SOI Based Low Power Thermocatalytic Sensor with Nanostructured Gas Sensitive Material

Abstract

Introduction Catalytic gas sensors are essential devices for detection of combustive gases near lower explosion limit (LEL). As the minimum power dissipation of Pt coil based sensors (pellistors) are 120 - 150 mW [1] intensive research is devoted to reduce it to be better compatible with portable devices while preserving sensitivity and stability. The reported microheater structures can operate up to 600 oC at a cost of 20-50 mW power consumption [2]. Nowadays the research activity is focused on development of stablenanostructured catalyst layer effective at low temperature, whereas compatible with MEMS thick film technology. Gas Sensitive Catalytic Materials The approach of fabrication gas sensitive material for coil and silicon membrane sensors is different. In first case the catalyst is bulky and forms bead or cylinder with diameter 400-500µm. In MEMS structures the catalyst is deposited on a microhotplate with a characteristic diameter of 100µm, de facto forming 2D surface. In the present work nanodespersed Al2O3 and ZrO2 ceramic carriers were prepared as presented on figs.5 and 6, respectively. Each material was divided into two equal parts - an active catalytic layer from one part and a comparative element from the second part were made exhibiting equal surface area. .In order to impregnate the catalyst support with the catalyst metal, salts of palladium chloride (PdCl2) and platinum acid (H2PtCl6) were used. Having annealed at high temperature metal clusters were formed in the catalyst support. Finally the active and reference materials were mixed with an organic binder to make the paste suitable for drop-coating deposition to MEMS silicone microheater. SOI Based MEMS Microheater Uniform and reproducible crystalline Si filaments were formed from SOI (silicon on insulator) wafers, because the buried oxide provides uniform thickness of the device layer and guarantees identical geometry. Cantilevers are suspended on stress compensated SiO2-Si3N4 membrane to increase their mechanical stability and eliminate their bending out of the original plane (fig. 1-4). Thereby the reduced stress provides longer lifetime. The higher resistivity of device silicon ensures higher filament resistance at the same temperature compared to its thin film metal reference, therefore the cross section of the current routes should be increased to achieve the sufficient resistance. A plausible advantage of the single crystalline filament material and the design is the minimized degradation effect of electromigration, thereby the lifetime of the heater is expected to achieve 6000-8000 hours. Moreover, the heated area of filament can be completely covered with catalyst or passive material, similarly to the coil-type filament devices.The opened side chip design facilitates catalyst deposition. Results and Conclusions Significant issues arise when the design of thermocatalytic sensors are transferred from the volumetric to the microplanar approach. First of all, the catalytic gas-sensitive layer must provide chemical activities:3∙10-6÷10-5mW/μm3. The solution of the problem is to choose classical materials alreadybeen used for many years in coil types pellistors - catalysts of platinum group metals on Al2O3 or ZrO2 ceramic carriers. The stability and behavior of these materials at high working temperatures has been already tested over tens of years in real working conditions (mains, gas line pipes, leakage alarm systems and etc.). The decrease in the quantity of the catalytic material deposited on the microheater leads to insufficient catalytic activity of the sensor as a whole. An increase in the operating temperature can correct the situation, but it is limited by the long-term stability of the microheater and the transformation of the crystallographic phase of the ceramic catalyst carrier. The critical temperature is around 550 °C. The Pt-Pd mixed-catalysts can be applied in the microplanar structure if uniform hotplate temperature is provided and the active and the reference sensing layers are deposited such as to minimize imbalance between the two elements. Acknowledgement This research was sponsored by the Sponsored by the National Research, Development and Innovation Office Foundation, Hungary, funding No. 2017-2.3.4-TeT-RU-2017-00006, and the Ministry of Science and Higher Education of the Russian Federation founding with unique identifier RFMEFI58718X0053.