A TSV-Structured Room Temperature p-Type TiO 2 Nitric Oxide Gas Sensor

: Planar MOS/MEMS gas sensors have been widely studied and applied, but the detection of exhaled gas has been little developed. The ﬂow rate of exhaled gas affects the suspension structure of the MEMS gas sensor and the operating temperature of the gas sensor. Therefore, this study uses the Bosch process and the atomic layer deposition (ALD) process to prepare a room-temperature (RT) TSV-structured TiO 2 gas sensor. The results indicated that the TiO 2 sensing ﬁlm is uniformed and covers the through-silicon via (TSV) structure and the TiO 2 sensing ﬁlm is conﬁrmed to be a p-type MOS. In terms of gas sensing at room temperature, the response of the sensor increases with the increasing NO concentration. The sensor response is 16.5% on average, with an inaccuracy of < ± 0.5% for ﬁve cycles at 4 ppm NO concentration. For gas at 10 ppm, the response of the sensor to NO is 24.4%, but the sensor produces almost no response to other gases (CO, CO 2 , SO 2 , and H 2 S). The RT TiO 2 gas sensor with a TSV structure exhibits good stability, reversibility, and selectivity to NO gas.


Introduction
Air quality is important since pollutant gases affect human health. Among them, nitrogen dioxide (NO 2 ) gas and nitric oxide (NO) gas are two types of nitrogen oxides (NO x ). The main source of these gases is as a natural product of fossil fuel combustion, thus improving the fuel's quality has an effect on production. Moreover, it was known that combustion facilities and automobiles are the major pollutants of NO x gas. They cause airway inflammation, acid rain, and photochemical smog [1][2][3][4]. In terms of the airway inflammation, NO has been a subject of research in the biomedical field due to its pivotal role in cell signaling and is implicated in the pathophysiology of various diseases [5,6]. In a study by Kim et al., it was reported that by conducting tests on children who exhaled NO, a link to allergic inflammation was observed [7]. Taylor et al. used exhaled NO measurements to guide the management of chronic asthma [8].
Several types of gas sensors are commercially available. They are categorized as electrochemical sensors, catalytic combustion sensors or optical sensors. In recent years, metal oxide semiconductor (MOS) gas sensors have been widely studied. For instance, Pour et al. reported the performance of gas nanosensor in 1-4% of hydrogen concentration [9]. In addition, Aval et al. reported the influence of oxide film surface morphology and thickness on the properties of gas sensitive nanostructure sensor [10]. These are categorized as n-type semiconductors, such as SnO 2 [11], ZnO [12], and WO 3 [2], and p-type semiconductors, such as CuO [13] and Co 3 O 4 [14]. However, due to the convenience of the circuit design and signal transmission, reducing gases are usually detected using n-type semiconductors and oxidizing gases are detected using p-type semiconductors. Titanium dioxide design and signal transmission, reducing gases are usually detected using n-type sem conductors and oxidizing gases are detected using p-type semiconductors. Titanium d oxide (TiO2) is an n-type semiconductor with a 3.0 eV energy gap and with the advanta of chemical and thermal stability at room temperature, which contributes to its suitabil for gas sensing [15]. Some p-type TiO2 have been produced using a different growth tec nology [16][17][18].
Three-dimensional (3D) through-silicon via (TSV) technology allows heterogeneo integration, low power consumption, and reduction in the size of component. Therefo it is widely used in integrated circuits (IC), humidity sensors [19], light-emitting diod [20,21], field emissions [22], and photo detectors [23]. Recently, various TSV process were widely developed, such as laser drilling [24], cryogenic etching [25], Bosch etchi [26], and wet anisotropic etching [27]. Among them, the Bosch process is widely used d to its high etch rate, better profile control, and mask selectivity [26]. In addition, the Bos process creates scallops on the sidewalls [26], which increase the sensing surface of t gas sensor. However, thin films are difficult to deposit on the surface of scallops. T atomic layer deposition (ALD) process was found to have good step coverage, especia for TSV structures [28].
This study uses a via-structured p-type TiO2 NO gas sensor that is produced by t Bosch and the ALD process. During the gas measurement, the sensor is operated at RT. via-structured gas sensor makes the airflow considerably smoother than a MOS gas sens with a general structure, which is more suitable for the detection of exhaled gas, especia for the detection of respiratory diseases, as shown in Figure 1. The via-structured fo mation, the TiO2 fabrication, and the sensor's sensing mechanism are described and d cussed.  Figure 2 displays the sensor's fabrication process diagram in this study. The fabric tion process has four stages. The first stage forms the via. The via's pattern is defined a 6" silicon (Si) wafer using standard exposure and development processes, then the Bos process [29] is utilized to etch Si for via's formation, as shown in Figure 2a. The seco stage produces the isolation layer. An 85 nm Al2O3 isolation layer is deposited using ALD process to cover the surface and sidewall of the via structure, as shown in Figure 2 The third stage produces the sensing layer. A 75 nm TiO2 is deposited on Al2O3 as a sen ing layer using identical ALD equipment, as shown in Figure 2c. During the deposition the TiO2 sensing layer, the precursors are TiCl4 and H2O vapor with an 80 sccm nitrog (N2) carrier gas. The pressure and temperature for the process are 300 mtorr and at 300 ° respectively. The sequence for each cycle is H2O (0.5 s), N2 (30 s), TiCl4 (0.5 s), and N2 ( s). The TiO2 sensing layer is continuously fabricated for 1000 cycles. The final stage pr duces the metal electrode. A 190 nm thick aluminum layer is prepared on the double si of the substrate using radio frequency (RF) magnetron sputtering, as shown in Figure 2 Figure 2 displays the sensor's fabrication process diagram in this study. The fabrication process has four stages. The first stage forms the via. The via's pattern is defined on a 6" silicon (Si) wafer using standard exposure and development processes, then the Bosch process [29] is utilized to etch Si for via's formation, as shown in Figure 2a. The second stage produces the isolation layer. An 85 nm Al 2 O 3 isolation layer is deposited using an ALD process to cover the surface and sidewall of the via structure, as shown in Figure 2b. The third stage produces the sensing layer. A 75 nm TiO 2 is deposited on Al 2 O 3 as a sensing layer using identical ALD equipment, as shown in Figure 2c. During the deposition of the TiO 2 sensing layer, the precursors are TiCl 4 and H 2 O vapor with an 80 sccm nitrogen (N 2 ) carrier gas. The pressure and temperature for the process are 300 mtorr and at 300 • C, respectively. The sequence for each cycle is H 2 O (0.5 s), N 2 (30 s), TiCl 4 (0.5 s), and N 2 (30 s). The TiO 2 sensing layer is continuously fabricated for 1000 cycles. The final stage produces the metal electrode. A 190 nm thick aluminum layer is prepared on the double side of the substrate using radio frequency (RF) magnetron sputtering, as shown in Figure 2d.

Materials and Methods
made instrument system was used to measure the sensor's response. The system in a Keithley 2400, a personal computer, a gas injection port, and an 8.8-L volume ch The NO gas used for measurement is a mixture of nitrogen and nitric oxide in a 97:3. When measuring, the applied voltage of the sensor is 8 V, and a 1-L gas bag the gas. Then, a micro syringe is used to inject the gas with an appropriate concen from the gas injection port into the test chamber [30].  Figure 3a shows the top-view SEM image of the via-structured TiO2 sensor. is almost square, with a side length of 270 um. Figure 3b shows the cross-sectio image of the via-structured TiO2 sensor. The via is 400 um deep.  The PlasmaPro 100 equipment was used to complete the Bosch process. The ALD was fabricated using a Picosun. The crystallinity of the TiO 2 sensing layer was analyzed using an X-ray diffraction (XRD). The thickness of the Al 2 O 3 and TiO 2 and via's structure was determined using field-emission scanning electron microscopy (FESEM, Hitachi SU8000) and a focus ion beam microscope (FIB). For gas sensor measurements, a home-made instrument system was used to measure the sensor's response. The system includes a Keithley 2400, a personal computer, a gas injection port, and an 8.8-L volume chamber. The NO gas used for measurement is a mixture of nitrogen and nitric oxide in a ratio of 97:3. When measuring, the applied voltage of the sensor is 8 V, and a 1-L gas bag collects the gas. Then, a micro syringe is used to inject the gas with an appropriate concentration from the gas injection port into the test chamber [30].  Figure 4 shows the micro-Raman spectrum for TiO 2 /Si at room temperature using a 532 nm solid laser excitation source. There is a strong and sharp peak at 522 cm −1 . This is attributed to the Si substrate. In addition, there is one weaker peak at 300 cm −1 , which is attributed to the SiO 2 that is generated by the water vapor and the surface of the Si substrate during the fabrication of the TiO 2 thin film [31]. Then, there is one minor peak at 144 cm −1 and three other significantly weaker peaks at 199, 400, and 638 cm −1 . These peaks are attributed to the TiO 2 thin film, which contains anatase crystals. The Raman peaks at 144, 199, and 638 cm −1 are attributed to the Raman mode position E g mode for TiO 2 . These Eg peaks are attributed to the symmetric stretching vibration of O-Ti-O [21]. The peak at 400 cm −1 is attributed to the B1g mode, which is probably related to the bending vibration of O−Ti−O [32]. 2022, 12, x FOR PEER REVIEW 4 of 10  Figure 4 shows the micro-Raman spectrum for TiO2/Si at room temperature using a 532 nm solid laser excitation source. There is a strong and sharp peak at 522 cm −1 . This is attributed to the Si substrate. In addition, there is one weaker peak at 300 cm −1 , which is attributed to the SiO2 that is generated by the water vapor and the surface of the Si substrate during the fabrication of the TiO2 thin film [31]. Then, there is one minor peak at 144 cm −1 and three other significantly weaker peaks at 199, 400, and 638 cm −1 . These peaks are attributed to the TiO2 thin film, which contains anatase crystals. The Raman peaks at 144, 199, and 638 cm −1 are attributed to the Raman mode position Eg mode for TiO2. These Eg peaks are attributed to the symmetric stretching vibration of O-Ti-O [21]. The peak at 400 cm −1 is attributed to the B1g mode, which is probably related to the bending vibration of O−Ti−O [32].  To confirm the structure of the TiO2 thin film, Figure 5 displays the XRD diffraction pattern of the TiO2 thin film prepared by ALD. It was found that the main diffraction plane and sub-main peak are the (101)   To confirm the structure of the TiO 2 thin film, Figure 5 displays the XRD diffraction pattern of the TiO 2 thin film prepared by ALD. It was found that the main diffraction plane and sub-main peak are the (101)   To confirm the structure of the TiO2 thin film, Figure 5 displays the XRD diffraction pattern of the TiO2 thin film prepared by ALD. It was found that the main diffraction plane and sub-main peak are the (101) and (200) planes, respectively. Moreover, it was found that the reflections from the (101)  Hall measurement is used to confirm the type of metal oxide semiconductor material. Figure 6 shows the I-V curve for the Hall measurement, which is linear. This shows that the probes at the sample and measurement are in ohmic contact. The carrier concentration is +3.84 × 10 18 cm −3 . The TiO2 thin film for this study that is produced using ALD is a ptype semiconductor material, as well as the experimental conclusions that can be drawn.

Results
Electrons have an important role in the gas sensing mechanism for a p-type MOS sensor. When the sensor is placed in air at room temperature, oxidizing gases (such as O2, nitric oxide (NO), etc.) are chemisorbed onto the surface of the MOS sensor. O2 traps an electron from the sensor's surface to form oxygen ions (O2 − ). The minority carriers in ptype MOS are electrons (e − ) and the majority carriers are holes (h + ), thus when O2 captures  Hall measurement is used to confirm the type of metal oxide semiconductor material. Figure 6 shows the I-V curve for the Hall measurement, which is linear. This shows that the probes at the sample and measurement are in ohmic contact. The carrier concentration is +3.84 × 10 18 cm −3 . The TiO 2 thin film for this study that is produced using ALD is a p-type semiconductor material, as well as the experimental conclusions that can be drawn.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 10 an electron, the number of hole carriers increases since the electron-hole (e − -h + ) pair recombination is inhibited and the resistance of the sensor is decreased. When a reducing gas (e.g., SO2, H2S, etc.) is injected, it reacts with oxygen ions, thus the trapped electrons return to the material and there is an increase in e --h + pair recombination and the resistance of the p-type MOS increases. With this sensing mechanism, ((Rair−Rgas)/Rair) × 100 [34] and ((Rgas−Rair)/Rair) × 100 [35], respectively define the response of the sensor to oxidizing gas and reducing gas, where Rair is the sensor's resistance and Rgas is the sensor's resistance in air containing an oxidizing or reducing gas. Using this definition, Figure 7 displays the sensor response of the TSV-structured TiO2 sensor. During the measurement, the NO concentration is increased from 0.5 to 8 ppm and the sensor operates at room temperature (RT). The respective sensor response for the TSV-structured TiO2 gas sensor is 6.4%, 8.2%, 10.9%, 16.7%, and 21.3% for NO concentrations of 0.5, 1, 2, 4, and 8 ppm. In other words, the response of the sensor is increased with the increasing NO concentration. The stability and reproducibility of the TSV-structured TiO2 NO gas sensor was measured over five cycles. Each cycle involves 5 min of NO gas injection and 5 min of pumping, as shown in Figure 8. During the measurement, the NO concentration is 4 ppm and the operating temperature is RT. The sensor's resistance rapidly decreases when NO Electrons have an important role in the gas sensing mechanism for a p-type MOS sensor. When the sensor is placed in air at room temperature, oxidizing gases (such as O 2 , nitric oxide (NO), etc.) are chemisorbed onto the surface of the MOS sensor. O 2 traps an electron from the sensor's surface to form oxygen ions (O 2 − ). The minority carriers in ptype MOS are electrons (e − ) and the majority carriers are holes (h + ), thus when O 2 captures an electron, the number of hole carriers increases since the electron-hole (e − -h + ) pair recombination is inhibited and the resistance of the sensor is decreased. When a reducing gas (e.g., SO 2 , H 2 S, etc.) is injected, it reacts with oxygen ions, thus the trapped electrons return to the material and there is an increase in e --h + pair recombination and the resistance of the p-type MOS increases. With this sensing mechanism, ((R air − R gas )/R air ) × 100 [34] and ((R gas − R air )/R air ) × 100 [35], respectively define the response of the sensor to oxidizing gas and reducing gas, where R air is the sensor's resistance and R gas is the sensor's resistance in air containing an oxidizing or reducing gas. Using this definition, Figure 7 displays the sensor response of the TSV-structured TiO 2 sensor. During the measurement, the NO concentration is increased from 0.5 to 8 ppm and the sensor operates at room temperature (RT). The respective sensor response for the TSV-structured TiO 2 gas sensor is 6.4%, 8.2%, 10.9%, 16.7%, and 21.3% for NO concentrations of 0.5, 1, 2, 4, and 8 ppm. In other words, the response of the sensor is increased with the increasing NO concentration. A MOS gas sensor must sense a specific gas selectively. Figure 9 shows the measurements for a TSV-structured RT TiO2 gas sensor. During this experiment, CO2, SO2, H2S, CO, and NO were injected at a flow rate of about 10 ppm. The sensor's response to NO is 24.4%. The sensor is only very slightly responsive to gases, such as CO, H2S, SO2, and CO2. The TiO2 gas sensor with TSV structure exhibits good selectivity to NO gas. This should be attributed to the lower activation energy of TiO2 film for NO gas. Furthermore, since the sensor operates at room temperature, the TiO2 surface is difficult to adsorb ionized oxygen species. As a result, the response of the sensor is poor when reducing gases, such as CO2, CO, H2S, and SO2, are introduced [41].  The stability and reproducibility of the TSV-structured TiO 2 NO gas sensor was measured over five cycles. Each cycle involves 5 min of NO gas injection and 5 min of pumping, as shown in Figure 8. During the measurement, the NO concentration is 4 ppm and the operating temperature is RT. The sensor's resistance rapidly decreases when NO gas is injected. The sensor's resistance increases when the test chamber is pumped. Of note, at this time, the ambient gas is also introduced. This dynamic behavior is consistent with the sensing mechanism for a p-type MOS that is previously described. Using the same definition for sensor response, the average sensor response is 16.5%, with an inaccuracy of <±0.5%. In other words, the sensor is stable and reversible at RT. Moreover, it was found that the average response and recovery time of the TSV-structured TiO 2 NO gas sensor are about 109 and 144 s. The response and recovery time are defined as the time required for a 90% change in the signal. Furthermore, this result indicated that the TSV-structured TiO 2 NO gas sensor had a better response and recovery time than the previous studies [36][37][38][39][40], as shown in Table 1.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 8 of 10 Figure 8. Resistance of a TSV-structured TiO2 gas sensor that is exposed to 4 ppm NO and at RT.

Conclusions
This study uses the Bosch process to form a TSV structure and then uses the ALD process to fabricate a TSV-structured TiO2 gas sensor. The FIB images show that the TiO2 The TSV-structured TiO 2 NO gas sensor Operting at RT 10 ppm gas concentration Figure 8. Resistance of a TSV-structured TiO 2 gas sensor that is exposed to 4 ppm NO and at RT. A MOS gas sensor must sense a specific gas selectively. Figure 9 shows the measurements for a TSV-structured RT TiO 2 gas sensor. During this experiment, CO 2 , SO 2 , H 2 S, CO, and NO were injected at a flow rate of about 10 ppm. The sensor's response to NO is 24.4%. The sensor is only very slightly responsive to gases, such as CO, H 2 S, SO 2 , and CO 2 . The TiO 2 gas sensor with TSV structure exhibits good selectivity to NO gas. This should be attributed to the lower activation energy of TiO 2 film for NO gas. Furthermore, since the sensor operates at room temperature, the TiO 2 surface is difficult to adsorb ionized oxygen species. As a result, the response of the sensor is poor when reducing gases, such as CO 2 , CO, H 2 S, and SO 2 , are introduced [41]. Resistance of a TSV-structured TiO2 gas sensor that is exposed to 4 ppm NO and at RT.

Conclusions
This study uses the Bosch process to form a TSV structure and then uses the ALD process to fabricate a TSV-structured TiO2 gas sensor. The FIB images show that the TiO2 sensing film uniformly covers the TSV structure. The Hall measurement confirms that the TiO2 sensing film is a p-type MOS. The respective sensor response of the TSV-structured TiO2 gas sensor to NO at room temperature is 6.4%, 8 The TSV-structured TiO 2 NO gas sensor Operting at RT 10 ppm gas concentration Figure 9. Gas measurements for a TSV-structured TiO 2 gas sensor.

Conclusions
This study uses the Bosch process to form a TSV structure and then uses the ALD process to fabricate a TSV-structured TiO 2 gas sensor. The FIB images show that the TiO 2 sensing film uniformly covers the TSV structure. The Hall measurement confirms that the TiO 2 sensing film is a p-type MOS. The respective sensor response of the TSV-structured TiO 2 gas sensor to NO at room temperature is 6.4%, 8.2%, 10.9%, 16.7%, and 21.3% for NO concentrations of 0.5, 1, 2, 4, and 8 ppm. In terms of the TSV-structured RT TiO 2 NO gas sensor's stability and reproducibility, the sensor response is 16.5% on average, with an inaccuracy of <±0.5%. In terms of selective gas measurement, the response of the sensor to NO is 24.4%. The sensor is only very slightly responsive to gases, such as SO 2 , CO, H 2 S, and CO 2 . In other words, a RT TiO 2 gas sensor with a TSV structure exhibits good stability, reversibility, and selectivity to NO gas.