Experimental approaches for analyzing the chemical composition of animal cells with spatial resolution are important for many fields of biomedical research. The analysis of threedimensional microstructures by time-of-flight secondary-ion mass spectrometry (TOF-SIMS) is an emerging technique to make the molecular architecture of biological samples accessible. In SIMS the sample surface is bombarded by primary ions. A fraction of the energy transported in the socalled collision cascade is directed back to the sample surface and causes the desorption of neutral and charged chemical species (secondary ions) from the uppermost molecular layer. These are subsequently collected and analyzed with respect to their mass/charge ratio.[1] Today, most state-of-the-art instruments for organic applications use TOF analyzers for mass determination of the desorbed secondary ions.[2] TOF-SIMS allows the detection of all elements as well as small organic molecules in parallel and has a sensitivity down to the ppm/ femtomole range.[3] Scanning the sample surface with the primary-ion beam provides a 2D image of the chemical surface composition. Moreover, prolonged ion bombardment of the sample at a constant position leads to sputter erosion. Mass analysis of the sputtered material then reveals the vertical composition of the sample.[1] The lateral distribution of organic material can be imaged with a resolution of about 150-400 nm, [4][5][6] whereas the vertical resolution in organic polymer films was shown to be better than 30 nm. [7] Application to biological samples like cells and tissues, however, has so far been hindered by the limited signal intensities obtained from organic materials and the fact that the collision cascade destroys organic molecules and, thus, molecular information. The low signal intensities in surface analysis and the loss of molecular information in sputter depth profiling have been improved by the use of polyatomic primary ions like Au 3 + and Bi 3 + . [3,8] Moreover, buckminsterfullerenes have become available as a new ion source for sputter erosion.[9] The impact of C 60 + ions was found to be less destructive to organic samples than the common sputter ions O 2 + and Cs + .[10] Even intact organic molecules survive the sputter process.[11] Thus, it was the objective of this study to reconstruct the molecular composition of animal cells in three dimensions by applying repeated cycles of SIMS analysis of the sample surface followed by sputter erosion that exposes a deeper layer of the sample to the next round of SIMS analysis (TOF-SIMS 3D microarea analysis). In a dual-beam setup Bi 3 + primary ions were used to determine the chemical composition of the surface, and C 60 + ions were used for intermittent sputter erosion. [12] Six confluent layers of normal rat kidney (NRK) cells, grown on cover slips under ordinary cell-culture conditions, were analyzed by TOF-SIMS 3D microarea analysis after the cells had been stabilized by chemical fixation. Chemical fixation is a routine procedure to preserve the struct...

Ar cluster sputtering of organic multilayers such as organic light-emitting diode model structures and Irganox delta layers is studied with time-of-flight secondary ion mass spectroscopy in the dual beam mode. Results for sputtering yield volumes and depth resolution are presented for Ar clusters with sizes from 500 to 5000 atoms in the energy range from 2.5 to 20 keV. The sputtering yield volume shows a linear dependence on the energy per atom for all materials in this study with a material-dependent threshold below 1 eV/atom. The sputtering yield volume at a given energy per atom increases with the cluster size. At constant beam energies, the sputtering yield volume decreases slightly with increasing cluster size. The depth resolution is investigated for the two model systems as a function of energy and cluster size, and it will be shown that the depth resolution depends mainly on the sample roughening. The depth resolution is approximately proportional to the depth of the impact crater at a given cluster size and energy. The optimum depth resolution achieved is in the range of 4-5 nm and is fairly constant with depth. At very low energies per atom close to the threshold energy, ripple formation is observed that leads to a fast degradation of the depth resolution with depth. This can be completely eliminated by fast sample rotation. Finally, the perspective of 3D analysis of organic devices with high depth resolution in the dual beam mode will be discussed. Figure 1. a) Sputtering yield volume of HTM-1 versus energy/atom for sputtering with Ar cluster sizes from 500 to 5000. b) Sputtering yield volume of HTM-1 for Ar cluster energies from 2.5 keV to 20 keV as a function of the cluster size.

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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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Molecular semiconductor devices, such as light-emitting diodes and photovoltaic cells, have recently received considerable attention because of their compatibility with flexible substrates and large-area applications. Because of the importance of the interfacial properties for the performance of the devices, these organic (multi)layers constitute an important field of application for molecular depth profiling by SIMS. In this contribution, we investigate the use of C 60 n+ and Ar 1000-2000 + cluster projectiles at different energies (ranging from 2.5 to 20 keV) as sputter ions for the organic depth profiling of fullerene-based films and heterojunctions. The bilayers consist of C 60 fullerenes on tin phthalocyanine (SnPc), deposited on silicon substrates. Our preliminary results showed that C 60 films could not be successfully profiled using C 60 n+ ions in regular analysis conditions (room temperature). In contrast, with Ar clusters, the depth profiling is successful (except for 20 keV Ar 1000 ) and the sputtered volume shows a linear relationship with the Ar cluster energy. Surprisingly, for a given total energy of the projectiles, Ar 2000 sputters approximately two times more than Ar 1000 . The observations are tentatively explained as being the result of a balance between the sputtering and the cross-linking efficiency for the different bombardment conditions, larger clusters being expected to naturally induce less cross-linking than smaller clusters with the same total energy.

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