Atomic Force Microscopy

1. How does Atomic Force Microscopy Work?

In an atomic force microscope (AFM) a sharp probe is mechanically scanned across a surface and the motion of the probe is captured with a computer. The probe’s motion is then used to create a three dimensional image of the surface. In the AFM either the probe can be scanned over a stationary surface (tip scanning AFM), or the sample can be scanned under a stationary probe (sample scanning AFM). The primary components in an AFM are illustrated in Figure 1.

Figure 1
This figure illustrates the primary components of a sample scanning atomic force microscope:

  • Control Computer
  • Z Feedback Electronics

  • XY Scan Electronics

  • Light lever force sensor with a laser, cantilever and photodetector

  • 3-D piezoelectric scanner.

2. What is the resolution of an Atomic Force Microscope?

Forces between the probe and surface can be as low as a few pico-Newtons, so very sharp probes with a few nanometers diameter can be used. Thus, in an atomic force microscope horizontal resolutions of a few nanometers are possible. In the vertical dimension, an atomic force microscope can have a resolution (or noise floor) of less than 35 picometers.

3. How are AFM images displayed?

Image processing software such as Gwyddion is used for displaying AFM images. Images can be displayed in a 2-dimensional or 3-dimensional format. Most AFM images are displayed with a color scale such that a specific color indicates the height of a feature on a surface. Alternatively, images can be displayed with a light shading format to give a photo-realistic appearance. Figure 2 illustrates a 2-D, 3-D and light shaded image of a SiC sample having atomic steps.

4. What is the difference between Contact and Tapping AFM?

Both Tapping and Contact modes measure the topography of a sample’s surface. In contact mode, the force between the probe and surface is established by the deflection of a cantilever with the probe on its end. In tapping or vibrating mode, the force between the probe and surface is measured by monitoring the reduction in cantilever vibrating amplitude. Figure 3 illustrates the operating principle of contact and vibrating mode. The minimum force contact mode is 0.1 nano-Newtons and in vibrating mode the minimum force is .001 nano-Newtons. Vibrating mode is used for measuring both hard and soft samples, while contact mode is used primarily for imaging hard samples like metals and ceramics.

Figure 3
Top – In contact mode AFM, when the probe interacts with a surface, the cantilever bends. The force between the probe and surface is proportional to the deflection of the cantilever. When scanning a sample, the cantilever deflection remains constant.
Bottom – In tapping(vibrating) mode AFM when the vibrating probe interacts with the surface, the vibrating amplitude is reduced. The amount of force between the probe and surface is related to the reduction in vibration amplitude. The vibration amplitude is held constant while scanning across a surface.

5. What is Phase Mode imaging?

Phase mode imaging is used to measure the differences in hardness of the sample surface and is commonly used for measuring images of polymer samples. A phase difference between oscillation of the cantilever and of the signal that drives cantilever oscillation (by, for example, piezoelectric crystal) is measured and visualized in phase imaging. There is no phase contrast when the surface is homogeneous, or when there is no interaction between the tip and surface (i.e., the cantilever is well above the surface). However, if specific regions of the surface have distinct mechanical properties, that could be captured with phase imaging. This is because the cantilever loses a different amount of energy as the probe contacts surface areas with differing mechanical properties. Hence, phase imaging could be helpful to detect variations in mechanical properties such as friction, adhesion, and viscoelasticity on surfaces. It can also be used to detect patterns of various materials such as polymers on the surface or to identify contaminants that cannot be distinguished with topography imaging.

Figure 4
In a phase mode image the phase of a vibrating cantilever is measured. A change in the phase signal is related to a change in the hardness in differing regions on a samples surface.

6. How does Lateral Force Microscopy work?

In lateral force or frictional force microscopy, lateral deflections of the cantilever, arising due to forces parallel to the plane of the sample surface such as friction force, are measured. This allows the technique to detect inhomogeneities on the material which gives rise to variations in surface friction.

Figure 5
A lateral force mode image is measured by measuring the twist in a cantilever as it moves across a surface. If an area has greater friction, the cantilever will twist as it passes over the area of greater friction.

7. Why are Force-Distance curves important?

With a force – distance(F/D) curve it is possible to understand the vertical forces between a probe and a surface. To measure a curve, the probe at the end of a cantilever is pushed into the surface of a sample. Once the probe begins to interact with the sample, the cantilever bends. The inbound part of a F/D curve (red in figure 6) is the deflection of the cantilever when the probe is being pushed into the sample, and the outbound (blue in figure 6) corresponds to when the probe and sample are moving apart. There are two regimes in a F/D curve: The attractive regime which occurs when the cantilever bends down, and the repulsive regime when the cantilever bends up.

Figure 6
To measure a force distance curve the sample is pushed into the probe/cantilever(called the inbound curve – blue) and then the sample is retracted from the probe/cantilever (called the outbound curve – red). When the probe is a long distance from the surface, the cantilever is not bent (no interaction). As the probe approaches the surface the cantilever bends down (attractive interaction). When the probe touches the surface the cantilever bends up (repulsive interaction).

8. Is an AFM easy to operate?

Learning to operate an AFM requires about the same amount of effort as learning how to drive a car. After a few hours of training on AFM operation and theory a new user can start measuring images.

For measuring extremely high resolution images, an operator must master the probe approach and PID optimization steps.

There are 7 steps required to measure an AFM:

  • 1
    Put a sample in the AFM
  • 2
    Put a probe in the AFM
  • 3
    Alight the laser
  • 4
    Align the photodetector
  • 5
    Probe Approach
  • 6
    Scan the Sample
  • 7
    Retract the probe from the sample

Changing AFM probes- AFM operators must learn how to change probes in an AFM. This video shows how to exchange the probe on an AFM. It typically takes less than 2 minutes to change a probe.

Measuring Images-As the probe is scanned back and forth across a surface, the motion of the probe is displayed on a computer screen. An image of a surface is created line by line with an AFM.

9. How long does an AFM probe last?

A new AFM operator that is learning how to change probes and measure AFM images may use one probe a day for the first few days. Sometimes, while learning, a probe gets dropped on the floor and cannot be used for measuring AFM images. Once the basic skills are mastered, for routine applications, a probe may last for a week or two. There are two times the probe tip can be broken: a) During probe approach if the probe crashes into the surface, and b) While scanning if the Z feedback parameters are not optimal.

10. What is the difference between AFM and SEM?

To address imaging surface structures, the scanning electron microscope (SEM) and atomic force microscope (AFM) have complementary capabilities. SEMs have fantastic depth of field, and are capable of imaging structures that have a strong vertical relief. AFMs have poor depth of field, but provide amazing contrast on flat samples. The unique capabilities of the SEM and AFM are demonstrated in such extreme examples as the SEM’s ability to image a fly’s head, and the AFM’s ability to image structures on polished silicon.

  • Images measured with an SEM give a direct representation of surface features, requiring no image processing. AFM images always require image processing before optimal viewing of surface structures. It takes time to learn how to avoid inadvertently adding artifacts to AFM images and other common processing errors.
  • AFMs have an advantage in their ability to operate in vacuum, air, and in liquids. The highest resolution AFM images of atomic structures typically require an ultra high vacuum environment. But in ambient air, AFM’s can routinely measure images with sub nanometer resolution. SEMs require a vacuum for optimal operation and do not permit high-quality imaging in either ambient air or in a liquid environment.
  • Beyond measuring sample topography, both SEM and AFM can measure surface properties. As advantage of the SEM is that it can measure the chemical composition of surface features, while an AFM can measure surface physical properties, such as magnetic fields (MFM), surface potential (SKPM), surface temperature (SThM), friction (LFM), and many other surface physical properties.
  • The SEM gives magnification in two dimensions: x and y. The AFM gives magnification in three dimensions: x,y and z. Users can directly measure the height of a sample feature from an AFM image, while typically the SEM sample must be cross-sectioned to obtain the height of a feature. AFMs also provide different magnifications in the x, y, and the z axis.
  • While SEMs scan a sample surface much faster than an AFM, they are not actually faster to use than AFM overall. One must account for the time involved in sample preparation, moving a sample into the SEM vacuum chamber, and the measurement session from start to finish. In the end, the SEM and the AFM require about the same amount of time to produce and measure images. It will generally take a few hours for a trained operator to measure images of an unknown sample on both an AFM and an SEM.
  • Thanks to advances in software automation, the learning curve for measuring images with modest resolution on an SEM and an AFM is not too steep. More important to gaining great images on both the AFM and the SEM is investing the time in learning how to correctly prepare samples for scanning and in how to establish specialized scan parameters. Great images are measured by operators on many types of samples.
Imaging Advantage High Depth of Field High Contrast
Dimensions 2-D 3-D
Measurements Chemical Composition Physical Properties
Environment Vacuum Air, liquids, vacuum

11. How much does an Atomic Force Microscope Cost?

An AFM for general laboratory use costs between $20,000 and $300,000. The amount that the AFM costs depends on the features and capabilities of the instruments.

$20K – 30K
  • Cell phone type Optics
  • Noise floor > 250 picometers
  • Limited modes available
  • Basic Atomic Force Microscope
30 K – 100 K
  • High Quality Research Type Optics
  • Noise floor < 150 picometers
  • Multiple Scanners
  • Several imaging modes
  • Table top (TT-2) Atomic Force Microscope
  • High Resolution Atomic Force Microscope
  • Life Sciences Atomic Force Microscope
  • Nano-Profiling Atomic Force Microscope
  • Standalone Atomic Force Microscope
100 K – 300 K
  • Motorized sample position stage
  • Automated Focus
  • Noise floor <50 picometers
  • Advanced options for specialized research
  • Automated software features
  • Life Sciences Atomic Force Microscope

Typically, AFMs costing less than $30,000 are used for educating students on the operation and application of AFM. The mid price systems are purchased by researchers wanting AFM imaging capabilities but do not want to be AFM experts. Instruments costing more than $100,000 are purchased by experts that require advance capabilities.