human body [31]. Other examples of implantable actuator systems are drug dosage devices

for hormones such as insulin. The first systems have been used as semi-implants in the form

of an insulin pump for the treatment of type I diabetes [13], but fully implantable systems

are also under development.

21.3 Biocompatibility

The insertion of microelectronic systems into the body of humans or other mammals is a

challenging task because the underlying material systems have emerged from different

development processes and are largely incompatible with each other. For example, the

tertiary and quaternary structure adopted for the function of proteins is generally de­

stroyed when they come into contact with engineered surfaces [32,33]. On the other hand,

the thin-film stacks from which microelectronic chips are constructed tend to corrode and

become defunctionalized when they are in contact with electrolyte solutions for extended

periods [27,34–36].

In particular, the metallic conductive electrode layers made of Al, Ti, W, and CoSi2 as

used in CMOS chips degrade within a short period [34,35]. Figure 21.4 shows the cor­

rosion of a meander-shaped conductor track made of Al:Cu on SiO2, which was stored

for a few days in saline. Of all the materials used in CMOS technologies, only the ceramic,

yet electrically highly conductive titanium nitride TiN is sufficiently stable to withstand

the harsh environmental conditions in body fluids over extended periods [34–36]. Depth-

resolved XPS studies have shown that the effect is associated with a 1–2 nm thin oxidized

surface, i.e., the presence of an ultrathin TiO2 layer [34]. Accordingly, thin TiN films are

well suited for coupling voltages into electrolyte solutions. However, currents can only

be introduced as dielectric displacement currents and not in the form of charges crossing

the interface because the surface TiO2 coating acts as an insulator. This explanatory

model correlates well with other observations such as (i) the very low charge transfers –

compared to gold electrodes – observed for TiN layers as working electrodes in an

electrochemical cell [34] or (ii) the use of 30 nm thin TiO2 layers for hermetic en­

capsulation of implantable needles for voltage measurement in neuronal tissues [37].

However, not only electrically conductive electrode surfaces but also structural mate­

rials such as the SiN-based passivation layer are corroded in body fluid. In the backend

FIGURE 21.4

Optical micrographs of metallic meander structures of TiN/Al:Cu/TiN multilayer stack. Both figures show the

same chip section, (top) initial (bottom) after five days in 0.9% saline.

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Bioelectronics