MICROBOLOMETERS

1. MICROBOLOMETERS KORAY POLAT 500612007 2013-SPRING

2. Outline • Theory of operation • Applications • Consequences • Referances

3. Microbolometer Thermal sensor convertradiationtoheatandmeasuretemperaturedifferencefor IR sensing. Andthistype of sensorsarecalledwiththe name BOLOMETER A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Theresistancechange is measuredandprocessedintotemperatureswhich can be usedtocreate an image. Unlikeothertypes of infrareddetectingequipment, microbolometers do not requirecooling.

4. Infraredradiationwithwavelengthsbetween 7.5-14 μmstrikesthedetectormaterial • Unwantedlightwavesfiltered • Remainingwaves hit the sensor • Temperatureincreases • Thischangestheelectricalresistance

5. Theory of Operation • A microbolometer is an uncooledthermalsensor. • Unlikeexpensivecoolingmethodsincludingstirlingcyclecoolersandliquidnitrogencoolersbolometers not needtoanycoolingmethod • Thissensorsalsogain 10 minutesunliketheotherthermalresolutionsensors.

6. Thediagram of microbolometer is in figurebelow; • Eachcompanythatmanufactures microbolometers has theirownuniqueprocedureforproducingthemandtheyevenuse a variety of differentabsorbingmaterials • Thebottomlayerconsists of a siliconsubstrateand a readoutintegratedcircuit (ROIC). • Electricalcontactsaredepositedandthenselectivelyetchedaway. • A reflector, forexample, a titaniummirror, is createdbeneaththe IR absorbingmaterial.

7. A sacrificiallayer is depositedsothatlater in theprocess a gap can be createdtothermallyisolatethe IR absorbingmaterialfromthe ROIC. • A layer of absorbingmaterial is thendepositedandselectivelyetchedsothatthe final contacts can be created. • Tocreatethe final bridgelikestructureshown in Figurebelow, thesacrificiallayer is removedsothattheabsorbingmaterial is suspendedapproximately 2 μmabovethereadoutcircuit. • Because microbolometers do not undergoanycooling • Theabsorbingmaterialmust be thermallyisolatedfromthebottom ROIC andthebridgelikestructureallowsforthistooccur

8. Themicrobolometerarray is commonlyfound in twosizes, 320×240 pixelsorlessexpensive 160×120 pixels. • Currenttechnology has ledtotheproduction of deviceswith 640×480 or 1024x768 pixels • There has alsobeen a decrease in theindividualpixeldimensions. Thepixel size wastypically 45 μm in olderdevicesand has beendecreasedto 17 μm in currentdevices. As thepixel size is decreasedandthenumber of pixelsperunitarea is increasedproportionally, an imagewithhigherresolution is created.

9. Detectingmaterialproperties • Thedevicesresponsivity is a mainfactor , howwellthe device willwork . • Responsivity is theability of the device toconverttheincomingradiationinto an electricalsignal. • Detectormaterialpropertieseffectthisvaluehenceseveralmainmaterialpropertiesshould be investigated: TCR, 1/f Noise, andResistance.

10. Temperaturecoefficient of resistance ( TCR ) • Thematerialused in thedetectormustdemonstratelargechanges in resistance as a result of minutechanges in temperature. • As thematerial is heated, duetotheincominginfraredradiation, theresistance of thematerialdecreases. • his is relatedtothematerial'stemperaturecoefficient of resistance (TCR) specificallyitsnegativetemperaturecoefficient. • Industrycurrentlymanufactures microbolometers thatcontainmaterialswithTCRsnear -2%. • Althoughmanymaterialsexistthathave far higherTCRs, thereareseveralotherfactorsthatneedto be takenintoconsiderationwhenproducingoptimized microbolometers.

11. 1/f noise • 1/f noise, likeothernoises, causes a disturbancethataffectsthesignalandthatmaydistorttheinformationcarriedbythesignal. • Changes in temperatureacrosstheabsorbingmaterialaredeterminedbychanges in thebiascurrentorvoltageflowingthroughthedetectingmaterial. • Ifthe noise is largethensmallchangesthatoccurmay not be seenclearlyandthe device is useless • a detectormaterialthat has a minimum amount of 1/f noise allowsfor a clearersignalto be maintainedbetween IR detectionandtheoutputthat is displayed.

12. Resistance Using a materialthat has lowroomtemperatureresistance is alsoimportant. resistanceacrossthedetectingmaterialmeanlesspowerwillneedto be used. Also, there is a relationshipbetweenresistanceand noise, thehighertheresistancethehigherthe noise. Thus, foreasierdetectionandtosatisfythelow noise requirement, resistanceshould be low.

13. Detectingmaterials Thetwomostcommonlyused IR radiationdetectingmaterials in microbolometers areamorphoussiliconandvanadiumoxide. Amorphous Si (a-Si) workswellmainlybecause it can easily be integratedintothe CMOS fabricationprocess. Tocreatethelayeredstructureandpatterning, theCMOSfabricationprocess can be used but it requirestemperaturestostaybelow 200˚C on average. A problem withsomepotentialmaterials is thattocreatethedesirablepropertiestheirdepositiontemperaturesmay be toohighalthoughthis is not a problem for a-Si thinfilms. a-Si alsopossessesreasonablevaluesfor TCR, 1/f noise andresistancewhenthedepositionparametersareoptimized.

14. Vanadiumoxidethinfilmsmayalso be integratedintothe CMOS fabricationprocessalthough not as easily as a-Si fortemperaturereasons. • VO2 has lowresistance but undergoes a metal-insulatorphasechangenear 67 °C andalso has a lowervalue of TCR. • On theotherhand, V2O5exhibitshighresistanceandalsohigh TCR. • Manyphases of VOxexistalthough it seemsthat x≈1.8 has becomethemost popular formicrobolometerapplications.

15. Active vs Passive microbolometers • Most microbolometers contain a temperaturesensitiveresistorwhichmakesthem a passiveelectronic device. • In 1994 onecompany, Electro-Optic Sensor Design (EOSD), beganlookingintoproducing microbolometers thatused a thin film transistor (TFT), which is a specialkind of fieldeffect transistor. • Mainchange in thesedeviceswould be theaddition of a gateelectrode. • Althoughthemainconcepts of thedevicesaresimilar, usingthisdesignallowsfortheadvantages of the TFT to be utilized.

16. Advantages • Theyaresmallandlightweight. Forapplicationsrequiringrelativelyshortranges, thephysicaldimensions of thecameraareevensmaller. Thispropertyenables, forexample, themounting of uncooledmicrobolometerthermalimagers on helmets. • Providereal video outputimmediatelyafterpower on. • Lowpowerconsumptionrelativetocooleddetectorthermalimagers. • VerylongMTBF. • Lessexpensivecomparedtocamerasbased on cooleddetectors.

17. Disadvantages • Lesssensitivethancooledthermalandphotondetectorimagers. • Cannot be usedformultispectralorhigh-speedinfraredapplications. • Have not beenabletomatchtheresolution of cooledsemiconductorbasedapproaches. • Higher noise thancooledsemiconductorbasedapproaches.

18. Performancelimits • Thesensitivity is partlylimitedbythethermalconductance of thepixel., • thespeed of response is limitedbythethermalheatcapacitydividedbythethermalconductance. • Reducingtheheatcapacityincreasesthespeed but alsoincreasesstatisticalmechanicalthermaltemperaturefluctuations (noise). • Increasingthethermalconductanceraisesthespeed, but decreasessensitivity.

19. Applications • The application areas of the uncooled detectors can be summarized as: • MilitaryApplications: • Simplesurveillance • This sensor type is a general bolometertechnologyandthenightvisiongetsforsecurityandsurveillance. This not alsouseformilitary but alsocivilsecuritysystems.

20. Riflesights • Inmilitary, forriflestherearenightvisioncameraaparatus. • Thisthermalsensing is veryimportantanddevelopingbythetechnology.

21. Advancedthreatwarning Forthenationalsecuritybolometersareusedformonitoringdangerousthreat, especiallyforbordermonitoring.

22. Unattendedgroundsensors Unattendedgroundsensorsare small ground-based sensors that collect intelligence through seismic, acoustic, Radiological Nuclear and Electro-Optic means. These sensors are networked devices that provide an early warning system to supplement a platoon size element and are capable of remote operation.

23. Longrangescouts Inmilitaryapplicationcurrentyearsprovidetolongscoutrange. Scanninglongrange is importantforcountriesborderlinesagainstenemies.

24. Missileseeker

25. CivilianApplications • Nightvisionenhancersfordrivers Todecreasetheaccidentswhichareresultwithdeath Theautomobiletechnologychoosebolometersforthermalsensingtodecraseaccidents

26. Satelliteinstruments Monitoringworldandgiveinformationaboutthethermalchangesbolometerschooseforsatallitesystems

27. Fire fighting Bolometersespeciallychoosebythe fire fighting . By a sensingcamera in a fire actionfightersmonitoringthedifferentthermalvaluesandmaybesavepeopleor an animal.

28. Medicalsensing Manyapplications of bolometersarethermalbasedandseemliketheothersforexample in militarynighvisionriflesightsand in civil life forsecuritysystemsworkwithsameprocedure. Howevever , in medicalapplicationsthermalsensing ( bolometer ) is veryimportantfor modern medicalsystems. Skin cancerdetection: Dentaluse:

29. Consequences Microbolometersarethermalsensingdevices. Bytheadvantages of bolometersandbythehelp of technologyusingareasareincreasing. Theuse of bolometersareplayed an important role on industryandarmy. Theadvent of uncooledmicrobolometers is set tochangehowtodiseasesaredetectedandmonitored. Overthenextdecadeincreasedreserch in theterahertzspectrumwillleadtomorebreakthroughsthatwillchangethefield of Biomems.

30. Referances • Uncooled Thermal Imaging Arrays, Systems, and Applications, Paul W Kruse • WeiguoLiu, Bin Jiang, andWeiguangZhu • MicroelectronicsCenter, School of ElectronicandElectricalEngineering, NanyangTechnologicalUniversity, Singapore 639798  • C. HANSON, H. BERATAN and S. MCKENNEY, Proc. SPIEInt. Soc. Opt. Eng., 1735 InfraredDetector • R. W. WHATMORE, Ferroelectrics • S. NOMURO and S. SAWADA, J. Phys. Soc. Japan • Microelectromechanicalsystemresearchandapplicationcenter METU