Application note
NANOSENSORS TM Insulated Conductive Probes for combined AFM and
ElectroChemical studies (EC-Probes)
Ilya V. Pobelova, Miklós Mohosa and Christoph Richterb*
a
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
b
NANOSENSORS™, Rue Jaquet-Droz 1, CH-2002 Neuchatel, Switzerland, Phone: +41 32 720 5085
fax: +41 32 720 5792, E-mail: info@nanosensors.com
* To whom correspondence should be addressed.
1) Description
The batch fabricated EC-Probes [1] feature a 70 nm thick platinum transmission line integrated into
the support chip, cantilever and the tip of a monolithic silicon SPM probe with PointProbe®Plus
geometry (Figure 1). The transmission line is hermetically sealed by an isolation layer which is only
opened at the last 1 µm of the tip apex (electrode) and at the opposite side of the support chip
(contact pad). The radius of the electrode is below 40 nm.
500 µm
500 µm
1.6 mm
1.6 mm
1
85 µm
85 µm
tip apex (SEM-picture)
tip apex (cross section)
3.4 mm
5 µm
3.4 mm
SiN
15 µm
< 1 µm
Pt
contact pad
100nm
Pt
SiN
200 – 250 µm
200 - 250 µm
300- –450
450µm
µm
300
10-15 µm
300 µm
200- –250
250µm
µm
200
Support chip thickness: 300µm
1
Figure 1: Geometry of the NANOSENSORSTM EC-Probe with integrated platinum electrode. The SEM-picture shows the
Cone-shaped electrode protruding the silicon nitride isolation layer only at the very end of the tip.
Table 1: Nominal geometry and corresponding mechanical properties of the EC-Probe cantilever (CONT-type)
Cantilever
Length
Width
Thickness
Force Constant
Resonance Frequency
CONT
500µm
85µm
5µm
2 N/m
25kHz
2) Creating an electric contact to the probe
To perform electrochemical measurements, the apex of an EC-Probe has to be electrically
connected with measurements electronics (e.g. a (bi-)potentiostat) by a customer. We recommend
the following procedure reported by Pobelov et al. [2].
Precautions: Always use tweezers made from a conductive polymer to handle probes. When
handling wired cantilevers, do not pull wire to detach a cantilever from the gel box. Firmly hold a chip
by tweezers when placing a probe into a desired position.
Materials: copper wire insulated with polyurethane BLOCK CUL, diameter 0.15 mm;
two-component conductive epoxy Circuit Works 2400.
Equipment: A soldering iron, an oven allowing a temperature of 110 °C.
Procedure:
1) Place an EC-Probe on a mechanically and thermally stable support. A standard black gel box
used to pack NANOSENSORS cantilevers without a plastic lid may serve this purpose.
2) Cut a piece of BLOCK CUL wire, which is sufficiently long for your purposes. Typically it should be
at least 5 cm long. Heat the very end of the copper wire (not more than 1 to 2 mm) with a
soldering iron working at 350 to 400 °C. This procedure removes polyurethane insulation. For a
better electric contact, the end of the wire can be covered with the soldering tin.
3) Thoroughly mix a very small amount of conductive epoxy (ratio 1:1). Use a disposable support
chip and a clean disposable probe (e.g., a plastic pipette tip) for mixing. Pick a very small drop
of conductive epoxy (1 to 2 mm in diameter) by the exposed end of the copper wire. Take the
wire by tweezers and hold it horizontally. Carefully place the wire on the support so that the drop
of epoxy is located above the contact pad. After this, the wire should rest firmly on the support.
Avoid using too much epoxy and moving the wire; this will increase the exposed conductive area
on top of the chip and make its insulation more complicated.
4) Place the support chip with cantilevers in the oven. Cure epoxy at 110 °C for one hour. This
curing procedure creates an electric contact between the inner conductive layer of the
EC-Probe and the insulated wire, which also have high mechanical stability.
The contacting procedure can be easily extended to multiple cantilevers by repeating steps 1 to
3. The wired cantilevers can be stored in a gel box.
Platinum line
Doped silicon
Isolation
Platinum nano-electrode
Figure 2: Sketch of a wired and insulated
NANOSENSORSTM EC-Probe.
3) Insulating the electric contact on the probe
Normally a whole probe is immersed into an electrolyte solution during electrochemical AFM
measurements. To avoid electrochemical currents originating from the contact area on the chip, the
latter one should be insulated from the working solution. To date, no reliable and well-tested insulation
procedure was reported. The following insulation strategies may be applied. In all cases, the
employed coating should not release any components into the working solution and thus
contaminate it.
1) Insulating with a drop of non-conductive epoxy:
In this strategy, a drop of insulating epoxy is placed on top of the conductive epoxy and exposed wire
and subsequently cured. An example of suitable epoxy is the two-component DEVCON® 5 Minute®
Epoxy. Do not use Araldit for this purpose.
Advantages: Easy procedure.
Disadvantages: A typical epoxy mixture has a rather high viscosity. It is difficult to create a thin
layer of insulation. Too thick insulation might prevent cantilever from contacting the sample (see
Troubleshooting 4).
2) Insulating with a thin layer:
In this strategy, a thin insulating layer is applied to the contact area. Examples include coating by an
electrophoretic paint [2], parylene [3] or fluorocarbon [4] films.
Advantages: No problems due to the high thickness of the insulation layer. The applied insulation
might also cover small defects on the chip.
Disadvantages: Relatively complicated procedures. A thin insulation layer might be scratched
during the handling of the cantilever and expose the contact area.
3) Insulating a mounted probe:
Unlike in the case of 1) and 2), in this strategy a wired probe is first mounted into an AFM scanner, then
all exposed conductive parts are insulated. Note: due to the obvious reasons, it should be possible
to remove the insulating film from the scanner. Examples of suitable coatings include polystyrene [5]
and Reprorubber® Thin Pour resin.
Advantages: Avoids problem of insulation damaging due to the handling of the cantilever. A
faulty coating can be reapplied.
Disadvantages: Cannot be applied in advance, cannot be paralleled.
4) Troubleshooting
Please take following hints into account to avoid any undesired measurement artefacts or probe
damage!
No.
Problem
Consequence
1
Conductive glue
contacts beside
the contact pad
and the wire also
the bare silicon of
the support chip
side wall and/or
back side
• Successful
insulation is hardly
possible
2
Insulating film
does not fully
cover the
contact area
• Bad insulation
• High leakage
current
3
Insulating film
touches the
cantilever
• Cantilever might
be broken or its
mechanical
properties
significantly altered
4
Insulating film is
too thick
• Insulation layer and
not the tip touches
the surface during
the probe approach
5) Acknowledgments
Ilya V. Pobelova and Miklós Mohosa acknowledge support of the Swiss National Science Foundation
(200020_144471, NFP 62, Sinergia CRSII2 126969/1), Swiss Commission for Technology and
Innovation (13696.1), COST Action TD 1002 and MOLESCO (606728).
6) References
[1] A Wain, A Pollard, C Richter. Anal. Chem. 86 (2014) 5143–5149
http://dx.doi.org/10.1021/ac500946v
[2] IV Pobelov, M Mohos, K Yoshida, V Kolivoska, A Avdic, A Lugstein, E Bertagnolli, K Leonhardt, G
Denuault, B Gollas, T Wandlowski. Nanotechnology 24 (2013) 115501
http://dx.doi.org/10.1088/0957-4484/24/11/115501
[3] A Kueng, C Kranz, A Lugstein, E Bertagnolli, B Mizaikoff. Angew. Chem. Int. Ed. 42 (2003)
3238–3240.
http://dx.doi.org/10.1002/anie.200351111
[4] J Wiedemair, B Balu, JS Moon, DW Hess, B Mizaikoff, C Kranz. Anal. Chem. 80 (2008) 5260–5265.
http://dx.doi.org/10.1021/ac800246q
[5] JV Macpherson, PR Unwin, AC Hillier, AJ Bard. J. Am. Chem. Soc. 118 (1996), 6445–6452.
http://dx.doi.org/10.1021/ja960842r
Ver.080914