Resonant Laser-SNMS

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

Resonant (R-) laser-SNMS [3.107-3.112] has almost all the advantages of SIMS, e-SNMS, and NR-laser-SNMS, with the additional advantage of using a resonance laser ionization process which selectively and efficiently ionizes the desired elemental species over a relatively large volume (Eig. 3.40 C). Eor over 80% of the elements in the periodic table, R-laser-SNMS has almost unity ionization efficiency over a large volume, so the overall efficiency is greater than that of NR-laser-SNMS. Quantification is also simpler because the unsaturated volume (where ionization is incom- 

N on reso nan laser-post- on izatio n ol secondary neuira-s NR-Laser-St IMS 

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Fig. 3.42. Non-resonant laser-SNMS spec- riety oftransition metals and hydrocarbons trum ofa Si wafer contaminated with a va- [3.105].

Element mapping with non-resonant laser- SNM S can be used to investigate the structure of electronic devices and to locate defects and microcontaminants [3.114]. Typical SNMS maps for a GaAs test pattern are shown in Fig. 3.43. In the subscript of each map the maximum number of counts obtained in one pixel is given. The images were acquired by use of a 25-keV Ga" liquid metal ion source with a spot size of approximately 150-200 nm. For the given images only 1.5 % of a monolayer was consumed -"static SNMS". 

Fig. 3.43. Non-resonant laser-SNMS mapping of a contact test structure on GaAs. Field of view 40 x 40 pm [3.114],...

A versatile Laser-SNMS instrument consists of a versatile microfocus ion gun, a sputtering ion gun, a liquid metal ion gun, a pulsed flood electron gun, a resonant laser system consisting of a pulsed Nd YAG laser pumping two dye lasers, a non-resonant laser system consisting of a high-power excimer or Nd YAG laser, a computer-controlled high-resolution sample manipulator on which samples can be cooled or heated, a video and electron imaging system, a vacuum lock for sample introduction, and a TOF mass spectrometer. 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

Kubota, N., Hayashi, S. (2008) Application of resonant laser postionization SNMS for quantitative depth profiling in stainless steel with oxide film. Appl Surf. Sci,255, 1516-1518. 

Some less common acronyms for laser SNMS are also used, such as sputter-initiated resonance ionization spectroscopy (SIRIS) [287] and surface analysis by laser ionization (SALI) [288]. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

An alternative for the low detection efficiencies of the emitted particles is to ionize them with a UV laser beam, either in a resonant or non-resonant way [37]. In this way the ionization efficiency increases about a thousandfold and the attractive prospect of doing SNMS under static conditions at sensitivities comparable to those of... 

The closely allied topics of secondary neutral mass spectrometry (SNMS), fast atom bombardment (FAB), and laser ablation SIMS are important, but are beyond the scope of this chapter. SNMS is a technique in which neutral atoms or molecules, sputtered by an ion beam, are ionized in an effort to improve sensitivity and to decouple ion formation from matrix chemical properties, making quantification easier. This ionization is commonly effected by electron beams or lasers. FAB uses a neutral atom beam to create ions on the surface. It is often useful for insulator analysis. Laser ablation creates ions in either resonant or nonresonant modes and can be quite sensitive and complex. 

The boron distribution inside the gate region of a transistor was investigated before and after diffusion with SNMS by employing a finely focused Ga ion gun for sputter removal, a so-called three-color resonance ionization of boron via three lasers with different wavelengths (249.7, 563, and 1064 nm) and mass analysis by TOR The beams were aimed at about 1.5 mm above the sample surface with intensities in the 10 -10 W/cm range and beam diameters of a few mm. With this setup the ionization of boron was saturated, but applied to phosphorus (with three UV photons), it was only about 10% [285]. 

A high percentage of the sputtered secondary particles are neutral and must be post-ionized for mass spectroscopy analysis (SNMS) (62). Post-ionization can be achieved by electron impact in a plasma or by an electron beam. Alternatively, resonant and nonresonant laser ionization can be applied. Applications of SNMS for catalyst characterization have still not been reported. 

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