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Prototypes of micro/nanotransducers based on a scaled down version of an electrochemical cell

Prototypes of micro/nanotransducers based on a scaling down of an electrochemical impedance spectroscopy cell have been developed.

The chips have been designed with a long rectangular shape (13.0 x 1.0mm{2}), to facilitate the electrical isolation of the sensing part, to be immersed in a liquid solution, from the electrical connection pads. Much of the chip area consists on micron-sized conducting elements and electrodes.

Various designs of microelectrodes have been produced, with either square or circular shapes. The circular electrodes have been designed with the following diameters: 80, 60, 40, 20, 15, 10, 5um and 500nm. This allows a scaling down of the electrochemical impedance spectroscopy measurements.

The microelectrode devices are fabricated on wafer level by using standard microelectronic processes. To obtain nanoelectrodes (500nm), the gold electrodes are left covered with a silicon dioxide layer. This is later opened by a nanolithography process. This can be done by electron beam lithography at a wafer level (a serial process but with 24 chips at a time), followed by oxide etching, or directly by focused ion beam etching each chip. Both individual nanoelectrodes and nanoelectrode arrays have been fabricated. Various final diameters from about 1 micron to less than 500 nm have been successfully obtained.

The platinum layer is used to define counter-electrodes and also reference microelectrodes on chip. To obtain a reference electrode, an additional layer of silver is deposited on top of one of the platinum electrodes. The silver is then electrochemically chlorinated in an HCl solution to obtain an Ag/AgCl reference electrode.

In relation to the chip packaging the requirement of usage in a liquid medium adds some constraints to the packaging design. The wires making electrical contact between the chip and the external system must be protected, but the micro/nano-electrode area should be in contact with the liquid. For this reason the micro- and nano-electrode chips have been defined with a long geometry, to separate the liquid from the electrically active area. The chips have been bonded to specific printed circuit boards (PCB), wire-bonded and covered with a polymer to protect the chip. The wire bonding is further protected by a polymeric (epoxy) cover, which is applied using a mould.

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Reported by

Centro Nacional de Microelectronica (IMB-CSIC)
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