INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 58

Figure 3-3. Although not obvious to the user, devices that indicate AFM cantilever
deflection (and thus force) are not linear over a wide range of operating voltages (circles).
This can be linearized through analysis of full nonlinear profile to increase accuracy and
range of AFM indentation forces. After Silva and Van Vliet. 8

3.1.5. Integration of AFM feedback and instrumented indentation probes

Most commercially available AFM cantilevers include probes that are
readily fabricated in silicon wafer platforms, such as pyramids of apex
angle α ~ 60° etched along close-packed planes of the wafer. These
probes are quoted to have radii on the order of 25 nm but, due to the
nature of this etching and filling process, range widely in size and facet
severity. For indentation into compliant surfaces to depths of several m,

this probe imperfection is minimized and one assumes the geometry of a
sharp cone. However, there are several cases in which biomaterial
indentation requires more well defined probe geometry, including very
thin, very compliant films that cannot be probed to μm depths; and
analyses that require low imposed strains. In such cases, two distinct
approaches have developed.
The first approach is to use colloidal probes, so named because
colloidal particles of silica, polystyrene and other materials can be glued
to AFM cantilever free-ends to better define the contact area. This
approach decreases the imposed strain of indentation, but also increases
the strained volume, such that finite thickness effects must be considered.

μ