The process of silicon deposition involves the application of thin layers of silicon onto substrates like silicon or glass through physical or chemical methods. The primary techniques used are Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). The thickness of these layers can vary from a few nanometers to several micrometers.
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), which results in solid silicon being deposited on the substrate with hydrogen as an exhaust gas. This 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, which could lead to roughening of the film due to silicon particles falling onto the growing film.
Deposition of Polysilicon: Polysilicon, which has a higher resistivity than single crystal silicon at the same doping level, is formed through this process. The higher resistivity is due to dopants segregating along grain boundaries, reducing the number of dopant atoms within the grains, and defects in these boundaries that decrease carrier mobility. The grain boundaries also contain many dangling bonds that can trap free carriers.
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.
Metal Deposition in CVD: CVD is also used for depositing metals like tungsten, aluminum, and copper, which 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, often forming useful silicides when deposited onto silicon.
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 and is crucial for preparing the surface chemistry and ensuring the purity of the deposited layer.
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 and heating to high temperatures for surface preparation. Substrate temperature control is critical not only during deposition but also during cooling, which 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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