1924 De Broglie wave theory: electrons can move as particles or waves

1930 Knoll and Ruska first electromagnetic lens

Sources of electrons

• 1. radioactivity Bemitters generate high energy electrons, can't control
  
• 2. metal emission a metal has a cloud around it of covalently bound electrons
• a. photo emission: light causes electrons to flow e.g.,cadmium sulphide, silicon solar cells

low electron emission

• b. field emission use a very positive field very high voltage to pull electrons from source

as electrons are removed, the metal becomes more positive and resists giving up more electrons


cold process

• c. thermionic emission heat a metal

heat tungsten 2200 Kelvin=>boils off electrons

3400K =>melts wire

FOR DIAGRAM OF BIASED CAP, SEE FIG 2.1



With resistor, makes the guard cap more negative, pushing the electrons closer to the filament, so more see the anode plate and come down column. Too much bias prevents the electrons from leaving the filament

Can increase the efficiency of current production compared to unbiased cap

==> isn't changing the number of electrons emitted, only the number that come down the column

Anode plate: electrons leave the filament with a charge usually less than 1 electron volt. See anode at very positive value compared to electron and accelerate down the column. The difference between the filament current and the anode plate is called the accelerating voltage. This ranges from less than 1 eV to 30,000 eVolts (1-30 kV)
 

Filament saturation/longevity curves (FIG 2.3). Saturation is the point of maximum electron emission at a minimum current or source temperature. Increasing the current to the source does not produce any more electrons--it sharply decreases the lifetime of the filament.

 
Can shape filament for different efficiencies:

The smaller the area from which the electrons are emitted, the smaller the spot to start and the brighter it is

--v-shaped standard, or pointed wire tip
--crystal tip

tungsten filament cheap and easy to use, replace

lanthanum hexaboride crystal on a wire (Fig2.6)

Pro	Con
longer life	very expensive to replace
smaller spot	harder to saturate
brighter emission	needs very high vacuum
better resolution

field emission cold cathode (Fig 2.7)

Pro	Con
very bright	needs ultra-high vacuum
smaller spot	can't be retrofitted
smaller energy spread	unstable beam current
long long life

Electrons entering a magnetic field in a lens are deflected in direction but there is no change in velocity.

wrap wire around tube, get magnetic field inside, strongest along sides, weakest in the middle. Electrons entering the field are delected perpendicular and end up spiraling down the tube (Fig. 2.8).

To increase lens power:

1. more wire wraps

2. more metal in casings

pole pieces to concentrate field into smaller area

3. more current

Astigmatism: non symmetrical lens--Circles look like ovals

Correction: add equal and opposite distortion from additional lenses, called stigmator coils. These are arranged around the lens in 2 sets of coils

Chromatic aberration: different wavelengths focus at different distances

Correction:

1) Stabilize electron source

2) Increase lens current

3) Use thinner sections in TEM since specimen interactions slow electrons down variably

Spherical aberration: electrons at edge don't focus at same spot as electrons in center

Correction:

1) Increase lens current

2) Use aperture to prevent electrons from contributing to image

1. Apertures limit the outer edge of lenses (can correct spherical aberration).

2. Reduce the amount of illumination on sample.

3. Increase contrast by removing inelastically-scattered electrons (IN TEM)

4. Effect the depth of field

Depth of field == This range of distance in object over which structures appear in focus.
• Determined by the core of light entering lens--as angle decreases, depth of field increases
Depth of field is not depth of focus (range of distance from lens to screen that produces a focussed image)
• TEM scope lens measures 0.0057 degrees, so depth of field in scope is measured in meters

 

1. condensers: focuses beam to a large spot
2. objective lens: forms image -------this is the primary image former (0-100x)
1. must place specimen close to or in lens for minimum focal length for best imaging

will cause asigmatism by cutting a hole in the magnet
3. intermediate lens: primary magnifier (variable, 100-200x)
4. projector lens: secondary magnfier (only about 300x)forms final real image




Column Configurations--SEM (Fig. 5.3)

5. condensers: focuses beam to small spot
1. relatively weak with focal length of several cm

may or may not have pole pieces
6. objective lens==focuses small spot onto sample
1. scan coils in lens cause beam to move in grid pattern over sample

TEM --lenses focus image on viewing screen

--lenses magnify image in floodbeam formed at sample and project on viewscreen within microscope column

SEM --lenses focus illumination spot on sample surface, image subsequently formed on view CRT outside column

--illuminating spot is moved by magnetic coils deflecting beam aimed at sample.
--Magnification changes as area scanned is changed