Vacuum Technology

The FEGSEM is only possible because some complex problems of vacuum engineering have been solved

Some basic knowledge of vacuum technology is useful in getting the best from the machine and maintaining the vacuum integrity

Qualitative Vacuum Ranges

Low or rough vacuum 760 to 1 Torr

Medium vacuum 1 to 10-3 Torr

High vacuum 10-3 to 10-6 Torr

Very high vacuum 10-6 to 10-9 Torr

Ultra-high vacuum 10-9 and lower

FEGSEM contains regions of each type

laminar

molecular

Vacuum pumps

For each of the vacuum ranges identified there is one or more type of pump that is best suited

Pumps are always used in combination with one pump used to start the next

The sequencing of the pump down is important. Now under computer control - do not try to do this by hand

Ion Pumps

Ionized molecules spiral in magnetic field and get buried in Ti wall coating

Diode pumps only handle some gases

Triodes pumps will handle most gases

Ion pump performance

“The” UHV pumpRequires no backing

- works best in a closed system

Requires periodic bake-out into rough pumped system to clean the pump

Vacuum Hygiene

Always keep vacuum systems runningUse LN2 and fore-line traps if fittedDon’t rough pump for too longKeep fingers away from chamberWear gloves when handling anything

that will go into the sample chamber!

Contamination and Cleaning samples

Try not to use solvents as these are always contaminated, even when fresh from a glass container

Never use squeeze or spray bottles

Carbon Dioxide ‘snow’ cleaning may be worth investigating - no residue and good solvent action

Use a plasma cleaner or an Active Oxygen system

Options available

Storing Samples

As soon as a specimen is prepared for observation it begins to get dirty again

Even storing the sample in a vacuum dessicator will not prevent the growth of surface contaminant films because the source of the problem is carried in by the specimen itself

Remedial action is

therefore required

As prepared

After one week

Plasma cleaning

Plasma cleaning provides a rapid and efficient way of removing the build-up of surface contaminants and restoring the sample to a pristine condition

Same sample after plasma cleaning

Unwanted Beam Interactions

Radiation Damage

IonizationDisplacementHeating

ContaminationEtching

Intrinsic to electron beam irradiation

Results fromvacuum problems

Both are usually important

Ionization Damage

Occurs when the beam generates high energy excitations lasting long enough for relaxation of ion cores to occur. This causes a bonding instability and the structure falls apart.

May also cause visible effects such as the formation of color centers

In metals and semiconductors the conduction band electrons delocalize the excitation and prevent damage

Radiolysis

Ionization damage is most important threat to organic, and some inorganic, materials.

Electrons are the most intense source of ionizing radiation available - the typical dose in an SEM is equivalent to standing 6 foot from a 10 megaton H-bomb

Compare SEM to Sun and SPEAR*

*Stanford Positron Electron Accelerating Ring

Effects of radiolysis

Direct effect - destroys the crystalline structure of polymers, and other organic crystals, leaving them amorphous

Probability of radiolysis is 10x to 100x bigger than the chance of generating an X-ray

Damage competes with signal generation - damage usually wins

Heating

Is not usually a serious problem as the energy deposited is quite small.

For a typical material of medium density and thermal diffusivity the temperature rise varies with energy, and beam dose

Magnification 5keV 15keV 30keV

400x 0.1C/nA 0.24C/nA 0.56C/nA

4000x 0.15C/nA 0.34C/nA 0.79C/nA

Contamination - EtchingContamination is beam induced polymerization of

hydrocarbons on the sample surface. The organic molecules come from the oil vapors of the vacuum pumps and the outgassing of any organic material present in the instrument.

Etching is removal of surface layer by impact of ions (C + OH - --> CO + H2 )

Both effects are affected by surface charging and often go together

Both are changed by temperature

Contamination and Etching

Electrons break down contamination film. The residue charges +ve and the field pulls in other contaminant. If water vapor is

present then OH- ions go to the + ve charge region and etch that area away

Low magnification

At low magnification the hydrocarbon film is polymerized into a thin sheet.

This will charge positive (and look dark) but is not a serious problem

High magnification

At high magnification the contamination grows a cone which prevents the beam reaching the surface

Avoid spot mode ! Try and pre-pump

samples before use Keep your hands off

the sample

Cones

Contamination cones can grow to a height of hundreds of angstroms and are very tough

Prevent growth by irradiating area at low magnification before going to a high magnification

Beam currents

The beam currents and current densities available in an FEG SEM are high even for small probe sizes

This can cause problems on radiation sensitive samples such as organic materials and biological tissue

Always try to minimize the radiation dose

Radiation doses

SEM dose is about 100 el/Å2

Typically at 1 -10el/Å2 loss of crystallinity

at 10-100 el/Å2 mass loss

and above100 el/Å2 limiting mass loss

Dose for a single photo scan

Temperature effects

Altering both the temperature of the sample and its surroundings will switch contamination to etching as the temperature falls

This is because water vapor condenses on sample.

Temperature Effects II

Holding the sample at RT but placing a cold surface close to it can dramatically reduce the contamination rate

Such a device is usually called a “Cold Finger”

The Cold Finger

The finger is held at LN2 temperatures, very close to the specimen surface

After filling the cold finger allow the sample enough time to reach thermal equilibrium before starting to image

Advantages of a Cold Finger

Organic molecules tend to collect on the colder surface

Reduced contamination

Better light-element quantitative analysis

Vacuum and Contamination Summary

Insure proper vacuumUse LN2 and fore-line traps if fittedReduce contamination of samplesProper sample preparationUse cold finger when necessary