New materials for uncooled IR imaging: Nickel manganite thin films grown by spin spray

Ceramic thermistors like VO_x, amorphous Si, and NiMn ₂O₄ are used for thermal sensing applications such as microbolometers and infrared sensors. These materials should have large temperature coefficient of resistance (TCR), high sensitivity, and low noise for these applications. Nickel manganite films have large TCR (>-3%/K) and good environmental stability, so that the properties are robust during subsequent processing. To improve the ability to prepare manganite spinels on pre-existing circuitry, new techniques that enable low-temperature depositions need to be developed. To address this, the spin spray technique was adopted in this work; this approach is both low cost and permits low process temperatures (<100°C). Spin spray deposition is accomplished using two dilute water-based solutions: a reaction solution and oxidizing solution. The reaction solution consists of metal salts like nickel chloride and manganese chloride while the oxidizing solution contains pH buffer, pH adjuster, and oxidizing agent. To grow films, the solution was nebulized by a nitrogen carrier gas and sprayed onto a rotating silicon substrate with a 1μm thick SiO₂ buffer layer. As-deposited nickel manganite films were identified as nanocrystalline spinel by TEM analysis. The TCR values of nickel manganite film and nickel copper manganite film were about -3.6 %/K and -2.9 %/K respectively. Adding Cu allowed the electric resistivity to be tuned to less than 1000 Ω-cm. The normalized Hooge parameter was around 1.7x10^-21 cm³.