Particle-beam Apparatus Provided With A Phase-shifting Foil Which Corrects For Wave Aberrations

Abstract

A particle-beam apparatus has a beam axis and a beam generator for issuing particle beams along the axis including a primary beam coincident with the axis. An electron-optical lens for focusing an image on an image plane causes wave aberrations that shift the respective phases of the particle beams. An foil is disposed in the path of the beams and shifts the phases of the beams with respect to the phase of the primary beam in dependence upon their respective radial distances from the axis so that the phases are shifted to correct for the shift caused by the wave aberrations. The foil has concentric discontinuities in its thickness at which the phase-shifting effect of the foil decreases in steplike fashion in radial distance, by values which correspond to whole number multiples of the wavelength of the particle beam, each adjacent pair of the discontinuities enclosing an annular foil portion having a thickness which increases continuously in radial direction.

Description

My invention relates to particle beam apparatus of the type used for studying a specimen, for example, an electron microscope. One such microscope is described with regard to construction and operation in an article which I coauthored, namely "ELMISKOP 101 - a new High-Power Electron Microscope," Siemens Review, Vol. 36, No. 2, Feb. 1969, pages 57 to 67.

In a particle-beam apparatus, a beam generator issues beams along a beam axis, the beams being diffracted in the specimen. The beam axis extends through an objective lens and through a phase-correcting foil having a phase-shifting characteristic that varies as a function of the radial distance from the beam axis. The objective lens causes wave aberrations which are responsible for undesirable shifts in the respective phases of the beams with respect to the phase of a primary beam coincident with the beam axis. The foil corrects for the path-dependent phase-shift of the diffracted beams that is due to the aberration of the objective lens, to give all diffracted beams an overall phase-shift of a predetermined value, and so provide the phase condition for achieving a sharp contrast in an image plane. Preferably, the foil is arranged in the rear focal plane of the objective lens. Wave aberration and its influence on diffracted beams is also considered in the copending U.S. application of Karl Josef Hanssen, entitled: "Corpuscular Ray Device for Phase or Amplitude Specimen with a Phase-Rotating Foil," Ser. No. 595,793, filed Nov. 21, 1966 which is now Pat. No. 3,469,096.

In the case of a specimen whose details predominantly influence the phase of the incoming waves, there are two diffracted beams disposed symmetrically to the incident direction of the beams behind the object, and these defracted beams exhibit a phase-shift of .lambda./4 with respect to the primary beam, .lambda. being the wavelength of the particle beam. To obtain a good contrast in projection, this phase-shifting must be made reversible by the projecting system. Thus, for projection with positive phase contrast, if W designates the total phase-shift (total aberration) which the particle beam must experience at a distance r from the primary beam, the two following relationships result:

W = 0 for r = 0 (1 )

W = (n+1/4). .lambda. for r 0 (2)

where n = 0, .+-.1, .+-.2, ...

For negative phase contrast, equation (2 ) is replaced by the following relationship:

W = (n+3/4). l ). .lambda. for r 0 (3 )

For positive amplitude contrast, equation (2 ) is replaced by:

W = n ..lambda. for r 0 (4 )

and for negative amplitude contrast, by equation (2 ) is replaced by:

W = (n=1/2 ). .lambda. for r 0 (5 )

It would thus appear that the condition for good contrast in projection can be fulfilled by the provision of a phase-shifting foil designed to give all diffracted beams the same phase-shift, i.e. .lambda./4 for positive phase contrast. However, such a solution would not take into account the fact that the objective lens itself causes a wave aberration in accordance with the following equation which is itself dependent on the aperture error constant C.sub.o of the lens, on the relevant defocusing z (i.e. on the relevant lens excitation), and on the radial distance r from the primary beam:

For this reason, the condition for a contrast-rich projection can only be fulfilled when a phase-shifting foil is provided whose phase-shift effect is dependent on each radius r, i.e. which for each distance from the primary beam satisfies the relationship:

W.sub. F = W - W .sub.L ( 7)

where W .sub.F is the wave aberration of the foil, W .sub.L is the wave aberration of the lens in accordance with equation (6 ), and W is the necessary total aberration through the projection system.

The requirements for a foil which will satisfy equation (7 ) will now be described with reference to FIG. 1, which corresponds to FIG. 2 of the above-mentioned application Ser. No. 595,793, which is now Pat. No. 3,469,096 FIG. 1 is a plot of wave aberration W (r ) against radial distance r of the diffracted beam from the primary beam.

Several curves are shown in FIG. 1 for various values of the defocusing z as parameter. If it is firstly assumed that the excitation of the lens has such a value that a course of the wave aberration W of the lens is set in dependence upon the radial distance r from the primary beam corresponding to the curve a, the phase-shifting foil must possess a phase-shift effect corresponding to the course characterized by the shaded area. From this graphical illustration, it can be seen that it is not sufficient to use a phase-shifting foil whose phase-shift effect is constant for all the pertinent values of r, but that the thickness of the foil, which as is known is generally very nearly proportional to the phase-shift effect, must vary, and becomes very great above a given value of the radial distance r from the primary beam. This causes few difficulties in the production of the foil, but more difficulties are presented by the distribution of the particles in such a thick material layer. As is known, this distribution leads to an impairment of contrast, particularly in an electron-microscope image. Here, the great differences in thickness can furthermore make their presence felt in disadvantageous fashion, leading to a different distribution in dependence upon radial distance. In this way, the beams responsible for a high resolving power cannot pass undisturbed to the image plane.

It is an object of my invention to provide a foil for use in a particle-beam apparatus which overcomes the foregoing disadvantages.

It is another object of my invention to provide a foil which shifts the phase of diffracted beams to correct for wave aberrations caused by an electro-optical lens.

It is still another object of my invention to provide a foil which establishes a phase condition in defracted beams for achieving a sharp contrast of an image in an image plane.

The invention consists in a particle beam apparatus of the type used for studying the phase or amplitude characteristics of specimen, in which a beam generator issues beams along a beam axis including a primary beam coincident with the beam axis, the beams being diffracted in the specimen. The axis of the beam extends through an electron-optical lens and through a phase-correcting foil having a phase-shifting characteristic that varies as a function the radial distance from the beam axis in such a manner that the path-dependent phase-shift of the diffracted beams due to the aberration of the lens is corrected to give all diffracted beams an overall phase-shift of a predetermined value with respect to the phase of the primary beam and so provide the phase condition for obtaining a good contrast image in the image plane.

According to a feature of the invention, the foil is provided with concentric discontinuities in its thickness at which the phase-shifting effect of the foil decreases in steplike fashion in the radial direction, by values which correspond to whole number multiples of the wavelength of the particle beam, and wherein each neighboring pair of discontinuities encloses an annular foil-portion whose thickness increases continuously in the radial direction.

The electron-optical lens may be the objective lens, in which case the correcting foil is adapted to shift the phase of diffracted beams to satisfy the phase-conditions for a contrast-rich projection in the image plane. The periodic nature of the phase-shifting corrections necessary in accordance with the equations (2 ) to (5 ) in conjunction with (7 ), by whole number multiples of the corpuscular beam wavelength, enables the maximum required thickness of a phase-correcting foil for compensating the wave aberration of a given lens to be considerably reduced. In this respect, it has proved particularly expedient, if each of the discontinuities lies at a radial distance from the axis of the primary beam, at which the minimum thickness of the annular foil portion surrounding that discontinuity has a thickness corresponding to the minimum thickness of the foil portion immediately surrounded by that discontinuity.

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

FIG. 1, as already mentioned, is a plot of wave aberration W (r) against radial distance r of the diffracted beam from the primary beam;

FIG. 2 is a graphical representation of the curve a of FIG. 1 on an enlarged scale, indicating how the required steplike discontinuities can be derived;

FIG. 3 is a radial cross section of a foil according to the invention which corresponds to the representation of FIG. 2.

FIG. 4 is a broken-out view of a microscope column in which is illustrated, in section, an imaging lens provided with an associated phase-shifting foil according to the invention.

Initially, a given lens excitation, i.e. a certain defocusing z, must be selected, at which equation (2) to (5) give a technically possible inner annular portion thickness of the foil, for n = o. The curve a in FIG. 1 is particularly favorable, since it possesses a producible thickness d.sub.m (see FIG. 2). If the foil thickness then varies with radial distance in a regular manner, the cross section indicated by shading in FIG. 1 would be required, laying between the curve a and the broken line at .lambda./4. with increasing distance r, the foil thickness would then increase very rapidly. For this reason, a discontinuity in the foil thickness is provided at those curve positions whose ordinate value differ from the broken line in each case by a value n..lambda., each discontinuity in the foil thickness being at a value which corresponds to a phase-shift by .lambda.. This steplike arrangement is shown in FIG. 3, and the relationship with the shaded portion of FIG. 2 is readily apparent. In FIG. 3, the thickness d is plotted against the distance r from the axis of the primary beam. The innermost annular portion of the foil, which surrounds an aperture 2 through which the primary beam can pass, has a minimum at 1, and then increases with increasing distance r. The second annular portion 3 surrounding the innermost portion commences with a minimum thickness equal to that of the innermost portion, and increases to a thickness equal to the maximum thickness of the innermost portion, and actually corresponds to the section of curve a between - (3/4.lambda. + d.sub.m) and - (7/4.lambda. + d.sub.m). This thickness course is therefore obtained for n = -1. A sharp discontinuity 4, in the foil thickness d, corresponds to the phase-shifting by .lambda.. Further annular portions 5, 6, etc. follow in a similar manner, each succeeding portion being narrower in width, corresponding to values of n (n= -2, -3,... ) so that the thickness changes of the foil necessary for achieving the relevant continuous course of the phase-shifting effect are obtained.

As can be seen, the thickness of the foil, and also the difference in thickness of the various areas are limited. Thus, the distribution of particles in the phase-correcting foil can be kept so low that they have no significant adverse influence upon the contrast in the image.

In FIG. 4, a region of an objective lens of an electron-microscope is shown. The microscope column 1' of the microscope is partly broken out in order to show the specimen stage and the objective lens. The objective lens comprises the lens winding 2' which develops a magnetic flux that closes over the iron core 3' and the lens gap 4'. The phase-shifting foil 7 is held in the lens gap between the pole shoes 5' and 6' of the objective lens by a holding rod 8, which is connected to an operating knob 9 disposed outside the microscope. This permits the phase-shifting foil 7 to be centered during the operation of the microscope without breaking the vacuum. The specimen is situated at the lower end of the specimen cartridge 10 which is inserted into a specimen-shifting table 11.

Apart from electron-microscopes, the invention can be employed in any particle beam instrument in which certain phase conditions must be fulfilled for the contrast-rich reproduction of specimen or object structures. The correction obtained can be applied for compensating the aperture error of any lens, and is not limited to use with an objective lens, i.e. a correcting foil can be allotted to any lens along the beam path, for example, to a fine beam lens in a raster scan microscope, in whose image plane the object lies. The object for this lens is, as is known, the crossover in front of the cathode.

The required foil shape can be manufactured by introducing a thin particle-beam-permeable foil into a hydrocarbon atmosphere, where it is irradiated in areawise fashion by means of an electron-beam, with intensities and/or times selected so as to grow a carbon layer whose distribution over the foil cross section gives the desired variations in the phase-shifting effect across the foil. Alternatively, a carbon layer having any distribution, preferably constant, can be applied to the foil surface and the coated foil introduced into an oxygen atmosphere where it is irradiated in areawise fashion by means of an electron-beam, with intensities and/or times selected such that decomposition of the carbon layer produces the required distribution over the foil surface.

It is particularly favorable for good contrast, if the discontinuities between neighboring annular foil portions are as sharp as possible.

To those skilled in the art it will be obvious upon a study of this disclosure that my invention permits of various modifications and may be given embodiments other than particularly illustrated herein, without departing from the essential features of the invention and within the scope of the claims annexed hereto.

Claims

I claim:

1. a particle-beam apparatus having a beam axis comprising beam generating means for issuing particle beams along said axis including a primary beam coincident with said axis, particle-beam-optical lens situated about said axis for focusing an image in an image plane, said lens causing wave aberrations that shift the respective phases of said particle beams, and foil means disposed in the path of said beams for shifting said phases of said beams with respect to the phase of said primary beam in dependence upon their respective radial distances from said axis so that said phases are shifted to correct for said shift caused by said wave aberrations, said foil means having concentric discontinuities in its thickness at which the phase-shifting effect of the foil decreases in steplike fashion in a radial direction, by values which correspond to whole number multiples of the wavelength of the particle beam, each adjacent pair of said discontinuities enclosing an annular foil portion having a thickness which increases continuously in radial direction.

2. A particle-beam apparatus according to claim 1, said apparatus comprising means for accommodating a specimen before said foil means in the path of said beams, the latter being defracted by the specimen, said particle-beam-optical lens being an objective lens, and said foil means being a foil disposed in said lens and shifting the phase of said defracted beams so that said image is focused in said image plane with sharp contrast.

3. A particle-beam apparatus according to claim 1, said apparatus comprising means for accommodating a specimen before said foil means in the path of said beams, said particle-beam-optical lens being a high resolution lens disposed so that its image plane is coincident with the specimen locality, said foil means being a foil disposed in said lens.

4. In a particle-beam apparatus according to claim 1, each one of said discontinuities lying at a radial distance from said primary beam at which its outer adjacent annular foil portion has a thickness equal to the minimum thickness of its inner adjacent annular foil portion.