Electron Microscopy
Part I
Hystorycal notes Anatomy of TEM
Resolution ≈ ½ λ
0.61 λ
R.P. = --- N.A.
In electron microscopy the refractive index cannot exceed 1.0, the half angle is very small, and thus the only thing that can be adjusted is decreasing the wavelength of illumination
Resolution is the minimum separation that still allows us to see two objects as separate
Bad lens
Average lens Good lens
mm
Transmission Electron Microscopy
Louis de Broglie 1923
Transmission Electron Microscopy
h = Planck's constant (6.624 X 10-27 erg/second)
m = mass of an electron (9.11 X 10-28 gram = 1/1837 of a proton) v = velocity of the electron
Louis de Broglie 1923
Transmission Electron Microscopy
λ
≈
( 150 / V )1/2 AngstromsSubstituting 200 eV for V gives λ a of 0.87 Angstroms For a beam of 100 KeV we get a wavelength of 0.0389 and a theoretical resolution of 0.0195 Angstroms!
But in actuality most TEMs will only have an actual resolution 2.4 Angstroms at 100KeV
mmmm
Final Resolution is affected by:
• Quality of the sample: well ordered, static samples can give the highest resolution.
• Characteristics of the data measurement (wavelength, diffraction angle): was high resolution data collected?
• Quality of the structure determination: poor phases/alignment give a low resolution
Ernst Ruska
Physics Nobel Laureate 1984
Transmission Electron Microscopy
Bill Ladd 1939
Transmission Electron Microscopy
James Hillier - RCA
EMB 1940
Electron Optics
Electron Sources
Thermionic Emitters
Field Emitters
Electron Sources
Thermionic Emitters
Field Emitters
Electron Sources
Thermionic Emitters
Field Emitters
Electron Sources
Work Function
Energy (or work) required to withdraw an electron
completely from a metal surface. This energy is a measure of how tightly a
particular metal holds its electrons
Electron Sources
Thermionic Emitters
utilize heat to overcome the work function of a material.
Tungsten Filament (W) Lanthanum Hexaboride LaB6
Electron Sources
Tungsten emitters
Wire bent into a loop of various dimensions.
W (m.t. 3410 degrees C.)
Electron Sources
Increasing the filament current will increase the beam current but only to the point of saturation at which point an increase in the filament current
will only shorten the life of the emitter
Electron Sources
Heat is applied by way of separate resistance
wire or ceramic mounts Filament current is
separate from heating current
Electron Sources
Similar in design to a tungsten filament
Electron Sources
Filament Current (Heating Current) Current running through the emitter Beam Current
Current generated by the emitter
Electron Sources
Filament Centering
Gun Horizontal
Gun Tilt
Electron Sources
Field Emitter
Single oriented
crystal of tungsten etched to a fine tip
Electron Sources
Field Emitter
Single oriented
crystal of tungsten etched to a fine tip
The emission of electrons that are stripped from parent atoms by a high electric field
Electron Sources
A Field Emission
tip can be “cold” or thermally assisted to help overcome the work function but
ultimately it is a high voltage field of 3 KeV that is needed
Electron Sources
Electron Sources
Other Factors to consider?
Cost W= $15 LaB6 = $400 F.E. = $6000 Lifetime 100 hr. 1000 hr 5-8,000 hr.
Electron Optics
Transmission Electron Microscope
Optical instrument in that it uses a lens to form an image
Scanning Electron Microscope
Not an optical instrument (no image forming lens) but uses electron optics.
Probe forming-Signal detecting device.
Electron Optics
Refraction, or
bending of a beam of illumination is
caused when the
wavelength enters a medium of a
different optical density.
Electron Optics
In light optics this is accomplished when a
wavelength of light moves from air into glass In EM there is only a vacuum with an optical density of 1.0 whereas glass is much higher
Electron Optics
In electron optics the beam cannot enter a
conventional lens of a different optical density.
Instead a “force” must be applied that has the same effect of causing the beam of illumination to bend.
Electron Optics
In electron optics the beam cannot enter a
conventional lens of a different optical density.
Instead a “force” must be applied that has the same effect of causing the beam of illumination to bend.
Electromagnetic Force or Electrostatic Force
Classical optics: The refractive index changes abruptly at a surface and is constant between the
surfaces. The refraction of light at surfaces separating media of different refractive indices makes it possible to construct imaging lenses. Glass surfaces can be
shaped.
2) Electron optics: Here, changes in the refractive
index are gradual so rays are continuous curves rather than broken straight lines. Refraction of electrons
must be accomplished by fields in space around
charged electrodes or solenoids, and these fields can assume only certain distributions consistent with
field theory.
Converging (positive) lens: bends rays toward the axis. It has a positive focal length. Forms a real
inverted image of an object placed to the left of the first focal point and an erect virtual image of an
object placed between the first focal point and the lens.
Diverging (negative) lens: bends the light rays away from the axis. It has a negative focal length.
An object placed anywhere to the left of a diverging lens results in an erect virtual image. It is not
possible to construct a negative magnetic lens
although negative electrostatic lenses can be made
Electromagnetic Lens
Passing a current through a single coil of wire will produce a strong magnetic field in the center of the coil
Electromagnetic Lens
Electromagnetic Lens
Pole Pieces of iron Concentrate lines of Magnetic force
