Ultra-high vacuum is a vacuum regime characterized by pressures lower than 10−7 pascal (10−9 approximately 10−9 tor). Ion pumps are used in some settings to establish an ultra-high vacuum. In an ion pump, an array of cylindrical anode tubes are arranged between two cathode plates such that the openings of each tube faces one of the cathode plates. An electrical potential is applied between the anode and the cathode. At the same time, magnets on opposite sides of the cathode plates generate a magnetic field that is aligned with the axes of the anode cylinders.
The ion pomp operates by trapping electrons within the cylindrical anodes through a combination of the electrical potential and the magnetic field. When a gas molecule drifts into one of the anodes, the trapped electrons strike the molecule causing the molecule to ionize. The resulting positively charged ion is accelerated by the electrical potential between the anode and the cathode toward one of the cathode plates leaving the stripped electron(s) in the cylindrical anode to be used for further ionization of other gas molecules. The positively charged ion is eventually trapped by the cathode and is thereby removed from the evacuated space. Typically, the positively charged ion is trapped through a sputtering event in which the positively charged ion causes material from the cathode to be sputtered into the vacuum chamber of the pump. This sputtered material coats surfaces within the pump and acts to trap additional particles moving within the pump.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.
A method includes assessing a plurality of Titanium plates to determine a grain size for each plate and removing all Titanium plates with an average grain size that is larger than a threshold size from the plurality of Titanium plates. One of the Titanium plates remaining in the plurality of Titanium plates after the removing step is then used to form a cathode for an ion pump.
In accordance with a further embodiment, a method includes requiring that a cathode plate have au average grain size that is smaller than a threshold size and constructing an ion pump from the cathode plate.
In accordance with a still further embodiment, a method includes setting a maximum average grain size for a cathode plate in an ion pump and building the ion pump using a cathode plate that has an average grain size that is less than the maximum gram size.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
Within vacuum chamber 102, an array cylindrical anodes 114 is positioned between two cathode plates 116 and 118 such that the openings of each anode cylinder face the cathode plates.
The cylindrical anodes 114 and chamber wall 104 are maintained at ground potential while cathode plates 116 and 118 are maintained at a negative potential by an external power supply 120 that is connected to ion pump 100 by a power cable 122. In accordance with some embodiments, the potential difference between cylindrical anode 114 and cathode plates 116 and 118 is 7 kV.
In operation, flange 106 is connected to a flange of a system to be evacuated. Once the flange is connected, particles within the system to be evacuated travel into vacuum chamber 102 and eventually move within the interior of one of the cylindrical anodes 114. The combination of the magnetic field B and the electrical potential between anodes 114 and cathode plates 116 and 118 cause electrons to be trapped within each of the cylindrical anodes 114. Although trapped within the cylindrical anodes 114, the electrons are in motion such that as particles enter a cylindrical anode 114, they are struck by the trapped elections causing the panicles to ionize. The resulting positively charged ions are accelerated by the potential difference between anode 114 and the cathode plates 116 and 118 causing the positively charged ions to move from the interior of cylindrical anodes 114 toward one of the cathode plates 116 and 118.
The ions strike the cathode plates 116/118 causing material from the cathode plates to sputter outwardly away from cathode plates 116/118 and to cause the ions to become embedded in cathode plates 116/118. This removes the ions from the pump thereby reducing the pressure in the ion pump.
In standard ion pumps, cathode plates 116 and 118 are both made of Titanium. However, it was found that having both cathode plates made a Titanium resulted in pump instability when pumping a large amount of Noble gases such as Argon. During pump instability, previously trapped Noble gases are re-released from the cathode plates into the pump at a rate that is faster than the ion pump can remove them. The result is a sudden rise in pump pressure by as much as 100,000%.
To address this problem, the prior art created Noble Diode ion pumps (DI pumps) where one of the cathode plates is constructed of Tantalum and the other is constructed of Titanium. While this reduced the occurrence of pump instability, some DI pumps continued to show pump instability.
As shown in
In the prior art, Noble gas instability appeared to be a random event. Some DI pumps experienced such instability while other DI pumps did not and there was no way to predict which DI pumps were more likely to become unstable. As a result, there was no way to reduce the occurrence of pump instability through the manufacturing process.
Embodiments of the present invention provide a method for reducing Noble gas instability by requiring that Titanium cathode plates have a maximum grain size in order to be used in the construction of an ion pump.
In metals, atoms are linked together in crystalline structures. Typically, multiple crystalline structures are present in a metal sample and have different orientations from each other. Each distinct crystalline structure is referred to as a grain and the locations where two different crystalline structures meet are referred to as grain boundaries. The distance between two grain boundaries along a line across a grain is referred to as the grain size. The grain size in a metal sample varies considerably from grain to grain. Nonetheless, some metal samples have larger average grain sizes than other metal samples. One technique for evaluating the grain size of a sample, known as ASTM test method E112, involves measuring the number of grain boundaries along a line. This number is then applied to a function that compensates for the magnification under which the measurement was taken and the length of the line that was used. The value computed by the function as typically an integer referred to as a grain number. Since the grain number is based on the number of grain boundaries that are encountered, samples with smaller average grain sizes have larger grain numbers since there will be more grain boundaries in a fixed length of as sample with smaller grain sizes than in a sample with larger grain sizes. Thus, a sample with a grain number of 2 has a larger grain size than a sample with a grain number of 10. Other methods of determining grain size determine the number of grain boundaries in a unit area at a particular magnification. In the embodiments described below, the ASTM standard for grain number is used however any standard may be used.
If the average grain size exceeds the maximum grain size threshold (or equally if the grain number is less than the minimum grain number threshold) the plate is removed at step 512 and is not used to build an ion pump. If at step 510, the average grain size does not exceed the maximum gram size threshold (or equally the grain number is not less than the minimum grain number threshold) the plate is used to build an ion pump at step 514.
After step 512 or step 514, the process determines if there are more plates to be evaluated at step 516. If there are more plates to be evaluated, the process returns to step 506 to select the next plate. Steps 508-516 are then repeated for the next plate. When all the plates have been processed, the method of
Although the embodiment above tests every plate for average grain size, in other embodiments, a single sample of a sheet of Titanium is tested and if the average grant size of the sample exceeds the threshold, the entire sheet of Titanium is removed so that cathode plates are not formed from the sheet. If the average grain size of the sample does not exceed the threshold, the sheet is cut into plates that are then used to build ion pumps.
In accordance with some embodiments, in addition to requiring that the average grain size be less than a threshold grain size, the Titanium plate is required to be of a specific ASTM grade, where the grade indicates the types and amounts of other elements present in the Titanium plate but does not by itself specify a grain size. In accordance with one particular embodiment, the cathode plate is required to be formed of Grade 2 Titanium with a maximum average grain size described by ASTM grain number 9.
In accordance with one embodiment, the threshold grain size is set such that each ion pump is built with Titanium plates containing average grain sizes with ASTM grain numbers that are no smaller than 9. By using Titanium plates with ASTM grain numbers of 9 or greater, the present inventors have discovered that the occurrence of Noble gas instability and in particular Argon instability can be reduced and in some cases completely removed from produced ion pumps. Thus, the likelihood of Noble gas and in particular Argon gas instability is reduced through the method of
To evaluate the performance of ion pumps constructed with Titanium plates with grain numbers of 9 or greater, the present inventors constructed ion pumps with Titanium plates of grain numbers 2, 8, 9, 10, and 14. The peak drift for each ion pump was then measured to determine a maximum peak drift, an average peak drift, and a standard deviation in the peak drift.
The root cause of the instability based on grain size appears to be related to the construction of vertical structures on the surface of the Titanium during sputtering. These structures encapsulate Argon as shown in
Thus, it appears the formation and perhaps destruction of these vertical surfaces contributes to Argon instability in that by using smaller grain sizes, the present in reduces either the size or frequency of these structures and thereby reduces the occurrence of Argon instability.
Although the discussion above refers to Argon and Argon instability, the present invention may be used with any Noble gases to reduce noble gas instability.
Although elements may have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms for implementing the claims.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
Number | Name | Date | Kind |
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6388385 | McGinn et al. | May 2002 | B1 |
Number | Date | Country |
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H09143704 | Jun 1997 | JP |
2007109666 | Sep 2007 | WO |
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