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

Localized actuation of temperature responsive hydrogel-layers with a PCB-based micro-heater array

Space-resolved stimulation of active hydrogel layers can be achieved for example by using a micro-heater array. In the current work, we present the interaction of (i) such a rigid array of heating elements that can be selectively activated and (ii) an active thermo-responsive hydrogel layer that responds to the local stimulus change. Due to the respective local actuation, (iii) the surface form of a passive top-layer can be manipulated. We present continuum-based simulative predictions based on the Temperature Expansion Model and compare them to experimental outcomes for the system

Novel High Temperature Materials for In-Situ Sensing Devices

The overriding goal of this project was to develop gas sensor materials and systems compatible with operation at temperatures from 500 to 700 C. Gas sensors operating at these temperatures would be compatible with placement in fossil-energy exhaust streams close to the combustion chamber, and therefore have advantages for process regulation, and feedback for emissions controls. The three thrusts of our work included investigating thin film gas sensor materials based on metal oxide materials and electroceramic materials, and also development of microhotplate devices to support the gas sensing films. The metal oxide materials NiO, In{sub 2}O{sub 3}, and Ga{sub 2}O{sub 3} were investigated for their sensitivity to H{sub 2}, NO{sub x}, and CO{sub 2}, respectively, at high temperatures (T > 500 C), where the sensing properties of these materials have received little attention. New ground was broken in achieving excellent gas sensor responses (>10) for temperatures up to 600 C for NiO and In{sub 2}O{sub 3} materials. The gas sensitivity of these materials was decreasing as temperatures increased above 500 C, which indicates that achieving strong sensitivities with these materials at very high temperatures (T {ge} 650 C) will be a further challenge. The sensitivity, selectivity, stability, and reliability of these materials were investigated across a wide range of deposition conditions, temperatures, film thickness, as using surface active promoter materials. We also proposed to study the electroceramic materials BaZr{sub (1-x)}Y{sub x}O{sub (3-x/2)} and BaCe{sub (2-x)}Ca{sub x}S{sub (4-x/2)} for their ability to detect H{sub 2}O and H{sub 2}S, respectively. This report focuses on the properties and gas sensing characteristics of BaZr{sub (1-x)}Y{sub x}O{sub (3-x/2)} (Y-doped BaZrO{sub 3}), as significant difficulties were encounter in generating BaCe{sub (2-x)}Ca{sub x}S{sub (4-x/2)} sensors. Significant new results were achieved for Y-doped BaZrO{sub 3}, including sensitivities of more than 60 atm{sup -1} for H{sub 2}O vapor at 400 C. These results were achieved despite significant difficulties with a strong Ba deficiency in the deposited films, and difficulties with stress in the targets and films. Ultimately, these films achieved good sensitivity, selectivity, and reliability in our gas sensing tests. The final thrust of our project was to develop microhotpates. We proposed the use of SiC thin films for the heater of the microhotplate, but despite extensive efforts we were not able to secure a reliable source of SiC. An alternative microhotplate architecture using SiO{sub 2} and Si{sub 3}N{sub 4} suspended membrane structures, and a polysilicon heater were developed, which could be fabricate at commercial MEMs foundries. These microhotplates were fabricated at Microtechnology Services Frankfurt (MSF) in Germany. The fabricated heaters were able to achieve temperatures > 600 C using {approx} 0.25 W, and when combined with In{sub 2}O{sub 3} films demonstrated sensor systems with sensor responses up to 50 for 25 ppm NO{sub x}, and time constants of less than 10 s

A Sensor Platform for Smart Hydrogels in Biomedical Applications

Smart hydrogels are inherently biocompatible hydrophilic three-dimensional polymer networks able to undergo a volume-phase transition in the presence of an analyte. By molecular imprinting and/or aptamer-based approaches they can be tailored for a wide range of analytes with high selectivity. In combination with the biocompatibility, this makes hydrogels very promising candidates for biomedical sensor applications. However, to date hydrogels are rarely used for that purpose as the reliable detection of their swelling state remains a challenge. Here we report on a newly developed biocompatible bending sensor platform which can be equipped with almost any smart hydrogel, thereby paving the way for biomedical applications

Low-Cost Microfluidic Sensors with Smart Hydrogel Patterned Arrays Using Electronic Resistive Channel Sensing for Readout

There is a strong commercial need for inexpensive point-of-use sensors for monitoring disease biomarkers or environmental contaminants in drinking water. Point-of-use sensors that employ smart polymer hydrogels as recognition elements can be tailored to detect almost any target analyte, but often suffer from long response times. Hence, we describe here a fabrication process that can be used to manufacture low-cost point-of-use hydrogel-based microfluidics sensors with short response times. In this process, mask-templated UV photopolymerization is used to produce arrays of smart hydrogel pillars inside sub-millimeter channels located upon microfluidics devices. When these pillars contact aqueous solutions containing a target analyte, they swell or shrink, thereby changing the resistance of the microfluidic channel to ionic current flow when a small bias voltage is applied to the system. Hence resistance measurements can be used to transduce hydrogel swelling changes into electrical signals. The only instrumentation required is a simple portable potentiostat that can be operated using a smartphone or a laptop, thus making the system suitable for point of use. Rapid hydrogel response rate is achieved by fabricating arrays of smart hydrogels that have large surface area-to-volume ratios

Smart Hydrogel Swelling State Detection Based on a Power-Transfer Transduction Principle

Stimulus-responsive (smart) hydrogels are a promising sensing material for biomedical contexts due to their reversible swelling change in response to target analytes. The design of application-specific sensors that utilize this behavior requires the development of suitable transduction concepts. The presented study investigates a power-transfer-based readout approach that is sensitive to small volumetric changes of the smart hydrogel. The concept employs two thin film polyimide substrates with embedded conductive strip lines, which are shielded from each other except at the tip region, where the smart hydrogel is sandwiched in between. The hydrogel’s volume change in response to a target analyte alters the distance and orientation of the thin films, affecting the amount of transferred power between the two transducer parts and, consequently, the measured sensor output voltage. With proper calibration, the output signal can be used to determine the swelling change of the hydrogel and, consequently, to quantify the stimulus. In proof-of-principle experiments with glucose- and pH-sensitive smart hydrogels, high sensitivity to small analyte concentration changes was found along with very good reproducibility and stability. The concept was tested with two exemplary hydrogels, but the transduction principle in general is independent of the specific hydrogel material, as long as it exhibits a stimulus-dependent volume change. The application vision of the presented research is to integrate in situ blood analyte monitoring capabilities into standard (micro)catheters. The developed sensor is designed to fit into a catheter without obstructing its normal use and, therefore, offers great potential for providing a universally applicable transducer platform for smart catheter-based sensing