U.S. patent number 3,569,698 [Application Number 04/834,316] was granted by the patent office on 1971-03-09 for particle-beam apparatus provided with a phase-shifting foil which corrects for wave aberrations.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Karl-Heinz Herrman.
United States Patent |
3,569,698 |
Herrman |
March 9, 1971 |
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.
Inventors: |
Herrman; Karl-Heinz (Berlin,
DT) |
Assignee: |
Siemens Aktiengesellschaft
(Berlin, DT)
|
Family
ID: |
4354970 |
Appl.
No.: |
04/834,316 |
Filed: |
June 18, 1969 |
Foreign Application Priority Data
|
|
|
|
|
Jun 28, 1968 [CH] |
|
|
9724/68 |
|
Current U.S.
Class: |
250/398;
335/210 |
Current CPC
Class: |
H01J
37/263 (20130101); H01J 37/266 (20130101); H01J
37/153 (20130101); H01J 2237/1534 (20130101); H01J
2237/2614 (20130101) |
Current International
Class: |
H01J
37/26 (20060101); H01J 37/04 (20060101); H01j
037/26 () |
Field of
Search: |
;250/49.5 (3)/ ;250/49.5
(4)/ ;335/210 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Birch; A. L.
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.
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.
* * * * *