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ScienceDirect Procedia Engineering 120 (2015) 736 – 739
EUROSENSORS 2015
Fabrication of Microhotplate by Selective Laser Sintering of Micropowder for Thermal Conductivity Measuring Sensors Konstantin Oblova, Anastasia Ivanovaa,Sergey Solovieva, Nikolay Samotaeva,*, Alexey Vasilievb,c, Andrey Sokolovb,Alexandr Pisliakovb a
Micro- and Nanoelectronics Department, National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe higway 31, Moscow 115409, Russia b Kuchatov Complex of Physical and Chemical Technology, NRC Kurchatov Institute, Academician Kurchatov square 1, Moscow, 123098, Russia c St. Petersburg ITMO University, Kronverkskiy pr. 49, St. Petersburg, Russia
Abstract The layout of experimental samples of microhotplate based on nanocomposite material consisting of ruthenium dioxide and borosilicate glass binder were formed by selective laser sintering method. The resulting structures were used as microhotplate to measure changes in thermal conductivity of gases. The comparison of power consumption of standard platinum wire coil element widely used in thermocatalytic (calorimetric) sensors and microhotplate produced by the new technology showed strong advantage of the latter one. ©2015 2015The The Authors. Published by Elsevier Ltd. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: laser; MEMS; microhotplate; gas sensor;
1. Introduction The aim of our work was to develop technological process allowing quick production of small-scale batches of microhotplate for thermoconductometric gas sensors. This type of sensing elements is widely used for measuring high concentrations of combustible gases, usually above LEL (Low Explosion Level) or in thermomagnetic oxygen
* Corresponding author. Tel.:+7-495-585-8273; fax: +7-499-324-2111. E-mail address:
[email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.787
Konstantin Oblov et al. / Procedia Engineering 120 (2015) 736 – 739
sensors. As a guideline for result acceptability (first, power consumption and mechanical robustness), we have taken platinum microwire coil microhotplate (e.g. applied in thermocatalytic sensors, widely used in Russia [1]). For proper operation, thermoconductometric gas sensors require typically moderate working temperature, close to 300°C or a little more. Following technologies are mainly used for the fabrication of microheater platforms usable for gas sensing applications: silicon technology (CVD [2], SOI wafers [3] and porous silicon [4]); ceramics (thin alumina films [5, 6], slip casting under load [7, 8] and LTCC [9]); thick film (screen printing [10, 11]); All these technologies are characterized by low production cost, only in the case, when sensors are fabricated in large batches (more than ca. 1 million units per year). Moreover, some of these technologies require expensive microelectronic technological equipment and clean rooms for efficient sensor manufacturing; this equipment and tools gives additional non-transparent contribution to the cost of production. Also, some technological processes have technological limitation. For example, it is very difficult to fabricate shadow masks with hole size below 50 μm for the usage in sputtering process in ceramic and silicon MEMS technologies. Sometimes, photolithographic process is impossible, because it is very difficult to clean up porous material after photoresist deposition (typically for ceramic and porous silicon based materials). Silicon technology and flexible polymeric substrates give membranes and platinum heaters characterized by insufficient long-term stability at high temperature. Thick film technology provides too low resolution of microhotplate layout. Therefore, it is possible to formulate some limitations for each of these technologies, suitable only for mass production of sensing elements. Nevertheless, one restriction is common for all these approaches. All of them require relatively expensive tools for the production (substrate, shadow masks, photomasks etc.), therefore, these technologies are not flexible and it is impossible to change anything in technology quickly, during several hours or days, what is necessary for the optimization of sensor layout and technology. As a result, the development of new product needs very long period for stabilization of technological process.
Fig. 1. (a) The microhotplate layout (sizes are given in mm). (b) Microhotplate with homogeneous structure obtained by laser sintering of the ruthenium dioxide/glass powder with the power of 20 W. (с) Microhotplate with inhomogeneous structure obtained by laser sintering of the ruthenium dioxide/glass powder with the power of 30 W.
In our work, we tried to develop a flexible, low cost and technology effective process for the production of microhotplates to be used in gas sensors orientated to the mass application, when where it is sufficient to produce 110k samples with different layout and resistance of heater. Of course, this technology can be used as well for medium-scale mass production of gas sensors, up to several thousand units per day. Our main goal in this work was to fabricate low power consuming microhotplate with short thermal response time comparable with the level typical of ceramic MEMS [5-9] or screen-print technologies [10, 11]. 2. Experiment As a material for the microhotplate a mixture of conductive powder consisting of ruthenium dioxide RuO 2 (with particle size of 100 nm) and borosilicate glass (particle size of 1 – 2 Pm) was used. RuO2 is stable in air at temperatures up to 950°C, which makes possible its use for high-temperature heating circuits used in gas sensors area as well. Selective laser sintering of 10 Pm layer of ruthenium dioxide/glass powder deposited on 500-Pm thick
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ceramic Al2O3 substrate was carried out using CO2 laser. Characteristics of the laser facility for sintering are presented in Table 1. Layout of microhotplate simply processed in the AutoCAD program (Fig. 1a). Then, using CO2 laser, selective laser sintering was carried out by in two regimes – low and high power. Homogeneous microhotplate (Fig. 1b) - meanders with a solid structure obtained by a laser sintering of a low power, whereby sintering occurs by fusing the borosilicate glass. Inhomogeneous microhotplate (Fig. 1c) - meanders obtained by sintering at high laser power, but mean Gaussian distribution intensively- center portion of a meander partially evaporated. The difference in the physical structure of the homogeneous and inhomogeneous microhotplate could be explained with the help of Fig. 2, which schematically shows the principle of laser irradiation influence on the micropowder, which have difference in distribution sintered and evaporated areas. Since the microhotplate with inhomogeneous structure has the highest ratio bulk to surface square and that case chosen as a further object of research. Table 1.Main characteristic of the laser facility. Laser source type
CO2 tube
Wavelength, μm
10,6
Power (max),W
80
Diameter of laser beam, μm
150
Laser position system - three-axis
Servomotor
Maximum speed of the laser beam, mm/s
300
Software and hardware solution, μm
2,5
Fig. 2. Gaussian distribution of the laser radiation intensity typical for the production of (I) inhomogeneous and (II) - homogeneous microhotplate structure. Literal “S” show the sintering area on the surface of substrate.
3. Equations Microhotplate obtained during technology testing were easily separated from the ceramic substrate, and without any further processing were mounted by soldering (Ag-Pb-Sn solder with flux) in the TO-18 package (Fig. 3b). Moreover, during soldering all manipulations with microhotplate were carried out manually using pincers, that indicating good mechanical robustness of the fabricated microhotplate. Microhotplate was mounted in the TO-18 package and examined for the power consumption and the heating characteristic time. We obtained that power consumption is comparable with classic pellistors with 10 μm glass-insulated platinum microwire consuming ca. 70 mW at 450°C [1], but microhotplate heating above 240°C destroyed it due to local overheating and mechanical stresses. Simulation using COMSOL program shows that temperature distribution in the three-dimensional model of microhotplate during heating are different in different areas and have changing in range of 80 – 240°C (Fig. 3a). But despite all these shortcomings, the laser sintering technology of microhotplate is much cheaper technology of platinum microwire.
Konstantin Oblov et al. / Procedia Engineering 120 (2015) 736 – 739
Fig. 3. (a) Group of the laser sintered inhomogeneous ruthenium dioxide microhotplate s before separation from the alumina substrate; (b) Microhotplate soldered in TO-14 package; (с) The temperature distribution in the three-dimensional model of microhotplate within the temperature range 80 - 240°C obtained in COMSOL program.
4. Conclusion Nowadays, 3-D rapid prototyping technologies become more and more widespread. They are generally characterized by low cost of equipment and materials, the high speed of the prototype design and manufacturing, reduced requirements to the personnel. With the use of simple equipment for selective laser sintering, widely available software like the AutoCAD and cheap ruthenium dioxide/glass nanopowder the working microhotplate for thermoconductometric gas sensors were obtained. The power consumption and response time of the microhotplates are close to level typical of ceramic MEMS technologies and pelistor type thermocatalityc sensors.
Acknowledgements The research was performed within a frame of Russian Federation President Grant for support young scientists №14.Y30.15.7910-MK from 16.02.2015.
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