Page 123 - Organic Electronics in Sensors and Biotechnology

100 Cha pte r T h ree

Kapton (50 μm) Au

Titanium p-doped pentacene
FIGURE 3.6 Cross section of the organic semiconductor strain sensor.
(Reprinted with permission from Ref. 12. Copyright 2003, IEEE.)

Next, a 50 nm thick pentacene layer was deposited, again by thermal
evaporation. The pentacene layer was then doped p-type by exposure
to a 1% solution of ferric chloride in water. The maximum process
temperature used to fabricate the organic strain sensors is 110°C.
The devices were tested using a Wheatstone bridge configura-
tion, and the results indicate that it is possible to fabricate at low tem-
perature a strain sensor with mechanical characteristics matched to
low-Young-modulus substrates using organic semiconductors.
13
Jung et al. have also demonstrated the possibility of combining
these sensors with pentacene-based thin-film transistors as temper-
ature sensors. The strain sensor consists of a Wheatstone bridge
structure where the pentacene film acts as sensing layer of a strain
gauge, while the temperature sensors adopt a bottom-contact penta-
cene transistor configuration in which the variations of the drain
currents in the subthreshold regime are measured vs. temperature.
The effects of strain on pentacene transistor characteristics while
changing the bending radius of the structure have been investigated
by Sekitani and coworkers. A cross section of their device structure
14
is shown in Fig. 3.7.
First, a gate electrode consisting of 5 nm Cr and 100 nm Au was
vacuum-evaporated on a 125 μm thick poly-ethylenenaphthalate
(PEN) film. Polyimide precursors were then spin-coated and cured
at 180°C to form 900 nm thick gate dielectric layers. A 50 nm thick

FET Capacitor
Au(s) Au(d)
Pentacene Au

Base film (PEN)

Polyimide Au(g) Au
FIGURE 3.7 Cross section of the organic FET and capacitor reported in Ref. 14.
(Reprinted with permission from Ref. 14. Copyright 2005, American Institute of
Physics.)