Instruction manual

SPM Fundamentals for the MultiMode
Hardware
26 MultiMode SPM Instruction Manual Rev. B
Cantilevered Probes
Most SPM work is done using cantilevered probes. These consist of a flexible cantilever extending
from a rigid substrate, to which a tip has been attached. In contact AFM, the cantilever flexibility
acts as a nanometric spring, allowing the tip to measure surface forces. In TappingMode, the probe
is oscillated up and down at its resonant frequency while its amplitude and phase are monitored.
Figure 2.1j Two Types of Cantilevered Probes
Cantilevered Probes—silicon nitride
Most contact AFM is conducted with silicon nitride tips. These tips exhibit excellent flexibility,
making them easier to use and more “forgiving” than stiffer crystal silicon cantilevers. They are
offered in a variety of sizes and coatings, allowing the user to match them to the sample being
imaged. One characteristic of silicon nitride tips is that they are easily captured by the sample’s
surface tension (capillary) properties; that is, trapped within a microscopic layer of condensed
atmospheric water vapor on the sample surface. This surface tension effect exerts considerable
force at the probe’s atomically sharp tip. Although this may prove unproblematic on harder
samples, it is frequently enough to deform softer samples. Adjustment to the Setpoint parameter
can offset much of this force; however, it may still prove troublesome on delicate samples. Silicon
nitride tips may also be operated in TappingMode, although they are not optimal for this purpose.
Cantilevered Probes—TappingMode
Veeco’s answer to minimizing contact AFM forces is TappingMode, a proprietary form of AFM. In
this instance, a stiff crystal silicon probe is oscillated to its resonant frequency. Because the tip
describes a high-frequency (e.g., 100-plus kHz), oscillating arc, it possesses sufficient energy to
break free of surface tension forces. The probe is considerably stiffer than silicon nitride, making it
more brittle and less forgiving. Thus, the operator must be more cautious while setting up the tip
and sample.
Siliscon Nitride Crystal Silicon