Thin film technology for RF filters

Fifth- and sixth-generation communications (5G and 6G) will benefit modern society, opening up vast possibilities for the Internet of Things, data transfer, extreme density of users and devices, real-time communication, intelligent transport (autonomous cars) and communication systems, robotics, complex manufacturing, smart cities and more.

By 2020, the next-generation smartphone will contain between 50 and 100 radio frequency (RF) filters. To intensify data transmission, we need to increase the number of RF bands dedicated to data transmission, the bandwidth, or working frequencies, and also reduce latency.

RF filter technologies are based on surface elastic wave (SAW, for Surface Acoustic Wave), or bulk elastic wave (BAW, for Bulk Acoustic Wave) devices. Piezoelectric materials are used, such as LiNbO ₃ and LiTaO ₃ in the form of single-crystal wafers, for SAW filters, and AlN in the form of thin films, for BAW filters.

The frequency of conventional SAW filters is currently limited to 3.5 GHz. Increasing this frequency requires increasing the phase velocity of available materials, as well as reducing the interdigitated electrode period (IDT). The latter is limited to 350 nm by the SAW industry's use of I-line lithography at the UV wavelength of 365 nm. Phase velocity, meanwhile, can be significantly increased by guided elastic waves in piezoelectric thin films on high acoustic velocity substrates (diamond, sapphire, etc.), but stability at high power densities is the main limitation of SAW technology.

In the case of BAW filters, thin-film acoustic resonators exploit elastic bulk wave (BAW) modes. To be used in broadband BAW filters, a piezoelectric material must be electromechanically coupled, $K^2$ ideally equal to twice the relative bandwidth of the filter (for example, $K^2=7\%$ for a relative bandwidth of 3.5%), and a quality factor, $Q$ maximum (preferably greater than 2,000 at 2 GHz). These parameters are generally given by the figure of merit, $K^{2}Q$ which we want to be as high as possible for a given frequency.

These parameters also depend on the design of...