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Opto-mechanical design of tuneable InP-based Fabry–Pérot filter for gas analysis
Sensors and Actuators A 94 (2001) 136±141
Opto-mechanical design of tuneable InP-based Fabry±PeÂrot ®lter for gas analysis P. Bondavallia, T. Benyattoub,*, M. Garriguesc, Jean Louis Leclercqc, S. Jourbac, C. Pautetd, X. Hugond a
Laboratoire Interconnexions Optiques, Laboratoire Central de Recherche, THALES, Route DeÂpartementale 128, 91404 Orsay Cedex, France b LPM, UMR CNRS 5511, INSA de Lyon, 20 Avenue Albert Einstein, F-69621 Villeurbanne Cedex, France c Laboratoire d'Electronique, OptoeÂlectronique et MicrosysteÁmes, EC-Lyon, 36 Avenue Guy de Collongue, BP163, Ecully 69131, France d Near Infrared Division Atmel-Grenoble, Avenue de Rochepleine, Saint-Egreve 38521, France Received 17 October 2000; received in revised form 16 July 2001; accepted 16 July 2001
Abstract This paper presents the opto-mechanical design of tuneable InP-based Fabry±PeÂrot ®lters for hydrocarbon gas metrological applications. These ®lters are composed by a top and a bottom InP/air Bragg mirrors with a central air micro-cavity. The tunability is achieved by changing electrostatically the cavity thickness. For this type of applications, the required performances are a large tunability (200 nm), a high selectivity (10 nm), a weak temperature dependence and a nearly constant transmission level over the tuning range. Firstly, we have obtained the device composition which permits to attain the optical speci®cations simulating its optical response (transmission and re¯ectivity). Secondly, we have developed a mechanical model using ®nite elements analysis (FEA) calculation method: this one permits to obtain the optimal mechanical design of the device in order to achieve a 200 nm tuning range with a bias voltage of around 15 V (usual value for this kind of device). Finally, preliminary experimental results on a ®rst generation of ®lters are presented. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas sensing; Fabry±PeÂrot ®lters; Opto-mechanical simulation; FEA calculation method; Optical transmission
1. Introduction Several kinds of techniques for gas measurements have been developed [1,2]. Optical measurement techniques have been considered more advanced than the use of chemical sensors. Actually, compared with other competing technologies, these methods have advantages in selectivity, a large range of sensitivity, ready compatibility with computer control and fail-to-safety [1,2]. Recently, it has been shown that it is possible to produce highly selective and continuously tunable ®lters based on InP material using surface micromachining [3]. One interesting issue for this kind of devices is the near infrared (NIR) absorption spectroscopy for metrological hydrocarbon gas analysis. The concentration *
Corresponding author. Tel.: 33-4-72-438907; fax: 33-4-72-438531. E-mail addresses: [email protected] (P. Bondavalli), [email protected], [email protected] (T. Benyattou), [email protected] (M. Garrigues), [email protected] (J.L. Leclercq), [email protected] (S. Jourba), [email protected] (C. Pautet), [email protected] (X. Hugon).
value of the gas could be obtained by the ratio of the measure of the absorption of the gas lines to the reference measure [4] or by their difference [5±8]. This analysis is performed selecting the Q absorption lines of the gas [1,2] with a tuneable ®lter. The device that we propose is a tuneable InPbased Fabry±PeÂrot ®lter composed by a top and a bottom InP/air Bragg mirrors with a central air micro-cavity. The mirrors are respectively p- and n-doped in order to obtain a p±i±n diode structure. The tunability is achieved by changing electrostatically the cavity thickness applying a bias voltage. We seek applications with hydrocarbon gas at atmospheric pressures (3 atm) where the absorption lines merge in one broad band with a full width at half maximum (FWHM) value of around 100 nm. We deduce that a 200 nm tunability around 1.7 mm (wavelength corresponding to the C±H stretching absorption frequency) and a selectivity of around 10 nm are needed. This last value is a good compromise between the measurement precision, considering the FWHM line value, and the intensity of the ®lter exit signal. A nearly constant level for the resonance peak transmission in the tuning range and a spectral thermal dependence of the resonance peak lower than 0.1 nm/8C
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 0 1 - 4
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between 40 and 608C, are also required for assuring the reproducibility of the device performances. In the next paragraphs, we deal with the optical and mechanical design parameters of the device that permit to obtain the expected performances. 2. Optical modeling The structure that we want to realize is a tuneable Fabry± PeÂrot ®lter composed by two Bragg mirrors constituted by several InP/air pairs. We have chosen this kind of mirrors because they permit to attain a large re¯ectivity value with only few InP/air pairs [9]. The ®rst objective of the optical modeling is to determinate the theoretical number of InP/air for the two mirrors necessary to obtain a 10 nm device selectivity. Actually, we know that the dependence of the FWHM of the resonance peak of the mirrors re¯ectivity is described, supposing to work with two identical mirrors, by Eq. (1) [10] Dl
1
RlR 2
(1)
2pd R1=2
where R is the mirror reflectivity, lR the resonance wavelength (1.7 mm) and d is the micro-cavity thickness. We deduce that to reach a 10 nm selectivity we need two Bragg mirrors with a reflectivity of 98% for a l/2 micro-cavity thickness (850 nm) or 96% for a l micro-cavity (1700 nm). We have examined only these two cases because a larger micro-cavity thickness is not realizable from a technological point of view. The first simulated structure is composed by a top and a bottom Bragg mirrors composed respectively by 1.5 and 2.5 nip/air quarter-wave stacks (Fig. 1). The electrostatic forces that, applying a bias voltage, takes place between the p- and n-doped InP layers adjacent the central
Fig. 1. Schematic view of the structure.
Fig. 2. Simulation of the optical transmission response of the structure of Fig. 1.
air micro-cavity (Fig. 1), leads to a reduction of its thickness. Actually, for this kind of structure, we cannot reduce electrostatically the micro-cavity thickness more than one-third of its initial value: beyond this limit, the micro-cavity collapses [4]. For this reason, the starting thickness of the micro-cavity has been chosen larger than l/2 or l in order to increase the tuning range of the device. The optical performances have been simulated using a multi-layers optical modeling software based on the Abeles matrix calculation method [11]. The optical transmission spectrum of the device for a l/2 micro-cavity thickness is shown in Fig. 2. We can observe the resonance peak at the center of a 400 nm Bragg Plateau. From this ®gure, we could expect that 200 nm tuning range can be easily reached. We have reported in the Table 1 the spectral response of this device when we tune the cavity. We can see that both structures (l and l/2 cavity) do not reach 200 nm of tuning. The problem is that when we reduce the micro-cavity thickness to tune the wavelength, we also change the air layers thickness delimiting the micro-cavity (Fig. 1). We deduce that the mirror re¯ectivity rapidly decreases because the air layers do not satisfy the ideal Bragg mirrors condition: a thickness value equal to l/4. Another effect is the very large variation of the FWHM of the transmission resonance peak that could give measurements interpretation problems. This is due to the asymmetrical re¯ectivity response of the two Bragg mirrors that is enhanced when we tune the device. In order to improve the optical device performances, we have reduced the starting thickness of the two air layers. In this way, we have tried to reduce their total thickness variation from the ideal condition (l/4) and so the decrease of the mirrors re¯ectivity. The optimized structure is shown in Fig. 3. As
Table 1 Summary of the optical simulation results on the structure of Fig. 1 Micro-cavity starting thickness
Variation of the resonant peak maximum in the tuning range
Tuning range (nm)
FWHM variation in the tuning range (nm)
Reduction of the microcavity thickness (nm)
0.6l 1.2l
0.87±1.00 0.81±1.00
168 180
7±21 6±15
340 476
138
P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141 Table 3 Thermal spectral displacement of the resonance peak: the reference temperature is 308C Air micro-cavity thickness (nm) 0.6 1.2
Fig. 3. Schematic view of the optimized structure.
we can see in Fig. 3, we have chosen two different values for the air thickness in order to balance the re¯ectivity response of the two Bragg mirrors. The spectral optical characteristics are reported in Table 2. We can see that the optimized structures attain the fundamental objective of around 200 nm tuning range. However, the maximum peak variation is over 20% even if by only a few percent. The FWHM variation in the tuning range is largely reduced compared to the non-optimized structures. We can conclude that the optical performances of the optimized structures are very near to the expected ones. 3. Thermal dependence of the resonance filter wavelength One of the aims of this work is to conceive a device with a weak thermal dependence of the central wavelength of the ®lter between 40 and 608C (industrial working temperature). Two effects contribute to the resonance wavelength displacement: the thermal dilation of the layers which changes the layers thickness and the thermal dependence of the refraction index of the materials. Considering these effects, we have evaluated the thermal dependence of the resonance wavelength for the optimized structures. Preliminary simulation results have showed that the thermal in¯uence on the mechanical behavior of the structure does not in¯uence the optical response of the device (too little temperature interval). The simulation results are reported in Table 3. We can see in Table 3 that the whole thermal wavelength displacement between 60 and 408C is lower than 0.1 nm/ 8C according to the starting device requirement. We have observed that the largest in¯uence is mainly related to the
Resonance wavelength displacement (nm) 408C 5 6
08C
608C
1 1
3 2
thermal dependence refractive index: around 80% of the total thermal displacement. These theoretical results have been con®rmed by experimental results obtained at THOMSON-CSF LCR laboratory on structures closely resembling these ones but used as WDM ®lters for telecommunications applications. 4. Mechanical analysis After having optically modeled the device, we have to de®ne its geometrical shape in order to achieve a 200 nm tuning range with a bias voltage of around 15 V. We have developed a mechanical FEA model of the structure using a three-dimensional (3D) ®nite element solver (ANSYS 5.4) in order to give an accurate description of the tuning performances of the device [12]. The compliance matrix elements that we have used, are reported in Table 4. The meshing of the mechanical model has been performed using the 3D element SOLID64 that permits to consider the mechanical anisotropy of the semiconductors materials. We have simulated only the central part of the device (Fig. 4): the only part that has to move. The liberty degrees of the points showed in Fig. 4 are locked in order to be as near as possible to the real starting conditions. The shape of the simulated device consists in a central circular platform with four arms. We can see in Fig. 5, a SEM image of this kind of device. We can observe the different InP/air layers with the air micro-cavity in the middle. The four arms con®guration has been chosen because when we actuate the structure the central platform deformation is lower than the two arms con®guration [13]. Actually, this parameter is very important in order to reduce the optical loss of the cavity [14]. We have chosen a circular plate-form because the micro-machining process is more ef®cient than a square membrane. Because the incident light spot dimension is 40 mm, we have chosen a 40 mm diameter central platform.
Table 2 Summary of the optical simulation results on the optimized structure Micro-cavity starting thickness
Variation of the resonant peak maximum in the tuning range
Tuning range (nm)
FWHM variation in the tuning range (nm)
Reduction of the microcavity thickness (nm)
0.6l 1.2l
0.75±0.97 0.74±1.00
194 219
14±16 12±18
340 550
P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141
139
Fig. 4. Schematic view of the simulated part of the device.
Table 4 Compliance matrix elements for <1 0 0> wafer growth directiona
5. Experimental results
<1 0 0>
S11 S33
In(0.53)Ga(0.47)As InP
1.583 1.650
A ®rst generation of ®lters has been realized. These ®lters are characterized by a l/2 air micro-cavity thickness. The schematic view of the ®lters is shown in Fig. 7. The thickness of the air layers adjacent to the central microcavity is lower than l/4 (425 nm) according to the optical simulation results. The air micro-cavity thickness is lower than 0.6l: this is related to an epitaxial growth calibration error. The device dimensions, according to the mechanical simulations results, are: 40 mm for the central plate-form
a
The values are expressed in 10
S12 S13 0.535 0.594 2
S44 S66 2.130 2.170
GPa 1.
The arms are 5 mm wide and their length depends on the air micro-cavity thickness: in the case of the l micro-cavity thickness, we need four longer arms than the l/2 case because, in order to reach the 200 nm tuning range, we need to reduce more largely the micro-cavity thickness (see Table 2). After having simulated the mechanical performances of the device for these two cases, we have found out that the ideal arms length is 40 and 20 mm, respectively for the l and l/2 micro-cavity structure. In the Fig. 6, the simulated mechanical performances of the devices are shown. We can see that we need a bias voltage of 21.3 and 15 V, respectively for the l and l/2 micro-cavity structures to reach a 200 nm tuning range according with the expected performances.
Fig. 5. SEM image of the device.
Fig. 6. Total InP layers displacement as a function of the applied bias voltage: the dashed lines represent the displacement needed to reach a 200 nm tuning range.
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P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141
6. Conclusions
Fig. 7. Schematic view of the realized structure.
Fig. 8. Optical transmission preliminary results.
In this paper, we have presented the optical and mechanical design of an InP-based Fabry±PeÂrot ®lter for gas sensing applications. Firstly, we have calculated the optimal layer thickness needed to reach the optical speci®cations. This has been achieved optimizing the re¯ectivity spectral response of the top and bottom Bragg mirrors. The thermal dependence displacement of the resonance peak of the device has been evaluated and it is lower than 0.1 nm/8C. Secondly, we have developed a FEA mechanical model which permits to simulate the mechanical performances of the device. This model allows us to de®ne the geometrical dimensions of the structure in order to reach the 200 nm tuning range with a 15 V bias voltage. Finally, optical transmission, measurements on a l/2 air micro-cavity ®lter have been performed. The experimental results are very promising: the ®lter attains a 100 nm tuning range with 11 V and the FWHM varies between 14 and 16 nm. These performances are very interesting and could be improved in the near future with the realization of a l air micro-cavity that will permit to attain 219 nm with a 21.3 V bias voltage. The experimental results are in very good agreement with the mechanical simulations: the FEA model that we have developed permit to realistically simulate the mechanical performances of this kind of surface micromachined devices used for gas sensing applications. Acknowledgements We would like to thank Prof. G. Guillot for critical reading of this manuscript.
References
Fig. 9. Variation of the applied bias voltage as a function of the resonance filter wavelength: compare between the FEA model simulation and the optical transmission results.
diameter, 20 and 5 mm, respectively for the arms length and width. The optical transmission results are reported in Fig. 8. We reach a 100 nm tuning range with a 11 V bias voltage. The FWHM varies between 14 and 16 nm according to the optical simulations. In order to validate our mechanical model, we have compared the experimental results with the mechanical simulations results in Fig. 9. We obtain a very good agreement between the theoretical and experimental results. We can conclude that our FEA mechanical model is very ef®cient to realistically simulate the mechanical performances of such device.
[1] K.R.M. Measures, Laser Remote Sensing, Wiley, New York, 1984, p. 483. [2] K.E.D. Hinkley, Laser Monitoring of the Atmosphere, Springer, Berlin, 1986, p. 369. [3] A. Spisser, R. LeDantec, C. Seassal, J.L. Leclercq, T. Benyattou, D. Rondi, R. Blondeau, G. Guillot, P. Viktorovitch, Highly selective and widely tuneable 1.55 mm InP/air-gap micromachined Fabry±PeÂrot filter for optical communications, IEEE Photonics Technol. Lett. 10 (1998) 1259. [4] M. Viitasalo, M. Hotola, M. Blomberg, Ch. Helelund, A. Torkkeli, A. Letho, Carbon Dioxide Sensor Based on Micromachined Fabry±PeÂrot interferometer, Sensor'97 Kongressband III, 1997. [5] K. Chan, H. Ito, H. Inaba, Remote sensing system for near-infrared differential absorption of CH4 gas using low-loss optical fiber link, Appl. Opt. 23 (1984) 3415. [6] K. Chan, H. Ito, H. Inaba, All optical remote monitoring of propane gas using a 5 km long, low loss optical fiber link and an InGaP lightemitting diode in the 1.68 mm region, Appl. Phys. Lett. 45 (1984) 220. [7] T. Kobayashi, M. Hirama, H. Inaba, Remote monitoring of NO2 molecules by differential asorption using optical fiber link, Appl. Opt. 20 (1981) 3279.
P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141 [8] K. Chan, H. Ito, H. Inaba, Absorption measurements of n2 2n3 band of CH4 at 1.33 mm using an InGaAs light-emitting diode, Appl. Opt. 22 (1983) 3802. [9] R. Lednatec, T. Benyattou, G. Guillot, A. Spisser, J.L. Leclercq, P. Viktorovitch, D. Rondi, R. Blondeau, Tuneable micro-cavity based on InP/air Bragg mirrors, J. Mater. Sci.: Mater. Electron. 10 (1999) 1±4. [10] W.H. McLeod, Thin-Film Optical Filters, Adam Hilger, Bristol, 1985. [11] F. AbeÂleÁs, Recherche sur la propagation des ondes eÂlectromagneÂtiques sinusoõÈdales dans les milieux stratifieÂs: applications aux couches minces, Annales de Physique 5 (1950) 596±706. [12] P. Bondavalli, R. LeDantec, T. Benyattou, Influence of the involuntary under-ectching on the mechanical properties of tuneable Fabry±PeÂrot filters for optical communications, J. microelectromech. Syst., in press. [13] P. Bondavalli, MicrosysteÁmes opto-meÂcaniques aÁ base d'InP pour l'analyse de gaz, Ph.D. thesis, INSA of Lyon, January 2000. [14] P. Tayebati, P. Wang, M. Azimi, L. Maflah, D. Vakhshoori, Microelectromechanical tuneable filters with stable half symmetric cavity, Electron. Lett. 34 (1998) 1967±1969.
Biographies Paolo Bondavalli obtained his MS degree in physics from the University of Parme, Italy, in November 1995 and PhD degree in engineering from the National Institute of Applied Sciences of Lyon, France, in January 2000. The PhD thesis was about the properties of microsystems for gas sensing applications. He has worked at the R&D Department of THOMSON-CSF LASER DIODES and is now working at the THALES Corporate Research Center in Orsay (France). He is currently involved in research on new kind of MEMS for telecommunications. Taha Benyattou is a CNRS researcher at the Laboratoire de Physique de La MatieÁre UMR CNRS 5511. He received his PhD degree from the Universite Claude BERNARD LYON 1 in 1987. He is now involved in the field of microphotonic devices, such as optical sensors, photonic bandgap structure for telecommunication and optical interconnect. Michel Garrigues was born in 1948 in France. He received the DocteurIngeÂnieur degree in 1976 from the University of Lyon. He is presently Directeur de Recherche at Laboratoire d'Electronique, OptoeÂlectronique et
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Micro-systeÁmes (LEOM), CNRS, Ecole Centrale de Lyon. From 1977 to 1986 he has co-ordinated a French research project on the instability phenomena due to hot electron injection in submicronic silicon devices. From 1986 to 1990, he has developed a novel technique of room temperature scanning photoluminescence (SPL) for controlling III-V compound semiconductor wafers. In 1989, he started the SCANTEK company for the industrial development of SPL equipment. Temporarily on leave from CNRS, he was appointed as General Manager by SCANTEK from 1991 to 1996. Since 1996, he is project leader for the development of tunable micro-machined filters and photodetectors in collaboration with French industrial partners. Jean-Louis Leclercq was born in 1964. He received his PhD in chimie des mateÂriaux in 1990 from the University of Montpellier II, France. His thesis work focused on PECVD processing for InP-based MISFET. He joined the French CNRS as permanent researcher in 1990. From 1990 to 1992, he worked at EM2 of University of Montpellier II by developing GaSb-based laser diodes. In 1992, he joined the LEOM at Ecole Centrale de Lyon where he is currently managing the III±V based microtechnologies and microsystems developments. His current interests include optoelectronics, micromachining and microfabrication related to III±V based materials for photonic systems. He is author and co-author of about 30 papers in international journals and 40 international conferences. Serguei Jourba was born in 1971. He received his MS degree in solid-state electronics from Kiev University, Ukraine in 1993, and his PhD degree in microelectronics from Ecole Centrale de Lyon, France in 1999. In 2000, he joined ATMEL Corporation as R&D engineer. His fields of interest are the physics of semiconductor devices, micro-opto-electro-mecanical systems (MOEMS), and resonant cavity enhanced light emitters and photodetectors operating in the near infrared spectral range. Christophe Pautet was born in Lyon in 1967. He received his MS and PhD degrees in materials science from the University Claude Bernard, Lyon, in 1991 and 1995, respectively. His thesis work was focused on the metal organic vapor phase epitaxy on characterization of InGaAsP compound using alternative arsenic precursors. He joined THOMSON-CSF SEMICONDUCTEURS SPECIFIQUES Saint-EgreÁve in 1996 (Atmel-Grenoble since 2000) where he is currently in charge of Epitaxy for near infrared devices. Xavier Hugon biography not available.
Opto-mechanical design of tuneable InP-based Fabry±PeÂrot ®lter for gas analysis P. Bondavallia, T. Benyattoub,*, M. Garriguesc, Jean Louis Leclercqc, S. Jourbac, C. Pautetd, X. Hugond a
Laboratoire Interconnexions Optiques, Laboratoire Central de Recherche, THALES, Route DeÂpartementale 128, 91404 Orsay Cedex, France b LPM, UMR CNRS 5511, INSA de Lyon, 20 Avenue Albert Einstein, F-69621 Villeurbanne Cedex, France c Laboratoire d'Electronique, OptoeÂlectronique et MicrosysteÁmes, EC-Lyon, 36 Avenue Guy de Collongue, BP163, Ecully 69131, France d Near Infrared Division Atmel-Grenoble, Avenue de Rochepleine, Saint-Egreve 38521, France Received 17 October 2000; received in revised form 16 July 2001; accepted 16 July 2001
Abstract This paper presents the opto-mechanical design of tuneable InP-based Fabry±PeÂrot ®lters for hydrocarbon gas metrological applications. These ®lters are composed by a top and a bottom InP/air Bragg mirrors with a central air micro-cavity. The tunability is achieved by changing electrostatically the cavity thickness. For this type of applications, the required performances are a large tunability (200 nm), a high selectivity (10 nm), a weak temperature dependence and a nearly constant transmission level over the tuning range. Firstly, we have obtained the device composition which permits to attain the optical speci®cations simulating its optical response (transmission and re¯ectivity). Secondly, we have developed a mechanical model using ®nite elements analysis (FEA) calculation method: this one permits to obtain the optimal mechanical design of the device in order to achieve a 200 nm tuning range with a bias voltage of around 15 V (usual value for this kind of device). Finally, preliminary experimental results on a ®rst generation of ®lters are presented. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas sensing; Fabry±PeÂrot ®lters; Opto-mechanical simulation; FEA calculation method; Optical transmission
1. Introduction Several kinds of techniques for gas measurements have been developed [1,2]. Optical measurement techniques have been considered more advanced than the use of chemical sensors. Actually, compared with other competing technologies, these methods have advantages in selectivity, a large range of sensitivity, ready compatibility with computer control and fail-to-safety [1,2]. Recently, it has been shown that it is possible to produce highly selective and continuously tunable ®lters based on InP material using surface micromachining [3]. One interesting issue for this kind of devices is the near infrared (NIR) absorption spectroscopy for metrological hydrocarbon gas analysis. The concentration *
Corresponding author. Tel.: 33-4-72-438907; fax: 33-4-72-438531. E-mail addresses: [email protected] (P. Bondavalli), [email protected], [email protected] (T. Benyattou), [email protected] (M. Garrigues), [email protected] (J.L. Leclercq), [email protected] (S. Jourba), [email protected] (C. Pautet), [email protected] (X. Hugon).
value of the gas could be obtained by the ratio of the measure of the absorption of the gas lines to the reference measure [4] or by their difference [5±8]. This analysis is performed selecting the Q absorption lines of the gas [1,2] with a tuneable ®lter. The device that we propose is a tuneable InPbased Fabry±PeÂrot ®lter composed by a top and a bottom InP/air Bragg mirrors with a central air micro-cavity. The mirrors are respectively p- and n-doped in order to obtain a p±i±n diode structure. The tunability is achieved by changing electrostatically the cavity thickness applying a bias voltage. We seek applications with hydrocarbon gas at atmospheric pressures (3 atm) where the absorption lines merge in one broad band with a full width at half maximum (FWHM) value of around 100 nm. We deduce that a 200 nm tunability around 1.7 mm (wavelength corresponding to the C±H stretching absorption frequency) and a selectivity of around 10 nm are needed. This last value is a good compromise between the measurement precision, considering the FWHM line value, and the intensity of the ®lter exit signal. A nearly constant level for the resonance peak transmission in the tuning range and a spectral thermal dependence of the resonance peak lower than 0.1 nm/8C
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 0 1 - 4
P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141
137
between 40 and 608C, are also required for assuring the reproducibility of the device performances. In the next paragraphs, we deal with the optical and mechanical design parameters of the device that permit to obtain the expected performances. 2. Optical modeling The structure that we want to realize is a tuneable Fabry± PeÂrot ®lter composed by two Bragg mirrors constituted by several InP/air pairs. We have chosen this kind of mirrors because they permit to attain a large re¯ectivity value with only few InP/air pairs [9]. The ®rst objective of the optical modeling is to determinate the theoretical number of InP/air for the two mirrors necessary to obtain a 10 nm device selectivity. Actually, we know that the dependence of the FWHM of the resonance peak of the mirrors re¯ectivity is described, supposing to work with two identical mirrors, by Eq. (1) [10] Dl
1
RlR 2
(1)
2pd R1=2
where R is the mirror reflectivity, lR the resonance wavelength (1.7 mm) and d is the micro-cavity thickness. We deduce that to reach a 10 nm selectivity we need two Bragg mirrors with a reflectivity of 98% for a l/2 micro-cavity thickness (850 nm) or 96% for a l micro-cavity (1700 nm). We have examined only these two cases because a larger micro-cavity thickness is not realizable from a technological point of view. The first simulated structure is composed by a top and a bottom Bragg mirrors composed respectively by 1.5 and 2.5 nip/air quarter-wave stacks (Fig. 1). The electrostatic forces that, applying a bias voltage, takes place between the p- and n-doped InP layers adjacent the central
Fig. 1. Schematic view of the structure.
Fig. 2. Simulation of the optical transmission response of the structure of Fig. 1.
air micro-cavity (Fig. 1), leads to a reduction of its thickness. Actually, for this kind of structure, we cannot reduce electrostatically the micro-cavity thickness more than one-third of its initial value: beyond this limit, the micro-cavity collapses [4]. For this reason, the starting thickness of the micro-cavity has been chosen larger than l/2 or l in order to increase the tuning range of the device. The optical performances have been simulated using a multi-layers optical modeling software based on the Abeles matrix calculation method [11]. The optical transmission spectrum of the device for a l/2 micro-cavity thickness is shown in Fig. 2. We can observe the resonance peak at the center of a 400 nm Bragg Plateau. From this ®gure, we could expect that 200 nm tuning range can be easily reached. We have reported in the Table 1 the spectral response of this device when we tune the cavity. We can see that both structures (l and l/2 cavity) do not reach 200 nm of tuning. The problem is that when we reduce the micro-cavity thickness to tune the wavelength, we also change the air layers thickness delimiting the micro-cavity (Fig. 1). We deduce that the mirror re¯ectivity rapidly decreases because the air layers do not satisfy the ideal Bragg mirrors condition: a thickness value equal to l/4. Another effect is the very large variation of the FWHM of the transmission resonance peak that could give measurements interpretation problems. This is due to the asymmetrical re¯ectivity response of the two Bragg mirrors that is enhanced when we tune the device. In order to improve the optical device performances, we have reduced the starting thickness of the two air layers. In this way, we have tried to reduce their total thickness variation from the ideal condition (l/4) and so the decrease of the mirrors re¯ectivity. The optimized structure is shown in Fig. 3. As
Table 1 Summary of the optical simulation results on the structure of Fig. 1 Micro-cavity starting thickness
Variation of the resonant peak maximum in the tuning range
Tuning range (nm)
FWHM variation in the tuning range (nm)
Reduction of the microcavity thickness (nm)
0.6l 1.2l
0.87±1.00 0.81±1.00
168 180
7±21 6±15
340 476
138
P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141 Table 3 Thermal spectral displacement of the resonance peak: the reference temperature is 308C Air micro-cavity thickness (nm) 0.6 1.2
Fig. 3. Schematic view of the optimized structure.
we can see in Fig. 3, we have chosen two different values for the air thickness in order to balance the re¯ectivity response of the two Bragg mirrors. The spectral optical characteristics are reported in Table 2. We can see that the optimized structures attain the fundamental objective of around 200 nm tuning range. However, the maximum peak variation is over 20% even if by only a few percent. The FWHM variation in the tuning range is largely reduced compared to the non-optimized structures. We can conclude that the optical performances of the optimized structures are very near to the expected ones. 3. Thermal dependence of the resonance filter wavelength One of the aims of this work is to conceive a device with a weak thermal dependence of the central wavelength of the ®lter between 40 and 608C (industrial working temperature). Two effects contribute to the resonance wavelength displacement: the thermal dilation of the layers which changes the layers thickness and the thermal dependence of the refraction index of the materials. Considering these effects, we have evaluated the thermal dependence of the resonance wavelength for the optimized structures. Preliminary simulation results have showed that the thermal in¯uence on the mechanical behavior of the structure does not in¯uence the optical response of the device (too little temperature interval). The simulation results are reported in Table 3. We can see in Table 3 that the whole thermal wavelength displacement between 60 and 408C is lower than 0.1 nm/ 8C according to the starting device requirement. We have observed that the largest in¯uence is mainly related to the
Resonance wavelength displacement (nm) 408C 5 6
08C
608C
1 1
3 2
thermal dependence refractive index: around 80% of the total thermal displacement. These theoretical results have been con®rmed by experimental results obtained at THOMSON-CSF LCR laboratory on structures closely resembling these ones but used as WDM ®lters for telecommunications applications. 4. Mechanical analysis After having optically modeled the device, we have to de®ne its geometrical shape in order to achieve a 200 nm tuning range with a bias voltage of around 15 V. We have developed a mechanical FEA model of the structure using a three-dimensional (3D) ®nite element solver (ANSYS 5.4) in order to give an accurate description of the tuning performances of the device [12]. The compliance matrix elements that we have used, are reported in Table 4. The meshing of the mechanical model has been performed using the 3D element SOLID64 that permits to consider the mechanical anisotropy of the semiconductors materials. We have simulated only the central part of the device (Fig. 4): the only part that has to move. The liberty degrees of the points showed in Fig. 4 are locked in order to be as near as possible to the real starting conditions. The shape of the simulated device consists in a central circular platform with four arms. We can see in Fig. 5, a SEM image of this kind of device. We can observe the different InP/air layers with the air micro-cavity in the middle. The four arms con®guration has been chosen because when we actuate the structure the central platform deformation is lower than the two arms con®guration [13]. Actually, this parameter is very important in order to reduce the optical loss of the cavity [14]. We have chosen a circular plate-form because the micro-machining process is more ef®cient than a square membrane. Because the incident light spot dimension is 40 mm, we have chosen a 40 mm diameter central platform.
Table 2 Summary of the optical simulation results on the optimized structure Micro-cavity starting thickness
Variation of the resonant peak maximum in the tuning range
Tuning range (nm)
FWHM variation in the tuning range (nm)
Reduction of the microcavity thickness (nm)
0.6l 1.2l
0.75±0.97 0.74±1.00
194 219
14±16 12±18
340 550
P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141
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Fig. 4. Schematic view of the simulated part of the device.
Table 4 Compliance matrix elements for <1 0 0> wafer growth directiona
5. Experimental results
<1 0 0>
S11 S33
In(0.53)Ga(0.47)As InP
1.583 1.650
A ®rst generation of ®lters has been realized. These ®lters are characterized by a l/2 air micro-cavity thickness. The schematic view of the ®lters is shown in Fig. 7. The thickness of the air layers adjacent to the central microcavity is lower than l/4 (425 nm) according to the optical simulation results. The air micro-cavity thickness is lower than 0.6l: this is related to an epitaxial growth calibration error. The device dimensions, according to the mechanical simulations results, are: 40 mm for the central plate-form
a
The values are expressed in 10
S12 S13 0.535 0.594 2
S44 S66 2.130 2.170
GPa 1.
The arms are 5 mm wide and their length depends on the air micro-cavity thickness: in the case of the l micro-cavity thickness, we need four longer arms than the l/2 case because, in order to reach the 200 nm tuning range, we need to reduce more largely the micro-cavity thickness (see Table 2). After having simulated the mechanical performances of the device for these two cases, we have found out that the ideal arms length is 40 and 20 mm, respectively for the l and l/2 micro-cavity structure. In the Fig. 6, the simulated mechanical performances of the devices are shown. We can see that we need a bias voltage of 21.3 and 15 V, respectively for the l and l/2 micro-cavity structures to reach a 200 nm tuning range according with the expected performances.
Fig. 5. SEM image of the device.
Fig. 6. Total InP layers displacement as a function of the applied bias voltage: the dashed lines represent the displacement needed to reach a 200 nm tuning range.
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P. Bondavalli et al. / Sensors and Actuators A 94 (2001) 136±141
6. Conclusions
Fig. 7. Schematic view of the realized structure.
Fig. 8. Optical transmission preliminary results.
In this paper, we have presented the optical and mechanical design of an InP-based Fabry±PeÂrot ®lter for gas sensing applications. Firstly, we have calculated the optimal layer thickness needed to reach the optical speci®cations. This has been achieved optimizing the re¯ectivity spectral response of the top and bottom Bragg mirrors. The thermal dependence displacement of the resonance peak of the device has been evaluated and it is lower than 0.1 nm/8C. Secondly, we have developed a FEA mechanical model which permits to simulate the mechanical performances of the device. This model allows us to de®ne the geometrical dimensions of the structure in order to reach the 200 nm tuning range with a 15 V bias voltage. Finally, optical transmission, measurements on a l/2 air micro-cavity ®lter have been performed. The experimental results are very promising: the ®lter attains a 100 nm tuning range with 11 V and the FWHM varies between 14 and 16 nm. These performances are very interesting and could be improved in the near future with the realization of a l air micro-cavity that will permit to attain 219 nm with a 21.3 V bias voltage. The experimental results are in very good agreement with the mechanical simulations: the FEA model that we have developed permit to realistically simulate the mechanical performances of this kind of surface micromachined devices used for gas sensing applications. Acknowledgements We would like to thank Prof. G. Guillot for critical reading of this manuscript.
References
Fig. 9. Variation of the applied bias voltage as a function of the resonance filter wavelength: compare between the FEA model simulation and the optical transmission results.
diameter, 20 and 5 mm, respectively for the arms length and width. The optical transmission results are reported in Fig. 8. We reach a 100 nm tuning range with a 11 V bias voltage. The FWHM varies between 14 and 16 nm according to the optical simulations. In order to validate our mechanical model, we have compared the experimental results with the mechanical simulations results in Fig. 9. We obtain a very good agreement between the theoretical and experimental results. We can conclude that our FEA mechanical model is very ef®cient to realistically simulate the mechanical performances of such device.
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Biographies Paolo Bondavalli obtained his MS degree in physics from the University of Parme, Italy, in November 1995 and PhD degree in engineering from the National Institute of Applied Sciences of Lyon, France, in January 2000. The PhD thesis was about the properties of microsystems for gas sensing applications. He has worked at the R&D Department of THOMSON-CSF LASER DIODES and is now working at the THALES Corporate Research Center in Orsay (France). He is currently involved in research on new kind of MEMS for telecommunications. Taha Benyattou is a CNRS researcher at the Laboratoire de Physique de La MatieÁre UMR CNRS 5511. He received his PhD degree from the Universite Claude BERNARD LYON 1 in 1987. He is now involved in the field of microphotonic devices, such as optical sensors, photonic bandgap structure for telecommunication and optical interconnect. Michel Garrigues was born in 1948 in France. He received the DocteurIngeÂnieur degree in 1976 from the University of Lyon. He is presently Directeur de Recherche at Laboratoire d'Electronique, OptoeÂlectronique et
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Micro-systeÁmes (LEOM), CNRS, Ecole Centrale de Lyon. From 1977 to 1986 he has co-ordinated a French research project on the instability phenomena due to hot electron injection in submicronic silicon devices. From 1986 to 1990, he has developed a novel technique of room temperature scanning photoluminescence (SPL) for controlling III-V compound semiconductor wafers. In 1989, he started the SCANTEK company for the industrial development of SPL equipment. Temporarily on leave from CNRS, he was appointed as General Manager by SCANTEK from 1991 to 1996. Since 1996, he is project leader for the development of tunable micro-machined filters and photodetectors in collaboration with French industrial partners. Jean-Louis Leclercq was born in 1964. He received his PhD in chimie des mateÂriaux in 1990 from the University of Montpellier II, France. His thesis work focused on PECVD processing for InP-based MISFET. He joined the French CNRS as permanent researcher in 1990. From 1990 to 1992, he worked at EM2 of University of Montpellier II by developing GaSb-based laser diodes. In 1992, he joined the LEOM at Ecole Centrale de Lyon where he is currently managing the III±V based microtechnologies and microsystems developments. His current interests include optoelectronics, micromachining and microfabrication related to III±V based materials for photonic systems. He is author and co-author of about 30 papers in international journals and 40 international conferences. Serguei Jourba was born in 1971. He received his MS degree in solid-state electronics from Kiev University, Ukraine in 1993, and his PhD degree in microelectronics from Ecole Centrale de Lyon, France in 1999. In 2000, he joined ATMEL Corporation as R&D engineer. His fields of interest are the physics of semiconductor devices, micro-opto-electro-mecanical systems (MOEMS), and resonant cavity enhanced light emitters and photodetectors operating in the near infrared spectral range. Christophe Pautet was born in Lyon in 1967. He received his MS and PhD degrees in materials science from the University Claude Bernard, Lyon, in 1991 and 1995, respectively. His thesis work was focused on the metal organic vapor phase epitaxy on characterization of InGaAsP compound using alternative arsenic precursors. He joined THOMSON-CSF SEMICONDUCTEURS SPECIFIQUES Saint-EgreÁve in 1996 (Atmel-Grenoble since 2000) where he is currently in charge of Epitaxy for near infrared devices. Xavier Hugon biography not available.