Printed Miniaturized Platinum Heater on Ultra-Thin Ceramic Membrane for Mox Gas Sensors

Abstract

Introduction The progress of the Internet of Things stimulates the development of sensors of small size and low power consumption. Miniaturized metal-oxide semiconductor (MOX) gas sensors (e.g. methane, hydrogen or carbon monoxide detection) can be integrated into agro-industrial facilities such as livestock facilities, fish farming, forestry, food-storage and horticulture, where they support future-oriented plant production (smart agriculture). The central part of a MOX gas sensor is a micro-hotplate, which is mainly responsible for the sensor power consumption at operating temperatures of 450 to 600°C. Under harsh environmental conductions, ceramic materials are the best choice for the micro-hotplate substrate and sensor housing (ceramic MEMS) in combination with platinum metallization for the heater. To realize such gas sensors with low power consumption (< 200 mW@4500C) the development of miniaturized printable heaters on ultra-thin ceramic membranes is needed. Methods Yttria-stabilized zirconia (3YSZ) powders were evaluated for the preparation of casting slurries and tape casting of thin tapes < 40 µm (present on Fig.1). After thermal treatment the substrates were characterized by density, thickness, flatness, roughness and mechanical bending stability. To reduce the thermal mass of the hotplate, the ceramic substrate was cut by a developed micro-milling process to realize free standing membranes of 280 µm size in diameter (present on Fig.2). Platinum particle (20 wt.-% and 30 wt.-%) as well as Pt-SiO2-composite inks were synthesized and ink properties like viscosity, surface tension and sedimentation stability characterized. Printing tests of miniaturized heater layouts (2.0x0.5 mm2 and 40 µm line width in hot spot) were performed by aerosol-jet (Optomec M3D 150 µm nozzle) and piezoDoD inkjet (Dimatix DMP 10 pL). The printed microheater were characterized (photo of experiment present on Fig.5) after sintering by film thickness, resistivity and microstructure (SEM). The applicability is demonstrated by heating tests, where temperature and power consumption is monitored in dependence of the heater driving voltage.The all parts of gas sensor was fabricated by totally digital technological flow with using pre-developed in STL format 3D model like as is customary in rapid prototyping way. For material of sensor package was used inexpensive Al2O3 monolithic ceramics. As a form factor for sensor was using SOT-23 package (3.0x1.4x1.0 mm), which give possibility to dissipate 350 mW heating power at room temperature. Parts of sensor package was fabricated by 20W fiber laser with a wavelength of 1.064 μm and tunable pulse duration from 50 to 200 ns with using especially developed software (present on Fig.7) combined process of micronilling and on-line comparisons fabricating geometrical parameters with 3D model. Results and Conclusions Mechanical flexible 3YSZ substrates of 5x5 cm2 size with thickness of 20 to 40 µm and a high density of 6.1 g/cm3 (equals theoretical density) were achieved (Fig.1a). The choice of raw material powders correlates to the substrates surface roughness (250 to 600 nm) and mechanical stability. The developed Pt-particle inks show a good print compatibility and a high sedimentation stability of < 0.2 mm per month. Miniaturized heater layouts with a narrow line width down to 40 µm were only achieved by aerosol-jet printing (Fig.4). Too large ink droplets and a pronounced ink wetting on the substrates leads to line width > 100 µm by using inkjet. The sintered Pt-heater show a suitable resistance of 10 to 30 Ohm and were successfully evaluated by heating tests up to 450-600°C by micro melting technique [1] with using micro powder of polyamide with dependence of power consumption vs. working temperature on surface of microhotplate present on Fig.6. Acknowledgement This research was sponsored by the Federal Ministry of Education and Research (BMBF) in Germany founding No. 02P15B520, the Israel Innovation Authority and the Ministry of Science and Higher Education of the Russian Federation founding with unique identifier RFMEFI58718X0054 in frame of MANUNET project MNET17/ADMA-1147.