Online proceedings - EDA Publishing Association

Tungstenmicro-heaterGas SensingMaterialNMOS24-26 September 2008, Rome, Italytemperature beyond which the saturation current becomescomparable with the drive current depend entirely on thedriving current. A high driving current results in lowerPMOSMembranethermodiodeReferencethermodiodeN+ N+ P+ P+P well N wellThermodiode+micro-heaterMembrane areaElectronics areaFig. 1. Cross-sectional view of suspended thermodiode, a referencethermodiode and the CMOS electronic cellsion etching (DRIE) step carried out at a micro electricalmechanical systems (MEMS) foundry. The SOI process wasbased on 1.0 μm technology with 0.25 μm SOI thickness. Theschematic cross-section of the suspended thermodiode, areference thermodiode and the CMOS electronic cells is shownin Fig. 1. The suspended thermodiode (diameter 34 μm) isembedded in a dielectric membrane with a total thickness ofabout 5 μm. Its role is to monitor the temperature of thesensing element which is placed right in the middle of themembrane (hot-spot of the IC). The membrane diameter is 300μm. The optical microscope picture of the fabricated microhotplate(with the suspended thromodiode and micro-heater) isshown in Fig. 2. A tungsten resistive micro-heater is used toheat up the membrane and was calibrated with a high precisionhot chuck (1°C resolution). A reference thermodiode wasplaced on the silicon substrate to monitor the ambienttemperature and asses the piezo-junction effect. The referencethermodiode is identical to the suspended thermodiode, exceptthat the latter is placed on the membrane.III.THERMODIODE PERFORMANCEThe forward voltage vs temperature (V-T) plots of thesuspended thermodiode and the reference thermodiode aregiven in Fig.3. One can see that there is very little differencebetween their characteristics indicating that there is minimalpiezo-junction effect and that the residual and thermallyinducedmechanical stress in the membrane plays minimal rolein the forward voltage temperature characteristics of thethermodiode. The slope of the thermodiodes is found to be -1.3mV/°C at 65μA current. As expected, the characteristicsbecome non-linear if we further increase the temperature. Toobtain such high temperatures, we have used tungstenmetallisation (instead of the standard aluminum) and thin SOIlayers (0.25 μm) to minimise the depletion region volume andthus obtain a very low value for the diode saturation current(I s ). The linearity maintained upto high temperatures is due tothis very low I s (I s ~ few fA at room temperature). To eliminatethe effect of the ambient temperature, a simple differentialcircuit, as shown in Fig. 4, can be used. The current mirrors areintegrated on-chip for good matching characteristics. Theinstrumentation amplifier (IA) is also integrated and placedoutside the membrane. The V-T slope and the maximumFig. 2. Fabricated micro-hotplate with SOI thermodiode temperature sensorForward voltage drop (V)0.900.850.800.750.700.650.600.550.50Reference thermodiodeSuspended thermodiode0 50 100 150 200 250 300Temperature ( o C)Fig. 3. Forward voltage drop vs temperature plot of the suspended andreference thermodiode.IReferencethermodiodeMembraneIMembranethermodiode50 μmFig. 4. Instrumentation amplifier (IA) circuit for temperature measurement .+- IA©EDA Publishing/THERMINIC 2008 196ISBN: 978-2-35500-008-9