Electronic Components
Probe Monitoring
AFM Position Sensing Components
Focused light emitted from an infrared LED is reflected onto a cantilever which, in
turn, reflects the onto a second mirror into a quadrant photodetector. When the LED
is properly aligned, an the cantilever is undeflected, the four quadrants will
produce equal current outputs. Deflecting the cantilever along the verticle and/or
horizontal axis will change the input power on the detector quadrants indicating the
direction and extent of the cantilever's deflection.
- Advanced Photonix
makes our photodector for AFM operation. Their site contain a description of the
position sensitive detector
as well as the specifications for our model, the
SD380-23-21-025.
- A quadrant's output is amplified by a
Burr-Brown OPA627AP op amp.
- This output is summed/differenced by an
Analog Devices AD620AN operational
amplifier. The difference of the upper quadrants sum to the low quadrant's sum
indicates the normal force, while the difference of the left and right sums
indicate latteral force. A third output is a summation of the quadrants and provides
normalization.
Non-Contact AFM
AFM in non-contact mode, the feedback signal is derived from the force induced shift in
resonance frequency of a vibrating cantilever. The driving frequency of the cantilever is
set by a piezo drive attached to the cantilever mount. The vibrational frequency of the tip
is monitored by the standard AFM detector, but with added feedback into the system to
control the piezo drive frequency and amplitude.
STM Positional Feedback Components
A voltage is applied between the sample and tip. The magnitude of the current
flowing through the circuit is strongly dependent upon the tip to sample distance. This
strong positional dependence is the heart of the STM feedback circuit.
- The tip current is amplified in-vacuo by a
Burr-Brown OPA602 op amp.
Z Piezo Control
The control of the Z piezo consists of two components: the Z piezo drive unit which
performs the actual positioning of the sample with respect to the tip and the Z piezo
regulation circuit.
The Z piezo is driven by a +/- 135 VDC supply.
Z Input
The control voltage for the Z piezo is set through a DAC and nominally has a range
of +/- 10 VDC which is further amplified by a piezo driver circuit to
obtain the full piezo drive voltage range.
The control circuit can be software modified to deliver a range of +/- 1 VDC
which provides a resolution enhancement of Z movements while limiting the piezo overall
range.
The detector output, FN, for AFM or the current, I, from STM operation along
with proportional and intergral gains provide the feedback for the control voltage circuit.
Z Output
The control voltage (Z) of the Z piezo (+/- 10 VDC) is differenced with an offset
voltage (Z0) and is passed into an ADC. The difference (Z-Z0) can
be amplified by: 1, 10 or 100 to provide enhanced Z resolution.
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