Experimental and computational studies of properties of thin films of cadmium-tin-oxide for manufacture of micro-electromechanical systems and for noise control

Abstract

The recent advancement in electronics have stimulated the high demand for semiconductor substrate materials for the printing of electronic circuits, some of which are used in the Micro-Electromechanical Systems (MEMS) devices. The most commonly used substrate for the printing of these devices is silicon (Si), which is preferred because, compared to the other semiconductor substrate materials like silicon nitride and gallium arsenide, it: Has better electrical, mechanical and thermal properties that are desirable for the fabrication of MEMS devices; is readily available, and is also relatively cheaper. Despite the advantages above, Si has serious drawbacks, including rarely occurring as a pure element and being brittle, which call for alternative substrate materials such as cadmium tin oxide (CTO), whose thin films (TFs) have been studied and proved to possess excellent optical and electrical properties, as well as flexibility. However, the mechanical and thermal properties of CTO have not been explored extensively. This study employed ab initio calculations to determine the mechanical and thermal properties of CTO. The specific objectives of the study were: i) to find out the effect of concentration of cadmium and tin on the structural properties of CTO, (ii) to develop a simplified method that applies stress and strain in calculating elastic constants of materials, (iii) to find out the effect of concentration of cadmium and tin on the mechanical properties of CTO, and (iv) to find out the effect of concentration of cadmium and tin on the thermal properties of CTO. The TFs of CTO were prepared by dissolving cadmium chloride and tin II chloride separately in distilled water to form 0.4 M of each solution. The two solutions were then mixed at ratios of 1:5, 2:4, 3:3 4:2 and 5:1 by volume. The final solutions were then sprayed onto the preheated microscope glass substrates by spray pyrolysis at a temperature of 450 °C. The measurement of film thickness was done using surface profiler, the X-Ray diffraction measurements were taken using Expert Pro from 2θ = 10-70° at a step of 0.02°, the chemical composition was measured using X-Ray Fluorescence. The ab initio calculations were done using density functional theory with PBESOL functionals as implemented in the Quantum Espresso code. Stress-strain method was developed for the study of mechanical properties. The outcome of the study showed that all the properties of CTO are very sensitive to the number of cadmium atoms. The optimum properties obtained were: crystallite size of 60.75 nm, density of 7.636 g/cm3, bulk modulus of 149.1 GPa, shear modulus of 53.2 GPa, Young’s modulus of 142.7 GPa, Vickers hardness of 3.757 GPa, Poisson’s ratio of 0.367, Pugh’s ratio of 3.422, Debye temperature of 377.68 K, melting temperature of 1558.07 K, vibrational energy of 8.557 x 10-2 Ry/cell, Gibbs free energy of 0.1584 Ry/cell, entropy of 7.984 x 10-4 Ry/cell/K, and specific heat at constant volume of 2.659 x 10-4 Ry/cell/K. Due to the comparable mechanical and thermal properties of CTO to Si, it can replace Si in the manufacture of MEMS. The ductile nature of CTO is ideal for manufacture of flexible MEMS such as biomedical MEMS, microbolometers, image sensors, and microbridges, which require softer and dultile substrates.