Pressure ultimate

A volume of gas is enclosed in the space bounded by the rotor, the stator and the two vanes (see Fig. 1.6). The pump removes the gas by compressing it to a pressure slightly higher than the atmospheric pressure. This overpressure opens the spring-loaded outlet valve, and the gas escapes to the atmosphere. A thin film of oil makes the final seal therefore the ultimate pressure depends also on the oil vapour pressure. With one stage, the lowest attainable pressure is about 10 2torr and with two stages in series 10-3 torr. 

If gases like H2, He and Ne are to be trapped, the pump must be cooled to 4.2 K. Detailed information on cryopumping can be found in ref. [10]. Ultimate pressure of the order of l(U5torr can be achieved. 

A cold cap is usually mounted on the top of the pump assembly to prevent vapour from reaching the vacuum chamber. A thermal protection switch is often used. The maximum working pressure of a diffusion pump is about 10-3 torr. The ultimate pressure of a diffusion pump can be around 10-9 torr and heavily depends on the oil vapour pressure (p < 10-8 torr at room temperature for very good oils). Pumps with very large pumping speed (up to 104l/s) are commercially available. 

Important note Particularly in rough vacuum technology, partial pressure in a mix of gas and vapor is often understood to be the sum of the partial pressures for all the non-condensable components present in the mix - in case of the partial ultimate pressure at a rotary vane pump, for example. 

The lowest pressure which can be achieved in a vacuum vessel. The so-called ultimate pressure Pgi j depends not only on the pump s suction speed but also upon the vapor pressure p j for the lubricants, sealants and propellants used in the pump. If a container is evacuated simply with an oil-sealed rotary (positive displacement) vacuum pump, then the ultimate pressure which can be attained will be determined primarily by the vapor pressure of the pump oil being used and, depending on the cleanliness of the vessel, also on the vapors released from the vessel walls and, of course, on the leak tightness of the vacuum vessel itself. 

At the beginning of a pump down process, the gas ballast pump should always be operated with the gas ballast valve open. In almost all cases a thin layer of water will be present on the wall of a vessel, which only evaporates gradually. In order to attain low ultimate pressures the gas ballast valve should only be closed after the vapor has been pumped out. LEYBOLD pumps generally offer a water vapor tolerance of between 33 and 66 mbar. Two-stage pumps may offer other levels of water vapor tolerance corresponding to the compression ratio between their stages -provided they have pumping chamber of different sizes. 

The ultimate pressure attainable with the DRYVAC 251 S or 501 S is -compared to the versions without integrated Roots pump - by approximately one order of magnitude lower (from 2 10 mbar to 3 10 mbar) and the attainable throughput is also considerably increased. It is of course possible to directly flange mount LEYBOLD RUVAC pumps on to the DRYVAC models (in the case of semiconductor processes also mostly with a PFPE oil filling for the bearing chambers). 

Fig. 2.33 Ultimate pressure of the DRYVAC 100S as a function of pure gas flow in stages 2 4...

Losses in pumping speed and a reduction in ultimate pressure can be kept very small due to the special way in which the gas is made to pass through the pump. 

Basically the ultimate pressure of fluid entrainment pumps is restricted by the value for the partial pressure of the fluid used at the operating temperature of the pump. In practice one tries to improve this by introducing baffles or cold traps. These are condensers" between fluid entrainment pump and vacuum chamber, so that the ultimate pressure which can be attained in the vacuum chamber is now only limited by the partial pressure of the fluid at the temperature of the baffle. 

DC 705 has an extremely low vapor pressure and is thus suited for use in diffusion pumps which are used to attain extremely low ultimate pressures of< 10- °mbar. 

After a certain time, the stay-down time , a continuous layer of oil molecules builds up, and the ultimate pressure is practically determined by the vapor pressure of the pump fluid at the temperature of the vessel walls. This stay-dovm time can even amount to several hours, indeed even te days, A/ith the use of low-temperature baffles. 

The ultimate pressure attainable with adsorption pumps is determined in the first instance by those gases that prevail in the vessel at the beginning of the pumping process and are poorly or not at all adsorbed (e.g. neon or helium) at the zeolite surface. In atmospheric air, a few parts per million of these gases are present. Therefore, pressures < 10 mbar can be obtained. 

To pump out larger vessels, several adsorption pumps are used in parallel or in series. First, the pressure is reduced from atmospheric pressure to a few millibars by the first stage in order to capture many noble gas molecules of helium and neon. After the pumps of this stage have been saturated, the valves to these pumps are closed and a previously closed valve to a further adsorption pump still containing clean adsorbent is opened so that this pump may pump down the vacuum chamber to the next lower pressure level. This procedure can be continued until the ultimate pressure cannot be further improved by adding further clean adsorption pumps. 

NEG pumps are mostly used in combination with other UHV pumps (turbomolecular and cryopumps). Such combinations are especially useful when wanting to further reduce the ultimate pressure of UHV systems. 

Ultimate pressure Pg For the case of cryocondensation (see Section 2.1.9.4) the ultimate pressure can be calculated by ... 

Table 2.6 Ultimate temperatures at a wall temperature of 300 K temperature of Tq= 300 K (i.e. when the cryopanel is exposed to the thermal radiation of the wall) sufficiently low ultimate pressures can be attained. Due to a number of interfering factors like desorption from the wall and leaks, the theoretical ultimate pressures are not attained in practice.

Pg j Ultimate pressure (see above) p Pressure in the vacuum chamber... 

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