Inorganic micro/nanostructures-based high-performance flexible electronics for electronic skin application

Electronics in the future will be printed on diverse substrates, benefiting several emerging applications such as electronic skin (e-skin) for robotics/prosthetics, flexible displays, flexible/conformable biosensors, large area electronics, and implantable devices. For such applications, electronics based on inorganic micro/nanostructures (IMNSs) from high mobility materials such as single crystal silicon and compound semiconductors in the form of ultrathin chips, membranes, nanoribbons (NRs), nanowires (NWs) etc., offer promising high-performance solutions compared to conventional organic materials. This thesis presents an investigation of the various forms of IMNSs for high-performance electronics. Active components (from Silicon) and sensor components (from indium tin oxide (ITO), vanadium pentaoxide (V2O5), and zinc oxide (ZnO)) were realised based on the IMNS for application in artificial tactile skin for prosthetics/robotics. Inspired by human tactile sensing, a capacitive-piezoelectric tandem architecture was realised with indium tin oxide (ITO) on a flexible polymer sheet for achieving static (upto 0.25 kPa-1 sensitivity) and dynamic (2.28 kPa-1 sensitivity) tactile sensing. These passive tactile sensors were interfaced in extended gate mode with flexible high-performance metal oxide semiconductor field effect transistors (MOSFETs) fabricated through a scalable process. The developed process enabled wafer scale transfer of ultrathin chips (UTCs) of silicon with various devices (ultrathin chip resistive samples, metal oxide semiconductor (MOS) capacitors and n‐channel MOSFETs) on flexible substrates up to 4″ diameter. The devices were capable of bending upto 1.437 mm radius of curvature and exhibited surface mobility above 330 cm2/V-s, on-to-off current ratios above 4.32 decades, and a subthreshold slope above 0.98 V/decade, under various bending conditions. While UTCs are useful for realizing high-density high-performance micro-electronics on small areas, high-performance electronics on large area flexible substrates along with low-cost fabrication techniques are also important for realizing e-skin. In this regard, two other IMNS forms are investigated in this thesis, namely, NWs and NRs. The controlled selective source/drain doping needed to obtain transistors from such structure remains a bottleneck during post transfer printing. An attractive solution to address this challenge based on junctionless FETs (JLFETs), is investigated in this thesis via technology computer-aided design (TCAD) simulation and practical fabrication. The TCAD optimization implies a current of 3.36 mA for a 15 μm channel length, 40 μm channel width with an on-to-off ratio of 4.02x 107. Similar to the NRs, NWs are also suitable for realizing high performance e-skin. NWs of various sizes, distribution and length have been fabricated using various nano-patterning methods followed by metal assisted chemical etching (MACE). Synthesis of Si NWs of diameter as low as 10 nm and of aspect ratio more than 200:1 was achieved. Apart from Si NWs, V2O5 and ZnO NWs were also explored for sensor applications. Two approaches were investigated for printing NWs on flexible substrates namely (i) contact printing and (ii) large-area dielectrophoresis (DEP) assisted transfer printing. Both approaches were used to realize electronic layers with high NW density. The former approach resulted in 7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs while the latter approach resulted in 7 NWs/μm with simultaneous assembly on 30x30 electrode patterns in a 3 cm x 3 cm area. The contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with a Wheatstone bridge (WB) configuration. The assembled V2O5 NWs were used to realize temperature sensors with sensitivity of 0.03% /K. The sensor arrays are suitable for tactile e-skin application. While the above focuses on realizing conventional sensing and addressing elements for e-skin, processing of a large amount of data from e-skin has remained a challenge, especially in the case of large area skin. A Neural NW Field Effect Transistors (υ-NWFETs) based hardware-implementable neural network (HNN) approach for tactile data processing in e-skin is presented in the final part of this thesis. The concept is evaluated by interfacing with a fabricated kirigami-inspired e-skin. Apart from e-skin for prosthetics and robotics, the presented research will also be useful for obtaining high performance flexible circuits needed in many futuristic flexible electronics applications such as smart surgical tools, biosensors, implantable electronics/electroceuticals and flexible mobile phones

Soft eSkin:distributed touch sensing with harmonized energy and computing

Inspired by biology, significant advances have been made in the field of electronic skin (eSkin) or tactile skin. Many of these advances have come through mimicking the morphology of human skin and by distributing few touch sensors in an area. However, the complexity of human skin goes beyond mimicking few morphological features or using few sensors. For example, embedded computing (e.g. processing of tactile data at the point of contact) is centric to the human skin as some neuroscience studies show. Likewise, distributed cell or molecular energy is a key feature of human skin. The eSkin with such features, along with distributed and embedded sensors/electronics on soft substrates, is an interesting topic to explore. These features also make eSkin significantly different from conventional computing. For example, unlike conventional centralized computing enabled by miniaturized chips, the eSkin could be seen as a flexible and wearable large area computer with distributed sensors and harmonized energy. This paper discusses these advanced features in eSkin, particularly the distributed sensing harmoniously integrated with energy harvesters, storage devices and distributed computing to read and locally process the tactile sensory data. Rapid advances in neuromorphic hardware, flexible energy generation, energy-conscious electronics, flexible and printed electronics are also discussed. This article is part of the theme issue ‘Harmonizing energy-autonomous computing and intelligence’

Large-Area Soft e-Skin: The Challenges Beyond Sensor Designs

Sensory feedback from touch is critical for many tasks carried out by robots and humans, such as grasping objects or identifying materials. Electronic skin (e-skin) is a crucial technology for these purposes. Artificial tactile skin that can play the roles of human skin remains a distant possibility because of hard issues in resilience, manufacturing, mechanics, sensorics, electronics, energetics, information processing, and transport. Taken together, these issues make it difficult to bestow robots, or prosthetic devices, with effective tactile skins. Nonetheless, progress over the past few years in relation with the above issues has been encouraging, and we have achieved close to providing some of the abilities of biological skin with the advent of deformable sensors and flexible electronics. The naive imitation of skin morphology and sensing an impoverished set of mechanical and thermal quantities are not sufficient. There is a need to find more efficient ways to extract tactile information from mechanical contact than those previously available. Renewed interest in neuromorphic tactile skin is expected to bring some fresh ideas in this field. This article reviews these new developments, particularly related to the handling of tactile data, energy autonomy, and large-area manufacturing. The challenges in relation with these advances for tactile sensing and haptics in robotics and prosthetics are discussed along with potential solutions

Three-dimensional field-effect transistors with top-down and bottom-up nanowire-array channels

This dissertation research effort explores new transistor topologies using three-dimensional nanowire (NW)-array channels formed by both bottom-up and top-down synthesis. The bottom-up NW research centers on the Au-catalyzed planar GaAs NW assembly discovered at the University of Illinois Urbana-Champaign (UIUC). The top-down NW research approach involves plasma etching of an emerging wide-bandgap material, Gallium Oxide (Ga2O3), to make arrays of NW channels (or fins) for high-power electronics. Bottom-up AlGaAs/GaAs heterostructure core-shell planar NWs are demonstrated on a wafer scale with excellent yield. Their placement is determined by lithographically patterning an array of Au seeds on semi-insulating GaAs substrate. The GaAs NWs assemble by lateral epitaxy via a vapor-liquid-solid mechanism and align in parallel arrays as a result of the (100) GaAs crystal plane orientation; then, a thin-film AlGaAs layer conforms to the GaAs NWs to form AlGaAs/GaAs NW high-electron mobility channels. Radio frequency (RF) transistors are fabricated and show excellent dc and high-frequency performance. An fmax > 75 GHz with 104 is measured which is superior compared to carbon-based nanoelectronics and “spin-on III-V NWs”. A comprehensive small-signal model is used to extract the contributing and limiting factors to the RF performance of AlGaAs/GaAs NW-array transistors and predict future performance. Finally, a process is developed to show that III-V NWs on sacrificial epitaxial templates can be transferred to arbitrary substrates. Top-down NWs were formed from Sn-doped Ga2O3 homoepitaxially grown on semi-insulating beta-phase Ga2O3 substrates by metal-organic vapor phase epitaxy. First, conventional planar transistors were fabricated from a sample set to characterize and understand the electrical performance as a function of Sn-doping and epitaxial channel thickness. Second, the high-critical field strength was evaluated to highlight the benefit of using Ga2O3 as a disruptive technology to GaN and SiC. Lastly, the planar transistor results feed into a design for a top-down NW-array transistor. The Ga2O3 NW-arrays were formed by BCl3 plasma etching. A new wrap-gate transistor demonstrates normally-off (enhancement-mode) operation with a high breakdown voltage exceeding 600 V which is superior to any transistor using a 3D channel

Wearable System with Integrated Passive Microfluidics for Real-Time Electrolyte Sensing in Human Sweat

Wearable systems embodied as patches could offer noninvasive and real-time solutions for monitoring of biomarkers in human sweat as an alternative to blood testing, with applications in personalized and preventive healthcare. Sweat is considered to be a biofluid of foremost interest for analysis due the numerous biomarkers it contains. Recent studies have demonstrated that the concentration of some of these biomarkers in sweat, such as the electrolytes studied in this work, can be directly correlated to their concentrations in blood, making sweat a trusted biofluid candidate for non-invasive diagnostics. Until now, the biggest impediment to onâbody sweat monitoring was the lack of technology to analyze sweat composition in realâtime and mainly to continuously collect it. The goal of this work was to develop the building blocks of such wearable system for sweat electrolyte monitoring, with main emphasis on the passive microfluidics, the integrated miniaturized quasi-reference electrode and the functionalization of the sensing devices. The basic sensor technology is formed by Ion Sensitive Field Effect Transistors (ISFET) realized in FinFET and ultra-thin body Silicon on Insulator technology. This thesis shows the development of a state-of-the-art microsystem that allows multisensing of pH, Na+, K+ electrolyte concentrations in sweat, with high selectivity and high sensitivities (â50 mV/dec for all electrolytes), in a wearable fashion. The microsystem comprises a biocompatible skin interface that collects even infinitesimal quantities of sweat (of the order of hundreds of picoliters to tenths of nanoliters), which the body produces in periods of low physical effort. One of the main achievements of this work is the integration of Ion Sensing Fully Depleted FETs and zero power consumption microfluidics, enabling low power (less than 50 nWatts/sensor) wearable biosensing. The thesis presents the needed technological processes and optimizations, together with their characterization, in order to achieve a Lab-On-Skin system

Simulation study of junctionless silicon nanoribbon FET for high-performance printable electronics

High-performance electronics on flexible substrates along with low-cost fabrication by printing has gained interest recently. For this purpose, the printing of inorganic semiconductors based micro/nanostructures such as nanowires etc. are being explored. However, due to thermal budget, the controlled selective source/drain doping needed to obtain transistors from such structure remains a bottleneck post transfer printing. This paper presents an attractive solution to address this challenge. The solution is based on junctionless FETs (JLFET), which do not require selective doping. Unlike conventional JLFETs, which use nanowires, the devices presented here are based on nanoribbons as this enable larger channel width and hence high drive current. Studied through simulation, the JLFETs presented here show high-performance with current high enough to drive micro-LED. The TCAD simulation has been carried out to study the effect of single and dual metal gate (top and bottom side) of JLFETs as well as that of doping and nanoribbon thickness on the electrical characteristics. The simulation results indicate that the proposed devices will be suitable for high performance printable electronics applications