US 3761708 A
A suppressor grid is positioned between an ionizer and 1n ion gauge in a mass spectrometer. The suppressor grid prevents electron interference with the ion gauge for operation at ultrahigh vacuum levels.
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Description (OCR text may contain errors)
United States Patent [1 1 Roepke et al.
[451 Sept. 25, 1973 1 ELECTRON SUPPRESSOR GRID FOR A MASS SPECTROMETER  Inventors: Wallace W. Roepke, Hopkins;
Kenneth G. Pung, Chuska, both of Minn.
173] Assignee: The United States of America as represented by the Secretary of the Interior, Washington, DC.
 Filed: Oct. 8, 1971  Appl. No.: 189,472
Related US. Application Data Continuation-in-part Ser No. 86.870, Oct. 20 1970,
 US. Cl. 250/419 G, 250/419 S, 324/33  Int. Cl. HOIj 39/34  Field of Search 250/419 G, 41.9 S; 324/33  References Cited UNITED STATES PATENTS 3,057,996 10/1962 Boyer 250/419 3,265,890 8/1966 Briggs 250/419 Primary ExaminerWilliam F. Lindquist Att0rney-Frank A. Lukasik and Thomas Zack [5 7] 7 ABSTRACT A suppressor grid is positioned between an ionizer and In ion gauge in a mass spectrometer. The suppressor grid prevents electron interference with the ion gauge for operation at ultrahigh vacuum levels.
4 Claims, 2 Drawing Figures ELECTRON SUPPRESSOR GRID FOR A MASS SPECTROMETER CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION In a residual gas analyzer, partial vapor pressures of a vacuum environment are analyzed. lons within the environment are accelerated and focused through a filter which passes only discrete charge-to-mass ratio ions. At a detector the quantity of ions passed by the filter is related to an electrical current output as an analogue representation of partial pressures. One common residual gas analyzer incorporates a mass spectrometer which includes both an ionizing section for ionizing the gas sample within the vacuum chamber, and an ion gauge for measuring total vapor pressure within the vacuum chamber.
The ionizing section of a mass spectrometer has a heated filament, an ionizing chamber, and electrostatic lenses for ion filtering and focusing. Electrons produced at the filament are accelerated through the ionizing chamber by a controllable electric field. A portion of these electrons strike neutral atoms and molecules. If the resulting energy transfer is adequate, positive ions are produced. From the ionizing chamber these positive ions are accelerated through the filter section by a focusing aperture, and into an electron multiplier or Faraday ion detector.
For total vapor pressure monitoring in an ultra high vacuum an ion gauge is used. An ion gauge usually includes a heated filament, a grid, and a collector. Electrons thermally ejected from the filament collide with residual gas molecules, forming positive ions. These positive ions collide with the collector, inducing positive current flow in the collector circuit. If the density of the residual gases decreases, fewer positively ionized molecules are produced, so the output current also decreases, indicating a vapor pressure drop in the chamber.
SUMMARY OF THE INVENTION When the total vapor pressure in an ultra high-vacuum chamber used with a mass spectrometer is below approximately 2.5(l) torr, as occurs when a mass spectrometer is used as a residual gas analyzer, electron interference produced in the ionizer section begins to interact with the vapor pressure responsive ion gauge. The electron interference appears to cause a space charge in the ion gauge, which acts as an effective shield against ionization. Since the ion gauge detects vapor pressure by measuring positive current flow generated by positive ions, the space charge causes an erroneous reduced pressure reading. As the pressure falls increasingly below 2.5( torr, this electron crosscoupling of the ionizer and ion gauge causes progressively lower readings. To overcome this defect of the prior art this invention was made.
This invention is an electron suppressor grid for use in an ultra-high vacuum chamber with a mass spectrometer or residual gas analyzer to prevent spurious vapor pressure readings. The suppressor grid consists of a negative or positive voltage wire screen, positioned on a line-of-sight path between the ionizer and ion gauge. The voltage of the suppressor grid, which is somewhat higher in absolute value than the negative voltage of the ion gauge collector, prevents electron interference between the ionizer and the ion gauge. Using a negative voltage, the suppressor grid repels stray electrons to prevent interference between the two ionizer systems. Using a positive voltage, the grid traps electrons while at the same time repelling positive ions back into the ionizer section of the spectrometer, thus increasing measurement sensitivity while preventing space charge interference. By eliminating electron interference between the ionizer and ion gauge, the suppressor grid enables accurate vapor pressure readings at pressures substantially less than 2.5( 10) torr.
Therefore, one object of this invention is an electron suppressor grid for improving vapor pressure measurement in a mass spectrometer.
This and other objects of the invention are apparent in the following specification and drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a lateral schematic view of an electron suppressor grid 10 in intermediate position between mass spectrometer ionizer 12 and a nude ion gauge 14.
FIG. 2 is a frontal view of an electron suppressor grid 10, shown laterally in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT An electron suppressor grid 10, together with associated components of a mass spectrometer, is shown in FIG. 1. The suppressor grid is positioned on a line-ofsight path between an ionizer section 12 and a nude ion gauge 14. Broken rectangles generally defined the bounds of components 10-14 for descriptive purposes. Extending across the lower portion of FIG. 1, a partition 16 divides a vacuum environment 18 above from an ambient environment 20 below. Electrical power for the components is transmitted by sealed conductors passing through the partition.
In ionizer section 14 a filament 22, supplied with current by a conductor 24, emits free electrons. The electrons are accelerated by a negative potential electrode 26 and positive potential electrode 28, connected by conductors 30 and 32, respectively, to appropriate potential sources (not shown). After acceleration, the free electrons enter an ionizing chamber 34, formed by the positive accelerator electrode 28, a Faraday cage 36, and an electron extractor 38, connected to a positive potential source by a conductor 40.
When free electrons are accelerated into ionizing chamber 34, some of them collide with neutral atoms and molecules. If sufficient energy transfer results, positive ions are formed. Positive ions of discrete mass-tocharge ratio are drawn from the ionizing chamber by a focusing lens 42, supplied with negative potential by a conductor 44. From the focusing lens, the particles pass through an exit lens 46, grounded by a conductor 48.
For monitoring total vapor pressure within the vacuum environment 18, a nude ion gauge 14 is used. Electrical power from an appropriate source (not shown) is transmitted, through conductors 50 and 5 2, to heat a filament 54. When heated, the filament emits electrons that collide with residual gas molecules, forming positively charged ions. Under the influence of a positive potential grid 56, the positive ions migrate toward and collide with collector electrode 58. The
collector is grounded by two conductors 60 and 62, through an electrometer amplifier 64. The collision of positive ions with collector 58 induces positive current flow in the amplifier circuit. Since the number of ions produced from residual gas molecules is directly related to vapor pressure, the higher the vapor pressure in vacuum environment 18 the greater the current flow in the amplifier circuit. When the pressure decreases, the current through the amplifier similarly decreases.
The ionizer section 12 and the nude ion gauge 14 described above are well known in the prior art. When operating in the prior art configuration in a vacuum environment such as occurs with a residual gas analyzer when used with a pressure below approximately 25(10) torr, cross-coupling between the ionizer section 12 and ion gauge 14 occurs causing erroneous vapor pressure readings. Cross-coupling is caused by electron interference generated in the ionizer section. The electron interference appears to cause a space charge around the ion gauge grid 56 which acts as an effective shield against ionization within the grid. Since the ion gauge detects vapor pressure by measuring positive current flow generated by positive ions within the grid, the space charge causes an erroneous reduced pressure reading. To prevent this reduction in indicated pressure, the electron suppressor grid is positioned on a line-of-sight path between the ionizer 12 and ion gauge 14.
Electron suppressor grid 10 is shown in H6. 2 as it would be viewed by sighting from the ionizer 12 toward the ion gauge 14. The grid consists of a planar, wire mesh 66, connected to a source potential by a conductor 68. The exact shape and position of the grid 10 depend upon the relative dimensions and positions of the ionizer and ion gauge, and are easily determined for common mass spectrometer and residual gas analyzer components. The potential of the grid is somewhat greater in absolute value than the negative voltage of the collector 58. In operation with a negative potential applied, electrons approaching suppressor grid 10 from the ionizer side are repelled by the negative potential, preventing interference between the two ionizer systems. With a positive grid potential, electrons are trapped by the suppressor grid, again preventing interference between the two systems. As vapor pressure within the system drops substantially below (10) torr, operation of the suppressor grid is relatively more effective, enabling ion gauge accuracy that was not obtainable in prior systems.
In addition to increasing ion gauge accuracy when operated with positive potential, the electron suppressor grid 10 increases measurement sensitivity by a factor of approximately two. As a probable explanation of this increased sensitivity, it appears that the suppressor grid repels positive ions back into the ionizer section. Because of the increased sensitivity, this invention is particularly useful for surface studies with low energy electron diffraction or Auger emission equipment at pressures less than l0 torr.
In one embodiment of the system shown in FIG. 1
and 2, the following parameters are effectively used. The electron suppressorgrid 10 is a two inch diameter, stainless steel screen of tensile bolt cloth, type NAM, 40 X 40 mesh with 54.8 percent open area. The screen is supported by a suitable insulated support (not shown) approximately 1 inch in front of the filament 22 in the ionizer section, although this distance is not critical. A 1 ma current energizes the filament 22 of the ionizer 12; negative electrode 26 has a potential of volts; positive electrode 28 has a potential of +10 volts; electron extractor 38 has a potential of +15 volts; focusing lens 42 has a potential of 60 volts. In the nude ion gauge 14, grid 56 has a potential of +1 50 volts, and the collector has a potential of 45 volts. With these operating parameters, a l45 volt potential on the anion suppressor grid effectively prevents crosscoupling between the ionizer and ion gauge.
In another embodiment of the invention, the above parameters are employed with a +245 volt potential substituted for the negative grid potential.
While electron suppressor grid 10 has been described by referring to a specific preferred embodiment, other forms of the invention are expected within the scope of this disclosure. Therefore, the extent of this invention is limited only by the scope of the following claims:
1. A mass spectrometer comprising in combination:
an ionizer section having a total vapor pressure less than approximately 2.5( 10)" torr for forming and accelerating positive ions in a vacuum environment;
a nude ion gauge for monitoring the total vapor pressure in the same environment; and
an electron suppressor grid including a negative or positive potential wire screen positioned on a line-ofsight path between said ionizer section and said ion gauge, and having sufficient potential to prevent electron interference between said ionizer and ion gauge.
2. A mass spectrometer as claimed in claim 1 in which:
the potential on the electron suppressor grid is on the order of minus volts.
3. A mass spectrometer as claimed in claim 1 in which:
the potential on the electron suppressor grid is on the order of plus 250 volts.
4. A method for preventing electron cross-coupling between an ionizer section and a nude ion gauge in a mass spectrometer having an ultra high vacuum chamber with a pressure less than approximately 25(10) torr compris-ing the steps of:
ionizing the neutral atoms and molecules in said ionizer section to produce positive ions;
measuring the total vapor pressure in said chamber by said nude ion gauge and preventing electron cross-coupling between said ionizer section and ion gauge by positioning a suppressor grid with an appropriate potential on a line-ofsight path therebetween.