Microprobe mode

In the scanning (or microprobe) mode the image is measured sequentially point-bypoint. Because the lateral resolution of the element mapping in scanning SIMS is dependent solely on the primary beam diameter, LMISs are usually used. Beam diameters down to 50 nm with high currents of 1 nA can be reached. 

Raman spectroscopy has been successfully applied to the investigation of oxidic catalysts. According to Wachs, the number of Raman publications rose to about 80-100 per year at the end of the nineties, with typically two thirds of the papers devoted to oxides [41]. Raman spectroscopy provides insight into the structure of oxides, their crystallinity, the coordination of metal oxide sites, and even the spatial distribution of phases through a sample when the technique is used in microprobe mode. As the frequencies of metal-oxygen vibrations found in a lattice are typically between a few hundred and 1000 cm 1 and are thus difficult to investigate in infrared, Raman spectroscopy is clearly the indicated technique for this purpose. 

Operation of a SIMS instrument resembles both that of an isotope ratio mass spectrometer and an electron microscope. Most SIMS instruments include an optical microscope so that the sample can be directly viewed during analysis, which allows for accurate positioning of the area of interest on the sample. Data can be in the standard mode used for other types of mass spectrometers in which ions are produced and the mass spectrum is analyzed by scanning or peak-hopping. This mode is sometimes called the microprobe mode in SIMS. Another application for SIMS is the acquisition of ion-images. This mode is called the microscope mode because the SIMS is operated as an ion microscope. 

Laser SNMS requires the operation with properly selected duty cycles that control the delay times between the primary ion pulse, a pulsed extraction voltage for separating the secondary ions from post-ionized neutrals, and the firing of the postionizing laser pulse. Such duty cycles have, in addition, to be synchronized with the stepwise motion of the pulsed primary ion beam across the sample surface in the microprobe mode of laser SNMS. The selection of appropriate duration and decay times of the ion and laser pulses, of the laser intensity, and beam shape is important to make the photoion yields independent on the sputtered particle velocities. The detection volume must be matched to the entrance ion optics of the TOP such that it becomes independent of the individual ionization process. Usually, laser intensities in the range from 10 to lO Wcm are applied. While the particle density in the detection volume is monitored at small laser intensities, the particle flux is measured at high photon densities. 

Besides TOP instruments, other mass analyzers such as quadrapole-orthogonal TOP instruments, ion traps, Pourier transform (PT) ion cyclotron resonance mass spectrometers and, most recently, orbital trapping mass spectrometers were employed for MALDI imaging in microprobe mode, as described later in this chapter (see also Chapter 2). 

Generally, SIMS instruments are operated in two modes microscope mode and microprobe mode. A defocused primary ion beam ( 5—300 pm) is used for investigating a large surface in the microscope mode. The secondary ions are then transmitted to the mass spectrometer and generally detected by imaging. In the microprobe mode, a focused primary ion beam (< 10 pm) is used to investigate a small portion of the surface and detected usually in an electron multiplier. 

The diameter of the laser beam used to probe the sample surface typically determines the effective spatial resolution of a measurement performed in microprobe mode. Obviously, the laser beam diameter can be reduced by focusing the beam to smaller dimensions. However, as the laser beam diameter is reduced, it illuminates a smaller area, fewer molecules of each analyte are present within the probe beam, and so fewer molecules are ionized at each location. Therefore, smallest diameter beams are rarely practical because the amount of analyte that can be desorbed and ionized from a smaller sample area is not sufficient for detection and high-accuracy mass measurement. Consequently, the laser probe diameter for the analyses of proteins and peptides usually is larger than 10 xm. 

Consequently, XPS has developed from a large area analysis method to one which has some degree of spatial resolution (selected area analysis). There are essentially only two ways in which such an improvement can be obtained, operating the spectrometer in a microprobe mode, in which the X-ray... 

Images in all three dimensions can be constructed by stacking the spatial images collected at every depth. Imaging can be carried out via either the microprobe or the microscope modes (both are discussed in Section 5.3.2.2). When carried out using the microprobe mode, an ultimate spatial resolution approaching 10 to 20 nm is possible (McPhail et al. 2010), although 50 nm and above is more common. This spatial resolution is, however, heavily dependent on primary ion spot size, and hence the primary ion current. When carried ont in the microscope mode, the spatial resolution is fixed at 1 pm irrespective of the primaiy ion spot size/cnrrent. Note Improved detection limits also allow for improved spatial resolntion. 

The sizable data files arise from the fact that these instruments generate mass spectra for every point the primary ion beam is scanned over when operated in the microprobe mode (this imaging mode is discussed in Section 532.2). As data is typically collected over 128 x 128 or 256 X 256 points, this translates to 16,384-65,536 mass spectra collected per layer. The number of data points per mass spectra can also be significant (this is a function of the mass resolution and mass range). This extensive information content often translates into increased data processing complexity. 

In the case of ion Magnetic Sector-based instruments operated in the microscope mode, the information on the point of secondary ion ejection is retained throughout the secondary ion column. As a result, both pulse counting and spatial resolving detectors are commonly found in such instruments. DD-EMs are used when maximum sensitivity is required (these also allow for imaging in the microprobe mode), PCs are used when increased dynamic range is needed, and MCPs combined with a phosphor screen or reactive anode encoder when imaging in the microscope mode. 

Secondary Ion Imaging Modes Imaging in SIMS refers to the reproduction of the distribntion of any element or molecule on or within the volume of interest. Reproduction of the spatial distribution is the most common form of imaging. This can be carried ont via one of two modes, these being the Microprobe mode or the Microscope mode. 

Figure 5.16 Pictorial illustrations (not to scale) of SIMS imaging via (a) the microscope mode and (b) the microprobe mode. Reproduced with permission from van der Heide and Fichter (1998) Copyright 1998 John WUey and Sons.

As this is carried out using the microprobe mode, the image quality is defined by the primary ion beam diameter at the substrate s surface, i.e. the spot size. 

Spatial imaging can be carried ont via one of two approaches, these being the microscope mode or the microprobe mode. These are specific to the secondary ion mass filter and the secondary ion optics nsed. The former translates the aerial position of the secondary ion emanating from the snbstrate s snrface to a position-sensitive detector, whereas the latter scans a finely focnsed beam across the substrate s surface with the beams position relayed to the detector. Three-dimensional imaging can be made possible by stacking spatial images collected as a function of sputtering time. 

As the current of an ion beam is increased, so too is the minimum spot size (this explains why imaging in the microprobe mode, whether in SIMS, SEM, or HIM, must be carried out at very low ion currents). 

Microprobe mode An imaging mode in which points a projected on to a detector... 

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