Atomic Force Microscopy (AFM)/Scanning Probe Microscopy (SPM)

In a Nutshell

Take Home Point:

Small area imaging providing roughness quantification

What It Provides:

Provides surface images, information on morphology and quantitative roughness values. Can also provide details on electrical, physical and mechanical properties.

Brief Description:

Measurement of Forces Between Tip and Sample

In AFM, a sharp tip located on a cantilever (a spring) is brought into close proximity of a sample surface and scanned. The deflection of the tip (caused by repulsive or attractive Van der Waal forces depending on the tip-sample distance) is measured as it is scanned over the surface. The resulting images have high resolution, particularly in the Z direction, giving detailed information on topography and surface roughness.

Variations in the tips used and the method of collecting data allow a wide variety of sample properties to be imaged such as mechanical (e.g. adhesion, elasticity, hardness), conductivity, capacitance, magnetic properties and spreading resistance/ carrier density.

What is Detected:

Not Applicable

Detection Limits:

Not Applicable

Information Depth:

Surface Imaging

Applications:

Surface topography
Roughness measurement of substrates (prior to coating) and coatings
Dimensions of features and defects
Mapping of mechanical and physical properties (e.g. adhesion, friction, etc…)
Mapping of magnetic and electrical properties
Surface manipulation

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Greater Detail

Overview

AFM is a type of SPM (Scanning Probe Microscopy) technique. In AFM, a sharp tip located on a cantilever (a spring) is brought into close proximity (<0.1-10nm) of a sample surface and scanned. A laser bounces a light beam off the back of the cantilever onto a position sensitive photodiode detector which measures the deflection of the tip as it is scanned over the surface. Tips are typically 1-20nm in diameter and commonly comprised of Si or Si3N4.

The tip experiences repulsive or attractive Van der Waal forces depending on the tip-sample distance. The tip is either allowed to come into contact with the surface (Contact Mode) or allowed to oscillate above the sample surface. In one mode of the latter case it is never allowed to contact the surface. This is defined as Non-Contact Mode. In another mode (Tapping) it only gently contacts the surface at the bottom of each oscillation. A feedback loop is used to adjust properties (e.g. tip-sample force, oscillation amplitude) and the amount of adjustment necessary at each scan point is used to map the height of the surface. Contact Mode is a fast imaging mode that is used on hard surfaces that are not easily damaged. Non-Contact Mode, on the other hand, is used for sensitive samples that may be susceptible to damage during conventional AFM imaging. However, it has lower resolution and is more susceptible to the fluid contaminant layer found on most surfaces under ambient conditions. Tapping Mode is the most common imaging mode and is useful for samples that may be easily damaged. It provides high resolution images although it is typically slower than Contact Mode.

The main advantages of AFM are:

1. Provides surface topography

2. Images have high Z resolution

3. Surface Roughness Quantification

The ability to quantify surface roughness is one of the key features of AFM that sets it apart from many other imaging techniques. Roughness is important due to its effect on many properties such as adhesion, appearance, friction coefficients and the 'feel' of a surface.

In addition to providing surface images, AFM can obtain Force Curves to measure the force experienced by the cantilever as the tip is brought close to and eventually indented into the sample surface and then pulled away from it. Force Curves help to measure chemical and mechanical properties such as adhesion, elasticity and hardness. In addition to providing this information from a single point, Quantitative Nanomechanical Mapping (QNM) allows for this information to be mapped over the sample surface while obtaining conventional topographical images. Thus, heterogeneity of these properties can be explored.

Other variations of AFM can be used to map conductivity (TUNA or Conductive AFM), capacitance (Scanning Capacitance Microscopy), magnetic properties (Magnetic Force Microscopy) and spreading resistance/carrier density (Scanning Spreading Resistance Microscopy). Many others variations not mentioned exist as well.

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