Metal coating thickness and image quality in scanning electron microscopy

In biological scanning electron microscopy (SEM), metals are deposited on specimen surfaces for different applications. In conventional microscopy, 10 - 20 nm thick coatings of gold are used to increase electrical conductivity and secondary electron (SE) emission. The - conventional - SE signal (SE-II) is generated by electrons of the probe scattered in the thick gold layer; useful magnification is limited to ∼25,000 x. At higher magnifications the metal coating is imaged by more than one pixel and contributes significantly to the size of particles. Topographic contrasts are produced at features ∼20 nm. High resolution is obtained through another type of signal which is generated by the electrons of the probe before they scatter within the specimen.