Magnetic Nanoferrites for RF CMOS: Enabling 5G and Beyond

Abstract

The upcoming 5th generation (5G) telecommunication technology will enable realization of the full potential of the Internet-of-things (IoT) by utilizing various frequency bands, ranging from <4 GHz to 100 GHz. A brief review of the technical requirements of RF CMOS at the unveiling of the 5G system is provided. Key challenges associated with 5G are illustrated, with a focus on the need for, and state of the art of, magnetic materials, especially ferrites, in enhancing the performance of RF integrated circuits. The suitability of a low-temperature, CMOS-compatible, on-chip ferrite thin film deposition technique, which was elusive until recently, is compared with traditional deposition processes for thin ferrite films. The uniqueness of this microwave-irradiation-assisted, far-from-equilibrium, solution-based deposition technique in achieving an unusually high ferromagnetic resonance frequency is discussed. The effect of magnetic ferrite films in niche applications such as in X-band (8~12 GHz) inductors and K-band (18~40 GHz) electromagnetic noise suppressors is discussed and demonstrated.

Nanostructured ZnFe2O4: An Exotic Energy Material

“…Recently, Saha et al synthesized [52] nano hollow spheres (NHSs) (shown in Figure 9a) instead of nanoparticles of Zn y Fe 3−y O 4 by template-free solvothermal method, which shows an increase in M S values with Zn doping, attaining a maximum at x = 0.2 (Ms = 92.52 emu/g at 300 K), similarly to the bulk Zn y Fe 3−y O 4 . Therefore, enhanced magnetism with a decrease in conductivity, permittivity, and dipolar interaction enables Zn y Fe 3−y O 4 NHSs to be a useful material for high-frequency applications [39,[53][54][55][56][57]. Porous ZnFe2O4 nanotubes (Figure 9b) have been fabricated by electrospinning followed by two-step calcination in the atmosphere [58].…”

“…Performance of on-chip X-band (8–12 GHz) inductor, integrated with magnetic film can enhance its inductance density as well as the quality factor, facilitating miniaturization of RF devices and reducing dependency on silicon [ 54 , 55 , 56 ]. To find a suitable candidate material, whose ferromagnetic resonance frequency ( f FMR ) is above 6 GHz, i.e., in the expected new spectrum for 5G mobile communication, is now indispensable.…”

“…To find a suitable candidate material, whose ferromagnetic resonance frequency ( f FMR ) is above 6 GHz, i.e., in the expected new spectrum for 5G mobile communication, is now indispensable. Among metallic alloys, amorphous films, granular films and soft-magnetic ferrites, the first three exhibits very high permeability that results in >20% enhancement of inductance, but their applicability is limited to a few GHz; eddy-current losses and ferromagnetic resonance losses become prohibitively large at higher frequencies [ 55 ]. However, the family of ferrites with very high electrical resistivity and high f FMR values can limit the aforesaid losses up to a few tens of GHz.…”

“…A crucial challenge is the growth of thick ferrite films on a silicon chip in a CMOS (complementary metal oxide semiconductor)-compatible manner [ 55 ]. Most PVD methods are either non-scalable or require high processing temperatures (in-situ or ex-situ annealing ≥ 500 °C).…”

“…On the other hand, the low-temperature chemical methods require strict control of the pH of the solution, which may otherwise corrode the on-chip metal wiring. Recently, Sai et al deposited partially inverted ZnFe 2 O 4 film with soft magnetic characteristics ( M S = 130 emu/cc and H C = 120 Oe) directly on a Si-CMOS integrated circuit by Microwave-Assisted Synthesis Technique (MAST) at 200 °C [ 54 , 55 ]. These films showed FMR frequency above 30 GHz, with negligible FMR loss below 15 GHz, therefore, they could be used as inductor core in the frequency range up to 15 GHz and as an electromagnetic noise suppressor around 30 GHz.…”