sensors Article Optimization of a Piezoelectric Energy Harvester and Design of a Charge Pump Converter for CMOS-MEMS Monolithic Integration † Marcos Duque 1 , Edgardo Leon-Salguero 2 , Jordi Sacristán 1 , Jaume Esteve 1 and Gonzalo Murillo 1, * 1 Department of Micro and Nanoengineering, Instituto de Microelectrónica de Barcelona IMB-CNM (CSIC), Campus UAB Bellaterra, 08193 Barcelona, Spain; Marcos.Duque@imb-cnm.csic.es (M.D.); Jordi.sacristan@imb-cnm.csic.es (J.S.); Jaume.Esteve@imb-cnm.csic.es (J.E.) 2 Postgrado en Nanotecnología, Universidad de Sonora (Unison), Hermosillo, Sonora 83000, Mexico; eleonsal@gmail.com * Correspondence: Gonzalo.murillo@csic.es; Tel.: +34-93-594-77-00 † This paper is an extended version of the conference paper: Towards the Monolithic Integration of Converter Circuitry and Piezoelectric MEMS Energy Harvesters. In Proceedings of the EUROSENSORS 2018, Graz, Austria, 9–12 September 2018.  Received: 11 February 2019; Accepted: 19 April 2019; Published: 21 April 2019  Abstract: The increasing interest in the Internet of Things (IoT) has led to the rapid development of low-power sensors and wireless networks. However, there are still several barriers that make a global deployment of the IoT difficult. One of these issues is the energy dependence, normally limited by the capacitance of the batteries. A promising solution to provide energy autonomy to the IoT nodes is to harvest residual energy from ambient sources, such as motion, vibrations, light, or heat. Mechanical energy can be converted into electrical energy by using piezoelectric transducers. The piezoelectric generators provide an alternating electrical signal that must be rectified and, therefore, needs a power management circuit to adapt the output to the operating voltage of the IoT devices. The bonding and packaging of the different components constitute a large part of the cost of the manufacturing process of microelectromechanical systems (MEMS) and integrated circuits. This could be reduced by using a monolithic integration of the generator together with the circuitry in a single chip. In this work, we report the optimization, fabrication, and characterization of a vibration-driven piezoelectric MEMS energy harvester, and the design and simulation of a charge-pump converter based on a standard complementary metal–oxide–semiconductor (CMOS) technology. Finally, we propose combining MEMS and CMOS technologies to obtain a fully integrated system that includes the piezoelectric generator device and the charge-pump converter circuit without the need of external components. This solution opens new doors to the development of low-cost autonomous smart dust devices. Keywords: IoT; MEMS; energy harvesting; monolithic integration; piezoelectric; CMOS; charge pump; self-powered; power management; AlN 1. Introduction The Internet of Things (IoT) has become a trending topic in the last years [1–4]. This concept allows the interconnection of thousands of wireless sensors to acquire ambient information and take decisions without human interaction. Nevertheless, the energy dependence of the wireless sensor nodes, limited by the capacity of batteries, is one of the most important barriers to the global implementation of the IoT. A promising alternative is to recover energy from the environment, in the way of heat, light, or mechanical energy [5–7]. This concept allows these devices to be self-sufficient. Ambient Sensors 2019, 19, 1895; doi:10.3390/s19081895 www.mdpi.com/journal/sensors Sensors 2019, 19, 1895 2 of 14 vibrations can be converted into electrical energy by using piezoelectric [8–13], triboelectric [14–16], electromagnetic [17], and electrostatic transducers [18–20]. Among them, piezoelectric transducers are considered an attractive option due to their high energy density, ease of integration, and durability. Piezoelectric MEMS allow device miniaturization, piezoelectric actuators with low operation voltages, easy implementation of high frequency, and temperature-stable resonant devices, and the possibility of harvesting energy from mechanical sources [21–24]. A piezoelectric generator provides an alternating-current (AC) electrical signal that needs to be rectified to get a direct current (DC). A simple energy harvesting circuit consists of a diode-based rectifier (AC-DC) and a DC-DC converter, such as the LTC3588 IC from Linear Technologies. The addition of a DC-DC converter has been shown to improve the conversion efficiency by a factor of seven. A more specialized power management system for piezoelectric generators is the “synchronized switch harvesting on inductor” (SSHI) circuit [25–27]. It consists of a switching device in parallel with a piezoelectric element. The device is composed of a switch and an inductor connected in series, and it allows a higher efficiency for the rectification stage. The inductance is by far the largest component of the power management circuit. This is a critical limitation for the circuit miniaturization; in addition, inductances of such high values are relatively expensive. A charge pump circuit is an alternative to the SSHI that only uses capacitors [28,29]. It is a switching regulator that delivers the power by only charging and discharging capacitors. It is appropriate for an application that has low currents and a moderate input-to-output voltage difference. From the energetic point of view, it is not as efficient as an SSHI converter, but it allows a full monolithic integration. In addition, device encapsulation and packaging usually represents a large part of the manufacturing costs of MEMS devices and integrated circuits [30–32]. For these reasons, our goal is to monolithically combine MEMS and CMOS technologies to obtain a fully integrated system comprising of a piezoelectric generator and its associated converter circuit [33–37]. 2. Piezoelectric MEMS Energy Harvester 2.1. Device Specifications and Fabrication Process There are different types of MEMS structures, such as cantilevers, clamped-clamped beams, or diaphragms. Among them, a cantilever generates a higher average strain for a given input force, which in turns results in a higher power output when used as a piezoelectric generator. Furthermore, a cantilever has the lowest resonance frequency for a given size [38]. For this reason, the cantilever structure was chosen for our piezoelectric transducer. The functional schematic of our piezoelectric MEMS energy harvester is shown in Figure 1a. Our energy harvesting devices are based on an silicon-on-insulator (SOI) silicon wafer (silicon: 15 µm—SiO2 : 1 µm—silicon: 500 µm) [39,40]. The device layer is used to define the cantilever beam and the bulk layer is used for the inertial mass (Figure 1b). On top of the cantilever beam, a 1-µm-thick layer of aluminium nitride (AlN) forms the piezoelectric transducer. The piezoelectric layer is sandwiched between two bilayer metal electrodes of Ti/Pt. In most applications, the frequency of the ambient vibrations is lower than 1 KHz [41]. For this reason, we have used an inertial tip mass to decrease the structure’s natural frequency. The fabrication process of our devices is depicted in Figure 1c. The thin layer of Ti/Pt is deposited by radio-frequency reactive sputtering to define the two device electrodes. This same technique is used to deposit the AlN layer of 1 µm. The device structure is defined by several steps of photolithography and deep reactive-ion etching (DRIE). Finally, a wet etch based on hydrofluoric acid (HF) is used to remove part of the buried oxide layer and, consequently, to release the cantilever beam [35,40]. In this work, we focused on the PEH-B1X1M2X1 device, shown in Figure 2a–c, with a mass area of 1 mm × 2 mm and a cantilever area of 1 mm × 1 mm. Sensors 2019, 19, x 3 of 14 beam Sensors[35,40]. 2019, 19,In this work, we focused on the PEH-B1X1M2X1 device, shown in Figure 2a–c, with 1895 3 ofa14 mass area of 1 mm × 2 mm and a cantilever area of 1 mm × 1 mm. (a) (b) (c) Figure 1. (a) Functional device configuration: A cantilever structure with a tip mass and a piezoelectric Figure 1. (a) Functional device configuration: A cantilever structure with a tip mass and a material deposited on top of the beam; (b) cross-section sketch of the cantilever structure; (c) different piezoelectric material deposited on top of the beam; (b) cross-section sketch of the steps of the fabrication process. cantilever structure; (c) different steps of the fabrication process. 2.2. Design and FEM Simulations 2.2. Design and FEM Simulations The designed devices have been simulated with COMSOL Multiphysics to compare the expected The with results designed devices characterization the electrical have been simulated performedwithafterCOMSOL Multiphysics manufacturing. to compare The used modulesthe for expected resultsare this simulation with the the AC/DC electrical modulecharacterization performed (electrostatics, electrical after and circuits) manufacturing. the structuralThe used mechanics modules for this module (solid simulation mechanics and are the AC/DC piezoelectric module devices). The(electrostatics, materials selected electrical circuits) are silicon and the (single-crystal, structural anisotropic)mechanics modulenitride, and aluminum (solid mechanics and piezoelectric with the default values provided devices). by theThe materials software. selected Figure are 2a shows silicon (single-crystal,device the PEH-B1X1M2X1 anisotropic) model andwithaluminum a swept mesh nitride, with for the the defaultmaterial piezoelectric values provided by the and a tetrahedral software. mesh forFigure the rest2aofshows the PEH-B1X1M2X1 structure. In this study, a finedevice mesh model withwith 20,925a swept elementsmesh wasforutilized. the piezoelectric The stress material and in distribution a tetrahedral the structure, mesh for the by using therest of structure.study, Eigenfrequency In thiswas study, a finetomesh computed achievewith 20,925 a uniform elements wasover distribution utilized. The stress the entire surfacedistribution in the structure, of the piezoelectric by using layer (Figure 2b,c).the Eigenfrequency In this way, a higherstudy,charge was computed density to achieve is generated a uniforma more and therefore, distribution efficientover devicethecan entire surface ofAfter be obtained. the piezoelectric layer some careful design (Figure 2b,c). In this and simulation way, a higher processes, beamscharge density is generated with trapezoidal shapes and androunded therefore, a morewere corners efficient deviceto designed can be obtained. achieve After some a homogeneous careful stress over design and simulation the entire surface andprocesses, avoid stress beams with trapezoidal concentration in theshapes corners and rounded (Figure corners 2c) [42]. Figurewere 2ddesigned shows thetoopen-circuit achieve a homogeneous voltage generated stressby over thethe entire surface structure submitted andto avoid stress concentration a frequency sweep of thein the corners excitation (Figureat2c) vibration [42]. Figure different values 2dofshows the open-circuit acceleration. voltage The open-circuit generated configurationby theis structure submitted used to measure the to a frequency maximum sweep voltage ofcan that the be excitation generated vibration at different by the piezoelectric values of acceleration. generator. The open-circuit Our simulation results show configuration the resonant is used to measure frequency of thethe maximum device and the voltage that maximum can be generated open-circuit by the voltage thatpiezoelectric can be generatedgenerator. by theOur simulation piezoelectric results show generator the resonant at different frequency input accelerations. of theUsing devicethe andsamethestructure, maximumbut open-circuit voltage thatshape without a trapezoidal can be andgenerated rounded by the piezoelectric corners, we obtained generator at different an open-circuit input voltage of accelerations. around 0.48 V for an acceleration of 0.1 G [39]. In our simulation, shown Using 2d, in Figure the asame structure, voltage of 0.625 but V without a trapezoidal was obtained shape and rounded with a homogeneous corners, stress over we obtained the entire surface. an open-circuit Therefore, voltage of around an increase 30% in the0.48voltage V for anis acceleration expected by of 0.1 G using [39]. this In our simulation, optimized design. shown in Figure 2d, a voltage of 0.625 V was obtained with a homogeneous stress over the entire surface. Therefore, an increase of 30% in the voltage is expected by using this optimized design. Sensors 2019, 19, x 4 of 14 Sensors 2019, 19, 1895 4 of 14 Sensors 2019, 19, x 4 of 14 (a) (b) (a) (b) (c) (d) Figure 2. Simulated results (c) of the piezoelectric device by using COMSOL (d) multiphysics: (a) Mesh used in the simulation; (b) 3D FEM simulation of the stress distribution in the Figure2.2.Simulated Figure Simulatedresults resultsof of thethe piezoelectric piezoelectric device device by using by using COMSOL COMSOL multiphysics: multiphysics: (a) (a) Mesh cantilever beam and (c) top view of the structure with the trapezoidal beam shape detailed used in the simulation; (b) 3D FEM simulation of the stress distribution in Mesh used in the simulation; (b) 3D FEM simulation of the stress distribution in thethe cantilever beam and (arbitrary units); (d) maximum open-circuit voltage that can be generated by the (c) top view beam cantilever of the structure and (c) topwith the trapezoidal view beam shape of the structure with thedetailed (arbitrary trapezoidal units); beam (d) maximum shape detailed piezoelectric generator submitted to a frequency sweep, at different values of the input open-circuit voltage that (arbitrary units); (d) can be generated maximum by the piezoelectric open-circuit generator voltage that can submitted to a frequency be generated by the acceleration sweep, at ranging different from values of 0.05 the G toacceleration input 0.20 G. ranging from 0.05 G to 0.20 G. piezoelectric generator submitted to a frequency sweep, at different values of the input acceleration 2.3. Device ranging from 0.05 G to 0.20 G. Manufacturing Several 2.3. Device different geometries devices, shown in Figure 3c, with different Manufacturing geometries and and designs designs were manufactured manufactured density. After manufacturing, every microgenerator was placed and bonded expecting a high power density. Several devices, shown in Figure 3c, with different geometries and designs were manufactured in aaprinted-circuit printed-circuit board board (PCB)(PCB) to perform to perform the electrical the electrical characterizations characterizations (Figure (Figure 4c). 4c). The The fabrication expecting a high power density. After manufacturing, every microgenerator was placed and bonded fabrication process for process for these these devices devices isand is patented patented andtolicensed licensed ENERGIOTto ENERGIOT Devices SLDevices [42–44],SLwhich [42–44], is in a printed-circuit board (PCB) to perform the electrical characterizations (Figure 4c). The which is commercially commercially exploiting exploiting this technology. this technology. The layoutTheoflayout of the PEH-B1X1M2X1D the PEH-B1X1M2X1D device device can can be be seen in fabrication process for these devices is patented and licensed to ENERGIOT Devices SL [42–44], seen in3a. Figure Figure 3a. The resonance The resonance motion ofmotion of this this device wasdevice waswith captured captured with a microscope a microscope camera camera (Figure 3b). which is commercially exploiting this technology. The layout of the PEH-B1X1M2X1D device can be (Figure 3c3b). Figure Figure shows some3cfabricated shows somedevicesfabricated devices with different with and designs different designs and sizes, sizes, demonstrating high seen in Figure 3a. The resonance motion of this device was captured with a microscope camera demonstrating high miniaturization miniaturizationcapabilities. and customization and customization capabilities. (Figure 3b). Figure 3c shows some fabricated devices with different designs and sizes, demonstrating high miniaturization and customization capabilities. (a) (b) (c) Figure3.3. Figure (a)(a) (a) Layout Layout of a piezoelectric of a piezoelectric (b)microgenerator microgenerator (5 mm (5 mm × 5 mm × 5mm); × 0.516 mm(b)×(c) 0.516 mm); resonance (b) motion resonance of motion the fabricated of the device fabricated captured withdevice captured a microscope with(c) camera; a microscope camera; different designs (c) different of manufactured Figure 3. (a) piezoelectric Layout of a on piezoelectric microgeneratorspiezoelectric top of a one microgenerator cent coin. (5 mm × 5 mm × 0.516 mm); (b) designs of manufactured microgenerators on top of a one cent coin. resonance motion of the fabricated device captured with a microscope camera; (c) different designs of manufactured piezoelectric microgenerators on top of a one cent coin. Sensors 2019, Sensors 2019, 19, 19, 1895 x 55 of of 14 14 2.4. Electromechanical Characterization 2.4. Electromechanical Characterization An electrodynamic shaker was used to emulate environmental vibrations at different input An electrodynamic acceleration magnitudes shaker was Testing (Vibration used to Controller emulate environmental vibrations VR9500, Vibration at different Research, Jenison,input MI, acceleration USA). This wasmagnitudes controlled (Vibration throughTesting Controller a MATLAB VR9500, program thatVibration Research, Jenison, allows automatizing, MI, USA). acquiring, and This was controlled processing the voltagethrough a MATLABatprogram measurements differentthat allows automatizing, frequencies acquiring, and acceleration valuesand processing (Figure 4a,b). the this All voltage datameasurements is measured byatan different frequencies acquisition and acceleration system (National values Instruments (Figure and PCI-6132 4a,b). All this BNC-2110, data is measured National by anAustin, Instruments, acquisition system TX, USA) (National with Instruments very high PCI-6132 and internal impedance BNC-2110, to avoid charge National leakages Instruments, (100 MΩ). AAustin,specialTX, USA) test PCBwith verywas layout highdesigned internal impedance to provideto physical avoid charge leakages support and (100 MΩ). electrical A special test connection toPCB layout the chip was designed (Figure 4c). Thetofabrication provide physical support was carried outand electrical using connection a circuit to the board plotter. chip (Figure Finally, 4c). The two wires werefabrication bonded was carried to the PCB to out usingan obtain a circuit board electrical plotter. Finally, connection two wiresdevice to the mounted were bondedtest under to the PCB to (DUT). A obtain an electrical dual in-line connection package to the (DIP) zero mounted insertion device force under (ZIF) testwas socket (DUT). A dual utilized to in-line package anchor the DUT(DIP)to thezero insertion force (ZIF) socket was utilized to anchor the DUT to the shaker. shaker. (a) (b) (c) Figure Figure 4.4.(a)(a) Characterization Characterization setup setup forpiezoelectric for the the piezoelectric MEMS (b) MEMS devices; devices; (b) microscope microscope camera and DIP ZIF socket to anchor the DUT; (c) fabricated piezoelectric microgenerator bonded camera and DIP ZIF socket to anchor the DUT; (c) fabricated piezoelectric microgenerator to a PCB and mounted on the shaker. bonded to a PCB and mounted on the shaker. Three different electrical characterizations were performed to demonstrate the correct operation of Three different electrical characterizations were performed to demonstrate the correct operation our piezoelectric MEMS generators (Figure 5). The open-circuit voltage of the piezoelectric generator of our piezoelectric MEMS generators (Figure 5). The open-circuit voltage of the piezoelectric was measured while performing a frequency sweep with accelerations ranging from 0.05 G to 0.2 G. generator was measured while performing a frequency sweep with accelerations ranging from 0.05 This characterization allows the measurement of the maximum voltage that the piezoelectric device can G to 0.2 G. This characterization allows the measurement of the maximum voltage that the generate at its resonance frequency (Figure 5a). The second characterization was performed by fixing a piezoelectric device can generate at its resonance frequency (Figure 5a). The second characterization certain acceleration magnitude and sweeping the value of the load resistor. In this way, the value of the was performed by fixing a certain acceleration magnitude and sweeping the value of the load load resistor for an optimal power generation and the maximum generated power at each acceleration resistor. In this way, the value of the load resistor for an optimal power generation and the magnitude can be found (Figure 5b,c). Finally, a Schottky diode bridge and a load capacitor of 10 µF maximum generated power at each acceleration magnitude can be found (Figure 5b,c). Finally, a were connected to the piezoelectric generator subject to a constant acceleration of 0.2 G. The increase of Schottky diode bridge and a load capacitor of 10 µF were connected to the piezoelectric generator the capacitor voltage by using this simple rectification circuit can be observed (Figure 5d). subject to a constant acceleration of 0.2 G. The increase of the capacitor voltage by using this simple As shown in Figure 5a, a resonance peak ranging from 594.5 Hz to 596.5 Hz, depending on the rectification circuit can be observed (Figure 5d). acceleration magnitude, is clearly identifiable. This variation in the natural frequency is due to a As shown in Figure 5a, a resonance peak ranging from 594.5 Hz to 596.5 Hz, depending on the non-linear response of the device, hypothetically caused by the very large amplitudes reached at higher acceleration magnitude, is clearly identifiable. This variation in the natural frequency is due to a accelerations. The quality factor, Q, is also affected by this non-linear effect, which also alters the non-linear response of the device, hypothetically caused by the very large amplitudes reached at structural damping [45–47]. The measured resonance frequencies are quite close to those simulated higher accelerations. The quality factor, Q, is also affected by this non-linear effect, which also alters (604 Hz) by FEM and are shown in Figure 2d. Different load resistors, from 50 KΩ to 950 KΩ, were the structural damping [45–47]. The measured resonance frequencies are quite close to those used to find the optimal value where the extracted power reaches a maximum value of 0.32 µW and simulated (604 Hz) by FEM and are shown in Figure 2d. Different load resistors, from 50 KΩ to 950 1.03 µW for a 0.1 G and 0.2 G of acceleration, respectively (Figure 5b,c). KΩ, were used to find the optimal value where the extracted power reaches a maximum value of Regarding our previous work that used a similar structure, but without trapezoidal shapes or 0.32 µW and 1.03 µW for a 0.1 G and 0.2 G of acceleration, respectively (Figure 5b,c). rounded corners [39], an increase in the power of 60% was obtained. The volume of this device was Regarding our previous work that used a similar structure, but without trapezoidal shapes or 12.9 mm3 (5 mm × 5 mm × 0.516 mm) for the complete chip, and 1.54 mm3 (1 mm × 3 mm × 0.516 mm) rounded corners [39], an increase in the power of 60% was obtained. The volume of this device was for the functional structure composed by the cantilever and the inertial mass (Figure 3a). Therefore, the 12.9 mm3 (5 mm × 5 mm × 0.516 mm) for the complete chip, and 1.54 mm3 (1 mm × 3 mm × 0.516 mm) effective power density for this energy harvester, using the data shown in Figure 5, is 665.4 µW/cm3 for for the functional structure composed by the cantilever and the inertial mass (Figure 3a). Therefore, a vibration acceleration of 0.2 G and a frequency of ~595 Hz. In previous works [48,49], the volume for the effective power density for this energy harvester, using the data shown in Figure 5, is 665.4 the functional structures were 32.2 mm3 (8.2 mm × 7 mm × 0.535 mm) and 18.85 mm3 (6 mm × 7.8 mm µW/cm3 for a vibration acceleration of 0.2 G and a frequency of ~595 Hz. In previous works [48,49], Sensors 2019, 19, x 6 of 14 the volume Sensors for1895 2019, 19, the functional structures were 32.2 mm3 (8.2 mm × 7 mm × 0.535 mm) and 18.85 mm 3 6 of 14 (6 mm × 7.8 mm × 0.402 mm), respectively. Their maximum power values generated were 3.0 µW and 29.7 µW for accelerations of 0.2 G and 0.5 G, respectively. Thence, the corresponding effective × 0.402densities power mm), respectively. Their maximum were 93.1 µW/cm power 3 and 1575.5 µW/cmvalues generated 3. Our deviceswere offer3.0 µW andfor potential 29.7 a µW largerfor accelerations of 0.2 G and 0.5 G, respectively. Thence, the corresponding effective power densities power density. Alternatively, they perform better in terms of the power density per unit of excitation were 93.1 µW/cm3 and 1575.5 µW/cm3 . Our devices offer potential for a larger power density. Alternatively, amplitude. they perform better in terms of the power density per unit of excitation amplitude. . (a) (b) (c) (d) Figure 5. Graphs Figure 5. Graphs ofofthe the electrical electrical characterization characterization of theof the piezoelectric piezoelectric generator:generator: (a) Maximum (a) open-circuit voltage that can be generated by the piezoelectric generator Maximum open-circuit voltage that can be generated by the piezoelectric generator submitted to a frequency sweep at different submitted acceleration to a frequency amplitudes; sweep (b) dependence at different accelerationofamplitudes; the generated(b) power with the value dependence of theof generated power with the value of the load resistor; (c) maximum power that can bevs. the load resistor; (c) maximum power that can be generated at each of the accelerations; (d) voltage time by using generated a Schottky at each diode bridge and of the accelerations; (d)avoltage charge capacitor vs. time of by10using µF with an acceleration a Schottky of 0.2 G, diode bridge showing the charge collection and the energy increase in the capacitor. and a charge capacitor of 10 µF with an acceleration of 0.2 G, showing the charge collection and the energy 3. Converter Circuitincrease in the capacitor. The purpose 3. Converter Circuitof this circuit is the conversion of the alternating current (AC) signal generated by the piezoelectric MEMS and the adaptation of the output voltage to power a utility (i.e., wireless The purpose of this circuit is the conversion of the alternating current (AC) signal generated by sensor node for IoT). In our case, contrary to typical macroscopic piezoelectric generators, the AC the piezoelectric MEMS and the adaptation of the output voltage to power a utility (i.e., wireless voltage generated is smaller than the one required by the final application. In addition, as the key sensor node for IoT). In our case, contrary to typical macroscopic piezoelectric generators, the AC goal, this circuit must be fully able to be integrated. For these reasons, our proposed circuit is a charge voltage generated is smaller than the one required by the final application. In addition, as the key pump, suitable to be implemented in a monolithic system comprised by the converter circuit for energy goal, this circuit must be fully able to be integrated. For these reasons, our proposed circuit is a collection and the piezoelectric resonant MEMS generator. charge pump, suitable to be implemented in a monolithic system comprised by the converter circuit The converter circuit, shown in Figure 6a, consists of several functional blocks. The generator for energy collection and the piezoelectric resonant MEMS generator. block is a model of the equivalent circuit of our piezoelectric. The function of the rectifier block is The converter circuit, shown in Figure 6a, consists of several functional blocks. The generator the conversion of the AC current to the direct current (DC) in order to charge the first capacitor (C1 ). block is a model of the equivalent circuit of our piezoelectric. The function of the rectifier block is the The level detector waits for a certain threshold voltage in the first capacitor to power the oscillator conversion of the AC current to the direct current (DC) in order to charge the first capacitor (C1). The once reached. Its function is to avoid energy consumption while charging the first storage capacitor. level detector waits for a certain threshold voltage in the first capacitor to power the oscillator once Then, the oscillator produces a periodic square wave of 5 kHz, by converting the DC voltage from reached. Its function is to avoid energy consumption while charging the first storage capacitor. Then, the first capacitor [50]. The outputs of the oscillator are connected to another rectifier through two the oscillator produces a periodic square wave of 5 kHz, by converting the DC voltage from the first capacitors, in order to double the voltage in the second capacitor. The counter block is utilized as a powered. For this to be achieved, we integrated a level detector and a counter. This working principle is compatible with the typical operation of the wireless sensor nodes used for IoT, where the duty cycle is very low (i.e., the device is normally in stand-by mode and only operates periodically for short cycles of time). The designs and simulations of the converter were performed by using CADENCE-Virtuoso Sensors 2019, 19, 1895 7 of 14 6.1.6-64b. We selected Silterra C18G—180 nm CMOS logic 1.8 V/3.3 V as the technology for this project. This is especially interesting because they can integrate AlN as piezoelectric material in its timer standardto letCMOS the capacitor, C1 , toFurthermore, processes. be charged again [51].technology Silterra Its function offers is to reset an the level detector optimal everyof combination n cycles of the oscillator. performance in terms of power, speed, and gate density. (a) (b) Figure6.6.(a) Figure (a)Schematic Schematicof of thethe converter converter circuit, circuit, showing showing the different the different functionfunction blocks; blocks; (b) graph(b) of the simulated graph of the open-circuit voltages at capacitor, simulated open-circuit voltagesCat1 and C2 , for the capacitor, piezoelectric C1 and MEMS C2, for the generator piezoelectric connected to the converter MEMS generator circuit. connected to the converter circuit. The The most model energy of the consuming equivalentpart of the converter piezoelectric circuitcircuit is the oscillator developed by S. Roundythat forms [23] the voltage consists in elevator stage, shown in Figure 6a. The entire converter circuit consumes modelling both the mechanical and electrical behaviour of the piezoelectric system as circuit 1.875 µA from which the oscillator elements. stage uses 825 This block nA.the solves This corresponds simplified to 44% of differential the total motion consumption. equation The instantaneous (i.e., the lumped parameter power model of the forced damped spring-mass system with piezoelectric transduction) oscillator. generated by the piezoelectric generator is lower than the consumption of the and generates For this an reason, we need a first block that rectifies and collects charge AC signal from a mechanical motion. The use of an ideal model containing only in a first capacitor (C ) and additional 1 an AC source, voltage elevator the which provides blocks that allow the corresponding increase of open-circuit the final voltage voltage at the (Figure resonance 6b). Once frequency, is the energy a common accumulated is sufficiently large, the remaining components of the charge pump alternative to model these energy harvesters. However, due to its simplicity, the simulation results are powered. For this to be achieved, we integrated a level detector and a counter. This are not realistic and the design of the converter circuit might not work properly. An ideal working principle is compatible with the typical transformer operation is used of the wireless by S. Roundy to model sensor and nodes usedthe represent forpiezoelectric IoT, where the duty cycle coupling. is very In this case,lowas (i.e., the device is normally in stand-by mode and only operates periodically shown in Figure 7a, this transformer was replaced by two dependent sources, thus avoiding for short cycles of time). The designs modelling issues and simulations in different of thesimulation electrical converter software, were performed where the by using ratio, n,CADENCE-Virtuoso of the transformer 6.1.6-64b. We selected Silterra C18G—180 nm CMOS logic 1.8 V/3.3 depends on the inverse of the Young's modulus, Ck, and the capacitance of the piezoelectricV as the technology for this project. layer, Cb. This is especially interesting because they can integrate AlN as piezoelectric These sources, E and F, represent the effects produced by the voltage on the mechanical response of material in its standard CMOS the deviceprocesses. and theFurthermore, Silterraastechnology generated current a result of offers an optimalstress, the mechanical combination of performance respectively. The voltage in terms of power, speed, and gate density. source, σin, models the mechanical stress produced by the ambient vibration. The inductive element, The modelthe Lm, represents of the equivalent inertial mass piezoelectric of the generator,circuitand developed by S. Roundy relates stress [23] consists to the second derivatein modelling of strain. both the mechanical and electrical behaviour Finally, Rb symbolizes the mechanical damping of the system. of the piezoelectric system as circuit elements. This block solves the simplified differential motion equation (i.e., the lumped parameter model of the forced damped spring-mass system with piezoelectric transduction) and generates an AC signal from a mechanical motion. The use of an ideal model containing only an AC source, which provides the corresponding open-circuit voltage at the resonance frequency, is a common alternative to model these energy harvesters. However, due to its simplicity, the simulation results are not realistic and the design of the converter circuit might not work properly. An ideal transformer is used by S. Roundy to model and represent the piezoelectric coupling. In this case, as shown in Figure 7a, this transformer was replaced by two dependent sources, thus avoiding modelling issues in different electrical simulation software, where the ratio, n, of the transformer depends on the inverse of the Young’s modulus, Ck , and the capacitance of the piezoelectric layer, Cb . These sources, E and F, represent the effects produced by the voltage on the mechanical response of the device and the generated current as a result of the mechanical stress, respectively. The voltage source, σin , models the mechanical stress produced by the ambient vibration. The inductive element, Lm , represents the inertial mass of the generator, and relates stress to the second derivate of strain. Finally, Rb symbolizes the mechanical damping of the system. Sensors 2019, 19, 1895 x 88of of 14 (a) (b) (c) (d) (e) Figure Figure 7.7.Schematic Schematic ofdifferent of the the different blocks blocks that that constituent constituent the converterthe converter circuit: circuit: (a) Generator (a) circuit; (b) rectifier; circuit; Generator (c) counter; (d) level detector; (b) rectifier; (e) oscillator. (c) counter; (d) level detector; (e) oscillator. For the For the rectifier rectifier circuit, circuit,shownshownininFigure Figure7b, 7b,asasinput, input,InInA, Ais, higher is higher thanthanInB,IntheB , the transistors, transistors, M1 M and and 1 M2, are M , are on and the input, 2 on and the input, InA, is connected In A , is connected to the output, to the output, OutHigh, and Out High , and In InB is connected B is connected to OutLow to. Out In Low . In contrast, contrast, as the input, as the InBinput, , is higherInB , than is higher than InA, the InA , the transistors, transistors, M0 and M3M 0 and , are on andM3 , areInA on andand InB InareA and connectedIn B are connected to OutLow and Out to Out and Out High, respectively. Low High , respectively. Therefore, a full-wave Therefore, a full-wave rectified sinusoidal signal is rectified sinusoidal signal is generated. generated. To prevent To prevent the the oscillator oscillator from from working working all all the the time time andand consuming consuming power, power, aa counter counter formed formed by by D flip-flops was implemented (Figure 7c). This counter receives D flip-flops was implemented (Figure 7c). This counter receives the input from the oscillator output the input from the oscillator output signal and signal and once once itit counts counts16 16level levelchanges changesatatthe theinput inputofofthe thefirst firstflip-flop, flip-flop, thethelast flip-flop last flip-flop changes changes its its level. The output of this counter (Rst_det) is connected to the reset input of the level detector (Rst_det).). level. The output of this counter (R st_det ) is connected to the reset input of the level detector (R st_det In this In this way, way, every every 16 16 cycles cycles of of the the oscillator, oscillator,the thedetector detectorwill willbe bereset. reset. Figure 7d shows the schematics of the level Figure 7d shows the schematics of the level detector circuit. The detector circuit. The transistors, transistors, M0M ,M 0 ,1,M M12,, M and2 , and M 3, M form , form a current source from which the reference for the oscillator 3 a current source from which the reference for the oscillator is taken. The D flip-flop always has is taken. The D flip-flop always hasdata its its data input,input, D, connected D, connected to to VinV(i.e., in (i.e., thethe power power supply supply correspondingtotothe corresponding thevoltage voltageof of the the first first capacitor, C ). In this way, D is always set to a logic “1”. From capacitor, C1). In this way, D is always set to a logic “1”. From the beginning, the D flip-flop starts 1 the beginning, the D flip-flop starts with with its its outputs outputs (Q, nQ) == (0,1), (Q, nQ) (0,1), therefore, therefore, V Vout2 = ”0” and the M transistor enables the operation of out2 = ”0” and the M44 transistor enables the operation of the current the current comparator, comparator, implemented implemented with with M M33andandM M5.5The . TheCC input input LKLK of ofthethe flip-flop flip-flop is connected is connected to to V Vc2, so so the flip-flop changes its output c2, the flip-flop changes its output state and Vout2 out2 state and V = ”1”. Hence, at the output = ”1”. Hence, at the output of the detector, the of the detector, the voltage voltage of the of the input, input, Vin,Visinobtained, , is obtained, andand thethe M4 M 4 transistor transistor goesgoesoff.off. TheThe factfact that that thethe M4Mtransistor 4 transistor is is off implies that the detector stops consuming power. Once the off implies that the detector stops consuming power. Once the detector is activated and the output is detector is activated and the output is connected to V (as shown in Figure 6a), the only way to change connected to Vinin (as shown in Figure 6a), the only way to change its state is to restart it through the its state is to restart it through the CLR C input. The LR input. TheCCLRLRinput inputresetsresetsthe theDDflip-flop flip-flopand andcauses causes the the detector detector output output to to be be set set to to “0”and “0”and M M44 goes on again to start a new goes on again to start a new detection cycle. detection cycle. The oscillator The oscillator circuit circuit is is shown shown in in Figure Figure 7e, 7e, where where the the schematic schematic of of the the circuit circuit thatthat generates generates V Vrcrc can be can be seen seen on on the the left left side. This signal side. This signal is is used used as as aa reference reference for for the the comparator comparator to to detect detect when when the the capacitor connected capacitor connected to to the the input inputof ofeach eachcomparator comparatorisischarged. charged.Once OncenV nVout_det becomes ”0”, out_det becomes ”0”, M starts M22 starts driving and the voltage driving and the voltage reference, Vrc reference, V , is obtained at the drain rc, is obtained at the drain of M4 of M . With this configuration, 4. With this configuration, we can we can obtain aa voltage obtain voltage reference reference and and the the transistors, transistors,M M00,, M M11,, MM22,, M M33,, and and M M44,,that that form form aa current current source source do do not consume power while the oscillator is not working. On the not consume power while the oscillator is not working. On the right side, we can see the schematic of right side, we can see the schematic of the circuit the circuit that that performs performs the charge and the charge and discharge discharge of of the the capacitors. capacitors. Four Four switches switches are are formed formed by by two two PMOS transistors (switches 2 and 3) and two NMOS transistors (switches 1 and 4). It is assumed that Sensors 2019, 19, 1895 9 of 14 PMOS transistors (switches 2 and 3) and two NMOS transistors (switches 1 and 4). It is assumed that at the initial operation state, the outputs of the SR flip-flop are (Q, nQ) = (0, 1). With this combination of outputs, switches 1 and 3 are closed, causing the capacitor, Cap2 , to charge and the capacitor, Cap1 , to discharge. When the capacitor, Cap2 , reaches the reference value (Vrc ), the output of the second comparator Sensors 2019, 19,changes x its state. Likewise, when the capacitor, Cap1 , is discharged and decreases below 9 of 14 Vrc , the output of the first comparator change its state. This forces the outputs to be (Q, nQ) = (1, 0) and switches 1 and at the initial 3 to open operation andthe state, switches outputs2 and 4 to of the SRclosed, theare flip-flop originating (Q, nQ) =capacitor, (0, 1). WithCap1 , tocombination this charge and the capacitor, of outputs, Cap2 , to1discharge. switches and 3 are When closed,the capacitor, causing Cap1 , reaches the capacitor, Cap2the , toreference charge and value, the V rc , the output capacitor, Cap1, of to the first comparator discharge. When the changes its state. capacitor, Cap2,In this way, reaches thethe outputsvalue reference of the (V comparators rc), the outputwill of oscillate from the second one state to another, comparator changes generating a squarewhen its state. Likewise, wavethe signal. The inverters capacitor, connected between Cap1, is discharged and decreasesthe outputs below of Vrcthe , theflip-flop output and of thethe output first of the oscillator comparator change its (nVstate. , Vout_osc This out_osc forces, the andoutputs CLK ) avoid to bethe(Q,current nQ) = (1,flow0) towards and switches to the1 output and 3 toofopenthe oscillator and therefore, and switches 2 and 4 to eliminates closed, thethe associated originating power consumption. capacitor, Cap1, to charge and the In order to increase capacitor, Cap2, the output voltage to discharge. When to the the desired capacitor,value, Cap1additional , reaches the rectifiers referenceand value, the associated Vrc, the capacitors output of the can first be added at the voltage comparator changeselevator its state. stage. Figure In this way,8 theshows the simulated outputs voltages at will of the comparators the output (V oscillate from C2 ) for different converter circuits with three and five stages. Each one state to another, generating a square wave signal. The inverters connected time a rectifier is added, the time to between thereach the of outputs desired outputand the flip-flop voltage is doubled. the output of the This is because oscillator each (nVout_osc , Vtime out_osc,we andtransfer CLK) avoidthe energy the currentfrom oneflowcapacitor towardstotoanother, half ofofthe the output theenergy is lostand oscillator in that transfer.eliminates therefore, This is a rather inefficient the associated strategy, but it is principally the only way of allowing a full monolithic integration. power consumption. (a) (b) (c) Figure8.8.(a)(a) Figure Schematic Schematic of theofconverter the converter circuit circuit with with stages; n rectifier n rectifier stages; (b,c) (b,c) simulated simulated output voltages with output voltages with a converter circuit with (b) three and (c) five stages and a valueC1ofand a converter circuit with (b) three and (c) five stages and a value of 20 nF for the capacitors, 20 C nF2 , when connected for the to the piezoelectric capacitors, C1 and C2, generator and submitted when connected to anpiezoelectric to the ambient vibration acceleration generator and of 0.1 G. submitted to an ambient vibration acceleration of 0.1 G. In order to increase the output voltage to the desired value, additional rectifiers and the associated capacitors can be added at the voltage elevator stage. Figure 8 shows the simulated voltages at the output (VC2) for different converter circuits with three and five stages. Each time a rectifier is added, the time to reach the desired output voltage is doubled. This is because each time we transfer the energy from one capacitor to another, half of the energy is lost in that transfer. This is a rather inefficient strategy, but it is principally the only way of allowing a full monolithic integration. Sensors 2019, Sensors 2019, 19, 19, 1895 x 10 10 of 14 of 14 4. Application Example 4. Application Example The proposed energy harvesting has potential applications related to the use of self-powered sensorThe proposed networks forenergy harvesting hasand smart environments potential applications predictive related maintenance. Onetospecific the usecaseof self-powered is connected sensor to the networks railway for smartinenvironments sector, and predictive order to extend maintenance. the maintenance One specific intervals and tocase is connected allow predictiveto the railway sector, maintenance of wheelin order to extend bearings, the maintenance gearboxes, and motors. intervals SKF, aand to allow leading globalpredictive technology maintenance provider of andwheel partnerbearings, of the gearboxes, EnSO project and(Energy motors.forSKF, a leading Smart Objects, global technology Contract provider No. 692482), hasand partner us provided of the EnSO project (Energy for Smart Objects, Contract No. 692482), has provided with different measurements of vibrations present in trains in operation. The vibration spectra of us with different measurements these measurementsof vibrations present in trains show accelerations fromin 0.05 operation. G to 0.75 TheGvibration spectra ofranging with frequencies these measurements from 100 Hz show to 1000accelerations Hz. Our devicesfrom 0.05 can G betodesigned 0.75 G with frequencies to work in this ranging range offrom 100 Hz to frequencies 1000 and Hz. Ourthey therefore, devices are can be designed to work in this range particularly suitable for this application. of frequencies and therefore, they are particularly suitable for this application. In order to simulate the voltages and currents in the internal components of the converter duringIn order to simulate the long the voltages charge periods, and a long currentscomputation transient in the internal components with a very fine ofresolution the converter during is needed. the For long this charge reason,periods, a long transient the previous simulationscomputation with a very were performed withfinea resolution value of 20 is needed. nF for the For two this reason, capacitorsthe(C previous 1 and Csimulations were performed 2) and an acceleration of 0.15withG. a value of 20 However, thisnFCfor the two capacitors 2 capacitance value is (C not1 and C enough ) and an acceleration of 0.15 G. However, 2 to power a wireless node sensor. A new simulation this C 2 capacitance value is not enough of the converter was carried out withto power a wireless node sensor. four rectifying stages A new and simulationvalues capacitance of theofconverter 20 nF and was carried 1 µF for Cout with 1 and C2four rectifying(Figure , respectively stages and 9a). capacitance values of 20required This single simulation nF and 1aµF for capacity high C1 and Cof 2 , respectively calculation (Figure (159 GB 9a). of This data) single and ansimulation intensive required a high use and continuous capacity of calculationresources of computational (159 GB (17of data) and an intensive h of simulation with a and continuous dedicated serveruse of of 10 computational cores and 128 GB resources of RAM). (17 h of simulation with a dedicated server of 10 cores and 128 GB of RAM). (a) (b) Figure9.9.(a) Figure (a)Simulated Simulatedoutputoutput voltages voltages withwith a converter a converter circuitcircuit with with four four rectifying rectifying stages, 20stages, nF for 20 nF for(C capacitor capacitor 1 ), and 1 (C µF 1 ), for and 1 µF capacitorfor C2 ;capacitor (b) typicalCduty 2; (b) typical cycle of a duty cycle wireless of sensor a wireless node sensor powered by energy harvesting. node powered by energy harvesting. It can be seen that once the capacitance value of the final capacitor (C2 ) is increased to 1 µF, It can be seen that once the capacitance value of the final capacitor (C2) is increased to 1 µF, the the final voltage does not change, demonstrating that the converter works correctly regardless of the final voltage does not change, demonstrating that the converter works correctly regardless of the capacitance value of the load capacitor. The energy accumulated in C2 at the end of the simulation for capacitance value of the load capacitor. The energy accumulated in C2 at the end of the simulation a capacitor of 1 µF and a potential difference of 3.325 V is 5.53 µJ. for a capacitor of 1 µF and a potential difference of 3.325 V is 5.53 µJ. A typical duty cycle of a wireless sensor node powered by energy harvesting is shown in A typical duty cycle of a wireless sensor node powered by energy harvesting is shown in Figure Figure 9b. Hypothetically, the input energy flow generated by a piezoelectric harvester is continuous. 9b. Hypothetically, the input energy flow generated by a piezoelectric harvester is continuous. The The harvested energy, at the elevated voltage, is accumulated in the capacitor, C2 , to power the harvested energy, at the elevated voltage, is accumulated in the capacitor, C2, to power the transmitter module once this energy is higher than that needed to power the transmitter module. transmitter module once this energy is higher than that needed to power the transmitter module. The transmitter module only consumes when it works every x cycles of time. As an application The transmitter module only consumes when it works every x cycles of time. As an application example, Enocean ECO200 is an energy harvesting module that can power the transmitter module, example, Enocean ECO200 is an energy harvesting module that can power the transmitter module, PTM330, in order to implement a wireless sensor. The energy output of the ECO200 (120–210 µJ) is PTM330, in order to implement a wireless sensor. The energy output of the ECO200 (120–210 µJ) is sufficient to transmit three sub-telegrams through the PTM330 module. In this way, when the final sufficient to transmit three sub-telegrams through the PTM330 module. In this way, when the final capacitor of the converter, C2 , is resized to 22 µF and four rectifying stages are used, our system capacitor of the converter, C2, is resized to 22 µF and four rectifying stages are used, our system can can accumulate 121 µJ of energy, which is compatible with the operation of the transmitter, PTM330, accumulate 121 µJ of energy, which is compatible with the operation of the transmitter, PTM330, every 22 min. every 22 min. To increase the energy generated or reduce the charging time of the capacitor, one possible solution To increase the energy generated or reduce the charging time of the capacitor, one possible is to connect n piezoelectric generators, with their respective rectifiers, in parallel. When connecting n solution is to connect n piezoelectric generators, with their respective rectifiers, in parallel. When Sensors 2019, 19, x 11 of 14 Sensors 2019, 19, 1895 11 of 14 connecting n devices in parallel, the final voltage at the capacitor, C2 (Figure 6a), will be the same, but the charge devices time the in parallel, willfinal be nvoltage times shorter. at the capacitor, C2 (Figure 6a), will be the same, but the charge time will be n times shorter. 5. Proposed Monolithic System and Fabrication Process 5. Proposed Monolithic System and Fabrication Process In this work, we propose the use of a single chip composed by the energy harvester, the power management circuit, In this work, we and the storage propose the use capacitors. Withcomposed of a single chip this monolithic by theintegrated circuit based energy harvester, on a the power CMOS-compatible management technology circuit, [33], the and the storage cost of the capacitors. energy With harvestingintegrated this monolithic system can be reduced. circuit based onIn a addition, taking advantage CMOS-compatible technology of [33], the silicon the costarea onenergy of the top of harvesting the cantilever andcan system thebe rest of the chip reduced. to add In addition, the integrated taking advantagecircuits of theand capacitors silicon (Figure area on top of the10a), the area cantilever andofthe the entire rest of thesystem chip tocan addbethereduced. In integrated this way, circuits andarrays of energy capacitors (Figure harvesting systems 10a), the area of thecan be system entire incorporated can be in the same reduced. area, In this obtaining way, arrays ofa higher power energy density. harvesting systems can be incorporated in the same area, obtaining a higher power density. . (a) (b) Figure10. Figure 10.(a) (a)Proposed Proposed monolithic monolithic system, system, composed composed by the by the converter converter circuit, circuit, the the load load capacitors, and the piezoelectric MEMS structure for energy harvesting; (b) cross-section sketch of capacitors, and the piezoelectric MEMS structure for energy harvesting; (b) cross-section the fabrication process forthe sketch of the fabrication proposed monolithic system. process for the proposed monolithic system. Our Our suggestion suggestionisisthe thefabrication fabrication ofof MEMS MEMS structures following structures a post-CMOS following a post-CMOS strategy (Figure strategy 10b). (Figure The 10b).fabrication process The fabrication starts with process startsa CMOS with a standard process on CMOS standard an SOI process onwafer an SOIto create wafer the converter to create the circuit. converter circuit. The following step consists in the definition of the piezoelectric transducerand The following step consists in the definition of the piezoelectric transducer and the the load capacitors. The piezoelectric transducer, as already mentioned, is created load capacitors. The piezoelectric transducer, as already mentioned, is created when depositing, bywhen depositing, by radio-frequency radio-frequency (RF) (RF) sputtering, sputtering, a piezoelectric a piezoelectric layerand layer andtwo twometal metallayers layersthat thatform formthe theelectrodes. electrodes. Finally, Finally, a wet etch based on a buffered solution of hydrofluoric acid (BHF) is used to release the a wet etch based on a buffered solution of hydrofluoric acid (BHF) is used to release the free-standing cantilever. free-standing cantilever. 6. Conclusions 5. Conclusions In this paper, we proposed the integration of a single chip composed of an energy harvester, In this paper, we proposed the integration of a single chip composed of an energy harvester, power management circuit, and storage capacitors. With this monolithically integrated circuit based power management circuit, and storage capacitors. With this monolithically integrated circuit based on a CMOS-compatible technology, the cost and the area of the entire system can be reduced. on a CMOS-compatible technology, the cost and the area of the entire system can be reduced. The stress distribution in the cantilever structure of the piezoelectric devices was optimized using The stress distribution in the cantilever structure of the piezoelectric devices was optimized beams with trapezoidal shapes and rounded corners to achieve a homogeneous stress over the entire using beams with trapezoidal shapes and rounded corners to achieve a homogeneous stress over the surface. With this design optimization, an increase of 60% in the power generated was obtained. entire surface. With this design optimization, an increase of 60% in the power generated was The correct operation of the charge pump was demonstrated by means of electrical simulations obtained. computed with CADENCE Virtuoso. We confirmed that a single piezoelectric generator can obtain a The correct operation of the charge pump was demonstrated by means of electrical simulations power density greater than 665 µW/cm3 for a vibration acceleration of 0.2 G at a resonance frequency computed with CADENCE Virtuoso. We confirmed that a single piezoelectric generator can obtain a of ~595 Hz. Furthermore, by connecting3 n piezoelectric devices in parallel, we obtained power outputs power density greater than 665 µW/cm for a vibration acceleration of 0.2 G at a resonance frequency in the order of mW in a volume smaller than a few mm3 . of ~595 Hz. Furthermore, by connecting n piezoelectric devices in parallel, we obtained power Our integrated system with a storage capacitor, C , of 22 µF allows self-powered operation in real outputs in the order of mW in a volume smaller than 2a few mm3. conditions of a transmission module and its associated sensors every ~20 min. Our integrated system with a storage capacitor, C2, of 22 µF allows self-powered operation in real conditions of a transmission module and its associated sensors every ~20 min. Sensors 2019, 19, 1895 12 of 14 Author Contributions: G.M. provided the main idea of the designs of the energy harvesters and the fully monolithically integrated system; E.L.-S. performed the simulations to optimize the designs of the energy harvesters; M.D. and J.S. designed and simulated the converter circuit; M.D. performed the characterization of the harvesters; M.D. and G.M. wrote and reviewed the document; G.M., M.D., J.E. and J.S. contributed to the discussion of the obtained data and to the final version of the manuscript. Funding: The authors M. Duque and G. Murillo from the Instituto de Microelectrónica de Barcelona would like to thank the financial support of EnSO project (Energy for Smart Objects), Contract No. 692482. In addition, G. 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