Page 30 - Science at the nanoscale

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RPS: PSP0007 - Science-at-Nanoscale
June 9, 2009
Classical Physics at the Nanoscale
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In the following sections, we will discuss how some impor-
tant physical properties such as mechanical frequency, viscosity
and motion of nanoscale objects differ from those of macro-sized
objects that we normally see.
2.1 MECHANICAL FREQUENCY
A cantilever is a beam anchored at one end and projecting into
space. Cantilevers are widely found in construction, notably in
cantilever bridges and balconies, as well as in aircraft wings. Civil
and aircraft engineers are very concerned about the mechanical
frequencies that these cantilever structures are subjected to,
since external frequencies around the resonant frequencies of
these structures can lead to catastrophic failures and major
disasters.
Cantilevered beams are very often found in micro-electro-
mechanical systems (MEMS). MEMS cantilevers are commonly
fabricated from silicon, silicon nitride or polymers. The fabrica-
tion process typically involves undercutting the cantilever struc-
ture to release it, often with an anisotropic wet or dry etching
technique.
In particular, the important technique of atomic
force microscopy (AFM) depends on small cantilever transduc-
ers. Other applications of micron-scale MEMS cantilevers are in
biosensing and radio frequency filters and resonators.
Mechanical resonance frequencies of cantilevers depend on
their dimensions; the smaller the cantilever the higher the
frequency. To do a simple analysis of the mathematical size
Figure 2.1. (left) Cavenagh bridge, Singapore’s oldest suspension (can- ch02
tilever) bridge; (right) SEM image of an AFM cantilever (from author’s
lab).