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Smart
textiles or Electronic textiles is the combination of electronics and
textiles into fabrics and clothing which are able to sense, compute,
communicate and so many other features. These days, fabrics are the
new silicon wafers;they have generated much interest due to the
advent of soft computing and portable devices. An applied approach to
fabricate electronic textiles is to combine textile substrates with
rigid printed circuit boards (PCBs), allowing the integration of
transistors, logic gates, sensors, microprocessors, storage units and
other communication interfaces into textiles.

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Amorphous
indium-gallium-zinc-oxide (IGZO) as semiconductor material has
attracted a lot of awareness as the material is quite flexible and
allows the fabrication of various transistors having increased
switching speed compared to amorphous silicon TFTs due to the higher
electron mobility of IGZO. The strips called e-strips fabricated with
these transistors are made and can be situated inside yarn or fabric
for circuit applications. The e-strips are woven into textiles in
weft direction using an industrial weaving machine. These e-strips
have Thin film transistors fabricated on it and can be used to
connect circuits allowing circuitry for various logic gates like
AND,NAND,EXOR etc. The application of flexible e- strips as
substrates for electronic devices allows the integration of a variety
of thin-film devices such as interconnect lines and integrated
circuits (ICs) based on silicon and surface mount devices (SMDs) .

INTRODUCTION

The
convergence of electronics with textiles offers variety of
applications in all scenarios. Electronic or smart textiles also
known as e-textiles assures to have a significant impact in areas
such as wearable computing, large-area electronics and fashion
textiles . Potential areas of application include health care,
sports, fashion industry . Here, the vision is of an e-textile
consisting of a fabric that maintain all the properties of textile
fibers, like comfort, washability, drapability ,softness or
stretchability and combines them with electronic functionality for
better results. The foregoing electronic functionality often refers
to different sensors like for temperature strain, posture, or other
physiological signals but also includes the associated conditioning
circuits, power supply, signal processing or transmission
electronics. To a certain extent, the span of e-textiles ranges from
conventional electronics attached to textiles to electronic
components build from active textile yarns. A measured approach for
fabrication of transistors on fabrics is the integration of flexible
electronics into a woven textile. Here, the use of flexible plastic
stripes as carriers for thin-film devices and standard silicon chips,
represents a good compromise between the electrical and mechanical
properties of the final textile device . Addition to that, the
integration of electronic fibers and conductive yarns in the weft and
warp direction of a woven fabric also enables the fabrication of more
complex systems inside a textile. We can also fabricate the
mechanically flexible active electronic devices directly on circular
fibers.

Additionally,
yarns usable for the fabrication of textiles exhibit diameters
significantly below 1 mm, which results in a highly curved surface.
These challenges can be addressed by new developments in the area of
flexible electronics. In particular the use of oxide semiconductors,
such as amorphous In-Ga-ZnO (IGZO) , promises to realize high
performance active electronic devices on a variety of substrates.
Here, we evaluated how IGZO thin-film transistors (TFTs),
representing the most important

and
basic building block of all electronic systems, can be fabricated on
a variety of different yarns. It is

shown
that high performance TFTs, on glass fibers with a radius of 62.5 ?m
and on polymer fibers with

a
radius of 125 ?m, are fully functional and can be integrated into
textiles for wearable or industrial

applications.

THIN
FILM TRANSISTORS (TFTs)

Flexible
electronics and in particular flexible thin-film transistors are key
candidates for integration into textiles. Amorphous Indium-
Gallium-Zinc-Oxide (IGZO) was presented for the first time in 2004

as a
semiconductor material for thin-film transistors (TFTs) on flexible
plastic substrates. IGZO TFTs are beneficial for fabrication on
flexible plastic substrates, because IGZO can be sputtered at room
temperature while the process temperatures to fabricate drain,
source, gate contact and gate insulators are below 150 C. Process
temperatures not exceeding 300 C are imperative due to temperature
stability of for example polyimide foils. Furthermore, the electron
mobility of room temperature sputtered IGZO is about 10 times larger
than the electron mobility of amorphous silicon . IGZO TFTs can be
designed to operate at 5 V with a threshold voltage around 0.5 V.
Since IGZO TFTs can be fabricated on flexible plastic foils, the
impact of bending and hence applying strain in the TFT layers is
investigated . Bending IGZO TFTs is mainly motivated to enable
flexible displays. Bending radii as small as 3 mm corresponding to
0.7 % strain in the TFT layers are reported. Due to the higher
electron mobility of IGZO TFTs compared to TFTs made of amorphous
silicon, faster switching speeds of the tran- sistors are possible
and the fabrication of circuits becomes attractive. To demonstrate
the switching speed of transistors, ring oscillators are

commonly
applied. In, five-stage and seven-stage ring oscillators were
fabricated and propagation delays per stage of 240 ns and 48 ns are
reported, while operating with supply voltages of 18 V and 25 V,
respectively. Basic circuits such as shift registers, inverters and
NAND gates are presented in 44, 45, 46. 44 demonstrates a shift
register operated at 20 V, while in 45 inverters operating at 10 V
are shown. NAND gates and inverters representing the basic building
blocks for digital circuits are demonstrated in 46. The circuits
operate at 5 V and can be bent to a radius of 3.5 mm corresponding to
a strain of 0.6 %. The system on panel concept, described in 7, to
integrate sensors, actuators and the required control electronics on
a single flexible foil, requires analog circuits and in particular
amplifier circuits. Since mechanical strain changes the
characteristics of single IGZO TFTs, it is important to characterize
the behavior of analog circuits subjected to mechanical strain.

A
thin-film transistor (TFT) is
a special kind of field-effect transistor made by depositing of an
active semiconductor layer as well as the dielectric layer and
metallic contacts over a supporting (but non-conducting) substrate. A
common substrate is glass, because the primary application of TFTs is
in liquid-crystal displays. This differs from the conventional
transistor, where the semiconductor material typically is
the substrate, such as a silicon wafer.

TFTs
can be made using a wide variety of semiconductor materials. A common
material is silicon. The characteristics of a silicon-based TFT
depend on the silicon’s crystalline state; that is, the semiconductor
layer can be either amorphous silicon, microcrystalline silicon,or it
can be annealed into polysilicon.

Other materials which have been used as
semiconductors in TFTs include
compound semiconductors such as cadmium selenide, or metal oxides
such as zinc oxide or hafnium oxide. An application for hafnium oxide
is as a high-? dielectric. TFTs have also been made using
organic materials, referred to as organic field-effect transistors or
OTFTs.

TFT
ON FIBRES

Rather than regular substrates used for the creation of electronic
thin-film devices, for example, semiconductor wafers, glass plates
etc. the mechanical and geometrical properties of fibres and yarns
are less advantageous. Hence, the successful fabrication of
transistors requires a modification of the fabrication process and a
proper selection of suitable yarns or fibers.

a range of possible substrate fibers e.g. steel and cotton yarns,
nylon fibers with different diameters, glass fibers, and thin
insulated metal Cu wire. All materials have certain advantages and
disadvantages concerning the fabrication of smart textiles but here
are most important parameters for the fabrication of TFTs in
electronic textiles:

Chemical
properties

The chemical stability of the fiber material is a key aspect since
the fibers have to resist the etchants and solvents used during the
fabrication process. In this respect the metal and glass fibers
exhibit the most beneficial properties.

Temperature
resistance

The melting or glass transition temperature of the evaluated
materials can significantly limit the choice of usable deposition
technologies. While the maximum temperature of cotton and nylon is in
the range of 200 ?C, the glass fiber can be processed at
temperatures above 1000 ?C.

Fiber
surface

Thin-film devices are made from active layers with thickness in the
nanometer range, hence the surface of the fibers has to be as flat as
possible. While the steel and cotton yarns do not exhibit a
continuous surface, also the surface roughness of the other fibers
varies strongly.

Conductivity

Non-conductive fibers (glass, cotton, nylon) have the advantage that
no additional insulation layer is needed, and all electronic devices
on their surface are decoupled from each other. Metallic substrate
fibers at the same time, could simplify the device structure by
providing electronic functionality themselves. Here an interesting
option could be the use the insulated Cu wire as substrate fiber,
gate contact and gate insulator simultaneously.

Textile
properties

Unobtrusive smart textiles needs electronic fibers which are soft,
bendable, and with dimensions comparable to the textile yarns of the
fabric. In this respect cotton but also steel yarns have beneficial
properties. Similarly, polymer fibers such as nylon are common.
Anyway, the diameter of the nylon fibers should not be too large (
750 ?m ).

Furthermore, thin Cu wires are bendable and can be imperceptible when
integrated into a textile . Glass fibers on the other hand exhibit a
small diameter, but their minimum bending radius is limited to ?5
cm.

FABRICATION
APPROACHES

To
determine the most appropriate manufacturing process, we evaluated
two different approaches to fabricate TFTs on fibers. The first is
direct fabrication of devices on nylon and glass fibers using
standard semiconductor manufacturing equipment. The second one is
transfer of TFTs, fabricated on flat and thin substrates, to
different fibers, and yarns.