U.S. patent number 3,847,689 [Application Number 05/374,424] was granted by the patent office on 1974-11-12 for method of forming aperture plate for electron microscope.
Invention is credited to James C. Administrator of the National Aeronautics and Space Fletcher, Klaus Heinemann, N/A.
United States Patent |
3,847,689 |
Fletcher , et al. |
November 12, 1974 |
METHOD OF FORMING APERTURE PLATE FOR ELECTRON MICROSCOPE
Abstract
An electron microscope including an electron source, a condenser
lens having either a circular aperture for focusing a solid cone of
electrons onto a specimen or an annular aperture for focusing a
hollow cone of electrons onto the specimen, and an objective elns
having an annular objective aperture, for focusing electrons
passing through the specimen onto an image plane. The invention
also entails a method of making the annular objective aperture
using electron imaging, electrolytic deposition and ion etching
techniques.
Inventors: |
Fletcher; James C. Administrator of
the National Aeronautics and Space (N/A), N/A
(Sunnyvale, CA), Heinemann; Klaus |
Family
ID: |
26916015 |
Appl.
No.: |
05/374,424 |
Filed: |
June 28, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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221670 |
Jan 28, 1972 |
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Current U.S.
Class: |
216/75; 850/9;
216/78 |
Current CPC
Class: |
H01J
37/04 (20130101); H01J 2237/0451 (20130101); H01J
2237/0453 (20130101) |
Current International
Class: |
H01J
37/04 (20060101); C23f 001/02 () |
Field of
Search: |
;117/215,217,71,107,107.2,119 ;204/23 ;156/3,7,16,18
;250/49.5R,49.5A ;313/64,74,83,85X,86 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Brekke; Darrell G. Morin, Sr.;
Armand G. Manning; John R.
Parent Case Text
The invention described herein was made in the performance of work
under a NASA contract and is subject to the provisions of Section
305 of the National Aeronautics and Space Act of 1958, Public Law
850,568 (72 Stat. 435; 42 U.S.C. 2457).
This is a division, of application Ser. No. 221,670 filed Jan. 28,
1972 now abandoned.
Claims
What is claimed is:
1. A method of forming an objective aperture plate for an electron
microscope having an annular aperture, comprising the steps of:
disposing an annular aperture in the condenser aperture plane to
form a hollow cone of electrons;
disposing a first metallic layer in the objective aperture plane of
the focused microscope and in the path of said electrons to cause a
contamination layer of residual gas molecules to form on first said
metallic layer in the area illuminated by said electrons;
disposing a second metallic layer over that portion of the surface
of the first metallic layer which is not covered by the
contamination layer; and
etching the composite structure on the side opposite said second
metallic layer to a depth transcending said first metallic layer
and said contamination layer.
2. A method as recited in claim 1 and further comprising the step
of evaporating a third metallic layer over said second metallic
layer to add stability to the resultant aperture plate.
3. A method as recited in claim 1 wherein said first metallic layer
is of silver and said second metallic layer is of copper
galvanically grown over said first metallic layer.
4. A method as recited in claim 1 wherein said first metallic layer
has a thickness of at least 300 A, and said second metallic layer
has a thickness of at least 5,000 A.
5. A method as recited in claim 1 wherein said etching step is
accomplished by bombarding said first metallic layer and said
contamination layer with ions.
Description
SUMMARY OF THE INVENTION
The present invention relates generally to corpuscular ray devices
and more particularly to an electron microscope having an annular
objective lens aperture for eliminating chromatic aberation and
inactivate spherical aberation, and a method of making the annular
objective lens aperture.
Briefly, the electron microscope of the present invention includes
an electron source, a condenser lens having either a circular
aperture for forming a solid cone of electrons onto a specimen or
an annular aperture for focusing a hollow cone of electrons onto
the specimen, and an objective lens having an annular objective
aperture for focusing the electrons passing through the specimen
onto an image plane. The circular and annular condenser apertures
can conveniently be made using conventional techniques. However,
the much smaller annular objective aperture cannot conveniently be
provided using prior art methods. The present invention includes a
process for making the objective aperture which involves electron
imaging, electrolytic deposition and ion etching techniques.
A primary advantage of the present invention is that the resolution
and brightness of the high performance electron microscope can be
substantially improved.
Other advantages of the present invention will no doubt become
apparent to those of ordinary skill in the art after having read
the following detailed description of the preferred embodiments
which are illustrated in the several figures of the drawings.
IN THE DRAWINGS
FIG. 1 is a diagram schematically illustrating an electron
microscope having a set of annular apertures in accordance with the
present invention;
FIG. 2 is a partial plan view of an annular condenser aperture of
the type used in the microscope illustrated in FIG. 1;
FIG. 3 is a partial plan view of an annular objective aperture made
in accordance with the present invention;
FIGS. 4-8 sequentially illustrate a method of making an annular
objective aperture in accordance with the present invention;
FIG. 9 is a diagram schematically illustrating an electron
microscope having a circular condenser aperture and an annular
objective aperture in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1 of the drawing, a schematic diagram is
shown illustrating an electron microscope including an electron
source 10, a condenser lens 12 having a condenser aperture plate 14
with an annular aperture 15 provided therein. Disposed on the
opposite side of the specimen 16 is an objective lens 18 having an
objective aperture plate 20 with an annular aperture 21 provided
therein. The beam 22 of electrons developed by source 10 is focused
by condenser lens 12 and passed through the annular aperture 15 to
provide a hollow cone of electrons which are focused to a point on
specimen 16. As the electrons pass through specimen 16 they are
again focused by objective lens 18 through the annular objective
aperture 21 and onto a point in image plane 24.
In using the annular condenser aperture to provide hollow cone
illumination, the zero order of diffraction passes the objective
lens in a respective annular zone. It can be shown mathematically
that any imaging process using electrons from a particular zone of
the objective lens is not subject to chromatic aberation in a
mathematical approximation which is far better than required for
experimental realization, and that all image information can be
given in only one particular optimal defocus setting.
It can be shown that any space frequency (reciprocal specimen
distances) below a maximum determined by the size of the annular
condenser aperture can be transferred with almost ideally even
contrast if conical specimen illumination is used in such a system
with an annular objective aperture and the illumination and
objective aperture cone angles are identical.
In the preferred embodiment, the size of the condenser aperture 15
is usually on the order of 2-3 millimeters in diameter with the
width of the open ring area being about 100 microns. As illustrated
in FIG. 2 of the drawing, the inner part of the aperture is
supported by three bars dividing the open ring into three areas.
Such apertures can be manufactured using conventional techniques,
mechanical or otherwise. The corresponding annular objective
aperture 21 (FIG. 3) however, is usually about 50 times smaller in
diameter than the condenser aperture 15 (e.g., approximately 50
.mu.m in diameter and 3 .mu.m in ring width) and is much more
difficult to manufacture. A preferred method of manufacturing an
aperture plate having a suitable objective aperture is illustrated
in stepwise fashion in FIGS. 4-8 and includes the following
steps:
1. First, a collodium film 30 of about 500 angstroms (A) thickness
is stretched over a copper specimen grid 32 which is supported on a
conventional aperture base 34.
2. A metallic layer or film 36 of several hundred A thickness is
then evaporated onto the upper surface of the collodium film 30 as
shown in FIG. 4. The metallic film 36 must be continuous in order
to be electrically conductive. If silver is chosen for film 36, a
thickness of approximately 300 A is adequate.
3. The prepared composite structure including film 36, film 30, and
grid 32 is thereafter inserted into the regular objective aperture
slider of an electron microscope, such as that illustrated in FIG.
1, having an annular condenser aperture. With the microscope
operated in the selected area diffraction mode, the back focal
plane of the objective lens, where the first image of the annular
condenser aperture occurs and where the objective aperture diaphram
is to be located, is imaged onto the image plane 24 (the microscope
screen). With the image of the condenser aperture 15 falling upon
the metal film 36, as illustrated in FIG. 5, a contamination layer
38, caused by the decomposition of residual gas molecules, forms on
top of the metal film 36 in the illuminated area. A sufficient
exposure of the electron beam is approximately 1 amp sec/cm.sup.2
with the microscope operating at 100kV. Contamination layer 38
provides a permanent image of the condenser aperture 15 and has
annular dimensions corresponding to those required for the future
annular objective aperture 21. In other words, the area occupied by
the contamination layer 38 will be open in the final aperture
plate. Note that during this step, the electron image can still be
observed on the image plane 24 because the films 30 and 36 are
still electron transparent.
4. The composite structure is then taken out of the microscope and
submerged in an electrolytic solution comprised of 250g CuSO.sub.4,
1,000 ml H.sub.2 O, and 15 ml H.sub.2 SO.sub.4, and a thin layer of
metallic film 40 is galvanically grown over the exposed surfaces of
film 36 as shown in FIG. 6. The metal layer 40 in the preferred
embodiment is of copper and has a thickness of approximately 10,000
A.
5. The composite structure is next inserted into an ion etching
device and is bombarded from beneath, i.e., the side opposite metal
layer 40, wtih ions from a gas discharge. The, preferrably Argon,
ions will first etch away the collodium film 30, then the first
metal film 36, and finally part of the second metal film 40
together with the contamination layer 38. During this etching
process, the aperture is observed with a light microscope to
determine when an etching depth sufficient to remove the
contamination layer 38 has been reached. As the contamination layer
38 has been etched away, the etching process is interrupted.
6. Finally, a third metal layer 42 may be evaporated onto the
surface of the composite structure for stabilization and
cleanliness purposes. In the preferred embodiment a layer of gold
metal of approximately 1,000 A thickness is provided. At this point
the aperture plate 20 is complete and may be inserted into the
electron microscope for use.
Since the characteristics of objective aperture plate 20 are
uniquely related to a particular condenser aperture plate 14 and
objective lens 18, it will be appreciated that the aperture 21 must
necessarily correspond identically to the aperture 15. Moreover, it
will be noted that since the primary determinant of dimensions of
the annular aperture 21 is the electron beam cross section, the
size of the objective aperture can be increased or diminished by
simply varying the focusing characteristics (focal length) of
either condenser lens 12 or objective lens 18. Since the
manufacturing process of the present invention involves operative
steps which are inherently highly accurate in the dimensional
sense, the resultant aperture is highly accurate. The production of
apertures is also highly reproducible.
Using the method of the present invention, it is easily possible to
print several contamination images on one aperture film so that a
multi-aperture diaphram can be manufactured featuring one or more
annular apertures in each grid opening. Furthermore, by varying the
objective lens current, images of various sizes can be imprinted
from the same original condenser aperture pattern.
The resolution limit of very high resolution electron microscopes
operating in phase contrast, which is the common mode of image
formation in high resolution operation and medium excelerating
voltage (up to 150kV) microscopes, is determined by chromatic
aberations in the objective lens. This limitation in resolution can
be eliminated if only those electrons are permitted to take part in
the image formation which have passed the objective lens in the
same zone, that is, those electrons having the same distance from
the optical axis. Such conditions can be achieved by the use of the
annular objective aperture.
Annular objective apertures of the type can be used in the electron
microscope in two basically different modes of operation which are
dependent on the kind of specimen illumination. The two modes of
illumination are (1) axial illumination and (2) complimentary
hollow conical illumination. Both methods can be used to obtain
ultra-high resolution with strongly reduced chromatic aberation,
since the resolution is basically effected only by the imaging
parts of the optical system which is unchanged and characterized by
the annular objective aperture in both cases. The difference
between the two modes of operation is in (a) image contrast and (b)
width of the transferrable space frequency band.
In accordance with the present invention, two possible combinations
of these modes are permitted, i.e., (1) hollow cone illumination --
annular objective aperture, and (2) axial illumination -- annular
objective aperture which are illustrated in FIGS. 1 and 9
respectively.
If only the annular objective aperture is used, under paraxial
illumination conditions, selected dark zone field microscopy (SZDF)
can be performed having (a) extremely high contrast because images
appear on a black (low noise) background, (b) high resolution
because the influence of chromatic aberation is eliminated, (c)
images of a selected range of reciprocal space frequencies only,
and thus the possibility of performing quantitatively an
orientation determination of crystal and specimens, and (d) defocus
dependent Bragg reflection image displacement phenomena, useful for
quantitative azimuthal orientation determination of small
individual crystallites.
The advantages of using an annular objective aperture in
conjunction with a normal circular condenser aperture as is done in
selected zone dark field microscopy are illustrated in the article
"Selected Zone Dark Field Electron Microscopy," by Klaus Heinemann
and Helmut Poppa, Applied Physics Letters, pp. Feb. 1, 1972.
In the first mode illustrated in FIG. 1, i.e., hollow conical
illumination-annular objective aperture, hollow cone illumination
is applied in an electron microscope using an annular condenser
aperture as shown in FIG. 1. In this case, the zero order of
diffraction passes the objective lens in an annular zone.
Characteristic of this method is that the annular zone within which
the zero order of diffraction passes the objective lens is
identical with the zone selected by the annular objective aperture
21. Thus, in this mode two complimentary annular aperture diaphrams
are used simultaneously, one in the illumination system and one in
the imaging system.
It can be shown that any space frequency below a maximum determined
by the size of the annular condenser aperture 15 (reciprocal
specimen differences) can be transferred with almost ideally even
contrast if conical specimen illumination is used with the annular
objective aperture. Since this is a bright field mode however,
there is no gain in contrast when compared to conventional bright
field modes of operation. There is, on the other hand, a
considerable increase in beam intensity resulting from the fact
that the open area of the annular condenser aperture 15 is much
larger (approximately two order of magnitude) than the open area of
a conventional comparable disc aperture. Since, accordingly, the
microscope can now be operated with a very low beam current,
anomalous beam energy broadening effects (Boersch-Effect) can be
avoided. This adds to the earlier mentioned significant decrease in
effective chromatic aberation.
The second mode (axial illumination-annular objective aperture) is
illustrated in FIG. 9. The apparatus for implementing this mode
includes an electron source 49, a condenser lens 50 and a condenser
aperture plate 52 having a circular aperture 54. Disposed on the
other side of the specimen 56 are the objective lens 58, an
objective aperture plate 60 having an annular aperture 62 and the
image plane 64.
In this mode the zero order of diffraction is blocked off (the
criterion for dark-field microscopy) by the center portion 63 of
aperture plate 60, and the annular objective aperture 62 selects a
special objective lens zone only for the image formation. This
method has been called selected-zone-dark-field-microscopy.
Contrary to the usual modes of microscopy, this is a typical phase
contrast dark field method. The image is an interference image
between two beams which have been diffracted at the specimen and
passed through the objective lens into azimuthally different
locations in the same zone (with the same aperture angle
.theta..sub.B). Consequently, a small aperture width restricts the
width of the space frequency band which is transferrable
considerably. This may be undesirable in the case where amorphous
specimens are being observed, where practically all distances
between specimen details occur and should be resolved. If, however,
crystalline specimens are observed, only discrete object distances,
the interplanar distances of the ordered atom planes, are necessary
and available to be imaged if the diameter of the annular objective
aperture is selected properly. such images occur wtih remarkable
high contrast, as can be expected in dark field microscopy.
It is possible that interference between two non-symmetrically
diffracted beams, i.e., two beams which have been Bragg diffracted
at two different overlapping sets of lattice planes with the same
separation but different azimuthal orientation, can occur. These
interferences will, for example, in a [110] oriented f.c.c.
crystal, result in simultaneous "pseudo" images of [200] and [220]
lattice planes together with the ordinary [111] lattice planes, if
the annular objective aperture was designed for [111] Bragg
diffraction at crystalline material of such or similar cell
dimensions.
The use of an annular objective aperture in the case of axial
illumination is not only valuable for high resolution images of
crystal or graphic lattice planes but can be advantageously applied
if the crystal or graphic orientation of small crystallite has to
be determined. This can be done without direct images of the
lattice planes of the cells.
Although it is further contemplated that additional modifications
of the above disclosed invention will no doubt become apparent to
those of ordinary skill in the art after having read the above
description of the preferred embodiment, it is to be understood
that this description is made for purposes of illustration only and
is in no way intended to be limiting. Accordingly, it is intended
that the appended claims be interpreted as including all
modifications which fall within the true spirit and scope of the
invention.
* * * * *