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Mirror electrostatic actuation of a medium-infrared tuneable Fabry-Perot interferometer based on a surface micromachining process
Sensors and Actuators A 123–124 (2005) 584–589
Mirror electrostatic actuation of a medium-infrared tuneable Fabry-Perot interferometer based on a surface micromachining process N. Sabat´e a , R. Rubio b , C. Calaza a , J. Santander b,∗ , L. Fonseca b , I. Gr`acia b , C. Can´e b , M. Moreno a , S. Marco a a
Departament d’Electr`onica, Universitat de Barcelona, Mart´ı i Franqu´es 1, 08028 Barcelona, Spain b Centre Nacional de Microelectr` onica (IMB-CSIC), Campus UAB, 08193 Barcelona, Spain Received 13 September 2004; received in revised form 22 February 2005; accepted 3 March 2005 Available online 18 April 2005
Abstract A tuneable Fabry-Perot interferometer fabricated with a tailored surface micromachining process is presented in this work. The device based on a movable flat micromirror driven by electrostatic actuation, is intended for gas sensing applications and operates in the medium-infrared spectral range. The electrostatic actuation features of the proposed structure are presented showing that a flat and parallel displacement of the mobile mirror is obtained. The characterization of the optical transmittance of the structure shows that this displacement allows covering a range of the transmitted spectra between 3.5 and 5.0 m. © 2005 Elsevier B.V. All rights reserved. Keywords: Fabry-Perot interferometer; Surface micromachining; Electrostatic actuation
1. Introduction The growing interest for low cost and long-term stable gas detectors in a great variety of fields, such as air-quality control and toxic gases detection, has made a great number of detection systems to appear (metal oxide sensors, SAW sensors, thermal conductivity sensors, resonant structures, optical gas sensors, . . .). Among them, optical detectors based on selective absorption offer significant advantages over other choices, specially those concerning selectivity and long-term stability [1]. Due to the fact that most pollutant gases have strong absorption bands at characteristic wavelengths in the medium-infrared spectral region (MIR), the gas sensors based on infrared radiation absorption appear to be the optimal choice when multicompound analysis is required. Conventional non-dispersive infrared systems (NDIR) consist of an IR source, a gas absorption chamber, an IR detector and an optical filter, which selects the wavelength ∗
Corresponding author. Tel.: +34 93 594 77 00; fax: +34 93 580 14 96. E-mail address: [email protected] (J. Santander).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.03.036
region corresponding to a particular absorption band. One or the main drawbacks in this kind of devices is their highcost, especially in low-end applications. In this sense, the miniaturization and integration by means of silicon fabrication technology not only can enhance significantly the price competitiveness of these detectors, but also improve their performance. In this work, a micromachined Fabry-Perot interferometer is presented. In particular, our interest will be focused on the electrostatic actuation features of the structure, which has been designed and fabricated to be integrated with a thermoelectric infrared detector to obtain an active detector module with spectral selectivity.
2. Principle of operation A Fabry-Perot interferometer consists of an optical resonant cavity composed of two plane parallel mirrors separated by a certain gap. In an ideal structure, that is, with no absorption losses, the optical transmittance T for a given wavelength
N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589
λ is a function of the reflectivity of the mirrors R1 and R2 , the gap thickness h and the refraction index n of the gap material: T (λ) =
1−
√
(1 − R1 )(1 − R2 ) 2 √ R1 R2 + R1 R2 sin2 2π nh λ
(1)
According to the last expression, the basic Fabry-Perot interferometer design rules can be derived. That is, the transmittance of the structure will be maximum when the gap distance h is an even entire multiple of λ/4n and minimum when h is an odd entire multiple of λ/4n [2]. Due to the periodicity of Eq. (1), for a given value of the gap, the interferometer shows a series of transmitted peaks located on wavelengths λi = 2nh/i with i = 1, 2, 3, . . . depending on the transmission order. The dimension of the gap thickness h used in the resonant cavity has a huge influence in other features of the Fabry-Perot transmittance as well, such as the free spectral range and the finesse of the transmitted lines. In addition to this, Eq. (1) shows that the maximum transmittance value that can be obtained with an ideal interferometer depends on the reflectivity of the mirrors that compose the optical resonant cavity. If the reflectivity of both mirrors is identical, R1 = R2 , the value of transmittance will reach its maximum value, which will be equal to one. Moreover, contrast between maximum and minimum will be improved if the reflectivity of both mirrors is as high as possible. The maximum reflectivity of a mirror of dielectric nature is limited by the index of refraction contrast between mirror material and surrounding media, and its obtention for a given wavelength tightly fixes the mirror thickness as well.
3. Implementation with silicon based microtechnologies The proposed arrangement for the Fabry-Perot consists of a vertically integrated optical resonator structure, formed by a dielectric moving micromirror suspended by cantilever beams over a second identical parallel mirror that is attached to the silicon substrate. Fig. 1 shows schematically the proposed arrangement for the interferometer. In this structure, changing the distance between mirrors results in a displacement of the transmission peaks, which can be tuned to a desired wavelength to perform an optical wavelength selection in the transmitted light. Both upper
Fig. 1. Schematical cross-section of the mobile mirror structure.
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and low micromirrors have been fabricated in polysilicon because of its good structural properties together with the possibility of making it electrically active, and because its high-index of refraction makes it a good dielectric mirror material [3]. The fabrication process is based on a standard surface micromachining process customized to reduce the stress-induced deflection in the polysilicon layers and to overcome the technological constrains of the optical thickness of mirrors (<1 m) and gap (>1 m) [4]. The antireflection coating depicted in the figure consists of an oxide layer, and has been laid between the silicon substrate and the bottom mirror in order to achieve a similar reflectivity for the upper and the bottom mirrors, condition that increases the total transmittance of the structure. A detailed description of the fabrication process features can be found elsewhere [5]. The transmission order selected in the Fabry-Perot to be tuned in a certain wavelength range will depend on the application requirements. In the present case, the structure is intended to detect CH4 , CO2 and CO using their absorption bands located in the medium-infrared spectral range (at 3.33, 4.26 and 4.67 m, respectively), and therefore a free spectral range higher than the 1.37 m separation between the limiting absorption bands is required. Maximizing the free spectral range forces the use of the lowest transmission order. In our case, this condition ensures that higher order transmitted peaks will not overlap with the desired bands during the tuning process. Taking into account all the structure and application requirements, a simulation of the filter behavior has been carried out in order to determine the gap dimensions. Fig. 2 shows the evolution of the reflected peaks for different values of the gap. The proposed application should have an initial gap of around 2400 nm and it should be actuated till a minimum gap of approximately 1500 nm. Fig. 3 shows a SEM micrograph of an array of interferometers once the fabrication process is completed. The area covered by a single micromirror is 2.25 × 104 m2 .
Fig. 2. Simulated reflectance peaks for the proposed Fabry-Perot structure depending on the gap distance.
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Fig. 3. A SEM micrograph of one of the fabricated interferometer arrays.
Fig. 4. Z-displacement of a polysilicon mirror obtained by FEM simulation.
4. Electrostatic actuation and IR response
5. Experimental results
Electrostatic actuation is carried out by applying a voltage to the chip electrodes. Due to the electrostatic force, the upper mirrors are attracted to the substrate, and thus they experience a vertical displacement that can be clearly observed by optical inspection. Flat and parallel movement of every upper mirror during the interferometer operation is important to assure the best shape of the transmitted wavelength. Moreover, the uniformity of this movement along the micromirror array has also to be ensured. The obtaining of a simultaneous and uniform actuation of the structures is conditioned by the uniformity of the electric field established between the chip electrodes, which rely in a good isolation of both electrodes through the supporting points of the upper one plate. In order to keep the desired planarity during the tuning process, the mirror must be stiffer than its suspensions. The stiffness of the suspensions can be significantly reduced by making the beams longer as well as reducing their width and thickness [6]. Keeping those suspensions close and parallel to the mirror body allows increasing the fill factor if a cell based design (as the one shown in Fig. 3) is needed. Fig. 4 shows the deflection that is originated in a polysiliconsimulated structure with 10 m-wide arms and a mirror area of 2.25 × 104 m2 when a voltage of 6 V is applied. It can be seen that the deformation is concentrated on the supports. The displacement of this kind of micromachined structures can be controlled with electrostatic actuation within a certain distance (that expands typically to 1/3 of the initial gap). If actuated beyond that point, the movable mirror snaps down onto the bottom electrode. In order to avoid the permanent sticking of upper and lower mirrors that could be occasioned when the threshold voltage is overcome, an overrange stopper 1500-nm high has been included in the mobile platform.
The released Fabry-Perot structures have been set in motion by means of electrostatic actuation. The vertical displacement of the structure has been observed with an optical microscope operating in the Mirau interferometric mode. Fig. 5 shows photographs of the mirror deflection obtained at different applied voltages. The first photograph shows the deflection of one of the mirrors as obtained after the release process. Two interferometric rings can be clearly observed in the membrane region, as well as between the tip and the fastening region of the supporting arms. Taking into account the accuracy given by the operating wavelength of the light source, this indicates an initial deflection close to 540 nm. The following two photographs show the displacement of the mirror for increasing voltage levels, that is, 6 and 9 V, respectively. In this image sequence, it can be observed how the number of interferometric rings appearing in the supporting arms grows with increasing voltages, whereas the inteferometric pattern in the mirror plate section remains almost unaltered. The mirror shows this behavior until the over-range stopper contacts with the underlying fixed mirror. As it will be shown in next figures, plane displacement turns into an inclination when this occurs. Fig. 6 shows the measurements of the upper mirror displacement performed by a PL non-contact confocal imaging profiler compared to the ideal electrostatic actuation predicted by simulation for a plane mirror without the over-range stopper. It can be seen that points 1 and 2 show an identical behavior in the range 0–9 V. This evidences the planarity of the upper mirror displacement. At the same time, it has to be noted that the mirror follows the non-linear relation between voltage and displacement caused by the non-linear dependence of the electrostatic force on both voltage and distance between mirrors. However, when the voltage is increased beyond 9 V, the behavior of the two measurement points becomes dissimilar, which means that the plane displacement
N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589
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Fig. 6. Displacement of two symmetrical points of the micromirror at different applied voltages compared to the simulated actuation curve of a mirror structure without over-range stopper.
Fig. 5. Interferometric images of a filter electrostatic actuation at 0 V (top), 6 V (middle) and 9 V (bottom).
turns to an inclination. As this happens when the theoretical collapse of the structure is about to take place, the inclination can be attributed to the blocking action of the over-range stopper that at this voltage contacts with the fixed mirror. Fig. 7 shows the comparison between confocal images of a mirror operated in the planar regime and the non-planarity achieved when a value of 9 V is surpassed. At higher voltages, the mirror inclination is more pronounced and leads to partial striction. Tests have shown that such striction event is temporary even at voltages as high as 15 V. In those cases, the structure recovers itself spontaneously shortly after being unbiased or biased below the pull-in voltage. This effect occurs simultaneously in the whole array pointing out that the voltage at each element is equivalent. Good isolation between the two electrodes is important. A thin ni-
tride layer had to be added on those points where the micromachined upper electrode came into contact with the bottom electrode plate. Without this nitride layer, there was a conduction path through both electrodes, thus preventing uniform actuation of the whole array, even though the upper electrode rested only on the undoped regions of the bottom polysilicon electrode. The optical transmittance of the Fabry-Perot structure has been tested with an IR-Plan Spectra Technology. Microscopy in the reflection mode. Reflection mode was chosen to avoid the influence of different layers still present in the backside of the wafer. The total wavelength range spanned from 400 to 4000 cm−1 . In order to obtain local measurements, the spot size was set to 30 m × 30 m with the aid of an adjustable window. In Fig. 8, the reflection measurements corresponding to point 1 of the micromirror are depicted. Since these measurements took place at open air, the presence of ambient CO2 can be seen in the range of interest. It can also be seen how during the actuation the main peak moved widely from one side to the other side of the CO2 line. These measurements show a good qualitative agreement with the estimated behavior obtained in numerical simulations. The electrostatic actuated mirrors produce a significant displacement of the transmitted spectra covering the
Fig. 7. Mechanical performance of the filter. Between 0 and 8 V vertical displacement is concentred at the arms (left), whereas at higher voltages the mirror loses its planarity (right).
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N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589 [2] K. Lizuka, Engineering Optics, Springer Series in Optical Sciences, 1987. [3] Y.H. Min, Y.K. Kim, Modelling, design, fabrication and measurement of a single layer polysilicon micromirror with initial curvature compensation, Sens. Actuators A 78 (1999) 8–17. [4] S. Greek, N. Chitica, Deflection of surface-micromachined devices due to internal, homogeneous or gradient stresses, Sens. Actuators A 78 (1999) 1–7. [5] C. Calaza, L. Fonseca, M. Moreno, S. Marco, C. Can´e, I. Gr`acia, A surface micromachining process for the development of a mediuminfrared tuneable FP interferometer, Sens. Actuators A 113 (2004) 39–47. [6] C. Calaza, M. Moreno, S. Marco, L. Fonseca, C. Can´e, I. Gr`acia, Modelling and simulation of an electrostatically actuated surfacemicromachined tuneable Fabry-Perot interferometer for the medium IR spectral range, in: Proceedings of the EuroSimE, Paris, France, April, 2001, pp. 241–247.
Fig. 8. FTIR spectra showing the position of transmission interferometer peaks at different applied voltages.
3.5–5.0 m range. However, reflected peaks get broader as λ increases as the polysilicon layers thickness has been optimized for the CO absorption peak. In addition, a diminution of the amplitude is observed for lower wavelengths.
Biographies Neus Sabat´e was born in Tarragona, Spain in 1975. She received her BSc degree on physics from Barcelona University, Spain in 1998. In 1999, she joined the microsystems department of CNM and she obtained her PhD in physics in 2003, working on the development of gas and flow sensor devices and microsystems. She is currently working for the electronics department of the University of Barcelona in MEMS applications for the gas sensing field.
6. Conclusions The electrostatical actuation features of a surface micromachined structure intended to work as a tuneable FabryPerot interferometer have been presented. With the proposed structure, a planar displacement of the upper mirror can be obtained in the operation range of 0–9 V. This behavior can lead to a accurate control of the trasmitted spectra. Optical characterization has shown a good agreement between numerical simulations and experimental results in terms of peaks position and displacement. However, nonideal features of the fabricated filter can be observed in the broadening and the decreasing amplitude of the reflected peaks. The presented structure allows covering a range between 3.5 and 5.0 m that is compatible with CO and CO2 detection. Nevertheless, the gap mirror range has to be expanded below 1600 nm if CH4 is to be detected more accurately, which can be done rearranging the height of the over-range stopper.
Acknowledgement This work has been financed by the Spanish CICYT project DPI-2001-3213-CO2-01.
References [1] D.I. Sebacher, in: J. Wormhoudt (Ed.), Infrared Methods for Gaseous Measurements, Marcel Dekker, New York, 1985, pp. 247–274 (Chapter 6).
R. Rubio was born in Madrid, Spain in 14 August 1978. He received his BS degree in physics from the University of Barcelona in 2001. In 2002, he joined the Microelectronics National Center, where he is pursuing his PhD. His main research activities are related to the infrared sensing technologies, but also with the pattern recognition techniques. C. Calaza was born in Mondo˜nedo, Spain, in 1974. He received his BS degree in physics from the University of Santiago de Compostela, Spain in 1996 and the degree in electronic engineering from the University of Barcelona, Spain in 2000. From 1999, he was with the department of electronics, University of Barcelona, where he was working in the field of silicon MOEMS for infrared gas sensing applications. Joaquin Santander received his PhD degree in physics from the Autonomous University of Barcelona, Spain in 1996. He is currently working at the Microelectronics National Center in Barcelona, as a responsible of the electrical characterization laboratory. His main research areas are related to different microelectronic technologies (CMOS, MCM, sensors, microsystems) and electrical parametric characterization using mainly test structures. Luis Fonseca was born in Barcelona, Spain in 1966. He received his BS and PhD degrees in physics from the Autonomous University of Barcelona in 1988 and 1992, respectively. In 1989, he joined the National Center of Microelectronics as a post-graduate student, working till 1992 on the growth and characterization of thin dielectric films for VLSI and ULSI applications. After this first research period, he has worked as a process engineer, currently leading the diffusion and deposition areas of the CNM production facilities. Isabel Gr`acia received the PhD degree in physics in 1993 from the Autonomous University of Barcelona, Spain, working on chemical sensors. She joined the National Microelectronics Center (CNM) working on photolithography, she is currently in the microsystems department working in the gas sensing field.
N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589 Carles Can´e received the BSc degree in telecommunication engineering in 1986 and the PhD degree in 1989 from the Universitat Polit`ecnica de Catalunya in Barcelona, Spain. Since 1990, he is permanent researcher at the National Microelectronics Center (CNM) in Barcelona. He is currently working in the fields of sensors and microsystems and their compatibility with standard CMOS technologies. M. Moreno was born in Barcelona, Spain. He received the degree in physics in 1989 from the University of Barcelona (UB), and the PhD degree in sciences in 1995 from the Polytechnic University of Catalonia (UPC), Spain. He has been an associate professor in the electronics department, UB, since 1997. He is involved in the design and test of
589
infrared thermopile detectors and the test of integrated optical devices in silicon technology for DWDM applications. S. Marco is associate professor at the department d’Electronica of Universitat de Barcelona since 1995. He received degree in physics from the Universitat de Barcelona in 1988. In 1993, he received his PhD (honor award) degree from the departament de F´ısica Aplicada i Electr`onica, Universitat de Barcelona, for the development of a novel silicon sensor for in vivo measurements of the blood pressure. His current research interests are two-fold: chemical instrumentation based on intelligent signal processing and microsystem modeling.
Mirror electrostatic actuation of a medium-infrared tuneable Fabry-Perot interferometer based on a surface micromachining process N. Sabat´e a , R. Rubio b , C. Calaza a , J. Santander b,∗ , L. Fonseca b , I. Gr`acia b , C. Can´e b , M. Moreno a , S. Marco a a
Departament d’Electr`onica, Universitat de Barcelona, Mart´ı i Franqu´es 1, 08028 Barcelona, Spain b Centre Nacional de Microelectr` onica (IMB-CSIC), Campus UAB, 08193 Barcelona, Spain Received 13 September 2004; received in revised form 22 February 2005; accepted 3 March 2005 Available online 18 April 2005
Abstract A tuneable Fabry-Perot interferometer fabricated with a tailored surface micromachining process is presented in this work. The device based on a movable flat micromirror driven by electrostatic actuation, is intended for gas sensing applications and operates in the medium-infrared spectral range. The electrostatic actuation features of the proposed structure are presented showing that a flat and parallel displacement of the mobile mirror is obtained. The characterization of the optical transmittance of the structure shows that this displacement allows covering a range of the transmitted spectra between 3.5 and 5.0 m. © 2005 Elsevier B.V. All rights reserved. Keywords: Fabry-Perot interferometer; Surface micromachining; Electrostatic actuation
1. Introduction The growing interest for low cost and long-term stable gas detectors in a great variety of fields, such as air-quality control and toxic gases detection, has made a great number of detection systems to appear (metal oxide sensors, SAW sensors, thermal conductivity sensors, resonant structures, optical gas sensors, . . .). Among them, optical detectors based on selective absorption offer significant advantages over other choices, specially those concerning selectivity and long-term stability [1]. Due to the fact that most pollutant gases have strong absorption bands at characteristic wavelengths in the medium-infrared spectral region (MIR), the gas sensors based on infrared radiation absorption appear to be the optimal choice when multicompound analysis is required. Conventional non-dispersive infrared systems (NDIR) consist of an IR source, a gas absorption chamber, an IR detector and an optical filter, which selects the wavelength ∗
Corresponding author. Tel.: +34 93 594 77 00; fax: +34 93 580 14 96. E-mail address: [email protected] (J. Santander).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.03.036
region corresponding to a particular absorption band. One or the main drawbacks in this kind of devices is their highcost, especially in low-end applications. In this sense, the miniaturization and integration by means of silicon fabrication technology not only can enhance significantly the price competitiveness of these detectors, but also improve their performance. In this work, a micromachined Fabry-Perot interferometer is presented. In particular, our interest will be focused on the electrostatic actuation features of the structure, which has been designed and fabricated to be integrated with a thermoelectric infrared detector to obtain an active detector module with spectral selectivity.
2. Principle of operation A Fabry-Perot interferometer consists of an optical resonant cavity composed of two plane parallel mirrors separated by a certain gap. In an ideal structure, that is, with no absorption losses, the optical transmittance T for a given wavelength
N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589
λ is a function of the reflectivity of the mirrors R1 and R2 , the gap thickness h and the refraction index n of the gap material: T (λ) =
1−
√
(1 − R1 )(1 − R2 ) 2 √ R1 R2 + R1 R2 sin2 2π nh λ
(1)
According to the last expression, the basic Fabry-Perot interferometer design rules can be derived. That is, the transmittance of the structure will be maximum when the gap distance h is an even entire multiple of λ/4n and minimum when h is an odd entire multiple of λ/4n [2]. Due to the periodicity of Eq. (1), for a given value of the gap, the interferometer shows a series of transmitted peaks located on wavelengths λi = 2nh/i with i = 1, 2, 3, . . . depending on the transmission order. The dimension of the gap thickness h used in the resonant cavity has a huge influence in other features of the Fabry-Perot transmittance as well, such as the free spectral range and the finesse of the transmitted lines. In addition to this, Eq. (1) shows that the maximum transmittance value that can be obtained with an ideal interferometer depends on the reflectivity of the mirrors that compose the optical resonant cavity. If the reflectivity of both mirrors is identical, R1 = R2 , the value of transmittance will reach its maximum value, which will be equal to one. Moreover, contrast between maximum and minimum will be improved if the reflectivity of both mirrors is as high as possible. The maximum reflectivity of a mirror of dielectric nature is limited by the index of refraction contrast between mirror material and surrounding media, and its obtention for a given wavelength tightly fixes the mirror thickness as well.
3. Implementation with silicon based microtechnologies The proposed arrangement for the Fabry-Perot consists of a vertically integrated optical resonator structure, formed by a dielectric moving micromirror suspended by cantilever beams over a second identical parallel mirror that is attached to the silicon substrate. Fig. 1 shows schematically the proposed arrangement for the interferometer. In this structure, changing the distance between mirrors results in a displacement of the transmission peaks, which can be tuned to a desired wavelength to perform an optical wavelength selection in the transmitted light. Both upper
Fig. 1. Schematical cross-section of the mobile mirror structure.
585
and low micromirrors have been fabricated in polysilicon because of its good structural properties together with the possibility of making it electrically active, and because its high-index of refraction makes it a good dielectric mirror material [3]. The fabrication process is based on a standard surface micromachining process customized to reduce the stress-induced deflection in the polysilicon layers and to overcome the technological constrains of the optical thickness of mirrors (<1 m) and gap (>1 m) [4]. The antireflection coating depicted in the figure consists of an oxide layer, and has been laid between the silicon substrate and the bottom mirror in order to achieve a similar reflectivity for the upper and the bottom mirrors, condition that increases the total transmittance of the structure. A detailed description of the fabrication process features can be found elsewhere [5]. The transmission order selected in the Fabry-Perot to be tuned in a certain wavelength range will depend on the application requirements. In the present case, the structure is intended to detect CH4 , CO2 and CO using their absorption bands located in the medium-infrared spectral range (at 3.33, 4.26 and 4.67 m, respectively), and therefore a free spectral range higher than the 1.37 m separation between the limiting absorption bands is required. Maximizing the free spectral range forces the use of the lowest transmission order. In our case, this condition ensures that higher order transmitted peaks will not overlap with the desired bands during the tuning process. Taking into account all the structure and application requirements, a simulation of the filter behavior has been carried out in order to determine the gap dimensions. Fig. 2 shows the evolution of the reflected peaks for different values of the gap. The proposed application should have an initial gap of around 2400 nm and it should be actuated till a minimum gap of approximately 1500 nm. Fig. 3 shows a SEM micrograph of an array of interferometers once the fabrication process is completed. The area covered by a single micromirror is 2.25 × 104 m2 .
Fig. 2. Simulated reflectance peaks for the proposed Fabry-Perot structure depending on the gap distance.
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Fig. 3. A SEM micrograph of one of the fabricated interferometer arrays.
Fig. 4. Z-displacement of a polysilicon mirror obtained by FEM simulation.
4. Electrostatic actuation and IR response
5. Experimental results
Electrostatic actuation is carried out by applying a voltage to the chip electrodes. Due to the electrostatic force, the upper mirrors are attracted to the substrate, and thus they experience a vertical displacement that can be clearly observed by optical inspection. Flat and parallel movement of every upper mirror during the interferometer operation is important to assure the best shape of the transmitted wavelength. Moreover, the uniformity of this movement along the micromirror array has also to be ensured. The obtaining of a simultaneous and uniform actuation of the structures is conditioned by the uniformity of the electric field established between the chip electrodes, which rely in a good isolation of both electrodes through the supporting points of the upper one plate. In order to keep the desired planarity during the tuning process, the mirror must be stiffer than its suspensions. The stiffness of the suspensions can be significantly reduced by making the beams longer as well as reducing their width and thickness [6]. Keeping those suspensions close and parallel to the mirror body allows increasing the fill factor if a cell based design (as the one shown in Fig. 3) is needed. Fig. 4 shows the deflection that is originated in a polysiliconsimulated structure with 10 m-wide arms and a mirror area of 2.25 × 104 m2 when a voltage of 6 V is applied. It can be seen that the deformation is concentrated on the supports. The displacement of this kind of micromachined structures can be controlled with electrostatic actuation within a certain distance (that expands typically to 1/3 of the initial gap). If actuated beyond that point, the movable mirror snaps down onto the bottom electrode. In order to avoid the permanent sticking of upper and lower mirrors that could be occasioned when the threshold voltage is overcome, an overrange stopper 1500-nm high has been included in the mobile platform.
The released Fabry-Perot structures have been set in motion by means of electrostatic actuation. The vertical displacement of the structure has been observed with an optical microscope operating in the Mirau interferometric mode. Fig. 5 shows photographs of the mirror deflection obtained at different applied voltages. The first photograph shows the deflection of one of the mirrors as obtained after the release process. Two interferometric rings can be clearly observed in the membrane region, as well as between the tip and the fastening region of the supporting arms. Taking into account the accuracy given by the operating wavelength of the light source, this indicates an initial deflection close to 540 nm. The following two photographs show the displacement of the mirror for increasing voltage levels, that is, 6 and 9 V, respectively. In this image sequence, it can be observed how the number of interferometric rings appearing in the supporting arms grows with increasing voltages, whereas the inteferometric pattern in the mirror plate section remains almost unaltered. The mirror shows this behavior until the over-range stopper contacts with the underlying fixed mirror. As it will be shown in next figures, plane displacement turns into an inclination when this occurs. Fig. 6 shows the measurements of the upper mirror displacement performed by a PL non-contact confocal imaging profiler compared to the ideal electrostatic actuation predicted by simulation for a plane mirror without the over-range stopper. It can be seen that points 1 and 2 show an identical behavior in the range 0–9 V. This evidences the planarity of the upper mirror displacement. At the same time, it has to be noted that the mirror follows the non-linear relation between voltage and displacement caused by the non-linear dependence of the electrostatic force on both voltage and distance between mirrors. However, when the voltage is increased beyond 9 V, the behavior of the two measurement points becomes dissimilar, which means that the plane displacement
N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589
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Fig. 6. Displacement of two symmetrical points of the micromirror at different applied voltages compared to the simulated actuation curve of a mirror structure without over-range stopper.
Fig. 5. Interferometric images of a filter electrostatic actuation at 0 V (top), 6 V (middle) and 9 V (bottom).
turns to an inclination. As this happens when the theoretical collapse of the structure is about to take place, the inclination can be attributed to the blocking action of the over-range stopper that at this voltage contacts with the fixed mirror. Fig. 7 shows the comparison between confocal images of a mirror operated in the planar regime and the non-planarity achieved when a value of 9 V is surpassed. At higher voltages, the mirror inclination is more pronounced and leads to partial striction. Tests have shown that such striction event is temporary even at voltages as high as 15 V. In those cases, the structure recovers itself spontaneously shortly after being unbiased or biased below the pull-in voltage. This effect occurs simultaneously in the whole array pointing out that the voltage at each element is equivalent. Good isolation between the two electrodes is important. A thin ni-
tride layer had to be added on those points where the micromachined upper electrode came into contact with the bottom electrode plate. Without this nitride layer, there was a conduction path through both electrodes, thus preventing uniform actuation of the whole array, even though the upper electrode rested only on the undoped regions of the bottom polysilicon electrode. The optical transmittance of the Fabry-Perot structure has been tested with an IR-Plan Spectra Technology. Microscopy in the reflection mode. Reflection mode was chosen to avoid the influence of different layers still present in the backside of the wafer. The total wavelength range spanned from 400 to 4000 cm−1 . In order to obtain local measurements, the spot size was set to 30 m × 30 m with the aid of an adjustable window. In Fig. 8, the reflection measurements corresponding to point 1 of the micromirror are depicted. Since these measurements took place at open air, the presence of ambient CO2 can be seen in the range of interest. It can also be seen how during the actuation the main peak moved widely from one side to the other side of the CO2 line. These measurements show a good qualitative agreement with the estimated behavior obtained in numerical simulations. The electrostatic actuated mirrors produce a significant displacement of the transmitted spectra covering the
Fig. 7. Mechanical performance of the filter. Between 0 and 8 V vertical displacement is concentred at the arms (left), whereas at higher voltages the mirror loses its planarity (right).
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N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589 [2] K. Lizuka, Engineering Optics, Springer Series in Optical Sciences, 1987. [3] Y.H. Min, Y.K. Kim, Modelling, design, fabrication and measurement of a single layer polysilicon micromirror with initial curvature compensation, Sens. Actuators A 78 (1999) 8–17. [4] S. Greek, N. Chitica, Deflection of surface-micromachined devices due to internal, homogeneous or gradient stresses, Sens. Actuators A 78 (1999) 1–7. [5] C. Calaza, L. Fonseca, M. Moreno, S. Marco, C. Can´e, I. Gr`acia, A surface micromachining process for the development of a mediuminfrared tuneable FP interferometer, Sens. Actuators A 113 (2004) 39–47. [6] C. Calaza, M. Moreno, S. Marco, L. Fonseca, C. Can´e, I. Gr`acia, Modelling and simulation of an electrostatically actuated surfacemicromachined tuneable Fabry-Perot interferometer for the medium IR spectral range, in: Proceedings of the EuroSimE, Paris, France, April, 2001, pp. 241–247.
Fig. 8. FTIR spectra showing the position of transmission interferometer peaks at different applied voltages.
3.5–5.0 m range. However, reflected peaks get broader as λ increases as the polysilicon layers thickness has been optimized for the CO absorption peak. In addition, a diminution of the amplitude is observed for lower wavelengths.
Biographies Neus Sabat´e was born in Tarragona, Spain in 1975. She received her BSc degree on physics from Barcelona University, Spain in 1998. In 1999, she joined the microsystems department of CNM and she obtained her PhD in physics in 2003, working on the development of gas and flow sensor devices and microsystems. She is currently working for the electronics department of the University of Barcelona in MEMS applications for the gas sensing field.
6. Conclusions The electrostatical actuation features of a surface micromachined structure intended to work as a tuneable FabryPerot interferometer have been presented. With the proposed structure, a planar displacement of the upper mirror can be obtained in the operation range of 0–9 V. This behavior can lead to a accurate control of the trasmitted spectra. Optical characterization has shown a good agreement between numerical simulations and experimental results in terms of peaks position and displacement. However, nonideal features of the fabricated filter can be observed in the broadening and the decreasing amplitude of the reflected peaks. The presented structure allows covering a range between 3.5 and 5.0 m that is compatible with CO and CO2 detection. Nevertheless, the gap mirror range has to be expanded below 1600 nm if CH4 is to be detected more accurately, which can be done rearranging the height of the over-range stopper.
Acknowledgement This work has been financed by the Spanish CICYT project DPI-2001-3213-CO2-01.
References [1] D.I. Sebacher, in: J. Wormhoudt (Ed.), Infrared Methods for Gaseous Measurements, Marcel Dekker, New York, 1985, pp. 247–274 (Chapter 6).
R. Rubio was born in Madrid, Spain in 14 August 1978. He received his BS degree in physics from the University of Barcelona in 2001. In 2002, he joined the Microelectronics National Center, where he is pursuing his PhD. His main research activities are related to the infrared sensing technologies, but also with the pattern recognition techniques. C. Calaza was born in Mondo˜nedo, Spain, in 1974. He received his BS degree in physics from the University of Santiago de Compostela, Spain in 1996 and the degree in electronic engineering from the University of Barcelona, Spain in 2000. From 1999, he was with the department of electronics, University of Barcelona, where he was working in the field of silicon MOEMS for infrared gas sensing applications. Joaquin Santander received his PhD degree in physics from the Autonomous University of Barcelona, Spain in 1996. He is currently working at the Microelectronics National Center in Barcelona, as a responsible of the electrical characterization laboratory. His main research areas are related to different microelectronic technologies (CMOS, MCM, sensors, microsystems) and electrical parametric characterization using mainly test structures. Luis Fonseca was born in Barcelona, Spain in 1966. He received his BS and PhD degrees in physics from the Autonomous University of Barcelona in 1988 and 1992, respectively. In 1989, he joined the National Center of Microelectronics as a post-graduate student, working till 1992 on the growth and characterization of thin dielectric films for VLSI and ULSI applications. After this first research period, he has worked as a process engineer, currently leading the diffusion and deposition areas of the CNM production facilities. Isabel Gr`acia received the PhD degree in physics in 1993 from the Autonomous University of Barcelona, Spain, working on chemical sensors. She joined the National Microelectronics Center (CNM) working on photolithography, she is currently in the microsystems department working in the gas sensing field.
N. Sabat´e et al. / Sensors and Actuators A 123–124 (2005) 584–589 Carles Can´e received the BSc degree in telecommunication engineering in 1986 and the PhD degree in 1989 from the Universitat Polit`ecnica de Catalunya in Barcelona, Spain. Since 1990, he is permanent researcher at the National Microelectronics Center (CNM) in Barcelona. He is currently working in the fields of sensors and microsystems and their compatibility with standard CMOS technologies. M. Moreno was born in Barcelona, Spain. He received the degree in physics in 1989 from the University of Barcelona (UB), and the PhD degree in sciences in 1995 from the Polytechnic University of Catalonia (UPC), Spain. He has been an associate professor in the electronics department, UB, since 1997. He is involved in the design and test of
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infrared thermopile detectors and the test of integrated optical devices in silicon technology for DWDM applications. S. Marco is associate professor at the department d’Electronica of Universitat de Barcelona since 1995. He received degree in physics from the Universitat de Barcelona in 1988. In 1993, he received his PhD (honor award) degree from the departament de F´ısica Aplicada i Electr`onica, Universitat de Barcelona, for the development of a novel silicon sensor for in vivo measurements of the blood pressure. His current research interests are two-fold: chemical instrumentation based on intelligent signal processing and microsystem modeling.