Effect of Film Thickness on H2S Sensing Characteristics of WO3-x Films

Abstract

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, WO3-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 WO3-x film thickness on H2S sensing characteristics has been demonstrated. Methodology WO3-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 WO3-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 WO3-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%=(|Ig-Ia|x100)/Ia , where Ig and Ia 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.