Biomedical Engineering Reference
In-Depth Information
modifications to the optics of the light lever [3, 84]. Nevertheless, the use of small levers
for both spectroscopic and fast scanning applications is growing, and recently a commer-
cial source of such levers became available, and commercial AFM instruments capable of
using laser spots sizes suitable to use small levers are now also available.
Various environmental factors can drastically affect the results obtained with AFM. These
include the environment the instrument is maintained in, for example the presence of
vibrations in the building in which the AFM is housed. An AFM is a very sensitive device,
and is highly susceptible to interference from vibration. The two main types of vibration
picked up by the AFM are acoustic and mechanical vibrations. Acoustic vibrations are
transmitted through the air, and are sometimes, but not always, also audible to humans. In
deciding where to place an AFM in your organization, it is always worth looking for the
quietest room you can find; even people talking quietly can show up in the AFM when
scanning at very high resolution, so it is ideally located in a place with few other activities
taking place. Section 6.4.2 shows an example of acoustic noise in an AFM image. In
general, locating the AFM in a room with little traffic and audible noise reduces these sorts
of interfering vibrations to a reasonable level. Some acoustic noise however, such as the
fans used to cool the AFM or computer system, or from air conditioning units are hard to
avoid. In addition, increasingly, AFM instruments are being installed in locations with
unfavourable characteristics for the operation of an AFM. In these cases, some form of
isolation from acoustic and floor vibrations is necessary. Typically, the AFM is easily
isolated from acoustic noise by placing it in a sealed cabinet constructed from acoustic
damping materials. One drawback of these cabinets is that such materials are commonly
made of polymer foam, and they tend to be good thermal insulators as well, so that a small
acoustic cabinet can often mean it is more difficult to disperse the heat generated by the
AFM, leading to poor thermal stability. An alternative to such cabinets is to clad the walls
of the room with sound-absorbing panels; these greatly decrease the transmission of
acoustic noise to the instrument without reducing the cooling potential.
Mechanical vibrations are transmitted to the AFM stage from mechanical contact.
There are many possible sources of such mechanical noise. Typically, however, the
noise reaches the AFM via floor contact. Therefore isolation of the AFM stage from the
floor is necessary. The most simple method to do this is by simply mounting the AFM
on a granite stage (often included as part of the AFM), and further isolating with a
metallic plate/rubber balls stack. Such systems work adequately for low-resolution work
or work with quite rough samples, but are generally inadequate for more demanding
applications. For better vibration isolations two types of solution exist; active and
passive vibration isolation.
Active vibration solutions generally take the form of a platform for the AFM, which
may be suspended, devices that measure the vibration transmitted from the base of the
platform (accelerometers), and active elements (transducers) that counter measured
vibrations, via a feedback loop including the signal from the accelerometers. Such
devices work extremely well in isolating AFM instruments, and are generally very
compact.
