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High flux, high energy photon sourceHigh flux, high energy photon source description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070278429, High flux, high energy photon source. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION FIELD OF THE INVENTION [0001] The invention relates generally to a high flux, high energy photon source, and in particular to a pulsed plasma source. [0002] It is desirable in various applications to produce photons with wavelengths in the extreme ultra-violet (EUV) range, in the region of around 1-50 nm. One such application, for example, is semi-conductor lithography where increasing demands on resolution require shorter wavelengths. [0003] Various pulsed plasma sources are known for producing EUV. These can be divided into two basic types: electrical/magnetic sources sometimes termed discharge produced plasma (DPP) sources embracing electrical excitation of the plasma using, for example, discharge, capacitative or inductive type systems; and laser produced plasma (LPP) sources. In either case an appropriate pulse is applied to a material, usually a target or working gas, and creates a plasma from which EUV radiation is generated. [0004] Various problems exist with known systems, however, including low efficiency--in the region of 0.25% for electrical/magnetic excitation and 0.5% for LPP (or about 1.5% for LPP with solid/liquid targets). This means that power inputs of 25-150 kW and 5-75 kW, respectively, are likely to be required for lithographic production systems. Attendant to that, high heat loading on the supporting system, bearing in mind that the temperatures generated in the plasma can be of the order of hundreds of thousands of degrees Kelvin in order to reach the optimum temperature for the generation of the desired EUV wavelength, and the creation of large amounts of debris--that is, high velocity, high energy matter created as the plasma expands--from the plasma and from secondary damage/heating are highly problematic for production tools. Scaling LPP and electrical/magnetic excitation to production levels is problematic not only in terms of heating and debris, but also vis-a-vis dose control, cost, reliability, and efficiency. In particular, higher conversion efficiencies could mitigate the thermal and debris issues. [0005] Debris is a significant problem in both LPP and electrical/magnetic excitation systems due to the need for a large and expensive collection optic to share the vacuum space in order to collect a high solid angle of emission. Primary debris from the plasma or secondary debris from chamber components significantly reduce the useable lifespan of such an optic. Heat loading is a problem in that the chamber components will distort and possibly be damaged if sufficient cooling is not available. Moreover, the large collection optic has a very precise shape, and heating easily distorts it beyond usability and/or causes direct damage to the coating. [0006] Cost of ownership is also a serious problem. Although LPP applied to solid/liquid targets currently holds out the possibility of higher conversion efficiencies than electrical/magnetic excitation, the capital cost of the lasers needed for a production level tool is expected to be much higher than that associated with electrical/magnetic excitation. It is not clear which route offers the most attractive solution in this respect, even if the problems of debris and heating can be overcome. [0007] One known electrical/magnetic excitation system described in U.S. Pat. No. 6,084,198 comprises an electrode column in which a plasma sheath is formed. The plasma sheath exits the column and forms a pinch at the end. Gas at the pinch generates EUV photons. The arrangement, however, requires close contact between the working electrodes and the working gas with concomitant issues of electrode damage. Moreover, it is difficult to control plasma parameters such as size, shape, and density and thereby maximize the conversion of electrical energy to the required wavelength of EUV radiation. [0008] U.S. Pat. No. 6,084,198 further discloses a plasma initiator in the form of spark plugs surrounding the electrode column, but this arrangement is inflexible and does not allow much variation in the operating parameters of the working gas/gasses. A further problem is that the spark plugs themselves are exposed to the plasma created and that additional components can be required to avoid damaging re-strikes. [0009] Another electrical/magnetic excitation system is described in U.S. Pat. No. 6,031,241, in which a discharge is formed in a capillary to create a plasma from material introduced into the capillary. This arrangement suffers from the further problem that the plasma is created within the capillary which must therefore be treated to resist the extreme conditions it is subjected to. Maintaining capillary integrity is difficult, and there is a limit to the EUV yield that can be achieved in a single capillary. [0010] A known arrangement is illustrated in FIG. 1 and is generally designated with the reference numeral 10. A gas, preferably an inert gas such as Xenon is fed at high pressure through a nozzle 12. The gas expands supersonically into a cone 14 on exiting the nozzle 12 and is subjected to excitation by a laser 16. A plasma is created which emits EUV radiation collected by a collector 20, which may for example be a parabolic reflector and channeled therefrom for the desired use, for example in semiconductor lithography. [0011] According to this arrangement, the use of a suitably shaped nozzle and supersonic beam of gas produces a high gas density. If for example an inert gas such as Xenon is used, clusters of gas atoms are formed which helps to maintain a high local gas density and is thought to assist EUV generation However various problems arise with the system. Because of the rapid expansion of the gas cone 14, the laser 16 must be focused as close as possible to the nozzle 12 to maintain an acceptable energy density in the plasma. As a result the nozzle 12 is at risk of damage because of its proximity to the plasma, and the arrangement as a whole is physically constrained. [0012] Yet further, the expansion of the gas in a cone away from the nozzle gives rise to debris filling the chamber in an broadly expansive beam. Despite the use of an inert gas as the source gas, this can be damaging to the components of the system and in particular the collector mirror, which is at risk of physical damage from collisions with more massive particles, and chemical damage such as oxidization. This in turn places constraints on the overall chamber design. [0013] Yet further, the gas density is not constant across the cone 14 as a result of which the laser pulse may be absorbed by the lower density periphery of the cone, providing unsatisfactory penetration to the high density center of the source gas cone. [0014] It is known in electrical/magnetic excitation systems that in order to improve the efficiency and stability of EUV generation, additional electrodes or coils can be introduced to pre-ionize source material by electrical/magnetic excitation. Such pre-ionization is known to allow creation of a more stable and well defined plasma as well as enhanced control of the initial conditions, in particular reducing fluctuation between pulses and enhancing dose control. However the pre-ionization electrodes are exposed to potentially damaging conditions. Moreover, spatially smooth pre-ionization is a necessary condition to avoid hot or cold spots in the final plasma. Such variations in temperature reduce the efficiency for emission in the required spectral bandwidth and therefore limit the effectiveness of the device. [0015] Background pressure in known systems can be detrimental as it can give rise to self-absorption, the source gas surrounding the gas cone absorbing some of the EUV radiation before it reaches the collector, reducing the efficiency of the system. In order to reduce the problem of self-absorption in known capillary systems, differential pumping is used. A significant pressure gradient is thus maintained in the known systems requiring costly and high maintenance pumps. SUMMARY OF THE INVENTION [0016] According to the invention, there is provided a high energy photon source comprising a source material emitter arranged to emit a stream of source material, an excitation component downstream thereof being arranged to create a plasma in the source material for emission of high energy photons and an apertured stop provided therebetween. [0017] The use of the apertured stop, for example in the form of a skimmer plate, gives rise to numerous advantages. Because the apertured stop collimates the beam, the debris created is greatly restricted and also channeled away in the collimation direction from the excitation component and collector, i.e., collimation of the molecular beam is found to restrict the flow of debris in the chamber to a narrow path. In turn, this allows more flexibility with the excitation component location and geometry. In a preferred embodiment, the excitation component comprises an electrical/magnetic excitation component in which case the coil, plate or electrode configuration can be easily varied. [0018] Preferably, the emitter emits source material at a supersonic velocity, yet further improving the channeling away of the beam from the excitation component as it takes place at high speed. [0019] In addition, EUV flux generation can be tuned at will into considerably more efficient conditions. In particular, the volume/cross-section, velocity, and density of the target gas stream can be varied over an extremely wide range to allow optimal efficiency of conversion to EUV radiation by altering the skimmer plate aperture dimensions appropriately. [0020] The variable geometry also allows the size and shape of the EUV emitting volume to be controlled so as to match the requirements of the large collection optic and the lithographic projection system which receives light from said optic. [0021] Preferably, the material emitter, which may be a nozzle, and excitation components are provided in separate chambers divided by the apertured stop. Accordingly, the nozzle side of the chamber can be maintained at a high pressure and the pumping side at a very low pressure to restrict self absorption without the need for differential pumping. Even further, a greater proportion of the gas can be recirculated and recycled. Continue reading about High flux, high energy photon source... 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