Secondary Ion optics

Fig. 3.19. Basic set-up of a direct imaging magnetic sector instrument. The stigmatic secondary ion optics consists of an electrostatic analyzer (ESA) and a magnet sector field.

The direct imaging magnetic sector mass analyzer (Fig. 3.19) has the unique property that all parts (lenses, electrostatic analyzer and magnetic sector field) of the secondary ion optics are stigmatic (comparable with light microscopes). This means that all points of the surface are simultaneously projected into the analyzer. 

Strictly speaking, the term "secondary Ion optics" refers to all those lon-optical components which transmit secondary Ions from the sample surface to the detector. For the sake of this discussion we will limit the scope of the term to those secondary lon-optical components between the sample surface and the mass spectrometer. With this restriction, we see that the purpose of the secondary ion optics is to extract the Ions efficiently from the sample surface and present them to the entrance of the mass spectrometer In a satisfactory manner. Recognition of three Important facts are necessary for such ion optics design ... 

The name, nanoSIMS 50, stands for a new-generation SIMS instrument. The unique feature of the instrument is that it combines high sensitivity with a high lateral resolution of 50 nm when operated with the Cs primary beam. This is achieved by using a specific design of ion optics where the primary and secondary ion optics are coaxial and the primary beam hits the sample surface at an angle of 90°. Compared to the old SIMS, the transmission of the new instrument for secondary ions is a factor of -20-30 better in typical measurement conditions. 

The different modes exist as it is impossible to attain data with all of the earlier mentioned factors being maximized at the same time, i.e. optimizing one tends to result in the deterioration of one or more of the others. As a result, the prior choice between the different modes of operation must be made according to the param-eter/s of greatest importance. The different modes, some of which are instrument type specific, entail not only control of the primary and/or secondary ion optics in a highly specific manner but also the vacuum conditions present during analysis. All are dependent on the specific secondary ion signal used. These are discussed henceforth for commercially available SIMS instruments. 

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. 

NIRMS = noble-gas-ion reflection mass spectrometry OSEE = optically stimulated exoelectron emission PES = photoelectron spectroscopy PhD = photoelectron diffraction SIMS = secondary ion mass spectroscopy UPS = ultraviolet photoelectron spectroscopy ... 

Several ion sources are particularly suited for SSIMS. The first produces positive noble gas ions (usually argon) either by electron impact (El) or in a plasma created by a discharge (see Fig. 3.18 in Sect. 3.2.2.). The ions are then extracted from the source region, accelerated to the chosen energy, and focused in an electrostatic ion-optical column. More recently it has been shown that the use of primary polyatomic ions, e. g. SF5, created in FI sources, can enhance the molecular secondary ion yield by several magnitudes [3.4, 3.5]. 

The Mattauch-Herzoggeometry (Fig. 3.20) enables detection of several masses simultaneously and is, therefore, ideal for scanning instruments [3.49]. Up to five detectors are adjusted mechanically to locations in the detection plane, and thus to masses of interest. Because of this it is possible to detect, e. g., all isotopes of one element simultaneously in a certain mass range. Also fast, sensitive, and precise measurements of the distributions of different isotopes are feasible. This enables calculation of isotope ratios of small particles visible in the image. The only commercial instrument of this type (Cameca Nanosims 50) uses an ion gun of coaxial optical design, and secondary ion extraction the lateral resolution is 50 nm. 

One problem with methods that produce polycrystalline or nanocrystalline material is that it is not feasible to characterize electrically dopants in such materials by the traditional four-point-probe contacts needed for Hall measurements. Other characterization methods such as optical absorption, photoluminescence (PL), Raman, X-ray and electron diffraction, X-ray rocking-curve widths to assess crystalline quality, secondary ion mass spectrometry (SIMS), scanning or transmission electron microscopy (SEM and TEM), cathodolumi-nescence (CL), and wet-chemical etching provide valuable information, but do not directly yield carrier concentrations. 

Optical examination of etched polished surfaces or small particles can often identify compounds or different minerals hy shape, color, optical properties, and the response to various etching attempts. A semi-quantitative elemental analysis can he used for elements with atomic number greater than four by SEM equipped with X-ray fluorescence and various electron detectors. The electron probe microanalyzer and Auer microprobe also provide elemental analysis of small areas. The secondary ion mass spectroscope, laser microprobe mass analyzer, and Raman microprobe analyzer can identify elements, compounds, and molecules. Electron diffraction patterns can be obtained with the TEM to determine which crystalline compounds are present. Ferrography is used for the identification of wear particles in lubricating oils. 

For resist exposure, the resolution limit will be set by the range over which the ions interact with the resist. As with electron beam exposure, ions create secondary electrons up to several nanometers away from the beam, and these electrons can travel further before their energy is absorbed. Ultimate resolution will probably be about 10-20 nm, as it is with electrons. At present this limit is beyond the capabilities of the ion optical systems. 

The secondary Ion extraction optics should Include an Immersion lens above the sample surface to maximize secondary Ion transmission from the sample surface to the mass spectrometer. An electrostatic analyzer should again be used to filter out high-energy neutral particles and photons from the secondary Ion beam. Energy analysis of the secondary Ions Is generally not necessary, but can be used to vary the amount of fragmentation observed In the mass spectrum (9). 

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