AFM Measurement Basics

AFM Measurement Basics

Atomic force microscopy (AFM) is a powerful measurement technique. As with any other measurement device, the operator needs to be familiar with the basic principle of operation of the Atomic Force Microscope in order to be able to take full advantage of the possibilities that AFM offers. Below is a short list of the most important steps for performing a successful AFM measurement on a system with optical cantilever deflection detection

Select a measurement sample

Check the maximum vertical movement that the piezoelectric scanner allows. Measure samples with topography that does not exceed the limitations of the scanner. Samples with excessively high features can lead to tip, sample and even piezoelectric scanner damage.

AFM Measurement Basics, Step 1
Select and install a probe

Choose the appropriate probe for the specific AFM application. AFM probes are different from one another in terms of tip geometry, cantilever and material. Place the AFM probe into the measurement head. The probe mounting procedure depends on the specific AFM system. Please note that probe tips are prone to electrostatic discharge (ESD) damage. Therefore it is recommended to wear an ESD wrist strap when manipulating the probes, like for instance during probe installation.

AFM Measurement Basics, Step 2
Align the laser optics

Focus the optical microscope system on the cantilever. Direct the incident laser beam onto the back of the cantilever. Adjust the optical detection system so that the reflected beam produces optimal signal. Pay attention to the precise position of the reflected light in the photodetector. The required position may vary depending on the AFM system and the measurement mode. For some systems in contact mode the vertical position of the laser spot in the photodetector relates directly to the force that the tip exerts on the sample. Check your system’s user manual.

AFM Measurement Basics, Step 3
Find the probe resonance frequency (for tapping mode operation)

If working in tapping or non-contact mode, set the drive frequency to a value that is equal or close to the resonance frequency of the probe cantilever. Adjust the drive amplitude. Higher amplitudes allow faster and more stable scanning, but also cause faster tip wear and more sample damage. Sometimes, more than one resonance peak can be seen on the frequency sweep. Usually the highest peak is the natural resonance frequency and it is the one that should be used. Secondary resonance frequencies may appear at frequencies below or above the primary

AFM Measurement Basics, Step 4
Adjust the interaction force

Choose the appropriate probe for the specific AFM application. AFM probes are different from one another in terms of tip geometry, cantilever and material. Place the AFM probe into the measurement head. The probe mounting procedure depends on the specific AFM system. Please note that probe tips are prone to electrostatic discharge (ESD) damage. Therefore it is recommended to wear an ESD wrist strap when manipulating the probes, like for instance during probe installation.

AFM Measurement Basics, Step 5
Adjust the scan parameters

Set the parameters related to the scan such as scan size, scan rate, feedback settings, etc. The combination of scan size and scan rate defines the velocity at which the tip scans the surface. High velocities can lead to improper surface tracking and faster tip wear. Low scan velocities prolong tip life and improve surface tracking at the expense of increased scan time. Feedback settings also affect the way the tip tracks the surface. Too low feedback gains result in inaccurate tracking, while too high gains cause instability of the feedback loop. For some systems scanning starts automatically directly after the tip touches the sample surface. For others, scanning starts after a command from the operator. For the latter, step 6 can be performed after the engage procedure (7).

AFM Measurement Basics, Step 6
Engage the probe

Coarse approach: Some systems require the operator to manually lower the probe to within parts of a millimeter from the surface. Other systems use an integrated camera for focusing on the sample surface in order to give the instrument a rough estimate of the distance between the tip and the sample. In all cases this coarse approach procedure should be performed with special care. Be careful not to lower the probe too far. If the probe tip hits the sample surface, both the probe tip and the sample will be damaged. Fine approach: Set the engage parameters and engage the probe onto the sample surface. Even the standard dual piezoelectric scanner - stepper motor engage can be harmful to the tip if the engage parameters are not set properly. Refer to your system’s operation manual for more details about the available engage parameters.

AFM Measurement Basics, Step 7
Scan the sample surface

Scan the sample surface and acquire an image

Scan the sample surface

Keep in mind that even with the optimal scan parameters, there will still be tip wear. Tip wear is something natural!

Tip wear is inevitable because the tip interacts with the surface and the tip is manufactured from a material with finite hardness. Nevertheless, the right scan parameters can significantly decrease tip wear. It is always best to observe the progress of the scan. Correct the scan parameters if necessary. To prolong tip life, retract the tip from the surface during long periods of inactivity.

AFM Measurement Basics, Step 8
Analyze the scan
Scan the sample surface

Please keep in mind that this is a brief and very general overview of how to perform AFM measurements. Your system’s operation manual provides a much more detailed description with specific details about your specific system.

‘Seeing is believing’. You can hear this phrase commonly in AFM related talks. An experienced AFM user must learn to interpret AFM images correctly, to understand tip sample convolution and to distinguish common measurement artifacts.

AFM Measurement Basics, Step 9
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