Method of selectively separating charged particles using a variable intensity non-uniform magnetic field

March 7, 1967 r. f. mathams 3,308,293

METHOD OF SELECTIVELY SEPARATING CHARGED PARTICLES USING

A VARIABLE INTENSITY NON-UNIFORM MAGNETIC'FIELD

Filed April 20, 1964 2 Sheets-Sheet 1

PAGT/CLR set/ect

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I 32

March 7, 1967 p p mathams 3 20A oqq

METHOD OF SELECTIVELY SEPARATING CHARGED PARTICLES USING

-,., , . ^A J^A?^IABLE INTENSITY NON-UNIFORM MAGNETIC FIELD

Filed April 20, 1964 2 Sheets-Sheet 2

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United States Patent Office

3,308,293

Patented Mar. 7, 1967

3,308,293

METHOD OF SELECTIVELY SEPARATING CHARGED PARTICLES USING A VARIABLE INTENSITY NON - UNIFORM MAGNETIC FIELD 5

Ronald Frank Mathams, Brisbane, Queensland, Australia, assignor to Associated Electrical Industries Limited, London, England, a British company

Filed Apr. 20, 1964, Ser. No. 360,968

Claims priority, application Great Britain, Apr. 24,1963, 16,172/63 ¹⁰

Claims. (CI. 250—41.9)

The present invention relates to spectrometers.

A spectrometer is an apparatus for forming a spectrum from a beam of charged particles having different 15 energies and/or different masses. The apparatus includes a magnetic field through which the charged particles travel. On travelling through the field each particle is deflected in accordance with its energy and its mass. Therefore, in general, each particle emerges from 20 the field travelling along a direction inclined at an angle to the direction along which it enters the field, and the size of this angle depends on the energy and the mass of the particle.

If a beam of particles of different energies and the same 25 mass enters the field along an initial direction, particles of different energies will emerge from the field along directions inclined at different angles to this initial direction, and therefore an energy spectrum of the beam of charged particles will be formed. Alternatively, if a 30 beam of particles of different masses and the same energy enters the field along an initial direction, particles of different masses will emerge from the field along directions inclined at different angles to this initial direction and therefore a mass spectrum of the beam of charged par- 35 tides will be formed.

In known spectrometers all charged particles are deflected on travelling through the apparatus and therefore the apparatus cannot be introduced and operated in other equipment without affecting the overall paths of the 40 charged particles. In particular the particles may be electrons produced in electron optical equipment, an electron microscope for example.

In an electron microscope a beam of electrons all having the same energy bombard the specimen and the elec- 45 trons are deflected or diffracted. The energy levels of the deflected or diffracted electrons are not all the same and therefore a beam of electrons having different energies is formed. It may be desirable to use a spectrometer as described above to form an energy spectrum of 50 the beam of electrons with different energies so that electrons having a selected energy can be detected.

With known types of spectrometer the introduction of the spectrometer would affect the optical path of the electrons relative to the remaining components of the ®5 electron optical system and this may be undesirable.

The object of the present invention is to provide a method of obtaining a spectrum which has particular use in electron optical equipment.

According to the present invention a spectrometer com- ⁶⁰prises means for providing a magnetic field of intensity which varies as a function of the distance from the axis of the spectrometer so that only charged particles having a selected energy and a selected mass entering said magnetic field along said axis are deflected in said field and ⁶⁵emerge along said axis.

Charged particles having energies or masses or other than the selected values will also be deflected on travelling through the magnetic field of the spectrometer but will not emerge along the axis of the spectrometer. 70 If such an energy spectrometer is inserted in electron optical equipment so that the axis of the spectrometer coincides with the electron optical axis of the equipment, a beam of charged particles travelling along the optical axis with said selected energy and mass will emerge from said spectrometer along said optical axis.

Preferably the intensity of the magnetic field increases with distance from the axis so that the charged particles having the selected energy and mass described a substantially superior trochoidal path. With this arrangement the intensity of the magnetic field must vary monotonically and the magnetic field must be in such a direction as to deflect the charged particles in the direction of increasing field strength.

In order that the invention may be more readily understood reference will now be made to the accompanying drawing, in which:

FIG. 1 is a side view partly in section of a spectrometer embodying the invention,

FIG. 2 is a part plan view of the spectrometer illustrated in FIG. 1 sectioned on the plane II—II,

FIG. 3 is another part plan view of the spectrometer illustrated in FIG. 1 sectioned on the plane III—III, and . FIGS. 4 and 5 are graphs illustrating the operation of the spectrometer.

With reference to FIGS. 1, 2 and 3 the spectrometer comprises a cylindrical body member 1 with one integral circular end plate 2 and another circular end plate 3 suitably secured to the body member 1. Apertures 4, 5 are formed in end plates 2, 3 respectively and serve as input and output apertures respectively for the spectrometer. Secured within the cylindrical body member 1 is a circular plate 6 having a central aperture 7 in which is sealed a tube 8. Plate 6 is removably secured to an inner annular flange 9 on body member 1 and the joint is sealed with an O-ring 11.

Two pole pieces 12, 13 are mounted on core members 14, 15 respectively and these core members extend through cooperating apertures in plate 6 so that the pole pieces are located between plate 6 and end plate 2. The core members 14, 15 extend through coils 16, 17 and are connected at their ends to an annular yoke member 18. The core members 14, 15 are formed with collars 19, 21 respectively. These collars 19, 21 are removably secured to the lower face of plate 6 so as to secure the core members in position and the joints are sealed by O-rings 22, 23.

Tube 8 extends between coils 16, 17 and through the central aperture of yoke member 18 and is removably secured in a central aperture 24 of a diaphragm 25 extending across and spaced from the inner face of end plate 3. An O-ring 26 is used to seal the end of tube 8 within the aperture 24.

A slit defining assembly is secured to the inner face of end member 3 and comprises two plates 27 movable relative to each other and defining a slit 28 which is variable in width and in position relative to the axis Z—Z of the apparatus, as will be seen more clearly in FIG. 3.

The pole faces 31, 32 of pole pieces 12, 13 are inclined to each other at a small angle, as will be seen more clearly in FIG. 2, and are spaced symmetrically from the axis Z—Ϊ of the apparatus. Means (not illustrated) may be provided for rotating core members 14, 15 relative to plate 6 so as to vary the angle of inclination of the pole faces 31, 32. The energising coils 16, 17 produce a magnetic flux in the core members 14, 15 the yoke 18 and the pole pieces 12, 13 so that a magnetic field is induced between the pole faces 31, 32. Due to the inclination of the pole faces 31, 32, the intensity of this magnetic field varies with distance from the axis Z—7-. Means are also provided for varying the energising currents in coils 16, 17 in order to vary the overall intensity of this magnetic field.

3,308,293 ³

The operation of the spectrometer described above with reference to FIGS. 1, 2 and 3 can best be explained with reference to FIGS. 4 and 5.

FIG. 4 illustrates the variation of the intensity of the magnetic field between the pole faces 31, 32 in a plane 5 through the axis Z—Z of the spectrometer and extending symmetrically between the pole faces. The intensity of the magnetic field at any point is given by the formula:

<img file="US3308293A_D0003.tif" />

where:

B=flux density of the field in the direction between the pole faces, k=a constant depending upon the excitation provided by 15 coils 16, 17, /=the distance from the axis Z—Z to the line in which the planes of the pole faces intersect, x=the distance from the axis Z—Z in the direction of increasing field intensity. ²⁰

The above formula indicates that the intensity B of the field increase monotonically. The direction of the field is indicated by the symbol in FIG. 4 and is into the plane of the drawing. The direction is chosen so that a charged 25 particle, for example an electron, entering the field along the axis Z—Z is deflected in the direction of increasing magnetic field strength.

FIG. 5 illustrates the path traversed by a charged particle travelling through the magnetic field which is con- 39 sidered to extend effectively from the x-axis to the line L—L in FIG. 5. The particle is considered to have entered the field along the axis Z—Z. If the particle has a certain energy eV and a certain mass m its path will be substantially superior trochoidal provided the distribution 35 of the intensity of the magnetic field is correct, and the particle will complete a full cycle of the superior trochoid and will emerge from the magnetic field along the axis Z—Z, its path being represented by the full line in FIG. 5. 40

If the particle has the same mass m and a lower energy e(V—8V) this path will approximate to a smaller superior trochoid as represented by the chain dotted line in FIG. 5 and this particle will reach the axis Z—Z before emerging from the magnetic field having completed 45 a full cycle of the superior trochoid. The particle will therefore be further deflected before emerging from the field and will emerge in a direction inclined at a small angle δα to the axis Z—Z given by:

where K is defined as the dispersion sensitivity of the spectrometer. K has a minimum value of about π/4.

If the particle has the same mass m and a higher energy 55 e(V-'-oV') its path will approximate to a larger superior trochoid as represented by the dashed line in FIG. 5. The particle will fail to complete a full cycle of the superior trochoid and will emerge from the magnetic field before reaching the axis Z—Z. The particle will therefore ₆₀emerge from the field in a direction inclined to the axis Z—Z at an angle δα of opposite polarity to that of the angle of inclination of the path of the particle of lower energy on emerging from the magnetic field.

Hence if a beam of charged particles of equal mass and &lt;55 of different energy levels passes into the magnetic field of the spectrometer along the axis Z—Z, the beam will be dispersed on emerging from the field in accordance with the different energy levels of the particles and thus an energy spectrum will be formed. By varying the intensity γθ of the magnetic field the value of the energy eV, for which particles having this energy emerge along the axis Z—Z, can be selected.

If a beam of particles of equal energy levels but different masses passes into the magnetic field of the spec-. 75 trometer along the axis Z—Z all the particles will travel along paths approximating to superior trochoids and the intensity of the magnetic field can be selected so that particles having a selected mass will travel ,a full cycle of a superior trochoid and will emerge from the field along the axis Z—Z while the particles of other masses will emerge travelling in different directions. Hence a mass spectrum of a beam of charged particles of equal energy levels will be formed.

It will be appreciated that the variation in the intensity of the magnetic field is chosen so that a particle of selected energy and mass will be deflected and will travel in a pafh corresponding to a cyclic wave form and will just complete a full cycle on traveling through the field. If the cycle is started and finished at a point at which the particle is travelling along the direction of the axis the directions of travel of the particle on entering and leaving the field will be collinear and the overall path of the particle is not changed. Particles of energy and mass values differing from the selected values will also be deflected on travelling through the field but will not just complete one full cycle.

It will also be appreciated that the selected particle need not move in a superior trochoidal path, but can move in other cyclic path shapes, an inferior trochoid or a sine wave for example. The intensity of the magnetic field provided by the pole pieces 12, 13 could be chosen so as to cause the particle to move in such a cyclic path.

By causing the particle of selected energy and mass to traverse a path of cyclic shape other than a superior trochoid the sensitivity of the spectrometer may be varied.

Referring to FIGS. 1, 2, and 3 the spectrometer described is adapted to be inserted in electron discharge apparatus so that a beam of electrons of different energy levels from a particle source 41 enters through the aperture 4. The chamber 33 defined by end member 2 and the plates 6 is connected to the vacuum system of the electron discharge apparatus. Chamber 33 is also connected by tube 8 to a chamber 34 defined by end member 3 and the diaphragm 25. The region containing the coils 16, 17 can be maintained at a higher pressure, the vacuum within components 33, 34 and 8 being maintained by the O-ring seals 11, 22, 23 and. 26. Tube 8 also acts as a magnetic screen for any electrons within the tube.

The electrons on passing between the pole pieces 12, 13 are deflected as described above and electrons having a selected energy level according to the values of the energising currents in coils 16, 17 travel on along axis Z—Z through slit 28 and out through aperture 5 to be received by an electron collector 42 which may be, for example, another component of the electron discharge apparatus, a screen for example. By varying the width of the slit 28 the width of the output beam and hence the selected energy range of the output beam can be varied. Also the mean energy level of electrons which passes through slit 28 can be varied by moving the slit defining plates 27 or by varying the currents through coils 16, 17.

If the energy spectrometer described above is used with electron optical apparatus shell as the electron microscope, the spectrometer can be used to select electrons of a certain energy level from a beam of electrons of various energy levels. The spectrometer would be inserted in the electron microscope so that the axis Z—Z coincides with the optical axis of the microscope and only electrons of a selected energy level would emerge through the slit 28 along the optical axis. The electron optical apparatus would include a powerful astignatism corrector and an electron optical aperture at the entrance to the spectrometer.

The energy spectrorheter has been described for use with electron discharge apparatus producing a beam of electrons. Alternatively the spectrometer could be used with a beam of other charged particles and could be connected into the equipment producing this beam of charged particles in a manner similar to that described above so

3,308,293 that the beam of charged particles passes through the magentic field. If the particles all have the same mass but have different levels an energy spectrum can be formed by the spectrometer. Alternatively, if the particles all have the same energy but have different masses, 5 a mass spectrum can be formed by the spectrometer.

It is expected that a magnetic field of such a type as to produce an inferior trochoidal trajectory for the selected particles would be about 6 times more sensitive than a field to produce a superior trochoidal trajectory. jq

A spectrometer of the type herein described can be used satisfactorily to form a mass spectrum of a beam of particles of equal energy levels provided the ratio of the heaviest to the lightest particles is of the order of 2.