Pump Types and theory – by Varian

 

Diffusion Pump

Historical Notes

Producing low pressures is the function of vacuum pumps, one type of which is the diffusion pump. Diffusion pumps were first conceived and constructed by W. Gaede (1915-Germany) and I. Langmuir (1916-U.S.A). They operate on the principle of transferring momentum from high velocity vapor molecules to the gas molecules that are to be moved out of the system. The vapor molecules are formed by heating a suitable condensable fluid. The early pumps used mercury for this purpose.

 

In the late 1920s, C.R. Burch (England) and K.C. Hickman (U.S.A.) found that certain high molecular weight oils having high boiling points and low vapor pressures could be used as pumping fluids. These oils, not generally synthetic hydrocarbons, were useful because they remained in the pump indefinitely and allowed lower pressures to be attained without the use of a cold trap (see section on Baffles and Traps). Today, with the exception of a few isolated applications like some analytical instruments, all diffusion pumps utilize some form of oil. For additional information in this area, see the discussion on pumping fluids, below.

 

As industrial and scientific requirements for rarefied atmospheres increased, research and development into the nature and production of high vacuum increased. By the early 1940s, a well-developed vacuum technology existed and was intensified both during World War ll and by the space effort of the 1960s. Engineering has continued in the vacuum field and in 1965, Varian's M.H. Hablanian, et al. made a significant contribution to diffusion pump design that markedly increased pumping speeds.

Applications

Due to its simplicity, high performance, and low initial cost, the diffusion pump remains as the primary industrial high vacuum pumping mechanism. Applications for this type of pump are found in such diverse areas as:

·                  Analytical instruments

·                  Coating, functional

·                  Coating, ornamental

·                  Electron tube manufacture

·                  Metallurgy

·                  Optics

·                  Outer space simulation

·                  Particle accelerators

·                  Petrochemicals

·                  Pharmaceuticals

·                  R&D laboratories

·                  Semiconductor manufacture

By the proper choice of motive fluids, traps, baffles, and valves, diffusion pumps can be used in a wide variety of applications and over pressure ranges from 1 x 10-3 Torr to 2 x 10-11 Torr.

Basic Performance Factors

1.               Pumping speed is volume per unit time. It is generally specified in liters/second and is an important parameter in determining the ultimate pressure of a system. This is expressed by the relationship
Q = PS
Where:

o                               Q is the system gas load in Torr-liters/second

o                               P is the attainable pressure in Torr

o                               S is the effective pump speed at the system

o                               'Q' is the total leakage of the system which includes vapors given off by dirt and outgassing of internal surfaces as well as holes to the outside world. Ultimate pressure is also affected by such factors as the compression ratio for light gases and the nature of the pumping fluid.

2.               Maximum throughput is the pump's maximum mass gas transfer capability pressure x volume per unit time. It is generally specified in Torr-liters/second.

3.               Tolerable forepressure is the maximum allowable pressure in the foreline. It is maintained at or below this value by a suitably-sized mechanical foreline (backing) pump. If this pressure increases above that specified for a given pump, gas will diffuse back through the pump and pumping will stop. It should be noted that the size of this mechanical pump can affect the maximum throughput value.

4.               Backstreaming rate is the rate at which the pumping fluid vapor leaves the inlet opening of the pump, moving back in the direction of the system being pumped. It is measured in milligrams per cm2 per unit time and will vary with the type of motive fluid employed.

Operation

Diffusion pumps are vapor jet pumps that work on the basis of momentum transfer from a heavy high speed vapor molecule to a gas molecule. This results in the gas molecules being moved through the pump. With reference to Figure 1, the bottom of the pump contains an electric heater which is used to produce the vapor by heating the pumping (motive) fluid to its boiling point at reduced pressure.

This means that before the pump is started, it must be 'rough pumped' down to and held at an acceptable pressure, typically 10-1 Torr. (For information on rough pumping, see section on Primary Pumps.) To do otherwise will result in no pumping actionand possible damage to the pumping fluids. Once boiling of the fluid has begun, the vapor is forced up the central columns of the jet assembly. It then exits at each downward-directed jet in the form of a molecular curtain that impacts the water-cooled pump body. Here, the vapor condenses and runs back down to the boiler. This refluxing action continues as long as proper heat and forepressure are maintained.

As gas molecules from the system randomly enter the pump (molecular flow conditions), they encounter the top jet. Some of them are correctly impacted and driven on to the next jet. Subsequently, they reach the foreline where they are exhausted to the atmosphere by the mechanical backing pump.

 

The diffusion pump is similar in character to other compression pumps in that it develops a relatively high exhaust pressure compared to the inlet pressure. This compression ratio for an inlet pressure of 2 x 10-7 Torr and a foreline pressure of 2.0 x 10-1 would be ten million to one for most gases. Figure 2 shows how the pumping speed varies with pressure. Note that the speed remains constant from the 10-3 Torr scale to the X-10 Torr scale and then falls off as a result of the compression ratio for hydrogen and helium plus the vapor pressure contribution of the pumping fluid.

Typical plot of diffusion pump performance. Four regions are evident: 1) Effect of the pressure ratio limit; 2) Normal operating range with constant speed; 3) Throughput limited condition; 4) Effect of backing pump.

In the same way that the pump must be rough pumped before starting, so must the system to be evacuated by rough pumping prior to exposure to the pump. Exposing a hot pump to a rush of air at atmospheric pressure could be catastrophic for the equipment and possibly explosive, depending upon the pump fluid being used. For further information in this area, see the discussion on pumping fluids, below, and the section on valves.

Design Features

Design features unique to Varian diffusion pumps provide positive benefits to the customer, such as:

1.               Varian oil diffusion pumps incorporate an ejector stage as well as the full fractionation jets. This feature assures the user of constantly purified pumping fluid and the capability of maintaining low pressures.

2.               Varian oil diffusion pumps incorporate insulated jet drip shields which prevent re-boiling of oil droplets outside the jet assembly. This feature assures the user of the lowest backstreaming rates attainable.

3.               Varian water cooling coils are attached by a proprietary weld/brace technique. This special technique means excellent thermal contact and no chance for coils to 'melt' away from the pump body in cases of accidental overheating.

4.               Varian pumps all incorporate a water-cooled cold cap which reduces 98 percent of the backstreaming common to most diffusion pumps. Thus, the user is assured of a cleaner system.

5.               Varian water-cooled pumps incorporate the quick cool boiler coils, allowing faster shutdown of the system with no damage to the oil.

6.               Varian pumps utilize standard ASA flanges. This feature permits wide flexibility for mating with systems and other hardware.

7.               Varian (4-inch and larger) pumps have a thermal protection switch as a standard feature. This device prevents damage to the pump and surroundings due to overheating.

Pumping Fluids

In an oil diffusion pump, high speed heated oil vapor provides the kinetic energy that moves gas molecules to the foreline and prevents their back-migration. These oils may be derived from a petroleum base but more typically are synthesized from phthalates, sebacates, phenyl groups, or siloxanes.

 

To be an effective pumping fluid, the compound must have a relatively high molecular weight and a low vapor pressure at elevated temperatures. Other desirable properties are inertness and stability in order to resist chemical reaction and disintegration into undesirable fractions.

Phenyl ethers such as Neovac-SY and Santovac-5 are fairly resistant to oxidation and are used successfully around electronic devices. These oils polymerize into a conducting film when bombarded with electrons and thus do not promote static charge build-up. In addition, they are quite soluble and 'clean up' easily. Neovac-SY has the advantage of economy while Santovac-5 is more durable and has a lower vapor pressure.

 

For additional oxidation resistance, many applications lend themselves to the use of silicone fluids. These are phenyl siloxane compounds that polymerize as a non-conducting film that can allow static charge buildup and are difficult to 'clean up'. Two common fluids of this type are DC-704 and DC-705; the former has four phenyl groups and the latter has five. The DC-705 is, therefore, a heavier molecule, and it has a lower vapor pressure, so it is highly suitable for achieving very low pressures. However, it is somewhat less effective under high throughput conditions than DC-704, due to the fact that fewer molecules emerge from the top jet.

Another extremely stable fluid under reactive conditions is the fluorinated polyphenyl ether (Fomblin or Krytox). This oil is widely used in mechanical oil-sealed pumps where large amounts of oxygen are pumped. It is also suitable as a diffusion pump fluid where large quantities of oxygen or other reactive gases may be encountered.

 

Varian Low-Profile Water-Cooled Baffles combine 100% optical density with high conductance and unusually low overall height. They are especially useful in applications where clean operation down to 10E-8 Torr is required but cryogenic traps are not. With M-series pumps, these traps retain approximately 50% of the pumping speed. Mechanical refrigeration can be used to reduce the re-evaporation of pump fluid and attain a partial trapping effect.

 

The liquid nitrogen Cryotraps provide optimum performance for diffusion pumps. These traps offer optical density intercepting 100% of primary backstreaming while giving additional pumping speed for condensables. Each trap has a large built-in reservoir that gives long, unattended service. Cryogenic temperatures are maintained even as liquid nitrogen level drops. High conductance internal geometry achieves the highest possible pumping speed at the inlet taking full advantage of the diffusion pump's speed. Varian's Halo Baffles are used instead of a standard cold cap and are therefore integral to the pump, adding no height to the pump. They reduce primary backstreaming by approximately 90% while only cutting the pump speed by less than 80%, about half that of opaque chevron baffles. Pumps can be ordered with halo baffles installed or can be retrofitted in the field.

 

Extended Cold Caps are used in place of the standard cold caps in the VHS-series diffusion pumps. They reduce primary backstreaming to levels that cannot be measured by the American Vacuum Society standard collection methods. They can be ordered installed in a new pump or can be retrofitted in the field.

 

 

Ion Pumps

Thin films of reactive materials have been used for 'gas cleanup' or 'gettering' for over a century. The early electron tube makers were only able to mechanically pump to about 1 x 10^-4 mbar, but through the use of 'getters' flashed on the internal surfaces, pressures in the low 10^-7 mbar scale were attained. These getters were typically metals like barium, titanium, zirconium, or thorium. Gettering materials are still used in tubes today even though pressures of 1 x 10^-8 mbar are readily attained by the pumps in the manufacturing process.

Gettering was not employed extensively in vacuum systems until the 1960s, when it was found to be highly compatible with ion pumping. Titanium was the metal commonly used because of its availability and its ability to sublime readily over a moderate temperature range.

Applications

Due to cleanliness, bakeability, low power consumption, vibration-free operation, long pumping life, and high pumping speed, Titanium Sublimation Pumping (TSP) is the ideal cost-effective companion to ion pumping in ultrahigh vacuum.

Applications for this pumping mode are found in many areas, such as:

1.               Auger electron spectrometry

2.               Electron spectroscopy for chemical analysis

3.               Electron tube manufacturing

4.               Mass spectrometers

5.               Materials science conductor R&D

6.               Nuclear physics

7.               Outer space simulation

8.               Particle accelerators

9.               Secondary ions mass spectroscopy

10.            Solid state semiconductors

Basic Performance Factors

1. Pumping Speed. The pumping speed of a Ti film is proportional to the film area and to the sticking coefficient, that is the probability that an impinging gas molecule reacts with Ti forming a stable compound. The pumping speeds per unit area of a fresh evaporated Titanium film are reported in Table 1. Using these coefficients the intrinsic pumping speed (Si) of a Ti film can be evaluated using the following equation:

 

Si[l/s] = Coefficient x Surface

As the gas molecules react with the surface Ti atoms, the number of active sites decrease and, as a consequence, the pumping speed decreases. A plot of the specific pumping speed vs time at different pressures is reported in Figure 1. Using this plot it is possible to estimate how frequently the Ti film has to be renewed.

 

It must be noted that the actual pumping speed S of a TSP depends on the conductance C between the active surface and the vacuum vessel according to the following equation:

1/S = 1/C + 1/Si

 

2. Throughput. When the impingement rate of the gas molecules on the active film becomes higher than the Ti sublimation rate (excess of gas molecules respect the available Ti atoms), the pumping speed does not depend any more on the sticking coefficient. It is simply controlled by the quantity of the available Ti atoms according to stoichiometric reaction.

If n Ti atoms need to pump a gas molecule

example: 2Ti + N2 -> 2TiN, n = 2

the gas throughput Q is given by:

Q [mbar l/s] =        (0.13)/n R[gr/h]

 

where R is the Ti sublimation rate. In this condition the pumping speed is not constant but it depends on the pressure and is directly proportional to the sublimation rate (Figure 2).

 

 

3. Other factors. The overall performance of a titanium sublimation pump is a function of several variables, including gas species, pressure, gas temperature, getter film temperature, get ter film area, the geometry of the area, sublimation rate,sticking coefficient, and the conductance from the film to the area being evacuated. For further information, write for 'Predicting and Evaluating Titanium Sublimation Pump Performance' by D.J. Harra, 1974 (Vacuum Report VR-88).

Operation

Titanium Sublimation Pumping is accomplished by coating the inner surfaces of a vacuum system with sublimed titanium films. Since it involves a chemical reaction, this kind of pumping is useful where mainly active gases are present. The pumping speed of a unit area varies with various reactive gas species as shown in the following table. It can also be seen that cooling the substrate to liquid nitrogen temperature markedly increases the speed for hydrogen and water.

The gases thus 'gettered' form stable compounds with titanium and are stored in the system as such until they are removed by cleaning. Since there is generally un-reacted pyrophoric titanium buried in the deposits, caution should be used in cleaning. If the desired gas throughput is known (Q = pumping speed x pressure) the maximum theoretical operating time is given by:

Operating time [h] =  0.13  T [gr]

                        ______      _______

                             n      Q [mbar l/s]

where T is the usable Titanium.

For example, using our cryopanel at 1 x 10-8 mbar with a three filaments cartridge Ti source, the theoretical operating time is given by:

0.13        3.6 [gr]    = 46,800 hrs = about 5 years

___         _________________

2             500 [l/s] x 10-8 [mbar]

 

After this time the filament cartridge should be replaced.

Design Features

·                  All sources are mounted on 23 4" Varian ConFlat Flanges and fit through 11 2" ports.

·                  The three-filament source contains 3.6 grams of useful titanium.

·                  The Varian Mini Ti-Ball source contains 15 grams of useful titanium.

·                  The Varian Standard Ti-Ball source contains 35 grams of useful titanium.

·                  Control units for the Ti-Ball source configuration automatically maintain the temperature dependent titanium phase change and thereby insure the most efficient use of the available getter.

 

Turbo Pumps

Turbomolecular Pump Bearings

The Turbo-V pumps incorporate Varian's innovative ceramic bearing design with a proprietary ultra-low vapor pressure solid lubricant, which enables these pumps to provide a long service life and a high degree of cleanliness under many operating conditions. The ceramic bearings utilize balls made of silicon nitride, a polycristalline material with an amorphous intergranular binder base that offers the following advantages.

Hardness This is a critical aspect of bearing design, and it closely relates to bearing performance and reliability. The silicon nitride material used in Varian's Turbo-V bearing system is twice as hard as conventional steel providing dramatic improvement in wear resistance while minimizing the effects of surface contact and stress.

 

Weight Silicon nitride is 40% less dense than conventional steels, which helps to reduce centrifugal loading and stress levels at high rotational speeds, especially in the bearing race area.

 

Friction Silicon nitride's low coefficient of friction enhances wear resistance and adds to the bearing's operational life.

 

Thermal stability With its low thermal expansion coefficient, the silicon nitride bearing material ensures that tight tolerances and mating component fit will be maintained over an extremely wide temperature range. In addition, silicon nitride has an outstanding resistance to fracture by thermal shock.

 

Chemical Stability Silicon nitride is virtually inert.

Another feature of the Turbo-V bearing system is its proprietary lubricant which has an extremely low vapor pressure and is virtually hydrocarbon-free. The use of this lubricant in the permanently sealed bearing system ensures clean, reliable operation without the need for any maintenance whatsoever.

General Care

There are some simple steps you can take that will further enhance the efficient operation of the Turbo-V pump. For example, sudden exposure to atmospheric pressure should be avoided because this will produce an overload condition. The bearing temperature (displayed on the control panel) should be kept within proper limits to avoid operation in poor lubrication conditions. This will be achieved if the cooling instructions are followed.

 

A vibration monitor can also be useful for preventive maintenance, as an increase in the bearing vibration levels may be an indication that replacement of the bearings is required.

When the pump is initially installed, or after a long period of inactivity, it is good practice to use a soft start sequence, which will gradually accelerate the rotor to full speed, allowing proper redistribution of the lubricant in the bearings.

The MacroTorr Concept

The Varian award-winning, patented MacroTorr design, which was developed in 1991, is the result of the improvement of the original design of the Gaede molecular pump.

·                  It is based on the idea of substituting (rather than adding) molecular impeller disks to some turbo-bladed stages.

·                  The molecular impellers consist of a disk rotating in a channel in which the inlet and outlet are divided by a stripper.

·                  The cross section of the channels decreases from the top to the bottom of the pump (from high vacuum to low vacuum or from the low pressure to the high pressure zone).

·                  Gas molecules gain momentum after each collision with the moving surface of the impellers.

·                  The gas is then forced to pass through a hole to the next stage due to the stripper.

 

How to Select a Turbo-V Pump

The right choice of a turbomolecular pump depends on the application. As a general rule, we can reduce the choice to two types of use:

·                  HV and UHV operation (low gas load)

·                  gas handling operation

The first choice includes most cases in which the turbomolecular pump is employed to create vacuum in systems where the gas load is produced only by outgassing. In this application, the choice is typically based on the desired base pressure within a desired time as a function of the foreseen outgassing rate, i.e

where:

·                  p is the desired base pressure (mbar)

·                  Q is the total outgassing rate at the desired time (mbar l/s)

·                  Seff is the effective pumping speed

The second choice relates to all operations where process gases must be used. The main parameters are, therefore, the desired operating pressure and the process gas flow

where Q' is the total gas flow and p' is the operating pressure.

How to Select the Backing Pump

The selection of a backing pump should be based on the required roughing time and the minimum size of the pump for properly backing the turbopump.

Roughing

Once the desired roughing time is established, the size of the forepump can be determined through the following formula:

where

·                  Sforeline is the pumping speed of the roughing pump (l/min)

·                  V is the volume of the chamber to be evacuated (l)

·                  t is the desired roughing time (min)

·                  p0 is the starting pressure (mbar)

·                  p1 is the end pressure (mbar)

When using a foreline pump much larger than the recommended size, a by-pass line might be necessary to achieve the calculated roughing time.

 

Backing

The backing pump must be big enough to achieve an effective pumping speed as close as possible to the nominal speed.

where

• Sforeline is the pumping speed of the foreline pump
• Q is the gas load
• p is the operating foreline pressure

It should be noted that Q is the total gas load on the pump and includes process gases and turbo purge gases when used.

The size of the backing pump can be calculated according to the following rule:

where

• S is the pumping speed of the turbopump
• Sforeline is the pumping speed of the backing pump
• K is the maximum compression ratio of the turbopump
  for a given gas (i.e. process gas) at the operating foreline pressure

The pumping speed of the backing pump should be the highest of the two values calculated as above (roughing and backing).

 

It is possible to use dry diaphragm pumps for hydrocarbon-free operation when MacroTorr-type pumps are used

Vacuum Gauges

Historical Notes

Early interest in pressure measurement was stimulated in the 17th century by engineers who were concerned about the inability of suction pumps to remove water from mines. The pumps were limited to about 30 feet. For example, the Duke of Tuscany (Italy) commissioned Galileo to investigate the problem.

 

Galileo, among others, devised a number of experiments to investigate the properties of air.

Among these experiments were pistons for measuring the force of vacuum' and a water barometer that stood about 34 feet tall.

 

After Galileo's death in 1642, the work was carried on by his associate, Evangelista Torricelli. Torricelli invented the mercury barometer (Figure 1); he concluded that atmospheric air forced water up to a height of 33.6 feet.

Figure 1 Notes

1. Air has weight and mass
   • 2 Ibs. per cubic yard, or
   • 1.293 grams per liter
2. Pressure = force per unit area
3. The Barometer At 'standard conditions', the height of the Hg column above the surface of the Hg In the dish will be 760 mm or 29.9". The density of mercury is 0.49 Ibs. per cubic inch and, if the column is 29.9" high, it would then exert a force per unit area of 0.49 x 29.9 = 14.7 pounds/In2.

 

The weight of the atmosphere exerts a force of 14.7 pounds per square inch on the surface of the Hg in the dish. The height of the mercury column is therefore a direct measure of the pressure and the unit of pressure is 1/760 of an atmosphere, which is called a Torr. The international pressure unit is Pascal, equal to one Newton per meter square.

 

In 1644, the French mathematician, Blaise Pascal, sent a group of mountaineers up into the Alps with a barometer and proved that air pressure decreased with altitude. The average height of the mercury column at sea level is 760mm, and this is defined as a standard atmosphere. This also is 1.01 x 10⁵ Pascals or 1.01 x 10⁵ dynes cm2. The 1/760 of this value is called a Torr in honor of Torricelli.

 

An extension of the mercury barometer was the mercury U-tube manometer (Figure 2). Varying atmospheric pressures causes the mercury level to rise and fall in the 'Torricellian Void.' Likewise, if the pressure at the other end of the tube is artificially reduced by a vacuum pump, the mercury in the tube falls drastically.

 

With both the barometer and the manometer, it is the difference in heights of the mercury levels that indicates the pressure, that is, the force (weight of Hg) per unit area that the air pressure will support.

 

As the pressure on the system side is reduced, the height of the columns on either side of the U-tube approaches the same, and any difference becomes very difficult to measure (Figure 2).

Many schemes were tried to magnify the very small differences that occurred at very low pressures. But the only one that really extended the range of the manometer was invented by H. McLeod in 1872.

 

This gauge is an application of Boyle's Law and is still in use today as a standard for calibrating secondary gauges (Figure 3).

 

P2 V2
(P1 + h) bh = P1V1
P1 bh + bh3 = P1V1
bh3 = P1V1 - P1 bh)
bh3 = P1 (V1 - bh)

V1 bh
V1 = Total volume, capillary plus bulb (cm3)
P1 = Pressure in system
b = Volume of capillary (in cubic cm)
mm length
h = Difference in height of mercury columns
V2 = bh (cm3) volume in capillary
P2 = Pressure in capillary = P1 + h

Applications

The vacuum gauges in use today mainly fall into three categories: mechanical, manometric, and electronic. Which gauge is used in a particular application generally depends on the pressure range it is intended to measure. Figure 4 shows useful pressure ranges of some typical gauges.

High pressures such as those found in the rough pumping of a vacuum system are generally measured with a thermocouple gauge. This instrument measures heat transfer rate from a heated wire. As gas is removed from the system, less heat is removed. The changes in temperature are measured by a thermocouple junction and its output is displayed as changes in pressure. The most useful pressure range for this gauge is from 5 Torr to 5 microns.

 

At lower pressures from 1 x 10^-2 Torr to 1 x 10^-7 Torr found in many industrial applications, the cold cathode gauge is very useful.

 

This instrument is basically a gaseous electric discharge cell which operates on the same principle as a diode-type ion pump. It is a rugged gauge that does not use a hot filament.

The most commonly used measurement device for high vacuum is the hot filament ionization gauge. This type of gauge can be designed to measure pressures as high as 5 x 10^-1 Torr, and as low as 5 x 10^-12 Torr. Since it is found in many industrial and scientific applications, it will be treated here in more detail.

Basic Performance Factors of Ionization Gauges

I+ = (I-)PK (1)

Where: (P) is the number of molecules present (Pressure)

And: (K) is the gauge constant which depends on the geometry of the device and the electrical parameters employed (K) is also referred to as the sensitivity (S), and: S = I+
PI-

Where: Both (1+) and (1-) are measured in amperes and (P) is in Torr.
Hence: S = (I+) Amps = (a number)Torr (I-) Amps Torr. For instance, the sensitivity of the Varian UHV-24 nude gauge is 25 per Torr.

Operation of Ionization Gauges

When an ionization gauge is used to measure pressure, two physically observable parameters are of interest; namely, emission current and ion current. These two currents must be observed simultaneously, and the pressure can be calculated by the following rule:

 

P = (I+) 1
(I-) S
I+ = Observed ion current
I- = Observed emission current
S = Gauge sensitivity (constant for any particular gauge)

 

Although the McLeod gauge uses mercury in a way different from the manometer, it still expresses pressure in terms of the height of a mercury column. At the pressures attained by modern vacuum systems, gauges that depend on the mechanical effects of pressure are ineffective. So, other means had to be found that could take advantage of other properties of atoms and molecules, such as heat conductivity or the ability to be ionized.

 

There are many ways to express pressure and some of the more common units are listed in Table 1.

The gauge sensitivity, S, is a function of the design and construction of the gauge. For the Varian 563 Bayard-Alpert gauge, S has a nominal value of 10 Torr. For the Varian 507 Triode gauge, S has a nominal value of 17 Torr.

 

One could use an instrument that measured both the I+ and I- currents with a high degree of accuracy. However, the absolute values of I+ and I- are unimportant in determining P; only their ratio must be measured. Therefore, entirely equivalent results can be obtained with an instrument which measures I+ as a fraction of I- (a ratiometric instrument).

 

It is convenient to maintain a constant emission current at a preselected value, rather than to observe it for each measurement of pressure. Thus, in some gauge controls, the emission current is regulated at a nominal value of 9 ma (Bayard-Alpert) or 6 ma (triode). This value of emission leads to ion currents equal to 0.1 amp/Torr. During the calibration procedure, emission current is sensed by the electrometer amplifier and displayed on the panel meter.

 

When the ion current is measured, the same amplifier and meter are used. Hence, the meter deflection observed during ion current measurement is automatically interpreted as a fraction of emission current, providing a true ratiometric measurement, even though the absolute value of emission current may be 20% different from nominal.

 

In conventional gauge controls, emission current is measured by the panel meter with suitable precision resistor shunts. The ion current is then amplified by an electrometer and displayed on the meter. Overall accuracy of this kind of system depends on the individual accuracies of the shunt resistors, the meter movement, and the electrometer gain. Since these items all function independently, the errors can add up.

 

However, in the Varian ionization gauge controls, the electrometer and meter are always used together, whether measuring emission or ion currents. Thus, some of the errors are cancelled in the calibration process. As a result, the accuracy of the Varian controls is not critically dependent on the tolerances of a large number of components, and long-term accuracy and repeatability are assured.

 

 

Some additional conversion factors for pressure and flow units are shown in Table 2.

Pressures

Standard atm = 1.01325 x 10⁶ Dynes cm^-2
= 760 mm Hg (at 0 C)
= 29.9213" of Hg (at 32 F)
Bar = 1 x 10⁶ Dynes cm^-2
75.0062 cm Hg (at 0 C)
0.986 atm
Torr = 1333 Dynes cm^-2
= 1 mm Hg (at 0 C)
= (760)-1 Standard atm
Micron = (length) 1000 Angstroms
(1A = 10-10 meter)
= (pressure) 1.33 Dyne cm^-2
= (pressure) 1 x 10^-3 Torr
Barye = 1 Dyne cm^-2
= 9.869 x 10^-7 atm
= 1 x 10^-6 Bar

Dyne = Force necessary to give a one-gram mass an acceleration of one cm/sec/sec
Flows
Cubic Foot = 28.3 liters
= 2.83 x 10⁴ cm³
CFM = 28.3 liters min-1
= 0.47 liter sec-1
Liter sec-1 = 2.12 cfm
= 3.53 x 10^-2 cubic feet sec-1

 

Vacuum Valves

Historical Notes

Vacuum valves of a crude type date back to the early 1600's, and were originally adapted from the stopcocks used to tap barrels of various fluids. Improvements in valve design by Robert Hooke in England and Otto Von Gureicke in Germany reduced leakage into the vacuum environment and thus permitted work at lower pressures and for longer time periods. No improvements on these designs were made until the 1800's, when the requirements for machinery in the industrial revolution became more sophisticated.

Applications

The field of vacuum can be roughly divided into three areas: Low vacuum, which is relatively high pressure (10-1 to 10-4 Torr); medium vacuum (10-4 to 10-7 Torr); and high vacuum (below 1 x 10-7 Torr). A further subdivision for pressures below 1 x 10-9 Torr is often included and called ultrahigh vacuum. The vacuum hardware for each area requires special design features to match the type of application, and this is naturally true of valves.

 

Valves used in low vacuum are assembled from castings of brass or aluminum and have elastomer seals, such as Viton O-Rings. Valves used in medium vacuum are usually brass or aluminum and will generally incorporate a hydroformed bronze bellows as a shaft seal. High vacuum valves are usually made from stainless steel (typically, series 304); use welded stainless steel or inconel bellows as a shaft seal; and use either Viton, Polyimide, or a metal gasket for the main seal. Valves used in ultrahigh vacuum applications will always use metal gaskets (typically, copper or gold) because of the baking requirement (250 to 400 C).

Common Applications

1. Low vacuum
   • Freeze drying
   • Food processing
   • Metal ore refining
   • Steam plant condensers
   • Vacuum Distillation
2. Medium vacuum
   • Decorative coatings
   • Functional coatings
   • Chemical processes
   • Electron microscopes
   • Microscopy sample processing
3. High/Ultrahigh vacuum
   • Physics research
     • Optics
     • High energy
   • Semiconductor manufacturing
   • Electron tube manufacturing
   • Surface analysis (Auger Spectroscopy)
   • Molecular beam epitaxy
   • Outer space simulation

The last three high vacuum applications must be able to attain pressures of 1 x 10-10 Torr or less.

Valve Choice Criteria

Once a chamber design is planned and the effective pumping speed needed in order to attain the required pressure is determined, the remaining plumbing configuration should be laid out according to the pressure range, space limitations, and process requirements. Based on the chamber pressure required, a valve and other components for the plumbing configuration should be assumed and tested for suitability in the next steps of the selection procedure.

 

Certain criteria have already been established for the choice of valve. The pressure range and cleanliness level required for the process of interest will determine the vacuum level, materials of construction, and bakeability needed for the valve. Since delivered pumping speed (Sd) will rely on conductance, and the conductance of the valve relies on pipe size, a larger size (higher conductance) should be chosen for higher chamber pumping speeds. Choice of pump and other components will help to determine how the valve will connect to the system (solder, weld, ConFlat flange, etc.), and space limitations and conductance will influence the flow pattern that is chosen. Contact a Varian sales engineer for assistance in selecting materials of construction, bakeability, conductance, flow patterns, and connections that are appropriate for your application.

Narrow Down Your Valve Choice

The next step in valve selection is to determine the means of actuation needed for the valve. The intended type of valve may be available with a choice of manual, air-operated, or electromagnetic actuation. In making this choice, parameters to be considered are frequency of use, utilities required, and whether operator attention is required during the stage of the process at which the valve is to be actuated.

 

Remember, Varian stocks many types of valves. Just contact your Varian sales engineer for immediate information on application, price, and delivery of any of our valves.

Varian Leak Detectors

Historical Notes

The helium mass spectrometer leak detector was born of necessity during the Manhattan Project in the early 1940s. Since then, it has become, through successive technical innovations, an increasingly useful device for quality control of a wide variety of systems and products. The mass spectrometer dates back to 1912 when the British physicist, J.J. Thompson, used a magnetic field to separate various gas ions by their mass-to-charge ratios. See Figure 1.

Development of nuclear devices under the Manhattan Project required the utmost integrity of gaseous diffusion processing systems, a tightness far beyond all existing means of measurement or detection. As in other phases of the project, the uniqueness of the requirement demanded a new and different technique for identifying and measuring leakage paths smaller than previously known to exist.

 

The technique and tracer gas selected were tailormade for this stringent requirement. Helium was selected as the tracer primarily because, as the lightest inert gas, it penetrated small leaks readily. Helium was also known to be nontoxic, nonhazardous, nondestructive, plentiful, relatively inexpensive, and present in the atmosphere only in minute quantities (5 ppm). These features enable it to go through small passages without affecting parts or processes, and be detected easily, reliably, and economically.

Applications

Some applications include testing for and measuring the vacuum integrity of:

1. Components

a. Automotive parts
b. Bellows
c. Beverage and food containers

• Pressurized fluids
• Vacuum-packed solids

d. Electronic feedthroughs
e. Electron tubes
f. Glass-to-metal seals
g. Heat exchangers
h. Hermetic seals
i. Medical devices
j. Nuclear components
k. Quartz crystal packages
l. Refrigeration parts
m. Relays
n. Semiconductor packages
o. Transducers
p. Vacuum hardware
q. Vacuum switches

2. Systems

a. Electron microscopes
b. Evaporation coils
c. Freeze dryers
d. Ion implanters
e. Nuclear reactors
f. Plumbing
g. Space simulation
h. Sputtering
i. Vacuum evaporators
j. Vacuum furnaces

 

Basic Performance Factors

1. Sensitivity is the minimum detectable partial pressure of helium in the spectrometer tube. This factor relates to the minimum detectable leak rate in std cc per sec for helium molecules (1 std cc/sec = 0.76 Torr-liter/sec). This number is sometimes specified in terms of air molecules, which results in a lower number, and at first glance, appears better than the number for helium molecules.

 

2. Cycle time is the minimum time constant for the machine between leak check cycles. This time includes pumpdown, crossover, response time, and cleanup. It basically relates to the design and efficiency of the vacuum system.

 

3. Maximum detectable leak is the gross leak detection capability and is limited by the maximum pressure that the system can tolerate.

 

4. Ease of operation and maintenance. This factor is basically the cost of training, errors, and maintenance. It relates directly to uptime.

 

5. Fail-safe operation is the machine's immunity to accidents. This can include utility failures and sudden exposure of the vacuum system to full atmospheric pressure. It relates directly to uptime and maintenance cost.

Operation

A mass spectrometer is a machine that can produce, sort, and detect gaseous ion species. As such, it requires a high vacuum to operate effectively.

 

A helium leak detector basically consists of a simple mass spectrometer tuned to helium and an associated high vacuum system.

 

Helium is extremely well-suited as a probe (test) gas because:
1. It is a small light atom that can penetrate small holes and move rapidly. This latter quality reduces both response and clean-up time.
2. It is inert, therefore non-reactive and non-toxic.
3. It has an isolated peak in the spectrum that does not require the use of a high resolution instrument.
4. It is readily available and inexpensive, and it also has a low atmospheric partial pressure, so it presents a low background signal.

 

A helium mass spectrometer leak detector (HMSLD) is a complete system for locating and/or measuring the size of leaks into or out of a device or a container. In use, this method of leak detection is initiated when the tracer gas, helium, is introduced to a test part that is connected to the HMSLD system. The helium from the test part leak travels through the system, its partial pressure is measured, and results are displayed on a leak rate measurement meter. The HMSLD operating principle consists of ionizing gases in a vacuum and accelerating the various ions through an electric and magnetic field. The helium ions are separated and collected, and the resulting ion current is amplified and measured.

 

The heart of the leak detector is the mass spectrometer tube (spec tube) together with its control circuitry. See Figure 2.

The spectrometer tube is typically designed and adjusted so that hydrogen ions are deflected 135, helium ions 90, and all heavier species less than 90 , as shown in Figure 2. Consequently, only helium ions pass through the field exit slits and arrive at the collector. The collector current is, therefore, proportional to the partial pressure of helium in the spectrometer tube and, within the normal operating pressure range of the HMSLD, is not affected by the pressure of other residual gases. The collector current is measured by an electrometer amplifier and displayed on the meter as a leak rate rather than as a partial pressure.

 

The Varian spectrometer tube, illustrated in Figure 2, shows a cold cathode ionization gauge as an integral part of the tube. This gauge serves two functions. First, it monitors total pressure within the tube to initiate protection of the filament in case of an excessive pressure rise. Second, because of its location between the filament and the vacuum system, it minimizes the quantity of gaseous hydrocarbons reaching and damaging the filament.

There are basically two accepted approaches to helium leak detection the conventional system shown in Figure 4, and the Contra-Flow method shown in Figure 5.

 

With an HMSLD such as this, the part to be tested is simply coupled to the test port. A switch is manually actuated, whereupon the part is automatically evacuated to a suitable pressure and then valved off from the rough vacuum system and onto the HMSLD spectrometer tube vacuum system. A lamp indicates when this process is complete, and leak testing can be started. At the conclusion of leak testing, the switch is returned to its original position, the test port is automatically isolated from the HMSLD and vented to air, and the test piece is ready to be uncoupled from the leak detector.

 

A newer development is the Contra-Flow leak detector, shown in Figure 5. The Contra-Flow technique takes advantage of the differences in compression ratios (outlet pressure divided by inlet pressure) produced by the diffusion pump for gases of different molecular weights. For example, the maximum compression ratio for helium may be 10 or 100, while for oxygen, nitrogen, and other gases contained in air, the ratios are normally far in excess of 1 million.

This principle is implemented in the leak detector by introducing helium (and other inlet gases, such as those resulting from a leak in the test piece) into the high vacuum pump outlet (foreline) rather than into the 'normal' pump inlet, as in conventional leak detectors. Helium, having a much lower maximum compression ratio than other gases contained in air, diffuses backwards through the pump to reach the spectrometer tube, where it is detected in the normal manner. Although the mechanical pump is also attached to the high vacuum pump foreline and removes all inlet gases, including helium, there is no appreciable loss of sensitivity in the Contra-Flow leak detector. (The sensitivity of the automatic conventional model is 10-11 std cc/sec, while the sensitivity of the Contra-Flow is 10-10 std cc/sec.)

 

One advantage of the Contra-Flow is that no liquid nitrogen is required, a feature that results in a substantial reduction in operating cost. Another advantage is the ability to begin leak detection at a higher (rough vacuum) pressure. This allows the presence of outgassing during leak detection. Conventional leak detectors must rough pump to a much lower pressure before they can be used to detect leaks or must use some sort of throttling device that admits only a small portion of the gas stream, with subsequent loss of sensitivity.