Non-resonant multiphoton ionization (NRMPI) of sputtered atoms as well as resonance-enhanced multiphoton ionization (REMPI) of sputtered organic molecules in combination with time-of-flight mass spectrometry has turned out to be one of the most sensitive techniques for surface analysis. If, additionally, high spatial resolution is demanded the ionization process has to be extremely efficient since only a very limited amount of sample material is available. In preliminary investigations we used ultrashort and intense UV laser pulses (500 fs pulse width) for both elemental and molecular surface imaging. Images of an Ag-coated Cu grid, an Au-pattern on Si and surface structures of a biomolecule (tryptophan) and a polymer (polystyrene) are presented. Useful yields of up to 1% are achieved. INTRODUCTIONThe last few years have brought an effort to improve the spatial resolution of traditional surface analytical techniques like XPS, AES and SIMS. In particular, AES' and SIMSZ*3 have reached lateral resolutions well below 1 pm by scanning a fine-focused primary beam across the sample. Auger electron spectroscopy provides quantitative elemental information but fails in revealing the molecular characteristics of the analysed surface. In contrast, SIMS provides detailed elemental and molecular information. Quantification, however, is hampered by the 'matrix effect': the ionization probability for a sputtered particle depends strongly on the chemical environment and may vary over several orders of magnitude for different matrices. This means that quantification in general needs calibration by external or internal standards.The sputtered species in most cases are predominantly neutral in charge. Therefore, post-ionization of these neutrals is a promising approach4 for quantitative elemental as well as molecular surface a n a l y~i s .~.~ An appropriate ionization scheme has to provide a high useful photoion yield and has to avoid the fragmentation of a considerable fraction of sputtered molecular species.Only very limited work has been published so far about surface imaging by post-ionization of sputtered neutrals.' Up to now, conventional (nanosecond) lasers have been used for post-ionization studies. The results reported here were obtained by the combination of a fine-focused Ga + ion beam and ultrashort UV laser pulses generated in a hybrid excimer-dye laser system. * Author to whom correspondence should be addressed. The use of such a short-pulse-length laser system increases the useful yield for both non-resonant multiphoton ionization (NRMPI) of sputtered atoms as well as for resonance-enhanced multiphoton ionization (REMPI) of sputtered organic molecular species. Two inorganic and two organic surface structures have been investigated and preliminary results are presented. EXPERIMENTALAll measurements presented here were performed with the time-of-flight (TOF) SIMS I11 instrument for which the imaging SIMS capabilities have been described in detail previ~usly.~ The 10 keV G a f ion beam is focused to a diameter below 1 ...
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