Growth Analysis of Thin Films of Aluminum and Copper Deposited through Evaporation - Essay Example

? Growth Analysis of Thin Films of Aluminum and Copper Deposited through Evaporation [You can change the of [Enter [Enter University Name] Abstract: Copper and Aluminum thin films were deposited on glass substrates using evaporation technique. For growth characterization, resistance measurements were made across the film at regular intervals to obtain rate of change of resistance. These readings were analyzed to determine time taken for continuous film formation for both copper and aluminum and their growth rate was determined through empirical methods. 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 essential 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 consecutive 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 the 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 proper assembly. 4. Separate copper contacts were made across the slides for resistance measurement, which was to be carried out later. 5. A grid was placed to collect falling material. 6. A rotating cover was used as a controller for the experiment conduction. 7. The dome was affixed over the whole assembly, fastened by a rubber washer. 8. A safety grid was placed around the dome. Preparation was followed by actual conduction of the experiment: 1. Rotary pump was used to create low vacuum 10-3 torr in the chamber. 2. This is the ultimate limit of the rotary pumps, after which turbo molecular pumps were used in conjunction to get the chamber pressure down to 10-6 torr. 3. Tungsten filament was connected across a high voltage to start resistance heating. 4. The copper was allowed to melt and wet all over the filament. 5. The control cover was removed to initiate deposition of copper on the slide. 6. Resistance measurements were taken for every two seconds. 7. After 2-3 minutes, the deposition was stopped. 8. The system was allowed to cool slowly, and the vacuum was gradually drained through the draining valve. 9. After sufficient cooling, the glass slide was removed and the chamber and the glass dome were cleaned. 10. The whole process was repeated to also obtain aluminum deposits. Care was taken not to reheat the brittle tungsten filament too quickly. In both copper and aluminum depositions, copper clips were used for resistance measurement. At the end of the operation, the chamber was left with a slight vacuum in it, to keep the instrument in working condition and to prevent contamination and wear. Results and Discussions: As has already been made clear, resistance measurements were the only readings taken for our sample with time. The readings obtained for copper sample have been graphed below: Figure 3: Results Obtained for Resistance Measurements of Deposited Copper Film The resistance readings show a high resistance initially, decreasing rapidly with time. Up till t = 25s, the film is not continuous, and is in the form of islands. Transformation of this non-continuous film to a continuous film followed the lines of Ostwald Ripening, sintering and cluster migration, shown schematically as 1, 2 and 3, respectively, in the figure below: Figure 4: Island to Film Growth Schematic, Showing Particle Coalescence Following is the graph for the same parameters, obtained for the copper coating: Figure 3: Results Obtained for Resistance Measurements of Deposited Aluminum Film The figure above shows that the aluminum film takes about 40 seconds to become continuous. A combination of this result with the van der Pauw method indicates that, at 200 seconds, the film had grown to about 5 times its thickness at 40 seconds. This is calculated keeping into account the linear growth that is shown in this figure and the empirical values obtained for growth rate in terms of resistance, related proportionally by the van der Pauw method (1958). Conclusions: The aim of this experiment was to learn evaporation deposition technique for thin films, and analyze the growth properties of the films obtained through this technique. The use of glass substrate for both aluminum and copper films negated any differentiable adhesion characteristics for both metals with the substrate, thus giving an equal chance of growth for the films depending upon their evaporation and nucleation/growth characteristics. It was shown that the growth of the continuous films follows a linear trend, before which the film growth takes place in the form of patches, confirmed by the resistance measurements. Also, the conformation of the resistance readings with van der Pauw method showed that evaporation technique can be used to grow films linearly, which enables the user to control growth of the films produced through technique. References Caswell, H.L. (1963). Physics of Thin Films. Vol. 1. Academic Press. New York. COMSOL. (1998-2011). The Joule Heating Effect. Retrieved from www.comsol.com Glang, R. (1970). Handbook of Thin Film Technology. McGraw Hill. New York. Holland, L. (1956). Vacuum Deposition of Thin Films. Wiley. New York. Nave, C.R. (2010). Hyperphysics. Retrieved from www.hyperphysics.phy-astr.gsu.edu Ohring, Milton. (1992). The Materials Science of Thin Films. Academic Press. London. Pulker, H.K. (1999). Coatings on Glass. Elsevier Science. Amsterdam. Thornton, J. (1988). Semiconductor Materials and Process Technology Handbook for Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI). Noyes Publications. Berkshire. Thornton, J.A. (1978). Thin Solid Films. 45. Vossen, J.L. and J.J. Cuomo. (1978). Thin Film Processes. Academic Press. New York. Van der Pauw, L.J. (1958). A method of measuring specific resistivity and Hall effect of discs of arbitrary shape. Philips Research Reports. Weisstein, Eric W. (1996-2007). Joule’s Law. Retrieved from www.scienceworld.wolfram.com Read More