(Invited) Lithium-Based Components Integrated on Silicon: Disruptive, Promising and Credible Solutions for 5G & Beyond
Authors: Lamy Yann , Dupont Florian , Rodriguez Guillaume , Bedjaoui Messaoud , Perreau Pierre , Bousquet Marie , Reinhardt Alexandre , Oukassi Sami 
Published: July 2022

Following a trend similar to Moore’s low which prevailed for decades for active circuits, RF integrated passive components have reinvented themselves over the years in order to sustain continuous performance and size requirements. Their roadmap is still unrolling, thanks to a wide variety of new materials integration: high-k dielectrics for capacitors[1]
,[2]
, magnetic material for inductors [3], aluminium nitride[4] (now scandium doped) for RF filters, or more recently phase-change materials for RF switches[5].
In the last few years, RF integrated passives built upon Lithium-based materials have attracted strong attention because of their state-of-the-art performances and their direct integration on silicon wafers. Lithium-based piezoelectric materials are used since 40 years by the SAW filters industry, which processes LiTaO3 (LTO) or LiNbO3 (LNO) bulk wafers in dedicated fabs. Recently, however, layered SAW devices exploiting thin films of these materials directly on a silicon wafer have exhibited dramatically improved performances. These devices leverage the latest developments in single crystal Li-based layer transfer, or in deposition techniques (PVD, ALD[6], Pulse-Laser-Deposition, ...) of epitaxial, textured, or amorphous Li-based thin films, all of which achievable in industrial grade semiconductor equipments.
In this presentation, we will give an overview of the potential of integrating lithium-based materials on silicon through different examples of promising RF components for 5G. First, we will show how the availability of Li-based Piezoelectric-on-Insulator (POI) wafers[7] is a game changer for 5G filtering. We will present very promising perspectives regarding the development of LNO-based Bulk Acoustics Wave filters (BAW)[8]
,[9],[10]
which aim at extending the application space of POI SAW filters towards the upper 5G bands and even Wi-Fi 6E [5-7 GHz] . Different examples of Li-based materials integrations will be given3,4,5,[11]
. Secondly, we will discuss the potential of a new type of Li-based hybrid micro supercapacitors integrated on silicon. LiPON thin films offer a unique combination of dual properties, being both a dielectric and an electrolyte[12]. Their integration on silicon is not only bringing potentially ultra-high capacitance densities, but also local on-chip energy storage for 5G components, opening a new paradigm in use of the device in a system[13]
,[14]
. After that, we will open the horizon of the potential of Li-based materials integration towards other types of RF devices, like RF switchs, and elaborate on their synergy with Li-transistors for neuromorphic applications[15] and with more conventional lithium microbatteries integrated on silicon[16]. Finally, the integration of Lithium in a silicon industrial environment and the remaining challenges will be discussed. The similarities and discrepancies of the different Li-based processes will be analyzed as well as the compatibility with a silicon CMOS and/or microsystem fab, and the potential for wafers size scaling. Risks like sensitivity to humidity and potential Li contamination will be outlined with some relevant preventive protocols in order to make the Lithium integration on silicon a real and credible disruptive solution regarding 5G challenges.

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[4] A. Reinhardt et al., IFCS 2011

[5] A. Leon et al., IEEE Trans. Microwave Theory and Techniques, vol. 68, n°1, pp. 60-73 (2020)”

[6] M. Bedjaoui et al. ECS Meeting (October 10-14, 2021).

[7] E. Butaud et al. IEDM 2020

[8] M. Bousquet et al., Proc. IEEE International Ultrasonics Symposium 2019.

[9] M. Bousquet et al., Proc. IEEE International Ultrasonics Symposium 2020.

[10] A. Reinhardt et al., Proc. Joint Conference of EFTF & IFCS 2021

[11] L. Sauze et al, Thin solid films, 726, may 2021

[12] L. Le Van-Jodin et al, Solid State ionic, 2013

[13] V. Sallaz et al, Journal of Power Source, 2020

[14] V. Sallaz et al. submitted to ECS 2021

[15] N-A Ngyuen et al,” submitted to ECS 2021

[16] S. Oukassi et al, IEDM 2019