HIGH-SPEED ATOMIC FORCE MICROSCOPY FOR DYNAMIC BIOLOGICAL IMAGING - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 163

developed are made of silicon nitride and coated with gold of thickness ~
20 nm. They have dimensions of length ~ 6 μm, width ~ 2 μm and thickness
~ 90 nm, which results in resonant frequencies of ~3.0 MHz in air and ~1.2
MHz in water, a spring constant of ~0.2 N/m and ~ 2.5 in water. The small

cantilevers with a sharp tip are not commercially available at present. We
therefore use electron beam deposition to grow an amorphous carbon tip on
the original tip, 11 which can be sharpened by a plasma etching in argon gas.

Q

8.2.2.2 Opcal beam deflecon detector

To focus an incident laser beam onto a small cantilever, a lens with a high
numerical aperture (resulting in a short working distance) has to be used.
An objective lens with a long working distance of 8 mm is used; a laser beam
relected back from the rear side of a cantilever is collected and collimated
using the same objective lens as that used for focusing the incident laser beam
onto the cantilever. 3 The focused spot is 3-4 μm in diameter. The incident
and relected beams can be separated using a quarter wavelength plate and
a polarization splitter.

8.2.2.3 Amplitude detecon

Conventional rms-to-dc converters use a rectiier circuit and a low-pass ilter,
which requires at least several oscillation cycles to output an accurate rms
value. To detect the cantilever oscillation amplitude at the periodicity of a
half oscillation cycle, we developed a peak-hold method; the peak and bottom
voltages are captured and then their difference is output as the amplitude.

This amplitude detector is the fastest detector, and the phase delay has a
minimum value of π, resulting in a bandwidth of

3

f c /4.

8.2.2.4 High-speed scanner

The scanner is the device most dificult to optimize for high-speed scanning.
High-speed scanning of mechanical devices with macroscopic dimensions
tends to produce unwanted vibrations. Several conditions are required to
establish a high-speed scanner: (a) high resonant frequencies, (b) a small
number of resonant peaks in a narrow frequency range, (c) suficient
maximum displacements, (d) small crosstalk between the three-dimensional
(3D) axes, (e) low quality factors. We employ lexure stages made of blade
springs for the -scanners. The lexure stages are made by monolithic

processing to minimize the number of resonant peaks. 3 The maximum
displacements of the

x - and y

x - and y -scanners at 100 V are 1 and 3 μm, respectively.