US3233098A

US3233098A - Mass spectrometer tube

Info

Publication number: US3233098A
Application number: US203311A
Authority: US
Inventors: Reich Gunter
Original assignee: Leybold Holding AG
Current assignee: Leybold Holding AG
Priority date: 1961-06-15
Filing date: 1962-06-15
Publication date: 1966-02-01
Anticipated expiration: 1983-02-01
Also published as: DE1448178B2; DE1448178A1; GB1005854A

Description

Feb. 1, 1966 REICH 3,233,098

MASS SPECTROMETER TUBE Filed June 15, 1962 2 Sheets-Sheet l BATTERY ION CURRENT MEASURING DEVICE ow SUPPLY Fig. I

Jnvenlar.

GUNTER RE/CH ,4 TTOR/VE'V Feb. 1, 1966 G. REICH MASS SPECTROMETER TUBE 2 Sheets-Sheet 2 Filed June 15, 1962 ION T CURRENT MEASURING DEVICE POWER SUPPLY BATTERY Fig. 2

v Mm w m n 7 J A United States Patent 4 Claims. oi. 250-419 The invention deals with a mass spectrometer tube for the detection of ions of various masses, especially for partial pressure measuring devices in the high-vacuum range, whereby the ions resulting from electrons produced from a cathode in an ion source are directed in the form of a stream of ions, through a sorting space in which a high frequency alternating field of variable frequency is applied to sorting electrodes, so that only ions having a mass corresponding to a given applied frequency can reach the target electrode, the current of which serves as a measure of the proportion of this specific ionic mass in the given mixture.

When a mass spectrometer of the above described type is used, then a residual current is superimposed on the current produced at the target electrode by the ions of a given mass. This residual current is present even in the case of a frequency far removed from the resonant condition of the ionic masses and is dependent both on pressure and frequency and can also have both positive and negative signs. This residual current diminishes the detecting efficiency, in respect to components of low intensity and can also, especially at higher pressures, assume such large values that a perfect analysis of the separate components will not be possible. Each decrease of the residual current will result, therefore, in an increase of the detecting efiiciency as well as a widening of the range of application of such measuring devices in the field of higher pressures.

Certain devices for the decrease of the residual current have been suggested already, which include the high frequency sorting system, that is, sorting electrodes positioned between grid electrodes with negative potential. In addition, solid apertured shield electrodes for screening of the ion source from the sorting system and which permit the stream of ions-to pass through, have been utilized. The mechanical screening between the ion source and the sorting system results in an additional disadvantage, since the areas enclosing the ion source and the sorting system must be evacuated separately. Such a device is especially disadvantageous when mass spectrometers are used as partial pressure measuring devices in high vacuum technique. Furthermore, devices built according to this principle cannot be designed as built-in measuring units, in which all of the electrodes are supported by one and the same flange, making it possible to install the measuring system directly in the vacuum container.

Tests carried out concerning the cause of the disturbing residual current have shown the following:

(1) Non-resonant charged particles may leave the sorting system sidewise by means of a velocity component in a direction which deviates from that of the stream of ions and find electric field forces which accelerate them in the direction of the target electrode.

(2) Resonant and non-resonant particles create secondary particles when hitting the surface of the sorting system. These secondary particles such as, for example, secondary electrons can have velocity components deviating from the direction of the beam of ions and similarly reach the target electrode other than by way of the sorting system.

(3) The ion source does not provide a concentrated "ice beam of ions with velocity components of all ions in parallel directions. Due to the finite generating angle, the ions may appear in the zone enclosing the sorting system and from there reach the target electrode without passing through these sorting systems.

The present invention intends to create a mass spectrometer tube which, by means of a special arrangement and design of screening elements, will allow only such ions as have passed through the sorting system reach the target electrode, while stray particles will be intercepted by the screening elements to a substantial extent and therefore will not influence the target electrode current. According to this invention this is achieved in such a way that tubular screening elements are provided for in the areas of both the ion source and the target electrode, whereby the axis of symmetry of these elements is parallel to the stream of ions, and that these tubular screening elements are arranged in relation both to the ion source and the target electrode in such a way that the particles can neither leave the ion source with a. velocity component which deviates to any considerable degree from the center line of the stream of ions nor reach the target electrode from the inside space of the tube. Different designs of such a total screening are possible as to their details. In an advantageous type of design one screening element, as an anode of the ion source, is shaped by two coaxial cylindrical pieces of different diameters, whereby the cylindrical piece with the smaller diameter is placed on the side of the sorting space and is equipped at both end faces with electrically conduct-ive grid electrodes. These grid electrodes may have a metallic connection with the cylindrical piece.

Another advantageous design provides for a hairpinshaped cathode for the ion source, whereby this hairpin cathode penetrates with its point an opening of a screening electrode, which in turn is placed inside that cy1indri cal piece of the anode which has the larger diameter. The potential applied to the screening electrode which adjoins the cathode is preferably negative in relation to the cathode potential.

In connection with the screening element constructed as the anode of the ion source and consisting of two cylindrical pieces, the cathode can also be enclosed by a further tubular screening electrode which is similarly equipped at its end adjacent the sorting space with a grid electrode. The cathode, the grid electrode and the further tubular screening electrode preferably operate at the same potential as that of the cathode.

In addition it may be of advantage to equip the target electrode with a tubular screening part which extends up to the range of the sorting space and which at least covers the last sorting grid of the sorting space. In a modified design the entire sorting space or a substantial part of it is encompassed by this screen. It might be expedient thereby, in the case of a device which has, in an otherwise known manner, both a retarding grid and an electrically biased screening grid, to provide an apertured disc electrode, preferably of the same potential as that of the tubular screening element, between this electrically biased screening grid and the target electrode.

The measure described will achieve a very considerable decrease of the residual current, while the detecting efficiency will be correspondingly increased. Apart from the separate useful designs of the screening elements it appears to be fundamentally essential to apply a total screening, that is, both as far as the ion source and the target electrode is concerned. Test results have shown that neither the screening of the ion source nor that of the target electrode in itself alone makes it possible to suppress satisfactorily the residual current, even in the case of a complicated geometrical and electrical design.

The drawing shows the diagrammatical view of forms of construction of a mass spectrometer tube in accordance with the invention, as follows:

FIGURE 1 a mass spectrometer tube with cathode cylinder,

FIGURE 2 a mass spectrometer tube with hairpin cathode.

Both figures show the cross-section of a cylindrical mass spectrometer tube 1, which can be connected by means of a connecting piece to a vacuum device not represented on the drawing.

Tube 1 of FIGURE 1 is equipped with a coiled heated cathode 310 and an anode 410 of an ion source,

screening grids

5 and 6, a retarding grid 7, a target electrode 811) and a sorting space consisting of sorting electrodes 9 through 19. The heated cathode 310 is enclosed by a cathode cylinder 311 which has the same potential as that of the cathode and the surface of which is shaped to form the grid 312, screening the cathode towards the side of the connecting piece 2.

The anode 410 of the ion source consists of two concentric tubular cylinders which have

different diameters

411 and 412 and which are connected with each other by means of a circular disc 413, whereby the cylinder with the larger diameter 411 partially overlaps the cathode cylinder 311 and also contains on its inside an entrance grid electrode 414. An exit grid 415 is placed on the tubular cylinder 412 on that side of the anode 411 which faces the sorting space.

The ion catcher electrode 81% is entirely surroundedwith the exception of the recess $11-by a screening 812 which has likewise the same potential as that of the cathode. The screening in the shape of a cylinder 813 may stretch along the entire length of the sorting space or at least overlap the sorting electrode 19, as can be seen on diagram 2.

FIGURE 2 shows inside the tube a hairpin cathode 320 which penetrates with its point an opening in the screening electrode 321. An anode 420 which corresponds to anode 4-11! in FIGURE 1 consists in a similar way of tube-shaped cylindrical pieces 421 and 422 with an inserted circular disc 423, whereby the cylinder 421 surrounds the screening electrode 321.

Grid electrodes

424 and 425 are provided on the cylindrical part 422 in the area of the end faces. The screening 813 of the target electrode 310 in the form of construction as shown in FIGURE 2 reaches only as far as the last sorting electrode 19 of the sorting space.

The heated cathodes 310 and 320 (FIGURES 1 and 2, respectively) are heated by means of a filament battery 20. The high frequency current for the grids 919 of the sorting space is provided by a tunable high-frequency generator 21. A one-side grounded measuring device 22 shows the ion current reaching the target electrode 810. All of the direct-current voltage potential as well as the comparison voltage for the control of the high frequency amplitude are supplied by a common power-supply unit 23.

The mass spectrometer tubes, in accordance with the invention, do not diifer from the known constructions as far as their basic operation is concerned. The ions are drawn out of the ion source through the screening grid 5 which is negatively biased to a high degree, whereby the cylinders 412 and 422 (FIGURES 1 and 2, respectively,) serve to insure an exit which is nearly parallel to the center line of the tube, so that a well focused beam Will enter the sorting space,

By means of a special design of ion sources, such as is made possible by electron-optical systems, a well focused stream of ions is directed into the sorting space, whereby the special forms of construction of the screening at the ion collector electrode 810 provide protection against the stray particles. The positively biased retarding grid 7 can be penetrated only by such particles as fulfill the resonant conditions in respect to the high frequency voltage in the sorting space and thus have absorbed sufficient energy. The negatively charged screening grid 6 suppresses disturbances which might appear due to secondary emissions of the surfaces within the range of the ion collector electrode 810.

The novel form of construction thus achieves a considerable increase of the detecting efiiciency and can be utilized in the field of higher pressures.

I claim:

1. A mass spectrometer tube of the type in which ions resulting from the electron beam of a cathode in an ion source are directed in the form of a beam of ions through a sorting space, in which a high frequency alternating field of variable frequency is present on a plurality of sorting electrodes so that only ions having a mass corresponding to a given frequency can reach a target electrode, the current of which serves as a measurement of the partial pressure of this given ionic mass in the given mixture; the improvement comprising a cylindrical anode element positioned adjacent the ion source and having a central axis parallel to the directed ion beam, a cylindrical exit element of smaller diameter than and concentric with said cylindrical anode element, said cylindrical exit element extending beyond said cylindrical anode element in the direction of the target electrode, an entrance grid electrode positioned at the end of said cylindrical anode element facing the ion source, an exit grid electrode positioned at the end of said cylindrical exit element facing the target electrode, a conductive tubular screening part enclosing the target electrode and extending into the sorting space so as to overlap at least one sorting grid therein, and means for establishing the same potential on said cylindrical anode element as on said cylindrical exit element. 7

2. A mass spectrometer tube according to claim 1 wherein the ion source comprises a hairpin-shaped cathode which extends into said cylindrical anode element.

3. A mass spectrometer tube according to claim 1 wherein the ion source is located within a cathode cylinder, and which has one end closed by an electrically conductive grid electrode and an open end facing said cylindrical anode element.

4. A mass spectrometer tube according to claim 1 including a retarding grid and an electrically biased screening grid, an apertured disc electrode positioned between said electrically biased screening grid and the target elecrode, and means for establishing said apertured disc electrode at the same potential is that of the tubular screening element.

References Cited by the Examiner UNITED STATES PATENTS 2,758,214 8/1956 Glenn 250-41.9 2,818,507 12/1957 Britten 250-41.9 2,847,574 8/1958 Donner 25041.9 X 2,975,277 3/1961 Von Ardenne 2504l.9

RALPH G. NELSON, Primary Examiner.

Claims

1. A MASS SPECTROMETER TUBE OF THE TYPE IN WHICH IONS RESULTING FROM THE ELECTRON BEAM OF A CATHODE IN AN ION SOURCE ARE DIRECTED IN THE FORM OF A BEAM OF IONS THROUGH A SORTING SPACE, IN WHICH A HIGH FREQUENCY ALTERNATING FIELD OF VARIABLE FREQUENCY IS PRESENT ON A PLURALITY OF SORTING ELECTRODES SO THAT ONLY IONS HAVING A MASS CORRESPONDING TO A GIVEN FREQUENCY CAN REACH A TARGET ELECTRODE, THE CURRENT OF WHICH SERVES AS A MEASUREMENT OF THE PARTIAL PRESSURE OF THIS GIVEN IONIC MASS IN THE GIVEN MIXTURE; THE IMPROVEMENT COMPRISING A CYLINDRICAL ANODE ELEMENT POSITIONED ADJACENT THE ION SOURCE AND HAVING A CENTRAL AXIS PARALLEL TO THE DIRECTED ION BEAM, A CYLINDRICAL EXIT ELEMENT OF SMALLER DIAMETER THAN AND CONCENTRIC WITH SAID CYLINDRICAL ANODE ELEMENT, SAID CYLINDRICAL EXIT ELEMENT EXTENDING BEYOND SAID CYLINDRICAL ANODE ELEMENT IN THE DIRECTION OF THE TARGET ELECTRODE, AN ENTRANCE GRID ELECTRODE POSITIONED AT THE END OF SAID CYLINDRICAL ANODE ELEMENT FACING THE ION SOURCE, AN EXIT GRID ELECTRODE POSITIONED AT THE END OF SAID CYLINDRICAL EXIT ELEMENT FACING THE TARGET ELECTRODE, A CONDUCTIVE TUBULAR SCREENING PART ENCLOSING THE TARGET ELECTRODE AND EXTENDING INTO THE SORTING SPACE SO AS TO OVERLAP AT LEAST ONE SORTING GRID THEREIN, AND MEANS FOR ESTABLISHING THE SAME POTENTIAL ON SAID CYLINDRICAL ANODE ELEMENT AS ON SAID CYLINDRICAL EXIT ELEMENT.