1. 毫巴（mbar）来自于气压的单位bar，1000 mbar = 1 bar = 1*105 Pa；
2. 托（Torr）来自于托里拆利实验中的毫米汞柱（mmHg），760 Torr =1 atm；
3. 帕（Pa）来自于国际单位制（SI），1 Pa = 1 N/m2；
备注：1 bar严格定义为105 Pa, 1 atm严格定义为101325 Pa，两者在实际使用中基本认为一致，但定义上有所不同。
1. 超高真空（ultra-high vacuum，UHV）一般定义为10-7-10-12 mbar；
2. 高真空（high vacuum，HV）一般定义为 > 10-7 mbar；
3. 极高真空（Extreme high vacuum，XHV）一般定义为 < 10-12 mbar。
表面科学中用L（Langmuir）定义样品表面的暴露情况，1 L = 10-6 Torr*s。我们可以看到，样品的暴露情况和气压是成反比关系。所以，为了提高样品的寿命，我们往往会尽可能地提高系统的真空度。
如果以室温条件下的N2分子进行计算，考虑所有碰撞表面的分子全部被吸附的话，在10-6 Torr的真空条件下，3秒钟会在样品表面吸附一层分子。在科普宣传中我们经常以10-6 Torr对应1 s单分子层覆盖时间来描述真空的重要性，这个提法比较形象，易于理解，但从事表面研究的同学们一定不要以此作为科学研究的依据。
每个气体分子相邻两次碰撞的距离统计平均值称为分子的平均自由程。分子平均自由程的大小和真空中分子的种类、密度和运动速度都有关系。在常温下，考虑N2的话，气体分子的平均自由程和气压成反比：大气压（105 Pa）下，平均自由程为59 nm，10-7 Pa的平均自由程则高达59 km。根据这一参数，我们可以估计磁控溅射生长所需要的最低真空度。
I. Common units for pressure
1. mbar comes rom the unit bar, 1000 mbar = 1 bar =1 x 105 Pa;
2. Torr comes from the mmHg in the Torricelli’s experiment, 760 Torr = 1 atm;
3. Pa from the International System of Units (SI), 1 Pa = 1 N/m2.
Note: Pa is an SI derived unit.
Note: 1 bar is strictly defined as 105 Pa and 1atm is strictly defined as 101325 Pa, which is generally considered to be the same in practice, but different in definition.
Note: In practice, the value of torr and mbar are generally considered equivalent when accuracy is not required.
II. The definition of UHV
1. UHV is generally defined as 10-7-10-12mbar;
2. High vacuum (HV) is generally defined as > 10-7 mbar;
3. Extreme high vacuum (XHV) is generally defined as < 10-12 mbar.
III. The characteristics of UHV
1. High cleanliness
Surface physics focused on the physical phenomena of several atomic layers on the surface. Even in high vacuum, the adsorption of gas molecules on the surface of the sample tends to significantly affect the experimental results. We often use "lifetime" to describe the time it takes for the clean sample surface to get contaminated. The "lifetime" varies greatly from sample to sample, due to the different adsorption capacity of the gas molecule. Even for the same sample, different experiments may have completely different definitions of "lifetime". In general, the "lifetime" of the surface state is much shorter than that of the body.
Surface science uses Langmuir (L) to define the exposure of sample surface, 1 L = 10-6 Torr*s. We can see that the exposure of sample surface is proportional to the air pressure. Therefore, in order to increase "lifetime" of the sample, we tend to improve the vacuum of the system as much as possible.
If calculated at room temperature and N2 molecules, considering that all molecules on the collision surface are all adsorbed, a layer of N2 molecules would be formed on the sample surface within 3 seconds in 10-6 Torr. In popular science propaganda, we often describe the importance of vacuum by “10-6 Torr corresponding to a single-molecule layer coverage time of 1 s”, which is more visual and easier to understand. However, for students who engaged to surface science, you should note that it is not an accurate description.
2. Mean free path
The statistical average distance traveled between two collisions is called mean free path. Mean free path can be affected by the type, density and speed of molecules in the vacuum. At room temperature, consider N2, the mean free path is proportional to the pressure: the mean free path is 59 nm in 105 Pa and reaches to 59 km in 10-7 Pa. Based on this parameter, we can estimate the minimum vacuum required for Physical vapor deposition (PVD).
The mean free path of electrons refers to the statistical average of the distance traveled between two consecutive collisions with electrons or gas molecules. This parameter is mainly applied to the photoelectron energy spectrum systems.
Under UHV condition, thermal convection is generally ignored, mainly considering thermal radiation and thermal conduction. Low temperature systems (such as liquid helium, liquid nitrogen) are mainly considered to prevent the introduction of external heat. For systems that use liquid nitrogen, thermal conduction is the main source of heat, and for systems that use liquid helium, external thermal radiation is not negligible and should be designed with care. For high temperature systems, the outgassing of materials caused by the heating from filament should be considered. At high temperature, heat conduction can significantly affect the temperature measurement of thermocouple. In addition, the heat radiation generated by the material after it is heated to a high temperature cannot be ignored.
IV. Applications of UHV systems
UHV is widely used in many fields. Here we list some technologies that are most closely related to the study of surface physics, including magnetron sputtering, pulsed laser deposition (PLD), molecular beam epitaxy (MBE), surface analysis and particle accelerator.
UHV technology is widely used in the field of MBE and surface analysis. Many preparation and characterization systems such as MBE, photoelectron spectrometer and scanning tunnel microscopes work in UHV condition. Since vacuum systems often account for a considerable proportion of the cost of system construction, how to choose the right pump set and quickly obtain the best vacuum is a common problem in the relevant fields.
Particle accelerator vacuum requirements are the most demanding. Since the overall system cost is very high, the vacuum system is not the main component of the cost. Therefore, they usually use the best vacuum pumps. In addition, the accelerator chamber is generally clean without source of pollution. So, particle accelerators usually reach to very high vacuum level, sometimes even in the range of XHV.
For magnetron sputtering, vacuum is limited due to the mechanism: a large pollution was produced in the evaporating process. Therefore, generally with molecular pump group can meet the conditions of use. In recent years, with the continuous progress of technology and the further development of research needs, the vacuum of magnetron sputtering system has been continuously improved, and ultra-high vacuum-related technology has also entered this field.
The demand for vacuum in the past of laser pulse deposition technology is between molecular beam extension and magnetron sputtering. In recent years, the vacuum requirement is constantly increasing due to the gradual fusion with molecular beam extension technology. Laser molecular beam epitaxy (LMBE) is a kind of PLD which integrates the UHV technology of MBE.