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Conductive AFM (C-AFM)
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TT-2, HR. HR-2D AFM - Model: CA-2011
SA, NP, LS AFM - Model: TSCA-2011
AFMWorkshop’s Conductive-AFM (C-AFM) accessory provides surface conductivity images when used as an accessory. The C-AFM creates images showing pixel-by- pixel conductivity. Conductivity is measured using any DC voltage (from -10 to +10 volts) with currents ranging from picoamps to 10 microamps.
(Below) Images of a test pattern measured with the C-AFM option. At the left is a topography image and at the right is the conductivity image. The features labeled 1 and 2 had a conductive path to ground, and thus were visualized in the conductivity image
The C-AFM measures topography and conductivity images simultaneously. This option allows measuring current-to-voltage curves at specific locations on a surface.
Below is a picture of the HR AFM stage with C-AFM option installed. The pre amplifier is mounted inside the black box, and a coax wire extends to the C-AFM probe holder. With this arrangement, at High Gain, the noise level with a probe installed is + 3 picoamps.
TECHNICAL DESCRIPTIONS
Below, a block diagram depicts the functionality of the Conductive AFM circuitry. Conductivity is measured by applying a fixed DC bias voltage between the probe and the sample, and then by measuring the resulting current.
The preamplifier is an I/V circuit, i.e. a current-to- voltage converter. It has 3 gains selected by a small slide switch on the preamplifier circuit board. These gains are designated “low”, “med” and “high”.
Multiplying the measured voltage from the C-AFM with the Scaling Factor (provided in the specifications section of this document) yields the current. Dividing the current by the applied voltage yields conductivity in mhos, or conversely, dividing the applied voltage by the measured current yields resistance in ohms.
The preamplifier circuit can be configured so that a fixed DC bias voltage is applied to the probe while the sample is held at ground, or the DC bias can be applied to the sample while the probe is held at virtual ground. In the former case, the DC bias voltage range is ± 5 volts.
SPECIFICATIONS
PROBE
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PROBE VOLTAGE
| Range | |
| LOW | + 1V |
| HIGH | + 10V |
PREAMPLIFIER GAINS
| Gains | Full Scale | Scaling | Transimpedance |
| LOW | 10µA | 2µA/V | 0.5MΩ |
| MED | 250µA | 50nA/V | 20MΩ/td> |
| HIGH | 10nA | 2nA/V | 500MΩ |
PROBE HOLDER
The probe holder for the C-AFM replaces the standard probe holder in the AFM. To assure minimal noise in the C-AFM measurement an U.FL connector is used to connect to the preamplifer, and the probe is held in place with a clip that has a ground shield.
SOFTWARE
Software for the C-AFM option is written in LabVIEW™. VI’s for the application are available to customers who want to modify the user interface. The application allows changing the bias on the sample and monitoring the current between the probe and surface.
Probe Signal Connection to T-B:
This drop down menu is set to Off for C-AFM.
ELECTRONIC MODE CONTROL UNIT
At the back of the electronic control box are connectors for a 50 pin ribbon cable, a USB cable, a cable to the preamplifier, and an extra DAC output BNC.
