Advances in oil-free vacuum pumps
03/01/1999
Advances in oil-free vacuum pumps
Eckhard Bez, Leybold Vakuum GmbH, Cologne, Germany
Minimizing pump failures and contamination from pump exhausts is imperative to tool efficiency and environmental performance. For the last two decades, vacuum pump manufacturers have been working toward the goal of producing low-cost, oil-free, and maintenance-free mechanical pumps for roughing and backing in industrial applications. The importance of totally oil-free evacuation increases as manufacturers strive to produce better and more complicated products, usually at lower cost. Lower cost of ownership often means lower maintenance, which an oil-free pump promises. There is no used oil to dispose of, no filters or oil to change, no smell during operation, and no oily mess to clean up.
Dry pumps are especially well suited for loadlock chambers, transfer chambers, metrology tools (e.g., wafer inspection tools, mass spectrometers, electron microscopes), and as forepumps for high-vacuum process chambers. Reliable dry forepumps with low cost of ownership and maintenance requirements are especially beneficial to LPCVD and PECVD silicon nitride systems. These deposition systems generate solid by-products (powders) from cross-reactions between the gases used for the deposition process and the gases used for the etch/clean process.
Realizing the savings from a dry pump has been difficult. Many of the dry vacuum pumps available are specifically designed with semiconductor processes in mind, but no totally satisfactory pumps have been on the market to replace oil-sealed pumps directly for clean and semiclean applications. It is important to note that these oil-free pumps still contain considerable amounts of oil in the gearboxes and only the compression spaces are said to be oil free. These process pumps are generally large, expensive, and complicated, and require water cooling and nitrogen in their operation.
In many general vacuum applications, particularly those associated with furnaces, loadlocks, and metrology instruments, the above "quasidry" process pumps are often used for lack of a more suitable alternative. Oil filters have been used for many years to try to limit the oil contamination of both the vacuum system and the environment, but they must ultimately be changed and disposed of. It would therefore be better if no oil existed in any part of the pump.
For reliable, maintenance-free operation under totally oil-free conditions, a pump must contain no liquid lubricants-even in the noncompression spaces-that could potentially contaminate the system or environment either during operation or when the pump is not in use . Completely sealing the parts becomes a challenge.
Without the use of liquid lubrication, the most rigorous requirements are those of achieving adequate ultimate pressure and pump speed, the tolerance to liquids and particles, and the low wear needed for long operation before service. In an oil-sealed pump, the oil is used not only to lubricate the mechanism, but also to increase the compression ratio by filling any "dead" spaces as well. It is quite easy for such pumps to reach pressures of 5 ? 10-3 torr, which is a compression ratio of about 106 using only two stages of compression. An oil-free pump cannot reach these pressures with only two compression stages - four are generally used. The problem of sufficient lifetime (10,000 hr between service) also presents vexing difficulties. The pump must be capable of continuous operation, particularly at atmospheric pressure, and again at its ultimate pressure without sacrificing its lifetime and without stopping suddenly due to internal mechanical problems.
To fulfill dry friction requirements, components used must be very hard and resistant to wear. They should absorb shock loads and tolerate some ingested dirt, some particles, and small quantities of liquid water and moisture without long-term effect on performance. Further, these materials must be chemically inert, easy to apply to the final configuration, and relatively inexpensive. Although diamond films meet some of these requirements, at their present stage of development, they are too expensive. The entire pump should be similar in weight and dimensions to an oil-sealed pump and must operate only on electricity.
The latest advances in dry pump design are based on a highly reliable, low-cost piston principle. Figure 1 shows a schematic of the gas flow of the new four-stage, oil-free pumps.
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Figure 1. Gas flow in the four-stage, oil-free pump.
The modular design allows the drive unit to be separate from the vacuum pump. A seal between the motor and the drive unit prevents air from entering the space around the crankshaft.
An alternative, but much more expensive solution, is to use a "canned" motor that requires no rotary seal. The inside of the drive unit is pumped to about 200 torr through an orifice. The ball bearings used throughout the pump are specially sealed and use nonhydrocarbon grease for permanent lubrication. The pistons, which have a large diameter head and smaller rear portion, are covered on the outside with a relatively soft, low-wear material with a low coefficient of friction.
There are always two stages of compression/identical piston. The aluminum cylinder insert in the pump body is covered with aluminum oxide in an electrolytic process and can be easily changed in case of damage. All the pistons and cylinders are similar as are the valves, allowing easy construction.
The heat generated by gas compression and friction of the mechanical components cannot be transported by oil to the exterior of the pump body, as is the case in oil-sealed pumps. While a vacuum pump must operate at an elevated temperature to avoid the condensation of liquids (often water), there is an upper limit to this temperature. The pistons use their large surface area to convey the heat generated in the depths of the pumping mechanism to the exterior body, thus providing easy cooling.
A further measure, which greatly limits heat generation, is to provide a pressure-sensitive bypass valve that limits the compression to only one stage at pressures above 200 torr (see Figure 2). It is possible to limit the temperature of the outside of the pump to approximately 60?C at an ambient temperature of 30?C. This pump can operate in either a vertical or horizontal position.
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Figure 2. Pressure-sensitive bypass valve used to limit compression and heat generation; a) low-pressure operation and b) high-pressure operation.
The pump uses the first two pistons in parallel to provide four pumping chambers, but other arrangements could be used. The next piston has both its working spaces arranged in series with the first two pistons, forming the second compression stage. In the fourth piston, the cylindrical working space provides the third compression stage, while the annular working space provides the final stage where the gas exits through the exhaust. This piston arrangement and the use of only mechanically operated low-stress valves guarantees very high reliability and long operating lifetimes.
During operation, gas entering the inlet port of the pump passes through internal passages to the inlet slot in the cylinder. Two double-acting pistons acting in parallel form the first stage of compression. When any piston reaches the bottom dead center, the slot in the sidewall of the cylinder opens and gas can enter the working space above the piston. As the piston leaves the bottom dead center, it acts as a valve blocking the intake slot on its upward stroke. As the gas above the piston is cut off from the inlet, it is compressed and can leave the cylinder via a valve located at the top of the cylinder. As the piston approaches the top dead center, bumpers on top of it force open the exhaust valve. At low pressure, the pressure of the gas will always be insufficient to overcome the spring force and the valve is always forced open by slight contact with the piston. A second slot located in the sidewall of the cylinder opens when the piston reaches the top dead center, and allows gas to enter the annular working space behind the piston head. As the piston descends, the gas is again cut off from the inlet exits through an exhaust valve located in the bottom of the annular working space.
Conclusion
Studies show that the next generation of dry pumps satisfies most of the requirements unfulfilled by the older pumps. They are also designed to measure performance changes available and to give ample warning of upcoming maintenance requirements.
ECKHARD BEZ is a vacuum technology consultant for Leybold Vakuum GmbH. Questions on the article can be sent to him at [email protected], on Leybold Vakuum to [email protected]