X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2001-10-12
2004-07-06
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C250S50400H
Reexamination Certificate
active
06760406
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to a method and an apparatus for generating X-ray or EUV radiation, i.e. radiation in the wavelength region of approximately 0.01-100 nm. The generated radiation can be used in any application requiring X-ray or EUV radiation, for example lithography, microscopy, materials science, or medical diagnostics.
BACKGROUND ART
EUV and X-ray sources of high intensity are applied in many fields, for instance surface physics, materials testing, crystal analysis, atomic physics, medical diagnostics, lithography and microscopy. Conventional X-ray sources, in which an electron beam is brought to impinge on an anode, generate a relatively low X-ray intensity. Large facilities, such as synchrotron light sources, produce a high average power. However, there are many applications that require compact, small-scale systems which produce a relatively high average power. Compact and more inexpensive systems yield better accessibility to the applied user and thus are of potentially greater value to science and society. An example of an application of particular industrial importance is future narrow-line-width lithography systems.
Ever since the 1960s, the size of the structures that constitute the basis of integrated electronic circuits has decreased continuously. The advantage thereof is faster and more complicated circuits needing less power. At present, photolithography is used to industrially produce such circuits having a line width of about 0.18 &mgr;m with projected extension towards 0.10-0.13 &mgr;m. In order to further reduce the line width, other methods will probably be necessary, of which EUV projection lithography is a very interesting candidate and X-ray lithography may become interesting for certain technological niches. In EUV projection lithography, use is made of a reducing extreme ultraviolet (EUV) objective system in the wavelength range around 10-20 nm (“EUV Lithography—The Successor to Optical Lithography?” by Bjorkholm, published in Intel Technology Journal Q3'98). Proximity X-ray lithography, employing a contact copy scheme, is carried out in the wavelength range around 1 nm (see for instance the article “X-ray Lithography” by Maldonado, published in J. Electronic Materials 19, p. 699, 1990).
Laser plasmas are attractive table-top X-ray and EUV sources due to their high brightness, high spatial stability and, potentially, high-repetition rate However, with conventional bulk or tape targets, the operating time is limited, especially when high-repetition-rate lasers are used, since fresh target material cannot be supplied at a sufficient rate. Furthermore, such conventional targets produce debris which may destroy or coat sensitive components such as X-ray optics or EUV multilayer mirrors positioned close to the plasma. Several methods have been designed to eliminate the effect of debris, i.e., preventing the already produced debris from reaching the sensitive components. As an alternative, the amount of debris actually produced can be limited by replacing conventional solid targets with for example gas targets, gas-cluster targets, liquid-droplet targets, or liquid-jet targets.
Targets in the form of microscopic liquid droplets, such as disclosed in the article “Droplet target for low-debris laser-plasma soft X-ray generation” by Rymell and Hertz, published in Opt. Commun. 103, p. 105, 1993, are attractive low-debris, high-density targets potentially capable of high repetition-rate laser-plasma operation with high-brightness emission. Such droplets are generated by stimulated breakup of a liquid jet which is formed at a nozzle in a low-pressure chamber. However, the hydrodynamic properties of certain fluids result in unstable drop formation. Furthermore, the operation of the laser must be carefully synchronized with the droplet formation. Another problem may arise in the use of liquid substances with rapid evaporation, namely that the jet freezes immediately upon generation so that drops cannot be formed. Such substances primarily include media that are in a gaseous state at normal pressure and temperature and that are cooled to a liquid state for generation of the droplet targets. To ensure droplet formation, it is necessary to provide a suitable gas atmosphere in the low-pressure chamber, or to raise the temperature of the jet above its freezing temperature by means of an electric heater provided around the jet, such as disclosed in the article “Apparatus for producing uniform solid spheres of hydrogen” by Foster et al., published in Rev. Sci. Instrum. 6, pp. 625-631, 1977.
As an alternative, as known from U.S. Pat. No. 6,002,744, which is incorporated herein by reference, the laser radiation is instead focused on a spatially continuous portion of a jet which is generated by urging a liquid substance through an outlet or nozzle. This liquid-jet approach alleviates the need for temporal synchronization of the laser with the generation of the target, while keeping the production of debris equally low as from droplet targets. Furthermore, liquid substances having unsuitable hydrodynamic properties for droplet formation can be used in this approach. Another advantage over the droplet-target approach is that the spatially continuous portion of the jet can be allowed to freeze. Such a liquid-jet laser-plasma source has been further demonstrated in the article “Cryogenic liquid-jet target for debris-free laser-plasma soft x-ray generation” by Berglund et al, published in Rev. Sci. Instrum. 69, p. 2361, 1998, and the article “Liquid-jet target laser-plasma sources for EUV and X-ray lithography” by Rymell et al, published in Microelectronic Engineering 46, p. 453, 1999, by using liquid nitrogen and xenon, respectively, as target material. In these cases, a high-density target is formed as a spatially continuous portion of the jet, wherein the spatially continuous portion can be in a liquid or a frozen state. Such laser-plasma sources have the advantage of being high-brightness, low-debris sources capable of continuous high-repetition-rate operation, and the plasma can be produced far from the outlet orifice, thereby limiting plasma-induced erosion of the outlet. Such erosion may be a source of damaging debris. Further, by producing the plasma far from the outlet, self-absorption of the generated radiation can be minimized. This is due to the fact that the temperature around the jet decreases with the distance from the outlet, resulting in a correspondingly decreasing evaporation rate. Thus, the local gas atmosphere around the jet also decreases with the distance from the outlet.
However, many substances, and in particular liquid substances formed by cooling normally gaseous substances, yield a jet that experiences stochastic changes in its direction from the jet-generating outlet. Typically the change in direction can be as large as about ±1° and occurs a few times per minute to a few times per second. This in turn results in a spatial instability at the focus of the laser beam, i.e. at the desired area of beam-jet-interaction, which should be as far away from the outlet orifice as possible for the reasons given above. The spatial instability leads to high pulse-to-pulse fluctuations in the emitted X-ray and EUV radiation flux and spatial instability of the radiating plasma. Furthermore, the average power is significantly lowered.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and an apparatus for stable and uncomplicated generation of X-ray or EUV radiation. More specifically, the invention should provide for low pulse-to-pulse fluctuations in the generated X-ray or EUV radiation flux, low erosion of the jet-generating outlet, as well as low self-absorption of the generated radiation.
It is also an object to provide an apparatus for generating X-ray or EUV radiation that is compact, inexpensive, generates radiation at a relatively high average power and has a minimum production of debris.
A further object is to provide a method and an apparatus which produces
Ĥemberg Oscar
Berglund Magnus
Hansson Björn A.M.
Hertz Hans Martin
Rymell Lars
Burns Doane Swecker & Mathis L.L.P.
Church Craig E.
Jettec AB
Yun Jurie
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