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Basic knowledge of Scanning Electron Microscopy (SEM)

What is a scanning electron microscope?

A scanning electron microscope (SEM) is a microscope that uses an electron beam to obtain magnified images of surfaces that are several tens of times to one million times larger than the original image. Compared to visible light, electron beams have a very short wavelength, making it possible to analyze microscopic irregularities and composition better than with an optical microscope.

Structure of the electron gun

The electron gun is the equivalent of the light source in an optical microscope. Depending on the material of the filament and the method of electron generation, electron beams with different characteristics are generated.

The tungsten-type thermionic emission gun Lanthanum hexaboride (Lab6) thermionic emission gun
Tungsten thermionic emission gun - Basic knowledge of scanning electron microscopy (SEM) Lanthanum hexaboride (Lab6) thermionic emission gun - Basic knowledge of scanning electron microscopy (SEM)
Emitter material: Tungsten wire
Luminance: 104 - 105 (A / cm2 / sr)
Vacuum: 10-3 (Pa)
Applications: benchtop SEM to general-purpose SEM
Emitter material: LaB6
Luminance: 106 (A / cm2 / sr)
Vacuum: 10-5 (Pa)
Applications: General-purpose SEM
Cold Field Emission Gun (Cold FEG) Schottky Field Emission Gun (Schottky FEG)
Cold Field Emission Gun (Cold FEG) - Basic knowledge of scanning electron microscopy (SEM) Schottky Field Emission Gun (Schottky FEG) - Basic knowledge of scanning electron microscopy (SEM)
Emitter material: Tungsten single crystal
Luminance: 109 (A / cm2 / sr)
Vacuum: 10-8 (Pa)
Applications: High resolution SEM
Emitter material: Zirconia/Tungsten
Luminance: 108 (A / cm2 / sr)
Vacuum: 10-7 (Pa)
Applications: Analytical SEM

Objective Lens Structure

The objective lens is the most important lens for focusing the image. There are three main types of objective lenses depending on the position of the sample and the shape of the pole piece.

Out-lens In-lens Semi-in-lens
out-lens - Basic knowledge of scanning electron microscopy (SEM) in-lens - Basic knowledge of scanning electron microscopy (SEM) emi-in-lens - Basic knowledge of scanning electron microscopy (SEM)
This is the most common objective lens shape. Place the sample under the objective lens and observe it with the detector. Since the leakage magnetic field is less than that of other objective lenses, it can observe a variety of samples, including magnetic materials. This objective lens has very low aberration because the sample is placed in the magnetic field of the objective lens. Although it is difficult to observe large samples and magnetic materials, the highest resolution can be achieved by combining it with a field emission electron gun (FE electron gun). This is an objective lens that is intermediate between the out-lens and in-lens systems. By devising the shape of the pole piece and using a leaky magnetic field, it is possible to observe with a shorter work distance than the out-lens method.

The difference in how the image looks due to the acceleration voltage

The appearance of the image varies greatly depending on the acceleration voltage. This is because the electron scattering region of the primary electron beam differs depending on the acceleration voltage and the composition, density, and crystal orientation of the sample. The heavier the element and the lower the acceleration voltage, the smaller the electron scattering area and the more sensitive the SEM image. Using a lower acceleration voltage makes it possible to observe the fine texture of light elements, which is not visible at higher acceleration voltages.

SEM image low accelerating
low accelerating - Basic knowledge of scanning electron microscopy (SEM)
Image at acceleration voltage of 3 kV Sample: Talc
SEM image High accelerating
High accelerating - Basic knowledge of scanning electron microscopy (SEM)
Image of acceleration voltage 15kV Sample: Talc

The penetration depth of primary electron beam due to the difference in acceleration voltage

The primary electron penetration depth of acceleration voltage 1kV / 5kV / 10kV calculated using Monte Carlo simulation (Sample: Si)

1 keV

Primary electron penetration depths calculated by Monte Carlo simulation at irradiation voltages of 1 kV

5 keV

Primary electron penetration depths calculated by Monte Carlo simulation at irradiation voltages of 5 kV

10 keV

Primary electron penetration depths calculated by Monte Carlo simulation at irradiation voltages of 10 kV

Retarding method (deceleration method)

The retarding method is a method in which a negative voltage is applied to the sample to decelerate the primary electron beam just in front of the sample. For example, if the primary electron beam is accelerated to 5.0 kV at the electron gun and -4.0 kV is applied to the sample, the primary electron beam is affected by the negative voltage and is decelerated to an energy of 1.0 kV (= 5.0 kV - 4.0 kV) before being irradiated to the sample. In this method, the primary electron beam passes from the electron source to the objective lens under conditions of high acceleration voltage, which reduces aberrations compared to acceleration to 1.0 kV at the electron gun, and enables high-resolution observation even at low irradiation voltage (low acceleration primary electron beam).

Retardation method (deceleration method) - Basic knowledge of scanning electron microscopy (SEM)
SEM image with retarding
Image by retarding method (deceleration method) - Basic knowledge of scanning electron microscopy (SEM)
Image with irradiation voltage of 1kV (5-4kV)
Sample: WS2 powder
SEM image No retarding
Image without retarding method (deceleration method) - Basic knowledge of scanning electron microscopy (SEM)
Image at acceleration voltage of 1 kV
Sample: WS2 powder

Major analysis using scanning electron microscopy

When a sample is irradiated with an electron beam, a lot of signal information is generated in addition to secondary electrons and reflected electrons. From there, various analyses can be performed, such as characteristic X-ray analysis (EDS and WDS), backscattered electron diffraction (EBSD), and electron beam induced current (EBIC). This article will discuss energy dispersive X-ray analysis (EDS), which is often used in SEM.

Energy dispersive X-ray analysis (EDS analysis)

When an electron beam is irradiated to a sample, X-rays are generated. An energy-dispersive X-ray spectrometer (EDS) is a device that detects these X-rays and obtains elemental information. In combination with SEM, it is possible to obtain micron-order elemental information.

Qualitative analysis (component composition) - Basic knowledge of scanning electron microscopy (SEM)
SEM image
Sample: black ore - Basic knowledge of scanning electron microscopy (SEM)
Sample: black ore
EDS image
Element: Zinc - Basic knowledge of scanning electron microscopy (SEM)
Element: Zinc
EDS image
Element: Oxygen - Basic knowledge of scanning electron microscopy (SEM)
Element: Oxygen
Optical microscope image
Sample: Black ore - Basic knowledge of scanning electron microscopy (SEM)
Sample: Black ore
EDS image
Element: Manganese - Basic knowledge of scanning electron microscopy (SEM)
Element: Manganese
EDS image
Element: Calcium - Basic knowledge of scanning electron microscopy (SEM)
Element: Calcium