Flexible Ultralow-Power Sensor Interfaces for E-Skin

Thin-film electronics has hugely benefitted from low-cost processes, large-area processability, and multi-functionality. This has not only stimulated innovation in display and sensor technology, but has also demonstrated great potential for integration of components for human-machine interfaces. For electronics to be deployed as sensor interfaces and signal processing, the quest for low power is compelling due to the inherently limited battery lifetime. This review will present the state-of-the-art in thin film electronics and demonstrate examples of low-cost printable transistors and biosensors that are flexible/stretchable for wearable and other applications. Ultralow power design for thin-film transistors will be discussed from the standpoint of reducing both operating voltage and operating current, taking into account the challenges in meeting frequency requirements. Compact models for circuit design will be reviewed along with new insights into ultralow power transistors and high gain amplifier circuits. Finally, a concept for an integrated system comprising sensors and interfacing circuits will be demonstrated, which has the potential to enable battery-less operation.EPSRC under Project EP/M013650/1 EU under Projects DOMINO 645760, 1D-NEON 685758 and BET-EU 692373 IEEE Electron Devices Society PhD Student Fellowship China Scholarship Counci

Development of Fully Printed Oxide Electronics for Flexible Substrates

Mit dem Erscheinen des Internets der Dinge (IoT) und dem Wunsch Alltagsgegenständeintelligenter zu machen, steigt die Nachfrage an mechanisch flexiblen oder in Textilien integrierbare Sensoren dieser nächsten Generation. Schon jetzt werden unzählige dieser Sensoren benötigt, um den immer größer werdenden Bedarf zu decken. Aus diesem Grund sind kostengünstige Herstellungsprozesse, die eine großflächige Herstellung auf einem flexiblen Substrat erlauben von Interesse. Das Drucken funktionaler Tinten erlaubt die Herstellung kostengünstiger elektronischer Bauteile auf flexiblen Substraten. Das vollständige Drucken von elektronischen Bauelementen hat in den letzten zwei Jahrzehnten immer mehr an Bedeutung gewonnen, dank einfacher und kostengünstiger Skalierung dieser gedruckten Bauelemente. Um den komplexen Anforderungen zukünftiger Sensoren gerecht zu werden müssen gedruckte elektronische Elemente wie beispielsweise Kondensatoren oder Transistoren auf flexiblen Substraten, integriert werden. Flexible, überwiegend p-leitende, organische Halbleiter sind bis zum jetzigen Zeitpunkt weit verbreitet, sind aber unter normalen Umgebungsbedingungen instabil und weisen eine geringe Ladungsträgerbeweglichkeit auf. Metalloxide, die ebenfalls n-leitende Halbleiter sind, können die Nachteile von organischen Materialien überwinden und zudem auf flexiblen Substraten, wie z.B. PET oder PEN aufgebracht werden. Nichtsdestotrotz wird die Herstellung von vollständig gedruckten Schaltungen auf flexiblen Substraten von vielen verschiedenen Faktoren beeinflusst. Insbesondere ist es wichtig, dass die Bauteile auf dem Substrat haften, kompatibel mit anderen elektronischen Bauelemente und die Prozesstemperaturen auf das Substrat abgestimmt sind. Auch der Einfluss von Leistungsparametern wie der Ladungsträgerbeweglichkeit, der mechanischen Stabilität oder der Reproduzierbarkeit sind nicht zu vernachlässigen. Die elektronischen Komponenten einer Schaltung beinhalten elektrisch leitende Elemente (Elektroden und elektrische Verbindungen), die leitende Tinten wie Silber oder Graphen), Halbleiter basierend auf Metalloxiden (ZnO, In2O3) und elektrische Isolatoren wie beispielsweise als Gate-Dielektrikum dienende Elektrolyte benötigen. Daher müssen die zuvor aufgeführten Materialien unter Berücksichtigung der physikalischen Gegebenheiten des Substrates in einem Druckprozess integriert werden. Des Weiteren müssen die chemische und elektrische Kompatibilität aller Komponenten berücksichtigt werden, um die Leistung der gedruckten Schaltung zu maximieren. Daher liegt der Fokus der vorliegenden Arbeit auf der Entwicklung von gedruckten (Metall)Oxid-Transistoren und Logikgattern auf flexiblen Substraten. Insbesondere auf die Architektur der Transistoren wird hierbei eingegangen. Durch ein Elektrolyt isolierte Transistoren, mit Indiumoxid als aktiven Semikonduktor und den durch konduktive Graphentinte realisierten Elektroden und elektrischen Verbindungen als passive Komponenten, werden vollständig auf ein Glassubstrat gedruckt. Verschiedene Transistorarchitekturen werden analysiert, um Kontaktwiderstände und chemische Reaktionen zu minimieren und gleichzeitig die aktive Fläche im Halbleiter zu maximieren. Ein besonderes Augenmerk liegt dabei auf den Schlüsselparametern der gedruckten Transistoren wie beispielsweise Kapazitäten, Schaltgeschwindigkeit, Ein-, Ausschaltströme und Schwellwertspannung, die für eine große Anzahl an Transistoren analysiert wird. Die Isolation der Transistoren mit einem Elektrolyten erlaubt es, die eingesetzten Versorgungsspannungen auf 1 V zu reduzieren und als gedruckte Batterien zur Spannungsversorgung einzusetzen. Diese Transistoren zeigen mit nur 0.33Ωcm einen deutlich niedrigeren Kontaktwiederstand als Transistoren mit konventionellem Dielektrikum. Um die Transistoren für den potentiellen Einsatz in elektronische Schaltungen zu untersuchen, werden Logikgatter in Form von Invertern in der Widerstands-Transistor-Logik (TRL) gedruckt und analysiert. Um diese Gatter auf ein mechanisch flexibles Polyamid Substrat zu drucken, wird besondere Aufmerksamkeit auf die Vorbereitung des Substrats und die Ermittlung kritischer Parameter wie die Signalverstärkung und Signallaufzeiten gelegt. Die Signalverstärkung von 3.5 ist für diese Inverter identisch zu Invertern welche auf unflexiblen Substrate gedruckt sind. Des Weiteren wird die mechanische Flexibilität von Indiumoxid auf dem Polyamidsubstrat über mehrere Zyklen hinweg untersucht. Abschließend, für die Kompatibilität mit den PEN-Substrate wird die Prozesstemperatur vom Indiumoxid reduziert. Das Formen eines Indiumoxid-Films wird dabei durch optische anstatt thermischen Methode erzielt, wodurch die Prozesszeit von 2 Stunden auf etwa 20 Millisekunden reduziert werden kann. Zusammenfassend beschäftigt sich die vorliegende Arbeit mit der Entwicklung von vollständig gedruckten elektronischen Bauelementen, die auf Metalloxiden basieren und die auf preisgünstigen Plastiksubstraten prozessiert werden., Diese Entwicklung ebnet den Weg für den zukünftigen Einsatz in digitalisierten und role-to-roll kompatiblen elektronischen Anwendungen wie Großraum Displays und tragbaren Sensoren

Patterning methods for organic electronics

Organic electronics is an exciting new avenue for low cost electronics. The unique properties of organic semiconductors may enable a new generation of electronic devices to be fabricated into flexible, large area, and even transparent consumer products. However, for this to become a reality, many challenges must first be overcome. As the performance of these materials continues to improve, it is now necessary to look to new manufacturing methods and materials that can fully exploit the advantages of organic materials. The work presented in this thesis is focused on the development of new and high resolution fabrication methods which are compatible with organic electronic materials. The findings presented in the first half of this thesis are based on the idea that fundamentally new forms of manufacturing are required to match the unique properties of organic materials. Initially the adhesion properties of several materials are analysed with a focus on how they interact at the nano-scale. Further work then outlines how adhesion forces can be manipulated and used to produce highly aligned nano-scale electronic devices, something that until now has required high cost and specialist equipment. The second part of this thesis describes how existing fabrication methods can be modified to produce high performance organic devices. By creating self-aligned organic transistors, higher frequency device operation and enhanced performance may be possible. New materials such as graphene and low voltage nano-scale dielectrics are tested in this configuration and compared with similar devices reported in the literature.Open Acces

Self-Aligned Patterning Methods for Large-Area Electronics

Printed electronics is studied as an alternative to conventional electrics, especially for large-area applications, such as organic light emitting diode (OLED) lighting panels. The whole technology, however, suffers from a low resolution and registration accuracy in the printing process, limitations that directly affect the performance of the applications. Photolithography can overcome these limitations and provide both good registration accuracy and resolution, but it is a challenging process in high throughput fabrication. Thus new fabrication methods are being studied intensively to replace expensive lithography steps in the fabrication chain.This thesis presents two alternative fabrication methods with a scale-up capacity for high volume production. The first combines fast low-resolution patterning with roll-to-roll scalable high resolution microcutting; the second was developed to accurately align dielectric patterns on conductors. The latter uses an electric current to heat metal lines and cure a polymer dielectric locally near the conductor. Uncured polymer is rinsed away, leaving an aligned dielectric on the lines. The method is well suited for passivating OLED anode grid lines, which require excellent registration accuracy to prevent significant losses in the device active area.The layer-to-layer registration accuracy of Joule heating is defined by heat conduction in the substrate. Thus the accuracy can be increased either by selecting a thermally low conductive substrate or by using short current pulses. The latter method allows more freedom to design the other process parameters and materials. An optimal pulse length depends on the substrate material in that materials with high thermal conductivity require short heating pulses. Here though the pulse lengths are of the order of milliseconds, which are easy to produce. In addition to increased registration accuracy, pulsed heating significantly cuts down the processing time and required energy.In this thesis, a dielectric registration accuracy of 2 µm was demonstrated on shadow- mask-evaporated silver lines on glass, a value similar to that reported for registration accuracy in roll-to-roll photolithography, 1 µm. Joule heating, however, does not require challenging alignment steps. To demonstrate the feasibility of Joule heating for passivation in an OLED device, a printed silver current distribution grid was passivated using pulsed Joule heating and fabricated as an OLED device.The Joule heating work constituted not only an experimental study but involved extensive finite element simulations to obtain design rules for the current distribution grid and to study the heating selectivity. The idea of the pulsed heating was also a result of this work

Technologies for printing sensors and electronics over large flexible substrates: a review

Printing sensors and electronics over flexible substrates is an area of significant interest due to low-cost fabrication and possibility of obtaining multifunctional electronics over large areas. Over the years, a number of printing technologies have been developed to pattern a wide range of electronic materials on diverse substrates. As further expansion of printed technologies is expected in future for sensors and electronics, it is opportune to review the common features, complementarities and the challenges associated with various printing technologies. This paper presents a comprehensive review of various printing technologies, commonly used substrates and electronic materials. Various solution/dry printing and contact/non-contact printing technologies have been assessed on the basis of technological, materials and process related developments in the field. Critical challenges in various printing techniques and potential research directions have been highlighted. Possibilities of merging various printing methodologies have been explored to extend the lab developed standalone systems to high-speed roll-to-roll (R2R) production lines for system level integration

Towards green 3D-microfabrication of Bio-MEMS devices using ADEX dry film photoresists

Current trends in miniaturized diagnostics indicate an increasing demand for large quantities of mobile devices for health monitoring and point-of-care diagnostics. This comes along with a need for rapid but preferably also green microfabrication. Dry film photoresists (DFPs) promise low-cost and greener microfabrication and can partly or fully replace conventional silicon-technologies being associated with high-energy demands and the intense use of toxic and climate-active chemicals. Due to their mechanical stability and superior film thickness homogeneity, DFPs outperform conventional spin-on photoresists, such as SU-8, especially when three-dimensional architectures are required for micro-analytical devices (e.g. microfluidics). In this study, we utilize the commercial epoxy-based DFP ADEX to demonstrate various application scenarios ranging from the direct modification of microcantilever beams via the assembly of microfluidic channels to lamination-free patterning of DFPs, which employs the DFP directly as a substrate material. Finally, kinked, bottom-up grown silicon nanowires were integrated in this manner as prospective ion-sensitive field-effect transistors in a bio-probe architecture directly on ADEX substrates. Hence, we have developed the required set of microfabrication protocols for such an assembly comprising metal thin film deposition, direct burn-in of lithography alignment markers, and polymer patterning on top of the DFP

Towards Green 3D-Microfabrication of Bio-MEMS Devices Using ADEX Dry Film Photoresists

Current trends in miniaturized diagnostics indicate an increasing demand for large quantities of mobile devices for health monitoring and point-of-care diagnostics. This comes along with a need for rapid but preferably also green microfabrication. Dry film photoresists (DFPs) promise low-cost and greener microfabrication and can partly or fully replace conventional silicon-technologies being associated with high-energy demands and the intense use of toxic and climate-active chemicals. Due to their mechanical stability and superior film thickness homogeneity, DFPs outperform conventional spin-on photoresists, such as SU-8, especially when three-dimensional architectures are required for micro-analytical devices (e.g. microfluidics). In this study, we utilize the commercial epoxy-based DFP ADEX to demonstrate various application scenarios ranging from the direct modification of microcantilever beams via the assembly of microfluidic channels to lamination-free patterning of DFPs, which employs the DFP directly as a substrate material. Finally, kinked, bottom-up grown silicon nanowires were integrated in this manner as prospective ion-sensitive field-effect transistors in a bio-probe architecture directly on ADEX substrates. Hence, we have developed the required set of microfabrication protocols for such an assembly comprising metal thin film deposition, direct burn-in of lithography alignment markers, and polymer patterning on top of the DFP

Flexible and Stretchable Electronics

Flexible and stretchable electronics are receiving tremendous attention as future electronics due to their flexibility and light weight, especially as applications in wearable electronics. Flexible electronics are usually fabricated on heat sensitive flexible substrates such as plastic, fabric or even paper, while stretchable electronics are usually fabricated from an elastomeric substrate to survive large deformation in their practical application. Therefore, successful fabrication of flexible electronics needs low temperature processable novel materials and a particular processing development because traditional materials and processes are not compatible with flexible/stretchable electronics. Huge technical challenges and opportunities surround these dramatic changes from the perspective of new material design and processing, new fabrication techniques, large deformation mechanics, new application development and so on. Here, we invited talented researchers to join us in this new vital field that holds the potential to reshape our future life, by contributing their words of wisdom from their particular perspective

hybrid materials for integrated photonics

In this review materials and technologies of the hybrid approach to integrated photonics (IP) are addressed. IP is nowadays a mature technology and is the most promising candidate to overcome the main limitations that electronics is facing due to the extreme level of integration it has achieved. IP will be based on silicon photonics in order to exploit the CMOS compatibility and the large infrastructures already available for the fabrication of devices. But silicon has severe limits especially concerning the development of active photonics: its low efficiency in photons emission and the limited capability to be used as modulator require finding suitable materials able to fulfill these fundamental tasks. Furthermore there is the need to define standardized processes to render these materials compatible with the CMOS process and to fully exploit their capabilities. This review describes the most promising materials and technological approaches that are either currently implemented or may be used in the coming future to develop next generations of hybrid IP devices

Design, Fabrication, and Characterization of Conjugated Polymeric Electrochemical Memristors as Neuromorphic/Integrated Circuits

Organic materials are promising candidates for future electronic devices compared to the complementing inorganic materials due to their ease of processability, use, and disposal, low cost of fabrication, energy efficiency, and flexible nature toward implementation as flexible and non-conformal devices.With that in mind, electrochemical materials have been widely demonstrated with commercial use as sensors, displays, and a variety of other electronic devices. As Moore\u27s law predicts the increase in the density of transistors on a chip, the requirement to create either smaller transistors or the replacement of the transistor device entirely is apparent. Memory resistors, coined ``memristor , are variable resistive tuning devices that are capable of information processing and data storage in one device. This work focuses on the embodiment of a non-volatile conjugated polymeric electrochemical memristor. Three-terminal memristive systems are fabricated and studied using various electrochemicals (a self-doped PEDOT derivative, a polypyrrole, and a dithienopyrrole derivative) and are tested for their electronic properties and biomimicking capabilities. Optical absorbance properties are studied in order to verify the electrochemical material\u27s redox tuning potential for their respective oxidized and reduced chemical forms. The three-terminal device employed a post-synaptic ``read\u27\u27 channel where conductivity of the electrochemical material was equated to synaptic weight and was electronically decoupled from the pre-synaptic programming electrode by means of a polymeric gel electrolyte. Basic electronic characteristics are exhibited for these three devices such as state stability and retention, non-volatile voltage-driven conductivity tuning, input parameter characteristic trends, and power consumption per input program. Biological synapses consume, on the order of, 1 - 100 fJ of energy per synaptic energy. The electrochemical materials used in this study, at their most optimized input parameters, were capable of demonstrating a 4.16 fJ/mm2 power consumption per input pulse and lead to a promising candidate for implementation as future artificial neural networks. Biological mimicry was displayed for these devices in the form of paired-pulse facilitation and paired-pulse depression, both a form of short term memory which observes the effect the timescale between two incoming inputs has on the change in the final output signal. Toward the indication for the replacement of transistors with three-terminal memristors, basic circuit operations are achieved and demonstrated for these devices. These operations include both Boolean and elementary algebra, key features that demonstrate data processing and storage in-memory where the physical states of the conjugated polymer film represent either logical statements or arithmetic counting variables. The Boolean algebra demonstrated the use of a single memristive device equal to a variety of single logic gates (AND, NAND, OR and NOR) where, by wiring several devices in series, more advanced combinational logic gates can be achieved. Furthermore, each device was capable of displaying elementary algebra for the basic arithmetic functions of addition, subtraction, multiplication, and division. In regards to thin film deposition techniques, the self-doped PEDOT device employed roll-to-roll gravure printing, a high speed and high resolution commercially used deposition technique. The polypyrrole device was fabricated implementing an in-situ polymerization technique, referred to as vapor phase polymerization, and demonstrated the use of this technique toward non-conformal devices. The dithienopyrrole derivative was polymerized through the same vapor phase polymerization technique as the polypyrrole and used in tandem with screen printing in order to construct the final device, including the oxidant film, the silver electrodes, and the polymeric gel electrolyte