Common units for ultra-high vacuum
1. Millibars (mbar) are units of air pressure, 1000 mbar=1 bar=1 * 105 Pa;
2. Torr comes from the millimeter mercury column (mmHg) in the Torricelli experiment, with 760 Torr=1 atm;
3. Pa comes from the International System of Units (SI), where 1 Pa equals 1 N/m2;
Note: Pa is the derived unit in the International System of Units, not the base unit.
Note: 1 bar is strictly defined as 105 Pa, and 1 atm is strictly defined as 101325 Pa. The two are generally considered consistent in practical use, but have different definitions.
Note: In practical use, due to the similar values of Torr and mbar, they are generally considered equivalent when accuracy is not required.
Note: Kilograms (kg/cm2) are often used as a unit of pressure in engineering, with a value close to 105 Pa.
Definition of ultra-high vacuum
1. Ultra high vacuum (UHV), generally defined as 10-7-10-12 mbar;
2. High vacuum (HV), generally defined as>10-7 mbar;
3. Extreme high vacuum (XHV), generally defined as<10-12 mbar.
Characteristics of ultra-high vacuum
High cleanliness is the fundamental reason why surface analysis requires ultra-high vacuum. Surface physics often studies the physical phenomena of several atomic layers on the surface. Therefore, even under vacuum conditions, the adsorption of gas molecules on the sample surface can significantly affect experimental results. We often use 'lifetime' to describe the time it takes for a sample surface to be cleaned and the experimental results to be affected by contamination. Due to the different adsorption abilities of gas molecules, there are significant differences in sample lifetimes among different samples. Even for the same sample, different experiments will have completely different definitions of sample lifespan. Generally speaking, the lifespan of surface states is much shorter than that of body states.
In surface science, L (Langmuir) is used to define the exposure of a sample surface, where 1 L=10-6 Torr * s. We can see that the exposure of the sample is inversely proportional to the air pressure. So, in order to improve the lifespan of the sample, we often try to increase the vacuum degree of the system as much as possible.
If calculated based on N2 molecules at room temperature, considering that all molecules on the collision surface are adsorbed, a layer of molecules will be adsorbed on the sample surface in 3 seconds under vacuum conditions of 10-6 Torr. In popular science propaganda, we often describe the importance of vacuum by using 10-6 Torr corresponding to 1 s monolayer coverage time. This term is quite vivid and easy to understand, but students engaged in surface research must not use it as a basis for scientific research.
The statistical average of the distance between two adjacent collisions of each gas molecule is called the average free path of the molecule. The size of the average free path of molecules is related to the type, density, and velocity of molecules in vacuum. At room temperature, considering N2, the average free path of gas molecules is inversely proportional to gas pressure: at atmospheric pressure (105 Pa), the average free path is 59 nm, and at 10-7 Pa, the average free path is as high as 59 km. Based on this parameter, we can estimate the minimum vacuum required for magnetron sputtering growth.
The average free path of electrons refers to the statistical average of the distance traveled between two consecutive collisions of electrons and gas molecules (ignoring collisions between electrons). This parameter is mainly applied to the photoelectric energy spectrum experimental system.
Under ultra-high vacuum conditions, thermal convection is generally ignored, and thermal radiation and conduction are mainly considered. Low temperature systems (liquid helium, liquid nitrogen) mainly consider preventing the transfer of external heat. For systems using liquid nitrogen, heat conduction is the main source of heat; For systems using liquid helium, external thermal radiation cannot be ignored, and special attention should be paid when designing the system. High temperature systems need to consider the material temperature rise and gas release caused by the thermal radiation generated by heating the filament. Heat conduction at high temperatures mainly affects the temperature measurement of thermocouples. In addition, the thermal radiation generated by the material itself after being heated to a higher temperature cannot be ignored.
The application field of ultra-high vacuum
The application field of ultra-high vacuum is very extensive, and here we list several that are most closely related to surface physics research, including magnetron sputtering, laser pulse deposition, molecular beam epitaxy, surface analysis, and particle accelerators.
Ultra high vacuum technology is widely used in the fields of molecular beam epitaxy and surface analysis, and various types of molecular beam epitaxy equipment, photoelectron spectroscopy, scanning tunneling microscopy, and other preparation characterization systems work within this range. Due to the fact that vacuum systems often account for a significant proportion of system construction costs, how to choose the appropriate pump set and quickly obtain the best possible vacuum degree through appropriate means is a common problem that troubles related fields.
Particle accelerators have the most stringent requirements for vacuum, but due to the high overall system cost, thevacuum pump unit is not the main component of the cost. Generally, better vacuum pumps are configured as much as possible. In addition, there is generally no pollution source in the accelerator chamber, and the vacuum degree usually reaches a very high vacuum range.
Magnetron sputtering generates significant pollution during the evaporation process due to mechanism issues, and usually does not pursue particularly high vacuum levels. Molecular pump units are generally sufficient to meet the usage conditions. In recent years, with the continuous advancement of technology and further development of research needs, the vacuum degree of magnetron sputtering systems has been continuously improved, and ultra-high vacuum related technologies are also constantly entering this field.
In the past, the demand for vacuum degree in laser pulse deposition (PLD) technology was between molecular beam epitaxy and magnetron sputtering. In recent years, due to the gradual integration with molecular beam epitaxy (MBE) technology, the requirement for vacuum degree has also been constantly increasing. Laser molecular beam epitaxy (LMBE) is an ultra-high vacuum technology that incorporates MBE into PLD.
