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Comb IDE Patterned Piezoelectric MEMS Cantilever for Sensing Applications Neha Mukhija, Nidhi Paliwal and Deepak Bhatia Department of Electronics Engineering, Rajasthan Technical University Kota-324010,India. Email [email protected] 978-1-4673-7231-2/15/$31.00 ©2015 IEEE

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Comb IDE Patterned Piezoelectric MEMS Cantilever for Sensing Applications

Neha Mukhija, Nidhi Paliwal and Deepak Bhatia

Department of Electronics Engineering, Rajasthan Technical University

Kota-324010,India.

Email [email protected]

978-1-4673-7231-2/15/$31.00 ©2015 IEEE

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Abstract---Power scavenging is a method of extracting energy from the surrounding and converting it into electrical energy. In this paper two element array of dimensions 30×4 µm2 with IDE pattern on top is proposed. An insulation layer of SiO2 is sandwiched in between Si and PZT layer. This layer provides the bow control of cantilever beam. Cantilever array is simulated using COMSOL multiphysics. The effect of various parameters on the performance of cantilever beam is studied. Effect of thickness of PZT layer and length of the cantilever on displacement and stress generated across the cantilever is studied.

Keywords—IDE;bow control; PZT.

I. INTRODUCTION

Batteries have been the foundation of power for most wireless, medical and mobile applications. Now, with chronic computing requirement in the field of medical, wireless sensors, embedded systems and low power mechanical systems like MEMS devices, an alternate source of power is essential. With inadequate capability of power sources and need for providing power for life span, there is obligation for self-powered devices. To operate these devices, the electric power needed can be acquired from thermal, acoustic, wind, mechanical or wave energies existing in the surroundings. Consequently the method of translating existing mechanical energy in surroundings into utilizable electrical energy is called energy scavenging or power harvesting. Devices which are energized by energy scavengers offer essential information and can be placed at the location which is not accessible later.

A. Sources of Energy

Different sources of energy harvester are available viz. thermoelectric, wind turbines, mechanical vibrations, piezoelectric, photovoltaic cells etc [1]. Among all these sources piezoelectric energy harvesters have maximum power density about 330µW/cm3. So, in this paper we have used piezoelectric energy scavengers. When a device is forced to vibrate, it starts moving [3]; this movement is converted to electrical energy using three methods-

(1) Piezoelectric (2) Electrostatic (3) Electromagnetic

Piezoelectric materials convert mechanical energy into electrical energy. This property of piezoelectric material is believed to develop piezoelectric scavengers. There are wide varieties of piezoelectric material available and according to their property of sensing and harvesting it is chosen. Lead Zirconate Titanate (PZT) is preferred as it shows the high efficiency of mechanical to electrical energy conversion.

For this conversion to be more proficient IDE pattern of electrodes is used [2].

B. Interdigitated Electrode Pattern

It is a comb like pattern of electrode. It is designed to boost up the energy scavenged from cantilever beam.

Fig.1. Comb like pattern of IDE.

There are two piezoelectric modes used in piezoelectric transducers- d31 and d33 mode.

These are distinguished as if E-field direction is perpendicular to input strain direction it is d31 mode and if E-field direction is parallel to input strain direction then it is d33

mode. Generated open circuit voltage of a d33 type device is much higher than that of d31 type device. d31 type mode has top and bottom electrode and d33 type mode eliminates the need for bottom electrode. IDE pattern has d33 mode of operation so in this paper IDE pattern is utilized.

To enhance the capability of single cantilever beam array is utilized [4], [5]. In this paper two element array of piezoelectric cantilever beam with IDE pattern is utilized. An insulation layer of SiO2 is inserted in between PZT and the structural steel material.

Insulating SiO2 layer is used to control the bow of the cantilever beam due to its property of residual stress. It act as a buffer layer.

Geometric Structure

A cantilever beam of size 30×4 µm2 dimensions is simulated. At the top of the beam an IDE pattern is used as shown in the figure below-

Fig.2. side view of the cantilever beam

Fig.3. Geometric structure of two element array.

As shown in the figure above array is comprised of four layers. First bottom layer of base material structural steel is of 1 µm thickness; above it an insulation layer of SiO2 of thickness 0.4 µm is added. Above the insulation layer PZT-5H material layer of thickness 1.5 µm is inserted. And at the top electrode of aluminium in finger like pattern of thickness

Al

PZT-5H

SiO2

Si

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0.2µm is added. The structure is simulated using COMSOL multiphysics.

A. Modelling Equations

As already mentioned that piezoelectric material has two modes of operation d31 and d33. The following equations show relation between stress ηyy or strain y3 generated and electric field Ej or voltage V3j.

(1)

(2)

Where d3j mode in V/m, g3j is piezoelectric constant in Vm/N, lj is piezoelectric thickness tPZT. d3j is directly proportional to piezoelectric constant g3j through dielectric coefficient of PZT.

Stiffness constant of the cantilever beam fixed at one end is given by-

(3)

Where E is young’s modulus, d is width, l is length and t is thickness of the beam.

Resonance frequency of the cantilever beam is-

(4)

Here ‘m’ is mass.

Displacement of the cantilever is given by-

(5)

Here ‘‘ld’ is surface area of cantilever and ‘q’ is accumulated charge which is given as-

(6)

Stored energy of the cantilever is presented as-

(7)

II. SIMULATION AND RESULTS

Proposed array structure was simulated using FEM (Finite Element Method) with the help of software tool COMSOL Multiphysics. Different simulation studies were carried out to observe the effect of various parameters on the harvester performance. Stationary solver analysis was carried out primarily. In the first step the effect of piezoelectric thickness on the stress was observed while keeping other layers parameters constant. From fig.4 it is observed that as the thickness of PZT increase stress generated also increases up to 1.5µm and then decreases with increase in PZT thickness. It shows the maximum stress at 1.5µm so the optimized value of PZT thickness is 1.5µm.

In next step variation of stress with respect to applied force is observed.

Fig.4. Plot between generated stress and piezoelectric thickness for applied force 15µN.

Fig.5. plot between generated stress and force applied at the tip of the cantilever for piezoelectric thickness 1.5µm.

It is illustrated from fig.5 for the PZT thickness of 1.5µm as the force at the tip of cantilever is increased generated stress across the cantilever also increases linearly. Now in subsequent step we draw plot between length and displacement.

As shown in fig.6 as length of cantilever beam increases displacement of the cantilever increases.

Fig.6. Graph between displacement and cantilever length for PZT thickness 1.5µm and force applied is 15µN

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Fig.7. Plot between generated stress and length of cantilever for PZT thickness 1.5µm and force applied is 15µN

Effect of cantilever length on generated stress is observed in following step. As shown in the fig.7, as the length of the cantilever increases stress generated across the cantilever decreases up to certain value and then after 29µm it gets saturated. There is no variation in stress after 29µm.

Finally eigen frequency analysis is performed. Total displacement of beam for various eigen frequencies is shown in fig.8 and fig.9.

Fig.8. displacement of the cantilever beam for eigen frequencies 2.75 MHz and 3.59 MHz.

Fig.9. displacement of the cantilever beam for eigen frequencies 16.27 MHz and 20.8 MHz.

III. CONCLUSION

A two element array of MEMS piezoelectric cantilever for sensing application is simulated and optimized using COMSOL multiphysics software tool. Stationary and eigen frequency analysis were performed using software. In first step stress generated across the cantilever for various value of PZT layer thickness for the force applied of 15µN is studied. According to this study piezoelectric layer thickness is optimized as 1.5µm as shown in the fig.4. Further the effect of force on stress is studied and according to fig.5 as force increases stress . Similarly the effect of length of cantilever on stress and displacement is studied as shown in the fig. 6 and fig.7. In fig.8 and 9 displacement of cantilever for various eigen frequencies is shown.

References[1] Sravanthi Chalasani, James M. Conrad. “A survey of Energy Harvesting

Sources for Embedded Systems” IEEE 978-1-4244-1884-8/08/$25.00 ©2008.

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[2] Alperent Toprak and Onur Tigli, “Intedigitated-Electrode-Based MEMS-Scale Piezoelectric Energy Harvester Modeling and Optimization Using Finite Element Method,” IEEE Transaction on Ultrasonic, Ferroelectrics and Frequency Control, vol. 60, No.10, October 2013.

[3] Neha Mukhuja, Nidhi Paliwal and Deepak Bhatia,”Design and simulation of piezoelectric energy scavanging system using IDE for wireless sensors,” in press.

[4] Rana N. Badran and Hani A. Ghali, “Geometric Optimization of Piezoelectric Energy Harvesting System,” Excerpt from the proceedings of COMSOL confrence, Milan, 2012.

[5] H. A. Sodano, D. J. Inman, and G. Park, “Comparison of piezoelectric energy harvesting devices for recharging batteries,” J. Intell. Mater. Syst. Struct., vol. 16, pp. 799–807, Oct. 2005.

[6] C . Mo, R. R. Knight, A. A. Frederick, and W. W. Clark, “Fabrication and energy harvesting measurements of vibrating MEMS piezoelectric benders,” J. Vib. Acoust., vol. 133, no. 1, art. no. 011006, Feb. 2011.

[7] M. Guizzeti, V. Ferrari, D Marioli and T. Zawda, “Thickness Optimization of a Piezoelectric Converter for Energy Harvesting.” University of Berscia, Milan, 2009.

[8] M. Raju, “Energy Harvesting, ULP meets energy harvesting: a game changing combination for design engineers,” Texam instruments, Texas, 2008.

[9] D. Shen, J.-H. Park, J. Ajitsaria, S.-Y. Choe, H. C. Wikle, and D.- J. Kim, “The design, fabrication and evaluation of a MEMS PZT cantilever with an integrated Si proof mass for vibration energy harvesting,” J. Micromech. Microeng., vol. 18, no. 5, art. no. 055017, 2008.

[10] S . Roundy and P. K. Wright, “A piezoelectric vibration based generator for wireless electronics,” Smart Mater. Struct., vol. 13, no. 5, pp. 1131–1142, 2004.

[11] A. A. Vives, “Piezelectric Transducer and Application,” valencia: Springer, 2008.

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