Conceptually, an atomic force microscope is a simple instrument. A probe is scanned over a surface, and by monitoring the motion of the probe with a photodetector, a surface topogram is measured. From this, images and data characterizing the surface can be obtained. Although an AFM is a simple device, there are a few common mistakes that new operators make that can lead to great frustration.
Often these common issues result in new AFM operators concluding the AFM itself is faulty. In the next several newsletters themed "AFM Tips", we will discuss these problems, their cause, and their solution. Being aware of these issues will give you a better understanding of the AFM, and make for less stressful scanning. In this newsletter, we discuss the common issue of false feedback.
AFM Tips: How to deal with "false feedback"
In an AFM, the probe is moved from a distance of about 1 millimeter above the surface of a sample to a distance where it interacts with the hard forces associated with the surface in order to obtain a topographic scan. In all modern AFM systems, an automated tip approach algorithm (see our automated tip approach video here) is typically employed. The algorithm stops tip approach when the probe is in contact with the surface (non-vibrating/contact mode) or interacting with surface forces (vibrating/tapping mode).
In vibrating (tapping) mode, a reduction in vibration amplitude is measured; in non-vibrating (contact) mode, the deflection of the cantilever is measured. Ideally, tip approach is completed when the probe interacts with the "hard" forces associated with the surface. As we will see, the automated tip approach can be "tricked" into thinking it is in feedback (in contact/interacting with) with the surface, which is called false feedback.
When the probe approach is stopped before the probe interacts with a surface's hard forces, it is called "false feedback". The AFM image that is measured when the probe is in "false feedback" appears blurry and out of focus. There are two common causes for false feedback: when there is a large layer of surface contamination, and when there is a substantial electrostatic force between the surface and the probe.
Surface Contamination Layer
One issue that often causes false feedback is a surface contamination layer. In ambient air, there is a layer of surface contamination on every surface. The thickness and density of the contamination layer depends on the environment and the type of sample being scanned. Samples left exposed for long periods of time, or operation in a humid environment typically results in thick contamination layers.
The probe can become trapped in the contamination layer before interacting with the hard surface forces of the sample. It is important to notice the difference. The AFM software can be tricked into thinking it is in feedback with the surface, resulting in the automated tip approach being stopped. False feedback due to surface contamination results in the scanned image looking blurry, and nanoscopic features cannot be visualized.
The most common solution for this type of false feedback is to increase the probe-surface interaction. In vibrating mode this is done by decreasing the setpoint value; in non-vibrating mode this is done by increasing the setpoint value. The setpoint refers to the z-piezoelectric ceramic extension (non-vibrating mode) and the amplitude of vibration (vibrating mode). By increasing the tip-sample interaction, the probe is forced through the contamination layer, and clear images can be obtained.
Fig 1. Illustration showing the probe interacting with the contamination layer, causing false feedback.
Another issue that can cause false feedback is surface charge on either the cantilever or sample surface. This causes electrostatic forces between the probe and surface. In vibrating mode, this force can cause the amplitude of vibration to be affected; in non-vibrating mode, the force can cause the probe/cantilever to bend.
The magnitude of the change and its effects depends on the stiffness of the cantilever as well as the amount of surface charge present. This problem is common when soft cantilevers are used in non-vibrating mode.
To reduce the effects of surface charge, a conductive path between the cantilever and the sample must be created. In cases where making a conductive path is not possible, the effects of electrostatic forces can be reduced by using a stiffer cantilever.