- Email: [email protected]
A tandem ion analyzer of large radius
International Journal of Mass Spectrometry and Ion Physics Elsevier Publishing Company, Amsterdam . Printed in the Netherlands
277
A TANDEM ION ANALYZER OF LARGE RADIUS
H. RASEKH! AND F. A. WHITE
Department of Nuclear Science, Rensselaer Polytechnic Institute, Troy, N. Y. 12180 (U.S.A .) (Received August 10th, 1971)
ABSTRACT A mass spectrometric facility has been designed and constructed comprising two electrostatic and two magnetic 90° lenses, each with a mean radius of curvature of 122 cm . loth the electrostatic and magnetic components can be operated independently or programmed in tandem, permitting mass and energy resolution of primary ions and secondary species produced in collision processes . A special iondetector system provides "on-line" monitoring of the primary ion beam and counting of secondary ions and neutral atoms at counting rates from 1/sec to 10'/sec . Special features of the instrument make it useful in investigations of ion energy loss, charge exchange, sputtering, and ion implantation, as well as for isotopic abundance measurements .
INTRODUCTION
Within the past decade many research areas have provided a stimulus for the development of new mass spectrometric instrumentation . The traditional areas of chemistry, geology and nuclear physics have continued to provide an impetus for constructing spectrometers of very high resolution . The areas of solid state physics and materials engineering have also generated a need for more versatile spectrometers and mass spectrometry is being utilized to explore plasmas, diffusion, impurities at grain boundries, catalytic reactions, the composition of the upper atmosphere and surface physics . Today, mass spectrometric methods are being increasingly employed in areas of ecology and environmental science, and many potential applications are evident in fields of biology and medicine' . The concept of this present instrument is to provide a very flexible apparatus which can contribute to many of the above-mentioned studies during the next decade. Special attention, however, has been focused on the interaction of ions with matter as detailed information on this topic will only be forthcoming from instruments which can analyze both primary and secondary ion species . Int. J. Mass Spectrum_ Ion Phys ., 9 (1972) 277291
H . RASEICHI, F. A. WHITE
27 8 DESIGN CRrrERIA
A decision was made early to build an apparatus which could analyze ions at high kinetic energies . This requirement led to the design of a magnetic analyzer of very large radius of curvature . A multiple magnet system was also envisioned in order that atomic collision experiments could be conducted in which both primary and secondary ion species could be mass and energy resolved . The detection of neutral atoms was also desired in order that phenomena of sputtering, charge exchange and molecular dissociation could be explored in some detail . In addition to the two magnets, electrostatic lenses were also required for the determination of ion energy and charge state . When such lenses are also employed in a double-focusing arrangement, high resolution can be achieved' and high abundance sensitivity" can be realized in isotopic abundance measurements' . In consideration of the potential research measurements that were initially anticipated, this mass spectrometer was designed to possess the following characteristics : 1 . Momentum analysis of heavy ions at 100 keV . 2 . Reasonable dispersion j(AM/M)R] . 3_ Adaptability for surface ionisation, arc, photoioni7a tion, electron bombardment, or ion impact source. 4. High abundance sensitivity, i.e . ability to measure large (M± I )IM isotopic ratios. 5 . Maximum ion detection sensitivity, at high or low counting rates . 6. Programmability for time-of-flight measurements, with a large source to detector distance . 7. Independent analysis of momentum, energy, or charge state . 8 . Independent control of primary ion beams and analysis of the secondary ion spectra generated in "transmission" or sputtering experiments . 9. Very high vacuum, with a minimum of hydrocarbon impurities, and a long mean free path . 10. Operation of magnets and electrostatic filters as single elements, or in a double-focusing configuration . 11 . Maximum stability of the tandem magnet system over extended periods . 12. Capability for ion implantation . In addition to these technical criteria, economic considerations required that the magnets should be constructed of readily available materials, and that many spectrometer components should be acquired from commercial vendors . A further requirement was that the instrument should be modified easily in future years with a minimum of difficulty and expense .
GENERAL DESCRIPTION
The configuration of the apparatus is patterned after the four-stage mass Int. J. Mass Spectrom. Ion Phys., 8 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
WIA
279
~,ocx stL°
waw
Fig . 1 _ Tandem magnetic analyzer system comprising two 90°, 122-cm radius of curvature magnets, thermal ionization source, thin foil target, and ion and neutral beam detectors .
spectrometer reported by White and Forman . The size of the instrument, however, makes this present apparatus closer to the category of isotope separators than to mass spectrometers_ The two electromagnets are represented schematically in Fig.1 . Each has a mean radius of curvature of 122 cni and an effective angle of 90' . The actual physical angular sector is approximately 86 ° ; this reduction from 90° is to account for the fringing field which extends substantially beyond the edge of the pole pieces . The basic magnet design is "C" shaped with a cylindrical core surrounded by exciting coils, and the massive top and bottom yokes are shaped to accommodate the pole pieces . The cores, represented by the dotted lines in the schematic, are low-carbon steel cylinders 76 cm in diameter and 63 .5 cm in height . Pole piece sectors are low-carbon, hot-rolled steel plate . These sectors were fabricated by flame cutting to an appropriate contour, and the horizontal surfaces were subsequently machined and finish ground . A pole-piece gap of 5 cm is maintained by spacer blocks of aluminum . The top and bottom yokes are also hot-rolled plate, flame cut and finish ground, joining the cylindrical cores and the 86° sectors . The pole-pieces were air annealed at 860° C ; yoke pieces were annealed 650 'C, after flame cutting and rough machining . Two exciting coils surround each magnet core . Each coil consists of an Int. J. Mass Spectrom. Ion Phys., 9 (1972) 277-291
280
H. RASEKHi, F . A . WHITE
aluminum-welded coil form, wound with 5,808 turns of 0 .206-cm copper conductor with heavy Formex insulation . The 5,808 turns are wrapped in four sub-coils . with output terminals being provided for each 1,452 turns . Thus, each magnet is excited by 8 sub-coils which can be connected in a series or parallel configuration . This arrangement permits operation with a single or independent power supply . The resistance of each sub-coil is 24 ohms ; parallel connections for the 8 sub-c :oils (2 coils each for the two magnets) is 3 ohms . This impedance was selected in order that both magnets could be powered by a commercially available 60 V', 20 A regulated power supply (Kepco, Model KS-60-20-M) . The coils require only air cooling and at 20 A (2 .5 A per sub-coil) a magnetic field strength of 7300 gauss is generated over a pole piece area of approximately 5600 em -. Figs . 2 and 3 are photographs of the analyzing system . The vacuum envelope was designed to accommodate a multiplicity of access ports and provisions were made so that an ion source could be inserted at every focal point of the instrument . The main analyzer sections were fabricated from stainless steel tubing and bent to appropriate shapes . Approximately half of the analyzing envelope was comprised of 20-cm diameter stainless steel . This tubing was selected to accommodate the rather large electrostatic lenses, and these sections were then welded to five rectangular boxes which serve as access ports for the ion sources and detectors . A stainless steel tubing of smaller diameter (10 cm and subsequently flattened) was used for the 90' sections which pass through the 5-cm gap of the electromagnets . Figs_ 2 and 3 also show the large number of flanges which were welded to this
Fig . 2 . Photograph of large electromagnets, and laboratory lay-out of the entire analyzing system ; curved tubing in the fo .
A TANDEM ION ANALYZER OF LARGE RADIUS
281
Fig . 3 . Photograph of section between the tandem magnets, including goniometer, vacuum pumps, ion collimating slits, isolation valves, and `on-line" ion detector .
vacuum envelope in order to accommodate ion pumps, several slit systems, ion :7ation gauges and the goniometer_ This goniometer was specially developed for orienting and positioning single crystal targets . The determination of the several focal points were calculated and the precise physical location was achieved by means of a transit . In addition, provision was made for determining a best focal position experimentally . The large rectangular marble blocks that support the two 15-ton magnets can be seen in Fig . 2. On top of these marble blocks was placed an assembly for moving the magnets, comprised of four roller bearings held within heavy-duty ball-bearing supports . Thus an adjustment can be made easily to correct for the effect of a fringing field at high or low magnet field strengths . Further, motion of either magnet can be utilized to bring the ion beam in perfect focus even when the magnets No . 1 and No . 2 are operated with their exciting coils connected in series and the magnets are not programmed independently . The electrostatic lenses were constructed with a mean radius of curvature of 122 cm, thus matching the radii of the electromagnets . The fabrication of these Int. J. Mass Spectrom . Ion Phys., 8 (1972) 277-291
282
H. RASEKHI, F . A . WHITE
Fig. 4 . Electrostatic analyzer assembly constructed with glass spheres that function both as insulators and precision spacers for the machined components . lenses was patterned after the manner previously described by Stein and Whites_ The interelectrode spacing of the lenses is 10 cm . With this spacing 100 keV ions can be analyzed utilizing a commercially available powersupply of 10 kV . A photograph of one of the electrostatic analyzers is shown in Fig . 4. The total source-to-detector distance for the entire assembly (the two ma7 nets and two electrostatic analyzers) is approximately 25 m . At present, however, experimental work has been limited to the use of the two magnet analyzer sections shown in Fig. I . In any event, the very Ion .- source-to-detector trajectory required construction of a very clean analyzing tube, utilizing metal-to-metal seals and the very high capacity pumping described in a following section .
ION SOURCE
Fig_ 5 is a schematic of the 30 kV thermal ionization source . Basically, it is patterned after the design reported by Dietz 6 _ The mechanical construction, however, includes the use of precision glass spheres which provide both precise interelectrode alignment of the collimating slits, and high voltage insulation . The actual source of ions is comprised of a capillary reservoir (2 mm stainless steel tubulation) connected to two 0_3 mm tungsten ribbons (baffles) . A helicoil filament, also of tungsten, provides radiation heating to the baffles which in turn Eat. J. Mass Spectrorn. Ton Phys ., 3 (1972) 277-291
283
A TANDEM ION ANALYZER OF LARGE RADIUS ESERVOIR
VI roeE D IONIZED
AFFLES
(LAMENT TOP ELECTRODE
DRAW-OUT ELECTRCO NORYDNTAL REAM EENTERM ELECTRODES AUXILIARY FOCUSING ELECTRODE
010[o \\\\\\\////J/
nI
FOCUSING ELECTRODE FRSTCOLLWATPID SLIT
VERTICAL BEAM
_y CENTERING ELECTRODES >~7•!•1sA! n
esl_~t9s SECOND COLL!&ATING
Stir
Fig . 5 . Schematic diagram of 30 kV thermal ionization source, comprising filament, focusing electrodes, glass sphere insulators, and collimating slits . raises the reservoir temperature . This slight rise in the temperature causes the material to evaporate at a reasonably constant rate . As the evaporated material establishes contact with the hot surfaces (the filament and the baffles), some ions are formed ; such ions are then subject to the electric field of the "drawing-out" electrodes . The construction of this diffusion-thermal ion source is displayed in Fig . 5 . This source has provided ion beams of alkali metals of 10 9 ions/sec for periods exceeding 100 hours . Potentials for the several electrodes of the ion source are obtained from a voltage divider network which is shown in Fig . 6. A combination of step-switching plus a continually variable resistance provides a wide range of voltage control . Furthermore, this circuit allows adjustment of the individual electrode potentials independent of other potentials .
ION-ATOM DETECTION SYSTEM The beam detection system is comprized of (1) a Faraday cup, (2) an "online" beam sampling device, (3) a neutral beam monitor, (4) an ion beam detector at the final focal point of the tandem analyzer, and (5) a PDP 8/1 computer .-The locations of these detectors are shown in Fig. 1 . The Faraday cup is attached to a linear mechanical feed-through which permits withdrawal of the cup after initial measurements of the primary ion beam . The "on-line" detector device is shown in Fig. 7. Ions are deflected by a strong Int. .1 Mass Spectrom. Ion Phys., 8 (1972) 277-291
H. RASEKHI, F. A. WHITE
284 30W TOP ELECTRODE
%%%--r
0
DRAY-OUT ELECTRODE
Fig. 6- Circuit diagram of the 30 kV voltage divider, indicating potentiometers for optimizing potentials applied to the ion sourceelectrostatic field (when the ion 'beam is actually monitored at this point) so that ions impinge upon a "converter" . The secondary electrons emitted by this converter are then focussed onto an electron multiplier . The deflection voltage applied to the "converter" is supplied from the ion source voltage divider (see Fig . 6) . This device has proven to be very useful in obtaining estimates of the primary ion beam at very low intensities ; the ion beam can then pass through this device by simply Inc. S Mars Spectrum. Ion Phys., 8 (1572) 277-291
A
TANDEM ION ANALYZER OF LARGE
RADIUS
285
Fig. 7 . Diagram of "on-line" beam monitor which intercepts ions when a potential is applied so as to deflect ions onto a ground plate; secondary electrons produced by ion impact are simultaneously focused onto an electron multiplier for pulse counting of ions .
turning off the "converter" deflection potential. The "converter" electrode can also be oriented for maximum secondary electron yields, or withdrawn completely via a linear motion feedthrough . The ion detector which is located at the focal point of the second magnet, Int. J Mass Spectrom . Ion Phys., 8 (1972) 277291 -
286
H . RASEKHI, F. A . WHITE
and the neutral beam monitor, consist of "venetian blind" type electron multipliers having eighteen stages with a gain of approximately 10' . Ions are monitored in a pulse counting mode' . The output pulses of the electron multipliers are amplified in the amplifier discriminator system which was originally developed by Sawada 8 . Two major improvements have been made . The first substantially reduced the nonlinearity in the discriminator circuit . Secondly, an addition of one more amplification stage to the preamplifier and the amplifier sections improved the input-output characteristics of this system . This improved system is now capable of sensing pulses with amplitudes as low as 3 mV and effective duration time of 8 nsec . The discriminator level can be adjusted between 3 mV to 150 mV . Its output is characterized by pulses with about 3.5 V in amplitude and 8 nsec in effective width. It is appropriate to make special mention of the simultaneous counting capability for measuring neutral atoms and positive ions . As indicated in Fig . 1, ions becoming neutralized in a collision process will proceed through the magnetic field and be intercepted by the neutral beam multiplier . Positive ions undergoing energy degradation in a target (i.e. in a thin-foil) must be focussed onto the final detector by reducing the magnitude of the magnetic field of magnet No . 2 with respect to that of magnet No . 1 . There is a distinct advantage in monitoring neutrals and ions simultaneously, inasmuch as the neutral beam monitor can be used to normalize variations in the primary ion beam . Specifically it has been possible to obtain consistent data at counting rates ranging from 10' atoms per second to 5 atoms per second. The data acquisition system is a PDP 8/I computer . Output pulses of the discriminator multipliers of both neutral and ion detectors are fed into individual scalers which are then interfaced to the computer . The computer is then programmed to furnish ratios, perform other desired mathematical operations on the detecting channels, and provide a suitable print-out .
VACUUM COMPONENTS
Stainless steel was selected as the vacuum envelope material in order to facilitate high temperature bake-out and permit the weldment of the many adjunct access ports, fittings, etc . Copper gaskets were used to join all sections of the vacuum envelope via high vacuum flanges of various sizes . All collimating slits and movable devices within the spectrometer were connected through metal bellows- Prior to assembly, the entire interior surface of the analyzing housing was treated chemically by the Diversy process' . The only conventional "0" rings in the systems were those used for internal valves which permit the isolation of several sections of the analyzing tube, i .e. ion-source region from magnetic analyzer, the section containing the goniometer where "targets" are mounted and interchanged, and the portion of the analyzing system housing the neutral and final ion beam detector. Inn J Mass Spectrum_ Ion Phys_, 9 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
287
The pumping system is comprised of two mechanical pumps, three liquid nitrogen sorption pumps, and four 1600 I/sec sublimation pumps . In order to reduce oil contamination from the mechanical pumps to a minimum, a standard operating procedure is followed . The section to be evacuated is first reduced to about I torr by a mechanical pump through a molecular sieve. A subsequent reduction to about 10 -5 torr is achieved by the sorption pumps . The high capacity sublimation pump is then activated . By this means pressures of 10 -9 ton are achieved without bake out, providing the overall system has not been exposed to the ambient atmosphere for more than short periods. The large diameter of the vacuum envelope provides a high conductance and a total of 6400 I/sec pumping capacity is available when all pumps are activated . A view of two sorption pumps and two sublimator pumps can be seen in Fig. 3. Monitoring of the pressure is achieved by meters of the sublimation pumps, and ionization gauges located in proximity to the ion source and at a point located mid-way between the two magnets.
STABILIZATION OF MAGNETIC FIELDS
For isotopic abundance measurements the magnets are cohered so as to produce identical magnetic field strengths . When used to explore collision phenomena, however, it is required that for a given primary ion energy, the field of magnet No . 1 must be kept invariant, but magnet No . 2 must be programmed over a wide range . Even though the magnets are several meters apart, a mutual magnetic coupling exists which limits the stability of the primary ion beam . The magnetic fields BI and B 2 associated with magnets No . I and No . 2 can be expressed as BI
=
B1o+IV12 B2
B_ = l21 BI +B20
where M,2' and M21 are the mutual magnetic field coupling coefficients and B10 and B>o are the magnetic fields of magnet No . 1 and No . 2 when only a single magnet is energized. Experimentally, it was determined that Ni I2
= M,
1
t - 450
This value is reasonably constant over a range of 0 to 6000 gauss_ Thus, M tz introduces a perturbation in the magnitude of BI of OBI = Mtz ABz =
1 ~B2 450 Int . J. Mass Spectraam_ Ion Phys., 8 (1972) 277-291
288
H . RASEKHI, F_ A. WHITE
Fig. 8 . Magnetic field control circuit for stabilizing the energizing current to the two tandem magnets_ Hence, a magnetic field control system was developed to perform the following functions : (1) effectively decouple the two magnetic fields ; (2) reduce the instabilities and thermal drifts associated with the energizing current ; and (3) reduce the time period needed for the magnets to reach an ultimate magnetic field intensity . This control system is shown in Fig . 8. A relatively high current source (the 0-20 A main current source) supplies the major portion of the energizing current . A smaller current source (0-i A, control current source-Kepco Model JQE-100-1M) with a much greater inherent stability, is utilized to correct for the main power supply current drifts and mutual coupling perturbation . Effective decoupling of the two fields is accomplished by controlling the smaller power supply with a feed-back signal produced by a Hall probe as indicated in Fig_ 8_ Further, this feed-back signal provides corrections for the long term stabilization time associated with the magnetic core . One can show that under a steady state condition :
Bt
(Rtt
V« _
t R,. =- 400
I
RL.L
1
I,,K - Rt.P,I
(h+Ij ),
Rh
Ie 5 1 amp
where K is the Hall constant and RLL is the Hall probe output termination resistanc . The output of the Hall probe, Vh , exhibits thermal drifts beyond a tolerable limit for this application . Thus, a temperature compensated network is introduced, as shown in Fig. 8. This network has an optimum output termination resistance,
Ins. L
Mass Spectrom. Ion Phys., 8 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
289
RLL , for which V„ is nearly a linear function of B_ It -can be shown that :
Z = t- R, (aRh - $(Rh+RLL))k Y where R, is the resistance of the thermistor at room temperature, R h is the output internal resistance, Z = R°R,JR° +R„ a = I/R h - dRb /dT, ;R = 1/VV, • d Vh/dT y = 1/R, - dR,/dT, RLL = RL +Z, and T is temperature. Usually, values of a, >4 and RLL are given by the manufacturer of the Hall
probe, and a value for y is given by the manufacturer of the thermistors . USE OF ANALYZER FOR DErERMD4ING ION ENERGY LOSS
As indicated above the instrument is especially useful for investigating atomic collisions and molecular dissociation phenomena" . As a single example, consider the measurement of atomic stopping power of 'Li ions traversing a thin film of nickel . Reference is made to the schematic diagram of Fig- 1 . We should like to determine the energy loss incurred by these singly charged ions in traversing the foil . Lithium ions are generated in the thermal ionization source, accelerated to a kinetic energy of 5 .1 KeV and focused on a very thin (250 A) foil . The actual shape of the energy loss curve is important inasmuch as it permits some analysis to be made with respect to the nuclear and the electronic component of atomic stopping, and transport phenomena generally l1 • 1 2_ The nuclear component is essentially Rutherford scattering, and the electronic component consists of atomic interactions that give rise to ionization and excited states of atoms within the solid_ 004-
7Li
17
KeV
Fig . 9. Energy spectrum of 7 Li having an initial energy of 5 .1 keV, after traversing a thin polycrystalline nickel foil (250 A) .
Ins. I. Mass Spectrom . Ion Phys., 8 (1972) 277-291
H . RASEKHI, F . A . WHITE
The primary analyzing magnet was used to determine the kinetic energy of the lithium ions incident upon the foil . The second analyzer was then programmed (decreased in field strength) so as to focus those ions, which had traversed the foil, onto the electron multiplier . Further, lithium ions which had undergone charge exchange in the foil and emerged as neutral atoms were detected with the neutral beam electron multiplier . A ratio was then taken between the ion and neutral atom counting rates . This procedure eliminated substantially all errors associated with variable ion emission from the source, and it permitted the acquisition of reliable data at very low counting rates . Fig. 9 is a single energy loss curve of 'Li ions, obtained at the lowest primary ion energy at which data could be obtained . A detailed analysis of the energy loss mechanism and a comparison with existing stopping power theory has been reported elsewhere" . It will be evident that this instrumentation provides flexibility for many investigations relating to beast foil spectroscopy, the interaction of ions with gases, as well as isotopic measurements where very high isotopic ratios are desired to monitor nuclear reactions_ Presently, the large electrostatic lenses are being attached to the system and further improvements are being made with respect to the entire anal zer. ACKNOWLEDGEMENTS
It is a pleasure to acknowledge the many sources of assistance that were essential to the construction of this instrument . Substantial mechanical design help was received from J. C. Sheffield, M . Trzepacz and J. Lewis, and R. Pendt performed alt the high vacuum welding . Dr. T. W. Whitehead performed computer calculations to locate the focal points and determine the 86° angle of the pole pieces . Dr. J . D. Stein assisted with the electrostatic lens design and G_ Struthoff designed the goniometer, collimating slits, and several vacuum valves . J. L . Segal and J_ D . Walling interfaced the detection system to the computer and G. M . Wood assisted with vacuum components . A grant from the General Electric Foundation made possible the acquisition of the exciting coils and we are indebted to several members of the General Electric staff for help in coil design and the actual fabrication .
REFERENCES I F. A. WHnt, Mass Spectrometry in Science and Technology, Wiley, New York, 1968, pp. 278, 317 . 2 K_ T. BAINBRIDGE AND E_ B. JORDe , Phys_ Rev_, 50 (1936) 2823 F. A_ Wmxr, F_ M. RouRxs N- J. C_ SIEFFIELD, App!. Spectros., 12 (1958) 46 . 4 F. A. Want A .vn L. Fowii.uN, Rev. Sci. Instrum., 38 (1967) 355 . 5 J. D. SmELV AND F. A. WxrrE, Int. J. Mass Spectrom . Ion Phys., 5 (1970) 205. 6 L_ A. DiErz, Rev. Sci. Lnstrum ., 32 (1961) 859. 7 F. A . Warn AND T. L. Cor * * a, App!_ Spectrom_, 8 (1954) 17 . Int. J Mass Spectrom. Ion Phys., 8 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
291
8 F. H . SAWADA, IEEE Trans. Nucl. Sci., NS-12, No . 1 (1965) 374. 9 AProcess for Producing a Microfinisln on Stainless Steel, Technical
Publication of the Diversy - Chemical Company, 212 West Monroe Street, Chicago, HL 10 F. M . RouRKE, J. C. SImFFiEIn, W . D. DAVIS AND F. A . WHITE, 7. Chem- Phys., 31 (1959) 19311 J. LINDIaRD, V. Name AND M . ScHARFF, Kgl. Dan . Yidensk . Selsk., Mat: Fyr. Medd., 36 (1968) 10.
12 S . M .
S=zEA AJZD M. J . BERGER . Nat. Aced. Sci. - Nat . Res. Coune., No . 1133 (1964) 187 . 13 H . P.. sEKHt, Doctoral Dissertation, Rennselaer Polytechnic Institute, Troy, N . Y., 1971 .
Int. 3- Mass Spectrum . Ion Phys., 8 (1972) 277-291
277
A TANDEM ION ANALYZER OF LARGE RADIUS
H. RASEKH! AND F. A. WHITE
Department of Nuclear Science, Rensselaer Polytechnic Institute, Troy, N. Y. 12180 (U.S.A .) (Received August 10th, 1971)
ABSTRACT A mass spectrometric facility has been designed and constructed comprising two electrostatic and two magnetic 90° lenses, each with a mean radius of curvature of 122 cm . loth the electrostatic and magnetic components can be operated independently or programmed in tandem, permitting mass and energy resolution of primary ions and secondary species produced in collision processes . A special iondetector system provides "on-line" monitoring of the primary ion beam and counting of secondary ions and neutral atoms at counting rates from 1/sec to 10'/sec . Special features of the instrument make it useful in investigations of ion energy loss, charge exchange, sputtering, and ion implantation, as well as for isotopic abundance measurements .
INTRODUCTION
Within the past decade many research areas have provided a stimulus for the development of new mass spectrometric instrumentation . The traditional areas of chemistry, geology and nuclear physics have continued to provide an impetus for constructing spectrometers of very high resolution . The areas of solid state physics and materials engineering have also generated a need for more versatile spectrometers and mass spectrometry is being utilized to explore plasmas, diffusion, impurities at grain boundries, catalytic reactions, the composition of the upper atmosphere and surface physics . Today, mass spectrometric methods are being increasingly employed in areas of ecology and environmental science, and many potential applications are evident in fields of biology and medicine' . The concept of this present instrument is to provide a very flexible apparatus which can contribute to many of the above-mentioned studies during the next decade. Special attention, however, has been focused on the interaction of ions with matter as detailed information on this topic will only be forthcoming from instruments which can analyze both primary and secondary ion species . Int. J. Mass Spectrum_ Ion Phys ., 9 (1972) 277291
H . RASEICHI, F. A. WHITE
27 8 DESIGN CRrrERIA
A decision was made early to build an apparatus which could analyze ions at high kinetic energies . This requirement led to the design of a magnetic analyzer of very large radius of curvature . A multiple magnet system was also envisioned in order that atomic collision experiments could be conducted in which both primary and secondary ion species could be mass and energy resolved . The detection of neutral atoms was also desired in order that phenomena of sputtering, charge exchange and molecular dissociation could be explored in some detail . In addition to the two magnets, electrostatic lenses were also required for the determination of ion energy and charge state . When such lenses are also employed in a double-focusing arrangement, high resolution can be achieved' and high abundance sensitivity" can be realized in isotopic abundance measurements' . In consideration of the potential research measurements that were initially anticipated, this mass spectrometer was designed to possess the following characteristics : 1 . Momentum analysis of heavy ions at 100 keV . 2 . Reasonable dispersion j(AM/M)R] . 3_ Adaptability for surface ionisation, arc, photoioni7a tion, electron bombardment, or ion impact source. 4. High abundance sensitivity, i.e . ability to measure large (M± I )IM isotopic ratios. 5 . Maximum ion detection sensitivity, at high or low counting rates . 6. Programmability for time-of-flight measurements, with a large source to detector distance . 7. Independent analysis of momentum, energy, or charge state . 8 . Independent control of primary ion beams and analysis of the secondary ion spectra generated in "transmission" or sputtering experiments . 9. Very high vacuum, with a minimum of hydrocarbon impurities, and a long mean free path . 10. Operation of magnets and electrostatic filters as single elements, or in a double-focusing configuration . 11 . Maximum stability of the tandem magnet system over extended periods . 12. Capability for ion implantation . In addition to these technical criteria, economic considerations required that the magnets should be constructed of readily available materials, and that many spectrometer components should be acquired from commercial vendors . A further requirement was that the instrument should be modified easily in future years with a minimum of difficulty and expense .
GENERAL DESCRIPTION
The configuration of the apparatus is patterned after the four-stage mass Int. J. Mass Spectrom. Ion Phys., 8 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
WIA
279
~,ocx stL°
waw
Fig . 1 _ Tandem magnetic analyzer system comprising two 90°, 122-cm radius of curvature magnets, thermal ionization source, thin foil target, and ion and neutral beam detectors .
spectrometer reported by White and Forman . The size of the instrument, however, makes this present apparatus closer to the category of isotope separators than to mass spectrometers_ The two electromagnets are represented schematically in Fig.1 . Each has a mean radius of curvature of 122 cni and an effective angle of 90' . The actual physical angular sector is approximately 86 ° ; this reduction from 90° is to account for the fringing field which extends substantially beyond the edge of the pole pieces . The basic magnet design is "C" shaped with a cylindrical core surrounded by exciting coils, and the massive top and bottom yokes are shaped to accommodate the pole pieces . The cores, represented by the dotted lines in the schematic, are low-carbon steel cylinders 76 cm in diameter and 63 .5 cm in height . Pole piece sectors are low-carbon, hot-rolled steel plate . These sectors were fabricated by flame cutting to an appropriate contour, and the horizontal surfaces were subsequently machined and finish ground . A pole-piece gap of 5 cm is maintained by spacer blocks of aluminum . The top and bottom yokes are also hot-rolled plate, flame cut and finish ground, joining the cylindrical cores and the 86° sectors . The pole-pieces were air annealed at 860° C ; yoke pieces were annealed 650 'C, after flame cutting and rough machining . Two exciting coils surround each magnet core . Each coil consists of an Int. J. Mass Spectrom. Ion Phys., 9 (1972) 277-291
280
H. RASEKHi, F . A . WHITE
aluminum-welded coil form, wound with 5,808 turns of 0 .206-cm copper conductor with heavy Formex insulation . The 5,808 turns are wrapped in four sub-coils . with output terminals being provided for each 1,452 turns . Thus, each magnet is excited by 8 sub-coils which can be connected in a series or parallel configuration . This arrangement permits operation with a single or independent power supply . The resistance of each sub-coil is 24 ohms ; parallel connections for the 8 sub-c :oils (2 coils each for the two magnets) is 3 ohms . This impedance was selected in order that both magnets could be powered by a commercially available 60 V', 20 A regulated power supply (Kepco, Model KS-60-20-M) . The coils require only air cooling and at 20 A (2 .5 A per sub-coil) a magnetic field strength of 7300 gauss is generated over a pole piece area of approximately 5600 em -. Figs . 2 and 3 are photographs of the analyzing system . The vacuum envelope was designed to accommodate a multiplicity of access ports and provisions were made so that an ion source could be inserted at every focal point of the instrument . The main analyzer sections were fabricated from stainless steel tubing and bent to appropriate shapes . Approximately half of the analyzing envelope was comprised of 20-cm diameter stainless steel . This tubing was selected to accommodate the rather large electrostatic lenses, and these sections were then welded to five rectangular boxes which serve as access ports for the ion sources and detectors . A stainless steel tubing of smaller diameter (10 cm and subsequently flattened) was used for the 90' sections which pass through the 5-cm gap of the electromagnets . Figs_ 2 and 3 also show the large number of flanges which were welded to this
Fig . 2 . Photograph of large electromagnets, and laboratory lay-out of the entire analyzing system ; curved tubing in the fo .
A TANDEM ION ANALYZER OF LARGE RADIUS
281
Fig . 3 . Photograph of section between the tandem magnets, including goniometer, vacuum pumps, ion collimating slits, isolation valves, and `on-line" ion detector .
vacuum envelope in order to accommodate ion pumps, several slit systems, ion :7ation gauges and the goniometer_ This goniometer was specially developed for orienting and positioning single crystal targets . The determination of the several focal points were calculated and the precise physical location was achieved by means of a transit . In addition, provision was made for determining a best focal position experimentally . The large rectangular marble blocks that support the two 15-ton magnets can be seen in Fig . 2. On top of these marble blocks was placed an assembly for moving the magnets, comprised of four roller bearings held within heavy-duty ball-bearing supports . Thus an adjustment can be made easily to correct for the effect of a fringing field at high or low magnet field strengths . Further, motion of either magnet can be utilized to bring the ion beam in perfect focus even when the magnets No . 1 and No . 2 are operated with their exciting coils connected in series and the magnets are not programmed independently . The electrostatic lenses were constructed with a mean radius of curvature of 122 cm, thus matching the radii of the electromagnets . The fabrication of these Int. J. Mass Spectrom . Ion Phys., 8 (1972) 277-291
282
H. RASEKHI, F . A . WHITE
Fig. 4 . Electrostatic analyzer assembly constructed with glass spheres that function both as insulators and precision spacers for the machined components . lenses was patterned after the manner previously described by Stein and Whites_ The interelectrode spacing of the lenses is 10 cm . With this spacing 100 keV ions can be analyzed utilizing a commercially available powersupply of 10 kV . A photograph of one of the electrostatic analyzers is shown in Fig . 4. The total source-to-detector distance for the entire assembly (the two ma7 nets and two electrostatic analyzers) is approximately 25 m . At present, however, experimental work has been limited to the use of the two magnet analyzer sections shown in Fig. I . In any event, the very Ion .- source-to-detector trajectory required construction of a very clean analyzing tube, utilizing metal-to-metal seals and the very high capacity pumping described in a following section .
ION SOURCE
Fig_ 5 is a schematic of the 30 kV thermal ionization source . Basically, it is patterned after the design reported by Dietz 6 _ The mechanical construction, however, includes the use of precision glass spheres which provide both precise interelectrode alignment of the collimating slits, and high voltage insulation . The actual source of ions is comprised of a capillary reservoir (2 mm stainless steel tubulation) connected to two 0_3 mm tungsten ribbons (baffles) . A helicoil filament, also of tungsten, provides radiation heating to the baffles which in turn Eat. J. Mass Spectrorn. Ton Phys ., 3 (1972) 277-291
283
A TANDEM ION ANALYZER OF LARGE RADIUS ESERVOIR
VI roeE D IONIZED
AFFLES
(LAMENT TOP ELECTRODE
DRAW-OUT ELECTRCO NORYDNTAL REAM EENTERM ELECTRODES AUXILIARY FOCUSING ELECTRODE
010[o \\\\\\\////J/
nI
FOCUSING ELECTRODE FRSTCOLLWATPID SLIT
VERTICAL BEAM
_y CENTERING ELECTRODES >~7•!•1sA! n
esl_~t9s SECOND COLL!&ATING
Stir
Fig . 5 . Schematic diagram of 30 kV thermal ionization source, comprising filament, focusing electrodes, glass sphere insulators, and collimating slits . raises the reservoir temperature . This slight rise in the temperature causes the material to evaporate at a reasonably constant rate . As the evaporated material establishes contact with the hot surfaces (the filament and the baffles), some ions are formed ; such ions are then subject to the electric field of the "drawing-out" electrodes . The construction of this diffusion-thermal ion source is displayed in Fig . 5 . This source has provided ion beams of alkali metals of 10 9 ions/sec for periods exceeding 100 hours . Potentials for the several electrodes of the ion source are obtained from a voltage divider network which is shown in Fig . 6. A combination of step-switching plus a continually variable resistance provides a wide range of voltage control . Furthermore, this circuit allows adjustment of the individual electrode potentials independent of other potentials .
ION-ATOM DETECTION SYSTEM The beam detection system is comprized of (1) a Faraday cup, (2) an "online" beam sampling device, (3) a neutral beam monitor, (4) an ion beam detector at the final focal point of the tandem analyzer, and (5) a PDP 8/1 computer .-The locations of these detectors are shown in Fig. 1 . The Faraday cup is attached to a linear mechanical feed-through which permits withdrawal of the cup after initial measurements of the primary ion beam . The "on-line" detector device is shown in Fig. 7. Ions are deflected by a strong Int. .1 Mass Spectrom. Ion Phys., 8 (1972) 277-291
H. RASEKHI, F. A. WHITE
284 30W TOP ELECTRODE
%%%--r
0
DRAY-OUT ELECTRODE
Fig. 6- Circuit diagram of the 30 kV voltage divider, indicating potentiometers for optimizing potentials applied to the ion sourceelectrostatic field (when the ion 'beam is actually monitored at this point) so that ions impinge upon a "converter" . The secondary electrons emitted by this converter are then focussed onto an electron multiplier . The deflection voltage applied to the "converter" is supplied from the ion source voltage divider (see Fig . 6) . This device has proven to be very useful in obtaining estimates of the primary ion beam at very low intensities ; the ion beam can then pass through this device by simply Inc. S Mars Spectrum. Ion Phys., 8 (1572) 277-291
A
TANDEM ION ANALYZER OF LARGE
RADIUS
285
Fig. 7 . Diagram of "on-line" beam monitor which intercepts ions when a potential is applied so as to deflect ions onto a ground plate; secondary electrons produced by ion impact are simultaneously focused onto an electron multiplier for pulse counting of ions .
turning off the "converter" deflection potential. The "converter" electrode can also be oriented for maximum secondary electron yields, or withdrawn completely via a linear motion feedthrough . The ion detector which is located at the focal point of the second magnet, Int. J Mass Spectrom . Ion Phys., 8 (1972) 277291 -
286
H . RASEKHI, F. A . WHITE
and the neutral beam monitor, consist of "venetian blind" type electron multipliers having eighteen stages with a gain of approximately 10' . Ions are monitored in a pulse counting mode' . The output pulses of the electron multipliers are amplified in the amplifier discriminator system which was originally developed by Sawada 8 . Two major improvements have been made . The first substantially reduced the nonlinearity in the discriminator circuit . Secondly, an addition of one more amplification stage to the preamplifier and the amplifier sections improved the input-output characteristics of this system . This improved system is now capable of sensing pulses with amplitudes as low as 3 mV and effective duration time of 8 nsec . The discriminator level can be adjusted between 3 mV to 150 mV . Its output is characterized by pulses with about 3.5 V in amplitude and 8 nsec in effective width. It is appropriate to make special mention of the simultaneous counting capability for measuring neutral atoms and positive ions . As indicated in Fig . 1, ions becoming neutralized in a collision process will proceed through the magnetic field and be intercepted by the neutral beam multiplier . Positive ions undergoing energy degradation in a target (i.e. in a thin-foil) must be focussed onto the final detector by reducing the magnitude of the magnetic field of magnet No . 2 with respect to that of magnet No . 1 . There is a distinct advantage in monitoring neutrals and ions simultaneously, inasmuch as the neutral beam monitor can be used to normalize variations in the primary ion beam . Specifically it has been possible to obtain consistent data at counting rates ranging from 10' atoms per second to 5 atoms per second. The data acquisition system is a PDP 8/I computer . Output pulses of the discriminator multipliers of both neutral and ion detectors are fed into individual scalers which are then interfaced to the computer . The computer is then programmed to furnish ratios, perform other desired mathematical operations on the detecting channels, and provide a suitable print-out .
VACUUM COMPONENTS
Stainless steel was selected as the vacuum envelope material in order to facilitate high temperature bake-out and permit the weldment of the many adjunct access ports, fittings, etc . Copper gaskets were used to join all sections of the vacuum envelope via high vacuum flanges of various sizes . All collimating slits and movable devices within the spectrometer were connected through metal bellows- Prior to assembly, the entire interior surface of the analyzing housing was treated chemically by the Diversy process' . The only conventional "0" rings in the systems were those used for internal valves which permit the isolation of several sections of the analyzing tube, i .e. ion-source region from magnetic analyzer, the section containing the goniometer where "targets" are mounted and interchanged, and the portion of the analyzing system housing the neutral and final ion beam detector. Inn J Mass Spectrum_ Ion Phys_, 9 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
287
The pumping system is comprised of two mechanical pumps, three liquid nitrogen sorption pumps, and four 1600 I/sec sublimation pumps . In order to reduce oil contamination from the mechanical pumps to a minimum, a standard operating procedure is followed . The section to be evacuated is first reduced to about I torr by a mechanical pump through a molecular sieve. A subsequent reduction to about 10 -5 torr is achieved by the sorption pumps . The high capacity sublimation pump is then activated . By this means pressures of 10 -9 ton are achieved without bake out, providing the overall system has not been exposed to the ambient atmosphere for more than short periods. The large diameter of the vacuum envelope provides a high conductance and a total of 6400 I/sec pumping capacity is available when all pumps are activated . A view of two sorption pumps and two sublimator pumps can be seen in Fig. 3. Monitoring of the pressure is achieved by meters of the sublimation pumps, and ionization gauges located in proximity to the ion source and at a point located mid-way between the two magnets.
STABILIZATION OF MAGNETIC FIELDS
For isotopic abundance measurements the magnets are cohered so as to produce identical magnetic field strengths . When used to explore collision phenomena, however, it is required that for a given primary ion energy, the field of magnet No . 1 must be kept invariant, but magnet No . 2 must be programmed over a wide range . Even though the magnets are several meters apart, a mutual magnetic coupling exists which limits the stability of the primary ion beam . The magnetic fields BI and B 2 associated with magnets No . I and No . 2 can be expressed as BI
=
B1o+IV12 B2
B_ = l21 BI +B20
where M,2' and M21 are the mutual magnetic field coupling coefficients and B10 and B>o are the magnetic fields of magnet No . 1 and No . 2 when only a single magnet is energized. Experimentally, it was determined that Ni I2
= M,
1
t - 450
This value is reasonably constant over a range of 0 to 6000 gauss_ Thus, M tz introduces a perturbation in the magnitude of BI of OBI = Mtz ABz =
1 ~B2 450 Int . J. Mass Spectraam_ Ion Phys., 8 (1972) 277-291
288
H . RASEKHI, F_ A. WHITE
Fig. 8 . Magnetic field control circuit for stabilizing the energizing current to the two tandem magnets_ Hence, a magnetic field control system was developed to perform the following functions : (1) effectively decouple the two magnetic fields ; (2) reduce the instabilities and thermal drifts associated with the energizing current ; and (3) reduce the time period needed for the magnets to reach an ultimate magnetic field intensity . This control system is shown in Fig . 8. A relatively high current source (the 0-20 A main current source) supplies the major portion of the energizing current . A smaller current source (0-i A, control current source-Kepco Model JQE-100-1M) with a much greater inherent stability, is utilized to correct for the main power supply current drifts and mutual coupling perturbation . Effective decoupling of the two fields is accomplished by controlling the smaller power supply with a feed-back signal produced by a Hall probe as indicated in Fig_ 8_ Further, this feed-back signal provides corrections for the long term stabilization time associated with the magnetic core . One can show that under a steady state condition :
Bt
(Rtt
V« _
t R,. =- 400
I
RL.L
1
I,,K - Rt.P,I
(h+Ij ),
Rh
Ie 5 1 amp
where K is the Hall constant and RLL is the Hall probe output termination resistanc . The output of the Hall probe, Vh , exhibits thermal drifts beyond a tolerable limit for this application . Thus, a temperature compensated network is introduced, as shown in Fig. 8. This network has an optimum output termination resistance,
Ins. L
Mass Spectrom. Ion Phys., 8 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
289
RLL , for which V„ is nearly a linear function of B_ It -can be shown that :
Z = t- R, (aRh - $(Rh+RLL))k Y where R, is the resistance of the thermistor at room temperature, R h is the output internal resistance, Z = R°R,JR° +R„ a = I/R h - dRb /dT, ;R = 1/VV, • d Vh/dT y = 1/R, - dR,/dT, RLL = RL +Z, and T is temperature. Usually, values of a, >4 and RLL are given by the manufacturer of the Hall
probe, and a value for y is given by the manufacturer of the thermistors . USE OF ANALYZER FOR DErERMD4ING ION ENERGY LOSS
As indicated above the instrument is especially useful for investigating atomic collisions and molecular dissociation phenomena" . As a single example, consider the measurement of atomic stopping power of 'Li ions traversing a thin film of nickel . Reference is made to the schematic diagram of Fig- 1 . We should like to determine the energy loss incurred by these singly charged ions in traversing the foil . Lithium ions are generated in the thermal ionization source, accelerated to a kinetic energy of 5 .1 KeV and focused on a very thin (250 A) foil . The actual shape of the energy loss curve is important inasmuch as it permits some analysis to be made with respect to the nuclear and the electronic component of atomic stopping, and transport phenomena generally l1 • 1 2_ The nuclear component is essentially Rutherford scattering, and the electronic component consists of atomic interactions that give rise to ionization and excited states of atoms within the solid_ 004-
7Li
17
KeV
Fig . 9. Energy spectrum of 7 Li having an initial energy of 5 .1 keV, after traversing a thin polycrystalline nickel foil (250 A) .
Ins. I. Mass Spectrom . Ion Phys., 8 (1972) 277-291
H . RASEKHI, F . A . WHITE
The primary analyzing magnet was used to determine the kinetic energy of the lithium ions incident upon the foil . The second analyzer was then programmed (decreased in field strength) so as to focus those ions, which had traversed the foil, onto the electron multiplier . Further, lithium ions which had undergone charge exchange in the foil and emerged as neutral atoms were detected with the neutral beam electron multiplier . A ratio was then taken between the ion and neutral atom counting rates . This procedure eliminated substantially all errors associated with variable ion emission from the source, and it permitted the acquisition of reliable data at very low counting rates . Fig. 9 is a single energy loss curve of 'Li ions, obtained at the lowest primary ion energy at which data could be obtained . A detailed analysis of the energy loss mechanism and a comparison with existing stopping power theory has been reported elsewhere" . It will be evident that this instrumentation provides flexibility for many investigations relating to beast foil spectroscopy, the interaction of ions with gases, as well as isotopic measurements where very high isotopic ratios are desired to monitor nuclear reactions_ Presently, the large electrostatic lenses are being attached to the system and further improvements are being made with respect to the entire anal zer. ACKNOWLEDGEMENTS
It is a pleasure to acknowledge the many sources of assistance that were essential to the construction of this instrument . Substantial mechanical design help was received from J. C. Sheffield, M . Trzepacz and J. Lewis, and R. Pendt performed alt the high vacuum welding . Dr. T. W. Whitehead performed computer calculations to locate the focal points and determine the 86° angle of the pole pieces . Dr. J . D. Stein assisted with the electrostatic lens design and G_ Struthoff designed the goniometer, collimating slits, and several vacuum valves . J. L . Segal and J_ D . Walling interfaced the detection system to the computer and G. M . Wood assisted with vacuum components . A grant from the General Electric Foundation made possible the acquisition of the exciting coils and we are indebted to several members of the General Electric staff for help in coil design and the actual fabrication .
REFERENCES I F. A. WHnt, Mass Spectrometry in Science and Technology, Wiley, New York, 1968, pp. 278, 317 . 2 K_ T. BAINBRIDGE AND E_ B. JORDe , Phys_ Rev_, 50 (1936) 2823 F. A_ Wmxr, F_ M. RouRxs N- J. C_ SIEFFIELD, App!. Spectros., 12 (1958) 46 . 4 F. A. Want A .vn L. Fowii.uN, Rev. Sci. Instrum., 38 (1967) 355 . 5 J. D. SmELV AND F. A. WxrrE, Int. J. Mass Spectrom . Ion Phys., 5 (1970) 205. 6 L_ A. DiErz, Rev. Sci. Lnstrum ., 32 (1961) 859. 7 F. A . Warn AND T. L. Cor * * a, App!_ Spectrom_, 8 (1954) 17 . Int. J Mass Spectrom. Ion Phys., 8 (1972) 277-291
A TANDEM ION ANALYZER OF LARGE RADIUS
291
8 F. H . SAWADA, IEEE Trans. Nucl. Sci., NS-12, No . 1 (1965) 374. 9 AProcess for Producing a Microfinisln on Stainless Steel, Technical
Publication of the Diversy - Chemical Company, 212 West Monroe Street, Chicago, HL 10 F. M . RouRKE, J. C. SImFFiEIn, W . D. DAVIS AND F. A . WHITE, 7. Chem- Phys., 31 (1959) 19311 J. LINDIaRD, V. Name AND M . ScHARFF, Kgl. Dan . Yidensk . Selsk., Mat: Fyr. Medd., 36 (1968) 10.
12 S . M .
S=zEA AJZD M. J . BERGER . Nat. Aced. Sci. - Nat . Res. Coune., No . 1133 (1964) 187 . 13 H . P.. sEKHt, Doctoral Dissertation, Rennselaer Polytechnic Institute, Troy, N . Y., 1971 .
Int. 3- Mass Spectrum . Ion Phys., 8 (1972) 277-291