Introduction

Abstract

The word microscope is derived from Greek micros (small) and skopeo (look at). Just like any microscope, the primary function of the scanning electron microscope (SEM) is to enlarge small features or objects otherwise invisible to human sight. It does that by way of using electron beam rather than light which is used to form images in optical light microscopes. The images are obtained by scanning an electron beam of high energy on the sample surface, hence the name scanning electron microscope. By virtue of its smaller wavelength, electrons are able to resolve finer features/details of materials to a much greater extent compared with optical light. A modern day SEM can magnify objects up to one million times their original size and can resolve features smaller than 1 nm in dimension. Similarly, electron beam interaction with the specimen emits x-rays with unique energy that can be detected to determine the composition of material under examination. The SEM is, therefore, a tool used for materials characterization that provides information about the surface or near surface structure, composition, and defects in bulk materials. It allows scientists to observe surfaces at submicron and nano-level to elaborate material properties. It has emerged as one of the most powerful and versatile instruments equally valuable to materials and life scientists working in wide-ranging industries.

1.1 What Is the SEM

The word microscope is derived from Greek micros (small) and skopeo (look at). Just like any microscope, the primary function of the scanning electron microscope (SEM) is to enlarge small features or objects otherwise invisible to human sight. It does that by way of using electron beam rather than light which is used to form images in optical light microscopes. The images are obtained by scanning an electron beam of high energy on the sample surface, hence the name scanning electron microscope. By virtue of its smaller wavelength, electrons are able to resolve finer features/details of materials to a much greater extent compared with optical light. A modern day SEM can magnify objects up to one million times their original size and can resolve features smaller than 1 nm in dimension. Similarly, electron beam interaction with the specimen emits x-rays with unique energy that can be detected to determine the composition of material under examination. The SEM is, therefore, a tool used for materials characterization that provides information about the surface or near surface structure, composition, and defects in bulk materials. It allows scientists to observe surfaces at submicron and nano-level to elaborate material properties. It has emerged as one of the most powerful and versatile instruments equally valuable to materials and life scientists working in wide-ranging industries.

1.2 Image Resolution in the SEM

A human eye cannot distinguish objects smaller than 200 μm (0.2 mm). In other words, the resolution of a human eye is 200 μm, while a light microscope can typically magnify images up to 1000× to resolve details down to 0.2 μm. Resolution limit is defined as the smallest distinguishable distance separating two objects, i.e., minimum resolvable distance. For instance, two objects separated by a distance of less than 200 μm will appear as one object to the human eye since the latter is not able to resolve details that have dimensions smaller than 200 μm. Hence, 200 μm can be considered to be the resolution limit of the human eye. The same objects viewed under a light microscope will appear as two distinct entities since the light microscope can easily differentiate distances less than 200 μm. In fact, the objects can be brought closer together further to a distance of 0.2 μm and still maintain their separate identities under a light microscope. However, if the distance between the objects is decreased further to less than 0.2 μm, the light microscope will no longer be able to discern them as two separate objects, which will then appear as a single entity. Thus 0.2 μm can be defined as the resolution limit of the light microscope. It follows that the smaller is the value of minimum resolvable distance, the higher is the resolution of a microscope.

Both light microscope and humans use visible light as a means to probe into or interact with an object. The increased ability to observe details in a light microscope compared to the unaided eye is attributed to the lens/aperture system used to magnify the image of an object. It is theoretically possible to keep enlarging the image by increasing the magnification indefinitely. However, it is not possible to keep revealing newer details in an object by simply increasing the magnification. Fine details in an image cannot be resolved beyond a certain magnification. This is due to limitations imposed by the resolving power of the imaging technique as well as that of the human eye. The maximum useful magnification beyond which no further details are revealed is determined by the resolving power of a microscope. The following equation can be used to determine the typical useful magnification of a microscope:

$$ \mathrm{Useful}\ \mathrm{Magnification}=\frac{\mathrm{Resolution}\ \mathrm{of}\ \mathrm{the}\ \mathrm{Human}\ \mathrm{Eye}}{\mathrm{Resolution}\ \mathrm{of}\ \mathrm{Microscope}} $$

(1.1)

For a light microscope, useful magnification $\left(\frac{200\ \upmu \mathrm{m}}{0.2\ \upmu \mathrm{m}}\right)$ is around 1000×. For a scanning electron microscope, useful magnification $\left(\frac{200\ \upmu \mathrm{m}}{1\ \mathrm{nm}}\right)$ is typically 200,000×. Increase in the resolution of the instrument results in the increase of its useful magnification.

The ability of visible light to resolve image details is limited by its relatively large wavelength (λ = 380–760 nm) (see Fig. 1.1). Use of light with a shorter wavelength (such as ultraviolet) and a lens immersed in oil (high refractive index) improves resolution to around 0.1 μm. If the image is formed by using a radiation with a smaller wavelength, such as an electron beam, higher resolution limit can be achieved since the smaller the wavelength, the greater the resolving power and the greater the detail revealed in an image. Due to this fact, techniques like the SEM and TEM employ an electron beam to probe the material resulting in an image far superior in resolution compared to that of the light microscope. For example, an electron beam (λ of 0.000004 μm) with an accelerating voltage of 100 kV can achieve a resolution of 0.24 nm. The practical limit to resolution is determined by lens aberrations and defects. Modern-day field emission SEM typically operated at 20–30 kV accelerating voltages can achieve image resolution in the order of 1 nm or better. It is worth noting here that resolving power or resolution (a more commonly used term) of an instrument is demonstrated by manufactures using a specimen ideally suited for that instrument. For instance, tin balls/powder is routinely employed for the SEM since the former is conductive and has strong contrast (see Fig. 1.2). Details in real samples, however, are not usually revealed to that level of resolution.

Fig. 1.1

Electromagnetic spectrum showing the size of the wavelength used in the light, scanning (SEM), and transmission electron microscope (TEM)

Fig. 1.2

Secondary electron images of tin balls showing good contrast at low to very high magnifications (100,000× to 1,000,000×)

1.3 Image Formation in the SEM

The SEM instrument can be considered to comprise of three major sections: the electron column, the specimen chamber, and the computer/electronic controls, as shown in Fig. 1.3. The topmost section of the electron column consists of an electron gun which generates an electron beam. Electromagnetic lenses located within the column focus the beam into a small diameter (few nanometers) probe. The scan coils in the column raster the probe over the surface of the sample present in the chamber that is located at the end of the column. The gun, the column, and the specimen chamber are kept under vacuum to allow electron beam generation and advancement. The electrons in the beam penetrate a few microns into the surface of a bulk sample, interact with its atoms and generate a variety of signals such as secondary and backscattered electrons and characteristic x-rays that are collected and processed to obtain images and chemistry of the specimen surface. The ultimate lateral resolution of the image obtained in the SEM corresponds to the diameter of the electron probe. Advances in the lens and electron gun design yield very fine probe diameters giving image resolutions of the order of <1 nm. In order to provide a perspective to the way the image is realized in the SEM, a comparison of how light and the transmission electron microscopes work compared to the SEM is shown in Table 1.1 and Fig. 1.4.

Fig. 1.3

A photograph showing three major sections of the SEM: the electron column, the specimen chamber, and the computer control system. (Courtesy of T. Siong, JEOL Ltd.)

Table 1.1 Comparison of various characteristics of the SEM with light and transmission electron microscope

Fig. 1.4

Schematic comparing the modes of image formation in the light, transmission, and scanning electron microscopes

1.4 Information Obtained Using the SEM

The scanning electron microscope is used to observe and image the micro- and nanostructural surface details of a wide range of materials such as metals, alloys, ceramics, polymers, rock minerals, corrosion deposits, filters, membranes, foils, fractured/rough surfaces, biological samples, etc. The materials can be conductive or non-conductive either in solid or powder form and can be examined in an as-received or prepared (sectioned, ground, polished, etched, coated, etc.) condition. The SEM has the ability to examine materials in the dry or wet state as well as obtain microchemical information from fine structural details. The SEM equipped with a field emission gun can distinguish surface features that are only 1 nm apart (i.e., lateral spatial resolution = 1 nm). Extraordinary ability to depict large depths of field (10–100% of horizontal field width) allows large areas of a sample to remain in focus at one time and thus yield 3-D characteristics in SEM images (see Fig. 1.5). Imaging can be performed using both secondary electrons (for topographic contrast) and backscattered electrons (for topographic and/or compositional contrast). Microchemical information is generally obtained using energy dispersive x-ray spectrometer (EDS) detector attached to the SEM. Both qualitative and quantitative elemental information from microstructural features can be obtained from beryllium to uranium with limits of detection of approximately 0.2–0.5 wt%. The electron beam in the SEM can penetrate as much as a few microns into the sample depending on sample density, beam accelerating voltage, etc. Typical applications include observation of metallographically prepared samples (such as steel) to study surface morphology, grain size/shape, inclusions, precipitates, dendrites, grain boundaries, etc. It is also employed to observe materials in an unprepared (as-received) condition, e.g., fracture surfaces for metallurgical failure analysis, electronic devices for electronic failure analysis, corrosion deposits, catalyst shape, size and surface structure, polymer additives, rock mineral samples, etc. Apart from bulk samples, it is also used to examine coatings and thin films deposited on substrates. Table 1.2 highlights various applications and the range of information obtained using SEM and related techniques.

Fig. 1.5

Secondary electron SEM image of an ant showing detailed three-dimensional details. (Image courtesy of TESCAN)

Table 1.2 Summary of capabilities of the SEM and related techniques

A combination of factors such as good resolution, large depth of field, compositional information, time-efficient analysis, as well as relative ease of use and image interpretation in both materials and life sciences has made the SEM one of the most heavily used instruments in academia, research, and industry. Although the SEM can generate rich morphological and compositional data for a wide variety of materials, it is often necessary to employ various other analytical tools to undertake complete materials characterization. Selection of these tools will depend on the type of material under study and the nature of information required. For instance, if the objective is to reveal true microstructure (not surface topography) of a material, transmission electron microscopy is employed. This technique can also extract chemical information from features at the nanometer scale. Likewise, if the bulk composition is desired, x-ray fluorescence is a better choice as it analyzes large volumes of material. Phase identification at a macro level is often carried out using x-ray powder diffraction analysis. Analysis of structure and composition of a thin surface constituting a few atomic monolayers is undertaken with Auger and x-ray photoelectron spectroscopy. All these techniques are tools available to a scientist to carry out essential materials characterization and information obtained from one complements the others. A brief comparison of the imaging and analytical abilities of these techniques is shown in Table 1.3.

Table 1.3 Comparison of imaging and analytical techniques generally used to study materials structure, chemical composition, and defects

1.5 Strengths and Limitations of the SEM

Strengths of the SEM include:

• 1. A wide variety of specimens can be examined.

• 2. Relatively easy and quick sample preparation.

• 3. Ease of use due to user-friendly and automated equipment.

• 4. Rapid imaging, quick results, time-efficient analysis, and fast turnaround time.

• 5. Relatively straightforward image interpretation.

• 6. Large depth of field (ability to focus large depths of samples at one time and produce 3-D like images)

• 7. Microchemical analysis capability from Be to U.

• 8. Samples can be dry or wet.

• 9. Nondestructive (some beam damage may result).

• 10. High spatial resolution (1 nm) achieved by modern equipment.

• 11. A versatile platform that lends support to other sophisticated tools, devices, and techniques.

• 12. Capable of several modes of imaging, spectroscopy, and diffraction analysis.

• 13. Affordability and availability compared to more expensive equipment.

Limitations can be summarized as follows:

• 1. The sample size is limited.

• 2. Samples are solid.

• 3. EDS detector cannot detect H, He, or Li.

• 4. Poor detectability limit of elements (with EDS) compared to wet analytical methods.

• 5. Samples need to be examined under vacuum.

• 6. The instrument typically requires an installation space of 5 × 5 m.

• 7. Non-conductive samples need to be coated.

1.6 Brief History of the SEM Development

Limitation of light microscope in resolving fine details of organics cells provided the impetus to develop an electron microscope in the early twentieth century. The first electron microscope was a transmission electron microscope developed by German scientist Max Knoll and electrical engineer Ernst Ruska in Germany in 1931. It employed a working model similar to light microscope except that a beam of electrons, instead of visible light, was made to pass through the body of a sample to form an image on a fluorescent screen. Use of electrons as an imaging medium afforded a resolution of 10 nm compared to a resolution of 200 nm achievable by a light microscope. The resolution achieved at the time might seem modest today, but the real breakthrough was the fact that for the first time in history, electrons had been successfully employed to create images of matter. Subsequent improvements in the accelerating voltage, lens technology, vacuum systems, electron guns, power supplies, and overall design of the microscopes in the next few decades led to the imaging of atoms (i.e., atomic resolution was achieved). Due to his “fundamental work in electron optics and for the design of the first electron microscope,” Ernst Ruska received a Nobel Prize in physics in 1986.

German physicist Max Knoll introduced the concept of a scanning electron microscope in 1935 [1]. He proposed that an image can be produced by scanning the surface of a sample with a finely focused electron beam. Another German physicist Manfred von Ardenne explained the principles of the technique and elaborated upon beam-specimen interactions. He went on to produce the earliest scanning electron microscope in 1937 [2,3,4] as shown in Fig. 1.6.

Fig. 1.6

First scanning electron microscope built by Manfred von Ardenne in 1937

Later, the SEM with a resolution of 50 nm was built by American scientists Zworykin, Hillier, and Snijder in 1942 [5] and later by Professor Sir Charles W. Oatley and his postgraduate student D. McMullan at the University of Cambridge in 1952 [6]. Scintillator-based secondary electron detector was developed by Everhart and Thornley in 1960 [7]. Further improvements in the technology led to the development of first commercial SEM known as “Stereoscan” by Cambridge Scientific Instruments in 1965 [8, 9]. The SEMs built in the 1960s had a resolution of about 15–20 nm. In the 1970s and 1980s, the resolution was improved to 7 nm and 5 nm (at 1 kV), respectively. Next couple of decades saw resolution improvements down to 3 nm and then to 1 nm. Currently, manufacturers claim resolutions of 0.5 nm in the SEM. Although the scanning electron microscope was developed subsequent to the transmission electron microscope, the former quickly became popular due to its ease of use, simple sample preparation, and ability to generate 3-D like images of the sample topography. A detailed history of the SEM development has been documented by some authors [10,11,12,13]. Main events in the development of SEM techniques and instrumentation are listed in a chronological order in Table 1.4 [10,11,12,13,14]:

Table 1.4 Development of SEM instrumentation and techniques in a chronological order [10,11,12,13,14]