JP3317957B2 - Plasma focus high energy photon source with blast shield - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] This application is related to US patent application Ser. No. 09 / 268,243, filed Mar. 15, 1999, and Ser. No. 08/85, now US Pat. No. 5,763,930.
U.S. patent application Ser. No. 09 / 093,416 filed on Jun. 8, 1998, which is a continuation-in-part of 4507, and U.S. Pat. Is a continuation-in-part application.

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high energy photon source, and more particularly to a highly reliable x-ray and high energy ultraviolet source.

[0003]

2. Description of the Related Art The semiconductor industry continues to evolve lithography technology, which allows the printing of increasingly smaller integrated circuits. These systems must have reliable, cost-effective throughput ratios and reasonable processing tolerances. The integrated circuit manufacturing industry currently has mercury G-lines (436 nanometers) and I
There is a shift from line (365 nanometer) exposure sources to 248 and 193 nanometer excimer laser photon sources. This transition has been fueled by the need for high lithographic resolution to minimize loss of depth of focus.

The demands of the integrated circuit industry will soon exceed the resolution of 193 nanometer exposure sources, creating a need for reliable exposure sources at wavelengths much shorter than 193 nanometers. One excimer line is at 157 nm, but it is difficult to obtain an optical material having sufficient transmittance at this wavelength and sufficient optical performance. Therefore, a total internal reflection imaging system would be required. Total internal reflection optical systems require a smaller numerical aperture than transmission systems.
The loss of resolution due to the small numerical aperture cannot be compensated for without significantly reducing the wavelength. Therefore, if the resolution of optical lithography had to be improved beyond that obtained at 193 or 157 nanometers, a light source in the range of 10 nanometers would be required.

[0005] The current state of the art in high energy ultraviolet and x-ray sources utilizes plasmas created by bombarding various target materials with laser beams, electrons and other particles.
Although solid targets have been used, debris created by ablation of solid targets has deleterious consequences on various components of the system intended for run-of-the-mill operations. One proposed solution to the debris problem is to use a frozen liquid or frozen gas target to avoid sticking to the surface of the optical device. However, none of these systems has proven to be practical in a line operation.

It has long been known that x-rays and high-energy ultraviolet radiation can be produced by a plasma focusing operation. In a plasma aperture, one of several possible configurations, for example, a magnetic field generated by a current, accelerates the electrons and ions in the plasma, stripping off most of the electrons outside the ions, resulting in x-rays and high-energy ultraviolet radiation. The current is passed through the plasma in such a configuration that it is formed into a small volume with sufficient energy to produce. Various prior art techniques for producing high energy radiation by plasma focusing, ie, focusing, are described in the following patents.・ Dawson (JM Dawson), “x-ray generator”
U.S. Patent No. 3,961,197, June 1, 1976-TG Roberts, et al., "X-ray Generator Assisted by Strong, High Energy Electron Beam" U.S. Patent No. 3,969,628 JH Lee, Adduction Cycloid Restriction Device, U.S. Pat. No. 4,042,848, 1977.
Aug. 16, 2013 L. Cartz et al., "Laser Beam Plasma Focused X-Ray System" U.S. Pat.
4,964, March 12, 1985 A. Weiss et al., "Plasma Focused X-Ray Apparatus," U.S. Pat. No. 4,536,884,198.
Aug. 20, 5 S. Iwamatsu "x-ray source" U.S. Pat. No. 4,538,291, Aug. 27, 1985 G. Herziger and W. Neff "x-rays" Apparatus for generating a plasma source with high radiation intensity in the region "US Pat. No. 4,596,03
0, June 17, 1986 A. Weiss et al., "X-Ray Lithography System", U.S. Pat. No. 4,618,971, 198.
October 21, 6 ・ A. Weiss et al., “Plasma Focus X-Ray Method” US Pat. No. 4,633,492, 1986
December 30, 2012-Okada (I. Okada) and Saito (Y. Sai)
toh) “X-ray source and x-ray lithography method” US Pat. No. 4,635,282, January 6, 1987 ・ RP Gupta et al., “Plasma focus x-ray source derived from multi-vacuum arc”, USA Patent number 4,75
1,723, June 14, 1988 • Gupta et al., “Ring Plasma Focused X-Ray Source Derived from Gas Emission”, US Pat.
2,946, June 21, 1988-JC Riodan. And JS Peariman, "Filtering devices for use with x-ray sources" U.S. Pat. No. 4,837,794,1
June 6, 989 ・ W. Neff et al., “Using a plasma source for x
Device for generating line radiation "U.S. Patent No. 5,023,
897, June 11, 1991 DA Hammer and DH Kalantar, "Methods and apparatus for microlithography using x-ray focused x-ray sources".
U.S. Pat. No. 5,102,776, Apr. 7, 1992 MW McGeoch "Plasma x-ray source" U.S. Pat. No. 5,504,795, 1996
April 2, 2010 ・ G. Schriever et al., “Laser-produced lithium plasma as a narrow-band broad-band ultraviolet radiation source for photoelectron spectroscopy”, Applied Optics No. 3
Vol. 7, Part 7, pages 1243-1248, September 1998. R. Lebert, et al., "Exhaust Gas-Based Radiation Sources for Extreme Ultraviolet Lithography", International Conference on Micro and Nano Engineering, 199.
September 2008 ・ W. Partlo, Fomenkov (I.
Fomenkov and D. Birx, "Ultraviolet (13.5 nm) Generation Using High-Density Plasma Focus Devices", Bulletin of the International Association of Optical Engineering (SPIE), Latest Lithography Technology III, 3676, 846- 858
, March 1999-WT Silfast et al., "High-Performance Plasma Emission Sources at 13.5 and 11.4 Nanometers for Ultra-Ultraviolet Lithography", International Institute of Optical Engineering (SPIE) ) Newsletter, Latest Lithography Technology III, 3
676, pp. 272-275, March 1999 ・ F. Wu et al., “Vacuum Spark and Spherical Focused X-ray / Ultraviolet Point Radiation Source”, Report of the International Association of Optical Engineering (SPIE), Latest Lithography Technology , 3676, 410-420, 199
March 9 ・ W. Partlo, Fomenkov (I.
Fomenkov and D. Birx "13.5 nanometer characterization for extreme ultraviolet lithography based on high density plasma focus and lithium emission", SEMATECH International Workshop on Extreme Ultraviolet Lithography, 199.
October 9

[0007] Typical prior art plasma focus devices have been able to generate large amounts of radiation suitable for near x-ray lithography, but due to the high electrical energy requirements per pulse and the short-lived internal components, the repetition rate is low. Speed is limited. Storage electrical energy requirements for these systems range from 1 kilojoule to 10 kilojoules. Generally, the repetition rate did not exceed 2-3 times per second.

[0008]

What is needed is a reliable run-of-the-mill capable of producing high energy ultraviolet and x-ray radiation that operates at high repetition rates and avoids the prior art problems associated with debris formation. It is a simple system with.

[0009]

SUMMARY OF THE INVENTION The present invention provides a high energy photon source. A pair of plasma focusing electrodes are mounted in the vacuum chamber. The chamber contains a functional gas, which contains a buffered noble gas and an active gas selected to obtain the required spectral lines. The pulsed power source supplies an electrical pulse of high voltage sufficient to cause a discharge between the electrodes, producing a very hot and dense plasma aperture that emits in the active gas spectral lines in the functional gas . A blast shield located directly behind the high-density plasma focus is a physical obstacle that limits the focusing and limits its axial extension. A small opening is provided in the blast shield through which radiation can pass but not plasma. In a preferred embodiment, the surface of the shield facing the plasma is dome-shaped.

[0010] In a preferred embodiment, an externally reflected radiation collecting director collects the radiation generated in the plasma focus and directs the radiation in a desired direction. Also, in a preferred embodiment, the active gas is lithium vapor, the buffer gas is helium, and the radiation collector is made of or coated with a material having high reflectivity for small angles of incidence. Good reflector materials are molybdenum, palladium, ruthenium, rhodium, gold or tungsten.

In another preferred embodiment, the buffer gas is argon and the lithium gas is generated by evaporation of solid or liquid lithium contained in a hole along the axis of the central electrode of the coaxial electrode structure. In a preferred embodiment, debris is collected by a conical nested debris collector having a surface that is aligned with the light rays that extend from the aperture and are directed to the radiation collection director. The nested debris collector and the radiation collection director are maintained in the range of about 400 ° C., which is above the melting temperature of lithium and substantially below the melting temperature of tungsten. Both tungsten and lithium vapor are collected on the debris collector, but the lithium evaporates away from the debris collector and collection director, while the tungsten remains permanently on the debris collector, Therefore, they do not collect and impair the reflection performance of the radiation collecting director. The reflected radiation collection director and the conical nested debris collector can be assembled together as one part, or they can be separate parts, aligned with each other and with the constriction You can also.

A unique room viewing window can be provided that transmits extreme ultraviolet light but reflects low energy light, including visible light. The window is preferably a very thin, small diameter window made of a material such as silicon, zirconium or beryllium.

The applicant now describes a high-density plasma focus (DPF) prototype device made by the applicant and his colleagues as a light source for extreme ultraviolet (EUV) lithography employing an all solid state pulsed power drive. I do. Utilizing the results obtained from a vacuum grating spectrometer, combined with measurements by silicon photodiodes, Applicants have determined that a significant amount of radiation in the reflection band of the molybdenum / silicon mirror is 13.5 of divalently ionized lithium. It has been discovered that it can be generated using the nanometer emission line spectrum. The prototype high-density plasma focus (DPF) converts 25 joules of stored electrical energy per pulse into approximately 0.76 joules in-band 13.5 nanometer radiation emitted into 4π steradians. The pulse repetition rate performance of this device has been observed to go up to the limit of the DC power supply of 200 Hz. Up to this repetition rate, no significant dip in extreme ultraviolet power per pulse has been observed. 200
At Hertz, the energy stability between the measured pulses is σ
= 6%, and no pulse is missing. The electrical circuit and operation of the prototype high-density plasma focus (DPF) device is shown along with a description of several preferred alternative embodiments intended to improve stability, efficiency and performance.

[0014] The present invention is directed to the use of extreme ultraviolet lithography in a reliable high intensity extreme ultraviolet source (EUV) with emission characteristics that closely match the reflection band of the molybdenum / silicon or molybdenum / beryllium mirror system. It is intended to provide actual use. Since the proposed total internal reflection ultra-violet (EUV) lithographic apparatus is a system based on slit scanning, the present invention provides an ultra-violet (EUV) light source with high repetition rate performance.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Preferred Embodiment) A simplified diagram of a high energy ultraviolet light source is shown in FIG. The main components are a plasma focusing unit 2, a high energy photon collector 4, and a light transmission tube 6. The plasma focusing light source is a low inductance pulse power circuit 10.
And has a coaxial electrode 8 powered by. The pulsed power circuit in the preferred embodiment is a high voltage, full energy circuit that delivers about 5 microsecond pulses in the range of 1 kilojoule to 2 kilojoules to 1,00.
It can be supplied to the coaxial electrode 8 at a rate of 0 Hertz.

A small amount of functional gas, such as a mixture of helium and lithium vapor, is present near the base of the electrode 8, as shown in FIG. Each time a high voltage pulse is applied, the coaxial electrode 8
Avalanche breakdown occurs between the internal and external electrodes. During the avalanche breakdown process that occurs in the buffer gas, the gas is ionized, creating a conductive plasma between the electrodes at the electrode base. Once the conductive plasma is generated, a current flows between the inner and outer electrodes. In this preferred embodiment, the internal electrodes are at a high positive potential and the external electrodes are at ground potential. Current flows from the inner electrode to the outer electrode, so that electrons flow toward the center and positive ions flow out of the center. This current flow generates a magnetic field which accelerates the flowing charge carriers away from the base of the coaxial electrode 8.

When the plasma reaches the tip of the center electrode,
The force of the electric and magnetic fields on the plasma narrows the plasma to a "focal point" along the center line of the central electrode and just ahead of the tip at point 10, and the pressure and temperature of the plasma rise rapidly. Extremely high temperatures, in some cases much higher than the sun's surface temperature. The dimensions of the electrodes and the total electrical energy of the circuit are preferably optimized to produce the required black body temperature in the plasma. To generate radiation in the 13 nanometer range, 20 to 1
Black body temperatures in excess of 00 eV are required. Generally, for a particular coaxial structure, the temperature will increase as the potential of the electrical pulse increases. The shape of the radiating point is somewhat irregular in the axial direction and almost Gaussian in the radial direction. The typical radial dimension of the light source is 300 microns and the length is approximately 4 millimeters.

In most prior art plasma focus units described in the technical literature, the emission point emits radiation in all directions with a spectrum very close to a blackbody. The purpose of lithium in the functional gas is to narrow the spectrum of radiation from the point of emission.

(Lithium vapor) Divalent ionized lithium exhibits an electronic transition at 13.5 nm, and functions as a radiation source atom in a helium buffer gas. Divalent ionized lithium is an excellent choice for two reasons. The first is the low melting point and high vapor pressure of lithium.
Lithium released from the emission point can be prevented from adhering to the vacuum chamber wall and the surface of the collection optical device by simply heating the surface to 180 ° C. or more.
The gas phase lithium can then be pumped out of the vacuum chamber with helium buffer gas using conventional turbo molecular pumping techniques. Also, lithium is simply these two
The gas can be easily separated from the helium by cooling.
Coating materials are used to provide good reflection at 13.5 nanometers. FIG. 8 shows the lithium emission peak in relation to the published molybdenum / silicon reflectivity.

A third advantage of utilizing lithium as the source atom is that non-ionized lithium has a low absorption cross section for 13.5 nanometer radiation. Further, the ionized lithium emitted from the emission point can be easily wiped out with a mild electric field. The remaining non-ionized lithium has little effect on 13.5 nanometer radiation. The most common radiation source currently proposed in the 13 nanometer range is a laser-ablated xenon frozen jet. Because of the large absorption cross section of xenon at 13 nanometers in such a system, virtually all emitted xenon must be captured before the next pulse.

(Radiation Collector) The radiation produced at the point of radiation is uniformly emitted into all 4π steradians. Certain types of collection optics require that this radiation be captured and directed to a lithographic apparatus. Previously proposed 13 nanometer light sources have suggested collection optics based on multilayer electrically insulating coated mirrors. Multilayer electrically insulated mirrors were used to obtain high collection efficiency in large angle areas. A debris-producing radiation source would coat the insulated mirror and reduce its reflectivity, thus reducing the collected power exiting the radiation source. The preferred system would have degraded any insulated mirrors placed near the emission point over time due to electrode erosion.

Several materials are available that have high reflectivity at small angles of incidence of ultraviolet light at 13.5 nanometers. Graphs for some of these are shown in FIG. Good options include molybdenum, rhodium and tungsten. The collector can be composed of these materials, and it is preferred to apply them as a coating to a substrate structural material such as nickel. This cone is
Prepared by nickel electroplating on a removable mandrel.

In order to make a collector that can accept large cone angles, several cone sections can be nested inside one another. Each conical section may reflect the radiation one or more times to redirect the radiation cone of that section to a desired direction. Designing a collection for operation at small angles of incidence will create a collector that will best withstand the deposition of eroded electrode material. The reflectivity of such mirrors at very small angles of incidence depends on the surface roughness of the mirror. The dependence on surface roughness decreases as the angle of incidence approaches the small angle of incidence. We estimate that the 13 nanometer radiation emitted over a fixed angle of at least 25 degrees can be collected and directed. A suitable collector for directing radiation to a light transmission tube is shown in FIGS.
It was shown to.

(Tungsten Electrode-Tungsten Coating of Collector) A preferred method of selecting the material of the external reflection collector is to make the coating material of the collector the same as the electrode material. Tungsten is a promising candidate because it has demonstrated performance as an electrode and the real property of a refractive index of 0.945 at 13 nanometers. Using the same material for the electrode and the mirror coating minimizes the loss of mirror reflectivity as the eroded electrode material coats over the collection mirror.

Silver (Silver Electrode and Coating) Silver is also an excellent choice for electrodes and coatings because it has a low refractive index at 13 nanometers and high thermal conductivity which allows high repetition rate operation.

Conical Nested Debris Collector In another preferred embodiment, the collection director is provided by a debris collector 5, which collects all the tungsten vapor before it reaches the collection director. Protected from contamination by materials. FIG. 9 shows a conical nested debris collector 5 that collects debris resulting from plasma narrowing. The debris collector 5 comprises a conical nested portion extending from the center of the aperture and having a surface aligned with the light beam directed to the collection director 4.

The collected debris contains evaporated tungsten and evaporated lithium from the tungsten electrode.
The debris collector is attached to or part of the radiation collection director 4. Both collectors are made of a nickel-plated substrate. The radiation collecting director 4
Are coated with molybdenum or rhodium to have very high reflectivity. Preferably, both collectors are heated to about 400 ° C., which is well above the melting point of lithium and substantially below the melting point of tungsten. Both the lithium vapor and the tungsten vapor collect on the surface of the debris collector 5, but the lithium evaporates away, and even if the lithium collects on the collection director 4, it evaporates immediately thereafter. Once collected in the debris collector 5, the tungsten remains there permanently.

FIG. 7 shows the optical features of the collector designed by the applicant. The collector consists of five nested, small angle of incidence parabolic mirrors, but only three of the five mirrors are shown. The two inner mirrors are not shown. In this design, the collection angle is 0.4 steradians. As discussed below, the collector surface is coated,
Heated to prevent lithium deposition. This design produces a parallel beam. Other preferred designs, such as those shown in FIGS. 1, 3, and 10, concentrate the beam. The collector should be coated with a material that has a high micro-incidence reflectance in the 13.5 nanometer wavelength region. Two such materials are palladium and ruthenium.

(Light Transmission Tube) It is important that the deposited material is kept away from the illumination optics of the lithographic apparatus. Therefore, it is desirable that the optical transmission tube 6 further ensures this separation. The light transmission pipe 6 is a hollow light transmission pipe whose inner surface is almost totally reflected outward. The basic collection optics can be designed to reduce the cone angle of the collected radiation to match the acceptance angle of the hollow light transmission tube. This concept is illustrated in FIG. As shown in FIG. 1, tungsten, silver or lithium atoms are
The insulated mirror of the lithographic apparatus would be very well protected from electrode debris, as it would diffuse upstream against the buffer gas flowing through the hollow light transmission tube.

(Pulse Power Unit) The preferred pulse power unit 10 is a solid state high frequency high voltage pulse power unit using a solid state trigger and a magnetic switch circuit such as the pulse power unit described in US Pat. No. 5,142,166. These units are reliable and can operate continuously for almost months and billions of pulses with little maintenance. The teachings of US Pat. No. 5,142,166 are incorporated herein by reference.

FIG. 4 shows a simple electric circuit for supplying pulse power. The preferred embodiment includes a DC power supply 40 of the command resonant charge supply type used for excimer lasers. C ₀ is a bank of off-the-shelf capacitors with a merged capacitance of 65 μFaraday, and peaking capacitor C ₁ is also a bank of off-the-shelf capacitors with a merged capacitance of 65 μFaraday. The saturable inductor 42 has a saturation excitation inductance of about 1.5 nanoFaraday. The trigger 44 is an insulated gate bipolar transistor. Diode 4
Functions 6 and the inductor 48, which forms an energy recovery circuit similar to the one described in U.S. Patent No. 5,729,562, to accumulate reflected electrical energy emitted from one pulse to the C ₀ prior to come next pulse do.

(System—First Preferred Embodiment) Thus, as illustrated in FIG. 1, in a first preferred embodiment, a mixed functional gas of helium and lithium vapor is released into the coaxial electrode 8. The electrical pulse from the pulse power unit 10 creates a dense plasma focus at a sufficiently high temperature and pressure to ionize the lithium atoms in the functional gas bivalently, producing ultraviolet radiation at a wavelength of about 13.5 nanometers.

This light is collected by an external total internal reflection collector 4 and directed to a hollow light transmission tube 6, where the light is further directed to a lithographic apparatus (not shown). Discharge chamber 1
Is maintained at a vacuum of about 4 Torr by the turbo suction pump 12. Some of the helium gas in the functional gas is separated by a helium separator 14 and used to clean the light transmission tube as shown in FIG. The pressure of helium in the light transmission tube is preferably balanced with the required pressure of the lithographic apparatus, which is usually kept at a low pressure or vacuum. The temperature of the functional gas is maintained at a desired temperature by the heat exchanger 20, and the gas is cleaned by the electrostatic filter 22. The gas is released into the coaxial electrode space as shown in FIG.

Prototype Unit A diagram of a prototype plasma focusing unit fabricated and tested by Applicants and colleagues is shown in FIG.
Major elements C ₁ capacitor decks, C ₀ capacitor deck, insulated gate bipolar transistor switches, saturable inductor 42, vacuum vessel 3, and coaxial electrode 8.

(Test Results) FIG. 6 shows a typical pulse shape measured by the applicant in the unit shown in FIG. Applicants have recorded the luminance of a current and 13.5 nm of the voltage C ₁ a C ₁ over 8 micro seconds. The total energy of this representative pulse was about 0.8 Joules. The pulse width was about 280 nanoseconds (at full width at half maximum). The voltage at C ₁ before breakdown was slightly less than 1 kilovolt. The prototype of this embodiment can operate at pulse rates up to 200 Hertz. The measured 13.5 nanometer radiation in the average band at 200 Hertz is 152 watts at 4π steradian. The energy stability at 1 sigma is 6%. Applicant has 3.2% of the energy
Estimate that it can be directed into a 13.5 nanometer beam useful with the collector 4 illustrated in FIG.

(Second Preferred Plasma Focusing Unit) FIG. 2 shows a second preferred plasma focusing unit. This unit is disclosed in U.S. Pat. No. 4,042,84
It is similar to the plasma focus device described in No. 8. This unit has two outer disc-shaped electrodes 30, 32 and an inner disc-shaped electrode 36. The aperture is formed from three directions, as described in patent 4,042,848 and pointed out in FIG. The constriction starts near the periphery of the electrode, progresses toward the center, and the emission point appears along the axis of symmetry at the center of the internal electrode, as shown at 34 in FIG. Radiation is collected and directed as described in connection with the embodiment of FIG. However, as shown in FIG. 2, radiation emerges from both sides of the unit.
It is possible to capture in any direction. Furthermore, it would be possible to place an insulating mirror at point 38 so that a significant proportion of the radiation initially reflected to the left is reflected through the radiation point. This should enhance radiation to the right.

(Third Preferred Embodiment) The third preferred embodiment can be described with reference to FIG. This embodiment is similar to the first preferred embodiment. In this embodiment, however, the buffer gas is argon. Helium has the desirable property of being relatively insensitive to 13 nanometer radiation, but has the undesirable property of low atomic weight. Due to the small atomic weight, we must operate the system in a 2 to 4 Torr pressure environment. Another disadvantage of the small atomic weight of helium is the length of the electrodes required to match the acceleration distance to the timing of the electrical excitation circuit. Helium is light,
The electrode must be longer than desired so that the helium moves away from the tip of the electrode as soon as the current through the excitation circuit peaks.

[0038] Heavier atoms, such as argon, are less conductive than helium at a given pressure, but their higher atomic weights can produce a stable refinement at lower pressures. The low operating pressure of argon is too much to compensate for the increased absorption propensity of argon. In addition, the required electrode length is reduced due to the high atomic weight. Short electrodes are advantageous for two reasons. The first is the reduction in circuit inductance that occurs when using short electrodes. The lower inductance makes the excitation circuit more efficient and thus reduces the required electric pump energy. A second advantage of short electrodes is a reduction in the heat transfer path distance from the tip to the base of the electrode. Most of the thermal energy imparted to the electrode occurs at the tip, and conduction cooling of the electrode occurs primarily at the base (radiative cooling also occurs). With short electrodes, the temperature drop along the length from the hot tip to the cold base is small. The system operates at a higher repetition rate, both due to the lower pump energy per pulse and the improved cooling path. Increasing the repetition rate directly increases the average optical power capability of the system. Assessing the output power with increasing repetition rate is the most desirable method for the average output power of a lithographic light source, as opposed to increasing the energy per pulse.

In this preferred embodiment, unlike the first and second embodiments, lithium is not injected into the vacuum chamber in the gas phase. Instead, solid lithium is placed in the center hole of the center electrode as shown in FIG. Heat emanating from the electrodes raises the lithium to the evaporation temperature. By adjusting the height of lithium relative to the hot tip of the electrode, the partial pressure of lithium near the tip of the electrode can be controlled. A preferred embodiment for doing this is shown in FIG. A mechanism is provided for adjusting the tip of the lithium bar relative to the tip of the electrode. Coaxial electrode 8
Preferably, the system is installed vertically so that the open end of the top is on top and the molten lithium need only be lumpy near the tip of the center electrode. The beam exits vertically vertically as shown in FIG. 5A. (An alternative is
Heating the electrode to a temperature above the melting point of lithium so that lithium is added in liquid). Very low speed fluid pumps are available to pump the liquid at the rate required for a particular repetition rate. Liquid lithium can also be pushed to the center electrode tip region using a tungsten push rod.

The hole through the center of the electrode offers another important advantage. Since the narrowing of the plasma is formed near the center of the tip of the center electrode, much of the energy dissipates in this region. The electrode material near this point is ablated and impairs other surfaces inside the pressure vessel. Employing an electrode with a central hole greatly reduces eroded material. In addition, Applicants' experiments indicate that the presence of lithium vapor in this region further reduces the erosion rate of the electrode material. Using a bellows or other suitable sealing method
A good seal should be maintained where the electrode device enters the vacuum chamber. A replacement electrode sufficiently equipped with solid lithium can be manufactured easily and inexpensively and can be easily replaced in the vacuum chamber.

(Small Window in Vacuum Chamber) The narrowing portion generates a large amount of visible light which must be separated from extreme ultraviolet (EUV). Preferably, the window additionally ensures that the lithographic optical element is not contaminated with lithium or tungsten. The extreme ultraviolet beam produced by the present invention is very well absorbed by solids. Therefore, windowing the beam is a difficult problem. A solution to the window problem that applicants prefer is to use a very thin foil that passes extreme ultraviolet (EUV) and reflects visible light. Applicants' preferred window is a beryllium foil (about 10 to 15 microns) inclined at an angle of incidence of about 10 ° with the axis of the incident beam. In this arrangement, almost all visible light is reflected and about 50-80% of extreme ultraviolet (EUV) is transmitted. Of course, such thin windows are not very strong.
Applicants therefore use very small diameter beryllium windows, through which the beam is focused. Preferably, the diameter of the thin beryllium window is about 10 millimeters. Heating of the small window must be considered, and high repetition rates require special cooling of the window. In some designs, this component is simply designed as a beam splitter, simplifying the design because there is no difference in pressure across the thin optical element.

FIG. 10 shows beam 9 having a thickness of 0.5 microns,
Figure 3 shows a preferred embodiment in which the radiation collector 4 is extended by a collector extension 4A for narrowing through a 1 millimeter diameter beryllium window 7;

(Play-On) Applicants' experiments have shown that good results can be obtained without play-on, but that performance improves with play-on. The prototype unit shown in FIG. 5 has a DC driven spark gap pre-on device for pre-oning the gas between the electrodes. Applicants could greatly improve these energy stability values and improve other performance parameters with improved play-on techniques. Play-on is a well-developed technique used by applicants and others to improve the performance of excimer lasers. Preferred pre-on techniques include: 1) DC driven spark gaps 2) High frequency driven spark gaps 3) High frequency driven surface discharges 4) Corona discharges 5) Spiker circuits in combination with pre-onization These techniques are described in the scientific literature on excimer lasers. And is well known.

Blast Shield FIG. 5B shows the location of two of a total of eight spark plugs 138 that provide play-on in the preferred embodiment. This figure also shows a cathode 111 and an anode 123 made of stainless steel on the outside and tungsten inside. An insulator coating surrounds the anode 123 and a 5 mil thick film insulator 125 completely separates the anode and cathode. FIG.
B's 1-6 show a typical pulse progression 1.5 nm after the beginning of the discharge to the narrowing that peaked in FIG. 5B5. During the discharge, the plasma
Toward the tip of the anode, it is accelerated by the Lawrence force acting on ions and electrons generated by the current flowing through the plasma. As soon as the tip of the electrode shown at 121 in FIG. 5B is reached, the radial force vector compresses and heats the plasma until it reaches a high temperature.

Once the plasma has been compressed, the axial forces there and acting on the plasma, especially in FIG.
As shown in FIG. 5, the plasma column tends to be stretched.
This stretching leads to instability. Once the plasma column grows beyond a point in the axial direction, the voltage drop across the region of the compressed plasma becomes too large to be supported by the low pressure gas near the anode tip. An arcover occurs and much or all of the current flows through a short low density gas region near the anode tip, as shown by the dashed line in FIG. 5B6. This arcover is harmful because it causes pulse instability and causes relatively fast electrode erosion.

A solution to this problem is to create a physical barrier to the movement of the plasma column in the axial direction. Such a barrier is shown as element number 143 in FIG.
It is called a blast shield by the applicant because it acts as a defense against plasma waste of the DF device. The blast shield must be made of an electrically insulating material with strong mechanical and heat resistance. in addition,
When operating with highly reactive elements such as lithium, the chemical compatibility of the blast shield material must be considered. Lithium, as its extreme ultraviolet (EUV) source, has its strong emission at 13.5 nanometers,
Proposed as a release element. Excellent candidate substances are single crystal alumina, sapphire or amorphous sapphire, such as the proprietary substance Lucalux, manufactured by General Electric Company.

The optimal shape of the blast shield has been found to be a dome with a radius equal to the diameter of the electrode and centered on the electrode, as shown in FIG. 5C. Such a shape closely matches the line of the naturally occurring plasma flow when the compression of the plasma is at its maximum. If the blast shield was made further away from the tip of the anode,
The plasma column will be too long and insufficient plasma heating will result in danger of arc over. If the position of the blast shield is too close to the anode tip, the flow of current from the central axis to the cathode will be limited, again resulting in insufficient heating of the plasma. At the upper point 144 of the blast shield 143, there must be a hole that can be collected for escape and use of extreme ultraviolet (EUV) radiation. This hole is
It must be as small as possible so that the plasma escapes from this hole and has the property of forming long, narrow columns on the blast shield. An umbrella cut into this hole, indicated by point 144, increases the off-axis collection of extreme ultraviolet (EUV) produced by the plasma focus device.

FIGS. 5C1-6 show how the blast shield encompasses the plasma focusing and prevents arc over.

The above-described embodiments are only a few of the many possible specific embodiments which may illustrate the application of the principles of the present invention.
It can be understood that it is only. For example, instead of recirculating the functional gas, it may be desirable to simply capture lithium and release helium. Instead of tungsten and silver, other electrode and coating combinations may be used. For example, copper or platinum electrodes and coatings could be used. Other techniques for generating the plasma constriction could be used in place of the particular embodiment described. Certain of these other technologies are described in the patents cited in the Background section of this specification, and all of these descriptions are incorporated herein by reference. Many methods of generating high frequency, high voltage electrical pulses are available and can be used. An alternative would be to keep the light pipe at room temperature and condense out the lithium and tungsten as they pass through the entire length of the light pipe. In addition, this decontamination concept will reduce the amount of debris that reaches the optics used in lithographic equipment, as the atoms are permanently attached by impact on the light transmission tube walls. Deposition of electrode material on the lithographic equipment optics can be avoided by designing the collection optics to re-image the emission point through a small hole in the first discharge chamber and to use a working pump facility. it can. Helium or argon can be supplied from the second chamber to the first chamber through the opening. This mechanism has been shown to be effective in preventing material deposition on the output window of a copper vapor laser. Lithium hydride can be used instead of lithium. The unit will also function using a static filling system that does not use the functional gas flowing through the electrode. not to mention,
Repetition rates ranging from a single pulse to 5 to hundreds of thousands of pulses per second are possible. If necessary, the adjustment mechanism for adjusting the position of the solid lithium could be modified so that the position of the tip of the central electrode is adjustable in view of the erosion of the tip.

Many electrode arrangements other than those described above are possible. For example, the outer electrode may have a conical shape with a large diameter on the narrowed side, instead of a cylindrical shape as shown. In some embodiments, the performance may be improved by projecting the internal electrode beyond the tip of the external electrode. This will be possible with spark plugs or other play-on devices known in the art. In another preferred embodiment, the outer electrode is replaced by an electrode rod arranged to form a generally cylindrical or conical shape. This method helps to maintain a symmetric constriction with a central axis along the electrode axis, as the resulting induction acts as a ballast.

Accordingly, it is necessary for the reader to determine the scope of the invention, regardless of the given embodiment, by the appended claims and their legal equivalents.

[Brief description of the drawings]

FIG. 1 is a diagram of a high energy photon source illustrating a preferred embodiment of the present invention.

FIG. 2 is a diagram of a three-dimensional plasma focusing device having a disk-shaped electrode.

FIG. 3 is a diagram of a third preferred embodiment of the present invention.

FIG. 4 is a preferred circuit diagram for a preferred embodiment of the present invention.

FIG. 5A is a diagram of a prototype unit made by the applicant and his colleagues.

FIG. 5B is a cross-sectional view showing a prototype electrode provided with a spark plug type pre-on device.

FIG. 5B1 is a diagram showing growth of a plasma focus.

FIG. 5B2 is a diagram showing growth of a plasma focus.

FIG. 5B3 is a diagram showing growth of a plasma focus.

FIG. 5B4 is a diagram showing growth of plasma focus.

FIG. 5B5 is a diagram showing growth of plasma focus.

FIG. 5B6 is a diagram showing the growth of plasma focus.

FIG. 5C is a cross-sectional view of an electrode region to which a blast shield has been added.

FIG. 5C1 illustrates the growth of a plasma focus with the appropriate addition of a blast shield.

FIG. 5C2 illustrates the growth of a plasma focus with the appropriate addition of a blast shield.

FIG. 5C3 illustrates the growth of a plasma focus with the appropriate addition of a blast shield.

FIG. 5C4 illustrates the growth of a plasma focus with the appropriate addition of a blast shield.

FIG. 5C5 illustrates the growth of a plasma focus with the appropriate addition of a blast shield.

FIG. 5C6 illustrates the growth of a plasma focus with the appropriate addition of a blast shield.

FIG. 6 is a diagram of a plasma generated in a prototype unit.

FIG. 7: Ultra-violet light (EU) produced by a hyperbolic collector
V) shows a part of the beam.

FIG. 8: Reflectance of molybdenum silicon coating versus 1
The peak of lithium at 3.5 nm is shown.

FIG. 9 shows a nested conical debris collector.

FIG. 10 shows a thin beryllium window that reflects visible light and transmits extreme ultraviolet (EUV).

FIG. 11 is a graph showing the reflectance of various materials for 13.5 nanometer ultraviolet radiation.

[Explanation of symbols]

 2 Refinement unit 4 High energy photon collector 6 Optical transmission tube 8 Coaxial electrode 10 Pulse power unit 12 Turbo suction pump 14 Helium separator 20 Heat exchanger 22 Electrostatic filter

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Daniel El Barks 94561 Oakley Route 175 Dee Chrismore Lane 3300 California (56) References JP-A-63-284744 (JP, A) JP-A-63-211598 , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H05G 2/00

Claims (40)

(57) [Claims] 1. A. First Embodiment Vacuum chamber, B. B. at least two electrodes installed in said vacuum chamber, defining an electric discharge area, and arranged such that a high-frequency plasma narrowing occurs at a narrowing portion on the discharge; B. an active gas selected to provide light having at least one spectral line, and a functional gas including a noble gas, a buffer gas; B. a functional gas supply system for supplying a functional gas to the discharge region; B. a pulsed power source that supplies an electrical pulse and a high voltage sufficient to cause a discharge between said at least one pair of electrodes; A high-energy photon source comprising a blast shield made of an electrically insulating material and installed to limit the extension of the plasma aperture. 2. The high-energy photon source according to claim 1, wherein the blast shield has a hole provided so that an ultraviolet ray can pass through the blast shield from the narrowing portion. 3. The method of claim 2, wherein the off-axis collection of the light beam is performed.
The high-energy photon source according to claim 2, wherein the hole has an umbrella shape. 4. The apparatus of claim 1, further comprising an externally-reflected radiation collection director for collecting the radiation produced by the plasma aperture and directing the radiation in a desired direction. High energy photon source. 5. The apparatus of claim 4, further comprising a conical nested debris collector having a surface aligned with the light rays extending from the aperture to the radiation collection director. The high energy photon source as described. 6. The high-energy photon source according to claim 5, wherein the conical nested debris collector is assembled as part of the radiation collection director. 7. The method according to claim 7, wherein the active gas is a vapor of a metal having a distinct melting point, and further comprising heating means for maintaining the radiation collecting director and the clastic collector at a temperature exceeding the melting point of the metal. The high energy photon source according to claim 5, characterized in that: 8. The high energy photon source according to claim 4, wherein said metal is lithium. 9. The high energy photon source according to claim 1, wherein the pulsed power source is programmable to provide electrical pulses at a frequency of at least 1000 Hertz. 10. The high energy photon source according to claim 1, wherein said active gas is produced by heating a material. 11. The high energy photon source according to claim 6, wherein said substance is lithium. 12. The high energy photon source according to claim 7, wherein said lithium is disposed in one of said two electrodes. 13. The method according to claim 8, wherein the two electrodes are formed concentrically so as to define a central electrode and a central tip that are axes, and the lithium is disposed along the axis. The high energy photon source as described. 14. The high energy photon source according to claim 10, further comprising a position adjusting means for adjusting the lithium with respect to the center electrode tip. 15. The high energy photon source according to claim 1, wherein said active gas is lithium vapor. 16. The high energy photon source according to claim 1, wherein said active gas is lithium hydride. 17. The high energy photon source according to claim 1, further comprising a light transmission tube arranged to transmit the radiation collected and directed by the collection director. 18. The high energy photon source according to claim 4, wherein said electrodes are made of an electrode material, and said collection director is coated with the same electrode material. 19. The high energy photon source according to claim 18, wherein the electrode material is tungsten. 20. The high energy photon source according to claim 19, wherein the electrode material is silver. 21. The high energy photon source according to claim 18, wherein said buffer gas is helium. 22. The high energy photon source according to claim 1, wherein said buffer gas is argon. 23. The high energy photon source according to claim 1, wherein said buffer gas is radon. 24. The source of high energy photons according to claim 1, wherein said at least two electrodes comprise one internal electrode and two external electrodes at the opposite pole during operation. 2. The electrode according to claim 1, wherein the electrode is a shaped electrode.
2. A high energy photon source according to claim 1. 25. The height according to claim 1, wherein the two electrodes are coaxially arranged so as to define a central electrode serving as an axis and an external electrode consisting of an array of electrode rods. Energy photon source. 26. The high energy photon source according to claim 25, wherein the array of electrode rods is arranged to form a generally cylindrical shape. 27. The arrangement of claim 1, wherein the array of electrode bars is arranged to form a generally conical shape.
2. A high energy photon source according to claim 1. 28. The high-energy photon source according to claim 1, further comprising a pre-on device for pre-on the functional gas. 29. The high energy photon source according to claim 28, wherein said play-on device is a DC spark gap ionizer. 30. The high energy photon source according to claim 28, wherein said play-on device is a high frequency driven spark gap. 31. The high energy photon source according to claim 29, wherein said pre-on device is a high frequency driven surface discharge. 32. The high energy photon source according to claim 29, wherein the play-on device is a corona discharge. 33. The high energy photon source according to claim 28, wherein said play-on device comprises a spiker circuit. 34. The high energy photon source according to claim 1, further comprising a vacuum chamber window for transmitting extreme ultraviolet radiation and reflecting visible light. 35. The high energy photon source according to claim 34, wherein the window comprises a flake of solid material having a thickness of 1 micron or less. 36. The material according to claim 3, wherein the material is selected from the group consisting of beryllium and silicon.
5. The high-energy photon source according to 4. 37. A high energy photon source according to claim 34, further comprising focusing means for focusing said radiation on said window. 38. The high-energy photon source according to claim 1, further comprising a beam splitting device for transmitting extreme ultraviolet light and reflecting visible light. 39. The high energy photon source according to claim 38, wherein said window comprises a flake of solid material less than 1 micron in thickness. 40. The source of claim 38, wherein the material is selected from the group consisting of beryllium, zirconium, and silicon.