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Chapter 5.Nanometrology > 5.2Chemical Surface Structure (Chemography) - Pg. 82

82 CHAPTER 5 Nanometrology Instead of electrons, ions can also be used as imaging particles (FIB, fast ion bom- bardment). Originally developed as a way of controllably etching parts of samples in order to be able to more completely investigate their inner structures using electron microscopy, the instrumentation is now sufficiently advanced to enable it to be used as an ultramicroscopy technique in its own right. 5.2 CHEMICAL SURFACE STRUCTURE (CHEMOGRAPHY) Many of the available techniques are essentially nonimaging approaches because they lack lateral resolution in the nanoscale. Often nanoscale resolution is only achiev- able for regular structures. The best-known approach of this type is probably X-ray diffraction (which could equally well be considered as a technique for determining structure). A beam of X-rays is made to impinge on the sample making an angle with a plane of atoms within it, and the spacial distribution of the scattered X-rays is measured. Because the wavelength of X-rays is of the order of interatomic- plane distance d (tenths of a nanometer), crystalline material, or at least material with some order in its atomic arrangement, diffracts the beam. The key condition for constructive interference of the reflected beam is Bragg's law: d sin D n , n D 1, 2, : : : . (5.4) This metrology technique was developed soon after the discovery of X-rays by R ontgen in 1895, in other words long before the era of nanotechnology. ¨ The main family of classical surface chemical analytical methods involves firing one kind of photon (or electron) at the sample and observing the energy of the photons (or electrons) whose emission is thereby triggered. In Auger electron spectroscopy (AES), an incident electron beam ejects an elec- tron from a core level; the resulting vacancy is unstable and is filled by an electron from a high level, releasing energy that is either transferred to another (Auger) elec- tron (from a yet higher level), or emitted as an X-ray photon. The measurement of the spectrum of the Auger electrons is called AES. In energy dispersive X-ray spec- troscopy (EDS, EDX) it is the X-ray photons whose spectrum is measured. Both these techniques are capable of good lateral resolution (within the nanoscale), because the incident electron beam can be finely focused. EDS is typically carried out within a scanning electron microscope equipped with a suitable X-ray detector. It yields quantitative elemental abundances with an accuracy of around 1 atom%. AES, on the other hand, can additionally identify the chemical state of the element. All these techniques yield an average composition within a certain depth from the surface of the sample, which is a complicated function of the scattering of the incident and emergent radiations. Typically AES samples only the first few nanometers from the surface, whereas EDS averages over a somewhat greater depth, which might be as much as 1 m.