Dear Jonnalagadda,

Since FESEM is used primarily to analyse surface morphology, generally the sample is spread over a substrate and analysed. As SEM cannot provide any data about   entities or texture from within the bulk of the sample, we should try to provide as much surface (by spreading the sample) possible for analysis. 

And this is done for example, let's say you have powdered sample, we prepare a suspension of it in a volatile solvent like methanol, ethanol, acetone etc and spread it over a substrate. After the solvent evaporates (your sample needs to be completely dry) it creates something like a "thin film" of the sample providing the "required surface" as mentioned earlier and also ensuring uniform thickness. Advantages:-

1) More surface area reveals more about the surface morphology of your samples so you can be sure of the analysis i.e. the texture is a representation of the entirety of the sample and not just of a particular location of the sample.

 2) Uniform thickness ensures uniform working distance between the gun and the sample. So you get images of good quality otherwise if there is very significant difference between the working distance of two points on your sample, the focusing becomes difficult and images become hazy i.e some parts are well focused and some are out of focus.

And also since FESEM operates in vacuum, you need to make sure that your sample stick firmly to the substrate or they will be  sucked out due to vacuum in the chamber. Generally carbon tapes are used as substrate (sometimes silicon wafers too). Hope that helps.

thank you very much srujan

That's probably for dispering your sample, so you can see more clearly to their morphology rather than a bunch of powder staying together.