Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, Italysensitivity, but maintains the linearity of the diode upto ahigher temperature. Experimental, numerical and analyticalcalculations are all in excellent agreement as one can see inFig. 5. Furthermore the results are found to be highly uniformin different locations across the wafer and are similar from onewafer to another. The maximum temperatures of linearoperation for three different driving currents were calculatedassuming the maximum error of 10% of the non-linear terms(shown in Fig. 6). It shows that the non-linearity startsdominating at lower temperature for the lower driving current.Fig. 7 shows the temperature distribution over the micro-heaterregions simulated using ANSYS. Infra red (IR) camera(Quantum Focus Instruments Infrascope II) measurementswere also performed to see the temperature distribution acrossthe surface of the membrane at 250°C (because it was themaximum temperature allowed by the IR camera). It was foundForward voltage drop (V)1.00.80.60.40.20.014 nA1 μA28565 μA385525ExperimentalNumericalAnalyticalMembrane25 200 350 600 780 °CFig. 7. Temperature distribution over the membrane as determined byANSYS simulations.TracksMembraneTungstenmicro-heaterThermodiode+micro-heater0 200 400 600 800Temperature ( o C)Nonlinearity relative error (%)Fig. 5. Experimental, numerical and analytical V-T plot.5040302010065 μA1 μA14 nA2853855250 100 200 300 400 500 600Temperature ( o C)Fig. 6. Non-linearity relative error vs temperature plot.Fig. 8. IR measurements of the microhotplate at 250°C. Note that thetemperature profile is more accurate in the micro-heater area as beyond this thedielectric membrane is transparent to IR.from both ANSYS simulation and IR measurements (shown inFig. 8) that the temperature distribution is uniform in the heaterregion (within 5%) while it decreases rapidly beyond that,across the rest of the membrane.IV.LONG TERM CONTINUOUS RUNThe thermodiodes were operated at 400°C and 500°C for 100hours using the tungsten micro-heater on the membrane tocheck the long term stability. The tungsten micro-heater (i.e.metal RTDs) was operated using a constant current while thethermodiode was driven at current of 65 µA. As the heatingoccurred in a localized isolated membrane area (within the onchiptungsten micro-heater), the on-chip electronic circuitswere not affected. At the same time, we considered the case ofan open cavity package (for gas sensing), and therefore weassumed that the materials for packaging are not exposed to thehigh temperature levels as it is the case with the sensor and the©EDA Publishing/THERMINIC 2008 197ISBN: 978-2-35500-008-9