Available online at ScienceDirect. Physics Procedia 72 (2015 )

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1 Available online at ScienceDirect Physics Procedia 72 (2015 ) Conference of Physics of Nonequilibrium Atomic Systems and Composites, PNASC 2015, February 2015 and the Conference of Heterostructures for microwave, power and optoelectronics: physics, technology and devices (Heterostructures), 19 February 2015 Fabrication of MEMS Platform for Sensors Applications by Laser Micro Engraving Konstantin Oblov a, Anastasia Ivanova a, Sergey Soloviev a, Nikolay Samotaev a, *, Alexey Vasiliev b, Andrey Sokolov b a Micro- and Nanoelectronics Department, National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe higway 31, Moscow , Russia b Center of Physical and Chemical Technology, NRC Kurchatov Institute, Academician Kurchatov square 1, Moscow , Russia Abstract The target of this work is the demonstration of advanced approaches able to provide rapid prototyping by using laser technology ceramic MEMS platforms for chemical sensor operating under harsh environmental conditions and, on the other hand, to assure microhotplate stable at high temperature, which can be used for the deposition of high working temperature gas sensing materials, for example, oxides of tin, gallium, zirconium and hafnium. As substrate ceramic material in work using alumina oxide 2015 The Authors. Published by Elsevier B.V. Peer-review 2015 The Authors. under responsibility Published by Elsevier ofthe B.V. National This is Research an open access Nuclear article under University the CC BY-NC-ND MEPhI (Moscow license Engineering ( Physics Institute) Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Keywords: laser ; MEMS; microhotplate; gas sensor; 1. Main text Many research groups in the World investigate now the possibility of the fabrication of metal oxide gas sensors based on micromachining technology, which present by Simon et al. (2001). All of these technologies have really * Corresponding author. Tel.: ; fax: address: nnsamotaev@mephi.ru The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi: /j.phpro

2 476 Konstantin Oblov et al. / Physics Procedia 72 ( 2015 ) low production cost starting only from large scale production (more than ~ 1 million units per year). Moreover, some of these technologies (first, silicon based approaches reported in Briand and Courbat (2013)) require expensive microelectronic technological equipment and clean rooms for efficient manufacturing; this equipment and tools give additional non-transparent contribution to the cost of production. In our work, we tried to develop an ultra fast, low cost and technology effective process for the production of MEMS microhotplates to be used in gas sensors orientated to the laboratory application, when where it is sufficient to produce samples with different layout of heater and membrane per day. Of course, this technology can be used as well for middle-scale mass production of gas sensors, up to one thousand units per day (under full load of equipment using in describing work). Using the philosophy of rapid prototyping technologies, it is possible to form two approaches to this problem add material or remove one for form the MEMS construction. As an example of technology with the materials addition (layer by layer) is LTCC (low temperature co-fired ceramic) technology. There is possible to avoid the process of etching of ceramic MEMS substrate using multi-layer ceramic system, for example stack of low LTCC sheets. These green tape sheets can be cut by laser or stamp and then the stack of these sheets can be co-fired to form rigid frame. The microhotplates based on LTCC were studied by several research groups from: Technical University of Wroclaw (Poland) reported by Teterycz et al (1998) and Kita et al. (2000), University of Bayreuth (Germany) reported by Rettig and Moos (2004), National Institute for R&D in Microtechnologies (Bucharest, Romania) reported by Moldovan et al. (2007), and others. LTCC ceramics is, obviously, very suitable material for the tooling and fabrication of all elements of microhotplate. It is possible to fabricate microhotplates suspended with ceramic beams in rigid ceramic frame, which can be much thicker than the microhotplate itself. This rigid frame provides easy manipulation with sensor during packaging. However, the microheaters made of LTCC ceramics are fabricated using commercial green tape. This green tape has minimum thickness of about 100 m. Because of this thickness and because of the application of screen-printing for the fabrication of microhotplate elements, the microhotplates are characterized by relatively high heating power consumption equal to 600 mw at C, as it was reported by Kita et al. (2000). This power consumption is even higher that power consumption of traditional thick film gas sensors reported by Kim et al. (1997). Lack of LTCC technology for rapid prototyping in the cost of the equipment set to work with the ceramic green sheet is several million Euros required clean room class ISO7 complex and expensive materials (ceramic tape and paste). Therefore, in our study, we chose an approach to the use of material removal by laser treatment monolithic ceramic aluminum oxide. The cost of a standard substrate 96% Al 2 O 3 with 60x48 mm size and a thickness of 0.5 mm used in the work is a few Euros. Prevalence and cost of this type of substrates to order more than green ceramics for LTCC technology. Platinum paste to create metallization can be prepared by hand in the presence of small skills in the field of chemistry or used from the list of LTCC materials. It is also desirable, but not necessary to have equipment for screen printing, to apply the paste uniformly to form a layout. Our main goal in this work was to fabricate MEMS microhotplate with thermal response time comparable with the level typical for ceramic MEMS reported like example in Karpov et al. (2013), Vasiliev et al. (2008) and Vasiliev et al. (2011) or screen-print technologies present in Samotaev et al. (2013) and Samotaev et al. (2007). Also additional requirements presented in Samotaev et al. (2014), Vasiliev et al. (2014) for material for microhotplate is stability up to С for using in harsh environmental conditions where low power consumption of MEMS platform is not critical. The aim of the present work is to develop a process allowing quick design and production of low-scale series of ceramic microhotplate for gas sensors. Microhotplate made by LTCC technology with power consumption of 480mW at 330 C and characteristic heating time below 10 seconds were taken as a guideline for acceptable results. 2. Experiment Laser micromachining using Minimarker installation with parameters given in Table 1 was taken as a basic technological tool. The base material for the microhotplate substrate was monolithic ceramics from aluminum oxide containing 96% alumina because of its cheapness (5 times cheaper than 99.9% alumina) and sufficient robust for sensor fabrication. As metallization base material, we took platinum containing composite with additives of organic binder components and borosilicate glass microparticles, which are used for the preparation of finely dispersed paste

3 Konstantin Oblov et al. / Physics Procedia 72 ( 2015 ) usable in screen printing process. Microhotplate layout was designed using AutoCAD program (Fig. 1a), and then loaded to the computer of laser engraving machine. According to the previously obtained relationship between the number of laser ablation passes (Fig. 2a), laser power and depth of engraving (Fig. 3a), the micromachining process was pre-set (Fig. 2b). The next steps of fabrication process were the thinning of the whole surface by engraving, cutting of the membrane obtained during the thinning process leading to the formation of beam suspended microhotplate, and dicing the substrate into chips. In the final microhotplate manufacturing step, platinum ink was deposited on its surface, and then its local laser firing was implemented. The residual, unfired, ink was removed by washing to form microhotplate. Or, alternatively, the ink was fired in oven, and the heater was formed by local laser ablation. Fig. 1. Production stage of microhotplate (a) Design layout in AutoCAD software, dimension given in mm, chip size of microhotplate is 6 6 mm. (b) Engraving by laser Al 2O 3 substrate. (с) Deposition and annealing of Pt paste on top of chip microhotplate. (d) Engraving by laser Pt paste on the Al 2O 3 substrate. Table 1. Main characteristic of the laser facility. MiniMarker -2 Laser pulsed source type ytterbium fiber Wavelength, μm 1,064 Pulse frequency (regulated), khz from 20 to 100 Power (average),w 20 Pulse energy, mj 1,0 Focused spot diameter, μm 50 Laser position system - two-axis scanner G325DT Maximum speed of the laser beam, mm/s 8700 line width with automatic filling, mm From 0,05 to 3 Software and hardware solution, μm 2,5 3. Result and Discussion Microhotplate obtained within technology tests was made during several hours; the top view of the microhotplate is presented in Fig. 1d. Microhotplate was packaged in TO-8 housing, and studied for the power consumption and heating characteristic time. The following results were obtained: power consumption 525mW at 350 C (Fig. 3b) and heating characteristic time less than 10 seconds. Subsequent studies showed that for more precise design, the simulation of microhotplate in the Ansys program, which gives more accurate heat and power parameters of the manufactured chip, should be carried out initially.

4 478 Konstantin Oblov et al. / Physics Procedia 72 ( 2015 ) Fig. 2. (a) Test structure for investigation relationship between the number of laser ablation passes, laser power and depth of engraving (Each step is related to 5 laser ablation passes for 20W laser source power (average) and bottom structure for 16W respectively ; (b) Photo of microhotplate s section with bottom side. Thickness of Al 2O 3 substrate is 500 μm. Thickness of engraving Al 2O 3 membrane is 110 μm. Roughness of engraving Al 2O 3 membrane measured by Veeco DEKTAK 150 Profilometer is 19 μm. Fig. 3. (a) Dependence deepness from numbers of engraving passes measured by Veeco DEKTAK 150 Profilometer. Black curve for 20W laser source power (average) and read carve for 16W respectively; (b) Thermal characteristics of microhotplate. 4. Conclusion Using combination of cheap monolithic ceramic substrate, thick film Platinum paste, laser engraving and AutoCAD software for sensor layout design, microhotplates with good mechanical properties were fabricated. These chips can be fixed and bonded in TO-8 packaging; power consumption and response time characteristics are close to level typical for LTCC ceramic MEMS technologies. Production speed (laser engraving) of single MEMS structure was the order of 1 minute and 15 minutes for group annealing of Platinum paste in belt furnace. Our experiments show that describing technology in experience hands gives fantastic speed the design and manufacture of ready-touse microhotplate in the order of 1 hour. The cost of materials (alumina substrate and Platinum paste) for the manufacturing single microhotplate size 6 6 mm is about 0.50 Euro. The concept of the fabrication process also enables the integration of an array of sensor elements with different sensing layers and/or operating temperatures on to one single ceramic chip to be used for enhanced gas mixture analysis.. Acknowledgements The research was performed within a frame of Russian Federation President Grant for support young scientists 14.Y MK from

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