Guns, surface analysis

Figure 6. Plan of the target preparation facilities consisting of UHV preparation chamber (a), (reactive) ion etching chamber (b), ion etching gun (c), laser (d), photon detector (e), transfer arms (f), Auger system for surface analysis (g), sample manipulator and annealing facility (h), load lock and optical microscope for viewing sample (i), evaporator (j), transmission diffractometer (k), and vacuum tank for main spectrometer (1).

The sample preparation chamber on a surface analysis system (AES, XPS) is fitted with an ion gun for depth profiling (see diagram). The gun can be differentially pumped to maintain the required pressure difference between the ion source and the preparation chamber, which is initially fitted with a TMP with 5eff = 50 L s 1. 

For SIMS analysis, the sample can be a solid material, a thin layer or particles supported on a substrate. It must be able to withstand ultra-vacuum, since volatile components may be redeposited during analysis. Because of the erosion of the sample during ion impact, any contamination layer can be eliminated prior to analysis. The preparation of the sample is thus subject to less severe conditions than for other surface analysis techniques. For the results of composition profiles to be significant the surface must, however, be flat at the scale of the ion probe. The use of a liquid metal ion gun will enable concentration profiles to be determined on grains of around 0.1 pm in diameter. 

Figure 7.6 Non-monochromatic X-ray radiation from an X-ray gun with an A1 target. The characteristic Alkaline is at about 1.5 keV. (Reproduced with permission from D. Briggs and J.T. Grant, Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications and Surface Spectra Ltd, Chichester. 2003 IM Publications.)...

Figure 7.7 An X-ray gun with two-anodes. Two tapered anode faces (one is A1 and the other is Mg) have semi-circular filaments, which are near ground potential. An acceleration voltage of about 15 kV between a filament and anode generates X-rays that exit through an A1 window. (Reproduced with permission from D. Briggs and M.P. Seah, Practical Surface Analysis, Volume 1, John Wiley Sons Ltd, Chichester. 1990 John Wiley Sons Ltd.)...

The transmission electron microscopy was done with a 100-kV accelerating potential (Hitachi 600). Powder samples were dispersed onto a carbon film on a Cu grid for TEM examination. The surface analysis techniques used, XPS and SIMS, were described earlier (7). X-ray photoelectron spectroscopy was done with a Du Pont 650 instrument and Mg K radiation (10 kV and 30 mA). The samples were held in a cup for XPS analysis. Secondary ion mass spectrometry and depth profiling was done with a modified 3M instrument that was equipped with an Extranuclear quadrupole mass spectrometer and used 2-kV Ne ions at a current density of 0.5 /zA/cm2. A low-energy electron flood gun was employed for charge compensation on these insulating samples. The secondary ions were detected at 90° from the primary ion direction. The powder was pressed into In foil for the SIMS work. 

As we will see, surface analysis equipment is very similar for several techniques, so most surface analysis instruments are configured for multiple surface analysis techniques. A schematic diagram of a commercial instrument showing the placement of the ion gun, energy analyzer, source, and so on is shown in Fig. 14.8. 

Why is an electron flood gun used in ESCA, Auger, and other surface analysis instruments ... 

Many applications of ion beams in surface analysis use a rastered ion beam to insure uniform erosion. The fraction of the ion beam that is neutral atoms is not affected by the potentials applied to the deflection plates of an ion gun and result in distortion in the erosion rate over the analysis area of the sample. Unfortunately, it is difficult to measure the composition of an ion beam to detect the presence of impurity ions, doubly charged or neutral species in most experimental situations. 

Another standard component is an ion gun providing ions which may impinge on a defined sample position seen by the Auger electron analyzen The ions gradually erode the sample surface so that when combined with the surface analysis intrinsic to ZkES an elemental in-depth profile is obtained from a well-defined small area of the sample. Inert gases, such as argon, are typically used so that the ions do not react with the sample surface. The erosion rate is material dependent but can be 50-100 nm min... 

XPS and AES instruments are often equipped with an argon ion gun. Ion impingement to sputter away material, and surface analysis, can be alternated to give a composition depth profile. 

The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electron-gun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 80—... 

In insulator analysis an electron gun is also necessary to compensate for the positive ion current at the sample surface. Two types of operation are typical. 

The control of materials purity and of environmental conditions requires to implement physico-chemical analysis tools like ESC A, RBS, AUGER, SEM, XTM, SIMS or others. The principle of SIMS (Secondary Ion Mass Spectroscopy) is shown in Eig. 31 an ion gun projects common ions (like 0+, Ar+, Cs+, Ga+,. ..) onto the sample to analyze. In the same time a flood gun projects an electron beam on the sample to neutralize the clusters. The sample surface ejects electrons, which are detected with a scintillator, and secondary ions which are detected by mass spectrometry with a magnetic quadrupole. 

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