Optimization of a Piezoelectric Energy Harvester and Design of a Charge Pump Converter for CMOS-MEMS Monolithic Integration

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: AlN; CMOS; IoT; MEMS; charge pump; energy harvesting; monolithic integration; piezoelectric; power management; self-powered.

Figure 1

(a) Functional device configuration: A cantilever structure with a tip mass and a piezoelectric material deposited on top of the beam; (b) cross-section sketch of the cantilever structure; (c) different steps of the fabrication process.

Figure 2

Simulated results of the piezoelectric device by using COMSOL multiphysics: (a) Mesh used in the simulation; (b) 3D FEM simulation of the stress distribution in the cantilever beam and (c) top view of the structure with the trapezoidal beam shape detailed (arbitrary units); (d) maximum open-circuit voltage that can be generated by the piezoelectric generator submitted to a frequency sweep, at different values of the input acceleration ranging from 0.05 G to 0.20 G.

Figure 3

(a) Layout of a piezoelectric microgenerator (5 mm × 5 mm × 0.516 mm); (b) 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.

Figure 4

(a) Characterization setup for the piezoelectric MEMS devices; (b) microscope camera and DIP ZIF socket to anchor the DUT; (c) fabricated piezoelectric microgenerator bonded to a PCB and mounted on the shaker.

Figure 5

Graphs of the electrical characterization of the piezoelectric generator: (a) Maximum open-circuit voltage that can be generated by the piezoelectric generator submitted to a frequency sweep at different acceleration amplitudes; (b) dependence of the generated power with the value of the load resistor; (c) maximum power that can be generated at each of the accelerations; (d) voltage vs. time by using a Schottky diode bridge and a charge capacitor of 10 µF with an acceleration of 0.2 G, showing the charge collection and the energy increase in the capacitor.

Figure 6

(a) Schematic of the converter circuit, showing the different function blocks; (b) graph of the simulated open-circuit voltages at capacitor, C₁ and C₂, for the piezoelectric MEMS generator connected to the converter circuit.

Figure 7

Schematic of the different blocks that constituent the converter circuit: (a) Generator circuit; (b) rectifier; (c) counter; (d) level detector; (e) oscillator.

Figure 8

(a) Schematic of the converter circuit with n rectifier stages; (b,c) simulated output voltages with a converter circuit with (b) three and (c) five stages and a value of 20 nF for the capacitors, C₁ and C₂, when connected to the piezoelectric generator and submitted to an ambient vibration acceleration of 0.1 G.

Figure 9

(a) Simulated output voltages with a converter circuit with four rectifying stages, 20 nF for capacitor (C₁), and 1 μF for capacitor C₂; (b) typical duty cycle of a wireless sensor node powered by energy harvesting.

Figure 10

(a) Proposed monolithic system, composed by the converter circuit, the load capacitors, and the piezoelectric MEMS structure for energy harvesting; (b) cross-section sketch of the fabrication process for the proposed monolithic system.