Imagine the chaos if your smartphone suddenly played your guilty-pleasure song on a stranger’s pair of wireless earphones. Or, in a more serious scenario, if ultrasound images were so noisy that doctors couldn’t tell a tumor apart from a muscle. In an increasingly connected world dominated by radio waves and frequencies, being able to pick out only the relevant signals has never been more important.
Smart devices use radiofrequency filters to receive signals and connect accurately with each other. In turn, these filters rely on layers of thin films, typically made of scandium-doped aluminum nitride (ScAlN), that allows only the right signal to pass through while blocking out everything else.
While doping with scandium helps improve the film’s filtering function, this process also often produces large, abnormal and misoriented grains on the film’s surface, leading to performance inefficiencies like choppy connectivity and low data transfer between devices.
In the hopes of minimizing the risk of these abnormal grains forming, a team of researchers led by Minghua Li and Yao Zhu, both from A*STAR’s Institute of Microelectronics (IME), conducted a complete characterization of the grains.
“This in-depth morphology and microstructural study aims to facilitate the understanding of how the abnormal grains form and grow,” said Zhu, an A*STAR Scholarship recipient.
On top of the usual scanning electron microscopy (SEM) and X-ray diffraction (XRD) imaging techniques, the team also deployed transmission electron microscopy (TEM), a technique that achieves better spatial resolution and contrast, to study the ScAlN films. Li and Zhu used TEM to obtain an in-plane, bird’s-eye view of the films, allowing them to see the abnormal grains from a new angle in addition to the usual cross-sectional images.
Their SEM and XRD analysis showed that the surfaces of ScAlN films were evenly textured, and TEM revealed the normal grains to be uniform hexagonal crystals. Meanwhile, the abnormal grains appeared as pyramid-shaped spikes visibly protruding above the film’s surface and exist as much larger crystals or crystal clusters.
The researchers also found that the malformed grains occupy larger area as the film grows thicker, suggesting that they grow vertically and laterally at the expense of their neighboring grains.
According to Zhu, the irregular shapes and non-uniform distribution of the grains across wafers could compromise the energy efficiency of the resulting ScAlN films, and might also cause obstacles in further ScAlN-based device fabrication.
“Moving forward, the team aims to produce high-quality ScAlN films by improving the sputtering process to significantly reduce or remove abnormal grains,” she said, adding that in the future, such films could lead to more efficient radiofrequency filters and better smart devices.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics (IME).