| dc.description.abstract | Flexible Electronics is a multi-disciplinary domain that creates intersections among elec-
tronics, materials-science, mechanics, packaging, sensor-design, etc., to build
exible /
stretchable / conformal electronic circuits and systems. The spectrum of this field is
very broad, and hence the approaches, materials/substrates di er significantly based on
applications and requirements. In the recent years, development of electronic circuits
and systems on wearable elastomeric substrates has gained a lot of research attentions
due to its possible applications in wearable bio-medical devices for clinical diagnostics,
electronic-skin for prosthetic implants, artificial-skin for robotics, etc. Traditionally, prob-
lems in
exible electronics have been handled using two distinct approaches. The former
approach involves fabrication of non-crystalline semiconductor based thin film transis-
tors (TFTs) and integrated circuits on mechanically
exible substrates. This approach
is well-suited for applications like large-area displays, image-sensor arrays, etc. How-
ever, this approach su ers from several issues like poor mobility, threshold voltage shifts,
degradation on exposure to ambience, etc. Hence, this approach is not well-suited for
applications requiring high-performance
exible electronics with wireless capabilities. The
latter approach to
exible electronics involves packaging of conventional crystalline semi-
conductor based integrated-circuit components on elastomeric substrates. This approach
involves heterogeneous integration of rigid circuit components on conformal elastomers
using stretchable interconnects, and hence involves addressing issues related to packag-
ing and reliability. The usage of conventional electronics based circuit elements helps to
build high-speed, wireless systems on elastomers, and hence this approach is well-suited
for wearable electronics applications like clinical diagnostic devices.
Motivated by such applications, the thesis concentrates on this latter approach of
solving packaging problems on elastomers. The thesis begins with an exhaustive literature
survey with regards to this packaging approach and finds some gaps/shortcomings which
lead to several possible research questions such as:
1. How to develop fabrication processes/techniques for conformal electronics, which are
in-line with conventional printed circuit board manufacturing processes, i.e. without
use of specialized clean-room infrastructure, thin-film deposition facilities?
2. How to retain metals like copper for interconnects-layer in
exible/conformal elec-
tronic circuits, so as to achieve an excellent high-frequency operation in these cir-
cuits?
3. How to design specialized meandered geometries for interconnect buses that satisfy
all user constraints relating to: mechanical stress, electrical impedance, band-width,
layout-area, etc.?
These questions, along with the thrust to explore novel end-user applications, form the
basic motivations for this thesis-work.
The objective of this thesis is to develop a generic platform to package electronic circuits
and sensors on conformal elastomeric substrates. The thesis focusses on developing and
understanding packaging techniques and design rules needed for this generic platform.
With regards to packaging of circuits on elastomers, the thesis discusses novel archi-
tectures and fabrication techniques for developing stretchable copper interconnects and
contacts to die-pads on PDMS (Poly Di-Methyl Siloxane) elastomer. The packaging tech-
niques are characterized through relevant experiments, where circuit demonstrations are
shown on elastomers for proof-of-concept.
With regards to design rules for
exible/conformal electronic circuits, an optimization
platform is developed to help choose the best design for stretchable interconnect-buses
used in these circuits. To achieve this, a generic meandered bus topology is proposed for
the interconnect design. The impact of the proposed meandered-geometry on parametric
functions such as mechanical stress, sti ness coe cient, electrical impedance, layout-area,
packaging density, etc. are thoroughly investigated through analysis, simulations and
experiments. The trade-o s relating to mechanical, electrical and layout-area functions
are studied. Intuitive design rules are evolved based on these analysis and trade-o s. An
optimization problem is formulated to help choose the best geometry for the meandered-
buses, that satisfies all the given user-constraints such as: strain, impedance, layout-area,
bus-width.
The final part of the thesis discusses the development of percolation-based sensors
on elastomeric substrates that can sense both strain and temperature. This sensor de-
velopment is targeted towards very specific aerospace applications, where strain range of
0-10,000 micro.strain is of much interest for measurement along surfaces of launch vehicles
and space-crafts.
To summarize, the primary objectives of the thesis involve developing this generic plat-
form for conformal electronic circuits/systems by engineering the building blocks, namely:
i. stretchable interconnects, ii. contacts to die-pads, iii. sensors on elastomeric substrates.
The thesis does not concentrate on specific system-building targeting a particular appli-
cation. Most of the focus is given to understand and engineer these constituent building
blocks for conformal electronics. This platform can be potentially applied for various
end-user applications like: wearable bio-medical devices, smart-textiles, artifical skin for
soft-robotics, electronic skin for prosthetic implants, energy harvesting on elastomers, etc. | en_US |
