Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyUltra-high temperature (>300°C) suspendedthermodiode in SOI CMOS technologyF. Udrea*, S. Santra, P. K. Guha, S. Z. Ali, and I. HaneefEngineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK,*Corresponding author: email-Address: fu@eng.cam.ac.uk, tel: + 44 1223 748319, fax: +44 1223 748348Abstract- This paper reports for the first time on theperformance and long term continuous operation of asuspended silicon on insulator (SOI) thermodiode with tungstenmetallisation at temperatures beyond 300°C. The thermodiodehas been designed and fabricated with minute saturationcurrents (due to both small size and the use of SOI technology)to allow an ultra-high temperature range and minimal nonlinearity.It was found that the thermodiode forward voltagedrop vs temperature plot remains linear upto 500°C, with anon-linearity error of less than 7%. Extensive experimentalresults on performance of the thermodiode, fabricated using aCMOS (complimentary metal oxide semiconductor) SOIprocess have been presented. These results are backed up byinfra red measurements and a range of 2D and 3D simulationsusing ANSYS and ISE software. The on-chip electronics forthermodiode and micro-heater drive, as well as the transducingcircuit for the sensor were placed adjacent to the membrane.Moreover, we demonstrate that the thermodiode isconsiderably more reliable in long-term direct currentoperation at high temperatures when compared to the moreclassical resistive temperature detectors (RTDs) using CMOSmetallisation layers (Tungsten or Aluminum). Finally, webelieve that the thermodiode suffers less of piezojunction/piezo-resistiveeffects when compared to silicon basedRTDs. For this we compare a membrane thermodiode with areference thermodiode placed on the silicon substrate andassess their relative performance at elevated temperatures.I. INTRODUCTIONTemperature sensors are one of the fastest growingsegment in sensors’ market. An integrated temperature sensorfor thermal management is a core component in power hungrycircuits that tend to operate close to the maximum junctiontemperature. In such systems, accurate monitoring of thejunction temperature is mandatory to optimize the integratecircuits (ICs) performance while maintaining high reliability.Most of the CMOS (complimentary metal oxidesemiconductor) processes target now higher junctiontemperatures to allow increased packing density of transistors,better cost-performance value and more powerful processing.Maximum junction temperatures in bulk CMOS ICs has movedfrom a conservative level of 125°C to 150°C [1-4] and even to175°C. By using Copper or Tungsten, which are more resistantto electro-migration, some of these processes can potentiallymove to 200°C, provided that they address issues such latch-upand low cross-talk, and overcome reliability problems such asnegative bias temperature instability (NBTI), time dependentdielectric break-down (TDDB), etc. Furthermore, the use ofsilicon on insulator (SOI), that not only suppresses the latchup,but provides excellent vertical and lateral isolation, andminimizes the leakage currents, can lead to maximum junctiontemperatures of 225 or even 250°C. Such ICs can be of use inautomotive electronics, power supplies, motor control or otherpower systems. Nevertheless, operation beyond 300°C insilicon technologies is very rare and to the authors’ knowledge[5-7], there is no study of any IC temperature sensors at suchhigh temperatures. However a new generation of silicon-basedsensors, using CMOS technology such as smart microcalorimeters[8], resistive gas sensors [9, 10] or automotiveengines, exhausts etc can operate at such ultra-hightemperatures, well beyond the standard junction temperaturesof standard ICs. For example, micro-calorimeters can operateat 400 or 500°C. Such smart sensors use membranetechnologies for thermal isolation. The on-chip electronics canoperate very close to the ambient temperature while the activesensing element (e.g. gas sensitive layer) suspended on a verythin dielectric membrane would operate at high temperaturesfor optimal sensing. For such sensors, accurate monitoring ofthe temperature in the hot-spot of the membrane is absolutelyessential for enhanced sensitivity and selectivity (as it is thecase in gas sensors) and last but not least, for reliabilityassessment. In this paper we will report on the use of asuspended thermodiode (i.e. a thermodiode embedded in adielectric membrane) at temperatures well beyond 300°C.Linearity is preserved up to 500°C and the maximumtemperature, beyond which the saturation current becomescomparable with the drive current of the thermodiode, isaround 750°C. We demonstrate for the first time that at veryhigh temperatures the suspended thermodiode offers betterreliability than equivalent metal resistive temperature detectors(RTDs) using CMOS metals such as Aluminum or Tungstenwhile preserving very high linearity. The thermodiode alsosuffers less from the piezo-junction/piezo-resistive effect,which tends to limit the operation of silicon-based resistivesensors at very high temperatures.II.THERMODIODE DESIGN AND FABRICATIONThe micro-hotplate containing the tungsten micro-heater, thethermodiode and adjacent on-chip electronics was fabricated ina CMOS foundry with an additional post-CMOS deep reactive©EDA Publishing/THERMINIC 2008 195ISBN: 978-2-35500-008-9