CHAPTER 21

Ultraclean Vacuum Systems

Treatises on vacuum technology traditionally include a section entitled “ultrahigh vacuum.” For those of us whose interest in vacuum dates to soon after the middle of the last century, that phrase connoted specific construction techniques, components, materials, and procedures, including a 450°C overnight bake. The goal was the achievement of an ultrahigh vacuum environment in which fundamental studies could be performed. Today, such pressures are required for myriad applications ranging from high-energy particle research to the manufacture of magnetic thin-film read-write heads, semiconductor, and opto-electronic devices. Systems have grown in size and complexity. In parallel, technical restrictions have limited the time available to reach ultimate pressure as well as the maximum allowed baking temperatures. Some systems can be baked only to 75-1 50”C, whereas others, by virtue of their size, cannot be baked at all. All these systems require base pressures in the low 1 0-8- to 1 O-9-Pa (1 0-l’- to lO-”-Torr) range. Base pressure requirements will continue to decrease to meet advanced processing needs. Additionally, contamination caused by environmental or process-generated particles as small as 10 nm may affect product yield or the ability to do research.

The term “ultrahigh vacuum” is no longer adequate to describe the requirements demanded by the highest technology applications. Not only must the process or experimental vessel reach the classical ultrahigh vacuum pressure range, but also it be ultraclean, and it must allow translational and rotational motion, electrical power, cooling fluids, and electrical and optical instrumentation signals through its walls without adding contamination. Metrology is not simple. UHV pressure gauges and residual gas analyzers may provide incorrect information, should they be mounted or operated incorrectly; misunderstanding of electron-stimulated desorption can result in inaccurate pressure measurements. The need for distributed pumping to evacuate extremely long, narrow accelerator or beam chambers is well understood; however, the designers of complex production systems with shielded interiors do not yet appreciate that they have a similar problem.

403

A User’s Guide to Vacuum Technology, 3rd Edition. John F. O’Hanlon Copyright 0 2003 John Wiley & Sons, Inc.

ISBN: 0-471-27052-0

404 ULTRACLEAN VACUUM SYSTEMS

There are many sources and types of contamination in any vacuum system. Gases can desorb from the chamber walls or evolve from a pump. We give here a modified version of a view first proposed by Santeler [l]. Figure 21.1 shows a generalized vacuum system. The total pressure in the system from vacuum chamber sources is composed of the partial pressure of each gas or vapor desorbing from the walls; it is equal to the rate of desorption of that species divided by its pumping speed.

P = C - Ql

i SI b (21 .l)

Gases that back diffuse from the pump will contribute to the chamber background pressure

P = C - Qpi

i si The total system pressure results from all sources. It is given by

(21.2)

(21.3)

Santeler’s argument states that the first term behaves normally (e.g., as the pumping speed increases, the pressure contribution decreases), but the second term does not behave in a like manner. For example, more hydrogein would backstream from a larger sputter-ion pump. If the backstreamed gas load and the pumping speed were increased in proportion, the base pressure would not change. The result would be an effectively constant base pressure Po, for a given pump. By this argument (2 1.3) may be simplified to read

P = C - + P o Qi

i Si b (2 1.4)

The production of ultraclean vacuum is therefore concerned with both the pump and chamber. The contamination from the chamber may be reduced by the intelligent choice of materials, fabrication techniques, and operating procedures; the gas load originating in the pump may be reduced by pump design and selection, as well as processing procedures.

This chapter discusses the roles of pumps (turbo, ion, TSP, NEG, and cryo) and chambers (design, materials, and operation) in achieving ultraclean vacuum. The chamber becomes important, because it is more effective to reduce the chamber sources than to increase S beyond certain limits. Pump choice is important, because increasing S will not decrease Po. Purnps are selected by matching performance with application.

21.1 ULTRACLEAN PUMPS 405

Vacuum Cham be r

Gas Source(s) Q From Chamber

Walls Gas Source(s) Q, From Pump

Pump

Fig. 21.1 High vacuum pumping schematic.

21.1 ULTRACLEAN PUMPS

Selection of the pumps for an ultraclean vacuum system is important. Equation (2 1.4) indicates that a clean vacuum pump should reduce pressure by providing high pumping speed (low Q/S) and low backstreamning (low Po). In addition, the vacuum should be free of particulate contamination. Here backstreaming is defined as the reverse flow of any gas, vapor, or material from the pump. To reduce the contaminant levels, rigorous adherence to exacting pumping procedures is necessary. A refrigerated trap that has been warmed for only a few minutes will release condensed vapors. Although recooling will prevent further contamination, it will take time to repump the vapors that have accidentally been released, especially those that were marginally pumped on cry0 surfaces.

Each pump type has its own characteristic class of gases and vapors that it will not pump and that it will generate as impurities. Each pump has its own characteristic particle source. The design and selection of pumps for ultraclean applications is done on an individual basis. There is more than one “best” solution. The remainder of this section reviews the selection and clean operation of some ultraclean configurations.

21.1.1 Turbomolecular Pumps

Turbo pumps can contribute two contaminants to the system; hydrogen and pump fluid vapors. The turbo-pump-generated hydrogen is primarily a result of the pump’s maximum compression ratio and the hydrogen partial pressure in the foreline. Turbo pumps are made with hydrogen compression ratios of 5000-15,000. Putting two turbo pumps in series can further increase the compression ratio for hydrogen. Magnetically levitated turbo pumps are required to eliminate vapors present in turbo pumps using oil-lubricated bearings. The drag stages attached to some pumps provide an

406 ULTRACLEAN VACUUM SYSTEMS

additional way of reducing the hydrogen compression limit. Oil-free scroll or four-stage diaphragm pumps can be used to back turbo-drag pumps. The ultimate pressure may vary within pumps of the same design, because of minute leaks between the stator and housing. The variation in ring-to- wall spacing could be large enough to cause variations in the ultimate pressure of the pump.

Turbomolecular pumps have a large internal surface area and consequently they must be baked, usually into an adjacent turbo pump. Unfortunately the construction of the pump is such that it is not possible to bake it much over 100°C. This bake cycle is necessary to remove the water from the interior of the stator and rotor surfaces. The roughing cycle can contaminate a turbomolecular pumped system just the same as any other system. For this reason, oil-fiee roughing pumps are required.

21.1.2 Cryogenic Pumps

Helium gas refiigerator pumps are able to reach the ultrahigh vacuum region required for rapid-cycle industrial processing systems. They are often augmented by large area cryo-cooled arrays designed only for pumping water vapor. Hydrogen adsorption isotherms show the greatest capacity at low temperatures. A most important consideration in reaching very low pressures is the isolation of thermal and optical radiation from the cold stage. Baffles have been designed to maximize molecular transmission and minimize photon transmission [2,3]. Gas-refrigerator- cooled pumps cannot be baked above 70-100°C because they use indium gaskets. Turbo pumps are used during the baking cycle. Cryogenic pumps easily pump large amounts of hydrogen, provided that the cold-stage temperature remains at -20 K. They use no high voltages and generate no hydrocarbons or metal flakes. However, the displacer generates some vibration and the charcoal used on the cold stage sheds dust that can be transported to the chamber. Iwasa and Ito [4] developed a helium gas refrigerator pump using an Er3Ni heat exchanger with which they were able to reach 3.6 K. Thus, they were able to produce a cry0 pump with no dust producing charcoal. Cry0 pumps must be careklly damped for use with sensitive surface-analysis equipment such as ESCA or SIMS. The Viton-sealed overpressure relief used in ordinary cry0 pumps should be replaced by its single-use, stainless steel foil, knife-relief counterpart.

21.1.3 Sputter-Ion, TSP, and NEG Pumps

Long particle beam lines require distributed pumping. Distributed pumping systems have been designed using continuous strip NEG pumps, in combination with sputter-ion pumps [ 5 ] . The distributed NEG pumps

21.1 ULTRACLEAN PUMPS 407

remove all except the noble gases, which are pumped by the sputter-ion pumps. Sputter-ion pumps require baking; they can be baked into the turbo pumps used for system roughing. The main background gas present in a sputter-ion pumped system at low pressures is hydrogen, but small amounts of methane ethane and carbon monoxide are observed.

When used in combination with helium gas cry0 pumps, NEG pumps were shown to improve pumping speed of an unbaked chamber and reduce the base pressure of a baked system [6]. NEG pumps have also been used in combination with turbo pumps to produce enhanced pumping of hydrogen and water vapor [7]. Benvenuti et al. [S] have demonstrated that hydrogen, the dominant gas remaining at base pressure, could be pumped with higher speeds by depositing thin-film coatings of nonevaporable getter material on the system walls. Although distributed pumps are necessary to maintain high hydrogen pumping speed at low pressures, TSP have been studied [9,10]. Hydrogen pumping in TSPs is improved if the surfaces are liquid nitrogen-cooled rather than water-cooled. Odaka and Ueda [l 11 have demonstrated that a 1000°C bake removed large quantities of gaseous impurities from within Ti-Mo filaments. The water vapor load in an ultraclean system is much smaller than it is in an unbaked rapid-cycle system; therefore the sublimed titanium film will not entrap enough water to make titanium flaking a problem. Momose, Saeki, and Ishimaru [12] identified Ti0 particles generated by ion pumps; they that postulated these particles were transported from insulating surfaces within the pump to the chamber by the nitrogen purge gas.

21.2 ULTRACLEAN CHAMBERS

21.2.1 Chambers Materials and Components

AISI grades 304L and 316L are the most frequently used in constructing ultraclean chambers. Stabilized grades such as 321 or 347 contain additives to reduce carbide precipitation; they are acceptable but are expensive. All joints are made by TIG welding or metal gasket flanges; O-rings are not used anywhere in the high vacuum portion of the system, including the high vacuum connection to the pump or trap. In some systems, O-rings are used for internal pass-through valves. After the initial air outdiffuses from these O-rings, they will no longer be a source of residual air contamination. They do shed particles in vast numbers, and they do release process gases from gasket compression during valve closure. See Problem 17.9.

Aluminum, processed by special extrusion or by the EX process, outgasses at the same rate as stainless steel. Ordinary aluminum is covered with a porous, hydrated aluminum oxide over 10 nm thick. This layer

408 ULTRACLEAN VACUUM SYSTEMS

becomes a source for water vapor and therefore is difficult to outgas. The special extrusion process results in a dense aluminum oxide layer of approximately 3-nm thickness; it is achieved by extruding the aluminum in an atmosphere of argon and oxygen [13]. The surface finish on large aluminum components is realized by lathe machining in a vacuum chamber filled with argon and oxygen [14,15]; the resulting oxide is identical in thickness and density to that made by special extrusion. Outgassing rates of lo-" Pa-m/s (

High-density alumina is a stable insulator, and it is stronger than quartz or machinable glass ceramic. The properties of this and other materials are discussed in Chapter 16. Metal-to-glass or metal-to-ceramic seals should not be subjected to temperatures higher than 400"C, and sealed copper- gasket flanges should not be baked at temperatures higher than 450°C. Unsealed flanges, however, will tolerate higher temperatures. The flange and knife-edge will suffer some loss of temper during an 800-950°C bake. The exact amount depends on the grade of stainless steels (grain growth retarders)t and fabrication steps used in flange construction. Most commercial copper-gasket flanges are forged fiom grade 304, and some contain trace additives that will retard grain growth during heat treatment. Small grain size is necessary to prevent significant loss of hardness. Not all flanges are forged in the same manner or have the same properties.

It is not recommended that parts be stored in plastic boxes or bags. Polymer storage bags contain plasticizers that readily outdiffuse; see, for example, Problem 16.10. Sulfur fiom rubber bands will diffuse quickly through the storage bags [17]. Aluminum foil is frequently used to cover open flange faces or to wrap cleaned parts until final assembly. Aluminum foil that has been specifically degreased for vacuum use will not contaminate the cleaned parts. Ordinary household aluminum foil will contaminate the parts with residues of the vegetable-based rolling oil [ 181. Precleaned aluminum foil is commercially available.

Internal fixtures held together with screws will have gas trapped between flat surfaces and within and under screw threads. Slotted screws, which allow trapped gases to be vented, should be used within ultraclean chambers. Gold-plated screws, used to secure copper gasket sealed flanges, will prevent galling of threads. Thread galling is a problem with systems that are fiequently baked at high temperatures.

Connecting electrical power, instrumentation wiring, process gases and liquids, and cooling fluids through an ultrahigh vacuum wall is not trivial. There are few techniques for accomplishing these objectives. If one requires only linear motion the welded bellows, described in Fig. 17 .23~ allows motion while maintaining vacuum integrity with a solid metal wall. If rotary motion is required, there are two choices. If no fluids or power are required, the magnetic follower described in Fig. 17.2 1 is the best solution.

Torr-L/s) are achieved with these treatments [ 161.

21.2 ULTRACLEAN CHAMBERS 409

A solid stainless steel interface separates vacuum from atmosphere. If cooling fluids, instrumentation, and electrical power must be provided to a rotating substrate holder, then the differentially pumped rotary motion feedthrough, illustrated in Fig. 17.19, is the only solution. Seals of the ferrofluidic type contain organic liquids that allow air permeation, and thus are not acceptable in the lowest pressure ranges of interest.

Personnel with cleanroom training must construct ultraclean chambers within clean facilities using clean assembly techniques. After pretreatment and fabrication, components, wire, hardware, subassemblies, and all assembly tools are cleaned with a 1:7 mixture of isopropyl alcohol and ultrapure water, before being moved to the assembly cleanroom. Clean tools remain within the assembly area. After assembly and testing is complete, systems are double-wrapped for customer shipment.

21.2.2 Chamber Pumping

The generic pressure-time behavior during pumping is sketched in Fig. 4.10. The volume gas is removed first, followed by surface desorption, outdiffusion from the solid, and, last, permeation through the solid wall. All of these processes except volume gas removal are greatly temperature- dependent. The dominant gas desorbing from the interior surfaces in this portion of the pumping cycle is water vapor. Water vapor will bounce, re- adsorb, and bounce again during the process of being pumped. Eventually, the molecule will reach a pump entrance, where it has a probability of being pumped. Equation (4.16) estimates the residence time of a molecule. This relation predicts that the residence time is dependent on the ratio of chamber wall area to pump entrance area. If the pump entrance area is small (low pumping speed) compared to the total internal area of the system, the residence time will be long. Figures 16.2 and 16.3 show the time dependence of the outgassing rate as the chamber is pumped from atmosphere. The rate is practically independent of surface treatment, because water vapor is the dominant surface contaminant. Multiplying the vertical axis of either of these figures by the internal surface area and dividing by the pumping speed will yield a typical pumping time. Systems are designed to pump from atmosphere to 10-7-10-8 Pa in a specified time, usually in the range 24-48 h.

Designers of particle-beam accelerators learned early that distributed pumping is needed to reach suitably low pressures. Locating a pump at one end of a long, low-conductance tubular chamber would not provide adequate pumping for the entire duct. For that reason these systems are constructed in the form of continuously interconnected extruded parallel tubes of cylindrical or oval cross section. Typically, NEG pumping is provided continuously, with intermittently located ion pumps.

410 ULTRACLEAN VACUUM SYSTEMS

t

24 22 19 28

27 26 11 12

11

13 15 21 I

Designers of complex processing chambers with many baffles or internal shields should consider the same concept in attempting to reach low pressures quickly. Consider the diagram in Fig. 21.2. Here we have drawn a generic chamber of rectangular cross section containing two pumps and two internal surfaces. These internal surfaces were added to simulate pumping a complex chamber. The numbers located around the interior were generated by the Monte Car10 technique and represent the number of bounces required by a molecule to reach a pump starting fiom that location. The number of bounces is dependent on the geometry of the chamber. If one were to install larger pumps at the existing locations, the number of bounces, and hence the time to remove molecules fiom the distant portion of the chamber, would be altered only marginally. Only by installing an additional pump or two in the upper chamber, can one reduce the pumping time for molecules originating in the upper chamber. Distributed pumping is required to improve the ultimate pressure and time required to reach the base pressure of processing systems containing complex and conductance-limited pumping paths.

Surface preparation methods, treatments and procedures used for reaching pressures in the Pa (lo-”) Torr region have undergone major changes since ultrahigh vacuum pressures were routinely attainable. The original procedures for Diversey cleaning chambers fabricated fiom 304 stainless steel, followed by a 450°C bake during each pumping cycle, are no longer used. Baking 304L or 316L stainless steel at 800°C in a vacuum hrnace is one common prefabrication treatment. Prior to this heat treatment, any surfaces subjected to grinding or similar procedures would have been machined to remove surface damage. After welding and assembly, a number of treatments may be used. One initial treatment involves baking the chamber at 200°C so as to produce an oxide layer on the system interior. After this treatment, the system may or not be baked. Some very large systems cannot be baked at all. In these systems, various

Fig. 21.2 Rectangular chamber containing arbitrary baffles and pumps. The numbers on the interior surface represent the number of bounces required by a molecule originating at that location before it reached a pump entrance.

REFERENCES 411

plasma discharge treatments such as those discussed in Chapter 16 are used to remove adsorbed gases and reduce the ultimate pressure. A chamber bake in the range 75-150°C is a part of the 24- to 48-hour pumping cycle in systems that can be baked.

Unbaked areas of the system can reduce system performance. The area of the system that cannot be baked depends on the pump and the efficiency with which interior surfaces can be baked. An ion pump can be completely baked. A turbomolecular pump and a gas refrigerator cryogenic pump can be baked at approximately 100°C. With the exception of the ion pump, all have some surface that cannot be subjected to a bake at a temperature in excess of 100°C. In some cases, interior surfaces are not completely baked. Consider again the illustration of Fig. 21.2. If the surfaces inside the chamber-shown as a double line-could not be baked, then the residence time of molecules that stick would be significantly longer than the residence time of molecules on the heated surface. Monte Car10 analyses of several chamber configurations show that molecules bounce fiom heated and unheated surfaces in roughly the proportion of the heated-to-unheated surface area ratio. Since residence time is exponentially reduced with increasing temperature, complete baking of interior surfaces is desirable. UV lamps have been used to assist in this goal. However, they are most useful in applications where large line-of-sight areas can be illuminated.

It was commonly thought that CO and HZ were the predominant residual gases found in all ultrahigh vacuum systems. The data presented in Fig. 5.15 confirm that CO is an artifact of the hot filament gauge. Hydrogen is the dominant gas remaining at ultrahigh vacuum pressures. The source of this remaining hydrogen outgassing has been the subject of considerable research. Hydrogen is soluble in many metals. Bills [19] noted that there was insufficient atmospheric hydrogen to account for permeation through the bulk of a stainless steel wall, but that other sources, such as the dissociation of water on the chamber exterior, could provide an adequate diffusion source. Redhead [20] has reviewed the status of hydrogen outgassing in ultrahigh vacuum region. In Section 16.3.1 we discussed the diffusion model [21], which assumed hydrogen of one activation energy permeated by a diffusion limited step with zero concentration at the vacuum interface. As Redhead noted, this predicted high outgassing rates well, but did not predict low levels of outgassing. Recently, Moore [22] applied an older recombination model that showed hydrogen atoms must recombine at the surface before desorption. Thus, the concentration of H at the vacuum surface cannot be zero. For low surface coverage the atoms must travel a distance before recombining and recombination becomes the limiting step. Moore solved this problem numerically by matching the diffusive flow of H fiom the near-surface concentration gradient with the

412 ULTRACLEAN VACUUM SYSTEMS

observed outgassing rates by adjusting the surface recombination coefficient. This concept was used to model hydrogen outgassing fkom the after-shot dose in a fusion device [23].

A portion of the hydrogen dissolved in stainless steel is trapped at sites such as precipitates and in the Cr-rich phase of stainless steel [24]. These traps have activation energies ranging fkom 73-1 80 MJkg-mol. Redhead notes that baking at temperatures near 800°C depopulates a large number of hydrogen atoms trapped in these sites, some of which then occupy sites of lower activation energy. For this reason, baking only at a low temperature, say 150"C, may yield lower outgassing rates than by first vacuum firing at 800°C. Additionally, oxygen plays a role in reducing hydrogen outgassing rates. An earlier study [25,26] had shown that the outgassing rate of stainless steel decreased with increasing concentration of Cr in the oxide, but was unchanged with increasing Fe content of the oxide. Thus the oxide can become a hydrogen surface trap. These studies shed light on why the outgassing rate of stainless varies widely with surface treatments such as electropolishing and oxidation.

An experimental technique for reducing hydrogen outgassing makes use of a coating applied to the exterior surface of stainless steel [27]. A 5 x reduction in outgassing was achieved, which reportedly resulted fiom the reduction of the dissociation of water on the exterior stainless steel surface. If this conclusion is verified, it would imply that hydrogen outgassing does not result solely fiom H within traps, but partly fiom interstitial permeation. Interstitial H has an activation energy of 56 MJkg-mole [25].

21.2.3 Pressure Measurement

The choice of gauge, where to mount it, and how to care for it during baking are relevant issues in ultraclean systems. No gauge tube should be mounted on the base of a system with its entrance facing upward, lest it become a sink for debris. The gauge tube should be kept warmer than the system during cooling after a bake cycle. Gauges and RGA heads should be the last items cooled otherwise they become a small capture pump.

Perhaps the most critical issue is the choice of gauge for measuring extremely low pressures. Hot filament gauges should be operated with emission currents of 10 mA so as to minimize ESD. As we noted in Section 5.2.3, few commercial gauges are available for operation in this region. One can choose between the extractor gauge and the cold cathode gauge. The number of commercially available ultrahigh vacuum gauges is not sufficient to meet future needs. Advanced gauges and RGAs, incorporating the designs of Watanabe [28] for reducing ESD and electrode outgassing will be required.

REFERENCES 413

REFERENCES

1. D. J. Santeler, J. Vac. Sci. Technol., 8,299 (1971). 2. C. Benvenuti and D. Blechschmidt, Jpn. J. AppZ. Phys. Suppl. 2, Pt. 1, 77 (1974). 3. H. J. Halama, and J. R. Aggus, J. Vac. Sci. TechnoZ., 12, 532 (1975). 4. Y. Iwasa and S. Ito, Vacuum, 47,675 (1 996). 5. C. Benvenuti, Nucl. Instr. Meth., 205,391 (1983). 6. R. Giannantonio, M. Succi, and C. Solcia, J. Vac. Sci. Technol. A, 15, 187, (1997). 7. A. Porn, C. Boffito and F. Mazza, Vacuum, 47, 783 (1996). 8. C. Benvenuti, J. M. Cazeneuve, F. Cicoira, A. Santana, V. Johanek, V. Ruzinov, and J.

Fraxedas, Vacuum, 53,219 (1 999). 9. C. Benvenuti and C. Hauer, Le Vide, Suppl. 2,201, 199 (1980). 10. V. Rao, Vacuum, 44,519 (1993). 11. K. Odaka and S. Ueda, Vacuum, 44,713 (1993). 12. T. Momose, H Saeki, and H. Ishimaru, Vacuum, 43, 189 (1992). 13. H. Ishimaru, J. Vac. Sci. Technol. A, 2, 11 70 (1 984). 14. J. R. Chen, K. Narushima, M. Miyamoto, and H. Ishimaru, J. Vac. Sci. Technol. A, 3,

15. M. Miyamoto, Y. Sumi, S. Komaki, K. Narushima, and H. Ishimam, J. Vac. Sci.

16. H. Ishimaru, J. Vac. Sci. TechnoZ. A , 7,2439 (1989). 17. T. Sigmond, Vacuum, 25,239 (1975). 18. V. Rao,J. Vac. Sci. Technol. A, 11, 1714 (1993). 19. D. G. Bills, J. Vac. Sci. Technol., 6, 166 (1969). 20. P. A. Redhead, Intl. Workshop on Hydrogen in Mat’s. Vac. as., G. R. Myneni and S.

21. R. Calder and G. Lewin, Br. J. Appl. Phys., 18, 1459 (1967) 22. B. C. Moore, J. Vac. Sci. Technol. A , 13,545 (1995). 23. K. Akaishi, M. Nakasuga, and Y. Funato, J. Vac. Sci. Technol. A , 20,848 (2002). 24. T. Yoshimura and Y. Ishikawa, J. Vac. Sci. Technol. A, 20, 1450 (2002). 25. Y. Ishikawa, T. Yoshimura, and M. Arai, Vacuum, 47,701 (1996). 26. K. Okuda and S. Ueda, Vacuum, 47,689 (1996). 27. C. Dong P. Mehrotra, and G. R Myneni, Intl. Worhhop on EIydogen in M a . Vuc. Sys., G. R

Myneni and S. Chattopadhyay, Ids., AIP Cod. Proc. 671, AIP, Melville, NY, 2003. p. 307. 28. F. Watanabe, J. Vac. Sci. Technol. A, 20, 1222 (2002).

2200 (1985).

TechnoZ. A, 4,25 15 (1 986).

Chattopadhyay, Eds., AIP Conf. Proc. 671, AIP Melville, NY, 2003. p. 243.

PROBLEMS

Table I. 1 classifies the low, medium, high, very high and ultrahigh vacuum ranges. Describe the suitability of the following materials in each range: Brass, cadmium, nickel-plated copper, household aluminum foil used to wrap cleaned parts, aluminum or copper castings, copper sheet, aluminum sheet, quartz, alumina ceramic, Buna-N, Nylon, Teflon, polyimide, and Viton. Assume materials used in the construction of ultrahigh and extreme high vacuum chamber walls might be preprocessed at temperatures as high as 800°C. Assume ultrahigh vacuum chambers may be baked to 75- 150°C during routine use.

Why should a pressure gauge be the last item cooled when baking?

414

21.3

21.4

21.5

21.6

21.7

21.8

21.9

ULTRACLEAN VACUUM SYSTEMS

A small vacuum chamber with an internal surface of 0.1 m2 is pumped to its ultimate pressure by a pump of speed 100 L/s. The net outgassing rate of the inner surface is reported to be 3x10-" Pa-m/s. (a) What is the ultimate pressure? (b) If the sticking coefficient of the surface is 0.1, what is the true outgassing rate of the surface?

A particular vacuum system has a total internal surface area of 50 m2 imd a water pumping speed of 50,000 L/s. Calculate the average number of bounces made by a water molecule at room temperature, during its sojourn to a pump.

Assuming that 20% of the system interior is not baked above room temperature, nor cleaned by any stimulated desorption method, calculate the increase in residence time for the average molecule during baking in the previous problem. (Assume 20% of the molecular bounces land on unbaked, room-temperature surfaces.)

Carefully estimate the outgassing rate of a fingerprint. What compounds are in a fingerprint?

0 2 enters an ion-pumped ultrahigh vacuum system. The pump current increases by 2x and the pressure, as indicated by a Bayard- Alpert gauge, increases lox. What is happening?

't After a rapid system backfill with argon, flakes from a titanium sublimation pump have been blown into the ion pump and the power supply is shorted. What do you do?

(a) Calculate the rate of pressure rise, due to helium in the atmosphere, in a sealed 7740 Pyrex glass system with a wall area of 1 m2 and a wall thickness of 1 mm which is initially pumped to lo-'' Pa? Assume the permeation constant for 7740 glass is m2/s at 25°C. (b) Calculate the pumping speed necessary to hold the chamber at 1 0-" Pa, if the permeating helium is the only gas source.

21.10 In a surface analysis system samples are introduced through a load Bock on a long cylindrical rod. (See Fig. 17.19.) The rod passing through the load lock is isolated at the atmospheric side by an O-ring seal and at the UHV side by a long closely fitting cylinder, which passes through the vacuum wall. Assume the rod diameter is 1 cm, the gap between the rod and cylinder is 0.003 in., and the cylinder is 8 cm long. (a) Calculate the gas leak into the main chamber through this load lock seal if the base pressure is 1 ~ 1 0 ' ~ Pa in the main chamber and 1 x104 Pa in the load lock. (b) If the main system pump has a speed of 400 L/s at the chamber, what fi-action of its gas load is entering via the concentric seal at the given base pressure?