Advanced Contact Mode AFM Operation

Data type is the first parameter set because the settings of other parameters depend on it. The Data type parameter in the Channel control panels selects the type of data collected by the system.

• Height data corresponds to the change in piezo voltage needed to keep the cantilever deflection constant.
• Height Sensor data corresponds to the sensor measured change in piezo height needed to keep the cantilever deflection constant.
• Deflection Error data comes from the differential signal off of the top and bottom photodiode segments relative to the Deflection setpointOperator-selected threshold used as the target of the feedback control loop..

The scan parameters required to collect accurate height data are different from the optimal parameters for deflection data. To collect height data, the feedback gains must be high so that the tip tracks the sample surface with minimal cantilever deflection. The position of the piezo during the scan reflects the height of the sample. To collect accurate topographical data, set the Data type parameter to Height Sensor.

 

Topographical Deflection Error data is only reasonable on very smooth, flat samples. Topographical Deflection Error data is used infrequently and only for high resolution work. Collect Deflection Error data with low feedback gains to ensure the piezo remains at a constant position relative to the sample. In this case, the tip and cantilever are deflected by the features on the sample surface. The output fluctuations in the cantilever deflection voltage from the top and bottom photodiode segments are recorded as a measure of the variation in the sample surface. Deflection Error data is not automatically calibrated in units of distance. You must measure the sensitivity using the Contact Mode Force Calibration procedure.

Deflection Error data collected with high feedback gains essentially equals the derivative of the height. This is commonly referred to as the error-signal. The error-signal provides a sensitive edge-detection technique and can be very helpful in visualizing fine details in topography that are difficult to see in regular height data. Using two channels, you must capture both height and deflection data simultaneously. Deflection Error (error-signal) data alone does not yield quantitative height information.

The Integral, Proportional and LookAhead gains in the Feedback panel determine the feedback on the piezo height. The feedback loop keeps the deflection signal constant by adjusting the height of the Z piezo. If the gainsThe gains are scaling factors for the error signal to determine how quickly corrections should be applied in the feedback loop. There are two important gain factors: the proportional gain scales to the current value of the error signal, and the integral gain scales the aggregate of the past values of the error signal (the "area under the curve"). are high, as they should be for Height data, the piezo height changes to keep the cantilever deflection nearly constant. If the gains are low, as they should be for topographical Deflection Error data, the cantilever deflects from its nominal position as it encounters features in the sample.

In general, set the gain settings as follows:

1. Set the Integral and Proportional gains to 2–5 to start scanning.
2. To optimize the gains for Height Sensor data, increase the Integral gain until the piezo begins to oscillate, then eliminate the oscillations by reducing the gain with 2–3 clicks of the left arrow key.
3. Repeat the process for the Proportional gain.

4. For Deflection Error data, engage the microscope with the gains high, then lower them as much as possible without losing contact with the sample once the system begins scanning.

In general, decrease the Scan Rate as the Scan Size increases. Use scan rates of 1.5–2.5 Hz for large scans on samples with tall features. High scan rates help reduce drift, but use high scan rates only on flat samples with small scan sizes. When first using the system, vary the scan rate enough to observe a change in the image quality.

The Deflection Setpoint parameter defines the desired voltage (and, therefore, the desired deflection or force of the cantilever) for the feedback loop. The setpoint voltage is constantly compared to the present vertical deflection signal of the photodiode to calculate the desired change in the piezo position. When the gain values are high the Z piezo position changes to keep the photodiode output signal close to the Deflection Setpoint; therefore, the cantilever deflection remains nearly constant.

Adjust the Deflection Setpoint to increase or decrease the cantilever deflection and, therefore, the contact force of the tip on the sample. The Ramp Mode menu allows you to adjust the setpoint while viewing a graph of the tip position versus the deflection voltage. Contact Mode Force Calibration minimizes the contact force of the tip on the sample. This is especially important on soft materials such as biological samples.

LP Deflection (Other > LP Deflection) invokes a user programmable low pass filter to remove high frequency noise from the real-time data. The filter operates on the collected data regardless of scan direction. The cutoff frequency can be set from 2–20 kHz. The filter affects feedback in Contact Mode. For faster scanning, set the LP Deflection BW parameter to higher values.

LP Friction (Other > LP Friction) invokes a user programmable low pass filter to remove high frequency noise from the friction data channel. The LP Friction filter does not affect feedback or scanning speed. The cutoff frequency can be set from 2–20 kHz.