As we increasingly depend on smartphones in daily life, the demand for faster, more stable and more energy-efficient mobile connectivity has only accelerated. Today, our phones tap into nearly one hundred of invisible radio waves, providing wireless links to a range of mobile networks.
To select the desired radio-frequency (RF) bands while rejecting unwanted signals and interference, bulk acoustic wave (BAW) filters are often included in their hardware, minimising signal loss and creating a sharp separation between closely spaced frequency bands. “A premium smartphone today may contain nearly 100 RF filters for cellular, Wi-Fi and other services,” said A*STAR Institute of Microelectronics (A*STAR IME) Senior Scientist Chen Liu.
However, each BAW filter often covers just one fixed frequency band. This means that as new cellular networks roll out, more filters and related electronics need to be squeezed into each mobile device to support maximum connectivity—leading to increasing component counts, chip areas, power consumption rates and risk of signal losses.
To address the issue, Liu and A*STAR IMRE colleagues recently developed a prototype for a switchable BAW filter based on scandium-doped aluminium nitride (ScAlN). “A switchable filter could cover several frequency bands with one device, making it especially valuable for increasingly complex 5G and future wireless systems,” said Liu.
Liu explained that ScAlN combines three advantages rarely found in a single material: a high electromechanical coupling, allowing a high efficiency at converting electrical signals into acoustic vibrations and enabling a wider operating bandwidths; ferroelectric properties, enabling changes to its polarization orientation using short electrical pulses; and a high compatibility with existing semiconductor manufacturing processes.
Leveraging ScAlN’s ferroelectric properties, the team fabricated a bilayer BAW resonator structure with four polarisation configurations on the Lab-in-Fab platform, a 200-mm piezoelectric microelectromechanical systems (piezoMEMS) R&D line formed in collaboration between A*STAR IME, ULVAC and STMicroelectronics.
Apart from ScAlN, their filter also relied on a two-step switching method newly proposed by the team. “Typically, a BAW filter contains several resonators working together. As every resonator needs its own electrical connection and switch, more complex filters need a larger number of control connections and switching operations,” said Liu.
To circumvent this, the team carefully grouped the resonators’ control electrodes together, significantly simplifying its control system and connections. “Our method allows the whole filter to move between frequency bands in two steps where intuitive resonator-by-resonator methods might need 10 or more,” Liu explained.
In testing, the team’s filter switched between 3 GHz and 6 GHz RF bands while maintaining higher fractional bandwidths than those previously reported switchable filters, which could translate to better data processing reliability. The team also found that their filter’s endurance could be extended over that of conventional designs through a dynamic pulse-amplitude method which adjusted electrical pulses as the device aged.
The team hopes to improve the filter’s performance by reducing signal loss, improving isolation between frequency bands, balancing bandwidths and extending long-term reliability. “We’re also seeking commercial partners to evaluate this technology in practical wireless systems and help it move from a prototype to a real-world product that can be manufactured at scale,” Liu added.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Microelectronics (A*STAR IME).
