FEI Quanta FEG 250, Руководство пользователя

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Operating Procedures: Optimising an Image
5-7
OBTAINING AN IMAGE ON SCREEN
The following assumes that the electron emission is on. Continue the
procedure (see above):
10. Select an appropriate detector (see below) and release the active
quad.
11. Click the Column module / Beam On button to ramp up the
accelerating voltage. An image appears in the active quad.
12. Focus the image and Link Z to FWD (see Chapter 4).
13. Adjust to a suitable magnification, optimize the image with
Contrast and Brightness, focus, Stigmator (see below).
Optimising an Image
PRINCIPLES OF SEM IMAGING
All scanning beam microscopes produce images with the same
fundamental technique. The primary beam is scanned across the
specimen surface in a regular pattern called a raster. Normally, this raster
consists of a series of lines in the horizontal (X) axis, shifted slightly from
one another in the vertical (Y) axis. The lines are made up of many dwell
points and the time of each dwell point can be shortened or prolonged
(dwell time). The number of points per line can be increased or
decreased as well as the number of effective lines (resolution). The result
is a picture point (pixel) array. Low or high resolution images can be
obtained by changing these factors. The larger the pixel array, the higher
the resolution of the image. The image is created pixel-by-pixel in the
computer memory and displayed on a monitor screen.
The signal emitted by the specimen surface as it is illuminated with
the primary beam is collected by the detector, amplified and used to
adjust the intensity of the corresponding image pixel. Because of this
direct correspondence, the image displayed on the monitor is directly
related to the specimen surface properties.
The raster consists of many (typically one million) individual locations
(pixels) that the beam visits. As the beam is scanned, the signal
emitted by the sample at each beam position is measured and stored
in the appropriate digital memory location. At any time after the beam
scan, the computer can access the data and process it to change its
properties, or use it to generate a display.
MAGNIFICATION
Magnification is calculated as the displayed image dimension (L)
divided by the sample scanned dimension (l).
If the observed sample point size is made smaller while the monitor
size remains constant, the magnification increases. At low
magnification, you get a large field of view. At high magnification, you
point only a tiny sample area.