This article continues the series of Five Reasons that vacuum is used in science and industry. The first “To Provide a Working Force” was published in December 2013. In addition to writing about the reason itself I will try to give one or more examples of the reason in practical applications. If you know of other applications that fit the reason, I will be pleased to hear from you.
2. To Remove Active & Reactive Constituents
Cyclic vacuum processes
|Fig. 1. Small crystal coater.|
For many vacuum applications the process is a cyclic one. The workload is placed in the chamber, the chamber is evacuated and the process takes place. The chamber is then let back up to atmospheric pressure and the process workload is removed. These cycles can be as short as a few seconds in the case of coating a small communications crystal (Fig.1) to give it a specific frequency, or it can take several hours in the case of a heat treating cycle that may include evacuation, heating, soaking, cooling and eventually back to atmospheric pressure. In this type of cycle the main “constituent” being removed is “air”.
However, “air” is a bit more complicated than it seems. Let’s look at what main gases are present in dry air, see Fig.2. There are two or three rare gases not included and the total will not add up to exactly 100%. In regular air that we breathe, there is also a small amount of water vapor. It varies due to the humidity at any specific place and can be from less than 0.01% up to over 5%. On average, at room temperature, it may be close to 1.57%.
Water vapor is lighter than most of the air molecules (nitrogen and oxygen) and the air can contain a percentage of moisture until it reaches its saturated vapor pressure, dependent also on temperature. If the water vapor content exceeds the saturated vapor pressure the excess moisture is given up outside in the form of rain or snow. Inside your home you may see excess water vapor (humidity) as condensation on a cold window pane.
In the industrial or scientific world similar phenomena exist inside the vacuum system. When we have a cyclic vacuum process and the chamber is opened to atmospheric pressure air, including dry gases and water vapor, enters the chamber. These very small invisible molecules are moving at very high speeds and are so densely packed together that they collide with each other and the surfaces inside the vacuum chamber many times each second. It is these collisions and the resulting forces that produce atmospheric pressure close to 14.7 psia (pounds per square inch absolute).
When “dry” air molecules collide with a surface they tend to stay on the surface for a very short time and then release off the surface in a completely random direction. The atomic bond between the molecule and the surface is very weak and the molecule easily moves away from the surface. At pressures near atmospheric pressure (760 Torr) the molecule will very quickly collide with other molecules due to the density of the air. This will be less than one ten thousandth of an inch. In this condition the molecules are said to be in viscous flow conditions.
At pressures around 0.01 Torr (10 microns) the molecule may move about one third of an inch before it collides with another molecule. As the pressure (and hence the density) is reduced in the vacuum system, the molecules will travel progressively longer distances before hitting other molecules or another surface. If the pressure is reduced further it becomes more likely that the gas molecule will collide with another surface inside the vacuum chamber rather than collide with another molecule. At this point and at lower pressures still, the gas molecules are said to be in molecular flow conditions.
So, from the above, most dry gas molecules are not much of a problem when we try to evacuate a vacuum system to run a process. Light molecules such as hydrogen and helium can be evacuated more slowly than the heavier molecules but they are rare in air and do not generally create much of a problem. The main dry gas molecule that requires removal from the vacuum chamber is oxygen. Oxygen is reactive, can combine with other molecules and most importantly will promote burning if the process happens to include heat.
|Fig. 2. Gases in Dry Air (%).|
In the heat treating industry products can become oxidized if there are too many oxygen molecules present once the product is heated for processing. The vacuum pumps are used to reduce the percentage of oxygen molecules to a low level where they do not cause any problem. I’m sure many operators have seen oxidized parts come out of the furnace when something went wrong with the pumpdown. The oxidation may be all over the product in the case of a major vacuum loss or it may be confined to certain areas of the chamber if a small leak developed during a process run.
Water vapor is the second constituent in the atmosphere that can create problems with your vacuum process. Water vapor has a stronger atomic bond than the dry gases and this causes water vapor molecules to “stick” to internal surfaces of the system longer before they release and can be evacuated. If we boil water in a kettle on the stove at home, we have to give it the energy delivered by heating it to 212 degrees F before the water molecules have enough energy to release from the liquid and leave the spout of the kettle as steam. That is due to the 14.7 pounds per square inch of atmospheric pressure pushing on the surface of the water.
In the vacuum system, once the pressure is reduce by the vacuum pumps to about 18 Torr, water molecules on the surface of the chamber interior will release or evaporate from the surface at about 68 degrees F (room temperature) and be pumped away. This is why on many large vacuum systems Roots boosters or Roots blowers are used in this pressure area. In a somewhat short period of time, as the pressure reduces through the 50 to 5 Torr region, a large volume of water vapor is released from the chamber and product surfaces inside the chamber and need to be evacuated (pumped away) quickly. The Roots booster pump is designed to operate in this pressure area and has a high pumping speed. It can evacuate the large volume of water vapor and pass it on to the roughing pump to be exhausted back to the atmosphere.
If the vacuum system uses heat as part of the process, the chamber can be preheated to give the water vapor more energy and encourage it to release off the surfaces. Clean and smooth surfaces are preferred inside the vacuum chamber but not always possible. Materials inside the chamber that are porous should be avoided, this type of material may have a tremendously large surface area due to the small holes in it, will adsorb water vapor when the chamber is open and is slow to release the water vapor. Some insulating materials may have this type of texture.
In humid weather large vacuum coating systems are particularly prone to capturing moisture unless the chamber is kept very clean. This is true especially for systems evaporating aluminum onto metals of plastics that may run a complete cycle every hour or so. Layer upon layer of coating material tends to adsorb moisture and cause the evacuation to take longer to reach process vacuum. Using liners inside the chamber walls is a way to reduce this problem, they will receive a major portion of the contamination and can be changed out for clean ones once the pump down time becomes extended. The clean liners will allow for a faster pump down while the contaminated liners can be cleaned in preparation for the next change over.
Some vacuum systems use cold traps to capture this water vapor before it reaches the oil sealed vacuum pump, as the lowest pressure attainable by the vacuum pump will be compromised if water vapor condenses in the oil. Cold traps work well but do require some service/maintenance to eliminate any trapped moisture either each cycle or perhaps at the end of a shift. Another option to reduce the amount of water vapor condensing in the vacuum pump oil is to use the gas ballast valve during the time that the water vapor is being pumped and perhaps additional gas ballasting between cycles to help keep the oil clear of water vapor contamination.
In vacuum systems that utilize glass bell jar chambers it is not unusual to see a cloud of water vapor for a few milliseconds at the point where the water vapor evolves from the internal surfaces.
In conclusion for this section, cyclic systems, the main “enemies” of the process are oxygen and water vapor.
Continuous vacuum processes.
|Fig. 3. Inline Vacuum System.|
This type of system has different problems when it comes to removing active and reactive constituents. Some systems can be in-line load lock systems and the center chamber or chambers stay under vacuum once the whole system is started up. Others may have multiple load lock and process chambers with a central automatic robot arm to move the product from one chamber to the next. This type of “cluster tool” is commonly used for processing silicon wafers that will eventually have many integrated circuits (computer chips) build up on them.
In line vacuum process systems
The in-line systems, Fig. 3, have load lock chambers that are used to move the product being processed into and out of the process chambers. They act in a similar fashion to the cyclic systems above but see many more air to vacuum cycles. In fact the pumps needed for load lock operation need to be selected carefully as they see frequent high pressure (atmospheric pressure) loads and that tends to make the pumps operate hotter than when mostly operating at low pressure. These systems can require an evacuation from atmospheric pressure to vacuum every two or three minutes which is tough on the vacuum pumps and they often need additional maintenance due to the difficult application. Each load lock and each process chamber will have its own set of vacuum pumps. In Fig. 3 the work racks or work holders may be on a looped track so that once unloaded they return to the inlet end to be reloaded for the next cycle.
The process chambers themselves however don’t see atmosphere once they are evacuated to the operating pressure and start the process. These systems often have gases bled into the process chamber as part of the process. These gases create a chemical reaction on the substrate and then the reacted gases have to be pumped away quickly by the vacuum pumps ready for the next process load entering through the load lock. Other processes may create solid effluent in the form of fine dust and this is evacuated towards the vacuum pumps. Oil sealed pump sets may use inlet traps to catch this contaminant before it reaches the pump or dry pumps may be used and the contaminants captured after passing through the pump mechanism.
Semiconductor processing tools
As stated above, cluster tools consist of a number of process chambers mounted around a central vacuum chamber called a transfer chamber. Each chamber will have its own vacuum pumps and may operate at different vacuum levels depending on the specific process. The load locks are fitted with a vacuum gate valve on the entry from outside and another to allow the load to be presented to the robot arm in the transfer chamber. A load may consist of a special wafer holder called a cassette that holds twenty five silicon wafers. The cassette may stay in the load lock but the inner gate valve is open to allow the robot arm to select a wafer and move it to the first process chamber. Each process chamber will be fitted with a valved entry to allow the robot arm to place a wafer inside it for processing and then remove it and deliver it to the next process chamber in turn.
|Fig. 4. Semiconductor “cluster tool”.|
Not shown in Fig. 4 are the larger dry pump/Roots pump sets that are situated in the basement under the tool. These tools require so many pumps that they are often stacked two high to save space. In the last few years manufacturers have started to use variable frequency drives to allow the pumps to run faster when needed. This has allowed the footprint of the pumps to be smaller and also reduces power requirements. This type of tool has very clever controls to make sure every step of the process in each chamber is done correctly. Thank goodness for computer controls!
From inside the clean rooms a semiconductor manufacturing facility “fab” looks very pristine. All the operators are wearing bunny suits, hair coverings and bootees. But, as the processes take place on the wafers – a number of coating and etching steps – there is a lot of dangerous gas mixture effluent being exhausted by the vacuum pumps. The inside of the process chambers, vacuum lines, vacuum pumps and even the exhaust lines are filled with gases that may kill you if you breathe them, may burst into spontaneous combustion if they see oxygen, can condense into solids and block the pipelines and probably several other nasty results that are not mentioned.
Hazardous gases used to create chemical reactions include silane, arsine, phosphine, nitrogen trifluoride and tungsten fluoride.
The vacuum pumps have to take the nasty mixtures of reactive gases and compress them through the pump mechanism and exhaust them to the exhaust abatement equipment. In many cases this effluent is so dangerous that it has to be chemically reacted to safe gases and/or incinerated before it can be exhausted back into the atmosphere.
In the case of condensable vapors it is sometimes necessary to run the vacuum pump at a specific temperature and heat the vacuum lines to prevent the condensable vapors from solidifying. Many semiconductor processes that use chemical reactions to coat wafers and then etch materials away in another chamber use inert nitrogen purges. The nitrogen doesn’t react with the dangerous chemical mixtures but protects the inner pump surfaces from corrosion, helps to blow the solid contaminants through the mechanism and dilutes the gases to less hazardous mixtures.
In conclusion, for continuous processes, the vacuum pumps remove the process gases from the chamber ready for the next process step and are also tuned to suit the specific gas mixture that is being reacted.
The chart in Figure 1 is from “Modern Vacuum Practice” written and published in the UK by Nigel Harris.
The three dimensional representation of a cluster tool in Figure 4 was modified from a slide in a BOC Edwards presentation. (BOC Edwards is now Edwards Vacuum, a division of Atlas Copco.)
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 December 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.