Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyT3Ster Master: cumulative structure function(s)0.1MgCl - 32.78 %NaCl - 75.29 %KCl - 84.34 %KNO3 - 93.58 %0.010.001Cth [Ws/K]1e-41e-51e-61e-7Fig. 3. Concept of the cumulative structure function.T3Ster Master: Smoothed response1e-80 5000 10000 15000 20000Rth [K/W]10MgCl - 32.78 %NaCl - 75.29 %KCl - 84.34 %KNO3 - 93.58%2.5403Fig. 5. Cumulative structure functions, generated by the captured transientcurves8Temperature rise [°C]64100008000MgCl - 32.78 %KNO3 - 93.58 %T3Ster Master: Tau intensityShift inthepeaks201e-4 0.001 0.01 0.1Time [s]Fig. 4. Temperature response curves of the same sensor in 4 different RHenvironmentsTime constant intensity [K/W/-]600040002000B. Structure function approachThe heat flow path in the sensor structures can bereconstructed using the structure functions. Note that theinterpretation of the structure functions is not straightforwarddue to these were not generated from steady state – to –steady state transient responses. However the results providevaluable information about the first structural layers of thesensor device, including the app. 60-80 µm thick poroussilicon. The highest thermal resistance between the Pt heaterand the bottom of the porous layer (bulk silicon) wasmeasured in the driest environment and the thermalresistance of the layer diminished as the surrounding relativehumidity content increased. These functions can be used ashumidity – proportional output of the sensing devices.01e-4 0.001 0.01 0.1Time [s]Fig. 6. Time constant spectrums of the sensors in case of the highest and lowestRH environments. The arrows indicate the time constants referring to theporous layer.Figure 6. presents the comparison of the time constantspectrums of the devices at the highest and lowest humiditylevels. The second peak refers to the main time constant ofthe porous layer. The intensity value of the given timeconstant in the spectrum is proportional to the actual thermalresistance of the active layer affected by the surroundinghumidity and observed higher at low RH level (KNO 3solution). What is even more interesting, that there is a timeshift between the two peaks relating to the thermalcapacitance change of the layer as the consequence of thewater content of the porous layer. However this effect israther small, it could open further opportunity to estimate theabsorbed amount of water in the porous matrix.©EDA Publishing/THERMINIC 2008 202ISBN: 978-2-35500-008-9