Development of a nanogap fabrication method for applications in nanoelectromechanical systems and nanoelectronics

There is a great need for a well-controlled nanogap fabrication technique compatible with NEMS applications. Theoretically, a displacement sensor based on vacuum tunnel junction or a nanogap can be capable of performing quantum-limited measurements in NEMS applications. Additionally, in the context of nanoelectronics, nanogaps are widely demanded to characterize nanostructures and to incorporate them into nanoscale electronic devices. Here, we have proposed and implemented a fabrication technique based on the controlled shrinkage of a lithographically defined gap between two suspended structures by thermal evaporation. We have consistently produced rigid and stable metallic vacuum tunneling junctions at nanometer or subnanometer sizes. The fabricated nanogaps were characterized by I-V measurements and their gap sizes and potential barrier heights were interrogated using the Simmons' model. Throughout this work, high tensile stress silicon nitride thin films were preferred for the fabrication of suspended structures because they have high resonance frequencies with low dissipation, they are mechanically stable, and they are resilient to stiction problem. However, high-stress nitride structures experience a complex shape deformation once they are suspended. The shape deformation is undesired when the precise positioning of the structures is required as in nanogap fabrication. We developed a new method in which the built in stress gradient is utilized to tune the distance between two suspended structures. The technique was simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability beyond the lithographic resolution limits

Engineering for a changing world: 60th Ilmenau Scientific Colloquium, Technische Universität Ilmenau, September 04-08, 2023 : programme

In 2023, the Ilmenau Scientific Colloquium is once more organised by the Department of Mechanical Engineering. The title of this year’s conference “Engineering for a Changing World” refers to limited natural resources of our planet, to massive changes in cooperation between continents, countries, institutions and people – enabled by the increased implementation of information technology as the probably most dominant driver in many fields. The Colloquium, supplemented by workshops, is characterised but not limited to the following topics: – Precision engineering and measurement technology Nanofabrication – Industry 4.0 and digitalisation in mechanical engineering – Mechatronics, biomechatronics and mechanism technology – Systems engineering – Productive teaming - Human-machine collaboration in the production environment The topics are oriented on key strategic aspects of research and teaching in Mechanical Engineering at our university

Integrated Nanoscale Tools for Interrogating Living Cells

The development of next-generation, nanoscale technologies that interface biological systems will pave the way towards new understanding of such complex systems. Nanowires – one-dimensional nanoscale structures – have shown unique potential as an ideal physical interface to biological systems. Herein, we focus on the development of nanowire-based devices that can enable a wide variety of biological studies. First, we built upon standard nanofabrication techniques to optimize nanowire devices, resulting in perfectly ordered arrays of both opaque (Silicon) and transparent (Silicon dioxide) nanowires with user defined structural profile, densities, and overall patterns, as well as high sample consistency and large scale production. The high-precision and well-controlled fabrication method in conjunction with additional technologies laid the foundation for the generation of highly specialized platforms for imaging, electrochemical interrogation, and molecular biology. Next, we utilized nanowires as the fundamental structure in the development of integrated nanoelectronic platforms to directly interrogate the electrical activity of biological systems. Initially, we generated a scalable intracellular electrode platform based on vertical nanowires that allows for parallel electrical interfacing to multiple mammalian neurons. Our prototype device consisted of 16 individually addressable stimulation/recording sites, each containing an array of 9 electrically active silicon nanowires. We showed that these vertical nanowire electrode arrays could intracellularly record and stimulate neuronal activity in dissociated cultures of rat cortical neurons similar to patch clamp electrodes. In addition, we used our intracellular electrode platform to measure multiple individual synaptic connections, which enables the reconstruction of the functional connectivity maps of neuronal circuits. In order to expand and improve the capability of this functional prototype device we designed and fabricated a new hybrid chip that combines a front-side nanowire-based interface for neuronal recording with backside complementary metal oxide semiconductor (CMOS) circuits for on-chip multiplexing, voltage control for stimulation, signal amplification, and signal processing. Individual chips contain 1024 stimulation/recording sites enabling large-scale interfacing of neuronal networks with single cell resolution. Through electrical and electrochemical characterization of the devices, we demonstrated their enhanced functionality at a massively parallel scale. In our initial cell experiments, we achieved intracellular stimulations and recordings of changes in the membrane potential in a variety of cells including: HEK293T, cardiomyocytes, and rat cortical neurons. This demonstrated the device capability for single-cell-resolution recording/stimulation which when extended to a large number of neurons in a massively parallel fashion will enable the functional mapping of a complex neuronal network.Physic

Nanogap capacitive biosensor for label-free aptamer-based protein detection

Recent advances in nanotechnology offer a new platform for the label free detection of biomolecules at ultra-low concentrations. Nano biosensors are emerging as a powerful method of improving device performance whilst minimizing device size, cost and fabrication times. Nanogap capacitive biosensors are an excellent approach for detecting biomolecular interactions due to the ease of measurement, low cost equipment needed and compatibility with multiplex formats.This thesis describes research into the fabrication of a nanogap capacitive biosensor and its detection results in label-free aptamer-based protein detection for proof of concept. Over the last four decades many research groups have worked on fabrication and applications of these type of biosensors, with different approaches, but there is much scope for the improvement of sensitivity and reliability. Additionally, the potential of these sensors for use in commercial markets and in everyday life has yet to be realized.Initial work in the field was limited to high frequency (>100 kHz) measurements only, since at low frequency there is significant electronic thermal noise (=4kBTR) from the electrical double layer (EDL). This was a significant drawback since this noise masked most of the important information from biomolecular interactions of interest. A novel approach to remove this parasitic noise is to minimize the EDL impedance by reducing the capacitor electrode separation to less than the EDL thickness. In the case of aptamer functionalized electrodes, this is particularly advantageous since device sensitivity is increased as the dielectric volume is better matched to the size of the biomolecules and their binding to the electrode surface. This work has demonstrated experimentally the concepts postulated theoretically.In this work we have fabricated a large area (100 x 5 μm x 5 μm) vertically oriented capacitive nanogap biosensor with a 40 nm electrode separation between two gold electrodes. A silicon dioxide support layer separates the two electrodes and this is partially etched (approximately 800 nm from both sides of each 5 μm x 5 μm capacitor), leaving an area of the gold electrodes available for thiol-aptamer functionalization.AC impedance spectroscopy measurements were performed with the biosensor in the presence of air, D.I. water, various ionic strength buffer solutions and aptamer/protein pairs inside the nanogap. Applied frequencies were from 1Hz to 500 kHz at 20 mV AC voltage with 0 DC. We obtained relative permittivity results as a function of frequency for air (ɛ=1) and DI water (ɛ~80) which compares very favorably with previous works done by different research groups.The sensitivity and response of the sensors to buffer solution (SSC buffer) with various ionic strengths (0.1x SSC, 0.2x SSC, 0.5x SSC and 1x SSC) was studied in detail. It was found that in the low frequency region (<1 kHz) the relative permittivity (capacitance) was broadly constant, that means it is independent from the applied frequency in this range. With increasing buffer concentration, the relative permittivity starts to increase (from ɛ=170 for 0.1x SSC to ɛ=260 for 1x SSC).The sensor performance was further investigated for aptamer-based protein detection, human alpha thrombin aptamers and human alpha thrombin protein pairs were selected for proof of concept. Aptamers were functionalized into the gold electrode surface with the Self-Assembly-Monolayer (SAM) method and measurements were performed in the presence of 0.5x SSC buffer solution (ɛ=180). Then the hybridization step was carried out with 1 μM of human alpha thrombin protein followed by measurements in the presence of the same buffer (ɛ=130). The response of the sensors with different solutions inside the nanogap was studied at room temperature (5 working devices were tested for each step). The replacement of the buffer solution (ɛ=250) with lower relative permittivity biomolecules (aptamer ɛ=180) and further binding proteins to immobilized aptamer (ɛ=130) was studied. To validate these results, a control experiment was carried out using different aptamers, in this case which are not able to bind to human alpha thrombin protein. It was found that the relative permittivity did not change after the hybridization step compared to the aptamer functionalization step, which indicates that the sensors performance is highly sensitive and reliable.This work serves as a proof of concept for a novel nanogap based biosensor with the potential to be used for many applications in environmental, food industry and medical industry. The fabrication method has been shown to be reliable and consistent with the possibility of being easily commercialized for mass production for use in laboratories for the analysis of a wide range of samples

Thermal characterisation of miniature hotplates used in gas sensing technology

The reliability of micro-electronic devices depends on the device operating temperature and therefore self-heating can have an adverse effect on the performance and reliability of these devices. Hence, thermal measurement is crucial including accurate maximum operating temperature measurements to ensure optimum reliability and good electrical performance. In the research presented in this thesis, the high temperature thermal characterisation of novel micro-electro-mechanical systems (MEMS) infra-red (IR) emitter chips for use in gas sensing technology for stable long-term operation were studied, using both IR and a novel thermo-incandescence microscopy. The IR emitters were fabricated using complementary-metal-oxide semiconductor (CMOS) based processing technology and consisted of a miniature micro-heater, fabricated using tungsten metallisation. There is a commercial drive to include MEMS micro-heaters in portable electronic applications including gas sensors and miniaturised IR spectrometers where low power consumption is required. IR thermal microscopy was used to thermally characterise these miniature MEMS micro-heaters to temperatures approaching 700 °C. The research work has also enabled further development of novel thermal measurement techniques, using carbon microparticle infra-red sensors (MPIRS) with the IR thermal microscopy. These microparticle sensors, for the first time, have been used to make more accurate high temperature (approaching 700 °C) spot measurements on the IR transparent semiconductor membrane of the micro-heater. To substantially extend the temperature measurement range of the IR thermal microscope, and to obtain the thermal profiles at elevated temperatures (> 700 °C), a novel thermal measurement approach has been developed by calibrating emitted incandescence radiation in the optical region as a function of temperature. The calibration was carried out using the known melting point (MP) of metal microparticles. The method has been utilised to obtain the high temperature thermo-optical characterisation of the MEMS micro-heaters to temperatures in excess of 1200 °C. The measured temperature results using thermo-incandescence microscopy were compared with calculated electrical temperature results. The results indicated the thermo-incandescence measurements are in reasonable agreement (± 3.5 %) with the electrical temperature approach. Thus, the measurement technique using optical incandescent radiation extends the range of conventional IR microscopy and shows a great potential for making very high temperature spot measurements on electronic devices. The high power (> 500mW) electrical characterisation of the MEMS micro-heaters were also analysed to assess the reliability. The electrical performance results on the MEMS micro-heaters indicated failures at temperatures greater than 1300 °C and Scanning Electron Microscope (SEM) was used to analyse the failure modes

Design, Fabrication, Testing of CNT Based ISFET and Characterization of Nano/Bio Materials Using AFM

A combination of Carbon Nanotubes (CNTs) and Ion Selective Field Effect Transistor (ISFET) is designed and experimentally verified in order to develop the next generation ion concentration sensing system. Micro Electro-Mechanical System (MEMS) fabrication techniques, such as photolithography, diffusion, evaporation, lift-off, packaging, etc., are required in the fabrication of the CNT-ISFET structure on p-type silicon wafers. In addition, Atomic Force Microscopy (AFM) based surface nanomachining is investigated and used for creating nanochannels on silicon surfaces. Since AFM based nanomanipulation and nanomachining is highly controllable, nanochannels are precisely scratched in the area between the source and drain of the FET where the inversion layer is after the ISFET is activated. Thus, a bundle of CNTs are able to be aligned inside a single nanochannel by Dielectrophoresis (DEP) and the drain current is improved greatly due to CNTs` remarkable and unique electrical properties, for example, high current carrying capacity. ISFET structures with or without CNTs are fabricated and tested with different pH solutions. Besides the CNT-ISFET pH sensing system, this dissertation also presents novel AFM-based nanotechnology for learning the properties of chemical or biomedical samples in micro or nano level. Dimensional and mechanical property behaviors of Vertically Aligned Carbon Nanofibers (VACNFs) are studied after temperature and humidity treatment using AFM. Furthermore, mechanical property testing of biomedical samples, such as microbubbles and engineered soft tissues, using AFM based nanoindentation is introduced, and the methodology is of great directional value in the area

Development and optimization of 3D advanced functional magnetic nanostructures grown by focused electron beam induced deposition

Esta tesis doctoral incluye diferentes estrategias para controlar la arquitectura, composición y magnetismo de nanohilos y nanotubos 3D ferromagnéticos fabricados mediante deposición inducida por haz de electrones focalizado (FEBID) [1], cuya versatilidad en el crecimiento de nanoestructuras 3D complejas abre nuevas perspectivas para el desarrollo de novedosos dispositivos magnéticos.En primer lugar, se ha desarrollado un nuevo método para el crecimiento de nanohilos funcionales sobre sustratos conductores y aislantes permitiendo la modulación in situ de su geometría mediante la aplicación de campos eléctricos [2]. En segundo lugar, se han explorado tratamientos térmicos ex situ e in situ posteriores al crecimiento, monitorizando los cambios estructurales, químicos y magnéticos en cada temperatura [3][4][5]. En tercer lugar, se ha llevado a cabo el crecimiento de materiales heteroestructurados en forma de nanohilos 3D con un núcleo y un recubrimiento [6]. Esta nueva estrategia ha sido aplicada para sintetizar nanohilos verticales con núcleos ferromagnéticos de Co o Fe recubiertos de una capa protectora de Pt-C, minimizando la degradación de las propiedades magnéticas causada por la oxidación superficial natural del núcleo que convierte esta capa externa en un material no ferromagnético. Mediante este mismo método de fabricación, se ha realizado la síntesis de nanotubos 3D ferromagnéticos compuestos por núcleos de Pt-C y recubrimientos de Co.Asimismo, dado que estas estructuras son candidatas potenciales para ser usadas en almacenamiento y procesamiento de información debido a una óptima conducción de paredes de dominio magnéticas, también se han fabricado nanohilos 3D de Co recubiertos con Pt-C con una morfología consistente en la formación de codos a lo largo de la longitud del nanohilo, actuando como sitios de anclaje donde las paredes de dominio pueden estar localizadas [7]. Finalmente, se ha llevado a cabo el crecimiento de puntas magnéticas con posibles aplicaciones en microscopía de fuerza magnética (MFM). Se han realizado experimentos destinados al crecimiento de nanohilos verticales de Co y Fe sobre puntas comerciales de microscopía de fuerza atómica, evaluando la optimización de las puntas MFM crecidas por FEBID [8] y realizando una comparación de su comportamiento con respecto a las puntas MFM estándar. Las puntas han sido analizadas en experimentos MFM, tanto en condiciones ambientales como en entorno líquido, comportándose apropiadamente en términos de estabilidad mecánica, resolución y sensibilidad. Los resultados han demostrado que las puntas crecidas por FEBID son superiores a las estándar, y pueden dar lugar a la siguiente generación de puntas MFM comerciales.<br /

Near room temperature self-assembly of nanostructures by reaction of gallium with metal thin films.

Liquid gallium (Ga) spontaneously alloys with thin films of metals such as Ag, Au, Pt Al, and Cu at near or even below room temperature resulting in rapid self-assembly of nanostructures. In this dissertation, studies of the formation of these nanostructures are reported together with application of the processes towards device fabrication. Ag 2 Ga needles, CoGa 3 rods, and Ga 6 Pt plates self-assemble at room temperature at the interface of Ga and thin films of Ag Co, and Pt. The Ag 2 Ga needles orient nearly vertical to the interface which suggests that an individual needle can be directed to grow in a desired direction by drawing a silver-coated surface from the Ga droplet. Needles from 25 nm to microns in diameter and up to 33 microns long were grown by this method. Needle-tipped cantilevers have been used to perform atomic force microscopy (AFM) and voltage lithography. Mechanical properties of the Ag 2 Ga needles are measured during bending, buckling, yielding, and AC electric excitation of vibrational modes. The rates of reactive spreading of Ga through thin films of Au and Ag from room temperature to 200°C are measured. A model of the reduction in spreading rate of Au-Ga over time describes the reduction in area for inter granular flow as the Ga 2 Au crystallites precipitates and grow together. Ga spreading on Au microelectrodes is used to perform time-resolved measurement of changes in the contact resistance of multiwall carbon nanotubes. Networks of Au-Ga nanowires form when a liquid Ga drop spreads and reacts on 10- to 100- nm-thick Au thin film at temperatures between 310°C and 400°C. Au suspended nanowires were fabricated by etching these networks in HCl followed by anisotropic etching of the Si substrate. Suspended nanowires as long as 6 Ìm and as narrow as 35 nm diameters have been produced. Superporous Au and Pt thin films with feature size as small as 5 nm are formed after HCI etching of metal thin films that have been reacted with gallium. Superporous Pt formed on a set of microelectrodes was evaluated for electrochemical sensing. These electrodes showed a 6 fold improvement in its limit of detection for H 2 O 2 over the nonporous Pt electrodes