Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyPossibilities of Humidity Sensing with ThermalTransient Testing on Porous StructuresAndrás Vass-Várnai 1,2 , Péter Fürjes 3 , Márta Rencz 1,2vassv@eet.bme.hu, furjes@mfa.kfki.hu, rencz@micred.com1 Budapest University of Technology and Economics (BUTE), Dept. of Electron Devices2 MicReD Ltd3 Research Institute for Technical Physics and Materials Science (MFA)Budapest, HungaryBoth the modern environmental monitoring and thehome automation applications require new tailor madehumidity sensor solutions.A significant group of humidity sensors is based on theproperty changes of porous thick and thin films [1].Capacitive sensors make use of the dielectric propertychanges of such films upon water vapor uptake whichdepends on the surrounding media’s relative humiditycontent [2].However as the water molecules enter the porous layerbeside the dielectric constant the thermal conductivity ofthe layer changes as well.Thermal transient testing, a well known technique forthermal characterization of IC packages [3] can be asuitable method for detecting humidity changes this way.In the paper this measuring technique is evaluated.Experiments were done on an appropriate sensorstructure at different environments and relativehumidity levels. Based on the results, a new method forhumidity sensing is introduced.I. INTRODUCTIONAs the technology of silicon gas sensors becomes moreand more developed, new humidity sensor solutions appearon the microelectronics’ market. Humidity sensing of aporous Si structure is based on the alteration of its thermalproperties resulted by the infiltrated water having muchbetter thermal conductivity coefficient (0.58W/m·K),compared to the air (0.0257 W/m·K) and relatively highspecific heat (2,25×10 6 J/kg). The typical pore size in p or p+type silicon is 2-4 nm and 12-15 nm, therefore, watertransport in porous Si is described by Knudsen diffusion. Inaddition to adsorption-chemisorption processes, capillarycondensation phenomena are involved when considering thefine structure of the porous matrix [4]. In order to be able tomeasure the effect based on the above mentioned facts, aporous layer is used for sensing purposes in the discusseddevice. The humidity content of the porous layer, the rate ofadsorption and desorption is highly dependent on the relativehumidity (RH) changes of the environment. As the RHincreases the number of the adsorbed molecules shouldincrease and vice versa, following the shape of theadsorption-desorption isotherms [5]. As the water fills thepores the overall thermal conductivity of the layer increases[6]. In case of high surface adsorbents the decrease of thethermal resistance can be significant.The change of the thermal conductivity of the porouslayer is measured by the thermal transient methodology [3]For this reason the sensing device should also contain aheater and temperature sensing element to make thetemperature excitation and measure the temperature responsein time.II.EXPERIMENTALA. Sensor fabricationHeater and temperature reading elements of thecalorimetric sensor structures have to fulfill strict structuraland thermal requirements, like adequate mechanical stabilityor excellent thermal isolation [7]. The manufactured andanalyzed heaters and temperature read-out elements consistof Pt resistor filaments embedded in silicon rich siliconnitride.The reduced stress silicon-nitride is deposited byLPCVD process at 800 °C from a SiH 2 Cl 2 :NH 3 = 4:1 gasmixture. The micro hotplates (100×100×1µm 3 ) aresuspended across a 60–80µm deep, electrochemically etchedporous silicon sensing layer.B. Thermal transient measurementsThe sensor structure was exposed to various different RHlevels. In order to set and maintain such RH levels, theclosed environment over saturated salt solutions was used.Following the fixed-point RH sensor characterizationmethodology, MgCl, NaCl, KCl, KNO 3 solutions wereapplied to set 4 different RH levels, 32.78%, 75.29%,84.34%, 93.58% respectively [8]. The temperature of thetest environments was maintained at 25 °C by a watercooledthermostat.©EDA Publishing/THERMINIC 2008 200ISBN: 978-2-35500-008-9