US3839655A

US3839655A - Bayard-alpert vacuum ionization tube

Info

Publication number: US3839655A
Application number: US00391392A
Authority: US
Inventors: W Helgeland; D Sheldon
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1973-08-24
Filing date: 1973-08-24
Publication date: 1974-10-01
Anticipated expiration: 1991-10-01
Also published as: DE2440090A1; FR2241779B3; IT1020185B; JPS5073684A; FR2241779A1

Abstract

An improved Bayard-Alpert type vacuum ionization tube includes an electron collecting grid surrounding and coaxial with a centrally located positive ion collector. The grid has an inner diameter of approximately 0.5 inches and an electron emitting filament is positioned outside of the grid and spaced from the grid by approximately 0.2 inches. A grounded shield screen surrounds the grid outside of the filament and is coaxial with the collector. The tube has constant vacuum gauging sensitivity over the range from 10 1 to 5 X 10 10 torr.

Description

United States Patent 119 Helgeland et al.

Oct. 1, '1974 BAYARD-ALPERT VACUUM IONIZATION TUBE Inventors: Walter Helgeland, Lexington; Deane Palmer Sheldon, Franklin, both of Mass.

Assignee: Varian Associates, Palo Alto, Calif.

Filed: Aug. 24, 1973 Appl. No.: 391,392

US. Cl. 313/7, 324/33 Int. Cl. H01j 2/16 Field of Search 313/7; 324/33 References Cited UNITED STATES PATENTS 7/1973 Redhead 313]? Primary Examiner-Herman Karl Saalbach Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or FirmStanley Z. Cole; Leon F. Herbert; John J. Morrissey [5 7] ABSTRACT An improved Bayard-Alpert type vacuum ionization tube includes an electron collecting grid surrounding and coaxial with a centrally located positive ion collector. The grid has an inner diameter of approximately 0.5 inches and an electron emitting filament is positioned outside of the grid and spaced from the grid by approximately 0.2 inches. A grounded shield screen surrounds the grid outside of the filament and is coaxial with the collector. The tube has constant vacuum gauging sensitivity over the range from l0 to 5 X 10 torr.

11 Claims, 7 Drawing Figures 1 BAYARD-ALPERT VACUUM IONIZATION TUBE FIELD OF INVENTION The present invention relates generally to vacuum ionization gauging tubes and more particularly to an improved Bayard-Alpert type vacuum ionization gauging tube.

BACKGROUND OF THE INVENTION Vacuum ionization tubes used for vacuum gauging, frequently referred to as ionization gauges, are well developed in the art. The tube includes a hot filament for emitting electrons which bombard gas in the atmosphere being gauged to form positive ions. The positive ions are collected on an electrode having a negative potential relative to the emitting filament and electrons are collected by a positively charged electrode.

Ionization tubes fall into two general geometry categories. In one type of geometry, an electron emitting cathode is located between a pair of parallel plates having opposite polarities. Generally, parallel plate geometry vacuum ionization tubes have a range between approximately 1 and 10' torr. A second type of vacuum ionization gauge tube has a cylindrical geometry including an axial filament surrounded by a coaxial helical grid which is polarized to collect electrons. The helical grid is surrounded by a larger outer cylinder coaxial with the filament and polarized to collect positive ions. Gauges of this type are useable over a pressure range of 10' to ltorr, a range that was found to be excessively narrow for advanced vacuum techniques. The lower pressure limit of the initially developed tube is due to X-ray effects of the ion collector, while the upper pressure limit is controlled, to a large extent, by space charge effects between the electrodes. The space charge effects limit the current flow to the positive ion collecting electrode.

The lower pressure limit has been extended to 10' torr by reducing the area of the ion collecting electrode exposed to X-rays. The geometry involves inverting the initially developed structure so that a positive ion collecting electrode is formed as a thin wire or needle on the axis of the tube structure. Coaxial with the ion collector is a one inch diameter electron collecting grid, outside of which is an electron emitting hot filament. This type of gauge has been widely used in the art and generally is referred to as a Bayard-Alpert gauge. It is reported in the Review of Scientific Instruments, Volume 21, page 571, 1950.

An attempt to extend the upper limit range of the Bayard-Alpert tube from 10' torr to 10 was reported by Nottingham in the Vacuum Symposium Transactions, 1954, page 76, and the Transactions of the Eighth Vacuum Symposium, 1961, page 494. In one structure reported by Nottingham, the grid is provided with end closures and the entire structure is surrounded by a screen grid which functions as a shield to reduce the current limiting effects of space charge between the electrodes. The reported work indicates that sensitivity was greatly enhanced and that the high pressure range was extended. While Nottingham concludes that his gauge is useable to l0 torr, an inspection of the reported results (FIG. 3 of the I961 article) shows highly non-linear operation above 10' torr in response to current derived only from positive ions striking the centrally located collector. While linearity can be extended to 10' torr by combining the currents derived from the electron and positive ion collecting electrodes, the Nottingham structure still does not appear to be linear to a high pressure limit of 10 torr. ln'Nottinghams articles, there is also a report of a gauge employing a screen grid sans end closures on the grid. While this gauge may be useable up to 10 torr, the sensitivity thereof is highly non-linear to this upper limit since there is a pronounced increase in sensitivity beginning at 10 torr, with a peak at 6 X 10' torr, and a rapid decrease for pressures above 10 torr.

Other investigators of vacuum ionization gauging tubes, attempting to extend the upper pressure limit of the Bayard-Alpert tube, are Redhead, Journal of Vacuum Science and Technology, Volume 8, page 848, I969, Blank et al, Vacuum, Volume 15, page 127, 1965, and Ramey, Vacuum Symposium Transactions, 1959, page 85. However, none of these investigators describes a vacuum ionization gauging tube having constant sensitivity over the desired range of from 10' to 10 torr without switching the mode of tube operation. Of course, mode switching is not desirable because of the complexity of the electronic devices associated therewith, as well as because of opportunity for operator error and confusion.

Most reported and manufactured Bayard-Alpert gauges employ electron collecting grid diameters of about one inch (25 millimeters) and utilize glass enclosures having diameters of approximately two inches. An exception is a miniature tube manufactured by the General Electric Company (Model 22GTI03), having dimensions of approximately one-half of these typical values and employing a relatively expensive mesh grid, as contrasted to the usual helical grid. While the General Electric tube is useable to an upper pressure limit of 2.5 X 10' torr, an examination of the tube calibration curve reveals a noticeable departure from linearity at 10 torr, with increasingly non-linear performance for increasing pressures. The calibration curve indicates that sensitivity decreases monotonically for increasing pressures. Our investigation of this tube indicates that glass charging effects on the glass wall of the tube cause the non-linear sensitivity for pressures above I0 torr. The glass charging effects appear to limit the current capable of flowing between the electrodes, whereby as the nunber of gas molecules increases, and therefore for higher gas pressures, the current has a tendency to remain constant and does not increase linearly with the increased number of gas molecules.

In an attempt to minimize or prevent the glass charging effects, we placed a shield on the interior of the glass wall of the General Electric tube. The shield was connected to ground and improved results were found to occur. In particular, sensitivity remained substantially constant to approximately 3 X 10' torr for monitored atmospheres of air and argon and the positive ion sensitivity at 10 torr was approximately eighty per cent of midrange sensitivity. However, a problem with the General Electric sub-miniature tube, even with the shield included therein, is that the filament is located approximately one millimeter from the electron collecting grid. With normal manufacturing tolerances, the one millimeter spacing is a potential quality control problem. Also, it has been found in the above-noted investigations by Nottingham and Redhead that gauge sensitivity is a function of filament-grid spacing, particularly as the spacing gets smaller into the region of the General Electric tube. Further, our investigations indicate that glass charging effects appear to become more critical with the smaller diameter glass envelope of the General Electric design. A further disadvantage of the General Electric design is that the miniature tube socket is not compatible with generally existing vacuum equipment sockets, necessitating a redesign of equipment.

In many commercially available gauges, the shielding screen is internally connected to the filament. Thereby, any current collected on the shielding screen comingles with the electron-emission current from the filament and the emission current is in error by the amount of co-mingling. If the gauge current, and therefore sensitivity is high, the error is not appreciable, even at low pressures. To determine the extent of the error due to the co-mingling of the currents, we modified a commercially available gauge so that the shielding screen has an isolated connection, to enable the shielding screen current to be monitored on a separate lead. The modified tube had increased sensitivity at the high pressure end of the range, with a peak at approximately 2.5 X 10' torr.

It is therefore apparent that the prior art devices do not have constant sensitivity over the range from 10 torr to 10 torr. Constant sensitivity over this range is highly desirable for certain applications, particularly for monitoring vacuum conditions during cathode sputtering processes. It was, therefore, necessary to investigate geometries different from those which had been utilized in the prior art in an attempt to provide a vacuum ionization gauging tube having linearity over the 10 to 10' torr range of interest.

BRIEF DESCRIPTION OF THE INVENTION After experimenting with the General Electric gauge in its original and modified forms and the commercially available gauge with the shield screen isolated, we built a vacuum ionization gauging tube intermediate in design between the General Electric tube and the modified tube in an attempt to achieve constant sensitivity from 10 to 10' torr, without mode switching. Another object in building the intermediate designed tube was to reduce the magnitude of the sensitivity hump of the modified tube, a result achieved by reducing the di ameter of the electron collecting grid to approximately that of the General Electric grid. A further object was to provide a larger grid to filament spacing than can be achieved in the miniature General Electric tube to enable the tube to be built without high tolerance requirements. A further object was to provide an ionization tube that is adaptable with the existing tubes, so that it can be easily substituted for them both physically and electrically.

The tube we invented to achieve these results is an improved Bayard-Alpert type vacuum ionization tube having constant vacuum gauging sensitivity from X torr to l X 10' torr. The result is achieved by providing the usual centrally located positive ion collecting electrode surrounded by a helical electron collecting grid coaxial with the centrally located ion collector. The electron collecting grid has a diameter of approximately 0.5 inches. The grid is spaced approximately 0.2 inches from an electron emitting filament which is located outside of the grid. The electron collecting grid is preferably formed as a helix having approximately 9.l5 turns per inch, twice the number of .tums per inch of conventional prior art tubes having helical collecting grids. The increased number of turns per inch provides greater metal surface area for the electron collecting grid and therefore a more nearly cylindrical equal potential surface for collecting electrons.

A grounded shield screen surrounds the grid outside of the filament and is coaxial with the centrally located collector, by being formed on a glass tube envelope having a diameter of approximately two inches. The grounded shield also assists in providing improved high pressure sensitivity since it neutralizes envelope glass space charge effects and current does not have a tendency to saturate at pressures considerably less than 10' torr.

The relatively small distance between the central ion collector and the electron collecting grid provides a greater electric field strength to cause ions to move at higher accelerations between the two electrodes. In the experiments we conducted, as noted supra, it was found that the lower ion accelerations of the one inch spacing between the two electrodes do not produce satisfactory results because of decreased sensitivity at the high pressure end of the range. Spacings between the electron and positive ion collecting electrodes of less than one-quarter of an inch are not acceptable because of the decreased volume that exists between the electrodes in such a confined configuration. Decreased volume results in lower sensitivity because there are fewer gas molecules to be intercepted by the electrons. In addition, a spacing of less than one-quarter of an inch between the electron and positive ion collecting electrodes results in a tube structure that is difficult and expensive to fabricate.

The relatively close spacing between the positive ion collecting electrode and the helical grid which results in approximately a four to one length to diameter ratio of the grid volume, in combination with the increased number of turns per inch of the helical grid, has a tendency to prevent ion migration from the ends of the structure, i.e., prevents ions from travelling axially out of the grid collector region. Therefore, ions travel radially to a greater extent with the structure of the present invention than the typical prior art structure. Thereby, the percentage of electron and ion currents reaching the current collecting electrodes is greater than in the typical prior art devices to maintain sensitivity constant over a wider pressure range.

While sensitivity is enhanced by one aspect of the invention, it is less than in most prior art tubes. Therefore, co-mingling of the shield screen and filament currents cannot be tolerated because of the significant percentage of the shield current to the positive ion collector current at low pressures, at which the positive ion current may be twenty microamperes. To provide accurate low pressure readings, the shield'is electrically separate from the remaining electrodes of the tube and is grounded. The filament is at a first dc. voltage above ground while the screen is at a higher dc. voltage to collect electrons from the filament. The collector is basically at ground potential, being connected to an input terminal of d.c. amplifier having a high input impedance and which drives an output meter.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side sectional view of a preferred embodiment of the invention, taken along the lines 1l, FIG. 2;

FIG. 2 is a top sectional view of the embodiment illustrated in FIG. 1, taken along the lines 2-2, FIG. 1;

FIG. 3 is a bottom view of the tube illustrated in FIG.

F IG. 4 is a side view of a helical grid assembly of FIG.

FIG. 5 is a side view of an electron emitting hot filament of the embodiment of FIG. 1;

FIG. 5a is a front view of the filament of FIG. 5; and

FIG. 6 is a plot of sensitivity vs. pressure of the gauge illustrated in FIGS. 1-5a.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to the drawing wherein there is illustrated in a glass envelope 11 for the vacuum ionization gauging tube of the present invention. Envelope 11 includes a main, substantially cylindrical portion 12 having a diameter on the order of two inches and a height that is typically approximately five and onequarter inches. Extending at right angles to main portion 12 is cylindrical arm 13 that is connected to a vacuum system being monitored. Arm 13 is in fluid flow relationship with the interior of main portion 12, whereby the distribution of molecules in main portion 12 is substantially the same as the distribution of molecules in the system being monitored. Typically, the monitored system is the interior of a sputtering chamber that has an air or argon atmosphere and a pressure in the range between l0 and torr.

Main portion 12 includes a longitudinal axis 14 on which lies a needle-like, positive ion, metal collecting electrode 15 that is secured in situ to the top of main section 12 and electrically connected to a single measuring circuit 16 including a high impedance shunt resistor 51, which is connected to a high input impedance terminal of variable gain d.c. amplifier 52, the output of which drives d.c. voltmeter 53. Collector 15 extends into main portion 12 along axis 14 so that the end of the needle is below longitudinal axis 17 of arm 13 and slightly above the lowest portion of arm 13. Typically, the lower end of collector 15 is slightly in excess of 3 inches below the upper end of main envelope portion 12. Collector 15 is secured to the top of main envelope portion 12 by a connection to a metal pin that extends through and is bonded to downwardly extending finger portion of envelope portion 12.

Surrounding and coaxial with needle 15 is a positive ion collecting, helical grid 18 having an inner diameter of approximately one-half inch, approximately 9.15 turns per inch and a typical length of 1.8 inches, which results in a length to diameter ratio of approximately four to one for the grid. The one-half inch diameter of the grid 18 is critical to the operation of the present invention to enable gauge sensitivity to be constant to within 10 percent of midrange sensitivity to the upper pressure limit of 10' torr. If the one-half inch diameter is exceeded by more than approximately one-tenth inch, sensitivity does not remain constant at the upper end of the pressure range, while diameters of grid 18 less than approximately 0.4 inch result in decreased overall sensitivity throughout the range of interest, as well as manufacturing difficulties. The decreased overall sensitivity for diameters less than 0.4 inch occurs because fewer positive ions can be formed in the volume between ion collector 15 and electron collector or grid 18, with the result being a material reduction in the output current of the positive ion collector at the low end of the pressure range of interest, approximately 10 torr. The 9.15 turns to the inch for grid 18 enables a very large percentage of electrons in the main section 12 to be collected by the grid and also provides a uniform equal potential cylinder in the grid cylindrical area.

The lower end of grid 18 is bonded and electrically connected to metal pin 19 which extends through and is bonded to a bulbous portion 21 at the lower end of main envelope portion 12. The upper end of grid 18 is connected to strut 22 that extends longitudinally of the section 12, parallel to axis 14. The lower end of strut 22 is fixedly connected to metal pin 23 that is secured in situ to and extends through bulbous portion 21.

Outside of and separated from grid 18 by approximately 0.2 inches (5 millimeters) is a hot electron emitting filament 26. Filament 26 includes an electron emitting thoria coated iridium, triangularly shaped, filiamentry wire 27 having substantially coplanar legs with equal lengths of approximately .8 inch. F ilamentry wire 27 is maintained in situ by an arrangement of metal wire struts 28-30. Strut 28 is connected to the upper apex of filamentry wire 27 by hook 31, while the opposite lower ends of the filamentry wire 27 are connected to struts 29 and 30. The ends of horizontally extending portions of

struts

29 and 30 are bonded and electrically connected to

metal pins

31 and 32 which extend through and are bonded to the bulbous portion 21.

The approximately 0.2 inch separation between triangularly shaped filamentry wire 27 and electron collecting grid 18 is also critical in enabling the device of the present invention to function satisfactorily and provide constant sensitivity over the 10' to 10 torr pressure range. The 0.2 inch separation is particularly critical for convenient manufacture of the device, whereby conventional manufacturing techniques can be utilized. In addition, it is believed that the 0.2 inch spacing between the filament 27 and shield 18 assists in preventing electrons from reaching positive ion collector 15 even though the ion and

electron collecting electrodes

15 and 18 are relatively closely spaced. The 0.2 inch filament to grid spacing is sufficient to enable electrons to have a substantially uniform penetration around the periphery of grid 18. In contrast, the one millimeter spacing of the prior art General Electric tube may result in greater electron penetration through the grid in the vicinity of the filament than through other portions of the grid.

Surrounding the segment of main envelope portion 12 where the ions are collected, and the electrons are emitted and collected is a metal shield 35 which prevents space charge accumulation on glass envelope 11. The shield 35 comprises a metal, preferably platinum, coated formed on the interior of main portion 12 and a slight portion of arm 13. The platinum coating 35 is thereby coaxial with longitudinal axis 14 and collecting electrode 15. Coating 35 extends longitudinally of axis 14 from a distance of approximately two and fiveeighths inches, with a lowest extent approximately one and one-half inches below center line 17.

To isolate currents which may flow in shield 35 from currents which flow in grid 18 or collector 15 and provide a ground connection for the shiled 35, the shield is provided with its own external connection to ground. To this end, a metal contact finger 37 is soldered to coating 35 and extends inwardly, generally toward axis 14, but is slightly off a radial line from the axis. Contact finger 37 is electrically connected to ground by metal pin 38 to which it is bonded and that extends through bulbous portion 21.

Electrical excitation is such that pin 38 is connected to ground to establish the grounded connection for coating 35. Filament 26 is maintained at a dc. voltage of +30 volts above ground by connecting the negative electrode of d.c. source 39 to pin 38 and the positive electrode of the dc. source to one of filament pins 31 or 32. An ac. voltage source 40, having a low d.c. impedance, is connected between

pins

31 and 32 to provide energization voltage for heating filament 26. Grid 18 is maintained at a voltage of approximately 180 volts above ground by connecting a negative electrode of 150 volt d.c. source 42 to the positive electrode of d.c. source 39. The positive electrode of d.c. source 42 is connected in parallel to

pins

19 and 23 and thence to the upper and lower ends of grid 18.

Typical operations of the device for air and argon vacuum atmospheres are respectively illustrated by the

curves

51 and 52 of FIG. 6, wherein pressure, in torr, is plotted against normalized sensitivity (S/S where:

S is sensitivity for the particular pressure, and

S is mid-range sensitivity for 10" torr. As indicated by the argon response curve 52, the sensitivity of the ionization gauging tube of the present invention for 10" torr is within ten percent of mid-range sensitivity and there is no pronounced increase in sensitivity for pressures less than 10' torr. Curve 51, the sensitivity response for air, shows that the sensitivity at 10 torr is within twenty percent of midrange sensitivity and there is no increase in sensitivity for pressures less than 10 torr. Similar results are provided at the low pressure end of the device, at a pressure of X torr.

While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A Bayard-Alpert type vacuum ionization tube having constant vacuum gauging sensitivity from approximately 5 X 10 to l X 10 torr comprising a centrally located positive ion collector, an electron collectimg grid surrounding and coaxial with the collector and having a diameter of approximately 0.5 inches, an electron emitting filament outside of the grid and spaced from the grid by approximately 0.2 inches, a grounded shield screen surrounding the grid outside of the filament and coaxial with the collector.

' 2. The tube of claim 1 wherein the grid has a length approximately four times its diameter.

3. The tube of claim 2 wherein the grid is a helix having on the order of nine turns to the inch.

4. The tube of claim 1 wherein the grid is a helix having on the order of nine turns to the inch.

5. The tube of claim 1 wherein the shield is electrically connected to a pin electrically isolated in the tube from each of the grid, filament, and positive ion collector.

6. The tube of claim 5 wherein the grid has a length approximately four times its diameter.

7. The tube of claim 6 wherein the grid is a helix having on the order of nine turns to the inch.

8. The tube of claim 7 wherein the tube is provided with a glass envelope having a cylindrical portion with a diameter of approximately two inches in which the filament, grid and collector are located, said shield being a metal coating on said envelope, said collector lying on the longitudinal axis of the envelope cylindrical portion.

9. The tube of claim 5 further including a dc. current measuring circuit responsive only to dc. current derived from the positive ion collector.

10. The tube of claim 1 wherein the tube is provided with a glass envelope having a cylindrical portion with a diameter of approximately two inches in which the filament, grid and collector are located, said shield being a metal coating on said envelope, said collector lying on the longitudinal axis of the envelope cylindrical portion.

11. The tube of claim 1 further including a dc. current measuring circuit responsive only to dc. current derived from the positive ion collector.

Claims

2. The tube of claim 1 wherein the grid has a length approximately four times its diameter.
3. The tube of claim 2 wherein the grid is a helix having on the order of nine turns to the inch.
4. The tube of claim 1 wherein the grid is a helix having on the order of nine turns to the inch.
5. The tube of claim 1 wherein the shield is electrically connected to a pin electrically isolated in the tube from each of the grid, filament, and positive ion collector.
6. The tube of claim 5 wherein the grid has a length approximately four times its diameter.
7. The tube of claim 6 wherein the grid is a helix having on the order of nine turns to the inch.
8. The tube of claim 7 wherein the tube is provided with a glass envelope having a cylindrical portion with a diameter of approximately two inches in which the filament, grid and collector are located, said shield being a metal coating on said envelope, said collector lying on the longitudinal axis of the envelope cylindrical portion.
9. The tube of claim 5 further including a d.c. current measuring circuit responsive only to d.c. current derived from the positive ion collector.
10. The tube of claim 1 wherein the tube is provided with a glass envelope having a cylindrical portion with a diameter of approximately two inches in which the filament, grid and collector are located, said shield being a metal coating on said envelope, said collector lying on the longitudinal axis of the envelope cylindrical portion.
11. The tube of claim 1 further including a d.c. current measuring circuit responsive only to d.c. current derived from the positive ion collector.