By Howard Tring

Fig. 1. MFP at different pressures

This article completes the series of Five Main Reasons that vacuum is used in science and industry.

1. To Provide a Working Force
2. To Remove Active and Reactive constituents
3. To Remove Trapped and Dissolved Gases
4. To Decrease Thermal Transfer
5. To increase the Mean Free Path to a useful dimension

The gas state

The article printed back in January this year talked about solid, liquid and gas states of matter. The following is a short excerpt from that article.

Fig. 2. Flow of Molecules

“In a gas the atoms and molecules are generally much further apart than in solids and liquids. In air at atmospheric pressure and room temperature the actual space occupied by atoms and molecules is about 0.01 per cent or one ten thousandth of the volume. The equivalent for solid copper is about 74 percent or close to three quarters. (So much for being called a “solid”).

In air the molecules are in constant random movement, typically in a straight line, and the interatomic forces have little effect due to the space between the molecules. The moving molecules will constantly collide with other molecules and then move away in a different direction. These collisions occur about 10,000,000,000 times per second at atmospheric pressure.”

Atmospheric pressure is always the starting point of any vacuum process, and we know that we can reduce that pressure in a closed vacuum chamber by using one or more vacuum pumps to reach whatever lower pressure (vacuum) is required for the process.

Fig. 3. Conductance in Pipes

In the excerpt above it states that molecules in the chamber are constantly colliding with other molecules and changing direction. Molecules of gas that collide with the inner chamber wall, or the surface of any fitment, work holder or product in that chamber will reside on that surface for a fraction of a second and then release off the surface in a completely random direction.

Mean free path

The term “mean free path” is the average distance that a molecule will move before colliding with another molecule, it is measured in inches or meters and is related to the pressure and density of the gas molecules in the chamber. We usually show the mean free path at a certain pressure because that is how we measure the vacuum level using some type of vacuum gauge (Fig.1). Density is related to the pressure but is rarely used as an indicator for vacuum processes.

From Fig. 1 we can “see” that as the pressure in the vacuum system drops the mean free path becomes longer. Molecules of gas are being evacuated by the pumps, the density of the gas is being reduced and the molecules have to move further before they collide with another molecule. I used the word “see” above but this is one of the most difficult things about understanding what is happening in a vacuum chamber, you can’t actually “see” the molecules.

Quick review of conductance

Fig. 4. Vacuum evaporation coating

The increase in the mean free path of gas molecules starts to become useful once it reaches several inches, but just before that point there is another interesting occurrence. This is the transition of the flow of molecules in the vacuum system piping from viscous or continuum flow to molecular flow (Fig. 2). This drawing tries to show the changes in the flow of molecules moving down a pipeline as the pressure is reduced. It is simplified and does not show the flow variations at the pipe wall, however, the reader will see these main differences.

When I delivered vacuum technology classes for Edwards I likened the changes to the crowd leaving a stadium at the end of a football game. (It assumes that the game kept the crowd in their seats until the end!)

Viscous flow, turbulent: When the final whistle blows most of the crowd attempts to move towards the exits and there is a lot of random movement as the crowd jostles around trying to move into any open space that may lead to a faster exit.

Viscous flow, laminar: After a few minutes the big rush has moved away and the rest of the crowd has more room to move in an orderly fashion towards the exits.

Fig. 5. Electron beam gun

Molecular flow: A bit later most of the crowd has left and the remaining few can meander all over the walkways going in a variety of directions without being jostled by anyone else.

Returning to the molecules, the big change in all this occurs between laminar flow and molecular flow, when the mean free path becomes longer than the inside diameter of the pipe. At and after that point the gas molecules are more likely to collide with the walls of the pipeline (or chamber) than collide with another molecule.

This change also affects the conductance in the pipeline. Conductance is the measure of the mass of gas flowing at the average pressure per meter of pipe length. It is measured in liters per second, per meter (Fig 3.). This graph shows how the conductance changes in a number of different diameters of pipe as the pressure changes. The interesting point to see is that as the pressure drops the conductance also drops until the gas flow changes to molecular flow. Once the molecules are in molecular flow the conductance becomes constant.

For example, look at the curve in Fig.3 for a 100 mm diameter pipeline. At a pressure of about 1 Torr the conductance is 10,000 liters/sec and steadily drops as the pressure drops. However, at around 0.002 Torr (2 x 10^-3 Torr) the conductance becomes constant at about 100 liters/sec and does not change as the pressure drops below that reading.

Vacuum applications due to a long MFP

Fig. 6. Sputter coating process

Going back to the discussion of “mean free path” there are a number of processes that become viable once the MFP is longer than a few inches. The reason is that once the MFP is relatively long, other small objects can move across the vacuum system with a good chance of not colliding with a gas molecule. If, for example, the vacuum chamber is at a pressure of about 0.00001 Torr (1 x 10^-5 Torr) the MFP will be around 190 inches or 16 feet. If the vacuum chamber has a diameter of only 5 feet it is unlikely that a molecule of gas will strike another when it moves across the chamber.

This allows systems such as vacuum coaters, electron microscopes, mass spectrometers, surface science instruments and particle accelerators to work successfully. Let’s see how each of these works under low pressure conditions.

Vacuum coating

Vacuum coating or vacuum deposition is a method of producing a finish coat on a substrate. The substrate may be paper, metal, plastic, glass or some of the fancy materials used in the manufacture of integrated circuits (computer chips). The actual deposition process is selected depending on the substrate material, the material being deposited, the thickness and surface finish of the coating and whether the substrate is stiff or flexible.  They are only successful processes because the material being deposited can move across the space without colliding with a gas molecule.

Typical deposition processes are:

Fig. 7. Electron beam microscope

Evaporation by a hot filament (Fig.4)

In this process metal is evaporated from a hot wound filament or boat filament. The wound filament is used for solid metals where small sections of wire are placed over the filament loops. Boats are used when the metal is in powder form. In both cases the metal vapor evaporates in straight lines and condenses on the first surface it touches. The item being coated (substrate) is placed at a suitable distance from the filament so that coating is as even as needed. If multiple parts are being coated they would be placed in a workholder. This process tends to contaminate all interior surfaces of the chamber in line of site with the evaporation material. Shields are often used so that they can be replaced or cleaned when the contamination builds up and makes the pumping cycle longer.

Evaporation using an electron beam gun (Fig. 5)

This system is also used for coating metals but the material is melted by a high powered beam of electrons emitted from the filament. This is often a longer process and the filament is protected from contamination by placing it under the water cooled crucible that contains the material being evaporated. A magnetic field bends the beam of electrons through 270 degrees and onto the material. The beam can be focused and also moved across the metal in the crucible to melt it somewhat evenly.     

Fig. 8. Picture from an electron beam microscope

Sputter coating using a plasma (Fig. 6)

Sputter coating is a more difficult process to understand. In this process the material being sputtered and the substrate are arranged as a cathode and anode. Argon gas is bled into the vacuum chamber and is ionized to produce a plasma between the two electrodes. The argon ions are accelerated into the cathode material, called the target, and they dislodge particles of the target material. The dislodged material is then attracted to the anode which the substrate is attached to and creates a coating on the substrate.

Electron beam microscopes (Fig. 7)

These instruments are used to look at items and surfaces under very high magnification. Electrons are fired from an electron gun and are either scanned over the item surface or transmitted through the surface. Please consult an expert to learn about the differences, I have only a simple knowledge of these instruments as it is the vacuum pumps that are my concern. Fig. 7 shows an electron microscope and you can see the white vertical column in the center which is the electron beam assembly. The electron gun is at the top and the item to be viewed is in the cube shaped chamber below it. The white column has to be evacuated to a low enough pressure that the mean free path of remaining gas molecules is substantially longer than the height of the column. This would generally be a turbomolecular pump and mechanical pump combination. The picture in Fig. 8 shows the head of a fly in great detail.

Fig. 9. Mass spectrometer

Mass spectrometers (Fig. 9)

It was difficult to find a simple picture of how a mass spectrometer works. It is a device to determine (analyze) what a gas or solid is made of by ionizing a small sample. There are two main types of mass spectrometer; one that uses gas chromatography, a GC-MS, and the other that uses liquid chromatography, an LC-MS. Again, this is not my area of expertise; consult an expert for more information. Both types of Mass Spec ionize the sample and separate the ions by magnetism so that the detector can determine what the constituents are. The different constituents are displayed on a screen by mass number. The whole detection system is under vacuum and only works because the ions do not collide with any gas molecules in the chamber under vacuum.

Other applications

There are other applications using low pressure conditions to allow ions, electrons and other small particles to move through a system without colliding with gas molecules. Surface science systems are one of these and physically large systems such as particle accelerators are included. The CERN accelerator in Europe has a circular vacuum chamber that is about one meter in diameter cross section in an underground circular tunnel. The circle itself is about 9 kilometers across.

Each of these applications is a much more complicated science than I have tried to explain in this short article. I hope it has “tweaked” your interest.

Howard Tring is the owner of Vacuum and Low Pressure Consulting, a company that supplies vacuum pump accessories such as reconditioned inlet traps and exhaust filters and new replacement elements for exhaust filters. Howard also offers on-site vacuum technology and oil sealed vacuum pump repair training and consulting services, customized to the needs of the client. Howard is a member of ASM International and the Heat Treat Society, the AVS, the SME, the SVC and the American Society for Training and Development.

Copyright September 2014, Tring Enterprises LLC – Comments on this article are welcome. I do not profess to know everything about any specific vacuum related subject. However, I have worked in the vacuum pump industry a long time and have seen good, bad and ugly. Please contact me with any comment or question. All messages related to the content of the article will be answered.