Spray Pyrolysis Thin Film Deposition Technique for Micro-Sensors and Devices

Introduction

Sensing films are a necessary part of sensors and electronic devices. Sensing films as a form of thin films are used heavily for the last 100 years because of their properties, technical value, and sensing performance. They have a huge range of applications, ranging from micrometer to a few square meters on the outer surface of window glasses to microsensors and devices. But thin film technology has progressed rapidly since the last 4-5 decades which in turn progress the research on microsensors and devices. Last 2-3 decades microsensors and device research gain attention to the researchers due to the advancement of fabrication technology. Different techniques like metalorganic chemical vapor deposition (MOCVD) (Myong, Baik, Lee, Cho, & Lim, 1997; Vispute et al., 1998), molecular beam epitaxy (MBE) (Look et al., 2002), radiofrequency magnetron sputtering (Chitanu & Ionita, 2010), pulsed laser deposition (PLD) (Wisz, Virt, Sagan, Potera, & Yavorskyi, 2017), sputtering (Van de Pol, Blom, & Popma, 1991), sol-gel process (Znaidi, Illia, Benyahia, Sanchez, & Kanaev, 2003), hydrothermal (Wang, Xie, Yan, & Duan, 2011) and spray pyrolysis (Nunes, Fernandes, Fortunato, Vilarinho, & Martins, 1999) are used for thin film deposition. These thin films can be grown on any substrate such as glass, polyamide, silicon, stainless steel etc. Various techniques have been studied to find the most reliable and inexpensive way to produce such thin films for sensing applications.

Spray pyrolysis is an inexpensive method compare to other available thin film deposition techniques. Spray technique is an established and widely used thin film technology. It is a one form of chemical vapor deposition which is successfully used to develop solar cells and various oxide-based materials (Ayouchi et al., 2003; Miki-Yoshida & Andrade, 1993; Nunes et al., 1999; Subbaramaiah & Raja, 1994). In this method, two significant advantages are: deposition happens at atmospheric pressure with a large deposition area and it is an inexpensive and fast deposition method. This is a fairly simple method because of the use of a vacuum free framework, whereas basic thin film deposition setups such as Sputtering, CVD, Evaporation, Ablation laser, etc. require vacuum environment for depositing the thin films.

Pyrolysis stands for decomposition at high temperature i.e. under the influence of heat (high temperature)/thermal treatment materials decomposed and it changes its chemical composition. In spray pyrolysis deposition techniques, fine droplets (size of 1-30 µm) are first generated from the precursor solution. Then these streams of droplets containing metal salts are showered onto a heated substrate to develop a solid/porous thin film. Subsequently, the property of the utilized solution and the thermodynamic at the interface of droplets and substrate are key parameters controlling the spray process and the properties of the deposited film. Important deposition parameter such as precursor molarity, flow rate, doping concentration, substrate temperature and properties of the substrate. In this chapter, we will discuss spray pyrolysis, its fundamentals and advantages, different types of spray pyrolysis and finally the application of spray pyrolysis in different sensors and devices.

First work on chemical spray pyrolysis happened on cadmium sulfide (CdS) thin-film solar cell (Chamberlin & Skarman, 1966). After that Researchers made extraordinary efforts to make chemical spray pyrolysis techniques better over about three decades for preparation of the thin film. Subsequently, the spray pyrolysis apparatus is very simple to handle and has good productivity in large scale production for the formation of thin films of metal oxides and metal composite. It has several advantages.