Patent Number: 061880760
Section: summary

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.sup.16 to 10.sup.21 cm.sup.-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 to conducting materials to serve as electrical interfaces between the electrical current within the capillary and the electrical current of the external circuit The capillary is filled with a gaseous medium that becomes ionized so as to provide a low resistance for conduction of the electrical discharge current within the capillary. The electrical discharge current excites the gas or vapor within the capillary which then provides the desired radiation in the spectral region between 11 nm and 14 nm. The gas or vapor within the capillary when ionized by the discharge current thus acts as both an electrically conducting medium and an EUV radiator. The following objectives relate to capillary discharge sources operating in the wavelength range of 11-14 nm and which, within that wavelength region, provide the necessary flux for their particular applications. The objectives relate to: debris formation, materials considerations, discharge geometry, and applications. The first objective of the present invention is to define the necessary capillary bore diameter and length ranges of a capillary discharge source. These dimensions are determined by experimental evidence in which strong EUV emission was observed. The second objective of the present invention is to define the currents and current densities of operation of a capillary discharge source containing a gas or liquid vapor or metal vapor such that it will not produce debris destructive to the optics for a duration of at least the industry-defined Lifetime of those optics. The third objective of the present invention is to describe a method of pre-treating the capillary bore region so as to make it resistant to erosion or other changes in the capillary during subsequent normal operation. The fourth objective of the present invention is to define the necessary operating pressure range of a gas or metal vapor or liquid vapor or other atomic or molecular species present within the capillary of a capillary discharge source. The fifth objective of the present invention is to describe the "differentially pumped capillary geometry." This geometry obviates the need for an EUV transmitting window which would provide a barrier between the vacuum within the condenser system and the gas required for the source plasma emission. The sixth objective of the present invention is to describe the "heat pipe capillary discharge" which contains a wick within a heat pipe configuration such that the wick is mounted only outside of the capillary discharge region. The seventh objective of the present invention is to describe various materials which may be used in the "differentially pumped capillary discharge" and/or the "heat pipe capillary discharge." The eighth objective of the present invention is to provide a capillary discharge source for use in any of the following applications: microscopy, interferometry, metrology, biological imaging, pathology, alignment, resist exposure testing for microlithography, and extreme ultraviolet lithography (EUVL). A preferred method of operating a capillary discharge source in the 11 nm to 14 nm wavelength region includes forming a discharge within a capillary source having a bore size of approximately 1 mm, and at least one radiating gas, with a discharge current of approximately 2000 to approximately 10,000 amperes, and radiating selected wavelength regions between approximately 11 to approximately 14 nm from the discharge source. The gases can include one radiating gas such as xenon or an oxygen containing molecule to provide oxygen as the one radiating gas, each having a pressure of approximately 0.1 to approximately 20 Torr. The gas can include a metal vapor such as lithium, to radiate the selected wavelength regions and has a pressure of approximately 0.1 to approximately 20 Torr. Besides the radiating gas, a buffer gas can be used, wherein the total pressure in the capillary can range from approximately 0.1 to approximately 50 Torr. The use of multiple plural gases can include lithium radiating the selected wavelength region between approximately 11 to approximately 14 nm, and helium as a buffer gas. Another preferred method of operating a capillary discharge source in the 11 nm to 14 nm wavelength region includes forming a discharge across a capillary source having a bore size diameter of approximately 0.5 to approximately 3 mm, and a length of approximately 1 to approximately 10 mm, and at least one radiating gas, with a discharge current density of approximately 250,000 to approximately 1,300,000 Amperes/cm.sup.2, and radiating selected wavelength regions between approximately 11 to approximately 14 nm from the discharge source. A method of constructing the capillary discharge lamp source operating in the ultraviolet wavelength region includes constructing a capillary from an electrically insulating material, inserting at least one gaseous species in the capillary, wherein the capillary is used to generate ultraviolet discharges. A metallic conductor such as molybdenum, Kovar, and stainless steel, can be used as electrodes on opposite sides of the capillary. A nonconducting and the insulating material can be used such as quartz, saphire, aluminum nitride, silicon carbide, and alumina Furthermore, the capillary can be a segmented bore of alternating conductive and nonconductive materials. Another preferred embodiment of the discharge lamp source operating the ultraviolet wavelength region can include a capillary, a first electrode on one side of the capillary, a second electrode on a second side of the capillary opposite to the first side, a pipe having a first end for supporting the second electrode and a second end, a discharge port connected to the second end of the pipe, a wick passing through the pipe from the discharge port to a portion of the pipe adjacent to but not within the capillary having a lithium wetted mesh for operation as a heat pipe, and means for operating the capillary as a discharge source for generating ultraviolet wavelengths signals. Pre-processing techinques of the capillary discharge bore source is when the bore is used with an optical element that operates in the ultraviolet region, prior to operating the source, in order to prevent rupturing of the optical element or contaminating mirrors that receive radiation, are disclosed. The pre-processing techniques include the steps of pre-conditioning interior bore surface walls of a capillary discharge source that operates in the ultraviolet region, and continuing the pre-conditioning until a selected impulse value is reached. The pre-processing technique can use a heat source, such as an excimer laser, a Nd:Yag laser, and a Copper Vapor laser. The laser can be focussed within the bore, and operated at a focussed intensity in the range of approximately 10.sup.7 to approximately 10.sup.11 Watts/cm.sup.2. Another version of the pre-processing technique has the selected value less than approximately 20 Torr-.mu.s, wherein initiating discharge current discharge pulses within the capillary with a second gas having a pressure range of approximately 1 to approximately 20 Torr., and the pre-operation pulses are approximately 3000 pulses.