Abstract:
An alignment tool for use in calibrating an optical bench and/or alignment of an optical system such as a collector optical system for EUV and X-ray applications is disclosed. The optical system includes multiple nested mirrors attached to a mechanical support. The tool includes a mechanical interface plate, a lower reference ring, an upper reference ring and a pinhole member disposed spaced apart axially in sequence; a first positioning device attached to the mechanical interface plate and to the lower reference ring; the first positioning device being adapted for precisely adjusting the position of the lower reference ring in two dimensions; a second positioning device attached to the mechanical interface plate and to the upper reference ring and adapted for precisely adjusting the position of the upper reference ring in two dimensions; a third positioning device attached to the upper reference plate and to the pinhole member and adapted for precisely adjusting the position of the pinhole member in three dimensions; a mechanical interface mounted on or integral with the mechanical interface plate and being substantially identical in form to that of the mechanical support of the optical system.

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of priority under 35 U.S.C. §365 of International Patent Application Serial No. PCT/EP2008/000872, filed on Feb. 04, 2008, designating the United States of America, which in turn claims the benefit of priority of European Patent Application Serial No. EP 07425059.8, Filed on Feb. 02, 2007, with both Applications being incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to optical systems, and more particularly to an alignment tool for such systems, especially collector optical systems, and to the use of such tools. 
     BACKGROUND ART 
     A well known optical design for a collector for X-ray applications is the type I Wolter telescope. The optical configuration of type I Wolter telescopes consists of nested double-reflection mirrors operating at grazing incidence angles. 
     More recently, a variation of the type I Wolter design already proposed for other applications, in which the parabolic surface is replaced by an ellipsoid, has found application for collecting the radiation at 13.5 nm emitted from a small hot plasma used as a source in Extreme Ultra-Violet (EUV) microlithography, currently considered a promising technology in the semiconductor industry for the next generation lithographic tools. 
     A simplified block diagram of an EUV lithography system is shown in  FIG. 1  (PRIOR ART). The ultra-violet source  102  is normally a hot plasma the emission of which is collected by the collector  104  and delivered to an illuminator  106 . The latter illuminates a mask or reticle  108  with the pattern to be transferred to the wafer  110 . The image of the mask or reticle is projected onto the wafer  110  by the projection optics box  112 . 
       FIG. 2  (PRIOR ART) depicts the conceptual optical layout of a known type I Wolter collector  104  for EUV plasma sources. In the nested Wolter I configuration, each mirror is a thin shell consisting of two sections (surfaces). Although many more nested mirrors in the collector optical system  104  may be illustrated, only two ( 202 ,  204 ) are shown. The radiation from the source  206  is first reflected by the hyperbolic surfaces  208 ,  210 , then reflected by the elliptical surfaces  212 ,  214 , and finally focused to an image or intermediate focus  216  on the optical axis  220 . As in the type I Wolter telescope mentioned above, the elliptical ( 212 ,  214 ) and the hyperbolic ( 208 ,  210 ) surfaces share a common focus  218 . For each of the mirrors  202 ,  204 , etc. the different sections on which the surfaces  208 ,  212  are disposed may be integral, or may be fixed or mounted together. 
     In the aforementioned optical systems (for EUV and X-ray applications, mainly in the medical, astronomical and lithographical fields), a series of nested grazing incidence mirrors (mainly elliptical and Wolter I) are co-aligned, one with respect to the other, and all with respect to their mechanical support. 
     The alignment respect the mechanical support is very important because, when met, it assures that the entire optical system, when positioned in the machine (e.g. lithography system) for which it is designed, is automatically aligned, and no complex additional alignment systems and no additional alignment processes are required. 
     This plug-in capability is particularly useful when lithographic applications are concerned because the optical system must be replaced at frequent intervals, and because the machine downtime must be minimized during 7 h/day, 7 days/week mass production cycles. 
     A problem with known systems is how to provide mounting of the mirrors of the optical system with respect to each other and to the mechanical support, so that the mirrors are fixed to the mechanical support in aligned configuration. 
     A further problem with existing systems is that once the mechanical support is mounted in the machine, further, post-mounting, alignment of the optical system is usually required. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to address the aforementioned and other issues. 
     According to one aspect of the invention there is provided an alignment tool for calibrating an optical bench and/or use in alignment of an optical system, the optical system including multiple nested mirrors attached to a mechanical support, the tool comprising: a mechanical interface plate, a lower reference ring, an upper reference ring and a pinhole member, the mechanical interface plate, lower reference ring, upper reference ring and pinhole member being disposed spaced apart axially in sequence; a first positioning device, attached to the mechanical interface plate and to the lower reference ring; the first positioning device being adapted for precisely adjusting the position of the lower reference ring in two dimensions; a second positioning device, attached to the mechanical interface plate and to the upper reference ring; the second positioning device being adapted for precisely adjusting the position of the upper reference ring in two dimensions; a third positioning device, attached to the upper reference ring and to the pinhole member; the third positioning device being adapted for precisely adjusting the position of the pinhole member in three dimensions; a mechanical interface, the mechanical interface being mounted on or integral with the mechanical interface plate and being substantially identical in form to that of the mechanical support of the optical system. 
     Preferably, in use, the axis of the tool is vertical. 
     Preferably, the first positioning device comprises an x,y translation stage. Preferably, the second positioning device comprises an x,y translation stage. Preferably, the third positioning device comprises an x,y,z translation stage. 
     Preferably, the mechanical interface comprises three or more interface elements, for mechanically abutting, in use, an optical bench or other optical equipment. Preferably, the interface elements comprise v-grooves or balls having integral projections. 
     Preferably, the spacing of the pinhole member to the mechanical interface is about 200 mm to about 700 mm, and is more preferably about 500 mm. Preferably, the spacing of the lower reference ring to the mechanical interface is about 50 mm to about 200 mm, and is more preferably about 100 mm. Preferably, the spacing between the two reference rings is about 100 mm to about 300 mm, and is more preferably about 200 mm. Preferably, the pinhole member includes a pinhole of about 5 um to about 100 um diameter, more preferably about 20 um diameter. Preferably, the two reference rings have a diameter of about 100 mm to about 800 mm diameter, more preferably about 500 mm (lower reference ring) and 400 mm diameter (upper reference ring). 
     According to another aspect of the invention there is provided an alignment tool for calibrating an optical bench and/or use in alignment of an optical system, the optical system including multiple nested mirrors attached to a mechanical support, the tool comprising: a first plate, an intermediate ring and a second plate, the first plate, intermediate ring and second plate being disposed spaced apart axially in sequence; a first positioning device, attached to the first plate and to a first pinhole member; the first positioning device being adapted for precisely adjusting the position of the first pinhole member in three dimensions; a second positioning device, attached to the second plate and to a second pinhole member; the second positioning device being adapted for precisely adjusting the position of the second pinhole member in two dimensions; a mechanical interface, the mechanical interface being mounted on or integral with the intermediate ring and being substantially identical in form to that of the mechanical support of the optical system. 
     Preferably, in use, the axis of the tool is vertical, and the first plate is disposed uppermost and the second plate is disposed lowermost. Preferably, the first positioning device comprises an x,y,z translation stage. Preferably, the second positioning device comprises an x,y translation stage. 
     Preferably, the mechanical interface comprises three or more interface elements, for mechanically abutting, in use, an optical bench or other optical equipment. Preferably, the interface elements comprise balls having integral projections. 
     Preferably, the spacing of the first pinhole member to the mechanical interface is about 100 mm to about 500 mm, and is more preferably about 300 mm. Preferably, the spacing of the first pinhole member to the second pinhole member is about 300 mm to about 1000 mm, and is more preferably about 500 mm. Preferably, the first pinhole member includes a pinhole of about 5 um to about 100 um diameter, more preferably about 20 um diameter. Preferably, the second pinhole member includes a pinhole of about 0.3 mm to about 2 mm diameter, more preferably about 1 mm diameter. Preferably, the intermediate ring has a central aperture of about 50 mm to about 300 mm diameter, more preferably about 100 mm diameter. 
     An advantage of the invention is that it assures plug-in mounting of the optical system on the lithography facility, avoiding additional adjustments. 
     A further advantage of the invention is that the optical system (that comprises many mirrors fixed to the mechanical interface) can be dismounted from the optical bench and plugged straight into the lithographic tool. It is automatically aligned within the required tolerances. 
     A further advantage of the invention is that the alignment tool affords essentially guaranteed alignment between the optical axes of each mirror (shell) and the interface of the mechanical support, within small tolerances. 
     Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (PRIOR ART) shows a simplified block diagram of an EUV lithography system; 
         FIG. 2  (PRIOR ART) depicts the conceptual optical layout of a known type I Wolter collector, for use in the EUV lithography system of  FIG. 1 ; 
         FIG. 3  is a perspective side view of the alignment tool according to a first embodiment of the invention; 
         FIG. 4  is a perspective plan view of the alignment tool of  FIG. 3 ; 
         FIG. 5  shows the alignment tool of  FIG. 3 , when mounted on an optical bench during the optical alignment method according to the invention; 
         FIG. 6  shows highly schematically in successive steps the method of alignment according to one embodiment of the invention; 
         FIG. 7  is a perspective side view of the alignment tool according to a second embodiment of the invention; 
         FIG. 8  is a perspective plan view of the alignment tool of  FIG. 7 ; 
         FIG. 9  shows the alignment tool of  FIG. 7 , when mounted on an optical bench during the optical alignment method according to the invention; and 
         FIG. 10  illustrates the actual form of the mechanical support in accordance with one embodiment of the invention, and having Wolter I mirrors attached thereto. 
     
    
    
     In the description and drawings, like numerals are used to designate like elements. Unless indicated otherwise, any individual design features and components may be used in combination with any other design features and components disclosed herein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the illustrations of optical elements or systems herein, unless indicated otherwise, cylindrical symmetry around the optical axis is assumed; and references to an “image focus” are references to an image focus or to an intermediate focus. 
     In one aspect, the invention consists in the alignment tool that is used during the process of assembling the mirrors of an optical system (optical co-alignment and fixation to the mechanical support), performed on a dedicated optical bench. 
       FIG. 3  is a perspective side view of the alignment tool  300  according to a first, preferred embodiment of the invention; and  FIG. 4  is a perspective plan view of the alignment tool  300  of  FIG. 3 . 
     Referring to  FIGS. 3 and 4 , the alignment tool  300  consists of a mechanical structure that supports the pinhole member  302  (pinhole member  302  with a upper pinhole  306  of diameter, e.g., 20 um). The pinhole member  302  is made of any metal (e.g. stainless steel or aluminum) suited to optical applications. The same applies for all other components (excluding the optical system, which may be made by electroforming nickel, or the like) discussed hereinafter, unless indicated otherwise. 
     Upper pinhole member  302  is mounted on a precise x,y,z translation stage  310 , which is in turn mounted on an upper reference ring  311  (outer diameter, e.g., 400 mm). In this embodiment, the pinhole member  302  is positioned such that the spacing between the pinhole  306  and the mechanical interface plate  314  is (vertically) 500 mm. 
     In use, the upper reference ring  311  is also mounted on a precise x,y translation stage  322  via base ring  304  and support rods  305 . The alignment tool  300  includes a lower reference ring  313  (outer diameter, e.g., 500 mm). In use, the lower reference ring  313  is also mounted on a precise x,y translation stage  312 . 
     The alignment tool  300  includes mechanical interface plate  314  (outer diameter, e.g., 600 mm). Provided on the mechanical interface plate  314  is the interface of the alignment tool  300 , e.g., comprised collectively of three reference v-grooves members  318 . The latter are formed such that the interface of the alignment tool  300  is identical to the interface (not shown) of the mechanical support of the mirrors (collector optical system). Typically, the v-grooves members  318  are mounted on blocks  319  on the mechanical interface plate  314 . A indicates the optical axis. It will be appreciated that, for example, reference balls, with or without projections, may be used in place of v-groove members. 
       FIG. 5  shows the alignment tool of  FIG. 3 , when mounted on an optical bench  500  during the optical alignment method according to the invention. The optical alignment method that involves the use of the alignment tool  300  will now be described, with reference to  FIGS. 5 and 6  (the latter being an extremely schematic illustration, for the purpose of explanation).
         1. As an initial step ( FIG. 6(   a )), the pinhole  306 , the lower reference ring  313  and the upper reference ring  311  are aligned to the three v-groove members  318  by means of their translation stages  310 ,  312  and  322 , respectively, and by the use, for example, of a 3D coordinate machine (the pinhole  306  is aligned in x,y,z; the lower reference ring  313  and the upper reference ring  311  in x,y).   2. Next ( FIG. 6(   b )), the alignment tool  300  is positioned on the optical bench  500  (i.e., the one that will be used later to co-align the mirrors of the collector optical system (not shown)).   3. Then, a laser source  602  ( FIG. 6(   c )) disposed above the optical bench  500 , and directing radiation substantially in the direction of arrow B, is aligned in x, y, z by maximizing the amount of light passing through the pinhole  306 .   4. Next ( FIG. 6(   d )), micrometers  506  and  508  (that will be used to measure and control the transverse alignment of the mirrors) are positioned on the optical bench  500  and are then aligned with respect to the reference rings  311  and  313  of the alignment tool  300 . The upper reference ring  311  and lower reference ring  313  are co-aligned to the v-grooves  318 . These rings  311 ,  313  are the reference used to calibrate the zero of the micrometers  506  and  508 : the micrometers  506  ensure the radial position of the upper reference ring  311  and micrometers  508  ensure the radial position of the lower reference ring  313 .   5. The alignment tool  300  is then removed from the optical bench  500 .   6. As a next step ( FIG. 6(   e )), the mechanical support  604  of the mirrors is placed on the optical bench  500 .   7. A CCD camera  608  is then placed in the area Z (below the optical bench  500 ) where the focus of the mirrors (not shown) will arrive (see  FIG. 6(   f )). It is aligned along the optical axis A by placing it at the nominal distance (e.g., 2 m) from the laser source  602 .   8. Then ( FIG. 6(   g )), a reference mirror  610  is placed on the optical bench  500 , aligned relatively to the micrometers  506  and  508  and relative to the laser source  602 .   9. The CCD camera  608  is then aligned in x, y directions by centering the focus of the reference mirror  610  in the center of the CCD sensor.   10. Next, the reference mirror  610  is removed from the optical bench  500 .   11. Then, the mirrors  612  that will form the collector optical system  614  are placed on the optical bench  500 , aligned to the mechanical interface (not shown) of the mechanical support  604 , and fixed to the mechanical support  604 ; see  FIG. 6(   h ).
           The mirrors  612  are attached in sequence, by any suitable means, to the mechanical support, starting with the smallest (innermost) mirror.  FIG. 10  illustrates the actual form of the mechanical support  604  in accordance with one embodiment of the invention, and having Wolter I mirrors attached thereto. The mechanical support  604 , usually called a “spider”, resembles a bicycle wheel. The mechanical support  604  is composed of a cylindrical external ring  1002  having arms  1004  connected to a smaller central ring  1006 . A set of v-grooves  1008  (here: three) is attached to the external ring  1002 . The outer diameter of the upper reference ring  311  (see  FIGS. 3-5 ) is equal to the diameter of the intermediate section  1010  of the largest (outer) mirror  612 ′ to be aligned, while the outer diameter of the lower reference ring  313  is equal to the diameter of the lower section  1012  of the largest (outer) mirror  612 ′ to be aligned. For the attachment of each mirror  612 , the mirror is supported from the bottom at three points; it is moved (e.g. millimetre distances or less) in x,y,z (3 translations) and theta, phi (2 rotations) by means of precise translation stages and micrometers attached directly to the optical bench  500 . The position of the mirror is controlled by the readings of the 4 micrometers  508  for the x,y values and by the readings of the focus image recorded by a ccd camera  608  for the z, theta, phi values.   
           12. Finally (returning to  FIG. 6(   i )), the collector optical system  614 , that now comprises many mirrors fixed to the mechanical support  604 , is dismounted from the optical bench  500  and may be plugged into the lithographic tool (not shown). The optical system  614  is automatically aligned within the required tolerances.       

     The tolerances are, for example:
         longitudinal position of source&lt;20 um;   transverse position of source&lt;20 um;   longitudinal position of mirror&lt;50 um;   transverse position of mirrors&lt;50 um;   tilt of mirror&lt;0.5 mrad;   longitudinal position of second focus of mirrors&lt;2.5 mm; and
 
transverse position of focus of mirrors&lt;0.5 mm.
       

       FIG. 7  is a perspective side view of the alignment tool  700  according to a second embodiment of the invention; and  FIG. 8  is a perspective plan view of the alignment tool  700  of  FIG. 3 . 
     Referring to  FIGS. 7 and 8 , the alignment tool  700  consists of a mechanical structure that supports two pinholes members  702 ,  704  (upper pinhole member  702  with a upper pinhole  706  of diameter, e.g., 20 um; lower pinhole member  704  with a lower pinhole  308  of diameter, e.g., 1 mm). 
     Upper pinhole member  702  is mounted on a precise x,y,z translation stage  710 , which is in turn mounted on an upper plate  711 ; and lower pinhole member  704  is mounted on a precise x,y translation stage  712 , which is in turn mounted on a lower plate  713 . In this embodiment, the pinholes members  702 ,  704  are positioned such that the pinholes are (vertically) 500 mm apart. 
     The alignment tool  700  also includes a reference ring  714  (inner diameter, e.g., 100 mm). In use, the reference ring  714  is also mounted on a precise x,y translation stage (not shown). Provided on the reference ring  714  is the interface  716  of the alignment tool  700 , e.g., comprised collectively of three reference balls  718 . The latter are formed such that the interface  716  of the alignment tool  700  is identical to the interface (not shown) of the mechanical support of the mirrors (collector optical system). Typically, the reference balls  718  are mounted on blocks  720  attached (e.g. by bolting) to the reference ring  714 , and include rods  722  projecting (parallel to the optical axis A) therefrom. It will be appreciated that, for example, v-groove members, with or without projections, may be used in place of reference balls. 
       FIG. 9  shows the alignment tool of  FIG. 7 , when mounted on an optical bench  900  during the optical alignment method according to a second embodiment of the invention. The optical alignment method that involves the use of the alignment tool  700  will now be described. This is the same as the method described with reference to  FIGS. 5 and 6 , except as described below.
         1. As an initial step, the upper pinhole  706 , lower pinhole  708  and the reference ring  714  are aligned to the three reference balls  716  by means of their translation stages  710 ,  712  and by the use, for example, of a 3D coordinate machine (upper pinhole  706  is aligned in x,y,z; lower pinhole  708  and reference ring  714  in x,y).   2. Next, the alignment tool  700  is positioned on the optical bench  900  (i.e., the one that will be used later to co-align the mirrors of the collector optical system (not shown)).   3. Then, a laser source (not shown) disposed above the optical bench  900 , and directing radiation substantially in the direction of arrow B, is aligned respect to the upper pinhole  706  by maximizing the amount of light passing through the upper pinhole  706 .   4. A CCD camera (not shown) is then placed in the area Z (below the optical bench  900 ) where the focus of the mirrors (not shown) will arrive. It is aligned by centering the light beam transmitted (substantially in the direction of arrow C) through the lower pinhole (not shown).   5. Micrometers (that will be used to measure and control the transverse alignment of the mirrors) positioned on the optical bench  900  are then aligned respect to the reference ring  714  of the alignment tool  700 .   6. Next, the alignment tool  700  is removed from the optical bench  900 ;   7. As a next step, the mechanical support (not shown) of the mirrors is placed on the optical bench  900 , the mechanical interface (not shown) of the mechanical support (not shown), engaging tags  902  formed integrally with support plate  904  of the optical bench  900 .   8. Then, the mirrors (not shown) are placed on the optical bench  900 , aligned to the mechanical interface (not shown) of the mechanical support (not shown), and fixed to the mechanical support (not shown).   9. Finally, the optical system (not shown), that now comprises many mirrors fixed to the mechanical support (not shown), can be dismounted from the optical bench  900  and may be plugged into the lithographic tool (not shown). The optical system is automatically aligned within the required tolerances.       

     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.