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Sensors and Actuators B 119 (2006) 255–261

Functionalized pyroelectric sensors for gas detection

Matthias Schreiter a,∗, Reinhard Gabl a, Johannes Lerchner b, Christian Hohlfeld b,Annekatrin Delan b, Gert Wolf b, Anja Bluher c, Beate Katzschner c,

Michael Mertig c, Wolfgang Pompe c

a Siemens AG, Otto Hahn-Ring 6, D-81739 Munchen, Germanyb TU Bergakademie Freiberg, Leipziger Str. 29, D-09596 Freiberg, Germany

c Max-Bergmann-Zentrum fur Biomaterialien und Institut fur Werkstoffwissenschaft,Technische Universitat Dresden, D-01069 Dresden, Germany

Received 5 October 2005; accepted 9 December 2005Available online 14 February 2006

bstract

A new calorimetric gas sensor based on functionalized integrated pyroelectric detector arrays was fabricated and tested. The advantages of thisensor are mainly based on its miniaturized design enabling a close-by arrangement of sensing and reference structures. The low heat capacity andigh thermal isolation of the sensor elements enable in principle the application of integrated low power heating structures. Lead–zirconate–titanatePZT)-based detector arrays were fabricated on silicon and mounted in test packages adapted to the printed circuit board (PCB) containing

simple readout circuitry. The performance of two different types of sensor surface functionalisations was studied. A polymer coating with

olydimethylsiloxane (PDMS) was chosen to detect heptane absorption. A detection limit of 10 ppm heptane was found. Bacterial surface layersunctionalized with small Pt clusters were applied to study catalytic oxidation of hydrogen. Within the investigated range from 0.5 up to 3.5 vol.%ydrogen, no saturation of the detection signal was observed.

2005 Elsevier B.V. All rights reserved.

ion; H

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eywords: Calorimetric sensor; Pyroelectric detector array; S-layer; H2-detect

. Introduction

The capability of pyroelectric transducers to detect tem-erature changes which are generated by chemical reactionsffers the possibility to utilize pyrochips as calorimetric chemi-al sensors. Generally, calorimetric sensors are widely used forhe detection of combustible gases like for instance hydrocar-ons, carbon monoxide and hydrogen, where the concentration-ependent thermal power of a catalyzed oxidation reactions converted into an electrical signal by appropriate tempera-ure sensing elements [1–6]. Furthermore, calorimetric sensorshich are functionalized with gas absorbing coatings are useful

or the recognition of volatile organic compounds (VOCs).

In a pellistor, which is the classical catalytic calorimetrical

ensor for combustible gases, temperature sensing is performedy platinum resistance wires. Nowadays, miniaturized planar

∗ Corresponding author. Tel.: +49 89 63648347; fax: +49 89 63648131.E-mail address: matthias.schreiter@siemens.com (M. Schreiter).

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eptane absorption

ilicon devices with thin film platinum heaters and temperatureensors (micro hot plates) are more common [1,2]. Alternativelyo resistive temperature detection, the importance of thermoelec-ric heat power transducers for calorimetric sensors is growing.hermoelectric metal oxide sensors [3,4], fabricated in thick film

echnology, as well as Si or SiGe thin film thermopile sensors5,6], prepared by conventional CMOS processes, provide, e.g.or hydrogen, detection limits in the lower ppm range. Becausef the small thermal inertia of silicon thermopile sensors and thenherent reference temperature compensation effects, they cane operated in a high-speed temperature scanning regime up to00 ◦C and with cycle times of a few milliseconds [5]. Withespect to differences in the activation energies of the involvedas components, the shape of the signals is a characteristic of thenvestigated mixture, i.e. the application of pattern recognition

ethods should lead to an enhanced selectivity of the sensor

5].

Pyroelectric-based heat power transducers, which are mainlytilized in practice for the detection of infrared radiation, showdvantages for chemical sensor applications, if fast temperature

2 d Actuators B 119 (2006) 255–261

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hanges can be expected. Thus, they play an outstanding rolen adsorption calorimetry, e.g. for the direct calorimetric mea-urement of the monolayer adsorption of metal atoms, oxygenr carbon monoxide on clean single crystal surfaces [7,8]. Ifhe rate of a sensor recognition reaction is modulated in timen order to increase the rate of the temperature changes, whichould be done, e.g. by periodic switching between analyte andeference gas flows or by temperature modulation, pyroelectricransducers should also have potentials for application as chem-cal sensor, as it was already demonstrated for the detection ofydrogen [9].

Here we focus on the chemical sensor application of Si-ntegrated pyroelectric detector arrays [10] enabling the applica-ion of different functionalisations on the same chip. The arraysere fabricated using thin film and bulk micro-machining tech-ologies. Two kinds of sensor surface functionalizations weresed to demonstrate their potentials for the detection of VOCss well as for pellistor-similar applications. For the latter anlevated sensor temperature according to the catalytic workingemperature of the particular gas of interest is necessary. Inte-rated low power heating structures adapted to the sensor designere successfully tested.

. Sensor design and surface functionalization

.1. Sensor device fabrication

The basic part of the sensor is formed by the pyroelectricetector chip carrying eight sensitive elements with an individ-al size of 720 �m × 360 �m. To compensate interfering signals,wo elements are series connected. The functional principle ofpyroelectric sensor is given by the conversion of a change of

emperature into an electrical signal as voltage or current. Origi-ally developed for infrared applications, these sensors exhibit apecific detectivity of about 108 cm Hz1/2/W for a frequency ofHz correlating with a noise equivalent power of <1 nW making

hem interesting also for calorimetric sensor applications.A schematic cross section of a sensitive element of the pyro-

lectric detector array is depicted in Fig. 1. The array is formedy a thin film capacitor deposited on a supporting SiO2/Si3N4-embrane. The dielectric is made of a thin film of ferroelectric

ead zirconate titanate (PZT). To provide a sufficiently high ther-

Fig. 1. Schematic sensor layout.

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Fig. 2. Assembled gas sensor on PCB with readout circuit.

al isolation, the underlying silicon is removed by bulk microachining.To enable elevated working temperatures a local low power

hin film heater arranged on bulk-micromachined membranesnd integrable with the pyroelectric sensor structure was devel-ped and tested.

For the application as calorimetric gas sensor, the individuallements were functionalized by covering them with appropriatehin films. Heat released by interactions between analyte gas andunctionalization will change the temperature of the pyroelectriclement and consequently result in an electrical signal. For theest of the sensors, two types of sensor surface functionalizationsere chosen as described in detail in the next section.The detector chip is mounted in a ceramic housing adapted

o the printed circuit board (PCB) carrying a simple readoutircuitry with an optional voltage amplification of 10 (Fig. 2).CB and housing are adapted to a gas flow cell enabling theupply of defined concentrations of the analyte.

.2. Functionalization of the active sensor surfaces

The functionalization of active elements of the pyroelectricetector array was accomplished by coating them with a chem-cally sensitive layer of PDMS (polydimethylsiloxan) for theetection of VOCs or with Pt-cluster arrays grown on bacte-ial surface layers (S-layers) serving as catalyst for hydrogenxidation.

For PDMS, a change of amount of gas absorbed by theunctional layer causes transient heat power effects leading toemperature changes within the sensor elements. Their ampli-ude depends on the enthalpy and the equilibrium constant ofhe absorption as well on the rate of the concentration changef the analyte. A micromanipulator served for precise position-ng of the polymer solution. The expected heat of absorptionelated to the mass of PDMS is 1.9 Ws/g at a heptane concen-

ration of 1000 ppm [11]. Based on the specific detectivity of08 cm Hz1/2/W at around 1 Hz and a polymer density of aboutg/cm3, a PDMS thickness of a few micrometers is estimated

o be necessary for a heptane detection in the low ppm-range.

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The second type of functionalization represents a sensor coat-ng which catalyzes the oxidation of combustible gases enablingn application in a pellistor-similar manner, i.e. the concentra-ion of the combustible gas can be measured due to the heatower effect generated in the course of the oxidation process.s catalyst, Pt clusters on S-layers were used. S-layers form

he outermost cell envelope component of many prokaryotes inlmost all phylogenetic branches of bacteria and archaea. Theegular protein crystals exhibit different lattice symmetries (p2,3, p4, and p6) and a center to center distance of neighboringorphological units in the range of 3–30 nm [12].Here the S-layer isolated from the bacterium Bacillus sphaer-

cus NCTC 9602 was used. This S-layer exhibits a p4 symmetryith a lattice constant of 12.5 nm and is frequently used as a

wo-dimensional template for the fabrication of highly-orientedetallic nanocluster arrays [13–15]. The conditions for cell cul-

ivation and purification of S-layer sheets were described previ-usly [16]. Pt cluster arrays were grown on the two-dimensionalrystalline protein layer by a method similar to that describedn [17]. Three different samples have been prepared. Sample Aas prepared by using dimethylaminoboran (DMAB) as reduc-

ng agent. To this end, 100 �l of a S-layer suspension with arotein concentration of 10 mg/ml was mixed with 300 �l of amM K2PtCl4 solution, which had been prepared 24 h before

o enable hydrolysis of the metal complexes [18], and with00 �l of 10 mM DMAB solution. For sample B, NaN3 wassed as reducing agent instead of DMAB. As reference (sample) a sample without S-layer was prepared by mixing 1 ml of

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Fig. 3. Experimental set-up of

uators B 119 (2006) 255–261 257

3 mM K2PtCl4 solution with 100 �l of 5 mM NaN3 solution.he metallized S-layer sheets and the reference where depositedy placing small drops of the metallized S-layer suspensionnto the active sensor surfaces by means of a micromanipulator-ontrolled micropipette system and a subsequent drying in air.he sensor-pixel surface was completely covered with the func-

ional film.

. Experiments

To verify the thermal and electrical properties of the mountedyroelectric sensor chips including the readout circuit, the indi-idual sensor elements were irradiated with a laser, power mod-lated in a frequency range from 50 mHz to 100 Hz, before theas measurements were taken. Averaged signal amplitudes ofhe sensor output were obtained by fast Fourier transformationFFT) data processing of a series of 300 signal periods for thendividual modulation frequencies.

Based on the nature of the pyroelectric effect, detecting tem-erature changes, a periodical modulation of the analyte con-entration is necessary for gas detection. The device with anntegrated gas modulation facility used is schematically depictedn Fig. 3. Here, a periodic change between analyte and referenceas is realized by a computer controlled valve system. To assure

hat both gases are at the same temperature a heat exchange units integrated.

For the PDMS as sensor functionalization, heptane was cho-en to demonstrate the sensor capabilities because of its high

the gas handling system.

258 M. Schreiter et al. / Sensors and Actuators B 119 (2006) 255–261

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habgfthe behavior of other known calorimetric hydrogen sensors [4].In general, the activities of the metallized S-layer prepared byDMAB reduction (see Section 2) and of the reference sample

ig. 4. Sensor signal vs. frequency of the laser beam for functionalisation test.

nthalpy of absorption in the polymer layer [19]. As prelimi-ary experiments have shown, a modulation frequency of 0.5 Hzs sufficient to establish an absorption equilibrium. Since thebsorption effects of the analyte do not supply a sinusoidal heatow, multi-frequency signals have been obtained. Therefore, theveraged amplitudes of ground and overmodes determined byFT data processing were cumulated. Measurements were taken

n a concentration range between 50 and 2000 ppm heptane atoom temperature.

For the sensors functionalized with metallized S-layers, oxi-ation of H2 was chosen as model reaction proceeding at roomemperature. The measurement procedure as well as the datarocessing is comparable to that of gas absorption experimentsith PDMS. The hydrogen concentration was varied between.5 and 3.5 vol.%.

Integrated thin film heating structures of different resis-ances, meander lengths and covering surfaces were character-zed. Based on V–I measurements the achieved temperature raiseersus input power was derived.

. Results

Considering the laser characterization of sensor pixels, theypical frequency dependence of the averaged non-amplified sig-al amplitudes obtained is given in Fig. 4. The result correspondsell to the typical frequency characteristics of a pyroelectric

ensor. The sensor transfer function is given by the followingelation:

ˆ s = SVΦ = pA

G

�√1 + (�H/G)2

R√1 + (�RC)2

Φ (1)

here us and Φ is the amplitude of the output voltage and thencoming heat flow, respectively, SV the voltage sensitivity and

the angular frequency of the incident laser radiation, p rep-esents the pyroelectric coefficient, A the pixel area, H and Cts thermal and electrical capacity, respectively, and R corre-ponds to the input resistance of the readout circuit. From thehermal cut-off frequency �th of about 55 Hz and a thermalapacity H of the sensor element including the carrying mem-

rane of 1.3 × 10−6 W s/K, which was estimated from the bulkalues of specific heat capacities of the individual layers, a ther-al conductance Gth of about 70 �W/K can be derived. This

orresponds to an area-related conductance of 270 W/(m2 K)

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ig. 5. Sensor signal vs. heptane concentration for pyro-chips coated withDMS.

hich is in a typical range for the isolation method used. Henceaking a capacity of about 320 pF, an electrical cut-off fre-uency �el of about 0.06 Hz and a pyroelectric coefficient of.8 × 10−4 C/(m2 K) of the PZT film, a maximum voltage sen-itivity of about 2 kV/W can be estimated from the maximumutput peak voltage of 11 mV between both cut-off frequencies.he chosen frequency for gas measurements of 0.5 Hz corre-ponding to an angular frequency of 3.14 Hz is well in the rangef the maximum sensitivity.

Results of the experiments on the heptane absorption usingDMS as the functional layer are summarized in Fig. 5. Theutput signals were amplified by a factor of 10. From the sensorutput signal over gas concentration, a voltage sensitivity of1 �V/ppm can be derived. Within the recorded range up to000 ppm no saturation effects were observed. Regarding theoise level of the sensor output, a detection limit of about 10 ppman be estimated, which is in a similar range as achieved in earliereasurements using thermopile sensors [11].In Fig. 6, the cumulated signal amplitudes obtained for the

ydrogen oxidation by Pt catalysts prepared in different waysre depicted. Again, the sensor output voltage was amplifiedy a factor of 10. A nearly linear dependence on the hydro-en concentration is shown and no saturation is found, evenor concentrations as high as 3.5 vol.%, which corresponds to

ig. 6. Sensor signal vs. H2 concentration obtained for differently prepared coat-ngs of the active sensor area with metallized S-layers. Sample A is prepared byeduction of a K2PtCl4 solution with DMAB. Sample B is obtained by chemicaleduction with NaN3. The reference (sample C) is prepared without S-layer (forreparation details see Section 2).

M. Schreiter et al. / Sensors and Actuators B 119 (2006) 255–261 259

Table 1Characteristics of different types of heating meanders

Resistance (�) @ 25 ◦C Number of windings Heater area (�m2) Gth/A (W/(m2 K)) @ 25 ◦C

T 300 × 300 654T 800 × 800 246T 1500 × 1500 252

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pure platinum without S-layer) are comparable. The metallized-layer sample prepared by NaN3 reduction shows a slightlyigher sensitivity. We anticipate that this effect is caused by theact that the Pt clusters obtained by NaN3 reduction are smallerhan those produced via the other fabrication routes [20], whichan turn into a higher surface activity for catalytic reactions.

To enlarge the detection spectrum of combustible gases,igher working temperatures of the sensor are necessary tonsure a sufficient catalyst activity. Investigations on chem-cal conversion versus temperature for different gases usingt-functionalized S-layers on different substrates are reported

n [21] revealing for example a T50-value for carbon monoxide,.e. temperature at 50% of the maximum conversion, of about60 ◦C. The maximum working temperature of the pyroelectrichip itself is limited to about 400 ◦C due to the Curie temperaturef the ferroelectric PZT thin film used.

Three different types of chip integrated Pt heating meandersoncerning resistance and area were investigated to study theirroperties depending on design parameters (Table 1). Whileypes 1 and 2 are suitable for single sensor elements, type 3llows heating up the complete array. The Pt thickness was 75 nmor all structures and the filling factor of the meander design was6%. The test set-up used allows a maximum heating voltagend current of 10 V and 10 mA, respectively. The determined–I characteristics for all three types are shown in Fig. 7. Theeander temperature raise can be calculated using the following

elation:

T = (V/I)/R0 − 1

α(2)

here R0 is the initial resistance at room temperature and a theesistance temperature coefficient. The latter was experimen-

ally determined at room temperature for the Pt thin film usedo 2.3 × 10−3/K and is assumed to be constant in the consid-red temperature range. Results are given in Fig. 8. Types 1nd 2 exhibit a temperature raise of more than 320 K with input

ig. 7. Measured V–I characteristics of different types of heating structures.

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ig. 8. Calculated temperature raise as a function of heating power for differentypes heating structures.

owers as low as 23 and 73 mW, respectively. For type 3 anncrease of only 80 K is achieved due to the limited input powerf about 50 mW for the significantly higher meander resistancen conjunction with an increased thermal conductance resultingrom the larger heater surface. Here, an adaptation of the powerource to higher voltages is necessary. To estimate the individualhermal conductance of the structures, Gth, was fitted within theollowing relation to the measurement results:

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t was found that Gth is constant up to a corresponding tempera-ure raise of about 70 K. The area-related values are summarisedn Table 1. While for the larger heater surfaces (types 2 and 3)

th is comparable to the value estimated from the laser experi-ent, type 1 indicates an increased thermal conductance. This

s obviously due to an additional heat flow to the silicon bulkhrough the contact lines of the heating structures. From geo-

etric facts, the portion of flowing off heat through the contactines is largest for type 1 possessing the smallest meander sur-ace and largest cross sections of the contact lines. This is to keepts electrical resistance low in comparison to the relatively lowesistance of the short meander. For a temperature raise higherhan 70 K an increase of the thermal conductance of about 1‰/Kas observed which might be explained by upcoming airflows

bove the hot heater surface.In future, optimised heating structures integrated with pyro-

lectric sensor arrays will enable to characterise the chips at ele-ated temperatures and thus enlarge the spectrum of detectableases.

. Conclusions

Functionalized pyroelectric sensors were investigated for gasetection as an alternative concept to well-known calorimet-

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ic sensors like pellistors and thermopiles. The specific coatingf miniaturized pyroelectric sensor pixels provides chemicalensors for the detection of volatile organic compounds or com-ustible gases. For example, heptane could be detected with aetection limit of 10 ppm using a polymer layer for recognition.n order to detect combustible gases like hydrogen, differentlatinum catalyst has been deposited successfully at the pixelurface. Sensor pixel coatings with metallized S-layers usingaN3 as reduction agent show an enhanced sensor activity for2 oxidation in comparison to the reference catalysts. An impor-

ant advantage of this new class of calorimetric sensors is theirreparation by micro-fabrication techniques, which furnishesensors in the low-cost range. By integrating heating struc-ures, applications at higher temperatures are possible, whichxtents the application range especially for the detection ofombustible gases such as hydrocarbons. Tested heating struc-ures adapted to the sensor design exhibit a temperature raisef more than 300 K at low heating powers of a few 10 mW.t present we are integrating the pyroelectric sensor arrayith heating elements to establish working temperatures up to00 ◦C.

cknowledgements

We would like to acknowledge the technical assistance ofana Pitzer, Robert Primig as well as Bettina Winzer, Alexanderirchner, Per Lothman and Albrecht Ullrich in the early stagef the experiments. This research is financially supported byhe German BMBF (project: 13N8142), the SMWK and the EUFP5, contract: G5RD-CT-2002-0750).

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[8] St.J. Dixon-Warren, M. Kovar, C.E. Wartnaby, D.A. King, Pyroelectricsingle crystal adsorption microcalorimetry at low temperatures, oxygen onNi{100}, Surf. Sci. 307–309 (1994) 16–22.

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iographies

atthias Schreiter has received his degree in Electrical Engineering from Dres-en University of Technology in 1996. He works for Siemens at the Corporateechnology department where he specialised in the development of tailoredunctional thin films for integrated sensor devices such as integrated pyro- andiezoelectric sensor arrays. His current research activities include the develop-ent of biosensor arrays based on thin film bulk acoustic resonators.

einhard Gabl received the Diploma in physics in 1995 and the PhD in physicsn 1999, both from the University of Innsbruck Austria. His thesis was on designnd development of high frequency silicon germanium transistors. Throughouthe last years he has conducted a number of interdisciplinary research projectsn the field of gas- and bio-detection. His main interests are semiconductorased sensors enabled by the incorporation of new materials including biologicalpecies.

ohannes Lerchner received his PhD and DSc degrees in physical chemistryrom the University of Leipzig in 1990 and 1997, respectively. Dr. Lerchneras worked upon problems in calorimetry and laboratory automation. He isurrently research and teaching assistant at the Institute of Physical Chemistry,U Bergakademie Freiberg.

hristian Hohlfeld received his PhD degree in physical chemistry from the TUergakademie Freiberg in 1979. His main research interests are thermodynamic

tudies of solids and solutions.

nnekatrin Delan is physics graduate of the TU Chemnitz. She has receiveder degree in 1992. She is working now in microelectronic technology projects.

ert Wolf is a chemistry graduate of the TU Bergakademie Freiberg. Cur-ently he is professor and director of the Institute of Physical Chemistry, TUergakademie Freiberg. His research interest include chemical thermodynam-

cs of solids and solutions, calorimetry and applications of calorimetry in sensorechnique and biochemistry.

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M. Schreiter et al. / Sensors an

nja Bluher is a biologist in the BioNanotechnology and Structure formationroup at the Max-Bergmann-Center of Biomaterials, TU Dresden. She prepareser PhD thesis in functionalization and integration of S-layers in to microelec-ronical structures.

eate Katzschner is a technician of the BioNanotechnology and Structure for-ation group at the Max-Bergmann-Center of Biomaterials, TU Dresden. She

upervises the microorganism laboratory and prepares the S-layer for researchrojects.

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uators B 119 (2006) 255–261 261

ichael Mertig is the head of the BioNanotechnology and Structure formationroup at the Max-Bergmann-Center of Biomaterials, TU Dresden. His mainesearch interest is the investigation of biomimetic processes and their applica-ion in an engineering context.

olfgang Pompe is Professor of Materials Science and Nanotechnology athe Technical University, Dresden. His main research fields are the mechanicalehaviour of ceramics and thin films, functional ceramics, formation of nanos-ructures, and hard tissue engineering.