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Microwave sensors: a new sensing principle. Application to humidity detection
Sensors and Actuators B 68 Ž2000. 88–93 www.elsevier.nlrlocatersensorb
Microwave sensors: a new sensing principle. Application to humidity detection Catherine Bernou ) , Dominique Rebiere, ` Jacques Pistre´ Laboratoire IXL-UMR 5818 CNRS, ENSERB-UniÕersite´ Bordeaux I-351, cours de la Liberation-F33405 Talence Cedex, France ´
Abstract We are presenting here a new approach to gas sensor, using the electromagnetic Že.m. properties variation of some sensitive materials in the presence of gas at ultrahigh frequencies Žca. 1 GHz.. The chemical sensor basically consists of a microwave resonator to which a sensitive coating is added. The e.m perturbation can be seen through a frequency variation measurement when the sensor serves as feedback element in an oscillator chain. After the development of a narrow-band-pass filter, a permittivity variation study is done, thanks to an e.m simulator HP-MDS. Then a humidity sensor is designed and tested. A good sensitivity as well as a great reversibility are obtained. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas sensor; Microwave; Planar resonator; Thick film technology; Humidity sensor
1. Introduction A new approach to gas sensor is presented here, based on the change of electromagnetic Že.m. properties of some materials at ultrahigh frequencies. This new sensor concept uses resonant structures at ca. 1 GHz. In microstrip technology, such resonant structures w1x are screen-printed lines on a substrate and this microwave element selects a frequency, the resonant frequency. The propagation on these transmission lines can be determined if the e.m characteristics of the substrate are known, that is the dielectric constant ´ and the permeability m w2,11x. If these parameters can be changed, the propagation would be disturbed and the resonant frequency is going to change. Considering then a sensitive material whose e.m properties change in the presence of chemical compounds. Indeed, there is an absorption of the chemical compounds in the sensitive layer. When this sensitive layer covers these microstrip lines, the propagation takes place in both the substrate, on which microstrips are laid, and in the overlay,
) Corresponding author. Tel.: q33-05-56-846540; fax: q33-05-56371545. E-mail address: [email protected] ŽC. Bernou..
so that the e.m characteristics for the propagation are an effective permittivity, ´eff Ždepending on the substrate permittivity, ´substrate , and on the overlay permittivity, ´overlay . and an effective permeability Ždepending on msubstrate and m overlay .. The e.m field distribution is different from the one without chemical compounds, and there will be a resonant frequency shift, for example. This is the basic idea of the microwave chemical sensor, but in order to realize and test such a sensor, we have to: Ø design a resonant microwave structure, Ø study the microwave propagation variations resulting from the overlay e.m properties change, Ø realize a prototype.
2. Design of a microwave resonant element With the aim of developing a low cost microwave device, the resonant structure uses the planar technology, and more particularly the microstrip line technique. We choose to design a narrow band-pass filter w3,4x that consists of a resonant structure and two ports for supply and measurements.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 6 6 - 4
C. Bernou et al.r Sensors and Actuators B 68 (2000) 88–93
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2.1. The resonant structures in microstrip technique In this microstrip technique, a resonant structure can be a quarter wavelength stub, a half-wavelength coupled line w8,9x or a ring resonator w10x ŽFig. 1.. To develop a band-pass filter, we choose to develop a structure using coupled lines because it is quite simple to study and optimize. 2.2. Excitation and coupling This resonator has to be excited and a second line is necessary to supply the resonator, and also to connect it in order to measure the frequency response. It can be shown that the optimum coupling length is equal to a quarter wavelength Ž lr4. w13x. Indeed, the resonator is the load and the supply line the generator and like in other electronic devices, to obtain a maximum power transfer, the generator and the load have to be matched. If the coupling is too weak, the load does not receive enough power and there will not be any frequency selection. If the coupling is too strong, the resonant phenomenon will be erase and not only a frequency but too wide a range of frequencies will be selected. 2.3. A filter design The designed filter consists of a half wavelength line coupled on a quarter wavelength with a supply line, so that the resonator has to be bent in a U form ŽFig. 2.. This structure is a cut-band filter and we would prefer to develop a band-pass filter that could serve as feedback element in an oscillator ŽFig. 3.. In order to have a band-pass behaviour, a gap is done in the middle of the supply line. A model for this gap is a capacitive coupling represented by Cg , C1 and C2 represent the open-end effect of the interrupted supply line. To conclude, the filter consists in a resonator bent into a U-form to have a quarter wavelength coupling with the supply line interrupted by a gap ŽFig. 4.. 2.4. Optimization of the filter dimensions An e.m simulator HP-MDS ŽMicrowave Design System. w14x is used to characterize and optimize the scattering parameters si j of the filter. These scattering parameters
Fig. 1. The microstrip resonant elements.
Fig. 2. Filter design with a l r2 resonator and a supply line.
give us the filter behaviour, like s21 , the transmission parameter Žinsertion loss. w15x. Indeed, these parameters are simply gains and reflection coefficients defined in Fig. 5 and by the following formulae: s21 s Ž b 2ra1 .< a 2 s 0 for example. s21 is the transmission gain with the output port is terminated in a matched load. On this frequency response ŽFig. 6: the transmission parameter s21 in magnitude Žand dB. vs. frequency f ., the resonant frequency, the pass-band width or the band-pass gain can easily be determined. 2.4.1. Electromagnetic simulator and quasi-static analysis The HP-MDS simulator is mainly based on approximated formulae, found on Conformal Mapping and experimental results, such as Wheeler’s formulae w5,16x. Similar to these methods, the e.m simulator does the quasi-Transverse Electromagnetic ŽTEM. approximation. More precisely, the microstrip line is for the propagation, a non-homogeneous problem, because the propagation takes place in the substrate under the lines and in the air above them. It is very difficult to know exactly the very accurate form of the e.m field Žwhereas it is known that two propagation modes exist.. But in the range extending into the low gigahertz region, and if the substrate dielectric constant ´substrate is large enough, the propagation mainly takes place in the substrate. The e.m analysis can then be done as a current-and-voltage analysis: this approximation is known as quasi-TEM approximation or quasi-static analysis. This approximation is enough to give us the main filter frequency-response features. 2.4.2. The dimensions optimization The filter dimensions have to be optimized to meet the following specifications: Ø The pass-band insertion loss should be close to 0 dB, Ø The 3 dB pass-band should be very narrow for a good frequency selection, Ø The phase near resonance should be continuous Ža phase jump in this region can be prejudicial to detection.. ŽThese parameters are defined in Fig. 6.. Some filter dimensions have influence on the frequency response, more precisely the resonator length and the gap.
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Fig. 3. Gap in the supply line and equivalent electric model.
The resonator length is theoretically equal to a halfwavelength. With the microstrip open-effect, a half-wavelength line has an effective length l, l s lr2 q l s where: l s s 0.412 h
ž
´eff q 0.3 ´eff y 0.258
/ž
wrh q 0.262 wrh q 0.813
/
where ´eff is the effective dielectric constant, w the transmission line width and h the substrate thickness w17x. Then the length has to be reduce to have a lr2 effective length. By means of a HP-MDS simulation, a study for the gap shows that the smaller the gap, the weaker the insertion loss. The smallest realization for g in the used screenprinted technique is 100 mm. 2.4.3. The filter performances With the optimized filter dimensions, the following frequency response is obtained ŽFig. 7.. The pass-band loss DG is about 1 dB; the 3 dB pass-band is f 2 y f 1 s 40 MHz wide for a resonant frequency of 1.5 GHz. The calculated quality factor Q is then of 20, which is quite good for this technology but not very high for a narrow band-pass filter. It is important to have a high quality factor because it is linked with the near-to-resonance phase variation, and consequently with the detection and the sensitivity. The sensor is electrically connected to an oscillator, so that the change in e.m parameters of the structure is converted into a frequency variation, easier to measure.
3. Sensor design. Realization and test of a humidity sensor
Fig. 5. Two-port network showing incident Ž a1 ,a2 . and reflected waves Ž b1 ,b 2 . waves used in s-parameter definitions.
3.1. PermittiÕity study thanks to an e.m simulator The microwave sensor consists of the above designed filter to which a chemical interface is added. So, the propagation medium for this structure is the alumina substrate, the sensitive coating and the air. We have to determine the sensitivity of our sensor to an e.m parameter variation. We choose to study in particular a permittivity variation. The HP-MDS simulator cannot describe this threelayered structure, but a part of this simulator, called HPMomentum w7x, can do the propagation analysis. HPMomentum does a full-wave analysis by using a numerical procedure, known as Moment Method w6x. In the non-homogeneous structure, the propagation can be described as a second order differential equation with boundary conditions :
™ E2 E ™ D E y ´m s 0 for the electrical field. Et 2
If the unknown equation solution u Ži.e. the electrical or the magnetic field. is expressed as a sum of n trial functions a i with a i weights, this method basically consists in projecting the trial functions on m test functions Õj . Mathematically speaking, the projection is a scalar product and the n = m products are the Moments of the unknown function u. At mesh nodes of the structure w7x, the Moments are computed and optimized to meet the boundaries. Then, the calculation of the weights a i give a numerical description of the e.m field. The HP-Momentum simulator is used to study the effect of a permittivity variation of the sensitive layer. After
Planning to use a sensitive coating whose dielectric constant varies in the presence of humidity, we begin to study the perturbations induced by a permittivity variation. Then a humidity sensor is designed and connected in an oscillator chain.
Fig. 4. Filter final design. l1 q2 l 2 s l r2.
Fig. 6. Filter response sample. Main frequency response features.
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Fig. 7. Ža. Magnitude G ŽdB. and Žb. phase of the transmission coefficient s21 ŽdB. vs. frequency f. Substrate: Al 2 O 3 ; ´ r s 9.8; h s 635 mm. Dimensions: l1 s 10 mm; l 2 s 5 mm; g s 100 mm.
computation, we can see that a permittivity increase of the sensitive coating induces a decrease of the filter resonant frequency, which can be seen on the transmission parameter frequency response. In Fig. 8, we present the frequency shift observed on the scattering parameter s21 of the band-pass filter Ždesigned above. due to the sensitive layer permittivity increase: when the dielectric constant increases from 4 to 40, the resonant frequency decreases by 270 MHz. 3.2. Humidity sensor design To prove the feasibility of the detection principle and to validate the permittivity variation study, a humidity sensor is realized. Indeed, this sensor consists of the narrow band-pass filter and a polyimid overlay. The polyimid is a polymer sensitive to wet air: when the water molecules are absorbed in the coating, the polymer dielectric constant increases. The narrow-band-pass filter is realized in microstrip lines by means of the screen-printed technology, that gives thick films. The screen-printing is done first for the ground
Fig. 8. Magnitude s21 ŽdB. vs. frequency. Effect of permittivity variation. Trace 1: ´ r s 40. Trace 2: ´ r s 4.
plane with an AgPd ink, with a final thickness between 10 and 17 mm, on very pure alumina substrate, the roughness of which is very low. Secondly, the microstrips are screen-printed with a high definition gold ink, with a thickness between 6 and 12 mm. The filter is then connected on an aluminum stand, the electrical ground plane connection being reinforced by the use of a conductive silver lacquer. Two Small Miniaturized Adaptator ŽSMA. connectors are placed on both ports of the supply line. The filter performances are then tested with a HP-8753C Network Analyzer. The pass-band insertion loss is about y7 dB, because of the connection losses, for a measured quality factor of 30. The polyimid is an Ultradel 4208 coating. A regular coating on the alumina substrate and on the filter strips is achieved by a spin coat operation. 3.3. Electrical connections The sensor is fixed in an airtight cell and is electrically connected to an oscillator. The electronic configuration of this oscillator is developed with a 20 dB gain low noise amplifier and an adjustable attenuator, which constitute an adjustable gain amplifier. The remaining oscillator chain consists of a manually adjustable phase shifter in order to
Fig. 9. Oscillator configuration.
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achieve the phase loop condition. With our oscillator configuration, the Barkhausen gain condition for oscillation is:
4.2. Discussion
Gfilter q Gampl q Gatten q Gphase - shifter q Gcoupler s 0 dB
The detection mechanism is attributed to the polyimid dielectric constant increase in the presence of wet air: it is a well known principle, used for capacitive transducers w18–21x. In our microwave structure, this permittivity increase induces a change of the structure dielectric constant. Indeed, the microstrip lines must be analyzed considering the non-homogeneous dielectric medium: ´ 1 s 9.8 for the alumina, ´ r2 for the polyimid and ´ r s 1 for air. To perform a suitable analysis, these three dielectric constant are replaced by the effective dielectric ´e constants w2,5,11,13,16x Žfor the whole structure. which has to be calculated, for example, thanks to a Variational method. This method determines the potential V on the microstripline, the charge Q of this conductor, and then the capacity of the line C0 when the calculation is done for a Žvirtual. free-air microstrip and C for the studied substrate w12x. Finally, the effective dielectric constant is given by:
and for phase condition:
w filter q wampl q watten q wphase - shifter q wcoupler s 0 Ž 2p . . A 20 dB directional coupler allows the measurement of the output frequency with a HP-5350B microwave electronic counter. Fig. 9 depicts the oscillator configuration. Measures for stability are done with the oscillator chain in ambient atmosphere at ca. 258C; the counter accuracy Žtime gate. is 1 Hz and the measured frequency is near 1.61206 GHz. The sensor short-time stability Žwithin 1 min. is 2 kHz and the longtime stability Žwithin 1 h or more. is 5 kHz. It is quite good as neither the ambient temperature nor the amplifier temperature are controlled.
4. Experimental results and discussion 4.1. Sensor response The set up in an airtight cell sensitive device is connected to an automatic gas line which can generate gas atmospheres at a constant flow Žfrom 100 to 1000 cm3rmin for dry air, the gas carrier. and at a controlled Relative Hygrometry ŽRH. from 0% to 80% RH. Insofar as we can consider the humidity stimulation as a 40% RH wet air step, the response ŽFig. 10. can be considered as a first order type and the 90% value is obtained within 2 min. As seen above, the frequency decreases as the relative humidity increases. Taking the gas line delay into account, the rising time Ž10% to 90%. of the response to the RH step is estimated at 68 s. The sensor sensitivity is about 5 kHz per %RH, which is rather good in comparison with the oscillator stability. As the frequency level at the end of desorption is the initial one, a very good reversibility is also obtained.
Fig. 10. Response to a 40% humidity step at 258C. Polyimide thickness: 6 mm. A H5000 Coreci humidity sensor gives the reference for %RH.
´e s
C C0
.
Based on this method, a program was built up to study the sensitive coating influence. Generally speaking Žwhen the polyimid permittivity ´ r2 is higher than the alumina one., the higher ´ r2 , the higher effective permittivity ´ r . This results in the concentration of both electric and magnetic fields in the double-layered substrate. If the fields are less scattered, the resonant frequency decreases in accordance with HP-Momentum simulation. The observed frequency shift has to be compared with the simulated one. Our problem is that we have no knowledge of the polyimid dielectric constant evolution law vs. RH at ultrahigh frequencies. For the simulation, we did an extrapolation, knowing that the water permittivity is 80 and that of the polyimide with dry air is 4. A decrease of 270 MHz was calculated by simulation, the experiment only shows a 160 kHz decrease. Several reasons could explain this result. Ž1. The sensitivity depends on the coating thickness. The optimum thickness found by simulation is about 10 mm, whereas with the spin operation, a 5 mm coating is obtained. Ž2. The manufacturer has modified this polymer because the permittivity variation bothers some users. So this second polyimid generation could have a lower sensitivity to wet air. Ž3. Finally, it would be interesting to study the diffusion process in the polyimid coating. An evaluation of the diffusion constant could tell us the polyimid behaviour in presense of humidity. Ž D is defined by Fick’s law: F s yDŽECrE x ., where F is the rate of transfer per unit area of section, C the concentration of diffusing substance, and x the space coordinate measured normal to the section..
C. Bernou et al.r Sensors and Actuators B 68 (2000) 88–93
5. Conclusion We have presented a new sensor working at ultra-high frequency formed with a microwave structure and a sensitive coating. CAD permits to evaluate the sensitivity of resonant structure to e.m parameter variations. To illustrate this new concept we have realized a humidity sensor with polyimid working at 1.5 GHz, by using thick film technology, and performed a humidity detection in a gas line. The results are in good accordance with the previsions on the detection principle. The sensor shows high sensitivity to humidity and good reversibility is also obtained. The evaluation of the diffusion coefficient D for the polyimid will improve our knowledge of the interaction between polymer and wet air. We have demonstrated the feasibility of a microwave chemical sensor. Various chemical sensors based on this principle can be developed. Perturbation mechanisms like conductivity or permeability perturbation can lead to the realization of other Žbio.chemical sensor types. Studies on sensors working in other media Žlike liquids. or at higher frequencies are ongoing.
References w1x W. Gopel, K.D. Schierbaum, SnO 2 sensors: current status and future ¨ prospects, Sens. Actuators, B 26–27 Ž1995. 1–12. w2x F. Gardiol, Traite´ d’electricite ´ ´ de l’EPFL vol.13 Georgi, Suisse, 1981. w3x C. Cenac, Etude de filtres actifs planaires accordables fonctionnant a` ´ 12 GHz, These ` no.13-1994, Universite´ de Limoges, France. w4x J.S. Hong, M.J. Lancaster, Theory and experiment of novel microstrip slow-wave open-loop resonator filters, IEEE MTT 45 Ž12. Ž1997. December. w5x H.A. Wheeler, Transmission-line properties of parallel wide strips by a conformal-mapping approximation, IEEE MTT 12 Ž1964. May. w6x The weighted residuals method, cours de D.E.A. de l’Universite´ de Besanc¸on, 1996. w7x Momentum Technical Document: HP-Momentum User’s Guide. w8x S.B. Cohn, Parallel-coupled transmission line resonator filters, IRE Trans. 6 Ž1958. 223–231. w9x C. Nguyen, Accurate equation for determining resonator length in half-wavelength parallel-coupled bandpass filter, Electron. Lett. 29 Ž6. Ž1993. 18th March. w10x J.S. Hong, M.J. Lancaster, Couplings of microstrip square open-loop resonators for cross-coupled planar microwave filters, IEEE MTT 44 Ž12. Ž1996. December. w11x P.F. Combes, Transmission en espace libre et sur les lignes, Ed. Dunod Ž1983. .
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w12x E. Yamashita, R. Mittra, Variational method for the analysis of microstrip lines, IEEE MTT 16 Ž4. Ž1968. 251–256, April. w13x D.M. Pozar, Microwave Engineering, Wiley Editors, 2nd edn. w14x HP 85150B ŽMDS Microwave and RF Design Systems., User’s Guide, Printed in USA 12r92. w15x HP-Application Note 95-1, S-Parameter Techniques for Faster, More Accurate Network Design, reprinted Compliments of HP-Journal, vol. 18, no. 6, Feb. 1967. w16x K.C. Gupta, R. Garg, I.J. Bahl, Microstrip Lines and Slotlines, Artech House, 1979. w17x E.O. Hammerstad, F. Bekkadal, A microstrip handbook, ELAB Report, STF 44A74169, N7034, University of Trondheim-NTH, Norway, 1975. w18x P.R. Story, D.W. Galipeau, R.D. Mileham, A study of low-cost sensors for measuring low relative humidity, Sens. Actuators, B 24–25 Ž1995. 681–685. w19x H. Shibata, M. Ito, M. Asakura, K. Watanabe, A digital Hygrometer using a polyimide film relative humidity sensor, IEEE Trans. Instrum. Meas. 45 Ž2. Ž1996. April. w20x T. Boltshauer, C. Azeredo Lame, H. Baltes, High sensitivity CMOS humidity sensors with on-chip absolute capacitance measurement system, Sens. Actuators, B 15–16 Ž1993. 75–80. w21x A. Denton, A. NgiapHo, A. SenGangHe, A solid-state relative humidity measurement system, IEEE Trans. Instrum. Meas. 39 Ž3. Ž1990. June.
BIographies Catherine Bernou was qualified as an Engineer in electronics from ENSEEIHT Engineering School in Toulouse in 1992. She is preparing a PhD thesis on microwave gas sensors at IXL Microelectronic Laboratory. Dominique Rebiere d’Electronique-Electrotechnique ` received his Maıtrise ˆ et d’Automatique, his Diplome ˆ d’Etudes Approfondies in electronics and his PhD from Bordeaux I University, France, in 1987, 1988 and 1992, respectively. He has been involved in research on acoustic wave sensors since 1989 at Bordeaux I University-IXL Microelectronic Laboratory, and is Maıtre at Bordeaux University in electronic engineering. ˆ de Conference ´ Jacques Pistre´ received his MEng Degree in Electronics from Bordeaux University, France, in 1968 and his These ` d’Etat degree in ionic conductivity of thin films in 1979. Since joining the IXL Laboratory Žformerly Laboratoire d’Electronique Appliquee ´ ., he has worked in several areas of thick-film microelectronics, including applications to microwave circuits and sensors. He was sent on secondment for 18 months to the French company Thomson, working in the divisions of Radars, Countermeasures, Missiles. In 1987, he returned to the IXL laboratory where he is in charge of the sensors group. This group is mainly involved in gas sensors, using either elastic waves in solids or microwave devices. He is a professor at ENSERB, a French ‘Grande Ecole’, where he teaches electronic systems and is responsible for international relations.
Microwave sensors: a new sensing principle. Application to humidity detection Catherine Bernou ) , Dominique Rebiere, ` Jacques Pistre´ Laboratoire IXL-UMR 5818 CNRS, ENSERB-UniÕersite´ Bordeaux I-351, cours de la Liberation-F33405 Talence Cedex, France ´
Abstract We are presenting here a new approach to gas sensor, using the electromagnetic Že.m. properties variation of some sensitive materials in the presence of gas at ultrahigh frequencies Žca. 1 GHz.. The chemical sensor basically consists of a microwave resonator to which a sensitive coating is added. The e.m perturbation can be seen through a frequency variation measurement when the sensor serves as feedback element in an oscillator chain. After the development of a narrow-band-pass filter, a permittivity variation study is done, thanks to an e.m simulator HP-MDS. Then a humidity sensor is designed and tested. A good sensitivity as well as a great reversibility are obtained. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas sensor; Microwave; Planar resonator; Thick film technology; Humidity sensor
1. Introduction A new approach to gas sensor is presented here, based on the change of electromagnetic Že.m. properties of some materials at ultrahigh frequencies. This new sensor concept uses resonant structures at ca. 1 GHz. In microstrip technology, such resonant structures w1x are screen-printed lines on a substrate and this microwave element selects a frequency, the resonant frequency. The propagation on these transmission lines can be determined if the e.m characteristics of the substrate are known, that is the dielectric constant ´ and the permeability m w2,11x. If these parameters can be changed, the propagation would be disturbed and the resonant frequency is going to change. Considering then a sensitive material whose e.m properties change in the presence of chemical compounds. Indeed, there is an absorption of the chemical compounds in the sensitive layer. When this sensitive layer covers these microstrip lines, the propagation takes place in both the substrate, on which microstrips are laid, and in the overlay,
) Corresponding author. Tel.: q33-05-56-846540; fax: q33-05-56371545. E-mail address: [email protected] ŽC. Bernou..
so that the e.m characteristics for the propagation are an effective permittivity, ´eff Ždepending on the substrate permittivity, ´substrate , and on the overlay permittivity, ´overlay . and an effective permeability Ždepending on msubstrate and m overlay .. The e.m field distribution is different from the one without chemical compounds, and there will be a resonant frequency shift, for example. This is the basic idea of the microwave chemical sensor, but in order to realize and test such a sensor, we have to: Ø design a resonant microwave structure, Ø study the microwave propagation variations resulting from the overlay e.m properties change, Ø realize a prototype.
2. Design of a microwave resonant element With the aim of developing a low cost microwave device, the resonant structure uses the planar technology, and more particularly the microstrip line technique. We choose to design a narrow band-pass filter w3,4x that consists of a resonant structure and two ports for supply and measurements.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 6 6 - 4
C. Bernou et al.r Sensors and Actuators B 68 (2000) 88–93
89
2.1. The resonant structures in microstrip technique In this microstrip technique, a resonant structure can be a quarter wavelength stub, a half-wavelength coupled line w8,9x or a ring resonator w10x ŽFig. 1.. To develop a band-pass filter, we choose to develop a structure using coupled lines because it is quite simple to study and optimize. 2.2. Excitation and coupling This resonator has to be excited and a second line is necessary to supply the resonator, and also to connect it in order to measure the frequency response. It can be shown that the optimum coupling length is equal to a quarter wavelength Ž lr4. w13x. Indeed, the resonator is the load and the supply line the generator and like in other electronic devices, to obtain a maximum power transfer, the generator and the load have to be matched. If the coupling is too weak, the load does not receive enough power and there will not be any frequency selection. If the coupling is too strong, the resonant phenomenon will be erase and not only a frequency but too wide a range of frequencies will be selected. 2.3. A filter design The designed filter consists of a half wavelength line coupled on a quarter wavelength with a supply line, so that the resonator has to be bent in a U form ŽFig. 2.. This structure is a cut-band filter and we would prefer to develop a band-pass filter that could serve as feedback element in an oscillator ŽFig. 3.. In order to have a band-pass behaviour, a gap is done in the middle of the supply line. A model for this gap is a capacitive coupling represented by Cg , C1 and C2 represent the open-end effect of the interrupted supply line. To conclude, the filter consists in a resonator bent into a U-form to have a quarter wavelength coupling with the supply line interrupted by a gap ŽFig. 4.. 2.4. Optimization of the filter dimensions An e.m simulator HP-MDS ŽMicrowave Design System. w14x is used to characterize and optimize the scattering parameters si j of the filter. These scattering parameters
Fig. 1. The microstrip resonant elements.
Fig. 2. Filter design with a l r2 resonator and a supply line.
give us the filter behaviour, like s21 , the transmission parameter Žinsertion loss. w15x. Indeed, these parameters are simply gains and reflection coefficients defined in Fig. 5 and by the following formulae: s21 s Ž b 2ra1 .< a 2 s 0 for example. s21 is the transmission gain with the output port is terminated in a matched load. On this frequency response ŽFig. 6: the transmission parameter s21 in magnitude Žand dB. vs. frequency f ., the resonant frequency, the pass-band width or the band-pass gain can easily be determined. 2.4.1. Electromagnetic simulator and quasi-static analysis The HP-MDS simulator is mainly based on approximated formulae, found on Conformal Mapping and experimental results, such as Wheeler’s formulae w5,16x. Similar to these methods, the e.m simulator does the quasi-Transverse Electromagnetic ŽTEM. approximation. More precisely, the microstrip line is for the propagation, a non-homogeneous problem, because the propagation takes place in the substrate under the lines and in the air above them. It is very difficult to know exactly the very accurate form of the e.m field Žwhereas it is known that two propagation modes exist.. But in the range extending into the low gigahertz region, and if the substrate dielectric constant ´substrate is large enough, the propagation mainly takes place in the substrate. The e.m analysis can then be done as a current-and-voltage analysis: this approximation is known as quasi-TEM approximation or quasi-static analysis. This approximation is enough to give us the main filter frequency-response features. 2.4.2. The dimensions optimization The filter dimensions have to be optimized to meet the following specifications: Ø The pass-band insertion loss should be close to 0 dB, Ø The 3 dB pass-band should be very narrow for a good frequency selection, Ø The phase near resonance should be continuous Ža phase jump in this region can be prejudicial to detection.. ŽThese parameters are defined in Fig. 6.. Some filter dimensions have influence on the frequency response, more precisely the resonator length and the gap.
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90
Fig. 3. Gap in the supply line and equivalent electric model.
The resonator length is theoretically equal to a halfwavelength. With the microstrip open-effect, a half-wavelength line has an effective length l, l s lr2 q l s where: l s s 0.412 h
ž
´eff q 0.3 ´eff y 0.258
/ž
wrh q 0.262 wrh q 0.813
/
where ´eff is the effective dielectric constant, w the transmission line width and h the substrate thickness w17x. Then the length has to be reduce to have a lr2 effective length. By means of a HP-MDS simulation, a study for the gap shows that the smaller the gap, the weaker the insertion loss. The smallest realization for g in the used screenprinted technique is 100 mm. 2.4.3. The filter performances With the optimized filter dimensions, the following frequency response is obtained ŽFig. 7.. The pass-band loss DG is about 1 dB; the 3 dB pass-band is f 2 y f 1 s 40 MHz wide for a resonant frequency of 1.5 GHz. The calculated quality factor Q is then of 20, which is quite good for this technology but not very high for a narrow band-pass filter. It is important to have a high quality factor because it is linked with the near-to-resonance phase variation, and consequently with the detection and the sensitivity. The sensor is electrically connected to an oscillator, so that the change in e.m parameters of the structure is converted into a frequency variation, easier to measure.
3. Sensor design. Realization and test of a humidity sensor
Fig. 5. Two-port network showing incident Ž a1 ,a2 . and reflected waves Ž b1 ,b 2 . waves used in s-parameter definitions.
3.1. PermittiÕity study thanks to an e.m simulator The microwave sensor consists of the above designed filter to which a chemical interface is added. So, the propagation medium for this structure is the alumina substrate, the sensitive coating and the air. We have to determine the sensitivity of our sensor to an e.m parameter variation. We choose to study in particular a permittivity variation. The HP-MDS simulator cannot describe this threelayered structure, but a part of this simulator, called HPMomentum w7x, can do the propagation analysis. HPMomentum does a full-wave analysis by using a numerical procedure, known as Moment Method w6x. In the non-homogeneous structure, the propagation can be described as a second order differential equation with boundary conditions :
™ E2 E ™ D E y ´m s 0 for the electrical field. Et 2
If the unknown equation solution u Ži.e. the electrical or the magnetic field. is expressed as a sum of n trial functions a i with a i weights, this method basically consists in projecting the trial functions on m test functions Õj . Mathematically speaking, the projection is a scalar product and the n = m products are the Moments of the unknown function u. At mesh nodes of the structure w7x, the Moments are computed and optimized to meet the boundaries. Then, the calculation of the weights a i give a numerical description of the e.m field. The HP-Momentum simulator is used to study the effect of a permittivity variation of the sensitive layer. After
Planning to use a sensitive coating whose dielectric constant varies in the presence of humidity, we begin to study the perturbations induced by a permittivity variation. Then a humidity sensor is designed and connected in an oscillator chain.
Fig. 4. Filter final design. l1 q2 l 2 s l r2.
Fig. 6. Filter response sample. Main frequency response features.
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Fig. 7. Ža. Magnitude G ŽdB. and Žb. phase of the transmission coefficient s21 ŽdB. vs. frequency f. Substrate: Al 2 O 3 ; ´ r s 9.8; h s 635 mm. Dimensions: l1 s 10 mm; l 2 s 5 mm; g s 100 mm.
computation, we can see that a permittivity increase of the sensitive coating induces a decrease of the filter resonant frequency, which can be seen on the transmission parameter frequency response. In Fig. 8, we present the frequency shift observed on the scattering parameter s21 of the band-pass filter Ždesigned above. due to the sensitive layer permittivity increase: when the dielectric constant increases from 4 to 40, the resonant frequency decreases by 270 MHz. 3.2. Humidity sensor design To prove the feasibility of the detection principle and to validate the permittivity variation study, a humidity sensor is realized. Indeed, this sensor consists of the narrow band-pass filter and a polyimid overlay. The polyimid is a polymer sensitive to wet air: when the water molecules are absorbed in the coating, the polymer dielectric constant increases. The narrow-band-pass filter is realized in microstrip lines by means of the screen-printed technology, that gives thick films. The screen-printing is done first for the ground
Fig. 8. Magnitude s21 ŽdB. vs. frequency. Effect of permittivity variation. Trace 1: ´ r s 40. Trace 2: ´ r s 4.
plane with an AgPd ink, with a final thickness between 10 and 17 mm, on very pure alumina substrate, the roughness of which is very low. Secondly, the microstrips are screen-printed with a high definition gold ink, with a thickness between 6 and 12 mm. The filter is then connected on an aluminum stand, the electrical ground plane connection being reinforced by the use of a conductive silver lacquer. Two Small Miniaturized Adaptator ŽSMA. connectors are placed on both ports of the supply line. The filter performances are then tested with a HP-8753C Network Analyzer. The pass-band insertion loss is about y7 dB, because of the connection losses, for a measured quality factor of 30. The polyimid is an Ultradel 4208 coating. A regular coating on the alumina substrate and on the filter strips is achieved by a spin coat operation. 3.3. Electrical connections The sensor is fixed in an airtight cell and is electrically connected to an oscillator. The electronic configuration of this oscillator is developed with a 20 dB gain low noise amplifier and an adjustable attenuator, which constitute an adjustable gain amplifier. The remaining oscillator chain consists of a manually adjustable phase shifter in order to
Fig. 9. Oscillator configuration.
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achieve the phase loop condition. With our oscillator configuration, the Barkhausen gain condition for oscillation is:
4.2. Discussion
Gfilter q Gampl q Gatten q Gphase - shifter q Gcoupler s 0 dB
The detection mechanism is attributed to the polyimid dielectric constant increase in the presence of wet air: it is a well known principle, used for capacitive transducers w18–21x. In our microwave structure, this permittivity increase induces a change of the structure dielectric constant. Indeed, the microstrip lines must be analyzed considering the non-homogeneous dielectric medium: ´ 1 s 9.8 for the alumina, ´ r2 for the polyimid and ´ r s 1 for air. To perform a suitable analysis, these three dielectric constant are replaced by the effective dielectric ´e constants w2,5,11,13,16x Žfor the whole structure. which has to be calculated, for example, thanks to a Variational method. This method determines the potential V on the microstripline, the charge Q of this conductor, and then the capacity of the line C0 when the calculation is done for a Žvirtual. free-air microstrip and C for the studied substrate w12x. Finally, the effective dielectric constant is given by:
and for phase condition:
w filter q wampl q watten q wphase - shifter q wcoupler s 0 Ž 2p . . A 20 dB directional coupler allows the measurement of the output frequency with a HP-5350B microwave electronic counter. Fig. 9 depicts the oscillator configuration. Measures for stability are done with the oscillator chain in ambient atmosphere at ca. 258C; the counter accuracy Žtime gate. is 1 Hz and the measured frequency is near 1.61206 GHz. The sensor short-time stability Žwithin 1 min. is 2 kHz and the longtime stability Žwithin 1 h or more. is 5 kHz. It is quite good as neither the ambient temperature nor the amplifier temperature are controlled.
4. Experimental results and discussion 4.1. Sensor response The set up in an airtight cell sensitive device is connected to an automatic gas line which can generate gas atmospheres at a constant flow Žfrom 100 to 1000 cm3rmin for dry air, the gas carrier. and at a controlled Relative Hygrometry ŽRH. from 0% to 80% RH. Insofar as we can consider the humidity stimulation as a 40% RH wet air step, the response ŽFig. 10. can be considered as a first order type and the 90% value is obtained within 2 min. As seen above, the frequency decreases as the relative humidity increases. Taking the gas line delay into account, the rising time Ž10% to 90%. of the response to the RH step is estimated at 68 s. The sensor sensitivity is about 5 kHz per %RH, which is rather good in comparison with the oscillator stability. As the frequency level at the end of desorption is the initial one, a very good reversibility is also obtained.
Fig. 10. Response to a 40% humidity step at 258C. Polyimide thickness: 6 mm. A H5000 Coreci humidity sensor gives the reference for %RH.
´e s
C C0
.
Based on this method, a program was built up to study the sensitive coating influence. Generally speaking Žwhen the polyimid permittivity ´ r2 is higher than the alumina one., the higher ´ r2 , the higher effective permittivity ´ r . This results in the concentration of both electric and magnetic fields in the double-layered substrate. If the fields are less scattered, the resonant frequency decreases in accordance with HP-Momentum simulation. The observed frequency shift has to be compared with the simulated one. Our problem is that we have no knowledge of the polyimid dielectric constant evolution law vs. RH at ultrahigh frequencies. For the simulation, we did an extrapolation, knowing that the water permittivity is 80 and that of the polyimide with dry air is 4. A decrease of 270 MHz was calculated by simulation, the experiment only shows a 160 kHz decrease. Several reasons could explain this result. Ž1. The sensitivity depends on the coating thickness. The optimum thickness found by simulation is about 10 mm, whereas with the spin operation, a 5 mm coating is obtained. Ž2. The manufacturer has modified this polymer because the permittivity variation bothers some users. So this second polyimid generation could have a lower sensitivity to wet air. Ž3. Finally, it would be interesting to study the diffusion process in the polyimid coating. An evaluation of the diffusion constant could tell us the polyimid behaviour in presense of humidity. Ž D is defined by Fick’s law: F s yDŽECrE x ., where F is the rate of transfer per unit area of section, C the concentration of diffusing substance, and x the space coordinate measured normal to the section..
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5. Conclusion We have presented a new sensor working at ultra-high frequency formed with a microwave structure and a sensitive coating. CAD permits to evaluate the sensitivity of resonant structure to e.m parameter variations. To illustrate this new concept we have realized a humidity sensor with polyimid working at 1.5 GHz, by using thick film technology, and performed a humidity detection in a gas line. The results are in good accordance with the previsions on the detection principle. The sensor shows high sensitivity to humidity and good reversibility is also obtained. The evaluation of the diffusion coefficient D for the polyimid will improve our knowledge of the interaction between polymer and wet air. We have demonstrated the feasibility of a microwave chemical sensor. Various chemical sensors based on this principle can be developed. Perturbation mechanisms like conductivity or permeability perturbation can lead to the realization of other Žbio.chemical sensor types. Studies on sensors working in other media Žlike liquids. or at higher frequencies are ongoing.
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w12x E. Yamashita, R. Mittra, Variational method for the analysis of microstrip lines, IEEE MTT 16 Ž4. Ž1968. 251–256, April. w13x D.M. Pozar, Microwave Engineering, Wiley Editors, 2nd edn. w14x HP 85150B ŽMDS Microwave and RF Design Systems., User’s Guide, Printed in USA 12r92. w15x HP-Application Note 95-1, S-Parameter Techniques for Faster, More Accurate Network Design, reprinted Compliments of HP-Journal, vol. 18, no. 6, Feb. 1967. w16x K.C. Gupta, R. Garg, I.J. Bahl, Microstrip Lines and Slotlines, Artech House, 1979. w17x E.O. Hammerstad, F. Bekkadal, A microstrip handbook, ELAB Report, STF 44A74169, N7034, University of Trondheim-NTH, Norway, 1975. w18x P.R. Story, D.W. Galipeau, R.D. Mileham, A study of low-cost sensors for measuring low relative humidity, Sens. Actuators, B 24–25 Ž1995. 681–685. w19x H. Shibata, M. Ito, M. Asakura, K. Watanabe, A digital Hygrometer using a polyimide film relative humidity sensor, IEEE Trans. Instrum. Meas. 45 Ž2. Ž1996. April. w20x T. Boltshauer, C. Azeredo Lame, H. Baltes, High sensitivity CMOS humidity sensors with on-chip absolute capacitance measurement system, Sens. Actuators, B 15–16 Ž1993. 75–80. w21x A. Denton, A. NgiapHo, A. SenGangHe, A solid-state relative humidity measurement system, IEEE Trans. Instrum. Meas. 39 Ž3. Ž1990. June.
BIographies Catherine Bernou was qualified as an Engineer in electronics from ENSEEIHT Engineering School in Toulouse in 1992. She is preparing a PhD thesis on microwave gas sensors at IXL Microelectronic Laboratory. Dominique Rebiere d’Electronique-Electrotechnique ` received his Maıtrise ˆ et d’Automatique, his Diplome ˆ d’Etudes Approfondies in electronics and his PhD from Bordeaux I University, France, in 1987, 1988 and 1992, respectively. He has been involved in research on acoustic wave sensors since 1989 at Bordeaux I University-IXL Microelectronic Laboratory, and is Maıtre at Bordeaux University in electronic engineering. ˆ de Conference ´ Jacques Pistre´ received his MEng Degree in Electronics from Bordeaux University, France, in 1968 and his These ` d’Etat degree in ionic conductivity of thin films in 1979. Since joining the IXL Laboratory Žformerly Laboratoire d’Electronique Appliquee ´ ., he has worked in several areas of thick-film microelectronics, including applications to microwave circuits and sensors. He was sent on secondment for 18 months to the French company Thomson, working in the divisions of Radars, Countermeasures, Missiles. In 1987, he returned to the IXL laboratory where he is in charge of the sensors group. This group is mainly involved in gas sensors, using either elastic waves in solids or microwave devices. He is a professor at ENSERB, a French ‘Grande Ecole’, where he teaches electronic systems and is responsible for international relations.