A Test Specimen For SEM

The Specimen Characteristics

The specimen is made up of polystyrene latex spheres that are allowed to dry from a liquid to form a solid white block.

During the drying process, provided the latex preparation is free from contaminants, the spheres will deposit in an array that is of a square packing in one direction and hexagonal in the other. If a piece of the solid material is fractured, by pricking with a fine point, this opens up the internal structure of the compacted material, displaying the two types of array. 

An alternative is to clean a stub and to deposit a drop of latex solution on the clean surface and allow this to dry.  The Latex will not need pricking but you may need to experiment with its concentration to obtain nice layered results.

Having a very well defined structure the hexagonal arrays make a very good subject for judging the performance of a scanning electron microscope. Any hexagonal area on the specimen is comparable with another set in the same orientation.

Another advantage of this specimen is that the latex are of a specific size which may act as an inbuilt calibration. In most cases of performance monitoring the operator simply needs to take a test picture at a specific magnification and use a comparative process to judge performance. The latex are a nominal 0.24µm but when compacted in an array they are visibly reduced to about 0.2µm.

Should the sample become damaged it is easily recovered by re coating or once again pricking it with a pin to open up new areas and then recoating.

Making A Test Specimen

To convert the latex specimen into a high resolution test specimen a metal coating is required. A sputter coating will make the specimen conducting but further coats will build a sub structure on the surface of the spheres. The sub structure may be used for high resolution performance monitoring.

The level of sub structure desired will depend upon the capabilities of the instruments to be investigated. For instruments with a conventional tungsten source multi coating the latex with gold is satisfactory. For more advanced instruments the finer coating of gold-palladium may be more desirable.

The coating procedure depends upon the efficiency of the coater being used. Sputter coaters that use relatively high voltages (1 to 3kV) will require the following procedure.

i. Set the coater at a 5cm target to specimen distance.

ii. Sputter at 20mA, 1kV for one minute, wait one minute and repeat the process.

iii. Coat for 4 one-minute periods and then check the specimen in the microscope.

iv. If you need more coats, because you cannot see the metal, repeat the "coat and wait" procedure until the structure is satisfactory.

The more metal you put down the coarser the structure will be on the spheres. Low levels of coat will require better operating techniques in order to resolve the coat. Do not expect a conventional instrument to be able to resolve less than 4 coats.

If you have a modern coating unit, which will be much more efficient at putting down the coat, use 10mA for 30 seconds per coat. Experimenting with coating procedures will enable you to tune the coating parameters and coating time to obtain the exact specimen that you require.

For field emission instruments a gold-palladium coat, if carefully applied, will give you grains in the region of 8nm and a spacing of less than 1nm.

If you intend to push yourself and your microscope to near its limits there are some basic operating procedures that will be required. Firstly the specimen must be placed in the instrument and the high voltage must be switched on for at least 45 minutes prior to trying to work at high magnifications (>30,000X). This period is required for the high voltage tank, and hence the high voltage, to reach stability. After this period the heat gained by the components is equal to the heat lost through the walls of the tank and the high voltage will be at its most stable.

Whilst stability is being achieved move the specimen to a short working distance (<5mm) and set the instruments alignment to the best of your ability. Find areas of the specimen that are in the hexagonal array and flat to your view. A slight tilt of the array is not as good for comparison as is a perfectly flat surface; you may need to tune your specimen tilt slightly.

Find a good area and run up to at least double your intended recording magnification. Run through focus but it there is any image movement return to your final aperture or "beam alignment" correction controls; the image should not move when focusing if you wish to have the best possible image. Gently focus to obtain the most contrasty image that is not in any way directional. Then take each stigmator in turn and adjust that for the highest contrast. Check the image at a slow scan at double your recording magnification. If the image is not sharp reduce your spot size and repeat the focus-stigmate procedure before checking again at a slow scan. If the image is sharp reduce the magnification, set your brightness and contrast and take your picture. It may be advisable to move the image sideways a little, with your electrical image shift controls, so that you are photographing a clean area.

The specimen in the hexagonal areas does not tolerate astigmatism and in general the specimen will not hide errors in operating technique. Too large a spot (spot size limited), poor astigmatism correction and incorrect focus are all indicated as a softening of the image. Too large a spot size is also displayed by the space between the spheres being enlarged. The nearer the spot size is adjusted to optimise the image the smaller the space between spheres becomes. At high levels of performance the touching points of the hexagons at Y junctions display themselves as 6 black spots. The Y junctions make good areas to adjust focus and astigmatism, in that the Y must be of equal density in all three directions for focus to be achieved on a flat array.