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Two-Level Bulk Microfabrication of a Mechanical Broadband Vibration Amplifier E-Mail: Michelle.Mueller@Micro.Mavt.Ethz.Ch Journal of Micromechanics and Microengineering PAPER • OPEN ACCESS Related content - Prototyping of a highly performant and Two-level bulk microfabrication of a mechanical integrated piezoresistive force sensor for microscale applications Bilal Komati, Joël Agnus, Cédric Clévy et broadband vibration amplitude-amplifier with ten al. coupled resonators - Torsional MEMS gyroscopes with non- resonant drive Cenk Acar and Andrei M Shkel To cite this article: Michelle Müller et al 2018 J. Micromech. Microeng. 28 045009 - Wide-bandwidth piezoelectric energy harvester with polymeric structure Mehdi Rezaeisaray, Mohamed El Gowini, Dan Sameoto et al. View the article online for updates and enhancements. This content was downloaded from IP address 82.130.103.57 on 19/11/2018 at 16:31 IOP Journal of Micromechanics and Microengineering Journal of Micromechanics and Microengineering J. Micromech. Microeng. J. Micromech. Microeng. 28 (2018) 045009 (10pp) https://doi.org/10.1088/1361-6439/aaabf6 28 Two-level bulk microfabrication of a 2018 mechanical broadband vibration amplitude- © 2018 IOP Publishing Ltd amplifier with ten coupled resonators JMMIEZ Michelle Müller1 , Verena Maiwald1, Lothar Thiele2, Jan Beutel2, Cosmin Roman1 and Christofer Hierold1 045009 1 Department of Mechanical and Process Engineering, Micro and Nanosystems Group, ETH Zurich, 8092 Zurich, Switzerland Michelle Müller et al 2 Department of Information Technology and Electrical Engineering, Computer Engineering and Networks Laboratory, ETH Zurich, 8092 Zurich, Switzerland Two-level bulk microfabrication of a mechanical broadband vibration amplifier E­mail: [email protected] Received 14 December 2017 Printed in the UK Accepted for publication 31 January 2018 Published 19 February 2018 JMM Abstract A micromechanical broadband vibration amplitude­amplifier for low power detection of 10.1088/1361-6439/aaabf6 acoustic emission signals is presented. It is based on a coupled mass­spring system and was fabricated in a two­level bulk microfabrication process. The device consists of ten resonators coupled in series, which decrease in mass by a factor of three each, to achieve a Paper high amplification over a broad bandwidth. The fabrication process for this multiscale device is based on front­ and backside etching of a silicon­on­insulator wafer. It enables coupling MEMS resonators of two different thicknesses with a weight ratio from largest to smallest 1361-6439 mass of 26’244 and reduces die size by resonator stacking. The first ten eigenmodes of the device are in­plane and unidirectional. Steady­state and transient response of the device in comparison to a 1D lumped element model is presented. An average amplitude amplification 4 of 295 over a bandwidth of 10.7 kHz (4.4–15.1 kHz) is achieved and can be reached in less than 1 ms. Applications are low­power detection of short broadband vibration signals e.g. for structural health monitoring (cliffs, pipelines, bridges). Keywords: multiscale MEMS, multi­degree of freedom, mechanical vibration amplification, two­level microfabrication, coupled masses, band­pass S Supplementary material for this article is available online (Some figures may appear in colour only in the online journal) 1. Introduction Compliant mechanisms are used to increase motion ampl­ itudes. In [2], the sensitivity of a MEMS accelerometer was High mechanical vibration amplification over a specific band­ improved by compliant microlevers. Similarly, displacement width is interesting for detecting weak broadband vibrations, amplification by a compliant mechanism was demonstrated such as acoustic emissions in structural health monitoring or in [3]. microseismic events [1]. Various solutions for mechanical Unlike compliant mechanisms, motion amplifiers based motion amplification have previously been demonstrated. on coupled mass­spring systems are frequency selective. Two degrees of freedom (DOF) micromachined resonators have been demonstrated by mechanically linking two mass­ Original content from this work may be used under the terms spring­damping units together [4]. A three DOF mechanically of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title coupled bandpass filter was applied for harvesting energy from of the work, journal citation and DOI. ambient vibrations [5]. Dual­mass vibration energy harvesters 1361-6439/18/045009+10$33.00 1 © 2018 IOP Publishing Ltd Printed in the UK J. Micromech. Microeng. 28 (2018) 045009 Michelle Müller et al have been implemented at the macroscale [6, 7], achieving 51 times power amplification [8]. Here, mechanical vibration amplitude­amplification is obtained by coupling multiple resonators in series, while spring stiffnesses and masses decrease towards the end of the chain. Low vibrations at the large outermost mass travel towards the last and smallest mass in the chain with increasing oscillation amplitude. Previously, we have demonstrated an in­plane system with 6 coupled resonators, where the masses Figure 1. Schematic of coupled mass­spring vibration amplifier increase by a factor of two, with average amplification of 15.8 with ten coupled resonators. The spring stiffnesses decrease by a [9, 10], and an out­of­plane system with 8 coupled resona­ factor α from the first resonator, which is anchored to the substrate, tors and an average amplification of 57.8 [11]. Coupling more towards the last resonator in the chain. The individual resonators have the same eigenfrequency. For the presented amplifier, the resonators and increasing the weight ratio between biggest to resonators are distributed over two Si layers (handle and device smallest mass increases the amplification. However, adding layer of an SOI wafer). masses becomes technologically very challenging beyond a certain point. fabrication and characterization of the device. Subsequently, A key challenge in up­scaling the coupled mass­spring fabrication and measurement results, including steady­state amplitude­amplifier concept we introduced earlier [10] is the and transient response of the device, are presented in sec­ exponential increase of the masses with the number of degrees tion four and discussed in section five. Finally, a conclusion is of freedom. Conventional planar microfabrication technolo­ given in section six. gies utilize mainly single functional layers, e.g. the device layer of an SOI wafer. Realizing large masses in a single thin 2. Device concept and design layer would consume prohibitively large surface area and is thus not practical. SOI microfabrication processes have been 2.1. Concept proposed where, additionally to the device layer, the handle layer was utilized to increase the seismic mass of a high sen­ There are two design rules for the coupled mass­spring ampli­ sitivity in­plane accelerometer [12, 13]. Further, multi­level fier. First, all individual mass­spring pairs have the same surface microfabrication processes have been presented [14, eigenfrequency: 15]. However, they are limited to thin layers of few μm. ki + ki+1 kn As solution, we propose to expand the device over two ω0 = 2πf0 = = , i = 1, ..., n 1. (1) silicon layers of varying thicknesses. The smaller masses are mi mn − defined in the thinner layer, while the larger masses are in Secondly, the spring stiffnesses decrease by a factor α: the thicker or both layers. This two­level architecture can be ki achieved for example by using the handle and device layer of ki+1 = , i = 1, ..., n 1. (2) a silicon­on­insulator (SOI) wafer. α − Even with a two­level approach, several challenges remain The basic design and corresponding rules are schematically in the bulk microfabrication of a multiscale device with many portrayed in figure 1. Once frequency ω0, factor α and the degrees of freedom. First, a design has to be developed where mass or spring stiffness of one resonator are chosen, all undesired eigenmodes (out­of­plane­, rotational­ and gimbal­ other masses and spring stiffnesses are determined by equa­ modes) are avoided within the bandwidth of interest, as they tions (1) and (2). The masses decrease similar to the spring may degrade device performance. Second, the multiscale stiffnesses with factor α, except for the last mass. With these device is susceptible to warping by intrinsic stress, which thus design rules, weak vibrations exciting the first resonator are has to be kept at a minimum. Further, matching natural frequen­ amplified while traveling towards the last resonator, if they are cies of the resonators over the two layers has to be achieved. within the allowed frequency band. In this paper, we overcome several major obstacles in the An example of a frequency response of the last resonator path of upscaling the previously presented proof­of­concept is given in figure 2. The amplitude of m10 (Y10) is divided by device [10]. The mechanical amplitude­amplifier introduced the amplitude of the sensor package (Y0). Corresponding to in [10] has a number of very desirable features, including the ten resonators with each one DOF, ten resonance peaks high­amplification over a wide bandwidth and off­resonant are visible. The bandwidth is defined from the first to the last amplification, which allows it to respond very fast. Yet, as observable resonance peak. Average and minimum amplifica­ outlined above, upscaling presents a number of technical tion of displacements are determined within this bandwidth. challenges, which are addressed in this paper. Besides that, The given simple design rules result in a Pareto­like to our knowledge, we achieve the highest purely mechanical solution for bandwidth and minimum amplification (see sup­ vibration amplitude­amplification (295 average) over a broad plementary data, section 1 (stacks.iop.org/JMM/28/045009/ bandwidth (10.7 kHz) so far. mmedia)). We do not rule out that other designs may yield The paper is structured as follows: in the second section, similar high amplification over a broad bandwidth, but it is the general device concept as well as the design and mod­ not clear to us how those designs could be found besides an eling is introduced. The third section covers methods such as exhaustive numerical search. 2 J.
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