Gas Sensors Based on MEMS Structures Made of Ceramic ZrO2/Y2O3 Material A.A.Vasiliev1, A.S.Lipilin2, A.M.Mozalev3, A.S.Lagutin1, A.V.Pisliakov1, N.P.Zaretskiy1, N.N.Samotaev1, A.V.Sokolov1 1
NRC “Kurchatov Institute”, IACPh, Kurchatov sq., 1, 123182, Moscow, Russia, email:
[email protected] 2 Institute of Electrophysics of Ural Branch of RAS, Amundsen street., 106, 620016, Ekaterinburg, Russia 3 Belarus State University of radioelectronics and informatics, P. Brovki street, 6, Minsk, 220013, Belarus Abstract The methods of the fabrication of MEMS platforms based on yttria stabilized zirconia (YSZ) and alumina membranes for gas sensors used in harsh environmental conditions are described. It is shown that the application of such membranes permits a decrease in MEMS power consumption at 4500C down to ~75 mW at continuous and down to ~ 1 mW at pulse heating of gas sensor.
Keywords: Gas Sensors, Harsh Applications, MEMS Platforms, Zirconium Dioxide, Aluminum Oxide. Introduction One of important requirements to the instruments used for the gas analysis based on semiconductor, thermocatalytic, and electrochemical detection principles and applied in autonomous and wireless systems of gas media monitoring is the minimization of gas sensor power consumption. Recently, the most prospective for this is the application of microelectromechanical (MEMS) structures as a basis for such sensors. In these structures, a dramatic decrease in sensor power consumption is reached due to very strong decrease in heat dissipation of hot sensing element of the sensor heated up to 300 – 6000C, which is located not on a massive substrate, but on thin dielectric membrane.
Fig. 1. The structure of the ceramic MEMS gas sensor5. 1 – ceramic substrate; 2 – glue layer; 3 – thin ceramic membrane; 4 – sensing layer; 5 – laser drilled hole in ceramic substrate; 6 – platinum heater of the gas sensor; 7 – contact pads to the heater and sensing layer; 8 – measuring electrode for the measurement of the resistance of semiconductor sensing layer.
In traditional MEMS instruments fabricated using silicon technology, thin dielectric membrane is applied as supporting and thermo-insulating element of sensor construction; it is made of several layers of silicon oxide and silicon nitride. Such multilayer structure enables a decrease in mechanical strains in the membrane because the signs of Smart Sensors, Actuators, and MEMS V, edited by Ulrich Schmid, José Luis Sánchez-Rojas, Monika Leester-Schaedel, Proc. of SPIE Vol. 8066, 80660N · © 2011 SPIE CCC code: 0277-786X/11/$18 · doi: 10.1117/12.887500 Proc. of SPIE Vol. 8066 80660N-1
mechanical strains in silicon oxide and silicon nitride are opposite. Thin dielectric membrane of the sensor is fixed on a frame made of silicon and is fabricated by successive CVD deposition of silicon oxide and silicon nitride layers on silicon wafer followed by anisotropic etching of silicon from the back side of silicon. This etching releases free dielectric membrane. In contrast to silicon MEMS technology, in ceramic MEMS structures thin membrane is fixed on the frame made of the same material as the material of the membrane (Fig. 1). Many research groups and R&D departments of the companies producing gas sensors try now to fabricate gas sensors based on MEMS platforms using silicon technology1-3. It is supposed that these platforms will be used for the fabrication of semiconductor, thermocatalytic, optical and electrochemical gas sensors operating at temperature up to 6000C and even more. However, in spite of all effort made in this field, multilayer membranes made of silicon oxide and silicon nitride have now several important drawbacks. • Very strong internal mechanical stresses inside the dielectric membrane of the MEMS structure due to big difference in the thermal expansion coefficients of silicon, silicon oxide, and silicon nitride. In the case of the application of multilayer membrane, these mechanical strains in the interface between silicon oxide and silicon nitride lead relatively fast to the destruction of the MEMS structure even at the operation at constant temperature. • Bad adhesion of platinum material of microheater to the surface of the membrane, that is to silicon oxide or silicon nitride. At room temperature, this adhesion can be improved by the application of intermediate adhesive layers of titanium4 tantalum, or chromium3. However, at working temperature of the sensor (~4500C) and especially at temperature necessary for the formation of the gas sensitive layers of semiconductor or thermocatalytic sensors (700 – 8000C), adhesion of platinum with sub-layers of these materials to the membrane material is lost, and platinum detaches from the membrane. • Low chemical stability of silicon nitride at high temperature in humid atmosphere. In this publication we suggest a method of the solution of these problems by the application of ceramic membranes fixed on the ceramic substrate with holes (Fig. 1).
MEMS based on thin alumina membranes
Earlier5, we investigated ceramic MEMS with membranes made of aluminum oxide. These membranes can be fabricated using so-called spark-electrolyte oxidation or by anodic oxidation of aluminum foil. It was shown in paper5 that the optimal thickness of the membrane fabricated using spark-electrolyte oxidation of aluminum is of about 20 – 30 microns. In this case, membrane has satisfactory mechanical strength and relatively low heat conductivity in superficial direction assuring good thermal isolation of the heater deposited on top of the membrane. Photo of this MEMS chip is shown in Fig. 2.
Fig. 2. Sensor chip with thin alumina membrane fabricated using spark electrolyte oxidation of aluminum5.
Disadvantage of these membranes is their insufficient mechanical robustness. In addition, the surface is crumbly and rough (Fig. 35), as a result, wire bonding of the sensor chips is difficult, because the tool of ultrasonic wire bonding machine deforms the membrane, and the quality of resulting contact is not perfect. To overcome the difficulties mentioned above, we fabricated and used in our ceramic MEMS devices thin membranes made by anodic oxidation of aluminum in acidic electrolyte. It is known that the size and density of pores in
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these membranes depend on electrical and electrolytic regimes of their formation. Details of the process of alumina films are described in the book6. The main disadvantage of anodic films is their mechanical instability at high-temperature annealing, which bends the film. Until now, this disadvantage restricted the application of anodic oxide films in thermocatalytic and semiconductor MEMS sensors with working and technological treatments at temperature higher than 600оС.
Fig. 3. SEM photo of the surface of the alumina membrane fabricated by spark electrolyte oxidation of aluminum5.
To solve this problem, we developed an original method of high-speed galvanostatic anodization of thin aluminum foil in diluted water solution organic acids. The thickness of foil was in the range between 9 and 14 microns. The optimization of the regimes of oxidation depending on foil size, material and geometry, permitted us to get uniform and high-quality oxidation of whole surface of the foil. Thermocatalytic and semiconductor gas sensors operate at temperature up to 5000C. Moreover, the formation of sensing layers of these sensors requires technological annealing of the device at temperature up to 9000C. Therefore, for the proper operation of the sensor, alumina membrane must be annealed before use at high temperature (up to 12000C). The morphology of anodic membranes after annealing at 11500C was studied using scanning electron microscope JEOL JSM 6500. Microphotographs in Fig. 4 show the view of the cross section of the membrane.
Fig. 4. Cross-section of the 20-micron membrane fabricated by anodic oxidation of aluminum foil. The photo was made after annealing of the membrane at 11500C.
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Photo in Fig 5 shows the structure of the superficial layer of the membrane after annealing.
Fig. 5. SEM photo of the superficial layer of the membrane fabricated by anodic oxidation of aluminum foil. The photo was made after annealing of the membrane at 11500C.
Membranes made of anodic alumina were glued with glass onto the ceramic alumina substrate with holes and ware used for the fabrication of MEMS devices. Photo of the substrate with membrane is shown in Fig. 5. The size of the substrate can be up to 48 x 60 mm, it contains up to 80 individual chips. It is easy to see that the membrane over the hole in ceramic substrate is almost flat; this is important for further fabrication of platinum microheater on the surface of the membrane formed by vacuum sputtering through shadow mask.
Fig. 5. Top and back side view of alumina substrate with anodic membrane glued with glass. The size of each individual chip is 6 x 6 mm, hole diameter is of 3 mm. The thickness of the membrane is of 10 microns.
Membranes made of annealed anodic alumina are characterized by good mechanical properties. This enables the fabrication of different ceramic MEMS devises operating at high temperature (thermoanemometers, bolometers, gas sensors, etc.). In addition, platinum and other metallic coating have perfect adhesion to the surface of the membrane.
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Membrane has minimal thickness (down to 10 microns and even less) and therefore, minimal possible heat conductivity in lateral direction. This last properties permits us to heat up to high temperature only a small part of the membrane with heater and functional elements of the sensor, and therefore to fabricate devices working in autonomous and wireless instruments. Disadvantage of these membranes made of annealed porous anodic alumina is their relatively high coefficient of thermal conductivity equal to about 25 W/m·K. Therefore we tried to look for materials with lower coefficient of thermal conductivity, which could be used for the fabrication of ceramic MEMS devices. One of these materials is zirconium dioxide stabilized with yttrium. Thermal conductivity of ZrO2 (about 2,5 W/m·K) is lower by one order of magnitude compared to thermal conductivity of aluminum oxide, this enables a considerable decrease in power consumption of microelectronic devices operating at high temperature, for example, of MEMS gas sensors.
MEMS based on membranes made of yttria stabilized zirconia We fabricated experimental samples of ceramic MEMS platforms with thin membranes of zirconium dioxide stabilized with yttrium (YSZ). Ceramic film was made by slip casting with consequent annealing under mechanical load. Ceramic slip consists of small particles with size of about 20 nm, therefore satisfactory quality of the film sintering was obtained at relatively low temperature of about 11500C and sintering time of 12 hours. Resulting membrane has thickness of about 10 microns, SEM photos of this membrane (top view and cleaving, respectively) are presented in Figs. 6a and 6b.
a.
b.
Fig. SEM photo of the ceramic membrane made of yttria stabilized zirconia. a. Top view of the membrane; b. cross section (cleaving) of the membrane.
After annealing the membrane consists of rounded particles approximately 0.2 micron in diameter, the roughness of the membrane surface does not exceed 0.1 micron. This simplifies the deposition of platinum for the formation of the heater compared to the films of aluminum oxide obtained by spark electrolyte oxidation of aluminum. The substitution of the spark electrolyte oxidation with ceramic technology enables a significant improvement of the purity of the membrane and its uniformity. YSZ membrane was glued with glass onto the 0.5 mm thick substrate with 3 mm holes. The substrate was made of the same material (YSZ) as the membrane. For the gluing, glass particle containing ink was distributed on the surface of the substrate using screen printing machine PT-100. The glass chosen for the fabrication of the ink has approximately the same thermal expansion coefficient as the YSZ material (about 100 ppm per degree). After ink deposition, the membrane was pressed carefully to the wet layer of the ink, dried at 1500C and fired at 9500C. Microheater is fabricated by magnetron sputtering of platinum through shadow mask; the thickness of platinum is of 0.3 micron. After platinum deposition, the substrate with membrane and heaters was annealed at 8000C for platinum
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properties stabilization and cut for the formation of sensor chips with dimension 6 x 6 mm. Layout of the heater is shown in Fig. 8.
Fig. 8. Layout of the MES heater of semiconductor gas sensor. The area of the heater is of about 400 x 250 microns, the width of platinum lines is of about 40 microns.
Chips were packaged in TO-8 packaging using gold wire bonding. Contact pads of the sensor chip remain at near room temperature; this assures high stability of contacts compared to the chips made using traditional thick film technology. Electrical characteristics of the heater were measured using Keithley 2400 source-measuring device, which permits voltage and measure electric current through the sensor heater stabilization. For the determination of heater temperature, TCR of thin film platinum was determined preliminarily by calibration in the oven. It was equal to 0.27 % per degree according to the results of the paper5. The main characteristics of the heater is its heating power as a function of temperature. This plot is presented in Fig. 9.
Heating power, mW
80
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0 0
100
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Temperature, C Fig. 9. Heating power of the heater presented in Fig. 8 as a function of its temperature.
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Results obtained in this paper show that the application of YSZ membranes permits us to decrease power consumption of gas sensors down to 75 mW at continuous heating at 4500C, and down to approximately 1 mW and even lower at the operation in pulsing heating mode. Thus, ceramic membrane made of zirconium dioxide stabilized with yttrium oxide has much lower superficial heat conductivity compared to the film made by anodic oxidation of aluminum. This heat conductivity is comparable with heat conductivity of very porous membrane fabricated by spark-electrolyte oxidation of aluminum. At the same time, low roughness of the YSZ membrane enable the application of much thinner (0.2 – 0.3 micron) platinum heater layers compared to 1 micron used in the case of spark electrolyte alumina membranes5. It is necessary to note that the roughness of anodic alumina membranes is comparable with those of YSZ membranes and can be improved by preliminary electropolishing of initial aluminum foil.
Conclusion The results described in this paper concerning a comparative study of MEMS platforms of gas sensors containing thin membranes made of annealed anodic alumina and yttria stabilized zirconia (YSZ) show that membranes made of YSZ have much lower heat transfer coefficient in whole temperature range of working temperatures (25 W/m·K and 2.5 W/m·K, respectively). The application of YSZ membranes enables significant decrease in power consumption of the sensor working at 4500C. This power is equal to about 75 mW at constant heating of the sensor and to about 1 mW in average at pulsing heating with duty cycle of 0.01. Taking into account evident technological and economical advantages of the fabrication of alumina membranes made by anodic oxidation of aluminum foil (technical simplicity, minimal time necessary for membrane fabrication, cheap aluminum foil used in the process, ecological compatibility, etc.) it is necessary to optimize the process of the formation of alumina membrane to minimize their power consumption. Both technologies (anodic alumina and YSZ) can be adapted successfully to mass production of ceramic MEMS platforms of semiconductor and thermocatalytic sensors of combustible gases, gas fire detectors, and electrochemical sensors of oxygen.
Acknowledgments The results of this work were obtained partially in the framework of the joint S3 project co-financed by the program FP-7 of the European Union (grant agreement # 247768) and Russian ministry of science and education (State contract # 02.527.11.0008).
References [1] http://www.silsens.ch/products.htm [2] http://www.appliedsensor.com [3] A.A.Vasiliev, R.G.Pavelko, X.Vilanova, N.N.Samotaev, A.V.Sokolov, V.G.Sevastianov, V.Guarnieri, L.Lorenzelli. Micromachined thermocatalytic gas sensor with improved selectivity based on Pd/Pt doped YSZ material. 11th International Meeting on Chemical Sensors, Brescia, Italy, July 11 – 19, 2006, Book of abstracts, p. 127 – 128. [4] A. Vila, J. Puigcorbe, D. Vogel, B. Michel, N. Sabate, I. Gracia, C. Cane, J.R. Morante. Reliability analysis of Pt-Ti micro-hotplates operated at high temperature. Eurosensors XVII, Portugal, 2003, book of abstracts, p. 250. [5] A.A.Vasiliev, R.G.Pavelko, S.Yu.Gogish-Klushin, D.Yu.Kharitonov, O.S.Gogish-Klushina, A.V.Sokolov, A.V.Pisliakov, N.N.Samotaev. Alumina MEMS platform for impulse semiconductor and IR optic gas sensors. Sensors and Actuators B, v. 132 (2008), p. 216–223. [6] L.M.Lynkov, N.I.Mukhurov. Microstructures based on anodic alumooxide technology. Book. Minsk, Bestprint, 2002.
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