General Information
Atomic force microscopy (AFM) is a type of scanning probe microscopy that is used to see and measure surface topography, conduct force measurements or manipulate a sample’s surface.
It can have nearly atomic resolution. The scanning environment can range from being operated at an ambient environment, or in a specific gas, liquid, or under vacuum. There are very few limitations on the types of samples that can be used as well; nearly anything that is solid and has a surface can be imaged. The wide array of operating conditions makes the AFM useful in many different fields such as biology, physics, chemistry, astronomy, medicine, and more.
Substrates
The nature of the substrate does not matter for most samples.
When dealing with biological samples, the chemical nature of the substrate should be selected so that cells can be anchored down and immobilized during scanning. Mica and highly oriented pyrolytic graphite (HOPG) are common substrates because they are both atomically flat and fairly inert. Silicon is popular for electronic applications and lithography. Glass and quartz work for large samples and films and are commonly used for cells.
Instrumentation
An AFM can generally be split into three main components: the computer, the stage, and control electronics.
The computer is just where all signals get translated into a format that can be understood by the researcher(s) using programs built for the instrument. Although all of the components are complicated and very involved in their own way, the stage and control electronics are really what make the AFM what it is.
The stage encompasses the main parts of the AFM, like the scanner, sample holder, cantilever, optical microscope, and so on. There are two different stage types for AFMs, and each has its own pros and cons. In a sample scanning stage, the AFM head is stationary and the sample is placed onto an XYZ scanner that moves it under the head. The mass of the sample is included in the feedback loop, which does put a limit on the size of the sample that can be probed, but this type of stage is easier to make.
The other type of stage is a probe scanning stage. With this stage the sample remains fixed and the probe moves. This type of stage can allow for much larger samples, but it is more difficult to make because of the extra vibrations that come from the assembly. What controls the motion of the scanner or the probe are piezoelectric transducers. In AFMs they are synthetic ceramic materials that convert electric potential into mechanical motion. When an electric potential is applied across opposite sides of the piezo device, that changes its geometry. This change in shape is dependent on the material and magnitude of the applied voltage, and general expansion is approximately 0.1 nm per applied volt. What makes this useful is the ability to accurately control small movements, such as the motion of the probe as it is scanned across the surface of the sample.
The control electronics are what connect the stage and computer, so without them the AFM could not work. These controls generate the signals that are used to drive the scanner. They digitize the signals coming from the AFM and control the feedback based on what the user sets in the imaging program.
Feedback Control
The sole purpose of the feedback control in the AFM is to maintain a set force between the tip and the sample.
It is called the setpoint and is determined by the user. The signal is taken from the laser reflection off the cantilever. It is used to drive the piezoelectric transducers to maintain the tip-sample distance. If there is an increase in force, the feedback will tell the piezos to move the tip slightly away from the surface—and vice versa if there is a decrease in force.
The user should pick nearly the smallest amount of force to be applied during scanning, while still maintaining a stable engagement on the surface of the sample. A lower force will reduce wear and tear on the tip and help prevent damage to the sample. If the setpoint is too low, the tip will not track the surface properly. There is no one setpoint value for a particular cantilever, and it varies from sample to sample as well. The setpoint will need to be optimized each time the user intends to image.
The constant movement of the tip with the piezos and the feedback are what allow it to track the surface. If there was no reaction to a force being applied to the tip, then it would continue at the same height and scrape through the sample. Although this may be useful for lithography applications, for general topography imaging this will damage the sample.
Within the feedback there are two important parameters called the proportional and the integral gain. These are the P and I parts of a common Proportional-Integral-Derivative (PID) controller. What the controller does is continuously calculates the error value between the setpoint and the measured value. It will then apply a correction that is based on a proportional or integral component. Increasing either of the gains will increase the amount of input signal from the photodiode that will be fed into the output signal. The higher values mean that the AFM can react faster to changes in the sample topography. If they are too high, then there will be feedback oscillations that can result in image artifacts.
Operational Modes
There are quite a few operational modes for the everyday, general AFM.
Topographic and Non-topographic
The most common are topographic imaging and non-topographic modes that include force spectroscopy, nanoindentation, magnetic force, electrochemical and thermal measurements. Surface modifications can also be performed. Topographic imaging is the most common use of the AFM and can be split into two subcategories: contact AFM and non-contact AFM. The latter can be further divided into non-contact imaging and oscillating modes. Contact AFM and oscillating AFM will be discussed here.
Contact Mode
Contact mode is the simplest AFM mode conceptually and is the basis for a variety of other modes. It is the fastest of all topographic methods, capable of obtaining some of the highest resolution images in AFM (operating conditions permitting, of course). In this mode, the tip is always in contact with the sample—hence the name—but due to this constant contact the normal and lateral forces on the tip can be great. This may reduce the spatial resolution and can damage soft samples. For this reason contact mode is often used with harder samples. Cantilever choice can help prevent sample damage through use of a softer tip.
Oscillating Modes
When the tip is held far from the surface, it is said to have zero deflection and is in an equilibrium position. As the tip is brought down and approaches the surface, it will begin to feel attractive forces such as electrostatic, Van der Waals, or capillary forces. As the tip continues downward, the interaction will start to become repulsive and the tip will be deflected. The size of the deflection is a measure of how much the cantilever is deflected before being corrected by the feedback setpoint. The signal is used with the feedback parameters to determine how the Z piezo must move in order to maintain a constant deflection. The amount the Z piezo moves is taken to be the sample topography.
Tapping Mode or AC Topography
Of the oscillating modes, the most common is called tapping mode (or ac topography). This mode is slightly slower than contact mode, but because there is less direct contact with the sample, there is a decrease in the lateral and normal forces acting on the tip. Tapping mode still maintains high sensitivity to the sample topography. This also makes tapping mode more ideal for softer samples.
In tapping mode the cantilever is oscillated, using a piezoelectric transducer, at or around its resonant frequency. When the probe approaches the surface, the oscillations due to the tip-sample interactions are a result of electrostatic effects, Van der Waals forces, and capillary forces. The dampening of the oscillations reduces both the frequency and the amplitude. As with the deflection in contact mode, the oscillations are being monitored and the Z height is adjusted via the feedback loop with the amplitude setpoint.
Noise
Noise can be very apparent on the AFM, especially when scanning small areas.
The two most common types of noise are acoustic vibrations and electronic noise. Acoustic vibrations appear as irregular lines forming horizontally across the image. They are generally a result of everyday things, such as talking, walking, using rolling chairs, or closing doors to the lab. The best ways to eliminate such vibrations are to have the AFM in a low traffic area, have it placed on an anti-vibration table, and if possible have it in an acoustic enclosure. Low traffic areas can sometimes be hard to come by with limited lab space, and an extra enclosure can cost a great deal, so another way to limit vibrations in images is to use the AFM during non peak hours.
The other type of noise most frequently seen is electronic noise, which appears as high frequency oscillations and generally looks like a “streaky” pattern all over the image. This type of noise originates from the instrument itself and can sometimes be difficult to eliminate. It can appear when the tip is scanning too quickly and causing extra vibrations, or when a setting such as the integral gain is set too high. The only real way to fix this is to adjust some of the scanning and feedback settings and see if it helps resolve the issue.
Probe Artifacts and Tip Shape Issues
One of the primary factors that affects the resolution of AFM images is the shape and condition of the cantilever tip.
The smaller the tip radius the smaller the features that can be resolved, but a smaller tip may be more prone to noise. The sidewall angles of the tip limit how accurate steep walls on a sample can be imaged, as the tip cannot profile sides that are steeper than its sidewall.
With the high accuracy and precision of AFM cantilevers also comes fragility. As cantilevers are used they become dull, break, or get contaminated—all of which can give rise to many different image artifacts. Dull probes generally result in larger features, smaller holes, or flatter profiles due to the increased tip radius. Contaminated or broken tips can result in image distortion, unexpected shapes, or repeating patterns. If you believe that your tip may be dull, broken, or contaminated, test the cantilever by scanning a sample with a known topography, such as a calibration grid that may have been supplied with the AFM.