AFM Modes

In Contact AFM, the tip is in perpetual contact with the sample. The tip is attached to the end of a cantilever with a low spring constant—lower than the effective spring constant of the force holding the atoms of most solids together. As the scanner gently traces the tip across the sample (or the sample under the tip), the contact force causes the cantilever to bend to accommodate changes in topography. The Contact Mode feedback loop maintains a constant deflection between the cantilever and the sample by vertically moving the scanner at each (x,y) data point to maintain a “setpoint” deflection. By maintaining a constant cantilever deflection, the force between the tip and the sample remains constant.

The force is calculated from Hooke's Law: F = –kx, where:

• F = Force
• k = spring constant
• x = cantilever deflection

Force constants usually range from 0.01 to 1.0 N/m, resulting in forces ranging from nN to μN in an ambient atmosphere. The distance the scanner moves vertically at each (x,y) data point is stored by the computer to form the topographic image of the sample surface. Operation can take place in ambient and liquid environments.

Advantages

• High scan speeds (throughput)
• Only AFM technique that can obtain “atomic resolution” images
• Rough samples with extreme changes in vertical topography can sometimes be scanned more easily

Disadvantages

• Lateral (shear) forces can distort features in the image
• Forces normal to the tip-sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface
• Combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples (i.e., biological samples, polymers, silicon) due to scraping between the tip and sample

Non-contact AFM (NC-AFM) is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and the sample for NC-AFM is generally on the order of tens to hundreds of Ångstroms. The inter-atomic force between the cantilever and sample in this regime is attractive (largely a result of van der Waals interactions).

In non-contact mode, the system vibrates a stiff cantilever near its resonant frequency (typically 100–400 kHz) with amplitude of a few tens to hundreds of Ångstroms. The system detects changes in the cantilever’s resonance frequency or vibration amplitude. The tip does not contact the sample surface, but moves above the adsorbed fluid layer (present on sample surfaces in ambient, non-vacuum conditions) during scanning.

The attractive non-contact force between tip and sample is typically smaller than and more difficult to maintain than the repulsive contact force. In addition, cantilevers used for NC-AFM must be stiffer than those used for contact AFM. These factors result in a very small NC-AFM detector signal. Consequently, a sensitive AC detection scheme is used for NC-AFM imaging.

In NC-AFM mode, Z feedback is turned on. The system monitors either the resonant frequency or vibration amplitude of the cantilever and keeps it constant by moving the scanner up or down in response to changes. In this way, the average tip-sample distance is also kept constant and the Z feedback signal is used to generate the data set.

Advantage

• No force exerted on the sample surface

Disadvantages

• Lower lateral resolution, limited by the tip-sample separation
• Slower scan speed than TappingMode and Contact Mode to avoid contacting the adsorbed fluid layer (resulting in the tip getting stuck in the fluid layer)
• Usually only works on extremely hydrophobic samples, where the adsorbed fluid layer is at a minimum. If the fluid layer is too thick, the tip becomes trapped in the adsorbed fluid layer causing unstable feedback and scraping of the sample.

TappingMode™ AFM, the most commonly used of all AFM modes, is a patented technique (Bruker) that maps topography by lightly tapping the surface with an oscillating probe tip. The cantilever's oscillation amplitude changes with sample surface topography. The topography image is obtained by monitoring these changes and closing the Z feedback loop to minimize them. TappingMode has become an important AFM technique, as it overcomes some of the limitations of both contact and non-contact AFM (also described in this chapter). By eliminating lateral forces that can damage samples and reduce image resolution, TappingMode allows routine imaging of samples once considered impossible to image with AFMs.

In general, TappingMode is much more effective than non-contact AFM, but especially for imaging larger areas on the sample that may include greater variation in topography. TappingMode can be performed in gases and non-corrosive liquids. In liquid, the oscillation need not be at the cantilever resonance. When imaging in air, the typical amplitude of the oscillation allows the tip to contact the surface through the adsorbed fluid layer without getting stuck.

Advantages

• Higher lateral resolution on most samples (1 nm to 5 nm)
• Lower forces and less damage to soft samples imaged in air
• Lateral forces are virtually eliminated, so there is no scraping

Disadvantages

• Slightly slower scan speed than Contact Mode AFM