MULTI ROOF TILE-SHAPED VIBRATION MODES IN MEMS CANTILEVER SENSORS FOR LIQUID MONITORING PURPOSES
Georg Pfusterschmied1, Martin Kucera1,2, Víctor Ruiz-Díez3, Achim Bittner1, José Luis Sánchez-Rojas3, Ulrich Schmid1 1 Institute of Sensor and Actuator Systems, Vienna University of Technology, AUSTRIA 2 Austrian Center of Competence for Tribology, AC2T research GmbH, AUSTRIA 3 Group of Microsystems, Actuators and Sensors, E.T.S.I. Industriales, Universidad de Castilla-La Mancha Ciudad Real, SPAIN
electrodes covering areas with opposite volume strain.
ABSTRACT We realized piezoelectrically self-actuated selfsensing cantilever sensors for liquid monitoring purposes excited in higher orders of the roof tile-shaped mode. This advanced class of vibration mode supports very high Qfactors in liquid media and high volume strain values which result in combination with an optimized electrode design in enhanced strain related conductance peaks. Therefore, precise fluid property measurements even for highly viscous liquids like D500 (~ 430 cP) are feasible.
INTRODUCTION Since many years, the demand of micromachined cantilever-based sensors, being capable to measure physical properties such as density and viscosity [1-3], is continuously increasing. The fabrication process used in this work is based on standard silicon technology, allowing smaller package size, lower costs due to largescale integration and low power consumption. This makes it well suited for providing sensors not only for laboratory equipment but even for complete new types of mobile low-power systems. Latest research has introduced a special class of excitation mode exceeding the overall performance of commonly used out-of-plane vibration modes, showing the highest Q-factor of a cantilever-based MEMS resonator in liquid media up to now [4]. The cantilever sensors realized in Ref. [4-5] use two electrode pairs, covering half of the sensor surface, allowing inparallel and anti-parallel actuation exciting apart from standard in-plane [6] and out-of-plane [7] modes either odd (e.g. 1st) or even (e.g. 2nd) roof-tile shaped modes [4]. This new class of vibration modes (see Figure 1) can be described as a transversal out-of-plane vibration mode with a free-free boundary condition along the length of a single-sided clamped beam. Considering Leissa’s nomenclature [8] by counting the number of nodal lines in x- and y-direction, the multi roof tile-shaped modes are named in the following text 1X-mode. The 12-mode (Figure 1 (a)) e.g. has two longitudinal nodal lines, whereas the 13-mode (Figure 1 (b)) has three nodal lines, the 14-mode (Figure 1 (c)) already four nodal lines and so on. When exciting higher order modes, due to the increase in nodal lines, the number of areas with different curvature and thus, different sign of volumen strain, grows. Consequently, a standard electrode design covering the complete cantilever surface used in Ref. [4] acts as a filter for higher modes [9]. This belongs to the partial cancellation of surface charges caused by
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Figure 1: Visualization of the (a) fundamental (1st) roof tile-shaped modal shape (12-mode), (b) the 2nd order of the roof tile-shaped modal shape (13-mode) and (c) the 4th order of the roof tile-shaped modal shape (15-mode). (d) Top view on the 15-mode with tailored electrode design. The colored areas on the cantilever surface represent the local volume strain distribution. In this work we present an optimized electrode design for micromachined self-sensing and self-actuated aluminum-nitride (AlN) cantilevers excited in higher orders of the 1X-mode. The electrode design allows an anti-parallel actuation (+-+-), which increases the deflection and prevents charge compensation resulting in very high strain related conductance peaks. A complete set of experimental results in combination with finite element method (FEM) simulations are presented, which provides design guidelines for adapting this method to other types of resonator-based sensors.
DEVICE FABRICATION The micromachined cantilevers (see Figure 2) used in this work are fabricated on 4 inch SOI wafers and are based on the fabrication procedure reported in Ref. [10]. The cantilevers with a length of L = 2524 µm and a width of W = 1274 µm are originally designed for an in-plane study [2] to achieve the same in-plane resonance frequency as a 50 x 500 µm2 cantilever, but with a different scaled aluminum-nitride area. More information on this electrode design is given in Kucera et al. [5]. The electrode design shown within this article is optimized for
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This follows from the charge cancellation due to almost equally strained areas of opposite sign, resulting in the presence of contrary charge polarities (see Figure 1 (c) and (d)). Beside the 15-mode at 990 kHz, the 25- and the 35-mode can be detected at 1020 kHz and 1070 kHz, respectively. From these results, it is concluded that even for the 25- and 35-modes, the optimized electrode actuation shows superior performance when compared to the non-optimized.
the 15-mode and results in four anti-parallel (+-+-) connected electrode stripes to ensure a collection of all charges without cancellation. After the finished fabrication process the die is packaged in a 24-DIP (Dual in-line package) and bonded via gold wires (see Figure 2).
Figure 2: Close up view of the in-house fabricated silicon die (6 x 6 mm²), containing a released cantilever and a non-released counterpart for parasitic effect compensation purposes (not used in this work) with the top area dimensions of 2524 x 1274 µm². This cantilever uses an optimized electrode patterning, considering the volume strain of the modal shape presented in Figure 1 (c) and (d).
Figure 3: Optical (a) and electrical (b) characterization of the 4th order (15-mode) in air, exciting the proposed cantilever device in optimized (+-+-) and non-optimized (++--) configuration. The deflection spectrum is measured with a Polytec laser Doppler vibrometer MSV400 whereas the electrical characterization is performed with an Agilent impedance analyzer 4294A. The side peaks show higher harmonics of the new vibration mode, which are the 25-mode and the 35-mode, respectively.
DEVICE CHARACTERIZATION Figure 1 presents the results of finite element method (FEM) eigenmode analyzes for higher orders of the roof tile-shaped mode, starting with the 12-mode (a), the 13mode (b) and the 15-mode (c). The color scale represents the volume strain and thus, the piezoelectrically generated surface charge distribution. This figure illustrates the increase of longitudinal nodal lines when increasing the order of the mode. The results from the FEM eigenmode analyzes are used for optimizing the electrode pattering. This is indicated for the 15-mode by the top view shown in Figure 1(d). Figure 2 depicts a typical die layout after packaging and wire-bonding including a released cantilever and a nonreleased counterpart, both with the optimized electrode stripes for 15-mode. The non-released counterpart is not used in this study. Figure 3 compares the cantilever deflection and electrical behavior in the "non-optimized" anti-parallel configuration (++--) and the "optimized" anti-parallel configuration (+-+-). It can be seen that a tailored electrode design, which considers the modal shape of the 15-mode, achieves a 10 times higher deflection value and a 100 times higher conductance peak. In contrast, the 15mode without “non-optimized” anti-parallel connection (++--) suppresses this vibration mode almost completely.
Figure 4 presents the quality factor as a function of the inverse square root of the viscosity-density product for several liquids (Isopropanol, N10, N100, D500) with dynamic viscosities up to ~ 430 cP (see Table 1). Table 1: Density and dynamic viscosity values for the used liquids, determined with Stabinger viscometer SVM3000 and with the Ubbelohde-Walther equation at 27°C. Name Dynamic Density viscosity [cP] [g/ml] Isopropanol 1.98 0.780 N10 13.50 0.846 N100 181.62 0.862 D500 432.96 0.867 For the following characterization, two different devices were used, one optimized for the 12/13-mode (2 electrodes) and one optimized for the 15-mode (4 electrodes). The results in Figure 4 show the increase in quality factor when exciting higher orders of the 1X-
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vibration mode, due to charge compensation, almost completely. The quality factor was evaluated for the 12-, 13- and 15-mode in several liquids (Isopropanol, N10, N100 and D500) showing superior performance of the 15mode (quality factor about 120 in isopropanol) followed by the 13-mode and the 12-mode. It was even possible to excite the 15-mode in a liquid of 430 cP (D500) with a quality factor greater than 10. The improved performance of the resonator, shown in this letter, predestines the use of the 15-mode for challenging chemical bio-sensing, but also for atomic force microscopy (AFM) in liquid media.
mode. This enhancement in quality factor is related to the increase in resonance frequency for higher modes, leading to a very high quality factor of 120 in isopropanol when excited in the 15-mode. Compared to literature, the slope in the quality factor for the 15-mode exceeds also the overall performance of standard out-of-plane actuated resonators [4, 11] promising an enhanced sensor sensitivity.
ACKNOWLEDGEMENTS This work has been supported by the Austrian Research Promotion Agency within the COMET-K2 Project XTribology (Project-No. 824187). The financial support given by the Spanish Ministerio de Economıa y Competitividad: Project Ref. DPI2009-31203, FPI Grant (Ref. BES-2010-030770) awarded to Tomas Manzaneque and FPU Grant (Ref. AP2010-6059) awarded to Vıctor Ruiz is gratefully acknowledged. Figure 4: Electrical characterization of two different cantilevers in different liquids (isopropanol and viscosity standards N10, N100 and D500). The 12- and 13-mode are excited with only 2 electrode stripes connected inparallel (++) as in Ref. [4] and in anti-parallel connection (+-) as in Ref. [5], respectively. The 15-mode is excited with optimized actuation (+-+-).
REFERENCES [1] C. Riesch, E.K. Reichel, A. Jachimowicz, J. Schalko, P. Hudek, B. Jakoby, F. Keplinger, ”A suspended plate viscosity sensor featuring in-plane vibration and piezoresistive readout”, in J. Micromech. Microeng., vol. 19, 2009. [2] M. Kucera, “Performance of cantilever-based piezoelectric MEMS resonators in liquid environment”, PhD-Thesis, Vienna University of Technology, 2014 [3] C. Riesch, E.K. Reichel, F. Keplinger, B. Jakoby, “Characterizing vibrating cantilevers for liquid viscosity and density sensing”, in J. Sens. Volume 2008. [4] M. Kucera, E. Wistrela, G. Pfusterschmied, V. RuizDíez, T. Manzaneque, J. L. Sánchez-Rojas, J. Schalko, A. Bittner, and U. Schmid, “Characterization of a roof tile-shaped out-of-plane vibrational mode in aluminum nitride-actuated selfsensing micro-resonators for liquid monitoring purposes”, in Appl. Phys. Lett., vol. 104, 2014. [5] M. Kucera, E. Wistrela, G. Pfusterschmied, V. RuizDíez, T. Manzaneque, J. Hernando-Garcia, J.L. Sánchez-Rojas, A. Jachimowicz, J. Schalko, A. Bittner and U. Schmid., ”Design-dependent performance of self-actuated and self-sensing piezoelectric-AlN cantilevers in liquid media oscillating in the fundamental in-plane bending mode.”, Sens. Actuator B, vol. 200, pp. 235-244, 2014. [6] L. A. Beardslee, A. M. Addous, S. Heinrich, F. Josse, I. Dufour, and O. Brand, “Thermal Excitation and Piezoresistive Detection of Cantilever In-Plane Resonance Modes for Sensing Applications”, in J. Microelectromech. Syst., vol. 19, pp. 1015-1017, 2010. [7] T. Manzaneque, J. Hernando, L. Rodrıguez-Aragon, A. Ababneh, H. Seidel, U. Schmid, and J. L. Sanchez-Rojas, “Analysis of the quality factor of
CONCLUSION This paper investigated for the first time higher orders of the roof tile-shaped out-of-plane vibration mode (1X-modes) in piezoelectrically actuated self-sensing MEMS cantilever sensors for liquid monitoring purposes. This advanced class of vibration modes supports very high Q-factors in liquid media and very high volume strain values, which results, in combination with an optimized electrode design, in very high strain related conductance peaks as demonstrated for the 4th order mode (15-mode). These features predestinated this superior class of vibration modes for a large variety of challenging resonator-based sensing applications in liquid media, exceeding the overall performance of commonly used outof-plane vibration modes. The generation of the conductance peak was discussed, showing a great potential for increasing by introducing electrode stripes which are optimized for the specific modal shape. Finite element method (FEM) eigenmode analyses for higher orders of the roof tiled-shaped mode were presented, starting with the 12-mode, the 13-mode and the 15-mode. The visualized partial cancellation of surface charges was presented for the 15-mode by a XY-plot with indicated electrode pairs. A comparison of the cantilever deflection and the electrical behavior in the "non-optimized" antiparallel configuration (++--) and in the "optimized" antiparallel configuration (+-+-) was shown. It could be revealed that a tailored electrode design, which considers the modal shape of the 15-mode, achieves a 10 times higher deflection value and a 100 times higher conductance peak. In contrast, the 15-mode with “nonoptimized” anti-parallel connection (++--) suppressed this
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AlN-actuated micro-resonators in air and liquid”, in Microsyst. Technol., vol. 16, pp. 837-845, 2010 [8] A. W. Leissa, “Vibration of Plates”, in Scientific and Technical Information Division, National Aeronautics and Space Administration, NASA SP160, 1969. [9] J. L. Sanchez-Rojas, J. Hernando, A. Donoso, J. C. Bellido, T. Manzaneque, A. Ababneh, H. Seidel, and U. Schmid, “Modal optimization and filtering in piezoelectric microplate resonators.”, in J. Micromech Microeng., vol. 20, 2010 [10] M. Kucera, F. Hofbauer, E. Wistrela, T. Manzaneque, V. Ruiz-Díez, J.L. Sánchez-Rojas, A. Bittner, U. Schmid. “Lock-in amplifier powered analogue Q control circuit for self-actuated self-sensing piezoelectric MEMS resonators.”, in Microsyst. Technol., vol. 20, pp. 615-625, 2014. [11] T. Manzaneque, V. Ruiz, J. Hernando-García, A. Ababneh, H. Seidel and J. L. Sánchez-Rojas, “Characterization and simulation of the first extensional mode of rectangular micro-plates in liquid media”, in Appl. Phys. Lett., vol. 101, 2010.
CONTACT *G. Pfusterschmied, tel: +43-58801-36649;
[email protected]
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