Chemical Vapor Deposition - Principles and Implementation - Research Paper Example

?Penn EE 541, Spring CHEMICAL VAPOUR DEPOSITION Omar Alzaabi Department The Pennsylvania Park, PA 16802 Abstract Chemical Vapor Deposition (CVD) is a versatile style of creating thin deposits of metal solids on substrates. Gases are allowed into a reaction chamber which is set to optimum level conditions to allow the reaction of the gases to take place. Various adjustments on the reaction chamber produce four types of CVD: Atmospheric Pressure CVD (APCVD); Low Pressure CVD (LPCVD); Plasma enhanced CVD (PECVD) and Metalorganic CVD (MOCVD). CVD has wide applications which include provision of wear and corrosion resistance, formation of barriers and net shape components. INTRODUCTION Chemical Vapor Deposition (CVD) is a versatile technique of applying required coats of metals on surfaces of components. Unlike other types of painting techniques, this technique applies a coat of the wanted solid on all the accessible surfaces of a component. This therefore makes this technique very appropriate for coat application on surfaces which are of unorthodox nature. In this report, the principles and implementation of CVD are examined. In examining the principles of CVD, the CVD process is closely examined by specifically discussing what takes place inside the reaction chamber of a CVD reactor. The types of CVDs are also discussed. On the second part of this report, the implementation of CVD is discussed whereby the complete equipment set for typical CVD set up is examined. The types of CVD reactors and the applications of CVD are also discussed. PRINCIPLES OF CVD Under this section, the principles of CVD are examined. Specifically, the CVD process is discussed and types of CVDs are also examined. CVD Process The basic principle of the CVD process is the reaction of gaseous precursors to form a solid coating on a heated substrate (ATL 1). This process takes place in the reaction chamber located inside the CVD reactor. The object to be coated called substrate is placed inside this chamber. Figure 1 (CVD 1) The process starts by heating the substrate in the reaction chamber to the required optimum temperature which will allow gaseous reaction on its surface. This temperature can be in the range of 200 to 2200 degree Celsius. The temperature will depend on the level of coating required, the gaseous precursors involved and the substrate type. After the substrate has been heated to the required temperature, a controlled gaseous precursor (from the left in the above diagram – Fig. 1) is introduced into the gas chamber. The type of gaseous precursor to be used will depend on the type of deposit required. Figure 2 (ULTRAMET 1) For illustration purpose if metal M is to be deposited on the surface of a substrate, then a gaseous precursor such as MCl2 can be used. It is imperative that the precursor has to be in a gaseous form. After introducing a controlled flow of MCl2 gas, a controlled flow of hydrogen is also introduced. The mixture conditions are adjusted in such a manner that the mixture only reacts when in contact with the substrate surface (ATL 2). MCl2 + H2 = M + 2HCl The reaction on the surface of the substrate releases the metal atom and exhaust gas hydrogen chloride. The metal atom is bonded on the surface of the substrate while the waste gas is drawn out of the reaction chamber by use of a vacuum pump. In the schematic diagram above (Fig 1) the gas is removed to the right. The vacuum pump creates a constant flow of reacting gases into the chamber and waste gases out of the chamber (ATL 3). At the start of the reaction process, the metal deposition is localized (deposited on particular points on the substrate) but as the process progresses the atoms are evenly deposited all over the surface of the substrate. The deposition of the metal applies to all the accessible surfaces of the substrate no matter what shape the substrate has. The process is allowed to progress until the desired thickness of the metal is achieved. In some cases the coating is left to achieve some substantial level of thickness such that the substrate can be removed to create a free standing component (ATL 3). Types of CVD The classification of CVD depends on the operating pressure: 1. Atmospheric Pressure CVD (APCVD) 2. Low Pressure CVD (LPCVD) 3. Plasma enhanced CVD (PECVD) 4. Metalorganic CVD (MOCVD) Atmospheric Pressure CVD In this process the substrate (wafer) is placed on a horizontal conveyer belt. The diagram below shows an atmospheric pressure CVD reactor. Figure 3 At low temperatures the deposition rate is high. High temperature atmospheric pressure CVD is appropriate for deposition of Titanium carbide and titanium nitride while low temperature atmospheric CVD is appropriate for deposition of silicon dioxide. The Atmospheric pressure CVD however displays a number of limitations: 1. The deposition process is not undertaken uniformly thus the film thickness is not uniform 2. The process is susceptible to pinhole defects 3. The process proceeds through step coverage which has not been viewed favourably 4. The deposit are susceptible to contamination because the process in executed at the atmosphere pressure Low pressure CVD This type of CVD uses high temperatures in low pressures. The resulting deposition is of high purity and uniformity but the rate of deposition is quite low (Yu 20). Figure 4 This type can allow more than one substrate to be loaded (Curley, Thomas and Matthew 2). Plasma Enhanced CVD The reaction chamber for plasma enhanced CVD is as shown below. Figure 5 The reaction chamber is fitted with two electrodes – one on the upper side and the other on the lower side. The substrate placed on the lower electrode is maintained at a temperature range of between 100 and 400 degree Celsius. The pressure level used is normally in the range of 0.1 to 0.5 Torr. The plasma is created in the space between the two electrodes and is responsible for breaking down the reacting gases in the reacting chamber into radicals, ions, neutral atoms and molecules (Curley, Thomas and Matthew 3). Metalorganic CVD (MOCVD) The substrate is heated and organic molecules are passed over it. The molecules decompose and deposit atoms on the surface of the substrate. This type of CVD produces pure III-V semiconductors. Figure 6 IMPLEMENTATION OF CVD Under this section the implementation of CVD is examined. Specifically, the types of equipment used in CVD are closely examined and the CVD applications are also discussed. CVD Equipment The CVD process is a highly mechanized process which requires close monitoring. A typical CVD set up will have the following equipment. The first requirement is a supply of reacting gases. The gas supply equipment may be gas cylinders or a gas tank as shown in figure 7. Gas tank is used for storing liquid gas while the gas cylinder is for storing gas in a gaseous state. The liquid gas has to be vaporized before entering the reaction chamber. The gases from the cylinder and tank are fed into the gas supply cabinet which vaporizes the liquid gases and regulates the mixture. From the gas supply unit the gaseous mixture is fed into the reaction CVD reactor (ATL 5). Figure 5 below shows this process. Figure 8 The CVD reactor contains the reaction chamber which is surrounded by a heat source. The source of the heat can be resistance, induction, UV, plasma, microwave or laser. It should be noted that it is the CVD reactor which is modified to give the various types of CVD that have already been discussed above. The CVD reactor and the gas supply cabinet are under a control system called Programmable Logic Controller (PLC) – the PLC doubles up as a safety system (ATL 5). Waste gases are removed from the CVD reactor by a series of pump system. The pump system controls the pressure inside the CVD. As seen under the pump system in the above figure 8, the last pump in the series is called a Liquid Ring Pump (LRP). This pump is responsible for feeding the waste gases into a scrubber. The scrubber is responsible for cleaning the waste gases and neutralizing acidic gases before being released. The Liquid Ring Pump also draws out the neutralizing liquid from the scrub and mixes it with the waste gases. It has been shown that the PLC controls the gas supply cabinet and the CVD reactor. These are the most vital parts of this system. The PLC is controlled by a computer (PC) – (on the diagram – Fig. 8 check on the left upper side). The PC is used to set the conditions for the process, monitor the system, carry out recording of data and gives a remote viewing of the progress of the process (ATL 5). CVD reactors are of two types – hot wall and cold wall. The hot wall CVD reactor type has its reaction chamber supplied with heat and therefore the reaction chamber walls grow hot in the process of the reaction. The hot wall CVD reactors are designed in such a manner that they can accommodate many substrates. In the course of the gaseous reaction taking place, deposition takes place on the substrate but also on the inner walls of the reaction chamber. Deposition on the inner walls of the reaction chamber necessitates frequent cleaning of reaction chamber to limit contamination of substrates. On the other hand, the cold wall CVD reactor supplies heat only to the substrate. Because of this, there is no deposition on the inner walls of the reactor. The substrate is not vulnerable to any contamination therefore (Creighton and Ho 8). CVD Applications CVD applications are wide. The common one are discussed below Wear Resistance CVD is used to coat components which require protection from erosion, for instance, tungsten carbide coated extrusion dies (ATL 6). Corrosion Resistance Coating is used for components which are used in corrosive system. For instance, silicon nitride is used as an insulator when there is a contact with liquid silicon and other high temperature applications. Tungsten insulates components against corrosion from plasmas (ATL 6). Barriers CVD is also used to form interface layers for the purpose of preventing contamination by preventing movement of materials through layers. For instance, Erbium oxide is used as a barrier layer for hydrogen diffusion (ATL 6). Net Shape Products CVD is used to create free components by removing substrates after a substantial level of layer has been formed. Components which formed in this manner include crucibles, tubes and cones (ATL 6). SUMMARY Chemical Vapor Deposition (CVD) is a versatile technique of applying required coats of metals on surfaces of components. The basic principle of the CVD process is the reaction of gaseous precursors to form a solid coating on a heated substrate. This process takes place in the reaction chamber located inside the CVD reactor. With the help of appropriate predetermined conditions, gases are induced to react inside the reacting chamber where a substrate is placed. Atoms of a metal are uniformly deposited on all the accessible surfaces of the substrate. There are four types of CVD: Atmospheric Pressure CVD (APCVD); Low Pressure CVD (LPCVD); Plasma enhanced CVD (PECVD); and Metalorganic CVD (MOCVD). The CVD process is highly mechanized. Typical CVD equipment set up consists of gas cylinders and tanks, a gas supply cabinet, a CVD reactor, programmable logic control, a pump system (with a liquid ring pump), a scrubber and a computer. CVD applications are many and they include wear resistance, corrosion resistance, barriers, and production of net shape components. REFERENCES ATL. An Introduction to CVD. Archer Technicoat Ltd, 2013. Web. 16 Apr. 2013. . Curley, Ronald., Thomas, McCormack, and Matthew, Phipps. Low-pressure CVD and Plasma-Enhanced CVD. University of Maryland: Department of Electrical & Computer Engineering, n.d. Web. 16 Apr. 2013. . Creighton, R and Ho, P. Introduction to Chemical Vapor Deposition. Sandia National Laboratories, 2001. Web. 16 Apr. 2013. . CVD. Chemical Vapor Deposition. Nanoscience & Nanotechnology, n.d. Web. 16 Apr. 2013. . ULTRAMET. Chemical Vapor Deposition. Advanced Materials Solutions, 2013. Web. 16 Apr. 2013. . Yu, Winnie. Thin Film Deposition: Chemical Vapor Deposition. . List of Figures Figure 1: Schematic diagram of a reaction chamber Figure 2: Schematic diagram of a substrate gas reaction inside the reaction chamber Figure 3: Schematic diagram of atmospheric pressure CVD Figure 4: Schematic diagram of a low pressure CVD Figure 5: Schematic diagram of a plasma enhanced CVD Figure 6: Schematic diagram of a metalorganic CVD Figure 7: Schematic diagram of gas cylinders and tank Figure 8: Schematic diagram of a typical CVD set up Read More