The Five Image Channels in an AFM

Fig. 1: Z Feedback control loop in an AFM

In AFMWorkshop's Atomic Force Microscopes (AFMs) - as with almost all AFMs - there are typically up to five image channels that may be displayed while scanning a sample. This article is written to help users gain a greater understanding of each channel. Each of the AFM's 5 image channels is derived from the Z feedback loop (Figure 1) that utilizes the output of a photodetector to control the motion of a piezoelectric ceramic.

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Figure 1: The Atomic Force Microscope Z feedback control loop

In Figure 1, the photodetector output measures the motion of a cantilever/probe. This output is then used to activate the Z piezoelectric ceramic which controls the spatial relationship between the probe and the sample. In non-vibrating (contact) mode, the photodetector's Top-Bottom signal (T-B) is sent to a differential amplifier, and in vibrating (tapping) mode, the average vibrating amplitude of the cantilever is sent to the differential amplifier. The output of the differential amplifier, called the error signal is then passed through a proportional, integral, derivative (PID) controller. The PID controller output activates the Z piezoelectric ceramic.

An Atomic Force Microscope's Five Image Channels

• Error Signal: This is the output of the differential amplifier and is a measure of the tracking of the probe on a surface. If the probe is directly tracking the surface, the error signal will be zero. The optimal PID parameters are established by minimizing the error signal while scanning. This image channel can be added to the Zdrive (often called the topography image) to provide images with greater spatial detail.
• Z Drive Signal: This is the voltage that drives the Z piezoelectric ceramic and is a direct measure of a sample's topography. The image is subject to distortions because of hysteresis in the Z piezoelectric ceramic. The Z drive signal gives the highest Z resolution images in an AFM.
• Z Sensor Signal: Attached to the Z piezoelectric ceramic is a motion measuring device. AFMWorkshop uses a strain gauge for the motion sensing device that is attached to the Z piezoelectric ceramic. The output of the Z strain gauge is very linear, but has a spatial noise floor of about 1 nm. The Z sensor signal is essential for measuring force distance curves, and may be used for Z metrology measurements.
• Phase Signal: While scanning a sample in vibrating (tapping) mode, the Z feeback loop keeps the cantilever vibrating amplitude constant. During a scan, the probe interacts with regions of differing mechanical properties which then causes a change in the phase signal. This is commonly called the phase image.
• Lateral Force Image: Chemical and/or physical interactions between a probe and a surface can cause the cantilever in the AFM to twist. The magnitude of the twist is measurable by subtracting the left from the right sections of the photodetectors.

For more information on the Z Feedback Loop, you might review AFMWorkshop's animated online tutorials, and for more details about an AFM's five image channels, we recommend Atomic Force Microscopy from Oxford University Press.