Electromechanical Lifting Actuation of a MEMS Cantilever and Nano-Scale Analysis of Diffusion in Semiconductor Device Dielectrics

This dissertation presents experimental and theoretical studies of physical phenomena in micro- and nano-electronic devices. Firstly, a novel and unproven means of electromechanical actuation in a micro-electro-mechanical system (MEMS) cantilever was investigated. In nearly all MEMS devices, electric forces cause suspended components to move toward the substrate. I demonstrated a design with the unusual and potentially very useful property of having a suspended MEMS cantilever lift away from the substrate. The effect was observed by optical micro-videography, by electrical sensing, and it was quantified by optical interferometry. The results agree with predictions of analytic and numerical calculations. One potential application is infrared sensing in which absorbed radiation changes the temperature of the cantilever, changing the duty cycle of an electrically-driven, repetitively closing micro-relay. Secondly, ultra-thin high-k gate dielectric layers in two 22 nm technology node semiconductor devices were studied. The purpose of the investigation was to characterize the morphology and composition of these layers as a means to verify whether the transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS) could sufficiently resolve the atomic diffusion at such small length scales. Results of analytic and Monte-Carlo numerical calculations were compared to empirical data to validate the ongoing viability of TEM EDS as a tool for nanoscale characterization of semiconductor devices in an era where transistor dimensions will soon be less than 10 nm

Establishment of surface functionalization methods for spore-based biosensors and implementation into sensor technologies for aseptic food processing

Aseptic processing has become a popular technology to increase the shelf-life of packaged products and to provide non-contaminated goods to the consumers. In 2017, the global aseptic market was evaluated to be about 39.5 billion USD. Many liquid food products, like juice or milk, are delivered to customers every day by employing aseptic filling machines. They can operate around 12,000 ready-packaged products per hour (e.g., Pure-Pak® Aseptic Filling Line E-PS120A). However, they need to be routinely validated to guarantee contamination-free goods. The state-of-the-art methods to validate such machines are by means of microbiological analyses, where bacterial spores are used as test organisms because of their high resistance against several sterilants (e.g., gaseous hydrogen peroxide). The main disadvantage of the aforementioned tests is time: it takes at least 36-48 hours to get the results, i.e., the products cannot be delivered to customers without the validation certificate. Just in this example, in 36 hours, 432,000 products would be on hold for dispatchment; if more machines are evaluated, this number would linearly grow and at the end, the costs (only for waiting for the results) would be considerably high. For this reason, it is very valuable to develop new sensor technologies to overcome this issue. Therefore, the main focus of this thesis is on the further development of a spore-based biosensor; this sensor can determine the viability of spores after being sterilized with hydrogen peroxide. However, the immobilization strategy as well as its implementation on sensing elements and a more detailed investigation regarding its operating principle are missing. In this thesis, an immobilization strategy is developed to withstand harsh conditions (high temperatures, oxidizing environment) for spore-based biosensors applied in aseptic processing. A systematic investigation of the surface functionalization’s effect (e.g., hydroxylation) on sensors (e.g., electrolyte-insulator semiconductor (EIS) chips) is presented. Later on, organosilanes are analyzed for the immobilization of bacterial spores on different sensor surfaces. The electrical properties of the immobilization layer are studied as well as its resistance to a sterilization process with gaseous hydrogen peroxide. In addition, a sensor array consisting of a calorimetric gas sensor and a spore-based biosensor to measure hydrogen peroxide concentrations and the spores’ viability at the same time is proposed to evaluate the efficacy of sterilization processes

Advanced AlGaN/GaN HEMT technology, design, fabrication and characterization

Nowadays, the microelectronics technology is based on the mature and very well established silicon (Si) technology. However, Si exhibits some important limitations regarding its voltage blocking capability, operation temperature and switching frequency. In this sense, Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) devices have the potential to make this change possible. The unique combination of the high-breakdown field, the high-channel electron mobility of the two dimensional electron gas (2DEG), and high-temperature of operation has attracted enormous interest from social, academia and industry and in this context this PhD dissertation has been made. This thesis has focused on improving the device performance through the advanced design, fabrication and characterization of AlGaN/GaN HEMTs, primarily grown on Si templates. The first milestone of this PhD dissertation has been the establishment of a know-how on GaN HEMT technology from several points of view: the device design, the device modeling, the process fabrication and the advanced characterization primarily using devices fabricated at Centre de Recherche sur l'Hétéro-Epitaxie (CRHEA-CNRS) (France) in the framework of a collaborative project. In this project, the main workhorse of this dissertation was the explorative analysis performed on the AlGaN/GaN HEMTs by innovative electrical and physical characterization methods. A relevant objective of this thesis was also to merge the nanotechnology approach with the conventional characterization techniques at the device scale to understand the device performance. A number of physical characterization techniques have been imaginatively used during this PhD determine the main physical parameters of our devices such as the morphology, the composition, the threading dislocations density, the nanoscale conductive pattern and others. The conductive atomic force microscopy (CAFM) tool have been widely described and used to understand the conduction mechanisms through the AlGaN/GaN Ohmic contact by performing simultaneously topography and electrical conductivity measurements. As it occurs with the most of the electronic switches, the gate stack is maybe the critical part of the device in terms of performance and longtime reliability. For this reason, how the AlGaN/GaN HEMT gate contact affects the overall HEMT behaviour by means of advanced characterization and modeling has been intensively investigated. It is worth mentioning that the high-temperature characterization is also a cornerstone of this PhD. It has been reported the elevated temperature impact on the forward and the reverse leakage currents for analogous Schottky gate HEMTs grown on different substrates: Si, sapphire and free-standing GaN (FS-GaN). The HEMT' forward-current temperature coefficients (T^a) as well as the thermal activation energies have been determined in the range of 25-300 ºC. Besides, the impact of the elevated temperature on the Ohmic and gate contacts has also been investigated. The main results of the gold-free AlGaN/GaN HEMTs high-voltage devices fabricated with a 4 inch Si CMOS compatible technology at the clean room of the CNM in the framework of the industrial contract with ON semiconductor were presented. We have shown that the fabricated devices are in the state-of-the-art (gold-free Ohmic and Schottky contacts) taking into account their power device figure-of-merit ((VB^2)/Ron) of 4.05×10^8 W/cm^2. Basically, two different families of AlGaN/GaN-on-Si MIS-HEMTs devices were fabricated on commercial 4 inch wafers: (i) using a thin ALD HfO2 (deposited on the CNM clean room) and (ii) thin in-situ grown Si3N4, as a gate insulator (grown by the vendor). The scientific impact of this PhD in terms of science indicators is of 17 journal papers (8 as first author) and 10 contributions at international conferences

Solution Processable Carbon-Based Electronics

Fabricating electronic devices using solution-based processing methods opens up a broad range of potential applications that are inaccessible to conventional semiconductor fabrication technologies. The chemically diverse family of carbon-based materials are suitable for this purpose with almost limitless possibilities for molecular tailoring. The present work is a study of some of the materials for and device physics of field-effect transistors based on solution processable layers. Each aspect of this work is chosen to address a current difficulty in the development solution-processable carbon-based electronics. For portable and battery-powered applications, low-power circuits are required. This can be achieved by using a complementary logic circuit architecture (that requires both electron and hole transporting semiconductors) where the discrete devices operate at low voltages. Practically, this requires a high capacitance gate dielectric which is compatible with solution processing of a range of semiconductor materials. One family of molecules suitable for this purpose are self-assembling phosphonic acids that can form molecular monolayers. In the present study, molecular tailoring of this family of molecules is investigated as a route towards improving the compatibility of these dielectrics with solution processed semiconductors. One of the difficulties with utilising a complementary logic circuit architecture is the requirement of a suitable electron transporting semiconductor. This semiconductor must be solution-processable, exhibit a high electron mobility and be stable against degradation upon atmospheric exposure. Although many p-channel semiconductors fulfil these requirements, equivalent performance in many families of n-channel semiconductors remains challenging. In the present study, the use of fullerenes, a widely used family of semiconductors, is explored for implementation as an n-channel material in field-effect transistors. Their electronic structure is controlled by chemical tailoring of each molecule and the impact of this parameter variation on the air-stability of these fullerenes is assessed. Graphene, potentially one of the most important materials for future electronics, is currently impractical to prepare over large areas. Chemical derivation routes are sought which allow processing of graphene from solution. One of the most important routes is solution phase exfoliation of graphene oxide followed by thermal or chemical reduction. Unfortunately this introduces a high density of defects within the final graphene layer which ultimately limits the charge-carrier mobility. Here, a milder oxidation with surfactant-assisted solution phase exfoliation is investigated as a route to improving the quality of graphene films following reduction. The electronic properties of thin- films of these chemically-derived graphene layers are explored as the active layer in field-effect transistors

Surface studies of organic thin films using scanning probe microscopy and nanofabrication

Porphyrins and metalloporphyrins have unique chemical and electronic properties and thus provide useful model structures for nanoscale studies of the role of chemical structure for electronic properties. Porphyrins have been proposed as viable materials for molecular-based information-storage devices, gas sensors, photovoltaic cells, organic light-emitting diodes and molecular wires. The function and efficiency of porphyrins in devices is largely attributable to molecular architecture and how the molecules are self-organized. Modifications of the porphyrin macrocycle, peripheral groups or bound metal ions can generate a range of electrical, photoelectrical or magnetic properties. The conductive properties are greatly influenced at the molecular level by the organization of porphyrins into supramolecular arrays, aggregates, and nanocrystals on surfaces. Conductive-probe atomic force microscopy (CP-AFM) has been used extensively for studies of alkanes, phenylalkanes and arenethiols; however, the conductive properties of porphyrins have not been studied as rigorously. Characterizations with CP-AFM are becoming prevalent for molecular electronics studies because of the dual capabilities for obtaining physical measurements and structural information with unprecedented sensitivity. For CP-AFM, the tip is placed directly on the sample surface, at a designated force. To acquire current-voltage (I-V) spectra, a conductive tip is grounded, and a bias is applied to the substrate. For this dissertation, cobaltcarborane porphyrins were synthesized using a ring-opening zwitterionic reaction to produce isomers with different numbers of carborane clusters per macrocycle. Particle lithography was used to prepare regular arrangements of well-defined nanopatterns of porphyrin nanocrystals on conductive substrates. Nanopatterned SAMs of alkanethiols and organosilanes were used successfully to direct the nanocrystals of porphyrins on the surface and characterized with contact and tapping mode imaging of AFM. Our goals were to elucidate the role of molecular structure, packing and orientation for the conductive properties of porphyrins. Understanding how the self-organization and surface assembly influence electrical properties and reliable measurements of conductive properties when these molecules are coordinated to different metals and surfaces will provide information for developing predictive models

CARBON NANOTUBE THIN FILM AS AN ELECTRONIC MATERIAL

Carbon nanotubes (CNT) are potential candidates for next-generation nanoelectronics devices. An individual CNT possesses excellent electrical properties, but it has been extremely challenging to integrate them on a large-scale. Alternatively, CNT thin films have shown great potential as electronic materials in low cost, large area transparent and flexible electronics. The primary focus of this dissertation is patterning, assembling, characterization and assessment of CNT thin films as electronic material. Since a CNT thin film contains both metallic and semiconducting CNTs, it can be used as an active layer as well as an electrode material by controlling the growth density and device geometry. The growth density is controlled by chemical vapor deposition and airbrushing methods. The device geometry is controlled by employing a transfer printing method to assemble CNT thin film transistors (TFT) on plastic substrates. Electrical transport properties of CNT TFTs are characterized by their conductance, transconductance and on/off ratio. Optimized device performance of CNT TFTs is realized by controlling percolation effects in a random network. Transport properties of CNTs are affected by the local environment. To study the intrinsic properties of CNTs, the environmental effects, such as those due to contact with the dielectric layer and processing chemicals, need to be eliminated. A facile fabrication method is used to mass produce as-grown suspended CNTs to study the transport properties of CNTs with minimal effects from the local environment. Transport and low-frequency noise measurements are conducted to probe the intrinsic properties of CNTs. Lastly, the unique contrast mechanism of the photoelectron emission microscopy (PEEM) is used to characterize the electric field effects in a CNT field effect transistor (FET). The voltage contrast mechanism in PEEM is first characterized by comparing measurements with simulations of a model system. Then the voltage contrast is used to probe the local field effects on a single CNT and a CNT thin film. This real-time imaging method is assessed for potential applications in testing of micron sized devices integrated in large scale

Impedance Sensing of Cancer Cells Directly on Sensory Bioscaffolds of Bioceramics Nanofibers

Cancer cell research has been growing for decades. In the field of cancer pathology, there is an increasing and long-unmet need to develop a new technology for low-cost, rapid, sensitive, selective, label-free (i.e. direct), simple and reliable screening, diagnosis, and monitoring of live cancer and normal cells in same shape and size from the same anatomic region. For the first time on using an impedance signal, the breast cancer and normal cells have been thus screened, diagnosed and monitored on a smart bioscaffold of entangled nanowires of bioceramics titanate grown directly on the surface of implantable Ti-metal and characterized by SEM, XRD, etc. following a technology patented by Tian-lab. In experiment in the aqueous solution of phosphate buffer saline (PBS), human breast benign (MCF7) and aggressive (MDA-MB231) cancer cells, normal (MCF10A) cells, and colon cancer cells (HCT116) showed characteristic impedance spectrum highly different than that of the blank sensor (i.e. no cells on the bioscaffold surface). For two sets of mixtures each containing the normal and cancer cells over a wide range of mixing ratios, the shift of impedance signals has been linearly correlated with the mixing ratios which supports the biosensor’s selectivity and reliability. After being treated with pure glucose and chemotherapeutic drug (i.e. doxorubicin of DOX) and with one after the other, the breast cancer cells showed different impedance signals corresponding to their difference in glucose metabolisms (i.e. Warburg Effect) and resistances to the Dox, thus-fingerprinting the cells easily. Based on the nanostructure chemistry, impedance equivalent circuitry and cancer cell biology, it’s the different cells surface binding on the nanowires, and different cancer cells metabolic wastes from the different treatments on the nanowires that changed the charge density on the scaffolding nanowire surface and in turn changed the impedance signals. This new method is believed expandable to quantifying and characterizing live cells and even biological tissues of different types in general

Development of a Novel Platform for in vitro Electrophysiological Recording

The accurate monitoring of cell electrical activity is of fundamental importance for pharmaceutical research and pre-clinical trials that impose to check the cardiotoxicity of all new drugs. Traditional methods for preclinical evaluation of drug cardiotoxicity exploit animal models, which tend to be expensive, low throughput, and exhibit species-specific differences in cardiac physiology (Mercola, Colas and Willems, 2013). Alternative approaches use heterologous expression of cardiac ion channels in non-cardiac cells transfected with genetic material. However, the use of these constructs and the inhibition of specific ionic currents alone is not predictive of cardiotoxicity. Drug toxicity evaluation based on the human ether-\ue0-go-go-related gene (hERG) channel, for example, leads to a high rate of false-positive cardiotoxic compounds, increasing drug attrition at the preclinical stage. Consequently, from 2013, the Comprehensive in Vitro Proarrhythmia Assay (CiPA) initiative focused on experimental methods that identify cardiotoxic drugs and to improve upon prior models that have largely used alterations in the hERG potassium ion channel. The most predictive models for drug cardiotoxicity must recapitulate the complex spatial distribution of the physiologically distinct myocytes of the intact adult human heart. However, intact human heart preparations are inherently too costly, difficult to maintain, and, hence, too low throughput to be implemented early in the drug development pipeline. For these reasons the optimization of methodologies to differentiate human induced Pluripotent Stem Cells (hiPSCs) into cardiomyocytes (CMs) enabled human CMs to be mass-produced in vitro for cardiovascular disease modeling and drug screening (Sharma, Wu and Wu, 2013). These hiPSC-CMs functionally express most of the ion channels and sarcomeric proteins found in adult human CMs and can spontaneously contract. Recent results from the CiPA initiative have confirmed that, if utilized appropriately, the hiPSC-CM platform can serve as a reliable alternative to existing hERG assays for evaluating arrhythmogenic compounds and can sensitively detect the action potential repolarization effects associated with ion channel\u2013blocking drugs (Millard et al., 2018). Data on drug-induced toxicity in hiPSC-CMs have already been successfully collected by using several functional readouts, such as field potential traces using multi-electrode array (MEA) technology (Clements, 2016), action potentials via voltage-sensitive dyes (VSD) (Blinova et al., 2017) and cellular impedance (Scott et al., 2014). Despite still under discussion, scientists reached a consensus on the value of using electrophysiological data from hiPSC-CM for predicting cardiotoxicity and how it\u2019s possible to further optimize hiPSC-CM-based in vitro assays for acute and chronic cardiotoxicity assessment. In line with CiPA, therefore, the use of hiPSC coupled with MEA technology has been selected as promising readout for these kind of experiments. These platforms are used as an experimental model for studying the cardiac Action Potentials (APs) dynamics and for understanding some fundamental principles about the APs propagation and synchronization in healthy heart tissue. MEA technology utilizes recordings from an array of electrodes embedded in the culture surface of a well. When cardiomyocytes are grown on these surfaces, spontaneous action potentials from a cluster of cardiomyocytes, the so called functional syncytium, can be detected as fluctuations in the extracellular field potential (FP). MEA measures the change in FP as the action potential propagates through the cell monolayer relative to the recording electrode, neverthless FP in the MEA do not allows to recapitualte properly the action potential features. It is clear, therefore, that a MEA technology itself is not enough to implement cardiotoxicity assays on hIPSCs-CMs. Under this issue, researchers spread in the world started to think about solutions to achieve a platform able to works both at the same time as a standard MEA and as a patch clamp, allowing the recording of extracellular signals as usual, with the opportunity to switch to intracellular-like signals from the cytosol. This strong interest stimulated the development of methods for intracellular recording of action potentials. Currently, the most promising results are represented by multi-electrode arrays (MEA) decorated with 3D nanostructures that were introduced in pioneering papers (Robinson et al., 2012; Xie et al., 2012), culminating with the recent work from the group of H. Park (Abbott et al., 2017) and of F. De Angelis (Dipalo et al., 2017). In these articles, they show intracellular recordings on electrodes refined with 3D nanopillars after electroporation and laser optoporation from different kind of cells. However, the requirement of 3D nanostructures set strong limitations to the practical spreading of these techniques. Thus, despite pioneering results have been obtained exploiting laser optoporation, these technologies neither been applied to practical cases nor reached the commercial phase. This PhD thesis introduces the concept of meta-electrodes coupled with laser optoporation for high quality intracellular signals from hiPSCs-CM. These signals can be recorded on high-density commercial CMOS-MEAs from 3Brain characterized by thousands of electrode covered by a thin film of porous Platinum without any rework of the devices, 3D nanostructures or circuitry for electroporation7. Subsequently, I attempted to translate these unique features of low invasiveness and reliability to other commercial MEA platforms, in order to develop a new tool for cardiac electrophysiological accurate recordings. The whole thesis is organized in three main sections: a first single chapters that will go deeper in the scientific and technological background, including an explanation of the cell biology of hiPSCs-CM followed by a full overview of MEA technology and devices. Then, I will move on state-of-the-art approaches of intracellular recording, discussing many works from the scientific literature. A second chapter will describe the main objectives of the whole work, and a last chapter with the main results of the activity. A final chapter will resume and recapitulate the conclusion of the work