Development of high-sensitivity ambient light sensor based on cadmium sulfide-deposited surface acoustic wave sensor

A highly sensitive ambient light sensor based on surface acoustic waves (SAWs) was investigated. The ambient light SAW sensor is fabricated by depositing sensitive film and its resonant frequency shift was calculated in response to a change in light intensities. The resonant frequency is generated between the piezoelectric substrate and a specifically designed transducer, which is usually called interdigitated transducer (IDT). The IDT design was determined to exhibit the resonant frequency at 244.5 MHz by considering the sensor size and fabrication process. We fabricated our ambient light SAW sensor by using a cadmium sulfide (CdS) thin film as a sensing material. Absorbance spectra of CdS thin film in visible light region were investigated. Then the lithium niobate (LiNbO₃) substrate is adopted as a substrate due to its high coupling coefficient. Fabrication of CdS thin films was conducted by thermal evaporator and common lithography process including lift-off. To increase the sensitivity of the sensor, we focused on increasing the thickness and area of the sensitive film. As a result, the sensitivity increased by approximately three times when the area doubled.