Chemistry Reference
In-Depth Information
12.5.1
Small Cantilevers and Related Devices
The small cantilevers that have most recently been developed by Olympus are
manufactured from silicon nitride and have dimensions of 6
m wide,
and 90 nm thick. The resonant frequency is 3.5MHz in air and 1.2MHz in water, the
spring constant is about 0.2 N/m, and the quality factor is 2
m
m long, 2
m
3 in water. The tip has a
beak-like shape. The apex radius,
17 nm, is not small enough to produce a high-
resolution image. We usually use electron-beam-deposition (EBD) to forma sharp tip
extending from the original tip. Small pieces of naphthalene crystals are placed into a
small container, the top of which is perforated with holes of
0.1mm in diameter.
The container is placed in the scanning electronmicroscope chamber and cantilevers
are placed just above the containers holes. The spot mode electron beam irradiates
the cantilever tip to produce a needle on the tip with a growth rate of
50 nm/s. The
EBD tip is further sharpened by plasma etching in argon gas, which reduces the apex
radius to
600
in a vacuum to strengthen
the EBD tip. We use the optical lever system developed in 2001 [2] for detecting
de
ection of a small cantilever. The previous photodiode was replaced with a four-
segmented Si PIN photodiode (3 pF, 40MHz). The photodiode ampli
er has a
bandwidth of 20MHz. We also use the fast amplitude detector developed in
2001 [2]. The peak and bottom voltages of the input sinusoidal signals are held by
a sample/hold circuit for every half-oscillation cycle. Their difference is output as an
amplitude signal. The sample/hold timing signals are usually derived from the input
signals themselves. However, external signals that are synchronized with the
cantilever excitation signals can be used, which is sometimes useful for maximizing
the detection sensitivity of the tip-sample interaction. Since the amplitude signals are
produced at every half-cycle of oscillation, the amplitude read time is 0.42
4 nm. Sometimes it is heated further at
m
s.
The cantilevers response time is about 0.66
m
is in water.
12.5.2
Scanner
The scanner is the device that is most dif
cult to optimize for high-speed scanning.
High-speed scans of mechanical devices with macroscopic dimensions tend to
produce unwanted vibrations. Three techniques are required to minimize
unwanted vibrations: (a) a technique to suppress the impulsive forces that are
produced by quick displacement of the actuators, (b) a technique to increase
the resonant frequencies, and (c) an active damping technique to reduce the quality
factor .
The
rst issue was solved by a counterbalancing technique [2, 3]. For example, for a
z-scanner that moves at much higher frequencies than the x- and y-scanners, two
identical piezoactuators are placed at a supporting base in the opposite direction and
displaced simultaneously with the same length. An alternative method is to support a
piezoactuator at both ends with
flexures. This method was recently and effectively
applied to the x- and z-scanners (unpublished data).
