An optical humidity sensor: A compact photonic chip integrated with artificial opal
Introduction
Humidity monitoring is of crucial importance for a vast range of industrial and commercial applications, such as food processing, pharmaceutical preparation, air conditioning, agriculture, and electronic manufacturing. Recently, increasing attention has been paid to in-situ monitoring of human respiration rate by tracking the humidity in the inhaled and exhaled air [1], [2], [3], [4]. For instance, the frequency and depth of humidity-related breathing signals can be analyzed to provide potential assistance in health assessment, such as noninvasive diagnosis of sleep apnea, asthma, and cardiac arrest [5], and physiological monitoring [6]. To achieve this goal, a humidity sensor with rapid response and high sensitivity is desired. Optical sensing based on fiber optics is an effective means to detect the variation of refractive index induced by humidity changes [7], [8], [9], [10]. To enhance the sensitivity of the optical sensor to humidity, lots of efforts have been devoted to surface modification and a variety of humidity-sensitive coating layers has been reported, such as gelatin [11], [12], polyvinyl alcohol [13], [14], agarose gel [15], [16], metal oxides [17], [18], and polymers [19], [20].
Attributed to the ease of one-step integration and the large-area manufacturability, artificial opal is a promising photonic material for humidity sensing. Typically, artificial opals are solid colloidal crystals composed of self-assembled spheres of uniform size [21], [22], or their reversed replicas as inverse opals [23], [24]. They behave as photonic crystals, in which the stop-band prohibits the propagation of light in a specific frequency range and shifts according to changes in relative humidity (RH). Despite their recent advances in rapid response and high sensitivity, the quantitative analysis of RH readings still heavily relies on expensive and bulky spectrometers to determine the spectral shifts [25], [26]. In addition, effective light coupling involves the assembly of optical devices with external components and precise optical alignment is required. Therefore, the resulted bulky system may be difficult for large-scale manufacturing and high-density integration.
With the growing demand for portable devices in lab-on-a-chip and wearable applications, there is a great need to develop a miniaturized and low-cost humidity sensor by eliminating external components. In this work, we propose a fully integrated chip-scale humidity sensor based on the integration of an opal film onto a III-nitride chip. Noticeably, GaN and its alloys have been considered to be ideal materials for constructing light-emitting devices because of their high efficiency, high stability, and long lifespan [27], [28], [29]. In addition to the light emitter, another indispensable light detector is integrated on the same chip through a monolithic design. Covered on the chip surface, the opal film is composed of amorphous silica particles with the hydrophilic feature which is conducive to the rapid absorption of water vapor. The reflectance characteristic of opal film is investigated to verify the feasibility of the chip-scale optical sensor for rapid probing of surrounding RH.
Section snippets
Experimental section
The epitaxial structure composed of an unintentionally doped GaN template layer, Si-doped n-type GaN, InGaN/GaN multi-quantum well (MQW), and Mg-doped p-type GaN is grown on a 4-inch c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The III-nitride chips consisting of light-emitting diode (LED) and photodetector (PD) are fabricated through wafer-scale manufacturing processes. Details of the process sequence are schematically described in Supporting Information S1.
Results and discussion
After dicing the fabricated wafer into small pieces, a 1 × 1 × 0.21 mm3 chip is flip-chip bonded on an aluminum-based printed circuit board and then integrated with an opal film, as schematically shown in Fig. 1(a). The hexagonal area at the chip center with a side length of 160 µm functions as a light emitter, while the outer region serves as a detector, as illustrated in Fig. 1(b). The opal powders are prepared by crushing an artificial opal bulk to small particles of 4–10 µm and then
Conclusion
To summarize, we have developed a novel optical humidity sensor based on the compact integration of III-nitride chip with artificial opal. Adopting monolithic design, the GaN chip provides two essential functions of photoemission and photodetection and its electrical and optical properties are comprehensively demonstrated. The hydrophilic nature of opal film facilitates fast water molecule exchange. The opal film shows a linear reflectance response to the adsorption and desorption of water
Declaration of Competing
Interest The authors declare no conflict of interest.
CRediT authorship contribution statement
Binlu Yu: Conceptualization, Data collection, Data analysis, Methodology, Writing – original draft. Yumeng Luo: Methodology, Data collection. Liang Chen: Methodology. Zhiqin Chu: Supervision, Writing – review & editing. Kwai Hei Li: Conceptualization, Methodology, analysis, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
L.K.H. acknowledges financial support from the National Natural Science Foundation of China (62004088 and 12074170) and the Shenzhen Natural Science Foundation Stability Support Program Project (20200925160044004). Z.Q.C. acknowledges financial support from the HKSAR Research Grants Council (RGC) Early Career Scheme (ECS, No. 27202919) and HKU Start-Up Grant.
Binlu Yu received the B.S. degree from Wuhan University of Technology (Wuhan, China) in 2015, and received the M.S. degree from Tianjin University (Tianjin, China) in 2018, now he is pursuing his PhD degree in the School of microelectronics, Southern University of Science and Technology (Shenzhen, China). His research interests include the GaN-based microsensor and nanosensor.
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2022, Sensors and Actuators A: PhysicalCitation Excerpt :Yu et al. [10] integrated the opal film made of hydrophilic silica powders and monitored the humidity-dependent light reflection precisely by an on-chip detector with a linear response in the RHS range of 3.8–90 % with a sensitivity of 0.04–0.05 μA/%RH [10]. Currently, the LSPR fiber structures extensively used the fiber Bragg grating (FBG), tilted fiber Bragg grating (TFBG), long-period fiber grating (FPG), D-typed fiber, U-shaped fiber, micro/nano-fiber, and photonic crystal fiber (PCF) to develop high performance sensors for varied applications [7–13]. These grating structures can establish the interaction between the optical signal and measuring parameters, thus providing excellent sensing performance of the sensors.
Binlu Yu received the B.S. degree from Wuhan University of Technology (Wuhan, China) in 2015, and received the M.S. degree from Tianjin University (Tianjin, China) in 2018, now he is pursuing his PhD degree in the School of microelectronics, Southern University of Science and Technology (Shenzhen, China). His research interests include the GaN-based microsensor and nanosensor.
Yumeng Luo is currently working toward the B.S. degree from Southern University of Science and Technology, Shenzhen, China. Her research is focused on GaN-based optoelectronic devices.
Liang Chen received the B.Eng. and M.Eng. degrees from Huazhong University of Science and Technology, Wuhan, China, in 2003 and 2011, respectively. He is currently working toward the Ph.D. degree with Huazhong University of Science and Technology, Wuhan China. Simultaneously as a visiting scholar at School of Microelectronics, Southern University of Science and Technology. His research interests include the design, fabrication, and packaging of GaN optoelectronic devices.
Zhiqin Chu received his B.S. and PhD degrees all in Physics from Northwest University (China) and The Chinese University of Hong Kong, in July 2008 and July 2012, respectively. After one year as postdoctoral fellow in the same group, Prof. Chu carried out his postdoctoral research at The University of Stuttgart (Germany). He moved back to Hong Kong in October 2016 and worked as a Research Assistant Professor in Department of Physics at The Chinese University of Hong Kong. He is currently an Assistant Professor in Department of Electrical and Electronic Engineering (Joint Appointment with School of Biomedical Sciences) at The University of Hong Kong. Prof. Chu has been actively engaged in the research of precision sensing and imaging, biomedical devices, and biophysics.
Kwai Hei Li received the B.Eng. and Ph.D. degrees from the University of Hong Kong in 2009 and 2013, respectively. After graduation, he received 1-year postdoctoral research training at McGill University in Canada. He worked as a Research Assistant Professor in the Department of Electrical and Electronic Engineering at HKU from 2016 to 2019. He is currently an Assistant Professor at the Southern University of Science and Technology. His current research interests include the design, fabrication, and characterization of III-nitride optoelectronic devices.