Silicon deposition is a process where thin layers of silicon are applied onto substrates like silicon or glass.
This is done through physical or chemical methods.
The main techniques used are Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD).
The thickness of these layers can range from a few nanometers to several micrometers.
What is the Process of Silicon Deposition? 7 Key Steps Explained
1. Chemical Vapor Deposition (CVD) for Silicon Deposition
CVD is a widely used method for depositing silicon layers.
It involves the pyrolysis or thermal decomposition of silane (SiH4).
This results in solid silicon being deposited on the substrate with hydrogen as an exhaust gas.
The process is typically conducted in a hot-wall Low Pressure Chemical Vapor Deposition (LPCVD) furnace.
Engineers often dilute the silane with a hydrogen carrier gas to suppress gas phase decomposition of silane.
This helps prevent roughening of the film due to silicon particles falling onto the growing film.
2. Deposition of Polysilicon
Polysilicon is formed through this process.
It has a higher resistivity than single crystal silicon at the same doping level.
The higher resistivity is due to dopants segregating along grain boundaries.
This reduces the number of dopant atoms within the grains.
Defects in these boundaries also decrease carrier mobility.
The grain boundaries contain many dangling bonds that can trap free carriers.
3. Alternative Reactions for Silicon Nitride (SiNH) Deposition
In plasma, silicon nitride can be deposited using two reactions involving silane (SiH4) and nitrogen (N2) or ammonia (NH3).
These films have lower tensile stress but exhibit poorer electrical properties in terms of resistivity and dielectric strength.
4. Metal Deposition in CVD
CVD is also used for depositing metals like tungsten, aluminum, and copper.
These metals are crucial for forming conductive contacts and plugs in semiconductor devices.
Tungsten deposition, for example, can be achieved using tungsten hexafluoride (WF6) through different reactions.
Other metals like molybdenum, tantalum, titanium, and nickel are also deposited using CVD.
They often form useful silicides when deposited onto silicon.
5. Silicon Dioxide Deposition
Silicon dioxide is deposited using a combination of silicon precursor gases like dichlorosilane or silane and oxygen precursors such as oxygen and nitrous oxide.
This process occurs at low pressures.
It is crucial for preparing the surface chemistry and ensuring the purity of the deposited layer.
6. Overall Process and Considerations
The CVD process begins with a silicon dioxide substrate that has been deposited on a stainless steel-backed membrane.
The process involves thermal dehydration to remove oxygen impurities.
Heating to high temperatures is necessary for surface preparation.
Substrate temperature control is critical not only during deposition but also during cooling.
Cooling can take 20-30 minutes depending on the substrate material.
This method is favored for its reproducibility and ability to produce high-quality thin films.
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