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`USUO69Tr'24-21. B2
`
`(12) United States Patent
`Melnychuk et al.
`
`(10) Patent No.:
`
`(45) Date of Patent:
`
`US 6,972,421 B2
`Dec. 6, 2005
`
`(54) EXTREME ULTRAVIOLET LIGHT SOURCE
`
`(75)
`
`Inventors: Stephan T. Melnychuk, Carlsbad, CA
`(US); William N. Partlo, Poway, CA
`(US); Igor V. Fomenkov, San Diego,
`(.‘A (US); 1. Roger Oliver. San Diego,
`CA (US); Richard M. Ness. San Diego,
`CA (US); Norbert flowering, San
`Diego, CA (US); Oleh Khodykin, San
`Diego, (TA (US); Curtis L. Rettig,
`Vista, (TA (US); Gerry M.
`Bluntenstock, San Diego, CA (US);
`Timotiiy S. Dyer, Oceanside, CA (US);
`Rodney I). Simmons, San Diego, CA
`(US); Jerzy R. Hofiman, Escondido,
`CA (US); R. Mark Johnson, Ramona,
`CA (US)
`
`(73) Assignee: Cymel‘, Inc., San Diego, (T/\(US)
`
`{ *) Notice:
`
`Subject to any disclaimer, the term of this
`patent
`is extended or adjusted under 35
`U.S.C. 154(b) by 107 days.
`
`(21) Appl. No.: l0,I"409,254
`
`(22
`
`Filed:
`
`Apr. 3, 2003
`
`(65)
`
`Prior Publication Data
`Us 2ora4;mns473 Al Jun. in, 2004
`
`Related U.S. Application Data
`
`(63)
`
`Continuatien—in—part of application No. 1U;'384.067. filed on
`Mar. 8, 2tItl3_. which is a continuation—in—pari of application
`No.
`lDt'18'3I_.824, tiled on Jul. 3, 21')02_. now Pat. No. 6.815,
`?U{J, which is a eontinuation—in—part of application No.
`l0;‘12ti.65S, filed on Apr. 10, 2002, now Pat. No. 6,?44_.06f,I_.
`which is a continuation-in-part of application No. D9;‘S'I5_.
`719, liled on Jun. 6, 2001, new Pat. No. 6,536,757, which is
`a continuation—in—part of application No. (l9}8?'5_.?2l_, filed
`on Jun. 6, 2001, now Pat. No. 6,566,668, which is a
`t:ontinuation—in—parl of application No. C|9,r‘696,{l84_. tiled on
`Oct. 16. 2000, now Pat. No. 6,566,667, which is a continu-
`ation-in-parl 01‘ application No. 0FJ,t590_.962_. tiled on Jun. 9,
`2000, now abandoned.
`
`(60)
`
`Provisional application No. 6U;‘422_.8D8_. filed on Oct. 31,
`2002, and provisional application No. 6W4l9,8U5, filed on
`om. 13, zoo).
`
`Int.C|.7
`(51)
`(52) U.S. Cl.
`
`(58) Field of Search
`
`H01] 35120
`250.1504 R; ?.50t'493.1;
`378E119
`250E504 R, 493.1;
`378E119; 37215, 87
`
`(56)
`
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`Priniary Exrrmther—Kict T. Nguyen
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`
`(57)
`
`ABSTRACT
`
`invention provides a reliable. high-repetition
`The present
`rate, production line compatible high energy photon source.
`Avery hot plasma containing an active material is produced
`in vacuum chamber. The active material is an atomic ele-
`ment having an emission line within a desired extreme
`ultraviolet (EUV) range. A pulse power source comprising a
`charging capacitor and a magnetic compression circuit com-
`prising a pulse transformer, provides electrical pulses having
`suflicient energy and electrical potential suflicient to pro-
`duce the EUV light at an intermediate focus at rates in excess
`o1‘ 5 Watts. In preferred embodiments designed by Appli-
`canLs in-band, LEUV light energy at the intermediate focus is
`45 Watts extendablc to 105.8 Watts.
`
`78 Claims, 50 Drawing Slleets
`
`
`
`ASML 1331
`ASML 1331
`
`

`
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`* cited by examiner
`
`

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`U.S. Patent
`
`bee. 6,2005
`
`Sheet 1 of 50
`
`US 6,972,421 B2
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`Dec. 6,2005
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`US 6,972,421 B2
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`U.S. Patent
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`Dec. 6, 2005
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`Sheet 12 of 50
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`US 6,972,421 B2
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`U.S. Patent
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`Dec. 6, 2005
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`Sheet 13 of 50
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`US 6,972,421 B2
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`U.S. Patent
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`Dec. 6, 2005
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`US 6,972,421 B2
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`Dec. 6,2005
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`US 6,972,421 B2
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`Dec. 6,2005
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`US 6,972,421 B2
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`Dec. 6,2005
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`US 6,972,421 B2
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`US 6,972,421 B2
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`US 6,972,421 B2
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`US 6,972,421 B2
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`US 6,972,421 B2
`
`1
`EXTREME ULTRAVIOLET LIGHT SOURCE
`
`This application is a continuation-in-part of U.S. Ser. No.
`10t384,96'r' filed Mar. 8, 2003, Ser. No. 10t189,824 filed Jul.
`3, 2002 now US. Pat. No. 6,815,700, U.S. Ser. No. 10,020,
`655 filed Apr. 10, 2002, now U.S Pat. No. 6,744,060, Us.
`Ser. No. 09r'875,719 filed Jun. 6, 2001 now U.S. Pat. No.
`6,586,757. and U.S. Ser. No. 09l875,721 filed Jun. 6, 2001
`now U.S. Pat No. 6,566,668, US. Ser. No. 09;’690,084 filed
`Oct. 16, 2000 now US. Pat. No. 6,566,667 ; and claims the
`benefit of patent application Ser. No. 60;’422,808 filed Oct.
`31, 2002 and patent application Ser. No. 60t4l9,805 filed
`Oct. 18, 2002; all of which is incorporated by reference
`herein. This invention relates to high—energy photon sources
`and in particular highly reliable x—ray and high-energy
`ultraviolet sources.
`
`BACKGROUND OF THE INVENTION
`
`The semiconductor industry continues to develop litho-
`graphic technologies, which can print ever—smallcr inte-
`grated circuit dimensions. These systems must have high
`reliability, cost ellective throughput, and reasonable process
`latitude. The integrated circuit
`fabrication industry has
`recently changed over from mercury G-line (436 nm) and
`I-line {.365 nm) exposure sources to 248 nm and 193 nm
`excimer laser sources. This transition was precipitated by the
`need for higher lithographic resolution with minimum loss
`in depth-of-focus.
`The demands of the integrated circuit industry will soon
`exceed the resolution capabilities of 193 nm exposure
`sou rees, thus creating a need for a reliable exposure source
`at a wavelength significantly shorter than 193 nm. An
`excimer line exists at 15? nm, but optical materials with
`sutficient transmission at this wavelength and sufficiently
`high optical quality are difficult to obtain. Therefore, all-
`reflective imaging systems may be required. An all reflective
`optical system requires a smaller numerical aperture (NA)
`than the transmissive systems. The loss in resolution caused
`by the smaller NA can only be made up by reducing the
`wavelength by a large factor. Thus, a light source in the
`range of 10 to 20 nm is required if the resolution of optical
`lithography is to be improved beyond that achieved with l93
`nm or 157 nm. Optical components for light at wavelengths
`below 157 nm are very limited. I-Iowever, effective incidents
`reflectors are available and good reflectors multi-layer at
`near normal angles of incidence can be made for light in the
`wavelength range of between about 10 and 14 nm. (Light in
`this wavelength range is within a spectral range known as
`extreme ultraviolet light and some would light in this range,
`soft x-rays.) For these reasons there is a need for a good
`reliable light source at wavelengths in this range such as of
`about 13.5 nm.
`
`3‘
`
`I0
`
`15
`
`30
`
`[Jon
`
`30
`
`40
`
`45
`
`S0
`
`The present state of the art in high energy ultraviolet and
`x-ray sources utilizes plasmas produced by bombarding _
`various target materials with laser beams, electrons or other
`particles. Solid targets have been used, but the debris created
`by ablation of the solid target has detrimental elfects on
`various components of a system intended for production line
`operation. A proposed solution to the debris problem is to
`use a frozen liquid or liquidfied or frozen gas target so that
`the debris will not plate out onto the optical equipment.
`However, none of these systems have so far provcn to be
`practical for prodttction line operation.
`It has been well known for many years that x—rays and
`high energy ultraviolet radiation could be produced in a
`plasma pinch operation. In a plasma pinch an electric current
`
`60
`
`2
`is passed through a plasma in one of several possible
`configuration such that
`the magnetic field created by the
`llowing electric current accelerates the electrons and ions in
`the plasma into a tiny volume with sufficient energy to cause
`substantial stripping of outer electrons from the ions and a
`consequent production of X-rays and high energy ultraviolet
`radiation. Various prior art techniques for generation of high
`energy radiation from focusing or pinching plasmas are
`described in the background section of US. Pat. No. 6,452,
`199.
`
`Typical prior art plasma focus devices can generate large
`amounts of radiation suitable for proximity x-ray
`lithography, but are limited in repetition rate due to large per
`pulse electrical energy requirements, and short lived internal
`components. The stored electrical energy requirements for
`these systems range from 1 kJ to l00 kl The repetition rates
`typically did not exceed a few pulses per second.
`What is needed are production line reliable, systems for
`producing collecting and directing high energy ultraviolet
`x—radiation within desired wavelengtl‘I ranges which can
`operate reliably at high repetition rates and avoid prior art
`problems associated with debris formation.
`SUMMARY OF THE IN\I'EN"I‘l0N
`
`The present invention provides a reliable, high-repetition
`rate, production line compatible high energy photon source.
`Avery hot plasma containing an active material is produced
`in vacuum chamber. The active material is an atomic ele-
`ment having an emission line within a desired extreme
`ultraviolet [EUV) wavelength range. A pulse power source,
`comprising a charging capacitor and a magnetic compres-
`sion circuit comprising a pulse transformer, provides elec-
`trical pulses having suffieient energy and electrical potential
`sufficient to produce the EUV light at an intermediate focus
`at rates in excess of 5 WatLs on a continuous basis and in
`
`excess of 20 Watts on a burst basis. In preferred embodi-
`ments designed by Applicants in-band, EUV light energy at
`the intermediate focus is 45 Watts extcndable to 105.8 Watts.
`
`In preferred embodiments the high energy photon source
`is a dense plasma focus device with co-axial electrodes. the
`electrodes are configured co-axially. The central electrode is
`preferably hollow and an active gas is introduced out of the
`hollow electrode. This permits an optimization of the spec-
`tral line source and a separate optimization of a buffer gas.
`In preferred embodiments the central electrode is pulsed
`with a high negative electrical pulse so that
`the central
`electrode functions as a hollow cathode. Preferred embodi-
`
`ments present optimization of capacitance values, anode
`length and shape and preferred active gas delivery systems
`are disclosed. Special techniques are described for cooling
`the central electrode. In one example, water is circulated
`through the walls of the hollow electrode.
`In another
`example, a heat pipe cooling system is described for cooling
`the central electrode.
`An external reflection radiation collector-director collects
`
`radiation produced in the plasma pinch and directs the
`radiation in a desired direction. Good choices for the reflec-
`tor material are molybdenum, palladium,
`ruthenium,
`rhodium, gold or tungsten. In preferred embodiments the
`active material may be xenon, lithium vapor, tin vapor and
`the buffer gas is helium and the radiation-collector is made
`of or coated with a material possessing high grazing inci-
`dence reflectivity. Other potential active materials are
`described.
`
`In preferred embodiments the buffer gas is helium or
`argon. Lithium vapor may be produced by vaporization of
`
`

`
`US 6,972,421 B2
`
`3
`solid or liquid lithium located in a hole along the axis of the
`central electrode of a coaxial electrode configuration.
`Lithium may also be provided in solutions since alkali
`metals dissolve in amines. A lithium solution in ammonia
`
`(NI-I3) is a good candidate. Lithium may also be provided by
`a sputtering process in which pre-ionization discharges
`serves the double purpose of providing lithium vapor and
`also prc-ionization.
`In preferred embodiments, debris is
`collected on a conical nested debris collector having sur-
`faces aligned with light rays extending out from the pinch
`site and directed toward the radiation collector-director. The
`retlection radiation collector-di rector and the conical nested
`
`debris collector could be fabricated together as one part or
`they could be separate parts aligned with each other and the
`pinch site.
`This prototype devices actually built and test by Appli-
`cants convert electrical pulses (either positive or negative) of
`about 10 J of stored electrical energy per pulse into approxi-
`mately S0 ml of in-band 13.5 1'1l'I'I radiation emitted into 2::
`steradians. Thus, these tests have demonstrated a conversion
`elliciency of about 0.5%, Applicants estimate that they can
`collect about 20 percent of the 50 mi 13.5 nm radiation so
`that this demonstrated collected energy per pulse will be in
`about of 10 ml. Applicants have demonstrated 1000 I-17.
`continuous operation and 4000 I-12 short burst operation.
`Thus, 10 Watt continuous and 40 Watt burst outputs have
`been demonstrated. Using collection techniques designed by
`Applicants about half of this energy can be delivered to an
`intermediate focus distant from the plasma source. Thus
`providing at
`least 5 Watts of in band EUV light at
`the
`intermediate focus on a continuous basis and at
`least 20
`
`Watts on a burst basis. Applicants have also shown that the
`techniques described herein can be applied to provide out-
`puts in the range of 60 Watts at repetition rates of5,()0(l H9:
`or greater. At fill Hz, the measured pulse-to-pulse energy
`stability, (standard deviation) was about 9.4% and no drop
`out pulses were observed. The electrical ci