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title:High-vacuum Technology : A Practical Guide Mechanical Engineering (Marcel Dekker, Inc.) ; 111author:Hablanian, M. H.publisher:CRC Pressisbn10 | asin:0824798341print isbn13:9780824798345ebook isbn13:9780585138756language:EnglishsubjectVacuum technology.publication date:1997lcc:TJ940.H33 1997ebddc:621.5/5subject:Vacuum technology.

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1

Introduction

1.1

Nature of Vacuum

The word vacuum is used to describe a very wide range of conditions. At one extreme, it refers to nearly complete emptiness, a space without matter, or more specifically, space in which air and other gases are absent.

At the other extreme, vacuum is any air or gas pressure less than a prevailing pressure in an environment or, specifically, any pressure lower than the atmospheric pressure. An example of conditions approaching the first meaning is intergalactic space. Examples of the second meaning are pressures existing at the inlet of an ordinary vacuum cleaner and in a straw used for drinking.

The basic property involved here is the density of the gas. The degree of vacuum can easily be described in terms of gas particle density instead of pressure. Scientific and engineering interest in vacuum spans an enormous range of gas densities15 orders of magnitudein other words, it involves density changes of a million billion times. Corresponding to this wide variation in density, there are industrial and laboratory processes in which certain ranges in this wide variety of conditions are used. Production of a high degree of vacuum and an understanding of high-vacuum technology were closely associated with the development of physics as a science beginning approximately 350 years ago.

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another mechanical pump, 120 cm3/s; Gaede's rotary mercury pump, 0.15 L/s maximum; Gaede's first molecular pump, 1.4 L/s; Holweck's molecular pump, 2.5 L/s; the first Gaede diffusion pump 80 cm3/s maximum; the Langmuir diffusion pump, less than 4 L/s; and Crawford's diffusion pump, 1.3 L/s.

Only in the 1930s did larger pumps appear producing pumping speeds of a few hundred liters per second. Characteristic of the time is a description of a high-capacity vacuum installation in 1929 with 12 glass and three steel Langmuir condensation pumps (used in parallel), producing a total pumping speed of 40 L/s at 5 10-3 torr.

1.6.2

Vapor Jet (Diffusion) Pumps

The advantage of the vapor jet pumps compared to the mechanical pump for the creation of high vacuum is that they can produce much higher pumping speed at low pressure for the same size, weight, and cost. This was not realized at first.

Gaede, thinking in terms of kinetic theory, correctly deduced that gases can be pumped by the molecular drag effect, only to be defeated by mechanical difficulties. In the case of vapor jet pumps, he assumed that the pumping action was due to the classical picture of the diffusion of air into a cloud of mercury vapor. Actually, if the mercury cloud were immobile, no pumping action would result. This confusion kept vapor jet pump speeds at very low levels, and the full realization of their capabilities was not achieved until 20 to 25 years later.

Langmuir followed with a better design (1916) but he overemphasized the condensation aspects of the pumping mechanism. In fact, condensation is basically not required for the pumping action, only for the separation of pumped gas and pumping fluid. The first correct design from the point of view of fluid mechanics was made by W. W. Crawford in 1917, but the maximum pumping speed was only about 2 L/s for a 7-cm-diameter pump (roughtly 50 times lower than modern designs). Such pumps were improved continuously during the next 20 years.

Low-vapor-pressure oils were introduced by C. R. Burch in 1928. In 1937, C. M. Van Atta reported a design with 200 L/s for an 18-cm-diameter pump, about five times lower than that for present pumps. Large pumps were designed in the early 1940s for large gas separation plants and later for vacuum metallurgy, industrial coating, and space simulation chambers. Modern designs date from about 1960.

1.6.3

Molecular Pumps

The first mention of Gaede's molecular pump occurs in 1912 in The Engineering Index Annual. The pump was not successful because in 1912 the art of


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Figure 4.8

Long tubular chamber with multiple pumps.

rately. This problem can be solved by integrating the outgassing rate of a differential element and the corresponding conductance, giving

(4.47)

where C is the conductance of the entire tube. Note that the same result would be obtained if the entire outgassing load were concentrated at the midpoint of the tube. The conductance between the inlet and the midpoint is 2C. The pressure distributions are parabolic; that is, the pressure at any distance X from the entrance to the pipe is

(4.48)

with the maximum occurring at the end of the tube.

Note that if the pressure is reduced at the entrance to the tube, the pressure difference will remain the same [Eq. (4.47)]. The pressure difference depends only on the outgassing rate and the conductance. This solution can be extended by symmetry to the case of a very long tube with many pumps placed at intervals along the tube. In that case, L should be taken to be half the distance between pumps. The maximum permissible pressure in the tube can then be obtained by appropriate design specifications for P1 and C. A somewhat different approach to the problem of pumping long ducts has been published by C. Henriot.

Conductances of short tubes can be obtained either by using the conductance of an orifice of the same diameter and applying a correction based on the L/D ratio or starting with the long tube and applying a different correction also according to the L/D ratio. Here the first method will be used. Using Clausing's


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solution for the probability of a molecule entering the tube at one end and escaping at the other, the conductance of the tube can be compared to the conductance of an orifice of the same diameter.

(4.49)

For air at 22C this will be [see Eq. (4.38)]

C = 11.6aA (4.50)

if C is in liters per second and A in square centimeters. The correction factor can be found from Figure 4.9.

When dealing with more complex shapes, the simplified formulas discussed previously cannot be used. To obtain accurate values, a higher level of mathematical or experimental modeling is required. Two common shapes will be noted here: elbows and conical ducts. Some additional shapes can be found in Ref. 5 (see Figures 4.10 and 12). A 90 elbow does not represent an additional impedance to molecular flow if the axial length is used in calculations. Some textbooks recommend adding a length to the axial length equivalent to 1.33 (q/180) D, where q is the angle of the elbow and D the diameter. However, this correction has not been found to be necessary by experimental and theoretical evidence (Ref. 5) at least for 90 elbows.

Figure 4.9

Correction factor for conductances of short tubes.


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Figure 4.10

Examples of conductance calculations by Monte Carlo

method. (From Ref. 5.)

Conical ducts are important because they often serve as transition pieces between conductance elements having different diameters. Transmission probabilities for truncated conical ducts are shown in Figure 4.11. In this case, the probability is associated with the inlet diameter. In other words, it is based on a comparison with an orifice having the same diameter as the R1 circle.


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Figure 4.11

Transmission probabilities for conical ducts. (From Ref. 4.)

There are several ways to determine the conductance of a duct with a complex geometric structure. First, the particular complex shape may be separated into component shapes for which simple solutions exist, and the results summarized as conductances in series. In some cases, the solutions may be obtained by employing classical differential calculus. In more complex systems, finite element analysis or Monte Carlo computations can be used. The latter consists of following individual histories of a molecule entering a given duct and assign-


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ing to it random interactions at the walls of the duct according to the laws of reflection. By following histories of many thousands of molecules, integrated statistical results are obtained. The technique may be laborious for complex three-dimensional shapes, but it has been successfully applied in many important cases.

When the system does not lend itself to convenient theoretical analysis, experimental measurements of models can be very useful. The value of conductance can be obtained from the transmission probability, which depends only on the geometric shape of the structure and is independent of its size. All that is necessary in such measurements is to compare the conductance of the model to that of an orifice of known size. The experimental setup is shown in Figure 4.12. A set of models (baffles) is placed on a rotating disk and each indexed into the position above the high-vacuum pump for measurement. The apparatus can be calibrated by placing one or more orifices of known diameter in place of models. To maintain the calibration conditions, one of the models on the disk should be an orifice.

To assign a transmission probability to a duct that has different entrance and exit diameters, a decision must be made as to which diameter is to be used as the base. Although conductance of a duct is always equal in both directions, the transmission probability is not. In other words,

Figure 4.12

System for measuring conductance.


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a1A2 = a2A1 (4.51)

where a is the transmission probability and A is the area of entrance or exit. For example, if the shape under consideration is a conical section, a1 can be taken to be the transmission probability when the larger diameter (A2) is the entrance and similarly, a2 and A1 in the opposite direction. In the test setup of Figure 4.12, the reference orifice was made equal to the exit area of the model and the transmission probability obtained directly from pressure measurements.

(4.52)

where Cm is the conductance of the model and subscript zero is for the reference orifice. The conductance then can be obtained from

Cm = aCA (4.53)

where C is 11.6 L/s cm2 for air at room temperature. Transmission probabilities of various shapes obtained with the system shown in Figure 4.12 are given in Figures 4.13 to 4.16. The experimental accuracy of these values is between 5 and 10%, as judged from the correspondence with the expected orifice conductances and the scatter of pressure measurement data.

4.5.2

Combining Conductances in Series.

The combination of conductance elements that are connected in a parallel arrangement does not represent a problem. The conductance values are simply added. When conductance elements are connected in series, the general rule is to add the resistance values (the reciprocals of conductances)

(4.54)

where Cn is the net conductance and C1 and C2 are the conductances of two elements connected in series. This relationship holds as long as end effects are not significant, for example, in the case of long ducts or tubes.

In the case of short ducts, it should be recalled that conductance is defined as

(4.55)

where the two pressures are measured in large chambers located at the two ends of the duct. The presence of such chambers between each element will assure that the conductance values are not affected by the geometrical matching of the


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Figure 4.13

Examples of transmission probability of elbows.


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Figure 4.14

Examples of transmission probabilities of conical ducts.

adjacent element. However, in practice such chambers are usually absent and significant errors can be made using Eq. (4.54) without correction.

Using the example of connecting two short tubes of equal diameter, Oatley analyzed the effect of omitting a large chamber between the two items and produced a correction as follows:


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Figure 4.15

Transmission probabilities of some common shapes.

(4.56)

Transmission probability a is defined as C/C0, the ratio of actual conductance to the conductance of the orifice of the same diameter. In case of conical ducts, the conductance in both direction is the same but the transmission probability is not (i.e., a12A2 = a2-1A1).

If Eq. (4.56) is rewritten in terms of conductances, it will become

(4.57)

Using an example of two elements, each having a transmission probability of 0.3, we would obtain a1-2 = 0.15 by direct reciprocal addition and 0.17 by Eq. (4.57). This would give a nearly 12% error. For three elements in series, the correction should be

(4.58)

and so on.

Additional difficulties will be encountered with abrupt changes in cross section. Consider the case shown in Figure 4.17, which shows three pipe sections


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Figure 4.16

Examples of transmission probabilites of baffles.


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making high-speed rotating machinery had not yet been developed. It took 45 to 50 years before molecular pumps were made as an industrial device to adequate reliability. Other attempts, by Holweck and Siegbahn, were hardly more successful. They utilized, respectively, a cylinder and a disk as the molecular drag surface. One advantage of the bladed, multistaged, axial-flow compressor-type vacuum pump is that each rotor surface is exposed only to a particular vacuum level. In cylindrical designs the same surface passes from the high-pressure to the low-pressure side.

The idea of using the axial turbine as a vacuum pump seems to have occurred simultaneously to at least two persons (H. A. Steinherz and W. Becker), the former attempting simply to utilize an existing compressor device as a vacuum pump. The closed cell design by Becker was published in 1958. A. Shapiro (Massachusetts Institute of Technology) and his associates analyzed the open-blade design, which was found to be entirely practical and efficient. Later, the SNECMA Company in France developed industrial versions of such pumps. The open-blade designs are generally used now by most manufacturers.

1.6.4

Ion-Getter and Sputter-Ion Pumps

Many phenomena utilized today for high- and ultrahigh-vacuum pumping have been known, and basically understood, for a long time. These include outgasing, adsorption, gettering, sputtering, and cryogenic temperatures. They were not used earlier because the technology was not ready and the need was not pressing.

A vacuum of a few micrometers of mercury (10-3 torr) was required in Edison's original electric lamps. In 1881, it took five hours to achieve. To handle the outgassing of water vapor, bulbs were heated to about 300C during evacuation. Gettering by phosphoric anhydride was introduced from the beginning. Sprengel pumps are associated with the initial successes of the lamp industry. Improvements in pumping speed reduced evacuation time to 30 minutes. This process was used until 1896, when Malignani (Udine, Italy) chemical exhaust processing was adopted. This consisted of placement inside the lamp of red phosphorus, which would vaporize when the filament was heated. The evacuation times was reduced to a minute. The effect of phosphorus is partly chemical and partly due to adsorption.

The cleanup of gases by an electrical discharge has been observed for over a century. Gettering for lamp making is at least that old. T. M. Penning mentions sputtering as a method for reducing gas pressure at least as early at 1939. The mention of ionic pumping appears at least as early as 1953, associated with the initial development of ultrahigh-vacuum techniques.

In the early 1950s, R. Herb (University of Wisconsin) became determined to eliminate oil vapors in vacuum systems. One of his associates suggested ti-


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Figure 4.17

Approximations in calculating conductances of ducts

with abrupt changes in dimensions.

in series. To estimate the net conductance of this section without resorting to a higher level of mathematical analysis, we may appreciate that in analogy to flow of electricity or distribution of stresses in elements of the same geometry, the corners of the larger tube do not provide much additional conductance compared to the modified lower figure. Thus, instead of adding the lengths L1, L2, and L3, we can, for the first approximation, add conductances of lengths L4, L5, and L6. A better approximation will be obtained by adding the conductances of sections L1, L5, and L3 and the two conical sections. Then, because there are no large chambers between the elements, Oatley's method should be used. The equivalence of shallow conical sections, and short tubes with an aperture, has been confirmed experimentally. For example, two geometries shown in Figure 4.15a and b produced nearly identical transmission probability (based on the smaller diameter), using the apparatus shown in Figure 4.12.

The case shown in Figure 4.17 is relatively simple. Ducts of more complex geometry can be analyzed to establish an equivalent case consisting of recognizable elements for which conductances (or transmission probabilities) are


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known. It is often not an easy task. To produce a reasonable equivalent system, both the internal geometry and the transition between adjacent components must be considered. An example of an incompatible arrangement is shown in Figure 4.18.

Two identical baffles (of commercially available design) are arranged in series. In Figure 4.18a the arrangement is compatible, permitting the use of basic conductance value of the baffle in Eq. (4.54) (or a modified corrected form of it). The arrangement in Figure 4.18b, which is used only as an illustrative example, has nearly zero conductance. To make an estimate of conductance in this incompatible case, one should completely disregard the actual individual conductance of the baffles and treat the narrow space between them as a rectangular slot.

Much of the difficulty that seems to persist in the addition of series conductances is due to the tendency of using concepts of flow in high vacuum taken from simple electrical analogy. In molecular flow and for short duct geometries, simplified treatment is not directly applicable. Consider, for example, the problem of specifying the conductance of a baffle. There is no single number that can be assigned to a given baffle and used in all situations because the value will depend on entrance and exit conditions (i.e., the geometry of adjacent elements). There may be three alternatives to treat this problem:

1. Insert the baffle between two large chambers having pressure P1 and P2 and specify conductance as C = Q/(P1 - P2), which is the classical method.

Figure 4.18

Example of compatibility of two adjacent elements.


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2. Insert the baffle between tubes of the same diameter as baffle entrance and exit, measure pressures very near the entrance and exit, and use the same formula as above.

3. Measure the net speed of the baffle and pump, or any two elements in combination, without any reference to classical conductance values.

Perhaps a system could be developed permitting direct use of the simple reciprocal equation for routine engineering computations not requiring accuracy better than 10%. This may necessitate a dual set of values for conductance elements, corresponding to cases 1 and 2 above. Case 1 obviously has very basic physical meaning but is not easily applied without end-effect corrections. Cases 1 and 2, however, will both give unique conductance values for a given component.

The values obtained in case 2 may permit direct use of Eq. (4.54) for compatible components. Case 3 has been used in commercial situations because of lack of a better method. After measuring the net speed of a baffle with a high-vacuum pump using the standard speed measuring dome, the value of nominal conductance is derived from Eq. (4.54). Tihs procedure has the obvious advantage of direct measurement but does not provide a unique value for specifying the conductance. But for a system of well-matched components, Eq. (4.57). can also give reasonably good approximations in the latter case.

In cases involving short elbows joined at various angles and including various baffle structures inside, we cannot expect great accuracy. In many practical cases, it is impossible to divide the structure into logical parts of simple geometry to assign transmission probability values to each part for even an approximate analysis. The simplest approach may be direct measurement with models of the entire system. In extreme cases where compatibility is absent, gross errors can occur through careless application of simplified equations.

The reduction of pumping speed of a high-vacuum pump due to the presence of inlet ducts can be established using the following expression:

(4.59)

where Sp is the speed of the pump, C the conductance of the inlet duct (or baffle, or valve), and Sn the net pumping speed. This expression, often quoted in textbooks, contains an error if the conductance of a baffle is defined in the classical way as associated with large chambers on both ends. Santeler recommends a correction similar to Oatley's method (Ref. 9). The correction consists of subtracting the pressure drop associated with the exit of the baffle (i.e., the difference in pressure inside the tube, its exit, and the pressure measured in the large chamber). Thus the first correction is obtained as


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(4.60)

and then

(4.61)

4.5.3

Conductance in the Transition-Viscous Flow Region

In the molecular flow range, the value of the conductance is independent of pressure. Since collisions between molecules are insignificant, the conductance depends only on molecular velocity and the geometry of the duct. At higher pressures, the value of the conductance depends on pressure. Resistance to mass flow (per given pressure difference) is reduced with rising pressure because fewer molecules interact with the stationary walls and the flow rate depends on the viscosity of the gas. In the viscous flow domain, the conductance values for long tubes can be obtained from

(4.62)

where C is in liters per second, Pav is the average pressure in torr, D the diameter in centimeters, h the viscosity in poise, and L the length in centimeters. For air at room temperature, this becomes

(4.63)

with units of liters per second, torr, and centimeters.

Figure 4.19 gives some examples of conductance values in the range of pressures comprising the transition between molecular and viscous flow domains. A more generalized graph is presented in Figure 4.20, where the ratio of conductance is plotted versus the product of average pressure and the diameter for air at room temperature. Equation (4.63) does not give accurate results at pressures above 10 torr, when pressure ratios are high. Conductance values, as shown in Figures 4.19 and 4.20, do not grow indefinitely. When bulk velocities reach values higher than half the speed of sound, the flow becomes choked (see Eq. 3.21).

Turbulent flows can also occur in vacuum systems during the initial evacuation and in flow of atmospheric air through air-release valves. However, the conductance of the pipes connecting the rough vacuum pumps to the chamber


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Figure 4.19

Example of conductance values in the transitional pressure regime.

is normally so high compared to the speed of the pump that the net pumping speed is not significantly affected by the presence of turbulence. In other words, if the piping is correctly dimensioned for the laminar flow regime, it will be sufficient even if turbulent flow occurs. In unusual cases, textbooks on gas flow and pneumatic pressure loss tables should be consulted. When pressure drops are high (higher than 20% of inlet pressure), the pipe should be divided in sections and then the separately computed resistances added.


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Figure 4.20

Generalized conductance relationship in the transitional pressure range.

Turbulent and choked flows can occur in systems with multiple, differentially pumped chambers when a continuous thin web substrate is passed through narrow slits from atmospheric air into a high-vacuum process chamber. Another example is the passage of an electron beam from high vacuum into atmospheric pressure for electron beam welding (in argon). Such cases are treated, for example, by A. H. Shapiro (Ref. 17), B. W. Schumacher (Ref. 16), and P. Shapiro (Ref. 15).

4.6

Outgassing Effects

4.6.1

General Considerations

Most pumping devices in their normal range of operation tend to have a more-or-less constant pumping speed. When the volumetric flow rate of a gas is constant, the mass flow depends on density. During evaluation of a vacuum chamber, as the pressure (and density) is lowered, fewer and fewer gas molecules are removed per unit time. Therefore, to achieve lower pressures, any unwanted source of gas must be minimized.


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There are several unintended sources of gas in vacuum chambers in addition to the adsorption effects discussed in Chapter 2. First, there may be leaks through the walls either in a concentrated capillary flow or by permeation in porous materials. Then there are the so-called virtual leaks. These are gas sources inside the chamber without any penetration through the walls.

These could be cavities or porous regions in internal welds, captured gas in plugged holes or under screws in blind holes, or even gas left between tightly clamped plates with fine surface finishes. Often, the source of addition gas evolution is the process performed in the vacuum system. For example, during evaporation of thin films, a thick deposit of the evaporated material eventually accumulates on the walls of the chamber. Usually, this deposit is porous and it adsorbs large quantities of water vapor when exposed to atmosphere. For this reason, the walls of the vacuum chamber should not be cold when it is open to air. To prevent condensation, the chamber should be a few degrees warmer than room temperature. In systems that contain water cooling, the cooling of the walls should be interrupted a few minutes before the system is exposed to air and cooling should be restarted a few minutes after the evacuation.

The amount of gas evolving from metals increases rapidly when the metal is heated. In vacuum furnaces, electron beam welding, and similar equipment, the vacuum system must be designed to maintain a desired pressure at the elevated temperature and not when it is cold. As noted before, the following very approximate rule may serve as an illustration. If, after the initial evacuation, common metals in a vacuum chamber are suddenly subjected to a thermal energy input, at least a 1000-L/s pumping speed may be required for each kilowatt of energy to maintain 1 10-6 torr pressure.

4.6.2

Water Vapor in Vacuum Systems

Water vapor in the vacuum system tends to accumulate in the colder areas, depending on location of its sources and sinks and rates of desorption and sorption (or outgassing and pumping speeds). Therefore, the temperatures of various parts of the vacuum system should be considered carefully. In principle, the behavior is similar to common occurrences of the fogging of windows of automobiles or homes, mirrors in bathrooms, and the cold-water pipes in cold basemenets during changes of temperature and humidity conditions. The major difference is that instead of dealing with bulk liquid condensation, we have a high-vacuum system with thin films of dense absorbed vapor that may display properties different from the liquid. For example, a water molecule will usually adhere to the metal surface more strongly than to its own liquid.

Generally, areas near the pump should be kept colder than the system. A liquid-nitrogen-cooled trap above a vapor jet pump or a cryopump will produce this requirement naturally. Pump-down procedures, baking, and system shut-


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down sequences should also be arranged carefully to prevent unnecessary contamination of the vacuum system with vapors previously pumped and adsorbed. In systems that are frequently exposed to the atmosphere, it is generally recommended not to cool the chamber walls below room temperature. Even the vapor jet pump cooling water should not be started until rough pumping is completed, to prevent condensation from the water vapor in the air before it is removed by the rough vacuum pump.

In ultrahigh-vacuum systems, the water vapor content is reduced by pumping the water during baking. The remaining major constituent is then hydrogen. In unbaked high-vacuum systems, the major residual gas content is due to water vapor (as much as 95%). One way to reduce the water vapor density is to use as high a pumping speed as possible. However, if a process sensitive to the presence of water vapor is conducted near its sources, the high pumping speed may not be entirely helpful. Even if the process area is surrounded by liquid-nitrogen-cooled surfaces, providing 100% sticking efficiency for water vapor molecules, the density of the vapor will be given by the rate of outgassing or desorption from the uncooled surfaces. This example indicates the possibility of controlling the presence of water vapor by careful consideration of vacuum system design, not only by providing a high pumping speed, but also by controlling the sources of the vapor.

4.7

Pumping System Design

High-vacuum pumps are used, together with mechanical pumps, in applications where system operating pressures of 10-3 torr and below are desired. The physical arrangement of system components depends on the characteristics of the process to be carried out, such as the pressure level, cycle time, cleanliness, and so on. To some extent the availability and compatibility of components influences system design. In some instances, the economic aspects of component selection may determine system layout. The following paragraphs illustrate briefly the most common component arrangements, referred to as valved and unvalved systems, and outline their major respective advantages and disadvantages. A recommended operating procedure to prevent misoperation and work chamber contamination is outlined for each type.

To furnish maximum effective pumping speed at the processing chamber, it is generally desirable to make the interconnecting ducting between the chamber and the pump inlet as large in diameter and as short in length as practical. The amount of baffling and trapping required depends on the desired level of cleanliness in the chamber, the necessary reduction of the inherent backstreaming characteristics of the pump, and the migration of the pumping, sealing, or lubricating fluids. The effects of this phenomenon, which is discussed later, can be reduced significantly by correct component selection and


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operating procedures. Baffles, traps, and valving must be selected for minimum obstruction to gas flow, without sacrifice of efficiency of their specific function.

The size of roughing and foreline manifolding is governed by the capacity of the mechanical pump, the length of the line, and the lowest-pressure region in which it is expected to function effectively. Additionally, the size of the foreline is influenced by the forepressure and backing requirements of the high-vacuum pump under full-load operating conditions.

Several design considerations for vacuum systems cannot easily be put into a single set of requirements. A practice that can be easily tolerated in one case must be completely forbidden in another. The requirements span over three or four orders of magnitude in pressure range and surface area. For a small device that must be evacuated rapidly to higher vacuum with a system of limited pumping speed, any trapped air pockets must be carefully eliminated. For example, a screw or bolt in a threaded hole can be a source of virtual gas leakage. It is good practice to provide an evacuation passage for the trapped gas. A hole can be drilled through the bolt, threads can be filed flat on one side, or the blind holes can generally be avoided. However, all this will be of no significance in a system that has a pumping speed of 50,000 L/s and is operated only at 10-5 torr. Obviously, vacuum level, pumping speed, size of chamber, and the desired time of evacuation must be considered to provide a balanced design.

In large vacuum systems, structural distortions during evacuation may be high enough to be troublesome. To preserve precise alignments of optical equipment, mechanical drives, and so on, it may be necessary to support the various components independent of the external vacuum vessel.

As noted earlier, for a 2.5-m-diameter door used in an industrial coating system or furnace, the total force due to 1 atm pressure difference in nearly 50 tons. This is certainly sufficient to seal an elastomer gasket without the use of any clamping devices. However, if the chamber is air-released with bottled gas, it is advisable to prevent an internal overpressure. A small overpressure may suddenly open the door if the rubber gasket has a tendency to stick. A spring-loaded catch can be used to prevent this.

Another common malfunction due to distortion occurs in large high-vacuum valves. The pneumatic pistons used to actuate such valves are usually not strong enough to provide the vacuum seal. The pressure from the atmospheric side is relied upon to provide the tight closure. Thus such valves cannot be opened against the force of atmospheric pressure. A malfunction can occur during the roughing cycle. As the atmospheric pressure in the chamber is reduced, the valve seat may lift slightly and begin to leak. If the valve is used above a high-vacuum pump system, the leak may be large enough to overload the pump and cause backstreaming, stalling, and pumping fluid-loss problems.


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4.7.1

Pump Choice

The selection of high-vacuum pumps is not as straightforward as the selection of mechanical pumps. The considerations for reaching a given pressure in a short time may not prove to be an adequate measure of the system's capability to handle steady gas load characteristics; the pumping curves of the high-vacuum pump will indicate the proper components. More frequently, gas loads are not known and selection of the pump must be based on experience with similar applications.

Vacuum-equipment suppliers generally will offer performance based on pump-down characteristics of an empty chamber. This can be misleading if it is construed to be a measure of pumping efficiency under full-load conditions. The vacuum equipment designers must relate performance to a set of conditions that are known and can be defined and duplicated.

The selection of high-vacuum pumps should be concerned with all factors involved in the system. These factors normally consist of pumping speed, tolerable forepressure, backstreaming rates, pressure at which maximum speed is reached, protection devices, ease of maintenance, backing pump capacity, roughing time, ultimate pressure of the roughing pump, baffles or cold traps employed, valve actuation, and so on.

Unless these factors are carefully weighed in the system design, the system may not perform satisfactorily as a production system. It is to the advantage of both the supplier and the user to consider these factors carefully before finalizing system design.

It is obvious from the previous discussion that pump overload should be avoided in system operation. As a general rule, high-vacuum pumps should not be used at inlet pressures above their maximum throughput capacity for prolonged periods. In a properly designed system with a conventional roughing and high-vacuum valve sequence, the period of operation in an overload condition should be measured in seconds. As a rule of thumb, it may be said that if this period exceeds half a minute, the pump is being misoperated.

It is clear, then, that in applications requiring only 10-4 torr process pressure, the mechanical pump roughing period is normally much longer than the high-vacuum-pump portion of the pumpdown cycle, or at least it should be for economic reasons.

4.7.2

Use of High-Vacuum Valves

For applications involving rapid recycling, a fully valved pumping system is essential. This type of system is shown schematically in Figure 4.21. It permits isolation of the high-vacuum pump for the work chamber at the conclusion of a pumpdown and prior to air admittance. The pump can therefore remain at operating condition during periods when the chamber is at atmosphere and


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tanium for gettering. A container was pumped only by a forepump and a titanium sponge evaporator. It was successful but had a low-pressure limit which we suspected was argon. Then an ionizer was built to ionize residual gas and drive ions into the wall receiving titanium. It worked and I believe we had the first practical getter ion pump. Such Evapor-ion pumps were rather difficult to operate and maintain. Sputter-ion pumps developed by L. D. Hall and associates at Varian Company were simpler and achieved universal acceptance a few years later.

The first ion pumps were single-cell appendage pumps developed for maintaining high vacuum in large, high-power microwave tubes. They resembled Penning gauges with the cathode made of titanium. Multicell designs and triode pumps for better argon pumping followed quickly. Later, orbitron pumps, also introduced by Herb and his associates, enjoyed a brief popularity but were withdrawn due mainly to problems with reliability.

Most modern ultrahigh-vacuum instruments would not be possible without the development of sputter-ion pumps. A group of engineers and scientists at Varian Associates pioneered industrial utilization of sputter-ion pumps and other ultrahigh-vacuum pumping devices, such as sorption pumps, as well as valves and metal gasket flanges.

1.6.5

Cryopumping

Cryopumping effects with and without high-surface-sorption materials were understood at least 100 years ago. Early in the twentieth century, one of the methods for obtaining vacuum was to use activated charcoal cooled with liquid air. With the invention of vapor jet pumps, liquid nitrogen traps were generally used. Although the intended use of the traps was for suppression of the mercury vapor, they did pump water vapor remaining in the chamber after initial evacuation.

During the late 1950s and 1960s, cryogenic shrouds were often used in the construction of space simulation chambers. Occasionally, liquid helium was used for enhanced cryopumping in early, extreme-high-vacuum chambers. The industrial art of cryopumping had to wait until the development of reliable closed-loop gaseous helium cryorefrigerators. These refrigerators were developed for other than high-vacuum uses, but have recently found wide acceptance in the vacuum industry.

References

1. E. N. Da C. Andrade, in Advances in Vacuum Science and Technology, Proc. 1st Int. Cong. Vac. Tech., Namur, Belgium, E. Thomas, ed., Pergamon Press, Elmford, N. Y., 1960.


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Figure 4.21

Schematic view of a typical vacuum system.

during the rough pumping portion of the cycle. The length of these periods may indicate the need for a separate holding pump. The main isolation valve also permits continuous operation of the cryobaffle between it and the high-vacuum-pump inlet. Neither this nor rapid cycling can be realized without the valving indicated, in view of the shutdown and startup time lapse inherent in high-vacuum-pump operation, and the cool-down and reheat time of the cryobaffle. Judicious operation of the main valve at the changeover phase from roughing to high-vacuum pumping can significantly reduce the backstreaming of contaminants to the work chamber.

Leak testing and leak hunting are considerably easier in valved systems, and repair procedures are also generally less time consuming. However, the following disadvantages can be noted. Valved systems are initially more expensive, especially when large valves are involved. In addition, the use of valves inevitably adds to the system complexity and generally results in lower effective pumping speed at the chamber.

Valveless vacuum pumping systems are generally considered for applications where the length of the pumpdown is of less importance, and process or test


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cycles are of extended duration. They may be found on baked ultrahigh-vacuum chambers using large high-vacuum pumps, due to lack of availability and/or prohibitive cost of valves compatible with the proposed operating pressure levels. The valveless system generally offers higher effective pumping speed at the chamber and, in view of its prolonged operation at very low throughputs, lends itself to the use of a smaller holding backing pump. This pump is sized to handle the continuous throughput of the high-vacuum pump at low inlet pressure, 10-6 torr and below, and allows economy of operation by permitting isolation and shutoff of the considerably larger main roughing and backing pump. The valveless system, however, also has a number of disadvantages. More complex operating procedures are necessary to ensure maximum cleanliness in the work chamber and minimum contamination from the pumps.

To minimize chamber contamination, it is recommended that at least one of the traps be kept operative while the vacuum chamber is being baked. When the inlet ducts are at elevated temperatures, the rate of contaminants migration into the system is accelerated. At temperatures near 200C the oil vapors are not condensed in the chamber and are subsequently pumped by the high-vacuum pump. But a very thin (invisible) film of oil will condense in the chamber during the cool-down. This film formation is minimized by continuous operation of at least one of the traps.

4.8

Operation of High-Vacuum Systems

4.8.1

General Considerations

Operation of high-vacuum systems requires certain care and attention to several items. General cleanliness can be extremely important, especially in small systems. It should be remembered that a small droplet of a substance that slowly evaporates, producing a pressure of 10-6 torr, may take many days to evaporate and be pumped away if the available pumping speed is 100 L/s. Humidity and temperature can be important in view of the constant presence of water vapor in the atmosphere. When vacuum systems are backfilled or air-released and then repumped, it will make a significant difference whether the air was dry or humid. The time of exposure is also significant. If necessary, the backfilling can be done with nitrogen or argon. For short exposures (30 min or less) this appears to reduce the amount of water vapor adsorption in the vacuum system.

It is extremely important to develop good habits in sequences of valve operations, especially in systems with manual valves. Graphic panels showing valve locations and functions are very useful. A single wrong operation may require costly maintenance procedures. It is well to pause a few seconds before operating any valves in a high-vacuum system unless a routine procedure


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is followed. Automatic process control sequences have been widely used in recent years to avoid operational errors.

The performance of displacement-type high-vacuum pumps is usually displayed in the form of a plot of pumping speed versus inlet pressure, as shown in Figure 4.22. Note that the graph consists of four distinct sections. In the first section on the left, the speed is seen to decrease due to the limit of maximum pressure ratio obtainable with the pump. This will vary with different gases, and in most applications only light gases such as helium and hydrogen will show this effect. The constant-speed section results from the constant arrival rate of gas into the pump at molecular flow conditions. In this section, the amount of gas being pumped is lower than the maximum mass flow capacity of the pump and the pump speed efficiency is also constant. The section marked overload is a constant throughput section which indicates that the maximum mass flow capacity of the pump has been reached. This information is important for knowing when to switch from roughing to the high-vacuum pump in the process of evacuating a chamber. Note that it makes little difference whether the inlet pressure is at 1 or 10 mtorr as far as the amount of pumped gas is concerned, because here the mass flow is nearly constant.

Figure 4.22

Typical performance of a high-vacuum pump (pumping speed versus inlet

pressure).


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In the last section on the right, the high-vacuum pump performance is dependent on the size of the backing pump. At this condition, the upper stages of a vapor jet pump, for example, are not doing any pumping, and most of the pumping is done by the mechanical pump. Strictly speaking, a high-vacuum pump should not be used above the critical point shown in Figure 4.22. If process pressure demands high pressure, the pump should be throttled to limit the throughput to the maximum permissible.

This performance feature can be illustrated much more clearly by displaying throughput of the pump instead of speed and by reversing the traditional position of inlet pressure in the horizontal axis of the graph. This is shown in Figure 4.23. This graph is somewhat idealized. The actual pumps will not produce a completely vertical midsection as shown. However, it should be clear

Figure 4.23

Typical performance of a high vacuum pump (inlet pressure,

increasing downward, versus mass flow).


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that to the left of the vertical line, representing the maximum mass flow rate for the high-vacuum pump, it is performing the primary pumping. To the right of the vertical section, only the mechanical pump is pumping; the high-vacuum pump is idling.

Thus, the correct condition for switching from roughing to high-vacuum pumping is clear. It is when the amount of gas from the chamber is below the maximum throughput of the high-vacuum pump! How to recognize this condition is the subject of the following section.

4.8.2

Switching from Rough to High-Vacuum Pumps.

During the evacuation of a vessel, the question arises regarding the proper time to switch from rough pumping to the high-vacuum pump. In other words, when should the high-vacuum valve be opened? No general answer can be given because it depends on the volume and the gas load of the chamber.

Ideally, the answer should be: when the gas flow from the high vacuum system is below the maximum throughput of the high-vacuum pump. This would imply an inlet pressure near 1 mtorr. However, it is not necessary to obtain this pressure with mechanical roughing pumps or even with mechanical booster pumps (Roots type). In practice, the transfer from roughing to high-vacuum pumping is normally made between 50 and 150 mtorr. Below this pressure region, the mechanical pumps rapidly lose their pumping effectiveness and the possibility of mechanical pump oil backstreaming increases. Although the throughput of the high-vacuum pump is nearly constant in the region of inlet pressure of 1 to 100 mtorr, the initial surge of air into the pump when the high-vacuum valve is opened may overload the high-vacuum pump temporarily.

The general recommendation is to keep the period of inlet pressure exceeding approximately 1 mtorr (0.5 mtorr for large pumps) very shorta few seconds, if possible. Consider the following example. For constant throughput, the evacuation time (typically between 1 and 100 mtorr) is

(4.64)

For a common 45-cm-diameter bell jar, the volume is about 120 L and using, for example, a 15-cm-diameter vapor jet pump with a maximum throughput of about 3 torr L/s, we obtain

(4.65)

If the high-vacuum valve were to open slowly to admit the gas into the diffusion pump at the maximum throughput rate, it would take only 4 s to reach the stable pumping region, from point B to point A as illustrated in Figure 4.24.


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Figure 4.24

Crossover region between rough and high-vacuum pumps.

Because points A and B lie on the same gas load line (constant throughput), either one may be called the crossover pressure. To avoid this confusion, we should not speak of crossover pressure but only of crossover throughput (or crossover gas load!). This maximum gas load limitation for a given high-vacuum pump exists regardless of the size of backing pump. As a further illustration, consider the system shown in Figure 4.25. In principle, a 100-L/s high-vacuum pump can be connected in series to a 100-L/s mechanical pump (212 ft3/min), obtaining a constant pumping speed from atmosphere to the high-vacuum region.

In this system, points A and B coincide. The high-vacuum pump must still be isolated from excessive gas loads to prevent increased backstreaming, stalling, pumping fluid loss, or even complete destructive failure. To the right of point A the mechanical pump is doing the pumping; to the left, the high-vacuum pump. The crossover point is still point B. If we persist in thinking in terms of crossover pressure rather than crossover throughput, we arrive at an incongruous situation where a smaller mechanical pump would seemingly tolerate a higher crossover pressure.


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Figure 4.25

Crossover between rough and high-vacuum pumps of equal speed.

4.8.3

Measure Gas Load

Most manufacturers of high-vacuum systems specify a pressure value for switching to the high-vacuum pump or for opening the high-vacuum valve. Such pressure value can only be specified for a given syste

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