preparation methods (polishing and sputtering) significantly perturbs the surface making it difficult to extract meaningful data. Although the data taken at SSRL has lower spatial resolution than the ALS data, the ability to fracture in situ greatly facilitates interpretation of the data. Results obtained using these techniques will be discussed in terms of the structure of the MMCS and the potential of synchrotron radiation based microscopies for studying composite materials will be examined
11:20 AM		AS-MoM-10 Combining Time of Flight SIMS with ESCA for Organic Surface Analysis G. Jones, B. McIntosh, K. Robinson, J. Wolstenholme (VG Scientific, United Kingdom) XPS and SIMS have long been known to be complementary surface analysis techniques and many XPS instruments have therefore been fitted with a SIMS facility. For the analysis of organic surfaces Time of Flight SIMS is preferable to either quadrupole or magnetic sector type SIMS instruments because of the low primary ion dose and the high mass range. The combination of both ToFSIMS and XPS in the same instrument is therefore a powerful tool for the analysis of organic surfaces, providing both the quantification and chemical state analysis from XPS and the structural information and sensitivity of ToF SIMS.Such a combination is now possible using the VG Scientific's ESCALAB 220i-XL instrument in which the XPS lens and analyser arrangement is also used as a ToFSIMS energy-compensated analyser. The novel features of the instrument will be described briefly.The performance of both the XPS and the SIMS facilities of the combined instrument will be illustrated and results shown to illustrate the complementary nature of the two techniques. Examples will be drawn from a number of areas including organic chemistry, polymer surface analysis and quality assurance.
11:40 AM		AS-MoM-11 Scattering and Recoiling Imaging Spectrometry (SARIS) C. Kim, C. Hofner, V. Bykov, J. Rabalais (University of Houston) A new ion scattering technique called scattering and recoiling imaging spectrometry (SARIS) has been developed. SARIS extends the technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) to include both spatial and time resolution of scattered and recoiled particles. It uses a gated 75 x 95 mm position sensitive microchannel plate (MCP) and time-of-flight methods to capture images of scattered and recoiled particles from a pulsed keV ion beam. A time-to-digital converter allows collection of the images in 256 frames, each being a minimum of 10 ns wide. The method collects both the ejected ions and fast neutrals and disperses scattered and recoiled particles according to their velocities as a function of projectile/target atom masses and deflection angle. The images combine the advantage of atomic scale microscopy and spatial averaging simultaneously since they are created from a macroscopic surface area but they are directly related to the short-range atomic arrangement of the surface. The 10 ns resolution of the frames provides spatial and time resolved images of the scattering and recoiling trajectories. The technique can be used to monitor surface dynamics and structural changes in real time and real space. The images are simulated by means of a scattering and recoiling imaging code (SARIC) developed in this laboratory. Examples to be discussed include images from Pt(111) and Au(110), order-disorder phase transition in Au(110), and oxygen adsorbate induced reconstruction of Ag(110).