In this abstract, a micro wireless vacuum sensor is presented to be operated between high vacuum and atmospheric pressure. The sensor combines the Pirani principle and Surface Acoustic Waves

Gian Luca MORINI

Jan G. KORVINK

Juergen J. BRANDNER

Michael RAPP

Pascal NICOLAY

Sofia TOTO

Archivio istituzionale della ricerca - Alma Mater Studiorum Università di Bologna

6th Micro and Nano Flows Conference Atlanta,USA 9-12 September 2018  SAW-Pirani vacuum sensor with extended range and sensitivity   Sofia TOTO 1,*, Pascal NICOLAY 2, Gian Luca MORINI 3, Michael RAPP 1, Jan G. KORVINK 1 and Juergen J. BRANDNER1  * Corresponding author: Tel.: +49 (0)721 608 23248; Email: sofia.toto@kit.edu 1. Institute of Microstructure Technology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany  2. Carinthian Tech Research, Villach, Austria 3. Department of Industrial Engineering, University of Bologna, Bologna, Italy   Keywords: Vacuum, Pirani, Sensors, Wireless, Surface Acoustic Waves  Pressure sensors for a broad pressure range are needed for pressure monitoring and control in different applications of vacuum technology. Miniaturization of sensors helps to produce minimally invasive devices that preserve the vacuum level within the installations [1]. Thanks to the wide use of silicon micromachining, significant progress has been made in the miniaturization of thermal vacuum sensors [2-4]. Various micro machined subatmospheric pressure sensors based on different physical principles have been proposed [5-7]. In most cases, these systems can only be operated in a very specific pressure range below atmospheric pressure [8-11]. Among them are Pirani sensors and Surface Acoustic Wave sensors.  With a Pirani sensor the pressure dependent thermal conductivity of the gas surrounding the sensor is measured. When a heated element such as a wire, a plate or a chip is in contact with gas molecules,  a heat flux will be established to those molecules being proportional to the molecular density of the gas and therefore to the pressure. Measuring the temperature variation of the element allows measuring the pressure.    A Surface Acoustic Wave (SAW) sensor is measuring the pressure dependent frequency shift of a wave exciting a piezoelectric substrate. SAW are elastic waves that propagate on the surface of piezoelectric crystals. Their propagation frequency and elastic constants are sensitive to the properties of the surrounding environment. The general design consists of an interdigitally structured transducer printed on the surface of a piezoelectric crystal which then converts a voltage into a SAW and vice versa. SAW devices offer a lot of opportunities of wireless operation [12-14], which are, in most cases, not yet explored.  In this paper, a micro wireless vacuum sensor is presented to be operated between high vacuum and atmospheric pressure. The sensor combines the Pirani principle and Surface Acoustic Waves.  The sensor design depicted in Figure 1 consists of a 1 cm3 compact polymer cube crossed at its center by a microchannel. A SAW-Pirani chip is inserted inside the microchannel having 500 µm hydraulic diameter and is used for pressure sensing. The chip is heated via a Joule resistance printed at its bottom. A wireless heating coil and an interrogation antenna are inserted inside the core of the sensor. Due to the compact geometry and the integration of commercially available electronic components, the device can be integrated into microsystems as well as macrosystems.   The new sensor will be operated as follows: the SAW-Pirani chip inside the microchannel is heated via the Joule resistance at its bottom. The energy for this is obtained wirelessly from the coil beneath. An interrogation signal is sent to the sensor via the interrogation antenna using a Network Analyzer. The reflected signal including a frequency shift is received back by the same antenna. A processing algorithm calculates the pressure from the frequency shift. The sensor was designed and simulated with COMSOL, and manufacturing and assembly of a prototype was performed. The SS2467BB3 sensing chips shown in Figure 2 have been manufactured by the company SAW Components (Dresden, Germany). These chips have been tested at atmospheric pressure, and their resonance frequency has been identified by a frequency sweep using an Agilent E5071C Network Analyzer. Figure 3 shows the S11 reflection coefficient close to the resonance frequency of 2.45 GHz.   Figure 2. SS2467BB3 Chip from SAW Components Figure 1. Design of sensor 6th Micro and Nano Flows Conference Atlanta,USA 9-12 September 2018 Afterwards, the frequency shift of the chips due to temperature variations was tested. The temperature was variated using heat plates. The chips were then introduced into a vacuum environment and wireless interrogation via antennae was tested. Finally the chip was connected to the heating coil and wireless heating was implemented on it. Figure 4 (a) shows the receiving coil inside the sensor and Figure 4 (b) shows the transmitting coil for the wireless heating.   (a)  (b) Figure 3. Heating power supply (a) Receiving coil (b) Transmitting coil A 2.45 GHz interrogation antenna has been designed to fit inside the sensor with adjusted power gain and correctly matched to the chips. The antenna allows interrogation from a distance of about 10 cm [15].   References   1. Plöchinger, H., Thermal conductivity measurement with a free floating molecule detector. Vakuum in Forschung und Praxis 2016, 28, (6), 38-41. 2. Weng, P. K.; Shie, J. S., Micro‐Pirani vacuum gauge. Review of Scientific Instruments 1994, 65, (2), 492-499. 3. Puers, R.; Reyntjens, S.; De Bruyker, D., The NanoPirani—an extremely miniaturized pressure sensor fabricated by focused ion beam rapid prototyping. Sensors and Actuators A: Physical 2002, 97-98, 208-214. 4. Piotto, M.; Del Cesta, S.; Bruschi, P., A Compact CMOS Compatible micro-Pirani Vacuum Sensor with Wide Operating Range and Low Power Consumption. Procedia Engineering 2016, 168, 766-769. 5. Grau, M.; Völklein, F.; Meier, A.; Kunz, C.; Kaufmann, I.; Woias, P., Optimized MEMS Pirani sensor with increased pressure measurement sensitivity in the fine and rough vacuum regimes. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2014, 33, (2), 021601. 6. Völklein, F.; Grau, M.; Meier, A.; Hemer, G.; Breuer, L.; Woias, P., Optimized MEMS Pirani sensor with increased pressure measurement sensitivity in the fine and high vacuum regime. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2013, 31, (6), 061604. 7. Jiang, W.; Wang, X.; Zhang, J., A single crystal silicon micro-Pirani vacuum gauge with high aspect ratio structure. Sensors and Actuators A: Physical 2010, 163, (1), 159-163. 8. Jitschin, W.; Ludwig, S., Dynamical behaviour of the Pirani sensor. Vacuum 2004, 75, (2), 169-176. 9. Legendre, O.; Bertin, H.; Garel, O.; Zhang, M.; Mathias, H.; Mergherbi, S.; Juillard, J.; Mailly, F., High-Resolution Micro-Pirani Pressure Sensor With Transient Response Processing and Time-Constant Evaluation. IEEE Sensors Journal 2012, 12, (10), 3090-3097. 10. Mitchell, J.; Lahiji, G. R.; Najafi, K., An Improved Performance Poly-Si Pirani Vacuum Gauge Using Heat-Distributing Structural Supports. Journal of Microelectromechanical Systems 2008, 17, (1), 93-102. 11. Zhang, F. T.; Tang, Z.; Yu, J.; Jin, R. C., A micro-Pirani vacuum gauge based on micro-hotplate technology. Sensors and Actuators A: Physical 2006, 126, (2), 300-305. 12. Nicolay, P.; Lenzhofer, M., A Wireless and Passive Low-Pressure Sensor. Sensors 2014, 14, (2). 13. Ye, X.; Fang, L.; Liang, B.; Wang, Q.; Wang, X.; He, L.; Bei, W.; Ko, W. H., Studies of a high-sensitive surface acoustic wave sensor for passive wireless blood pressure measurement. Sensors and Actuators A: Physical 2011, 169, (1), 74-82. 14. Kim, J.; Luis, R.; Smith, M. S.; Figueroa, J. A.; Malocha, D. C.; Nam, B. H., Concrete temperature monitoring using passive wireless surface acoustic wave sensor system. Sensors and Actuators A: Physical 2015, 224, 131-139. 15. Krivec, M.; Roshanghias, A.; Abram, A.; Binder, A., Exploiting the combination of 3D polymer printing and inkjet Ag-nanoparticle printing for advanced packaging. Microelectronic Engineering 2017, 176, 1-5.  Figure 2. S11 of the chip at atmospheric pressure and room temperature near the resonance 

SAW-Pirani vacuum sensor with extended range and sensitivity

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