electrode (the cathode) attracts positive charges and the other electrode (the anode) at­

tracts negative charges. As these charges separate, they create an electric field across the

insulator. The capacitance of this system is established by the degree to which the charges

remain stored, and this depends on the permittivity of the insulator, the separation

distance between the two conductors (i.e., the insulator’s thickness), and the area of the

electrodes.

In capacitance biosensing, this two-electrode system is interfaced with cells, biomole­

cules, and/or liquid samples. Introducing these species within the system leads to local

perturbations in effective permittivity at the interface due to the species’ dielectric prop­

erties. These perturbations manifest themselves as a change in capacitance, and this change

is the basis for detecting events at the interface between the electrodes. For this reason, we

term this method “interfacial capacitance sensing.”

Capacitance sensing has been shown in a variety of bioassays ranging from pH mea­

surement [14], DNA detection, biological cell sensing [15], particle migration [16], and

length estimation of artificial muscles [17]. When implemented in CMOS processes, ca­

pacitance biosensing provides a compact means for label-free sensing. In the following

subsections, we introduce several constitutive elements of a CMOS interfacial capacitance

sensor, and we discuss an example architecture from our work.

6.4.1 Transducers for Interfacial Capacitance Sensing

In integrated circuit settings, capacitance transducers are typically implemented as in­

terdigitated electrodes formed using the chip’s top metal layer. The electrodes are pas­

sivated, i.e., they remain covered by the final insulating coating applied to the silicon

wafer during the CMOS fabrication process [18,19]. This geometry results in a significant

fringe field that passes through the sample when the electrodes are energized.

Perturbations at the interface are thus mapped directly to the capacitance of the electrode

system, and an underlying circuit in the CMOS substrate senses the induced change in

capacitance. Figure 6.4 illustrates this example configuration. The electrodes can trans­

duce cell activity by monitoring the coupled interfacial capacitance which is modeled as a

FIGURE 6.4

Schematic of a CMOS capacitance sensor configured for cell analysis. The electrodes form an interdigitated pair

of conductors. The capacitance at the interface may be read by a circuit in the silicon substrate (denoted NMOS

and PMOS for simplicity). The capacitors CPi and CPj are parasitic capacitances.

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Bioelectronics