Note: Descriptions are shown in the official language in which they were submitted.
1333426
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1 69912-140
X-RAY WAVE DIFFRACTION OPTICS CONSTRUCTED
BY ATOHIC LAYER EPITAXY
This invention relates to optics for diffracting x-rays
of short wavelength and a method of constructing such optics.
In the drawings:
Figure 1 shows side, cross-sectional views of two
multilayer structures, one of which serves as a good x-ray
reflector and the other of which is a poor x-ray reflector;
Figure 2 graphically illustrates the steps in forming
one layer of a multilayer structure using chemical vapor
deposition atomic layer epitaxy, on the lefthand side, and
molecular beam atomic layer epitaxy, on the righthand side;
Figure 3 is a diagrammatic illustration of the chemical
vapor deposition atomic layer epitaxy process, showing the various
steps in the process;
Figure 4 is a diagrammatical side cross-sectional view
of a multilayer device made in accordance with the principles of
the present invention and showing alternate layers of ZnSe (2 1/2
layers) and CdS (3 1/2 layers);
Figure 5 is a diagrammatical side cross-sectional view
of a multilayer device having curved layers for focusing incident
x-rays;
Figure 6 is a diagrammatical side cross-sectional view
of a grating having grooves with rectangular-shaped cross-sections
and coated with multilayers in accordance with the principles of
the present invention;
Figures 7A and 7B show respectively a diagrammatical
side cross-sectional view of multilayer device whose layers are
q~
133~425
2 69912-140
slanted with respect to the substrate surface by using photo-
assisted atomic layer epitaxy, and a diagrammatical side cross-
sectional view of a device having slanted multilayers illustrating
the reflection of x-rays from the layers;
Figures 8A, 8B and 8C shown respectively a
diagrammatical, side cross-sectional view, taken along lines A--A
of Figure 8B, of the construction of a slotted frame coated with
multilayers of material in accordance with the present invention,
a front, elevational view of the slotted frame, and side cross-
sectional views showing the pathway of x-rays impinging on the
slotted frames and being reflected therefrom;
Figure 9 is a diagrammatical side cross-sectional view
of a multilayer, x-ray holograph made in accordance with the
principles of the present invention.
Figure 10 is a diagrammatical side cross-sectional view
of a Fabray-Perot etalon made in accordance with the principles of
the present invention and showing pathways of reflected and
nonreflected electromagnetic waves; and
Figure 11 is a diagrammatical side cross-sectional view
of a multilayer beam splitter made in accordance with the
principles of the present invention and showing pathways of an
electromagnetic wave split into two waves.
All materials have refractive indices very close to one
(1.00) in the x-ray portion of the electromagnetic spectrum. As a
result, optic structures and instruments for refraction and
specular reflection of x-rays cannot be made from a single
material. Some reflection of x-ray waves, however, does occur at
the interface of two materials having different refractive
~33~2~
3 69912-140
indices. If such a pair of materials is formed into a large
number of alternating layers, with the layered pairs having
substantially the same thickness (or at least the corresponding
layers of each pair having substantially the same thickness), the
reflectances accumulate if the wavelength L and the angle of
incidence i (measured from the plane of the reflective surface)
meet the requirements of the Bragg equation: nL = 2d sin i, where
n is the order of diffraction and is an integral number and d is
the layer pair thickness. Such multilayer structures, constructed
using conventional sputtering (the ejection of atoms or groups of
atoms from a source onto another surface to form a thin layer
thereof) or vacuum deposition
133342~
`. ~
.. ~,
(producing a surface film of metal on a heated
surface, typically in a vacuum, either by
decomposition of the vapor of a compound at the work
surface, or by direct reaction between the work
surface and the vapor), have been utilized for
reflecting, focusing, dispersing, etc.,
electromagnetic waves.
However, with the currently used methods of producing
the multilayer structures (sputtering and vacuum
deposition), it is difficult to develop layers having
a thickness of less than about twelve angstroms and so
a certain range of x-ray wavelengths cannot be
diffracted. At least, it is difficult, if not
impossible, to develop layers having a uniform
thickness of twelve angstroms or less. The reason for
this is apparently because the materials used to
construct the layers tend to clump together when
physical vapor deposition methods are utilized. That
is, the first few atoms deposited tend to aggregate on
the surface to form islands several angstroms thick
before they begin to grow laterally. When such clumps
or islands finally grow together, their crystal
orientations are unlikely to match so that a grain
boundary is formed. Such imperfections not only
themselves degrade x-ray reflection but they also
serve as "chimneys" where one material diffuses into
another and this, in turn, further degrades x-ray
reflection because the refractive indices of the
diffused materials are subsequently closer together.
In addition to the above problems with currently used
physical deposition techniques for constructing
multilayer devices, sputtering, and especially
evaporation, oftentimes tend to blur the interface
between layers by driving fast moving vapor of one
layer into the adjacent layer of material.
`~ - t ~3~4~
Furthermore, physical vapor deposition methods tend to
be statistical in nature, i.e., atoms arrive at the
surface on which they are being deposited at random
times and in random places and usually "freeze" into
the surface where they first touch. Physical
deposition cannot be stopped at the precise time when
enough atoms have been added to complete the last
layer to thereby even out the variations in coverage
of the layer. As a result, one or more of the top
layers will be incomplete and this leads to rough
interfaces between layers and variations in layer of
thickness, both of which decrease reflectance of
electromagnetic waves. Also, random fluctuations (in
location and quantity) in arrival of the atoms lead to
roughness at the interfaces.
Although the imperfections described above are not a
serious problem in multilayer structures having thick
layer pairs, they can be fatal to the usefulness of
structures whose layers are thinner than about twenty
angstroms. An illustration of this problem is shown
in FIG. 1 of the drawings, where a good x-ray
multilayer reflector, shown at 4, has layers of
uniform thickness and smooth interfaces, and a poor x-
ray reflector, shown at 8, has layers of nonuniformthickness and rough interfaces. Since very thin
layers are needed for diffracting short x-ray
wavelengths used, for example, in medicine and
nondestructive testing, useful diffracting instruments
in these fields cannot be constructed with currently
available physical deposition techniques.
Crystals have also been used to diffract
electromagnetic radiation, including x-rays, and they
present diffracting planes which are more nearly
perfect. However, crystals are also usually quite
small, cannot be bent or shaped much without
1~ 3 3 3 ~
fracturing, and the range of layer of thicknesses
available is limited (all are too small for soft x-
rays). Multilayer structures, of course, if they
could be constructed with layers whose interfaces are
smooth and whose thicknesses are uniform, would
provide a solution to these shortcomings since such
structures can be specifically designed with (1) areas
of a predetermined size (larger than many crystals),
(2) layers having a predetermined thickness, and (3)
reflecting surfaces which are shaped in a
predetermined way, for example curved to focus
reflected radiation. The areas of application for a
high quality, controllable sur~ace shape and thin
multilayer structure are legion, including x-ray
microlithography for the production of integrated
circuits, and x-ray focusing optics for use in medical
diagnostics, biological research, and nondestructive
testing (for example, weld inspection, fatigue,
cracks, etc., in metal or metal alloys).
It is an object of the invention to provide a
multilayer structure for diffracting x-ray waves, and
a method of constructing such structure.
It is a further object of the invention to provide
such a structure whose layers are generally of uniform
thickness and whose layer interfaces are smooth,
abrupt and consistent.
It is also an object of the invention to provide such
a structure whose layers may be constructed to a
thickness of twelve angstroms or less.
It is a further object of the invention to provide a
method for constructing optical elements capable of
reflecting, focusing or dispersing incident x-rays.
133~12~
7 6g912-140
It is an additional object of the invention to provide a
method of fabricating x-ray optics whose parameters and
characteristics can be carefully controlled.
According to a broad aspect of the invention there is
provided a method of constructing an x-ray wave diffraction
structure comprising the steps of
(a) providing a substrate having a surface area, and
(b) growing a first layer of material on the substrate, the
first layer of material having a first index of refraction and a
first thickness,
(c) growing a second layer of material on top of the first
layer of material, the second layer having a second thickness and
a second index of refraction which is different than the index of
refraction of the material of the first layer and
(d) forming a plurality of pairs of first layers and second
layers with each of the second layers being grown on the first
layers such that the combined thickness of each pair of layers
formed by the first and second layers is about twelve angstroms or
less and the thickness of all the first layers each being
substantially the same as that of the corresponding layer of every
other pair and the thickness of all the second layers each being
substantially the same as that of the corresponding layer of every
other pair and the first or second indices of refraction being a
high index of refraction with the other of the first or second
indices of refraction being a low index of refraction such that x-
ray waves impinging thereon are reflected.
According to another broad aspect of the invention there
is provided an x-ray wave diffraction apparatus comprising
13~3426
7a 69912-140
a substrate having a surface area, and
a plurality of pairs of layers of material disposed one on
top of another on the surface area of said substrate, where the
material of one layer of each pair has an index of diffraction
which is different from that of the material of the other layer of
each pair, and where the thickness of each layer pair is
substantially uniform and is selected to enable diffraction of x-
rays impinging on the layers.
In accordance with one aspect of the invention ,the
layer pairs are formed to have a thickness of about twelve
angstroms or less so that short x-ray waves may be diffracted.
In accordance with another aspect of the invention, the
layer pairs are formed out of materials having indices of
refraction which are substantially different to thus achieve high
reflectance.
In accordance with still another aspect of the
invention, the layer pairs are formed out of materials having
indices of refraction which are close in value to achieve high
resolution of diffracted x-rays, i.e., reflect a narrow band of
wavelengths and have low absorbtion of the waves.
8 1 ~33~2~
Referring now to the drawings:
In accordance with the present invention, multilayer
x-ray wave diffraction structure capable of
reflecting, focusing, dispersing or otherwise handling
short x-ray wavelengths of from about one to twelve
angstroms is constructed using atomic layer epitaxy
(ALE). Using ALE, it is possible to deposit one and
only one layer of atoms during a process cycle so that
layer thicknesses may be carefully controlled and
smooth layer interfaces, with atomic abruptness,
achieved.
ALE can be carried out by either of two processes, one
known as molecular beam epitaxy and the other known as
chemical vapor deposition ALE. As is known, the
latter process is performed by passing a metal-organic
vapor over a heated substrate, typically from about
300 to 450 degrees centigrade. In a matter of a
second or two, a single layer (monolayer) of molecules
or, in some cases, only the atoms to be included in
the multilayer are chemically bound to the substrate.
If the layer is a chemical compound, it can be
considered to be composed of cations of the low
electronegativity element and anions of the high
electronegativity element. Zn++ and Cd++ are examples
of cations; and Se-- and S-- are examples of anions.
Referring ahead to FIG. 4 for a moment, a single
monolayer of cations (Zn++) is deposited on a
substrate 58 first as a monolayer of metal-organic
molecules from a vapor, and then it is converted to
monolayer of metal by removing the excess vapor and
replacing it with a reactive gas which removes the
organic portion of the molecules. After the gaseous
reaction products have been removed, a vapor of
molecules bearing the anions is allowed to contact the
13~
newly deposited metal. (The anion vapor, in some
instances, can be the reactive gas used for removing
the organic portion of the cation molecules discussed
above and thus a separate reactive gas would not be
needed.) A monolayer of anions is deposited by this
process. Depending on the vapor used, the same
procedure described above involving an appropriate
reactive gas could be utilized to leave only the
monolayer of anions on the surface. The steps are
summarized (without reactive gases) in the lefthand
side of FIG. 2. Subsequent deposition of cations and
anions is repeated by this process until the desired
thickness is achieved. This constitutes one layer of
the layer pair.
The second compound is deposited in the same way using
appropriate vapors. Note that the second compound may
have a different anion and a different cation than the
first compound and the second layer of the pair will
have a thickness different from the first. This whole
process is repeated until the desired number of layer
pairs has been deposited. With some types of vapors,
only part of a monolayer is deposited during the first
exposure. However, the fraction of the monolayer
deposited always has a small integer as the
denominator (e.g. 1/2, 1/4, etc.) if the temperature
and pressure are carefully selected. The monolayer
can be completed by using the appropriate number of
vapor exposure and reactive gas cycles.
FIG. 2 shows diagrammatically the steps in atomic
layer epitaxy with chemical vapor deposition shown on
the lefthand side and molecular beam epitaxy shown on
the righthand side. The layer of material deposited
is zinc selenide and in the chemical vapor deposition
process of ALE, the beginning materials from which the
layers are formed are a volatile zinc alkyl compound
13~2,~
such as diethyl zinc and a volatile selenide such as
selenium hydride, both of which are depicted in FIG.
2. As also depicted in FIG. 2 at 22, a monolayer of
diethyl zinc 24 is deposited on a substrate 26.
Further exposure of the substrate 26 and layer 24 to a
vapor of selenium hydride, depicted at 28, at the
appropriate temperature (between 300 and 500 degrees
centigrade) results in the ethyl radicals being
replaced by selenium atoms (and the giving off of
ethane) yields the monolayer of zinc selenide
depicted at 32. This could also be achieved using
molecular beam epitaxy depicted on the righthand side
of FIG. 2, but such process will not be discussed
here.
FIG. 3 depicts the steps in the well-known physical
and chemical processes that occur in ALE. In
these processes, a substrate is exposed to a metal-
organic vapor A(g) that contains one of the elements
desired in the layer (say the cation). A monolayer of
this material is physically adsorbed on the solid
surface. A(ad) refers to adsorbed molecules which
migrate over the surface until they reach reactive
sites and form chemical bonds to the substrate and to
each other (step #1). These bonded molecules, A(s),
form one and only one (crystalline solid monolayer.
However additional molecules may be physically
adsorbed on top of this monolayer.
In step #2, the excess A(ad) molecules are removed by
decreasing the vapor pressure of A(g). These
molecules are pumped out of the reactor.
In step #3, the reactive gas (B(g) is admitted to the
reactor and is adsorbed on the monolayer of A(s)
molecules. A chemical reaction occurs in step #4
which removes the organic part of the A(s) molecules,
11 13~3~2~
leaving the solid elemental monolayer C(s) and
gaseous reaction products D(g) (which are pumped out
of the reactor). The multilayer is now ready for a
similar process with a gas containing the anion and an
appropriate reactive gas.
Interfaces between layers made by ALE are atomically
abrupt. Therefore, as electromagnetic radiation
crosses the interfaces, it experiences a change in
refractive index over a distance that is short
compared to the layer thicknesses. This improves
optical performance, particularly the intensity of
high order diffraction.
FIG. 4, which was earlier briefly described, shows a
cross section of alternate layers of zinc selenide and
cadmium sulphide, with the different atoms of the
elements shown at the bottom of the figure. The
representation of FIG. 4 is of a typical multilayer
structure which can be formed using ALE, showing the
position of the atoms of the different materials of
which the layers are composed. The topmost group of
layers 52 consist of 2 1/2 layers of the compound zinc
selenide, with the half layer consisting only of zinc
atoms. The middle group 54 consists of 3 1/2 layers
of the compound cadmium sulfide, with the half layer
consisting only of sulfur atoms. The bottommost group
of layers 56 again comprises 2 1/2 layers of zinc
selenide deposited directly on a substrate 58.
The multilayer structure of FIG. 4 would be fabricated
at a temperature of between 300 and 400 degrees
centigrade. The substrate 58 might illustratively be
crystalline germanium, sodium chloride, silicon,
gallium arsenide, cadmium telluride, lithium
fluoride, or mica. Such crystalline materials provide
smooth beginning deposition surface areas on which to
12 1 3 3 ^~ &
form the layers of material.
FIGS. 5, 6, 7, 8, 9 and 10 all show various
implementations of x-ray wave diffraction structures
made in accordance with the present invention. FIG. 5
is a graphic representation of a side cross-sectional
view of an x-ray diffraction instrument having a
plurality of curved (concavely) multilayers 72 formed
on the curved (concavely) upper surface of a substrate
74. The multilayer curvature is selected to reflect
parallel incoming x-rays toward a focal point 76 to
enable using the instrument as an x-ray focusing
device. To focus incident x-rays to a point 76
would, of course, require spherical curvature whereas
if the curvature were cylindrical, the incident x-rays
would be focused along a line. The representation of
FIG. 5 is either of spherical curvature or cylindrical
curvature to illustrate the focusing of x-rays.
FIG. 6 iS a representation of a cross-sectional view
of a rectangular-groove grating formed in a substrate
84 and coated with multiple pairs of layers 82 using
atomic layer epitaxy. The upper surface of the
substrate 84 is formed with grooves 86, where such
grooves have rectangular cross-sections. The side
walls and bottom wall of each groove is coated with
the layers of material as shown to enable reflection
of x-rays in various directions, one of which is
illustrated by path 88 of FIG. 6. As illustrated,
there are three reflections in path 88 and this
narrows the band of wavelengths that are reflected in
accordance with a well known successive reflection
principle. Narrowing of the band width serves to
provide better resolution of the reflected waves.
FIG. 7A is a graphic cross-sectional view of a
multilayer structure in which the layers 92 are formed
~ 33~26
13
on a slant or angle with the surface of a substrate 94
using photo-assisted ALE. Photo-assisted ALE utilizes
the phenomenon that certain gases do not deposit on a
substrate unless light having certain wavelength
bands is used to illuminate the substrate. For
example, a mixture of certain gases will deposit two
materials on those areas of a substrate illuminated by
light, but only one material on those areas not so
illuminated. The phenomenon can be utilized to
develop layers having various configurations,
curvatures, etc.
In the FIG. 7A representation of the slanted layers,
the layers would be deposited on the substrate 94 as
light is moved to the right as successive layers are
deposited. For example, for the first layer 96, light
would illuminate the area designated by 98 so that
material having a high atomic number would be
deposited on that area and material having a low
atomic number would be deposited on the remaining
areas. For the next layer 100, light would illuminate
the area designated as 102 and the shaded material
would be deposited on that area, whereas the unshaded
material would be deposited on the remaining areas,
etc. In this manner, slanted layers of material are
formed on the substrate 94 (the squares in FIG. 7A
represent lattice sites of atoms of the layers.)
FIG. 7B depicts the path of x-rays 112 reflected from
a slanted multilayer structure 114. Path 116 of other
radiation traveling in parallel with the x-ray waves,
but reflected from the structure 114 in a different
direction, is also shown. As can be seen from FIG.
7B, the structure 114 serves to separate reflected x-
rays from specularly reflected radiation that does not
meet the Bragg conditions of wavelength, for example,
visible ultraviolet and infrared radiation. Such
14 1 333~2~
structure is useful in the field of solar astronomy
where intense visible radiation must not be allowed to
strike x-ray detectors and so the visible radiation
could be separated from the x-ray waves so that only
the x-ray waves would be directed to the x-ray
detector.
Photo-assisted ~LE can also be used to produce curved
layers on generally flat substrates so that initial
provision of a substrate having a curved upper
surface, as in FIG. 5, would be unnecessary. In such
case, the substrate 74 of FIG. 5 could simply have a
flat or planar upper surface (which most crystalline
substrates would have) and then the deposition of the
layers on the substrate could be carried out by photo-
assisted ALE to produce curved layers of any desired
shape.
FIGS. 8A, 8B and 8C show the structure and use of
slotted frames or mirrors, such as frame 122 of FIG.
8B, having a plurality of ribs 126 separating a
plurality of slots 124. FIG. 8A is a cross-
sectional view of frame 122 of FIG. 8B taken along
lines A--A. FIG. 8C shows the use of a pair of
slotted frames 142 and 144 in guiding x-rays emanating
from a point source 146 onto generally parallel
pathways 148. The frame 122 is formed from a
substrate by etching the slots 124 entirely through
the substrate, and then depositing multilayers 128
onto the sides of the ribs 126. This type of
structure intercepts a large solid angle of the
radiation emitted by a source and is therefore an
efficient collector (similar to a condenser lens).
However, the slotted multilayer mosaic can be thin and
light, which is advantageous in applications such as
x-ray astronomy from satellites and in x-ray radar.
FIG. 9 is a graphic representation of a cross-section
of an x-ray holograph, having multilayers 162 of spots
164 and stripes 166 and 168 of differing length. The
multilayers 162 are formed on the surface of a
5 substrate 170 using photo-assisted ALE, as previously
described. In the construction of the x-ray
holograph, the locations of the spots 164 and stripes
166 and 168 would be designated by illuminating those
areas with light as the deposition of material
10 proceeded. Then, two material layers would be
developed on the lighted areas of the substrate 170
whereas single material layers would be developed
elsewhere. In this fashion, an x-ray holograph having
spots, stripes or other shapes may be produced.
X-ray holographs find application in diffracting and
monochromatizing electromagnetic radiation in ways
that diEfraction gratings and etalons cannot match.
For example, holographic gratings can be more
20 efficient at reflecting a desired band of wavelengths
than can convention optical elements. Holographic
gratings can be used to project desired fine-featured
patterns on a substrate, an operation that would be of
value in x-ray microlithography for the production of
25 integrated circuits.
FIG. 10 shows a graphic side cross-sectional view of a
multilayer Fabray-Perot etalon. With this device, one
wavelength, represented by path 180, is reflected
30 strongly whereas all other wavelengths are transmitted
or absorbed. The thickness of a spacer layer 188,
made of a material having a low atomic number,
determines the wavelengths which will be reflected.
35 Constructing a Fabray-Perot etalon as shown in FIG. 10
using ALE allows for precise control of the thickness
of the spacer layer 188 and thus precise control of
16 13~3~2~
which wavelengths will be reflected.
FIG. 11 shows a diagrammatic side cross-sectional view
of an x-ray beam splitter 200 in which multilayers 204
are deposited on the upper surface of a substrate 208.
A portion of the substrate 204 is etched away leaving
a window 212 surrounded by a frame 216 (the frame
would completely surround the window 212). Such
etching of the window 212 would take place after
deposition of the multilayers 204. A thin portion 220
of the substrate could be left for additional support
of the multilayers 204 provided that x-rays could be
readily transmitted therethrough.
As indicated in FIG. 11, some portion of an incident
x-ray wave is reflected (indicated by path 224) and
some portion is not reflected (indicated by path 228)
but rather travels through the multilayers 204 and
through the window 212. With ALE, the number and
thicknesses of the multilayers 204 can be precisely
determined to thus determine the portion of incident
x-rays which is to be reflected and the portion which
is to be transmitted through the window 212.
In order to achieve high reflectance in a multilayer
x-ray diffraction structure, at least one of the
component elements in the multilayers should have a
high atomic number Z, and at least one should have a
low atomic number. An example of a highly reflective
multilayer structure is one in which lead telluride
and aluminum nitride make up the layer pairs. Such
multilayer structure is fabricated at temperatures
between 300 and 600 degree centigrade and preferably
between 400 and 500 degree centigrade using the
following gases: a volatile lead compound, preferably
tetraethyl lead; hydrogen; a volatile tellurium
compound such as diethyl tellurium or tellurium
~3~26
17
hydride; aluminum trialkyl compounds especially
trimethyl aluminum; and ammonia or hydrazine.
To achieve high resolution, all of the elements of a
multilayer x-ray diffraction device should have low
atomic numbers Z, or more precisely low absorbtion
over a given wavelength range. An example of such a
structure for the nitrogen K-line wavelength range is
one composed of zinc selenide and cadmium sulfide
layer pavis, which was briefly described earlier.
Such a structure is suitable for diffracting x-rays
having a wavelength of about ten angstroms, at an
angle near 30 degrees. The multilayer structure is
made by forming alternate monolayers of each compound
on a smooth, single-crystal, preferably germanium
substrate maintained at 350+5O centigrade. At a
pressure of 10 Torr, the feed gases (diethyl zinc and
selenium hydride as one pair, and diethyl cadmium and
hydrogen sulfide as the other pair) are alternated
every 30 seconds with argon purge gas. ~ore than 100
layer pairs should be laid down to achieve maximum
reflectance and resolution.
Single elements such as silicon (as opposed to
compounds) can also be deposited by ALE to produce
devices which achieve high resolution. Two other
multilayer x-ray diffraction structures for achieving
high resolution are a silicon and boron nitride layer
construction and a carbon and boron nitride layer
construction.
Gallium arsenide and zinc are preferred materials for
use in photo-assisted ~LE since the materials are well
suited for use in X-ray holograph, slanted multilayer
and curved multilayer devices. If the photo-assisted
layer is zinc, then preferably diethyl zinc gas vapor
is selected for the fabrication process. Temperature
13~3i~ 26
18
is not a factor in photo-assisted ALE and so it can be
selected and adjusted for the other material in the
layer pairs, for example, cadmium sulfide at 450
degrees centigrade. In this example, the temperature
should not be greater than 500 degrees centigrade
since at that temperature diethyl zinc decomposes.
In the manner described, precisely constructed x-ray
wave diffraction instruments can be fabricated using
atomic layer epitaxy. The ALE produces crystalline
layers having no or very little lateral variation in
thickness and few, if any, grain boundaries and
defects. The deposition of the layers can be
controlled to be almost any desired thickness and as
thin as one or two angstroms. Such precisely
constructed devices yield excellent x-ray handling
capabilities of use in a variety of fields.
It is to be understood that the above-described
arrangements are only illustrative of the application
of the principles of the present invention. Numerous
modifications and alternative arrangements may be
devised by those skilled in the art without departing
from the spirit and scope of the present invention and
the appended claims are intended to cover such
modifications and arrangements.
