Conformal Electronics on Elastomers: Packaging Methods and Design Rules
Lakshminarayanan, Oppili Prasad
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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.