Biomedical Engineering Reference
In-Depth Information
is hard to perform without the danger of damaging the tip, and relies on having well-
characterized masses readily available.
The third method commonly applied to obtain the normal spring constant is known as
the thermal noise method, first presented by Hutter
et al
.[
72, 73
]. This method requires
measuring the thermal noise spectrum when the cantilever is in contact with a surface. The
thermal noise spectrum can be very weak, and it is necessary to use a high performance
spectrum analyser to collect the data, pressing the cantilever against the surface means it
can potentially change the tip shape. However, the technique has been shown to be both
precise and accurate [62].
Finally, the reference cantilever method [74, 75] consists of pressing the cantilever
against a pre-calibrated spring, or reference cantilever. This is relatively easy to perform,
but relies on the availability of a well-characterized cantilever, and again, could be
considered to alter the tip shape.
All the methods mentioned above have focussed on the normal force constants of
cantilevers. However, for certain applications torsional or lateral force constants must be
calibrated [76, 77]. This is discussed further in Chapter 3, in the sections on lateral- and
torsional-bending based modes.
2.5.6
Improved probing systems
The scan rate of an AFM limits the sizes of areas that can be analysed to a few hundred
microns at best. It would be highly desirable to create an AFM with multiple probes that
could scan many areas simultaneously. Several efforts to create 'multiple probe' atomic
force microscopes demonstrated that it is possible. In the first approach, several AFM
scanners were positioned above a silicon wafer sample and scanned independently. In the
second approach, several probes on one silicon wafer were used to scan a sample
simultaneously [7]. The greatest challenge for creating multiple probe AFM instrumenta-
tion is to get all of the probes to be as sharp as is required for high-resolution scanning. In
most multiple probe AFM designs, a simplified cantilever actuation/sensing system is
used, to avoid the complication of fitting multiple optical lever set-ups around closely-
spaced cantilevers [78]. These set-ups also simplify the operation of the instruments greatly,
although they typically mean that standard cost-effective cantilevers cannot be used.
Another approach to improve the overall productivity of AFM is to maintain a single
probe, but to drive it at much higher speeds than are available by normal AFM. As shown
in Figure 2.29, standard AFM cantilevers have lengths of the order of several hundred
micrometers, typically 200-400
m. However, there are
advantages to be gained in specific applications in using much shorter and smaller
cantilevers [3, 79-81]. Fast scanning AFM benefits from the use of small cantilevers,
specifically because they can have much higher resonant frequencies than larger canti-
levers, without having very high spring constants, and can cause less sample damage when
scanning at higher speeds [3, 34, 81]. Small cantilevers also have advantages for force
spectroscopy, because the noise floor is typically controlled by perturbations of the
cantilever by the surrounding medium (air or liquid), which has a smaller effect for
smaller cantilevers, leading to higher signal-to-noise ratio [82]. Small cantilevers for
fast scanning AFM were first proposed in 1996 [83], but their use is far from widespread.
The main problem is that use of such cantilevers in a normal optical lever AFM head needs
m, and widths of the order of 40
