How Does A Vacuum Turbo Pump Work?
Resembling a jet engine, a turbo pump has a stack of rotors,
each with multiple, angled blades which drive at very high
tangential speed. Gas molecules, hit by the underside of the angled
blades, move with momentum in the direction of the higher pressure
exhaust.
Turbo pumps come in two basic designs. In the SNECMA design
(named after a French jet-engine manufacturer) all gas enters
through the main flange at the visible single-end of the pump; the
Pfeiffer (named after the original turbo pump's manufacturer)
double-ended design, has two rotors set-mounted on a common axle.
Gas enters the pump through a right angle port between the two
rotor sets and exits into an exhaust manifold connecting the two
ends of the pump.
For normal commercially-available pumps, pumping speeds range
from approximately 20 L/s to 3,000 L/s (although some turbo pumps
manufactured in Russia are quoted with pumping speeds of 20,000
L/s). All gases are pumped at roughly the same rate. One pump, for
example, listed with 450 L/s for nitrogen, has a pumping speed for
hydrogen of 310 L/s.
When selecting a turbomolecular pump, consider not only the rate
of pumping, but also compression ratio (CR). Let's look at an
example:
- The pump we mentioned above (with a rate of 450L/s for
Nitrogen) has a CR for nitrogen listed 4 x 108 and a CR for
hydrogen of 630 (compression ratio determines the partial pressure
of any specific component gas in the chamber). If the nitrogen
partial pressure in the foreline measures 1 x 10-4 Torr, the
chamber may reach a partial pressure of 2.5 x 10-13 Torr (a factor
of 4 x 108 lower). The actual partial pressure depends on the
nitrogen gas load caused by wall outgassing, virtual and real
leaks, etc. and the effective pumping speed. However, if the
foreline has hydrogen at the same partial pressure as nitrogen, the
chamber will not reach less than 1.6 x 10-7 Torr of hydrogen. We
can understand the differences in CR for the various gases,
qualitatively, by comparing the most probable velocity at 25°C for
hydrogen (1,700 m/s) and nitrogen (400 m/s) with the tangential
velocity of a 6" diam. turbo rotor rotating at 36,000 rpm (approx.
280 m/s). The fraction of the lighter gas that will "backstream"
through the turbo rotors, without blades striking it, will be
higher because of the gas's faster rate of speed.
- By making the foreline partial pressure low for light gases, a
turbo pump yields an excellent ultimate vacuum. In one method, a
smaller turbo or hybrid pump, backed in turn by a mechanical pump,
backs the primary turbo pump. To calculate the effect of two pumps
on the chamber's partial pressures, multiply the CRs. For example,
if the two turbo pumps each have CRs of 103 for hydrogen, then in
series their combined CR is 106.
Turbo pumps reach full operating speed within a few minutes of
switch-on, making a separate roughing line unnecessary since the
accelerating turbo can rough the chamber. The trick is to match
chamber volume and the effective pumping speed, and to time the
turbo pump's start so its rotational speed is high enough to
prevent backstreaming when the chamber reaches 10-1 Torr (that is,
when an oil-sealed mechanical pump's relative backstreaming rate
starts its rapid rise).
With proper venting, a turbo pump can be entirely halted in
under one minute. This slight delay before the chamber reaches
atmospheric pressure and can be opened usually works for most
applications. The benefit is clear. If the pump does not run while
venting the chamber, we have no need for a high vacuum valve
between pump and chamber. Further, in a correctly designed and
operated system, the turbo pump does not allow oil vapor to
backstream so that the system does not require an LN2 trap between
chamber and pump.
A turbo pump's high rotational speeds (some small diameter units
operate at 60,000 rpm) put serious strain on the shaft bearings.
Most manufacturers now offer light-weight ceramic ball bearings
with grease lubrication since this combination is both light,
lowering the momentum of the bearing, and has a low
bearing-lubricant vapor pressure at the pump's working
temperature.
More recently, manufacturers have begun offering magnetically
levitated bearings which are either permanent magnets for the small
sized pumps or a combination of permanent and dynamic magnetic
fields, supporting the shaft without contact. Magnetically
levitated pumps (such as those manufactured by Shimadzu) include
non-contact bearings, resulting in a turbo pump that has true zero
oil-vapor backstreaming with bearings that never wear down.
Futher information on High Vacuum Pumps, pump systems, and the
way vacuum technology works
Applications
Turbomolecular pumps are used in a wide variety of high vacuum
applications -- any that demand clean, truly oil-free vacuum
between 10-4 and 10-10 Torr. The single-ended pump is frequently
chosen to replace a comparably sized diffusion pump in existing
systems. Versions made from materials that withstand chemical
attack when pumping corrosive gases find extensive application in
semiconductor processing, particularly those using reactive gas
plasmas. For truly oil-free systems, choose magnetically levitated
turbo pumps. They can work with an oil-free piston pump or in some
circumstances, with a diaphragm pump. The initial cost of a turbo
pump is higher than a diffusion pump with similar pumping speed.
But the lack of need for external components (see Technical Notes
), the turbo pump's lower running costs, and the almost negligible
chance that clumsy operation will cause an expensive vacuum
disaster, make the turbo pump the first choice for many users. (We
do not recommend turbo pumps for systems generating dust or
particulate matter, nor where the induced micro-vibrations might
upset precise positioning, e.g. electron microscopes, micro-surface
analysis, atomic force and scanning tunneling microscopes.) Turbo
pumps find applications in processes involving:
- metal etching
- dielectric etching
- interconnect etching
- ion implantation
- sputtering, and
- plasma deposition.
Drag Pumps
The drag pump has three distinct stages for moving gas through
the chamber:
- First, a single-stage impeller, similar to the top row of
blades in a turbomolecular pump ensures the highest possible
pumping speed for the open area of the pump's inlet.
- In the second stage a high speed rotor spins between two
closely-spaced, inner- and outer-cylindrical walls with helical
grooves facing the rotor. When the rotor's tangential velocity
approaches that of the average gas molecule's velocity, momentum
transfer causes flow toward the exhaust port. To assist that flow,
the spiral grooves on the outer wall have a downward shape while
the ones on the inner wall have an upward shape.
- In the third stage, a dynamic seal allows the pump to operate
with a high exhaust pressure.
The compression ratios for drag pumps typically measure: 109 for
N2, 104 for He, 103 for H2. Use drag pumps in applications
requiring clean, low pumping speeds (less than 10 L/s) and modest
ultimate pressures (10-6 Torr). The drag pump accepts continuous
inlet pressures below 0.1 Torr and discharges with a foreline
pressure of 10 to 40 Torr. The latter pressure indicates that
diaphragm pumps, not normally suitable for backing high vacuum
pumps, could work in some applications with drag pumps.
The grease lubricated, ceramic ball bearings used to suspend the
rotor have a light weight with a low coefficient of friction. Low
forces on the bearing cage and low bearing temperatures, give these
pumps high reliability with field maintenance intervals of over one
year. These bearings have two major benefits over oil lubricated
designs: they allow for easy user replacement; and they allow the
pump to be mounted in any orientation.
