Contamination Rate -  contamination is a cracking of vapours within the vacuum by the heat of the beam on a surface, hydrocarbons and silicons being deposited hence the low signal level dark lines or rectangles.  SEM contamination rate is very much specimen dependant but by taking a constant approach this may be a useful test.  I use sputter coated latex spheres the specimen being in the microscope one hour prior to the test.  A typical rough tough microscope used without any care gives 10nm/min over my 20 minute test period.  Under similar (emphasise similar) conditions a well kept air locked instrument will come down to 2.5nm/min.  Add a cold finger around the final lens similar to that used in a cryo system and you are down to <1.5nm/min. 

Drift Rate - I thought SEM stages were very poor however testing a good number of instruments over a wide price range I found that over a twenty minute period the amount of drift was less than the instruments resolution, in other words the sample did not move.  If it did I always found an earth problem not a stage drift problem. I no longer bother with this test unless I have a worry about a particular stage stability.

 Most of my work has been on run of the mill instruments with the best results from the modern twin detector FEG systems. In these instruments a good cold finger sitting around the final lens is the difference between good and amazing results - contamination IS the killer of high resolution microscopy in my mind.

 Vacuum System.   You should know the pump down time from air, and the time each pump is operating during the pump down procedure.  These values should be obtained for each air locked area of the instrument.  The exact valve changes during each pump down cycle should be documented if a mimic display is provided.

  ROUTINE MAINTENANCE PROCEDURES

 During use the tungsten filament will thin, and eventually break, due to evaporation and oxidation.   Filaments will break in the following fashion:-

1.  A normal break occurs to one side of the tip, the break should be between two tapered ends.

2.  An overheated filament will break as above, except the ends will not taper, they will either be in the form of blobs of metal, or if the blobs have fallen off, two blunt ends.

3.  A filament effected by a very poor vacuum will break due to severe oxidation.  The filament will seem to break with the normal taper, but its life will have been very short.  You will notice how the filament is very thin, far thinner than for a normal break.  Even if the gauge indicates the vacuum is good, this is not often a true indication of the gun vacuum, as the gauge is normally positioned near to the high vacuum pump. 

 The filament ceramic also acts as an indication of the filament's life.

1.  If the filament has been carefully saturated, not overheated, the base will be a light blue in colour.  The colour comes from a light coating of evaporated tungsten.

2.  If the filament has been run very hard, either due to overheating in error, or to the filament being placed very close to the cathode, in order to obtain improved emission levels, the ceramic will be a dark blue in colour.

3.  An orange to brown colouration is due to contamination, the filament had been operating in a poor vacuum environment. This colouration would be expected if the filament had failed due to excess oxidation.

 The cathode assembly is made of stainless steel, which when dirty may be coated with tungsten (blue to black in colour), and general contamination from the vacuum (orange yellow colour).  Prior to fitting a new filament the components must be polished perfectly clean.  A number of cleaning methods are available, the choice being whether to clean by hand or to use an ultrasonic cleaner.

 Cleaning by hand, you may use any metal polishing media that is suitable for brass or stainless steel.  The most important point is, to remove ALL of the polishing media, when the cleaning process is complete.  Each cleaned component should be stored in a lint free tissue or aluminium foil until ready for use.  Prior to fitting a new filament, each component should be checked with a hand lens, to ensure there are no debris remaining.

 The o-rings that are disturbed, when entering the gun chamber, should be washed in hot soapy water.  The o-ring seat is best cleaned with a solvent prior to refitting the ring.  Do NOT grease the rings.  The only o-rings that require grease are those that make a moving seal, e.g. aperture drives, and mechanical gun alignment seals.  Grease is the largest contributor to vacuum contamination, the less grease you use the better the column and specimen contamination rates will be.

 The electron column will require cleaning, in part, or as a whole, under the following circumstances.

1.  If you are unable to correct the objective astigmatism, in the first case the final aperture should be changed.

2.  If the above action does not remove the problem, then the aperture holder should be cleaned.

3.       Should the problem remain after following the above two sections, then the remainder of the column liner should be cleaned, and the apertures cleaned or replaced

 When cleaning the aperture holder or column liner tubes, any of the techniques suggested for cleaning the cathode assembly should be more than adequate.  In the SEM case it does no harm placing a number of components in an ultrasonic cleaner, the external surfaces are of no consequence.

 Thin metal apertures, usually silver in colour, require cleaning techniques that use heat to drive off the contamination.  Two metals are used for thin metal apertures, platinum or molybdenum.  If the manufacturer's instruction book does not have details of which type of aperture are fitted to your microscope, there is a simple test.  Platinum apertures may be heated in a Bunsen flame, but molybdenum apertures will turn black.

 To clean a platinum aperture you will need to purchase a small platinum boat, or a pair of platinum tipped tweezers.  The aperture is placed on the boat, or held in the tweezers, and heated in a flame until it is orange to red heat.  Check the aperture under a microscope and if it does not look perfectly clean repeat the procedure.  Molybdenum apertures require heating in a high vacuum environment, to prevent oxidation. A high vacuum evaporating system, with a molybdenum boat linked between two low voltage terminals, is ideal.  The aperture is heated to orange-red heat until the dark patch, the contamination, is removed.   After cleaning apertures of any material, they should be checked with a microscope, to see that they are both perfectly clean, and perfectly round.  Throw away apertures that do not respond to cleaning, or become distorted by the cleaning procedure.

 The SEM specimen area should be cleaned each week with a vacuum cleaner, and the door o-ring checked for debris or damage.  If the specimen stubs fit into a removable cup, or cups, these should be cleaned with a solvent.   For both SEM and TEM do try to encourage operators to wear light weight gloves when handling specimens.  They cut down the transfer of finger grease and considerably reduce both specimen and column contamination.

 The vacuum system of the SEM should not be ignored when routine maintenance is being considered.  The first priority with any rotary pump is to remove the oil fumes.  The cheapest technique is to vent the pump outlet to the outside world through the use of a plastic pipe.  If your microscope is deep within a building you should use a filter fitted to the pump exhaust.  The fluid in the rotary pump works very hard and therefore contaminates rather rapidly.  If you have a pump filled with one of the very expensive low contamination oils, drain and refill the pump every two years.  If you use a pump that contains a conventional fluid, change this every year.  A pump that is constantly being used to pump a system from air takes in a high level of moisture.   The initial surge of other vapours from the specimen.  These contaminants gradually reduce the efficiency of the pump fluid, the pumping speed is reduced, and the exhaust emits rather nasty vapours.  Three very good reasons for regularly changing the fluid.

 Diffusion pumps require no maintenance if correctly operated.  The pump is most damaged by condensation, moisture building up on its outer surface.  This leads to corrosion, water leaks, and damaged heaters.   Diffusion pump should be operated with a water temperature within two degrees of room temperature. The best evaluation of the pump temperature is to check the temperature of the SECOND COIL from the base of the pumps.  This should be very slightly warm to the touch.  Vary the water flow until the correct temperature is attained. The only reason for changing the fluid in the diffusion pump is if the pumping speed is being impaired.  Check that your microscope does fail-safe by switching off the water once a month.  Fail safe devices may jam and cause an expensive pump melt down!

 The general vacuum performance of the microscope, and therefore the contamination rate, may be improved through the use of a nitrogen bleed system.  If a dry nitrogen gas cylinder is connected with the air inlet for the column, with a very low pressure setting, the dry gas will preserve a much cleaner column environment.  Under these circumstances, filament life, column cleanliness, pump down time, and specimen contamination rate all improve.