research activities in 2007 - CSEM

ArrayFM – An Atomic Force Microscope Using 2-Dimensional Probe Arrays
A. Meister, J. Polesel-Maris, S. Dasen, G. Gruener, M. Schnieper, T. Overstolz, A. Vuillemin, C. Gimkiewicz, R. Ischer, P. Vettiger,
H. Heinzelmann
In this multidisciplinary integrated project, an atomic force microscope able to investigate large sample surfaces with nanometric resolution was
developed. Instead of a single probe, this novel microscope uses a 2-dimensional array of probes operating in parallel. Applications of this
microscope include the biology domain, quality control, as well as material and surface characterization.
Since the emergence of the atomic force microscopy (AFM) in
the eighties, the topographic investigation of a sample surface
at a nanometric scale has become a standard technique. AFM
techniques can also be used to measure various kinds of local
interactions, such as magnetic, electrostatic, or binding forces,
electrical conductivity, or to determine mechanical properties
such as elasticity or friction. Standard AFMs use a single
probe, and, due to the scanning process, are rather slow in
terms of data acquisition. The aim of this project is to develop
an AFM functioning with a large probe-array instead of a
single probe, increasing thus the throughput of the instrument.
The realized instrument is shown in Figure 1. The
implementation of arrays instead of a single probe requires
new functionalities compared to a standard AFM instrument,
such as the parallel read-out of each probe, the spatial
alignment of the probe-array above the sample surface, and a
dedicated software to drive the instrument and for the user
interface. The correct positioning of the probe-array above the
surface is realized using a micropositioning stage with 6 axis
of freedom (Hexapod) with micrometric accuracy. The sample
is mounted on a piezoelectric nanopositioning stage, and is
scanned with a nanometric precision while the array probes
the sample surface.
Figure 1: Developed AFM instrument that is able to operate with an
array of probes in parallel
In contrast to standard AFMs, where the read-out of the
cantilever deflection is detected using a reflected laser beam,
in this instrument the parallel read-out of the cantilever-array
is based on optical interferometry using a Linnik
interferometer. The interferogram, which arises from the
combination of both reference and measuring optical beams,
is detected by a CMOS camera and analyzed by the software.
The characterization of the optical set-up showed an ability to
measure cantilever deflections as small as 1 nanometer.
The development and fabrication of the probe arrays made of
micro-cantilevers is another important issue. Such arrays are
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not today commercially available, and have therefore also
been developed within this project. Since the cantilever
deflection detection is not integrated in the probe array, this
latter can be passive, and thus be produced in a cheap way.
Two different processes leading to two different and
complementary kinds of probes were developed. The first
process relies on a sol-gel replication of the probe arrays in a
polymeric structure, which are foreseen as disposable probe
arrays to be used for parallel force spectroscopy in biology.
The second process is based on micromachining of silicon
wafer, and enables the production of cantilever with sharp tips
dedicated to high resolution imaging. Examples of realized
probe arrays are shown in Figure 2.
Figure 2: Left: Optical micrograph of a probe array fabricated by solgel
replication process (scale bar: 500 µm). Right: Scanning electron
microscope micrograph of a silicon probe array fabricated by
micromachining.
The ability of this instrument to operate AFM cantilever arrays
opens new application domains, such as multi-parameter
surface investigation using a probe array with different
cantilever functionalities, parallel force spectroscopy with
improved statistics, or large scale topographic imaging. The
application field covers:
• Quality control: metrology, surface roughness, defect
analysis.
• Biological and medical applications: parallel cell
indentation (determination of the cell elasticity), parallel
force spectroscopy to measure cell-cell interaction or
antibody-antigen binding (to detect the presence of the
target molecule on the receptor molecule in affinity
assays).
• Large scale imaging: topographic characterization at a
nanometric range, large scale surface studies such as
friction or elasticity with picoNewton resolution.
The partial support of the Swiss Federal Office for Education
and Science (OFES) in the framework of the EC-funded
project NaPa (Contract no. NMP4-CT-2003-500120) is
gratefully acknowledged.