Optimizing Imaging; Principles Of Sem Imaging; Magnification - Thermo Scientific Apreo User's Operation Manual

Optimizing Imaging

Principles of SEM Imaging

All scanning beam microscopes share the same fundamental technique. The primary beam is scanned across the
specimen surface in a regular pattern called 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. The result is a picture point (pixel) array.
Low or high resolution imaging can be obtained by changing these factors. The larger the pixel array, the higher the
image resolution. The image is created pixel-by-pixel in the computer memory and shown 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 pixel. Because of this direct correspondence, the
pixels shown on the monitor are directly related to the specimen's 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 an image.

Magnification

Magnification is calculated as the shown dimension (L) divided by the sample scanned dimension (l).
If the observed sample area is made smaller while the monitor size remains constant, the magnification increases.
At low magnification, one gets a large field of view. At high magnification, only a tiny sample area is imaged.
It is possible to set a corresponding databar magnification readout in the Quad Image and Single Image modes and
in the saved image (see the Preferences / Magnification section).
Scanned sample and monitor imaging
FIGURE 5-6:
Changing Magnification
• Toolbar list box – used to choose from predefined values
• Keyboard control (+ / - / *) – the numeric pad plus key (+) / the minus key (-) increases /
decreases the magnification 2× and rounds the value. The star (*) key rounds the
magnification value (e.g. 10 063× becomes 10 000×).
• Mouse wheel control – coarse / fine control can be operated by the Ctrl / Shift key and rolling
the mouse wheel up / down to increase / decrease the magnification.
• Selected Area Zooming In / Out – a quick way of zooming in / out on an area of interest. Click
inside the imaging area & drag to make a dotted box over the area of interest (the cursor
changes to a magnifying glass with a + sign). Release the mouse button, and the selected
area increases to fill the whole imaging area with respect to the sides ratio. Clicking on &
dragging + Shift consecutively reverses the above described technique (the cursor changes to
a magnifying glass with a – sign). The Esc key cancels the operation at any time.
• Magnification module
• Digital Zoom module
C O N F I D E N T I A L – limited rights
Revision A Feb 2018
Operating Procedures: Optimizing Imaging
User Manual 5-9