Abstract:
A moving photoconverter device that converts an incident light image into an equivalent electron or other charged particle beam image. The moving photoconverter is ring shaped and is rotated by using a motor such that the incident light image exposes a moving photoconverter surface. The photoconverter may additionally or alternatively move in an X-Y motion or radially. Continuous regeneration is provided at a site remote from the region of moving photoconverter device that converts an incident light image into an equivalent electron or other charged particle beam image.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to charged particle beam columns, and more specifically to a system for generating charged particle beams from a light image. 
     2. Description of The Related Art 
     In the field of electron beam (or charged particle beam) lithography, a beam of, e.g., electrons from an electron source is directed onto a substrate. The electrons expose a resist layer (in this case an electron sensitive resist) on the substrate surface. Typically electron beam lithography is used for making masks; however it can also be used for direct exposure of semiconductor wafers. 
     One technique to generate an electron beam image is to expose a photocathode with a light image, whereby the photocathode converts the light image into a demagnified electron beam image. One problem with the use of a photocathode is variations in the photon-to-electron conversion ratio of the photocathode, which in turn cause dose errors in a charged particle image written on the mask or wafer. Hereafter “electron conversion factor” means photon-to-electron conversion ratio of the photocathode. Such variations are due, for example, to particulate contamination, chemical contamination, and overexposure of a local area of the photocathode. 
     Thus what is needed is a method and apparatus to generate charged particle beam images using a photocathode with reduced variations in the electron conversion factor. 
     SUMMARY 
     One embodiment of the present invention provides a beam conversion system that includes: a photoconverter on which is incident a light image, that converts the light image into a charged particle beam; and a displacement device coupled to move the photoconverter, where the light image illuminates a moving surface of the photoconverter. 
     Thereby an embodiment of the present invention provides a method of generating a light image, the method including the acts of: generating a light image; directing the image onto a moving photoconverter, whereby the image is incident on a moving surface of the photoconverter device; and converting, at the photoconverter device, the image into an electron emission pattern which is imaged on the surface of a mask or wafer. 
     Various embodiments of the present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 depicts schematically an electron beam lithography system  100  in accordance with one embodiment of the present invention. 
     FIG. 2 depicts in side view, one embodiment of photoconverter system  104 , as well as electron optics system  107 , and a substrate  106 . 
     FIG. 3 depicts a ring shaped photocathode  304  in accordance with one embodiment of the present invention. 
     FIG. 4 depicts a cross sectional view of a photocathode  304  of FIG. 3 along line A—A in accordance with one embodiment of the present invention. 
     FIG. 5 depicts schematically a differentially pumped regeneration source  370 , used in place of regeneration source  308 , in one embodiment. 
     FIG. 6 depicts a relationship between a top side view of photocathode  304  and the opening of nozzle  360 . 
     Note that use of the same reference numbers in different figures indicates the same or like elements. 
    
    
     DETAILED DESCRIPTION 
     System Overview 
     FIG. 1 depicts schematically an electron beam lithography system  100  in accordance with one embodiment of the present invention. System  100  includes a light illumination system  102  and a photoconverter system  104 . Light illumination system  102  outputs a light image  103  by, for example, exposing an optical mask with light. In accordance with one embodiment of the present invention, photoconverter system  104  converts the light image  103  into an electron emission pattern  105 , whose demagnified image  108  is projected onto substrate  106  by electron optical system  107 . 
     Suitable Illumination System  102   
     A suitable illumination system  102  is any device that generates a light image. For example, a suitable conventional illumination system includes an illumination source that exposes an image defining mask and a lens that focuses the defined image onto the photoconverter system  104 . 
     Another suitable illumination system  102  is a laser system, i.e., a photolithographic device which creates one or more focused and modulated laser beams. One such laser system is described in U.S. Pat. Application Ser. No. 08/769,169, entitled “Short Wavelength Pulsed Laser Scanner”, inventors Paul C. Allen et al., filed Dec. 18, 1996, attorney docket no. M-4485 US; and U.S. Pat. Nos. 5,255,051, issued Oct. 19, 1993, to Paul C. Allen, 5,327,338, issued Jul. 5, 1994, to Paul Allen, et al., and 5,386,221, issued Jan. 31, 1995, to Paul C. Allen, et al., all incorporated herein by reference in their entirety. 
     Photoconverter System  104   
     In accordance with one embodiment of the present invention, FIG. 2 depicts in side view, one embodiment of photoconverter system  104 , as well as electron optics system  107 , and a substrate  106 . Photoconverter system  104  includes a photocathode  304  that converts light image  103  into electron emission pattern  105 ; a motor  306  that revolves photocathode  304 ; extraction electrode  320 ; and a regeneration source  308  that regenerates a portion of photocathode  304 . Electron optics system  107  projects a demagnified image  108  of emission pattern  105  onto substrate  106 . 
     In this embodiment, a vacuum casing  318  encloses photoconverter system  104 , electron optics system  107 , and substrate  106 . A conventional vacuum pump device (not depicted) controls the pressure within the vacuum casing  318 . The vacuum casing  318  includes a transparent window  324  that is aligned coaxial with light image  103  and allows light image  103  to expose photocathode  304 . 
     FIG. 3 depicts a ring shaped photocathode  304  in accordance with this embodiment. A suitable radius R of photocathode  304  is approximately 3 to 5.5 cm. FIG.  4  depicts a cross sectional view of a photocathode  304  along line A—A of FIG.  3 . Photocathode  304  includes a conventional substrate layer  330 , being a transparent material such as, e.g., sapphire or quartz, on a photoemission layer  332  being, e.g., gold, tin oxide, or cesium iodide (CsI). A suitable thickness T of photocathode  304  is approximately 1 mm to 5 mm. 
     Referring to FIG. 2, photocathode  304  absorbs the photons of incident image  103  from light illumination system  102  and thereby causes electrons present in the photoemission layer  332  to be excited above the vacuum level. Electrons which gain sufficient energy to escape from the surface of photoemission layer  332  are emitted in the -Z direction from the photoemission layer  332 . With respect to photoconverter system  104 , the term “downstream” means along the (-)Z direction from photocathode  304 . 
     Extraction electrode  320  is positioned in the -Z direction from photocathode  304 , and coaxial with the path of beam  103 . In other embodiments, a conventional field lens could be used in conjunction with extraction electrode  310 . Hereafter the term “extraction device” refers to extraction electrode  320  with or without a field lens unless otherwise specified. A suitable implementation of the extraction device is described in U.S. patent application Ser. No. 09/272,086, entitled “Compact Photoemission Source, Field And Objective Lens Arrangement For High Throughput Electron Beam Lithography”, filed Mar. 18, 1999, inventors Veneklasen and Mankos, now U.S. Pat. No. 6,315,128, which is incorporated by reference in its entirety. 
     When a voltage (typically tens of kilovolts) is applied to the extraction device, the extraction device extracts the electrons which have escaped from the photoemission layer of photocathode  304  and accelerates them to generate electron image  108  of the emission pattern  105  on the photocathode. 
     In this embodiment, photocathode  304  is mounted to a rotating spindle  326  driven by motor  306 , located within the vacuum casing  318  of photoconverter system  104 . Motor  306  rotates ring-shaped photocathode  304  about axis  322 , so that incident light image  103  exposes a ring-shaped area of the rotating photocathode  304 . 
     In one embodiment, motor  306  is located outside of the vacuum casing  318  of photoconverter  104 . Spindle  326  of motor  306  is divided into two portions. A portion of spindle  326  is external to the vacuum casing  318  (“external portion”) and directly coupled to motor  306  and a portion of spindle  326  is inside the vacuum casing (not shown) (“internal portion”) The external and internal portions are separated by a non-magnetic thin membrane being, e.g., stainless steel foil. The external and internal portions are coupled by a permanent magnet so that they rotate at the same rate. Thereby, motor  306 , located outside of the vacuum casing, rotates the photocathode  304 . 
     The purpose of rotating photocathode  304  is to average variations in the level of electron emissions from photocathode  304 . Such variations are due, e.g., to local defects on the photoemission layer  332  such as particulate or chemical contamination and overexposure of a local area. Such defects cause variations in the electron conversion factor of the photocathode  304 , which in turn cause dose errors in an electron image  108  written on the substrate  106 . When the rotating photocathode is illuminated by light image  103  to generate a single pixel imaged on the substrate  106  (hereafter image pixel), the effective electron conversion factor to generate the image pixel is the average of the electron conversion factors of the pixel areas of the photocathode illuminated by light image  103  during exposure of the pixel. 
     The photocathode  304  should move fast enough so that each image pixel on the substrate receives its exposure from many corresponding pixel areas on the photocathode  304 , where a pixel area on the photocathode  304  is the larger than an image pixel by, e.g., the square of the demagnification factor of the electron optics system  107 . A typical range of a demagnification factor of the electron optics system  107  is 3 to 10 times. 
     For example, if photocathode  304  moves at linear speed of 5 cm/s and a pixel on the photocathode  304  is exposed for a duration of 5 ms, then the photocathode has moved 0.025 cm. If the size of an image pixel on the photocathode is 1 μm and is demagnified 10 times to expose a 0.1 μm size pixel on the substrate  106 , 250 corresponding pixel areas on photocathode  304  are used to expose each image pixel. 
     Conventional regeneration source  308  (FIG. 2) is located in the -Z direction from photocathode  304 , but is offset from the path of image  103 , but interior to the outer edge of photocathode  304 . The region coaxial and immediately downstream of emission pattern  105  becomes available to include additional componentry such as extraction electrode  320  and does not have to accommodate a regeneration source. The length of the photoconverter system  104  can be reduced, thereby reducing electron-electron interactions which can cause blurring of the electron image  108 . 
     Regeneration source  308  continuously or periodically regenerates a portion of photoemission layer  332  of photocathode  304 . Regenerating the photocathode stabilizes the photocathode&#39;s “electron conversion factor” at an optimum value. In this embodiment, regeneration source  308  regenerates a part of the photoemission layer  332  by, e.g., sputtering; depositing of additional photoemission layer material; molecular beam epitaxial deposition; ion beam deposition; condensation from gas; exposure to gas; exposure to a molecular beam; or plasma exposure. A suitable implementation of regeneration source  308  is a source  309  that outputs molecules  311  of, for example, Cesium. In one embodiment, regeneration source  308  includes a nozzle  310  that controls the shape of the area exposed by the regeneration source  308 . 
     In one embodiment, an electron optical lens system  107 , shown schematically in FIG. 2, is used to demagnify the emission pattern  105  and focus the demagnified emission pattern  105  to an image  108  on the substrate  106 . In this embodiment, the electron optical lens system  107  is positioned in the -Z direction from the photocathode  304  and coaxial with emission pattern  105 . In this embodiment, electron optical lens system  107  and the substrate  106  are located within vacuum casing  318 . 
     In another embodiment, electron optical lens system  107  is positioned within the vacuum casing while the substrate  106  is positioned within a separately pumped vacuum casing. A suitable implementation of electron optical lens system  107  is described in U.S. patent application Ser. No. 09/272,086, filed Mar. 18, 1999, entitled “Compact Photoemission Source, Field And Objective Lens Arrangement For High Throughput Electron Beam Lithography”. 
     FIG. 5 depicts schematically a differentially pumped regeneration source  370 , used in place of regeneration source  308 . In this embodiment, there is a small gap between the regeneration source nozzle  360  and photocathode  304 . Another conventional vacuum pumping device  362  is coupled to pump the regeneration region  364 . This differential pumping arrangement allows the pressure in the regeneration region  364  to be higher than that in region  358 ; within vacuum casing  318  (FIG.  2 ), in which the electron emission pattern  105  is formed. This is desirable when byproducts of the regeneration source  370 , e.g., water and gas, are undesirable on the rest of the photocathode surface  304 . For example, regeneration source  370  is used where plasma deposition or condensation are applied. In this embodiment a suitable pressure in regeneration region  364  and region  358  are respective 1×10 −8  Torr and 1×10 −10  Torr. 
     In one embodiment, the opening of nozzle  360  is trapezoid shaped to compensate for unequal exposure dose of the photocathode due to differing linear velocities along increasing radii from the axis of rotation. FIG. 6 depicts a top side view of photocathode  304  (from the -Z direction in FIG.  2 ), the trapezoid shaped opening  366  of nozzle  360 , and emission pattern  105 . 
     When photocathode  304  moves, it is possible to regenerate photocathode  304  while simultaneously using another segment of photocathode  304  to convert incident light into an electron beam. Thereby, by both moving photocathode  304  and regenerating the photocathode;, the photocathode&#39;s “electron conversion factor” is continuously stabilized at an optimum value and the photocathode&#39;s electron conversion factor is averaged. 
     In one embodiment, photocathode  304  is simultaneously rotated and rotating spindle  326  moved within the plane of photocathode  304  so that photocathode  304  moves radially to and from the beam axis and incident image  103  exposes regions at varying radii from the center of photocathode  304 . This allows emission to be averaged over a larger area, providing a longer cathode life. 
     In one embodiment, photocathode  304  is moved in an X-Y direction, within the plane of the photocathode  304 , by for example a conventional X-Y stage. 
     Thus by use of a displacement device such as motor  306  or a conventional stage, photocathode  304  is moved while illuminated. 
     The above-described embodiments are illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as fall within the scope of this invention.