Optimized integrated micro-hotplates in CMOS technology

Abstract

Within this work the development of integrated Micro-HotPlates (μHPs) for gas sensing applications as a System-On Chip (SOC) is presented. As gas sensors exploit resistance variations of sensing materials like SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at high operating temperatures, integrated μHPs are required for the dynamic and low power operation of these sensors. The optimized μHP structures consist of fully released membranes with polysilicon heaters in the oxide stack and suspension arms to the bulk silicon. Thanks to the optimized μHP design very low power consumption of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">el</sub> ~20mW at high temperatures up to T=400°C together with a thermal uniformity of only ΔT~1K across the active area ending up in the highest reported efficiency of =26-20K/mW for standard CMOS hotplates is achieved. Further a rise/fall time t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rise</sub> /t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fall</sub> =4.5/5.4ms was measured. Long term stability of the μHP has been proven applying ten million measurement cycles. Thermography confirmed the temperature distribution and functionality. The realized hotplates cover a heating area of A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μHP</sub> =100×100μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /70×70μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at arm lengths of l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">arm</sub> =70μm/50μm respectively. The chips have been realized in 0.35μm standard CMOS technology and released in a post process MEMS-etching step.