physica status solidi (a)) Volume 185 issue 1 2001 [doi...

7/28/2019 physica status solidi (a)) Volume 185 issue 1 2001 [doi 10.1002/1521-396x(200105)185:11::aid-pssa13.0.co;2-u] G

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Editors Choice

New Sensors for Automotive and Aerospace Applications

G. Muller, G. Krotz, and J. Schalk

EADS-Deutschland GmbH, D-81663 Munchen, GermanyTel.: ++49/(0)89-607-27847, Fax: ++49/(0)89-607-24001

(Received January 19, 2001; accepted March 8, 2001)

Subject classification: 68.47.b; 73.40.Qv; 73.50.Dn; S5.11; S6

In modern cars, sensors, actuators and electronic components play an increasing role. Drivingforces are the demand for improved safety, higher comfort as well as increased economy and ecol-

ogy. The paper presents an overview on semiconductor sensors that had been designed to reachsuch objectives. Examples for micromachined silicon sensors as well as highly innovative devicesbased on wide-bandgap semiconductor materials are presented. Spin-offs to aerospace applicationsare also discussed.

1. Introduction

Electronic sensors have been used in road vehicles almost from their inception. Theearliest sensor devices were essentially switches used to measure the crankshaft positionfor ignition timing purposes. These control circuits combined sensor and actuator func-tions within a single rigidly coupled mechanical device. Nowadays control circuits con-

sist of electronic sensors to acquire information about the process to be controlled, amicroprocessor to decide what action needs to be taken and actuators to bring aboutthose changes that are required by the microprocessor.

The introduction of microprocessors between sensors and actuators represents anenormous enhancement in the flexibility of designing and adapting control circuits totheir particular application environments. These innovations could be introduced be-cause of a huge increase in the priceperformance ratio of the control equipment [1].During the last two decades this ratio increased by about 20 in the case of actuatorsand about 1000 in the case of microcontrollers. Progress in sensors, however, wascomparatively small. There the average priceperformance ratio increased by about a

factor of 3 only during this same period. Micromachined silicon sensors are likely toimprove this situation because their production technology can greatly benefit from theadvances made within the field of silicon microelectronics [2]. Several types of suchsensors have already been successfully introduced into the automotive market.

Considering semiconductor sensors, however, it has to be kept in mind that manyautomotive sensor applications require sensor operation in harsh, i.e. in hot and/or cor-rosive environments. Examples are powertrain or exhaust gas sensors which requiresensor operation in high-temperature or chemically reactive environments in which sili-con devices cannot successfully be operated. In this latter area wide-bandgap semicon-ductor materials such as diamond, silicon carbide (SiC) and gallium nitride (GaN) are

1) e-mail: [email protected]

phys. stat. sol. (a) 185, No. 1, 114 (2001)



likely to make useful contributions [35]. Such harsh environment situations also arisein the aerospace field, particularly in the field of large commercial aircrafts. Both theautomotive and aerospace fields can benefit from each other with the aerospace fieldbeing in the role of a technology and the much larger automotive field in the role of a

cost driver. Making semiconductor sensor technologies ultimately available to the hugeautomotive market is an economic need because of the high development and invest-ment costs typically encountered in the semiconductor industry.

On the following pages a number of examples of semiconductor sensors are pre-sented which had recently been developed in our laboratory both for the automotiveand the aerospace fields.

2. Semiconductor Sensors and Application Examples

2.1 Sensors for safety, comfort and navigation

Measuring the vehicle dynamics of a car is a very obvious problem in the automotiveindustry. Inertial sensors and sensor systems therefore play an essential and ever in-creasing role in modern cars. With a variety of silicon micromachining technologies

2 G. Muller et al.: New Sensors for Automotive and Aerospace Applications

Fig. 1. A dc-stable low-g acceleration sensor for active suspension control fabricated using bulk

micromachining and silicon-glass bonding technologies. a) Sensor architecture, b) scanning elec-tron micrograph, c) sensor response upon rotation in earth gravitational field

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becoming available [6, 7] the field of inertial sensors has experienced a very rapiddevelopment within the last decade. To date micromachined silicon sensors are beingintroduced in large quantities in cars for detecting crashes and for triggering airbaginflators. Potential follow-on applications are in anti-blocking and in active suspension

systems. Higher-level applications become feasible when accelerometers are combinedwith angular rate sensors. Such inertial sensor systems allow for applications in anti-skidding, in vehicle dynamics control and in drive-by-wire systems. In the near futureadvances in the performance of individual sensor elements as well as in system integra-tion technologies will also allow demanding applications to be addressed. Examples are:intelligent predictive systems, electronic guidance systems as well as inertial navigationsystems updated by GPS data.

Examples of silicon micromachined inertial sensors are presented in Figs. 1 and 2. InFig. 1 a dc-stable low-g acceleration sensor is shown as fabricated using a combinationof bulk micromachining and silicon-glass bonding techniques. A characteristic of this

device is that silicon is used as a purely mechanical material: upon acceleration alongthe input axis (F) the displacement of the seismic silicon mass induces a torsional dis-tortion in the suspensions that fix this mass within the surrounding chip frame. This

phys. stat. sol. (a) 185, No. 1 (2001) 3

Fig. 2. Inertial sensor for automotive anti-skidding systems fabricated from SOI wafers using dry

etching techniques. a) Sensor architecture, b) scanning electron micrograph, c) sensor assemblywith peripheral electronics

piezo-

electricactuators

piezoresistiveread-out

torsional

stem with

slot

tines

input axis

excitation modedetection mode

control oftine amplitude

(a)

(b) (c)

Silicon micromachined angular rate sensor



distortion in turn produces a measurable asymmetric change in the two capacitancesformed between the pairs of electrodes evaporated onto the seismic mass, on the onehand, and the neighboring glass cover lid, on the other hand. Figure 2 shows an angularrate sensor whose operation is based on a tuning-fork principle [8]. In this latter exam-

ple more innovative micromachining technologies based on silicon-on-insulator (SOI)wafers and on dry etching techniques have been used to realize the tuning-fork geome-try. During operation this tuning fork is piezo-electrically excited by AlN layers depos-ited on the tines of the tuning fork. Upon angular motion of the sensor chip around theinput axis the resulting Coriolis force generates a torsional stress on the stems of thetuning fork which in turn can be detected by the piezo-resistors integrated into thesestems.

2.2 Powertrain sensors

2.2.1 Air flow sensors

An example where micromachined silicon sensors have successfully been introducedinto the powertrain system are micromachined high-speed mass flow sensors [9]. Suchsensors monitor the influx of fresh air into the engine. In Otto engines this influx isregulated by a so-called throttle valve situated directly at the air input side of the en-gine. In modern combustion engines the opening angle of the valve flap is operated insuch a way that stoichiometric airfuel mixtures are injected into the cylinders. In thisway an optimum exhaust gas after-treatment can be carried out using three-way cataly-tic converters. Demands on optimized fuel consumption and/or demands on downsizingand longevity of catalytic converter beads require that the air-to-fuel ratio l is mea-sured and regulated to l 1 at a very high temporal rate, ideally with single-cylinder

resolution. Besides fast l-probes, such a high-rate engine management also requireshigh speed mass flow sensors with response times in the range of milliseconds.

In order to approach such measurement speeds, sensors based on the heated-wireanemometer principle were miniaturized to the extent that the thermal capacity of theheated-wire structures is reduced to attain millisecond response times [9, 10]. Examplesof such sensors are shown in Fig. 3. The upper two panels show that such sensors con-sist of thin dielectric membranes suspended over an etch hole in a micromachinedsilicon substrate. These membranes in turn carry a pair of thin-film platinum resistorswhich are heated to constant temperature by an external electronic control circuit. Inorder to assess the speed of an air column passing such a heated membrane the elec-

trical power input is measured that is required for maintaining a constant heater tem-perature. The inserts in the upper two panels show the time-variation of this controlsignal in response to an oscillating air column generated by a loud speaker. The com-parison of the two inserts further shows that operating two heated wires in series, bi-directional measurements of the flow velocity can be made. This latter possibility isextremely important for ensuring a proper air filling of the cylinders in view of theoscillatory phenomena that occur during engine operation in the air intake system ofautomobiles [9].

Anemometers as such measure the speed of air passing the sensor position. In orderto obtain proper measurements of the flowing air mass, additional measurements of air

pressure and temperature are required. The present state of the art is to place indivi-dual sensors at optimized positions within the air intake system of a car and to transmit




all the sensor signals to the engine control unit for calculating the actual air flow andfor generating the control signal needed to actuate the valve flap. Figure 4 gives anexample how in future all these sensors, actuators and electronic control functions can

be integrated and decentralized within a single mechatronic component. This latter ex-ample is of general interest for the entirety of sensor applications in automobiles: dif-ferent types of sensors are clustered into multi-functional arrays and merged with themechanical and electronic structures of the control systems they have to cooperatewith. In this way increased levels of functionality can be reached. In addition financialbenefits can be generated both for the car manufacturers and their component sup-pliers.

The level of integration demonstrated in Fig. 4 is easily achieved using conventionalsilicon micromachining techniques. This is possible because the air intake system ofautomobiles is not a particularly harsh environment for sensor applications. This situa-

tion, however, changes when exhaust gas recirculation is used to reduce the raw emis-sion of nitrous oxides from internal combustion engines [9]. Under such conditions a

phys. stat. sol. (a) 185, No. 1 (2001) 5

burning ofsoot particles

possible

free standing SiCbridgesabove Si etch

groove

-4

-3

-2

-1

0

1

2

3

4

0 10 20 30 40 50 60 70

unidirectional measurement

-4

-3

-2

-1

0

1

2

3

4

0 5 10 15 20 25

bi-directional measurement

Thermal mass flow sensors

Fig. 3. Thermal mass flow detection. Upper panel: principle of uni- and bi-directional measure-ments of air flows using micromachined silicon sensors (see also text). Lower panel: mass flowsensor with micromachined silicon carbide bridges. Due to the possibility of high-temperature heat-ing, such sensors can also be operated under conditions of exhaust gas recirculation



fraction of the exhaust gas is taken from the exhaust gas side and fed back into the airintake side of the engine. This recirculated exhaust gas carries soot particles and corro-sive gases which represent a severe threat to the silicon micromachined temperatureand flow sensors which are needed for reasons of measurement rate. In this contextmicromachining technologies become attractive which involve chemically inert materialslike for example SiC, diamond and GaN [35]. An example of a bi-directional massflow sensor for harsh environment operation is shown in the lower panel of Fig. 3.There hetero-epitaxial silicon carbide films had been deposited on top of silicon sub-strates and structured to form SiC heater bridges across a micromachined flow channel

[10]. As can be seen in the bottom of Fig. 3 such sensors can be operated under condi-tions of white glow (T> 1200 C) in ambient air for periods of several hours. Reducingthe electrical power input to obtain red glow (T 600 C), lifetimes of several thou-sands of hours can be attained which is comparable to the useful life of a car. Undersuch red-glow conditions self-cleaning sensor operation becomes feasible under condi-tions of exhaust gas recirculation.

2.2.2 High-temperature pressure sensors

Another example of a harsh environment application is cylinder pressure monitoring in

automobiles. Monitoring the cylinder pressure as a function of the crank angle, interest-ing pieces of information can be obtained such as air filling and compression of the


micromachined

flow sensor

anemometer

pressure

sensor

temperature

sensor

Integrated

air intake

manifold

Fig. 4. Prototype of an integrated air intake manifold. Sensors, actuators and electronic controlcircuits form a single autonomously operating mechatronic unit. The commercial macroscopic sen-sors indicated on the left-hand side are integrated in the form of silicon microsensor chips insidethe plug-in module shown in the center



load, time of ignition, resulting combustion pressure and last-not-least knocking. Froma technology point-of-view, cylinder pressure monitoring is far from being trivial be-cause there pressure sensors need to be exposed to hot and corrosive atmospheres athigh peak pressures of the order of 200 bar [5]. Commercially available silicon pres-

sure sensors cannot be directly exposed to such an environment. Limiting factors ofthe silicon technology are failure of the junction isolation of implanted piezo-resistorsat temperatures above 150 C and plastic deformation of the micromachined siliconmembranes at temperatures above 500 C [11]. Using sophisticated mechanical packa-ging techniques the semiconductor limitations of silicon pressure sensor chips could bealleviated by using stainless steel membranes and suitably designed mechanical separa-tion structures which transmit the measurand to the Si sensor chips (see the icons andthe first row in Fig. 5). In this way a direct exposure of the silicon chips can beavoided and their limits of application extended into the range of increasingly harshenvironments.

Due to the serial production and the complicated assembly of the mechanical separa-tion structures, the fabrication of such sensors cannot comply with the competitive pricegoals of the automotive industry. For this reason we have developed sensor architec-tures which lend themselves more easily to mass production. These latter technologiesas well as their high-temperature limitations are displayed in the two lower rows inFig. 5. These innovative technologies are firstly an all-silicon technology based on com-

phys. stat. sol. (a) 185, No. 1 (2001) 7

Price

Usefullife

550C - 600C 800C - 850C500C

150C 200C - 250C 450C - 500C

350C 400C - 450C 650C - 700C

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logy

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SOI

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exposureSteel

membrane

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High- temperature pressure sensors:

Trade- offs between price, reliability and useful life

Fig. 5. Price-performance trade-offs for different high-temperature pressure sensor technologies.Temperature ranges stand for media temperatures



mercial silicon-on-insulator (SOI) substrates which are increasingly used for microma-chining purposes. Secondly we developed a mixed silicon-carbide-on-silicon (SiCOIN)technology also based on commercial SOI substrates [12]. In the first sensor architec-ture (middle row of Fig. 5) junction isolation of the piezo-resistors is replaced by the

dielectric isolation provided by a buried SiO2 layer. In this way the high-temperaturelimit of sensor operation can in principle be extended towards the plastic deformationlimit of the micromachined silicon membranes. In practice, however, this limit cannotbe reached due to the lessening of the piezo-resistive effect in silicon and parasiticcurrent flows at elevated temperatures. In order to reach the plastic deformation limitof micromachined Si membranes the second sensor architecture uses hetero-epitaxialSiC piezo-resistors deposited onto the thin top Si layer of the SOI substrates (bottomrow of Fig. 5). Structuring of the SiC piezo-resistors was performed by a SiO 2 maskingand selective growth technique [12] (see Fig. 6). With this latter technology the high-temperature limit of cylinder pressure sensors could be extended to about 500 C and

the possibility of a cylinder-pressure-based engine management could be successfullydemonstrated in a prototype car.Even higher temperatures, beyond the plastic deformation limit of micromachined Si

substrates, are reached in jet turbines. Pressure sensors operated in the high-pressuredensifier of a jet engine reach temperatures of the order of 650 C at the membrane


SOI-substrate SiCOIN

sensor chip

selectively deposited

SiC piezo resistors

Si

Si SiO2

-SiC

(b)

(c)

(a)

Fig. 6. a) Cylinder pressure sensor; b) application environment; c) chip technology based on selec-tively deposited b-SiC on SOI silicon



position. Under such conditions all-silicon-carbide technologies have to be used. Ahigh-temperature pressure chip fabricated in our laboratory from a 6H bulk-SiC sub-strate is shown in Fig. 7. A problem with such substrates is that membrane formationby wet chemical etching is no longer possible and that therefore precision machining

with diamond tools is necessary to process such devices. Such serial mechanical proces-sing steps are not compatible with low-price mass production which limits such devicesto high-price, low-volume markets, as in aerospace. An interesting development in thiscontext is the observation of large piezo-resistive effects in high-electron-mobility tran-sistors (HEMT) based on GaN/AlGaN heterostructures [13]. Due to their large band-gaps these latter materials clearly qualify as candidate materials for high-temperaturepressure sensors. Due to their thermal, mechanical and chemical stability also the sap-phire substrates, onto which such heterostructures are routinely deposited, qualify. Anopen problem in connection with such substrates is the present unavailability of micro-machining techniques for such substrates.

2.2.3 Exhaust gas sensors

A third example of harsh environment sensors are exhaust gas sensors. The currentstate-of-the art in exhaust gas sensing are zirconia-based l-probes [1, 8, 14]. Such

phys. stat. sol. (a) 185, No. 1 (2001) 9

-SiC sensor chipwith sensor

membrane and

piezoresistors

Ceramic feedthrough

(AlN or Al2O3)

Carrier chip madefrom poly-SiC

Fig. 7. High-temperature pressure sensor for jet turbines fabricated in all-SiC technology using

bulk 6H-SiC substrates. The upper part shows the chip mounting within the sensor package andthe lower part a view onto the 6H-SiC pressure sensor chip



l-probes are solid electrolyte oxygen ion conductors. In a car they measure the ratio ofthe oxygen partial pressures in the exhaust gas, on the one hand, and in the ambientair, on the other hand. When operated in a potentiometric mode such probes generatea step function response when the composition of the exhaust gas changes from fuel-

rich to lean conditions, i.e. at l 1. In Otto engines this signal is used to control thefuel injection to maintain the air-to-fuel ratio at l 1, i.e. at a l-value which is opti-mum for the exhaust gas after-treatment using three-way catalytic converters.

In the near future demands on reduced CO2 emissions require internal combustionengines to be operated at their optimum fuel efficiency point, i.e. under slightly leanconditions [9]. Under such conditions the raw emission of unburned hydrocarbons(HC) and of CO is reduced relative to the emission of nitrous oxides (NOx). With theconcentrations of NOx overwhelming those of HC and CO, conventional three-way cat-alytic converters can no longer be used as a major fraction of the NO raw emissionwould no longer be eliminated. As a consequence completely new concepts of engine

operation and exhaust gas after-treatment are being developed. In this context also newsensors and sensor principles need to be investigated.A particularly interesting approach towards high-speed exhaust gas sensors has

been taken by the Lundstrom group [15]. These sensors are based on metal insula-torsilicon carbide structures. Such sensors have been shown to respond to changes


SiC

SiO2Metal Gate

100 200 300 400 500 600

-500

-400

-300

-200

-100

0

100

200

300

400

500

1500ppm Ethene

2000ppm Propane

1% CO

1000ppm NO

2000ppm NO2

Vo

ltage[mV]

Temperature [C]

-300

-200

-100

0

100

200

300

400

500

Voltage[mV]

NO2

0.1%

NO

0.1%

CO

1.0%

Propane

0.2%

Butane

0.3%

H2O

10%

Ethene

0.2%

Sensitivity and cross sensitivity of MOSiC sensors

(a)(c)

(b)

T = 400C

Fig. 8. Sensitivity of MOSiC exhaust gas sensors towards a number of relevant exhaust gas compo-

nents as measured in a background of 4% O2 in N2: a) chip layout; b) sensitivities as measured atan operation temperature of 400 C; c) variation of the gas sensitivity with operation temperature



in the exhaust gas atmosphere within milliseconds and to be able to be operated attemperatures up to 800 C. In our laboratory we have investigated such sensors insome detail. Measuring the sensor response to various reducing and oxidizing pollu-tants in an oxygen-containing atmosphere with the concentrations of the test gases

chosen similar to those expected under lean-burn engine conditions, such sensors re-spond relatively unselectively to a range of different hydrocarbons, to CO and also toNO2 (see Fig. 8). Such problems are normally encountered with gas sensors and canbe dealt with building arrays of sensors with different cross sensitivities. The data inFig. 8 show that such differences in cross sensitivity can be produced by operatingone and the same type of sensor at a number of different operating temperatures.Another established approach is using different catalytic noble metals on top of suchdevices [15].

A practical problem with such arrays is that the mounting and packaging of the SiCchips is delicate (Fig. 9) considering the harsh environments of an exhaust pipe in terms

of temperature, particulate emissions and mechanical vibration. In this latter context itis relevant to note that high-temperature gas sensing devices with similar properties asSiC ones can also be realized on the basis of GaN Schottky diodes and/or GaN/AlNhigh-electron-mobility transistors deposited in thin-film form on insulating sapphire sub-strates [15]. Such technologies might prove useful in the future to alleviate bonding andpackaging problems associated with bulk-SiC gas sensor arrays.

phys. stat. sol. (a) 185, No. 1 (2001) 11

Au-Wire-Bond

Au-

bond

pad

MOSiC-

Chip

150m

Packaging of MOSICexhaust gas sensors

(b) (c)

(a)

Fig. 9. Housing and assembly of MOSiC exhaust gas sensors; a) mounting onto ceramic heater

substrates by means of thermo-compression die bonding, b) ceramic substrates with back-side Ptheaters; c) final assembly



2.3 Liquid property sensors

In addition to fuel efficiency and ecology a major concern in the car industry is serviceand maintenance. Nowadays maintenance schemes are based on rigid time schedules atwhich particular service and maintenance actions need to be taken. Future servicingschedules are likely to be based on maintenance-on-condition concepts. Within suchconcepts the monitoring of liquid properties is an issue. Examples are monitoring thequality of the lubricant oil or of the brake fluids. The quality of both fluids can degradewith very different rates depending on driving and environmental conditions.

Recently we could show that the viscosity of engine oil can be monitored using mi-cromachined flexural plate-wave sensors. As shown in Fig. 10 such sensors consist of amicromachined silicon membrane with an AlN thin film deposited on top of this mem-brane. Interdigital electrodes launch a bulk acoustic wave which propagates across thismembrane suffering multiple reflections at the stiff silicon support structure producinga standing transverse wave. This transverse wave interacts with the liquid contacting the

membrane within the etch trough. Wave propagation across the membrane is influ-enced by both the mass loading as well as by the viscous damping in the liquid. Thedata of Fig. 10 clearly show that the degradation of engine oil increases both the massloading of the membrane as well as its damping. In comparison to existing surfaceacoustic wave (SAW) devices FPWs allow the sensitive electronic parts of the device to


liquid

electronics

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

-95

-90

-85

-80

-75

-70

-65

-60

oil 15W40 new

oil 15W40 used

Gain - Frequency

Frequency [MHz]

Gain[

dB

]

Liquid-property FPW sensor

(FPW: Flexural Plate Wave)

Fig. 10. Flexural plate wave (FPW) sensor for monitoring specific weight and viscosity of engine

oils. Degradation of engine oil is seen to increase both the mass loading and the damping of thevibrating silicon membrane



be mechanically separated from the corrosive liquids to be tested. Due to the fact thatbulk acoustic waves, i.e. relatively large masses are involved, relatively low vibrationfrequencies result which are easier to handle than the high SAW frequencies.

The problem associated with the degradation of brake fluids is the take-up of water

in the course of time. Formation of steam bubbles within the brake fluid can cause theproblem of soft brakes a serious safety problem. Problems with the take-up ofwater also exist in hydraulic oils. Water take-up makes such oils corrosive leading tomalfunction and failure of hydraulic actuators in aircrafts. In this latter context it isagain relevant to note that high-electron-mobility transistors based on GaN/AlGaN het-erostructures allow for a sensitive monitoring of polar contaminations in unpolar liquidslike hydraulic and brake fluids [17]. Another important point is that such sensing de-vices are easily combined with SAW structures which can be interrogated over wirelessdata links (for a review see [18]).

3. Conclusions and OutlookPrototypes of semiconductor sensors have been developed and tested in all relevantapplication areas in automobiles. Micromachined silicon sensors have started to becomeintroduced into commercial vehicles in significant quantities. Successful examples areinertial sensors, pressure and airflow sensors. Harsh environment sensors for applica-tions in the powertrain and/or maintenance and service systems have developed up tothe prototype level. Successful introduction of such sensors still requires major effortsin the down-stream industrialization of wide-bandgap semiconductor technologies. Theautomotive and aerospace fields can benefit from each other in their roles as cost andtechnology drivers.

Acknowledgements The work reported above is a collaborative effort of many indivi-duals. Special thanks are due to Dr. R. Vo. Financial support was provided by Prof.Dr. G. Hertel, director FT2 DaimlerChrysler Central Research.

References

[1] M.H. Westbrook and J.D. Turner, Automotive Sensors, Institute of Physics Publishing, Bristol/Philadelphia 1994.

[2] S.M. Sze (Ed.), Semiconductor Sensors, John Wiley & Sons, New York 1994.[3] G. Muller, G. Krotz, and E. Niemann, Sens. Actuators A 43, 259 (1994).

[4] G. Krotz, Ch. Wagner, W. Legner, H. Sonntag, H. Moller, and G. Muller, Proc. Internat.Conf. on Silicon Carbide and Related Materials, Inst. Phys. Conf. Ser. No. 142, Chap. 4, IOPPubl. Ltd., Bristol 1996 (p. 829).

[5] G. Krotz, M. Eickhoff, and H. Moller, Sens. Actuators A 74, 182 (1999).[6] W. Lang, Mater. Sci. Eng. Rep. 17, 1 (1996).[7] K.A. Shaw, Z.L. Zhang, and N.C. MacDonald, Sens. Actuators A 40, 63 (1994).[8] S. Sassen, R. Voss, J. Schalk, E. Stenzel, T. Gleissner, R. Gruenberger, F. Neubauer, W. Ficker,

W. Kupke, K. Bauer, and M. Rose, Sens. Actuators A 83, 80 (2000).[9] Autoelektrik, Autoelektronik am Ottomotor, Ed. Robert Bosch GmbH, VDI-Verlag, Dussel-

dorf 1994.[10] C. Lyons, A. Friedberger, W. Welser, G. Muller, G. Krotz, and R. Kassing, Proc. 11th Annual

Internat. Workshop on Micro Electro Mechanical Systems, Heidelberg (Germany), January2529, 1998 (p. 356).

[11] M. Eickhoff, P. Reinhardt, and G. Krotz, Proc. 2nd Micro Materials, Berlin 1997 (p. 955).[12] M. Eickhoff, H. Moller, G. Krotz, J. v. Berg, and R. Ziermann, Sens. Actuators A 74, 56 (1999).

phys. stat. sol. (a) 185, No. 1 (2001) 13



[13] M. Eickhoff, G. Krotz, O. Ambacher, and M. Stutzmann, Presented at 247th Heraeus Semi-nar, Tutzing, Germany, December 1315, 2000.

[14] P.T. Moseley, J. Norris, and D. Williams (Eds.), Techniques and Mechanisms in Gas Sensing,Adam Hilger Ltd., Bristol/Boston 1991.

[15] A. Lloyd Spetz, A. Baranzahi, P. Tobias, and I. Lundstrom, phys. stat. sol. (a) 162, 493

(1997).[16] J. Schalwig, G. Muller, O. Ambacher, and M. Stutzmann, phys. stat. sol. (a) 185, 39 (2001)

(this issue).[17] R. Neuberger, G. Muller, O. Ambacher, and M. Stutzmann, phys. stat. sol. (a) 185, 85 (2001)

(this issue).[18] G. Scholl, phys. stat. sol. (a) 185, 47 (2001) (this issue).


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