Biomedical Engineering Reference
In-Depth Information
measure several images on pre-set locations on the sample. It is possible to drive the stage
to pre-established locations for inspection applications with registration software.
2.4.3
x-y scan control
The exact scan control parameters that are used for scanning a sample depend on the
particular application. There are a few variables that must be selected to scan a sample.
They are:
(a) Image size: This is the window that is selected for viewing the features on a surface.
The image size should be at least as large as the features that are to be visualized. Often a
large scan is measured and then the operator 'zooms' in on a feature of interest.
(b) Number of lines in the image: The digital resolution of the image is established with
the number of lines selected for the image. For example, if the scan size is 10
10 microns
and the number of lines selected is 256 then the digital resolution is 39 nanometres. The
number of lines in the image may range from less than one hundred to several thousand.
Most AFM software limits the images to square or rectangular dimensions.
(c) Image rotation angle: The image scan angle may be changed with software. Rotation
angles between 0
8
8
can usually be selected. Rotating the image scan axis usually
means that the largest scan range cannot be achieved.
(d) Scanning speed: This is typically specified in hertz (i.e. lines scanned per second),
and in normal circumstances varies from around 0.5 Hz to 4 Hz. Combining this parameter
with the number of lines in the image, gives the time required to collect an image. Along
with the feedback parameters discussed in the next section, scanning speed can affect image
quality. This is discussed further in Chapter 4. Realistically, the important parameter in
terms of imaging quality is distance covered over time rather than frequency of line
collection, so scanning speed can also be expressed in micrometres/nanometres per second.
and 360
2.4.4
z control
Software is required for controlling the feedback control electronics, see Section 2.3.
There are two functions that are controlled; the set-point voltage and the PID parameters.
(a) Set-point voltage: This is the voltage that goes into the differential amplifier, so this
voltage is compared with the force sensor output voltage and an error signal is generated.
The set-point voltage controls the 'relative' force. A calibration of the specific cantilever is
required to convert the set-point voltage to a force (see Section 2.5).
(b) PID parameters: These parameters control the 'responsiveness' of the feedback
control electronics. These parameters must be adjusted such that the probe tracks the
surface while scanning. Sections 4.2-4.3 provide a description of optimization of the
feedback control parameters.
2.4.5 Force-distance curves
Force-distance (F-D) curves are used to measure the forces experienced by the probe as a
function of distance from the surface. In F-D measurements, the probe is moved toward the
sample surface to a pre-selected position, and then retracted. The extent of cantilever
deflection over the course of this movement is expressed by the
Z
(deflection) signal which
