SU-8 waveguiding interferometric micro-sensor for gage pressure measurement

Sensors and Actuators A 135 (2007) 179–184

SU-8 waveguiding interferometric micro-sensor for gage pressure measurement N. Pelletier a,∗ , B. Bˆeche b , N. Tahani a , J. Zyss c , L. Camberlein a , E. Gaviot a a

Laboratoire d’Acoustique de l’Universit´e du Maine, Micro Cap Ouest, LAUM UMR CNRS 6613, 72085 Le Mans, France b Institut de Physique, Universit´ e de Rennes1, GMCM-PALMS UMR CNRS 6626-6627, 35042 Rennes, France c Laboratoire de Photonique Quantique et Mol´ eculaire, LPQM UMR CNRS 8537, ENS-Cachan, 94235 Cachan, France Received 2 February 2006; received in revised form 14 June 2006; accepted 5 July 2006 Available online 5 September 2006

Abstract The authors present a successful modeling, realization and characterization of a new micro-sensor based on a convenient optical principle, namely an integrated Mach–Zehnder interferometer (MZI). This MZI device is designed with a view to measuring pressure disturbances due to optical path variations. Such a system is arranged in order to work in intensity modulation scheme. Moreover, the MZI is made up of straight and bent rib optical waveguides composed of SU-8 polymer. The mainstay of the device is based on differential measurements performed by a sensing arm arranged with a micromachined membrane and actuated by a given pressure disturbance, while the second arm of the interferometer is considered as a reference one. The main parameters of each element are given by way of two modeling approaches: an optical modeling with a semi-vectorial finite difference method together with a conformal transformation, and a mechanical modeling with a finite-element method associated to the mechanical theory of membranes. So, as the pressure to be measured is applied upon the diaphragm, an optical path variation of the acting arm is induced. After the combination of both signals, the variation at the output of the system is measured. A prototype is characterized by way of a micro-optical injection bench specifically designed to allow an efficient end-fire coupling into the waveguides. © 2006 Elsevier B.V. All rights reserved. Keywords: Microtechnologies; Integrated optics; SU-8 polymer; Optical sensor; Mach–Zehnder interferometer; Micromachined membrane; Gage pressure sensor

1. Introduction For many years, there has been a growing interest in the micro-opto-electro-mechanical systems (MOEMS), with the development of sundry micro-devices in respectively telecommunications and sensors fields [1,2]. The introduction of polymers in MOEMS fabrication presents several advantages, such as large volume and low-cost production [3–5]. Focusing on micro-optical applications, the polymer SU-8, featuring a high transparency above 400 nm, exhibits low propagation losses close to 1.5 dB cm−1 as used in waveguiding; so, SU-8 comes up as one of the most attractive polymers to develop low-cost MOEMS [6,7]. Furthermore, interferometer-based sensors add substantial advantages to miniaturization due to their high sensitivity [8–10]. As an example, the Mach–Zehnder structure presents a specific interest in the sensor research and industry

∗

Corresponding author. Fax: +33 243833794. E-mail address: [email protected] (N. Pelletier).

0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.07.012

given that it is well controlled in fabrication, and can be quite immune from manufacturing errors. For example, the refractiveindex measurements of gases or liquids can be performed by way of this principle, according to the interaction between the medium under analysis and the evanescent electromagnetic field [11]. Moreover, chemical and biochemical reactions can be monitored by means of this structure by building up proteins multilayers on the sensor surface [12]. This paper deals with an integrated two-wave interferometer (Mach–Zehnder) on silicon substrate. Our device is composed of straight and bent rib waveguides fabricated on the SU-8 polymer. Moreover, such a micro-sensor works with a modulation scheme in order to measure a given surrounding pressure disturbance by way of a bulk-micromachined membrane under the sensing arm of the interferometer. Hence, a phase-shift is produced between the signals from the respectively sensing and reference arms that induces a variation of the optical intensity at the output of the device. Firstly, a description of the system will be developed so as to introduce the basic principle of measurement with the presentation of the main elements making up

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the sensor. Secondly, the modeling aspects will be presented, especially optical and mechanical studies, in order to highlight the steps to be followed to improve and validate the confinement of optical modes and their propagation in the whole structure, allowing us to design the optimized micromachined membrane to make the sensor more sensitive. Finally, the realization and the characterization of the system will be introduced, showing a real improvement in the field of gage pressure measurement. 2. Presentation of the micro-system The micro-sensor presented here consists of an integrated Mach–Zehnder interferometer (MZI) based on single-mode TE00 –TM00 rib waveguides associated with a micromachined membrane located under the sensing arm of the MZI. The basic principle of this device relies on the intensity modulation observed at the output of the MZI, due to the combination of optical modes proceeded through both respectively sensing and reference arms, as the first one is subjected to a given pressure disturbance. Consequently, according to the well-known theory of the interferometry, the intensity modulation scheme should be characterized by an output signal (intensity value at the end of the MZI). The intensity signal in both arms behaves like a cosine; hence detection is possible as a consequence of the phase variation. As an illustration, Fig. 1 represents a schematic crosssectional view of the system. The device is mounted onto a (1 0 0)-oriented silicon substrate. Two silica (SiO2 ) layers

(1.2 ␮m each) are grown on both sides of the Si wafer. On the lower side, an appropriate etching allows us to realize of a sensing thin diaphragm. The straight and bent waveguides are placed on the upper side. Their core is made of SU-8 (Micro Chem. Inc.—negative photoresist formulation 2002) and so does the silica for cladding, and are arranged so as to realize a MZ interferometer. For a differential behavior, the membrane is centered under the sensing arm with a view to taking advantage of a lengthened optical path stemming from the pressure applied on the bottom side of the device. 3. Optical modeling The main waveguiding properties of these integrated optical waveguides structures, such as dimensions and effective index, have been studied by way of computing methods, in order to achieve a strong confinement of both optical modes TE00 and TM00 in the, respectively, straight and bent waveguides. Firstly, a numerical method based on semi-vectorial finite difference (herein after SVFD) simulation is used in order to optimize the opto-geometric parameters of the straight waveguides (i.e. refractive indexes, dimensions). The SVFD method is detailed in Ref. [13] and requires an apt discretization of the transverse section of the waveguide. So, the determining of eigenvalues and eigenvectors relative to the specific inhomogeneous vector wave equation, for both TE and TM modes, leads to optimize and validate the structural parameters; namely the width (wSU-8 ), the height (hSU-8 ) of the rib, the thickness of the different layers and the effective index of the design. The opto-geometric parameters of each material making up the waveguide and necessary to the simulation are summarized in Table 1. As an example, Fig. 2 represents the single-mode TE00 optical distribution for a specific Si/SiO2 /SU-8 rib waveguide with an effective index value evaluated by way of the SVFD method to neffTE00 = 1.5182. Secondly, the bent waveguides properties are worked out in order to improve the waveguiding of the optical modes in such structures [14,15]. Indeed, the bent waveguides may exhibit substantial radiation losses due to distortions of the optical field inherently occurring as guided waves travel through a bend increasing then the bending losses. In fact, the main factor which plays a key role in the quantity of loss is the radius of curvature of the bent rib waveguide. Then, to minimize such losses, an appropriate analysis is carried out while using the conformal transformation theory [16,17]. The latter one is used to calculate, in the complex plane, the index profile of the waveguide bend as an equivalent index profile of a straight waveguide. So, the transformation of a bend in the complex plane (z = x + jy) by the Table 1 Opto-geometric parameters of the structure necessary to the SVFD simulation

Fig. 1. Schematic cross-sectional view of the micro-sensor.

Width, w (␮m) Thickness, h (␮m) Refractive, n index Operating wavelength, λ(nm)

SiO2

SU-8

1.2 1.46

4 1 1.56 980

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stress in the material, due to the different thermal dilation coefficient between the silicon and the silica layer can be neglected. Indeed, as the principle of this pressure sensor consists of a differential measurement between both arms which are together under this permanent stress, this quantity is naturally screened out. Consider now the establishing of a relationship between the pressure and the deflection of the membrane; a straightforward way consists in solving the following equation of rectangular clamped rigidly plates [18]: D∇ 4 u(x1 , x2 , t) = P

(2)

with D the rigidity of the plate to bending: D= Fig. 2. TE00 optical mode, calculated by SVFD, on Si/SiO2 /SU-8 rib waveguides.

function w(z) = R2 ln (z/R2 ) brings out the consideration of an equivalent rib waveguide in a new complex plan (w = u + jv). So, if n(ρ) is the refractive index of the bent waveguide, R2 the radius of curvature and ρ the transverse coordinate, one can infer from the conformal transformation:     u u n(u) = n(ρ) exp ≈ n(ρ) 1 + (1) R2 R2 Thus, the bent waveguide with an index profile n(ρ) behaves like a straight waveguide with an index profile n(u). In this way, a numerical simulation has been carried out with a view to determining the best radius of curvature according to a minimum of loss. At first, this analysis is based on the determination of the effective index of the rib waveguide using the effective index method. Then, considering these results, a discretization according to the transverse section is carried out by way of the multi-layer theory [2]. Thus, an optimal radius of curvature of 40 mm is confirmed so as to reduce any optical loss and improve the optical confinement and propagation of the mode according to an energy confinement factor higher than 0.9. 4. Mechanical modeling As it is shown in the presentation of the micro-system, the main active element of the sensor is a micromachined membrane located only under one arm of the Mach–Zehnder interferometer, consequently the sensing one. Thus, the mechanical properties of this membrane have been studied so as to optimize the behavior of such a gage pressure sensor. In order to obtain simple analytic relations regarding applied pressure expressed as a function of a given deflection of the diaphragm, relevant assumptions have been established: according to the microtechnology processes and especially the anisotropic etching of silicon, the membrane is shaped as a rectangle, a in width, and b in length, with its four edges clamped rigidly. Moreover, the thickness e of the membrane is insignificant compared to the other dimensions. Hence, the permanent

Ee3 12(1 − ν2 )

(3)

where 4 is the bilaplacian, P the applied pressure, u(x1 , x2 , t) the transverse movement, E the Young’s modulus (E = 131 GPa for the silicon), e the thickness of the plate, and ν is the Poisson’s ratio (ν = 0.266 for the silicon). So, the value of the pressure P can be expected as a non-linear function of the deflection h of the membrane:     E e e3 σ0 e E 3 h + h + C∗ 2 h (4) P =C 4 2 4 1−νa 1 − ν 12αa a σ 0 is the initial stress in the silicon, and the three parameters α, C and C* depend of both Poisson and b/a ratios. To be more accurate, the third term of this relation was proved to be negligible due to the absence of permanent stress in the material. Furthermore, the coefficient α is given for instance by Timoshenko and Woinowsky-Krieger [18], with a value equal to 2.55 × 10−3 for a specified ratio b/a = 2. The parameter C is calculated by Tabata et al. [19] with a value equal to 10.94 considering our design (b/a = 2). From such cases, a numerical simulation based on the finite element method has been carried out for a given deformation upon a thin micromachined membrane with specific parameters: 1 mm in width, 2 mm in length and 25 ␮m in thickness. As can be observed on Fig. 3, a simulated absolute pressure value equal to 3 × 105 Pa being applied on the bottom of the system, the deflection induced on the diaphragm averages 25 ␮m. 5. Realization and characterization The modeling relative to both aspects of the micro-sensor, namely optical and mechanical considerations, was carried out, and each stage of the microtechnology process developed and validated in a clean room. To this end, two main steps have been taken in order to realize the micro-system. Firstly, the fabrication of the micromachined membrane at the bottom of the Si substrate is carried out by means of reactive ion etching (RIE) and wet anisotropic etching methods. Secondly, the MZI and its associated rib waveguide are fabricated on top of the substrate by way of SU-8 UV photolithography. Considering a (1 0 0)-oriented silicon substrate 260 ␮m in thickness, a SiO2 layer cladding is obtained by thermal oxida-

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Fig. 3. Example of simulation of the deformation upon a micromachined membrane.

tion on both sides of the wafer, yielding a 1.2 ␮m thickness. At first, the process consists on defining an opening in the SiO2 layer of the lower side of the wafer by RIE. Thus, the silica layer is considered as a mask while the pattern is etched with a potassium hydroxide KOH solution (30 wt.%). The etching process of such structures is highly dependent on the considered crystal orientation of the silicon wafer. Si exhibits planes being etched preferently (i.e. faster) such as the (1 0 0), instead of the (1 1 1), hence a 25 ␮m thick membrane can be achieved by wet etching of the (1 0 0) plane after a 9 h time at 60 ◦ C temperature. On the upper side of the stack, a 1 ␮m thick SU-8 layer is deposited by photolithography techniques. Following to a spin coating step, the soft baking (SB) is performed in order to remove solvents and get the film dried. SB is carried out in conventional hot plates in a two-step process. The waveguide pattern relative to the MZI is so patterned on the SU-8 layer by way of UV-lithography. After a post exposure bake carried out in the same way as the SB treatment, a development process allows us to obtain both straight and bent rib waveguides 4 ␮m in width featuring a radius of curvature of 40 mm. A hard bake (2 h at 200 ◦ C in drying oven) is carried out so as to stabilize the material, to ensure further dimensional stability. Fig. 4 shows a fabricated micro-chip including three devices with their respective micromachined membranes and MZI structures. After having cleaved both input and output ends of the waveguide, the latter is mounted within an optical bench so as to test

Fig. 4. Photograph of silicon wafer including three pressure sensors with micromachined membranes (highlighted on the lower side) and Mach–Zehnder structures.

the performance of the design. As shown in Fig. 5, this microoptical injection bench consists of a laser source operating at 980 nm (Opton Laser International) and associated objectives and polarizers (Newport Inc.). Hence, the excitation of the fundamental mode of the structure is allowed. The end of this bench is fitted with a highly sensitive powermeter (Ophir Optronics Inc.—photodiode allowing a power range until 3 W with supplied filter, resolution 1 nW, response time 0.2 s, automatic background subtraction) and a video system with CCD camera to visualize and quantify the set up output signal (Jai Inc.—625 lines, 25 frames/s, resolution 752 × 582 pixels, sensitivity 0.02 Lux) So, the end-fire coupling into the device allows us to analyze the sensor’s behavior subjected to a given gage pressure value. As an illustration, a zoom in Fig. 5 represents a relevant part of the optical bench showing the injection of light into the MZI structure to be calibrated by means of two 40× microscope objectives, and subjected to a pressure disturbance applied on the lower side. As shown before, the principle of the sensor relies on a modulation scheme induced by the application of a controlled pressure that yields a mechanical deformation of the membrane, the result being a phase change between the two optical modes of both sensing and reference arms. So, the end-fire coupling between the laser source and the device is carried out considering an

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Fig. 5. Schematic diagram of the micro-optical bench set up for the characterization of the pressure sensor and photograph of the pressure sensor between two 40× microscope objectives and subjected to a pressure disturbance.

Fig. 6. Optical intensity values observed at the output of the Mach–Zehnder Interferometer vs. applied gage pressure: optical response of the sensor considering a 80 mA stabilized laser operating current measured at room temperature (21 ± 0.1 ◦ C).

accurate positioning of the micro-sensor, as a relative pressure disturbance is imposed. For example, variation of the optical response at the output of the device can be observed in Fig. 6. The two-wave interferometric behavior of the Mach–Zehnder interferometer is highlighted in the graph plotted, and as a maximum applied pressure averaging 2 × 105 Pa is imposed, a π radians phase-shift on the optical response can be observed. Despite stringent difficulties for not to tear nor damage the membrane under pressure, such a series of measurements point out the nonlinear behavior of the sensor, according to the nonlinear variation of the intensity at the output of the device versus the gage pressure applied. 6. Conclusion In this paper, an integrated SU-8 based micro-sensor is presented. The modeling of the optical and mechanical principles is reviewed, considering an appropriate design for gage pressure measurement in a range averaging 2 × 105 Pa above

atmospheric pressure. Such a new device relies on miniaturized SU-8 Mach–Zehnder interferometer, and consequently works in a modulation scheme induced by the recombination of both optical modes from respectively, a sensing arm actuated by a micromachined membrane, and a reference one. The generated phase-shift entails a relevant change of the optical intensity at the output of the sensor. So as to optimize the design, we have modeled the optical and mechanical aspects of our system. First, the propagation of single-mode TE00 –TM00 in the whole structure making up the interferometer, straight and bent rib waveguides composed of SU-8 polymer, have been optimized by way of, respectively, semi-vectorial finite difference method (SVFD) and conformal transformation. Secondly, the mechanical properties of the sensing element connecting the pressure disturbance to the acting arm, namely the micromachined membrane, has been studied and validated in order to improve and optimize the response of the sensor. Our first results clearly show that such a system introduces a significant deflection of the membrane versus the applied pressure. It is expected that such a new approach regarding pressure measurement with be instrumental in many an industrial field in the next coming future. References [1] D. Marcuse, Theory of Dielectric Optical Waveguides, Academic press, 1974. [2] T. Tamir, Guided-wave Optoelectronics, Springer-Verlag, 1990. [3] A. Donval, E. Toussaere, R. Hierle, J. Zyss, Polarization insensitive electrooptic polymer modulator, J. Appl. Phys. 87-7 (2000) 3258–3262. [4] P. Labbe, A. Donval, R. Hierle, E. Toussaere, J. Zyss, Electro-optic polymer based devices and technology for optical telecommunication, C.R. Physique 3 Acad´emie des Sciences-Fascicule 4-3 (2002) 543– 554. [5] J.S. Kim, J.W. Kang, J.J. Kim, Simple and low cost fabrication of thermally stable polymeric multi-mode waveguides using a UV-curable epoxy, Jpn. J. Appl. Phys. 42 (2003) 1277–1279. [6] B. Bˆeche, N. Pelletier, E. Gaviot, J. Zyss, Single-mode TE00 –TM00 optical waveguides on SU-8 polymer, Opt. Commun. 230 (2004) 91–94.

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Biographies N. Pelletier was born in Poitiers, France, on 9th December 1980. In 2003, he graduated from the “Ecole Nationale Sup´erieure d’Ing´enieurs du Mans” (ENSIM), France, and received a MS Degree in “Micro-sensors and Photonics”. He is currently working as a PhD Student at the Laboratory of Acoustics at the University of Maine (LAUM, UMR CNRS 6613), and is involved in the program “Micro Cap Ouest”. His current research interests consist in the design, modelling and realization of microsystems based on optical effects for heat flow rate and pressure measurements. B. Bˆeche was born in 1970, in Lons-Le-Saunier, France. He worked on III–V semiconductors integrated optical components like tunable filters, electroop-

tic modulators in the Optics Laboratory of Besanc¸on and received in 1999 the PhD from the University of Franche-Comt´e, France. He has worked at Basics Research Labs of NTT, Tokyo in 1998, before being appointed Lecturer in Physics with the LAUM (Laboratory of Acoustics at the University of Maine): then, he worked on the Microtechnology development program “Micro Cap Ouest”. From 2004 to 2006, and was involved in the development and characterization of versatile polymer components for MOEMS applications within the Plasma and Thin Films Laboratory at the Institute of Materials in Nantes (IMN, UMR CNRS 6502). He was appointed as a Professor in 2006, and is currently working in the development of biosensors at the Physics Institute GMCM-PALMS, UMR CNRS 6626-6627 in Rennes, France. N. Tahani was born in Oujda, Morocco, in 1961. She graduated from Nancy University, France, with a degree in Mechanical Engineering in 1985. She received her Doctorat de 3`eme cycle degree in Mechanical Engineering in 1988, with the “Institut National Polytechnique de Loraine”, before being appointed Lecturer with the Universit´e du Maine. Her current interest, with the LAUM “Laboratoire d’Acoustique de l’Universit´e du Maine”, is in the behaviour of acoustic waves, and the development of dedicated microsystems, especially gyrometers. J. Zyss was born in 1950 in Neuilly/s Seine, France. He graduated from the “Ecole Polytechnique” and Paris VI University. He joined France Telecom Research Center in 1975 and obtained a Science Doctorate in 1982 in Molecular Nonlinear Optics. His contributions encompass molecular engineering for nonlinear optics, applications of molecular crystals to nonlinear optics, polymer-based optoelectronics and the control of molecular order by coherent control techniques. He is currently working at the “Ecole Normale Sup´erieure de Cachan”, as a Professor of Physics. He is also the head of the CNRS “Molecular Quantum Photonics” Laboratory, that he founded in 1998. L. Camberlein was born in Hazebrouck, France on 4th January 1969. He received his MS degree in “Electronique, Electrotechnique and Automatisme” in 1991. He joined the “Institut d’Electronique and de Micro´electronique du Nord” (UMR-CNRS 9929) in 1992, where he received his PhD relative to the design of new microradiometers. He worked as an Engineer with the Maritime Safety Agency, before joining the Micro Cap Ouest Institute in 2002, as a Scientist Engineer in charge of the equipment of microtechnologies. His current interest is in the development of thermoelectric and optical microsystems. E. Gaviot was born in Melun, France, in 1957. He received a MS degree in 1979 from the Orl´eans University, France. He joined the Lille University in 1981, and received his PhD in Electronics in 1985 for his work regarding Signal Processing and Entropy Generation. Working with the “Institut d’Electronique et de Micro´electronique du Nord” (UMR-CNRS 9929), he was appointed as a Lecturer in 1988, HDR in 1998, and a Professor in 1999. Since 1986 he has been involved in the design of thermal transducers, new sensors, and microsystems based on thermoelectric effects. Since 2000, he is in charge of a specific state program (Micro Cap Ouest, CER M 18024) devoted to the development of new microsystems.