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CHAPTER 19

Rough Vacuum Pumping

Characterizing vacuum systems during their initial pumping from atmosphere is not a subject of fiequent study; however, it is an important phase of the pumping cycle. If the initial pumping is done improperly, it can seriously affect system performance or product quality. No one recipe fits all situations. For accelerator applications, removal of atmospheric gas is but a short part of an extended time at ultrahigh vacuum. For high volume production applications that use load locks to transfer product from atmosphere to vacuum, the load lock pumping and venting times can be a major portion of the brief time the product spends in the process chamber. The significant gas loads released from roll stock and debris collected between cleaning cycles must be considered carehlly when specifying roughing pumps and pumping cycles for large coating systems.

How fast should a system be evacuated? To what pressure should it be evacuated? If we pump too slowly, product throughput will be reduced, if we pump too rapidly, water aerosols will form and capital expenditures may increase. If we pump to an unacceptably low pressure, oil-sealed pumps can backstream oil vapors, whereas if we pump to an unacceptably high pressure, the high vacuum pump will not accept the gas load at crossover. Traditionally, these issues were not always discussed in sufficient depth; a one-size-fits all solution was often suggested; however, that no longer applies for the diverse and specialized systems currently used.

Pumping time is dependent on pump and pipe sizes, which in turn affect system cost, return on investment, product cycle time, product quality, and surface contamination. Crossover pressure is a variable that can be adjusted by the operator. If set too low, the product may become contaminated. If set too high, the high vacuum pump may not perform properly. Most importantly, it is a variable that can be changed easily and without consideration of the consequences. The two fundamental questions that we address in this chapter are the rate at which air is pumped during roughmg and the condition chosen for crossover fiom the roughing pump to the high vacuum Pump.

359

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

360 ROUGH VACUUM PUMPING

19.1 PUMPING RATE

Economics and product quality are the two issues that determine the speed at which atmosphere is removed from the chamber. In general we wish to finish the process as quickly as possible, in a system that was purchased for the lowest possible cost, and without affecting product quality. In some cases these requirements conflict. By understanding vacuum basics, we can best meet the objectives.

19.1.1 Pump Size

Pumping from an initial pressure PInIl typically atmosphere, to crossover pressure can be modeled by a chamber of volume V connected to a pump of speed S', through a conductance C, as depicted in Fig. 19.1. The chamber will be assumed to have an internal outgassing of Q/S = P,,and that will remain approximately constant during the rough pumping cycle. The time dependence of the chamber pressure is described by

_ _ Sct

P, (0 = (Ed - pull ) e + PU,I (19.1)

The pumping speed at the chamber exit that is required to reach a desired chamber pressure P, is dependent on the system volume V, the initial pressure Pmrh the outgassing rate, and the time specified by the process or customer.

(19.2)

Fig. 19.1 Model for calculating the initial pumping time of a vacuum chamber from atmosphere to crossover. The rough pumping speed used to predict the pressure decrease is the speed measured at the chamber entrance.

19.1 PUMPING RATE 361

Note that the assumed short-term ultimate pressure Pulz is the quotient of the outgassing rate and chamber speed that is being calculated PuI, = Q,,&. One must either estimate this correctly or iterate to obtain the solution. For empty chambers, this will be a small quantity; however, when roughing chambers containing large product gas loads e.g., large rolls of paper or plastic, that term is significant. We desire to know the speed at the pump and it can be obtained from

(19.3)

Assuming we are pumping from atmosphere, the flow is viscous, and therefore C is pressure dependent. The conductance will be large at atmosphere and will decrease as the final pressure is reached. In practice, this means the pumping speed used in (19.1) is pressure-dependent and numerical methods are required to solve the problem exactly. One can obtain a simple approximate solution by assuming the conductance C to have a constant value-its smallest value-the value at the lowest pressure of interest. Using this value of conductance in (19.3) yields a worst-case or largest value of pumping speed required. The actual exhaust time will be less than that calculated with (19.2) and (19.3). The speed of an oil-sealed vane or piston pump can be considered to be constant for all practical purposes, as one rarely operates an oil-sealed pump in its compression- limiting region where the speed drops to zero. Large systems will have compound roughing pumps consisting of perhaps one or more vane or piston pumps and one or more lobe blowers. Small systems may use a scroll pump, screw pump, claw/lobe pump, or vane pump to rough from one atmosphere. The choice of pump depends, in part, on its base pressure.

Speed of roughing is also a consideration. If the pumping time cannot be met economically or quickly with a normal roughing system, an alternative solution can be considered. One such special-purpose roughing system uses one, two, or three sequentially operated pressure dividing tanks configured to remove most of the atmospheric gas in an extremely short time. This design is analogous to that described in Section 15.4.1 for multiple sorption pumps. There are four elements of the load lock cycle. Product is first loaded in the lock, then it is pumped. Next product is transferred to the adjacent process or holding chamber, and lastly the lock is vented to atmosphere. By taking advantage of ballast tanks, as well as the pressure drop produced when the process or holding chamber is opened, the pumping time portion of the load lock cycle can be reduced. During the remaining three parts of the load lock cycle, the ballast tanks can be re-evacuated. In particular situations, this can increase product throughput and profit for a nominal increase in system cost. Because water


aerosols ire formed during rapid pumping, this technique cannot be used in sensitive processes, unless aerosol generation is either unimportant or is dealt with by use of internal surfaces spaced close to the product surface. See below.

19.1.2 Aerosol Formation

Water aerosol is formed within a chamber when the pressure is suddenly reduced. The rapidly decreasing pressure causes the temperature to drop and causes the water vapor to become supersaturated. Supersaturated vapor quickly condenses, usually on micrometer- or nanometer-sized dust particles. A numerical simulation showed that water droplets grew to 45 pm diameter in 25 ms at 0.1 atm, and grew to 10 pm in 25 ms at a 1 atm [I]. Growth ceases when the source is depleted. The aerosol droplets collide with surfaces. Nucleating dust and any other atmospheric materials, such as ammonium or sulfur salts [2], remain on the surface after the water droplets evaporate. One may observe the aerosol with a flashlight in a darkened room.

In an isothermal system, the temperature of the gas is held constant by heat flow into the system. In an isentropic system, no heat flows into the system and the gas temperature drops abruptly. Over forty years ago, operators of large space chambers understood that an expanding gas does work on its surroundings, loses energy, and cools. They found that the temperature in the center of these large chambers tended to follow an isentropic expansion [3]. The relation between the pressure drop and temperature drop in an isentropic system is given by

(1 9.4)

where y is the ratio of specific heats of the gas. T and P are, respectively, the absolute temperature and absolute pressure of the gas.

In a real system, the behavior is neither isentropic nor isothermal, but lies somewhere between these two regimes; the temperature will drop, then relax to ambient temperature. A system with a spherical or cubical shape-that is, a system with a small surface-to-volume ratio-will behave more like an adiabatic system; its temperature will drop sharply and take a long time to return to ambient. A system containing many interior walls, with small, local volumes (large surface-to-volume ratio) will transfer heat rapidly to the gas during wall collisions; its gas temperature will drop only slightly and recover rapidly. Systems with high pumping speeds exhibit adiabatic behavior, whereas systems with low pumping speed tend to be isothermal.

Figure 19.2 illustrates the temperature changes recorded by several small-diameter thermocouples during the roughing of a 30-cm-diameter

19.1 PUMPING RATE

20

10

-30

363

I I 1 I I 1 I I 0 1 2 3 4 5 6 7

Time (s)

Fig. 19.2 Temperature versus time of fine thermocouples mounted in the center of a vacuum chamber and filled with either nitrogen or air. Reprinted with permission from J. Vuc. Sci. Technol. A, 9, p. 2802, J. F. O'Hanlon and J-J. Shieh. Copyright 1991, AVS-The Science and Technology Society.

chamber, 30 cm high. The thermocouples were located in the center of the volume; 55% RH air or dry N2 were individually pumped from atmospheric pressure. Expanding gas does work on its surroundings, cools, and then is warmed as gas-wall collisions transfer heat to the gas. The thermocouples consistently recorded the oscillations shown in Fig. 19.2, when surrounded by air. This was due to the formation of an insulating ice layer on the couples, and turbulent convection currents. Early particle counting measurement suggested that the particles observed during pumping from atmosphere were water droplets, which were formed while the temperature decreased [4].

During cooling, the air becomes supersaturated-that is, the relative humidity exceeds 100%-permitting one of several processes to nucleate an aerosol. Table 19.1 summarizes three critical nucleation processes described by Zhao et al. [2,5]. The presence of submicrometer dust particles will nucleate water condensation at just over 100% RH (critical saturation SC = 1 ), whereas homogeneous condensation requires a critical saturation of Sc = 3-8. These concepts were first recognized by Sir John Aiken [6] and later used by Charles T. R. Wilson in the development of his cloud chamber [7].


Table 19.1 Critical Saturation Ratio (Sc) for Given Condensation Process and Nucleia

Condensation Condensation Process Nuclei SC

Heterogeneous 20.1-0.002 JNn 1-3

A 100 8

?? E 60-

= 40- z 2 0 -

a

Y

i2 8O:

3

Q) >

Q)

.- -

Condensation on ions Negative ions (singly charged) Positive ions

-

I , . , . . -

4 6

Homogeneous Molecular clusters 3-8

Reprinted with permission fiom nermal m a m i c s and Particle Formation During Vacuum Pump-hwn, Ph.D. Dissertation, University of Minnesota, page 172. Copyright 1990, Jun Zhao.

Since water aerosol can transfer particles from the gas to the surface, it is important to develop a criterion by which supersaturation, and thus particle transport to a surface, can be avoided. This criterion uses a relationship between the initial relative humidity, critical supersaturation SC, and the degree of isothermal behavior Z. The expression for the Zhao factor [2.,5], requires knowledge of the pumping speed and chamber design.

T O z=- 5

(19.5)

o is the rate of heat flow fiom the wall; for stainless steel walls and air at STP, o = 0.0673 m / s [5] . z is the usual chamber time constant V/S (s). 5 = V/A the chamber volume-to-surface ratio, is given here in units of meters.

L

Fig. 19.3 Relationship between relative humidity, critical saturation ratio and the degree to which the chamber is isothermal or adiabatic. An aerosol will not be formed if the intersection of RH and Z lies to the right of a given saturation curve. Reprinted with permission from nermal Qmamics and Particle Formation During Vacuum Pump-Down, Ph.D. Dissertation, University of Minnesota, page 138. Copyright 1990, Jun Zhao.

19.1 PUMPING RATE 365

Figure 19.3 describes the relationship between these variables. It illustrates how the Zhao factor can be used to design an aerosol-free pumping process. First, one must know the initial relative humidity in the chamber to be pumped. Second, one assumes a nucleation mechanism, typically, this is the presence of ultrafine particles on which water can heterogeneously nucleate (the SC = 1 curve in Fig. 19.3). Next, one projects the intersection of the relative humidity and the SC = 1 curve to the horizontal axis and determines the minimum Zhao factor. To prevent heterogeneous nucleation, Z must be greater than the minimum value. If the chamber is known to be fiee of particles greater than 2-m diameter, then negative ions could nucleate particles and one should use the Sc = 4 curve to prevent nucleation on ions. (This is the basis of the Wilson Cloud Chamber.)

Zhao’s method provides a simple, direct way for designing a chamber and rough pumping speed combination, which will not produce aerosol- induced contamination during initial exhaust from atmosphere. If the chamber design is fixed, reducing the pumping speed may prevent aerosol formation-a process referred to as “soft roughing,” or “slow roughing.” One technique for reducing the roughing speed is based on a valve that opens in two steps. When opened the first step, the roughing begins at a reduced pumping speed. At a reduced pressure, say 0.1 atmosphere, the valve opens completely. This scheme may also be implemented with two valves: one that is the diameter of the roughing line connected in parallel with a small valve containing a choke. Another technique used a valve control algorithm that calculated the critical condensation parameters in real time; the pump throttle valve could then be opened dynamically [9]. This algorithm provided an optimally short roughing time.

If slow roughing adversely affects product throughput, other solutions may be considered. For example, one might purge the chamber with nitrogen to reduce the initial relative humidity; however, low-dew-point NZ must be used. One might locate the critical surfaces a few millimeters fiom a chamber wall, such that the conditions of the local volume between the surface and wall met the Zhao criterion. It has been shown that the gas between closely stacked silicon wafers could be exhausted radially at high speed without formation of an aerosol over the wafer surface [8].

19.2 CROSSOVER

System crossover-the point at which the roughing phase is ended and high vacuum phase commences-has been the subject of much over- simplification. The reason for this is straightforward; historically, the overwhelmin majority of vacuum systems have been small, say, a volume of a 0.1-.5 m , with a 4-cm-diameter roughing line. Many texts, including 8


earlier editions of this work, stated that one should not rough a chamber with an oil-sealed mechanical pump below a pressure of circa 10-15 Pa (1 00-1 50 mTorr), otherwise oil backstreaming would contaminate the chamber. In reality, there are two criteria that must be met to determine an acceptable crossover. (1): The crossover pressure must be above the pressure at which back-difising contaminants from an oil-sealed roughing pump could reach the chamber. (2): The pressure must be below the value at which the high vacuum pump would overload when connected to the chamber gas load. For the prototypical “small” vacuum system, the “100- mTorr rule” is valid, because at that pressure the chamber gas flow or quantity is less than the overload condition of the high vacuum pump.

Widespread use of large vacuum systems in myriad manufacturing applications such as semiconductors, coated paper and plastic films, security., decorative, and architectural glass coating, as well as the introduction of new types of roughing and high vacuum pumps has rendered the “1 00-mTorr rule” meaningless. Small dimision-pumped systems are no longer the norm.

Here we review the two relevant issues: oil backstreaming from oil- sealed roughing pumps and the operating condition or property that defines when a diffusion, turbomolecular, cryogenic, or ion pump is overloaded. The physical principle that defines overload is not the same for all pumps.

19.2.1 Oil Backstreaming

Oil backstreaming defines the minimum crossover pressure of an oil-sealed backing pump. Backstreaming of oil vapor been well-studied [10-131. In one case: the backstreaming of oil through a 30-cm-long x 25-mm-diameter roughing line connected to a 4.5-m3/h rotary vane mechanical pump was found to be small (-lo4 mg/min) at high pressures because of the viscous flushing action of the flowing air. At pressures below approximately 15 Pa the viscous flushing action was diminished until at a pressure of 1.3 Pa the backstreaming was 7x10” mg/min, or 70 times greater than at high pressures [ll]. Experimental data for a 9-cm-diameter x 220-cm-long pipe attached to an oil-sealed mechanical pump are illustrated in Fig. 19.4.

The counterflow dimision rate of the oil vapor is dependent on a number of variables including the gas flow rate, pressure, pipe diameter, mass of the exhaust gas and, most neglected, the roughing pipe length. Horikoshi and Yamaguchi [12] quantified the back dimision of molecular contaminants from an oil-sealed pump. They model the backstreamed concentration in the pipe as

(1 9.6)

19.2 CROSSOVER 367

10-1 F r-0 2

E z

10” 2

I O - ~ - 3

2 E“

Q A

Q: - 3 P

m 0 C m

L 3 0 Q: (3 a

- .-

-5 I o-’ s

lo-’oO 1 2 3 4 5 6 Nitrogen Flow Rate (sccm)

Fig. 19.4 Relation between partial pressures of backstreamed residual gases and the flow rate of nitrogen purge gas. Pipe length = 220 cm, pipe diameter = 9 cm. Reprinted with permission from J. Vuc. Sci. Technology A , 8, p. 2764, Y. Tsutsumi, S. Ueda, M. Ikegawa, and J. Kobayashi. Copyright 1990, AVSThe Science and Technology Society.

N , (x) and N , (0) are, respectively, the impurity concentsations at distances x

and 0 fiom the pump. LD is the impurity back diffusion length [ 121 given by

(19.7)

DI and DI-G are the diffision constants of the impurity molecule, respectively, when its mean free path is greater than the pipe diameter (2.30) and when the carrier gas is in viscous flow (2.35). N = hmol.l&o and VG is the average carrier gas velocity. Figure 19.5 sketches the fraction of a model impurity that would backstream in a 5.4-cm-diameter (2-in.) pipe connected to a 0.008 m3/s (17 c h ) pipe as a function of its internal pressure and length. Argon was used as the model impurity, as it is close in mass to the Wz=39 oil fragment; nitrogen was used as the carrier gas. These conditions and a pipe length of 0.5-to-1 m length represent those of the


Roughing Line Pressure (Pa) 1 0-2 10-1 1 10’ 102

- S = 0.008 m3/s

Roughing Line Pressure (Torr)

Fig. 19.5 Concentration of a model impurity, argon, counterflowing against a N2 flow using a relation developed by Horikoshi and Yamaguchi [ 121.

prototypical “small” vacuum system. The model demonstrates that such a system does not backstream oil when rough pumping is limited to pressures >10-15 Pa (100-150 mTorr). Note that this model does not account for surface migration. That can be reduced by lining a section of the pipe with a material such as Teflon, which is not wet by mineral oil.

Contamination from the roughing pump may be further reduced by use of a liquid nitrogen [14] or ambient temperature trap and low-vapor- pressure oil. The liquid nitrogen trap with closely spaced surfaces is the most effective, but the most difficult to maintain and consequently infrequently used. Ambient temperature traps do not require constant refrigeration but eventually saturate. Zeolite [ 15,161, alumina [ 16-1 81, and bronze or copper wool have been used for this purpose. Water vapor will soon saturate a Zeolite trap [11,19] and slow the roughing cycle. Zeolite traps can remove more than 99% of the contamination [15], but they generate particles that can drift into valve seats and into the pump and hasten wear. Kendall [20] designed a thermoelectrically cooled (-40°C) zeolite trap, which included a bypass valve to avoid water vapor saturation during the initial portion of the pumping cycle. Catalytic traps [22], which operate on the principle of oxidation and reduction of copper oxide, have been developed [21,22]. Hydrocarbons were oxidized to C02 and H20 in a heated catalyst. The catalyst was regenerated on exposure to air. All traps except LN2-cooled traps saturate. Experience has shown that traps are inadequately maintained and provide a false sense of protection.

Santeler [23] developed a gas purge technique to prevent mechanical pump vapors from contaminating a process chamber. The design is shown

19.2 CROSSOVER 369

in Fig. 19.6. Nitrogen or dry air is admitted to the roughing line at a point near the chamber isolation valve. The flow is adjusted so as to produce a pressure somewhat above that at which oil vapors could backstream, say 10 Pa for the example of the 54-mm-diameter pipe illustrated in Fig. 19.5. When crossover pressure is reached, the roughing valve is closed, and the preset flow prevents oil vapor back flow. Furthermore, it is impossible to pump the chamber below the set pressure.

Oil-free or “dry” roughing pumps do not have the concern of gross oil- vapor backstreaming. However, each has a distinct contamination issue with which to deal. For example, scroll pump users often add a 0.02-pn particle filter in the roughing line at the scroll pump entrance.

19.2.2 Overload Criteria

The crossover condition for a displacement pump is determined by its maximum flow. Cry0 pumps have a maximum impulsive heat load, and ion pumps have a maximum pressure above which they will not operate.

Diffusion Pumps

The maximum pressure at which a difksion pump may be connected to a process chamber corresponds to the point where the gas flow from the chamber Q = ddt(PV), just equals the pump’s maximum flow capacity. We noted in Chapter 12 that the rate at which energy is imparted to gas molecules is proportional to the electrical power delivered to the boiler. When the flow capacity of the pump is exceeded, the top jets will fail, because oil-gas scattering destroys the supersonic vapor stream. Continued

Purge Gas

Purge Gas High

Conta mi nated ,f Vacuum

Fig. 19.6 Preventing hydrocarbon contamination from the roughing pump from reaching the process chamber by use of a purge gas. Adapted with permission from J. Vac. Sci. Technol., 8, p. 299, D. J. Santeler. Copyright 1971, The American Vacuum Society.


L

Gas Flow -+

Fig. 19.7 Gas-flow-pressure characteristic of a difision pump and roughing pump. Adapted with permission from J. Vuc, Sci. Technol. A , 10, p. 2629, M. H. Hablanian. Copyright 1992, AVS-The Science and Technology Society.

increase of the inlet gas flow will cause the lower jets to fail in succession. The pressure-flow characteristics of a diffusion pump and roughing pump are shown in Fig. 19.7. Hablanian noted that the traditional pressure-speed plot masks the dependent variable [24]. The flow-pressure plot reinforces the concept that displacement pumps overload at a limiting gas flow. The sharpness of the flow-pressure behavior in the overload region is related to the safely factor that is used in the jet design. The gradual transition from under- to overloaded operation hides the fact that the top jet has failed and is backstreaming oil. It is best to crossover somewhat below the maximum gas flow (maximum throughput) provided by the pump manufacturer.

The actual gas flow fiom the chamber can be measured without additional metrology. Figure 19.8 illustrates the necessary measurement procedure. First, one fills the chamber with all product, substrate holders, deposition sources, and so on. Next, one pumps to an intended crossover pressure and quickly closes the roughing system valve. Last, one calculates the gas load released by the chamber Q = VAPlAt. This process is repeated for pressures above or below the first attempt until one arrives at a pressure where the loaded chamber outgassing rate is less than the maximum flow capacity of the high vacuum pump. If one knows the outgassing rates of the materials in the chamber and assumes that they are constant during the roughing cycle, the maximum crossover pressure will be

19.2 CROSSOVER 371

Fig. 19.8 Finding the proper crossover condition by performing a rate of rise measurement. Reprinted with permission fiom J. Vac, Sci. Technol. A, 10, p. 2629, M. H. Hablanian. Copyright 1992, AVS-The Science and Technology Society.

- Qmm

'net

-- Pcrossover, rnm (19.8)

where Sne, is the actual speed of the roughing system at the chamber entrance. When closing the roughing valve and opening the high vacuum valve, the pressure should drop quickly as the ratio of the speeds of high vacuum to roughing pump

ehrnber (after crossover) = Snet ehrnber (before crossover) ( 19.9)

If the pressure does not drop to this value immediately after crossover, then the diffusion pump is operating in its overload region [24]. In such a case, either a lower crossover pressure or a larger diffusion pump would be in order. This subject has been considered in detail by Hablanian [25].

'hzgh vacuum

Turbomolecular Pumps

Turbomolecular pumps like diffusion pumps, are displacement pumps. They overload at a maximum gas flow. However, at overload they behave differently than a diffusion pump. The rotational velocity of the blades will decrease, because the power to the drive motor is constant. Power is the product of rotational velocity and torque. Nesseldreher [26] has observed


gross backstreaming of heavy oil fragments through turbomolecular pumps when their rotational velocity decreased to 40% of maximum. However, most pumps will automatically remove drive power when o < 80% omm

The safe maximum crossover pressure corresponds to the maximum gas flow at which the rotational velocity o just begins to decrease. The speed decrease: will not be sudden, because momentum transfer comes from blades whose stages rotate as one. The shape of the overload region is dependent on the staging ratio, or relative sizes of the turbo and roughing pump. Figure 19.9 illustrates this dependence. One can consider the manufacturer's stated value of Qmm (the point at which the speed begins to decrease) to be the maximum gas flow. The maximum pressure at crossover is then given by

10

102 a

z 2 10' z

Y

3

LL

a, L

100 E m r 0

lo"

10"

1

Nitrogen Flow Rate (sccm) 10 ' 102 10 104 10

10

I I 1 I 1 102 10 10 1 0 5

Nitrogen Flow Rate (Pa-Us)

( 1 9.1 0)

Fig. 19.9 Chamber pressure as a function of gas throughput in a 3OO-Lh turbomolecular pump, measured individually for backing pumps with speeds of 4.17 L/s, 20 L/s, and 30 L/s. Dotted lines represent measured data. Solid lines are constant pumping-speed lines. Reprinted with permission from J. Vac. Sci. Technol. A, 14, p. 2858, N. Konishi, T. Shibata, and T. O h i . Copyright 1996, AVS-The Science and Technology Society.

19.2 CROSSOVER 373

Using the example in Fig. 19.9, one can see the effect of maximum crossover pressure on the roughing pump size. One must examine the pressure-speed performance characteristic of the roughing pump and ensure that its speed is retained at the lowest pressure-the actual crossover pressure-and not simply at the maximum crossover pressure.

Changing from an oil-sealed roughing pump to an oil-free roughing pump is of no value unless the replacement pump has adequate speed at the crossover pressure. For this reason, oil-free roughing pumps such as screw or scroll pumps are best suited for use with turbo-drag pumps and are not the best choice for use with conventional turbomolecular pumps.

Oil backstreaming of a turbomolecular pump at high values of inlet gas flow is a separate issue and is discussed in Chapter 22. Preventing foreline contamination from reaching the process chamber during power failure, when the rotational velocity decreases to zero, is described in Chapter 20.

Cryogenic Pumps

During crossover, the thermal load from the incoming gas from the chamber will cause the temperature of the two refrigerator stages to increase. The thermal load is proportional to the quantity of the gas impulse and its heat content. The heat capacity of the second (cold) stage is less than that of the first stage; it pumps everything except water vapor. Its temperature increase is a sensitive measure of pump capacity. Note the major distinction between a cry0 pump and the displacement pumps discussed previously. Displacement pumps overload at a critical gas flow (Q). Cry0 pumps overload at a critical instantaneous gas quantity (PV).

One recommended practice defines cry0 pump crossover as the maximum quantity of nitrogen (PV),, that can be condensed in a short time on the second stage of a newly regenerated cry0 pump, without the second-stage temperature exceeding 20 K [27]. The relation between this maximum impulsive heat loud (not gas flow) and the crossover pressure for small pump volumes is given by

(19.11)

Measured results for one cry0 pump are presented in Fig. 19.10. The first curve in this figure, no loading, shows that this pump has a heat load capacity of (PV),, x 20 Pa-m3 (150 Ton-L). This definition applies to an “unloaded,” or freshly regenerated pump. However, in normal service, the loading increases with time; eventually, the pump must be regenerated. Figure 19.10 illustrates the effect of gas (heat) loading on the second-stage temperature behavior during successive crossovers. As the loading


Fig. 19.10 Temperature of the cryopump second stage versus time for five different values of previously pumped nitrogen gas load on the second stage. In each case the nitrogen gas impulse load was P V = 20 Pa-m3 (150 Torr-L or 200 mbar-L) of nitrogen. Reprinted from Vacuum, 44, C. Juhnke, H. H. Klein, S. Schreck, U. Timm, H. U. Hafner, M. Mattem- Klosson arid H-J. Mundinger, 71 7-719, copyright 1993, with permission fiom Elsevier.

increases, the peak temperature decreases, but the time to recover to base temperaawe increases [28]. This interrelation between recovery time and accumulated gas load affects the design of cryo-pumped, rapid-cycle load locks. If the required load lock cycle time cannot be met, then either the gas load at crossover must be reduced (reduced crossover pressure) or the refrigeration capacity of the pump must be increased (increased pump size).

The actual crossover pressure must consider the prior gas loading when the system is cycled rapidly. The variable temperature recovery time of the cold stage is a consequence of the rate at which heat in the incoming gas can be removed and transported across the accumulated ice deposit.

Bentley [29] states a rule of thumb for the maximum gas load is Pv,lW2 I 4 Pa-m3/W (40 mbar-L/W or 30 Torr-L/W), where W2 is the refkigeration capacity (watts) of the second stage. If the gas burst is too large, water vapor in the gas burst could reach the adsorbent stage during viscous or transition flow and hinder the pumping of hydrogen and helium.

Ion Pumps

Ion pump crossover must take place below a maximum pressure, typically -1 0” Pa. Usually, small ion-pumped systems are rough-pumped with liquid-nitrogen-cooled sorption pumps followed by TSPs. Large systems might be roughed with a turbomolecular pump. As noted in Chapter 14, a plasma will form in an ion pump, if it is ignited at a pressure greater than

19.2 CROSSOVER 315

-10" Pa. The heat released by the plasma will warm the electrodes and release adsorbed gas. This causes the pressure to increase additionally, resulting in thermal runaway and a pump that no longer fbnctions.

The crossover-limiting criterion for an ion pump is a maximum pressure. The crossover criterion for turbo and diffusion pumps is a maximum gas flow. For a cry0 pump it is maximum impulsive heat load; thus, the maximum crossover pressure of a cry0 pump depends on gas heat capacity.

RERERENCES

1. J. Wu, D. W. Cooper, and R. J. Miller, J. Vac. Sci. Technol. A , 8, 1961 (1990). 2. J. Zhao, B. Y. H. Liu, and T. H. Kuehn, Solid State Techn., 33, 85 (1 990). 3. D. J. Santeler, D. W. Jones, D. H. Holkeboer, and F. Pagano, Vacuum Technologv and

Space Simulation, NASA SP-105, National Aeronautics and Space Administration, Washington, DC, 1966. p 80.

4. D. Chen, T. Seidel, S. E. Belinski, and S. Hackwood, J. Vac. Sci. Technol. A, 7, 3105 (1 989).

5. J. Zhao, Thermodynamics and Particle Formation During Vacuum Pump-Down, Ph.D. Dissertation, University of Minnesota, 1990.

6. John Aitken, Proc. Royal Society Edinburgh, 11, 1880; Trans. R SOC. Edinburgh, 30, 1880; and Nature, 23, 1880-1881.

7. C. T. R. Wilson, Proc. R. SOC., 86A, 285 (1911); Proc. R. Soc., 87A, 277 (1912). 8. J. F. O'Hanlon and J-J Shieh, J. Vac. Sci. Technol. A , 9,2802 (1991). 9. J. J. Wu, D. W. Copper, R. J. Miller, and J. E. Stem, Microcontamination, 8,27 (1990). 10. D. W. Jones and C. A. Tsonis, J. Vac. Sci. Technol., 1, 19 (1964). 11. M. A. Baker, L. Holland, and D. A. G. Stanton, J. Vac. Sci. Technol., 9,412 (1972). 12. G. Horikoshi and H. Yamaguchi, J. Vac. SOC. Japan, 25, 161 (1982). 13, Y. Tsutsumi, S. Ueda, M. Ikegawa, and J. Kobayashi, J. Vac. Sci. Technol. A , 8,2764

14. For example, see P. M. Danielson and F. C. Mrazek, J. Vac. Sci. Technol., 6, 423

15. M. J. Fulker, Vacuum, 18,445 (1968). 16. R. D. Craig, Vacuum, 20, 139 (1970). 17. L. Holland, Vacuum, 21,45 (1971). 18. M. A. Baker and G. H. Staniforth, Vacuum, 18,17 1968). 19. J. H. Singleton, J. Phys. E., 6, 685 (1973). 20. B. R. F. Kendall, Vacuum, 18,275 (1968). 21. R. Buhl, Vak-Tech., 30, 166, (1981). 22. T. Kraus, F. R. G. Patent No. 1,022,349, Aug. 28, 1956. 23. D. J. Santeler, J. Vac. Sci. Technol., 8,299 (1971). 24. M. H. Hablanian, J. Vac. Sci. Technol. A, 10,2629 (1992). 25. M. H. Hablanian, High Vacuum Technology: A Practical Guide, 2nd ed., Marcel

Dekker, New York, 1997. Chapter 10. 26. W. Nesseldreher, Vacuum, 26,281 (1976). 27. PNEUROP: Vacuum Pumps, Acceptance Specifications, Refrigerator Cooled

28. C. Juhnke, H. H. Klein, S. Schreck, U. Thimm, H. U. Htlfher, M. Mattem-Klosson, and

29. P. D. Bentley, Vacuum, 30, 145 (1980).

(1 989).

(1969).

Cryopumps, Part 5, PNRASRCC/5. Frankfort (1989).

H-J. Mundinger, Vacuum, 44,717 (1993).


PROBLEMS

19.1 Examine Fig. 10.9, curves C and D, which describe the pressure- speed dependence of a Roots-rotary pump set. Assume that this pump set is connected to a chamber of volume 5 m3 with a total outgassing rate (walls plus product load) of 100 Pa-L/s. Assume that all pumping lines have a large diameter, are short, and do not significantly reduce the speed of the pump set. Model the pumping speed of the set as two constant-speed segments. One segment Erom atmosphere until the Roots pump starts and a second for the Roots pump. (a) Calculate and plot the chamber pressure versus pumping time from atmosphere to a final pressure of 1 Pa. (b) Graphically determine the inlet pressure of the rotary pump at the point where the chamber pressure equals 1 Pa.

19.2 The nitrogen gas purge technique is illustrated in Fig. 19.6. Assume the mechanical pump has a pumping speed of 17 cfm, and the chamber has a volume of 0.8 m3. (a) Calculate the required flow of purge gas. (b) Calculate and plot the chamber pressure versus time from atmosphere to 10 Pa.

19.3 A diffusion pumped system consists of a 1-m3 chamber remotely connected to a 100 cfm roughing pump by a 10-m-long, 100-mm- diameter roughing line. The outgassing rate in the chamber was 2.4 Torr-L/s (mainly water vapor fi-om the product) at maximum crossover pressure. The diffusion pump has a limiting or maximum throughput of 3 Torr-L/s. Assume that the pumping speed of the two-stage mechanical pump is constant for the pressure range of interest. (a) Calculate the time to pump to the maximum crossover pressure. (b) With time, debris collects in the chamber, and increases the outgassing to 4 Torr-L/s. Assume that the initially measured outgassing rate had a time dependence off ' . Assume the value of 4 Tom-L applied at the initial crossover time calculated in (a). How much longer would the roughing have to operate before reaching maximum crossover pressure?

19.4 When crossing over &om rough to high vacuum pumping, the pressure in a small chamber will suddenly drop Erom -10 Pa to lo-* Pa when the gate valve is opened. The sudden drop is due mainly to the large pumping speed of the high vacuum pump and, to a small extent, the pressure divider effect of the evacuated volume between the gate valve and the pump entrance. (a) Show qualitatively that a water aerosol can form in the chamber during that brief instant. (b) How did early researchers observe this event?

PROBLEMS 377

19.5

19.6

19.7

Figure 19.9 illustrates the measured performance of a 300 L/s turbomolecular pump connected individually to one of three backing pumps. From this graphical data, determine the maximum permissible crossover pressure so that the turbo pump does not over load for mechanical pumps of (a) 4.17 L/s, (b) 20 L/s, (c) 30 L/s.

Assuming that the chamber in Problem 19.3 forms a water aerosol during roughing, comment on the suitability of attempting to reduce the relative humidity in the chamber using nitrogen gas to flush water vapor from the chamber prior to beginning the roughing cycle.

Determine whether a water aerosol will form when the chamber of Problem 19.2 is pumped from atmosphere if the R.H. is 50%. (a) If a water aerosol were found to form, what initial pumping speed would be required to prevent its formation? (b) Alternatively, would flushing the chamber with nitrogen gas be able to reduce the initial relative humidity sufficiently, so that the chamber could be pumped at the same rate?

Pressure dividers are used in some large, specialized vacuum systems to reduce pumping time and increase product throughput and economics. The roughing system, shown in Fig. 19.1 1, is part of an architectural glass coating system, and is used for Problems 19.8-19.10.

104 L

Fig. 19.11

19.8 The large ballast tank is first pumped to 6600 Pa (50 Torr). Load lock pumping begins by opening the connecting valve between the ballast tank and the load lock. At this time, the pressure in the lock suddenly drops to an intermediate value. (a) Assuming that the load lock was initially at atmospheric pressure, calculate the intermediate pressure after opening the connecting valve. (The holding tank pass-

378

19.9

ROUGH VACUUM PUMPING

through valve is closed.) (b) The ballast tank valve is then closed and the load lock pumped to a pressure of 13 Pa (100 mTorr) using only the pump set directly attached to the lock. The roughing valve connecting the pump to the lock is next closed and the isolation valve between the lock and the holding chamber is opened. Assuming that the pressure in the holding chamber was lo” Pa before opening the isolation valve, what is the pressure immediately after the isolation valve is opened, while product is being transferred from the load lock to the holding chamber? (c) If the holding chamber difksion pump will overload at a gas flow corresponding to 4.7 Pa (35 mTorr), is the pressure reduced sufficiently with the pressure divider effect so that the diffusion pump can be operated without overloading?

This system would not be efficient unless it operated on a rapid cycle. In order for the cycle time to be maintained, each pressure- dividing chamber must be repumped to its original pressure quickly. (a) After the ballast tank has been used, it must be pumped again to its starting pressure of 6600 Pa in 20 s to be ready for the next product cycle. What ballast tank pumping speed is required? (b) After the load lock reaches the intermediate pressure, it must be pumped quickly to 13 Pa (100 mTorr) using the load lock pump set. What pumping speed is required of the load lock pump set, if this is to be done in 8 s?

19.10 Rapidly pumping a large chamber may generate a water aerosol. The importance, if any, of water aerosol formation would depend on the nature of the product. Assume for this system a 3-m-long roughing line of 200-mm diameter. Assume that the length and width of the load lock are equal and its internal height is 125-mm. Assume the total internal surface area is 2x the wall area and 50% R.H. (a) Estimate the initial pumping speed of the load lock when it is first connected to the ballast tank. Hint: Use the method of Santeler described in Section 3.3.3 to deal with choked flow in a roughing line. (b) Estimate the degree of aerosol formation, based on where this falls on Zhao’s saturation curves, assuming nucleation begins on dust particles. (c) For economic reasons the pumping speed and product throughput cannot be reduced. What other options may be realistically considered to reduce water aerosol formation the load lock, assuming that the product consists of very-large-area (- 100-in wide x 144-in. long) glass sheets?

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