## 5 Results and Discussion
### Sensor Characteristics Using Developed Interface Electronics  
The characteristics of the HfO\({}_{2}\) thin-film-based SAW UV-C sensor were measured using newly developed custom interface electronics. These electronics incorporate an inter  
Figure 10: (**a**) Sensor characteristic at 220 nm UV-C wavelength based on HfO\({}_{2}\) thickness, (**b**) variation in sensor response time with different HfO\({}_{2}\) thicknesses, (**c**) response and recovery times of the 30 nm HfO\({}_{2}\) sensor, (**d**) repeatability of the sensor under more than seven cycles of 20 \(\upmu\)W/cm\({}^{2}\) UV-C irradiation, (**e**) frequency response to 220 nm UV-C light at varying intensities from 2 to 20 \(\upmu\)W/cm\({}^{2}\), (**f**) calibration plot showing sensitivity to UV-C irradiation, (**g**) wavelength selectivity analysis of UV-C compared to UV-A and UV-B, and (**h**) frequency response and recovery of the sensor measured after 1 day, 1 week, and 1 month post-fabrication. face system that defines the operating mechanism of the developed sensor, as illustrated in Figure 1b. This advanced interface system demonstrates high efficiency, enabling the detection of minimal frequency variations as small as 1 Hz within a 1 s interval. The sensor's frequency was continuously monitored in real time, with integrated compensation for environmental factors such as humidity and temperature through the use of a reference SAW device. Upon exposure to UV light, the sensor displayed real-time frequency changes, ensuring precise and reliable measurements under varying operational conditions. Figure 10a illustrates the effect of HfO\({}_{2}\) thin film thickness on the sensor's frequency shift under UV-C exposure. Films with a thickness of approximately 30 nm demonstrate significant frequency shifts, reaching a maximum of 2.5 kHz under 20 uW/cm\({}^{2}\) UV-C exposure. This can be attributed to an increase in the interaction volume and the density of absorption sites with greater thickness, which enhances the material's conductivity and leads to larger frequency shifts under UV-C exposure. Moreover, the response time of the sensor was observed to increase with film thickness, as depicted in Figure 10b, which provides a magnified view of the response time data from Figure 10a. This highlights the correlation between thickness and response time. A 10 nm HfO\({}_{2}\) film exhibited a response time of approximately 1.5 s, whereas a 30 nm film required about 2.8 s. This variation arises because thinner films absorb photons more rapidly, while thicker films require slightly more time to achieve maximum photon absorption. In this study, achieving high sensitivity was identified as a critical factor for optimal sensor performance. Although the response time increased to 2.8 s for the 30 nm film, it was selected as the optimal thickness for the sensing material due to its high sensitivity and significant measurable frequency shift under very low UV-C exposure. The comparison of previously conducted studies on SAW-based UV sensors regarding response recovery time and sensitivity calculations is presented in Table 2.  
Figure 10c illustrates the response and recovery times of the sensor with the optimally chosen thickness of the sensing material. The response and recovery times were determined to be 3 s and 4 s, respectively, which demonstrate significant improvement compared to previous studies on SAW-based UV-C sensors. Figure 10d presents the cyclic repeatability measurements under 20 uW UV-C exposure. The sensor demonstrated a maximum response of approximately 2.5 kHz with a standard deviation of 48.56 Hz, confirming its high repeatability and consistent performance over multiple cycles. Figure 10e illustrates the frequency shifts of the sensor system under different UV-C intensities, repeated over five measurements.