Patent Number: 062326139
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. FIG. 1 is a side cross-sectional view 1 of an end-emitting differentially pumped capillary (DPC) discharge source. The DPC has metal electrode 10 having a port through-hole 15 for allowing gas G such as xenon 2 to enter through port 15 in the direction of arrow I from a high pressure region 5. On the opposite side of an electrically insulating or partially insulating capillary 20 is a second electrode 30. Electrodes 10 and 30 serve as electrical connections to the flowing gas G, that is located between those electrodes 10 and 30 within the capillary 20. When a voltage V, is applied between the electrodes 10, 30, an electric field is produced within the gas G between the electrodes 10, 30, and particularly within the capillary 20, that causes electrons to be accelerated and collide with the gaseous atoms to highly excited and ionized states that radiate the desired light for use in various applications that are describe above. An example of the differentially pumped capillary (DPC) discharge source of FIG. 1 along with operational ranges of current, pressure, repetition rate, and the like, is described and shown in U.S. Ser. No. 09/001,696 filed on Dec. 31, 1997, now U.S. Pat. No. 6,031,241, entitled: Capillary Discharge Extreme Ultraviolet Lamp Source for EUV Microlithography and other Related Applications now issued as U.S. Pat. No. 6,031,241, by the same assignee, which is incorporated by reference. Referring to FIG. 1, gas G is flowed into the electrode region 10 at a selected pressure between approximately 0.1 and approximately 50 Torr and is pumped out at the radiation emitting end as described above such that the pressure beyond the emitting end 35 of the capillary 20 is less than approximately 0.1 to approximately 0.01 Torr (depending upon the absorption path length to the collecting optic) to avoid absorption of the EUV light emitted from the capillary. Debris 40 is produced when the current pulse is initiated within the capillary 20 and is ejected from the emitting end 35 of the capillary 20 and can be propelled toward optic components 50 (such as a multilayer concave reflecting mirror with alternating layers of molybdenum and silicon) that are used to collect the radiation E emitted from the end 35 of the capillary 20, thereby damaging the optics 50 either by pitting it with particle chunks of debris or by coating it with a layer of absorbing material. Experimentally, a range of debris sizes was observed. A distribution was measured by microphotographing sample regions of silicon witness plates which have been exposed to many discharge pulses of the lamp at close proximity of approximately 5 cm. The observed debris sizes range from a maximum of 40 microns in diameter down to the diffraction limit of high power optical microscopes of approximately 0.5 microns. The relative distribution of the particle sizes was seen to depend strongly on the operating characteristics of the discharge such as but not limited to pressure, bore diameter, current, gas flow rate, and the like. Under low magnification, the particle deposition field on the witness plate was observed to be localized and centered on the projection of the capillary bore axis. The debris field observed was approximately 1 cm in extent (full width), which means that the overwhelming majority of the particulate debris was ejected from the capillary at angles from 0 to 6 degrees with respect to the capillary axis. These observations form the physical foundation for the usefulness of the APEC geometry in producing EUV radiation with greatly reduced debris reaching regions beyond the lamp. All debris exiting from the lamp region is potentially damaging to EUV collecting optics facing the output of the capillary discharge. Submicron-sized particles down to single atoms produce a coating on the surface of the optics which leads to partial absorption of the EUV light. Larger particles, especially those greater than approximately 10 microns in diameter, can crater and dig into the surface of the optics, thereby reducing the useful EUV flux. FIG. 2 is a side cross-sectional view 100 of a first embodiment of an angular pumped and emitting capillary (APEC) discharge source of the subject invention. The uniqueness of the APEC is the geometry of the capillary 120 and electrode 130 at the light emitting end 127 of the capillary 120. Referring to FIG. 2, the APEC 100 overcomes the debris problem of the FIG. 1 embodiment as well as to allow for more collection of light from the capillary. The APEC 100 differs from the DPC 1 of FIG. 1 in that the radiation E is emitted in an angular direction E1 (because the pressure is typically higher than the ordinary DPC 1 of FIG. 1), symmetrically around the capillary 120 at the low pressure end 127. The capillary end region 127 is tapered as is the end 133 of the electrode 130 with an adjustable space S (approximately 0.1 mm to approximately 5 mm for DPC 1 and approximately 0.1 mm and up if only the collecting trap is used), between them. The radiation E1 that is collected at optics 150 (shown in FIG. 1) comes primarily from the mouth 127, 133 of the cathode as well as from the area between electrodes 110, 130. This angular tapered region 127 allows the light E1 to be collected in a large solid angle which in FIG. 2 can range from approximately 15 degrees or greater with respect to the axis of capillary 120. This geometry also allows differential pumping to continue through the tapered region so that the pressure in the emitting region 127 at the end of the capillary bore 125 is still at sufficiently high pressure to generate high radiation flux and also to provide sufficient gas to allow conduction of the discharge current to the electrode 130. Referring to FIG. 2, the electrode 130 also provides a direct blocking path for any debris that might be generated within the bore region 125 as the discharge current passes through the capillary 120. Different angles can be used within the angular region as well as different gaps between the capillary bore mouth 127 and the electrode 130 to allow for optimization of the radiation flux output. The flux output can be measured with a calibrated EUV diode type meter, so that the separation space is adjusted between the end of the capillary and the blocking means, until a maximum radiation is achieved. At the high pressure end 122 of the capillary bore 125 where the gas G is flowed into the capillary 120, the electrode 110 can be of several configurations including the hollow cylinder shape as shown in FIG. 2 or a solid cylinder shape that is inserted within the capillary bore region with the gas flowed is around the cylinder or flowed through a hole in the cylinder electrode. Another version might be a heated filament as a cathode. FIG. 3 is a side cross-sectional view 200 of a second embodiment of an angular pumped and emitting capillary (APEC) discharge source of the subject invention incorporating a window 150 around the emitting region and having a constant pressure of the gas within that region, rather than operate with differential pumping. The APEC device of FIG. 3 is for obtaining intense visible, ultraviolet, or vacuum ultraviolet emission. This version incorporates a window 150 around the emitting region E2 and has a constant pressure of the gas within that region, rather than operate with differential pumping. Here the insulating capillary would be simple in shape, with the end face of the capillary normal to the bore axis. In this case the large electrode 130 would serve to block and collect debris and there would be a much larger angular admitting region because differential pumping would not be required to avoid absorption of the emission E2 by the emitting gas outside of the bore region. FIG. 4A is a side cross-sectional view 300 of a third embodiment of an angular pumped and emitting capillary(APEC) discharge source of the subject invention. This embodiment is a variation on the APEC design shown in FIG. 2. Here the principal functional difference is that the gas is admitted to the system from the same end at which the useful eight is emitted. Discharge conditions and parameters are identical to the APEC 100. Referring to FIG. 4A, the angular pumped and emitting capillary 320 of embodiment 300, has metal electrodes 310 and 330 at opposite ends of an insulating capillary 320 whose bore 325 is filled with gas (i.e., Xenon helium, neon, argon, and krypton, which were referred to in U.S. Pat. No. 6.031,241 to the same inventors and same assignee, which has been incorporated by reference) under electrical discharge conditions. Both the metal electrodes 310 and 330 are hollow with axial bores 315 and 335-337 respectively. Gas G is flowed into the discharge region through the axial bore hole 315 in the metal electrode 310 located at the end of the capillary from which the useful radiation is emitted. Gas is admitted to this electrode by a gas inlet 311 connected to plumbing (not shown in FIG. 4A) in a similar fashion to the APEC 100. Outflowing gas enters both the capillary bore 325 and the annular gap between the electrode face 317 and the capillary face 327, which bound the line-of-site of the emitted useful radiation. This results in a region 321 of high gas density in the region of the discharge seen directly along the line-of-sight, which increases radiated output relative to the simpler APEC 100. Gas is pumped away both in the low pressure region into which the radiation is emitted, and also through the vacuum exhaust bore hole 339 in the metal electrode 330 on the opposite side of the capillary. Additionally, the holes 315, 339 in both electrodes 310, 330 serve as "shock tubes", which guide the discharge-induced gas pressure pulse by allowing an unimpeded path for axial gas to flow. Much of the particulate debris shot out the radiating end 317, 327 of the capillary bore would simply travel down the gas inlet line and come to rest deep in the gas reservoir behind the electrode 310. Finally, the flowing gas may serve to cool and protect the components. A tube of flowing gas exhausting into vacuum forms a Mach 1 nozzle. The kinetic temperature in a Mach 1 expansion is for a monatomic ideal gas, three-fourths of the reservoir temperature. If the inlet gas is cooled nearly to its freezing point temperature (to less than 4/3 its freezing temperature in Kelvins) then the expansion should cause gas to freeze out on the tip of the electrode and inner wall of the capillary bore, to serve as an ablative buffer which may reduce bore erosion and debris formation in the first place. Gas that does not freeze out would flow more slowly and have a higher atom density for a given inlet pressure, which also would be salutory from the standpoint of maximizing the radiator density at the radiating end of the capillary. Finally, it cools the capillary material making it a better insulator. Another variation on the modified APEC design is shown in FIG. 4B. Here, the radiating gas G flowed into the capillary 320 through both metal electrodes 310 and 340. Electrode 340 has a C-cross-sectional shape with interior 341 and gas inlet 349. Gas exhaust and useful radiation E are removed by the vacuum region containing the optics as for the simpler APEC 100. This configuration maintains a more nearly uniform high density of gas throughout the length of the capillary than any other design. FIG. 5 is a side cross-sectional view 400 of a fourth embodiment 400 of a capillary discharge lamp with a debris collecting device attachment 405. The assembly consisting of electrodes 410 and 430, capillary 420, and gas flow G7 is functionally identical to the DPC 1, FIG. 1. The debris collection device 405, in its most simple form a metal cup, kinetically intercepts the debris particles (40, in FIG. 1) while allowing the useful radiation E7 at greater axial angles to escape and be collected by optics (50, in FIG. 1). The debris collector 405 would subtend a full angle of at least 12 degrees along the capillary axis; its size would therefore depend on its distance from the end of the capillary. The collector 405 can be at the same or different voltage as the electrode 410. Choice of material for the debris collector 405 would include but not be limited to stainless steel, aluminum, brass, copper and the like materials. The use of insulators as collector materials would be problematic, as electrical charging in the presence of plasma could affect debris trajectories in an uncontrolled way. Shape of the collector is probably unimportant. A U-shaped device 405 as illustrated in FIG. 5 would intercept most secondary debris coming off the collector surface itself. FIG. 6 represents another embodiment of the discharge device with a U-shaped debris blocker/collector 505 that does not use a capillary associated with generating the discharge. Referring to FIG. 6, high pressure gas G9 is flowed through one cylindrical electrode 510 and a debris collecting (or just blocking) device 505 also serves as the other electrode 505. The EUV emission E9 is produced in the region where the gas exits the between electrode ends 507 of the debris catching electrode 505 and the ends 511 of the cylindrical electrode 510. The shape of the collector 505 in this case is more important than for the collector 405 (FIG. 5) since collector 505 also serves as an electrode. Electrical impedance between electrode faces 507 and 511 will determine distance between electrodes, and angular intercept requirements will in turn constrain the collectors size. Surface figure on end faces 507 will also be important. Collector material will require tolerance to high current throughput and high temperatures, and like before, the concave/U shape 505 will help to decrease secondary debris. The discharge lamp operating at wavelengths longer than approximately 100 nm can be used for materials processing, medical treatment such as photodynamic therapy, and other applications where pulsed high flux vacuum ultraviolet, ultraviolet, visible and near infrared wavelengths of light are required. This source can have applications for an EUV microscope. Such a microscope could be used to observe features as small as 0.05 microns (50 nm) and have very large depth of focus. One application would be as an inspection tool on a microlithography fab line in which great depth of focus is required to observe the resist or chip feature side-walls for uniformity and wall slope. It might also be used in hospitals, for example in pathology labs, where a tissue sample (biopsy) needs to be inspected immediately after it is taken from a patient. The microscope can also be used for general high resolution analysis in chemical and pharmaceutical labs. While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.