Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 265
The SEM has a much higher depth of field as well as spatial resolution. Depth of field,
DOF, in the SEM is given by equation (7.8).
0.
2W
DOF =
r × m
d
g
mm (7.8)
where Wd is the working distance. A typical value of DOF in the SEM is 40 µm
compared to 1 µm for an optical microscope [5].
Ultimate resolution
The best resolution the SEM can achieve is given by the sample pixel size, PS, which in
turn depends on the magnification. This resolution is only achieved if the sampling
volume is no bigger than PS. For X-rays, this volume can be of the order of microns, and
for SEs, of the order of the beam diameter. Therefore, spatial resolution better than this
cannot be achieved [5]. Lowering the beam current reduces the beam diameter, and hence
sampling volume, but this may result in insufficient signal. Ultimate resolution can
therefore be defined as the smallest probe diameter which provides a useable signal from
the sample [5].
Energy dispersive X-ray (EDX) analysis
When electrons with energy of the order of a few keV strike a sample, X-rays are
produced. These X-rays are characteristic of the atoms within the sample, and can