Methods for the Integration of New Architectural Nanostructures with MEMS Systems for Sensors and Harvester Devices
Authors: Gartner Mariuca , Chelu Mariana , Stroescu Hermine , Calderon Moreno Jose Maria , Moldovan Carmen , Firtat Bogdan , Anghelescu Adrian , Brasoveanu Costin , Dinescu Maria , Andrei Florin 
Published: May 2020

Introduction

This work is addressing the development of new, improved piezomaterials and devices with the goal to build high efficiency energy microharvesters as alternative sources of sustainable energy, and resonant piezo sensors for bio and chemical sensing. Integration of piezolayers in MEMS devices is the key issue of the successful harvester and sensors fabrication. An integrable piezomaterial appeared as a stringent need of our days in answering the environmental problems such as climate change, having as consequence the need to decrease the non-renewable fuels consumption.
Piezoelectric MEMS have been an attractive technology for the generation of a small amount of energy from the vibrations and movements of the environment. This technology can eliminate the replacement of chemical batteries or complex cables from microsensors / microsystems, bringing us closer to autonomous sensor systems and networks. To achieve this, a fully assembled energy piezoelectric micro generator should be able to provide approximately 200 µW of continuous power from ambient vibration and its cost should be low enough for large-scale manufacturing.
On the other hand, a piezolayer integrated into a resonant device like a thin silicon membrane working at high frequency would be a very good solution for sensitive chemo and biosensors dedicated to environment monitoring.

Device structure

We are focusing on the structure of a MEMS harvester which demonstrates the technology of new piezolayers with high piezocoefficients. The harvester includes a MEMS device based on an array of released silicon micrometric structures (cantilevers, bridges) electrically connected together for increasing the power density. Each MEMS structure was covered with a thick piezoelectric film having the role of converting mechanical energy into electrical energy (direct piezoelectric effect). The best piezoelectric film known until now is PZT. Replacing PZT films with lead-free ones (for environment protection) was one of our main goals.
For this purpose, we prepared on the one hand PZT thin film (deposited by Pulsed Laser Deposition) and on the other hand ZnO-based structures, and we studied comparatively their piezoelectric properties. It was found that the piezoelectric coefficients d33 for both type of films are similar.

Preparation Method of the complex structure based on ZnO

A new approach was to obtain a sample of stack layer consisting of hierarchical PMMA / CuO NSs / ZnO NWs / ZnO seed layer (as shown in the figure below).

Fig.1 Stack layer of PMMA / CuO NSs / ZnO NWs / ZnO seed layer
The structures consist of a ZnO seed layer deposited by the Sol-Gel method, over which a ZnO film was hydrothermally grown. Vertically ZnO nano wires (NWs) were formed after this step. One layer of nanostructured (NS) CuO was deposited on the ZnO-NWs and on top two layers of poly (methyl methacrylate) (PMMA) were added.
The results highlight the influence of the thickness of the ZnO seed layer on the distribution and density of the nanowires grown by hydrothermal synthesis. Also, the temperature chosen for annealing plays a major role in the formation of well-aligned and strongly oriented ZnO NWs along the c axis, a role confirmed by the SEM and TEM images
The piezoelectric response of this compound was highly dependent on the substrate. For this purpose, we used simple flexible metal substrate, such as Ti, and silicon substrates with several layers, lithographically patterned, as Pt/Ti/SiO2/Si or Au/Ti/SiO2/Si. In the case of silicon substrate, the MEMS test structures needed to be thinned up to 10 microns and this was done by back side DRIE etching of silicon wafer.

Results and Conclusion

The thickness of the substrate contributes to the mechanical deformation of the entire structure and therefore a piezoelectric response is generated. All films were characterized (structurally, morphologically), compared and optimized in order to obtain the best piezoelectric response (evaluated by the d
33
parameter). To verify the stability of the results, the piezoelectric measurements were repeated daily, for one week.
We concluded that the best piezofilm without lead, able to be integrated and technological compatible with a MEMS device is a stack layer consisting of PMMA / CuO NSs / ZnO NWs / ZnO seed layer

Acknowledgements: The financial support of the project PN-III-P1-1.2-PCCDI-2017-0419 and EU (ERDF) and Romanian Government for the research infrastructure under POS-CCE O 2.2.1 project INFRANANOCHEM – No. 19/01.03.2009 is gratefully acknowledged.

Figure 1