Online proceedings - EDA Publishing Association

Porous Si layer50µm24-26 September 2008, Rome, ItalyElectronicswitch4mA/1µsDrivingcurrentsourceSensorcurrentsourceElectricresistanceof the Pt,about 130 ΩFig. 2. Schematic of the measurement setupT3SterTransient capture12µV voltage and1µs time resolutionsFig. 1. The Pt filament [6]The change of the thermal conductivity of the poroussilicon layer under the Pt heater was measured electrically,applying the static thermal transient testing methodology, asit is defined in the JEDEC JESD 51-1 standard. Themeasurements were carried out by the T3Ster thermaltransient tester equipment and evaluated by the T3Ster-Master software [9]. After proper temperature sensitivitycalibration, the Pt heater served both the heater andtemperature sensing purposes.Cooling transient measurements were carried out. Asensor current source was used to maintain a small but wellmeasurable threshold voltage on the platinum heater. Thedriving current source can be switched on and off veryquickly in 1 µs to produce the power step excitation of the Ptheater. The temperature induced voltage change of the Ptresistor is measured and recorded continuously by themeasurement channels of the T3Ster, see fig.2. This realtransient was evaluated and transferred into other systemdescriptive functions by the T3ster-Master software tool.The driving current source was also used to heat up thesensor devices after each change of the RH environment, inorder to make the adsorbed water molecules evaporate fromthe porous structure and to set the output offset of thesensors.C. Thermal simulationsThe new humidity sensing methodology was alsoevaluated by thermal simulations using the SUNREDsoftware, which is based on successive network reduction. Ascaled model of the sensor structure was built including theporous silicon layer with a number of artificially designedpores. Both steady state and transient simulations werecarried out in order to show the difference between twoextreme conditions, empty pores and pores filled with water.In all simulation cases the ambient temperature was set to0 °C. The transient simulations were started at 1µs and thetime sampling was logarithmic. The simulation results werecompared to the results of the measurement on the samestructure.into a temperature transient curve. The resulting transientcurve is a unit-step response function, representing themicromechanical structure inside the package and itsenvironment. From the transient curve a Cauer type thermalimpedance model can be calculated by applying the NIDmethod (Network Identification with Deconvolution), whichis a ladder-like model of the heat flow path consisting ofthermal resistances and thermal capacitances [10]. Structurefunctions can be generated on the basis of the Cauer model.The cumulative structure function provides a map of thecumulative thermal capacitances respecting the thermalresistances measured from the heat source to the ambient[11]. The changes in the cumulative structure functionindicate the boundaries between the different structuralmaterials applied along the heat flow path, and their partialthermal resistances respectively, as presented in Fig. 3.A. Short thermal transientsSince the most important sensor response are the thermalconductivity changes of the porous adsorbent layer, thethermal transient measurements were stopped beforereaching thermal equilibrium. Each measurement lasted foronly 500 ms, which is sufficient to characterize the poroussilicon under the Pt heater, but not enough to consider thethermal effects of the whole package.The resulting temperature response curves are shown infigure 4. The highest temperature value was reached at lowRH environment referring the best thermal isolation of theheater structure. The final temperature reached in transientregime strictly decreases with raising RH, indicating animprovement in the thermal conductivity of the porous layer.The difference between the highest and lowest valuesresulted in 2.54 °C, in this particular case. The transientcurves run together until app. 30 µs, indicating thecharacteristic time of the heat propagation from the source tothe porous layer along the main trajectory of the heat flow inthe heater structure. As the front of the heat reaches theporous silicon with different humidity contents, the functionsstart to diverge.III.RESULTS AND DISCUSSIONThe temperature sensitivity of the device was measuredbetween 20 °C and 90 °C in a thermostated bath. Thesensitivity curve was linear as we expected, with theelevation of 1.293mV/°C. Knowing the sensitivity (k-factor)of the device, the voltage function can be easily transformed©EDA Publishing/THERMINIC 2008 201ISBN: 978-2-35500-008-9