X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
2001-07-23
2004-03-23
Church, Craig E (Department: 2882)
X-ray or gamma ray systems or devices
Source
Target
C378S119000, C378S124000, C378S125000
Reexamination Certificate
active
06711233
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to a method and an apparatus for generating X-ray or extreme ultraviolet (EUV) radiation, especially with high brilliance. The generated radiation can for example be used in medical diagnostics, non-destructive testing, lithography, microscopy, materials science, or in some other X-ray or EUV application.
BACKGROUND ART
X-ray sources of high power and brilliance are applied in many fields, for instance medical diagnostics, non-destructive testing, crystal structural analysis, surface physics, lithography, X-ray fluorescence, and microscopy.
In some applications, X-rays are used for imaging the interior of objects that are opaque to visible light, for example in medical diagnostics and material inspection, where 10-1000 keV X-ray radiation is utilized, i.e. hard X-ray radiation. Conventional hard X-ray sources, in which an electron beam is accelerated towards a solid anode, generate X-ray radiation of relatively low brilliance. In hard X-ray imaging, the resolution of the obtained image basically depends on the distance to the X-ray source and the size of the source. The exposure time depends on the distance to the source and the power of the source. In practice, this makes X-ray imaging a trade-off between resolution and exposure time. The challenge has always been to extract as much X-ray power as possible from as small a source as possible, i.e. to achieve high brilliance. In conventional solid-target sources, X-rays are emitted both as continuous Bremsstrahlung and characteristic line emission, wherein the specific emission characteristics depend on the target material used. The energy that is not converted into X-ray radiation is primarily deposited as heat in the solid target. The primary factor limiting the power, and the brilliance, of the X-ray radiation emitted from a conventional X-ray tube is the heating of the anode. More specifically, the electron-beam power must be limited to the extent that the anode material does not melt. Several different schemes have been introduced to increase the power limit. One such scheme includes cooling and rotating the anode, see for example Chapters 3 and 7 in “Imaging Systems for Medical Diagnostics”, E. Krestel, Siemens Aktiengesellschaft, Berlin and Munich, 1990. Although the cooled rotating anode can sustain a higher electron-beam power, its brilliance is still limited by the localized heating of the electron-beam focal spot. Also the average power load is limited since the same target material is used on every revolution. Typically, very high intensity sources for medical diagnostics operate at 100 kW/mm.sup.2, and state of the art low-power micro-focus devices operate at 150 kW/mm.sup.2.
Applications in the soft X-ray and EUV wavelength region (a few tens of eV to a few keV) include, e.g., next generation lithography and X-ray microscopy 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 complex circuits requiring less power. At present, photolithography is used to industrially produce such circuits having a line width of about 0.13 &mgr;m. This technique can be expected to be applicable down to about 0.1-0.07 &mgr;m. In order to further reduce the line width, other methods will probably be necessary, of which EUV projection lithography is a strong candidate, see for example “International Technology Roadmap for Semiconductors”, International SEMATECH, Austin Tex., 1999. In EUV projection lithography use is made of a reducing EUV objective system in the wavelength range around 10-20 nm.
In the soft X-ray and EUV region, compared to the conventional generation of hard X-ray radiation as discussed above, a different scheme for generation of radiation is normally used since the conversion efficiency from electron-beam energy into soft X-ray radiation, in solid targets, generally is too low to be useful. A common technique for generation of soft X-ray and EUV radiation is instead based on heating of the target material for production of a hot, dense plasma using intense (around 10
10
-10
13
W/cm
2
) laser radiation, such as disclosed in Chapter 6 in “Soft X-rays and Extreme Ultraviolet Radiation: principles and application”, D. T. Attwood, Cambridge University Press, 1999. These so-called laser produced plasmas (LPP) emit both continuous radiation and characteristic line emission, wherein the specific emission characteristics depend on target material and plasma temperature. Traditional LPP X-ray sources, using a solid target material, are hampered by unwanted emission of debris as well as limitations on repetition rate and uninterrupted usage, since the delivery of target material becomes a limiting factor. This has lead to the development of regenerative, low debris targets including gas jets (see for example U.S. Pat. No. 5, 577,092, and the article “Debris-free EUVL sources based on gas jets” by Kubiak et al, published in OSA Trends in Optics and Photonics, No. 4, p. 66, 1996), and liquid jets (see for example U.S. Pat. No. 6,002,744, and the article “Liquid-jet target for laser-plasma soft x-ray generation” by Malmqvist et al, published in Review of Scientific Instruments, No. 67, p. 4150, 1996). These targets have been extensively used in LPP soft X-ray and EUV sources. However, the applicability of LPP sources is limited by the relatively low conversion efficiency of electrical energy into laser light and then of laser light into X-ray radiation, necessitating the use of expensive high-power lasers.
Quite recently, electron-beam excitation of a gas-jet target has been tested for direct, non-thermal generation of soft X-ray radiation, albeit with relatively low power and brilliance of the resulting radiation, see Ter-Avetisyan et al, Proceedings of the SPIE, No. 4060, pp 204-208, 2000.
There are also large facilities such as synchrotron light sources, which produce X-ray radiation with high average power and brilliance. However, there are many applications that require compact, small-scale systems that produce X-ray radiation with a relatively high average power and brilliance. Compact and more inexpensive systems yield better accessibility to the applied user and thus are of potentially greater value to science and society.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve or alleviate the problems described above. More specifically, the invention aims at providing a method and an apparatus for generation of X-ray or EUV radiation with very high brilliance in combination with relatively high average power.
It is also an object of the invention to provide a compact and relatively inexpensive apparatus for generation of X-ray or EUV radiation.
The inventive technique should also provide for stable and uncomplicated generation of X-ray or EUV radiation, with minimum production of debris.
A further objective is to provide a method and an apparatus generating radiation suitable for medical diagnostics and material inspection.
Still another object of the invention is to provide a method and an apparatus suitable for use in lithography, non-destructive testing, microscopy, crystal analysis, surface physics, materials science, X-ray photo spectroscopy (XPS), X-ray fluorescence, protein structure determination by X-ray diffraction, and other X-ray applications.
These and other objectives, which will be apparent from the following description, are wholly or partially achieved by the method and the apparatus according to the appended independent claims. The dependent claims define preferred embodiments.
Accordingly, the invention provides a method for generating X-ray or EUV radiation, comprising the steps of forming a target jet by urging a liquid substance under pressure through an outlet opening, which target jet propagates through an area of interaction; and directing at least one electron beam onto the target jet in the area of interaction such that the electron beam interacts with the target jet to generate X-ray
Hemberg Oscar
Hertz Hans Martin
Burns Doane Swecker & Mathis L.L.P.
Church Craig E
Jettec AB
Yun Jurie
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