Since vacuum means no material, what is actually measured as vacuum, is the residual gas pressure in the chamber. Three different units of expression are usually used to express the pressure: Pascal (Pa), Torr, and bar. Vacuum quality is also usually classified into three categories: Rough Vacuum, High Vacuum and Ultra-High Vacuum.
Ultra-High Vacuum (UHV) is called pressure range less than 10-7 Pascal or 10-9 Torr.
When an ionic or electron beam collides with particles (residual gas inside the chamber), it may deviate from its path, divide or even react with that particle. As a result, the presence of unwanted particles in the system will reduce its efficiency. Mean Free Path (MFP) is the average distance traveled by a gas molecule before colliding with another gas molecule. In extremely high vacuum conditions, the average free path of gas molecules is approximately 40 km. So the gas molecules have very little contact with each other and will collide with the chamber walls and surfaces in the vacuum chamber many times before they collide with each other.
From the point of view of application, the main issue is not the value of the chamber pressure but it is the contamination of the sample in the vacuum chamber. The more residual gas molecules are present in the chamber the more the sample surface will absorb some atmospheric particles, decreasing the analysis quality over the time. For this reason, if the vacuum is as high as possible (the chamber pressure is low) the study and analysis of the specimens will be more accurate and accurate. Ultra-high vacuum (UHV) provides suitable conditions for surface analysis processes. Indeed, in the HV range a monolayer deposits every 4 seconds, and in the UHV range a monolayer deposits every 4 days.
How to reach Ultra high vacuum?
To reach this level of vacuum requires the use of special materials and various stages of pumping. Seals and gaskets used in the UHV system should prevent even minor leaks. Almost all of these seals are made of metallic materials with knife-edges on both sides cutting into a soft gasket, typically copper. These all-metal seals can maintain integrity to UHV ranges. The materials used in UHV condition need to stand heating at more than 120°C for many hours/ days but also shouldn’t present high vapor pressure. So, it is not possible to use plastics, PTFE, PEEK neither glue (screws are used instead), lead (soldering). This is why UHV systems are more expensive than HV systems. The baking process causes the gas atoms to exit the surface of the chamber wall. During the vacuum process, the gas atoms absorbed by the chamber wall are slowly released from the surface of the chamber wall (Outgassing phenomenon), and if the chamber is not baked, it will literally take months to reach UHV conditions.
Outgassing is one of the problems that need to be addressed in order to reach UHV conditions. All materials, even materials that are not usually absorbent, also exhibit the Outgassing phenomenon, such as some metals and plastics. For example, vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.
Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by selection of materials with low vapor pressures (such as glass, stainless steel, and ceramics) for everything inside the system.
At very low pressures, the gases absorbed by the surfaces in the vacuum chamber are gradually released, preventing the pressure reaching the UHV range. Water is one of the things that strongly cause Outgassing. As the vacuum chamber door opens and air adjacent its surfaces, a thin layer of water vapor absorbs the surfaces and enters the vacuum chamber during low pressure processes. Removal of water and similar gases generally requires baking the UHV system while vacuum pumps are running. During chamber use, the walls of the chamber may be chilled using liquid nitrogen to reduce outgassing further.
UHV systems are usually 100 percent dry and there should be no water or moisture in them. The most common gas remaining in UHV systems is hydrogen. Hydrogen is a light, mobile gas that is hard to pump. Pumping this gas requires special UHV pumps and reducing the amount of hydrogen released from the inner surface of the vacuum chamber is one of the things that should be taken into consideration.
Two or more pumps must be used to reach the UHV. There is no pump that can reduce pressure from the atmosphere to the UHV alone. n the first step, a rough pump, called a backing pump, pushes the pressure to an approximate vacuum (Rough Vacuum). Then by one or more low pressure pumps, the pressure reaches the UHV range. Pumps commonly used in the second stage include turbomolecular pumps, ion pumps, Getter pumps and cryo pumps.
For measuring the pressure in the HV and UHV ranges, the use of conventional gauge due to the Outgassing phenomenon is not suitable and instead of ionization gauges are used. These use the probability of gas ionization to determine the particle number density. There are two types: cold and hot cathode ionization gauges. A cold cathode ionization gauge often referred to as a Penning gauge. The mechanism of action of these pressure gauges is that by applying an electric field between the cathode and anode, the electrons accelerate to the cathode and collide with the gas atoms in the chamber in their path. As a result of this collision, the gas atoms become positive ions. As the pressure inside the chamber increases, the number of charge carriers also increases. By measuring the electric charge, the pressure inside the chamber is determined. These gauges can measure the pressure in the range of 10-2 to 10-5 mill bars.
In hot cathode pressure gauge, the cathode acts as an electron emitting source. The electrons are sent from the cathode to the anode and in their path collide with the gas atoms and ionize them. The measurement of the number of ions in the ion collector results in the determination of the pressure inside the chamber. These gauges can measure the pressure in the range of 10-2 to 10-11 mill bars.
If the particle density inside the vacuum chamber is high, the ions cannot reach the ion collector. This is why cold cathode gauges are used at high pressures and in the early stages of vacuum and in lower vacuum hot cathode gauges are used.
The electrons emitted from the cathode affect the anode and produce X-rays. The resulting X-rays also emit electrons from the ion collector, resulting in an offset current. Shields are used to protect the ion collector from X-rays in recent years.
For more information on ultra-high vacuum systems, see the below links.
- Strong, John (1938). Procedures in Experimental Physics. Bradley, IL: Lindsay Publications., Chapter
- B. Schläppi, et al. (2010), Influence of spacecraft outgassing on the exploration of tenuous atmospheres with in situ mass spectrometry, J. Geophys. Res., 115, A12313, doi:10.1029/2010JA015734.