Radiofrequency bulk elastic wave devices Simulation tools

Bandpass filters using elastic wave resonators have become indispensable components of mobile telecommunications systems. The successive deployment of four generations of mobile telephony systems, and the active preparation of the fifth generation (known as 5G) destined for deployment in the early 2020s, have led to the emergence of multiple frequency bands in order to ensure downward compatibility with older protocols, respond to the multiplication of applications, and take account of geographical disparities in the allocation of the radio frequency spectrum. This translates into the inclusion of several dozen miniaturized bandpass filters in a single mobile telephony terminal, as well as a race for performance: duplexer modules, or even recently up to five or six duplexers, to enable up to six transmit/receive channels to operate simultaneously on as many frequency bands while connected to the same antenna. Such modules must ensure isolation of the order of 50 to 60 dB between these different channels, while offering insertion losses of less than 2 dB in the passband of each channel, with increasingly narrow transition zones, thus requiring extremely precise frequency positioning, of the order of 0.1% of the center frequency. What's more, the radio-frequency filter industry has to keep up with the pace imposed by cell phone manufacturers, and ensure that their products are renewed over periods of the order of a year.

To meet all these constraints, it has been necessary to develop design models that are both accurate and fast, enabling iterations between design and manufacture to be minimized, and thus speeding up the time-to-market for new components. In this article, we focus on just one of these elastic wave filter technologies: volume elastic wave filters, which exploit wave propagation through the thickness of sub-micrometer layer stacks. Indeed, the other elastic wave filter technology employed, based on the propagation of surface waves under periodic arrays of electrodes, presents a number of characteristics that are relatively distinct from volume wave components, and are therefore the subject of a dedicated article [E 3 211] .

Here, we follow the design flow usually employed for volume elastic wave filters. The first part deals with the synthesis of a filter function, respecting the imposed specifications, from pseudo-resonators providing a purely mathematical description of the response expected by these components. The second part deals with resonator dimensioning using physical and therefore predictive models. The latter must be inherently multi-physical, combining mechanical aspects linked to the propagation of elastic waves and electrical aspects linked to the desired...