News Column

Patent Issued for EUV Radiation Source and Method of Generating EUV Radiation

June 4, 2014

By a News Reporter-Staff News Editor at Journal of Engineering -- A patent by the inventor Klaassen, Michel Francois Hubert (Eindhoven, NL), filed on June 24, 2011, was published online on May 20, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8730454 is assigned to ASML Netherlands B.V. (Veldhoven, NL).

The following quote was obtained by the news editors from the background information supplied by the inventors: "The present invention relates to an EUV radiation source and a method of generating EUV radiation. The EUV radiation source may form part of a lithographic apparatus.

"A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

"Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

"A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in the following equation:

".lamda. ##EQU00001## where .lamda. is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k.sub.1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength .lamda., by increasing the numerical aperture NA or by decreasing the value of k.sub.1.

"In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, or example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

"EUV radiation may be produced using a plasma. An EUV radiation source for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin). The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a collector. The collector may be a mirrored normal incidence collector, which receives the radiation and focuses the radiation into a beam. The EUV radiation source may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such an EUV radiation source is typically termed a laser produced plasma (LPP) source.

"A problem associated with LPP sources is that there is considerable non-uniformity in the intensity of radiation that they generate.

"It is desirable to provide an EUV radiation source that provides radiation with an improved intensity uniformity compared with at least some known EUV radiation sources."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventor's summary information for this patent: "According to a first aspect of the invention, there is provided an EUV radiation source comprising a fuel supply configured to deliver droplets of fuel to a plasma formation location, and a collector configured to collect EUV radiation emitted by a plasma at the plasma formation location, wherein the collector has a reflective surface that is a modified ellipsoid shape, the modified ellipsoid shape providing improved intensity uniformity of collected EUV radiation in the far field compared with a perfect ellipsoid shape.

"According to a second aspect of the invention there is provided a method of generating EUV radiation, the method comprising delivering droplets of fuel to a plasma formation location using a fuel supply, and collecting EUV radiation emitted by the plasma at the plasma formation location using a collector, wherein the collector has a reflective surface that is a modified ellipsoid shape, the modified ellipsoid shape providing the collected EUV radiation with improved intensity uniformity in the far field compared with a perfect ellipsoid shape.

"Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein."

URL and more information on this patent, see: Klaassen, Michel Francois Hubert. EUV Radiation Source and Method of Generating EUV Radiation. U.S. Patent Number 8730454, filed June 24, 2011, and published online on May 20, 2014. Patent URL:

Keywords for this news article include: ASML Netherlands B.V.

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Source: Journal of Engineering

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