US2670450A - Electron microscope - Google Patents

Electron microscope Download PDF

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Publication number
US2670450A
US2670450A US289685A US28968552A US2670450A US 2670450 A US2670450 A US 2670450A US 289685 A US289685 A US 289685A US 28968552 A US28968552 A US 28968552A US 2670450 A US2670450 A US 2670450A
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United States
Prior art keywords
lens
bore
pole
point
distance
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Expired - Lifetime
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US289685A
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English (en)
Inventor
Poole Jan Bart Le
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Hartford National Bank and Trust Co
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Hartford National Bank and Trust Co
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
    • H01J37/10—Lenses
    • H01J37/14—Lenses magnetic
    • H01J37/141—Electromagnetic lenses

Definitions

  • the invention relates to electron microscopes, more particularly to an improvement in the objective lens of such instruments.
  • the bore of the magnetic electron lens has been made materially wider than is required to allow the electron beam to pass.
  • the character of the lens is furthermore determined by the spacing between the pole shoes (referred to hereinafter as the pole spacing).
  • the pole spacing By an increase in this spacing the magnetic force which urges the electrons towards the axis becomes operative through a longer distance and the strength of the lens increases, provided that the field strength does not vary.
  • the pole spacing is increased, the maintenance of a definite field strength requires an increase in the number of ampere turns and therefore the pole spacin is chosen to be comparatively large, it is true, but it does not much exceed the diameter of the bore.
  • An electron microscope according to the invention comprises an objective lens having great field strength and a bore diameter which is smaller with respect to the pole spacing than has hitherto been used. With the construction of this new objective lens the requirements with respect to the constance of the operating voltage and the energizing current are extremely low.
  • Fig. 1 shows, diagrammatically, an electron lens with electron paths for the purpose of illustratin the invention
  • Fig. 2 is a curve showing the variation of field strength with distance along the axis in the pole space of an electron lens
  • Fig. 3 shows the path of electrons in an electron lens exhibiting the field strength curve shown in Fig. 12;
  • Fig. .4 shows a cross-sectional view of one sin- 2 Claims. (01. 313-84).
  • the focal distance of the electromagnetic lens varies in the first place with the field strength.
  • the increase in field strength is, however, limited by the magnetic saturation in the pole shoes. If this is reached, the field strength can be increased only by means of a large number of ampere turns.
  • the reference numeral l designates the optical axis of a magnetic electron lens and 2 a path of an electron parallel to this axis.
  • the magnetic field of the lens is operative on the strength AB.
  • the ray is subjected to a bending force. If the ray 2 enters the field of magnetic force from the left, it is curved towards the axis beyond the point A.
  • the field strength or the distance AB is so great that the ray under consideration intersects the axis I at a point F, situated between the pole shoes. Apart from the spherical aberration each ray which is incident from the left parallel to the axis will intersect this txis at the same point F, which, consequently, is a focal point of the lens.
  • the ray is curved in the opposite direction and if the portion FB of the stretch AB exceeds the portion AF, the ray emerges from the lens in-a direction towards the axis and intersects the axis again at the point 0.
  • the ray consequently has a straight portion 2, a curved portion 2' and again a straight portion 2".
  • the conditions may, as an alternative, be such that th ray intersects the axis 1 again at a point between F and B.
  • the electron microscope With the electron microscope the conditions are as described above, with this difference that the electron does not originate from the left, but from the right from point 0, where, for example, the electron source may be arranged.
  • the point F is the image point associated with the lens formed by the portion of the magnetic field which operates on the stretch FE.
  • F is the image point of the point 0' associated with this lens, which is located in a plane I through point 0 at right angles to the axis I.
  • the figure shows an arbitrary ray, which emanates as a straight line 3" from point 0' and winds as a curved line 3' through the magnetic field in a manner such that it passes through the point F and emerges from the lens as a straight line 3.
  • the magnetic field does not contribute to the amplification, but it only constitutes an undesired condenser, which, moreover, requires ampere turns. If the pole spacing of the objective lens is chosen so as not to exceed the distance AF plus the distance FP from the focal point to the object, the maximum obtainable lens stretch is obtained by means of a minimum number of ampere turns.
  • the objective lens has a focal point between the pole shoes and a pole spacing which exceeds the distance AF by so little that the field between the first pole shoe and the focus F has no adverse effeet. This is rendered possible by reducing the diameter of the lens bore to one third or even a smaller part of the pole spacing.
  • the magnetic electron lens may be assumed to have a focal distance.
  • the distance i between this point of intersection and the plane II is now defined as the focal distance of the lens.
  • This focal distance is determined not only by the magnetic field strength, but also by the diameter of the bore.
  • the bore has hitherto been chosen so as to be not much smaller than the pole spacing, since the narrower the bore, the more difficult it is to maintain the circular shape of the bore, a non-circular bore giving rise to astigmatism.
  • the magnetic field has a much greater refractive power than on the stretches a and c.
  • the strength of a lens element included between two planes at right angles to the axis, spaced apart by a distance ds is proportional to H ds with a given number of ampere turns, i. c. with a given value of the surface included between the curve Ic and the axis the portions a and c are restricted as much as possible, that is to say, if the surface is concentrated as much as possible on the stretch b, a stronger lens is obtained by means of the same number of ampere turns.
  • Fig. 2 relates to a lens, the rays of which are incident from the right-hand. the por' tion 0 does not contribute to the amplification, if it is located in front of the object, as is evident from the description with reference to Fig. 1.
  • a similar phenomenon is known from optics.
  • a weak lens which is spaced apart from a stronger lens by more than a definite distance has the effect that the focal distance of the combination exceeds that of the Strong lens alone. For thin lenses this definite distance is the focal distance of the weaker lens.
  • Fig. 3 of the accompanying drawings illustrates the adverse eifect of the offshoot of the curve K shown in Fig. 2.
  • Fig. 3 shows the path 5 of an electron entering the magnetic field at A1. This field is first weak, so that on the stretch a the ray (51) is curved only slightly. Beyond this stretch the electron path is curved more strongly and it intersects the axis at point F1. The focal distance is indicated in this case by ii,
  • the ray (A2) would be curved more strongly, so that it would intersect the axis at point F2 at a larger angle to the axis.
  • the focal distance would have been smaller, i. e. f2.
  • the distance of the ray from the axis, at the area where the ray enters the strong field is smaller and to this distance the radius of the curvature of the path is inversely proportional.
  • the objective lens of an electron microscope according to the invention thus has a combination of properties, resulting ultimately in an improved quality of the lens; these properties may be summarized as follows:
  • the maximum field strength is so great that the magnetisation of the iron touches the limit of saturation.
  • the value of the maximum field strength varies with the quality of the magnetic material.
  • For the conventional iron it amounts to 21,400 Gauss, for an alloy made substantially of iron and cobalt in a ratio of 3:2 it may be increased to 23,000 Gauss.
  • the pole distance is suificiently large to arrange the object between the focus located between the pole shoes and the first pole shoe, but it is so small that the magnetic forces operating in front of the focus have no adverse effect. It has been found that for this purpose it need not be chosen greater than the value dmax, which is expressed in millimetres by the formula:
  • E designates the voltage in kvs., by which the electrons are accelerated.
  • the diameter of the bore is not more than one third of the pole spacing.
  • FIG. 4 of the accompanying drawings shows one embodiment of an objective lens for use in an electron microscope according to the invention in a sectional view taken in a plane through the axis.
  • ll designates part of the iron wallof the microscope.
  • An annular chamber,-'formed by the two yoke plates. l2. and I3 and the intermediate piece [4, all three of ferromagnetic material, comprises a coil cylinder [5 of brass. The winding space of this coil cylinder is divided into two portions I! and 18 by an intermediate flange l6, each of these portions comprising one excitation coil.
  • pole shoes 23 and 24 To the tongues I 9 and 20 of the yoke plates are secured pole shoes 23 and 24 by means of the nuts 2
  • the plates I2 and I3 and the pole shoes 23 and 24 have a central bore, so that continuous channels 25 and 26 are formed, opening into the object chamber 21.
  • the intermediate flange l 6 has a hole 28, which accommodates an object holder (not shown).
  • the channels 25 and 26 and the object chamber 21 together form a space which may be evacuated. This space is shut in an airtight manner from the open air by means of stufling boxes 29 and 30 and the rings 3
  • An electron ray is passed through the evacuated space exactly along the axis, i. e. from the channel 26 across the object chamber 21 to the channel 25.
  • the front surface of the pole shoes is shaped in the form of a truncated cone. Their fiat parts are spaced apart by adistance of 1.4 mms. With a bore in the pole shoes of a diameter of 0.3 mm. at the surface the lens has a focal distance of 0.7 mm. with an electron velocity obtained by means of an acceleration voltage of kvs. This requires a number of ampere turns of about 3000.
  • the distance of the first focus from the front surface of the pole shoe 24 is, in this case, 0.3 mm.
  • Fig. 5 of the accompanying drawings is a detail view on an enlarged scale.
  • the object surface 34 is indicated by a broken line. It may be located at a distance of 0.2 mm. from the front surface 35 of the pole shoe 24, i. e. at a distance of 0.1 mm. from the focus 36. At a distance of 10 cms. the lens described above may produce an amplification of times.
  • this lens With a variation of the operating voltage of A2 to 1% and of the energizing current of A to /z% (these values vary slightly with the contrast sharpness of the image) this lens still has a resolving power of 60 A., owing to the small focal distance of 0.7 mm., which is obtained by the suppression of the offshoots of the field.
  • This has the important advantage that the supply device for the microscope can be considerably simpler, since the electron-technical precision device for stabilisation is dispensed with.
  • An electron microscope comprising an electromagnetic objective lens including a pair of pole shoes each of which have a bore and including means to produce a magnetomotive force which increases the magnetisation in the pole 7 shoes to'the limit otsaturation, said pole slices being spaced apart a. distance at which the foci of the lens are located between the: pole shoes and which does not exceed a value dmax, which is expressed in millimetres hy the formula:
  • E designates the voltage in kvs., by means of which the electrons are accelerated, the diameter of the bore being not more than onethird the spacing between the pole shoes.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Electron Sources, Ion Sources (AREA)
US289685A 1951-06-15 1952-05-23 Electron microscope Expired - Lifetime US2670450A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
NL302722X 1951-06-15

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US2670450A true US2670450A (en) 1954-02-23

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US (1) US2670450A (cs)
BE (1) BE512056A (cs)
CH (1) CH302722A (cs)
DE (1) DE918464C (cs)
FR (1) FR1071047A (cs)
GB (1) GB711672A (cs)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3035198A (en) * 1957-03-13 1962-05-15 Philips Corp Deflection and focusing apparatus for cathode ray tubes

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1614407B1 (de) * 1967-01-13 1972-08-03 Siemens Ag Anordnung zur erzeugung eines den korpuskularstrahl in einem korpuskulargeraet, insbesondere elektronenmikroskop, beeinflussenden elektromagnetischen feldes

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2418349A (en) * 1945-12-13 1947-04-01 Rca Corp Method of and means for correcting for distortion in electron lens systems
US2438971A (en) * 1946-10-31 1948-04-06 Rca Corp Compound electron objective lens
US2472315A (en) * 1948-03-30 1949-06-07 Rca Corp Varying the gap spacing of pole pieces for electron optical apparatus

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2418349A (en) * 1945-12-13 1947-04-01 Rca Corp Method of and means for correcting for distortion in electron lens systems
US2438971A (en) * 1946-10-31 1948-04-06 Rca Corp Compound electron objective lens
US2472315A (en) * 1948-03-30 1949-06-07 Rca Corp Varying the gap spacing of pole pieces for electron optical apparatus

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3035198A (en) * 1957-03-13 1962-05-15 Philips Corp Deflection and focusing apparatus for cathode ray tubes

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Publication number Publication date
CH302722A (de) 1954-10-31
DE918464C (de) 1954-09-27
GB711672A (en) 1954-07-07
BE512056A (cs)
FR1071047A (fr) 1954-08-24

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