A recent study shows the radio frequency (RF) filter market growing steadily by nearly $16 billion from 2019 to 2024 at a compound annual growth rate (CAGR) of approximately 20%, according to Technavio. The strong growth in the RF filter market is driven by the increased adoption of 5G technology, the surge in smartphones using 5G, and commercial and consumer devices dependent on internet of things (IoT) applications. Together, these factors are some of the most significant players driving society’s digital transformation.
However, the RF filter market is faced with many of the same challenges the semiconductor industry as a whole is experiencing, including the need to pack more into increasingly smaller spaces. In each successive generation of RF filters, the number of filters has not only steadily increased, the rising number of filters has led to a need for more stringent process monitoring and control. A frequency accuracy, 3σ of 0.1%, requires film thickness control within the same accuracy or better.
Let’s look at one RF filter component, a bulk acoustic wave (BAW) resonator. A BAW is a piezoelectric structure sandwiched between the top and bottom electrodes. The resonant frequency depends on the acoustic velocity and the thickness of the piezoelectric film, and the thickness of the electrode. The thickness of the top electrode as a mass loading layer can be dialed in to generate a frequency shift, which is often used to form a filter passband.
Since the RF filter process is directly correlated to thickness, extremely uniform films (~0.1% or better) need to be deposited. With the additional requirements of 5G to support higher frequencies and increased bandwidth, RF filter device manufacturers employ several different process knobs to tune the devices. For example, we see an increasing trend toward thinner layers to support higher frequencies, the adoption of Sc-doped piezoelectric materials to improve piezoelectric coupling and the addition of temperature compensation SiO2 layers to the stack to improve the temperature coefficient of the resonator.