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Available online at www.sciencedirect.com

Sensors and Actuators B 132 (2008) 404–410

Capacitive humidity sensors on flexible RFID labels

Alexandru Oprea a,∗, Nicolae Barsan a, Udo Weimar a, Marie-Luise Bauersfeld b,Dirk Ebling b, Jurgen Wollenstein b

a Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 8, 72076 Tubingen, Germanyb Fraunhofer Institute for Physical Measurement Technique, Heidenhofstrasse 8, 79110 Freiburg, Germany

Available online 16 October 2007

bstract

Capacitive humidity sensors intended to be used with intelligent RFID tags have been produced from polymer foils and/or thin films. They were

uccessfully tested on experimental platforms well reproducing the real operation conditions. Adequate sensing properties (enough sensitivity,ood selectivity and linearity and reasonable response and recovery times) have been achieved. The main requirements for a good match to thearget applications, that are also fulfilled, are discussed in connection with the readout options and other practical issues.

2007 Elsevier B.V. All rights reserved.

eywords: Humidity; RFID; Sensor

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. Introduction

Nowadays there is a lot of interest for the development ofmart and low cost radio frequency identification (RFID) tags toeplace bar codes. The monitoring of temperature and humidityuring shipment of, e.g. automotive parts, chemicals, aircraftngines, medical supplies, ammunition, foodstuffs and othererishables/vulnerable-to-environmental-conditions goods isne important application that could be solved by such devices;ne can imagine that the users will set acceptable temperaturend humidity ranges for their products and, at the end of thehipment, the environmental conformity can be checked. Anverview of the trends and latest achievements in this fieldringing in play “smart RFID” with increased connectivitynd additional facilities can be found in [1]. Nowadays oneemarks an increasing interest for intelligent tags, providedith sensing capabilities and able to save and store the acquired

nformation related to both identity and measured parameterssee for example [2–6]). Commercial RFID tags that measure

emperature are available [7].

Here, we present the development of novel polymer-basedapacitive humidity sensors integrated onto flexible RFID label

∗ Corresponding author. Tel.: +49 707 1297 7633; fax: +49 7071 29 59 60.E-mail address: alexandru.oprea@ipc.uni-tuebingen.de (A. Oprea).

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925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2007.10.010

ubstrates. The main ideas behind the development approachre:

For this type of applications it is crucial to have the possi-bility to measure with low power consumption and by usingstandard, cheap, off-shelf circuitry.The devices should be low cost and easy to manufacture;a limited number of simple technological steps are to beused.The design and realization should be compatible with thefuture goal of the “full plastic RFID tag” and extensible tothe sensing of other gases/sensing tasks.

. Concept

In order to meet the application requirements, capacitiveeadout sensing [8] was chosen because of the almost linearesponse, simple structure and low power consumption ensuredy its non-dissipative operation mode. Moreover, it is compatibleith the last generation of capacitance to digital integrated cir-

uits available on the market for other capacitive sensors (stress,ccelerations) and in the same time allows for standard ac &/ hf

mpedance readout electronics.

The realization of capacitive sensors on plastic substrates isven more attractive because of their chemo-mechanical prop-rties and price. Here, one has to point out that the substrate

Actuators B 132 (2008) 404–410 405

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A. Oprea et al. / Sensors and

tself has the potential for humidity sensing, allowing for theanufacture of self-supported devices.The capacitive transducing principle has another major

dvantage: it ensures significant selectivity for the humidityecause of the large value of water permittivity when comparedith one of the possible interferents. Using sensing materialsith nonlinear or strongly nonlinear response to humidity, new

ensor features (as condensation/dew point detection) can bemplemented.

. Experimental

The samples were realised from/onto commercially availableolyethylene-naphthalate (PEN) and polyimide (PI) foils with0–100 �m thicknesses.

The electrodes (Fig. 1), with an active area of 1 cm2 haveeen structured either by chemical etching of a pre-existing5 �m thick Cu film or by lifting off a 5 �m thick evaporated Cuayer. Usual interdigital traces and gaps of 20–100 �m lead tonominal capacity (NC) between 4 pF and 20 pF at 0% relativeumidity (r.h.).

Thin/thick poly cellulose acetate (CA), poly cellulose-cetate-butyrate (CAB), poly methyl-methacrylate (PMMA)nd polyvinylpyrrolidone (PVP) sensing layers were realised onifferent substrates by spray deposition or screen printing fromuitable polymer solutions/gels. Also, polymer mixtures, suchs PMMA&CA, PMMA&CAB, CAB&CA and PMMA&PVP,ave been investigated.

Fig. 2 is showing the surface morphology of a CAB layerprayed on a reference sapphire substrate.

The responses of the sensors to gases/vapours mixtures haveeen recorded in computer controlled dosing and measuringystems by using suitable instrumentation (HP 4263A and HP285A precision impedance bridges, Autolab Eco Chemie BV

mpedance spectrometer, Bruker Equinox 55 FT-IR spectrom-ter and home designed & made readout electronics basedn commercially available �� capacitance-to-digital converterCs).

ig. 1. Five micrometers thick copper-electrodes (brighter areas) on polyimide-oil for the capacitive readout of the humidity.

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Fig. 2. Optical micrograph of a 20 �m CAB (at 300× magnification).

. Results and discussion

Different types of sensors have been manufactured and inves-igated: single foil with readout electrodes, bimorph structuresonsisting on a sensing layer (single or composite polymer)eposited on foil substrates and multilayers placed on the sameype of substrates. The direct experimental results are the depen-ences of the sensor electrical parameters (parallel equivalentapacitance and resistance, complex impedance, etc.) on thembient atmosphere composition, as Fig. 3 shows for a PIKapton®) sample.

One observes a reduced influence of the operating frequencyn the capacitive response of the sensor, and a huge effect onhe conductance (actually the loss factor), leading to increasedower dissipation; this aspect is essential when designing mobilepplications, expected to run for weeks with only a small bat-

e chosen. It is important to point out here that the leakagedc) conductance of the samples is very low (�100 pS between% and 70% relative humidity) and that, to a large extent, it is

ig. 3. The response of the PI sensor to humidity (from 0% to 100% and backo 0% relative humidity in 10% steps). The capacitance response is displayedn the upper graph while the ac conductance in the down graph. Measurementerformed with HP 4263A and HP 4285A, at 25 ◦C and 200 sccm synthetic air.

406 A. Oprea et al. / Sensors and Actuators B 132 (2008) 404–410

Fig. 4. Comparison of the humidity responses from a CAB/PI humidity sensorand its supporting PI (Kapton®) foil. The open symbols stem from exposureeaH

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Fig. 6. The IS spectra (Cole–Cole plot) of a 30 �m PI foil (Kapton®). Mea-s23

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vents performed during increasing humidity sequences while the full symbolsre representing the decreasing humidity ones. Measurement performed withP 4285A, at 25 ◦C and 200 sccm synthetic air.

ue to the surface charge transport under humid ambient con-itions. Therefore, the sample cannot be directly utilised for aesistive readout of the humidity level even if the associatedould be appropriate. Fig. 3 (conductivity panel, 100 Hz curve)ives a suggestive image of the previous assertions. From theonductivity panel of Fig. 3 it is possible to subtract the meanissipated power under continuous operation conditions (50%.h. and 1 V excitation voltage assumed): 30 pW (100 Hz sinave)–30 �W (10 MHz sin wave). At frequencies of few kHz,here the capacitance-to-digital converters usually operate, theean power continuously dissipated is ∼1 �W. Reading rates

f 1 min−1 with active periods of 50 ms will result in averagedissipations of 1 pW.

From the raw data the calibration curves are usually inferred.ig. 4 presents the characteristics of a CAB sensing layer

eposited on PI (Kapton®) substrate while Fig. 5 makes a com-arison between PI and PEN foils. The presence of the substraten the structure of a complex sensor (at least a bimorph) is not aimple mechanical one. As almost all materials, the supporting

ig. 5. Comparison of the humidity responses from a 30 �m PI (Kapton®) and00 �m PEN humidity foil-sensors. The open symbols stem from exposurevents performed during increasing humidity sequences while the full symbolsre representing the decreasing humidity ones. Measurement performed withP 4285A, at 25 ◦C and 200 sccm synthetic air.

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urement performed with Autolab Eco Chemie BV impedance spectrometer at5 ◦C and 200 sccm synthetic air (the 0% relative humidity semicircle ends at0 G�).

oil is also absorbing the water vapour and, by that, is modify-ng its own permittivity. Fig. 4 exemplifies such a situation forhe case of PI (Kapton®). In the figure one can remark a goodinearity up to the saturation condition for the PI capacitor andp to 80% relative humidity for the CAB @ Kapton® bimorphthe CAB film being as prepared, without additional annealing).ome thermally treated polymer composites (PMMA&CA andMMA&CAB) are as linear as the PI substrates.

The choice of electrical readout type has a significant rolen the information acquired from the capacitive sensors; thiss especially true when using capacitance-to-digital convert-rs, which also respond to the active component of the sensormpedance and combine different operating frequency regions.or this reason, a more systematic investigation using the

mpedance spectroscopy (IS) method was performed [9]. Thepectra acquired for foils have “normal” shapes [10]. Theole–Cole plots of the complex impedance (Z) are practically

emicircular in the frequency range of interest (Fig. 6), indicat-ng standard dielectric relaxation mechanisms. They are usually

odelled with simple RC parallel equivalent circuits. The equiv-lent capacitance weakly depends on the humidity while thequivalent resistance, representing the material dielectric losses,ramatically decrease with it, in good agreement with the datacquired with the RLC bridges and depicted in Fig. 3. At lowrequencies, well below the sensor operation region, importantpectrogram sections appear that are linear, making an angle ofbout 45◦ with the real Z-axis. In the common IS cases [10]his dependency type indicates the control exercised by dif-usion over the electrochemical processes near the electrodes.ecause its description with few discrete circuit elements isot, in principle, possible, a model parameter, the Warburgmpedance (Zw =�(1 − j)ω−1/2, σ, a constant, ω, angular fre-

uency, j = (−1)1/2), is usually employed [10] for numericalvaluations. In our case, as in general in the case of thin films,he origin of the Warburg or Warburg-like impedance is morentricate and ambiguous. The extent and the structure of the

A. Oprea et al. / Sensors and Actuators B 132 (2008) 404–410 407

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Fig. 8. The relaxation time and equivalent parallel resistance associated witht3B

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oitsoha3stwatsample, as revealed by IS. Thus, the effects of the humidity

ig. 7. The IS spectra (Cole–Cole plot) of a 15 �m CAB deposited on a 30 �mI foil (Kapton®). Measurement performed with Autolab Eco Chemie BV

mpedance spectrometer at 25 ◦C and 200 sccm synthetic air.

esults we got during the present study, focused on RFID sen-or applications, did not allow us to fully investigate that topic;owever we think that an ion-diffusion component of the elec-rical processes taking place in the sample, in the vicinity of thelectrodes, should be involved [9].

For the bimorph sensing structures including foil-substratesoated with polymer sensing films, the IS spectra (see Fig. 7)annot be any more fitted with simple circuit elements (equiv-lent capacitor with parallel resistor). The deviation from idealielectric behaviour is always enhanced by the water condensa-ion in the sensing material [11] and by the additional swellingrocesses. In some cases, depending on the polymer structurend the value of the electrode interdigital gap, a second semicir-le, with no obvious significance, appears in the IS spectrogramt high humidity. The Warburg impedance is present this timeoo. A standard origin for its occurrence, that is, ion diffusion,s now more plausible since the CAB sensing layer presents a

ore pronounced electrolytic character.The huge variation of the equivalent resistance (Fig. 8) associ-

ted with the first semicircle reproduces quite well the behaviour

ig. 9. Comparison between the humidity effects determined by IR spectroscopy androm exposure events performed during increasing humidity sequences while the full

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he first semicircle (at high frequencies) for a 15 �m CAB deposited on a0 �m PI foil (Kapton®). Measurement performed with Autolab Eco ChemieV impedance spectrometer at 25 ◦C and 200 sccm synthetic air.

f the conductivity (shown for example in Fig. 3 for PI only).ooking to the comparative plots in Fig. 8, one easily observes

he tendency already remarked for the PI case: the loss factor isuch strongly dependent on the humidity than the capacitance,

etermining the overall behaviour of the device.The transmission IR investigations (see Fig. 9 for the case

f 30 �m thick Kapton foil) have put in evidence only a lim-ted equivalence between the optical and electrical responses:he capacitance is linearly and possible over-linearly (aboveome threshold, >60% r.h.) depending on the humidity; on thepposite, the ac conductance linearly follows the humidity atigh levels, but, at low levels, behaves under-linearly; the IRbsorption, as revealed by the “water bands” (1630 cm−1 and630 cm−1), is under-linear at increased humidity, suggesting aaturation trend of the absorption process. If one assumes thathe results of the IR measurements accurately reproduce theater content of the polymer film, then, one has to implicitly

gree on the role played by additional electrical and/or elec-rochemical effects in the overall behaviour of the investigated

impedance measurements (data from Figs. 3 and 10). The open symbols stemsymbols are representing the decreasing humidity ones.

bsorption go, very probably, beyond the stage of simple per-ittivity and loss factor enhancements resulting in adsorbedater state, ion concentration and ion mobility changes.

408 A. Oprea et al. / Sensors and Actuators B 132 (2008) 404–410

Fig. 10. Evolution of the IR spectrum of a 30 �m PI (Kapton®) foil exposed todMa

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ifferent levels of humidity; in the legend “r.h.” symbolizes the relative humidity.easurement performed with Bruker Equinox 55 FT-IR spectrometer at 25 ◦C

nd 200 sccm synthetic air.

hese preliminary data do not, however, allow for definitiveonclusions.

Due to the significant thickness of the foils important inter-erence effects appear in the spectra (Fig. 10). On one hand, theyeduce the precision with which the positions and the intensi-ies of the adsorption bands can be directly determined, but, onhe other hand, they can give valuable information concerninghe dependency of the layer geometry on the ambient humidity.ubsequent evaluations, including the interference fringes sub-

raction, will have to be performed in order to obtain more precisealues. The examination of the interference patterns superposedn the recorded spectra could make possible, in a future eval-ation step, the detection of the swelling processes and thevaluation of their contribution to the capacitive response.

From the analysis presented in the IS section results that, byaking advantage of both sensor structure [12,13] and readout

eculiarities, strongly nonlinear responses to humidity can beeliberately obtained; they can be exploited for special appli-ation, e.g. condensation determination, as Fig. 11 shows for

ig. 11. Highly nonlinear response of the PVP&PMMA sensor on PEN substrateo humidity. Exposure to incrementing (open symbols) and decrementing (fullymbols) humidity pulses and stairs (10%, 20%, 30%, 40%, 50%, 60%, 70%,0%, 90%, 100% relative humidity). The responses for 100% relative humidityverloaded the capacitance-to-digital-converter readout (limited to ±4 pF signalpan). Measurement performed with an AD 7745-based readout, at 30 ◦C and00 sccm synthetic air.

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easurement performed with an AD 7745-based readout, at 200 sccm syntheticir. The electronics was introduced in the measuring chamber having the sensoremperature.

PVP&PMMA sensor on PEN substrate driven with a ��

apacitance-to-digital converter.The influence of the temperature on the sensitivity of

he foils/layers/complex structures was investigated in lim-ted ranges and for some combinations only. At temperaturesetween 20 ◦C and 75 ◦C devices’ sensitivity and baselinehanges of about 10–30% have been observed (Fig. 12). Fortu-ately, many commercial capacitance-to-digital converters haventegrated thermometers that allow for temperature compensa-ion and, by that, solving this inconvenient.

The layers response time constant is between 1 min and 3 minnd for foils between 1 min and 7 min. The recovery time con-tants are usually longer (up to a factor 3). The reaction speedf the sensors also depends on the readout frequency, decreas-ng with increasing frequency. Fig. 13 displays the response of

CAB/PI (Kapton) sensor to a 50% humidity pulse recordedy using a commercial �� capacitance-to-digital converterAD7745) at 30 ◦C @ 200 sccm synthetic air. The measuringet-up time constant of about 30 s, included in the given values,

etermines an artificial loss of performance.

In any case, for the contemplated RFID applications, theynamic characteristics of the sensor are not restrictive as long

ig. 13. Response and recovery time constants evaluation for CAB/PI sensor.he signal was recorded with an AD 7745-based readout, at 30 ◦C and 200 sccmynthetic air.

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s the monitored items present themselves a significant inert-ess in absorbing humidity from the ambient (because of ownackaging and mass). Additionally, the limitations in the supply-ng battery lifetime and storage capacity, inherent for an activeag, drastically reduce the data acquiring and saving rates. Theserawbacks of the active transponders determined some authors4–6] to focus on passive RFID sensors and to come up withnteresting solution, which, however, did not solve the problemf long time watching goal. We tried to maintain the advantagesf continuous surveillance by reducing the power consumptionf the humidity sensors through high dielectric quality of theensing materials, suitable readout electronics and exploitationcenarios.

. Conclusion

The concept of “low power”, “self-supported”, planar, flex-ble and “directly integrable” sensor for “smart RFIDs” wasmplemented in the form of interdigitated chemo-capacitancerovided with humidity sensitive polymers and shows promisingreliminary results. In the same time, as underlined in Section, different technological alternatives such as simple polymeroil (which can be the substrate of the RFID tag itself) sin-le or composite polymer layers deposited onto polymer foilr complicated multilayer/foil structures have been designednd produced. The devices, manufactured on 30–100 �m PInd PEN substrates, with ∼1 cm2 active area, 30–40�m inter-igital traces and gaps and 4–20 pF nominal capacitance, haveood sensitivities, of about 100–1000 ppm/% r.h. The readoutith the capacitance-to-digital converters ensures a resolutionf about 100 aF at a full scale of ±4 pF. They offer a large rangef response types, from linear (better than ±3%) up to stronglyonlinear (more than a factor 5 for 100% r.h.), being suitableor the wide spectrum of practical applications required by theotential users (humidity monitoring, humidity threshold alarm,ew point detection, etc.).

The developed sensor is versatile, dissipate very low power<1 pW at 1 reading/min @ 1 kHz @ 50 ms active period pereading), has good response time constants (∼1–2 min) andan be extended to other classes of applications for whichensing materials with appropriate characteristics are alreadyr will be soon available. Through electrical, electrochemi-al and optical measurements, performed over wide rangesf operating conditions, the devices and their sensing mate-ials have been extensively characterised. In this context, thequivalence of the investigation methods employed to evaluatehe specific sensitivity towards humidity of the deposited lay-rs and self-supported films has been additionally addressed.omplementary experiments are still required to definitely con-lude on the detail differences encountered in the evaluations’esults.

cknowledgment

The authors acknowledge the financial support from theMBF project No. 16SV2038, TRACK.

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eferences

[1] P.G. Ranky, An introduction to radio frequency identification (RFID) meth-ods and solutions, Assembly Autom. 26 (2006) 28–33.

[2] O. Vermesan, N. Pesonen, C. Rusu, A. Oja, P. Enoksson, H. Rustad, Intel-liSense RFID—An RFID Platform for Ambient Intelligence with SensorsIntegration Capabilities, Ercim-News (online), vol. 67, 2006.

[3] N. Cho, S.-J. Song, S. Kim, S. Kim, H.-J. Yoo, A 5.1-�W UHF RFIDtag chip integrated with sensors for wireless environmental monitoring,in: Presented at 31st European Solid-State Circuits Conference, Grenoble,2005.

[4] P. Pursula, J. Marjonen, K. Jaakkola, Wirelessly powered sensor transpon-der for UHF RFID, in: Presented at Transducers’07 14th InternationalConference on Solid-State Sensors, Actuators and Microsystems, Lyon,2007.

[5] R.A. Potyrallo, W.G. Morris, Multianalyte chemical identification andquantitation using a single radio frequency identification sensor, Anal.Chem. 79 (2007) 45–51.

[6] K. Chang, Y.H. Kim, Y. Kim, Y.J. Yoon, Functional antenna integratedwith relative humidity sensor using synthesised polyimide for passive RFIDsensing, Electron. Lett. 43 (2007) 259–260.

[7] h.w.k.-m.d. produkte variosens en.php./.[8] N. Yamazoe, Y. Shimizu, Humidity sensors: principles and applications,

Sens. Actuator 10 (1986) 379–398.[9] H.-E. Endres, S. Drost, F. Hutter, Impedance spectroscopy on dielectric gas

sensors, Sens. Actuator B: Chem. 22 (1994) 7–11.10] E. Barsoukov, J.R. Macdonald, et al., Impedance Spectroscopy Theory,

Experiment and Applications, 2nd ed., Wiley & Sons, Inc., Hoboken, NJ,2005.

11] M. Sahm, A. Oprea, N. Barsan, U. Weimar, Water and ammonia influenceon the conduction mechanisms in polyacrylic acid films, Sens. Actuator B127 (2007) 204–209.

12] A.M. Kummer, A. Hierlemann, H. Baltes, Tuning sensitivity and selectivityof complementary metal oxide semiconductor-based capacitive chemicalmicrosensors, Anal. Chem. 76 (2004) 2470–2477.

13] R. Igreja, C.J. Dias, Dielectric response of interdigital chemocapacitors: therole of the sensitive layer thickness, Sens. Actuator B: Chem. 115 (2006)69–78.

iographies

lexandru Oprea received the diploma in physics from the University ofucharest in 1976 and the PhD in solid-state physics from the Central Institute ofhysics, Bucharest, Romania, in 1996. He got successively the positions of scien-

ific researcher, senior researcher and principal senior researcher at the Nationalnstitute for Materials Physics, Bucharest. He joint for 3 years (1997–2000) thepplied Physics-Sensors Department of the Technical University of Cottbus,ermany, and since 2002 is senior scientist in the Gas Sensor Group of theniversity of Tuebingen, Germany. The research fields: thin films solar cells,igh-field electroluminescent devices, polymer and metal oxide gas sensors.

icolae Barsan received in 1982 his diploma in physics from the Faculty ofhysics of the Bucharest University and in 1993 his PhD in solid-state physicsrom the Institute of Atomic Physics, Bucharest, Romania. He was a senioresearcher at the Institute of Physics and Technology of Materials, Bucharest,etween 1984 and 1995. Since 1995 he is a researcher at the Institute of Physicalhemistry of the University of Tubingen and actually is in charge with theevelopments in the field of metal oxides-based gas sensors. He published about00 papers and contributions to international conferences.

do Weimar received his diploma in physics 1989, his PhD in chemistry 1993nd his Habilitation 2002 from the University of Tubingen. He is currently the

ead of Gas Sensors Group at the University of Tubingen. His research interestocuses on chemical sensors as well as on multicomponent analysis and patternecognition. He is author of about 180 scientific papers and short notes. He isesponsible for several European projects and for co-ordinating the Networks ofxcellence NOSE and GOSPEL (see also http://ipc.uni-tuebingen.de/weimar/).

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arie-Luise Bauersfeld received her diploma in physical engineeringrom the University of Applied Sciences, Merseburg, Germany, in 2004.ince then she is engaged in the development of thin film gas sensoricrosystems on the Fraunhofer Institute of Physical Measurement, Freiburg,ermany.

irk G. Ebling received the MSc and PhD degrees in chemistry from the Univer-ity of Duesseldorf, Germany. His thesis research concerned the fabrication of

ubmicrometer structures in alumina and their electrical and optical properties.fter graduation in 1991, his research focused on the characterization and growthf compound semiconductor materials at the University of Freiburg, Germany.eside the main focus of his research, development of X-ray radiation detectorsnd advanced ultraviolet sensors from wide band gap materials, he was also

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tors B 132 (2008) 404–410

oncerned with the development of lead selenide native oxides for IR-LASERpplications. From 2001 to 2005, he worked on the epitaxial growth and devel-pment of optical devices from III/V materials for ultra fast laser applicationst the FIRST Center for Micro- and Nano-sciences, ETH, Zurich, Switzerland.ince early 2006, he is working in the field of thermoelectric materials and theirpplications at the Fraunhofer Institute for Physical Measurement Techniquesn Freiburg, Germany.

urgen Wollenstein received his diploma in electrical engineering from theniversity of Kassel, Germany, in 1994. In 2003 he finished his PhD Thesis

t the same university. In 1994 he joined the Fraunhofer Institute of Physicaleasurement, Freiburg, Germany, where he is engaged in development of laser

pectrometry and microsystems.