I. INTRODUCTION

Currently, there is a rapid evolution in semiconductor technology. Research in the packaging domain is becoming increasingly important, not just focusing on scaling down transistors ^[1-3]. Recently, difficulties have been encountered due to the limits of scaling down, leading to increased research into advanced packaging to enhance the performance and efficiency of semiconductors. In this situation, the concept of Monolithic 3D structures, which involve stacking multiple layers of semiconductor circuits such as CMOS circuits for analog-digital mixed signal circuits directly on a single wafer, is emerging ^[4]. Monolithic 3D IC offers significant advantages in terms of integration, but temperature restrictions are necessary during the upper-level processes. High-temperature processes at the upper levels can degrade the characteristics and reliability of the components fabricated at the lower levels. Therefore, it is essential to strictly proceed with low-temperature processes as we move towards the upper layers.

Next-generation intelligent semiconductors refer to systems that can rapidly perform the recognition, storage, and processing of information. Therefore, the implementation of a fused system, effectively combining each of these aspects through the previously mentioned packaging technologies, is essential. Monolithic 3D IC are also considered as strong candidates for realizing such integration. To achieve this, it is essential to have information sensing components that are produced through low-temperature processes while being capable of recognition under typical conditions. In this study, experiments will be conducted to apply a sensory device utilizing electrical signals that vary according to the detection of UV-C wavelengths (100 nm~280 nm), as shown in Fig. 1, to a monolithic 3D structure. UV-C wavelength is observed as part of the light and heat generated from a phenomenon known as an arc flash, which occurs as a kind of electrical explosion or discharge in electrical systems, typically connected to ground or other voltage levels through air and this is commonly observed in devices used for sanitization purposes ^[5]. However, direct exposure of UV-C wavelengths to the human body can be extremely harmful. So proper sensing of UV-C is necessary.

This study will apply beta phase-$\mathrm{Ga}_{2}\mathrm{O}_{3}$ for UV-C detection sensors ^[6,7], which can be deposited at temperatures suitable for Monolithic 3D structures, typically around 450$\mathrm{℃}$ to 500$\mathrm{℃}.$ There is also GaN, which can detect UV-C, but it requires Epi processes at temperatures exceeding 1000$^{\circ}$C, making it unsuitable for research requiring low-temperature processes. Through this study, we aim to expand the potential of Monolithic 3D structures and open up new avenues for improving the performance and efficiency of UV-C detection sensors. This is expected to bring about new technological innovations in the semiconductor industry.

Fig. 1. Schematic of UV-C sensor device Structure for Monolithic 3D IC of semiconductor system.

II. EXPERIMENTAL DETAILS

Fig. 2 illustrates the fabrication flow of the UV-C sensory device in this study. As shown in Fig. 2, a ${\beta}$- Ga$_{2}$O$_{3}$-based photodetector with metal-semiconductor-metal (MSM) structure was fabricated on SiO$_{2}$(300 nm)/Si substrate. First, a 60 nm thick amorphous Ga$_{2}$O$_{3}$ thin film was deposited by radio frequency (RF) sputtering using a polycrystalline Ga$_{2}$O$_{3}$ target. This deposition process was performed in an Ar gas atmosphere of 1.3 mTorr at a power of 70 W in. The amorphous Ga$_{2}$O$_{3}$ was crystallized into monoclinic ${\beta}$- Ga$_{2}$O$_{3}$ after annealing at 900$^{\circ}$C for 1 h in air. The formation of n-type ${\beta}$-Ga$_{2}$O$_{3}$ thin film was achieved via Sn dopants using the spin-on-glass (SOG) approach. To diffuse Sn into the Ga$_{2}$O$_{3}$ thin film, annealing is carried out at 450$^{\circ}$C in an N$_{2}$ gas atmosphere for one hour.

Source/drain (S/D) electrodes of Ti(5~nm)/ TiN(200~nm) were deposited by RF sputtering and patterned by conventional lithography. Source/drain (S/D) electrodes of Ti(5~nm)/TiN(200~nm) were deposited by RF sputtering and patterned by conventional lithography. The resulting MSM device was annealed at 400$^{\circ}$C for 1 h to reduce the contact resistance, the melting points of Ti and TiN are over 1000$^{\circ}$C and over 2000$^{\circ}$C, respectively.)

In this study, experiments were conducted using a commonly available UV-C sanitizer, which emits wavelengths ranging from 260 nm to 280 nm and has a light intensity of $4\,\,\mu \mathrm{W}/\mathrm{cm}^{2}$. Currently, there are many commercialized products of UV-C sanitizers for sterilization purposes in the surrounding environment. The reason for utilizing these products is that the detected light intensity required for their operation is significant enough to warrant safety features to prevent direct exposure to the human body. Typically, in this study focusing on the investigation of material properties, high intensity is used to evaluate the characteristics of the material itself. However, since the objective is to discern and detect light intensity at the level of commercialized products, it was deemed more practical to utilize commercialized products as sources for development. This is crucial for developing components with practical applications.

We plan to observe three aspects through this research. First, we will observe the difference in current levels from 0 V to 5 V during a Voltage Sweep when exposed to UV-C wavelengths and when not exposed. For this purpose, we utilize the Metal electrode of size 1010 ${\mu}$m ${\times}$830 ${\mu}$m as shown in Fig. 3(a) for the convenience of the experiment. Second, to examine the impact of Metal electrode size on current levels, we will use two 1010 ${\mu}$m ${\times}$830 ${\mu}$m electrodes and two 530 ${\mu}$m${\times}$490 ${\mu}$m. electrodes from Fig. 3, configuring them as input/output pairs of the same size, and sweep from 0 V to 5 V for measurement. Third, we observe the time and magnitude at which the current reaches saturation when applying a constant voltage and current while exposing to UV-C. For this, we will apply a constant voltage of 2 V and 3 V, and a current of 0.1 A, and measure the current level.

The reason for examining these results is that the sensor is not used in isolation but needs to be integrated and utilized in conjunction with a processor for information processing. The on/off current ratio responsive to the light source, current density based on component specifications, and reaction speeds are valuable outcomes that can be utilized in the configuration of the processor.

Fig. 2. Schematic of UV-C sensor device fabrication flow: (a) Substrate Preparation; (b) $\mathrm{Ga}_{2}\mathrm{O}_{3}$ Deposition; (c) PR Coating; (d) Exposure & Development; (e) Ti/TiN Deposition; (f) Lift-Off.

Fig. 3. Verification of Metal Electrode(Ti/TiN) Pattern using Optical Microscope. Black region is Metal electrode and green region is $\mathrm{Ga}_{2}\mathrm{O}_{3}$ : (a) Large size metal electrode (1010μm x 830μm); (b) Small size metal electrode (530μm x 490μm).

III. RESULTS AND DISCUSSIONS

Fig. 4 presents the results of analyzing the component characteristics of the sensor using $\mathrm{Ga}_{2}\mathrm{O}_{3}$. As mentioned earlier, a commercialized product with an intensity of $4\,\,\mu \mathrm{W}/\mathrm{cm}^{2}$ was used as the light source, and it was positioned together with the probe station in a dark box for measurements.

The setup was arranged to provide indirect illumination in the form of backlighting to the component from a distance of approximately 30 cm. Fig. 4(a) illustrates the operational characteristics of the component based on the presence or absence of UV-C wavelengths. The UV-C wavelengths correspond to the range of 260 nm to 280 nm, and the voltage of the component was swept from 0 V to 5 V. The reason for setting the sweep range in this manner is that the system voltage typically applied to semiconductor systems is 3 V or less. While it might be possible to apply higher voltages for enhanced material characterization, it is more crucial to evaluate the operational characteristics under conditions reflecting the actual system configuration. Therefore, the sweep was conducted only up to 5~V to closely align with the operational environment of practical semiconductor systems. As evident from the results, it is confirmed that a higher current flows when exposed to UV-C wavelengths, and this trend becomes more pronounced with an increase in the applied voltage.

Fig. 4(b) illustrates the impact of the component size on the current. As previously shown in Fig. 3, two different sizes of components were simultaneously fabricated in this experiment. By utilizing these, it is possible to verify whether the current generated in response to UV-C wavelengths increases with the electrode area. If the reaction to UV-C occurs primarily on the front surface of $\mathrm{Ga}_{2}\mathrm{O}_{3}$, the results should show an increase in current size proportional to the electrode area. Conversely, if there is a constant specification for the current path independent of the electrode area, the results may not exhibit a proportional relationship with the electrode area. The results depicted in Fig. 4(b) indicate that both the on and off currents are proportional to the electrode size, revealing the formation of a current path in response to the front surface of $\mathrm{Ga}_{2}\mathrm{O}_{3}$. In this case, it can be observed that the on/off ratio increases proportionally with the size of the electrode. In other words, increasing the size of the electrode can result in an increase in the output current. Additionally, it signifies that the on/off ratio, which represents the degree of reaction to light, can also be enhanced with the increase in electrode size.

Fig. 5 shows the Current-Time characteristics when applying a constant voltage bias of 2 V and 3 V, along with a compliance current of 0.1 A. The confirmation of a twofold increase in current values at 3 V compared to the results at 2 V voltage is consistent with the earlier experimental findings. Furthermore, Each on/off ratio has a value of approximately 8, and it takes about 60 seconds to reach the saturation value.

In prior research on other UV-C sensory devices, significant current on/off ratios were only achieved with a 10 V voltage bias and exposure to an intensity around $45\,\,\mu \mathrm{W}/\mathrm{cm}^{2},$ approximately 11 times higher ^[8]. However, in this study, significant current levels were detected despite applying much lower 2-3 V voltage biases and exposure to an intensity of $4\,\,\mu \mathrm{W}/\mathrm{cm}^{2}$. While the magnitude of the applied voltage and the light intensity can alter the current level and on/off characteristics, as evident from this experiment, the size of the applied voltage does not significantly accelerate the time to reach the saturation current. As mentioned earlier, a sensor is a component that cannot exist independently. The information acquired by the sensor ultimately needs to be processed through a processor. While the absolute quantity of on/off current can be used for processing, the detection of light presence can also be handled by utilizing changes in the slope of the current curve, such as using a differentiator. The results indicate that even under typical operating conditions of semiconductor systems (low operating voltage), there is no degradation in the time it takes for the current response to reach saturation. Therefore, it implies that the sensor using$\mathrm{~ Ga}_{2}\mathrm{O}_{3}.$ can be sufficiently utilized in integrated semiconductor systems.

Fig. 4. (a) Comparison of I-V characteristics with and without UV-C Radiation (260 nm~280 nm); (b) Comparison of Voltage-Current Characteristics with and without UV-C Radiation (260 nm~280 nm) on Electrodes of Different Sizes.

Fig. 5. The time correlated photo response at a bias of 2 V and 3 V for UV-C senso device.

IV. CONCLUSIONS

In this study, we developed a UV-C detection sensor using $\mathrm{Ga}_{2}\mathrm{O}_{3}$and successfully demonstrated its functionality. Unlike conventional methods that relied on high voltage and intense UV-C radiation for sensing, our research proved that sensing is achievable with significantly lower voltage and intensity. This implies that commercialization as a product is entirely feasible, and we have validated its applicability to monolithic IC-based integrated semiconductor systems. These innovative results are not limited to advanced packaging and sensor fields but can also be applied across various research areas within the semiconductor industry. For instance, this technology is expected to contribute to the enhancement of performance in various semiconductor devices. Furthermore, the flexible manufacturing approach presented in this study holds great potential not only for industrial applications but also for experimentation and research in academia and research institutions. Therefore, the discoveries from this study are expected to lead to significant progress in the semiconductor field, impacting society and the economy across many areas of application.