Abstract:
An imaging apparatus for DUV beams is provided which includes a lens for receiving a DUV beam and producing an image of the DUV beam, wherein the lens has an aplanatic surface and a hemispheric surface, and wherein at least the aplanatic surface is made from or otherwise has a down-converting medium for producing a down-converted beam; an image sensing member for viewing an image of the down-converted beam; and optics for relaying the image of the down-converted beam from the lens to the image sensing member. A processor can be communicated with the image sensing member for analyzing the image of the DUV beam.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to an apparatus for viewing and analyzing high spatial resolution structure in ultraviolet (UV, DUV and EUV) beams. 
   Microlithography and nano-fabrication technologies rely on ultraviolet laser beams for fabricating structures with features only a few tens of nanometers in size. Fabricating such structures requires beams and masks with sub micron sized features. Paramount in constructing beam delivery systems for manufacturing nano structures is the ability to view and quantify the physical dimensions of sub micron features of the beam at the work surface. 
   There are no known devices for in-situ imaging of high resolution DUV beams in real time. 
   It is the object of the present invention to provide an economical apparatus for in-situ viewing and quantification of high resolution DUV beam patterns in real time. 
   Other objects and advantages of the present invention will be presented below. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, the foregoing objects and advantages have been readily obtained. 
   The invention is a high resolution imaging system, which comprises a medium for down-converting the DUV beam into a visible image and suitable optics or other means for relaying the visible image to an imaging sensor or screen for viewing and analysis. 
   According to the invention, an imaging apparatus for DUV beams is provided which comprises a lens for receiving a DUV beam and producing an image of the DUV beam, wherein the lens has an aplanatic surface and a hemispheric surface, and wherein at least the aplanatic surface comprises a down-converting medium for producing a down-converted beam; an image sensing member for viewing an image of the down-converted beam; and optics for relaying the image of the down-converted beam from the lens to the image sensing member. 
   A processor can advantageously be communicated with the image sensing member for analyzing the image as desired. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of preferred embodiments of the present invention follows, with reference to the attached drawings, wherein: 
       FIG. 1  schematically illustrates an apparatus in accordance with the present invention; 
       FIG. 2   a  illustrates an Amici lens made entirely from D-C medium; 
       FIG. 2   b  illustrates an Amici lens having a disk of D-C medium optically coupled to the aplanatic surface; and 
       FIG. 2   c  illustrates an Amici lens having a layer of D-C medium deposited on the aplanatic surface. 
   

   DETAILED DESCRIPTION 
   The invention relates to an apparatus which accurately and reliably images high resolution ultraviolet (UV), deep ultraviolet (DUV) and extreme ultraviolet (EUV) beams without performance degradation due to long-term exposure to the DUV. The term “DUV beams” herein collectively refers to any UV, DUV or EUV source. 
   In accordance with the present invention, the DUV beam is down-converted to longer wavelength radiation. Down conversion (the down-conversion process is hereinafter referred to as D-C) is the process whereby light of one wavelength (e.g., UV, DUV, EUV radiation) is converted to light at a longer wavelength using a medium referred to as a down-converter. The down-converted, longer wavelength light (e.g., visible, near-IR, etc.) is herein collectively referred to as “visible” light. The down-converter medium must be carefully chosen to produce an accurate spatial distribution and intensity distribution “copy” of the DUV beam. 
   The “visible” light is relayed using suitable optics to an imaging sensor or screen for viewing and analysis. 
   The down-converter is comprised of a suitable medium. As will be further described below, the D-C medium can be provided in the form of a disk; a layer deposited on a substrate or, an entire optical element made of a D-C medium. Disks and layers of D-C medium on a substrate are substantially contained in prior disclosures. 
   In a preferred embodiment, the optical element is an Amici lens made from a D-C medium. An Amici lens has unique properties that can be advantageously used for the present invention. An Amici lens is an aplanatic hyper-hemispherical lens, which produces a magnified virtual image of an object located at the aplanatic point. The virtual image can then be relayed using suitable optics to form a real image of high magnification with few aberrations. The image is free of all orders of spherical aberration, third order coma, and third order astigmatism. It can be further shown that if the aplanatic point is a distance d=(r/n) from the center of the sphere of radius, r, then a virtual image of the object will be produced a distance d′=r×n from the center of the sphere. The magnification of the Amici lens (the ratio of the height of the virtual image to the height of the object at the aplanatic point) is (n/n o ) 2 , where n is the index of refraction of the lens medium and n o  is the index of refraction of the surrounding medium, e.g., air. High numerical apertures (NA) are possible using this technique (NA=n sin□, where □ is the angle of ray propagation from the optical axis inside the medium surrounding the object). Since the limiting spatial resolution of an optical system is [0.61□/NA], the advantage of this technique for viewing DUV beams is significant. Limiting spatial resolution is defined as the distance between two point sources such that the first minimum of the Airy diffraction pattern of one source in the focal plane of a lens of numerical aperture NA lies at the location of the peak of the Airy distribution of the other source. 
     FIG. 1  schematically illustrates the apparatus in accordance with the present invention. A high-resolution DUV pattern can be imparted to the DUV beam by passing the DUV beam  11  through a mask  10  containing the pattern. The high resolution pattern in the DUV beam at the aplanatic point  13  of aplanatic surface  21  is an accurate representation of the features of the mask. The down converting medium  14  which in this embodiment is also the medium used to make the Amici lens  15 , converts the high resolution DUV pattern into a high resolution “visible” pattern. The hyper-hemispherical surface  16  of the Amici lens redirects a portion of the “visible” light  12  to relay optics  17 , producing a “visible” image of the DUV beam  18 . The “visible” image of the DUV beam can be viewed with an imaging sensor array, such as a CCD camera, or with a screen  19 . A computer  20  or other suitable processor can capture and analyze images from the CCD camera or other suitable device, typically using image analysis software which is well known to persons of ordinary skill in the art. 
   It should be noted that the general structure of an Amici lens, relay optics, imaging sensors and image analysis software are all devices which themselves are well known to a person of ordinary skill in the art. Thus, further disclosure in connection with each of these individual components is not presented herein. 
     FIG. 2   a  illustrates an Amici lens  15  in accordance with a preferred embodiment of the invention, wherein the entire lens is made of D-C material. In this embodiment, and advantageously, down-conversion is carried out entirely within the lens itself. 
     FIG. 2   b  illustrates an alternative embodiment wherein lens  15  can be provided from any suitable optical element material, and wherein the D-C medium is provided in the form of a disk  30  of D-C medium material, wherein the disk is optically coupled with the aplanatic surface  21 . In this case the disk is positioned over the aplanatic surface. 
     FIG. 2   c  show a further embodiment of the invention wherein lens  15  can be provided from any suitable optical element material and wherein the D/C medium is deposited in a layer  32  on the aplanatic surface  21  of the lens. 
   It should be noted that the Amici lens referred to herein has been shown having a hyper-hemispherical surface, that is, a surface which is spherical through more than 180 degrees. It is within the broad scope of the present invention to utilize a lens wherein the spherical portion is hemispherical, or even slightly less than hemispherical, and these configurations, along with preferred hyper-hemispherical surfaces, are considered to be included within the term hemispherical as used herein. 
   Suitable D-C medium material is well known to a person of ordinary skill in the art. Examples of acceptable materials include, but are not limited to rare earth doped materials, such as CE:YAG, and the like. 
   This apparatus can advantageously be used in numerous industrial, medical and like procedures wherein high resolution images of DUV beams are required for material processing. 
   Specific examples of various applications wherein the apparatus of the present invention can find useful application include microlithography, micromachining, and the like. 
   It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications, which are within its spirit and scope as defined by the claims.