Sensors and Actuators A 68 ( 1998) 399-403
Directly heated quartz crystal microbalance with an integrated dielectric sensor Mathias Roth, Thomas Dera, Andreas Drost, Ralf Hartinger, Frank Wendler, Hanns-Erik Endres *, Bernd Hillerich Fruunhofer Institute for Solid State Technology, HansastraJe 27d, D-80686 Munich, Germany
Abstract Cross-sensitivity, especially to humidity, is a common problem in the analysis of chemical gas-sensor signals. To identify the contributions of two gas components at least two suitable signals are needed. This article presents a new chemical sensor unit which is a combination of a quartz crystal microbalance and a dielectric sensor. Thus it yields information about both the mass and the dielectricity of the same chemically sensitive film. The differences in absorption isotherms and drift behaviour are discussed. To make this a universal element for gas sensing, a heater and a temperature gauge are also integrated directly on the quartz by thin-film technology. This allows the application of temperature modulation techniques to the quartz microbalance principle. 0 1998 Elsevier Science S.A. All rights reserved. Keywords: Interdigital capacitors: Quartz crystal microbalances; Carbon dioxide; Water; Heaters; Isotherms; Quartz
Quartz crystal microbalances ( QMBs) [ I] and interdigital capacitors (IDCs) [ 21 are well-known transducer elements for gas sensing. Both employ different physical principles with their specific advantages and disadvantages caused by physics and technology. IDCs from the point of view of technology are simple elements, generally studied, and they have been designed in various ways. Mostly these elements are designed to be heatable, so a wider range of chemically sensitive coatings can be applied. A drawback of IDCs is their measurement principle. For a variety of gas-sensing reactions it is hard to predict whether the resulting dielectric change is large enough to be measured. Additionally water is a highly polar molecule and causes cross-sensitivities to humidity for almost all gas sensors based on IDCs. The QMB is an attractive transducer because it directly weighs the absorbed gas in a first approximation. Due to this measurement principle the cross-sensitivity to water can often be neglected, especially if the.element is used at an elevated temperature. In this paper, we describe the design, fabrication and testing of a novel combined IDC-QMB transducer element, which is furthermore directly heatable. The schematic drawing of this device is shown in Fig. 1. The goal is a device that * Corresponding author. Tel.: +49-X9-54759223; 100.
0924-4247/98/$19.00 8 1998 Elsevier Science S.A. All rights reserved. PrrSO924-4247(98)00094-6
Fig. 1. Schematicview of the top and bottom sides of a directly heatable IDC-QMB
allows simultaneous measurements of dielectric and mass changes on exactly the same coating at various temperatures. The device is directly heatable and has a low thermal constant. Thus it is able to apply the advantages of temperature modulation techniques  for drift suppression to the QMB measurement principle.
2. Thermal simulation The heating is intended to give a temperature distribution which is as homogeneous as possible in the sensitive regions. Unfortunately the heater is not allowed to overlap the microbalance electrodes, so the temperature will decrease towards
M. Roth et al. /Sensors and AImatom A 68 (1998) 399403
the centre. For multiple heater structures analytical and numerical simulations of the resulting temperaturewerecarried out. Using the analytical solution, the geometry and the numberof heaterresistorcircuits were optimized. Following the limits of the technology, the best results were achieved with threeheatercircuits to reduce’theheatingpower andget a homogeneoustemperaturedistribution. A more realistic simulation requires numerical simulation by the finite-element method (FEE/I) to verify the analytical results. Detailed heater structures, anisotropy and temperature-dependentmaterial properties are now considered.The bond pad of the microbalanceelectrodeextendsto the edge of the quartz substrateandforcesa gap in the heatermeander, as shown in Fig. 1. The width of each of the three loops is calculated from the heat generation per unit length of the analytic solution. The heat-generationparametersare indirectly proportional, i.e., the middle loop that hasthe leastheat generationis also the widest. Iterative optimization yields the width of the radial connections of the meander.The large metallization of the quartz ensuresa flat temperaturedistribution in the middle of the transducer.Fig. 2 showsthe result of a numerical simulation of the heat distribution of the sensor. Using an IR camerathe calculated valuescould be qualitatively verified without the metallizedsurface.The emission parameterof Au was too small for the measurementset-up used.
Flat quartz crystal blanks are available with various parameters.As for thicknessshearresonators,the basicresonance frequency is in inverse proportion to the thickness, so a 10 MHz quartz crystal with a thickness shear of 167 p,rn is chosenas a good compromisebetweenfrequency resolution and mechanicalstability. To avoid the risk of an unwantedspurious resonance,the diametersof the microbalanceelectrodesand the quartz crystal itself are taken from commercialmodels: 10 mm for the quartz diameter and 4 mm for the diameter of the microbalanceelectrodes. The transducer is well suited for transient temperature measurements.The heating time can be minimized to a few secondswith an appropriatecontrol algorithm. The limiting time constant for such an application is the cooling time, which was determined as tgO= 60 s ( 100°C+ 3O’C). This time constant dependsmainly on the sensordimensions and is acceptablefor many applications. Fig. 3 shows the time responseof the sensorfollowing a stepof the heating voltage. Simple metallic structureslike themicrobalanceelectrodes areusually madewith shadowmasks.For more detailedstructures photolithography is needed.Following the restrictions of the photolithography, a processing near the edge of the quartz substrateis not possible.Consequentlythe other strucANSYS 5.0 J-UN 1s 1996 10:30:26 PLOT NO. 1 NODAL SOLIPPION STEP=1 SUB =I. TIME=1 TEMP TEPC=2.235 SKN =69.461 Emx =?0.025
Fig. 2. FEM simulation of the temperaturedistribution (A.NSYS 5.0).
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A 68 (1998) 399103
Table 1 Overview about the technical data of the sensor
Fig. 3. Time responseof the sensorfollowing a step of the heating voltage (fit with an exponential decay function).
tures like the IDC, the heaterand the temperaturegaugehave to be designedin a ring areabetween2 and 4 mm radius. To minimize the number of processing steps,all structuresare madeof the samemetallic layer. The heaterand temperature gauge are resistive meanders.An overview about the technical data of the sensoris given in Table 1. 4. Technology The processingwasdoneaccordingto standardtechnology used for quartz-basedIDCs and is describedin our previous work . However, in contrast to IDCs the QMBs cannot be processedas a wafer yet; they have to be manually fabricated one by one. Thin gold bond wires connect the bond pads of the structures and the pins of the socket.The quartz crystal has to be fixed on a hard base,e.g., the socket,to standthe stresswhen being bonded. A special socket, shown in Fig. 4, has been developedfor this purpose:a non-conductingplastic ring with an inner diameter of 8 mm contains the pins. The structures on the quartz surfacearenot covered,andboth sidesarefreely accessible.The pins arearrangedin order to fit into a standard TO8 socket.Bond padsfor oneof themicrobalanceelectrodes and the temperature gauge are supported on the sensitive, easyaccessibleupper surface,but the contacting of the heater and the other electrode causesproblems. They have to be bonded first, but with loose bond wires. Then the quartz crystal is glued with silicone rubber onto the plastic socket, which already contains the pins. Silicone rubber is thermally isolating, very heat resistive and soft enough not to disturb the piezo-vibrations. 5. Gas-sensitive coating For the first tests, the DC-QMB combination was spincoatedwith a mixture of 3-aminopropyl-trimethoxysiloxane (.AMO) and propyltrimethoxysilane (PTMS) . The
Fig. 4. Photographof the IDC-QIMB mountedon a TO&compatible special socket.
thickness of the sensitive coating is about 0.5 km. The well of the sensitive material on the edge of the quartz is outside of the sensitive area.The main sensitivity of this coating is to CO2in a rangefrom 100to 10 000 ppm. The main adsorption reaction is the formation of a hydrogen carbonate: NH, + CO* + H,O tf NH,+ + HC03-
This reactionresultsin an increaseof the massof the sensor (and therefore in a decreaseof the QMB’s resonancefrequency). The CO, molecule hasonly a small dipole moment. Therefore, the formation of a carbonateresults in a decrease
M. Roth et al. /Sensors and Acruators A 68 (1998) 399403
of the permittivity of the indicator material and in the IDC’s capacitance. Additionally a swelling of the indicator’s thickness under adsorption of CO2 was observed and may contribute to the sensing effect. The sensitivity range fulfils the requirements for monitoring the CO, level of air, but the cross-sensitivity to water of this coating, for the IDC and also for the QMB, cannot be left uncompensated. Additionally this coating needs to be heated to about 60°C to show a good reversible gas adsorption behaviour.
IDC signal [pFI
Fig. 5 shows the adsorption isotherm of an AMO/PTMScoated IDC-QMB at about 60°C for C02. The transducers were heated using their built-in heater structure. Both the isotherms are well explained by a Langmuir model modified with an additional linear term to describe a two-stage absorption process [ 4,5] :
[CO21 S=So+k Lco,l +A, +&inear[COd where S is the sensor signal, Sothe basic signal, the next term corresponds to the Langmuir isotherm and the last term is a linear completion of the formula. It is evident that all constants depend on the sensor’s temperature. The second stage of the adsorption process of COz may not contribute to the dielectric isotherm. Also an increase of the capacitance signal, as observed in former studies with an SO, indicator material, was not observed. Following the main detection mechanism ( 1) such a behaviour was not expected. The overall behaviour is in good agreement with results reported earlier on separate QMB and IDC devices [ 61. Fig. 6 depicts a measurement of the material with different CO2 and humidity concentrations. The signals of the IDC and the QMB differ widely in their relative sensitivities to CO2 and H20, which was expected from earlier investigations. At a certain gas concentration the comparison of the QMB and IDC sensor signals (concentration step CO,: 300 + 1000 ppm, 30% r.h. and H,O: 30 + 60% r.h. at 300 ppm CO,, humidity related to 25°C) gives the relative humidity inter-
250 !$ 200 2 s -150 2 Y - 100 P -50 &
DewpoinilK1 iJ.d.dI-.lJ~ 0
Time [min] Fig. 6. Different responses of the QMB and IDC parts of the combined IDCQMB transducer. The coating is AMOIPTMS (see text). The sensor is directly heated to 60°C using 220 mW. The dashed lines are connecting points of the same CO1 content at different humidity values (intended as a guide to the eye).
ference on the mass signal as about 7% and 100% for the dielectric signal. Using these two signals, the responses to CO, and H,O can easily be separated by classical signal processing or by an artificial neural network algorithm.
7. Conclusions The design, fabrication and testing of a novel transducer structure, a combination of a quartz microbalance and an interdigital sensor, are described. This sensor allows two different physical properties, the dielectric change as well as the mass loading of exactly the same coatings, to be examined. The integrated direct heater allows not only the heating of the element, thus eliminating the need for a heated measuring chamber, but also the application of temperature modulation techniques to QMBs for the first time. Applications of the IDC-QMB can be found in any field where it is necessary or advantageous to extract more than one independent sensor signal from a certain sensor coating, e.g., in various electronic nose techniques based on polymer coatings. The fabrication process of the device contains manual steps on the singlesensor level. This limits the applications of the IDC-QMB at the moment to research and high-price sensor systems like electronic noses. However, this is not an intrinsic drawback and can be overcome by future designs.
References CO,Ippml Fig. 5. Absorption isotherms for the dielectric and mass-sensitive signals of the IDC-QMB device.
[ I] C. Lu, A.W. Czanderna, Applications of Piezoelectric Quartz Crystal Microbalance, Methods and Phenomena, Their Applications in Science and Technology, vol. 7, Elsevier, Amsterdam, 1993.
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Endres, S. Drost, Optimization of the geometry of gas-sensitive interdigital capacitors, Sensors and Actuators B 4 ( 1991) 95-98. [31 M. Roth, R. Hartinger, R. Faul, H.-E. Endres, Performance enhancement of organic coated gas sensors by temperature modulation, Sensors and Actuators B 35-36 ( 1996) 1-5 and references cited therein. [41 M. Horn, A new theory of adsorption for the quantitative description of gas sensors, Sensors and Actuators B 26-27 ( 1995) 217-219. [51 M. Roth, Ph.D. Thesis, in preparation. [61 H.-E. Endres, L.D. Mickle, C. K&linger, S. Drost, F. Hutter, A gassensor system with dielectric and mass sensors, Sensors and Actuators B 6 (1992) 285-288.
with a diploma thesis on a long-term study of metal oxide gas sensors.This work was carried out at the Fraunhofer Institute for Solid StateTechnology, where he continued his research,focusing on environmental monitoring and statistical signal analysis. In 1997he starteda Ph.D. thesis on the surfacestructureanalysisof metal oxides at the University of Munich (Institut fur Krisstalographie und angewandte Mineralogy). Hnnns-Erik Endres received his diploma in general physics
Biographies Mathias Roth studied physics and physical chemistry at the University of Karlsruhe and the University of Oregon,USA. In 1994 he received his MS degreefrom the University of Karlsruhe with a thesis on the electronic structure of doped fullerenes. In the sameyear hejoined the FraunhoferInstitute for Solid State Technology. His main researchinterests are the developmentof transducerstructuresand signal evaluation algorithms for gas sensing.He is also completing work on a Ph.D. thesis. Andreas Drost studied technical physics at the Technical
University of Munich andjoined the Fraunhofer Institute for Solid State Technology in 1987. He is working on technological developmentsin the field of MEMS. His specialinterestsare the deposition of metals and glasses,wafer-bonding techniquesand lithography. Ralf Hartinger studied technical physics at the Technical
College of Munich. He completed his study in 1994 with a thesis about characterizationof SAW gassensors.Sincethat time he hasworked at the Fraunhofer Institute for Solid State Physics. His fields of activity include the maintenanceand developmentof a testsystemfor gassensorsandthe improvement in signal evaluation methodsfor gassensorswith neural networks. Frank Weizdler studied physics at the Technical University
of Munich, Germany, and obtained an MS degree in 1995
from the Technical University of Munich (gamma ray spectroscopy) and his Ph.D. with a work about capacitive gas sensors in 1989 from the Universitat der Bundeswehr Munich. He joined the Fraunhofer Institute for Solid State Technology in 1985.He is responsiblefor the laboratory for gassensors.His main researchand developmentinterestsare chemical sensorsystemsfor gases,signal evaluationmethods and olfactometry. received the diploma in physics from the University of Bonn and the Ph.D. in electrical engineering from the Technical University of Berlin, Germany. From 1975to 1979he was with AEG-Cable, Mtilheim, to develop optical fibre cablesand measuringmethods.Subsequently,at Philips Communications Industrie, Cologne, Germany, he developed measuring equipment for optical fibre links. In 1981 he joined AEG ResearchCentre, Ulm. There he was engaged in research and development projects in various fields of micro-optics, optoelectronicsand communications. In 1993he becameheadof product developmentin the Sensor Division of Weidmiiller Interface,Gaggenau.In thesuccessor company, Precitec GmbH, he was responsible for development and production. In 1995 he transferredto Quartzkeramik GmbH, Stockdorf (near Munich) and led the R&D department, which was engaged in crystal oscillators and quartz resonatortechnology. SinceOctober 1995he hasbeen head of the MEMS Department at the Fraunhofer Institute for Solid StateTechnology in Munich, Germany.Dr Hillerich hasauthoredabout 60 technical papersand holds six patents. Bernd Hillerich