01 High cleanliness

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, with 1L=10-6Torr * 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 the 1-second monolayer coverage time corresponding to 10-6Torr. 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.

02 Average free path length

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 (105Pa), the average free path is 59nm, and at 10-7Pa, the average free path can reach up to 59km. 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.

03 Insulation

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.