## 2 Optimal Device Design Consideration and Simulations  
Figure 1a illustrates the overall view of the SAW UV-C sensor along with the reference SAW device. Figure 1b depicts the operating mechanism of the sensor system, which incorporates integrated interface electronics for real-time data extraction. The system includes two SAW devices, two oscillators, a mixer, a low-pass filter (LPF), a comparator, a field-programmable gate array (FPGA), and a PC. Both the sensor and reference devices utilize a two-port SAW delay line with a center frequency of 222 MHz, providing an adequate area for the sensing material. It was constructed on a 128\({}^{\circ}\) YX LiNbO\({}_{3}\) substrate. A 30 nm-thick layer of HfO\({}_{2}\) sensing material was deposited in the cavity between two inter-digital transducers (IDTs) to achieve acceptable insertion loss. To mitigate environmental interference perturbations, the reference and sensor devices share identical structures, with the outer surface of the reference device sealed by an encapsulation layer to block the target light from passing through. The HfO\({}_{2}\) sensing material generates electron-hole pairs when illuminated by UV-C light, altering its conductivity and causing a frequency shift at the output IDT, which depends on the incident UV-C power. In contrast, the substrate does not generate electron-hole pairs. The equation below is commonly used to calculate the frequency change when a perturbation is applied to the sensing materials in SAW devices. \[\frac{\Delta f}{f_{0}}=k\frac{\Delta V}{V_{0}}=-C_{m}f_{0}\Delta(\rho_{s})+C_{ e}f_{0}h\Delta\Bigg{(}\Bigg{(}\frac{4\mu}{V_{0}^{2}}\Bigg{)}\times\bigg{(} \frac{\mu+\lambda}{\mu+2\lambda}\bigg{)}\Bigg{)}-\frac{K^{2}}{2}\Delta\Bigg{(} \frac{1}{1+\big{(}\frac{V_{0}C_{S}}{\rho_{S}}\big{)}^{2}}\Bigg{)} \tag{1}\]  
where \(\Delta f\) refers to the frequency change observed in the SAW sensor, with \(f_{0}\) indicating its initial center frequency. \(\Delta V\) represents the velocity shift, and \(V_{0}\) is the velocity of the SAW traveling on the LiNbO\({}_{3}\) piezoelectric substrate. Furthermore, \(K^{2}\) represents the electromechanical coupling coefficient, \(\sigma s\) denotes the sheet conductivity of the film, and \(Cs\) is the capacitance per unit length of the SAW substrate material. This frequency change is attributed to variations in conductivity, mass loading, and viscosity of the sensing material. An increase in mass loading and conductivity results in a downshift in frequency, while an increase in viscosity leads to an upshift [21]. By calibrating the frequency change relative to the UV-C power intensity of the applied light, we can determine the power density of UV-C. We selected a two-port delay line on a LiNbO\({}_{3}\) substrate to ensure sufficient sensing material area, enabling a significant output change from the sensor. The LiNbO\({}_{3}\) substrate was chosen due to its high electromechanical coefficient, \(K^{2}\), which enables a significant frequency change even with small variations in conductivity. HfO\({}_{2}\) was selected as the sensing material because of its energy bandgap of 5.71 eV, which provides an optimal spectrum for solar-blind UV-C ranges. Furthermore, high-quality films of HfO\({}_{2}\) can be deposited using atomic layer deposition (ALD) methods, facilitating ease of deposition and the potential for mass production. The mixer in our developed interface electronics requires a larger voltage amplitude from the oscillator than a specific threshold to effectively read the signal. Various thicknesses were tested to determine the optimal value, and through these trials, a thickness of 30 nm was established as optimal. While a greater thickness can yield a larger frequency change, it also results in longer response times due to the increased time required to reach saturation. Additionally, greater thickness can lead to higher loss in S\({}_{21}\), making it challenging to read the signal from the filter.