Design, Simulation and Modeling of a Micromachined High Temperature Microhotplate for Application in Trace Gas Detection

A microhotplate (MHP) is a basic Microelectromechanical System (MEMS) structure that is used in many applications such as a platform for metal oxide gas sensors, microfluidics and infrared emission. Semiconductor gas sensors usually require high power because of their elevated operating temperatures. The uniformity of the temperature distribution over the sensing area is an important factor in gas detection. There are several silicon micromachined MHP that can easily withstand temperatures between 200°C and 500°C for long periods. However there is no systematic study on the effect of the thickness of the various layers of the MHP on its characteristics at high operating temperatures of up to 700oC with lower power dissipation, lower mechanical displacement and good uniformity of the temperature distribution on the MHP. The MHP for the present study consists of a 100 μm × 100 μm membrane supported by four microbridges of length 113 μm and width 20 μm designed and simulated using CoventorWare. Tetrahedron mesh with 80μm element size is applied to the solid model, while the membrane area is meshed with 5μm element size to obtain accurate FEM simulation results. In the characterization of the MHP, the length and width of the various layers (membrane, heat distributor and sensing film) are fixed while their thicknesses are varied from 0.3 μm to 3 μm to investigate the effect of thickness on the MHP characteristics. At the fixed operation temperature of 700°C, it is shown that as membrane thickness increases, power dissipation, current density, time constant and heat transfer to the silicon substrate increases, while mechanical displacement of the membrane remains constant. When the SiC heat distributor thickness increases, a small increase in power dissipation is observed while the displacement decreases. The temperature gradient on the MHP is found to decrease with increasing thickness of the SiC and is a minimum with a value of 0.005°C/μm for a thickness of 2 μm and above. An optimized MHP device at an operating temperature of 700°C was found to have a low power dissipation of about 9.25 mW, maximum mechanical displacement of 1.2 μm, a temperature gradient of 0.005°C/μm and a short time constant of 0.17 ms

Design, Simulation and Modeling of a Micromachined High Temperature Microhotplate for Application in Trace Gas Detection

A microhotplate (MHP) is a basic Microelectromechanical System (MEMS) structure that is used in many applications such as a platform for metal oxide gas sensors, microfluidics and infrared emission. Semiconductor gas sensors usually require high power because of their elevated operating temperatures. The uniformity of the temperature distribution over the sensing area is an important factor in gas detection. There are several silicon micromachined MHP that can easily withstand temperatures between 200°C and 500°C for long periods. However there is no systematic study on the effect of the thickness of the various layers of the MHP on its characteristics at high operating temperatures of up to 700oC with lower power dissipation, lower mechanical displacement and good uniformity of the temperature distribution on the MHP. The MHP for the present study consists of a 100 μm × 100 μm membrane supported by four microbridges of length 113 μm and width 20 μm designed and simulated using CoventorWare. Tetrahedron mesh with 80μm element size is applied to the solid model, while the membrane area is meshed with 5μm element size to obtain accurate FEM simulation results. In the characterization of the MHP, the length and width of the various layers (membrane, heat distributor and sensing film) are fixed while their thicknesses are varied from 0.3 μm to 3 μm to investigate the effect of thickness on the MHP characteristics. At the fixed operation temperature of 700°C, it is shown that as membrane thickness increases, power dissipation, current density, time constant and heat transfer to the silicon substrate increases, while mechanical displacement of the membrane remains constant. When the SiC heat distributor thickness increases, a small increase in power dissipation is observed while the displacement decreases. The temperature gradient on the MHP is found to decrease with increasing thickness of the SiC and is a minimum with a value of 0.005°C/μm for a thickness of 2 μm and above. An optimized MHP device at an operating temperature of 700°C was found to have a low power dissipation of about 9.25 mW, maximum mechanical displacement of 1.2 μm, a temperature gradient of 0.005°C/μm and a short time constant of 0.17 ms

Modelization, simulation and design of micro-electro-mechanicazed systems (mems) preconcentrators for gas sensing

Hoy en día, la normativa sobre gases que son tóxicos a muy bajas concentraciones se está haciendo muy estricta. Por esta razón, los dispositivos como los preconcentradores que permiten la detección de estos gases a bajas concentraciones se están investigando cada vez más.En esta tesis se hace un estudio sobre preconcentradores planos fabricados en silicio, con la idea de poder integrarlos con el sistema sensor. Estos preconcentradores se han simulado para comprender mejor su funcionamiento. Se ha hecho un estudio para conseguir una óptima homogenización de temperatura en el área calentada. Se han fabricado nuevos diseños con calefactores optimizados. También se han fabricado membranas grandes para poder depositar más material adsorbente y así incrementar la capacidad de concentración. Los nuevos diseños han sido caracterizados para validar las simulaciones y poder crear un modelo que sirva para probar nuevas ideas de diseño evitando los largos y costosos procesos de fabricación.Nowadays the allowed limits of volatiles that are toxic at very low concentrations are becoming restrictive. For this reason, preconcentrators which let detection at low levels are becoming more important and its study is increasing.In this thesis, our aim is to design a planar preconcentrator in silicon technology, in order to be able to design in the same substrate the preconcentrator and the sensor system. Simulations have been developed to study its behaviour. A good homogenisation temperature is needed in order to obtain big concentrations in a narrow desorption peak. We will develop new designs which improve homogenisation temperature. Also, a large area is needed in order to have more adsorbent material which ensures more concentration. New design have been fabricated and characterized. Experimental results validate our simulations and let us to develop future designs avoiding time and cost fabrication

An overview of micromachined platforms for thermal sensing and gas detection

Micromachined hotplates, membranes, filaments, and cantilevers have all been used as platforms for thermal sensing and gas detection. Compared with conventional devices, micromachined sensors are characterized by low power consumption, high sensitivity, and fast response time. Much of these gains can be attributed to the size reductions achieved by micromachining. In addition, micromachining permits easy, yet precise tailoring of the heat transfer characteristics of these devices. By simple alterations in device geometry and materials used, the relative magnitudes of radiation, convection and conduction losses and Joule heat gains can be adjusted, and in this way device response can be optimized for specific applications. The free-standing design of micromachined platforms, for example, reduces heat conduction losses to the substrate, thereby making them attractive as low power, fast-response heaters suitable for a number of applications. However, while micromachining solves some of the heat transfer problems typical of conventionally produced devices, it introduces some of its own. These trade-offs will be discussed in the context of several micromachined thermal and gas sensors present in the literature. These include micromachined flow sensors, gas thermal conductivity sensors, pressure sensors, uncooled IR sensors, metal-oxide and catalytic/calorimetric gas sensors. Recent results obtained for a microbridge-based catalytic/calorimetric gas sensor will also be presented as a means of further illustrating the concepts of thermal design in micromachined sensors

Gas Sensors on Plastic Foil with Reduced Power Consumption for Wireless Applications

Recently, there is a growing interest in developing so-called "smart" RFID tags for logistic applications. These smart tags incorporate sensing devices to monitor environmental parameters such as humidity and temperature throughout the supply chain. To fulfill these requirements cost-effectively, RFID tags were produced on plastic foil through large scale manufacturing techniques. To benefit from sensing capabilities on these systems, the integration of gas sensors directly produced on plastic foil was explored. Their gas sensing performances were investigated when fabricated on same polymeric substrates than the labels. To be compatible with wireless applications, all sensors were designed to operate in the sub-milliwatt power range. The integration of three different transducers on plastic foil for the detection of different gaseous species was investigated. First, the direct use of the PET or PEN foil as an optical waveguide for the fabrication of a selective colorimetric ammonia gas sensor was carried out. It led to a simplified processing based on additive fabrication techniques compatible with large scale manufacturing. Second, the impact of miniaturization on drop-coated metal-oxide gas sensors when fabricated on polyimide foil on their sensing performances was investigated. They took advantage from the low thermal conductivity of the substrate to reduce the power consumption with a simplified processing. The detection of oxidizing and reducing gases was achieved at low power consumption when pulsing the sensors. Lastly, the benefits brought by the gas absorption in a polyimide foil were exploited with the design of a simple capacitive structure. By operating it in a differential mode with a second functionalized capacitor, the discrimination between low-concentrations of volatile organic compounds and humidity was achieved. The design and fabrication of these sensors were developed with a vision of their future production performed by large scale manufacturing techniques. The gas sensing performances of all three transducers were assessed and revealed sensitivities comparable to standard devices made on silicon. Each sensor was associated with low-power electronics targeting an integration on wireless systems. The concept of a smart gas sensing system was demonstrated with the interfacing of a capacitive humidity sensor on a passive RFID label

Microheaters based on ultrasonic actuation of piezoceramic elements

This paper describes the use of micromachined lead zirconate titanate (PZT) piezoceramic elements for heat generation by ultrasonic energy dissipated within the elements and surrounding media. Simulations based on three-dimensional finite-element models suggest that circular disk-shaped elements provide superior steady-state temperature rise for a given cross-sectional area, volume of the PZT element and drive voltage. Experimental validation is performed using PZT-5A heaters of 3.2 mm diameter and 0.191 mm thickness. Single-element heaters and dual-element stacks are evaluated. Although the steady-state temperature generated by these heaters reaches the maximum value at the frequency of maximum electromechanical conductance, the heating effectiveness is maximized at the frequency of maximum electromechanical impedance. Stacked PZT heaters provide 3.5 times the temperature rise and 3 times greater heating effectiveness than single elements. Furthermore, the heaters attain the maximum heating effectiveness when bonded to highly damping and non-conducting substrates. A maximum temperature of 120 °C is achieved at 160 mW input power. Experiments are performed using porcine tissue samples to show the feasibility of using PZT heaters in tissue cauterization. A PZT heater probe brands a porcine tissue in 2–3 s with 10 V RMS drive voltage. The interface temperature is ≈150 °C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90803/1/0960-1317_21_8_085030.pd