A new SiC/HfB2 based micro hotplate for metal oxide gassensors

Abstract Solzbacher, Florian A new SiC/HfB2 based modular concept of micro hotplates for metal oxide gassensors Im Rahmen dieser Arbeit wurde ein neuer SiC/HfB2-basierter Mikroheizer mit niedrigster Leistungsaufnahme für die Anwendung in Metalloxid-Gassensoren entwickelt und demonstriert. Erstmals wurden Siliziumkarbid (SiC) und Hafniumdiborid (HfB2) als Werkstoffe für einen Mikroheizer eingesetzt. Durch geringe Modifikation der Herstellungsprozesse lässt sich der Heizer so variieren, dass der Einsatz sowohl für den automobilen Anwendungsbereich (12V- 24V) als auch für tragbare Geräte (1V-2V) für eine Vielzahl unterschiedlicher Messgase möglich ist. Es ist der erste Mikroheizer für Gassensoren überhaupt, der den Batteriebetrieb bei nur 1-2 V erlaubt. Der modulare Fertigungsansatz ermöglicht die Reduzierung der Entwicklungs- und Fertigungskosten für die unterschiedlichen Anwendungsbereiche. Aus der Marktentwicklung in der Sensorik, den industriellen Anforderungen und den zu den Metalloxid-Gassensoren im Wettbewerb stehenden alternativen Technologien ergeben sich das Anforderungsprofil des Sensors. Die Wahl der Materialien spielt eine Schlüsselrolle für die Heizereigenschaften. Der Mikroheizer besteht aus einer 1 (m dicken, an 150 (m langen und 10 bis 40 (m breiten Stegen aufgehängten Membran mit Außenmaßen von 100 (m x 100 (m. Alternativ kommen eine HfB2 - Dünnfilm-Widerstandsheizung oder ein dotierter SiC-Heizer zum Einsatz. Mit Leistungsaufnahmen von 32 mW werden Temperaturen von 600°C erreicht, was einer Effizienz von ca. 19 K/mW entspricht. Die verwendeten hexagonalen Strukturen ermöglichen dichtes Packen der Sensoren in Arrays bei hoher mechanischer Stabilität. Erste NO2 Sensoren mit gassensitiver In2O3 Schicht konnten gezeigt werden.A new SiC/HfB2-based micro hotplate with ultra low power consumption for the application in metal oxide micro gas sensors is developed and demonstrated. For the first time, silicon carbide (SiC) and Hafniumdiboride (HfB2) are used as materials for a micro hotplate structure. Using only slight modifications of the fabrication process, the device can be used either for automotive applications with operating voltages of 12V-24V or for battery operated handheld detectors with operating voltages of 1V-2V for a variety of different gases. It is the first micro hotplate device ever designed to work for low battery voltages of 1V-2V. The modular approach towards the processing allows easy modification for a variety of application fields and thus also reduces market entrance barriers. Based on the market development of micro sensors, the industrial requirements, and competing metal oxide gas sensors using alternative technologies, technical specifications for the hotplate as well as the state of the art's limits are determined. The new material choice plays a key role in the device properties. The micro hotplate consists of a 100 ?m x 100 ?m membrane supported by thin beams of 1 ?m thickness, 150 ?m length and 10 to 40 ?m width. Alternatively, an HfB2 ? thin film resistive heater or a doped SiC heater are used. Temperatures of 600°C are achieved using a power consumption of only 32 mW resulting in a thermal heater efficiency of ~19 K/mW. The hexagonal geometry allows close packing of the hotplates in array structures with high mechanical strength. NO2 sensors with gas sensitive In2O3 layer are presented

Microheater: material, design, fabrication, temperature control, and applications—a role in COVID‑19

Heating plays a vital role in science, engineering, mining, and space, where heating can be achieved via electrical, induction, infrared, or microwave radiation. For fast switching and continuous applications, hotplate or Peltier elements can be employed. However, due to bulkiness, they are inefective for portable applications or operation at remote locations. Miniaturization of heaters reduces power consumption and bulkiness, enhances the thermal response, and integrates with several sensors or microfuidic chips. The microheater has a thickness of~100 nm to~100 μm and ofers a temperature range up to 1900℃ with precise control. In recent years, due to the escalating demand for fexible electronics, thin-flm microheaters have emerged as an imperative research area. This review provides an overview of recent advancements in microheater as well as analyses diferent microheater designs, materials, fabrication, and temperature control. In addition, the applications of microheaters in gas sensing, biological, and electrical and mechanical sectors are emphasized. Moreover, the maximum temperature, voltage, power consumption, response time, and heating rate of each microheater are tabulated. Finally, we addressed the specifc key considerations for designing and fabricating a microheater as well as the importance of microheater integration in COVID-19 diagnostic kits. This review thereby provides general guidelines to researchers to integrate microheater in micro-electromechanical systems (MEMS), which may pave the way for developing rapid and large-scale SARS-CoV-2 diagnostic kits in resource-constrained clinical or home-based environments

Synthesis and integration of one-dimensional nanostructures for chemical gas sensing applications

The need for improved measurement technology for the detection and monitoring of gases has increased tremendously for maintenance of domestic and industrial health and safety, environmental surveys, national security, food-processing, medical diagnostics and various other industrial applications. Among the several varieties of gas sensors available in the market, solid-state sensors are the most popular owing to their excellent sensitivity, ruggedness, versatility and low cost. Semiconducting metal oxides such as tin oxide (SnO2), zinc oxide (ZnO), and tungsten oxide (WO3) are routinely employed as active materials in these sensors. Since their performance is directly linked to the exposed surface area of the sensing material, one-dimensional nanostructures possessing very high surface to volume ratios are attractive candidates for designing the next generation of sensors. Such nano-sensors also enable miniaturization thereby reducing power consumption. The key to achieve success in one-dimensional nanotechnologies lies in assembly. While synthesis techniques and capabilities continue to expand rapidly, progress in controlled assembly has been sluggish due to numerous technical challenges. In this doctoral thesis work, synthesis and characterization of various one-dimensional nanostructures including nanotubes of SnO2, and nanowires of WO3 and ZnO, as well as their direct integration into miniature sensor platforms called microhotplates have been demonstrated. The key highlights of this research include devising elegant strategies for growing metal oxide nanotubes using carbon nanotubes as templates, substantially reducing process temperatures to enable growth of WO3 nanowires on microhotplates, and successfully fabricating a ZnO nanowire array based sensor using a hybrid nanowire-nanoparticle assembly approach. In every process, the gas-sensing properties of one-dimensional nanostructures were observed to be far superior in comparison with thin films of the same material. Essentially, we have formulated simple processes for improving current thin film sensors as well as a means of incorporating nanostructures directly into miniature sensing devices. Apart from gas sensing applications, the approaches described in this work are suitable for designing future nanoelectronic devices such as gas-ionization, capacitive and calorimetric sensors, miniature sensor arrays for electronic nose applications, field emitters, as well as photonic devices such as nanoscale LEDs and lasers

Design And Fabrication Of Chemiresistor Typemicro/nano Hydrogen Gas Sensors Usinginterdigitated Electrodes

Hydrogen sensors have obtained increased interest with the widened application of hydrogen energy in recent years. Among them, various chemiresistor based hydrogen sensors have been studied due to their relatively simple structure and well-established detection mechanism. The recent progress in micro/nanotechnology has accelerated the development of small-scale chemical sensors. In this work, MEMS (Micro-Electro-Mechanical Systems) sensor platforms with interdigitated electrodes have been designed and fabricated. Integrating indium doped tin dioxide nanoparticles, these hydrogen sensors showed improved sensor characteristics such as sensitivity, response and selectivity at room temperature. Design parameters of interdigitated electrodes have been studied in association with sensor characteristics. It was observed that these parameters (gap between the electrodes, width and length of the fingers, and the number of the fingers) imposed different impacts on the sensor performance. In order to achieve small, robust, low cost and fast hydrogen micro/nano sensors with high sensitivity and selectivity, the modeling and process optimization was performed. The effect of humidity and the influence of the applied voltage were also studied. The sensor could be tuned to have high sensitivity (105), fast response time (10 seconds) and low energy consumption (19 nW). Finally, a portable hydrogen instrument integrated with a micro sensor, display, sound warning system, and measurement circuitry was fabricated based on the calibration data of the sensor

New Fabrication Methodologies for the Development of Low Power Gas Sensors Based on Semiconducting Nanowires

[eng] The air and environment quality is, nowadays, one of the main political concerns of the governmental institutions. Suitable gas detection is nowadays an important requirement, which is provided by gas sensors. Metal oxide semiconductors are the most common materials used as semiconducting gas sensors. They can be used with different type of operational mechanisms, like resistive, capacitive or optical based sensors, among others. The main objective of this dissertation is to contribute towards the improvement of gas sensors based on semiconducting nanowires. The easy integration of nanowires in low consumption devices is the fundamental idea that guides this dissertation, and the subsequent characterization of the sensors. Furthermore, the low power consumption of the device is a constant condition of the presented procedures, implemented by means of MEMS substrate that has been used as platform for all the fabricated sensors. The easy integration of nanowire-based devices by using localized growth on top of sensing platforms has been developed, by a site-specific growth of SnO2 and Ge nanowires on top of micromembranes and microhotplates, on the sensing area of the gas sensors. The fabrication procedure allows in single process the growth of NW networks on top of the electronic platforms that will be used for the gas sensing, an important step forward towards the integration of nanowires on electronic devices. On the other hand, the fabricated devices can be used as gas sensors readily after the growth. The gas sensing behaviour of SnO2 networks have been characterized towards different gases; specifically, the kinetics of ammonia response in dry and humid air has been analysed in detail. In addition, the influence of water vapour is analysed, and thus, the chemical paths of the interaction with ammonia have been related to the operating temperature. Furthermore, the synthesized meshes of Ge NWs have been for the first time, at the best of our knowledge, studied as a gas sensor. The chemical interactions towards oxidizing and reducing gases are analysed, paying important attention on the structural characterization, which results primordial for the analysis of sensing behaviour. A procedure based on electron beam lithography is explored in order to fabricate individual nanowire-based devices on top of microhotplates and micromembranes. The experimental procedure for that is detailed in the different steps. The individual nanowires have also studied as a gas sensor, whose results are discussed and compared to their mesh counterpart.[spa] La tesis titulada New Fabrication Methodologies for the Development of Low Power Gas Sensors Based on Semiconducting Nanowires, se enmarca dentro de los sensores de gas para la monitorización ambiental de la calidad del aire, con el objetivo de detectar la presencia de gases nocivos para la salud humana. El trabajo desarrollado se basa en el uso de sensores de gas resistivos, es decir, que la adsorción de un gas en la superficie del sensor da lugar a un cambio en la conductividad del sensor, el cual repercute en un cambio de su resistencia medible experimentalmente. Los materiales utilizados son los óxidos metálicos, materiales semiconductores de banda prohibida ancha (entre 2 y 4 eV). Generalmente, los óxidos metálicos necesitan ser calentados a una temperatura por encima de 150 ºC para promover la interacción con los gases adsorbidos, que se lleva a cabo mediante la denominada quimisorción, una interacción localizada que implica una transferencia de carga entre el semiconductor y la especie de gas. Dado que el gas se adsorbe en la superficie del material, el mecanismo es considerado como superficial, cuya sensibilidad depende en gran medida, entre otros efectos, del ratio superficie/volumen. Así pues, las nanoestructuras aparecen como candidatas óptimas para ser utilizadas como sensor de gas. En la tesis mencionada, se utilizan estructuras unidimensionales, concretamente nanohilos semiconductores como parte activa. El principal desafío para la comercialización de sensores basados en nanohilos es, actualmente, la integración de estos en un dispositivo de forma eficiente y escalable. Un nuevo método de fabricación que mejor la integración de los nanohilos en sus plataformas electrónicas ha sido desarrollado, basado en el crecimiento localizado de nanohilos monocristalinos sobre microplacas calefactoras, es decir, membranas de dimensiones micrométricas que contienen un microcalefactor enterrado. El calefactor es utilizado para proporcionar la temperatura necesaria para la termólisis del precursor durante el crecimiento, y para calentar y promover la reacción durante el sensado de gas. Mediante este proceso han sido crecidos nanohilos de SnO2 y de Ge, en forma de redes con múltiples nanohilos conectados entre sí. Los dispositivos fabricados han sido caracterizados como sensores de gas. Los nanohilos de dióxido de estaño presentan la máxima respuesta ante amoníaco. Los mecanismos químicos que se dan lugar durante la interacción entre el amoníaco y el semiconductor han sido analizados a partir de la respuesta cinética y los distintos fenómenos observados, en aire seco y húmedo. Los nanohilos de Ge han sido estudiados por primera vez como sensores de gas, manteniendo una temperatura de 100 ºC. El comportamiento de tipo p de los nanohilos ha sido determinado a partir de medidas de efecto campo. Los nanohilos presentan una capa de óxido de Ge en la superficie, de alrededor de 1 nm, que posee un papel clave en la interacción, ya que contiene sitos de adsorción para el sensado de gas. El mecanismo de sensado de Ge se concluye como consistente con un óxido metálico de tipo p. Por otro lado, han sido fabricados sensores de gas basados en nanohilos individuales de SnO2 mediante un proceso basado en litografía por haz de electrones, sobre micromembranas calefactores suspendidas. El proceso ha sido integrado y adaptado para el tipo de micromembranas, que presentan rugosidades en la superficie. Los dispositivos fabricados han sido caracterizados como sensor de gas y comparados con las redes de nanohilos del mismo material

Properties and sensor performance of zinc oxide thin films

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003.Includes bibliographical references (p. 144-152).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Reactively sputtered ZnO thin film gas sensors were fabricated onto Si wafers. The atmosphere dependent electrical response of the ZnO micro arrays was examined. The effects of processing conditions on the properties and sensor performance of ZnO films were investigated. Using AFM, SEM, XRD and WDS, the 02/Ar ratios during sputtering and Al dopant were found to control the property of ZnO films. Subsequent annealing at 700 C improved the sensor response of the films considerably although it had only minor effects on the microstructure. DC resistance, I-V curves and AC impedance were utilized to investigate the gas response of ZnO sensors. ZnO films prepared with high O2/Ar ratios showed better sensitivity to various gases, a feature believed to be related to their lower carrier density. Al doped ZnO showed measurable sensitivity even with lower resistance attributable to their porous microstructure. AC impedance identified two major components of the total resistance including Schottky barriers at the Pt-ZnO interfaces and a DC bias induced constriction resistance within the ZnO films. Time dependent drift in resistance of ZnO films has been observed. Without applied bias, the ZnO films showed a fast and a slow resistance change response when exposed to gases with varying oxygen partial pressure with both response components dependent on operating temperature. Even at the relatively low operating temperatures of these thin film sensors, bulk diffusion cannot be discounted. The oxygen partial pressure dependence of the sensor resistance and its corresponding activation energy were related to defect process controlling the reduction/oxidation behavior of the ZnO.(cont.) In this study, time dependent DC bias effects on resistance drift were first discovered and characterized. The DC bias creates particularly high electric fields in these micro devices given that the spacing of the interdigited electrodes falls in the range of microns. The high electric field is believed to initiate ion migration and/or modulate grain boundary barrier heights, inducing resistance drift with time. Such DC bias resistance induced drift is expected to contribute to the instability of thin film micro array sensors designed for practical applications. Suggestions for stabilizing sensor response are provided.by Yongki Min.Ph.D

Langasite bulk acoustic wave resonant sensor for high temperature applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references (p. 175-188).(cont.) The self consistent defect model established the defect chemistry of langasite, enabling important parameters describing reduction (Er = 5.70± -0.06eV and 6.57±-0.24eV for acceptor and donor doped langasite respectively) and oxidation (Eo = 2.18±0.08eV), intrinsic electron-hole generation (Eg [approx. equals] 4.0-4.4eV) and defect ionization (ED-ion = 52±0.06eV for Nb ionization), to be extracted. The predictive defect model was used to calculate the dependence of the partial ionic and electronic conductivities and mass change as functions of temperature, dopant level and pO₂. Given that the magnitudes of conductivity and mass change directly affect the resolution and sensitivity limits of langasite resonators, their predictions allowed for the definition of acceptable operating limits and/or the design of properties for optimum resolution and sensitivity. Two high temperature applications of resonant sensors were studied. Praseodymiumcerium oxide was selected for oxygen partial pressure monitoring and is representative of films which change mass upon absorption or desorption of gaseous species. Barium carbonate film was selected for NO₂ sensing and is representative of films which change mass upon reaction with the gas phase to form a new product phase. Both sensors showed sensitivity to their respective target chemicals and demonstrated the feasibility of high temperature sensor applications. The performance of each sensor was discussed and suggestions for improving sensor performance were presented.The high temperature transport properties of langasite, La₃Ga₅SiO₁₄, were investigated with special attention focused on their potential impact on the utilization of langasite as a mass sensitive resonant platform for high temperature sensor applications. The electrical properties of acceptor and donor doped langasite were examined at temperatures ranging from 700 to 1000 ⁰C, and pO₂ of 1 to 10-25atm. Acceptor doped langasite was shown to exhibit mixed ionic-electronic conductivity behavior, with predominant ionic conduction due to mobile oxygen vacancies at high pO₂, and n-type electronic conduction due to electrons at low pO₂. Increasing acceptor level resulted in the appearance of p-type hole conduction at high pO₂ and increased ionic conductivity, while the n-type electron conduction was depressed. Donor doped langasite was shown to be electronic at all temperatures and pO₂. The electron mobility of langasite was found to be activated (polaron hopping) with an activation energy of 0.15(±0.01)eV, whereas the holes were assumed to be quasi free carriers. The activation energy for oxygen vacancy migration was estimated to be 0.91(±0.01)eV under dilute solution conditions and 1.27(±0.02)eV for 1% Sr level under concentrated solution conditions. Both values of activation energy of ionic conductivity-temperature product are consistent with activation energy of oxygen self-diffusivity in the respective materials. The electrical properties were related to the underlying defect and transport processes using defect modeling.by Huankiat Seh.Ph.D

Innovative ozone sensors for environmental monitoring working at low temperature

L'abstract è presente nell'allegato / the abstract is in the attachmen

Nanostructuring of indium tin oxide with sub-15 femtosecond laser pulses for technical and biomedical applications

Modern laser technologies enable the production of nanostructures on a relatively large area for a variety of applications. Nonlinear effects occurring at high light intensities, such as multiphoton absorption, allow structuring in dimensions below the diffraction limit. While existing applications for 2D confined nanostructures are essentially based on selforganizing structures without a well definable profile or in combination with complex processing steps, precisely flexible defined nanostructures were fabricated in this work. It is about the interaction of a tightly focused near infrared sub-15 femtosecond laser (repetition rate: 85 MHz) with sputtered polycrystalline ITO thin films exhibiting different electrical conductivities. Depending on the choice of parameters, total ablation, periodic nano-cuts or else crystal modifications can be generated, rendering the ITO layer resistant to a subsequent etching step. Using this innovative approach, nanowires attached to the substrate or even freestanding nanowires (in combination with a selectively etchable sacrificial layer) with a length-to-width ratio of more than one hundred were prepared. The applicability of such nanowires for self-heating resistive gas sensors for detection of oxidizing gases was demonstrated. As another demonstrator, ITO bioelectrodes were trimmed with respect to their impedance by coherent sub-20 nm wide and several microns long nano-cuts.Moderne Lasertechnologien ermöglichen eine relativ großflächige Herstellung von Nanostrukturen für eine Vielzahl von Anwendungen. Bei hohen Lichtintensitäten auftretende nichtlineare Effekte, wie z.B. Multiphotonenabsorption, erlauben das Unterschreiten der Beugungsgrenze. Während existierende Anwendungen für räumlich 2D beschränkte Nanostrukturen wesentlich auf selbstorganisierenden, wenig steuerbaren Profile in Kombination mit aufwändigen Prozessen basieren, konnten in der vorliegenden Arbeit präzise definierbare Nanostrukturen hergestellt werden. Zentraler Punkt dieser Arbeit ist die Wechselwirkung eines eng fokussierten nahinfraroten Sub-15-Femtosekunden-Laserstrahls (Wiederholrate: 85 MHz) mit gesputterten ITO-Dünnschichten, die unterschiedliche elektrische Leitfähigkeiten aufweisen. Je nach Wahl der Parameter können totale Ablation, periodische Nanoschnitte aber auch Kristallmodifikationen erreicht werden, die die ITO-Schicht resistent gegenüber einem nachfolgenden Ätzschritt machen. Mit diesem innovativen Prozessansatz konnten substratgebundene bzw. freistehende (in Kombination mit einer selektiv ätzbaren Opferschicht) Nanodrähte mit einem Längen-zu-Breiten-Verhältnis von mehr als einhundert hergestellt werden. Die Verwendbarkeit solcher selbstheizbarer Drähte zur resistiven Detektion von oxidierenden Gasen konnte demonstriert werden. Als weiterer Demonstrator wurden ITO-Bioelektroden mit kohärenten sub-20 nm-breiten und mehrere Mikrometer langen Nanoschnitten bezüglich ihrer Impedanz modifiziert