This patent application is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "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 equation (1):
"CD = k 1 * .lamda. NA ( 1 ) ##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, for 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. A radiation system 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 droplets of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
"It can be difficult to consistently and accurately hit a series of moving droplets with a pulsed laser beam. For example, some high-volume EUV radiation sources may require the irradiation of droplets having a diameter of about 20-50 .mu.m and moving at a velocity of about 50-100 m/s.
"With the above in mind, systems and methods have been proposed for effectively delivering and focusing a laser beam to a selected location in an EUV radiation source.
"U.S. Pat. No. 7,491,954 describes an EUV radiation source which comprises an optical gain medium and a lens which is arranged to direct radiation generated by the optical gain medium onto a droplet of fuel material. The optical gain medium and lens are arranged such that the optical gain medium generates laser radiation when the droplet of fuel material is at a predetermined location, thereby causing the droplet of fuel material to produce an EUV radiation emitting plasma. Since optical gain medium is triggered by the presence of the droplet of fuel material at the predetermined location, a seed laser is not required to trigger operation of the optical gain medium.
"A problem associated with the type of system described in U.S. Pat. No. 7,491,954 is that because the lasing process starts by photons being reflected by droplets of fuel material such that the rays are reflected into themselves, the mode that builds-up is strongly dependent upon and confined around the initial trigger process. This in turn induces the following problems: the cavity is only used locally with the result that saturation effects in the gain medium limit the absolute power obtainable; and the moving droplet of fuel material flies by the initial trigger point, to which the laser fires back, with the result that the next reflection is less than optimal, which can lead to the development of an undesirable asymmetric mode."
In addition to the background information obtained for this patent application, VerticalNews journalists also obtained the inventors' summary information for this patent application: "It is desirable to provide a radiation source and lithographic apparatus which is novel and inventive compared with known radiation sources.
"According to a first aspect of the present invention there is provided a radiation source comprising a nozzle configured to direct a stream of fuel droplets along a trajectory towards a plasma formation location and a laser configured to direct laser radiation to the plasma formation location to convert the fuel droplets at the plasma formation location into a plasma, wherein the laser comprises an amplifier and an optical element configured to define a divergent beam path for radiation passing through the amplifier.
"The laser may be configured to generate a pulse of laser radiation when photons emitted from the amplifier are reflected along the divergent beam path by a fuel droplet. The laser may comprise a cavity mirror arranged to reflect photons reflected by fuel droplets, and the optical element may be provided in between the amplifier and the cavity mirror.
"The amplifier may comprise a plurality of amplifier chambers. The optical element may be provided in between the cavity mirror and the amplifier chamber closest to the cavity minor.
"In a first embodiment the optical element comprises a phase grating.
"In a second embodiment the optical element comprises a scatter plate.
"The radiation source may further comprise a collector mirror configured to collect and focus radiation generated by the plasma formed from the fuel droplets.
"The plasma produced by conversion of the fuel droplets is preferably EUV radiation emitting plasma.
"The laser radiation may have a wavelength of between about 9 .mu.m and about 11 .mu.m.
"The nozzle may be configured to emit fuel droplets as single droplets. Alternatively, the nozzle may be configured to emit fuel droplets as clouds of fuel which subsequently coalesce into droplets.
"The fuel droplets may comprise or consist of Xe, Li or Sn.
"The laser is preferably a CO.sub.2 laser.
"According to a second aspect of the present invention there is provided a lithographic apparatus comprising the radiation source of the preceding aspect of the present invention, and further comprising an illumination system configured to condition a radiation beam, a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.
"According to a third aspect of the present invention there is provided a method comprising emitting a stream of fuel droplets from a nozzle along a trajectory towards a plasma formation location and using a laser to direct laser radiation to the plasma formation location to convert the fuel droplets at the plasma formation location into a plasma, wherein the laser comprises an amplifier and an optical element and the method further comprises using the optical element to define a divergent beam path for radiation passing through the amplifier.
"Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present 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.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
"The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the present invention and to enable a person skilled in the relevant art(s) to make and use the present invention.
"FIG. 1 schematically depicts a lithographic apparatus according to an aspect of the present invention.
"FIG. 2 is a more detailed view of the apparatus of FIG. 1, including an LPP source collector module.
"FIG. 3 schematically depicts a radiation source according to the prior art.
"FIG. 4 schematically depicts steps in the operation of the radiation source of FIG. 3.
"FIG. 5 schematically depicts a radiation source according to a first embodiment of an aspect of the present invention, and
"FIG. 6 schematically depicts a radiation source according to a second embodiment of an aspect of the present invention.
"The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number."
URL and more information on this patent application, see: Wagner, Christian; Loopstra,
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