Physicists at HSE University and FIAN Discover Way to 'Photograph' Sound for Testing Materials Used in 6G Communications

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Researchers at HSE University, in collaboration with colleagues from the Lebedev Physical Institute of the Russian Academy of Sciences (FIAN), have developed a method for rapidly determining how firmly a film is bonded to a substrate. This is important for the creation of ultrahigh-frequency acoustic filters, which are key components of next-generation 5G and 6G communications. For the first time, researchers have succeeded in measuring the lateral rigidity of the bond between a two-dimensional material film and a substrate in this way. The study results have been published in Applied Physics Letters.

To transmit data at high speed, modern smartphones use filters that convert electromagnetic signals into ultrasound and back again. This helps eliminate interference. As communication frequencies continue to increase, new types of such filters are being developed, including those based on acoustoelectric effects. Almost any modern electronic device contains components made from thin films deposited on a substrate. The challenge is that at frequencies of several or even tens of gigahertz, the behaviour of ultrasound at the interface between these materials becomes almost impossible to predict. It is precisely in this frequency range that 5G and future 6G communications operate. 

At lower frequencies, the contact between the film and the substrate can be assumed to be ideal for sound propagation. However, as the frequency increases, microscopic slippage of the film caused by insufficient lateral rigidity at the interface can prevent the filter from transmitting the signal properly—something that chip developers may only discover after spending millions of dollars on development.

Scientists from FIAN, together with Alexander Kuntsevich, Leading Research Fellow at the Laboratory for Condensed Matter Physics and Professor at the HSE Faculty of Physics, have proposed a way to assess the quality of the contact even before device assembly. Instead of creating expensive prototypes, they suggest testing materials using short laser pulses.

The scientists worked with a quartz glass sample onto which they transferred a 600-nanometre-thick boron nitride flake. They then focused an infrared laser pulse onto its surface. The laser heated a tiny area, generating a surface acoustic wave that propagated through the material.

Alexander Kuntsevich

'When pebbles are thrown into water, waves spread across the surface in the form of concentric circles. These are surface waves. Similar waves can also propagate along the surface of solids; they are known as surface acoustic Rayleigh waves,' explains Alexander Kuntsevich.

Such waves are difficult to observe with the naked eye because they propagate at high speeds and typically have very small amplitudes. Nevertheless, they carry a great deal of important physical information about the material in which they travel. For example, changes in wave velocity and shape can be used to infer the material’s elastic properties and the strength of the bond between a thin film and its substrate—precisely the parameters the scientists needed to determine.

'Using the second beam, we took a snapshot of the wave. The first pulse, like a strike on the surface, generated the sound. We then directed the second pulse onto the same surface a fraction of a nanosecond later. This beam scanned the surface in 0.5-micrometre increments. The reflected signal varied depending on whether a given region of the surface was raised or lowered by the passing wave. By combining this data, we reconstructed an accurate map of vertical displacements—in effect, we obtained a frozen image of a traveling wave,' the scientist said.

The structure itself remained intact: the method did not damage either the film or the substrate. The scientists analysed the resulting image using a mathematical model that allowed them to determine how the speed of sound depends on wavelength. From the way the wave velocity changes with frequency, it is possible to infer how the film and substrate are coupled. By analysing the degree of wave distortion, the authors calculated two parameters of interfacial rigidity: vertical (related to separation) and lateral (related to shear). They found that the most significant parameter is the shear rigidity, which governs the sideways slipping of the film and had not previously been measurable.

By determining the parameters of interfacial rigidity, engineers will be able to identify unsuitable materials at an early stage and refine fabrication processes for ultrahigh-frequency filters. The method is also useful for the development of acoustic metamaterials—engineered structures designed to control sound in specific ways.

'Creating a device only to later discover that it does not perform due to poor acoustic matching is both frustrating and extremely costly. It is far better to test the interaction of different material pairs in advance and then proceed to device fabrication. Our technique is fast, fully optical, and non-destructive. It enables the measurement of interfacial contact properties before the materials are assembled into a device, allowing the optimal pair to be selected for operation at gigahertz frequencies,' summarised Prof. Kuntsevich.

The study was conducted with support from the Russian Science Foundation.