Atomic Force Microscopy
Atomic Force Microscopy (AFM) belongs to the family of scanning probe microscopy (SPM), but with high resolution for imaging features in nanometer or even sub-nm scale level.
After the Nobel Prize winning development of Scanning Tunneling Microscopy (STM), Gerd Binning together with Calvin Quate and Christoph Gerber developed Atomic Force Microscopy. which was designed for the increasing needs of nanometer scale features imaging, profiling, and characterizations.
The AFM consists of a cantilever with a very sharp tip (<10nm) at its end that is used to scan the sample surface. The tip material is typically doped silicon, but could be made of other materials with higher stiffness and hardness (such as SiN, SiC, and diamond). Depending on the scanning modes, AFM measures either contact force (contact mode) or van del waals force (non-contact mode) that deflects the cantilever by bringing the tip into proximity of a sample surface, As the surfaces have features with various heights and waviness, the degrees of AFM cantilever deflection alter as well. Thus the 3D surface topography can be profiled based on AFM cantliever deflection.
Imaging is not the only function AFM can provide, the system can be used for many other properties measurements, such as nanomechanical properties, electrical properties, magnetic properties, and chemical/bio properties, etc.
An AFM Imaging involves three steps:
1. Surface Sensing
An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip cause the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, increasingly repulsive force takes over and causes the cantilever to deflect away from the surface.
2. Detection Method
A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection (and the subsequent change in direction of reflected beam) is recorded by the PSPD.
An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the surface features.
At Mechaction, we always focus on nanomechanical properties measurements, AFM tool is a great addition to our lab functions, especially when the required test scale is much smaller than what a Nanoindenter can handle or the material is too soft to be measured by indenter force, such as soft polymer and hydrogel.
Typically, the minimum force a nanoindenter can provide is in uN range, while AFM nanoindentation force can easily go down to nN or even pN, making it ideal for extremely small feature mechanical assessment.
A specific mode of AFM called PinPoint can be utilized to mechanically map the Young's modulus or stiffness of the materials across a small area, assessing mechanical differences between phases, features, defects, and particles. During pinpoint mode, the XY scanner scan across a certain area. While at each pixel, the high speed force-distance curve is taken with well-defined control of contact force and contact time between the tip and the sample. Due to controllable data acquisition time, PinPoint mode allows optimized nanomechanical measurement with high signal-to-noise ratio over various sample surfaces, thus results in mapping of modulus, stiffness, surface adhesion force, and adhesion energy as well as the surface 3D topography. The images below represent three different maps (topography, modulus and surface adhesion) over the exact same area on a polymer surface, which clearly shows the existence of weak spots (phases) on the sample surface. Such mechanical discrepancy is not viewed under optical microscope or profiler, which is commonly used for polymer surface characterizations.
Pinpoint Surface Topography
Pinpoint Modulus Mapping
Pinpoint Adhesion Mapping
AFM has numerous advantages over optical microscope/profiler, and the scanning electronic microscope (SEM). Compared to any optical profiling methods, AFM provides a physical contact across sample surface, which is not affected by surface optical reflection, optical interference, and surface transparency, thus it can provide the most accurate measurements of feature heights in the range of 1nm to 1um, Compared to SEM which can only provide quantitative measurements in X and Y directions, the AFM provides a three-dimensional surface profile which includes the measurement in Z height. Furthermore, AFM scan can provide completely non-destructive imaging or sample surface, while SEM imaging usually requires sputtering a thin layer of gold which might change the surface conditions.
1. 3D topography Imaging
2. Surface Profiling
3. Height measurement (from 0.1nm to 15um)
4. Mechanical Mapping
5. Modulus and Stiffness
6. Surface Adhesion
7. Surface Friction
8. Electrical/Magnetic Properties
9. Chemical/Bio Properties