Z-pinch soft x-ray source using diluent gas

A plasma x-ray source includes a chamber defining a pinch region having a central axis, a gas supply for introducing a gas mixture into the pinch region, a preionizing device disposed around the pinch region for preionizing the gas mixture in the pinch region, and a pinch anode and a pinch cathode disposed at opposite ends of the pinch region. The gas mixture includes a primary X-radiating gas, such as xenon, and a low atomic number diluent gas, such as helium. The pinch anode and the pinch cathode produce a current through the plasma shell in an axial direction and produce an azimuthal magnetic field in the pinch region in response to application of a high energy electrical pulse to the pinch anode and the pinch cathode. The azimuthal magnetic field causes the plasma shell to collapse to the central axis and to generate X-rays. The gas mixture provides enhanced radiation intensity and reduced cost for the primary X-radiating gas.

FIELD OF THE INVENTION 
This invention relates to a plasma X-ray source of the Z-pinch type and, 
more particularly, to an X-ray source that utilizes a gas mixture 
including a primary X-radiating gas and a low atomic number diluent gas 
for improved axial radiation intensity and reduced cost. 
BACKGROUND OF THE INVENTION 
A Z-pinch plasma X-ray source that utilizes the collapse of a precisely 
controlled, low density plasma shell to produce intense pulses of soft 
X-rays is disclosed in U.S. Pat. No. 5,504,795 issued Apr. 2, 1996 to 
McGeoch. The X-ray source includes a chamber defining a pinch region 
having a central axis, an RF electrode disposed around the pinch region 
for pre-ionizing the gas in the pinch region to form a plasma shell that 
is symmetrical around the central axis in response to application of RF 
energy to the RF electrode, and a pinch anode and a cathode disposed at 
opposite ends of the pinch region. An X-radiating gas is introduced into 
the chamber at a typical pressure level between 0.1 torr and 10 torr. The 
pinch anode and the pinch cathode produce a current through the plasma 
shell in an axial direction and produce an azimuthal magnetic field in the 
pinch region in response to application of a high energy electrical pulse 
to the pinch anode and the pinch cathode. The azimuthal magnetic field 
causes the plasma shell to collapse to the central axis and to generate 
X-rays. 
X-ray measurements using different gases and gas mixtures in the disclosed 
x-ray source have shown that there is often more radiation intensity in 
directions close to the pinch axis than in the more radial directions. In 
the rapidly recombining plasma that exists within a few tens of 
nanoseconds after the pinch has reached peak density and temperature, the 
radiation field of emitted X-rays is converging on the Planck equilibrium 
distribution for a plasma at the recombination temperature. However, in 
such high aspect ratio plasmas, (aspect ratios, defined as length divided 
by diameter, of between 50 and 100 are typical in this device), it often 
happens that the radiation field cannot reach equilibrium in non-axial 
directions due to the limited optical depth of the plasma in these 
directions. As a consequence, it appears that the equilibrium intensity in 
the axial direction is able to overshoot the Planck value. This Planckian 
overshoot factor has been measured to exceed 6 for radiation at the 
wavelength of 100 angstroms in the case of the recombination of 
lithium-like oxygen (O VI). 
A method for exciting the 134 angstrom xenon band of interest for 
lithography, using laser excitation of xenon clusters in a high pressure 
expansion, is disclosed in U.S. Pat. No. 5,577,092 issued Nov. 19, 1996 to 
Kubiak et al. The disclosed method uses a continuous flow of xenon, 
accompanied by other gases, through a nozzle, and results in substantial 
xenon usage. An XUV radiation source, based on the electron beam 
excitation of a xenon gas jet, that is stated to be useful in lithography 
applications is disclosed in U.S. Pat. No. 5,637,962 issued Jun. 10, 1997 
to Prono et al. 
It is desirable to provide plasma X-ray sources and methods of operating 
such sources which produce increased radiation intensity and reduced 
operating costs in comparison with prior art X-ray sources. 
SUMMARY OF THE INVENTION 
According to a first aspect of the invention, a plasma X-ray source is 
provided. The plasma X-ray source comprises a chamber defining a pinch 
region having a central axis, a gas supply for introducing a gas mixture 
into the pinch region, a device disposed in proximity to the pinch region 
for preionizing the gas mixture in the pinch region, and a pinch anode and 
a pinch cathode disposed at opposite ends of the pinch region. The gas 
mixture comprises a primary X-radiating gas and a low atomic number 
diluent gas. The pinch anode and the pinch cathode produce a current 
through the plasma shell in an axial direction and produce an azimuthal 
magnetic field in the pinch region in response to application of a high 
energy electrical pulse to the pinch anode and the pinch cathode. The 
azimuthal magnetic field causes the plasma shell to collapse to the 
central axis and to generate X-rays. 
The diluent gas may be selected from the group consisting of helium, 
hydrogen, deuterium, nitrogen and combinations thereof. The primary 
X-radiating gas may be selected from the group consisting of xenon, argon, 
krypton, neon and oxygen, but is not limited to this group. The gas 
mixture preferably has a total pressure in the pinch region in a range of 
about 0.1 torr to 1.0 torr. 
In one embodiment, the primary X-radiating gas is xenon for generation of 
134 angstrom xenon band radiation and the diluent gas is helium. Radiation 
intensity enhancements of between 20% and 40% relative to the use of 
undiluted xenon have been achieved in this embodiment. 
The preionizing device may comprise an RF electrode for preionizing the gas 
mixture in the pinch region in response to application of RF energy to the 
RF electrode. The chamber may define a substantially cylindrical pinch 
region. The preionizing device preferably produces an axially uniform 
discharge in the pinch region.

DETAILED DESCRIPTION 
An example of a plasma x-ray source in accordance with the present 
invention is shown in FIG. 1. An enclosed chamber 10 defines a pinch 
region 12 having a central axis 14. The chamber 10 may include an x-ray 
transmitting window 16 located on axis 14. A gas inlet 20 and a gas outlet 
22 permit a gas at a prescribed pressure to be introduced into the pinch 
region 12. The example of FIG. 1 has a generally cylindrical pinch region 
12. 
A cylindrical dielectric liner 24, which can be a ceramic material, 
surrounds pinch region 12. An RF electrode 26 is disposed on the outside 
surface of dielectric liner 24. A pinch anode 30 is disposed at one end of 
the pinch region 12, and a pinch cathode 32 is disposed at the opposite 
end of pinch region 12. The portion of pinch anode 30 adjacent to pinch 
region 12 has an annular configuration disposed on the inside surface of 
the dielectric liner 24. Similarly, the portion of cathode 32 adjacent to 
pinch region 12 has an annular configuration inside dielectric liner 24 
and spaced from dielectric liner 24. In a preferred embodiment, the pinch 
cathode 32 includes an annular groove 50 which controls the location at 
which the plasma shell attaches to cathode 32. Preferably, the anode 30 
has an axial hole 31, and the cathode 32 has an axial hole 33 to prevent 
vaporization by the collapsed plasma, as described below. The anode 30 and 
the cathode 32 are connected to an electrical drive circuit 36 and are 
separated by an insulator 40. The anode 30 is connected through a 
cylindrical conductor 42 to the drive circuit 36. The cylindrical 
conductor 42 surrounds pinch region 12. As described below, a high current 
pulse through cylindrical conductor 42 contributes to an azimuthal 
magnetic field in pinch region 12. An elastomer ring 44 is positioned 
between anode 30 and one end of dielectric liner 24, and an elastomer ring 
46 is positioned between cathode 32 and the other end of dielectric liner 
24 to ensure that the chamber 10 is sealed vacuum tight. 
In the example of FIG. 1, the chamber 10 is defined by cylindrical 
conductor 42, an end wall 47 and an end wall 48. The cylindrical conductor 
42 and end wall 47 are electrically connected to anode 30, and end wall 48 
is electrically connected to cathode 32. It will be understood that 
different chamber configurations can be used within the scope of the 
invention. 
The RF electrode 26 is connected through an RF power feed 52 to an RF 
generator 200 which supplies RF power for preionizing the gas in a 
cylindrical shell of pinch region 12. The RF power preferably has a power 
level greater than one kilowatt. In a preferred embodiment, the RF power 
is 5 kilowatts at 1 GHz. It will be understood that different RF 
frequencies and power levels can be used within the scope of the present 
invention. In a preferred embodiment, the RF electrode 26 comprises a 
center-fed spiral antenna wrapped around the dielectric liner 24, with a 
total angular span of +/-200.degree.. It will be understood that different 
spiral configurations and different RF electrode configurations can be 
utilized for preionizing the gas in the pinch region 12. The spiral 
configuration described above has been found to provide satisfactory 
results. 
The drive circuit 36 supplies a high energy, short duration of electrical 
pulse to anode 30 and cathode 32. In a preferred embodiment, the pulse is 
25 kilovolts at a current of 300 kiloamps and a duration of 200-250 
nanoseconds. 
The inside wall of dielectric liner 24, the anode 30 and the cathode 32 
define a cylinder of low density gas. RF power is applied to the RF 
electrode 26 to cause ionization within the gas cylinder. It is a property 
of the application of intense RF power to a gas surface that the 
ionization is concentrated in a surface layer. This is exactly what is 
needed to create a precise cylindrical plasma shell 56 for the subsequent 
passage of current. Once the gas has been preionized by RF energy, the 
drive circuit 36 is activated to apply a high energy electrical pulse 
between anode 30 and cathode 32. Typically, the RF power is applied 1-100 
microseconds before the drive circuit 36 is activated. The high energy 
pulse causes electrons to flow from the pinch cathode 32 to the pinch 
anode 30. Initially, the current flows in the preionized outer layer of 
the gas cylinder and forms plasma shell 56. The return current flows back 
to the drive circuit 36 through the outer cylindrical conductor 42. An 
intense azimuthal magnetic field is generated between the outer current 
sheet through cylindrical conductor 42 and the current sheet in the plasma 
shell 56. The magnetic field applies a pressure which pushes the plasma 
shell 56 inward toward the axis 14. After approximately 200-250 
nanoseconds, the drive circuit 36 is discharged and the current drops to a 
lower value. At approximately the same time, the plasma shell reaches the 
axis 14 with high velocity, where its motion is arrested by collisions 
with the incoming plasma shell from the opposite radial direction. The 
result of this stagnation process is the conversion of kinetic energy into 
heat, which further ionizes the gas into high ionization states that 
radiate x-rays strongly in all directions. In the case of population 
inversion on an x-ray transition and in cases when the plasma is optically 
dense in the axial direction but optically thin in radial directions, the 
radiation is concentrated in the two axial directions via amplified 
spontaneous emission. Thus with reference to FIG. 1, the plasma shell 56 
collapses to form a collapsed plasma 60 on axis 14 in approximately 
200-250 nanoseconds. 
RF generator 200 supplies RF energy to RF electrode 26 through RF power 
feed 52. The RF generator 200 may be any suitable source of the required 
frequency and power level. A regulated gas supply 202 is connected to gas 
inlet 20, and a vacuum pump 204 is connected to gas outlet 22. The gas 
supply 202 and the vacuum pump 204 introduce gas into pinch region 12 and 
control the pressure at the desired pressure level. 
In drive circuit 36, multiple circuits are connected in parallel to the 
pinch anode 30 and the pinch cathode 32 to achieve the required current 
level. A preferred embodiment utilizes six to eight drive circuits 
connected in parallel, each generating about 20 to 40 kiloamps. As shown 
in FIG. 1, each drive circuit includes a voltage source 210 connected to 
an energy storage capacitor 212. A switch 214 is connected in parallel 
with storage capacitor 212. The switch 214 may comprise a multiple channel 
pseudospark switch as described in U.S. Pat. No. 5,502,356 issued Mar. 26, 
1996 to McGeoch, which is hereby incorporated by reference. The switch 214 
may also comprise a hydrogen thyratron. The switches 214 in the parallel 
circuits are closed simultaneously to generate a high energy pulse for 
application to the anode 30 and cathode 32. Additional information 
regarding the Z-pinch plasma X-ray source is disclosed in U.S. Pat. No. 
5,504,795, which is hereby incorporated by reference. 
According to the present invention, the gas introduced into the pinch 
region 12 is a gas mixture including a diluent gas and a primary X-ray 
emitting gas. The gas mixture renders radiating transitions of the primary 
gas optically thin in directions other than axial, thereby enhancing the 
axial radiation intensity that is achievable during recombination. 
Typically, the diluent gas is a substantial fraction of the gas mixture 
introduced into the pinch region prior to electrical excitation of the 
source. Because a smaller volume of the relatively expensive primary 
X-radiating gas is used, the cost of operating the X-ray source is 
reduced. 
The diluent gas should have low atomic number (preferably less than Z=8) in 
order to completely ionize without requiring too great an energy input, 
which would otherwise detract from the energy available for ionization of 
the primary radiating gas. The diluent gas typically can be, but is not 
limited to, helium, hydrogen, deuterium, nitrogen and combinations 
thereof. An example of the invention is the enhanced Z-pinch axial 
emission of xenon in the 134 angstrom band useful for lithography using 
helium as the diluent gas. 
Data from a 4 centimeter long Z-pinch region indicates an approximate 40% 
increase in the xenon band axial intensity at 134 angstroms as the helium 
diluent fraction is increased from 0% to 75% of a helium-xenon mixture. 
The typical evolution of the xenon band spectrum with helium dilution is 
shown in FIG. 2, with a spectral range from 100 angstroms to 150 angstroms 
as shown. Curves 300, 302 and 304 represent xenon percentages of 17%, 25% 
and 35%, respectively, in the gas mixture, with the balance being helium. 
In FIG. 2, the total gas density in the pinch region has been adjusted in 
each case to yield optimum spectral intensity at 134 angstroms. 
A corresponding set of data from an 8 centimeter Z-pinch region is shown as 
curve 320 in FIG. 3. Although the enhancement with dilution appears to be 
less for the longer pinch, it amounts to a 20% increase, with the optimum 
again being observed for the 25% Xe/75% He mixture. 
It has also been shown that both hydrogen and nitrogen can be substituted 
for helium with very little change in axial radiation efficiency. It is 
presumed that deuterium would perform in a similar manner. 
The use of helium as a diluent is preferred over more chemically active 
elements, such as hydrogen or nitrogen, in order to give the source 
maximum compatibility with user systems that might be exposed to low 
concentrations of the pinch gas mixture at remote locations down an 
evacuated X-ray beamline. 
Very low xenon concentrations can be employed in helium diluent with little 
loss of efficiency. FIG. 3 shows that as little as 0.7% Xe in helium will 
yield 80% of the intensity that occurs with 25% Xe in helium. This 
circumstance allows very efficient photon production per flowing xenon 
atom, although it is to be noted that approximately two times the total 
gas pressure is required for the lowest xenon cases, in order to optimize 
the spectral intensity in the band at 134 angstroms. 
The primary X-radiating gas contained within pinch region 12 can be any gas 
having suitable transitions for X-ray generation. Examples include, but 
are not limited to xenon, argon, krypton, neon and oxygen. The total gas 
pressure is selected to give high enough gas density to ensure a high 
collision rate as the gas stagnates on the axis, but not so high a density 
that the motion is slow and the incoming kinetic energy is too low to 
create the high temperature for needed for X-ray emission. Typically, the 
total gas pressure of the X-radiating gas and the diluent gas is in a 
range of about 0.1 torr to 1.0 torr. Gas may be caused to flow through 
pinch region 12 continuously or may be pulsed with a relatively long time 
constant. The pressure in the pinch region 12 should be substantially 
uniform when the high current electrical pulse is applied to the source. 
As described above, a higher total gas pressure is required when the 
primary X-radiating gas is a small fraction of the gas mixture. 
While there have been shown and described what are at present considered 
the preferred embodiments of the present invention, it will be obvious to 
those skilled in the art that various changes and modifications may be 
made therein without departing from the scope of the invention as defined 
by the appended claims.