Abstract:
A method and system are presented for use in optical processing of an article by VUV radiation. The method comprises: localizing incident VUV radiation propagation from an optical head assembly towards a processing site on the article outside the optical head assembly and localizing reflected VUV radiation propagation from said processing site towards the optical head assembly by localizing a medium, non-absorbing with respect to VUV radiation, in within the light propagation path in the vicinity of said site outside the optical head assembly. The level of the medium is controlled by measuring the reflected VUV radiation.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     The present application is continuation of co-pending parent application Ser. No. 10/958,665, filed Oct. 6, 2004 and claiming priority from Israel application No. 158344, filed Oct. 9, 2003. 
     FIELD OF THE INVENTION 
     This invention is generally in the field of optical measurement/inspection techniques, and relates to a vacuum UV optical system and method particularly useful in integrated metrology. 
     BACKGROUND OF THE INVENTION 
     Optical measurements of the thickness of thin films, as well as processing of small pattern features, require an optical system operation with shorter wavelengths. The most traditional technique is limited by DUV (Deep Ultra Violet) spectral range (down to 190 nm, or, due to some sources, to 200 nm). The use of shorter wavelengths enables many metrology advantages for thin film applications, but suffers from strong absorption of light by air (and water). 
     The above problem can be overcome by operating with such short wavelengths under vacuum conditions (the so-called “Vacuum UV” or “VUV”) or in the environment of VUV non-absorbing gas like Nitrogen. For example, the VUV ellipsometer, commercially available from J. A. Woollam Co., Inc., operates in the spectral range down to 150 nm. The configuration of this system requires both an article under measurements and an optical system to be located within a sealed enclosure filled by Nitrogen. 
     This technique, however, is unsuitable for integrated metrology systems, because of the need for a small system size and high throughput. Indeed, such procedures as loading and unloading of an article (wafer) to and from such a sealed enclosure are time consuming, requiring sealing/unsealing of a processing tool, while maintaining a required gas pressure inside. 
     SUMMARY OF THE INVENTION 
     There is a need in the art to facilitate VUV processing of articles by providing a novel optical system and method that eliminates the requirement for a sealed enclosure. 
     The main idea of the present invention consists of localizing a medium non-absorbing for VUV (e.g. noble gas, such as Nitrogen) along an optical path of light propagation towards and from an illuminated location (point) at the time this location is processed (measured). This enables localizing incident VUV radiation propagating from an optical head assembly towards a processing site on the article outside the optical head assembly and localizing reflected VUV radiation propagation from this site towards the optical head assembly. 
     The term “VUV radiation” signifies radiation of a wavelength shorter than 190 nm. The term “processing” used herein signifies, measuring (e.g., thickness measurements of at least a top layer of the article), inspection, patterning, data reading/recording, etc. A medium non-absorbing VUV radiation will be termed here for simplicity as “noble gas”, but it should be understood that this is a non-limiting example. 
     Thus, according to one aspect of the present invention, there is provided a method for use in optical processing of an article by vacuum UV (VUV) radiation, the method comprising: localizing incident VUV radiation propagation from an optical head assembly towards a processing site on the article outside the optical head assembly and localizing reflected VUV radiation propagation from said processing site towards the optical head assembly by localizing a medium, non-absorbing with respect to VUV radiation, in within the light propagation path in the vicinity of said site outside the optical head assembly. 
     The medium localization (noble gas environment, e.g., Nitrogen) within the light propagation path in the vicinity of the processing site is achieved by continuously substituting air environment by said medium environment within a space between the optical head assembly and the processed site. This is implemented by continuously passing the medium (noble gas) through a hollow tip-like housing accommodated between the optical head assembly and the article with a small gap (e.g., of about 0.5 mm) between said hollow housing and the article. 
     The VUV radiation propagates in the optical head assembly through vacuum or noble gas environment. 
     The level of the VUV non-absorbing medium in the vicinity of the processing site may be controlled by measuring the intensity of the reflected VUV radiation. 
     The sequential processing of successive sites of the article is achieved by providing a relative displacement between the article and the hollow housing of the light guiding assembly. 
     According to another aspect of the present invention there is provided an optical system for use in processing an article, the system comprising:
         an optical head assembly configured for propagating vacuum UV (VUV) radiation from a source of the radiation towards the article and propagating the VUV radiation collected from the article towards a detection unit; and   a VUV radiation guiding assembly that is configured and operable to receive the VUV radiation from the optical head assembly and guide it towards a processing site on the article, and to receive the VUV radiation reflected from the processing site and guide it into the optical head assembly, said guiding assembly comprising inlet and outlet means for a medium, non-absorbing with respect to VUV radiation, to thereby enable continuous passage of said medium therethrough to substitute air environment by said medium environment along a VUV radiation propagation path in between the optical head assembly and the article.       

     Preferably, the optical head assembly comprises a sealed enclosure. The latter may be filled or flushed with a VUV non-absorbing gas, or may be evacuated. The filling/flushed gas is the same as the gas in the VUV radiation guiding assembly. 
     More specifically, the present invention is used for measuring in semiconductor wafers and is therefore described below with reference to this specific application. It should however be understood that the principles of the present invention can be used in various other applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of an optical system according to one embodiment of the invention; and 
         FIG. 2  schematically illustrates an optical system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is schematically illustrated an optical system  10  for use in processing (e.g., measuring) an article  12  (e.g., wafer) by VUV radiation (of less than 190 nm wavelength). The article  12  is located on a stage  13  (e.g., X,Y stage or R-Theta stage) and a suitable drive means DM are provided for relative displacement between the article  12  and the system  10 . In the present example, the drive means are associated with the stage  13  to move it in a horizontal plane relative to the optical system  10 . It should be noted that wafer  12  may be stationary, while the optical system  10  being movable in the horizontal plane. 
     The system  10  comprises an optical head assembly  14 , and a light guiding assembly  16  that is attachable to or integral with the optical head assembly  14 . The optical head assembly  14  includes a sealed enclosure  18  which is either filled or flushed with a noble gas (e.g., Nitrogen) via inlet and outlet means  18 A and  18 B, or evacuated, and includes a light source  20  (e.g., a Deuterium lamp), a light detection unit  22 , and a light directing optics (reflective or refractive). The enclosure  18  at its distal end (closer to the article) is formed with an optical window  19  that allows the light propagation towards and from the article  12 . The light source may be located inside the sealed enclosure  18 , or outside thereof (as shown in the figure in dashed lines) in which case the light is guided from the light source towards the inside of the enclosure via an appropriate waveguide  21 . The detection unit  22  comprises one or more photodetectors, for example, a spectrometric detector and an imaging detector, and may also comprise imaging and/or auto-focusing optics. In the present example, the optical head assembly utilizes the normal incidence, i.e., the same optical path for the incident and reflected light propagation. To this end, a beam splitter  23  is used to spatially separate the incident and reflected light beans. It should, however, be noted that the optical head may alternatively utilize the light incidence and detection at a non-zero angle. 
     The light guiding assembly  16  is associated (attachable or coupled) with the distal end of the sealed enclosure  18 , and includes a hollow housing  24  with an noble gas inlet means  24 A and an opening  24 B at its distal end serving for light propagation and as a gas outlet. The light guiding assembly  16  presents a tip for the local supply and maintenance of noble gas environment along the optical path of light propagation between the optical head assembly  14  and a measurement site S on the article  12 . Preferably, the housing  24  has a conical shape (with an open distal end  24 B) that fits the geometry of incident and reflected light beams. The housing  24  is preferably made of a material that is non-transparent for UV radiation and does not produce particles that may contaminate the wafer. 
     The optical system  10  is located relative to the article&#39;s plane so as to define a small gap (e.g., of about 0.5 mm) between the housing  24  and the article. To this end, as well as for the light focusing purposes, the stage  13  is also preferably mounted for movement along the Z-axis. 
     The system  10  operates in the following manner. During the article displacement relative to the optical system, the noble gas environment is maintained in the sealed enclosure  18 . The noble gas is continuously flushed into the housing  24  to substitute air and flow through the volume of the housing  24  to be output into the gap through the opening  24 B. The noble gas inside the housing  24  thus allows the incident UV light propagation from the optical head to the article and the propagation of the reflected UV radiation to the optical head to be detected. Hence, there is no air above the measurement site S. It should be understood that the dimensions of the housing  24 , as well as those of the inlet and outlet openings and gap between the housing and the article, and the operational mode of gas supply, are appropriately selected to desirably replace air environment by noble gas environment within the region between the optical head and the article along the optical path associated with the currently measured site. It should also be noted that the gas environment conditions may be controlled for example by the intensity of detected light signal. 
       FIG. 2  exemplifies an optical system  100  according to another embodiment of the invention. The system  100 , similarly to system  10 , includes an optical head assembly  14  and a light guiding assembly  16 , and distinguishes from system  10  in that the optical head assembly is a two part assembly defining separate illuminating and detection channels associated with separate sealed enclosures  118 A and  118 B, respectively. Each of the sealed enclosures is either evacuated or has noble gas inlet  18 A and outlet  18 B. One of the sealed enclosures— 118 A in the present example—serves for the propagation of illuminating radiation and is associated with a light source  20 , either internal or external as described above, and the other enclosure  118 B contains a detection unit  22  (e.g., spectrometer, ellipsometer, etc.). The light guiding assembly  16  has a hollow housing  124  appropriately configured to define two portions  124 A and  124 B (separate or integral) that are at one side attachable to or integral with the distal ends of the enclosures  118 A and  188 B, respectively, and approach the measurement site by the other distal side, where an opening  24 B is provided. The housing  124  has one or more gas inlet means  24 A. 
     The system ( 10  or  100 ) may also include a gas collection means in the vicinity of the gap between the article and the light guiding assembly. The system of the present invention can be used as a stand alone measurement station and as an integrated measurement machine for processing articles progressing on a production line, e.g., semiconductor wafers, flat panels, etc. The system may utilize the central Nitrogen supply arrangement of a clean room in the semiconductor FAB. 
     Those skilled in the art will readily appreciate that various modifications and changes can be applied to the embodiments of the invention as hereinbefore described without departing from its scope defined in and by the appended claims.