25 Scanning Electron Microscopy (SEM)

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Scanning electron microscopy is central to microstructural analysis and therefore important to any investigation relating to the processing, properties, and behavior of materials that involves their microstructure. The SEM provides information relating to topographical features, morphology, phase distribution, compositional differences, crystal structure, crystal orientation, and the presence and location of electrical defects. The SEM is also capable of determining elemental composition of microvolumes with the addition of an x-ray or electron spectrometer and phase identification through analysis of electron diffraction patterns. The strength of the SEM lies in its inherent versatility due to the multiple signals generated, simple image formation process, wide magnification range, and excellent depth of field.

 

In 1931 Max Knoll and Ernst Ruska at the university of Berlin built the first electron microscope that use accelerated electrons as a source instead of light source. However, the first scanning electron microscope (SEM) was built in 1938 due to the difficulties of scanning the electrons through the sample. Electron microscope is working exactly the same as the optical microscope expects it use a focused accelerated electron beam [1].

 

Fundamental principles of electron microscopy

 

The principal of electron microscope is the same as a light microscope but instead of using visible light it use very energetic electrons as a source. However, the resolution of the optical microscope is limited by its wavelength compared to accelerated electrons which have very short wavelength. This is what makes it possible to see very small features. In electron microscopes, electrons have very small wavelength λ . This wavelength can be changed according to the applied high voltage. Hence, according to Rayleigh’s criterion the wavelength λ of an electron is related to the momentum p=mv of the electron by:

where h = 6×10-34 J s is the Planck constant, m and v are the mass and velocity of the electron respectively.

 

Since the electron can reach nearly the velocity of light c then we can use the relativistic equations. In this case the electron mass is changing according to:

 

 

where me is the rest mass of the electron.

 

The energy eV transmitted to an electron is giving by:

By using equations 1,2 and 3 the electron wavelength can be written as function of accelerated voltage: [3]

for example an accelerated voltage of 10 kV will yield a wavelength of 0.0122 nm. The extremely small wavelengths make it possible to see atomic structures using accelerated electrons.

 

Interaction of accelerated electrons with the specimen

 

The electron beam interacts with the specimen reveal useful information about the sample including: its surface features, size and shape of the features, composition and crystalline structure. The interaction of the electron beam with the specimen can be in different ways.

 

i)  Secondary electrons : If the incident electrons come close enough to the atom then these electrons will give some of their energy to the specimen electrons mainly in the K-shell. As a result, these electrons will change their path and will ionize the electrons in the specimen atoms. These ionized electrons that escape the atoms are called secondary electrons. These electrons will move to the surface of the specimen and undergoing to elastics and inelastic collision until reaching the surface. However, due to their low energy ~ 5eV only those electrons that are close to the surface (~ 10 nm) will escape the surface and then can be detected and can used for imaging the topography of the specimen.

 

2.   Backscattered electrons : When the incident electrons hit an atom directly, then they will be reflected or backscattered. Different atomic type of atoms will result in a different rate of backscattered electrons and hence the contrast of the image will vary as the atomic number of the specimen change, usually atoms with higher atomic number will appear brighter than those have lower atomic number.

 

3.  Transmitted electrons: If the incident electrons pass through the specimen without any interaction with their atoms, then these electrons called transmitted electrons, these electrons are used to get an image of www.intechopen.com FE-SEM Characterization of Some Nanomaterial 465 thin specimen. Another scattering mechanism called elastic scattered where electrons don’t loss their energy these scattered electrons can be used to get information about orientation and arrangement of atoms.

 

4.  Other interactions : When the atoms bombarded with incident electrons, electrons will released from these atoms and this will leave the atom in the excited state. In order for the atom to return to the ground state, it needs to release the excess energy Auger electrons, X-Rays,and cathodoluminescence are three ways of relaxation. The x-ray is used to identify the elements and their concentrations in the specimen by using a technique called Energy –dispersive Xray analysis (EDX) technique. Chemical analysis can be done by using Auger electrons.

 

Types of electron microscopes

 

There are two types of electron microscopes. Scanning Electron Microscopes (SEM), and Transmission Electron Microscope (TEM), these types of microscopes detect electrons that emitted from the surface of the sample.

 

The accelerated voltage is ranging from 10kV to 40kV for the SEM. The thickness of the specimen in this case is not important. In addition, the samples to be tested have to be electrically conductive; otherwise they would be overcharged with electrons. However, they can be coated with a conductive layer of metal or carbon.

 

In TEM the transmitted electrons are detected, and in this case the specimen thickness is important and typically should not exceed 150 nm. The accelerated voltage in this case > 100kV. Since the electrons are easily scattered in air all electron microscopes should operate under a high vacuum.

 

All types of electron microscopes are basically consist of three basic components: Electron Gun which is used to provide and supply electrons with the required energy. There are different types of electron gun;the old type was a bent piece of Tungsten wire with 100 micro-metresin diameter. Higher performance electron emitters consist of either single crystals of lanthanum hexaboride (LaB6) or from field emission guns.

 

Instrumentation of SEM

 

  • The basic components used in electron optical system are:
  • A source of electrons, called electron gun
  • Lenses
  • Scanning Coils
  • Detectors to collect signals
  • Sample Stage
  • Display/Data output devices

 

Infrastructure Requirement

  • Power supply
  • Vacuum system
  • Cooling system
  • Vibration free floor
  • Room free of ambient electric and magnetic fields

 

SEM INSTRUMENTATION

1) Electron Beam: It has two variables i.e. energy and current. The voltage is variable from about 1 – 60keV and the current from 1e-7 to 1e-12 A. These values are specific to the instrument model.

2)  Electron Gun: It is used to produce fine electron beam (also called as electron probe). Several different types of electron guns used are:

 

a)            TE (Thermionic- Emission) gun

b)            FE (Field- Emission) gun

c)            SE (Schottky- Emission) gun

 

TE (Thermionic- Emission) gun –

  • A thin tungsten wire filament act as cathode to generate thermo electrons by heating the filament at 2800K.
  • By applying positive voltage of about 1 to 30 KV to the metal plate acting as anode, in order to collect these thermo electrons.
  • By applying negative voltage to the Wehnelt electrode placed between the anode and the cathode, current of the electron beam is adjusted. This electrode also helps in focussing the electron beam.
  • Thinnest point of beam known as cross-over (15-20μm Diameter), regarded as actual electron source.
  • LaB6 crystal is used as a cathode. It used to reduce the spot size. It requires high vacuum due to its higher activity.

 

FE (Field- Emission) gun

  • Provides high resolution.
  • Works on field-emission effect when high electric field is applied to the metal surface.
  • A thin tungsten wire act as cathode wielded to the tungsten single crystal whose tip is curved with the radius of about 100nm known as emitter.
  • Electrons are emitted from emitter through tunneling effect when positive voltage was applied to the extracting electrode.
  • Hole created in the extracting electrode to allow emitted electrons to flow through it. Then electron beam containing some energy is obtained by applying voltage to the accelerating electrode present beneath the extracting electrode.
  • In FE gun energy spread is less because no heating is required and also electron beam diameter is 5-10nm.
  • Requires ultra-high vacuum of the order of 10-8 Pa.

 

SE (Schottky- Emission) gun –

  • Works on schottky emission effect when high electric field is applied to heated metal surface.
  • A tungsten single crystal (tip radius – few hundred nm) coated with ZrO acting as cathode.
  • ZrO coating reduces the work function to enhance the emission current at low cathode temperature.
  • Thermo electrons are shielded from emitter by applying negative voltage to the suppressor electrode.
  • Advantage: electron beam current is highly stable because emitter is placed in ultra high vacuum of the order of 10-7 Pa.
  • Produces larger probe current

 

3. Lenses

 

To produce finest beam of electron with desired crossoverdiameter therefore two- level lens system used are condenser and objective lens made of metal cylinders with cylindrical hole, which operate in vacuum.These lenses are located beneath the electron gun. Magnetic field is generated in the inner part of the lenses to focus or de-focus the beam.

 

Role of condenser lens:

 

Condenser lens action is related to the probe size. If it is strengthened then probe size is narrowed with a smaller ratio of b/a, whereas it is weakened then probe size is broadened. C1 and C2 lenses controls the beam current by varying size and intensity of beam spot. Aperture with a small hole in it made of metal placed between two condenser and objective lensto allow the beam to pass through it to reach the objective lens. Resolution is dependent upon aperture as it controls the spot size.

 

Role of objective lens:

 

It is used for focusing and determines the final diameter of probe.

 

4. Scanning Coils–

 

These coils deflect the beam in X or Y directions in order to scan the sample surface in a raster pattern.

 

5. Sample Stage-

 

It is a motorized plate which has movement in three directions X, Y and Z controlled by feeding value in the software. The samples are supported on it and move smoothly in the required direction. X and Y, the two horizontal movement are used to change the field of view whereas Z, the vertical movement is required for image resolution as well as depth of focus. Along with these movements Rotation and Tilting are also possible. Also, stage movement can be controlled manually through single click of mouse.

 

6. Detector–

 

Characteristics of sample are measured at different beam position to form image. Secondary electrons emitted from the sample are measured using secondary electron detector.

 

7. Display Unit and Recording system-

 

The output in the form of amplified electronic signal is send to the display unit. To form SEM image, scanning is synchronized with electron beam scan and brightness (which depends upon number of secondary electrons emitted) on the display unit appearing on the monitor screen. Previously, CRT (Cathode Ray Tube) was used as a display unit but these days it is replaced by LCD (Liquid – Crystal Display). Extremely fast scan speed is used while focusing for observation and slow speed used for capturing or saving image.

 

8. Vacuum System-

 

The microscope column and the specimen chamber is kept under high vacuum i.e. 10-3 to 10-4 Pa. Diffusion pump is used to evacuate these components. For oil- free environment – Turbo molecular pump is used, for FE-SEM – sputter ion pump is used as Fe-SEM is ultra-high vacuumed.

 

SEM Sample-

 

Conducting samples provide a path to earth for the beam electrons, and therefore require no special preparation. Insulating materials, however, require a thin coating of a conductor (often carbon or gold) in order to prevent charging. Sample Preparation-

 

It is done in order to eliminate the sample charging few steps are followed:

 

a)  Charging: A thin noble metal coating of about 10nm is done on the sample because metal film is highly stable and its secondary electron yield is higher. Too thin coating is not preferred because continuity is lost.

b)   Low accelerating voltage: Low KV value of about 1KV can even scan insulating samples because number of incident electrons becomes equals to the number of emitted secondary electrons that means sample is not charged.

c)Tilt Observation:In this case secondary electrons yield is higher as electron beam is entering at an angle.

d) Low Vacuum SEM observation: Ondecreasing the vacuum increases the gas molecules within the sample chamber which gets ionized due to electrons and thus, on reaching the specimen as positive ions neutralizes the charging.

you can view video on Scanning Electron Microscopy (SEM)

REFERENCES

  1. Stokes, Debbie J. (2008). Principles and Practice of Variable Pressure Environmental Scanning Electron Microscopy (VP-ESEM). Chichester: John Wiley & Sons.
  2. McMullan, D. (2006). “Scanning electron microscopy 1928–1965”. Scanning. 17 (3): 175–185.
  3. McMullan, D. (1988). “Von Ardenne and the scanning electron microscope”. Proc Roy Microsc Soc. 23: 283–288.
  4. de Jonge, N.; Ross, F.M. (2011). “Electron microscopy of specimens in liquid”. Nature Nanotechnology. 6: 695–704.
  5. Hitachi Launches World’s Highest Resolution FE-SEM. Nanotech Now. 31 May 2011.
  6. Takaku, Yasuharu; Suzuki, Hiroshi; Ohta, Isao; Tsutsui, Takami; Matsumoto, Haruko; Shimomura, Masatsugu; Hariyama, Takahiko (7 March 2015). “A ‘NanoSuit’ surface shield successfully protects organisms in high vacuum: observations on living organisms in an FE-SEM”. Proceedings of the Royal Society of London B: Biological Sciences. 282 (1802): 20142857.
  7. Danilatos, G. D. (1988). “Foundations of environmental scanning electron microscopy”. Advances in Electronics and Electron Physics. Advances in Electronics and Electron Physics. 71: 109-250
  8. US patent 4823006, Danilatos, Gerasimos D. and Lewis, George C., “Integrated electron optical/differential pumping/imaging signal detection system for an environmental scanning electron microscope”, issued 18 April 1989.