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Development of high-sensitivity ambient light sensor based on cadmium sulfide-deposited surface acoustic wave sensor
Sensors and Actuators A 293 (2019) 145–149
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Development of high-sensitivity ambient light sensor based on cadmium sulfide-deposited surface acoustic wave sensor Byungmoon Lee a,b , Jinkee Hong b , Jong Woo Kim a,b , Yeon Hwa Kwak c , Kunnyun Kim c , Jin-Woo Lee a,b,∗ , Byeong-Kwon Ju a,∗ a
Display and Nanosystem Laboratory, School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea New Business Group, Haesung DS Co., Ltd 508, Teheran-ro, Gangnam-gu, Seoul 06178, Republic of Korea c Smart Sensor Research Center, Korea Electronics Technology Institute, 25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do 13509, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 July 2018 Received in revised form 24 December 2018 Accepted 29 March 2019 Available online 30 March 2019 Keywords: Surface acoustic wave Ambient light sensor Photosensor Cadmium sulfide Absorbance spectrum
a b s t r a c t A highly sensitive ambient light sensor based on surface acoustic waves (SAWs) was investigated. The ambient light SAW sensor is fabricated by depositing sensitive film and its resonant frequency shift was calculated in response to a change in light intensities. The resonant frequency is generated between the piezoelectric substrate and a specifically designed transducer, which is usually called interdigitated transducer (IDT). The IDT design was determined to exhibit the resonant frequency at 244.5 MHz by considering the sensor size and fabrication process. We fabricated our ambient light SAW sensor by using a cadmium sulfide (CdS) thin film as a sensing material. Absorbance spectra of CdS thin film in visible light region were investigated. Then the lithium niobate (LiNbO3 ) substrate is adopted as a substrate due to its high coupling coefficient. Fabrication of CdS thin films was conducted by thermal evaporator and common lithography process including lift-off. To increase the sensitivity of the sensor, we focused on increasing the thickness and area of the sensitive film. As a result, the sensitivity increased by approximately three times when the area doubled. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ambient light sensors have been increasing in demand with the development of IoT devices and mobile applications. They are used for gathering information on the surrounding environment to reinforce interaction with users [1,2]. Recently, surface acoustic wave (SAW)-based sensors have been extensively studied because of their high sensitivity, low manufacturing cost, and low power consumption [3–7]. By applying specific designs and sensing materials, various types of SAW sensors—humidity sensors [8], ultraviolet (UV) sensors [9], strain sensors [10,11], etc [12–14].—have been investigated. The SAW humidity sensor uses graphene oxide as a sensitive material. Graphene oxide is reported to have a high hydrophilicity. When moisture in the air penetrates into graphene oxide, the velocity of the acoustic wave changes while the waves pass through the sensing film; in this way, the humidity is sensed. Regarding photosensors, various materials have been used as sen-
∗ Corresponding authors at: Display and Nanosystem Laboratory, School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea. E-mail addresses: [email protected] (J.-W. Lee), [email protected] (B.-K. Ju). https://doi.org/10.1016/j.sna.2019.03.048 0924-4247/© 2019 Elsevier B.V. All rights reserved.
sitive materials in SAW sensors to detect a certain range of the light spectrum, such as zinc ox ide (ZnO) nanowire thin films for UV detection. For the SAW strain sensor, it has no sensitive materials but utilizes substrate properties to detect strain. When the substrate is bent, the strain induces change of elastic properties of material and then changes its elastic velocity. In this work, we chose CdS as the sensitive material and fabricated an ambient light SAW sensor. CdS, which is already used in solar cells in the industry, is a well-known material that reacts with visible light. We deposited a CdS thin film on the piezoelectric substrate using a common thermal evaporation technique to achieve good optical properties [15]. We investigated CdS in-depth via scanning electron microscopy (SEM), focused ion beam (FIB), and a UV–visible–near-infrared (UV–vis–NIR) spectrophotometer to understand the material properties. From the result of spectrophotometry, CdS thin film absorbance to visible light region could have achieved. For the substrate, 128◦ YX lithium niobate (LiNbO3 ), which has a good coupling coefficient, was used. Using the properties of the substrate itself, the thermal response was measured. The fabricated sensors were tested by measuring the frequency shifts in the transmission spectrum (S21 ). The experiment was conducted by changing the thickness and area of the CdS thin film in order to improve the sensitivity of the sensor.
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B. Lee et al. / Sensors and Actuators A 293 (2019) 145–149 Table 1 Design parameters of the SAW sensor.
Fig. 1. Schematic view of the CdS-based SAW ambient light sensor.
Parameters
Value
Unit
Number of input and output IDT fingers Wavelength [] Space between fingers Finger width [d] Aperture Length of delay line Theoretical resonant frequency IDT finger height
70 × 70 16 2 2 480 560 244.5 200
m m m m m MHz nm
In a single sensor chip, two tracks of SAW sensors are deployed. One track is a reference track for stabilizing thermal effects. No sensitive film is deposited in this track. The other track is used as an ambient light sensor, which is the main purpose of the device. The size of one track of the sensor is 1.80 mm × 0.64 mm. Fig. 2(a) and (b) show microscopic images of the sensor.
2. Experimental 2.1. Design of the sensor A schematic view of the SAW sensor is presented in Fig. 1. The fabricated SAW sensor in this work has adopted the common delayline model. There are two interdigitated transducers (IDTs)—input and output IDTs—and between them is a delay line where SAW propagates through the surface of the substrate. Each IDT consists of 35 pairs of interdigitated electrodes, and each electrode finger is of the double-electrode type owing to the advantage of redundancy [16]. The width of the electrode and the space between the electrodes are equivalently matched to 2 m for ease of fabrication. Both the input and output IDTs have an aperture of 480 m (30 ), and the delay line is 560 m long (35). The IDTs were fabricated by depositing Al via DC sputtering, and a pattern was formed through a common photolithography process. To increase the efficiency of energy conversion via the piezoelectric effect, 128◦ YX lithium niobate (LiNbO3 ) with a high coupling factor of 5.5% was employed as a substrate. The LiNbO3 substrate exhibits linearity with respect to thermal changes, allowing the sensor to offset thermal effects. Table 1 shows the detailed parameters of the sensor design.
2.2. Cadmium sulfide deposition and fabrication of the sensor The pattern to deposit cadmium sulfide (CdS) was formed via photolithography after the IDTs were fabricated on the substrate. Then, the CdS was deposited by the thermal evaporator with CdS powder, and the CdS thin film was fabricated through the lift-off process. Fig. 2(c) shows a FIB image of the CdS thin film. We used FIB to check accurate thickness of CdS thin film, which is measured to be 185.9 nm. We expected the film thickness to be 190 nm, and we successfully deposited the thin film without significant difference as a result. Fig. 3 shows the frequency response of the fabricated ambient light SAW sensor. The resonant frequency is 243.5 MHz, with an insertion loss of -17.5 dB. In addition, to measure the sensitivity of the sensor depending on the volume of the sensitive film, sensors having various designs of CdS thin films were fabricated. CdS thin films with thicknesses of 140 nm and 190 nm were deposited. To increase the area of the sensitive film, we also fabricated the sensors shown in Fig. 4. Fig. 4(a) shows the “half-covered” sample with the CdS thin film covering
Fig. 2. Microscopic image of the sensor (a); FIB image of an IDT (b); measuring the thickness of the CdS thin film (c).
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half of the IDTs, and Fig. 4(b) shows the “full-covered” sample with the IDTs completely covered. 2.3. Measurement environment
Fig. 3. Frequency response of the SAW sensor.
Fig. 5 presents a concept view of the ambient light SAW sensor measurement system. To prevent unintentional light from affecting the sensor, we built a dark box to house the sensor. In the dark box, the sensor can be fixed, i.e., its position is unchanged, during repeated experiments. The sensor is connected to an external network analyzer (Protek A333) through an SMA cable. In network analyzer, the resonant frequency can be obtained in the transmission spectrum (S21 ) with maximum of amplitude in insertion loss. Normally it is observed in frequency around 244.5 MHz as intended. Once the resonant frequency is set, we checked how much resonant frequency shifted during irradiation in real time. The white LED light is applied to the sensor through a light guide, and it can be manually adjusted by an external light source from 0 lx to 20,000 lx. This light is monitored in real time by an illuminometer inside the dark box. The illuminometer is separated from the light source by the same distance as the SAW sensor, so that the same amount of light can be applied to the sensor.
Fig. 4. Microscopic images of the half-covered sensor (a) and full-covered sensor (b).
Fig. 5. Concept view of the ambient light SAW sensor measurement system.
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Fig. 6. Absorbance spectra (300–800 nm) for CdS thin films of different thicknesses deposited on the LiNbO3 substrate. Fig. 8. Resonant frequency shift with respect to the light intensity for sensors of different sensitive film areas.
Fig. 7. Resonant frequency shift due to the temperature change in the reference track.
3. Results and discussion Fig. 9. Sensing performance with the variation in the sensitive-film thicknesses.
The absorbance spectra for CdS thin films of different thicknesses are plotted in Fig. 6. To confirm the tendency of absorbance depends on CdS thicknesses, the tested samples are deposited with different thicknesses of 700 A˚ and 1700 A˚ CdS thin films on bare LiNbO3 substrates. Each sample is prepared in size of 10 mm × 10 mm and only CdS thin film is deposited on bare LN wafer. The light absorbance of the samples is measured using a UV–vis–NIR spectrophotometer (Agilent Cary 5000) in the wavelength range of 300 nm to 800 nm. According to the measurement, the absorbance of both samples increased under light of <550 nm. This is expected owing to the CdS bandgap, which is 2.45 eV at room temperature. Additionally, the absorbance increased with the thickness, indicating that it follows the Beer–Lambert law. Fig. 7 shows the shift in the resonant frequency of the reference track due to thermal changes. As described above, the reference track consists of IDTs and the bare LiNbO3 substrate, without the deposition of a sensitive film. The experiment was conducted in a commercial vacuum oven with the temperature varying from −30 ◦ to 150 ◦ . After setting up the temperature of the vacuum oven, the temperature of the sensor was confirmed using an infrared thermometer and the frequency was measured. The result shows good linearity. As expected by considering the TCF of substrate, the resonant frequency shifted by 3.51 MHz for a variation of 190 ◦ . This data can be used as a reference for offsetting thermal parameters when the sensor is mounted on mobile devices.
Fig. 8 shows the resonant frequency shift with the variation in the intensity of light for samples with different areas of sensitive films. Unlike the samples shown in Fig. 4, the “Non-covered sensor” means that the sensitive film was deposited only on the delay line, without covering the IDTs, as shown in Fig. 2(a). The result shows that the sensitivity of the SAW sensor increased with the area of the sensitive films. Under irradiation with 20,000 lx of visible light, the resonant frequencies of the non-, half-, and full-covered sensors varied by 30.54 kHz, 100.96 kHz, and 182.73 kHz, respectively. Thus, compared with the non-covered sensor, the half- and the fullcovered sensors are approximately 2.3 times and 4.98 times more sensitive, respectively. Each experiment was repeated three times for each sample, and the data were averaged. The results of the experiment with regard to the thickness are plotted in Fig. 9. Similar to the previously described absorbance spectra, the sensitivity of the sensor was higher for samples of thicker sensitive films. Under irradiation of 20,000 lx, the resonant frequency of a sample with a 140 nm sensitive film changed by 82.17 kHz. Under the same conditions, the resonant frequency of a sample with a sensitive film thickness of 190 nm changed by 431.79 kHz, which is approximately 4.25 times better than the previous sample. To theoretically understand these results, the following formula was used. The three main operational theories for
B. Lee et al. / Sensors and Actuators A 293 (2019) 145–149
the SAW sensor are the mass-loading, acoustoelectric, and viscoelastic effects, which are expressed according to this formula [17]: v
v0
= −cm f0 m/A + 4ce
f0
(hG’) − 2
v0
02 K2 2 2 0 + v20 C02
Here, cm and ce are the coefficients of the mass sensitivity and elasticity of the substrate, respectively, and f0 is the fundamental frequency of the SAW sensor. The term h represents the film thickness, and we can assume that increases with the film thickness. Depending on the relationship between the wave speed and the frequency, v = f, it can be observed that the frequency changes more drastically at higher wave speeds. In summary, the thickness and area of the CdS thin film affect both the sensitivity of the SAW sensor and the absorbance data. With a larger mass of the sensitive film, we can expect better response performance of the SAW sensor. 4. Conclusion This paper reports the achievement of a high sensitivity in a CdS-deposited ambient light SAW sensor. The detailed design parameters were introduced, and the fabrication process was described. The absorbance data for CdS thin films with various thicknesses were measured using a spectrophotometer, and the structural characteristics of the SAW sensor were observed via several microscopes. Using a LiNbO3 substrate, the sensor exhibited linearity in its thermal changes. By increasing the thickness and area of the CdS thin film, the sensitivity of the sensor was improved. This result is expected to be a practical example of mounting in mobile devices. Acknowledgement This work was supported by the R&D program of MOTIE/KEIT [10064078, Development of the Multi-Sensor for UV, Ambient Light, and Proximity for Next Smart Device]. References [1] M. Magno, A low cost, highly scalable wireless sensor network solution to achieve smart LED light control for green buildings, IEEE Sens. J. 15 (2015) 2963–2973. [2] H. Yoon, S.H. Park, K.T. Lee, Exploiting ambient light sensor for authentication on wearable devices, IEEE (2015) 95–100. [3] E.R. Hirst, W.L. Xu, J.E. Bronlund, Y.J. Yuan, Surface acoustic wave delay line for biosensor application, in: International Conference on Mechatronics and Machine Vision in Practice, 2008, pp. 40–44. [4] J. Filipiak, L. Solarz, G. Steczko, SAW delay line for vibration sensors, Acta Phys. Pol. A. 122 (2012) 808–813. [5] T. Wu, Y. Chen, T. Chou, A high sensitivity nanomaterial based SAW humidity sensor, Appl. Phys. 41 (2008), 085101. [6] S. Li, S. Sankaranarayanan, C. Fan, Y. Su, V. Bhethanabotla, Achieving lower insertion loss and higher sensitivity in a SAW biosensor via optimization of waveguide and microcavity structures, IEEE Sens. J. 17 (2017) 1608–1616. [7] J. Chen, H. Guo, X. He, W. Wang, W. Xuan, H. Jin, S. Dong, X. Wang, Y. Xu, S. Lin, S. Garner, J. Luo, Development of flexible ZnO thin film surface acoustic wave strain sensors on ultrathin glass substrate, J. Micromech. Microeng. 25 (2015) 115005. [8] X. Le, H. Ding, J. Pang, Y. Wang, J. Xie, A humidity sensor with high sensitivity and low temperature coefficient of frequency based on AlN surface acoustic wave and graphene oxide sensing layer, IEEE Transducers (2017) 210–213.
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[9] W. Peng, Y. He, C. Wen, K. Ma, Surface acoustic wave ultraviolet detector based on zinc oxide nanowire sensing layer, Sens. Actuators A 184 (2012) 34–40. [10] H. Oh, K. Lee, K. Eun, S. Choa, S. Yang, Development of a high-sensitivity strain measurement system based on a SH SAW sensor, J. Micromech. Microeng. 22 (2012), 025002. [11] R. Stoney, B. Donohoe, D. Geraghty, G.E. O’Donnell, The development of surface acoustic wave sensors (SAWs) for process monitoring, Procedia CIRP 1 (2012) 569–574. [12] H. Oh, C. Fu, S. Yang, W. Wang, K. Lee, A novel shock and heat tolerant gyrosensor utilizing a one-port surface acoustic wave reflective delay line, J. Micromech. Microeng. 22 (2012), 045007. [13] Y.J. Lee, H.B. Kim, Y.R. Roh, H.M. Cho, S. Baik, Development of a SAW gas sensor for monitoring SO2 gas, Sens. Actuators A 64 (1998) 173–178. [14] H. Oh, W. Wang, S. Yang, K. Lee, Development of SAW based gyroscope with high shock and thermal stability, Sens. Actuators A 165 (2011) 8–15. [15] P.P. Sahay, R.K. Nath, S. Tewari, Optical properties of thermally evaporated CdS thin films, Cryst. Res. Technol. 42 (2007) 275–280. [16] K. Inagawa, M. Koshiba, Equivalent networks for SAW interdigital transducers, IEEE Trans. 41 (1994) 402–411. [17] J. Cheeke, N. Tashtoush, N. Eddy, Surface acoustic wave humidity sensor based on the changes in the viscoelastic properties of a polymer film, in: IEEE Ultrasonics Symposium, 1996, pp. 449–452.
Biographies Byungmoon Lee received his B.S. from the School of Electrical and Electronics Engineering of Chung-Ang University, Seoul, Republic of Korea, in 2017. Since 2017, he has been a candidate for a M.S. degree from the Department of Micro/Nano Systems Engineering at Korea University under the supervision of Prof. Byeong-Kwon Ju. He is also working for HAESUNG DS CO., Ltd as a researcher under the supervision of principal research engineer Jin-Woo Lee. Jinkee Hong received his B.S. degree from the Department of English Language and Literature of Ajou University, Suwon, Republic of Korea, in 2015. He has been working for HAESUNG DS CO., Ltd as a researcher in the New Business Group. Jong Woo Kim received his B.S. degree from the School of Physics Department of Andong National University, Andong, Republic of Korea, in 2009 and his M.S. degree in Electrical Engineering in 2011 from Korea University. He has been a candidate for a Ph.D. in Electrical Engineering from Korea University under the supervision of Prof. Byeong-Kwon Ju. He is also working for HAESUNG DS CO., Ltd as a researcher under the supervision of principal research engineer Jin-Woo Lee. Yeon Hwa Kwak received her B.S. degree in Electronics Engineering in 1999 from Kyungpook National University (Daegu, Korea) and her M.S. and Ph.D. in 2004 and 2017, respectively, from Korea University. She is now working for Korea Electronics Technology Institute (Seongnam, Korea) as a managerial researcher. Kunnyun Kim received his B.S. and M.S. degrees in control and instrumentation engineering in 1991 and 1993, respectively, from Ajou University (Suwon, Korea) and his Ph.D. in Electrical Engineering in 2011 from Korea University. He has been working for Korea Electronics Technology Institute (Seongnam, Korea) as a principal researcher since 1993. His research interests include various sensors and input devices. Jin-Woo Lee received his B.S. and M.S. degrees in Chemical Engineering in 1998 and 2000, respectively, from Hong-ik University, Seoul, Republic of Korea. In 2000, he joined for Samsung Techwin, where he had been researching semiconductor materials and developing LED components. He had also conducted research on surface treatment. From 2014, he has been working for HAESUNG DS CO., Ltd as a principal research engineer. Currently, his main interests are developing new sensors, including optical sensors and thermometers. Byeong-kwon Ju received his Ph.D. in semiconductor engineering from Korea University in 1995. In 1988, he joined the Korea Institute of Science and Technology (KIST), Seoul, where he was mainly engaged in the development of flat-panel displays and microelectromechanical systems technology as a principal research scientist. In 1996, he spent six months as a visiting research fellow at the Microelectronics Centre, University of South Australia, Australia. Since 2005, he has been an associate professor in the Dept. EE of Korea University, with his main interest being flexible electronics (OLED, OTFT), field-emission devices, Si-micromachining, and carbon nanotube-based nanosystems.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Development of high-sensitivity ambient light sensor based on cadmium sulfide-deposited surface acoustic wave sensor Byungmoon Lee a,b , Jinkee Hong b , Jong Woo Kim a,b , Yeon Hwa Kwak c , Kunnyun Kim c , Jin-Woo Lee a,b,∗ , Byeong-Kwon Ju a,∗ a
Display and Nanosystem Laboratory, School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea New Business Group, Haesung DS Co., Ltd 508, Teheran-ro, Gangnam-gu, Seoul 06178, Republic of Korea c Smart Sensor Research Center, Korea Electronics Technology Institute, 25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do 13509, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 July 2018 Received in revised form 24 December 2018 Accepted 29 March 2019 Available online 30 March 2019 Keywords: Surface acoustic wave Ambient light sensor Photosensor Cadmium sulfide Absorbance spectrum
a b s t r a c t A highly sensitive ambient light sensor based on surface acoustic waves (SAWs) was investigated. The ambient light SAW sensor is fabricated by depositing sensitive film and its resonant frequency shift was calculated in response to a change in light intensities. The resonant frequency is generated between the piezoelectric substrate and a specifically designed transducer, which is usually called interdigitated transducer (IDT). The IDT design was determined to exhibit the resonant frequency at 244.5 MHz by considering the sensor size and fabrication process. We fabricated our ambient light SAW sensor by using a cadmium sulfide (CdS) thin film as a sensing material. Absorbance spectra of CdS thin film in visible light region were investigated. Then the lithium niobate (LiNbO3 ) substrate is adopted as a substrate due to its high coupling coefficient. Fabrication of CdS thin films was conducted by thermal evaporator and common lithography process including lift-off. To increase the sensitivity of the sensor, we focused on increasing the thickness and area of the sensitive film. As a result, the sensitivity increased by approximately three times when the area doubled. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ambient light sensors have been increasing in demand with the development of IoT devices and mobile applications. They are used for gathering information on the surrounding environment to reinforce interaction with users [1,2]. Recently, surface acoustic wave (SAW)-based sensors have been extensively studied because of their high sensitivity, low manufacturing cost, and low power consumption [3–7]. By applying specific designs and sensing materials, various types of SAW sensors—humidity sensors [8], ultraviolet (UV) sensors [9], strain sensors [10,11], etc [12–14].—have been investigated. The SAW humidity sensor uses graphene oxide as a sensitive material. Graphene oxide is reported to have a high hydrophilicity. When moisture in the air penetrates into graphene oxide, the velocity of the acoustic wave changes while the waves pass through the sensing film; in this way, the humidity is sensed. Regarding photosensors, various materials have been used as sen-
∗ Corresponding authors at: Display and Nanosystem Laboratory, School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea. E-mail addresses: [email protected] (J.-W. Lee), [email protected] (B.-K. Ju). https://doi.org/10.1016/j.sna.2019.03.048 0924-4247/© 2019 Elsevier B.V. All rights reserved.
sitive materials in SAW sensors to detect a certain range of the light spectrum, such as zinc ox ide (ZnO) nanowire thin films for UV detection. For the SAW strain sensor, it has no sensitive materials but utilizes substrate properties to detect strain. When the substrate is bent, the strain induces change of elastic properties of material and then changes its elastic velocity. In this work, we chose CdS as the sensitive material and fabricated an ambient light SAW sensor. CdS, which is already used in solar cells in the industry, is a well-known material that reacts with visible light. We deposited a CdS thin film on the piezoelectric substrate using a common thermal evaporation technique to achieve good optical properties [15]. We investigated CdS in-depth via scanning electron microscopy (SEM), focused ion beam (FIB), and a UV–visible–near-infrared (UV–vis–NIR) spectrophotometer to understand the material properties. From the result of spectrophotometry, CdS thin film absorbance to visible light region could have achieved. For the substrate, 128◦ YX lithium niobate (LiNbO3 ), which has a good coupling coefficient, was used. Using the properties of the substrate itself, the thermal response was measured. The fabricated sensors were tested by measuring the frequency shifts in the transmission spectrum (S21 ). The experiment was conducted by changing the thickness and area of the CdS thin film in order to improve the sensitivity of the sensor.
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B. Lee et al. / Sensors and Actuators A 293 (2019) 145–149 Table 1 Design parameters of the SAW sensor.
Fig. 1. Schematic view of the CdS-based SAW ambient light sensor.
Parameters
Value
Unit
Number of input and output IDT fingers Wavelength [] Space between fingers Finger width [d] Aperture Length of delay line Theoretical resonant frequency IDT finger height
70 × 70 16 2 2 480 560 244.5 200
m m m m m MHz nm
In a single sensor chip, two tracks of SAW sensors are deployed. One track is a reference track for stabilizing thermal effects. No sensitive film is deposited in this track. The other track is used as an ambient light sensor, which is the main purpose of the device. The size of one track of the sensor is 1.80 mm × 0.64 mm. Fig. 2(a) and (b) show microscopic images of the sensor.
2. Experimental 2.1. Design of the sensor A schematic view of the SAW sensor is presented in Fig. 1. The fabricated SAW sensor in this work has adopted the common delayline model. There are two interdigitated transducers (IDTs)—input and output IDTs—and between them is a delay line where SAW propagates through the surface of the substrate. Each IDT consists of 35 pairs of interdigitated electrodes, and each electrode finger is of the double-electrode type owing to the advantage of redundancy [16]. The width of the electrode and the space between the electrodes are equivalently matched to 2 m for ease of fabrication. Both the input and output IDTs have an aperture of 480 m (30 ), and the delay line is 560 m long (35). The IDTs were fabricated by depositing Al via DC sputtering, and a pattern was formed through a common photolithography process. To increase the efficiency of energy conversion via the piezoelectric effect, 128◦ YX lithium niobate (LiNbO3 ) with a high coupling factor of 5.5% was employed as a substrate. The LiNbO3 substrate exhibits linearity with respect to thermal changes, allowing the sensor to offset thermal effects. Table 1 shows the detailed parameters of the sensor design.
2.2. Cadmium sulfide deposition and fabrication of the sensor The pattern to deposit cadmium sulfide (CdS) was formed via photolithography after the IDTs were fabricated on the substrate. Then, the CdS was deposited by the thermal evaporator with CdS powder, and the CdS thin film was fabricated through the lift-off process. Fig. 2(c) shows a FIB image of the CdS thin film. We used FIB to check accurate thickness of CdS thin film, which is measured to be 185.9 nm. We expected the film thickness to be 190 nm, and we successfully deposited the thin film without significant difference as a result. Fig. 3 shows the frequency response of the fabricated ambient light SAW sensor. The resonant frequency is 243.5 MHz, with an insertion loss of -17.5 dB. In addition, to measure the sensitivity of the sensor depending on the volume of the sensitive film, sensors having various designs of CdS thin films were fabricated. CdS thin films with thicknesses of 140 nm and 190 nm were deposited. To increase the area of the sensitive film, we also fabricated the sensors shown in Fig. 4. Fig. 4(a) shows the “half-covered” sample with the CdS thin film covering
Fig. 2. Microscopic image of the sensor (a); FIB image of an IDT (b); measuring the thickness of the CdS thin film (c).
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half of the IDTs, and Fig. 4(b) shows the “full-covered” sample with the IDTs completely covered. 2.3. Measurement environment
Fig. 3. Frequency response of the SAW sensor.
Fig. 5 presents a concept view of the ambient light SAW sensor measurement system. To prevent unintentional light from affecting the sensor, we built a dark box to house the sensor. In the dark box, the sensor can be fixed, i.e., its position is unchanged, during repeated experiments. The sensor is connected to an external network analyzer (Protek A333) through an SMA cable. In network analyzer, the resonant frequency can be obtained in the transmission spectrum (S21 ) with maximum of amplitude in insertion loss. Normally it is observed in frequency around 244.5 MHz as intended. Once the resonant frequency is set, we checked how much resonant frequency shifted during irradiation in real time. The white LED light is applied to the sensor through a light guide, and it can be manually adjusted by an external light source from 0 lx to 20,000 lx. This light is monitored in real time by an illuminometer inside the dark box. The illuminometer is separated from the light source by the same distance as the SAW sensor, so that the same amount of light can be applied to the sensor.
Fig. 4. Microscopic images of the half-covered sensor (a) and full-covered sensor (b).
Fig. 5. Concept view of the ambient light SAW sensor measurement system.
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Fig. 6. Absorbance spectra (300–800 nm) for CdS thin films of different thicknesses deposited on the LiNbO3 substrate. Fig. 8. Resonant frequency shift with respect to the light intensity for sensors of different sensitive film areas.
Fig. 7. Resonant frequency shift due to the temperature change in the reference track.
3. Results and discussion Fig. 9. Sensing performance with the variation in the sensitive-film thicknesses.
The absorbance spectra for CdS thin films of different thicknesses are plotted in Fig. 6. To confirm the tendency of absorbance depends on CdS thicknesses, the tested samples are deposited with different thicknesses of 700 A˚ and 1700 A˚ CdS thin films on bare LiNbO3 substrates. Each sample is prepared in size of 10 mm × 10 mm and only CdS thin film is deposited on bare LN wafer. The light absorbance of the samples is measured using a UV–vis–NIR spectrophotometer (Agilent Cary 5000) in the wavelength range of 300 nm to 800 nm. According to the measurement, the absorbance of both samples increased under light of <550 nm. This is expected owing to the CdS bandgap, which is 2.45 eV at room temperature. Additionally, the absorbance increased with the thickness, indicating that it follows the Beer–Lambert law. Fig. 7 shows the shift in the resonant frequency of the reference track due to thermal changes. As described above, the reference track consists of IDTs and the bare LiNbO3 substrate, without the deposition of a sensitive film. The experiment was conducted in a commercial vacuum oven with the temperature varying from −30 ◦ to 150 ◦ . After setting up the temperature of the vacuum oven, the temperature of the sensor was confirmed using an infrared thermometer and the frequency was measured. The result shows good linearity. As expected by considering the TCF of substrate, the resonant frequency shifted by 3.51 MHz for a variation of 190 ◦ . This data can be used as a reference for offsetting thermal parameters when the sensor is mounted on mobile devices.
Fig. 8 shows the resonant frequency shift with the variation in the intensity of light for samples with different areas of sensitive films. Unlike the samples shown in Fig. 4, the “Non-covered sensor” means that the sensitive film was deposited only on the delay line, without covering the IDTs, as shown in Fig. 2(a). The result shows that the sensitivity of the SAW sensor increased with the area of the sensitive films. Under irradiation with 20,000 lx of visible light, the resonant frequencies of the non-, half-, and full-covered sensors varied by 30.54 kHz, 100.96 kHz, and 182.73 kHz, respectively. Thus, compared with the non-covered sensor, the half- and the fullcovered sensors are approximately 2.3 times and 4.98 times more sensitive, respectively. Each experiment was repeated three times for each sample, and the data were averaged. The results of the experiment with regard to the thickness are plotted in Fig. 9. Similar to the previously described absorbance spectra, the sensitivity of the sensor was higher for samples of thicker sensitive films. Under irradiation of 20,000 lx, the resonant frequency of a sample with a 140 nm sensitive film changed by 82.17 kHz. Under the same conditions, the resonant frequency of a sample with a sensitive film thickness of 190 nm changed by 431.79 kHz, which is approximately 4.25 times better than the previous sample. To theoretically understand these results, the following formula was used. The three main operational theories for
B. Lee et al. / Sensors and Actuators A 293 (2019) 145–149
the SAW sensor are the mass-loading, acoustoelectric, and viscoelastic effects, which are expressed according to this formula [17]: v
v0
= −cm f0 m/A + 4ce
f0
(hG’) − 2
v0
02 K2 2 2 0 + v20 C02
Here, cm and ce are the coefficients of the mass sensitivity and elasticity of the substrate, respectively, and f0 is the fundamental frequency of the SAW sensor. The term h represents the film thickness, and we can assume that increases with the film thickness. Depending on the relationship between the wave speed and the frequency, v = f, it can be observed that the frequency changes more drastically at higher wave speeds. In summary, the thickness and area of the CdS thin film affect both the sensitivity of the SAW sensor and the absorbance data. With a larger mass of the sensitive film, we can expect better response performance of the SAW sensor. 4. Conclusion This paper reports the achievement of a high sensitivity in a CdS-deposited ambient light SAW sensor. The detailed design parameters were introduced, and the fabrication process was described. The absorbance data for CdS thin films with various thicknesses were measured using a spectrophotometer, and the structural characteristics of the SAW sensor were observed via several microscopes. Using a LiNbO3 substrate, the sensor exhibited linearity in its thermal changes. By increasing the thickness and area of the CdS thin film, the sensitivity of the sensor was improved. This result is expected to be a practical example of mounting in mobile devices. Acknowledgement This work was supported by the R&D program of MOTIE/KEIT [10064078, Development of the Multi-Sensor for UV, Ambient Light, and Proximity for Next Smart Device]. References [1] M. Magno, A low cost, highly scalable wireless sensor network solution to achieve smart LED light control for green buildings, IEEE Sens. J. 15 (2015) 2963–2973. [2] H. Yoon, S.H. Park, K.T. Lee, Exploiting ambient light sensor for authentication on wearable devices, IEEE (2015) 95–100. [3] E.R. Hirst, W.L. Xu, J.E. Bronlund, Y.J. Yuan, Surface acoustic wave delay line for biosensor application, in: International Conference on Mechatronics and Machine Vision in Practice, 2008, pp. 40–44. [4] J. Filipiak, L. Solarz, G. Steczko, SAW delay line for vibration sensors, Acta Phys. Pol. A. 122 (2012) 808–813. [5] T. Wu, Y. Chen, T. Chou, A high sensitivity nanomaterial based SAW humidity sensor, Appl. Phys. 41 (2008), 085101. [6] S. Li, S. Sankaranarayanan, C. Fan, Y. Su, V. Bhethanabotla, Achieving lower insertion loss and higher sensitivity in a SAW biosensor via optimization of waveguide and microcavity structures, IEEE Sens. J. 17 (2017) 1608–1616. [7] J. Chen, H. Guo, X. He, W. Wang, W. Xuan, H. Jin, S. Dong, X. Wang, Y. Xu, S. Lin, S. Garner, J. Luo, Development of flexible ZnO thin film surface acoustic wave strain sensors on ultrathin glass substrate, J. Micromech. Microeng. 25 (2015) 115005. [8] X. Le, H. Ding, J. Pang, Y. Wang, J. Xie, A humidity sensor with high sensitivity and low temperature coefficient of frequency based on AlN surface acoustic wave and graphene oxide sensing layer, IEEE Transducers (2017) 210–213.
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Biographies Byungmoon Lee received his B.S. from the School of Electrical and Electronics Engineering of Chung-Ang University, Seoul, Republic of Korea, in 2017. Since 2017, he has been a candidate for a M.S. degree from the Department of Micro/Nano Systems Engineering at Korea University under the supervision of Prof. Byeong-Kwon Ju. He is also working for HAESUNG DS CO., Ltd as a researcher under the supervision of principal research engineer Jin-Woo Lee. Jinkee Hong received his B.S. degree from the Department of English Language and Literature of Ajou University, Suwon, Republic of Korea, in 2015. He has been working for HAESUNG DS CO., Ltd as a researcher in the New Business Group. Jong Woo Kim received his B.S. degree from the School of Physics Department of Andong National University, Andong, Republic of Korea, in 2009 and his M.S. degree in Electrical Engineering in 2011 from Korea University. He has been a candidate for a Ph.D. in Electrical Engineering from Korea University under the supervision of Prof. Byeong-Kwon Ju. He is also working for HAESUNG DS CO., Ltd as a researcher under the supervision of principal research engineer Jin-Woo Lee. Yeon Hwa Kwak received her B.S. degree in Electronics Engineering in 1999 from Kyungpook National University (Daegu, Korea) and her M.S. and Ph.D. in 2004 and 2017, respectively, from Korea University. She is now working for Korea Electronics Technology Institute (Seongnam, Korea) as a managerial researcher. Kunnyun Kim received his B.S. and M.S. degrees in control and instrumentation engineering in 1991 and 1993, respectively, from Ajou University (Suwon, Korea) and his Ph.D. in Electrical Engineering in 2011 from Korea University. He has been working for Korea Electronics Technology Institute (Seongnam, Korea) as a principal researcher since 1993. His research interests include various sensors and input devices. Jin-Woo Lee received his B.S. and M.S. degrees in Chemical Engineering in 1998 and 2000, respectively, from Hong-ik University, Seoul, Republic of Korea. In 2000, he joined for Samsung Techwin, where he had been researching semiconductor materials and developing LED components. He had also conducted research on surface treatment. From 2014, he has been working for HAESUNG DS CO., Ltd as a principal research engineer. Currently, his main interests are developing new sensors, including optical sensors and thermometers. Byeong-kwon Ju received his Ph.D. in semiconductor engineering from Korea University in 1995. In 1988, he joined the Korea Institute of Science and Technology (KIST), Seoul, where he was mainly engaged in the development of flat-panel displays and microelectromechanical systems technology as a principal research scientist. In 1996, he spent six months as a visiting research fellow at the Microelectronics Centre, University of South Australia, Australia. Since 2005, he has been an associate professor in the Dept. EE of Korea University, with his main interest being flexible electronics (OLED, OTFT), field-emission devices, Si-micromachining, and carbon nanotube-based nanosystems.