1. make a coherent beam

near free passage of electrons at atmospheric pressure is 1 cm.

at 10^-4 Pa, they travel several meters (about 6.5 m)

2. insulator=> no interaction of beam and gas molecules==

no 'electrical storms'

3. filament life is much longer

4. no interaction between gas molecules, e-beam, and sample

Mechanical vacuum pumpè "roughing pump" or "forepump" (Fig.3.1)

Oil diffusion pump (works with rotary prepump)(Fig 3.2)

1. boil oil

2. oil vapor rises up the CHIMNEY and out at different levels at supersonic speed

3. directed out to sides and condenses on the water-cooled side walls and runs down

oil vapor pressure forces air molecules down toward base

when enough molecules get accumulated , they can be removed by the mechanical pump

can improve vacuum an order of magnitude, by installing a cold finger(molecules are trapped on the cold surface) at the top of pump

drawback is the cold finger reduces pumping speed

Turbomolecular pump (Fig. 3.3)

goes from atmosphere to high vacuum in minutes, no warm-up

alternating stack of spinning blades moving at 20-60K rpm (rotors) and stationary blades (stators) spinning blades

can also use mechanical pump to help exhaust gases from turbine to improve performance

no oil in system is advantage

Titanium sublimation pump (a getter pump)(Fig. 3.4)

forms sorptive and chemical bonds on inside surface of pump

the heated titanium filament sublimes a new layer of titanium over old surface, creating new surface to trap molecules on

Sputter-ion pumps (Fig 3.6)

a high voltage field creates a current flow of electrons, and a magnetic field lengthens their pathlength

the electrons collide with gas molecules to ionize them

the ions knock particles of metal out of the titanium cathode (sputter them)

the ions are trapped on the surface walls and coated with titanium

Cryopumps and cold traps (used to enhance vacuum, not as the primary mode any more)(Fig 3.6)

molecules stick to cold surface of adsorptive material (e.g., activated charcoal)

has minimum trapping capacity at room temperature

use LN or helium

Cold traps are used to help lower the pressure, but cryopumps are not used any more in any but the most specialized microscopes.

must avoid venting to atmosphere== would get water condensation in system. Need to warm system to room temp, pump out, then cool down again
 

Pirani and thermocouple gauges for low vacuum --> only good to 10³Torr.
 

Pirani

Thermocouple (similar operation to the Pirani)

Has a heated wire attached to mixed metal thermocouple

As vacuum and temperature change, current flow changes and is read out directly.

Ion discharge gauges

Cold cathode, Penning (Fig 3.9)

Get current flow between anode and cathode (kept at several thousand volt difference relative to each other, which ionizes gas molecules in instrument. As electrons hit gas molecules, collisions form more ions. The more gas molecules present, the more collisions to generate more ions which leads to increased current measured by the gauge

Hot cathode (Fig 3.10)

Heat cathode to thermionic level to generate many more electrons. These electrons are more likely to hit any gas molecules present, ionizing them. The ions are preferentially attracted to an ion collector which is more negative than the cathode. These ions, moving to ion collector, generate a current. This gauge is 2 orders of magnitude more sensitive, but more likely to melt if brought in contact with air.

The turbomolecular pump is the only high vacuum pump that can function from atmosphere to high vacuum

High vacuum pumps, such as the oil diffusion pump and the ion-getter and titanium sublimation pump, cannot operate without pre-pumping by a low vacuum pump.

As a result, when removing samples from the SEM or camera plates from the TEM,or replacing the illumination source in either microscope, a series of valves and a specific valving sequence is necessary to prevent the high vacuum pumps from being exposed to atmospheric conditions.

The instruments start off with all valved closed.

The Roughing valve opens to evacuate the microscope column from atmosphere to low vacuum (a 'rough vacuum'). After reaching low vacuum, the chamber is sealed off as the roughing valve closes.

The Main valve separates the high vacuum pump from the microscope column. Once low vacuum is reached in the instrument, the main valve opens to bring the column from low vacuum to high vacuum.

In the case of the oil diffusion pump, the backing valve connects the rotary low vacuum pump to the base of the oil diffusion pump. It needs to be open for the rotary pump to remove the accumulating gases (which are 'backing up' ) at the base of the oil diffusion pump.