Electron Microscopy

3.3.3.1

Background

The scanning electron microscope is an observational instrument that irradiates samples confined in a vacuum chamber with a focused accelerated rastering electron beam. Rastering produces high depth of field micrographs with magnifications typically ranging between 10-10,000X (Goldstein et al., 2003). A general schematic of a SEM is pictured in Figure 3-1. First concepts of field emission optics were brought to light by Knoll (1935) in ‘Aufladepotentiel und Sekundäremission elektronenbestrahlter Körper’ German for ‘Static potential and secondary emission of bodies under electron irradiation’. Later, von Ardenne (1938) added transmission coils creating the scanning transmission electron microscope (STEM) while Zworykin et al. (1942) discovered the benefits of secondary electron imaging that delivered high topographic contrast. In total six types of electron-photon emissions occur during irradiation of matter in a SEM.

Various technologies, manufacturers and models exist and have different operating voltages and parameters. The following text summarizes common tungsten filament SEM operation and the instruments used in this thesis are described in the subsequent sections. SEMs are composed generally of four components; electron gun, lenses, electron collectors and associated image capturing electronics. The electron gun

is composed of a filament negatively charged (approximately 0.1-30 kV) in a high pressure vacuum chamber. Temperatures ranging from 2000-2700° K cause thermionic emission of electrons (Goldstein et al., 2003). Regeneration of filament current is replaced by a resistor and the voltage differences force electrons to crossover (Goldstein et al., 2003). Electrons are then accelerated by the lower anode aperture (Goldstein et al., 2003). The surrounding negatively charged Wehnelt cylinder focuses electrons within the electron gun chamber and controls electron emission (Goldstein et al., 2003). Accelerated electrons are directed through demagnifying lenses to the final spot size (~100 nanometres) (Oatley, 1972). Pairs of transmission coils located in the column deflect the focused electron beam in the X-Y plane by altering the frequency and strength of the electromagnetic coils as a function of time (Oatley, 1972; Goldstein et al., 2003). The frequency applied to the transmission coils creates the scanning effect. Diffracted and emitted electrons, resulting from bombarding the sample, are collected by sensors in the sample chamber. Digital processing takes signals obtained from detectors and produces images.

3.3.3.2

Hitachi SU6600 SEM

The Hitachi SU6600 SEM is a state of the art field emission variable pressure SEM housing a Schottky electron emission source. The Schottky field emission electron gun differs from traditional tungsten “hairpin” filaments as it does not rely on high temperatures to cause thermionic emission but precision shape to create a high applied electron field at a much higher vacuum allowing electrons to funnel down (Goldstein et al., 2003). Compared to hot filaments the spot size is less than half and 1000 times brighter (Crewe et al., 1968). The instrument is also furnished with wave length dispersive and energy dispersive spectrometers as well as a Gatan CL image detection system. The instrument is housed in the ZAPLab at the University of Western Ontario under the supervision of Dr. Desmond Moser and operated by Ivan Barker.

3.3.3.3

Secondary Electron Imaging

Secondary electrons (SE) are electrons ejected from inelastic scattering of the incident electron beam and sample material. Inelastic scattering is due to low energy conductive or valance state electrons interacting with the incident electron beam. SEs are low energy electrons ejected from outer atomic valence states between 5-10 KeV

(Goldstein et al., 2003). The SE detector itself is a variation of the Everhart-Thornely detector comprised of a positively charged Faraday cup attached to a scintillator (Figure 3-2). As secondary electrons pass into the Faraday cup they are accelerated to the scintillator producing light with every bombardment, which is transferred by a light pipe to a photomultiplier. Carbon coated thin and thick sections underwent SE imaging. Higher magnifications were used to identify the morphologies of datable phases.

Figure 3-2 Illustrated diagram of Everhart-Thornely detector taken from Reed (2005).

3.3.3.4

Backscatter Electron Imaging

Backscatter electrons are electrons ejected via elastic scattering (deflected) through electron beam interaction with the atom nuclei of the sample material with no loss of kinetic energy. Ejected electrons charge a silicon diode. Through electron-hole principle, the diode lets an electron leave while forming a conduction band while another electron drops back into the valance state (Goldstein et al., 2003). An applied current across the detector surface allows the “hole” and free electrons to move oppositely (Goldstein et al., 2003). The current is measured and is proportional to the number of BSE hitting the detector. Higher atomic number elements produce more BSEs because larger nuclei create a greater effect on the incident electron beam. Carbon coated thin sections containing selected datable phases were imaged using BSE. Brightness controls were dimmed down inversely to contrast levels to identify mineral grain textures and chemical zoning.

3.3.3.5

Cathodoluminescence Imaging

Luminescence is s characteristic of matter defined as the “transition of anion, molecule, or a crystal from an excited electronic state to a ground or other state with lesser energy” by Marfunin (1979). This transition creates electromagnetic photon emission. The emission occurs as two time-based categories: fluorescence (<10-8s) and phosphorescence (>10-8s)(Pagel et al., 2000). The electromagnetic and photon-electron emissions are within the visible light spectrum, as a result from electrical, mechanical or chemical excitation energy (Marfunin, 1979). Seven types of excitation energy are

classified: photo-, cathode-, thermo-, electro-, chemo-, bio-, tribo-, crystallo-, and X-ray (Gucsik, 2009). Cathodoluminescence develops from the emission of electrons from an accelerated electron beam typically from an SEM or TEM irradiating material 2-8 microns deep (Gucsik, 2009). The long wave length spectrum is influenced by numerous intrinsic and extrinsic variables including chemical composition, crystal structure, crystal defects, strain and temperature (Edwards et al., 2007). These intrinsic or extrinsic

characteristics act as “traps” for electrons returning to the valance band from the

conduction band. The energy emitted or lost from the electron leaving the trap is emitted in the 400-700 nm spectrum. Less energy is emitted in the UV and infrared spectrum. CL imaging in relation to minerals’ intrinsic and extrinsic characteristics typically show growth zones. Zircon crystals selected for geochronology analysis were subjected to cathodoluminescence excitation energy and imaged using a simultaneous Gatan ChromaCL detector housed within the Hitachi SU6600 FE-SEM.