DGIST Researchers Develop Single-Chip Optical Sensor for Air-Coupled Ultrasonic Testing

Engineers at the Daegu Gyeongbuk Institute of Science and Technology (DGIST) in South Korea have developed a microscopic optical ultrasound sensor thinner than a human hair that can detect megahertz-frequency sound waves through the air without requiring physical contact with the inspected object. The technological advance, published in the academic journal Photonics Research, promises to modernize non-destructive testing for delicate hardware products including semiconductor wafers and secondary battery cells.

Non-destructive testing is standard practice in industrial manufacturing to identify internal flaws and structural voids without disassembling machinery or damaging finished goods. Conventional ultrasonic systems rely on contact testing, which uses liquid or gel coupling agents applied to the component surface to conduct sound waves. However, these liquid mediums risk contaminating or physically damaging sensitive electronic components, making direct-contact methods unsuitable for advanced technology manufacturing.

Air-coupled non-contact inspection has long served as an alternative concept, but sound waves weaken rapidly when traveling through atmospheric gas. This attenuation worsens significantly at higher megahertz frequencies, which are necessary to detect microscopic defects inside high-density materials. Combining small physical size, high sensitivity, and megahertz-frequency operation inside an airborne acoustic sensor has remained an ongoing challenge for industrial instrumentation, as first reported by TechXplore.

To resolve this trade-off, a research team led by Professors Sangyoon Han and Jaesok Yu from DGIST's Department of Robotics and Mechatronics Engineering utilized silicon photonics, a manufacturing approach widely used in microelectronics. The team integrated both an optical waveguide circuit and a flexible microelectromechanical systems (MEMS) cantilever onto a single semiconductor substrate. The complete optomechanical receiver is manufactured in a unified semiconductor fabrication process, eliminating the need for external optical lenses or complex mechanical alignment.

The sensor operates by converting acoustic vibrations into measurable optical signals. When airborne ultrasound waves strike the chip, they cause the ultrathin cantilever structure to oscillate vertically. These microscopic movements produce slight shifts in the wavelength and intensity of the light traveling along the adjacent waveguide. By measuring these optical shifts, the sensor accurately converts sound wave variations into calibrated digital readings.

Despite having an active sensing footprint of just 0.0004 square millimeters—measuring 30 micrometers by 13.5 micrometers—the prototype demonstrated exceptional performance in experimental testing. The sensor achieved a receiving responsivity per unit area more than 25,000 times higher than that of current commercial ultrasonic sensors, overcoming the signal loss that typically limits airborne high-frequency acoustic monitoring.

A key commercial advantage of the DGIST design is its compatibility with existing semiconductor manufacturing supply chains. The sensors can be mass-produced on standard 8-inch (20-centimeter) silicon wafer contract manufacturing foundry lines without needing specialized hardware upgrades. Additionally, by making simple adjustments to the design layout, foundries can fabricate sensors tuned to different operating frequencies simultaneously on a single wafer, extending silicon photonics applications beyond data communications into industrial and medical sensing.

Reflecting on the milestone, Professor Han noted that the project proves high-performance ultrasonic receivers can be constructed as single monolithic chips without intricate multi-component assembly. He added that the team plans to scale the architecture into dense array systems that host dozens of optical receivers on a single microchip. Professor Yu stated that the researchers also intend to expand the technology beyond semiconductor and battery testing into high-resolution biological and medical imaging applications.