Page 204 - An Introduction to Microelectromechanical Systems Engineering

Microelectrode Arrays 183

operation in electrolytic capacitors. A simple electrical model for the microelec-
trode consists of a capacitor in series with a small resistor that reflects the resis-
tance of the electrolyte in the vicinity.
The fabrication of microelectrode arrays first involves the deposition of an insu-
lating layer, typically silicon dioxide, on a silicon substrate (see Figure 6.9). Alterna-
tively, an insulating glass substrate is equally suitable. A thin metal film is sputtered
or evaporated and then patterned to define the electrical interconnects and elec-
trodes. Gold, iridium, and platinum, being very chemically inert, are excellent
choices for measuring biopotentials as well as for electrochemistry. Silver is also
important in electrochemistry because many published electrochemical potentials
are referred to silver/silver-chloride electrode. It should be noted that wire bonding
to platinum or iridium is very difficult. If the microelectrode must be made of such
metals, it is necessary to deposit an additional layer of gold over the bond pads for
wire bonding. The deposition of a silicon nitride layer seals and protects the metal
structures. Openings in this layer define the microelectrodes and the bond pads. The
following sections describe two instances where microelectrodes show promise as a
diagnostics tool in biochemistry and biology.

DNA Addressing with Microelectrodes
A unique and novel application patented by Nanogen of San Diego, California [25],
makes use of microelectrode arrays in the analysis of DNA fragments of unknown
sequences. The approach exploits the polar property of DNA molecules to attract
them to positively charged microelectrodes in an array. The analysis consists of two
sequential operations, beginning first with building an array of known DNA cap-
ture probes over the electrode array, followed by hybridization of the unknown
DNA fragments. DNA capture probes are synthetic short chains of nucleotides of
known specific sequence.
Applying a positive voltage to a selection of microelectrodes in the array attracts
previously synthesized DNA capture probes to these biased electrodes, where they
chemically bind in permeable hydrogel layer that had been impregnated with a cou-
pling agent (see Figure 6.10) [26]. Microelectrodes in the array that are negatively
biased remain clear. Subsequent washing removes only unbound probes. Immersion

Microelectrode (e.g., Au, Pt, Ir, Ag)
Silicon nitride
Metal bondpad (e.g., Au)
R
C

Silicon oxide

Silicon

Figure 6.9 Cross section of a microelectrode array showing two different metals for the elec-
trodes and for the bond pads. The schematic also illustrates a basic electrical equivalent circuit that
emphasizes the capacitive behavior of a microelectrode. The silicon substrate and the silicon diox-
ide dielectric layer may be substituted by an insulating glass substrate.