A thin film is a layer of material that is applied to a substrate to give its surface a different property, including color, light reflection or refraction, hardness, electrical insulation and conductance. Thin layers of materials are used to develop filters, increase insulation or conduction, protect them from lights or create reflective surfaces. Thin film thickness ranges from fractions of a nanometer (1 nm = 0.000000001 m = 10 angstroms) to several micrometers (1 μm = 0.000001 m). For reference, the diameter of a human hair ranges from 17-181 μm. A strand of spider web silk has a width of 3 to 8 μm.
Manufacturers seeking to apply thin films have a host of deposition methods available to them, but the ideal method for a given application depends on the purpose of the deposition, the surface makeup of the substrate, and the thickness desired. There are basically two methods of deposition: chemical or physical.
Chemical deposition is a process by which a substrate is fully submerged in a chemical fluid and then the material is deposited on the surface is a conformal pattern. This means every surface of the substrate is equally coated. The most common types of chemical deposition are:
In the plating process, a substrate is submerged in a chemical bath, often composed of water mixed with metal salts destined for deposition. The metal salts adhere to the substrate in a uniform pattern, building up a thicker film the longer the substrate is submerged. A more specialized form of plating, known as electroplating, requires the use of electricity. The substrate is connected to an anode powered by an external battery or rectifier. When the electricity is turned on, the metal particles in the liquid begin adhering to the substrate surface through oxidation. Although different metals adhere at varying speeds and thicknesses, the general rule for electroplating is the higher the charge and the longer the current is connected, the thicker the deposition coating.
2. Chemical Solution Deposition (CSD)
CSD is a process very similar to plating, except that instead of metal salts in a water bath, organometallic powders in an organic solvent carry out the deposition procedure. CSD is cheaper and simpler than plating techniques, although its results are comparable.
3. Chemical Vapor Deposition (CVD) CVD does not use a liquid bath, but rather involves a substrate placed in a pressurized chamber full of organometallic gas. Typical organometallic gasses include polysilicon, silicon dioxide and silicon nitride. The gas either reacts with the substrate surface or slowly dissolves over it, depositing the thin film evenly. A more specialized variant of CVD is plasma enhanced CVD. Plasma is an ionized vapor, and is attracted to the suface of a substrate when excited by electric currents or microwaves.
Physical deposition techniques do not include chemical reactions. They rely on mechanical or thermodynamic methods to produce thin films instead. Generally, they require low-pressure environments for accurate and functional results.
1. Thermal evaporation
In a high vacuum, the deposition material is melted by an electric resistance heater until it covers the surface of the substrate. The vacuum is required to make sure there is not an unwanted reaction between the film material atoms and gas atoms. A variant of thermal evaporation uses an electron beam evaporator to melt materials on a substrate. Because of the added precision, materials of lower vapor pressure can be melted than in standard thermal evaporation.
Sputtering occurs when a noble gas plasma is shot at a substrate in atom-sized particles. The impact of the particles triggers a collision cascade, which results in many of the particles passing through the substrate and adhering to the opposite side. In this manner, the entire surface of a substrate is coated.
3. Pulsed Laser Deposition
Pulsed laser deposition involves a substrate and a block of film material in an ultra high vacuum chamber. A laser fires pulsed bursts of light at the block of material, which vaporizes and transfers to the substrate facing it. Sometimes the pulsed laser deposition takes place in an oxidated chamber so that oxygen can aid in oxide deposition.
4. Cathodic Arc Deposition (Arc-PVD)
Arc-PVD is very similar to pulsed laser deposition, except an electric arc is used instead of a pulsed laser. An electric arc is an electrical charge of a gas between an anode and a cathode, with the substance material serving as the cathode in Arc-PVD. As the substance material vaporizes, it condenses on the substrate and forms a thin film.
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