Abstract:
One embodiment disclosed relates to an apparatus for inspecting or revieiwing a mask or reticle. The apparatus includes at least an optical system, a converter plate, and an electron system. The optical system projects an optical illumination onto the mask. The optical signal from the mask is received by the converter plate. The converter plate transforms the optical signal to a corresponding electron signal. The electron signal is imaged by the electron system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims the benefit of provisional patent application No. 60/499,156, filed Aug. 29, 2003, entitled “Aerial Reticle Inspection with Particle Beam Conversion”, by inventor David L. Adler, the disclosure of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to apparatus and methods for inspection or review. More particularly, the invention relates to apparatus and methods for inspection or review of reticles or masks used to manufacture semiconductor devices. 
   2. Description of the Background Art 
   One of the major sources of yield loss in the manufacture of very large scale integrated (VLSI) circuits is random defect in the photomasks. As chip sizes and geometry densities increase more attention must be given to mask quality in order to reduce defects to a level commensurate with acceptable yields. 
   SUMMARY 
   One embodiment of the invention pertains to an apparatus for inspecting or revieiwing a mask or reticle. The apparatus includes at least an optical system, a converter plate, and an electron system. The optical system projects an optical illumination onto the mask. The optical signal from the mask is received by the converter plate. The converter plate transforms the optical signal to a corresponding electron signal. The electron signal is imaged by the electron system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram depicting a conventional apparatus for illuminating a wafer with a pattern from a mask or reticle. 
       FIG. 2  is a schematic diagram depicting an apparatus for inspecting or reviewing a mask or reticle in accordance with an embodiment of the invention. 
       FIG. 3  illustrates such a backside illumination converter plate. 
       FIG. 4  illustrates such a front-side illumination converter plate. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram depicting a conventional lithography apparatus for illuminating a wafer with a pattern from a mask or reticle. An optical source  102  generates light for illuminating the mask  108 . An optical lens system  104  directs and focuses the optical illumination  106  onto the mask  108 . 
   The mask  108  may comprise, for example, a phase shift type mask. The mask  108  transforms the illumination depending upon the patterning thereon. Typically, the patterning on the mask  108  is designed to form a particular pattern on a photoresist layer on the wafer  112 . 
   The illumination passing through and transformed by the mask  108  is focused by an objective lens  110  onto the surface of the wafer  112 . The image projected on the wafer may be a fraction in scale (for example, one fourth in scale) compared with the scale of the pattern on the mask  108 . 
     FIG. 2  is a schematic diagram depicting an apparatus for inspecting or reviewing a mask or reticle in accordance with an embodiment of the invention. The inspection apparatus depicted in  FIG. 2  enables the inspection of the mask  108  to be performed at a one times (1×) magnification in comparison to the eventual patterning on a wafer. In other words, the inspection or review may be performed using the same optical illumination and focusing system as that used in the lithography apparatus. This advantageously eliminates the need to perform complex phase shift related calculations and the errors and inaccuracies that may arise therefrom. Further, the use of a particle beam subsystem in accordance with an embodiment of the invention enables higher resolution inspection than may be obtained by optical means. 
   The upper optical portion of the inspection apparatus in  FIG. 2  may have the same configuration as the corresponding portion of the lithography apparatus in  FIG. 1 . An optical (or x-ray) source  102  generates light for illuminating the mask  108 . An optical lens system  104  directs and focuses the optical illumination  106  onto the mask  108 . The illumination passing through and transformed by the mask  108  is focused by an objective lens  110 . However, instead of being focused onto the surface of a wafer  112 , the illumination is focused onto a converter plate  202 . 
   The converter plate  202  comprises a device that converts the optical-based signal into an electron-based signal. For example, the converter plate  202  may be constructed using commercially available photoelectron material. Night vision goggles may also be constructed using such photoelectron material. Such photoelectron material receives photons  203  and emits electrons  205  in response thereto. 
   The electron signal  205  is directed and focused by an electron lens system  206  onto a fluorescent device  208 . The fluorescent device  208  may include a phosphor based material which receives the electrons and emits light in response thereto. In other words, while the converter plate  202  converts a light signal to an electron signal, the fluorescent device  208  converts an electron signal back to a light signal. Coupled to the fluorescent device  208  is an optical detector or camera  210 . The detector  210  may comprise, for example, an array of detector elements, such as a charge-coupled device (CCD) array. In one embodiment, the detector  210  may comprise a time-delay integration (TDI) detector. Such a TDI detector may be used to efficiently inspect a mask  108  while the mask  108  is in linear motion. 
   In an alternate embodiment, instead of using both a fluorescent device  208  and an array detector  210 , a back-thinned array detector may replace both devices. Such a back-thinned array detector may be configured to directly detect electrons, such that the fluorescent conversion of electrons to light is no longer needed. Advantageously, this would eliminate the need for the fluorescent device  208 . Disadvantageously, such a back-thinned array detector may be more prone to damage from the electrons and so may not be as robust. 
   In one embodiment, the detected image data may be compared to images taken from other mask regions to detect defects or process variations. In another embodiment, the detected image data may be compared to a rendered database to detect defects or process variations. 
   In the configuration depicted in  FIG. 2 , the converter plate  202  comprises a backside illumination type device.  FIG. 3  illustrates such a backside illumination converter plate. As shown in  FIG. 3 , the backside illumination converter plate  302  receives an optical signal  304  on one side of the plate  302  and emits a corresponding electron signal  306  on the opposite side of the plate  302 . Advantageously, the backside illumination converter plate  302  may comprise part of a vacuum interface  204 . The vacuum interface  204  separates the optical portion of the system which does not need to be in vacuum from the electron portion of the system which needs to be in vacuum. 
   In contrast, a front-side illumination converter plate  402  is depicted in  FIG. 4 . As shown in  FIG. 4 , the front-side illumination converter plate  402  receives an optical signal  404  on one side of the plate  402  and emits a corresponding electron signal  406  on the same side of the plate  402 . 
   An alternate embodiment of the invention may utilize a front-side illumination converter plate  402 . In such an embodiment, the electron lens system  206  and the electron imaging devices would be configured on the same side of the converter plate  202  as the optical illumination and focusing components. A field (such as a magnetic field) may be utilized to bend the emitted electrons towards the electron lens system  206 . In addition, the apparatus would be configured such that the emitted electrons traveled through a volume pumped by a vacuum. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. The above-described invention may be used in an automatic inspection or review system and applied to the inspection or review of optical or X-ray masks and similar substrates in a production environment. 
   In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
   These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.