Friday, October 18, 2019

Thin film lab Essay Example | Topics and Well Written Essays - 1750 words - 1

Thin film lab - Essay Example Introduction: Evaporation is a physical vapor deposition process, which involves vaporization of material to be coated through thermo-mechanical treatments, transport of that material in vapor phase to the substrate surface, and consequent deposition on the surface of the substrate based on adhesion (Pulker, 1999). Evaporation is a very common technique for film deposition, which has been used since ages to coat glass, silicon and other substrates with coatings of a very wide range of materials. The most common technique used to characterize film growth for evaporation deposited films employs measurement of surface film resistivity, which depicts the behavior of formation of islands of nuclei of various sizes, followed by Ostwald Ripening, sintering and cluster migration, leading to continuous film growth (Ohring, 1992). Using these vastly used deposition and characterization techniques, we deposited and analyzed copper films. Theory: Evaporation deposition technique involves three e ssential steps (Thornton, 1988): 1. Evaporation of material to be coated to obtain vapors for deposition. 2. Transport of vapors to substrate for deposition. 3. Physisorption of vapors on substrate surface, leading to nucleation and film growth. Resistance heating is a method to carry out evaporation of the target material. This is done simply by using wires or plates of high resistance, which have high resistance heating in accordance with Joule’s Law (COMSOL, 1998-2011). Wires, filaments, boats, plates or other shapes of these heaters may be used in accordance with the shape of the element to be evaporated. Refractory metals such as tungsten, tantalum and others are used for this purpose. Sublimation furnaces, crucible sources or electron beam evaporators can also be used as the heating technique (Ohring, 1992). Vapor phase transport depends on mean free path of the gas used. Mean free path is the average distance that a molecule travels in a gas chamber between two consecu tive collisions (Weisstein, 1996-2007). Pressure is the main factor controlling the mean free path. Depending upon the pressure and mean free path, different pressure ranges are termed as low vacuum, medium vacuum, high vacuum or ultrahigh vacuum as shown below: Figure 1: Vacuum Ranges Shown with Pressure Ranges for Comparison. Illustration from Ohring, 1992. Surface preparation is an essential part of surface deposition processes, which include surface cleanliness, substrate preheating, plasma assistance with evaporation and other factors. All these factors and the processing parameters of the evaporation technique combine to produce the morphology, growth rate and microstructure of the deposited films (Thornton, 1978; Holland, 1956; Caswell, 1963). Experimental Method: As has been mentioned, surface cleaning plays an important role in the deposition. To make sure the substrate is not contaminated – which may lead to deposit contamination – we used gloves throughout t he handling process. The four glass slides were agitated in a vibration tank to loosen contaminants, cleaned with alcohol, followed by blow drying of the cleansed slides with argon. These glass slides were then placed in marked containers. The evaporator preparation consisted of the following steps: 1. Tungsten wire was wound to form a filament. 2. Copper pieces, which were to be evaporated later, were placed on the tungsten filament at three places. 3. Glass slides were placed in premade stencils to ensure

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