{"id":350,"date":"2018-12-11T12:32:55","date_gmt":"2018-12-11T12:32:55","guid":{"rendered":"http:\/\/msp11.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=350"},"modified":"2018-12-12T05:35:31","modified_gmt":"2018-12-12T05:35:31","slug":"scanning-electron-microscopy","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp11\/chapter\/scanning-electron-microscopy\/","title":{"rendered":"Scanning Electron Microscopy"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n\r\n

Learning Objectives<\/strong><\/p>\r\n

From this module students may get to know about the following<\/strong><\/p>\r\n

i.<\/strong> Surface morphology and topography<\/em><\/p>\r\n

ii.<\/strong> Secondary and backscattered electrons<\/em><\/p>\r\n

iii.<\/strong> Electron-beam interactions<\/em><\/p>\r\n \r\n

1. Introduction to surface morphology and topography<\/strong><\/p>\r\n

Morphology is a qualitative evaluation of the three dimensional shape of a surface and topography provides the quantitative dimensions of the feature. Morphology captures the shape, texture, and distribution of materials at a surface and is best-evaluated using imaging techniques, such as optical microscopy or electron microscopy (EM). These methods can also provide layer thickness using optical profilometer or taking image in the transverse section of the specimen. However, estimation for the roughness, step heights, and feature size can be addressed using atomic force microscopy (AFM), stylus profilometry or optical profilometry. Few analytical imaging techniques are,<\/p>\r\n

\u2022 High resolution optical microscopy<\/p>\r\n

\u2022 Scanning electron microscope (SEM)<\/p>\r\n

\u2022 Fie ld emission scanning electron microscopy (FE-SEM)<\/p>\r\n

\u2022 Ultra high resolution-SEM (UHR-SEM)<\/p>\r\n

\u2022 Focused ion beam \u2013 scanning electron microscopy (FIB-SEM)<\/p>\r\n

\u2022 Scanning transmission e lectron microscopy (STEM)<\/p>\r\n

\u2022 Transmission electron microscopy (TEM)<\/p>\r\n

1.1 Electron Microscopy<\/strong><\/p>\r\n

Waves in electron beam can be coherent (waves of same wavelengths) or incoherent (waves of different wavelengths). The electron source in the microscope generates a beam of coherent electron waves, which when fall on the surface of the specimen, it interacts with the atoms of the specimen and can form beams consisting of either coherent or incoherent scattered electrons. Here, the electron beam generated from the thermionic electron gun passes through the electromagnetic lenses and directed towards the column of the microscope\u00a0 \u00a0and\u00a0 falls\u00a0 \u00a0onto the sample surface. Any changes in the electromagnetic wave after interacting with the sample surface could affect the movement of the electrons. The positive charges of the atom are strongly concentrated at the nucleus, whereas the negatively charged electrons in the atoms are much more dispersed in the shell structure. Electrons entering a material interact as a negatively\u00a0 charged\u00a0 particle with\u00a0 the\u00a0 electric\u00a0 field\u00a0 near\u00a0 the nucleus of the atoms in the specimen. These interactions can be classified into e lastic or inelastic interactions and are discussed briefly in the following sections.<\/p>\r\n

\"\"<\/p>\r\n

Figure 1.<\/strong> Schematic diagram of the optical microscope, TEM, and SEM.<\/em><\/p>\r\n \r\n

1.2 Optical and Electron Microscope<\/strong><\/p>\r\n

The schematic diagram with details of components for the optical and electron microscopies is shown in in Figure 1.<\/p>\r\n

The salient features of the optical and electron microscopes are tabulated below<\/strong><\/p>\r\n

\"\"Generally, the objects with dimensions less than half the wavelength of the microscope\u2019s illumination source can not be visible under the microscope. The optical microscopes use visible light (which has a minimum wavelength of 400 nm) and can not image the objects smaller than 200 nm (about the width of an average-sized bacterium). The wavelength of electron is thousands of times shorter than the visible light, enabling the electron microscopes to be able to resolve this issue and provide better magnification to image the objects at low dimensions. Hence, the electron microscopes can be used to visualize the inorganic, metallic , organic, and bio-molecular entities at atomic scale resolution.<\/p>\r\n \r\n

2. Origin of Scattering Events<\/strong><\/p>\r\n

2.1 Introduction<\/strong><\/p>\r\n

Electron microscopy is a technique that offers unique possibilities to capture images at nanometer resolution for understanding the structure, topology, morphology, and composition of a materia l. The primary electrons after collision with atoms within the specimen impart energy to the specimen, resulting loss of energy (or change in momentum) of the primary beam electrons that generates equivalent amount of energy in other form. These various forms of energy phenomena produced during electron - specimen interaction have the potential to provide important information about the nature of the specimen. When an electron hits a material, different interactions can occur, as summarized in Figure 2.<\/p>\r\n

\"\"\r\nFigure 2. <\/strong>Schematics for the electron-matter interactions arising from the impact of an electron beam onto a specimen surface.<\/em><\/p>\r\n \r\n

The electron-specimen interactions can provide information on:<\/p>\r\n

\u2022 Specimen composition<\/p>\r\n

\u2022 Topography<\/p>\r\n

\u2022 Crystallography<\/p>\r\n

\u2022 Electrical Potential<\/p>\r\n

\u2022 Local Magnetic Field<\/p>\r\n

As an electron travels through the specimen interaction volume , it scatters, loses energy, and changes the direction with each atomic interaction. Scattering events can be divided into two general classes: Elastic scattering and Inelastic scattering.<\/p>\r\n \r\n

2.2 Elastic scattering<\/strong><\/em><\/p>\r\n

When an electron beam approaches towards specimen, the electrons (of the beam) with various energies interact with the loca lized atoms (of the specimen), and during this process the electron trajectory changes and scatters. The process, in which the kinetic energy and velocity remain constant, is known as elastic scattering or electron backscattering (as shown in Figure 3). Here, there is no energy transfer takes place from\u00a0the electron to the sample and the electron leaving the sample (with energy El) is having identica l initial energy E0,<\/sub> i.e., El<\/sub>\u00a0 = E0<\/sub>.<\/p>\r\n

\"\"Figure 3. <\/strong>Illustration for the elastic scattering.<\/em><\/p>\r\n

A strong elastic scatterer very near to the nucleus may result the electrons leaving the specimen through backscattering and referred as the backscattered electrons (BSE). These electrons provide important information that is useful for the SEM imaging.<\/p>\r\n

The probability of elastic scattering increases strongly with the square of the atomic number (Z2<\/sup>), since heavier atoms have much stronger positive charge at nucleus and decreases as square of the energy of the electron (1\/E2<\/sup>).<\/p>\r\n

If the electron passes through the sample without any interaction, no energy is transferred and travels in the same direction as the incident beam without deviation (Figure 4).<\/p>\r\n

\"\"<\/p>\r\n

Figure 4.<\/strong> Scattering of an electron within the electron cloud in an atom.<\/em><\/p>\r\n \r\n

2.3 Inelastic scattering<\/strong><\/p>\r\n

Inelastic scattering\u00a0 is a process in which the energy\u00a0 is transferred from incident electrons to sample, resulting a reduction in the electron energy after interaction,<\/p>\r\n

EI\u00a0 < E0<\/strong><\/p>\r\n

In inelastic scattering, the trajectory of the incident electron is only slightly perturbed, but energy is lost through interactions with the orbital electrons of the atoms in the specimen. The process is sketched in Figure 5).<\/p>\r\n

\"\"\r\nFigure 5.<\/strong> Illustration for the inelastic scattering.<\/em><\/p>\r\n \r\n

The inelastic interactions (Figure 5) produce diverse effects including:<\/p>\r\n

\u2022 Secondary electrons<\/p>\r\n

\u2022 Phonon excitation (heating)<\/p>\r\n

\u2022 Cathodoluminescence (visible\u00a0 light fluorescence)<\/p>\r\n

\u2022 Continuum radiation (bremsstrahlung or \u201cbraking\u201d radiation)<\/p>\r\n

\u2022 Characteristic x-ray radiation<\/p>\r\n

\u2022 Plasmon production (secondary electrons)<\/p>\r\n

\u2022 Auger electron production (ejection of outer shell electrons)<\/p>\r\n

The signals caused by inelastic electron-matter interactions are predominantly utilized in the methods of analytical electron microscopy.<\/p>\r\n \r\n

2.4. Secondary Electrons<\/strong><\/p>\r\n

Several\u00a0 mechanisms\u00a0 of\u00a0 \u00a0inelastic\u00a0 electron-matter\u00a0 can\u00a0 lead\u00a0 to\u00a0 the\u00a0 ejection\u00a0 of\u00a0 secondary\u00a0 electrons, abbreviated as SE:<\/p>\r\n\r\n