Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 264
becomes smaller. Probe size and current are very important in determining the ultimate
resolution of the SEM.
Pixels and depth of field
To understand fully image formation in the SEM, which is entirely different to that of an
optical and transmission electron microscope (TEM), pixels (picture elements) must be
considered. The amplified signal from the detector is transferred to the CRT with a
diameter of around 0.1 mm. Therefore, a CRT 10 × 10 cm displays 1000 × 1000 = 10 5
pixels [5]. These units are the smallest element of the image, 0.1 × 0.1 mm square of
uniform intensity. As the electron beam mimics the movement of the CRT, there is a
corresponding pixel on the sample for each one on the screen. The size of the pixel, PS,
on the sample is given by equation (7.7) [5].
100
PS = µm (7.7)
m
g
where mg is magnification. To resolve two features in the image, they must occupy
separate pixels. Therefore, the working resolution of the SEM is limited by the sample
pixel size [5]. If the electron beam is larger than the pixel size, then the signal from
adjacent pixels will be merged, and this will degrade the signal. If the beam size is
smaller than the pixel size, the signal will be weak and noisy. To achieve optimum
performance in the SEM, the beam diameter should be equal to the pixel size, which will
vary with magnification [5].