The scanning electron microscope (SEM) is a multipurpose and commonly employed electron beam mechanism. Its reputation in scientific world developed from its simple interpretation methods of the generated micrographs, variety of information types that it can produce and combination of images with their analytical information counterpart (306) (307) (308). SEM's are used for material characterization involving image and quantitative data representation. It offered an insight into the two dimensional and three dimensional imaging of the microstructure, chemical composition, crystallography and electronic properties (306) (307) (308)(309)
Page | 109 Light microscopes (LM) operate using light to illuminate the surface to observe the structure; this limits resolution of these microscopes to the wavelength of light (308). Optical microscopes generally observe an optical limit around 300nm, whereas electron microscopes (EM) offer atomic resolution (306) (308). De Broglie theorised in 1925 that all particles assume a corresponding wave like property dubbed wave-particle duality. Particles like the electron therefore had a momentum which could be related to its wavelength λ via a constant, Planck's constant (306) (308).
(5. 4)
λ – Wavelength – Momentum - Planck‟s constant
Davisson, Germer, Thomson and Reid experimentally verified the existence of the electrons wave nature properties during the electron diffraction research (310). Knoll and Ruska in 1932 constructed a practical example of an electron lens which leads to the original electron microscope, the transmission electron microscope (TEM) (311).
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Figure 38 - Optical, Transmitting Electron, Scanning Electron Microscope Instrument Comparison
They were awarded the Nobel Prize in Physics for this magnificent achievement in 1986 (312). Scanning electron microscopy was unveiled in 1938 with its first wave of commercial instruments available from 1965 (308) (311). The delay in release was due to the advancements in electronics which permitted the scanning of the electron beam across a sample (311). Figure 38 shows the simplified schematics for OM, TEM & SEM. Several key improvements evolved through SEM characterization analysis compared to existing methods (308);
Topography - Surface texture and features.
Morphology - Size and shape of the material composing particles.
Composition - Elements and compounds that compose the material with the relative quantity.
Crystallographic - Atomic arrangement
Analysis of these areas allows the interpretation of their direct effect on the materials properties, resulting in an atomic comprehension of the material. We find significant benefits from using the SEM over traditional optical microscopes.
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Type Magnitude Depth of Field Resolution
OM 4x – 1400x 0.5mm ~ 0.2mm
SEM
10x –
500Kx 30mm 1.5nm
Table 9 - Optical Microscope Vs. Scanning Electron Microscope (306)
SEM operates with a large depth of field permitting a large expanse of the sample to be focussed on (308). This results in an overall better image that allows a representation of the three-dimensional facets of the sample. Combining the higher magnification with the greater resolution, over a larger depth of field in conjunction with compositional and crystallographic information endorses SEM as a very versatile tool for sample analysis.
Figure 39 – Interaction Signals Generated by Electron Beam (307) (313)
As the incident electron beam strikes and interacts with the surface sample, many signals are generated based on the interaction invents. Figure 39 illustrates the interaction of the electron with the sample and its consequences, of which is utilized to generate useful information based on the sample. Broadly the signals generated may be divided into two categories, electron and proton (306). The central point of the electrons interaction is within the titled “Interaction Volume”, which is affected by several properties (307)
. The atomic number of the examined sample affects the volume, as higher atomic numbers absorb or prohibit more electrons reducing the interaction volume (307). Higher acceleration voltages allow deeper penetration into the sample yielding a larger interaction volume (308). The incident angle of the
Page | 112 electrons determines the introduced interaction volume, as greater deviations from the normal produce a smaller volume (308).
Vitally to the employment of scanning electron microscopy is the mechanism of electromagnetic lenses (EML) in order to manipulate the trajectory of the incident electrons (307) (311)
. Traditional microscopy utilizes optical lenses to guide the light path for source focus and illumination. The employment of electromagnetic lenses permits the refinement of focus and affective illumination of the surface details for SEM. A basic depiction of the EML is a tunnelling configuration whereby the electron beam is projected between an array of magnets and balanced forces between the magnetic field stabilises their trajectory (307). The foundation of this is reliant on the Lorentz force principle (5. 3)(306) (311).
(5. 5) – Force - Electron Charge - Velocity - Magnetic Field
Several lenses are operated to help refine the electron flight path, the condenser and objective lens. Primarily the condenser lens helps collimate the electron beam whilst the object lens seeks to control the final focus (307). Manipulation of the electrons is achieved by variation of the magnetic field, enabling refinement of the focus. Contained within the lens is the stigmator and aperture with enables greater control over the final focus (311). The stigmator helps correct minor imperfections experienced in the objective lens and the aperture is employed to reduce the “spray “of electrons through the lens aiding refinement (306) (311)
. SEMS allow for penetration into greater resolution of microscopy, offering several considerations due to the lens configuration when compared with traditional optical lenses (308)
Page | 113 Electrons never touch the lens therefore there is no definite interface.
Electrons rotate in the magnetic field. Electrons repel each other.
Focus and magnification controlled electronically, therefore containing no moving parts.
Lenses may only comprise of positive elements in order to operate (always converging).
Cannot correct the electron lens aberration (unlike compound optical lenses). Electron lenses always operate with a small aperture.
Resolution is the ability to distinctly resolve two closely spaced points. This is different to magnification, as this is the ability to observe objects of smaller size whilst resolution offers the ability to focus on the object at the set distance (311). A method employed to improve the resolution of the SEM is to reduce the size of the electron beam projected on the sample surface (306). This may be calculated from the following set of equations (5. 6) (5. 3)(314);
⁄ ⁄ , ( ⁄ ) -
⁄
(5. 6)
This may be reduced to, for low currents;
⁄ ⁄ (5. 7)
– Minimum electron beam width - Spherical aberration - Current density of the source
– Electron wavelength - Current – Temperature
Page | 114 Several other mechanisms may be utilized to further improve the resolution of the SEM. Increasing the strength of the condenser lens aids beam refinement (314). Decreasing the objective aperture and working distance between the sample and objective lens aids resolution (307). The height of which the sample is within clear focus is known as the depth of field. SEM has a natively large depth of field with permits the three dimensional texturing nature visible on the images. Depth of field is improved by introducing a larger working distance, smaller objective aperture and by introducing lower magnifications (308). A consequence of improving the depth of field by working distance variation follows a reduction in the resolution possible, due to reciprocal nature between resolution and depth of field (306) (308). In certain cases it may be beneficial to sacrifice the resolution to gain crucial information held within the depth of field.
6.3 Four Point Probe and Hall Effect Measurement