One of the fundamental steps in microelectromechanical systems (MEMS) processing is the deposition of thin films of material with thicknesses ranging from a few nanometers to about 100 micrometers.
Deposition techniques in MEMS processing fall into two categories, the first being techniques that deposit a thin film based on chemical reactions. These include chemical vapor deposition (CVD), epitaxy, thermal oxidation, and electrodeposition.
All of these processes utilize the production of solid materials from chemical reactions in a gas or liquid matrix or through reactions with the substrate material.
The second category is the direct physical deposition of materials on the substrate, such as casting and physical vapor deposition (PVD).
Chemical deposition technologies
Among chemical deposition techniques, CVD is one of the most important.
The basic principle of CVD is that the substrate is placed inside a reactor where source gases are supplied, and when the chemical reaction between the gases takes place, the product of that reaction is a solid material that condenses on the surface of the substrate inside the reactor. CVD is ideally used when the thin film to be produced should have good step coverage. One drawback of CVD is that it can produce hazardous products during the process.
Epitaxy is a chemical process that is highly similar to CVD, the only difference being that it uses ordered semiconductor crystals as the substrate and that the thin film grows with the same crystallographic orientation as the substrate. Epitaxy has the advantage of high growth rate of the material, enabling the formation of films that are much thicker than usual. Itâ€™s a technology that is usually used for the deposition of silicon used in producing silicon on insulator substrates.
One of the most basic chemical deposition techniques is thermal oxidation, where the substrate surface is simply oxidized in an oxygen-rich atmosphere at high temperatures.
Thermal oxidation is also the only deposition method where the substrate is consumed, this means that the formation of the film is downwards into the substrate and that the oxidation time is increases as the film becomes thicker.
Thermal oxidation is a simple process that is typically used to produce films for electrical insulation, but it has the drawback of being of somewhat limited use in MEMS components.
Electrodeposition is a method that is limited only to electrically conductive materials. There are two kinds of electrodeposition: electroplating and electroless plating: electroplating involves the application of an electric potential between two electrodes in an electrolyte solution, while electroless plating utilizes a more complex chemical solution.
Electrodeposition is suitable for producing films of metals such as copper, nickel, and gold.
Physical Deposition Technologies
One of the physical deposition technologies is casting.
Casting involves the dissolution of the material to be deposited in a solvent, then applying it to the substrate by spraying it.
A thin film of the target material remains on the substrate after the solvent has evaporated.
Casting is particularly useful for polymeric materials that can be dissolved in organic solvents, it is typically used to apply photoresistive materials to substrates in photolithography.
PVD is a broad term for a number of deposition techniques where a material is released or removed from a source and then transferred to the substrate. It is the more common technology used for the deposition of metals as it can be performed at lower process risk and is cheaper, as compared to CVD.
PVD does have drawbacks, such as the inferior quality of its films as compared to films produced by CVD as well as lower step coverage.
There are several deposition methods that fall under the category of PVD, but the two most important techniques are evaporation and sputtering.
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