Photonic crystals in biology - NanoTR-VI

PPPP Hans-EliasP andPoster Session, Thursday, June 17Theme F686 - N1123Hydroflown: A MEMS Based Underwater Acoustic Particle Velocity Sensor123M. Berke Gür,Pde BreePUTuncay AkalUP P*PDepartment of Mechatronics Engineering, Bahçeehir University, stanbul 34353, TurkeyPMicroflown Technologies, Han University of Applied Sciences, Arhhem 6826, The Netherlands3PSUASIS Ltd., TUBITAK MRC Campus, Gebze-Kocaeli 41470, Turkey21Abstract-An acoustic field is fully described by two different but related variables: the scalar acoustic pressure and the vector valued acousticparticle velocity. Conventional hydrophones measure the acoustic pressure variable only. However, it is the particle velocity that carriesdirectional information and provides a more complete description of the acoustic field. Hydroflown is a novel MEMS based underwater acousticparticle velocity sensor under development, when combined with a MEMS hydrophone is capable of completely measuring the underwateracoustic field. Unlike conventional accelerometer based underwater acoustic particle velocity sensors, the sensor will not exhibit resonance andis expected to have a flat frequncy response from dc to 20 kHz.An underwater acoustic wave is a propagating energypacket which causes a disturbance in the ambient pressure andinduces a small volumetric motion (whose rate is termed theparticle velocity). Although the amplitudes of the acousticpressure and particle velocity are related, the latter is a vectorvalued and hence, carries directional information regardingacoustic energy propagation. While the scalar valued acousticpressure is easily measured using a hydrophone, low costsensors capable of accurately measuring the particle velocityhas remained a major challenge. Several underwater sensorsdesigned for this purpose (all which are not commerciallyavailable) rely on accelerometers that are packaged such thatthe sensor is naturally buoyant in water [1,2,3].Recently, a novel MEMS based acoustic particle velocitysensor, called the Microflown, capable of measuring theparticle velocity in air was developed [4]. This paper outlinesthe preliminary concepts and work done for extending theMicroflown sensor to underwater applications. The newsensor is termed appropriately the Hydroflown. TheMicroflown measures the particle velocity using two paralleloriented platinum wire resistances. The wires are 200 nm thinand 5m wide (see Figure 1). When voltage is applied acrossthe wire terminals the wires heat up to 300°C. An acousticwave propagating perpendicular to the wires results in atemperature difference between the wires. The upstream wirewill cool down more compared to the downstream wire due toconvective heat transfer, which will result in a change in theresistance of the wires. The particle velocity is proportional tothe voltage change induced by the change in the resistance.The sensor has a flat frequency response from dc up to 20kHz.The Hydroflown sensor is to be manufactured by firstdepositing a 300nm layer of Silicon Nitride (SiR3RNR4R) on aSilicon wafer followed by the deposition of a photoresist. ThisSilicon Nitride layer is used as a mask for the wet chemicaletching process in the following steps. Next, a 200nmPlatinum layer is added using the sputter technique. Theresistive wires and the terminals are patterned from thisPlatinum layer using the lift-off technique. Afterphotolithography, the Silicon Nitride layer is removed usingreactive ion etching. Finally, the Silicon wafer is etched withanisotropic wet chemical etching using Potassium Hydroxide(KOH) to create the channel and the bridge.Although the working principle of the Hydroflown sensor isTable 1. A comparison of the acoustic properties of air andseawater.Property Air SeawaterSpeed of Sound (m/s) 340 1500Density (kg/m 3 ) 1,2 1025Acoustic Impedence (Pa s/m) 415 1,54×10 6Specific Heat (kJ/kg/K) 1,0 4,0Thermal Conductivity (W/m/K) 0,025 0,596similar to the Microflown sensor, the former is designed towork underwater. Since water will start to boil above 100°C,the sensor is required to be encapsulated in an acousticallytransparent package filled with another fluid with a higherboiling temperature. A comparison of the acoustic propertiesof air and seawater are presented in Table 1.In summary, Hydroflown, a novel underwater sensor formeasuring the acoustic particle velocity is introduced. Theworking principles of the sensor are defined. The lab tests ofthe sensor are currently in progress and the first sea trials arescheduled for the summer of 2010. The Hydroflown sensor isexpected to provide higher quality measurements with fewersensors compared to conventional pressure measuringhydrophone based systems. Initial applications of the sensorare planned for uniform line arrays. This work is supported bythe EUROSTARS Programme grant E!-4224 Hydroflown.*Corresponding author: tuakal@suasis.comFigure 1. A scanning electron microscope image of a bridgetype Microflown MEMS acoustic particle velocity sensor clearlyshowing the Platinum resistive wires and the terminals.[1] K. J. Bastyr et al., J. Acoust. Soc. Am. 106, 6 (1999).[2] W. D. Zhang et al., Sensors 9 (2009).[3] V. Shchurov, Vector Acoustics of the Ocean, engl. transl. (2006).[4] H.-E. de Bree, Acoust. Aust. 31 (2003).6th Nanoscience and Nanotechnology Conference, zmir, 2010 687