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Reception and ultra high vacuum maintenance

 

Reception and ultra high vacuum maintenance

     It is well known that the most popular ultrahigh-vacuum pumps used to reach clean oil-free vacuum down to 1 . 10 -11 torr both in industrial process systems and research are turbomolecular pumps with a magnetic suspension, sublimation, cryogenic, getter (with either sputtered or unsputtered getters) pumps.

     A major disadvantage of the above pumps is that their ultimate low pressure is restricted by the physical limit of hydrogen evacuation speed, which approaches zero already at a hydrogen partial pressure of 10-10 - 10-11 torr. The residual gas spectrum at a limiting pressure for those pumps shows 95 - 99% of molecular and atomic hydrogen.

     The only pumps which do not suffer from the ultimate vacuum - related physical limitations and provide partial pressures below 10-13 torr for all gases including hydrogen and helium are condensation, condensation-sorption and sorption pumps cooled by liquid helium at 4.2 K to 2.2 K and below.

     The most simple in design, reliable in operation and relatively cheap pumps are so-called priming helium cryopumps. If a certain metallic vessel originally filled with atmospheric air is evacuated with any ejection pump (forevacuum mechanical or turbo molecular) down to 10-2 - 10-5 torr and then is fully immersed into liquid helium, inside the vessel there will be established a pressure of 10-8 - 10-11 torr, respectively, that will be mainly defined by partial pressures of residual helium and hydrogen whose content in air is about 5.10-4 and 5.10-5 vol.%.

     The remaining air components will condense on the interior surface of the vessel and will have very low partial pressures (see Table 1).

     If, however, this vessel is preliminarily filled with, for example, pure nitrogen at atmospheric or lower pressure and after that immersed in liquid helium, then the pressure in the vessel will be reduced to a much lower value. In fact, calculations show that the equilibrium pressure of the nitrogen condensate at 4.2 K turns to be below 10-70 torr.

     If a cryopanel, e.g., in the form of a copper vessel filled with liquid helium, is placed inside a vacuum chamber, it will constitute a prototype of the priming helium cryopump, and its exterior surface will serve as an evacuating element. Here the pumping velocity per cm2 of its surface for nitrogen will be 11.6 ls.

     Already for three decades a number of researchers including us have been involved in the design, investigation, optimization, development and operating of priming helium cryopumps [1-4]. Considerable experience has been already gained in using the cryopumps in electron technology, solid-state physics, space simulation, thermonuclear fusion and other fields.

     The condensation-sorption priming helium cryopumps are found to be the most efficient by many parameters [5].

     The cryopumps of this type produced by the world-known companies [6, 7] could not find wide application for a long time because of the high operating cost due to high evaporability of liquid helium. The continuous service time before helium refilling of the vessel-cryopanel in those cryopumps was only several days or weeks.

     A number of new designs, technological and methodological decisions have been derived and realized in several lines of priming condensation-sorption helium cryopumps with helium evaporability reduced 5-10 times [4,5].

     The continuous service time before refilling was increased to 2-4 months. The cost of operation was decreased several times and the efficiency turned to be even higher than that of any other type of ultrahigh-vacuum oil-free pumps.

     Fig.1 presents a general view drawing of one of such condensation-sorption cryopumps cooled by liquid helium.

     The pump consists of a housing (1) incorporating a vessel (2) filled with liquid nitrogen in the operating mode through a pipe (3). Below vessel (2) there is a heat screen (4), whose lower part contacts an optically dense chevron screen (5). In the space bounded by the bottom of vessel (2), screens (4) and (5) there is a vessel (6) filled with liquid helium in the operating mode. The vessel (6) is suspended by a tube-suspension (7) made of a thin-wall corrugated tube with helical profile corrugations. This tube also serves to fill vessel (6) with liquid helium and as a helium vapor outlet.

     In the space between the wall of housing (1) and the surface of vessel (2) and screen (4) is located a so-called "floating" thin-wall screen (8).

     In the lower part of the pump there is a flange NW 400 and a reducer (10) with a NW 250 Conflat flange (11).

     Any of these flanges may be used to connect the pump with the evacuated chamber either directly y or through a lock valve.

     All the surfaces of chevron screen (5) are coated with chromium and titanium by plasma spraying and have a blackness of about 0.99 at 78K.

     The surfaces of parts 1,2,4,6,8,10 on the vacuum side are coated with an aluminum film 1-3 µm thick using special technology. This film ensures substantially lower blackness of heat exchange surfaces for these parts as compared with even better polished and baked-out materials such as copper and aluminum. In fact, the surface blackness of any materials coated with the above mentioned film at 300K, 78K and 4.2K is ε 300=0.015, ε 78=0.010 and  ε4.2=0.002, respectively (see Table II).

     These coatings have substantially reduced vaporability of liquid helium and nitrogen as compared to cryopumps without surface coating [8].

     The pump shown in Fig.1 is usually started-up as follows. Upon its connection to the chamber, the space being evacuated is pumped down to a fore-vacuum pressure of 10-4 torr max by either a sorption pump cooled with liquid nitrogen or some other type oil-free pump. Then vessel (2) is filled with liquid nitrogen through one of the pipes (3), which usually leads to an order of magnitude reduction of pressure in the vacuum space of the pump and inside the chamber.

     After that vessel (6) can be filled with liquid helium, but to save liquid helium it is first cooled with liquid nitrogen down to 90-100K.

     Next, condensation of nitrogen or argon at the bottom of vessel (6) filled with liquid nitrogen is effected by leaking them in at (1-4) . 10-4 torr for 20 - 40 min or longer. The condensate thus formed is 103 - 105 monolayers thick and has an optimal fine-pored structure of the gas cryosorbate with pore sizes ranging from several tens to several tenths of nanometer. Such cryosorbate efficiently sorbs hydrogen molecules and helium atoms. The equilibrium pressures of these adsorbed gases may be below 10-12 - 10-13 torr if the adsorbed hydrogen-to-condensed adsorbent amount ratio is no less than 1: (101 - 102) and 1: (102 - 103), respectively  [1]. As mentioned above, the partial pressure of such gas cryosorbate itself at T=4.2K will be below 10-70 torr. The sorption capacity of this cryosorbate at T=4.2K is comparable with that of conventional solid sorbents such as active carbon or zeolite. The essential advantage of condensed gas cryosorbents, especially argon, is that the condensate 105-106 monolayers thick remains transparent for infrared radiation coming from the chevron screen [9-10].

Therefore the evaporability of liquid helium from vessel (6) after argon condensation increases little, while the conventional solid sorbents usually used on cryopanels with T=4.2K in sorption helium pumps exhibit a blackness of ε 4.2=0.5 even at a minimum thickness of several tens or hundreds μm. This makes the use of sorption pumps cooled by liquid helium non-cost-effective.

     The Fig.2 and 3 show the constructions condensation-sorption cryopumps cooled by liquid helium with top connective flange (Fig.2) and pump of scope-type (Fig.3).

     Thus, the condensation-sorption cryopumps cooled by liquid helium may provide ultimate pressures substantially lower than 10-12-10-13 torr, maintaining nearly constant pumping speed almost for all gases. However the particular pressure in the working chamber of ultrahigh vacuum bake able systems is known to be set up when the speed of gas release from the walls of the chamber and working parts inside it becomes equal to that of a given pump.

     To attain the Co west possible working pressure in ultrahigh - vacuum chambers, they are usually baked -out at about 350oC and over for several tens of hours. This will lead to a 3-5 orders of magnitude decrease in gas desorption from the chamber walls. At the same time this will release earlier dissolved and chemisorbed atomic hydrogen from the bulk of the stainless steel chamber walls, which later diffuses into the evacuated space even at room temperature.

     The hydrogen atoms that have escaped from the bulk of stainless steel and migrated into the vacuum space form micro channels through which the hydrogen atoms (from the atmospheric air as well) can then diffuse. As mentioned above, at reaching an ultimate vacuum of 10-10- 10-11 torr, the residual gas spectrum shows 95-99% content of atomic and molecular hydrogen even if the pump with an ultimate pressure for hydrogen below 10-11 torr is used.

     To substantially reduce gas desorption, the amount of hydrogen penetrating from the stainless steel chamber walls into the evacuated space, the aluminum coatings similar to those applied onto the heat exchange surfaces of cryopumps were used.

     A series of investigations and experiments have shown that the films of aluminum as well as of copper, gold, silver and other metals in a flow of very clean helium at (1-5) . 10-2 torr produced from a liquid phase form a coarse-crystalline phase with very low content of structural defects [11]. At a film thickness of about 1-3 µm the size of some tightly knitted crystals, e.g., of aluminum, reaches 3 to 30 µm. The defect area relative to the surface of crystalline features was 10-2 - 10-4%. So the number of adsorption centers, i.e., unsaturated Van-der-Waals forces on such film turns to be 4-6 orders of magnitude less than at the stainless steel surface without the above mentioned coating.

     Besides, a thin Al2O3 film about a few nanometers thick formed on the aluminum coating is found to possess a potential barrier too high for hydrogen to migrate from the bulk of the stainless steel chamber walls to their surface and penetrate into the vacuum space.

     These factors have allowed for reducing the total gas release from the chamber walls 104-105 times and also for attaining pressures of 10-11-10-13 torr and lower with cryopumps for coated chambers without bake-out even if they were exposed to air for a long time.

     Fig. 5a shows a sectional view of the stainless steel surface with a roughness of 2-3 µm typical for the interior surface of most ultrahigh-vacuum chambers. Such surfaces usually adsorb several hundreds or thousands of monolayers of different gases, especially of water and high-molecular hydrocarbons. Fig.5b shows the same surface but polished to a roughness of 0.2-0.3 µm. Such a surface adsorbs already 1-2 orders of magnitude less gas than that shown in Fig.5a, because the number of defects in the subsurface structure of stainless steel in the latter case is reduced 1-2 orders of magnitude.

     The stainless steel surface shown in Fig.5c is polished to a roughness of 0.2-0.3µm and, besides, is coated by an aluminum film deposited by the techniques described above. The film thickness is about 1 µm and the size of individual tightly knitted single-crystal reaches about 3 µm. The adsorption centers are now small-area defects, the boundaries of crystal knitting. The amount of gases adsorbed on the surface of such film is found to be 3-4 orders of magnitude less than that on the surface shown in Fig. 2b and 4-6 orders of magnitude less than on the surface of Fig.5a.

     The quality of aluminum or any other metal films obtained in conventional unbakeable    high-vacuum deposition units is worse than that achieved by deposition in a helium flow. The point is that under a vacuum of, for instance, (1-2). 10-6 torr the statistical number of gas molecules hitting the substrate surface every 2 seconds is such that, with the adhesion coefficient close to 1, this would yield a (1-2) monomolecular layer. When the deposited metal is evaporated  from an evaporator (for aluminum, we usually use helices made of lanthanide or thoriated tungsten wire), 1µm (~3.103 monolayers) films are deposited in 5 minutes, i.e., at a rate of ~ 10 monolayers per second. So every second atoms of the evaporated metal can burry in the substrate surface active atoms (molecules of H2O, CO, CO2, O2, hydrocarbons and others) in a proportion of 20: (1-2). This means that after vacuum deposition a metallic film may contain about (5-10) % and more impurities i.e., chemi - and physically adsorbed gases which produce a rather great number of structural defects and micropores. This will significantly deteriorate  optical properties of high electrictric conductance metal films and increase their adsorption capacitance. For deposition in a flow of very clean helium at the above-mentioned pressures free path lengths of helium atoms are found to be of about several millimeters to several centimeters. At such helium atom flow density active gas molecules desorbing from the vacuum chamber walls in the deposition unit are actually do not reach the space between the evaporator and substrate. The partial pressures of active gases in this zone prove to be below

10-9 _  10-10 torr. Helium atoms, due to their high mobility, very low adsorption heat and small size do not impede condensation of evaporated metal atoms into a high-quality coarse-crystalline film with a minimum number of structural defects.

     The optical properties of such film, its reflectivity in the infrared region in particular, at low temperatures depend on the quality of its subsurface layer (see Fig.3a). Hitting a free electron in a skin-layer, a photon excites it and gives its energy to it. The excited electron lifetime in metal at the liquid helium temperature is estimated to be about 10-7 s. During this time, moving with a velocity of about 30 m/s, the electron travels a distance of about 3 µm. If the electron does not encounter any obstacle ( boundary of a grain, pore or any other structural defect) in this path portion, then upon transition from the excited to the normal state the electron will give away the energy received from the photon in the form of the photon (see Fig.3b), i.e., the total mirror reflection of infrared radiation will take place. If, however, the excited electron encounters such an obstacle in its path, it will give over its energy to the crystal lattice of the film, and this energy will be absorbed by the lattice in the form of a phonon. Thus, the fewer structural defects and the larger individual single-crystals will be in the film, the more the total reflection effect will predominate over the absorption effect.

        Fig.4 presents a diagram of the deposition unit used for metal film deposition including that in a flow of helium evaporated from the liquid phase.

     The unit comprises a cylindrical chamber 1 m in diameter, 1.5 m in height, which can be lifted or lowered by means of a hoist and sealed at the plate 2. Below it there is a NW 250 slide gate (3) at the bottom of which a nitrogen trap (4) is attached, which is joined to a turbomolecular pump (5), whose exhaust is connected with a mechanical fore-vacuum pump (7) via a valve (6).

     In the upper part of the bell (1) there is a leak (8) through which evaporated helium is delivered to the vacuum chamber via a pipe (9) immersed into a Dewar flask (10) with liquid helium. At the bottom of bell 1 there is a valve (11) to admit dry gaseous nitrogen inside the bell until the atmospheric pressure is reached before the bell can be lifted after completing the deposition process.

     Plate 2 has a valve (12) attached to it for preliminary bypass evacuation of the space under the bell from the atmospheric pressure down to 1 torr via manifold 13 by fore-pump 7.

     At the top of the bell there is a rotation assembly (14) from which the piece to be coated (15) can be suspended. It may be subjected to surface heating with a number of quartz halogen lamps (16) located under the bell. For metal evaporation a set of evaporators - tungsten helices (17) is employed. To solve the problem of reliable measurement of pressures below 10-11 torr we used transducers of inverse-magnetron type with a cold cathode and a direct current amplifier capable of measuring currents down to 10-15 A.

     The hot cathode transducers of ionization type are unsuitable here. Thus, to summarize, we can conclude that priming condensation - sorption cryopumps cooled by liquid helium of the design shown above have demonstrated their efficiency in applications aimed at solving a number of scientific, technological and industrial problems. Some types of such pumps work at universities of Germany, in the electron accelerator in Japan, at industrial plants of Russia.

     Combination of aluminum film deposition in a helium flow onto the interior walls of working chambers with cryopumps allows one to reach ultimate pressures down to 10-13 torr and below.

     The photographs of some versions of helium priming condensation-sorption cryopumps and ultrahigh vacuum units evacuated by them are given in the appendix (Fig. 9-15). The units are made of stainless steel and titanium and their interior surfaces are coated with aluminum films deposited in a helium flow using the techniques outlined above.

Reception and ultra high vacuum maintenance Saint-Petersburg
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