Rotary kinetic fluid motors or pumps – Smooth runner surface for working fluid frictional contact
Reexamination Certificate
1999-09-09
2001-05-01
Lopez, F. Daniel (Department: 3745)
Rotary kinetic fluid motors or pumps
Smooth runner surface for working fluid frictional contact
Reexamination Certificate
active
06224326
ABSTRACT:
The present invention relates to turbomolecular pumps which, associated with primary pumps, make it possible to generate and sustain a hard vacuum in an enclosure.
BACKGROUND OF THE INVENTION
In general, a turbomolecular pump comprises a rotor and a stator having a plurality of stages, the rotor being carried by bearings. Going from the inside of the enclosure towards the outside, the gas pressure increases progressively from one stage to the next, and the stages close to the inside of the enclosure are considered to be low-pressure stages, while the stages close to the outlet are considered to be high-pressure stages.
When it includes magnetic or gas bearings for supporting the rotor, such a turbomolecular pump has the characteristic that its rotor is isolated physically and therefore electrically from the stator and from the body of the pump which finds itself at reference potential, i.e. at the ground potential of the equipment.
Turbomolecular pumps are frequently used in plasma deposition or etching equipment in the semiconductor industry.
While such turbomolecular pumps are being used in plasma deposition or etching processes, it has been observed that deposits tend to form of materials coming from the reaction products. For example, in a turbomolecular pump used in a plasma etching machine for etching semiconductor materials, residues coming from the etching of the resin masks tend to deposit on the inside surfaces of the rotor and of the stator, and to do so preferentially in the high-pressure stages of the turbomolecular pump.
In addition, in plasma deposition or etching methods, the turbomolecular pump and in particular its rotor are in direct contact with the plasma. As a result, the rotor, which is electrically isolated, is taken to a potential that is different from ground potential.
The pressure conditions in the high-pressure portion of the turbomolecular pump, combined with the short distance between the rotor and the stator and also with the potential difference between the rotor and the stator cause electrical discharges to occur between the rotor and the stator. In the absence of deposits, such discharges are distributed uniformly between the surfaces of the rotor and of the stator, and they do not do any damage.
Unfortunately, when deposits form, they disturb the discharges by creating preferential paths, and zones are created in which arc conditions occur in which high current densities are set up, thereby rapidly damaging the rotor.
In the prior art, attempts have already been made to remedy this problem in various ways.
In a first solution, a grid connected to ground has been interposed between the turbomolecular pump and the plasma in the enclosure. Unfortunately, the high density of the plasmas used requires very fine-mesh grids, sometimes of mesh size less than 100 &mgr;m. Under such conditions, the presence of grids reduces the conductance of the pump considerably, and significantly reduces its pumping speed. In addition, such a very fine-mesh grid is a site on which deposits form that can then generate particles detrimental to the industrial process that is performed in the enclosure.
In another solution, attempts have been made to prevent deposits from forming by increasing the temperature of the turbomolecular pump so as to avoid deposition by condensation. However, given the nature of the materials used and the high speed of rotation of the rotor, temperatures are rapidly reached that generate phenomena of material creep, thereby destroying the pump without even being effective in preventing deposits from forming.
OBJECTS AND SUMMARY OF THE INVENTION
The problem that the present invention proposes to solve is that of preventing the formation of deposits that would disturb the electrical discharges inside a turbomolecular pump having magnetic or gas bearings and connected to an enclosure containing plasma, without significantly adversely affecting pumping speed or the industrial process inside the enclosure.
Thus, an object of the invention is to design other means which, without interposing an intermediate grid or significantly increasing the temperature of the materials, prevents discharge-disturbing deposits from forming in a turbomolecular pump having magnetic or gas bearings and connected to a plasma enclosure.
To achieve these objects and others, the invention provides a method of preventing deposits from forming that would disturb electrical discharges between the rotor and the stator inside a turbomolecular pump with a plurality of stages on magnetic or gas bearings; in this method, an active gas is injected at at least one suitable location inside the turbomolecular pump, which active gas reacts with the deposit-generating molecules and forms gaseous compounds that are removed by the turbomolecular pump.
Excellent effectiveness is obtained by choosing an active gas that is dissociated and/or activated under the action of the electrical discharges between the rotor and the stator so as to react with the deposit-generating molecules.
In the method, the active gas is injected only into those stages in which the deposits are likely to occur. Generally, these stages are the high-pressure stages of the turbomolecular pump. In practice, it is possible to inject the active gas into the last turbomolecular stages, and/or into the Holweck stage when the pump is provided with such a stage.
In numerous applications, the active gas that is injected may advantageously be oxygen or contain oxygen. When it dissociates under the action of the electrical discharges, the oxygen produces oxygen atoms that are highly reactive and that combine effectively, in particular with the organic residues generated by the industrial processes, so as to form volatile molecules of the carbon monoxide, carbon dioxide, or water types, which are then removed along with the other gaseous compounds coming from the plasma enclosure.
The invention provides a turbomolecular pump for implementing such a method and having a rotor and a stator with a plurality of stages on magnetic or gas bearings; the turbomolecular pump includes means for injecting an active gas at at least one suitable location inside the turbomolecular pump, which active gas reacts with the deposit-generating molecules and forms gaseous compounds that are removed by the turbomolecular pump.
The turbomolecular pump of the invention may advantageously be provided with at least one gas intake pipe positioned so as to bring the active gas into the path of the gases flowing between the rotor and the stator in the last turbomolecular stages, and/or in the Holweck stage when the pump is provided with such a stage.
The turbomolecular pump of the invention may advantageously be associated with an external active gas source and with control means for delivering the active gas in a quantity that is sufficient to prevent deposits from forming.
REFERENCES:
patent: 4512725 (1985-04-01), Saulgeot
patent: 4904155 (1990-02-01), Nagaoka et al.
patent: 5443368 (1995-08-01), Weeks et al.
patent: 5522694 (1996-06-01), Bernhardt et al.
patent: 5577883 (1996-11-01), Schutz et al.
patent: 5688106 (1997-11-01), Cerruti et al.
patent: 0 408 792 A1 (1991-01-01), None
patent: 0 451 708 A2 (1991-10-01), None
patent: 0 695 873 A1 (1996-02-01), None
Alcatel
Lopez F. Daniel
Nguyen Ninh
Sughrue Mion Zinn Macpeak & Seas, PLLC
LandOfFree
Method and apparatus for preventing deposits from forming in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for preventing deposits from forming in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for preventing deposits from forming in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2566316