Note: Descriptions are shown in the official language in which they were submitted.
PROVIDING X-RAYS
FIELD
This invention relates to apparatus for providing
X-rays to an object that may be in an ordinaxy environ-
ment such as air at approximately atmospheric pressure.
Apparatus according to the present invention is
especially useful for applications wherein it is
expensive, tim~ consuming, or otherwise inconvenient to
move objects that are to receive soft X-xays into and
out of a special environment, such as a vacuum chamber
in which the X-rays are produced. ~ypical applications
of this type include laser produced X-ray systems for
high resolution lithography, for extended X-ray absorption
fine structure ~EXAFS) spectroscopy, and for X-ray
microscopy.
BACKGROUND
X-rays usually are produced in a vacuum, but for
many purposes it is desirable to apply them in air.
For soft X-rays, especially those having photon energies
of less than about 5 keV, a problem arises in bringing
the X-rays from the vacuum into air, because a window
that is thick enough and strong enough to withstand the
pressure difference between the vacuum and the air is
opaque to the X-rays, except in ~ery small windows. The
problem is especially serious in X-ray lithography, where
it is desirabl~ to illuminate large areas.
The present invention provides simple, inexpensive,
convenient means for overcoming the problem.
It is shown in Vnited State Patent 4,058,486,
November 15, 1977, of P.J. Mallozzi, H.M. Epstein, R.G.
Jung, D.C. Applebaum, B.P. Fairand, and W.J. Gallagher,
for Producing X-rays, that an intense point source of
X-rays can be generated by focusing a laser beam onto a
solid target. Neodymium laser light focused onto a
solid slab target has been converted into X-rays wi~h an
efficiency greater than `25 percent, with several tens of
joules of X-rays emanating from an essentially point
source (about 100 microns diameter) in a nanosec~nd.
The X-ray pattern produced with iron targets irradiated
with about 100-joule laser pulses ~t a 45 degxee angle
of incidence is substantially omnidirectional. The con-
version efficiency of greater than 25 percent refers to
X-rays which are radiated away from the slab and pass
perpendicularly through 3000 Angstroms of plastic
(paraline) coated with 2000 Angstroms of aluminum. This
conversion efficiency is thus a lower bound and refers
only to the portion of the spectrum above about 300
electron volts. Most of the observed X-rays lie betwePn
about 0.3 and 1.5 keV, with a small but useful fraction
having energies as high as 10 to 100 keV. In a densi-
tometer tracing of a bent crystal spectrograph taken witha KAP crystal, the radiation appears to be mostly lines
in the spectral interval of about 0.7 to 1.2 keV. The
unusual sharpness of the spectral detail is due to the
tiny dimensi.ons of the source. This novel point source
of X-rays provides a spectrum tuneable throughout a
range of about 0.1 to 100 ke~.
Apparatus according ~o the present invention
typically employs X-ray producing means of the type
described above~ It may, however, use other somewhat
similar means~ such as equipment that uses an electron
beam, rather than a laser beam~ for producing the X-rays.
DISCLOSURE
Typical apparatus according to the present
invention for providing X-rays to an object that may be
in an ordinary environment such as air at approxima~ely
atmospheric pressure comprises means for directing energy
onto a target to produce X-rays of a selected spectrum
and intensity at the target, a substantially fluid-tight
first enclosure around the target, means for reducing
the quantity of gas in the first enclosure to maintain
the pressure therein substantially below atmospheric
pressure, a substantially fluid-tight second enclosure
adjoining the first enclosure, with the two enclosures
having at least a portion of one wall in con~non, the
common wall portion having therein an opening large
enough to permit X-rays to pass through it and yet small
enough that the pressure reducing means can evacuate gas
from the first enclosure at least as fast as it enters
through the opening, the target being located close
enough to the opening and so positioned as to emit a
substantial portion of the X-rays produced toward the
opening, to pass through it and on toward a wall of the
second enclosure located opposlte the opening, means for
conveying into the second enclosure a gas that is highly
transparent to X-rays, to the substantial exclusion of
other gases, and the wall of the second enclosure to
which the X-rays travel having a portion that is highly
transparent to them, so that the object to which the
X-rays are to be provided may be located outside the
second enclosure and adjacent thereto and thus receive
the X-rays substantially unimpeded by air or other
undesired intervening matter.
The energy directing means comprises means for
directing energy from a laser onto the target, as by
focusing the energy onto a spot on the target having a
diameter of about 1 to 200 micrometers. Typically the
opening in the common wall portion is about 0.2 to 2
millimeters in diameter, and the distance between the
opening and the spot on the target is about 0.2 to 5
centimeters.
Typically the gas conveyed into the second
enclosure is helium, hydrogen, or a hydrocarbon;
preferably helium; maintained, at least in the vicinity
of the highly transparent portion of the wall thereof,
at a pressure of about 0.9 to 1 atmosphere. Typically
the highly transparent portion of the wall of the second
enclosure comprises a thin foil that typically compxises
essentially beryllium or a plastic material. The
thickness of the foil typically is about 2 to 20 micro-
meters. The X-rays produced at the target typically
have energies predominantly of about 0.3 to 2 keV.
Where the gas in the second enclosure, at least in
the vicinity of the highly transparent portion of the
wall thereof, is maintained at approximately the
pressure of the ambient air, the highly transparent
portion of the wall of the second enclosure may comprise
an opening therein; and the gas inside the second
enclosure can be substantially separated from the air
around it; either by a gas curtain passing along the
opening; or by the object to which the X-rays are to be
provided, or a component associated with the object,
placed against the wall and coYering the opening.
The second enclosure may have an intermediate
compartment between the common wall portion and the
wall having the highly transparent portion; the gas in
the intermediate compartment being maintained at a
pressure less than the pressure in the vicinity of the
highly transparent portion of the wall of the second
enclosure and yreater than the pressure in the first
enclosure.
Apparatus according to the in~ention for obtaining
EXAFS data of a material, typically comprises also
spectral dispersive means in the second enclosure so
located as to receive X-rays that pass through the
opening and to direct the spectrally resolved X-rays on
toward the highly transparent portion of the wall
adjacent to the object to which the X-rays are to be
provided, and the object typica:Lly comprises recording
means. Such apparatus typically comprises also means
for positioning a sample of material in the optical path
of the X-rays/ either in the second enclosure or outside
of the second enclosure and between the highly trans-
parent portion of the wall and the recording means.
7~
DRAWINGS
Figure 1 is a schematic plan view of typical
apparatus according to the present invention.
Figure 2 is a similar view of a typical embodiment
of the invention for obtaining EX~FS data of a material~
CARRYING OVT THE INVENTION
Referring to the drawings, and now especially to
~igure 1, typical apparatus according to the present
invention for provid.ing X-rays 11 to an object 12 that
may be in an ordinary environment such as air at
appraximately atmospheric pressure comprises means
such as a lens 13 for directing energy 14 onto a target
15 to produce X-rays 11 of a selected spectxum and
intensity at the target 15, a substantially fluid-tight
first enclosure 16 around the target 15, means as
indicated by the arrow 17 (such as a vacuum pump, not
shown) for reducing the ~uantity of gas in the first
enclosure 16 to maintain the pressure therein substan-
tially below a~mospheric pressure (typically less than
about 1 torr), a substantially fluid-tight second
enclosure 18 adjoining the first enclosure 16, with the
two enclosures 16,18 having at least a portion of one
wall 19 in common, the common wall portion 19 having
therein an opening 20 large enough to permit X-rays 11
to pass through it (20) and yet small enough that the
pressure reducing means can evacuate gas 21 from the first
enclosure 16 at least as fast as it enters through the
opening 20, the target 15 being located close enough to
the opening 20 and so positioned as to emit a substantial
portion of the X-rays 11 produced toward the opening 20,
to pass through it (20) and on toward a wall 22 of the
second enclosure lB located opposite the opening 20,
means as indicated by the arrow 23 (such as a pump, not
shown) fox conveying into the second enclosure 18 a gas
24 that is highly transparent to X-rays 11, to the
substantial exclusion of other yases, and the wall 22 of
the second enclosure 18 to which the X-rays 11 travel
having a portion 25 that is highly transparent to them
(11), so that the object 12 to which the X~rays 11 are
to be provided may be located outside the second
enclosure 18 and adjacent thereto and thus receive the
X-rays 11 substantially unimpeded by air or other
undesired intervening mattex.
Where only specific regions of the object 12 are
to receive the X-rays 11, as in X-ray lithography, a mask
26 may be placed between the highly transparent portion
25 of the wall 22 and the object 12 to block the X-rays
~ proceeding toward the other regions of the object 12.
~ he energy directing means typically comprises a
lens 13 for directing energy 14, passing through a window
29 in the first enclosure 16, from a laser 27, onto the
target 15, as by focusing the energy 14 onto a spot 28
on the target 15 having a diameter of about 1 to 200
micrometers. Typically the opening 20 in the common wall
portion 19 is about 0.2 to 2 millimeters in diameter, and
the distance between the opening 20 and the spot 28 on
the target 15 is about 0.2 to 5 centimeters.
Typically the gas 24 conveyed into the second
enclosure 18 is helium, hydrogen, or a hydrocarbon, such
as methane; maintained at a pressure of about 0.9 to 1
atmosphere,at least in the vicinity of the highly
transparent portion of the wall thereof. Preferably the
gas 24 comprises essentially helium, which is known to
be highly transparent to X-rays as well as substantially
inert.
Typically the highly transparent portion 25 of the
wall 22 of the second enclosure 18 comprises a thin foil
25 that typically comprises essentially beryllium or a
plastic material. The thickness of the foil 25 typically
is about 2 to 20 micrometers. Other materials, preferably
having atomic numbers of not more than about 8, may also
be used. Where a less transparent material is used it
must be very thin. The X-rays 11 produced at the
target 15 typically have energies predominantly of about
0.3 to 2 keV.
Where the pressure of the gas 24 in the second
5 enclosure 18 is maintained at approximately atmospheric
pressure, the highly transparent portion 25 of the wall
22 may be very thin, because the pressure on each side
of it is approximately the same. It may even comprise
only a gas curtain, rather than a solid material; or the
mask 26 in Figure 1 or the sample 32 in Figure 2 may be
placed against the thick "frame" formed by the wall 22
to substantially separate the gas 24 inside the second
enclosure 18 from the air around it. Where an adjacen~
mask or sample is not used~ the object 12 may be placed
against the wall 22 to substantially separate the gas 24
inside the second enclosure 18 from the air around it.
In some embodiments of the invention it may be
desirable to form at least one intermediate compartment
34 in the second enclosure 18, as shown in Figure 1
between the wall 19' (having an opening 20' therein for
the X-rays 11 to pass through) and the wall 19~ The
pressure in each such compartment is maintained between
the pressures in the ad~acent enclosed regions. As
indicated by the arrow 17', means such as a vacuum pump
(not ~shown) can maintain the proper pressure. Where more
than one intermediate compartment 34 is provided in the
second enclosure 18, a differential evacuation system of
the type used for the emission of electron beams into
the atmosphere may be desirable.
Where the gas 24 in the second enclosure 18, at
least in the vicinity of the highly transparent portion
25 of the wall 22 thereof, is maintained at approximately
the pressure of the ambient air, the highly transparent
portion 25 of the wall 22 of the second enclosure 18 may
comprise an opening therein; and the gas 24 inslde the
second enclosure 18 can be substantlally separated from
7~
the air around it; either by a gas curtain passing along
the opening at 25; or by the object 12 to which the X-rays
11 are to be provided, or a CQmpOnent associated with the
object 12 (such as the mask 25 in Figure 1 or the sample
32 in Figure 2), placed against the wall 22 and covering
the opening at 25.
The second enclosure 18 may have an intermediate
compartment 34, as in Figure 1, between the common wall
portion 19 and the wall 22 having the highly transparent
portion 25; the gas in the intermediate compar~ment 34
being maintained at a pressure less than ~he pressure in
the vicinity of the highly transp~rent portion 25 of the
wall 22 of the second enclosure 18 and greater than the
pressure in the first enclosure 16.
As is shown in Figure 2I typical apparatus accord-
ing to the invention for obtaining EXAFS data of a
material~ comprises also spectral dispersive means such
as a monochromator 30 in the second enclosure 18 so
located as to receive X-rays 11 that pass through the
opening and to direct the spectrally resolved X-rays
llR on toward the highly transparent portion 25 of the
wall 22 adjacent to the ob~ect 12 to which the X-rays
llR axe to be pro~ided, and the object 12 typically
comprises recording means such as a photographic film 12.
Such apparatus t~pically comprises also means such as
a support (not shown) for positioning a sample of
material 31 in the ~ptical path of the X-rays ll,llR,
either in th~ second enclosure 18 as indicated by the
dashed line 31, or outside of the second enclosure 18
and between the highly transparent portion 25 of the wall
22 and the recording means 12, as indicated at 32O The
latter position 32 usually is more convenient than
positions (such as 31~ in the second enclosure 1~.
Typically the radiant energy 14 is directed to the
target 15 in a single pulse in such manner as to produce
soft X-rays 11 ~rom the target 15 in a ~ingle pulse in
72~
such manner as to produce soft X-rays 11 from the
target 15 suitable for obtaining the EXA~S spectrum of
the material 32, which typically is an element having an
atomic number of less than 40~
EXAFS apparatus as in Figure 2 may comprise also
means for moving the surface of the target 15 typically
in a rotating and advancing motion (not shown3 to
pro~ide a helical locus of points on a cylindrical
surface of the target 15 travelling through the location
of the focal spot 28 where the laser light energy 14
strikes the surface. In such a case the energy 14
typically is directed to the moving target surface at 28
in a series of pulses in such manner as to produce soft
X-rays 11 from the target 15 suitable for obtaining the
EXAFS spectrum of the material 32.
The X-rays frorll the taxget 15 preferably comprise
continuum radiation in a selected EXA~S spectral regime
of the sample 32. Typically the target 15 comprises
essentially an element having a continuum just above the
L-lines that includes a selected EXAFS spectr~l regime
of the sample 32. Or the target 15 may comprise a plural-
ity of elements whose lines are spaced ~losely enough to
form virtual.ly a co~tinuum in a selected EXA~S spectral
regime of the sample 11. Such a target 15 typically
comprises a mixture of elements of adjacent atomic
numbers.
The radiant energy typically comprises a laser
pulse 14 with a power density of at least about 1013
watts per square centimeter, and the target 15 typically
comprises a solid (typically metal) surface, whereby a
sur~ace plasma is formed and raised to the kilovolt
temperature regime. Some EXA~S can be obtained, however,
in the ul~raviolet and ultrasoft X-ray regime using
lower power densities down to about 1011 watts per square
centimeter. The laser pulse 14 typically is focused to
strike the focal spot 28 on the taget 15 abQut 1 to 200
micrometers in diameter.
Further typical and preferred details of apparatus
of the type shown in Figure 2 for obtaining ~XAFS data
of a material are contained in the ~nited States pa~ent
application of Philip J. Mallozzi, Harold M. Epstein,
Rober E. Schwerzel, and Bernerd E. Campbell, for Laser
EXAFS; Serial No. 105,816, filed December 20, 1979; now
United States Patent ~,3/?,~ issued ~A~C* Z~ 8Z.
As is explained in detail in the United States
patent of Mallozzi et al., referred to in the Background
section herein, a typical method of producing X-rays for
use in the present invention comprises directing radiant
energy from a laser onto a target, and conversion effi-
ciency of at least about 3 percent is obtained by provid~
ing the radiant energy in a low-power precursor pulse
of approximately uniform effective intensity focused onto
the surface of the target for about 1 to 30 nanoseconds
so as to generate an e~panding unconfined coronal plasma
having less than normal solid density throughout and
comprising a low-density (underdense) region wherein the
plasma frequency is less than the laser radiation fre-
quency and a higher-density (overdense) region wherein the
plasma frequency is greater than the laser radiation fre-
quency and, about 1 to 30 nanoseconds after the precursor
pulse stri~es the target, a higher-power main pulse focused
onto the plasma for about 10-3 to 30 nanoseconds and having
such power density and total energy that the radiant energy
is absorbed in the underdense region and conducted into the
overdense region to heat it and thus to produce X-rays
therefrom with the plasma remaining substantially below
normal solid density and thus facilitating the substantial
emission o~ X-rays in the form of spectral lines arising
from nonequilibrium ionization states.
The target typically consists essentially of an
element having a high atomic number Z, i.e., an atomic
number Z greater than 10. Typically the target consists
essentially of iron, calcium, chromium, nickel, aluminum,
lead, tungsten, or cJold.
The amplitude, duration, and shape of the precursor
pulse typically are adjusted to control the intensity and
spectral content of the X-rays. The precursor pulse
typically comprises about 0.01 to 5 joules (a~out 101
to 1012 watts per square centimeter) in about 1 to 30
nanoseconds, and strikes the target at an angle of about
20 to 70 degrees from its surface.
The main pulse typically comprises at least 0.1
joule, preferably about 10 to 200 joules in about 1 to 3
nanoseconds.
In a typical emhodiment, the target consists essen-
tially of iron and the duration of the precursor pulse
is about 8 to 10 nanoseconds.
The electron density in the low-density region of
the plasma typically is about 1016 to 1021 per cubic
centimeter, and in the higher-density region about 1019
to 1025 per cubic centimeter. The radiant energy
typically is focused onto a spot on the target having a
diametex of about 1 to 1000 micrometers~ The volume of
the plasma typically is about 10 6 to 10-3 cubic centi-
meter, the thickness of the plasma in any direction
being about 0.001 to 0.1 ~entimeter.
For low energy applications the X-rays are emitted
predominantly in the form of spectral lines.
Th~ radiant energy may be focused onto a spot on
the target having a diameter of about 1 to 100 micro-
meters, generating a plasma of about the same diameter,
to form sub~tantially a point source of X-rays and thus
to provide substantially the advantages of stimulated
emission of X~rays.
In some embodiments of the invention the composi-
t:ion of the target and the temperature of the plasma are
selected to provide a substantial amount of stimulated
emission of X-rays.
In other embodiments Y-rays are directed to impinge
upon a f1uorescent target so as to remove inner shell
electrons from atoms thereof and thereby create a
population inversion.
In a typical method of providing stimulated
emission of X-rays by directing radiant energy onto a
target to create by means of a pumping mechanism some
upper and lower laser levels, the required population
inversion is not established by the pumping mechanism
alone, but by the combined action of the pumping
mechanism and a quenchiny mechanism that extinguishes
the lower laser level at a rate sufficient to establish
and continuously maintain the inversion. The pumping
mechanism typically comprises excitation by collisions
of electrons and ions or by dielectronic recombination.
The ~uenching mechanism typically comprises Auger
transitions, Coster-Krohig transitions, or collisions.
The radiant energy may be from a laser, or it may
comprise a beam of electrons. The pumping mechanism
may comprise a beam o~ electrons.
APPLICABILITY
Apparatus according to the present invention is
especially useful for applications wherein it is
expensive, time consuming, or otherwise inconvenient to
move objects that are to receive soft X-rays into and
out of a special environment, such as a vacuum ch~mber
in which the X-rays are produced. Typical applications
o' this type include laser produced X-ray systems for
hish resolution lithography, for extended X-ray absorption
rine structure (EXAFS) spectroscopy, and for X-ray
microscopy.
X rzys usually are produced in a ~acuum, but for
many purposes it is desirable to apply them in air.
~or soft X-rays, especially those having photon energies
of less than about 5 keV, a problem arises in bringing
~he X-rays ~rom the vaeuum into air, because a window
that is thick enough and strong enough to withstand the
pressure difference between the vacuum and the air is
~ 3
opaque to the X-rays, except in very small windows. I'he
problem is especially serious in X-ray lithography, where
it is desirable to illuminate large areas.
The present invention provides simple, inexpensive
convenient; means for overcoming the problem of providing
X-rays to an object that may be in an ordinary environ-
ment such as air at approximately atmospheric pressure.
Appaxatus according to this invention is useful
and advantageous not only in X-ray lithography but also
in laser EXAFS, and especially in fast EXAFS spectroscopy
with a single pulse of laser-produced X-rays, or with a
plurality of such pulses.
The technique of Extended X-ray Absorption Pine
Structure (E~AFS) spectroscopy is becoming an increas-
ingly important tool for the study of chemical structurein samples which lack long-range order, such as amor?hous
solids, solutions of biologically important materials,
and gases. These studies have gained impetus in recent
years by virtue of the availability o~ synchrotron
radiation, which provides a continuous and intense spec-
trum of the soft X-rays required for EXAFS. A synchrotron,
however, is an expensive, cumbersome source of X-rays,
to which scientists must travel in order to perform their
experiments. A laser X-ray source, on the other hand,
is relatively compact, inexpensive, and simple to opPrate
and maintain. Furthermore, there are a variety of novel
EXAFS experiments which are inherently beyond tne capa-
bilities of synchrotron radiation sources. These
experiments, which require short pulse width, intense
fluxes of low-energy tc4 keV) X-rays, and/or a continuum
or a closely packed spectral line structure, are ideally
suited to laser-produced X-rays.
The E~AFS spectrum of aluminum has been measured
with a nanosecond pulse of soft X-rays generated by a
laser-produced plasma. This technique provides a
?ractical alternative to synchrotron radiation for the
~ 6'~ l`3
14
acquisition of EX~FS data. It also provides a unique
capability for the analysis of molecular structure in
highly transient chemical species.
While the forms of the invention herein disclosed
constitute presently preferred embodiments, many others
are possible. It is not intended herein to mention all
of the possible e~uivalent forms of ramifications of
the invention. It is to be understood that the terms
used herein are merely descriptive rather than limiting,
and that various changes may be made without departing
from the spirit or scope of the invention.
