Micro-hotplates are MEMS structures of interest for low-power gas sensing, lab-on-chips and space applications, such as micro-thrusters. Micro-hotplates usually consist in a Joule heater suspended on a thin-film membrane while thermopiles or thermodiodes are added as temperature sensors and for feedback control. The implementation of micro-hotplates using a Silicon-On-Insulator technology makes them suited for co-integration with analog integrated circuits and operation at elevated environmental temperatures in a range from 200 to 300 °C, while the heater allows thermal cycling in the kHz regime up to 700 °C, e.g. necessary for the activation of gas sensitive metal-oxide on top of the membrane, with mWrange electrical power. The demonstrated resistance of micro-hotplates to gamma radiations can extend their use in nuclear plants, biomedical sterilization and space applications. In this work, we present results from electrical tests on micro-hotplates during their irradiation by Cobalt-60 gamma rays with total doses up to 18.90 kGy. The micro-hotplates are fabricated using a commercial 1.0 μm Silicon-On-Insulator technology with a tungsten Joule heater, which allows power-controlled operation above 600 °C with less than 60 mW, and temperature sensing silicon diodes located on the membrane and on the bulk. We show the immunity of the sensing platform to the harsh radiation environment. Beside the good tolerance of the thermodiodes and the membrane materials to the total radiation dose, the thermodiode located on the heating membrane is constantly annealed during irradiation and keeps a constant sensitivity while post-irradiation annealing can restore the thermodiode.
An argument for the geometry based frequency range extension of tunable MEMS capacitors is presented. It is shown that, besides reducing the length of the feed arms, the parasitic inductances in a parallel-plate MEMS capacitor can be reduced further by optimising the plate geometry. Extension of the self resonance frequency is demonstrated with reduced circumference of the plate, due to high-frequency currents travelling around the edge of the plate and acting as a major component affecting the self-resonance frequency (SRF) of the capacitor. Full-wave 2.5-D electromagnetic simulation results using Agilent EEsof's ADS Momentum are presented that demonstrate the improvement in self-resonance frequency of circular and symmetrically fed structures. It is shown that efforts in shortening current paths by means of slots did not yield significant further improvement.
KEYWORDS: Field effect transistors, Silicon, Sensors, Oxides, Standards development, Gas sensors, CMOS technology, Semiconducting wafers, Solid state electronics, Temperature metrology
This paper describes coupled-effect simulations of smart micro gas-sensors based on standard BiCMOS technology. The smart sensor features very low power consumption, high sensitivity and potential low fabrication cost achieved through full CMOS integration. For the first time the micro heaters are made of active CMOS elements (i.e. MOSFET transistors) and embedded in a thin SOI membrane consisting of Si and SiO2 thin layers. Micro gas-sensors such as chemoresistive, microcalorimeteric and Pd/polymer gate FET sensors can be made using this technology. Full numerical analyses including 3D electro- thermo-mechanical simulations, in particular stress and deflection studies on the SOI membranes are presented. The transducer circuit design and the post-CMOS fabrication process, which includes single sided back-etching, are also reported.
Gas sensors fabricated using conventional silicon microtechnology can suffer from a number of significant disadvantages when compared with commercially available thick-film, screen-printed devices. For example, platinum gate MOSFET devices normally operate only at a temperature of up to 180 degree(s)C and this limits the catalyst activity, and hence their sensitivity and response time. In addition, the fabrication of an integrated, resistive heater poses interesting problems; thus whilst polysilicon heaters are CMOS compatible, they tend to suffer from non-linearity, poor reproducibility and stability; whereas platinum resistive heaters are incompatible with a CMOS process and thus difficult and expensive to manufacture. Here we propose the use of SOI technology leading to a new generation of high-temperature, silicon smart gas sensors (patent pending). Numerical simulations of an n-channel MOSFET structure on a thin SOI membrane have been performed in non- isothermal conditions using a MEDICI simulator. Our results demonstrate that SOI-based devices can operate at temperatures of up to 350 degree(s)C without causing a problem for neighboring CMOS I.C. circuitry. The power consumption of our SOI-based designs may be as low as ca. 10 mW at 300 degree(s)C and so compares favorably with previously reported values for non-SOI based silicon micromachined gas sensors. In conclusion, SOI technology may be used to fabricate novel high-temperature, micropower resistive and catalytic-gate MOSFET gas/odor sensors. These devices can be fabricated in a standard SOI CMOS process at low unit cost and should offer an excellent degree of reproducibility. Applications envisaged are in air quality sensors for the automotive industry and odor sensors for electronic noses.
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