27 Scanning Electron Microscope (SEM)

Dr. Anchal Srivastava

Content of this Unit

1. INTRODUCTION

1.1 History

1.2 Why Electron Microscope?

1.3 Scanning Electron Microscope (SEM)

1.4 Modes of Imaging

2. Construction and Working Principle

2.1 Structure of Instrument

2.2 Electron Gun

2.3 Condenser and Objective Lenses

2.4 Sample Stage

2.5 Secondary Electron Detector

2.6 Imaging and Recording of Signal

2.7 Vacuum Process

3.      Focusing of SEM

4.      Magnification of SEM

5.      Elemental Analysis

6.      Summary

Learning Outcomes

• After studying this module, you shall be able to learn
• The basic principles of SEM operation; The origin of images and their formation;
• Procedures for collecting and interpreting images; Preparing samples for investigation
• Result of scanning electron microscopy Fields that rely on electron microscopy

INTRODUCTION

Since the Scanning Electron Microscope (SEM) was first commercialized about 40 years ago, the SEM has shown a remarkable progress. Now, many types of SEMs are being used, and their performance and functions are greatly different from each other. To utilize these different SEMs, it is essential to recognize their features, as well as to understand the reasons for the contrast of SEM images. Thus, this document material is aimed at helping to understand the basics of the SEM, including the instrument principles, specimen preparation and elemental analysis.

The combination of higher magnification, larger depth of field, greater resolution, compositional and crystallographic information makes the SEM one of the most heavily used instruments in academic/national lab research areas and industry.

1.1 HISTORY

The development of a SEM began a few years after the invention of a TEM by Ruska in 1931. But the commercialization of the SEM required about 30 years. Figure1 shows the history of the initial development stage of the SEM.

Fig 1: History of the initial development stage of the SEM.

The first SEM image was obtained by Max Knoll, who in 1935 obtained an image of silicon steel showing electron channeling contrast. Further pioneering work on the physical principles of the SEM and beam specimen interactions was performed by Manfred von Ardenne in 1937, who produced a British patent but never made a practical instrument. The SEM was further developed by Professor Sir Charles Oatley and his postgraduate student Gary Stewart and was first marketed in 1965 by the Cambridge Instrument Company.

1.2 WHY ELECTRON MICROSCOPE?

Electron microscopes are scientific instruments that use a beam of energetic electrons to examine objects on a very fine scale. Electron microscopes were developed due to the limitations of Light Microscopes which are limited by the physics of light. This required 10,000x plus magnification which was not possible using current optical microscopes.

Electron wave is a unique medium that can be used in imaging. By accelerating the electrons into high energy beam (via high voltage), the wavelength thus created is far shorter than white light. For example, for an electron beam produced from a 20 kV gun, the wavelength is only 1240.7/20,000 (eV) = 0.06 nm = 0.6 Å, corresponding to a resolution limit of λ/2 = 0.3 Å — theoretically, it can be used to image a species as small as 0.3 Å. Most atoms are in size of 2-3 Å.

1.3 SCANNING ELECTRON MICROSCOPE (SEM)

Fig 2: Scanning Electron Microscope (SEM)

The Scanning Electron Microscope (SEM) is used for observation of specimen surfaces. When the specimen is irradiated with a fine electron beam (called an electron probe), secondary electrons are emitted from the specimen surface. Topography of the surface can be observed by two-dimensional scanning of the electron probe over the surface and acquisition of an image from the detected secondary electrons.

Characteristic Information of sample using SEM

Topography

The surface features of an object or “how it looks”, its texture; direct relation between these features and materials properties

Morphology

The shape and size of the particles making up the object; direct relation between these structures and materials properties

Composition

The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties

Crystallographic Information

How the atoms are arranged in the object; direct relation between these arrangements and material properties

1.4 MODES OF IMAGING

The SEM image appears as if we observe an object with the naked eye, we may intuitively understand the features of the object. However, the SEM image often produces a contrast that is difficult to explain. To fully understand the contrast of the SEM image, we must understand the principle of the formation of the SEM image.

Fig 3: Emission of various electrons and electromagnetic waves from the sample

Originally microscopy was based on the use of the light microscope and could provide specimen resolutions on the order of 0.2 microns. To achieve higher resolutions, an electron source is required instead of light as the illumination source, which allows for resolutions of about 25 Angstroms. The use of electrons not only gives better resolution but, due to the nature electron beam specimen interactions there are a variety of signals that can be used to provide information regarding characteristics at and near the surface of a specimen

In scanning electron microscopy visual inspection of the surface of a material utilizes signals of two types, secondary and backscattered electrons. Secondary and backscattered electrons are constantly being produced from the surface of the specimen while under the electron beam however they are a result of two separate types of interaction. Secondary electrons are a result of the inelastic collision and scattering of incident electrons with specimen electrons. They are generally characterized by possessing energies of less than 50 eV. They are used to reveal the surface structure of a material with a resolution of ~10 nm or better

Backscattered electrons are a result of an elastic collision and scattering event between incident electrons and specimen nuclei or electrons. Backscattered electrons can be generated further from the surface of the material and help to resolve topographical contrast and atomic number contrast with a resolution of >1 micron. While there are several types of signals that are generated from a specimen under an electron beam the x-ray signal is typically the only other signal that is used for scanning electron microscopy. The x-ray signal is a result of recombination interactions between free electrons and positive electron holes that are generated within the material. The x-ray signal can originate from further down into the surface of the specimen surface and allows for determination of elemental composition through EDS (energy dispersive x-ray spectroscopy) analysis of characteristic x-ray signals.

2. CONSTRUCTION AND WORKING PRINCIPLE

2.1 Structure of Scanning Electron Microscope (SEM)

The SEM requires an electron optical system to produce an electron probe, a specimen stage to place the specimen, a secondary-electron detector to collect secondary electrons, an image display unit, and an operation system to perform various operations. The electron optical system consists of an electron gun, a condenser lens and an objective lens to produce an electron probe, a scanning coil to scan the electron probe, and other components. The electron optical system (inside of the microscope column) and a space surrounding the specimen are kept at vacuum.

Fig 4: Basic construction of a SEM.

2.2 Electron Gun

The electron gun produces an electron beam. this is a thermionic emission gun (TE gun). Thermoelectrons are emitted from a filament (cathode) made of a thin tungsten wire (about 0.1 mm) by heating the filament at high temperature (about 2800K). These thermoelectrons are gathered as an electron beam, flowing into the metal plate (anode) by applying a positive voltage (1 to 30 kV) to the anode. If a hole is made at the center of the anode, the electron beam flows through this hole. When you place an electrode (called a Wehnelt electrode) between the cathode and anode and apply a negative voltage to it, you can adjust the current of the electron beam. At this time, the electron beam is finely focused by the action of the Wehnelt electrode. The finest point of the beam is called the crossover, and this is regarded as an actual electron source with a diameter of 15 to 20 μm. The TE gun, explained here, is most generally used. An LaB6 single crystal is also used as a cathode, but it requires a higher vacuum because of its high activity. Other electron guns are the field-emission electron gun (FE gun) or the Schottky-emission electron gun (SE gun)

2.3 Condenser and Objective lenses

An electron microscope generally uses a magnetic lens. When you pass a direct electric current through a coil wound electric wire, a rotationally-symmetric magnetic field is formed and a lens action is produced on an electron beam. To make a strong magnetic lens (with a short focal length), it is necessary to increase the density of the magnetic line. The surroundings of the coil are enclosed by yokes so that part of the magnetic field leaks from a narrow gap. A portion with a narrow gap, called “polepiece,” is fabricated with a high accuracy. The main feature of the magnetic lens is that when you change the current passing through the coil, the strength of the lens is also changed. This is not achieved by an optical lens.

The “aperture” is placed between the condenser lens and objective lens. The “aperture,” made of a thin metal plate, has a small hole. The electron beam, which passed through the condenser lens, illuminates this aperture-plate. The aperture allows a part of the electron beam to reach the objective lens. If the excitation of the condenser lens is increased, the electron beam greatly broadens on the aperture and therefore, the number of the electrons (amount of probe current) reaching the objective lens is decreased. To the contrary, if the excitation of the condenser lens is decreased, the el electron beam does not broaden very much and therefore, most of the electrons pass through the aperture and many electrons reach the objective lens. That is, the adjustment of the excitation of the condenser lens enables you to change the electron-probe diameter and the probe current. However, even if the excitation of the condenser lens is infinitely increased, the diameter of the electron probe does not become infinitely small

The objective lens is used for focusing, and this lens is a very important lens that determines the final diameter of the electron probe. If the performance of the objective lens is not good, an optimally fine electron probe cannot be produced despite all of the efforts before the action of the objective lens. Thus, it is crucial to make the objective lens with the best performance.

2.4 Sample Stage

In general, the specimen is observed at a high magnification in an electron microscope. Thus, a specimen stage, which stably supports the specimen and moves smoothly, is required. The specimen stage for a SEM can perform the following movements: horizontal movement, vertical movement, specimen tilting, and rotation. The horizontal movements are used for the selection of a field of view. While the vertical movement provides the change of image resolution and the depth of focus.

2.5 Secondary Electron Detector

The secondary electron detector is used for detecting the secondary electrons emitted from the specimen. A scintillator (fluorescent substance) is coated on the tip of the detector and a high voltage of about 10 kV is applied to it. The secondary electrons from the specimen are attracted to this high voltage and then generate light when they hit the scintillator. This light is directed to a photo-multiplier tube (PMT) through a light guide. Then, the light is converted to electrons, and these electrons are amplified as an electric signal. A supplementary electrode, called the collector, is placed before the scintillator. In general, in order to help the scintillator acquire secondary electrons, a few hundred volts is applied to this collector. By changing this voltage, you can control the number of secondary electrons to be collected. This type of the detector was originally developed by Everhart and Thornley, so this detector can be called the E-T detector. Many SEMs incorporate this detector in the specimen chamber; however, when a SEM is equipped with a strongly excited objective lens for higher resolution. Secondary electron detector is placed above the objective lens and secondary electrons are detected by utilizing the lens magnetic fields. This detector is often called the TTL (Through The Lens) detector.

2.6 Imaging and Recording of Signal

The output signals from the secondary electron detector are amplified and then transferred to the display unit. Since the scanning on the display unit is synchronized with the electron-probe scan, brightness variation, which depends on the number of the secondary electrons, appears on the monitor screen on the display unit, thus forming a SEM image. A cathode-ray tube (CRT) was used for many years as a display unit; however in recent years, a liquid-crystal display (LCD) has been widely used. In general, the scan speed of the electron probe can be changed in several steps, An extremely fast scan speed is used for observation and a slow scan speed is used for acquisition or saving of images.

To record an SEM image, in the past, the SEM image appearing on the CRT was photographed with a camera. But recently, the image has been recorded in a digital format (electronic file). This is because it is now difficult to get a high-resolution CRT and there are many advantages of electronic file and it is easier to process images and convenient to send or receive image information. An image format with 1M pixels is generally used for the electronic file.

2.7 Vacuum Process

The inside of the electron optical system and the specimen chamber must be kept at a high vacuum of 10-3 to 10-4 Pa. Thus, these components are evacuated generally by a diffusion pump. If a user desires an oil-free environment, a turbo molecular pump may be used. When a SEM incorporates an FE gun (explained later), a sputter ion pump is used because the FE gun needs an ultrahigh vacuum.

To exchange a specimen, either of two methods is applied. One vents the entire specimen chamber at the time of specimen exchange. The other uses a specimen pre-evacuation chamber (airlock chamber) while keeping a high vacuum in the specimen chamber.

3.  FOCUSING OF SEM

In the observation of a specimen with a substantial depth, if the focus is adjusted to the top side, the bottom side may be out of focus. In such a case, if the range between upper and lower image blur is large, it is said that “the depth of focus is large.” Whereas if the range between upper and lower image blur is small, it is said that “the depth of focus is small.”, when the electron probe is considerably parallel (aperture angle is small), the image stays in focus even if the focus is changed by a large amount. Whereas when the electron probe is substantially angular (aperture angle is large), the image goes out of focus even if the focus is only slightly changed. In the case of an optical microscope (OM) where the probe scanning is not used for imaging, when the angle subtended by the objective lens from the specimen (aperture angle) is small, the depth of focus is large. Whereas when this angle is large, the depth of focus is small. Note that even when the image is blurred, this cannot be seen at a low magnification. However, when the magnification is increased, the image blur is found to appear. That is, the depth of focus is changed also by the magnification.

Figure 5 shows a difference of the depth of focus between the SEM and OM. Although a stereoscopic microscope provides an image with a relatively large depth of focus between OMs, you can obtain a much larger depth of focus wi the SEM. This is because the aperture angle of the electron probe in the SEM is much smaller than that of the objective lens in the OM. Note that the depth of focus for the SEM is different depending on the observation conditions.

Fig5: OM image and SEM image of the same field of view.

The OM and SEM images of a fractured surface of a screw. This fractured surface has large irregularity, leading to a fact that only a small part of the surface is actually focused with the OM. But, due to the large depth of focus of the SEM, the entire observed surface is in sharp focus

4.  MAGNIFICATION OF SEM

Magnification in a SEM can be controlled over a range of up to 6 orders of magnitude from about 10 to 500,000 times. Unlike optical and transmission electron microscopes, image magnification in the SEM is not a function of the power of the objective lens. SEMs may have condenser and objective lenses, but their function is to focus the beam to a spot, and not to image the specimen. Provided the electron gun can generate a beam with sufficiently small diameter, a SEM could in principle work entirely without condenser or objective lenses, although it might not be very versatile or achieve very high resolution. In a SEM, as in scanning probe microscopy, magnification results from the ratio of the dimensions of the raster on the specimen and the raster on the display device. Assuming that the display screen has a fixed size, higher magnification results from reducing the size of the raster on the specimen, and vice versa. Magnification is therefore controlled by the current supplied to the scanning coils, or the voltage supplied to the deflector plates, and not by objective lens power.

5 ELEMENTAL ANALYSES

Analysis by EDS

The Energy Dispersive X-ray Spectrometer (EDS) is used to analyze characteristic X-ray spectra by measuring the energies of the X-rays. when the X-rays emitted from the specimen enter the semiconductor detector, electron-hole pairs are generated whose quantities correspond to the X-ray energy. Measuring these quantities (electric current) enables you to obtain the values of X-ray energy. The detector is cooled by liquid nitrogen, in order to reduce the electric noise..

Analysis by WDS

The Wavelength Dispersive X-ray Spectrometer (WDS) is used to analyze characteristic X-ray spectra by measuring the wavelengths of the X-rays. First, the X-rays emitted from the specimen hit the analyzing crystal, next, this crystal diffracts the X-rays, and finally the X-rays enter the detector and their wavelengths are measured. The analyzing crystal and detector must move on a so-called “Rowland circle” with a constant radius. In order to cover (measure) all of wavelengths, a driving mechanism for multiple analyzing crystals is needed, requiring a lot of time to acquire all of the corresponding X-ray spectra.

Analysis of Non-conductive Specimen

When analyzing a nonconductive specimen, metal coating is required as in SEM observation. However, we must use a metal that is different from that may be contained in a specimen. When we want to detect light elements, a coated film must be thin enough because a thick heavy-metal coated film may prevent the emission of X-rays from light elements.

If we try to analyze a nonconductive specimen without coating, this specimen is analyzed with a low accelerating voltage caused by to the charging effect. Thus, some problems may occur; for example, characteristic X-rays with high excitation energies cannot be detected, and the accuracy of quantitative analysis is degraded. In addition, positional shift of the incident electron probe may occur when line analysis or X-ray mapping is performed.

6 SUMMARY

In this module you study

• SEM, has been presented as an powerful imaging technique
• The electrons interact with the atoms that make up the sample producing signals that contain information about the sample’s surface topography, composition and other properties such as electrical conductivity.
• During SEM inspection, a beam of electrons is focused on a spot volume of the specimen, resulting in the transfer of energy to the spot.
• To produce the SEM image, the electron beam is swept across the area being inspected, producing many such signals. These signals are then amplified, analyzed, and translated into images of the topography being inspected. Finally, the image is shown on a CRT.

Suggested Reading

• Belk, J. A. (ed). Electron microscopy and microanalysis of crystalline materials. London: Applied Science Publishers, 1979.
• Goldstein,Joseph. Scanning electron microscopy and x-ray microanalysis, 3rd ed.
  New York : Kluwer Academic/Plenum Pub., 2003.
• Reimer, L. Scanning electron microscopy: physics of image formation and microanalysis (2nd ed.).
  Berlin : Springer, 2000.
• Williams, David B. Practical analytical electron microscopy in materials science. [Basel]: Verlag Chemie International, 1984.