US3596090A

US3596090A - Particle beam apparatus having an imaging lens which is provided with an associated phase-displacing foil

Info

Publication number: US3596090A
Application number: US813629A
Authority: US
Inventors: Walter Hoppe
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-04-16
Filing date: 1969-04-04
Publication date: 1971-07-27
Anticipated expiration: 1988-07-27
Also published as: DE1810818B2; NL170900B; DE1810818A1; GB1259352A; NL6902406A; NL170900C

Abstract

A particle beam device has a longitudinal axis and a beamgenerating portion for issuing particle beams along the axis. A holder is provided for accommodating a specimen in the path of the beams and a particle beam imaging lens is disposed beyond the specimen locality coaxial with the axis. A foil is disposed in the lens in the path of the particle beams for shifting the respective phases of the latter and scattering the incident particles of the beams in bunches in distinct directions. The beam particles scattered in at least one of these directions are blocked by a diaphragm disposed beyond the foil.

Description

United States Patent inventor Walter Iloppe Schiller-stun: 46, 8000 Munich I5, Germany Appl. Nov 813,629 Filed Apr. 4, I969 Patented July 27, I97] Priority Apr. 16, I968 Switzerland 5,586/68 PARTICLE BEAM APPARATUS HAVING AN IMAGING LENS WHICH IS PROVIDED WITH AN ASSOCIATED I'IIASE DISPLACING FOIL l8 Claims, 2 Drawing Figs.

US. Cl 250/495 A [56] References Cited UNITED STATES PATENTS 3,469,096 9/ I 969 Hanssen 250/49. 5 3,500,043 3/1970 Hanssen 250/495 Primary ExaminerWilliam F. Lindquist Attorneys-Curt M. Avery, Arthur E. Wilfond, Herbert L.

Lerner and Daniel .I. Tick ABSTRACT: A particle beam device has a longitudinal axis and a beam-generating portion for issuing particle beams along the axis. A holder is provided for accommodating a specimen in the path of the beams and a particle beam imaging lens is disposed beyond the specimen locality coaxial with the axis. A foil is disposed in the lens in the path of the particle beams for shifting the respective phases of the latter and scatlering the incident particles of the beams in bunches in distinct directions. The beam particles scattered in at least one of these directions are blocked by a diaphragm disposed beyond the foil.

PATENIEU JUL27I97| 3 596 090 SHEEI 1 BF 2 Fig. 1

PATENTFJ] JUL27 ISH SHEET E OF 2 Fig. 2

PARTICLE BEAM APPARATUS HAVING AN IMAGING LENS WHICH IS PROVIDED WITH AN ASSOCIATED PHASE-DISPLACING FOIL My invention relates to particle beam apparatus having a radiation portion normally incorporating beam-generating and condenser systems. The apparatus is also equipped with at least one imaging lens arranged in the beam path beyond the '0 specimen, and associated with the lens is a foil for displacing or shifting the phases of the imaging particle beams. The foil can be commonly assigned to several lenses. However, as a rule, it is preferable to eliminate any disturbance in the particle beam caused by errors in the first image lens by arranging a foil directly behind the lens. This arrangement is preferred because image lenses in beam direction magnify the influences caused by errors of the first lens, these errors being especially pronounced relative to the quality of the image.

In light optics as well as in particle-beam apparatus it is known that certain phase conditions occurring during the imaging of objects or specimens are established by using phase-displacing devices, which for particle beam apparatus are phase displacing foils. In this connection reference may be had to: The proceedings of the European Regional Conference on Electron Microscopy," Delft 1960, pages 18 to 24. These phase-displacement devices do not create any fundamentul problems when used in light optics. However, this is not the case with particle beam apparatus of which the principal representatives are the electron microscope and diffraction devices.

It is known that the phase-displacing foils produced from the materials customarily used for this purpose not only provide the desired phase displacement, but, also diffusely scatter the particles such that the scattered particles are superimposed upon the particles which contain the actual information at the image locality thereby reducing the quality of the image.

However, in the production of electron-microscopic recordings or photographs of objects or specimens at high resolving powers, for example, it is necessary to ensure that the beams diffracted in the specimen have precisely defined phase conditions relative to the primary beam. This is of particular importance in connection with the utilization of the phase-contrast effect during electron-microscopic investigations which are conducted on thin specimens carried at high resolution and small radiation aperture. In such specimens, the particle beam which is used to investigate or form the image is not influenced with respect to the amplitude of but rather, with respect to the phase of the impinging waves. This occurs because the specimen details produce a potential distribution within the specimen itself.

According to the rule of Zernicke, the beams diffracted in the specimen of at least a positive and negative first order must have a phase displacement relative to the primary beam of n'180 if the phase specimen is to provide an amplitude image, the quantity :1 being a whole number including zero.

On the other hand, the diffracted beams in any case have a phase displacement of 90 with respect to the primary beam. Therefore, in order to fulfill the aforementioned condition, the diffracted beams must be phase-displaced by an additional 90, either positively or negatively. With a large aperture for individual lens zones, such phase displacements can be produced merely by wave aberrations of the lens which are caused by apertural errors and any defocusing which may be present.

With reference to the undesired phenomenon already referred to, namely, that relating to the occurrence of omnidirectional scattering of particles when using phase-displacement foils in particle beam apparatus, an additional phase displacement is dispersed with and according to German Pat. No. l,222,603, a diaphragm having several electronperrneable and electron-nonpermeable zones is placed in the path of the beam passing from the objective lens of an electron microscope and arranged in such a manner that the diaphragm permits only the elementary waves issuing from the image-side wave surface of the lens system which are of unidirectional phase to arrive at an arbitrary point of incidence. The word unidirectional relates to the phases of those waves whose associated space frequencies are imaged or reproduced either with positive or with negative contrasts only. With this diaphragm arrangement, known as a zone diaphragm, the waves which do not meet the phase conditions required for forming an image can therefore not arrive at the image plane.

A certain disadvantage of the zone diaphragm becomes evident when the principles of imaging which utilize the phase-contrast effect, discovered during the past few years, are considered. It was found convenient in theoretical investigation to treat the specimen according to Fourier as being combined of sinusoidal phase lattices of various space frequencies. The total of all space frequencies represents the total of all specimen points. In the present case, it is of special importance that different zones of the image lens plane are responsible for transmitting various space frequencies into the image plane. With respect the known zone diaphragm, this fact indicates that the masking of specific waves can sometimes result in certain losses of information. This is recognized in the publication of the Sixth International Congress for Electron Microscopy held in Kyoto, I966, wherein page 39 there is suggested an imaging method that uses such zone diaphragms, In this connection, a plurality of zone diaphragms of respectively different dimensions are used at different focusing conditions to make photographs in a sequence of the same specimen regions. These photographs were superimposed to form a composite in which as many as possible of the space frequencies contribute to produce the resulting image.

It is an object of my invention to provide a foil which eliminates the above disadvantages associated with the known phase-displacing devices.

It is another object of my invention to provide a foil for shifting the phases of imaging particle beams. More specifically, it is an object of my invention to provide a foil which scatters the particles impinging on the specimen in bunches in discrete directions.

To achieve these objects and according to a feature of the invention, I provide a foil made of a material which scatters the impinging particles, in bunches, in distinct directions. In addition, l arrange beyond the foil in the beam path diaphragms which block the scattered particles.

Hitherto, the foil materials used produced diffuse scattering of the beam particles, whereas the present invention employs a foil with a preferential structure so that although there is inevitably some scatter, this is in discrete spatial directions and is also accompanied by bunching of the beam particles. In contrast to known foils for this kind of application, therefore it is now possible to block the scattered beam particles, so that only the central beam is effective in generating the image, the particular appropriate phase conditions still being maintained.

Generally speaking, the correcting foil will be of crystal so that the bunching and scattering of the corpuscles is depen dent upon the crystal structure. A monocrystal foil has been found to be particularly suitable, especially since foils of this kind can be produced with a particularly flat surface.

However, this does not exclude the use of other materials for the foils, for example synthetic materials having a structure such that the scatter angles are adequate for blocking purposes.

If a crystalline foil is used, this may be of graphite or silicon, for example. The correcting foil can be arranged in the rear focal plane of the lens in just the same way as the known zone diaphragm. Alternatively, it can be located in the plane of an aperture diaphragm associated with the lens, this even if the diaphragm is not located in the focal plane, so that the provi sion of an additional mounting for the foil is rendered unnecessary. The aperture diaphragm can be constructed to contain the foil.

The Zernike condition has already been referred to hereinbefore, in accordance with which the beams diffracted in the specimen should have a phase of H with respect to the primary beam, and it was also explained that fundamentally there is only a phase difference of 90. It is possible, as those skilled in the art will appreciate, to make the thickness of the correcting foil used in accordance with the invention constant, and therefore to achieve a constant phase-shifting effect in all zones of the foil, with the exception of the central zone through which the primary ray passes without undergoing any phase shift. in accordance with the condition just referred to the foil thickness will be so chosen that the phase shift of 90 is positive or negative.

This kind of dimensioning of the foil, however, generally only approximately satisfies the phase condition because the beams have already undergone phase shift as a consequence of the wave aberration of the particular lens involved, which phase shift will be dependent upon the distance from the lens axis. Thus, if the above phase condition is to be satisfied exactly, then the thickness of the foil and therefore the phaseshifting efi'ect which it produces, must be chosen differently in different areas of the foil in order that the phases (determined by the wave aberration of the lens and the phase-shifting effect of the foil) of all the particle beams have approximately the same value relative to the primary beam. In accordance with what has been said in the foregoing, this value will be n-lSO where n=0, 1, 2,...etc.

The known type of zone diaphragm may be modified in such a way that the zones thereof which are impermeable or opaque to the beam particles are replaced by zones which produce a phase-shifting effect relative to the beams passing through the permeable or transmissive zones, the modification being provided by having the foil of the invention contain a pattern corresponding to that of the zone diaphragm. The phase shift in these zones must, to accord with the laws governing exclusively positive or negative contrast in reproduction, be l80 or multiple thereof. Outside these zones, the foil will be so dimensioned that on transition to the neighboring impermeable zones, no changes of phase occur. As is well known, in the conventional form of zone diaphragm, penneable and impermeable annular zones alternate with one another provided that the associated lens has no axial astigmatism; failing this, there are departures from the circular form, to configurations which are roughly elliptical in a first approximation. In order to produce a zone diaphragm correcting foil first of all a negative in the form ofa metal foil can be produced and then, by using ion etching through said negative, a monocrystalline foil can be etched away at those areas to which the negative allows the ion beam to pass. This process can be applied in a corresponding manner to the manufacture of a variable thickness foil.

Up to now, the chief emphasis has been placed upon the design and dimensioning of the phase-shifting foil. The diaphragms provided to block the scattered beam particles can be located anywhere behind the foil, considered in the direction of the radiation, provided they are intended simply to block the scattered particles without disturbing the main image, i.e. the image of zero order. In the embodiment to be illustrated presently there is provided, in the plane of the inter mediate images produced by the beam particles scattered by the foil, a field-of-view diaphragm which blocks these intermediate images but passes the main image. Although, when using a polycrystalline foil, the secondary images are superimposed upon one another in the Deby-Scherror ring, as those skilled in the art will appreciate, this is of no significance here because the secondary images are blocked out.

lt is of course essential that the main image and the secondary images should not overlap one another. For this reason, the beam-generating section of the apparatus should be arranged to direct onto the specimen a particle beam of such a slight cross section (fine zone illumination), that the different images are located in separate areas of the intermediate image plane in which the diaphragm is located.

The invention will now be described with reference to the accompanying drawings in which:

FIG. 1 schematically illustrates those pans of the particle beam apparatus which are essential to the invention; and

FIG. 2 is a broken-out view of a microscope column in which is illustrated, in section, an imaging lens provided with an associated phase-displacing foil according to my invention.

In FIG. 1, an object 1 is irradiated by a fine electron beam. After the object 1, the beam passes through an objective lens 2, illustrated in purely schematic fashion, which may be an electrostatic or electromagnetic lens, and which may be followed by other lenses, not shown. in the image-side focal plane of the lens 2 a phase-shifting correcting foil 3 is located, and in this particular example this is a monocrystalline foil. In the intermediate image plane there is a main image 4 produced by the central beam, and also

secondary images

5 and 6 produced by electrons scattered in the foil 3. Due to the structure of the particular material used for the foil 3, the electrons are scattered in a bunched way in discrete spatial directions. However, the

secondary images

5 and 6, and any other secondary images which may occur, but which have not been shown in the figure, can be blocked and in this embodi ment this is achieved by a ficld-of-view diaphragm 7 arranged in the intermediate image plane, which passes the central beam, said diaphragm 7 having individual zones which, under the influence of the phase-shifting effect of the foil 3, present the requisite phase values for image production, and being employed to obtain the electron microscopic image of the specimen.

PK]. 2 illustrates the relevant part of an exemplary electron microscope embodiment. The microscope has a column ll in which there is located an objective lens 12, for the magnified reproduction of an object held in vacuum-sealed fashion in an object cartridge 13. The essential components of the objective lens 12 are an upper pole piece 14 and a lower pole piece 15 between which the lens gap is located. In this case an elec tromagnetic lens is utilized having winding 16 which develops a flux that passes through an iron circuit l7, the two pole pieces 14 and I5 and the lens gap.

In the vicinity of the lens gap, a nonmagnetic perforated plate 18 is provided for the passage of a diaphragm drive system 19 carrying an aperture diaphragm 110. The aperture diaphragm is so constructed that it also functions as the mounting for a phase-shifting foil 111, for example a monocrystalline foil, which foil may be designed as a zone diaphragm. A drive 112 provides for the transverse displace ment of the diaphragm 110.

After the objective lens 12 in beam direction, there is disposed a further nonmagnetic ring 113, for example of brass, which locates a mounting "4 for a field-of-view diaphragm 115, which in this embodiment blocks the electrons scattered in the foil "1. This mounting 114 is provided with a drive arrangement "6 which extends in vacuum-tight fashion through the wall ofthe column 11.

The diaphragms serving to block the beam particles scattered in the foil, can comprise parts of the corpuscular beam apparatus which are already present, such as suitable flanges or projections.

Upon studying this disclosure it will be obvious to those skilled in the art that my invention is amenable to various modifications with respect to details and can be given embodi ments other than that particularly illustrated and described herein, without departing from the essential features of my invention and within the scope of the claims annexed hereto.

lclaim:

t. In a particle beam device which has a longitudinal axis, beam-generating means for issuing a particle beam along said axis, means for accommodating a specimen in the path of said beam, an electrooptical imaging lens disposed beyond the specimen locality coaxial with said axis, foil means having a crystalline structure and being disposed beyond the specimen locality in the path of said particle beam for shifting the phases of the resulting diffracted beams and for scattering the incident particles of said diffracted beams in bunches in distinct directions, and blocking means disposed beyond said foil means for blocking the beam particles scattered in at least one of said distinct directions.

2. The combination of claim 1 wherein said particle beam is an electron beam.

3. The combination of claim 2 wherein said particle beam device is an electron microscope.

4. The combination of claim 1 wherein said foil means is a monocrystalline foil.

5. Tile combination of claim 1 wherein said foil means ifa foil consisting of crystalline graphite.

6 The combination of claim 1 wherein said foil means is a foil consisting of crystalline silicon.

7. The combination of claim 1 wherein said imaging lens has two focal planes, one of said focal planes being spaced from said beam-generating means a larger distance than the other of said focal planes, said foil means being disposed in said one focal plane.

8. The combination of claim 1 wherein said imaging lens has an aperture diaphragm disposed therein, said foil means being arranged in the plane of said diaphragm.

9. The combination of claim 1 wherein said particle beam issuing from said beam-generating means has beyond the specimen locality a primary central portion on said axis and portions diffracted by a specimen placeable at said specimen locality, and wherein said foil means is a foil of constant thickness so that the phase of said diffracted portions incident thereon are shifted in phase the same amount except for a cen' tral region in said foil through which said central portion of said particle beam passes without being shifted in phase.

10. The combination of claim 9 wherein said thickness of said foil is selected for shifting the phase of said diffracted portions incident thereon by 90.

11. The combination of claim 1 wherein said particle beam issuing from said beam-generating means has beyond the specimen locality a primary central portion on said axis and portions diffracted by a specimen placeable at said specimen locality, said diffracted portions surrounding said central portion and having different phases resulting from wave aberration in said lens and phase shifting in said foil, and wherein said foil means is a foil having a thickness varying along directions radial of said axis, so that said different phases have approximately the same value relative to said central portion.

The combination of claim 11 wherein said phase value is substantially nl where n is a whole number or zero.

13. The combination of claim l and wherein said scattered beam particles form a main and secondary images in a plane beyond said foil means, said blocking means being disposed in said plane for blocking said secondary images and passing said main image.

14. The combination of claim 13 wherein said beamgenerating means issues a fine-zone illumination beam, and said main and said secondary images are situated in respective mutually separate regions of said plane.

15. The combination of claim 1 wherein said foil means is a diaphragm having mutually separate beamtransmissive and beam'opaque zones.

16. The combination of claim 1 wherein said foil means is a diaphragm having mutually separate beam-transmissive and beam-phaseshifting zones.

17. The combination of claim 16 wherein at least one of said beam-phase-shifting zones has a thickness corresponding to a shift of the phase of a beam incident thereon by 18. The combination of claim 1 wherein said blocking means includes an auxiliary diaphragm for blocking said beam particles scattered in at least one of said distinct directions.

Claims

1. In a particle beam device which has a longitudinal axis, beam-generating means for issuing a particle beam along said axis, means for accommodating a specimen in the path of said beam, an electro-optical imaging lens disposed beyond the specimen locality coaxial with said axis, foil means having a crystalline structure and being disposed beyond the specimen locality in the path of said particle beam for shifting the phases of the resulting diffracted beams and for scattering the incident particles of said diffracted beams in bunches in distinct directions, and blocking means disposed beyond said foil means for blocking the beam particles scattered in at least one of said distinct directions.

2. The combination of claim 1 wherein said particle beam is an electron beam.

5. THe combination of claim 1 wherein said foil means if a foil consisting of crystalline graphite.

6. The combination of claim 1 wherein said foil means is a foil consisting of crystalline silicon.

9. The combination of claim 1 wherein said particle beam issuing from said beam-generating means has beyond the specimen locality a primary central portion on said axis and portions diffracted by a specimen placeable at said specimen locality, and wherein said foil means is a foil of constant thickness so that the phase of said diffracted portions incident thereon are shifted in phase the same amount except for a central region in said foil through which said central portion of said particle beam passes without being shifted in phase.

10. The combination of claim 9 wherein said thickness of said foil is selected for shifting the phase of said diffracted portions incident thereon by 90*.

11. The combination of claim 1 wherein said particle beam issuing from said beam-generating means has beyond the specimen locality a primary central portion on said axis and portions diffracted by a specimen placEable at said specimen locality, said diffracted portions surrounding said central portion and having different phases resulting from wave aberration in said lens and phase shifting in said foil, and wherein said foil means is a foil having a thickness varying along directions radial of said axis, so that said different phases have approximately the same value relative to said central portion. The combination of claim 11 wherein said phase value is substantially n.180*, where n is a whole number or zero.

13. The combination of claim 1 and wherein said scattered beam particles form a main and secondary images in a plane beyond said foil means, said blocking means being disposed in said plane for blocking said secondary images and passing said main image.

14. The combination of claim 13 wherein said beam-generating means issues a fine-zone illumination beam, and said main and said secondary images are situated in respective mutually separate regions of said plane.

15. The combination of claim 1 wherein said foil means is a diaphragm having mutually separate beam-transmissive and beam-opaque zones.

16. The combination of claim 1 wherein said foil means is a diaphragm having mutually separate beam-transmissive and beam-phase-shifting zones.

17. The combination of claim 16 wherein at least one of said beam-phase-shifting zones has a thickness corresponding to a shift of the phase of a beam incident thereon by 180*.

18. The combination of claim 1 wherein said blocking means includes an auxiliary diaphragm for blocking said beam particles scattered in at least one of said distinct directions.