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Chemical Vapor Deposition (CVD) In this thin film deposition process, the substrate is placed inside a reactor to which a number of gases are supplied. The fundamental principle of the process is that a chemical reaction takes place between the source gases. The product of that reaction is a solid material with condenses on all surfaces inside the reactor. The two most important CVD technologies are the Low Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process produces layers with excellent uniformity of thickness and material characteristics. The main problems with the process are the high deposition temperature (higher than 600°C) and the relatively slow deposition rate. The PECVD process can operate at lower temperatures (down to 300° C) thanks to the extra energy supplied to the gas molecules by the plasma in the reactor. However, the quality of the films tend to be inferior to processes running at higher temperatures. Secondly, most PECVD deposition systems can only deposit the material on one side of the wafers on 1 to 4 wafers at a time. LPCVD systems deposit films on both sides of at least 25 wafers at a time. When do I want to use CVD? CVD processes are ideal to use when you want a thin film with good step coverage. A variety of materials can be deposited with this technology, however, some of them are less popular with fabs because of hazardous byproducts formed during processing. The quality of the material varies from process to process, however a good rule of thumb is that higher process temperature yields a material with higher quality and less defects. Electrodeposition This thin film deposition process is also known as "electroplating" and is typically restricted to electrically conductive materials. There are basically two technologies for plating: Electroplating and Electroless plating. In the electroplating process the substrate is placed in a liquid solution (electrolyte). When an electrical potential is applied between a conducting area on the substrate and a counter electrode (usually platinum) in the liquid, a chemical redox process takes place resulting in the formation of a layer of material on the substrate and usually some gas generation at the counter electrode. In the electroless plating process a more complex chemical solution is used, in which deposition happens spontaneously on any surface which forms a sufficiently high electrochemical potential with the solution. This process is desirable since it does not require any external electrical potential and contact to the substrate during processing. Unfortunately, it is also more difficult to control with regards to film thickness and uniformity. When do I want to use electrodeposition? The electrodeposition process is well suited to make films of metals such as copper, gold and nickel. The films can be made in any thickness from ~1µm to >100µm. The deposition is best controlled when used with an external electrical potential, however, it requires electrical contact to the substrate when immersed in the liquid bath. In any process, the surface of the substrate must have an electrically conducting coating before the deposition can be done. MEMS Exchange. “Thin Film Deposition Processes.” 25 Feb. 2010 [http://www.memsnet.org/mems/processes/deposition.html].
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