## 5 Results and Discussion  
### Sensor Characteristics Using a Network Analyzer with LabVIEW  
Before conducting sensor measurements via the interface electronics, the fabricated sensor was tested using a network analyzer with LabVIEW software (Ver. 2024) to check the center frequency and observe real-time frequency changes in response to UV-C illumination. A completed single sensor is connected to the PCB pads using wire bonding. To minimize changes in center frequency and insertion loss caused by the capacitance, inductance, and resistance generated by the wires, the length of the wires is kept to a minimum. Figure 9a shows the S\({}_{21}\) response of the fabricated sensor with a 30 nm-thick sensing material. The center frequency was observed at 221.3 MHz, with an insertion loss of 18 dB and a sidelobe level of 50 dB. As the thickness of the HfO\({}_{2}\) layer increases, the insertion loss also increases. For a thickness of 40 nm, the insertion loss rose to 20 dB, which negatively impacts sensitivity and stability, making it unsuitable for achieving high oscillations with the current interface electronics setup. Based on these results, the optimal HfO\({}_{2}\) thickness was set at 30 nm to enhance sensitivity while ensuring appropriate insertion loss for the operation of the interface electronics. In the S\({}_{21}\) measurement results, conductance due to capacitance between the two IDTs was minimized even at frequencies above 400 MHz. The distance between the two IDTs was designed to be 2 mm, providing sufficient sensing material area while minimizing conductance in the higher frequency region and reducing insertion loss. An LED serves as the UV-C source, providing a 220 nm single pulse. The  
Figure 8: UV–vis optical absorption spectra and Tauc plot of the HfO\({}_{2}\) thin film.  
UV-C power is adjusted by varying the distance between the UV-C source and the sensor, with power decreasing according to the inverse square law (\(1/r^{2}\)) based on this distance. A 220 nm, 20 \(\mu\)W/cm\({}^{2}\) UV-C light was applied to the sensor surface, with the UV-C power adjusted by varying the distance between the UV-C source and the sensor, using pre-calibrated values. A downshift in the center frequency is observed due to an increase in conductivity in the HfO\({}_{2}\) sensing material when exposed to UV-C light (Figure 9b). At the same time, an increase in insertion loss is confirmed. Despite the rise in insertion loss with increasing UV-C power, the sensor maintains its ability to detect characteristics and provides a wide measurement range. By adjusting the distance between the UV-C source and the sensor, the UV-C irradiation power is calibrated. As the distance increases, the extent of the downshift in the center frequency decreases. Using LabVIEW, real-time changes in the center frequency are monitored. A frequency sweep from 200 to 250 MHz occurs with a 0.5 s interval, and the data collected during this period are used to analyze the dynamic characteristics of the center frequency change, including response time and recovery time. As shown in the Figure 9c, it takes 2 s to reach 90% of the saturation value, while the recovery time is observed to be 4 s.