Achieving higher sensitivity of CdS-based SAW sensor detecting visual light

Introduction: SAW sensors have been studied for various applications in mobile devices and achieving higher sensitivity has become a major issue. In this paper, CdS thin film with various thicknesses and areas were... [ view full abstract ]

Introduction: SAW sensors have been studied for various applications in mobile devices and achieving higher sensitivity has become a major issue. In this paper, CdS thin film with various thicknesses and areas were investigated and the proper design was proposed and we found that higher sensitivity could be achieved by changing thicknesses and areas of cadmium sulfide (CdS) thin films deposited on surface acoustic wave (SAW) sensors. Methods: We used 128˚ Y-X lithium niobate (LiNbO3) as a substrate. Aluminum was DC sputtered on cleaned substrate to fabricate inter-digitated transducers (IDTs). IDTs consist of 70 pairs of fingers with the width of 4 μm and the same spatial length. Thermal evaporation process was used to deposit CdS on prepared substrate. CdS thin films were deposited with different thickness of 140 nm and 190 nm to confirm the thickness effect. Roughness by thickness was characterized using atomic force microscope (AFM). To investigate the area dependency, SAW sensors were fabricated in two ways, one covered with CdS thin films in the whole IDT area, and the other covered in half of the IDT. The sensors were exposed under the light from 0 lux to 20,000 lux, in 2,000 lux increment, in the completely dark room. In each step, the sensors absorb the light for 10 seconds having 100 seconds of intervals. Resonant frequency responded to the intensity of light and was measured by network analyzer (Protek A333). Results/Discussion: The test comparing 140nm thin films with 190nm shows that thicker one results in higher sensitivity of the sensor. It was also confirmed, through area dependency test, that sensitivity grows higher as the area becomes larger. Using these two results, we can expect the design rules for developing CdS-based SAW sensors to be evaluated.