SOI SENSING TECHNOLOGIES FOR HARSH ENVIRONMENT

F. Udrea1,2, S.Z. Ali1, M. Brezeanu3, V. Dumitru3, O. Buiu3, I.Poenaru1,

M.F. Chowdhury1, A. De Luca2 and J.W. Gardner1,4

1Cambridge CMOS Sensors Ltd, 59 St Andrew's Street, Cambridge CB2 3BZ, UK2 Univ. of Cambridge, CAPE Building, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK

3Honeywell Romania SRL, ACS Sensors & Wireless Laboratory Bucharest, 169A Calea Floreasca, Building A, 014459, Bucharest, Romania

4University of Warwick, School of Engineering, Coventry CV4 7AL, UKflorin.udrea@ccmoss.com

Abstract – This paper reviews and addressescertain aspects of Silicon-On-Insulator (SOI)technologies for a harsh environment. The paperfirst describes the need for specialized sensors inapplications such as (i) domestic and other small-scale boilers, (ii) CO2 Capture and Sequestration,(iii) oil & gas storage and transportation, and (iv)automotive. We describe in brief the advantagesand special features of SOI technology for sensingapplications requiring temperatures in excess ofthe typical bulk silicon junction temperatures of150oC. Finally we present the concepts, structuresand prototypes of simple and smart micro-hotplate and Infra Red (IR) based emitters forNDIR (Non Dispersive IR) gas sensors in harshenvironments.

Keywords: Silicon on Insulator (SOI), CMOStechnology for sensing applications, System inPackage (SiP)

1. INTRODUCTION - HARSHENVIRONMENT APPLICATIONS

Sensors operating in a harshenvironment need to cope with oneor more of the following extremeconditions: high operatingtemperature (>150 oC), highpressure (>10 bar), significantvibration, high humidity, highradiation levels, aggressive

media (corrosive, toxic,explosive), electromagneticspikes. Research in harsh environment

sensors is driven both by boththe current market needs and bythe strong legislationrequirements regarding thequality of the environmentalambient air. The upper safetylimits for emission gases havegradually decreased in the lastdecade. For this reason,industrial process, automotive,aerospace and marine combustioncontrol for efficient energygeneration are performed based onmonitoring the input and outputconcentration of the gasesparticipating (and resultingfrom) the combustion reaction.Thus, measuring gases such as O2,CO2, CO, CXHY, NOX, together withhumidity (i.e. water vapour) areessential. At the same time,measuring toxic gases such as H2Shas become an important issue foravoiding pollution and poisoningand in general for environmentsafety.

Combustion optimization indomestic and other small-scaleboilers, CO2 Capture andSequestration (CCS) and deep-welloil and gas exploration, oil andgas storage and transportation,combustion optimization andemission control in automotive,aerospace, and marine areexamples of applications thatrequire sensors operating invarious harsh environmentconditions.

(i) Combustion optimization in domestic and other small-scale boilers

Currently, the domestic boilersmarket is divided between the HE(Highly Efficient – PremixCondensing) and the SE (StandardEfficient - Atmospheric) systems.The HE boilers are based on thepremix system (1:1 gas/airratio), which is expected tobecome the dominant system in thenear future. The 1:1 gas/aircontrol ensures safe handling (ifthere is no air, then no gas willbe pulled out from the system), afast dynamic response, low costand modulation with premix burner[1]. It also leads to lower gasemissions. The novelty of the HEboilers is their auto-adaptability feature. This isachieved, on the one hand, byinserting a CO2 sensor above theburner (Fig. 1), which isemployed to measure or detect thecombustion quality, and on theother, by trimming the gas flowvia a motor driven throttle.

Besides the self adaptive control(at installation and overlifetime), the combustion qualitysensing that is performed by theCO2 sensor also leads to higherefficiency and to a wider gasrange measurement. The CO2 sensor has to cope with

operating temperatures of up to225oC and water vapour presence inthe gas composition above 10% involume. HE boilers, takingbenefit of the flue condensation,have also the option for gasmeasurement after water vapourcondensation. Thus, it ispossible to reduce the workingtemperature, but the relativehumidity can be as high as 100%.Therefore, the requirementemerges to measure temperatureand water vapour level inparallel with CO2 concentration.At the same time, the total gasflow also needs to be measured,for maximum improvement in thecombustion process efficiency.Fig.1 shows schematically theplacement of different sensorswithin a domestic boiler.

Fig. 1. Schematic view of a HE domesticboiler and the possible positioning of

CO2 and flow sensors

Furthermore, low powerconsumption is also arequirement, which again islinked to overall systemefficiency. Sensing the gas flow,water vapour and temperaturevalues, together with detectingthe CO2 level, is of course asignificant benefit provided thata single platform technology candeal with it, otherwise it canresult in increased cost and as aresult less market appeal. Sofar, no multi-measurand solution,able to cope to the harshenvironment conditions mentionedabove, is currently on themarket. Boilers that employ theHE system now are currently usingonly a CO2 sensor to achieve theirauto-adaptability feature. Amongother drawbacks, current sensorssuffer from poor reliability andreduced lifetime (3 years onaverage).SOI could be an ideal solution

for multi-measurand sensorsresulting in a dramatic increasein the combustion efficiency ofthe HE system with thecorresponding benefit ofsignificant decrease in naturalgas consumption and in boileremission. These sensors could beassembled in dedicated hightemperatures SIPs (System inPackages) with a lifetime of 5-10years, and significantlyincreased reliability. Moreover,due to its full CMOScompatibility and owing to itsintelligent design, the SOI SIPwill offer low power consumption

and will decrease the overallboiler cost. It is interesting tonote that the boiler market ishuge and increasing. More than 5million domestic boilers arecurrently sold per year in EU andthe growth rate is estimated tobe around 15% annually.

(ii) CO 2 Capture andSequestration (CCS)

CO2 Capture and Sequestration(CCS) is a technology that allowsthe storage of CO2 highlyundesired emissions in secure,deep underground reservoirs. Thisis a large-scale application,with tremendous potential. From the market point of view,

the potential is impressive whenone considers the increasinglytough legislative demands and thesignificant decrease in the oiland natural gas reserves. It isimportant to mention that CCS ispart of the European Energy 2020strategy [2]. Currently, coal-based power plants are thelargest contributor to CO2

emissions. Since renewable energysources are still struggling tomeet global energy demand, coalis expected to remain a majorplayer in the future. Thus,reducing its emission levels isof crucial importance. CCS hasthe potential to reduce coal-bases power plants CO2 emissionsby 80– 90%. The currently available CCS

sites worldwide have thepotential of sequestrating 20million tones of CO2 per year. By

2015, 6 new projects will beactive, thus increasing thesequestered CO2 amount to about 33million tones annually [3].A typical CCS chain contains

capture and separation of CO2 (infossil fuel-based power plants,iron and steel, cement or paperfactories, etc.), compression anddehydration, transportation(pipelines, ships), sequestration(in saline, depleted oil and gasreservoirs, coal mines, etc.). Inall these stages, gas sensors(especially for CO2 and H2Sconcentrations monitoring), watervapour and temperature sensorsare essential components(especially in tanks, pipelinesand ships transportation, butalso in storage reservoirs). Theconditions in which such sensorsare expected to operate for thistype of application are harsh(high temperature – up to 170ºC,high pressure – up 10-80 MPa,i.e. highly corrosive). An SOISIP able to reliably measure the4 measurands mentioned above(i.e. temperature, water vapour,CO2 and H2S) would make asignificant impact, both in termsof CO2 transportation performanceand for increasing safety andsecurity in the other stages ofthe CCS chain.

(iii) Oil & Gas Storage and Transportation

Similar to the above mentionedapplication, the Oil & GasStorage and Transportationbusiness requires increased

performance and safety. A multi-measurand SOI-based sensingsolution will help monitoring theoil & gas conditions within tanksand pipes. When filling a tankwith gas or oil, it is essentialto monitor the flow of gas oroil, respectively. At the sametime, measuring the temperature,pressure and flow within the tankand transportation pipe willincrease the safety of theoperation [4]. For similar safetyreasons, it is compulsory tomeasure precisely the temperatureand H2S level within the tankduring ship transportation. Whenstoring oil and gas in tanks,monitoring temperature and watervapour level provides essentialinformation with respect to thestorage efficiency. Harshenvironment conditions, such asrelative humidity up to 100% andhigh corrosion rates, are againexpected.

(iv) Automotive (combustion optimization and emissionmonitoring)

Gas sensors can be mountedeither in the engine or in theexhaust system, where detectingthe levels of CO2 and CO isessential both for the optimumcombustion of the engine and forreducing as much as possible thelevel of the emission gases. Theconditions in which such devicesare expected to operate are harshdue to the high temperaturelevels (up to 550 oC in the

engine, up to 225 ºC in theexhaust system).

2. The argument for SOItechnology

SOI is one of the most advancedCMOS technologies today. Its mainadvantages compared to bulksilicon technologies are [5-7]:

low leakage currents (oftenby one to two order ofmagnitude)

enhanced hardness againstradiation and cosmic rays

latch-up free due toeffective isolation

less parasitic componentsand low substrate leakageagain due to its highlyimproved isolation

enhanced CMOS performance(better sub threshold/lessinfluence of the parasiticbipolar transistors/lesscharge wasted in thedepletion/strongerinversion)

higher operating temperature(due to reduced leakage andless parasitic bipolaraction)

the buried oxide can be usedas a very effective etchstop to form membranes forpressure sensors, gassensors, IR emitters, IRdetectors etc.

the buried oxide can beetched under the SOI layerto leave a free standingmonocrystaline siliconstructure for

resonators/pressure sensors,accelero-meters etc.

the buried oxide canwithstand high electricfields in lateral highvoltage structures

excellent for integratingmore than one power or highvoltage device and allowingfast recovery diodes, and

excellent for lateralbipolar transistors asplasma is constricted to theSOI region resulting in veryhigh commutation speed.

These remarkable features makeSOI a very attractive platformfor application in four mainareas:

sensors high voltage integrated

circuits electronics for harsh

environment (hightemperature/ high radiation)

high speed electronics

Interestingly, here we areaddressing two of these areas insensing technologies for harshenvironments. An important questionis: ‘are there othersemiconductor technologies thatcan cover such areas?’. Theanswer is both yes and no. Bulksilicon struggles to operateabove 150-200 oC, but its reducedcost makes it highly attractiveand therefore there has been andwill always be a strongindustrial/commercial interest toextend its limits of operation.

For example several bulk silicondevices and IC technologies areapproaching the 200 oC limit,while isolation techniques usingDTI (deep trench isolation) andhighly doped buried layers havebeen developed. SiC and Diamondon the other hand have some veryattractive properties, such as avery wide bandgap, low intrinsiccarrier concentration, greatmechanical resistance but theirsuccess is or will be limited toniche and high end applications,as they suffer from high waferand processing costs, low yieldand poor availability ofmaterials from suppliers.

3. Building blocks in SOItechnology

In this section we present themain structures/devices used inSOI sensors at high temperatures:MOSFETs, thermo-diodes, micro-hotplates, smart micro-hotplatesand micro-wires.The typical I-V characteristic

of an n-channel SOI MOSFEToperating from room temperatureto 300 oC is shown in Fig. 2.Tungsten metallization is usedinstead of the usual aluminium inorder to allow higher operatingtemperatures. The leakage currentand the parasitic bipolartransistor action are minimizedthrough extensive use of bodyshorts in the third dimension.The MOSFET is an importantcomponent of any electronics andtherefore its demonstration and

stability at high temperatures inexcess of 200 oC is compulsory inharsh environments. An SOI MOSFETcan also be used as a micro-heater driver (in series with themicro-heater) or as a micro-heater itself. In the latter casethe MOSFET is embedded within aSOI membrane [8, 9]. Fig. 3 showsa photo of a micro-heater using ap-channel MOSFET and its self-heating characteristics. TheMOSFET can reach 550 oC before theparasitic p-n-p transistor kicksin.

Instead of a MOSFET, one canuse a resistive heater [10-12].The heater can be made oftungsten as either a micro-wire(for thermal flow sensors) or amicro-hotplate (for resistive gassensors, pellistors or IRemitters) [13]. Fig. 4 shows aphotograph of a micro-wiresuspended on a SOI membrane foruse as a flow or sheer stresssensor. The wire will dissipatemore heat due to convection tothe flow and therefore it iscooled down when operated atconstant power [14, 15]. Thetypical power consumption is0.1mW/oC. Given that the wire ismuch thinner (~ 2 µm × 0.3 µmcross section) than in state-of-the-art devices fabricated viascreen printing, the response isextremely fast (60 kHz frequency)with a sensitivity >50 mV/Pa.Typically, microwires operate ata maximum temperature of 300 oC.

Fig. 2 I-V characteristics of an n-channel SOI MOSFET at different

temperatures (up to 300 oC) and Vgs=5 V.

#58_PMO S

-0.0350

-0.0300

-0.0250

-0.0200

-0.0150

-0.0100

-0.0050

0.0000

-5 -4.5

-4 -3.5

-3 -2.5

-2 -1.5

-1 -0.5

0

Vheater (V)

Iheate

r (A) Vg=-5V

Vg=-4VVg=-3VVg=-2VVg=-1VVg= 0V

Fig. 3 Photograph (top) of an SOImicro-heater using a p-channel FET andthe self-heating characteristics showingnormal operation up to about 550 oCfollowed by the turn-on of the parasiticp-n-p bipolar transistor (below).

Fig. 4. Photograph of a micro-wire madeof tungsten in SOI technology for use as

a flow or sheer stress sensor

Micro-hotplates use a largerheater for applications in gassensors. Fig. 5 shows the cross-section of a micro-hotplate inSOI technology with the circuitryon the same chip. Depending onthe area of the heater and themembrane, the DC powerconsumption can vary between 0.05mW/oC and 0.2 mW/oC.100µm

Membrane

P-MOSFET Heater

TemperatureSensing

MOSFET Tracks

Current

(A)

550⁰

PMOS NMOS

CMOSSensor area

P-N+ N+N-P+ P+

N+ N- N+P+ P+N-

Reference Diode off-membrane

Substrate

Buried Silicon Dioxide

PassivationTungsten Heater

Sensing Electrodes Tungsten Heat

Spreading Plate

Gas sensing material

On-membrane diode

Membrane

This of course can be loweredsignificantly if the micro-hotplate is operated in pulseconditions, taking into accountthat typical thermal timeconstants are around 10-20 ms.Furthermore, the ambienttemperature (outside themembrane) can reach 225 oC,without impacting the operationof the micro-hotplate. The CMOSprocess is qualified at thistemperature against electro-migration or TDDB (time dependentdielectric breakdown). Thus,this SOI micro-hotplate is anessential platform for harsh-environments. A top-view photo ofasmart micro-hotplate usingfeedback electronics to maintainthe temperature constant is shownin Fig. 6.

Fig. 6 Photograph of a smart SOI micro-hotplate with integrated electronics, MOSFET drive and temperature sensor.

An important aspect in harshenvironments is the stability ofthe micro-hotplate and the otherbuilding blocks at extremetemperatures. In Fig. 7(a) thestability of the micro-heater inpulse conditions at 10 Hz with50% duty cycle is shown. In Fig.7(b) the Mean Time To Failure(MTTF) function of temperature isextracted using an Arrheniusrelationship. In order to obtaina very high MTTF the design ofthe heater has been carefullyoptimized to minimize mechanicalstress and concomitantly reduceelectro-migration. Thecomposition of the passivationlayer and the residual stress inthe ILDs and the buried oxideplay an important role inbalancing the overall stress inthe membrane.

100µm

Heater

Circuitry

(a)

(b)Fig. 7. Stability of the micro-heater in(a) DC and (b) pulse conditions (over 3

million pulses).

The thermo-diode is also animportant building block for anysensor as we rely on it foraccurate temperature detection ortemperature compensation in somecases. The thermo-diodes are notknown to operate commerciallyabove 200 oC and there are veryfew research studies [16, 17]reporting their behaviour beyondthis temperature. Here we showthat a carefully designed thermo-diode, with extremely low leakagecurrent (specific to SOI) can beoperated up to 600 oC reliablygiving a linear response of(approx 1.3 mV/oC), as shown in

see Fig. 8. Moreover the diode isstable, and for over 500 hours at500 oC, the change in the outputvoltage was below 1%. Furthermorethe diode was found to have aminimal piezo-junction responseand therefore the effect of thestress induced by the hightemperatures in the membrane onthe output voltage of the diodecan be neglected.

Fig. 8. The output voltage across an SOIdiode in the forward bias mode operationvs temperature. The forward currentsinjected were 20 µA, 65µA and 100 µA. Thediode shows a linear response with aslope of (1.3mV/oC) up to 600 oC.

4. NDIR based SOI gas sensorsuitable for harsh environments

IR sensors operate using thermalradiation from hot devices todetect the presence of heat or touse radiometric conversions toobtain the actual temperature ofthe device. The principle ofoperation is to detect IRwavelengths in the range of 1 to15 µm using sensitive materials,such a pyro-electric crystals,resistive devices such as

bolometers, thin-film or siliconbased planar thermopiles. Theprinciple of a Non-dispersive IRgas sensor (NDIR) is based on anIR emitter at one end of anoptical path while at the otheran IR detector with an opticalfilter is inserted (Fig. 9). Thegas is allowed in through someholes on top of the optical pathand depending on itsconcentration it absorbs acertain amount of radiation at aspecific wavelength. This isdetected by the IR sensor,amplified and eventually read outin a suitable format through atransducing circuit. The filteris used to enhance selectivity toa specific gas absorption band(e.g. 4.3 and 15 µm for CO2).Generally all the NDIR systemsoperate below 100 oC. The use of amicro-bulb as an IR emitter athigher ambient temperatures (i.e.in harsh conditions) isprohibited due to its poorlifetime. Its glass cap alsolimits the absorption wavelengthsof the gases that can be detectedto below 5 µm. Furthermore, thebulbs are very slow and cannot beoperated above 5 Hz, making thesystem sensitive to high 1/fnoise. LEDs cannot be used eitherin harsh environments as thetemperature variation would leadto high variations in the opticalpower and alter the wavelength atwhich the radiation peak occurs.

Fig. 9. A schematic drawing of a NDIR gassensor

0 2 4 6 8 10 12 14 16 18 200

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

x 10-10

W avelength [um ]

Powe

r [W/m2 srad]

Black Body radiation

T=300 KT=500 KT=700 KT=800 KT=900 KT=1000 K

Fig. 10. Emission power vs wavelength fordifferent micro-hotplate temperatures(the markers indicate the CO2 absorptionlengths). For increased sensitivitytemperatures > 500oC are needed.

The micro-hotplate, with its widespectrum and very high lifetimeis ideal for use as an IRemitter. In addition, the micro-hotplate can be driven in pulsemode by an internal MOSFET and/orcan have intelligenceincorporated on-chip.Furthermore, the SOI micro-hotplate with tungstenmetallization can operate atambient temperatures of 225 oCmaking it ideally suitable forharsh environments. Theelectronics and detectors mustalso be designed to operate atsuch temperatures (this is one of

the aims of the SOI-HITS EU FP7project [18]). The micro-hotplategives a spectrum close to that ofa black body. This is adjusted bya unitless factor called‘emissivity’. The closer theemissivity to one, the closer theemission is to the ideal black-body radiator. The emissivityhowever varies with temperatureand often time. Fig. 10 shows thetypical emission spectrum for amicro-hotplate operated atdifferent temperatures. For CO2,strong radiation absorption peaksoccur at 4.3 and 15 µmwavelengths. For CO theabsorption peak is at 4.6 µm. Toincrease the emissivity one cangrow or deposit nanomaterials,such as carbon nanotubes CNTs[19], or silver and goldparticles.

Fig. 11. SEM showing the CNTs grown self-aligned on top of one of the SOI micro-

hotplates

Fig. 12. Emission enhancement usingdifferent nano-materials. The signalshown is the output voltage on a

thermopile for different temperatures ofthe IR emitter microheater.

Fig. 11 shows a picture of singlewalled CNTs grown locally on topof the micro-hotplate using aPECVD technique and the hightemperature (~700°C) generated bythe micro-heater itself. Theseparticles or nanomaterials canenhance significantly theemissivity (or indeed theabsorption on the detector), butthe most important issues arereproducibility and stability forprolonged periods at hightemperatures. Fig. 12 shows theenhancement of the emission whenusing different type ofnanomaterials on top of the CMOSSOI micro-hotplate. The signalshown is the detector voltage (athermopile) for differenttemperatures of the IR emittermicroheater.

CONCLUSIONS

In this paper we have describeddifferent harsh environmentapplications (HE boilers and

carbon capture systems) thatdemand new advanced sensingtechnologies. We have shown herethat SOI CMOS technology providesan ideal platform for suchapplications, owing to its higheroperating temperature, ease ofMEMS manufacturability,possibility of integrating hightemperature electronics and, lastbut not least, highreproducibility, yield andreliability. We have showndifferent components that havebeen fabricated in SOICMOStechnology such as resistive andFET micro-heaters, thermo-diodes,and flow sensors. Finally, wehave described a prototype NDIRgas sensor with a suitable IRemitter for CO2 and possibly COdetection for application inharsh environments.

ACKNOWLEDGMENTS

This work has been partially fundedby the EU FP7 SOI-HITS (SmartSilicon-on-Insulator Sensing SystemsOperating at High Temperature) – seewww.soi-hits.eu. We would also liketo thank our partners in the SOI-HITSproject Microsemi, IREC, CISSOID andUCL, and Dr. A. Fasoli, Dr. M.T. Coleand Dr. V. Pathirana from theUniversity of Cambridge for advicerelated to this work.

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