High stability and spectral purity oscillators based on guided elastic wave resonators built on nano-crystal carbon-diamond composite substrates – Phase #2 – OSCCAR-2

Surface acoustic wave (SAW) resonators and assimilated devices are used for the frequency stabilization of electric oscillators in the frequency range 50 MHz - 4 GHz. The need for such resonators operating at frequency equal or above 1 GHz exhibiting low insertion losses for the fabrication of oscillators with a controlled phase noise at 10 kHz from the carrier is in line with the development of on-board high resolution detection systems (RADAR). Considering the state-of-the-art and technical specifications and dimensional constraints generally required for such an application, this kind of oscillator can only be achieved using passive devices based on guided elastic waves, therefore allowing for the development of an actual industrial solution. Until now, only very high structural quality single-crystal materials with a perfectly controlled inner structure have enabled one to reach the expected quality coefficient of the resonance, i.e QF (quality factor times frequency) products equal or even larger than 10 to the 13. However, increasing the operation frequency of such devices remains a hard task and the exploitation of the device qualities at SHF (Super-High-Frequency) range is still beyond reach for two principal reasons. First, propagation velocity of waves used in that purpose (Rayleigh or pure shear waves) rarely overcomes 5000 m/s. Second, accounting for known industrial technology limits, particularly concerning the lithography step needed for the fabricatio nof the inter-digited transducer, operational resonance frequencies remains below 3 GHz which is quite far from the above-mentioned band (3-30 GHz) which actually requires high stability source for various detection and timekeeping purposes. Therefore, the opportunity to develop frequency sources directly adjusted to the operating frequency is of particular interest and is addressed in this project, as it would dramatically simplify the system architecture and also reduce the additional noise due to frequency multiplication. During a first project supported by the Direction Générale de l'Armement (REI contract #636974 0680238016), resonators built on composite substrates associating nano-crystal carbon-diamond layers and a ZnO piezolayer for the excitation/.detection of elastic waves have been developed to answer this challenge with high
spectral purity and stability for on-board radar applications. The exploited waves exhibit a phase velocity equal or higher than 10 km/s, directly doubling the reachable frequency range. The so-called OSCCAR project was organised in two principal steps, the first being achieved under the above-mentioned frame. This first piece of work allowed for demondtrating the possibility to fabricate resonators at 4 GHz with QF products in excess of 3.10^12. An oscillator also was developed and characterized at 3 GHz. In the second step of the project, we propose to demonstrate the industrial viability of oscillators stabilized with nano-crystal diamond-carbon based acoustic resonators operating in the SHF band. In that matter, we develop the design and fabrication on 4-inch wafers of such resonators operating in the range 5-10 GHz for spatialm and defense applications, The efforts will be focused on the fabrication of a 5 GHz oscillator with a phase noise target of -115 dBc at 10 kHz from the carrier, and then on the fabrication of a resonator operating in the X band. A particular care will be devoted to parametric sensitivity optimisation and back-end operations (dicing, packaging) to provide the best operating conditions for these devices. A comprhenesive characterization of these devices will allow for setting the state-of-the-art of such devices for spatial and defense applications.