(Invited) Wafer Level Fabrication Process of High Performance Micro-Supercapacitors
Authors: Robert Kevin , Eustache Etienne , Brousse Kevin , Huang Peihua , Douard Camille , Brachet Mylène , Le Bideau Jean , Daffos Barbara , Taberna Pierre-Louis , Simon Patrice , Brousse Thierry , Lethien Christophe 
Published: February 2020

Nowadays, miniaturized power sources are key devices for providing autonomy to smart and connected sensors. To reach this goal, the current trend is to fabricate micro-storage unit such as microbatteries (MB) or microsupercapacitors (MSC) at the wafer level. According to the limited surface of such device, the metrics should be reported as normalized to the footprint area (mF, µWh and mW per cm2). With such metrics, the average energy density of most of MSC1,2 (Onions like carbon, graphene) is found to be close to 1 µWh/cm2which is not enough to get autonomous miniaturized and connected sensors. Within this study, two different approaches will be reported to significantly improve the performances of MSC.
One attractive way to improve the areal energy density of MSC is to enhance the material mass loading while keeping low the footprint area. To meet this requirement3, on chip fabrication of 3D MSC is proposed. Wafer level fabrication process on high area enlargement factor 3D scaffold (AEF # 50) is achieved. Step conformal deposition of MnO2 thin film (electrophoretic deposition) on this 3D template is performed and the MSC are tested either in aqueous electrolyte (0.5M Na2SO4) or in EMI-TFSI aprotic ionic liquids (AILs). The areal energy density of the proposed micro-device is close to 10 µWh/cm2 while maintaining high areal power density of 10 mW/cm2.
Carbon derived carbide (CDC) based MSC is the second followed way to propose on chip high performance device. The fine-tuned chlorination of TiC thin films deposited and patterned as interdigitated planar electrodes is performed in the framework of this study4. CDC based MSC were directly fabricated on silicon wafer. The corresponding areal energy density is around 1 µWh/cm2 for aqueous based device (1M H2SO4) and higher than 20 µWh/cm2 for MSC tested in organic electrolyte (2M EMI-BF4 in ACN). The power density is measured to be close to 100 mW/cm2.
Finally, MSC based on vanadium nitride5 thin films grown either by sputtering technique (for planar MSC) or by atomic layer deposition technology (for 3D MSC) will be reported. The objective of these studies is to clearly find the suitable technology allowing to propose the fabrication of high areal energy density, small footprint area MSC and to move toward solid state configuration.

References

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Acknowledgments: The authors want to thank the French network of the electrochemical energy storage (RS2E) for this support. The French RENATECH network is greatly acknowledged.