2 Diffusion Pump: Principle of operation

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Intoduction

 

The necessary condition for the scientific research and modern high technology lies in vacuum physics. “Vacuum” comes from the Latin word “vacua”, which means “nothing or empty”. However, there is no “ideal vacuum” since a totally empty space does not exist in nature. Thus a partially empty space is considered as vacuum, where some of the air and other gases have been removed from a gas containing volume (“gas” comes from the Greek word “chaos” = infinite, empty space). In other words, vacuum means any volume containing less gas particles, atoms and molecules (a lower particle density and gas pressure), than there are in the surrounding outside atmosphere. Therefore, gaseous environment below atmospheric pressures is known as vacuum. Existence of high vacuum is the essential need for the thin film deposition processes such that the source material particles face less obstruction due to the air and other gas particles inside the deposition chamber and can move over a large distance. There are a large number of national and international units for the measurement of vacuum. The most commonly used units are Pascal (Pa) , bar and Torr. By dropping the air or gas pressure inside a closed chamber via reducing the quantity of gas particles, vacuum can be generated. There are basically two ways of doing this:

(i) By displacing the gas molecules from the closed chamber either into the atmosphere or into an external space.

(ii)  Gas particles may be condensed, absorbed or chemically combined within the vacuum system.

 

An extensive range of vacuum generators is known these days in which all of them work accordingly based on the different technical principles and methods. These vacuum generators are categorised under the term “vacuum pumps”.

 

According to their mode of operation vacuum pumps are classified into three distinct types:

 

1.  Positive displacement pumps: Capturing, condensing and expelling the gas particles outside the chamber.

–  Mechanical pumps.

 

2.       Momentum transfer pumps: Gas particles gain a preferential direction.

–  Diffusion pump, Turbo-molecular pump,

 

3.  Adsorption or Reaction pump: Capturing and keeping the gas particles

–  Cryopumps, Sorption pumps, Ion pumps,

 

According to the grouping of pressure ranges vacuum systems are classified into five categories:

 

  • Rough/Low Vacuum: > Atmosphere to 1 Torr
  • Medium Vacuum: 1 Torr to 10-3 Torr
  • High Vacuum: 10-3 Torr to 10-7 Torr
  • Ultra-High Vacuum: 10-7 Torr to 10-11 Torr
  • Extreme High Vacuum: < 10-11 Torr

The different types of pumps for these ranges of vacuum can then be divided into the following category:

 

  • Primary (Backing) Pumps: Rough and low vacuum pressure ranges.
  • Booster Pumps: Rough and low vacuum pressure ranges.
  • Secondary (High Vacuum) Pumps: High, very high and ultra-high vacuum pressure ranges.

The fundamental principle of operation of working of all the vacuum pumps is same apart from their design. The function of the vacuum pump is to remove the molecules of air and other gases from the vacuum chamber (or from the outlet side of a higher vacuum pump if connected in series). At the same time as the pressure in the vacuum chamber is reduced, the task of removing additional molecules becomes exponentially harder. As a consequence, a developed vacuum system containing deposition chamber and vacuum pumps (Fig. 1) must be able to operate over a portion of an extraordinarily large pressure range, typically varying from 1 to 10-7 Torr of pressure.

 

In the upcoming sections we discuss the fundamental principle and working of the Diffusion Pump, which is a secondary high vacuum pump (ranges from 10-3 Torr to 10-7 Torr) worked on the principle of momentum transfer.

 

Diffusion pump: principle of operation

 

Diffusion pump is classified as a momentum transfer pump also known as molecular pump, the fundamental principle of working lying on the use of high speed jets of dense fluid which helps to move the gas molecules out of the deposition chamber. Positive displacement pumps commonly called as mechanical pumps are used in conjunction with the momentum transfer pumps to achieve high vacuum. Momentum transfer pumps in conjunction with displacement pumps comes under the category of gas transfer pumps, operated by transferring the gas molecules by either momentum exchange (kinetic action) or positive displacement. When the gas is expelled from the pump it is slightly above the atmospheric pressure as the same number of gas molecules is discharged from the pump as they enter it. Compression ratio of the gas particles is defined as the ratio of the exhaust pressure (outlet) to the lowest pressure obtained (inlet).

 

Mechanical pumps commonly referred as positive displacement transfer pumps are based on the mechanically trapping of a volume of gas particles and moving them from beginning to end of the interior of the pump. Mechanical pumps are commonly designed in compound stages on a common drive shaft. In order to expelled the gas particles outside into the atmosphere, the total gas volume is compressed to a smaller volume at a higher pressure (or to the next pump fig.2). Thus the diffusion pump and mechanical pump are used in series to provide a higher vacuum with high flow rate.

Using high speed blades or by introducing vapors, the kinetic transfer pumps which works on the principle of momentum transfer directs the gas particles towards the pump outlet and thus provide the increased probability of gas particles moving towards the outlet of the pump. Kinetic pumps can achieve high compression ratios at low pressures in spite of the fact that they do not have any sealed volumes.

 

Diffusion Pump : working principle

 

Diffusion pumps fundamentally work on the principle of momentum transfer by creating the jet of oil vapours. In the interior of the pump the high speed vapour molecule of the created jet collides with a gas particle and transfer their momentum to the gas particle thus directed it towards the preferred direction through the pump. The detailed assembly of the diffusion pump is shown in fig.3.

 

The bottom of the diffusion pump contains an oil reservoir below which an electric heater is placed to boil the oil. The oil is boiled to a significant limit so that it vaporize, the vapours of the oil rises up through the center assembly of the pump towards the upper direction and exit from the nozzles via jet assembly in a downward approach. The process of vaporising the oil and formation of jet must be performed at a reduced pressure. This implies that for the operation of a diffusion pump to get started a significant value of vacuum must be present or the assembly must be “rough pumped” down to an acceptable pressure, typically 100 millitorr. In the absence of the fore vacuum no pumping action by the diffusion pump is performed and the pumping fluid will get damaged. The multistage assembly of the jet is present in the form of ring and form a jet of vapor that extends from the central part of the pump exit from the nozzles and end up after colliding with the pump wall. The gaseous particles that are roaming into the inlet collides with these vapors particles are confined. After colliding with the water-cooled walls of the pump the oil vapours cools down and settles back at the bottom of the pump running through the walls of the pump and finally reaches the reservoir. Through this phenomenon the oil vapour jet completes its one cycle and releases off its confined gas particles thus ready to begin the next cycle. Therefore, the gas particles which are approaching towards the upper direction is confined by the oil vapour jet and exhausted towards the downward direction again. After certain number of cycles an area of higher pressure is created at the bottom of the pump than the top of the pump. Therefore, during the operation of the diffusion pump higher pressure is created below the oil vapour jet as compared to above that jet. At the bottom of the pump, the pressure is high enough for the gas to be pumped out by a standard mechanical pump. In the complete process the oil vapor jet is always preferred to move in the downward direction but it is quite possible for some of it to approach toward the upper direction at top of the pump. In order to avoid this situation of migration of oil vapours into the chamber, a cold trap is situated at the top of the jet assembly so that the vapours can be condensed in that area. The chamber is evacuated through the pump via concentric circular baffle, which is placed in between the cold trap and the pump. Baffle allows gas particles to wander in, but traps the oil vapors. Commonly silicone oil is used as the diffusion pump oil due to its high stability and vapor point. The performance of the silicone oil is reported well over a long periods of time with very low degradation. Silicon oil is also relatively much safe than any other in case of toxicity and flammability. The foremost criteria of using silicon oil is that the operation of vaporising it is performed in low pressure and it must not be exposed to air when heated otherwise the oil will get spoiled.

 

Parameters influencing the functioning of diffusion Pump

 

1. Compression Ratio

 

Compression ratio of the diffusion pump is the ratio of the exhaust pressure to the inlet pressure. For most of the gases the compression ratio is very high. For example, the compression ratio created is one million, for an inlet pressure of 2×10-7 Torr and a fore-line pressure of 2.0×10-1Torr. Since the compression ratio is different for gases having different molecular weights thus, in a mixture of gases, each gas may be pumped with different effects and can have different flow rates. For example, the compression ratio for hydrogen will differ greatly from the compression ratio for argon.

 

2. Critical Discharge Pressure

 

The critical discharge pressure of a diffusion pump is the maximum allowed pressure at the fore-line during normal pumping operation. The diffusion pump doesn’t perform efficiently if the critical discharge pressure is not appropriate. As a consequence, the discharged gas particles do not have sufficient energy and density to provide a barrier for the air in the fore-line, and the particles will carry the oil vapours in the wrong direction. The maximum allowable fore-line pressure for the normal functioning of diffusion pump is about 0.5 Torr. Thus, the most important rule for the functioning of the diffusion pump is to maintain a significant amount of critical discharge pressure.

 

3. Backstreaming

 

The pumping fluid can pass through the inlet port of the pump and also in the opposite direction to the favoured direction of gas flow, this process of undesirable flow of pumping fluid is called backstreaming. The process of backstreaming also must include the trap, baffle, and plumbing as well in addition with pump because all these parameters has a great influence on the rate of transfer of pumping fluid vapors from the pump assembly to the deposition chamber. Following are the conditions that can cause backstreaming: (i) The fore-line pressure exceeds the critical discharge pressure (ii) exceeding maximum throughput capacity for long periods of time, and (iii) Wrong procedure of start-up or shutdown also causes backstreaming. In case of thin film deposition, the process of backstreaming is a terrible phenomenon. When backstreaming oil comes in contact with the hot filaments or electrical discharges, it result in carbonaceous or siliceous deposits. Diffusion pumps are not considered as a suitable pumps for use with highly sensitive analytical equipment or other applications which require an extremely clean vacuum environment due to the process of backstreaming. Baffles and liquid nitrogen cold traps are used to minimize backstreaming.

 

4.  Baffles

 

Baffles are placed in between the cold trap and the pump to reduce the backstreaming of oil vapours into the deposition chamber. Baffles do obstruct the flow of pumped gases, but well designed units can retain about 60% of the pumping speed. Baffles are placed in conjunction with the water cooled traps which can reduce the rate of re-evaporation of condensed oil vapours and thus reducing the density of vapor in the between the baffle and the trap.

 

5. Traps

 

The cooled traps are placed above the baffle and can serve two purposes. Firstly, act as a wall in the path of the oil vapours from the diffusion pump to deposition chamber. Secondly, they also provide as cryopumps for condensable vapors (primarily water vapor) derive from the system. The pumping speed for water vapours can be increased by using liquid nitrogen traps and thus lower the base pressure.

 

6.  Fluids

 

Within the last three decades many of the pumping fluids have been developed. In the early development of the fluids, most of them can create the vapour pressure of about 10-7 Torr thus the chamber pressure of the system was limited to this range. The market revised upto great extent after Hickman exposed the used of polyphenyl ethers as a fluid, which shows exceptionally high thermal and chemical stability. Later on the characteristics of silicone fluid (DC705) was also found to be admirable as another low vapor pressure fluid. The operational feature of these fluids will allow the base pressures to reach 10-9 Torr limit. Recently, fluorinated oils have been also developed as a fluid to use in diffusion pumps as they have the advantage of compatibility with corrosive gases used in some deposition processes.

 

Advantages

 

  • Simple geometry of assembly
  • Comparatively lower in cost than other high vacuum pumps
  • Pumping rate is linear
  • Much easier to operate
  • No vibration or noise
  • Low maintenance

 

Disadvantages

  • Require oil as a fluid to pump.
  • Require significant value of fore-vacuum to operate.
  • In case of backstreaming the deposition chamber can be contaminated.
  • Require cold trap.
  • Require ample of time to vaporize the oil.

 

Start-Up of Diffusion Pump

1.  All the valves should be closed.

2.  Start the mechanical pump (rotary pump).

3.  Open the fore-line valve, rough pumped the dffusion pump to about 20 mTorr.

4.  Fill the liquid nitrogen in the cold trap placed above the baffle.

5.  Turn on the heater of the diffusion pump to boil the oil so that it can vaporize.

6.  Close the fore-line valve.

7.  Open the rough valve of the deposition chamber, evacuate it to about 100 mTorr.

8.  Now close the rough valve and open the fore-line valve.

9.  Open the baffle after checking the level of liquid nitrogen in cold trap.

10.  Switch on the high vacuum gage (ionization gage).

 

Shut-Down of Diffusion Pump

  1.  Close the baffle first and then turn off the ionization gage.
  2. Turn off the heater and allow 30 minutes to cool the oil.
  3. Warm the cold trap to room temp.
  4. Close the fore-line valve.
  5. Vent the diffusion pump via vent valve.
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REFERENCES

  1. D. G. Avery and R. Witty (1947). “Diffusion pumps: a critical discussion of existing theories”. Proc. Phys. Soc. 59 (6): 1016–1030.
  2. C. R. Burch (1928). “Oils, greases and high vacua”. Nature. 122 (3080): 729.
  3. Van Atta, C. M.; M. Hablanian (1991). “Vacuum and Vacuum Technology”. In Rita G. Lerner; George L. Trigg. Encyclopedia of Physics (Second ed.). VCH Publishers Inc. pp. 1330–1333.
  4. J. F. O’Hanlon, “A user’s guide to Vacuum Technology”, 2nd. Ed. , Wiley, NY, 1989.
  5. Armand Berman, “Total Pressure Measurements in Vacuum Technology”, Academic Press, Orlando, FL, 1985.