Vacuum Basics

VACUUM CONCEPTS
What is Pressure?
Imagine a closed container with air, or some other gas, inside. Gases are composed of molecules that you can imagine as round elastic balls. Molecules move in straight lines until they collide with neighboring molecules or the container wall. Molecules of gas hitting the wall impose a force on the wall. The amount of this impact force per area of the container inner walls is called pressure. Since gases are considered fluids, the “pressure” exerted by these collisions applies to all surfaces of the container no matter what their orientation.

At room temperature and normal atmospheric pressure, one cubic foot of air contains approximately 7×1023 molecules moving in random directions and at speeds of around 1,000 miles per hour. The momentum exchange imparted to the walls is equal to a force of 14.7 pounds for every square inch of wall area. This atmospheric pressure can be expressed in a number of different units, but is commonly expressed in terms of the weight of a column of mercury of unit cross section and 29.92 inches (760 mm) high. Thus, one standard atmosphere equals 29.92 in. Hg, but to avoid the anomaly of equating apparently different units, a term, torr, has been postulated. One standard atmosphere = 29.92 in. or 760 torr (1 torr =1 mm Hg).

The Table below shows conversion factors for the most common units of pressure:

From To Mult by:
atmosphere in. Hg 29.9213
atmosphere torr or mm Hg 760.00
in. Hg atmosphere 3.342 x 10-2
in. Hg torr or mm Hg 25.4
in. Hg mbar 33.85
mbar in. Hg 2.954 x 10-2
torr or mm Hg atmosphere 1.3158 x 10-3
torr or mm Hg in. Hg 3.937 x 10-2
torr or mm Hg mbar 1.3328

What is Vacuum?
Vacuum is defined as a space that is entirely devoid of matter; i. e., an enclosed volume that is not filled with air or any other gases. Ideal vacuum conditions can be found in interstellar space, where there is a particle density of one atom per cm3.

It is important to note that vacuum is NOT a “sucking” process. A molecule is only removed from a chamber when it enters the pump via random collisions. It is a common mistake to think a vacuum pump sucks gas from a chamber. There is no such force as suction. If gas molecules in one “section” of an enclosed volume are removed, then molecules from the remaining volume, in their normal random, high-speed flight, collide and bounce off walls until they fill the total space at a lower pressure. Expressing it differently, until a molecule, propelled by random collisions, enters the pumping mechanism of a pump, it cannot be removed from the chamber. The pump does not reach out, grab a molecule from the chamber, and suck it in. Grasping that basic fact makes all other aspects of vacuum easy to understand.

Vacuum Technology
Vacuum Technology is the term applied to all processes and physical measurement carried out under conditions of below-normal atmospheric pressure.
A process or physical measurement is generally performed under vacuum for one of the following reasons:
1. Low grade vacuum where a vacuum serves only as a source of pressure, as for example the application of a ‘suction’ at one end of a pipe to cause the same flow which could be produced by a pressure at the other end.

2. Air avoidance applications where it is merely desired to avoid some undesirable physical or chemical property of one or more of the constituents of air such as friction, convection currents, heat conduction, radiation absorption, or oxidation. (e.g., vacuum melting of reactive metals such as titanium)

3. High purity environments where any foreign material at all is an impurity. Gases dissolve in liquids and solids in amounts proportional to their pressure and contaminants settle on surfaces are a rate that is dependent upon the molecular density of the gas above the surface. Vacuum chambers can be used to disturb this equilibrium condition that exists at normal room conditions, such as the removal of occluded or dissolved gas or volatile liquid from the bulk of material (e.g., degassing of oils, freeze-drying) or desorption of gas from surfaces (e.g., the cleanup of microwave tubes and linear accelerators during manufacture)

4. Atomic and molecular beam applications. As the distance that a molecular or atomic particle can travel is directly dependent upon the space between the stray molecules in its surroundings, beams of these particles will move in an increasingly unimpeded fashion as the ambient pressure is lowered and the mean free path increases. Thus, a vacuum chamber is used to extend the distance that a particle must travel before it collides with another, thereby helping the particles in a process to move without collision between source and target (e.g., in vacuum coating, particle accelerators, television picture tubes)

5. To reduce the number of molecular impacts per second, thus reducing chances of contamination of surfaces prepared in vacuum (e.g., in clean-surface studies and preparation of pure, thin films).

6. Thermodynamic applications where the temperature at which a chemical or physical process proceeds depends upon the absolute pressure of the system.

Vacuum Terminology Mean Free Path –
Reduction in pressure results in a lower density of gas molecules. Given a certain average velocity for each constituent molecule of air at a given temperature (at room temperature this is about 1673 km/hr) an average molecule will travel a certain distance before it interacts (collides) with another at any given pressure. This average distance between collisions is the mean free path. At 1 Torr in air this distance is about 0.005 cm, a value that scales directly with pressure. Thus the mean free path would be 5 cm at 0.001 Torr and 50 meters at 1×10-6 Torr. The lengthening of mean free path at low pressures is a key enabler for devices such as vacuum tubes and particle accelerators as well as for processes such as vacuum coating where microscopic particles such as electrons, ions or molecules must traverse considerable distances with minimal interference.

Liquids and solids are characterized by their density or specific gravity, which answers the question — what does one cubic centimeter volume of this liquid/solid weigh in grams? The units, in this case, are gm/cc. But gases fill the volume that contains them and a density measurement wouldn’t mean much. However, there is an analog to density called number density — how many molecules are contained in one cubic centimeter volume. This term allows us to describe a ‘quantity’ of any gas without knowing: its composition, the molecular weight of the components, or the mass of the molecules. It is known that any gas under the conditions of atmospheric pressure and zero degrees Celsius has the same number density. It does not matter if it’s pure nitrogen, pure oxygen, pure argon, pure hydrogen or a mixture of all four, if it is at atmospheric pressure and 0°C, the number density is 2.5 x 1019 cm-3. The huge number density at atmospheric pressure and the high velocities of the gas molecules means there are many, many collisions every second. Expressed another way, even with its high speed a molecule can’t travel far before hitting another. The distance the average molecule travels before colliding with another is called the Mean Free Path (MFP).

Particle Flux –
In addition to colliding with each other in the gas phase, molecules of a gas hit the surfaces of the containing vessel. The rate at which they hit a surface, called the Particle Flux, depends on the gas’s number density and its molecular weight. The particle flux of molecules present in air on a surface at atmospheric pressure is 2.9 x 1023 cm-2 sec-1.

Outgassing and Vapor Pressure –
Assuming that a system is tight, as the pressure gets lower most of the load is from gases evolving from the surfaces of the materials in the system. This becomes significant below pressures of around 0.1 Torr. Outgassing will be the main limiting factor with regard to the ultimate pressure which any particular system may reach, assuming that leaks are absent. Leaks may be either real leaks, like holes in the chamber, or virtual leaks that are caused by gas escaping from, for example, screw threads within the system or porous surfaces that contain volatile materials. The level of outgassing is reduced by keeping the system clean and dry and with a proper selection of materials.

Base Pressure –
When you have cured all the leaks, eliminated all the virtual leaks, stopped all deliberate gas flows into your vacuum chamber, and pumped on it for several days or weeks, the pressure reaches an equilibrium value called the Base Pressure. In truth, since the pressure approaches equilibrium asymptotically and outgassing rate usually decays (very slowly) after a long time under vacuum, the chamber may never quite reach a stable pressure. But variations in vacuum gauge calibration, changes in room temperature, variations in pumping speed, backstreaming from the pump, etc., mask any real pressure reduction and the chamber appears to have reached a steady state.
Often, the intent is changed slightly, the user pumps the chamber for an hour, grows tired of waiting and claims that the chamber is at Base Pressure. That’s not necessarily wrong. After all, if the pressure fell from 5 x 10-7 torr to 4 x 10-7 torr by waiting another hour, is all that much gained? Perhaps it doesn’t conform to the AVS definition, but in one sense the Base Pressure is reached whenever the operator says it is and starts using the chamber for real work.

Working Pressure –
The term base pressure defines conditions where no gas is deliberately flowing into the system. But sometimes the chamber is first pumped to its base pressure (to exclude the possibility of new leaks or remove contamination) and then back-filled to an intermediate pressure with a gas. This is how processes such as sputter deposition, plasma etching, and CVD are done. This intermediate pressure is called the Working Pressure. But to establish and maintain a working pressure, it is rarely sufficient to just close the pumping port, back-fill with gas, and forget. The vast majority of back-filled chambers have some conductance limited pumping; fresh gas is being constantly added, and a fairly elaborate pressure control system (either downstream between chamber and pump, or upstream between gas source and chamber) to maintain the desired working pressure. This reduces the contamination caused by wall outgassing.

Ultimate Vacuum or Pressure –
When selling a vacuum pump, the manufacturer gives two specifications: pumping speed; and ultimate vacuum (also called ultimate pressure). The ultimate vacuum is measured by capping the pump’s inlet and measuring the equilibrium pressure after operating the pump for many hours. Because it is measured under “ideal” circumstances, you must not expect your chamber to reach this ultimate vacuum with just this pump attached. It never will! Another way to interpret a pump’s ultimate vacuum is: if the chamber reaches this ultimate vacuum by using other pumps or traps, then that pump’s characteristics (throughput, pumping speed, outgassing, backstreaming) will not cause any problems. However, if you attempt to reach a lower pressure, the pump may act as a gas source. For example, suppose we have forced a chamber to a mechanical pump’s ultimate pressure of 10-3 torr by using a second type of pump. The mechanical pump will let this happen. But if we then try to force the chamber to 10-6 torr, the mechanical pump’s oil vapor backstreaming and air leak back will make it look like a gas source.

Applications of Vacuum

Industrial
Industrial vacuum applications range from mechanical handling (such as the manipulation of heavy and light items by suction pads) to the deposition of integrated electronic circuits on silicon chips. Obviously, vacuum requirements are as widely varied as the particular processes using vacuums. In the rough vacuum range from about one torr to near atmosphere, typical applications are mechanical handling, vacuum packing and forming, gas sampling, filtration, degassing of oils, concentration of aqueous solutions, impregnation of electrical components, distillation, and steel stream degassing.

At lower pressures down to about 10-4 torr, many metallurgical processes such as melting, casting, sintering, heat treatment, and brazing can derive benefit. Chemical processes such as vacuum distillation and freeze-drying also need this range of vacuum. Freeze-drying is used extensively in the pharmaceutical industry to prepare vaccines and antibiotics and to store skin and blood plasma. The food industry freeze-dries coffee mainly, although most foods can be stored without refrigeration after freeze-drying, and the technique is receiving widespread acceptance.

The pressure range down to about 10-6 torr is used for cryogenic (low-temperature) and electrical insulation. It is used in the production of lamps; television picture tubes, X-ray tubes; decorative, optical, and electrical thin-film coatings; and mass spectrometer leak detectors.

In thin-film coating, a metal or compound is evaporated under high vacuum from a source onto a base material or substrate. The base material is generally plastic for decorative coatings; glass for optical coatings; and glass ceramic, or silica for electrical coatings. Thickness of the film can vary from about 1/4 wavelength of visible light to 0.001 inches or more. In the optical field, antireflection coatings are deposited on lenses for cameras, telescopes, eyeglasses, and other optical devices, considerably reducing the amount of light reflected by the lenses and thus giving a brighter transmitted image.

To achieve vacuum high enough for thin-film coating and for other industrial uses requiring pressures down to 10-6 torr, a pumping system consisting of an oil-sealed rotary pump and a diffusion pump is used. The oil-sealed rotary pump (sometimes referred to as forepump) “roughs” the chamber down to a pressure of about 0.1 torr, after which the roughing valve is closed. The fore valve and high-vacuum baffle valve are then opened so that the chamber is evacuated by the diffusion pump and rotary pump in series.

Research
Almost every research laboratory uses vacuum directly in its experiments or employs equipment that depends on vacuum for its operation. The lowest pressures are obtained in the research laboratories, where equipment is generally similar to, but smaller than that used by industry.

Typical of the research equipment using vacuum down to about 10-6 torr are the electron microscope, analytical mass spectrometer, particle accelerator, and large space simulation equipment. Particle accelerators range from small van de Graaff machines to large proton synchrotrons.

In space simulation, large units that simulate space around a complete vehicle require a vacuum of 10-6 torr or below. Such vessels incorporate a complete shroud at liquid nitrogen temperature and a port through which high-intensity light can be beamed to simulate the sun’s radiation. In the pressure region down to and below 10-9 torr, research applications include electrical insulation, thermonuclear energy conversion experiments, microwave tubes, field ion microscopes, field emission microscopes, storage rings for particle accelerators, specialized space simulator experiments, and clean-surface studies. In many experiments it is not only necessary to reach such pressures of 10-9 torr but to reduce the hydrocarbons in the residual gases to an absolute minimum. Even small traces of hydrocarbons can render the results unreliable. To achieve a vacuum of this order the vacuum vessel and the equipment inside must be cleared of residual gas (degassed) to the greatest extent possible. A common solution is to bake the whole apparatus for a number of hours at about 350oC while maintaining a vacuum in the 10-5 torr region. Baking at this temperature requires the use of all-metal sealing rings. To eliminate hydrocarbons, the unit is pumped down to about 10-3 torr using sorption pumps; and from there, sputter ion pumps and titanium sublimation pumps complete the task down to 10-9 torr or below.

Vacuum Equipment Oil-sealed Rotary Pump –
Capacities are available from 1/2 to 1,000 cubic feet per minute, operating from atmospheric pressure down to as low as 2 x 10-2 torr for single-stage pumps and less than 5 x 10-3 torr for two-stage pumps. The pumps develop their full speed in the range from atmosphere to about one torr. The speed then decreases to zero at their ultimate pressures. Two of the most common designs are useful for pumping both liquids and gases. One is a two-bladed pump in which the rotor is eccentric to the stator, forming a crescent-shaped volume swept by the blades through the outlet valve. The second, a rotary piston pump, similar to a single blade, is part of the sleeve fitting around the rotor. The blade is hollow and acts as an inlet valve, closing off the pump from the system when the rotor is at top center.
Ultimate pressures attainable are limited by leakage between the high and low-pressure sides of the pump (due mainly to carry over of gases and vapors dissolved in the sealing oil that flash off when exposed to the low inlet pressure) and decomposition of the oil exposed to high temperature spots generated by friction.
Gas ballasting helps to prolong pump life because it removes the chief source of pump contamination, condensable vapors. The gas ballast is a vented exhaust that admits a small amount of air at atmospheric pressure to the compression side of the pump, thus permitting most condensable vapors to pass through the pump without condensing.
Typical applications of this pump are in food packaging, high-speed centrifuges, and ultraviolet spectrometers. It is also widely used as a forepump or a roughing pump, or both, for most of the other pumps described.

Mechanical Booster –
Capacities are available from 100 to 70,000 cubic feet per minute, operating usually in the pressure range from 10 to 10-3torr. The peak speed of the pump is developed in the pressure range from 1 to 10-2 torr. The speed at the lower end of the pressure range depends on the type of forepump used. A typical mechanical booster uses two figure-eight-shaped impellers, synchronized by external gears, rotating in opposite directions inside the stator. The gas is trapped from between the impellers and the stator wall and transferred from the high vacuum to the fore vacuum side of the pump. The gears are oil-lubricated but are external to the pump, so that the impellers run dry. Clearance between the impellers and the stator wall is generally about .002 to .010-inch. As a consequence, back leaking of gas occurs at a rate governed by the pressure difference between input and output and the type of gas being pumped. Under normal running conditions, a pressure difference of about ten to one is obtained. The mechanical booster must be backed by another pump in series when working in its normal pressure range. The most frequently used type of forepump is the oil-sealed rotary pump. Typically, the mechanical booster is employed for pumping vacuum-melting furnaces, in an impregnation plant for electrical equipment, and in low density wind tunnels.

Molecular Pump –
Capacities are available up to 20,000 cubic feet per minute, with an operating range of 10-1 to 10-10 torr, when backed by an oil-sealed rotary pump. The full speed of the pump is developed in the very wide pressure range from 10-2 to 10-9 torr. In the molecular pump, a high rotational speed rotor (up to 32,000 revolutions per minute) imparts momentum to the gas molecules, moving them along the small clearance between the rotor and stator. The molecular drag pump also employs this principle of operation. For ultimate base pressures it has been almost entirely displaced by the faster and simpler turbo molecular pump, in which radial slots in both rotor and stator fins actuate the pump. A number of compression stages are employed but because of its design, larger clearances can be tolerated between rotor and stator than were possible in the molecular drag pump. The molecular drag pump is sometimes built integral with a turbo molecular pump to allow the use of vert clean “dry” backing pumps. These hybrids are often used in semiconductor processing where oil vapor backstreaming would contaminate processes but high pumping speeds are needed.

Vapor Diffusion Pump –
This pump is mainly used on equipment for the study of clean surfaces and in radio frequency sputtering. Pumping speeds are available up to 190,000 cubic feet per minute with an operating pressure range of 10-2 to less than 10-9 torr when water-cooled baffles are used and less than 10-11 torr when refrigerated baffles are employed. The pumping speed for a vapor pump remains constant from about 10-3 torr to well below the ultimate pressure limitations of the pump fluid. The best fluids allow pressures of better than 10-9 torr. The diffusion pump is initially evacuated by an oil-sealed rotary pump to a pressure of about 0.1 torr or less. When the pump fluid in the boiler is heated, it generates a boiler pressure of a few torr within the jet assembly. High-velocity vapor streams emerge from the jet assembly, impinge and condense on the water or air-cooled pump walls, and return to the boiler. In normal operation part of any gas arriving at the inlet jet is entrained, compressed, and transferred to the next stage. This process is repeated until the gas is removed by the mechanical forepump.
The oil-vapor booster pump works on the same principles as the diffusion pump, but it employs a higher boiler pressure. Normal operating pressure range is 1 to 10-4 torr. When backed by an oil-sealed rotary pump, this pump is widely used for achieving high vacuum in thin-film evaporation units, accelerators, and in TV tube pumping.

Sputter Ion Pump –
Capacities are available up to 14,000 cubic feet per minute, with an operating pressure range of 10-11 torr. The full speed of the pump is developed in the pressure range from about 10-6 to 10-8 torr, although the characteristic at the lower pressure is dependent on the pump design. This pump uses a cathode material such as titanium vaporized or sputtered by bombardment with high velocity ions. The active gasses are pumped by chemical combination with the sputtered titanium, the inert gasses by ionization and burial in the cathode, and the light gasses by diffusion into the cathode.
A typical pump consists of two flat rectangular cathodes with a stainless steel anode between them made up of many open-ended boxes. This assembly, mounted inside a narrow box attached to the vacuum system, is surrounded by a permanent magnet. The anode is operated at a potential of about seven kilovolts (kV), whereas the cathodes are at ground potential.
The sputter ion pump has low speeds and sometimes instability when pumping inert gases. To improve its characteristics other types of sputter ion pumps have been developed: the slotted cathode, triode, differential, and magnetron pumps.
To start up a sputter ion pump it is necessary to reduce the pressure to at least 2 x 10-2 torr, and preferably much lower, by means of a roughing pump. Sputter ion pumps can operate in any position and do not need water or liquid nitrogen supplies. They have a long life and can provide very clean, ultrahigh vacuum, free of organic contamination and vibration. They are employed mainly for the clean-surface studies and in those applications where any organic contamination will give unsatisfactory results.

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