Constant of Proportionality
For the production of microstructures
Microcontact Printing  is a simple an cheap alternative to the rather
complicated and expensive Photolithography. By means of Microcontact Printing
(fig.1) SAMs (selfassembled monolayers), e.g. a thiol can be printed on a
suitable substrate, e.g. gold.
The SAM protects the surface from being
etched when in a next step the surface is etched to produce a lateral structure.
The chemical function of a thiol depends on its functional groups, which can be
modified. In that way the surface can be modified to bind particular classes of
molecules. Microcontact Printing of SAMs can produce Microarrays carrying
thousands of different sensor properties within one cm2.
Microarrays are applicable in Genomics and Proteomics in biotechnology. Imaging
ellipsometry is cheap, fast, and marker-free detection method on Microarrays
Figure 1. Microcontact Printing
Microcontact Printed SAMs of two different
thiols on gold (50 nm film on glass slide)
Ellipsometer EP3-SW (532 nm) optional Scanning Probe
Ellipsometric Microscope (SPEM) from Accurion
including scanning probe microscope (SPM)
Quality control of the Microcontact Printed
SAMs by means of thickness maps
- Optimize the ellipsometric image contrast
with the angles of analyzer and polarizer
- Search for points of interest in the
ellipsometric contrast image in real time
- Let the EP3 record a map of Delta, from which the map of the thickness of thiol (fig.2) is
- Set the Region of Interest (white box,
fig.2 A) where to zoom in with the scanning probe microscope (fig.2 B,C)
Ellipsometer has the advantage that it can identify thickness variations in
structured SAMs with sub-monolayer vertical resolution (typ. 0.01 nm) in real
time, while the sample can be moved by the operator with an automatic
translation stage. By contrast scanning probe micrographs need many minutes for
preparation, optimization, and recording. SPMs produce artefacts when scanning a
large field of view (80 ¦Ìm as in fig.2 B) due to sample curvature and nonlinear
piezoelectric response. In fig.2 B the increasing signal towards the left and
right edges is an artefact, which makes the search for structures with nm-step
size very difficult with SPM. In order to identify nm steps in the layer with
SPM it is also necessary to eliminate vibrations e.g. by an antivibration stage,
which is not needed for ellipsometry.
The SPM offers a sub-micron lateral
resolution (fig.2 C), where the imaging ellipsometer
is limited by the optical diffraction limit (around 1 ¦Ìm). A calibrated SPM and
the ellipsometer, both measure step sizes in layers very
precisely. Only the ellipsometer can measure thickness absolutely without need for
a step in the layer. To this end the ellipsometer
measures a phase shift Delta, which is proportional to the monolayer
The constant of proportionality can be
calculated from the optical properties of the sample (refractive indices of
layer and substrate).
cannot measure the thickness absolutely when the difference of the refractive
indices is smaller than 0.01, which can be the case at SAMs on ordinary glass.
Absolute thickness measurement is always possible for SAMs on absorbing
substrates, i.e. gold and silicon, when the refractive indices of the SAM and of
the substrate are known. In order to calibrate the thickness scale of the map
(fig.2 A) typical values have been assumed for the refractive
Figure 2. Microcontact-Printed thiol on gold, thickness map (A, 5x objective,
1 min. recording time), scanning probe micrographs with 15 min. recording time
(B, 80 ¦Ìm x 80 ¦Ìm, corresponding to the white box in A, and C, 10 ¦Ìm x 10 ¦Ìm,
corresponding to white box in B)
Sub-monolayer-sized steps of 0.3 nm are
observed between two different types of thiols (blue and green in fig.2 A) in
Microcontact Printed SAMs.
Ellipsometer EP3 identifies steps in monolayers much
faster and with less effort than a SPM. Only the imaging ellipsometer
can record a thickness map with a field of view between 0.1 and 2 mm depending
on the objective. To zoom into the ellipsometric image with sub-micron lateral resolution a SPM
is well suitable. The scanning probe ellipsometric
microscope (SPEM) unifies SPM and EP3 in
one instrument. The EP3 and the SPEM are
perfectly suitable to characterize structured SAMs.
Figure 3. Microcontact-Printed thiol on gold, thickness profile according to
the thickness map (fig.2 A)
We would like to thank Professor John Green
Alberta, Canada) for
 J.L. Wilbur, A. Kumar, E. Kim, G.M.
Whitesides, Microfabrication by Microcontact Printing of Self-Assembled
Monolayers, Adv. Mater., Vol. 6, 600 (1994)
 Marker-free Detection on Microarrays by
M.Vaupel et al., p. 181-207, in Microarray Technology and Its Applications, U.R.
M¨¹ller and D.V. Nicolau (Eds.), Springer (2005),ISBN
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