eTraining Introduction

Specimen Preparation

Leica Ultracut UCT

Histology

JEOL JSM-6400 SEM

Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB

Microanalysis

Veeco Dim 3000 AFM

Fluorescence Microscopes

Support

Coating

Reasons for Coating

Charging

Beam electrons that strike a non-conductive specimen will "pool" at the beam interaction site. Incoming beam electron will be deflected by that negative charge producing image distortion called "charging". The distortion takes the form anomolously bright areas on the screen image, accompanied by dark horizontal lines across the acquired picture. Another form charging can take is "image drift", where a feature slowly drifts across the screen image. The coating on a specimen can break. This results in rapid movement of a feature in the screen image.

Coating Choice: Carbon or Metal?

Simply put, metal (Au/Pd and Pt/Pd) coating is used primarily for imaging, and carbon is used for either imaging or X-ray microanalysis. For this reason many people choose carbon exclusively. Metal coatings produce a high secondary electron signal, meaning a high quality image, an advantage for SEM studies. In X-ray analysis, the X-ray spectrum from a metal-coated specimen will contain a contribution from the metal. Often the metal peaks will obscure an element of interest. Carbon X-ray peaks occur at very low energy, which generally does not interfere with peak identification.

Another consideration in coating choice is the microscope you plan to use. Carbon and metal work equally well in the 6400 SEM, but carbon and Au/Pd have coarse grain morphology. The phenomenon is called "decoration", and that pattern can be seen in high resolution images produced in the FESEM.

Coating Thickness

Fears of decorating the specimen surface suggest that Pt/Pd be used for FESEM work and never more than a 5 nm as measured with the coating thickness monitor. Carbon is generally deposited to a 20 nm thickness, as measured on polished brass. A 20 nm carbon coat will produce an easily observable blue color on the brass. Metal coating for the 6400 SEM is generally applied at 20 nm as well, again measured with the film thickness monitor.

Reference: D. M. Kerrick, et al, "The Role of Carbon Film Thickness in Electron Microprobe Analysis," The American Mineralogist, 1973, 58, 920-925.

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