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Chemical Vapor Deposition (CVD)
- Reactants in the gas phare are mixed to produce the desired molecule to coat the surface. In general, this method is quite fast and produces thick coatings, however it is not very precise or pure.
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Physical Vapor Deposition
- is performed by ablating a solid sample of the desired coating with an ion beam. There is no chemical reaction that takes place. Energy from ions shot at it (typically noble gasses) ionizes the desired material which is scattered towards the substrate. This method is extremely pure and can deposit nonreactive pure metals, but it is rather slow.
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Atomic Layer Deposition
- is performed via chemical reactions on the substrate surface. One material must be very non cohesive, but stick to the substrate. The chamber is them purged. The second gaseous reactant is added, held, then purged. Because the first material will not stick to itself, only a single molecular layer was present on the substrate, hence ALD. Note this method occurs in steps, it is not continuous, is rather expensive, and does not work for many coatings. However thickness control and uniformity is phenomenal.
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Photolithography
- A material called photoresist is applied in a complete blanket coating on the surface. This material is sensitive to either UV light or electron beams. However, that sensitivity can mean it becomes less soluble (negative resist) or more soluble (positive resist). These will produce inverted patterns of each other.
- Light or electrons are applied in the desired pattern, or its inverse. A solvent is then applied to wash away the soluble areas of the coating. At this point the wafer is now ready for another operation (deposition or etching)
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Etching
- Can either be liquid (wet) or plasma (dry) etching. Liquid is generally an older, less useful method compared to plasma.
- Etchants with large bias are called isotropic, because they erode the substrate equally in all directions. Modern processes greatly prefer anisotropic etches, because they produce sharp, well-controlled features.
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Plasma etching
- Deep Reactive Ion Etching (DRIE) is a pulsed technique with two different phases. One bombards with free radicals and ions, most of the material removal is done via chemical reactions. The second phase covers the entire surface in inhibitor that can only be degraded by bombardment, not chemical reaction. So when the chamber is purged and the first step is repeated, only the anisotropic surface is exposed by the bombardment, allowing the reaction to take place.