Function & size
Vacuum pumps generate the vacuum in the vacuum chamber of the plasma system. The following vacuum pumps are used most frequently in plasma technology:
The rotary slide vane pump consists of a housing in which a rotor rotates eccentrically. In it, there are sliders loaded by springs, which are pressed against the inner wall of the housing and slide along it. The gas trapped on the intake side is compressed until it exceeds the ambient pressure and the outlet valve on the discharge side opens.
The pump runs in an oil bath. This has the following advantages:
1. Sealing of intake and pressure side
2. Reduction of friction
Rotary slide vane pumps are particularly well suited for our standard systems. The larger the pump, the shorter the process time.
The pump can be selected individually to match the chamber volume and desired process time.
Some treated products emit large amounts of gas (e.g. POM parts, silicone rubber or moist parts). In this case we recommended to use a stronger pump. This must be clarified in preliminary tests.
If oxygen is to be used, the pumps must be prepared for this. Oil mist is produced in the pump housing. If mineral oil is used, this mist is explosive in combination with pure oxygen.
There are two ways to solve this problem:
1. The pumps are filled with PFPE oil. This oil is non-combustible. But is has some major setbacks:
- PFPE oil is very expensive
- PFPE oil contains fluoride. This oil may decompose in plasma processes, extremely toxic compounds (e.g. perfluoroisobutylene) may form. Serious accidents have already occurred due to these oil compounds.
- PFPE oil must be disposed of as hazardous waste.
2. The pumps are flushed with air or N2. This solution is preferable.
Working with corrosive gases
Special vacuum pumps have been developed to work with corrosive gases (CF4 / O2, SF6, …). The rotary slide vane pumps described below are suitable for many applications.
However, using these pumps often with corrosive gases reduces their useful life. Usually, one pump of this type is sufficient for use in the lab. For frequent use on the shop floor, however, we recommend to purchase a dry-running pump.
Purging corrosive gas with dry air or nitrogen.
Dry-running pumps for corrosive gas processes
For processes with corrosive gases, dry-running pumps can also be used. However, these must then be specially designed for this high load. These pumps are designed for the highest loads. They can also cope with particle, condensation or corrosion by-products. The benefit of the dry-running pump is its optimised utilities consumption. The pumps require no preventive maintenance (oil change).
The rotary slide vane pump can produce limited pressure only. To increase its throughput, a combination with a Roots vacuum pump is advisable. Together, the pumps form a so-called pump stand. A typical combination comprises the following:
1. The first pump (e.g. a rotary slide vane pump) generates a pre-vacuum. It is described as “pre-pump”.
2. A Roots vacuum pump is used as second pump.
We will gladly advise you which pump best suits your processes.
Roots pumps, also named Roots-type blowers, are rotary lobe pumps in which two symmetrically designed rotary lobes rotate in the opposite direction in the feed housing.
The rotors have a roughly 8-shaped cross-section and are synchronised by a gear mechanism so that they move past each other and the housing wall with little play without mutual contact.
At piston positions I and II, the volume in the intake flange is increased. As the pistons continue to rotate in position lll, part of the volume is closed off from the intake side.
In position IV, this volume is opened towards the outlet side and gas under pre-vacuum pressure (higher than the intake pressure) flows in. The incoming gas compresses the gas volume conveyed from the intake side. As the piston continues to rotate, the compressed gas is discharged via the outlet flange.
This process is repeated twice per full revolution for each of the two pistons. Roots pumps can be operated at high speeds due to the non-contact running in the conveying chamber. Thus, a comparatively high intake capacity can be reached with small pumps.
The pressure difference and the compression ratio between intake and outlet side are limited in Roots pumps.
(Text and figures courtesy of Oerlikon Vacuum GmbH)