EVALUACIÓN Y ACEPTACIÓN DE OFERTAS

Vacuum pumps are the most indispensable part of any vacuum system which is

required to attain and maintain a low pressure inside the vacuum systems. The principle

of operation of different pumps is described in the following.

2.3.2.1 Mechanical (Rotary) Pump

Mechanical pumps work by the process of positive gas displacement.3-4 During their operation, the pump (Figure 2-2) periodically creates increasing and decreasing

volumes to remove the gases from the system, and exhaust them to the atmosphere. A

rotating vane traps the gas on the suction side of the pump forcing it towards the

and sealant. The ultimate pressure achieved by a mechanical pump is in the range of few

millitorrs. Although UHV cannot be achieved by using a mechanical pump, it is an

indispensable component of a UHV system. It is used as a roughing pump for the initial

pump down of the chamber from atmosphere, and also as a backing pump for the

turbomolecular and oil diffusion pumps.

Figure 2-2 Mechanical pump schematics and operation

They can run for years with very little maintenance and are cheaper in comparison

to other pumps. However there is a major disadvantage due to the use of oil which

sometimes back stream and contaminates the chamber and they are very noisy and

provide mechanical vibration to the system.

2.3.2.2 Turbomolecular Pump

The turbo molecular pumps operate by the principle of momentum transfer 5 similar to that of a jet engine. They consist of multiple stages of rotor-stator pairs

organized in series. The rotor blades spin at a very high speed (typically 75 KRPM). As

the gas molecules enter the inlet, the rotor blades collide with the molecules. Thus the

acquired momentum, the gas molecules enter the gas transfer holes in the stator. This

leads them to the next stage where they again collide with the rotor surface, and this

process is continued, finally leading the molecules outwards through the exhaust. The

relationship of orientation between blades of the rotor/stator pair is such that it increases

the probability of molecules going towards the outlet, generating a pressure difference

with respect to the inlet. The pump requires a backing pump during the operation.

The pump operates over a wide variety of pressure range starting from 10-4 to 10-10 Torr and it works extremely well to pump heavier gases. They are clean because they do not use any oil for the operation. However, they are noisy, provide vibration to

the system and are expensive.

2.3.2.3 Diffusion Pump

Diffusion pumps are another variety of momentum transfer pumps.6,7 They are widely used in vacuum systems to achieve a base pressure of ~1×10-10 Torr or lower after the system is initially pump down by the mechanical pump. The diffusion pump

comprises of a stainless steel cylinder containing a vertically stacked, cone-shaped jet

assembly. There is a heater on the bottom of the pump which heats the oil to its boiling

point. This step is done after the system is rough pumped to the range of militorrs,

otherwise there will be no pumping action occurs by the pump due to oxidation of the

pump oil. Once the oil starts to boil, the vapor is forced up through the central column of

the jet assembly and then expelled downwards through a downward directed jet

assembly, displacing molecules on its way and forcing them to leave the chamber. As the

molecules from the system enter the pump they encounter the top jet and are gradually

pump. Since backstreaming of the oil can be a serious problem, diffusion pumps use

specialized oil with a low vapor pressure and a high molecular weight. For the same

reason, UHV systems have a cryotrap located between the pump and the chamber. This

minimizes the back-streaming of oil molecules, preventing them from entering the UHV

system. Some advantages of diffusion pumps are: they are noise free, reliable, have a

simple design, relatively inexpensive and have high pumping speeds. On the other hand,

the major disadvantage of these pumps is that they can back stream some oil vapor.

Figure 2-3 Schematics of (a) Diffusion pump (b) Turbomolecular pump 2.3.2.4 Ion Pump

Ion pumps are the primary choice in UHV systems. They are clean, vibration free,

components of an ion pump are: a parallel array of short stainless-steel tubes (anode), two

titanium plates (cathode) and a magnet (Figure 2-4). The cathode plates are positioned on

both sides of the anode tubes. A strong magnetic field is oriented along the axis of the

anode, which is generated by permanent magnets located outside the vacuum. Electrons

are emitted from the cathode due to the action of an electric field and due to the presence

of magnetic field they move in a long helical trajectory (a penning trap). This trajectory

enhances the collision of the electrons with the incoming gas molecules. The result of this

collision creates a positive ion which accelerates towards the cathode with sufficiently

high kinetic energy to be buried in the cathode. When heavier gases and molecules are

pumped they often have high enough kinetic energy to sputter some of the cathode

material away and deposit them on the other parts of the pump providing a continuous

supply of gettering material and thus increasing the pump efficiency. Reactive molecules

like CO, O2, N2 etc. form stable complexes with titanium8 and thus are permanently removed from the system. When pumping inert gases for example Ar, the molecules are

buried deep inside the cathode by a sorption process. Since Ti does not form a stable

complex with Ar, this pumping effect is not permanent since, because of the erosion of

the cathode material, previously implanted molecules can be released which is often

referred to as “argon instability”. Therefore ion pumps are not ideal for pumping inert

gases.

Since ion pump works by either chemisorbing or physiosorbing ions into the

electrodes, it does not require a backing pump. Ion pumps require very low maintenance

and form a closed vacuum system with the vacuum chamber. It is also clean and vibration

Figure 2-4 Schematic diagram of an ion pump