SIMS AND SCANNING ION MICROSCOPY

Transcription

1 SIMS AND SCANNING ION MICROSCOPY G. Allen, I. Brown To cite this version: G. Allen, I. Brown. SIMS AND SCANNING ION MICROSCOPY. Journal de Physique Colloques, 1989, 50 (C2), pp.c2-121-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1989 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C2, supplhment au n02, Tome 50, f6vrier 1989 SIMS AND SCANNING ION MICROSCOPY G.C. ALLEN and I.T. BROWN Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, GB-Gloucestershire GL13 9PB, Great-Britain -- Resume - Un microscope a ballayage ionique developpe aux "Berkeley Nuclear Laboratories" du C.E.G.B., est decri t. Par bombardement ioniaue a partir d'une source de gallium metal 1 iquide et analyses consecutives des ions secondaires et des electrons emi s, sel on des techniques conventionnell e de spectroscopi e de masse et de microscopie electronique, cet appareil permet d'obtenir le spectre de masse des ions secondaires et de visualiser 1 a surf ace exami nee. Des resultats de microscopie a ballayage appliquees a des etudes de materiaux metalliques, inorqaniques et organique seront presentes. Abstract - A scanning ion microscope developed at the CEGB's Berkeley Nuclear Laboratories is described. By ion bombardment from a liquid metal gallium source, and subsequent analysis of both the resultant secondary ions and secondary electrons using conventional techniques of mass spectrometry and electron microscopy, this instrument offers both secondary ion mass analysis (SIMS) and the facility for imaging the surface under investigation. Results from the application of the scanning ion microscope to the study of metal1 icy inorqanic and orqanic materials are presented. The techniques used for the examination and analysis of solids are many and varied but their development may be traced usi nq scanning electron microscopy as a starting point. Following the introduction of this method came the desire to determine the chemical composition of the features revealed. A t first this was possible through the analysis of X-radiation produced during the process of bombardment of the specimen with the high energy electron beam, but two decades ago techniques offering information more representative of the surface such as X-ray photoelectron spectroscopy, Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) Article published online by EDP Sciences and available at

3 C2-122 JOURNAL DE PHYSIQUE were developed. Of these methods acceptance was gained most slowly by the latter but microscopic analysis by SIMS has been made possible by the use of liquid metal sources to produce the primary ion beaml. These hiqh brightness sources permit the formation of high intensity submicron ion probes and modern instrumentation allows the collection of secondary electron and secondary ion images which provide surface topography of one monolayer 2. The Berkeley instrument, shown schematically in Figure 1, is therefore, a combination of a microscope and a submicron analytical probe 3. DETECTOR r * MASS 25 VACUUM SYSTEM DISPLAY LIQUID METAL ION GUN Dl SPLAY i--i COPIER / PRINTER Figure 1: Schematic diagram of Berkeley Scanning Ion Microscope (SIM) Typical images from a number of specimens difficult to examine without prior coating techniques which mask or modify important surface chemical properties in conventional electron microscopy are shown in Figures 2 and 3. Figure 2: Ion induced electron micrograph from foot of horsefly (Tabanus)

4 Figure 3: Scanning ion micrograph of (a) pulverised fuel ash (PFA) particle(b) area of particle at higher magnification Organic materials are normally degraded in an electron beam and insulating samples such as the PFA sample (Figure 3) which is mainly composed of silica, may be subject to charging '+. In the ion microscope however, good images with a remarkable depth of field were obtained after several minutes; presumably the implantation of gal 1 ium ions a1 leviates the build up of surface charge during the bombardment process. Bombardment of the surface with i-ons not only produces secondary electrons but also stimulates the emission of secondary ions. If the primary ion beam is raster scanned over the surface and specific secondary ions monitored at each point, a chemical image can be generated: This is done by computer processinq the data to yield maps or spatial images of the secondary ions analysed which may then be related to electron images from the same surface (Figure 4). Figure 4: (a) (b) (a) Ion induced electron image of wear scars on the surface of a stain1 ess steel sampl e. (b) Map of the OH- ion taken from the same sample.

5 C2-124 JOURNAL DE PHYSIQUE To demonstrate the use of the instrument in secondary ion mass analysis (SIMS) th superconducting compound YBa2Cu307-x has been examined 5. The positive ion spectrum from a freshly prepared sample is shown i n Figure 5(a) and may be compared with that from the same material following lengthy exposure to the atmosphere Figure 5(b). The presence of a at the surface of the sample exposed to air is readily apparent. hydroxide layer Although this layer is readily removed by sputtering the presence of yttrium and barium hydroxides could play a s-ignificant role in surface conduction behaviour. Figure 5: Positive SIMS Spectra for YBa2Cu307-x (a) Freshly Sintered (b) Exposed to the Atmosphere The intensities of the various cluster ions in the spectra are not related to the composition of the sample. They are a result of the thermodynamic/chemical equi 1 i bria involved in the sputtering process 5. bond dissociation energies for metal oxide ions are plotted against the function log (MO+/M+) in Figure 6. This relationship suggests that as the dissociation energy of the metal oxide decreases, the observed metal-oxi de peak becomes less prominent. The

6 ; 2 3 L S I O Dlssociat~on energy 0: (M-0)' (ev) Figure 6: Relationship between metal oxide SIMS intensities a.nd stability of metal oxide ion. ACKNOWLEDGEMENT The work was carried out at the Berkeley Nuclear Laboratories of the Research Division and the paper is published with the permission of the Central Electricity Generating Board. REFERENCES 1 P.D. Prewett, D.K. Jeffries. Inst. Phys. Conf. Ser. Vol. 54. Liquid metal field emission ion sources and their applications. Institute of Physics, Bristol (1980) p R. Levi-Setti, J.M. Bhabala, Y.L. Wang. Ultramicroscopy (19881, 24, 97 G.C. Allen, I.T. Brown. CEGB Research (1988) in press. 4 G.C. Allen, A.R. Jones, A.G. Warner. Part. Charact. (19861, 3, G.C. Allen, I.T. Brown. Phil. Mag. Letters (1988) in press.