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Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressures lower than about 1 × 10 −6 pascals (1.0 × 10 −8 mbar; 7.5 × 10 −9 Torr). UHV conditions are created by pumping the gas out of a UHV chamber.
This is also called the regime of high vacuum, or even ultra-high vacuum. This is opposed to viscous flow encountered at higher pressures. [1] The presence of free molecular flow can be calculated, at least in estimation, with the Knudsen number (Kn). [2] If Kn > 10, the system is in free molecular flow, [3] also known as Knudsen flow. [4]
High to ultra-high vacuum removes the obstruction of air, allowing particle beams to deposit or remove materials without contamination. This is the principle behind chemical vapor deposition , physical vapor deposition , and dry etching which are essential to the fabrication of semiconductors and optical coatings , and to surface science .
The langmuir (symbol: L) is a unit of exposure (or dosage) to a surface (e.g. of a crystal) and is used in ultra-high vacuum (UHV) surface physics to study the adsorption of gases. It is a practical unit, and is not dimensionally homogeneous, and so is used only in this field. It is named after American physicist Irving Langmuir.
A roughing pump is any vacuum pump (typically mechanical) used to initially evacuate a vacuum system, as a first stage towards achieving high vacuum or ultra high vacuum.The term "roughing pump" derives from the vacuum range it works in, "rough vacuum", above 1x10 −3 torr (0.1 Pa).
Molecular-beam epitaxy takes place in high vacuum or ultra-high vacuum (10 −8 –10 −12 Torr). The most important aspect of an MBE process is the deposition rate (typically less than 3,000 nm per hour) that allows the films to grow epitaxially (in layers on top of the existing crystal).
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Note that in other fields, a lower division between high and ultra-high vacuum is common, often 10 −7 Pa. Sub-atmospheric CVD (SACVD) – CVD at sub-atmospheric pressures. Uses tetraethyl orthosilicate (TEOS) and ozone to fill high aspect ratio Si structures with silicon dioxide (SiO 2). [4] Most modern CVD is either LPCVD or UHVCVD.