Vacuum Technology Explained
Vacuum Technology (high vacuum pumps - systems) 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:
- to remove the constituents of the atmosphere that could cause a
physical or chemical reaction during the process (e.g., vacuum
melting of reactive metals such as titanium)
- to disturb an 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)
- 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
- 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).
For any vacuum process, the limiting parameter for the maximum
permissible pressure can be defined by:
- the number of molecules per unit volume (reasons 1 and 2)
- the mean free path (reason 3),or the time required
- to form a monolayer (reason 4).
At room temperature and normal atmospheric pressure, one cubic
foot (0.03 cubic metre) of air contains approximately 7x1023
molecules moving in random directions and at speeds of around 1,000
miles per hour (1,600 kilometers 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 (see Table), but until
recently it was commonly expressed in terms of the weight of a
column of mercury of unit cross section and 760 milimetres (mm)
high. Thus, one standard atmosphere equals 760 mm Hg, but to avoid
the anomaly of equating apparently different units, a term, torr,
has been postulated. One standard atmosphere = 760 torr (1 torr =1
mm Hg). This term was replaced in 1971 by SI unit defined as the
newton per square metre (N/m2), and called the pascal (one
pascal=7.5x10-3 torr).
The first major use of vacuum technology in industry occurred
about 1900 in the manufacture of electric light bulbs. Other
devices requiring a vacuum for their operation followed, such as
various types of electron tube. Furthermore, it was discovered that
certain processes carried out in a vacuum achieved either superior
results or ends actually unattainable under normal atmospheric
conditions. Such developments included the "blooming" of lens
surfaces to increase the light transmission, the preparation of
blood plasma for blood banks, and the production of reactive metals
such as titanium. The advent of nuclear energy in the 1950s
provided impetus for development of vacuum equipment on a large
scale. Increasing applications for vacuum processes were steadily
discovered, as in space simulation and microelectronics.
Applications of Vacuum
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.
In 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.
