Design, Fabrication and Characterization of a Piezoelectric Microgenerator Including a Power Management Circuit

DESIGN, FABRICATION AND CHARACTERIZATION OF A PIEZOELECTRIC MICROGENERATOR INCLUDING A POWER MANAGEMENT CIRCUIT

M. Marzencki, Y. Ammar and S. Basrour

TIMA laboratory, 46, avenue Félix Viallet, 38031 Grenoble FRANCE E-mail: marcin.marzencki@imag.fr

ABSTRACT We report in this paper the design, fabrication and experimental characterization of a piezoelectric MEMS microgenerator. This device scavenges the energy of ambient mechanical vibrations characterized by frequencies in the range of 1 kHz. This component is made with Aluminum Nitride thin film deposited with a CMOS compatible process. Moreover we analyze two possible solutions for the signal rectification: a discrete doubler-rectifier and a full custom power management circuit. The ASIC developed for this application takes advantage of diodes with very low threshold voltage and therefore allows the conversion of extremely low input voltages corresponding to very weak input accelerations. The volume of the proposed generator is inferior to 1mm3 and the generated powers are in the range of 1µW. This system is intended to supply power to autonomous wireless sensor nodes.

1. INTRODUCTION Nowadays, a great effort is devoted to the development of Self Powered Micro Systems (SPMS). These devices are commonly used as nodes in Wireless Sensor Networks, where small size and extended autonomy are essential [1]. Up to now, the only solution was to use an electrochemical battery that would supply power to the system. Nevertheless, this solution has a main drawback – the life span of the device is directly linked with the capacity of the energy reservoir and therefore with its size. A trade off has to be made between the miniaturization and the longevity of a device. Ambient power harvesting is a possible breakthrough in this domain. Several research teams have already analysed this subject and it has been found that mechanical vibrations propose very interesting power densities [1]. Furthermore, this kind of energy can be transferred to the device by the means of a simple mechanical coupling. We propose in this paper such an approach consisting in an electromechanical transducer using the piezoelectric effect to convert mechanical vibrations into useful electrical energy. We implement these devices using microfabrication techniques. These MEMS power generators deliver very small powers (in the nW range) at voltages often inferior to 200mV. To overcome the problem of rectification of such weak amplitude AC signals we designed and fabricated diodes with ultra low threshold voltage. These components have been used in a voltage multiplier (VM) which boosts and rectifies the voltage provided by the piezoelectric micro generator. The use of this system was compared with a standard approach using discrete Schottky diodes, similar as presented for a macro prototype by M. Ferrari et al. [2].
   The system is used to charge a storage capacitor from which energy is delivered for one cycle of operation of a very low power wireless sensor node. All components of the system are created using CMOS compatible batch microfabrication techniques. In the future it can be realised as a System On a Package (SoP) or even System On a Chip (SoC) in order to reduce its size and cost.
2. MICROGENERATOR 2.1. Fabrication process The presented system incorporates a MEMS micro power generator (µPG) that uses the piezoelectric effect for converting the energy of ambient mechanical vibrations into useful electrical energy. The structures are fabricated using an SOI wafer. The Aluminium Nitride thin layer is deposited directly on the heavily doped top silicon layer that serves as the bottom electrode for the piezoelectric capacitor. The piezoelectric layer is sputtered at low temperature and then wet etched in order to define the shape of the mobile structure and the openings for the bottom electrode contact. Then the top aluminium layer is deposited and patterned. Afterwards the top and bottom silicon layers are etched using Deep Reactive Ion Etching (DRIE) in order to create the mobile structure. The use of the AlN layer makes the process compatible with CMOS fabrication because of the lack of high temperature ©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3

Marcin Marzencki, Yasser Ammar and Skandar Basrour Design, fabrication and characterization of a piezoelectric microgenerator including a power management circuit treatment. Furthermore, this material is relatively easy to deposit and does not need to be polarised in order to gain piezoelectric properties. Thanks to these advantages, it is a good candidate for industrialisation. The process employed imposes that the seismic mass thickness is equal to the wafer thickness and the cantilever beam thickness is equal to the top silicon layer thickness. Furthermore the AlN layer thickness was fixed a

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