High‐spatial‐resolution surface imaging of inorganic and organic structures by multiphoton post‐ionization of sputtered neutrals and time‐of‐flight mass spectrometry

Abstract

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 ...

Postionization of molecules desorbed from surfaces by keV ion bombardment with femtosecond laser pulses

“…11,12 Femtosecond postionization studies showing a significant reduction in the degree of fragmentation when compared to nanosecond excitation have been reported. [13][14][15] For example, Benninghoven and co-workers report ionization efficiencies for producing intact molecular ions from sinapinic acid and tryptophan are comparable to those found for atomic photoionization. 14 In this paper we discuss the pertinent experimental factors associated with postionization of desorbed neutral molecules and how they relate to chemical imaging of surfaces.…”

Alkaline earth metal salts of CaCO3, BaCO3, and SrCO3 as matrix for tryptophan SIMS analysis

“…One reasonable explanation is that Trp molecules contain basic groups, which allow formation of protonated species. In addition to this observation, a number of studies have noted that the parent M+ ion is either absent, or a very small component of the spectrum of sputtered Trp, whether generated by SIMS or postionization 19–21. At higher masses of 400–800 u, Trp clusters are observed.…”

“…Tryptophan, as D‐tryptophan (Trp), C 11 H 12 N 2 O 2 , is the largest amino acid and it serves as a building block in protein synthesis. Due to the fact that indol functional group absorbs strongly in the near ultraviolet part of the spectrum, Trp photoionization has been studied under various conditions 18–21. MALDI mass spectrometry of Trp ions was demonstrated by using alkaline earth metal salts to enhance stability of metal amino acid clusters over pure amino acid clusters 22.…”

Characterization of cell cultures with ToF‐SIMS and laser‐SNMS

“…4 A pulsed 30 keV Ga C ion gun with a spot diameter of ¾200 nm and an ion current of 1.4 nA was used to bombard the sample. The ToF-SIMS and laser-SNMS modes require specific conditions for primary ion bombardment and particle registration.…”