Chandra Shekhar Prajapati

In this work, the effect of humidity on WO₃ thin films is studied. Two thin films of higher and lower oxidation states are characterized and studied for their interaction with water molecules on the metal oxide sensing surface. The film with higher oxidation is found to be more immune to humidity effect and hence, further used to reduce the same effect using absorbent filters and Nichrome deposited heater mesh. With the use of a Nichrome heater mesh, by increasing mesh temperature from 30 to 90 °C, there is a 63% reduction in absolute humidity at the sensor die. This in turn stabilizes the NO₂ sensor response, by decreasing the humidity dependent (40% RH to 90% RH) peak current variation for 3 ppm NO₂ exposure, from 65% to 26%. High‐temperature ageing at a temperature of 90 °C and 90% RH is found to be effective in significantly reducing humidity effect on sensor current output and only 8.4% change in peak current is observed for 3 ppm NO₂ at variable humidity, which stabilizes the sensor current drift due to humidity variation.

The exponentially increasing global population has led to environmental pollution which drastically affects human health; emphasizing the need for personal environmental monitoring. This demands the development of wearable devices capable of sensing the local environment and wirelessly transmitting data to cloud for spatial pollution tracking.In this paper, we have demonstrated the integration of MEMS gas sensors on flexible PDMS substrate using the fan out wafer packaging technique called FlexTrate™. One of the main issues in FOWLP involving the integration of MEMS sensors with a released membrane is the stability of the membrane during the molding process which results in poor yield. We have optimized the process for integrating released MEMS devices by protecting the membrane prior to the molding process and thus improving the stability of the released membranes and improving the yield by >80%. If the membrane is not protected, during curing the cavity which is filled by PDMS leads to membrane cracking due to generation of stresses. Simulation studies on the temperature profile of the microheater after protecting the membrane shows that the power consumption for 300 ^o C of heater temperature is 0.1W as compared to 0.091W where the PDMS fills the cavity of the membrane, which is <10% increase. Thus, this proves that the membrane protection process improves stability without affecting the thermal characteristics of the heater. Furthermore, there is an effort to integrate rechargeable flexible batteries to power the system wirelessly. Adding to this, is the capability of wireless communication achieved by integrating a Bluetooth die in the system to transmit data to a mobile phone.The MEMS sensors along with the other electronic components such as transimpedance amplifiers, analog-to-digital converters and Bluetooth will be integrated on the same PDMS platform with interconnect pitches of 40 μm. This greatly reduces the form factor of the system and enables higher density of sensor arrays resulting in reliable and repeatable output. The intention of the work is to demonstrate a wearable gas sensor for local pollution monitoring with wireless communication capability using Bluetooth. This will provide a means to spatially map the pollution levels globally.

Introduction H₂S is colorless, highly flammable and a toxic gas. It has a short (10 minutes) and long term (8 hours) exposure limit of 15 and 10 ppm respectively. Although, H₂S odor can be detected by human nose at a very low concentration (0.02 ppm) but the sense of smell is lost in just a few minutes after exposure due to olfactory fatigue [1]. This makes it very difficult to sense dangerous concentrations of H₂S which could be sometimes lethal. Hence, it is crucial to detect H₂S in ppm or sub ppm range. Various metal oxides-based materials have been employed for H₂S detection owing to their simplicity, ease of production and low cost [2-6]. Among such oxide materials, WO_3-x is versatile material which has been used for detection of various toxic gases including H₂S owing to mixed oxidation state of W (W⁴⁺, W⁵⁺, W⁶⁺) [7,8]. The optimization of the sensing material has to go through a series of steps in which film thickness, morphology, grain size etc. are the key aspects which require considerable attention. In the present work, an attempt has been made to optimize film thickness and the effect of WO_3-x film thickness on H₂S sensing characteristics has been demonstrated. Methodology WO_3-x thin films were deposited using magnetron sputtering technique. For this purpose, Ti/Pt (10/90 nm) based Inter-digitated electrodes (IDE’s) were first patterned on Si/SiO₂ substrate and thereafter WO_3-x was deposited above these IDE’s under optimized Ar:O₂ environment of 1:4. and deposition pressure of 1x 10^-4 torr. IDE’s with width and gap of 5 µm were patterned using optical lithography and were deposited using DC magnetron sputtering while WO_3-x films with a thickness of 25, 50, 90 and 150 nm were deposited using RF magnetron sputtering at 100 W power and were post annealed at 400℃ in N₂ ambient for one hour. The film thickness was estimated using a Dectek surface profilometer, morphological characterization was done using FESEM. Electrical characterization was done using a DC probe station and sensing measurement was performed by making use of a dynamic gas sensing system. The sensor response is defined as R%=(|I_g-I_a|x100)/I_a , where I_g and I_a are measured sensor current in gas and air respectively. Results and Conclusions The IV measurement results indicated that with increase in film thickness the resistance first increases till 50 nm and thereafter it decreases for 90 and 150 nm films. Though, highest response was observed for 50 nm thick film but it doesn’t completely recover whereas a comparable response with complete recovery is observed for 25 nm thick film. A plot of the normalized response is shown in Fig. 1 and from the plot it is inferred that H₂S response for all the films is comparable but only 25 nm films were found to recover to their base current value at a low operating temperature of 150°C and H₂S concentration of 1 ppm. Thus, it is concluded that 25 nm thick films annealed at 400°C were best for H₂S detection and can be exploited to fabricate an integrated H₂S sensor. References [1] Vishal Balouria, Niranjan S. Ramgir, A. Singh, A. K. Debnath, Aman Mahajan, R. K. Bedi, D. K. Aswal, S. K. Gupta, Enhanced H₂S sensing characteristics of Au modified Fe₂O₃ thin films, Sensors & Actuators B 219. (2015) 125-132. doi: 10.1016/j.snb.2015.04.113 [2] N.V. Toan, N. V. Chien, N. V. Duy, D. D. Vuong, N. H. Lam, N. D. Hoa, N. V. Hieu, N. D. Chien, Scalable fabrication of SnO₂ thin films sensitized with CuO islands for enhanced H₂S gas sensing performance, Applied Surface Science 324 (2015) 280-285. doi: 10.1016/j.apsusc.2014.10.134 [3] N. Kaur, D. Zappa, N Poli, and E. Comini, Integration of VLS-Grown WO₃ Nanowires into Sensing Devices for the Detection of H₂S and O₃. ACS omega, 4(15), pp.16336-16343.doi: 10.1021/acsomega.9b01792 [4] D.K. Aswal, and S.K. Gupta, Science and Technology of Chemiresistor Gas Sensors. 2007: Nova Science Publishers [5] G. Korotcenkov, Metal oxides for solid-state gas sensors: What determines our choice?, Materials Science and Engineering: B, 139. (2007) 1-2. doi: 10.1016/j.mseb.2007.01.044 [6] A. Mirzaei, S. S. Kim, H. W. Kim, Resistance-based H₂S gas sensors using metal oxide nanostructures: A review of recent advances, Journal of Hazardous Materials. 357. (2018) 314-331. doi: 10.1016/j.jhazmat.2018.06.015 [7] Cezarina C. Mardare and Achim W. Hassel, Review on the Versatility of Tungsten Oxide Coatings, Phys. Status Solidi A 216, (2019) 1900047. doi: 10.1002/pssa.201900047 [8] M. Stankova, X. Vilanova, E. Llobet, J. Calderer, C. Bittencourt, J.J. Pireaux , X. Correig, Influence of the annealing and operating temperatures on the gas-sensing properties of rf sputtered WO₃ thin-film sensors, Sensors and Actuators B 105 (2005) 271–277. doi: 10.1016/j.snb.2004.06.009 Figure 1

Semiconducting metal oxide gas sensors typically operate at a high temperature and consume hundreds of milliwatts of power. Therefore there is great demand for the development of a low-power gas-sensing technology that can sensitively and selectively detect the gas analytes present in the atmosphere. We report an ultralow-power nanosensor array platform, integrated with an independently controlled nanoheater of size 4 μ m × 100 nm, which consumes ∼1.8 mW power when operated continuously at 300 °C. The heaters exhibit a fast thermal response time of less than 1 μ s, and can be utilized to operate in duty cycle mode, leading to power saving. The active area of the nanosensor is 1 μ m × 200 nm, defined by sensing electrodes with a nanogap of ∼200nm, leading to small form factor. As a proof of concept, each of the sensing elements in the array is functionalized with different sensing materials to demonstrate a low-power, sensitive and selective multiplexed gas-sensing technology for the simultaneous detection of CO (∼93.2% for 3 ppm at 300 °C), CO ₂ (∼76.3% for 1000 ppm at 265 °C), NO ₂ (∼2301% for 3 ppm at 150 °C) and SO ₂ (∼94% for 3 ppm at 265 °C). The technology described here uses scalable crossbar architecture for sensor elements, thus enabling the integration of additional sensing materials and making it customizable for specific applications.

In this report, the thickness‐dependent carbon monooxide (CO)‐sensing characteristics of sputtered zinc oxide (ZnO) films are investigated, followed by their morphological and electrical characterizations. ZnO films, ≈21 nm, show repeatable and high change in their resistances on exposure to CO and the observed maximum response is ≈52.2%, whereas ≈219 nm films fail to show reproducibility in sensing performance. Response and recovery time of ≈21 nm films are ≈28 and 250 s, which are faster than the time observed in case of ≈219 nm films. An ≈21 nm film is able to resolve 100 parts‐per‐billions (ppb) CO concentration in air. Later, on‐chip integration of ≈21 nm film is successfully implemented on specifically designed Micro‐Electro‐Mechanical‐System (MEMS) platform, inbuilt with microheater and utilizes power of ≈23.7 Milliwatts (mW) (375 °C). The packaged sensor chip is further tested and qualified for indoor and outdoor detection of CO gas in the atmosphere.

In this study, we have investigated the thickness-dependent nitrogen dioxide (NO₂) sensing characteristics of a reactive-ion magnetron sputtered tungsten trioxide (WO₃) film, followed by morphological and electrical characterizations.

This paper describes the fabrication of a four element gas sensor array for monitoring air pollutants, namely CO, CO ₂ , NO ₂ , and SO ₂ . The four micro-heaters share a single suspended SiO ₂ diaphragm, utilizing thermal proximity to achieve low power consumption (~10 mW for 300 °C). Plasma enhanced chemical vapour deposition SiO ₂ diaphragm is demonstrated to give higher yield compared with thermally grown SiO ₂ . Sensor array elements are fabricated by customizing each element to sense a specific gas, using different sensing materials. Optimized thin films of ZnO, BaTiO ₃ -CuO doped with 1% Ag, WO ₃ , and V ₂ O ₅ are used for selective sensing of CO, CO ₂ , NO ₂ , and SO ₂ . Four sensors can be independently controlled to operate at different temperatures to get high selectivity for test gases. The sensor array is packaged on Kovar header, and then characterized for gas sensing. It is demonstrated that the sensors exhibit good sensitivity and selectivity. We report a maximum repeatable response to CO (~78.3% for 4.75 ppm), CO ₂ (~65% for 900 ppm), NO ₂ (~1948.8% for 0.9 ppm), and SO ₂ (~77% for 3 ppm) at operating temperatures of 330 °C, 298 °C, 150 °C, and 405 °C, respectively.