Flexible electronics | EPFL Graph Search

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use. Manufacturing Flexible printed circuits (FPC) are made with a photolithographic technology. An alternative way of making flexible foil circuits or flexible flat cables (FFCs) is laminating very thin (0.07 mm) copper strips in between two layers of PET. These PET layers, typically 0.05 mm thick, are coated with an adhesive which is thermosetting, and will be activated during the lamination process. FPCs and FFCs have several advantages in many applications:

Tightly

Engineered substrates with embedded strain relief for stretchable thin-film electronics

Alessia Romeo

Stretchable electronics are integrated circuitry that can reversibly expand and relax while retaining their functionality. This emerging technology has great potential in unconventional electronic application areas, especially in biomedical sector. To attain stretchability, new class of substrates, based on highly elastic polymers, have to be used. The resulting stretchable circuits are hybrid systems, combining hard functional materials on soft substrates. However, manufacturing of stretchable electronics sets new challenges due to significantly different material properties between the stretchable substrates and electronic devices, and,therefore, optimized mechanical architectures are required. In this thesis a simple and robust approach to design and manufacture stretchable substrate for stretchable electronic applications is reported. The proposed solution is based on engineered stretchable substrates with embedded strain relief. The substrate is engineered by embedding stiff platforms within a soft elastomer. Since the platforms are signicantly stiffer than the surrounding silicone matrix, the stiff regions distributed across the substrate allow for large global strains with local areas that remain un-strained. These local, un-strained regions act as non-stretchable areas on the plain top surface of the elastomer, which can host brittle devices and protect them from exceeding their fracture strain. Optimization of the platforms geometry and layout, as well as, grading of the mechanical compliance at the rigid-to-soft transition zones have been performed to adjust the strain distribution on the top surface. Associated design rules to produce stretchable circuits based on experimental as well as modeling data are presented. This innovative approach is compatible with conventional, additive thin-film processing. Direct integration of metal oxide thin film transistors onto the planar but mechanically engineered heterogeneous elastic substrate is demonstrated. IGZO TFTs interconnected with stretchable metallization spanning across the rigid platform were manufactured directly on the non deformable elastomer regions, using standard, dry and low temperature processing. IGZO TFT could sustained applied tensile strain up to 20% without electrical degradation and mechanical fracture. Elastomeric substrates with engineered strain relief are, thus, a promising solution to carry reliable and durable stretchable electronics. Additionally, concurrent mechanical and structural photopatterning in photosensitive elastomer was discovered. To date, photosensitive elastomers have mainly been implemented in soft lithography or to locally modulate the elasticity. This thesis demonstrates that shape and stiffness can be engineered in a single elastomeric membrane using simple UV exposure through standard photolithography mask. Modulating the UV dose defines mechanical stiffness gradient within the elastomer as well as topography across the elastomer surface. This process eliminates lithography wet development step therefore offers a new technique to silicone microstructuring, allowing for mold and development free 3D patterning of microfluidic channels. Therefore, single-step photopatterning of photosensitve elastomers enables for rapid prototyping approach for soft MEMS, micro uidics and stretchable electronic.

EPFL2015

Thermal scanning probe lithography for 2D material-based gas sensing

Berke Erbas

Recently, two-dimensional (2D) material based gas sensing, especially transition metal dichalcogenide-based sensing, has been widely investigated thanks to its room temperature sensing ability. Unlike metal oxide based sensors, 2D material-based sensing can be carried out without a heater, therefore it reduces the power consumption and makes them promising candidates for smartphones and wearable electronic devices. Besides gas sensing ability, these 2D materials are interesting and promising candidates for electronics and optoelectronics applications. For example, monolayer molybdenum disulfide (MoS2), which is a 2D semiconductor with direct bandgap, is one of the promising candidates for transistor but it has low mobility compared to silicon. To replace silicon-based devices, strain engineering of MoS2 is widely investigated. It is observed that tensile strain tunes the bandgap of MoS2, enhances carrier mobility and improves NO2 gas sensing by piezotronic effect. Different strategies have been used to strain MoS2 such as flexible substrates to stretch the exfoliated material on top of them, silicon/ dielectric substrates with surface roughness or silicon/dielectric substrates patterned with nanocone structure to locally strain transferred MoS2. Herein, field effect transistor with three-dimensionally patterned gate oxide is designed, fabricated and electrically characterized for sensing application. Thermal scanning probe lithography, which is a direct three-dimensional nanofabrication technique, is used for the fabrication of sinusoidal surfaces. By transferring the exfoliated MoS2 flakes on top of these sinusoidal gate oxide, biaxial strain is induced. Unlike counterparts, thermal scanning probe lithography-based patterning provides us design flexibility, which controls the direction of strain and the strain rate. As a result of Raman spectroscopy, the shifts in out-of-plane vibration (A1g) and in-plane vibration (E1 2g) modes are up to 0.87 cm−1 and 2.60 cm−1, respectively. According to the theoretical Raman spectra calculations of strained MoS2 [1], these shifts correspond to an average strain of 0.58% in a circular area with 1 μm diameter. The strain of 0.58% is obtained for a sinusoidal pattern having a 23 nm amplitude and a 440 nm period. Besides, as a result of photoluminescence characterization, it is shown that an average strain of 0.39% leads to a 35 meV redshift for A excitation peak position. Consequently, the strain of MoS2 on the surface patterned by thermal scanning probe lithography tunes the bandgap of MoS2 by 90.11 meV/%.

2021