Scanning Electron Microscopy (SEM)

3.2.1 Introduction

The scanning electron microscope creates various high magnification images by focusing a high energy beam of electrons onto the surface of a sample in a raster scan pattern and detecting signals from the interaction of the incident electrons with the sample's surface. The electrons interact with the atoms and produce the signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. In 1935, Max Knoll obtained the first SEM image of silicon steel showing electron channeling contrast [8]. Following this, in 1937 Manfred von Ardenne worked on the physical principles of the SEM and beam specimen interactions [9, 10]. In 1965, Professor Sir Charles Oatley and his student Gary Stewart further developed the SEM and it was subsequently marketed by the Cambridge Instrument Company as the "Stereoscan". The first instrument was delivered to DuPont [10].

Figure 3.4: Signals produced when an electron beam is incident on a sample [11].

3.2.2 X-ray Microanalysis

When the sample is bombarded by an electron beam the following signals/products may occur as shown in Figure 3.4 backscattered electrons, secondary electrons, x- rays, cathodoluminescence and Auger electrons. Backscattered and secondary electrons are used to obtain a detailed topographical image of the surface of the sample. Besides these, scanning electron microscopes are often equipped with Energy Dispersed Spectroscopy (EDS) or Energy Dispersed Analysis of X-rays (EDAX) detectors that analyse the emitted X-ray energies. With such instruments, it is possible to determine which elements are present in the surface layer of the sample (at a depth in the micrometre range) and where these elements are present. This particular microscope also allows one to capture directly reflected electrons, the so- called backscattered electrons, from which one can obtain a global appreciation whether one or several elements are present in the surface layer of the sample. Also the Auger electrons, which are emitted just under the surface, provide information about the nature of the atoms in the sample.

Incident Electron Beam Characteristic X-Ray photons

Bremsstrahlung Visible light (Cathodoluminescence)

Transmitted and Inelastically Scattered Electrons Backscattered Electrons Secondary Electrons Auger Electrons Elastically Scattered Electrons

SPECIMEN

Figure 3.5: Schematic of (a) typical SEM and (b) path of the electron beam in this

system [12]

3.2.3 Scanning Process

In a typical SEM, electrons are thermionically emitted from a tungsten or lanthanum hexaboride (LaB6) cathode and are accelerated towards an anode; alternatively, electrons can be emitted via field emission (FE). Tungsten is used because it has the highest melting point and lowest vapour pressure of all metals, thereby allowing it to

Detector Stigmator Objective Electron Source Condenser Signal in Scan Coils X, Viewing Monitor Specimen

Output to monitor fixed.

Output to Scan coils Varies Detector Stigmator Objective Electron Source Condenser Signal in Scan Coils X, Viewing Monitor Specimen

Output to monitor fixed.

Output to Scan coils Varies Detector Stigmator Objective Electron Source Condenser Signal in Scan Coils X, Viewing Monitor Specimen

Output to monitor fixed.

Output to Scan coils Varies

Output to monitor fixed.

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be heated for electron emission. The electron beam, which typically has an energy ranging from a few hundred eV to 100 keV, is focused by one or two condenser lenses into a beam with a very fine focal spot sized 0.4 nm to 5 nm, typically. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron optical column, typically in the objective lens, which deflect the beam horizontally and vertically so that it scans in a raster fashion over a rectangular area of the sample surface. Figure 3.5 shows the schematic of a typical SEM system. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into the surface. The size of the interaction volume depends on the electron's arrival energy, the atomic number of the specimen and the specimen's density. The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, each of which can be detected by specialized detectors. The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current. Electronic amplifiers of various types are used to amplify the signals which are displayed as variations in brightness on a screen. The raster scanning of the display is synchronized with that of the beam on the specimen in the microscope, and the resulting image is therefore a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. The image may be captured by photography from a high resolution cathode ray tube, but in modern machines it is digitally captured and displayed on a computer monitor and saved to a computer's hard disc. In this study, the surface morphology and composition of as- deposited and heat treated CuCl hybrid films were investigated using an ‘EVO LS 15’ scanning electron microscope developed by Carl Zeiss. An accelerating voltage of 15-19 keV and probe current of ~800 pA was used for a qualitative comparison of the CuCl composition as a function of time and heat treatment.