27 Instrumental Features of SEM and TEM

Vinay Gupta

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Important Instrumental Features of SEM and TEM

 

 

Scanning Electron Microscope

  1. Electron emitting gun
  2. Electro-magnetic lenses in order to direct and focus the electron beam path inside the column
  3. Pumps system for Vacuum
  4. Opening to insert the sample into the high-vacuum observation chamber in conventional SEM mode.
  5. Operation panel with focus, alignment and magnification tools and a joystick for positioning of the sample.
  6. Screen for menu and image display.
  7. Cryo-unit to prepare (break, coat and sublimate) frozen material before insertion in the observation chamber in Cryo-SEM mode.
  8. Electronics stored in cupboards under the desk

Transmission Electron Microscope

 

Basic electron optics:

 

Electrons and ions are charged particles; they can be accelerated in an electric field. The trajectory of an accelerated charged particle can be changed (deflected) by applying an electrical or magnetic field. The accelerated particles also behave like waves (de Broglie). A schematic of the transmission electron microscope is shown in Fig. 1.

Fig. 1: Schematic of the transmission electron microscope

 

The basic components of the transmission electron microscope are the electron source (electron gun) with associated accelerating facility for energizing the electrons, a set of lenses (condenser, objective, intermediate and projector), a fluorescent screen on which the final image is observed and photographic facilities for recording the images. In addition there are apertures introduced in the vicinity of the condenser, objective and intermediate lenses and stigmator. The specimen stage normally takes in a sample of 2.3 or 3 mm dia, either unmounted or supported by copper, carbon or beryllium grids. The specimen stage has facilities for tilting about one or two axes and rotation; it is also possible to heat, cool or strain the sample while under observation (heating, cooling and straining stages).

 

The functions of the lenses are as follows:

 

Condenser lens:         to control beam intensity, density, convergence

Objective lens:            first stage of magnification, facilitates the formation of bright field

                                      and dark field images

Intermediate lens:     further magnification, imaging or diffraction mode

Projector lens:           final magnification

 

A schematic of the scanning electron microscope is shown in Fig. 2.

 

 

Fig. 2: (a) Schematic of the scanning electron microscope

 

 

Fig. 2: (b) Schematic of the scanning electron microscope

 

The basic components are the electron gun, lenses, double deflection coil, detectors and stigmator along with the scan generator and cathode ray tube for observing the image. A light microscope and scanning electron microscope are similar as both are used in the reflection mode. The main difference in these microscopes is that in light microscope whole of imaging the entire specimen is performed at once, while in SEM the electron beam is scanned back and forth over the specimen and thus imaging is performed at only one point at a time. The interactions of the electrons with the surface are registered, and from this data an image can be constructed. The relationship between the area scanned on the specimen and that on the monitor is shown in Fig. 3 and the concept of magnification illustrated in Fig. 4.

 

Magnification level in a SEM can be varied over a range of about 5 orders of magnitude from x25 or less to x 250,000 or more. The image magnification in the SEM is not dependent on the power of the objective lens which is unlike optical and transmission electron microscopes. The function of condenser and objective lenses in SEMs is to bring the focus of the beam to a spot, and not to image the whole specimen. Provided the electron gun can generate a beam with sufficiently small diameter, an SEM could in principle work entirely without condenser or objective lenses, although it might not be very versatile or achieve very high resolution. In an SEM 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 x,y scanning coils, and not by objective lens power.

 

 

Fig. 3: Illustration of the principle of rastering

 

 

Fig. 4: Magnification in the SEM

 

A block diagram of the Scanning electron microscope is shown in Fig. 5 with the various parts labeled. As with TEM, it is possible to tilt the sample in the SEM along one or two axes.

 

The various components in the SEM can be summarized as follows:

  • A source (electron gun) of the electron beam which is accelerated down the column
  • A series of lenses (condenser and objective) which act to control the diameter of the beam as well as to focus the beam on the specimen
  • A series of apertures (micron-scale holes in a metal strip or film) through which the beam passes and which affect the properties of the beam
  • Controls for specimen posi tion (x, y, Z-height) and orientation (tilt, rotation)
  • An area of beam/specimen interaction that generates a variety of signals that can be detected and processed to produce an image or spectra
  • All the above components maintained at high vacuum levels (the value of the upper part of the column being greater than the specimen chamber.

 

Fig. 5: Block diagram of the scanning electron microscope

 

Electron optical elements &attachments

  • Electron source
  • Lenses
  • Deflection coils
  • Stigmators
  • Electron detectors
  • Photon/X-ray detectors

    Electron Source:

 

Generation of electrons that can be accelerated by high tension to obtain the illuminating electron beam. The sources are of two types – Thermionic gun:triode or self-biasing gun, W, LaB6, CeB6 and Field Emission Gun:Single crystal W. Photographs of the two types of guns are shown in Fig. 6.

 

                                          (a)                                                                                   (b)

 

Fig. 6: (a) Thermionic emitter (W filament) and (b) Field emitter

 

Thermionic emitter:

 

A schematic of the thermionic emitter is shown in Fig. 7. Electrons are emitted from a heated tungsten filament (temperature 2700K) and then accelerated towards an anode which is a flat plate with an aperture and is earthed. A large negative potential is applied between the filament and the anode. The Wehnelt cylinder is biased slightly negative ( 0 to 500V) with respect to the filament. The electric field of th gun causes the focusing of the electrons at the crossover region. This region has a size of ~ 50 µm. The current density Jc of the electrons from the cathode is given by (Richardson’s law)

 

Jc = AT2 exp-(Ew/kT) Acm-2

 

where A is the Richardson constant, T is the emission temperature, Ew is the work function and k is the Boltzmann’s constant. For the tungsten filament the values of A and Ew are 60 A/cm2K2 and 4.5 eV respectively. For a typical temperature of 2700K the value of Jc is 1.73A/cm2. The maximum value for beam brightness, β, is given by the expression

 

β                                                                                        = JceV/пkT

 

where e is the electronic charge and V is the accelerating voltage. Using the value of 1.73 A/cm2 for Jc and with the accelerating voltage of 100 kV, the beam brightness is found to be 2.37×105 A/cm2. The anode plate assembly (Plate with a hole and with a potential impressed upon it) acts as an electrostatic lens. A divergent beam of electrons emerges from the anode hole. W filament is the most commonly used electron source; it is robust, cheap and does not require relatively high vacuum

 

 

Fig. 7: Schematic of thermionic electron gun

 

The relationship between beam brightness and filament heating current for a tungsten filament gun is shown in Fig. 8. Brightness goes up with increase in filament current up to a certain limit beyond which no further gain is observed. Increasing the current beyond the saturation limit results in excessive heating of the filament leading to filament failure, an effect that is clearly seen in Fig. 9.

 

Fig. 8: Relationship between beam brightness and filament heating current in a Wcathode
self-biasing gun

 

Fig. 9: Scanning electron micrograph of W filament: (a) unused, (b) failed

 

There are three configurations for the LaB6 cathode depending on the heating method. The Broers type employs a tungsten coil enveloped around the sharp end of a stretched (around 2 cm) LaB6 Heat radiation and electron bombing from the tungsten coil heat up the tip end; transference of heat through the structural support assists to reduce reactivity of LaB6. In the Vogel configuration a short LaB6 rod is heated directly by transient current, perpendicular to the length of the rod using firm electrical connectors that also offer the provision for the rod. Pyrolytic graphite is used in between the conductors and the rod to evade the difficulties of chemical reactivity of LaB6 and the conductors. In the Ferris configuration a short LaB6 rod is maintained by a ribbon or belt through which an electrical current is passed for heating. The rod is heated by conduction from the ribbon. The ribbon material is chosen to be chemically inactive with the LaB6, such as graphite or tantalum. The details of these configurations are shown in Fig. 10.

 

 

Fig. 10: Illustration of the three configurations for the LaB6 cathode

 

The  work  function  and  Richardson’s  constant  for  LaB6  are  2.66  eV  and  29  A/cm2K2 respectively. Therefore the brightness of the LaB6 is an order of magnitude higher than that of W filament. Energy Spread in thermionic filaments is due to filament imperfections, high tension instability, variations in surface temperature and Boersch effect (mutual interaction). Typical source spot sizes are 30 µm for W and 5 µm for LaB6.

 

Field Emission Gun

 

Field emission refers to the emission of electrons that are stripped from parent atoms by a high electric field (typically 109V/m). The field emitter is a single oriented crystal of tungsten electrolytically etched to a fine tip (tip diameter of 100 to 1000 Å). A Field Emission tip can be “cold” or thermally assisted to help overcome the work function. The temperatures are lower than those required for thermionic emission. The process requires very good vacuum. A simple tungsten tip can be very sensitive to surface contamination. More than any other cathode design, the field emission tip is extremely sensitive to the size, shape and surface condition. The work function of the metal is responsible for the emission process. This work function can be affected by the adsorbed gases. This is the reason for the very high vacuum requirement. Sustaining high electrical field gradients is similarly important to emission, so a tip that is well damaged might not produce electrons at all. As with the tungsten filament gun, the voltage alteration or bias among the first anode and the accelerating voltage on the cathode regulates the emission current. The second anode is at ground potential and the voltage difference from here to the cathode determines the acceleration given the electrons. The shape of the anodes is carefully selected to minimize aberrations.

 

The thermionic and field emission guns are schematically shown in Fig. 11. The basic difference is the presence of an extra anode which applies the high electrical field on the filament tip and the considerably higher level of vacuum in the field emission gun.

 

 

Fig. 11: Schematic illustration of thermionic and field emission guns

 

The important points to note are the considerably higher level of brightness, lower energy spread (and therefore minimized chromatic aberration effects) and the longer life of the FEG source. Stringent vacuum requirements and the considerably higher cost are the limiting factors.

 

The tungsten filament requires a considerably higher probe size for carrying the same probe current compared to the FEG. LaB6 and FEG are preferred for carrying out high resolution electron microscopy and when using very fine probes in X-ray analysis. The W filament is good enough for most of the routine applications.

 

Questionnaire

  1. Describe the various parts of SEM.
  2. Describe the basic electron optics of TEM.
  3. Describe in detail the working of SEM.
  4. Describe Field Emission gun.
  5. Describe Thermionic gun.
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    References

  1. Fundamentals of Light Microscopy and Electronic Imaging by Douglas B. Murphy.
  2. Electron Microscopy: Principles and Fundamentals by S. Amelinckx (Editor), Dirk van Dyck (Editor), J. van Landuyt (Editor), Gustaaf van Tendeloo (Editor).
  3. Scanning Electron Microscopy and X-ray Microanalysis: Third Edition by Dale E. Newbury, David C. Joy, and Joseph I. Goldstein.
  4. Electron microscopy by John J. Bozzola.
  5. Transmission Electron Microscopy: A Textbook for Materials Science, Authors: Williams, David B., Carter, C. Barry