Fabrication of Impedance Sensor with Hydrophilic Property to Monitor Soil Water Content for Slope Failure Prognostics

1Department of Electrical and Electronic Engineering, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan 2Graduate School of Science and Technology, Research Division, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan 3Device Integration Division, LAPIS Semiconductor Co., Ltd. Miyagi Plant, 1 Okinodaira, Ohira-mura, Kurokawa-gun, Miyagi 981-3693, Japan 4Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano City, Nagano 380-8553, Japan 5Integrated Intelligence Center, Okayama University, 3-1-1 Tsushimanaka, Okayama City, Okayama 700-8530, Japan


Introduction
Natural disasters due to rainfall-induced slope failure and soil fall happen all over the world. Peoples' lives and their houses suffer serious damage in the disasters. (1) Wired sensors, tilt sensors, (2) and GPS sensors, which can detect the start of soil sliding, are useful for facilitating This version has been created for advance publication by formatting the accepted manuscript. Some editorial changes may be made to this version. the announcement of an evacuation signal just before the slope fails. When the degree of hazard can be detected well before a slope failure, evacuees have enough time to evacuate. Therefore, a prognostic of slope failure is needed. On the other hand, some groups have been studying the monitoring of water content in soil. (3)(4)(5) When the water content in soil is increased, the soil frictional force is decreased and the soil weight is increased. Measurement of the water content can thus be used to evaluate the danger level of slope failure.
Water content sensors have several types of devices: a tensiometer (6)(7)(8) to measure the suction force, and electrical impedance sensors, which are mainly of the capacitance measurement type, (9)(10)(11) time domain reflectometry (TDR) type, (12)(13)(14)(15)(16) or electrical conductivity type. (17,18) Sensors for electrical impedance measurements of soil are generally superior in terms of longevity and they lack the need for periodic maintenance. Our group has been studying miniaturized impedance sensors fabricated by Si LSI technology (18,19) for the measurement of soil water content. Additionally, we have been conducting research on slope failure prognostics using water content sensors. An electrical-conductivity-type sensor with low-power and lowfrequency operation succeeded in monitoring the changes in water content in soil on a mountain slope, corresponding to the amount of rainfall. (20) Before installing the sensor on the mountain, its measurement sensitivity was confirmed using model soil with several different water contents in our laboratory. In this experiment, it was also confirmed that each water content was detected correctly. Therefore, the sensor was installed on the slope of the mountain. However, sometimes the sensor failed to respond to rainfall, as shown in Fig. 1. In order to achieve stable measurements, it was imperative to determine the cause of the problem and devise solutions. In this study, we confirmed that the use of SiN x and SiO x films, which have the property of water retention, can serve as a solution. After experiments, a new sensor chip with a SiO x film to solve the problem of measurement failure was fabricated. The new sensor chip and conventional sensor chip measured model soils with different water contents to verify their effectiveness. Finally, on-site and continuous measurements using the chips embedded in a mountain slope were performed.   Figure 1 shows a photograph of the previous sensor system (20) and measurement results of the sensor. The conventional sensor sometimes could not detect changes in the water content in the soil when it rained. We speculated on the causes of this and proposed solutions.

Conventional sensor chip and failure mechanism
To understand the cause of nondetection, the measurement setup and a cross-sectional view of the measurement space are shown in Fig. 2. The measurement current flowed in the space above the sensor surface between the Pt electrodes. The height of the measurement range is approximately 5 mm from the surface because the distance between the Pt electrodes is 2.27 mm. The surface material of the space between the Pt electrodes is a SiN x film. As the soil particles are hydrophilic, it is estimated that free water does not reach the sensor surface when the sensor surface is more hydrophobic than the soil, as shown in Fig. 3(a). In this situation, there are air pockets on the sensor surface. Therefore, conventional sensors sometimes cannot  detect changes in water content. To solve this problem, we proposed a new sensor chip in which a SiO x film is added on the SiN x film as shown in Fig. 3(b). When the SiO x film is almost as hydrophilic as the soil, the sensor can measure changes in water content.

Comparing the hydrophilicity of SiN x and SiO x films
The test chips with SiN x and SiO x films were fabricated on SiO 2 /Si substrates. The films were made by the plasma CVD method. To confirm the hydrophilic properties, we added 10 μL of the water dropwise on the test chips, which were tilted at an angle of 50º from the horizontal, as shown in Fig. 4(a). Then, the slide distances of the chips were compared. We confirmed that we could visually capture the ability of free water to be retained on the surface of the chip. Figure 4(b) shows the water drops 10 s after adding the water. The slide distance of the SiO x film was shorter than that of the SiN x film. These results suggest that the SiO x film is more hydrophilic than the SiN x film. Therefore, we propose a new sensor chip with a SiO x film added on top of the SiN x film of the previous sensor chip.

Fabrication of New Sensor Chip
The new sensor chip was fabricated by Si LSI technology and is shown in Fig. 5. Figure 5   for impedance measurement and a PN junction diode for a temperature sensor were integrated into the chip. Figure 5(b) shows a cross-sectional view of the chip. The thickness of the top film of SiO x placed on the SiN x film is 200 nm. When there was no SiN x under the SiO x , the Al electrode corroded and electric leakage occurred, so the SiN x film was not removed from the new chip. The thickness of the Pt electrodes, to which voltage and current were applied for the impedance measurement, was 100 nm.

Model soil experiment
When an air pocket exists on the sensor surface, the measured impedance becomes larger than the measurement results under a stable contact condition without the air pocket. In our experiment, the SiN x sensor of a conventional sensor and the SiO x sensor of the proposed sensor were used to measure the model granite soil, as shown in Fig. 6. A photograph of the model soil experiment is shown in Fig. 6(a). To make the water content of the soil uniform, the soil was mixed and packed in the container to the same density. The water mixed with the model soils was made by mixing NaCl with pure water. The ion concentrations of the water were 0.16, 10, and 100 mS/m. By using the water with the three concentrations, model soils with three different water contents, 0.15, 0.3, and 0.45 m 3 /m 3 , were prepared. The impedances of the model soils were measured using the previous and proposed sensors with the measurement circuit shown in Fig. 6(b). The impedance Z of the soil was calculated by dividing the voltage V Sens by the current I R as shown in Fig. 6(b). The measurement results of the SiN x and SiO x sensors were compared. Figure 7 shows the ratio of the impedance measurement results using model soils with water contents of 0.15, 0.3, and 0.45 m 3 /m 3 and ion concentrations of 0.16, 10, and 100 mS/m in the water. The graph shows the ratio of impedances measured by the SiN x sensor and the SiO x sensor. When the ratio was one, the measured impedances of the two sensors were equal. When an air pocket existed above the SiN x sensor surface, the ratio was larger than one. These results show that the ratio increased with the water content. This finding suggests that the SiO x chip is more capable of stable measurements owing to its better contact with free water.

On-site and long-term measurement on slope of mountain
For on-site and continuous monitoring on a mountain slope, a wireless sensor system was fabricated, as shown in Fig. 8. As shown in Fig. 8(a), a hole was dug in the slope of the mountain, into which a sensor was inserted. The SiO x and SiN x sensors were inserted into separate holes. The sensor control circuit applied a voltage and measured the current of the Pt electrodes of the sensors. The circuits also converted the signal from analog to digital. The circuits were covered by a plastic tube for waterproofing and were buried in the soil. Figure  8(b) shows the area where the sensors were inserted and the waterproof box used to store the wireless transmitter and batteries. The wireless transmitter (21,22) used a radio frequency of 429 MHz because the influence of trees on a 429 MHz radio signal is smaller than that if a higher radio frequency is used. (23) The SiO x and SiN x sensors performed measurements for around five months. Figure 9 shows the conductivity, the reciprocal of impedance, of the two sensors plotted over the five months. When the water content in the soil became large, the conductivity signal also became large. The changes in the SiN x sensor signal were small when it rained, as shown in Fig. 9(a). In contrast, the changes in the SiO x sensor signal were large, as shown in Fig. 9(b). The measurement results in Fig. 9(b) were converted into volumetric water content information using the calibration curve, as shown in Fig. 10. The graph was obtained by using model soils with uniform water content. (20) The approximate curve is represented by Eq. (1). The coefficient of determination R 2 of the curve is 0.9441, indicating satisfactory measurement accuracy.
Here, WC is volumetric water content and EC is electric conductivity. The converted results are shown in Fig. 11. We confirmed that the water content values were consistent with the expected values. These results confirm that the SiO x sensor can measure changes in the soil water content resulting from precipitation.

Conclusions
Our group has been studying miniaturized impedance sensors to minimize the damage caused by slope failure. Our conventional sensor with a SiN x top film sometimes failed to detect changes in the water content in soil resulting from rain. It was speculated that the problem was due to the fact that the SiN x film was more hydrophobic than the soil. Experiments on the slide distances of water droplets were performed using SiO x and SiN x films. Because the slide distance of the SiO x film was shorter than that of the SiN x film, we confirmed that the SiO x film was more hydrophilic than the SiN x film. To improve the contact characteristics between the sensor surface and free water in soil, a new sensor chip was fabricated with a SiO x film in the space between the Pt electrodes of the conventional sensor chip using LSI technology. To confirm the contact condition between free water and the sensor surface, impedance measurements were performed in model soils using the conventional SiN x and proposed SiO x sensors. We confirmed that the impedance using the SiO x sensor was smaller than that using the SiN x sensor. The results showed that the contact condition of the SiO x sensor was better than that of the SiN x sensor. On-site and long-term measurements on a mountain slope were performed using these sensors for five months. The changes in the SiN x sensor signal were small when it rained, whereas the changes in the SiO x sensor signal were large. The water content values obtained using the SiO x sensor conformed to the expected results. Thus, we succeeded in fabricating a stable soil monitor sensor by using a hydrophilic SiO x film.
Yasushi Fuwa received his B.E. and M.E. degrees from Shinshu University in 1981 and 1983, respectively. From 1983 to 1992, he was a research associate in the Faculty of Engineering, Shinshu University. He received his Ph.D. degree from Nagoya Institute of Technology in 1992. He is now a professor in the Graduate School of Science and Technology, Shinshu University. His current research areas include ad-hoc networks, sensor networks, computer networks, and e-learning. He is a member of JSiSE, IEICE, IPSJ, and IEEE.
Mitsuru Komatsu was born in Kumamoto, Japan, in 1972. He received his B.A. and M.S. degrees and his Ph.D. degree in civil engineering, all from Okayama University, Okayama, Japan, in 1995, 1997, and 2000, respectively. He is currently an associate professor with Okayama University. His current research interests focus on methods of measuring hydraulic properties in unsaturated soil.