Thin film lab Essay Example | Topics and Well Written Essays - 1750 words

Thin film lab - Essay Example Introduction: This experiment involves the usage of a physical vapor deposition technique known as evaporation. Physical vapor deposition, or PVD for short, is a term used to denote vacuum deposition processes where the coating material is passed into a vapor transport phase by a physical mechanism such as evaporation, sputtering or others, and the coating material then gets deposited onto a substrate surface to which it is transported (Thornton, 1988). Evaporation is one of the PVD techniques which have been used for film deposition since Faraday’s (1857) time. The age of this technique dictates clearly that a lot of work has been done in this field, specifically on developing coatings on glass. This involves substrate cleaning, followed by choosing a method for evaporation of target, and running the test under vacuum for a calculated time to obtain a certain thickness of film growth (Bach and Krause, 1997; Jackson and Ford, 1981). The intricacies of the method and the theory behind it are discussed in the next section. Theory: Evaporation deposition technique involves three essential steps (Pulker, 1999): 1. Evaporation of target material to obtain vapors for deposition. 2. Transport of vapors to substrate for deposition. This requires very low pressures as high mean free paths are required to assure the close-to-linear deposition of the film. 3. Adsorption of vapors on substrate surface leading to heterogeneous nucleation film growth. This section will deal with the theory behind the methods used to conduct the above mentioned three steps in the experiment that we conducted in the lab. The first step, evaporation, was carried out through Joule heating using a tungsten filament. Joule heating, in simple terms, is resistance heating, done through using wires of high resistance R which generate heat Q when current I is passed through them for time t. They follow the mathematical relation (Weisstein, 1996-2007) given below: This heat is utilized for heati ng our target material. The choice of the heating wire depends on resistance, which needs to be high for high heat generation. Tungsten filaments are usually used for this purpose, though tantalum or molybdenum may also be used (Ohring, 1992). Special purpose evaporation techniques also use electron beam evaporation. The target to be evaporated is wrapped around the filaments or shaped in hollow cylinders to place the filament inside them. Once the evaporation is carried out, the next important step is to get the material transported to the substrate surface. This depends on the mean free path of the gas molecules in the chamber. Mean free path is the average distance that a molecule travels in a gas chamber between two consecutive collisions (Nave, 2010). This is mainly dependent on the pressure and the kinetics of flow of the gas. Both pressure and temperature adversely affect the mean free path of a gas. For uniform deposition, we require high mean free path, so that the particle s can get transported to the surface of the substrate evenly (Ohring, 1992). Adsorption of the transported species on the substrate surface is a very interesting science, involving both thermodynamics and kinetics, indicating how the type of the growth of the surface layer, its morphology, the nucleation rate, the growth rate and the microstructure can be influenced by small changes in the conditions. These may involve surface cleanliness, substrate preheating, plasma