High Frequency Filters
CDF has been working on high frequency filters since its inception, focusing on Surface Acoustic Wave (SAW) devices fabricated on diamond. SAW devices are generally fabricated on piezoelectric materials, materials which move on the application of a voltage. Acoustic waves are generated by applying an alternating voltage across electrodes on a piezoelectric. As the piezoelectric starts to oscillate, an acoustic wave is generated in the material. These devices resonate at frequencies which are a function of the spacing between the electrodes and the acoustic wave velocity of the material.
SAW Filters are key components in communication systems and are commercially available from Toshiba, Murata, Seiko Epson etc. Most SAW devices are fabricated on bulk Quartz and Lithium Niobate due to the ready availability of high-quality crystalline material. However these materials are generally limited to frequencies of a couple of GHz and low powers. The recent development of 5G with new frequency bands above 10 GHz has led to a need for more compact high - power handling super high frequency filters for base station transceivers. Current SAW filters cannot offer the power levels required for base stations and therefore new material combinations need to be developed and deployed to enable the next generation of SAW filters.
Diamond has the highest acoustic wave velocity of all materials (~ 20000 m/s), which leads to very high resonant frequencies. It also has the highest thermal conductivity of any electrical insulator (~ 2000 W/mK), which is ideal for high power handling. Unfortunately, diamond is not a piezoelectric and thus must be combined with other materials in order to generate the acoustic wave electrically. In collaboration with Dr Fuentes Iriarte at ISOM/UPM, we have shown very high frequency - Q products are possible by combining diamond with AlN as the piezoelectric. Recently, ISON/UPM have been developing AlScN films on diamond to improve the coupling coefficient.
Left: An example of a AlN / diamond SAW (diagram from Rodríguez-Madrid PhD thesis). SAW devices with resonant frequencies exceeding 14 GHz have been demonstrated. This performance is approximately twice that of competing materials. Quality factors exceed 40000. In this collaboration we have also demonstrated SAW filters at frequencies approaching 30 GHz and shown their potential as pressure sensors in harsh environments.
Below: Quality factor of competing SAW technologies plotted as a function of frequency. Diamond / AlN clearly exhibits the highest quality factor at high frequencies. The pink regions illustrate the 5G frequency bands.