Design and simulation of Bulk acoustic wave MEMS resonator

Transcription

1 Design and simulation of Bulk acoustic wave MEMS resonator 1 Wahengbam Kanan Kumar, 2 Anuj Goel 1 M.Tech. (VLSI Design), ECE Deptt., MMEC,Mullana,Haryana 2 Assistant Professor, ECE Deptt., MMEC,Mullana,Haryana Abstract:- A brief review on the use of acoustic waves in designing MEMS based resonator is described in this paper. Acoustic devices can be further classified into two basic types Surface acoustic wave (SAW) and Bulk acoustic wave (BAW) devices. A BAW resonator is modeled and simulated using COMSOL Multiphysics 4.3. In this model variation in thickness and piezoelectric material is the prime focus for studying the basic variation in series resonance and surface deformation of the device. Keywords: Acoustic wave resonator, SAW, BAW, Comsol Multiphysics 4.3, Piezoelectric material. I. INTRODUCTION Acoustics is the study of time-varying deformations or vibrations within a given material medium. In case of solid, it is the result of deformation of the material. Deformation occurs when atoms within the material move from their equilibrium position which results in internal restoring forces that return the material back to equilibrium. Figure 1 depicts the equilibrium and deformed states of particles in an arbitrary solid body- the equilibrium state is shown by solid dots and the deformed state is shown by circles [16]. Figure 1: Equilibrium and deformed states of particles in a solid body Plane wave is the most general type of acoustic wave which propagates in an infinite homogenous medium. It is further of two types: Longitudinal and shear waves, depending on the direction of propagation and polarization of the vibrating atoms within the propagating medium. In the former, the particles vibrate in the direction of propagation, while in the latter case the particles vibrate in a plane normal to the direction of propagation. However due to boundary restrictions on the propagation medium, it is no longer an infinite medium and consequently the nature of the wave changes. A graphical representation of the shear wave propagation is presented in the Figure 2 below. An acoustic wave can be described in terms of both its propagation and polarization directions. Figure 2: Acoustic shear waves in a cubic crystal medium [16] Acoustic wave can be induced by a variety of methods such as mechanical impact, pulsed thermal energy, and inverse piezoelectric effect. Acoustic devices are employed in manufacturing transducer which can be broadly classified into two groups: surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Piezoelectric crystals play crucial role in the communication and electronics industry in areas All Rights Reserved 2014 IJEETE Page 14

2 like filters, precision timers and frequency At this frequency, the transducer efficiency in control in oscillator circuits. This effect is converting electrical energy to acoustical, or vice demonstrated in the Figure 3. Material tends to versa, is maximised. get deformed due to the applied potential across its two surfaces. This is due to coupling of linear elasticity and electrostatic charge. Hooke s law for elasticity may be written as : S = s.t (1) D = ε.t (2) Piezoelectric materials combine the above two equation into one coupled equation: S = s E.T + d t.e (3) D = d.t + ε T.E (4) Where, S=strain, s=compliance, T=stress, D=charge density, ε=permittivity, E=electric field, d=coupling matrix, ε T =relative permittivity, s E =compliance matrix. Figure 3: Finger-spacings and their role in the determination of the acoustic wavelength [16] The simplest SAW device is the non-dispersive delay line depicted in Figure 4. One IDT is connected to an electrical source and the other to a detector. The source IDT sets up an electric field in the substrate that launches a SAW by means of the piezoelectric effect, and the receiving transducer converts the surface wave to an electrical signal. II. SENSOR STRUCTURE Surface acoustic wave (SAW) device SAW devices make use of mechanical wave that propagates along the surface of a piezoelectric material and whose amplitude decreases exponentially with depth of the material. They are mostly employed in electronic devices such as sensors, actuators, oscillators and filters. [9][13]. The SAW uses two metal comb-shaped electrodes placed on a piezoelectric substrate. An electric potential V applied to the electrodes creates dynamic strains in the piezoelectric substrate, which launches elastic waves. These waves contain the Rayleigh waves that run perpendicular to the electrodes with a velocity. To ensure constructive interference and in-phase stress, the distance d between two neighbouring fingers should be equal to half the elastic wavelength λ R [9]-[13] : d = λ R /2 (5) The associated frequency is known as synchronous frequency, f 0 f 0 = V R / λ R (6) Figure 4: Schematic of a SAW device with IDTs metallised onto the surface [16] Bulk acoustic wave (BAW) device: Bulk acoustic wave promises frequency in GHz range when integrated with RF circuits along with small size resonator and filters [1-5][7][8]. The resonator is of thin film [1],[3],[8] type in which the substrate is etched away on the back side. The natural frequency of the material and the thickness are used as design parameters to obtain the desired operating frequency. It is modelled by sandwiching a piezoelectric layer in between two electrodes as shown in Figue 5. A thick silicon layer is etched as the bottom layer and a potential drop is applied in between the two electrodes and the admittance in the piezoelectric layer is calculated. The bottom electrode is made the ground layer. All Rights Reserved 2014 IJEETE Page 15

3 III. SIMULATED RESULTS The admittance Y is obtained by dividing total charge Q on an electrode by the amplitude of the driving voltage [1]: (7) The total charge on electrode is calculated as the sum of the nodal charges. If F F is the nodal force with a constant displacement and Q Q is the nodal charge with a fixed potential, the total charge on an electrode can be computed by taking summation of all the nodal charges which is written as: (8) Where I el is a unity vector at positions corresponding to those electrical DOFs for an electrode. Figure 5: Equivalent Butterworth Van Dyke lumped element equivalent circuit of BAW resonator [6] Considering the BVD model [6] shown in the figure 5 admittance of the equivalent circuit is: (9) When an unrestrained piezoelectric ceramic element is exposed to a high frequency alternating field, an impedance minimum, planar frequency coincides with the series resonance frequency, fs. At higher resonance, another impedance minimum (i.e the axial resonance frequency) is encountered. The thickness mode frequency constant, NT, is related to the thickness of the ceramic element, h by [17]: N T = fs h (10) Figure 6: Bulk acoustic wave resonator used in this paper A pictorial representation of the modelled BAW resonator is displayed in the figure above. All simulations are performed in COMSOL Multiphysics 4.3 environment. It is a finite element analysis; solver and simulation software packaged for various physics and engineering applications, especially coupled phenomena, or Multiphysics. In addition it also enables user to enter coupled systems of partial differential equations (PDEs). It also offers extensive interface to MATLAB and its toolboxes for a variety of pre-processing and post processing applications [18][19]. All the geometrical measurements were carried out using the MEMS module present in the software tool. The model consists of two thin film metal layers, piezoelectric layer and a silicon substrate as displayed graphically in Figure 6. The bottom layer is the silicon substrate layer on top which is a thin metal layer serving as the ground electrode. A thin piezoelectric layer is sandwiched between two metal layers. It may be noted that the top metal serves as the positive electrode while the bottom is the ground electrode, a potential is applied between them. The whole simulation is done by dividing the readings into two broad categories i.e. thickness of the piezoelectric layer is increased in the second reading. The physical deformation along with the admittance, Eigen frequency and All Rights Reserved 2014 IJEETE Page 16

4 series resonant frequency of the device body are tabled in accordance with the simulated results as shown below. Four different piezoelectric materials are used for each simulation Zinc Oxide, PZT 5H, LiNbO 3, Barium titanate. Reading I: Thickness (µm) of: PZD = 9.6, Metal (Al) = 0.2, Silicon = 6.9. Figure 7: Admittance, Eigen frequency and series resonance frequency with Zinc Oxide as PZD Piezoele ctric material (PZD) Table 1: Result 1 Thickness(µm) PZD Met al Sili con Defo rmat ion n(n m) Re son ant fre q (M hz) Ad mitta nce ZnO PZT 5H Lithium niobate Barium titanate Reading II: Thickness (µm) of : PZD = 3, Metal= 0.4, Silicon = 1.6, top metal = Al, bottom metal = Mb All Rights Reserved 2014 IJEETE Page 17

5 Piezoelectri c material (PZD) ZnO Table 2: Result 2 Thickness(µm) PZ D Metal PZT 5H Lithium niobate Barium titanate Sili con Defor matio n (nm) Reso nant freq (Mhz ) Adm ittanc e (S) 10 1 > IV. CONCLUSION The simulated model has the capability to sustain different types of oscillations, where the lowest and highest series resonant frequencies were found in Lithium niobate (165 MHz) and PZT 5H (977.5 MHz) respectively within the investigated range of 100 MHz to 1 GHz. The frequency response analysis shown in figure 7 and figure 8 shows the deformation in surface with the change in frequency. The highest value of deformation was observed when PZT 5H was used as the PZD material, i.e nm. The admittance plot in each reading is used to determine the resonant and anti-resonant frequency. It may also be observed that the absolute value of admittance is highest in PZT 5H, i.e. of the order of REFERENCES [1]Tapani Makkonen, Antti Holappa, Juha Ella, and Martti M. Salomaa, Finite element simulations of Thin-Film Composite BAW Resonators, IEEE Trans. On Ultrason, Ferroelectrics, and Frequency Cotrol, VOL. 48, no5, pp , 2001 Figure 8: Admittance, Eigen frequency and series resonance frequency with LiNbO 3 as PZD [2]H.Satoh, H.Suzuki, C. Takahashi, C. Narahara, and Y. Ebata, A 400 MHz one-chip oscillator using and air gap type thin film resonator, in Proc. IEEE Ultrason. Symp., pp , 1987 All Rights Reserved 2014 IJEETE Page 18

6 [3] W.A. Burkland, A.R. Landlin, G.R. Kline, and R.S. Ketcham, A thin-film-bulk-acousticwave resonator-controlled oscillator on silicon, IEEE Electron. Device Lett., vol. EDL-8, no. 11, pp , 1987 [4] R.B. Stokes, J.D. Crawford, and D. Cushman, Monolithic bulk acoustic filters to X- band in GaAs, in Proc. IEEE Ultrason. Symp., pp , [5] T.W. Grudkowski, J.F. Black, T.M. Reeder, D.E. Cullen, and R.A. Wagner, Fundamentalmode VHF/UHF miniature acoustic resonators and filters on silicon, Appl. Phys. Lett., vol. 37, no. 11, pp , 1980 [6] R.F. Milsom and others, Effect of mesashaping on spurious modes in ZnO/Si bulk-wave composite resonators, in Proc. IEEE Ultrason. Symp., pp [7] K.M. Lakin, Modelling of thin film resonators and filters, IEEE MTT-S Digest, vol. 1, pp , [8] Frederic Nabki, Karim Allidina, Faisal Ahmad, Paul-Vahe Cicek, and Mourad N. El- Gamal, A Highly Integrated 1.8 GHz Frequency Synthesizer Based on a MEMS Resonator, IEEE Journal of solid-state circuits, VOL. 44, no. 8, pp , August 2009 [9] Clifford K. Ho, Eric R. Lindgren, K. Scott Rawlinson, Lucas K. McGrath and Jerome L, Development of a Surface Acoustic Wave Sensor for In-Situ Monitoring of Volatile Organic Compounds, Sensors vol. 3, pp , [10] Staline Johnson, Dr. T. Shanmuganantham, Design and Analysis of SAW Based MEMS Gas Sensor for the Detection of Volatile Organic Gases, Int. Journal of Engineering Research and Applications, Vol. 4, Issue 3( Version 1), March 2014, pp ,, ISSN : [11] Jerzy Filipiak, Lech Solarz, and Grzegorz Steczko, Surface Acoustic Wave (SAW) Vibration Sensors, Sensors 2011, 11, pp ; doi: /s [12] Donald C. Malocha, Mark Gallagher, Brian Fisher, James Humphries, Daniel Gallagher and Nikolai Kozlovski, A Passive Wireless Multi- Sensor SAW Technology Device and System Perspectives, Sensors 2013, 13, pp ; doi: /s [13] Ahmadi, F. Hassani, C. Korman, M. Rahaman, and M. Zaghlou, Characterization of Multi- and Single-layer Structure SAW Sensor, Sensors 2004, Proceedings of IEEE, vol. 3, pp , [14] Stephen Beeby, Graham Ensell, Michael Kraft, Neil White, MEMS Mechanical Sensors, 2004, Artech house Inc., ISBN [15] Waldner, Jean-Baptiste (2008). Nanocomputers and Swarm Intelligence. London: ISTE John Wiley & Sons. ISBN [16] Julian W. Gardner, Vijay K. Varadan, Osama O. Awadelkarim, Microsensors, MEMS, and Smart Devices, 2013, Copyright 2001 by John Wiley & Sons Ltd., ISBN: [17] [18] iphysics [19] All Rights Reserved 2014 IJEETE Page 19

7 AUTHOR S BIBLOGRAPHY Wahengbam Kanan Kumar was born in Manipur, India in Presently he is pursuing M.Tech in VLSI Design at Maharishi Markandeshwar University. After completing B.E. in 2012 he was a guest lecturer at NERIST (Deemed University). His research interests include SAW & BAW physics, MEMS based design, Digital circuit design, Memory design, Fuzzy logic, Neural network, Image processing. Anuj Goel was born in Haryana, India in He is presently working as Assistant Professor in ECE Department, MMEC, M.M.University, India. His research interests include MEMS Modelling, VLSI Design etc. All Rights Reserved 2014 IJEETE Page 20