Evaporation Deposition of Thin Films - Essay Example

? Evaporation Deposition of Thin Films – An Analysis of Growth Characteristics of Aluminum and Copper Films [You can change the of [Enter Your Name] [Enter University Name] Abstract: A vacuum chamber was used to deposit aluminum and copper, which were evaporated through Joule heating, as thin films on glass sheets. The film growth was analyzed through resistance measurement for after every two seconds. Discussions on the results obtained for growth rate of films indicated by the resistance measurements was made in the light of Van der Pauw’s method for resistance measurement of thin films. 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 heating 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 particles 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 assistance with evaporation and other factors. All of these are discussed in great detail in many books and research articles, just a few of which have been referenced (Ohring, 1992; Glang, 1970; Vossen and Cuomo, 1978). The growth rate of the film is not measured directly; rather indirect methods like film resistance are used to evaluate the thickness, and thus the growth rate of the film. This science is dictated by van der Pauw method (1958), which shows that sheet resistivity is inversely proportional to the thickness of the sheet. Our results will use this critical information to evaluate whether the growth of the film is linear with time, exponential, or has some other trend. A comparison of the copper and aluminum sample results will also be made. Experimental Method: All film deposition techniques involve surface cleaning of the substrate i.e. glass sheet. Of all the different choices that we had, we used high alcohol-water solutions along with agitation in a vibration tank, followed by argon blow drying of the cleansed surface. Gloves were worn to prevent contamination of the glass sheets by skin debris, and the sheets were placed in labeled storage boxes. 4 slides were thus prepared. After surface cleaning, we moved on to the preparation of our evaporator. For that purpose we had tungsten filament, which was wound with pliers in a manner so that little arcs could be obtained. Three pieces of copper were fitted to this filament. The tungsten coil was then affixed to a cathode and anode. A grid was used to remove excess fallen material. A rotating cover was used to give a mode of termination or initiation for the coating process. Glass slides were then placed in appropriate stencils so as to obtain two separated copper contacts. Two wire clips ensured rigidity of the assembly. This completed our assembly for the inner chamber. Once this was done, a dome was placed over the assembly and fastened with a rubber washer. A safety protection grid was placed around the dome. This was followed by creation of vacuum in the chamber through a set of pumps. A rotary pump brought the chamber to low vacuum and a turbo pump took low vacuum conditions to high vacuum. This procedure took ten minutes. After vacuum was established, current passage through the tungsten filament caused the copper to melt and uniformly distribute itself over the tungsten filament. This uniform distribution was then allowed to evaporate as the initiation/termination cover was removed to allow deposition of copper on the glass slide. Resistance measurements were made across the copper film for every two second interval. After the coating resistance readings stabilized to a linear pattern, we stopped the coating process and allowed the system to cool before taking results. For consequent film depositions, the glass dome was cleaned of the evaporated coating. 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: The technique for film growth characterization used in this method was measurement of resistance of the growing film with time. The results of the measurements have been graphed below: Figure 1: Results Obtained for Resistance Measurements of Deposited Aluminum Film The resistance readings show a high resistance initially, decreasing rapidly with time. This initial resistance reading till t = 40s represents the growth phenomenon of film in the form of islands i.e. the film is not yet continuous, rather it has nucleated at some places on the glass slide and is growing around it. After the mentioned point in time, the film shows quite a low resistance, which decreases linearly with time. If we compare this to the van der Pauw sheet resistance relation with thickness, we can safely conclude that the film growth is linear with time. This is a very interesting conclusion, as it shows that evaporation has a very controlled growth phenomenon, even though it has a fast growth rate as compared to sputtering (Ohring, 1992), another common PVD process. Following is the graph for the same parameters, obtained for the copper coating: Figure 2: Results Obtained for Resistance Measurements of Deposited Copper Film The figure above shows that the copper film takes about 25 seconds to become continuous. 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 Thornton, J. (1988). Semiconductor Materials and Process Technology Handbook for Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI). Noyes Publications. Berkshire. Faraday, M. (1857). Experimental Relation of Gold (and Other Metals) to Light. Philosophical Transactions. London. Bach, H. and D. Krause. (1997). Thin Films on Glass. Springer. Berlin. Jackson, N. and J. Ford. (1981). Thin Solid Films. Pulker, H.K. (1999). Coatings on Glass. Elsevier Science. Amsterdam. Weisstein, Eric W. (1996-2007). Joule’s Law. Retrieved from www.scienceworld.wolfram.com Ohring, Milton. (1992). The Materials Science of Thin Films. Academic Press. London. Nave, C.R. (2010). Hyperphysics. Retrieved from www.hyperphysics.phy-astr.gsu.edu Glang, R. (1970). Handbook of Thin Film Technology. McGraw Hill. New York. 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. Read More