Integration of LiNbO3 films to the silicon technology for ultra-wide band and high-frequency RF filters – Lilit

LiNbO3 (LN) presents orientation exhibiting highly-coupled BAW. Thus, the acoustical performance of epitaxial LN films will be explored in the frame of the LiLit project. To implement LN films based BAW devices, the LiLit project has to address these scientific and technical challenges:
- To obtain textured LN films with oblique orientation of polarization axis with respect to the surface normal , which offers high electromechanical coupling, on Si substrates with amorphous buffer layers (SiO2 or Bragg mirrors), which prohibit controlled textured growth of LN;
- To avoid the formation of silicates due to interaction between LN and SiO2 or Si at high temperatures;
- To reduce the thermal stresses in LN films on heterostructures;
- To ensure the high acoustical performance of films;
- To propose a technology with minor changes with respect to standard processing and compatible with industrial fabrication, which enables the implementation of highly coupled LN films not only for FBAR filters (selected as demonstrators) but also for SMR-BAW filters or devices based on SAW or Lamb waves;
- To offer wide range of precisely controlled thicknesses of high quality LN films for ultra-wide band or frequency agile BAW filters working at low or high frequencies;
- To design FBAR resonators/filters offering extreme performance (wide-band and/or high frequencies) and reasonable drift of frequency with temperature.
It was shown by the results obtained during our previous projects, that it is difficult to address all these challenges together by direct growth of LN thin films or by using the fabrication technology of thin films from single crystal wafers. Thus, LiLit proposes to develop an original approach, in which LN layers with homogenous and precisely controlled thicknesses will be integrated onto electrode/SiO2/Si structure.

In the frame of the LiLit project, we decided to target applications V2V (vehicle to vehicle) and V2X (vehicle to everything) communications at 5.89 GHz. The orientation of LiNbO3 films with promising properties for BAW devices operating at these frequencies were identified by simulations. The design of BAW resonators and filters was optimized, as well. Different approaches are considered for the integration of LN films in heterostructures on Si substrates. The growth of highly coupled LN orientations on Si substrates was demonstrated. The chemical compatibility of LN films with different oxides and metals has been studied in details experimentally and by means of thermodynamical simulations (in collaboration with SIMAP, Grenoble). High acoustical performance LN layers on sapphire substrates were demonstrated. The 5th generation communications requires industry-compatible solutions for high frequency filters. Thus, a new collaboration with Annealsys was started around a 4 inch DLI-CVD machine for LN growth which was installed at FEMTO-ST in May 2017. We have started the transfer of LN growth from home-made small-scale PI MOCVD reactor to industrial 4 inch DLI-CVD machine (this implements reoptimization of precursor evaporation conditions, deposition pressure and gas composition, single phase thin films – ratio of Li and Nb precursors). In autumn 2017, we were able to grow high quality epitaxial films on sapphire substrates by DLI-CVD. The bench for high temperature (up to 600 °C) electrical poling was installed at FEMTO-ST.

Further developments in the LiLit project will be focused on the optimisation of the LN layer integration to the electrode/SiO2/Si structures, electrical poling procedure adapted to the LN thin films, fabrication and characterization of TFBARs and filters. A patent on the fabrication technology is under preparation.

Articles in international peer reviewed journals:
1. Review “Toward high-quality epitaxial LiNbO3 and LiTaO3 thin films for acoustical and optical applications” A. Bartasyte et al., Adv. Mater. Interfaces 1600998, (36 pages) (2017).
2. “Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal qualtiy : from nano to micrometers” M. Apreutesei et al., Sci. Technol. Adv. Mater. 18, 430, (2017).
3. “Epitaxy of SrTiO3 on silicon: the knitting machine strategy” G. Saint-Girons et al., Chem. Mater. 28, 5347, (2016).
4. (Chapter) « Direct Liquid Injection Chemical Vapor Deposition » V. Astié, et al. in « Chemical Vapour Deposition » Ed. P. Mandracci, IntechOpen (accepted)

Communications:
1. (Invited) EMRS-2018 Spring meeting, June 2018, Strasbourg, France, Engineering the properties of functional oxides and integrating them on Si and GaAs thanks to molecular beam epitaxy, G. Saint-Girons et al.
2. IC-MBE 2016 international conference, Sept 2016, Montpellier (France), Interface reactivity and epitaxial growth of SrTiO3 and other functional oxides on Si and GaAs, B. Meunier et al.
3. (Tutorial) International Summer School on “Sustainability and Nanotechnology”, February 2018, Jaipur, India, Alkaline niobates - alternative lead-free materials for vibrational energy micro-harvesters, A. Bartasyte.
4. (Invited) ISEPD 2018, February 2018, Jaipur India, LiNbO3 thin films for high-temperature vibrational/thermal energy harvesters, A. Bartasyte et al.
5. (Tutorial) ANF JNCO, Ecole thématique du réseau national CMDO+, Septembre, 2017, Paris, France, Guides optiques à base des couches minces de LiNbO3, A. Bartasyte.
6. (Best presentation award) ISIF, Delhi, India, December 2017, Epitaxial lift-off and 3D structuring of Rutile TiO2 films, S. Kuprenaite et al.
7. (Oral, Proceeding) IEEE-EFTF, 516, Jul. 2017, Besançon, France, Characterization of single-port SAW resonators at 3.7 GHz based on epitaxial LiNbO3 layers, A. Clairet et al.

Radio frequency (RF) filter products (market of €4.3BN in 2016) are used in the fields of information & communication, automotive navigation/toll systems, medical instruments, industry, household appliances, military applications, etc. Evolution of communication systems and continuously increasing transmission of information through different channels (4G) require on one hand increasing the number of bands assigned for transmission of data (up to 8) and on the other hand increasing the width of pass band and/or the working frequency in order to intensify the transmission of data. This requires more and more RF bands and more complex RF circuits without increasing the total size of the systems. Conventional RF filters based on surface acoustic waves (SAW) are limited to the frequencies up to 3.5 GHz and so far there are few perspectives for improving of this technology based on single-crystals. Thin film bulk acoustic resonators (TFBARs), based on AlN films, are limited by their low electromechanical coupling (7.5%), which restricts the bandwidth that can be achieved and hence limits the frequency to 4 GHz and the widespread use of this technology. Thus, new suitable low loss materials are needed with larger electromechanical coupling to achieve larger bandwidth.
The proposed project entitled "Integration of LiNbO3 films to Silicon technology for ultra-wide band and high-frequency RF filters (LiLit)" will focus on the FBAR filters, based on highly coupled LiNbO3 films, operating at 5-6 GHz and/or with relative bandwidth in excess of 10 %. In the framework of LiLit project, a cost- and time-effective technology, with up-scaling possibility and compatibility with standard TFBAR processing will be developed, which will be able to offer a wide range of thicknesses, controlled with precision, of highly piezoelectrically coupled LiNbO3 films on electrode/SiO2/Si substrates. The proposed technology will be adaptable to the fabrication of SMR-BAW filters or devices based on SAW or Lamb waves, as well. Particular effort will be done (i) to achieve high acoustical performance of LiNbO3 films (electromechanical coupling > 20 %, high quality factor and stability at high power densities) through interface/surface and chemical engineering; (ii) to design FBAR resonators/filters with parasitic-free response offering extreme performance (large electromechanical coupling factor, i.e. wide-band at high frequencies), losses < 3 dB within the pass band and drift of frequency with temperature, acceptable for filter applications.
The integration of LiNbO3 films onto silicon technology would be a breakthrough in the information & communication industry (5G mobile phone infrastructures, data treatment, Wi-Fi, etc.). It would permit increasing the telecommunication frequencies from 2-3 GHz to 6 GHz or trade the increased electromechanical coupling factor of resonators for improved temperature compensation, quality factors or even to make them tuneable. This will allow ameliorated communication efficiency through increased and intensified transmission of information and reduced number of components.
LiLit links France-leading research groups at academic institutions and industry to give a combined integrated approach of advanced synthesis/cutting-edge micro-fabrication, characterization, modelling linked to concepts for materials integration in smart devices and systems. Such a science-supported total engineering approach will lead towards efficient next generation BAW filters viable for the RF communication industry. LiLit also seeks to intensify the relationship between academic and private sectors, which is essential to provide a strong European and French lead in this highly competitive RF industry.