Radiant energy – Radiant energy generation and sources – With radiation modifying member
Reexamination Certificate
1999-12-17
2001-02-13
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Radiant energy generation and sources
With radiation modifying member
C250S493100
Reexamination Certificate
active
06188076
ABSTRACT:
This invention relates to capillary discharges for use as imaging sources in Extreme Ultraviolet Lithography (EUVL) and other technologies such as EUV microscopy, interferometry, inspection, metrology, and the like. The invention describes characteristics of sources that radiate intense light in the wavelength region between 10 and 14 nm. The operation of these sources can be determined by: (1) the gas or vapor pressure within the capillary which generates optimum emission flux; (2) the range of discharge currents at which sufficient radiation flux occurs but above which significant detrimental debris and bore erosion begins; (3) the desired range of capillary bore sizes and lengths, some specific gaseous media that radiate effectively in the capillary discharges under the conditions described above, and (4) two specific configurations for housing the capillary discharge system.
BACKGROUND AND PRIOR ART
A commercially suitable Soft-X-Ray (or EUV) lithography facility will require an intense soft x-ray/EUV light source that can radiate within a specific wavelength region of approximately 11 to 14 nm in the EUV part of the electromagnetic spectrum. This region is determined by the wavelength range over which high reflectivity multilayer coatings exist. The multilayer coatings can be used to manufacture mirrors which can be integrated into EUVL stepper machines. Specifically, these coatings are either Mo:Be multilayer reflective coatings (consisting of alternate ultrathin layers of molybdenum and beryllium) which provide high reflectivity between 11.2 and 12.4 nm, or Mo:Si multilayer reflective coatings (consisting of alternate ultrathin layers of molybdenum and silicon) which provide high reflectivity between 12.4 nm and 14 nm. Thus any intense EUV source emitting in the wavelength range of 11-14 nm may be applicable to lithography. Two proposed EUV sources are synchrotrons which generate synchrotron radiation and soft-x-ray emitting laser-produced plasmas (LPP's). Synchrotron sources have the following drawbacks: the synchrotron and synchrotron support facilities cost up to $100 million or more: together they occupy a space of approximately 1,000,000 cubic feel Such a volume is incompatible with a typical microlithography fabrication line. Laser produced plasmas that have the necessary wavelength and flux for a microlithography system require a high power laser to be focused onto a target material such that sufficient plasma density can be produced to efficiently absorb the incident laser radiation. Laser produced plasmas have the following drawback: if a solid target material is used, the interaction of the focused laser beam with the target produces an abundant quantity of debris which are ejected from the laser focal region in the form of atoms, ions, and particulates. Such eject a can accumulate on and thereby damage the optics that are used in collecting the light emitted from the plasma The use of volatile target materials in LPP sources has been successful in overcoming the debris problem. A volatile target material is simply a material which is unstable to evaporation in a room temperature vacuum, examples of these are liquefied or solidified gases such as oxygen or xenon, and also liquids such as water. For these materials any bulk mass not directly vaporized by the laser pulse will evaporate and will be subsequently pumped away. Thus the excess target material does not collect or condense on the optics.
Although such laser-produced plasma sources have been developed for EUVL using oxygen and xenon as radiating species, there still exist two prohibitive drawbacks for which no realistic scenarios of significant improvement have been proposed. First, the total electrical efficiency of such sources is of the order of only 0.005-0.025%. This results from considering the multiplicative combination of the laser efficiency, which is of the order of 1-5%. and the conversion efficiency of laser light to useful EUV radiation (within the reflectivity bandwidth of a multilayer-coated reflecting mirror) of approximately 0.5%. Second, the cost of a laser that would necessarily operate at repetition rates of over 1 kHz would be a minimum of several million dollars.
To overcome the unique problems specific to the synchrotron sources and to the LPP sources we have invented a compact electrically produced intense capillary discharge plasma source which could be incorporated into an EUV lithography machine. Compared to synchrotrons and LPP's this source would be significantly more efficient, compact, and of lower cost (both to manufacture and to operate). We envision that one of these sources (along with all the necessary support equipment) would occupy the space of less than 10 cubic feet and would cost less than $ 100,000. One such embodiment of the proposed capillary discharge source was first described in U.S. Pat. No 5,499,282 by William T. Silfvast issued on Mar. 12, 1996. That particular proposed source would operate in a lithium vapor electrically excited to within specific ranges of plasma electron temperatures (10-20 eV) and electron densities (10
16
to 10
21
cm
−3
) which are required for optimally operating a lithium vapor discharge lamp at 13.5 nm. That same patent also proposed soft-x-ray lamps at wavelength of 7.6, 4.86, and 3.38 nm in beryllium, boron, and carbon plasmas. These wavelengths, however, are not within the range of wavelengths required for EUV lithography. Although that patent described the general features of these lamps, it did not give the specific discharge current operating range that would minimize bore erosion and the emission of debris from the lithium lamp, or the appropriate range of bore sizes for operating such a lamp. That patent did not mention the use of other materials, such as atomic or molecular gases that could be successfully operated in the lamp configurations described in that patent; it naturally follows that neither could it have mentioned what are the preferred operating pressure ranges of those gases that would be suitable for EUV lithography.
SUMMARY OF THE INVENTION
Although gaseous plasma discharge sources have been produced previously in many different kinds of gases for use as light sources and as laser gain media, none have been demonstrated to have sufficient flux at appropriate EUV wavelengths for operating a commercial EUV lithography machine. Consequently the necessary plasma discharge current and gas pressure necessary to obtain the required flux for use in an EUV lithography system and/or related applications have not previously been identified and described. Likewise the required capillary discharge bore size range for EUV lithography, as well as some specific capillary discharge configurations for use with gases and metal vapors have not been previously identified. The subject invention specifically indicates the range of gas pressures the range of discharge currents and/or current densities under which debris ejected from the capillary is minimized, as well as some specific gases to be used under those conditions. Also described, are two specific discharge configurations one of which is designed specifically for gases or vapors and requires no vacuum window. We have termed this the “differentially pumped capillary discharge”. The other is designed specifically for metal vapors or liquid vapors. We have termed this the “heat pipe capillary discharge.” It contains a wick which is located only beyond the discharge capillary (unlike that described in U.S. Pat. No. 5,499,282 by William T. Silfvast issued on Mar. 12, 1996, in which the wick is located inside the capillary).
For purposes of definition of a capillary discharge, we are operating an electrical current within an open channel of an insulating material where the open channel is filled with a gas or vapor that allows for electrical conduction within the capillary. The channel or capillary is typically of cylindrical shape with a diameter in the range of 0.5 mm to 3 mm and a length varying from 0.5 mm to 10 mm. The ends of the capillary are attached
Klosner Marc A.
Silfvast William T.
Law Offices of Brians S. Steinberger
Nguyen Kiet T.
Steinberger Brian S.
University of Central Florida
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