Advances in Systems Science and Application (2016) Vol.16 No.3 76-93 Optical MEMS Sensor for measurement of Low Stress using Ptolemy II I. Mala Serene, Rajasekhara Babu M, Zachariah.C.Alex School of Computing Science and Engineering, VIT University, School of Electronics Engineering, VIT University, Vellore-632 014, Tamil Nadu, India Abstract Modeling and simulation plays vital role in the Micro Electro Mechanical Sys- tems (MEMS) field. Optical MEMS comprise of three domains namely optical, electrical and mechanical. The existing MEMS software for modeling is very ex- pensive. This cost of modeling software increases the design and development of optical MEMS sensors. This paper proposes the design and development of a novel optical read out mechanism. This mechanism is used to measure the maximum stress applied on the cantilever and its corresponding deflection of the cantilever. The experiments have been carried out using Ptolemy II software for design and simulation of MEMS optical sensors. Laser Actor, a Photo detector and Force Actor have been created using Ptolemy II. COMSOL software has been used to model cantilever. A comparative study has been done for cantilever with three modes of Eigen frequencies using COMSOL. The experimental result shows that the Parylene optical MEMS force sensor can sense less range of stress 0.0003 N/m to 0.272 N/m when compared to the Polyimide optical MEMS sensor. Keywords Microcantilever, Optical MEMS, Ptolemy, Sensor, Laserdiode, COM- SOL 1 Introduction Optical MEMS can be defined as micro devices with three functionalities like electrical, mechanical and optical at the same time and can be fabricated using batch processing techniques developed from microelectronic fabrication [1]. For integrated micro-systems composed of electrical, optical and mechanical compo- nents, the need to model large numbers of linear and non-linear components with sufficient accuracy to analyze cross-talk, noise and tolerance in an interactive en- vironment leads to the requirement of an efficient yet accurate mixed-technology simulation technique[2]. Stevan P. Levitan et al reported a computer aided de- sign tool for free-space optoelectronic systems and achieved system-level model- ing[3]. The advantages of Optical MEMS sensors over Electrical sensors are high adaptability in harsh environments high temperature, chemical corrosion, strong electromagnetic interference and high-energy radiation exposure[4]. Currently, no single CAD tool completely models the complexity of these mixed tools to model, simulate, and analyze each stage of the design[4] . Hence we have chosen Advances in Systems Science and Application (2016) Vol.16 No.3 77 Ptolemy as our framework for developing Optical MEMS based sensors. Ptolemy II is a system level design environment that supports heterogeneous modeling and design of concurrent systems. For simulating Optical MEMS devices it is essential to integrate tools with different models of computation to simulate the whole system [5]. The Ptolemy II software provides an infrastructure that allows designers explore and integrate the different models of computation [6]. It is sys- tem levels tool it. It does not provide the functionality for implementation-level simulation. But external tools based on different model of computation can be integrated into each domain and Ptolemy II can serve as semantic glue. In this present work, the simple component of MEMS, a microcantilever is used to sense the stress. It can be operated in two modes: static and dynamic mode. In static mode, the bending of microcantilever depends upon the force or stress on the cantilever. In dynamic mode, the resonant frequency of microcan- tilever changes when the mass added to it. The different read out mechanisms of the microcantilever are optical readout, piezoelectric and piezoresistive [7]. Many researchers reported that the microcantilever is made of materials like Sil- icon, Silicon nitride and PolySilicon [8-9]. But the fabrication cost of the silicon based cantilevers is expensive. So Silicon can be replaced by a polymer which offers a shining future for the development of chemical and biological sensors. The merits of the Polymer microcantilever over silicon microcantilever are low cost, more flexibility, transparency to visible UV, easily mouldable capability, improved bio-compatibility[10]. In this paper, Polyimide and Parylene are iden- tified as suitable polymers for microcantilevers given their low Youngs modulus, high planarity, chemical resistance and biocompatibility [11]. 2 Expermental and Simulation Laser source emits the light of wavelength (λ=850nanaometers).This laser beam is then passing through the two optical fibers separated apart axially. The can- tilever structure is fixed at one end and free at other end. A slit is connected at the free end of the cantilever moves between the two optical fibers when force is applied. The deflection of the beam will be in Y direction and by virtue of this deflection the output power detected at one of the fiber ends is varied continu- ously from maximum to minimum though the slit arrangement as shown below. This output power variation can be calibrated according to change in minute force variation over the cantilever which in turn will constitute an accurate Op- tical MEMS sensor. The light coming out of the second optical fiber is detected by the photo detector. 78 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Fig. 1 Flowchart of Individual Particle Update 3 Details of Software development In the present investigation, we have developed software codes for various actors that make an Optical MEMS Sensor in a software platform called Ptolemy. The various actors are Laser, a photo detector and a Force Actor. The individual figures of the various actors like Laser actor, Force Actor and Photodiode actor are given in the fig 2 (a)-(c) below: Fig. 2 (a) Laser Actor (b) Force Actor and (c) Photodiode Actor 3.1 Laser actor The abbreviation of Laser is Light Amplification by Stimulated Emission of Ra- diation. Laser operates on the principle called Stimulated emission. It was postu- lated by Albert Einstein before 1920. This is a semiconductor laser diode (GaAs) which emits light when we apply a forward biased across the p-n junction. The laser diode actor is modelled using the mathematical equations which include Internal Power of the Laser, External Power of the Laser and Reverse Leakage current of the diode. The External Power of the laser diode is given by Po = Pint n(n+ 1)2 (1) where n, Pint, Po is refractive index of the GaAs, internal Power and external power of the Laser. Advances in Systems Science and Application (2016) Vol.16 No.3 79 3.2 Force Actor The Force actor made of a cantilever beam and two optical fibers. The deflection of the cantilever is modelled using stoneys equation, spring constant and the three modes of the resonant frequency. The optical fiber actor is created using the power output detected at the second fiber and the loss of light due to the force applied on the cantilever. 3.2.1 Cantilever Beam Micro cantilever is a widely used component in micro electro mechanical system devices [12]. Cantilever is a type of beam fixed at one end and suspended freely at the other end and the beam is originally straight. The equation (2) is the Stoneys formula [13], which relates cantilever end deflection δ to applied stress σ: δ = 3σ(1− ν) E ( L t )2 (2) where δ,σ,L,t,E,ν are deflection, stress, length of the Cantilever beam, Youngs Modulus, Poissons ratio. The spring constant (k) of the cantilever beam is given by k = Ewt3 4L3 (3) where E, w, t and L are the Youngs modulus, width , thickness and length of the cantilever beam. The frequency at which a cantilever tends to oscillate in the absence of any force is the eigen frequency .The eigen frequency of a cantilever beam [14] can be find out from the optimized cantilever geometry for the L and t and density , for the two sensors is given by f = αn t L2 √ E ρ (4) αn = 1 4π √ ε λ2 n (5) where λn=1.8751, 4.6941, 7.8547 ...... 3.2.2 Optical Fibre We have designed two fibres with core diameter 2a= 175m coupled longitudinally such that the free end of the cantilever will move the slit vertically down between the fiber ends as force is applied on it. As a result the light coupled from fiber1 to fiber2 decreases gradually as the amount of force increases. There are two formulas used for calculating loss and power detected at the second at the second fiber is given below. Pout = Pin [ 1− ( w 2a )] (6) 80 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Loss = 20log10A (7) where A stands for ratio of Pout/Pin. Loss is a function f(w), where w is the cantilever deflection, which is numerically equal to w/2a, where 2a is the fiber diameter. 3.3 Detector Actor A photodiode is a semiconductor device, with a p-n junction and an intrinsic layer between p and n layers. The photo detector used is a reverse biased photodiode (PD) which converts the input optical power into the photo current (Ip). The following formulas are applied to create a detector actor: The Photocurrent is given by I = RPout (8) The Responsivity measure the electrical output per optical input of the photodi- ode is given by R = ηqλ hc (9) where η, q, h, c, λ are Internal quantum efficiency, Charge of electron, Plancks constant, Velocity of light in vacuum, Wavelength of light. Using the above actors, the Optical MEMS sensor model are created in the Ptolemy framework as shown in the fig 3 and fig 4. In the present work, two Force actors were created using the same geometrical parameters but the cantilever beam is made of different polymer materials like Polyimide and Parylene. The maximum stress sensed by the cantilever is measured for two different materials of the cantilever beam. The material properties of the Cantilever beam include the Youngs Modulus (E), Poisson ratio (ν) and density (ρ) is given in the table 1 : Table 1 Material Properties of the Cantilever beam Material Properties Polyimide Parylene Youngs Modulus (GPa) 3.2 2.8 Poison Ratio 0.42 0.4 Density (Kg/m3) 1300 1289 Advances in Systems Science and Application (2016) Vol.16 No.3 81 Fig. 3 Model of the Optical MEMS Sensor(Polyimide material) using Ptolemy II Fig. 4 Model of the Optical MEMS Sensor (Parylene material) using Ptolemy II 3.4 Model the MEMS Cantilever beam using COMSOL COMSOL Multiphysics version 5.0, a commercial FEM tool for MEMS was used to develop a finite element model [15] of the Polymer cantilevers. In the present work, the cantilever beam modelled using cost effective open source Ptolemy Software and its eigen frequency of the first three modes are compared with the rectangular beam of two different materials Polyimide and Parylene using the using Comsol Software. The free tetrahedral meshing is applied. 82 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Fig. 5 5(a) Model of the rectangular Cantilever Beam using COMSOL, Fig 5(b) Mesh model of the Cantilever Beam 4 Results and Discussion 4.1 Optical MEMS Sensor Using Ptolemy In the optical MEMS sensor model, the Laser diode is modelled the fig.6 (a) and fig 7(a) represents the output power of the Laser and Output current of Laser Diode. The output power increases linearly with the applied current, when the applied current is larger than the threshold current. When the force applied on the cantilever, the cantilever bends and light passing from the optical fiber 1 to optical 2 is blocked based on the amount of force applied. The range of the force applied and the deflection of the cantilever is recorded for the two Optical Sensors are tabulated in table 2. In fig.6 (b)-(d) and fig. 7(b)-(d), the sample of the force applied in the cantilever and corresponding deflection of the cantilever is recorded, then the deflected Laser Power is converted into current by the photodiode and plotted in the graph. 4.2 Comsol Cantilever Beam Result The results of the first three modes of the cantilever beam of two materials modeled using COMSOL software are shown in the fig 8(a)-(f). The analytical values of the eigen frequencies are compared with Eigen frequencies of the two cantilevers modelled using Comsol are tabulated in the table 3 and the same is represented using bar chart is shown in fig 9(a)-(b). 5 Optimization of the Geometrical Parameters The different lengths (200 µm, 300 µm, 400 µm, 450 µm, 500 µm) of the two different materials of the cantilever are kept constant and the thickness of the cantilever is varied from 0.5 µm to 3.0 µm. For each length and the maximum stress/force is recorded for each simulation is shown in table 4 and table 6 and the results are plotted is shown in figure 10. (a)-(f). For different thickness (t=0.5 µm, 1.0 µm,1.5 µm,2.0 µm,2.5 µm and 3.0 µm), the length is varied from 200 µm to 500 µm for each thickness and the maximum stress/force is recorded for each Advances in Systems Science and Application (2016) Vol.16 No.3 83 Fig. 6 Simulation result of the Polyimide Optical MEMS Force Sensor (a) Output Power of Laser diode (b), (c) and (d) Deflected Laser Power Vs Output Current at the photodetector Fig. 7 Simulation result of the Parylene Optical MEMS sensor1 (a) Output Power of Laser diode (b), (c) and (d) Deflected Laser Power Vs Output Current at the photo detector 84 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Table 2 Stress/Force Applied vs. Cantilever deflection of the Two Optical MEMS Sensor using Ptolemy Polyimide Optical MEMS Sensor Parylene Optical MEMS Sensor Stress/force Applied Cantilever Stress/force Applied Cantilever (N/m) Deflection (N/m) Deflection 0.0005 2.72E-07 0.0003 1.93E-07 0.05 2.72E-05 0.05 3.21E-05 0.2 1.09E-04 0.1 6.43E-05 0.321 1.75E-04 0.272 1.75E-04 0.322 1.75E-04 0.273 1.76E-04 Fig. 8 Eigen frequency of the result of the rectangular Cantilever beam1 (a)-(c) and (d)-(f)Cantilever beam2 using Comsol Table 3 Comparison of Eigen frequency values usingPtolemy andComsol software Modes of Polyimide Cantilever Parylene Cantilever Eigen frequency Ptolemy(KHz) COMSOL(KHz) Ptolemy(KHz) COMSOL(KHz) 1 0.5071 0.5074 0.4764 0.4678 2 3.178 3.2392 2.986 3.037 3 8.899 9.1789 8.3597 8.602 Advances in Systems Science and Application (2016) Vol.16 No.3 85 Fig. 9 (a-b): Comparison of the modes Vs. Eigen frequency of the Polyimide and Parylene Cantilever beam using Ptolemy and COMSOL Table 4 Different length of the Cantilever beam Vs. Maximum Stress/force applied at constant thickness of the Polyimide Optical MEMS Force Sensor Polyimide Optical MEMS Sensor Cantilever Max. stress/force applied (N/m) Thickness (µm) L=200 µm L=300 µm L=400 µm L=450 µm L=500 µm 0.5 2.01 0.88 0.502 0.397 0.321 1 8.045 3.57 2.011 1.588 1.287 1.5 18.1 8.04 4.52 3.575 2.89 2 32.1 14.29 8.04 6.35 5.14 2.5 50 22.3 12.56 9.93 8.04 3 72.3 32.17 18.08 14.3 11.58 Table 5 Different thickness of the Cantilever beam Vs. Maximum stress/force applied at constant Length of the Polyimide Optical MEMS Sensor Polyimide Optical MEMS Sensor Cantilever Max. stress/force applied (N/m) Length (µm) t=0.5µm t=1.0µm t=1.5µm t=2.0µm t=2.5µm t=3.0µm 200µm 2.01 8.045 18.1 32.1 50 72.3 300µm 0.88 3.57 8.04 14.29 22.3 32.17 400µm 0.502 2.011 4.52 8.04 12.56 18.08 450µm 0.397 1.588 3.575 6.35 9.93 8.04 500µm 0.321 1.287 2.89 5.14 8.04 11.58 86 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Fig. 10 (a)-(f) Different length of the Cantilever beam Vs Maximum stress/force applied of the Polyimide Optical MEMS Sensor at constant thickness Advances in Systems Science and Application (2016) Vol.16 No.3 87 Fig. 11 (a)-(f) Different length of the Cantilever beam Vs Maximum stress/force applied of the Polyimide Optical MEMS Sensor at constant thickness 88 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Fig. 12 (a)-(f) Different thickness of the Cantilever beam Vs. Maximum stress/force applied of the Polyimide Optical MEMS Sensor at constant Length Advances in Systems Science and Application (2016) Vol.16 No.3 89 Fig. 13 (a)-(f) Different length of the Cantilever beam Vs. Maximum stress/force Applied at constant thickness of the Parylene Optical MEMS Sensor 90 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... Fig. 14 (a)-(e) Different thickness of the Cantilever beam Vs. Maximum stress/Force Applied at constant length of the Parylene Optical MEMS Force Sensor Advances in Systems Science and Application (2016) Vol.16 No.3 91 Table 6 Different length of the Cantilever beam Vs. Maximum stress / force applied at constant thickness of the Parylene Optical MEMS Sensor Parylene Optical MEMS Sensor Cantilever Max. stress/force applied (N/m) Thickness (µm) L=200µm L=300µm L=400µm L=450µm L=500µm 0.5 1.7 0.755 0.425 0.336 0.272 1 6.78 3.02 1.7 1.342 1.088 1.5 15.3 6.8 3.824 3.02 2.448 2 27.2 12.07 6.8 5.37 4.35 2.5 42.5 18.84 10.63 8.39 6.8 3 61.1 27.16 15.3 12.09 9.79 Table 7 Different thickness of the Cantilever beam Vs. Maximum stress/force applied at constant Length of the Parylene Optical MEMS Sensor Parylene Optical MEMS Sensor Cantilever Max. stress/force applied (N/m) Length (µm) t=0.5µm t=1.0µm t=1.5µm t=2.0µm t=2.5µm t=3.0µm 200 µm 1.7 6.78 15.3 27.2 42.5 61.1 300 µm 0.755 3.02 6.8 12.07 18.84 27.16 400 µm 0.425 1.7 3.824 6.8 10.63 15.3 450 µm 0.336 1.342 3.02 5.37 8.39 12.09 500 µm 0.272 1.088 2.448 4.35 6.8 9.79 simulation is shown in table 5 and table 7 and the results are plotted is shown in figure 11. (a)-(f). From the recorded values, low stress/force is achieved at the length 500 µm and the thickness is 0.5m for both the sensors. 6 Conclusion Different actors like Laser actor, Force actor and photodetector have been devel- oped and added in Ptolemy framework. The physical functioning of each compo- nent of the Optical MEMS Force Sensor device has been simulated using these actors. The results have been presented. The two optical MEMS force sensor are simulated in Ptolemy II. The Parylene Optical MEMS sensor can sense the low stress/force in the range of 0.0003 N/m to 0.272 N/m with the present fiber optic setup. The Eigen frequency of the two cantilever beam is modelled using open source Ptolemy II and the three modes of Eigen frequencies are compared with 92 I. Mala Serene, Rajasekhara Babu M and Zachariah.C.Alex:Optical MEMS Sensor for ... the results of the COMSOL. The low stress/force is measured for the optimized length (500 µm) and the thickness (0.5 µm) is found by varying the thickness and length of the Cantilever using Ptolemy. 7 Acknowledgement We are thankful for support by the NPMASS MEMS Design Centre, SENSE, VIT University for doing this project. References [1] E. Ollier(2002), “Optical MEMS Devices Based on Moving Waveguides”, IEEE J.Sel. Topics Quantum Elect, vol. 8, no. 1, pp. 155C62. [2] Selvarajan, A and Pattnaik, Prasant Kumar and Badrinarayana, T and S- rinivas, T (2006), “A comparative study of MOEM pressure sensors using MZI, DC, and racetrack resonator IO structures”, In Proceedings SPIE In- ternational Conference on Smart Structures and Materials 2006:Smart Elec- tronics, MEMS, BioMEMS, and Nanotechnology, pages Vol.61726, 61721A, San Diego, California, USA. 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Cuenot(2008), “ Finite Element Modelling of Micro- cantilevers Used as Chemical Sensors”, Recent Advances in Modelling and Simulation,pp.207-222. Corresponding author Rajasekhara Babu M can be contacted at: rajababu.m1@gmail.com