US2976413A - Mass spectrometer - Google Patents

Description

March 21, 1961 c. F. ROBINSON 2,976,413

MASS SPECTROMETER Filed June 25, 1956 2 Sheets-Sheet 1 SENS/NG C/RCU/T fl M4 l /o/v sou/70E /0 28 L INVENTOR.

CHA/PL Es f.' Roa//vsoN A TTORNEVS March 21, 1961 Filed June 25, 1956 C. F. ROBINSON 2 Sheets-Sheet 2 /NTEGRA To@ /A//RATOR //v TEGRATOR SENS /NG srs TEM W 44 f s,1-,I

V0 :fo/ TA G5 SUPP/ y IN V EN TOR. CHARLES ROB/NSON m me M ATTORNEYS United States Patent MASS SPECTROMETER Charles F. Robinson, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporatlon, Pasadena, Calif., a corporation of California Filed June 25, 1956, Ser..N0. 593,560 8 Claims. (Cl. Z50-41.9)

exceedingly diiicult. I have now developed an instruf ment in which this problem is overcome and by means of which this heretofore objectionable characteristic is used to advantage. Further, and of particular importance, the instrument of the invention permits selective analysis of ions originating in dilerent regions of such a source. In a spark source, for example, I am able to detect small differences in the composition of the two electrodes as a result of the ability to selectively analyze ions originating iirst from one and then Athe other of the electrodes.

Whereas certain ion sources inherently produce ions of widely dierent energy in different regions of the source, other ion sources are characterized by the absence of lany substantial energy spread in the ionized particles. This invention depends upon and hence contemplates inducing a significant energy spread if applied to an ion source which does not inherently develop this required energy spread.

'In its general application the invention comprises a mass spectrometer having an ion source, -a mass resolving system, and ion collecting means, the combination comprising means for imparting dilerent energies to ions formed in different regions of the ion source and means for adjusting the mass resolvingsystem to pass only ions occurring within a given energy band, `such band including less than the totality of energiesproduced within the source. In a preferred form the instrument may include a spark source, an energy iilter, means for causing ions to enter the filter from the source, means operable to enable the lter to pass only ions of an energy band less than the energy spread of the ions developed in the source, means for varying the operation of the source to cause ions originating within different regions thereof to fall in the pre-selected energy band, a mass analyzer into which ions passing the filter are injected, and collecting means for collecting ions focused thereon by the analyzer.

In the above described instrument, for example, the high voltage D.C. supply for accelerating ions into the resolving system can be connected alternately to the two electrodes of the source whereupon the ions formed rst from one electrode and than the other will possess a denite fixed kinetic energy upon emergence from the l ion source, ions from the other electrode emerging from the ion source Vwith energies distributed through ya very wide range. By introducing these ions to zan electrostatic energy lter in which the voltages are suitably adjusted, it is possible to arrange that the electrostatic filter will pass most of the ions formed on the electrode ,whose potential is xed, and reject most of the ions formed through the aperture, in electrode 28, but'will strike the surface thereof and be discharged thereon. .The

on the electrode whose potential is varying through a wide range.

The invention will be understood in conjunction with the accompanying drawing, in which:

Fig. 1 is -a schematic diagram of a simplified embodiment of the invention;

Fig. 2 is a schematic diagram of apparatus in accordance with the invention involving a spark source;

Fig. 3 is an enlarged perspective view of the electrode array disposed between the electrostatic filter and the analyzer in the apparatus of Fig. 2; and

Fig. 4 is a graphic diagram of the operation of the apparatus of'Fig. 2. p Y

The apparatus of Fig. 1 includes an ion Vsource 10, an electrostatic Ifilter 11, a magnetic mass analyzer 12,

a collector system 13 and a sensing and recording cir-V cuit 14. The ion vsource 10 is intended to represent any ion source characterized by production of ions of Widelyv varying energy. The particular source shown includesy an ion gun 16 oriented to direct yan ionizing beam against the surface of a sample 17 contained in the face of |a repeller electrode` 18. The ion gun may be any of aA variety of conventional ion sources such, for example, as the ion gun described by Moak, Reese and Good in Nucleonics, volume 9, page 18 (1951). -Ions formed at the .sample 17 are'collimated and accelerated by electrodes 19, 20. A voltage supply circuit 22 for the accelerating electrodes is shown schematically. Platesg Y 24, 2S of the electrostatic filter 11 areV ,connected` to op-` posite sides of a VD.C. supply 26 so that a positive potential is imposed on the plate 25 and the negative potential on the plate 24. Of the ions entering the` ilter 11, only those of a given energy spread determined by the potential applied between plates 24 and 25 and by the width of the aperture in electrode 28 will traverse the aperture in a first resolving electrode 2-8.`

for entry into the magnetic analyzer 12.'

The analyzer 12 is represented by a magnet structure defining the outlines of a conventional ion resolving transverse magnetic iield and showing the course of ion travel therein. 'In the analyzer ions of a certain mass depending on ion velocity and mganetic eld strength are focused through the aperture inra second resolving electrode 29 onto the tirst dynode of an electron multiplier 30 which, in this particular instance, forms the collection system 13.

It is the purpose of this invention to permit the retention for analysis of ions of a given mean energy, as for example those formed at the surface of 'the sample 17, at the same timerejecting ions of a different meanenergy; such as those formed from background gases in the space between the sample 17 and rst slit 19.

electrode as illustrated. An ionizing ion beam is directed against the 'surface of the sample with sufficient energy to achieve nonfractionating ionization. These ions together with others'fo-rmed in the'sourc'e are ye`x pelled therefrom at a velocity dependent upon thepotential established across the source. In the filterv 11 only ions of a given mean energy, in this exampleA the mean energy exhibited by ions formedby'bombardment at the sample surface, are transmitted. All-ionscf energy diering substantially from those originatingat p Y deflected by the electrostatic t the sample surface will be energy filter plates '24 and 25, so as to fail tolpass ions passing through the resolving aperture intojthe analyzer are resolved by the magnetic iield, ,selectively focused on the resolving aperture of the analyzer,and

sensed in conventional' fashion. In the instrument o1 Fig. 1, selectivity achieved by the invention is desirable to permit an accurate analysis of ions formed at a surface having a given potential and to discriminate against ions formed at other potentials, as for example ions formed from background gases. This is made possible in the apparatus of Fig. l by imposing `a substantial voltage between electrodes 1S, 19 and 20, this imposed voltage causing the potential in most of the free space surrounding electrode 18 to be substantially dilerent from the potential of eletcrode 18 so that ions formed in that space possess substantially dilerent kinetic energies from ions formed at electrode 18. In the apparatus of Fig. 2, ions are inherently formed with large energy spread, and the circuits are arranged so as to take advantage of that fact.

In the instrument of Fig. l, selectivity achieved by the invention is desirable to permit an accurate analysis of` ions developed at a surface having a given potential and to exclude ions formed at other potentials, as for example ions formed from background gases. In the apparatus of Fig. 2, a large energy spread in the ion source is purposely induced so that comparative analysis may be made with the mass resolving system of the invention.

The instrument shown in Fig. 2 comprises an ion source 40, an electrostatic tilter 41, an analyzer 4Z, collecting system 43, and sensing circuit 44. The ion source consists of a pair of electrodes 46, 47 to which a high voltage D.C. supply 48 may be alternately connected through switch 49 and potentiometers 50, 51. An RF. oscillator 52 is connected across the electrodes through a transformer S3. A grounded electrode 54 accelerates ions formed in the spark developed between electrodes 46, 47 into the electrostatic lter 41.

A D.C. supply 56 is connected to develop a positive potential at plate S7 and a negative potential at plate 58 of the filter. A so-called wiper electrode array consisting of a grounded lirst resolving electrode 68 and a sampling electrode 69 is preferably interposed between the lter 41 and the analyzer 42, for purposes which will beldescribed.

The settings of the energy iilter are well known in the mass spectrometer art. As an illustrative example we may say that where plates 57 and 58 are spaced l centimeter apart and held at a diierence of potential of 500 volts, having a mean radius of curvature of 30 centimeters, ions having a kinetic energy of 7,500 electron volts are passed at optimum eliciency and if aperture 68A is 2 l02 cm. wide and is placed immediately adjacent plates 57 and 58, ions having an energy spread of l volts (that is to say, ions having a kinetic energy of 7,500i7.5 volts) are passed by the electrostatic lilter.

The analyzer 42 is illustrated schematically as a'magnet pole 72 defining the outline of a transverse magnetic field, and a second resolving electrode 73 through which ions of a particular mass are focused on the first dynode of the electron multiplier which forms the collection system 43. The electron multiplier is connected to a switch 76 which in turn is connected to two integrating circuits 77, 78, the outputs of which are connected to the sensing system 44. The wiper electrode 69 is connected to an integrating network 80 which in turn is connected to operate switches 49 and 76" on the ion source high voltage supply and on the output integrators respectively. I

The electrodes 68, 69 formingthe wiper system intermediate the electrostatic iilter and the analyzer are shown in perspective in Fig. 3. The electrode 68has a vertically oriented aperture 68A. The Wiperrelectrode 69 has a vertically restricted aperture 69A so ,that the vertical extremities of an ion beam, illustrated at 82, form shadows 82A, 82B on the wiper electrode 69. The width of the aperture 68A deiines the energy spreadof the ions admitted to the magnetic analyzer, whereas the vertical height of the aperture 69Alimits the height4 of the beam admitted to the analyzer.

The integrating network 77, 78 and 80 may be of the type of the Weston Model 1473, available from the Weston Electric Instrument Corporation, Newark, New Jersey, and described in Weston Engineering Notes, vol. 10, No. 2, page 6, December 1955.

The operation of the apparatus of Fig. 2 may be described with relation to an actual analysis and with the aid of the graph of Fig. 4. Electrode 46 of the ion source may constitute a zone-refined silicon standard and electrode 47 a silicon of unknown purity. The electrostatic deflection system and its associated aperture 68A are set to pass ions whose energies lie in a narrow band centered on the applied high voltage V0 (this being the output voltage ofthe high voltage supply 48) that is, at or near the potential of whichever of electrodes '46 and 47 is directly connected to the high voltage supply. By switching the high voltage supply from electrode 46 to electrode 47 by means of switch 49, it is possible for the electrostatic deection system to sample selectively ions originating respectively from either electrode. This comes about because it the electrostatic deflection system is set so as to` pass ions of energy V0 and the high voltage supply 48 is connected to (for` example) electrode 46, ions from electrode 46 are passed.

reely by the electrostatic energy lilter while ions formed at electrode 47, whose potential with respect to ground varies widely with time according to the equation V47=V0+V53 sin wt, where V53 is the peak voltage amplitude on the secondary of transformer 53 and w is the frequency of oscillation of R.F. oscillator 52, are passed only during the instants that V47 lies within the narrow band of energies passed by the electroastic energy filter. In this mode of operation it is understood that V53 is large compared to the energy pass band of the electrostatic filter.

The ions passed by aperture 68A are further resolved in the magnetic resolving system 42 and second resolving electrode 73, and the output of collector 43 is fed selectively to one or the other of integrators 77, 78 through switch 76 which is ganged to switch 49. Thus, integrator 77 may be made to give the integrated ion current with respect'to a certain atomic species of ions originating at electrode 46 and integrator 78 to give similar intelligence with respect to ions of the same atomic species originating in electrode 47. Thus, the difference inr reading between integrators 77 and 78 constitutes a measurement of theabundance of the selected impurity on which the system is focused which, for instance, may be boron. This difference reading is obtained even though the instrument background in the neighborhoodof the boron peaks may be much higher than the dilerence to be detected.

The operation is illustrated graphically in Fig. 4 in which curve. B, represents the electrical signal applied, for example, to electrode 46 of the source, and curve a representsthe D.C. signal applied to electrode 47 of the source with switch 49 in the dotted lne position. If the electrostatic filter 41 is set to pass a band represented by the dotted lines on either side of D C. voltage a, only those ions originating at electrode 46 in the fractional cyclesl represented by the coincidence of curve B and the band pass of the electrostatic filter will reach the analyzer section 42. Substantially all of the ions produced at. electroder47, however, will be passed to the analyzer section while, as indicated, virtually all ofthe noise represented by ions produced at electrode 46 will be excluded. The converse is true, as illustrated by the second curve of Fig. 4, and with switch 49 in the solid line position as illustrated.

Potentiometers S0, 51, are not essential to the operation of the apparatus of Fig. 2 but they do enable a selectivity control. By setting the potentiometers at their respective end points, one 'or the other of the electrodes is` connected directly to the source and a maximum selectivity is` achieved.` At the` other extreme, nif the two potentiometers are.set at mid-point, there no` selectivity. Since the selectivity of the source is a function of the potential diiference between the electrodes 46, 47, manipulation of the potentometers 50, 51 as described permits a control on this selectivity.

In the detection of trace impurities the ion detector is preferably an electron multiplier as shown, and integrators 77, 78 may be simple decade counters counting pulses from the multiplier which in eiect constitute the count of individual ions. In this mode of operation, switch 49 can be used to reverse the direction of the count in a single counter, so that a single counter can be used to show directly the difference between the count corresponding to electrode 46 and that corresponding to electrode 47.

It is preferable that the reading intervals on electrodes 46 and 47 be equivalent not in terms of clock time but in terms of total usable ion current produced by the ion source. It is for this purpose that wiper electrode 69 is preferably embodied in the apparatus. The current developed at this electrode through impingement of a portion of the ion beam thereon, as shown in Fig. 3, is integrated in integrator 80, and switches 49 and '76 are controlled as a function of the integrated current developed at the wiper electrode. From a practical standpoint it is desirable for the switching action to take place several times during an analysis so that the tripping level of the wiper electrode current integrator 80 should correspond to about one-tenth or less of the total current necessary for obtaining the desired measurement.

The sensing system 44 may consist of any of the many typical indicating or recording systems suitable for accommodating the output of the integrating system as illustrated.

I claim:

1. A mass spectrometer comprising a spark ion source having at least two electrodes, an A.C source and a D.C. source connected across the electrodes, an energy filter, means for causing ions to enter the lter from the source, means operable to enable the filter to pass only ions of a given energy band, means for reversing the connection of the electrodes to the D.C. source to alternately cause ions originating at different ones of the electrodes to fall in said energy band, a mass analyzer into which ions passing the lter are injected, and collecting and measuring means for collecting and measuring ions focused thereon by the analyzer.

2. Apparatus according to claim 1 wherein the means for reversing the connection of the electrodes to the D.C. source is operable responsive to the integrated output of the collecting means.

3. Apparatus according to claim 1 wherein the means for reversing the connection of the electrodes to the D.C. source is operable as a function of the ion beam intensity passing from the lter into the analyzer.

4. A mass spectrometer comprising an ion source, a mass resolving system, ion collecting and measuring means, energy selecting means, means operable to energize the energe selecting means for selectively passing to the collecting means ions originating at a given predetermined position in the source and which are within a narrow band of kinetic energies and representing substantially less than the totality of ions formed in the source, and means for varying the potential distribution within the source so that ions formed in diierent parts of the source lie within the band of energies passed by the energy selecting means.

5. A mass spectrometer comprising an ion source, a

mass resolving system, ion collecting and collecting means ions originating at a given predetermined position in the source and which are Within a narrow band 'l of kinetic energies and representing substantially less than the totality of ions formed in the sourceand means for comparing the output of the collecting and measuring means at a given mass for different potential distributions within the ion source.

6. A mass spectrometer comprising an ion source, a mass resolving system, ion collecting and measuring means, energy selecting means, means operable to energize the energy selecting means for selectively passing toA the collecting means ions originating at a given predetermined position in the source and which are within a narrow band of kinetic energies and representing substantially less than the totality of ions formed in the source, and means for automatically and continuously comparing the output of the collecting and measuring means at a given mass for different potential distributions within the ion source.

7. A mass spectrometer comprising an ion source, a mass resolving system, ion collecting and measuring means, energy selecting' means, means operable to energize the energy selecting means for selectively passing to the collecting means ions originating at a given predetermined position in the source and which are within a naryraw band of kinetic energies and representing substantially less than the totality of ions formed in the source means for varying the potential distribution within the source, means for comparing the output of the collecting and measuring means at a given mass for different potential distributionsv within the ion source, and means automatically recording the difference output through a predetermined period of time.

8. A mass spectrometer comprising an ion source, a mass resolving system, ion collecting and measuring means, energy selecting means, means operable to energize the energy selecting means for selectively passing to the collecting means ions originating at a given predetermined position in the source land which are within a narrow band of kinetic energies and representing substantially less than the totality of ions formed in the source means for varying the potential distribution within the source,

-means for comparing the output of the collecting and measuring means at a given mass for different potential distributions Within the ion source and means automatically recording the difference output through a period of time suliicient for the ion source to deliver a predetermined total charge.

References Cited in the file of this patent UNITED STATES PATENTS 2,622,204 Shaw et al Dec. 16, 1952. 2,659,821 Hipple Nov. 17, 1953 2,685,035 Wiley v July 27, 1954 2,694,151 Berry Nov. 9, 1954 OTHER REFERENCES Semat: Introduction to Atomic and Nuclear Physics, 3d ed., published 1954 by Reinhart & Co. (pp. 53-55

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